MATERIAL PROPERTIES OF BILAMINAR
POLYMETHYLMETHACRYLATE CEMENT
MANTLES IN REVISION HIP
ARTHROPLASTY
By
P. C. Weinrauch M.B., B.S. (Qld)
A Thesis for the Degree of Master of Engineering
School of Engineering Systems
Queensland University of Technology
August, 2006
ii
Keywords Hip, Arthroplasty, Revision, Cement, Polymethylmethacrylate, Adhesion.
iii
Abstract Cement – within – Cement (C-C) revision techniques have been demonstrated to
reduce the complications associated with removal of secure cement from the femoral
canal during revision hip joint arthroplasty. Material failure at the interface between new
and old cement mantles represents a theoretical limitation of this technique. The
objectives of this thesis are to describe the variability in material properties of uniform
and bilaminar polymethylmethacrylate (PMMA) cement mantles in shear with respect to
duration of post-cure and the influence of commercial inclusion of antibiotics on
bilaminar cement mantle interfacial shear strength.
Uniform mantles of Surgical Simplex P and Antibiotic Simplex PMMA cements
demonstrated variability in ultimate shear stress to failure with respect to duration of
post-cure (p<0.001), however the variations were quantitatively small and unlikely to be
of clinical relevance. Bilaminar cement mantles were 15 – 20 percent weaker than
uniform mantles (p<0.001) and demonstrated similar time dependant material property
variations in shear (p<0.001). Bilaminar PMMA test specimens manufactured using
Antibiotic Simplex cement demonstrated equivalent ultimate shear stress to failure as
bilaminar specimens manufactured from Surgical Simplex (p=0.52). High C-C
interfacial strengths are demonstrated as early as one hour after cement application.
Interfacial adhesion by mechanisms other than mechanical interlock significantly
influence the bond formed between layered PMMA cements, with an important
contribution by diffusion based molecular interdigitation.
In the presence of a secure cement-bone interface, C-C femoral revision can be
recommended as a viable technique on the basis of the strong interfacial bond formed
between new and old cement mantles. The use of Antibiotic Simplex in C-C revision is
recommended as detrimental effects on the interfacial shear properties have not been
demonstrated with the commercial addition of Tobramycin.
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Table of Contents Keywords ........................................................................................................................... ii
Abstract............................................................................................................................. iii
Table of Contents ............................................................................................................. iv
List of Figures.................................................................................................................. vii
List of Tables .................................................................................................................. xiv
List of Equations ............................................................................................................ xvi
List of Symbols .............................................................................................................. xvii
List of Abbreviations ...................................................................................................xviii
Glossary of Terms ........................................................................................................... xx
Statement of Originality ..............................................................................................xxiii
Acknowledgements....................................................................................................... xxiv
Dedication ...................................................................................................................... xxv
Chapter 1 Introduction .................................................................................................... 1
1.1 Overview of Research and Clinical Application............................................... 1
1.2 Research Objectives and Hypotheses................................................................ 3
1.3 Thesis Overview................................................................................................ 4
Chapter 2 Literature Review........................................................................................... 7
2.1 PMMA Cement in Primary and Revision Hip Joint Arthroplasty .................... 7
2.1.1 PMMA cement in primary total hip arthroplasty ........................................ 7
2.1.2 PMMA cement in hip joint hemiarthroplasty............................................ 10
2.1.3 Revision hip arthroplasty and cement removal ......................................... 16
2.2 Cement – Within - Cement Femoral Revision................................................ 23
2.2.1 Clinical application.................................................................................... 23
2.2.2 Radiographic assessment of the bone – cement interface ......................... 28
2.2.3 Prosthetic femoral components designed for C-C revision....................... 38
2.3 Composition and Material Properties of PMMA Cement............................... 41
2.4 Properties of PMMA Cement and Bilaminar Mantles in Shear...................... 50
2.5 Summary of Literature Review and Relevance to Thesis............................... 63
Chapter 3 Experimental Evaluation of the Shear Properties of PMMA cements ... 67
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3.1 Introduction ..................................................................................................... 67
3.2 Hypotheses ...................................................................................................... 67
3.3 Method ............................................................................................................ 67
3.3.1 Equipment design considerations and objectives ...................................... 67
3.3.2 Overview of experimental equipment and outline of applications............ 68
3.3.3 Test specimen manufacture ....................................................................... 69
3.3.4 Evaluation of PMMA shear properties...................................................... 73
3.3.5 Testing schedule and data analysis............................................................ 74
3.3.6 Testing method validation and shear modulus calibration ........................ 77
3.4 Results ............................................................................................................. 78
3.4.1 Sawbones reference and shear modulus calibration.................................. 78
3.4.2 Uniform PMMA test specimen shear properties ....................................... 79
3.5 Discussion ....................................................................................................... 81
3.6 Conclusions ..................................................................................................... 83
Chapter 4 Experimental Evaluation of the Shear Properties of Bilaminar Cement
Mantles ................................................................................................................. 85
4.1 Introduction ..................................................................................................... 85
4.2 Hypotheses ...................................................................................................... 85
4.3 Method ............................................................................................................ 86
4.3.1 Bilaminar test specimen manufacture and testing schedule ...................... 86
4.3.2 Bilaminar test specimen evaluation of shear properties ............................ 89
4.4 Results ............................................................................................................. 89
4.4.1 Surface characteristics & fracture evaluation by LM and SEM................ 89
4.4.2 Bilaminar PMMA test specimen shear properties..................................... 94
4.5 Discussion ....................................................................................................... 97
4.6 Conclusions ................................................................................................... 101
Chapter 5 Conclusions and Recommendations ......................................................... 105
5.1 Application of Research Findings to Clinical Practice ................................. 105
5.2 Further Research ........................................................................................... 105
Appendix 1 ..................................................................................................................... 109
Appendix 2 ..................................................................................................................... 117
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Appendix 3 ..................................................................................................................... 119
References ...................................................................................................................... 131
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List of Figures
Figure 2-1 Cumulative risk for revision of THA by cement status excluding infection
(Australian Orthopaedic Association National Joint Replacement Registry 2004)...9
Figure 2-2 Age and sex distribution for hip joint hemiarthroplasty (Australian
Orthopaedic Association National Joint Replacement Registry 2004)....................11
Figure 2-3 Cumulative risk for revision of UAM and CT prostheses (Australian
Orthopaedic Association National Joint Replacement Registry 2004)....................13
Figure 2-4 Increasing incidence of acetabular disease over time with patients managed
with hip hemiarthroplasty (Yamagata et al. 1987)...................................................15
Figure 2-5 Diagnosis requiring revision hip replacement (Canadian Institute for Health
Information 2004). ...................................................................................................17
Figure 2-6 Extraction of cemented femoral component in line of implant long axis
(Hozack 1998)..........................................................................................................19
Figure 2-7 Surgical technique of C-C femoral revision (Stryker International)..............24
Figure 2-8 Coronal plane Gruen Zones for the standardised assessment of the femoral
component bone – cement interface (Gruen et al. 1979). ........................................30
Figure 2-9 Gruen modes of radiographic failure of cemented femoral THA components
(for explanation see text) (Harkess 1999). ...............................................................31
Figure 2-10 Radiograph demonstrating normal subsidence of an Exeter stem, with
cement-prosthesis debonding visible in Gruen Zone 1 (courtesy of R. Crawford). 32
Figure 2-11 Normal subsidence of a collarless cemented polished double taper femoral
component (Yates et al. 2002). ................................................................................32
Figure 2-12 Gruen Mode IB failure (compound subsidence). Radiolucency in Zone 1
representing distal migration of the cement – prosthesis composite, with distal
radiolucency and pedestal formation (Harkess 1999)..............................................33
Figure 2-13 Gruen Mode II failure (medial stem pivot). Note varus migration of the stem
with separation of the cement – prosthesis interface in zones 1 and 2 (Harkess
1999). .......................................................................................................................34
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Figure 2-14 Gruen Mode III failure (calcar pivot). Initial postoperative radiograph
depicted on the left for comparison. Progressive varus migration of the distal stem
demonstrated and osteolysis of the greater trochanter however minimal migration
of the proximal prosthesis consistent with pivoting about a proximal (calcar)
location (Harkess 1999). ..........................................................................................34
Figure 2-15 Gruen Mode IV failure (bending cantilever). Serial radiographs from time of
implantation from left to right. Left: Varus malposition of the stem at initial surgery.
Centre: Proximal cement – prosthesis lucency in zone 1 without abnormality in
distal cement fixation. Right: Fragmentation of proximal cement mantle in Gruen
Zone 7 with fatigue failure of prosthesis (Harkess 1999). .......................................35
Figure 2-16 Digital subtraction arthrogram demonstrating loosening of the femoral
implant with extensive extravasation of radio-opaque contrast about the cement –
bone interface and distal contrast leak to cortical perforation (Ovesen 2003).........37
Figure 2-17 Standard length Exeter stem with winged centraliser (left) and Exeter short
revision stem (125mm, +44 offset) with revision wingless centraliser (right)
(Stryker International 2005).....................................................................................40
Figure 2-18 Exeter Trauma Stem (ETS) monoblock hemiarthroplasty. Polished double
taper geometry without a prosthetic collar...............................................................40
Figure 2-19 Presentation of Antibiotic (Tobramycin) Simplex PMMA cement. ............42
Figure 2-20 Differences in molecular weight of PMMA chains due to alternate
sterilisation methods. Variation in the constituents of different cement preparations
produces differences in the material properties of the polymerised products (Kuehn
et al. 2005)................................................................................................................43
Figure 2-21 Residual monomer in Surgical Simplex PMMA Cement after initial
polymerisation (Kuhn 2000). ...................................................................................44
Figure 2-22 Increase in shear strength of Surgical Simplex P cement related to duration
of post-cure (Wilde et al. 1975). ..............................................................................45
Figure 2-23 Working curves for Simplex P Cement (Kuhn 2000). .................................47
Figure 2-24 Fatigue life of PMMA cements to cyclic compression are improved with
porosity reduction by vacuum preparation (Kuehn et al. 2005). .............................48
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Figure 2-25 Schematic diagram of the PMMA shear testing apparatus used by
Greenwald et al. (1978). Cast PMMA cylinders with a known cross-sectional area
are subjected to shear between two plates similar to a guillotine. ...........................52
Figure 2-26 ASTM D732 - 02 Shear punch tool .............................................................53
Figure 2-27 Variables influencing the calculation of material properties by shear punch
tool (Guduru et al. 2005)..........................................................................................54
Figure 2-28 Shear strength of four orthopaedic PMMA cements with variable
concentrations of added gentamicin (Moran et al. 1979).........................................55
Figure 2-29 PMMA cement shear properties determination conducted by Black et al.
(1982). Interfacial shear strength was not assessed with this experiment................57
Figure 2-30 Porosity observed in the experimental PMMA cement mantle applied onto
concurrently polymerising cement. Heat is transferred to the experimental cement
mass during early polymerisation and porosity is induced by the trapping of
vaporised monomer (Black et al. 1982). ..................................................................58
Figure 2-31 Method of interfacial shear stress evaluation (Greenwald et al. 1978). .......58
Figure 2-32 ASTM C 273 – 00 Standard test method for shear properties of sandwich
core materials. ..........................................................................................................59
Figure 2-33 Interfacial ultimate shear strength shows marked weakening with blood
present at the interface prior to application of the second cement mantle, however
approaches the strength of a uniform mantle with suitable preparation (Greenwald
et al. 1978)................................................................................................................60
Figure 2-34 Interfacial ultimate shear strength between PMMA mantles decreases when
the second mantle is applied late. Late application is associated with a decreased
mixture weight due to the evaporation of monomer from the cement surface prior to
polymerisation (Greenwald et al. 1978)...................................................................62
Figure 2-35 Interfacial ultimate shear strength between PMMA cement mantles
decreases in proportion to monomer loss from the second mix of cement applied
(Greenwald et al. 1978)............................................................................................62
Figure 3-1 SPT demonstrating casting and testing modes of application. The central
casting rod is removed for the manufacture of solid PMMA cast cylinders............68
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Figure 3-2 Parting of PMMA discs from solid cylinder. With the use of suitable lathe tip
allowing adequate swarf clearance, temperature of the cutting tool typically
measured at 35.0 – 37.0oC, well below the glass transition temperature of
polymerised PMMA (see text). ................................................................................71
Figure 3-3 Facing of the test specimens was performed on both sides. With the use of
suitable lathe tip allowing adequate swarf clearance, maximum temperature of the
cutting tool typically measured 25.0-26.5oC, well below the glass transition
temperature of polymerised PMMA (see text).........................................................71
Figure 3-4 Confirmatory measurement of specimen thickness using digital verniers
calliper with precision of 10μm. ..............................................................................72
Figure 3-5 Dry post-curing of PMMA specimens at 37oC. .............................................72
Figure 3-6 Shear punch testing using Hounsfield unit.....................................................73
Figure 3-7 Sectioned isometric view of SPT device in testing mode. 2mm gap between
top plate and base plate are demonstrated................................................................74
Figure 3-8 Illustration of R1 and R2 measurements used in Equation 3.3, where R1 is the
radius of the SPT die and R2 is the punch radius. ....................................................76
Figure 3-9 Ultimate shear stress to failure of Antibiotic Simplex and Surgical Simplex P
cements with 95% confidence intervals marked. Similar variations in shear
properties related to duration of post-cure are observed for both cement
preparations. .............................................................................................................79
Figure 3-10 Calculated shear modulus of Antibiotic Simplex and Surgical Simplex P
cements with 95% confidence intervals marked......................................................80
Figure 3-11 Above: Schematic representation of SPT device in testing mode. R1 =
radius of die. R2 = radius of punch. Below: Magnification of the zone of shearing.
The portion of material undergoing shear is represented in light blue. Die-Punch
clearance was estimated to be 20μm by use of micrometers, however inaccuracy in
this clearance measurement would potentially create large errors in the assessment
of shear modulus by direct calculation. Indirect methods for shear modulus
determination were therefore employed...................................................................82
Figure 4-1 Simplex PMMA cement cylinder with central hole after removal of casting
rod. After post-curing of the PMMA cylinder, fresh PMMA cement was poured
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into the central hole, therefore creating a solid cement cylinder with outer “old”
cement and inner “new” cement. .............................................................................87
Figure 4-2 Completed 5mm thickness bilaminar PMMA test specimen. For illustration
purposes, coloured cement is used to differentiate the outer (old) cement mantle
from the inner (new) cement mantle. .......................................................................88
Figure 4-3 SEM of the interfacial surface of the old cement mantle prior to the
application of new cement. Smooth surface with minimal porosity is demonstrated.
Surface roughness of this interface measured by profilometry was 0.22μm Ra.
(Original magnification x27). ..................................................................................90
Figure 4-4 SEM of the interfacial surface of the old cement mantle prior to the
application of new cement. At high power magnification the surface profile of the
cement secondary to defects on the casting rod and irregularities created during the
process of rod extraction are appreciated on careful inspection (Original
magnification x1500). ..............................................................................................90
Figure 4-5 Interfacial region between inner PMMA and outer PMMA cement mantles
after SPT testing. Clean separation between the cement mantles is observed
(Original magnification x10). ..................................................................................91
Figure 4-6 Interfacial region of Surgical Simplex P after punch testing. Brittle fracture at
the interface is demonstrated (Original magnification x80). ...................................91
Figure 4-7 Transverse section SEM of a bilaminar Simplex – Simplex test specimen at
the interfacial region after SPT evaluation. Linear separation between new and old
cement mantles is demonstrated. Outer (old) cement mantle is located upper left
and inner (new) cement mantle is located lower right. (Original magnification
x110). .......................................................................................................................92
Figure 4-8 Florescent dye penetrant examination of transverse section of bilaminar
PMMA test specimen after SPT testing demonstrating linear cement through the
interfacial region. .....................................................................................................93
Figure 4-9 Florescent dye penetrant examination of bilaminar test specimen after SPT
testing demonstrating cement fracture through the interfacial region. ....................93
Figure 4-10 Ultimate shear strength of Surgical Simplex P compared to bilaminar test
specimens manufactured from the same material with 95% confidence intervals
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marked. Outer mantle of the bilaminar test specimens was post-cured for 14 days
prior to application of the new mantle and tested 0 – 90 days after new mantle
application. Bilaminar PMMA mantles demonstrate time dependant shear
properties similar to that of uniform mantles, however a comparative reduction in
ultimate strength by 15 to 20 percent was observed. ...............................................94
Figure 4-11 Calculated shear modulus for uniform mantle and bilaminar Surgical
Simplex P PMMA samples. 95% confidence intervals marked. .............................95
Figure 4-12 Interfacial shear strength bilaminar test specimens comparing cement
preparations with and without the commercial inclusion of Tobramicin antibiotic,
95% confidence intervals marked. ...........................................................................96
Figure 4-13 Interfacial shear modulus of bilaminar test specimens comparing cement
preparations with and without the commercial inclusion of Tobramicin antibiotic.
95% confidence intervals marked. ...........................................................................96
Figure 4-14 Contributions to adhesion between polymer solids: (a) mechanical
interlocking; (b) interdiffusion of chains; (c) electrical interactions; (d) chemical
interactions (Garbassi et al. 1998)............................................................................97
Figure 4-15 Hypothetical components of the adhesion force between PMMA bone
cement and metal surfaces (Bundy et al. 1987). ......................................................99
Figure A3.1 Exeter PMMA Cement Reamer……………………………. ……………………110
Figure A3.2 Reamer Tip. Note the difference between negative rake angle established at
the centre cutting area of the drill compared to positive helix angle of the drill
body………………………………………………………………………………..…………….110
Figure A3.3 Swarf repeatedly accumulates in front of leading edge of drill tip when
performing end cutting procedures. Note the flutes running down the body of the
bit are clear of debris due to favourable helix angle………………………..………….112
Figure A3.4 Macroscopic appearance of mantle reamed to depth to allow revision with
standard length stem. Reaming was conducted without use of irrigation or regular
clearing of swarf. Note the delaminated appearance of reamed surface…………..113
Figure A3.5 Fluorescent Dye Penetrant study on mantle reamed with irrigation and
regular swarf clearance. Well finished surface with increased roughness compared
xiii
to unreamed. Fine circumferential surface marks from the drilling tip are held by
the material. No mantle cracks identified………………………………………………..114
Figure A3.6 Fluorescent dye penetrant study on same mantle reamed without adequate
irrigation or swarf clearance. Very rough surface with appearance of cement
delamination. No mantle cracks identified………………………………………...…….114
Figure A3.7 SEM (Magnification x27) PMMA Cement mantle unreamed……….……..115
Figure A3.8 SEM (Magnification x27) PMMA Cement mantle reamed according to
manufacturer recommendations with use of irrigation and regular swarf clearance.
Roughened surface but well finished……………………………………………………..115
Figure A3.9 SEM (Magnification x27) PMMA Cement mantle reamed without irrigation
or adequate swarf clearance. Very rough surface produced by smearing of melted
PMMA produced during drilling procedure……………………………...……………..116
Figure A3.10 SEM (Magnification x1500) Unreamed PMMA cement mantle……..….116
Figure A3.11 SEM (Magnification x 1500) Reamed mantle using irrigation and regular
swarf clearance. Roughened surface with mantle integrity maintained………..….117
Figure A3.12 SEM (Magnification x1500) PMMA Cement mantle reamed without
adequate irrigation or swarf clearance. Smearing of melted PMMA can be
appreciated……………………………………………………………………………...…….. 117
Figure A3.13 SEM (Magnification x400) of same mantle displayed in Figure A3.12..118
xiv
List of Tables
Table 2-1 Swedish National Hip Arthroplasty Register percentage survivorship for
cemented and uncemented THA implants after 12 years with failure defined as
revision for any cause. ^ = indicates 10 year follow up data. * = insufficient data.
Statistical analysis by two tailed χ2 test (The Swedish National Hip Arthroplasty
Register 2004). ...........................................................................................................9
Table 2-2 Diagnosis requiring revision hip replacement for primary THR performed
1979 - 2003 (The Swedish National Hip Arthroplasty Register 2004)....................17
Table 2-3 Diagnosis requiring revision hip replacement (Australian Orthopaedic
Association National Joint Replacement Registry 2004).........................................18
Table 2-4 Components used in major THA revision (Australian Orthopaedic Association
National Joint Replacement Registry 2004). ...........................................................21
Table 2-5 Indications and requirements for C-C Revision Hip Arthroplasty. Refer to
Figure 2-9 and text for explanation of Gruen modes of cemented femoral
component failure.....................................................................................................26
Table 2-6 Advantages of C-C femoral revision. ..............................................................27
Table 2-7 Contraindications and disadvantages of C-C femoral revision. ......................28
Table 2-8 Radiographic features of femoral component loosening in cemented THA
(Harkess 1999). See Figure 2-8 and text for explanation of Gruen zones. ..............30
Table 2-9 Composition of four commonly used orthopaedic PMMA cements. MA =
Methacrylic acid; PS = Polystyrene; MMA = Methylmethacrylate (monomer).
Values expressed in wt% (Wright et al. 2001).........................................................43
Table 2-10 Factors influencing the extent and distribution of porosity within PMMA
cements.....................................................................................................................48
Table 2-11 Properties of a typical surgical PMMA cement preparation (Greenwald et al.
1978; Harper et al. 2000; Kuhn 2000; Kuhn et al. 2001; Kuehn et al. 2005). .........51
Table 2-12 Summary of literature: PMMA ultimate shear strength. Refer to text for
details. ......................................................................................................................56
Table 2-13 Summary of literature: PMMA C-C interfacial ultimate shear strength .......61
xv
Table 3-1 Testing schedule for determination of PMMA cement shear properties.........75
Table 3-2 Results of Sawbones reference sample testing. Expected shear strength and
modulus values obtained from manufacturer. * Values +/- 10%. (James 2005;
Pacific Research Laboratories 2005). ......................................................................78
Table 3-3 Tabulated data from Figures 3-9 and 3-10. .....................................................80
Table 4-1 Testing schedule of bilaminar test specimens for the determination of PMMA
interfacial shear properties. ......................................................................................88
Table 4-2 Tabulated data for C-C interfacial shear properties displayed in Figures 4-10
to 4-13. .....................................................................................................................97
xvi
List of Equations
Equation 3-1 Calculation of ultimate shear strength of the test specimens. A is the
surface area of the test specimens subjected to shearing force within the SPT device.
..................................................................................................................................74
Equation 3-2 Calculation of test specimen shear area. T is the thickness of the test
specimen (5.00mm) and Rave is the average of radius of the punch and die. ...........74
Equation 3-3 Average radius of punch and die. Measurements R1 and R2 are depicted in
Figure 3-8. ................................................................................................................74
Equation 3-4 Calculation of the shear modulus calibration ratio, x. Gradient of the force/
displacement data obtained by SPT is represented by ΔF/ΔD…………………………75
Equation 3-5 Calculation of shear modulus of test specimens using calibration ratio.....75
xvii
List of Symbols
A Area of test specimen subjected to shear stress..
D Displacement..
F Force. .
ΔF/ΔD Force/ Displacement gradient. .
G Shear modulus...
Ra Arithmetic mean surface roughness...
R1 Radius of shear punch apparatus die..
R2 Radius of shear punch apparatus punch..
Rave Average radius shear punch apparatus punch and die..
T Test specimen thickness..
τult Ultimate shear stress..
x Shear modulus calibration ratio..
xviii
List of Abbreviations
AB Antibiotic (Tobramycin)
ANOVA Analysis of variance
ASTM American Society of Testing Methods
BPO Dibenzoyl peroxide
C-C Cement-within-cement (revision technique)
CDH Congenital Dysplasia of Hip
CI Confidence Interval
CT Cemented Thompson (monoblock hemiarthroplasty)
DMpT Dimethyl-para-toluidine
ESRS Exeter Short Revision Stem (125mm; +44 Offset)
ETS Exeter Trauma Stem (monoblock hemiarthroplasty)
FEA Finite Element Analysis
Ga-67 Gallium 67 citrate radio – isotope
g/cc Grams per cubic centimetre
HR Hazard Ratio
In-111 DTPA Indium -111 diethylenetriaminepentacetic acid radio –isotope
LM Light Microscopy
LSD Least Significant Difference
LV Low Viscosity
MA Methacrylic Acid
Mag Magnification
MMA Methylmethacrylate (monomer)
MPa Mega Pascals
PCF Pounds per Cubic Feet (measurement of density)
PMMA Polymethylmethacylate
PPM Parts Per Million
PS Polystyrene
PSI Pounds per Square Inch
xix
RR Risk Ratio
SEM Scanning Electron Microscopy
SPT Shear Punch Testing
Tc-99 Technetium 99 phosphate radio-isotope
THA Total Hip Arthroplasty
TKA Total Knee Arthroplasty
RPM Revolutions Per Minute
UAM Uncemented Austin Moore (monoblock hemiarthroplasty)
W/W Weight/ Weight proportion
xx
Glossary of Terms
Axial Perpendicular to the long axis.
Bilaminar Comprising two mantles; applied to C-C revision THA relates to a
cement mantle with layered new and old cement components.
Cancellous Bone with lattice like structure found within the central part of the
proximal femur and other anatomical regions. Surrounded by
cortical bone.
Centraliser Cap placed on the distal end of a femoral prosthesis which assists
in positioning the tip of the prosthesis within the middle of the
bone and allows the stem to subside.
Coronal Anatomical plane situated in the direction of the coronal suture
and at right angles to both the sagittal and axial planes.
Cortex Outer layer of dense hard bone.
Distal Remote; farther away from any point of reference; opposed to
proximal.
Glass transition (Tg) Temperature at which the amorphous phase of a polymer is
converted between glass and rubbery states.
Hazard Ratio (HR) The hazard ratio in survival analysis is a summary of the
difference between two survival curves, representing the
reduction in the risk of death on treatment compared to control,
over the period of follow-up. It is a form of relative risk.
Helix angle Angle made by the advance of the thread as it wraps around a
cylinder; Applied to drill bits and machine tools relates to the
angle between the long axis of the tool and spiral of the flutes.
Hemiarthroplasty Prosthetic hip replacement where the acetabulum is not resurfaced.
The metal femoral component articulates with cartilage.
Interfacial Boundary between two solid masses; Applied to C-C revision
relates to the boundary between new and old cement mantles.
In Vitro Outside the body; within an artificial environment.
xxi
In Vivo Within the body.
Liner Articulating insert for an uncemented acetabular prosthesis.
Housed within a metal case referred to as a shell.
Loosening Mechanical instability of prosthetic components.
Modular Composed by separate pieces which can be assembled or
disassembled; applied to hip arthroplasty relates to femoral
prostheses with separate stem and head components or an
uncemented acetabular prosthesis with separate shell and liner
components. Opposite of monoblock.
Monoblock Prosthesis unable to be assembled or disassembled. Opposite of
modular.
Necrosis Sum of the morphological changes indicative of cell death.
Point angle Angle between cutting edges at the tip of a drill bit or machine
tool. Standard point angle for an all-purpose drill bit is 118
degrees.
Proximal Nearest; closer to any point of reference; opposed to distal.
Rake angle Angle between the leading edge of a cutting tool and a
perpendicular to the surface being cut.
Ream Dilation of a canal by means of a power or hand instrument.
Risk Ratio (RR) Probability of the disease in the risk group divided by the
probability of the disease in the non-risk group.
Shell Outer metal portion of an uncemented THA acetabular component
which interfaces with bone on the outer aspect and accepts an
articulating insert known as a liner.
Stem Femoral prosthesis.
Swarf Particulate debris or shavings created during procedures such as
drilling, reaming or turning.
Tobramycin Fractionated and purified aminoglycoside antibiotic which has
bactericidal activity against many gram-negative and some gram-
positive organisms. Heat stable antibiotic constituent of some
Antibiotic Simplex PMMA cement preparations.
xxii
Uniform mantle PMMA cement mantle manufactured from a single cement type in
a single preparation. As opposed to bilaminar.
Unipolar Single articulation; Applied to hip joint hemiarthroplasty relates
to a prosthesis which articulates between the prosthetic head and
acetabulum only. Compared to a bipolar prosthesis which has a
second articulation built into the prosthetic head.
Van Der Waals A weak attractive force between atoms or non-polar molecules
caused by a temporary change in dipole moment arising from a
brief shift of orbital electrons to one side of one atom or molecule,
creating a similar shift in adjacent atoms or molecules.
Varus Deformity in which angulation of the limb or part is toward the
midline of the body.
xxiii
Statement of Originality
The work contained in this thesis has not been previously submitted for a degree
or diploma at any other higher institution. This thesis contains no material previously
published or written by another person except where due reference is made.
Signature: _______________________________________
Patrick Weinrauch
Date: _______________________________________
xxiv
Acknowledgements
I owe gratitude to many for their assistance in preparing this manuscript. My
family Megan, Lachlan and Harrison have again supported me through yet another
endeavour, and provided me with perspective to my efforts. My supervisor Professor
Ross Crawford has remained an exceptional role model who has demonstrated trust in
my ability and enthusiasm for my research project. The “engine room” vis. Cameron
Bell and Lance Wilson – I have thoroughly enjoyed working with, both from
professional and personal perspectives. Between us, the gap between clinical
orthopaedics and engineering is small. My old friend Dr O has provided practical advice
and comic entertainment during the year which has proven invaluable. Cameron Lutton
and Ben Goss have consistently provided me with answers regarding polymer chemistry
and the properties of bone cement and I am indebted to them for their patience and
tuition. Jon James I have to thank for assisting in the design and then expeditiously
manufacturing the testing apparatus used for this research. Sarah Whitehouse has
assisted with statistical analysis.
I am also indebted to the Queensland Training Committee of the Australian
Orthopaedic Association for supporting my research endeavours. I wish also to thank the
Orthopaedic Research Foundation of the Australian Orthopaedic Association for
financial support via the Foundation Fellowship. I acknowledge support from Stryker
Australia by the provision of cements used in this research.
xxv
Dedication
To my great mate Paul McCarthy.
Wish you were here.
1
Chapter 1
Introduction
1.1 Overview of Research and Clinical Application
Aseptic loosening of prosthetic components is the leading cause for revision of
Total Hip Arthroplasty (THA) (Australian Orthopaedic Association National Joint
Replacement Registry 2004; Canadian Institute for Health Information 2004; The
Swedish National Hip Arthroplasty Register 2004). Revision of a cemented hip
arthroplasty traditionally requires the removal of failed prosthetic components and
extraction of both loose and secure cement. While extraction of cemented femoral stems
may be conducted by selective cement removal and the application of a retrograde force,
removal of retained cement from the femoral canal during revision THA remains a time-
consuming and hazardous procedure (Halawa et al. 1978; Choy et al. 1987; Turner et al.
1987; Larson et al. 1988; Gray 1992; Rosenstein et al. 1992; Gardiner et al. 1993;
Hozack 1998; Harkess 1999) . Proximally the cement may be easily accessible, however
the surrounding cortex may be thin and fragile thereby susceptible to damage during
cement extraction. Access to the cement mantle distal to the metaphyseal flare becomes
increasingly difficult, sometimes necessitating exposures such as an extended
trochanteric osteotomy or distal cortical windowing (Hozack 1998; Harkess 1999). Even
within major arthroplasty centres, cement removal results in intraoperative femoral
fracture in approximately 8% of revisions (Gardiner et al. 1993). Other risks associated
with cement removal include cortical perforation, loss of bone stock, excessive blood
loss and increased duration of anaesthesia (Gray 1992; Hozack 1998).
Multiple techniques and instrumentation systems have been developed to assist
in the removal of retained cement from the femoral canal, including the use of hand and
power tools, ultrasonic devices and lasers (Choy et al. 1987; Gray 1992; Schurman et al.
1992; Brooks et al. 1993; Gardiner et al. 1993; Cautilli et al. 1994; Hozack 1998).
Femoral cortical perforations occur in approximately 10% of cases using high speed
cement removal instruments (Turner et al. 1987). Such perforations often occur in an
uncontrolled manner producing large gaps in the cortex, rendering the femur at risk of
2
postoperative fracture (Larson et al. 1988). Cortical perforations commonly occur
distally and posteriorly within the femur, representing a difficult region to reconstruct
(Gray 1992; Harkess 1999).
In addition to the intraoperative risks associated with the removal of secure
cement, extraction of a well fixed mantle is associated with the unavoidable
simultaneous removal of cancellous bone, thereby compromising the bone quality
required for stable implantation of the revision THA stem. The ultimate shear stress to
failure at the interfacial region between bone and cement is determined by the strength
of the cancellous bone, and damage sustained during cement removal results in a 30
percent reduction in the ultimate shear strength of bone-cement interface in revision
femoral arthroplasty (Halawa et al. 1978; Rosenstein et al. 1992). Traditional cemented
revision by unwarranted extraction of a stable cement mantle may therefore compromise
the performance of the subsequent revision implant.
C-C femoral revision refers to the technique of extracting the femoral prosthesis
with retention of stable cement and subsequent re-cementing of the revision stem into
the old mantle. Retention of the primary cement may be undertaken with no mantle
modification, or alternatively portions of the cement mantle may be removed as required
either proximally or distally (Hozack 1998; Harkess 1999; Hubble et al. 2005).
Excellent early to mid term clinical results have been described using this technique for
appropriate indications (Lieberman et al. 1993; Barrack et al. 1995; McCallum et al.
1995; Hubble et al. 2005).
Laboratory testing of the interfacial shear strength between new and old cement
mantles has been used to evaluate the mechanical integrity of bilaminar cement mantles
created in C-C revision THA. With correct surgical technique including the provision of
a clean and dry old cement mantle with exclusion of blood and debris at the interfacial
region, acceptable interfacial strength may be obtained between layered cement mantles
(Gruen et al. 1976; Greenwald et al. 1978; Li et al. 1996). Duration of post-cure between
PMMA cement mantles is important as PMMA exhibits variability in material properties
related to time (Wilde et al. 1975; Lee et al. 1977; Lee et al. 1978) and the energy
required to separate adhesive bonds formed between solid polymer masses is known to
increase with duration of contact (Wool 1991). It would be reasonable therefore to
3
postulate that the interfacial region of a bilaminar cement mantle will demonstrate
variability in interfacial shear strength related to duration of post-cure.
1.2 Research Objectives and Hypotheses
Objectives of this research are to describe:
1. Shear properties of Surgical Simplex P and Tobramycin Antibiotic Simplex
PMMA cements (Stryker International; Rutherford, NJ), and to monitor the
changes with respect to duration of post-cure;
2. Material properties of the interfacial region between new and old cement
mantles in shear, and to monitor the changes with respect to duration of
interfacial post-cure;
3. Influence of Tobramycin antibiotic present within Antibiotic Simplex on C-C
interfacial shear properties; and
4. Mechanisms of adhesion between layered PMMA cement mantles.
Specific hypotheses relating to the shear properties of uniform and bilaminar PMMA
cement mantles discussed within this thesis are:
1. Surgical Simplex and Tobramycin Antibiotic Simplex PMMA cements have
material shear properties which demonstrate variability with respect to duration
of post-cure.
2. Antibiotic Simplex PMMA cement demonstrates equivalent shear properties to
Surgical Simplex P.
3. The ultimate shear strength of the interfacial region between bilaminar cement
mantles approximates that of uniform mantles.
4. Bilaminar PMMA cement mantles demonstrate variation in the interfacial shear
properties with duration of interfacial post-cure.
5. Antibiotic Simplex when used in bilaminar cement mantles results in weakening
at the interfacial region.
6. Interfacial adhesion by mechanisms other than mechanical interlock play a
significant role in the bond formed between new and old PMMA cements.
4
1.3 Thesis Overview
Chapter 2 Literature Review is separated in to five main topics vis. PMMA
Cement in Primary and Revision Hip Joint Arthroplasty, Cement-Within-Cement
Femoral Revision, Composition and Material Properties of PMMA cement, Evaluation
of the Material Properties of PMMA Cement and Bilaminar Mantles in Shear and
Context of Thesis to Literature Review. Section 2.1 PMMA Cement in Primary and
Revision Hip Joint Arthroplasty describes the clinical application of PMMA cements in
current orthopaedic practice. Rationale is provided for the use of cemented femoral
components in preference to uncemented in both primary THA and hip joint
hemiarthroplasty. The aetiology of revision THA and issues relating to cement removal
from the femur during revision are discussed. Section 2.2 Cement-Within-Cement
Femoral Revision describes the clinical indications and requirements for the conduct of
C-C femoral prosthesis revision. Preoperative planning for C-C revision by radiographic
assessment of the bone-cement interface is discussed. Operative techniques and implants
facilitating the conduct of C-C femoral revision are presented. Section 2.3 Composition
and Material Properties of PMMA Cement provides an overview of commercial PMMA
acrylic cements used in hip joint arthroplasty, with particular reference to Surgical
Simplex P. Intraoperative factors influencing the material properties of PMMA cements
are discussed. Section 2.4. Evaluation of the Material Properties of PMMA Cement and
Bilaminar Mantles in Shear describes die-punch testing and other techniques for the
investigation of cements in shear. Previous research relating to the shear properties of
PMMA cement and bilaminar interfaces is presented. Section 2.5 Summary of Literature
Review and Relevance to Thesis discusses the rationale of the selected thesis objectives
and hypotheses with respect to the available literature.
Chapter 3 Experimental Evaluation of the Shear Properties of PMMA Cement
describes material properties of PMMA cements in shear, with specific reference to the
variation in material properties with respect to duration of post-cure. These experiments
provide data to validate the die-punch testing method and to supply reference data for
interpretation of results obtained for bilaminar PMMA cement test experimentation.
5
Chapter 4 Experimental Evaluation of the Shear Properties of Bilaminar Cement
Mantles describes the material properties of cement – cement interfaces in shear.
Objectives selected for this experimentation are based on issues relevant to the clinical
practice of C-C revision arthroplasty. Variation in the shear properties of C-C interfaces
with respect to duration of post-cure and the influence of the commercial inclusion of
antibiotics on interfacial shear properties is evaluated. Mechanisms of interfacial
adhesion between layered PMMA cement mantles are discussed.
Main conclusions and recommendations of the thesis are provided in Chapter 5.
6
7
Chapter 2
Literature Review
2.1 PMMA Cement in Primary and Revision Hip Joint Arthroplasty
Section 2.1 describes the clinical application of PMMA cements in current orthopaedic
practice. Rationale is provided for the use of cemented femoral components in
preference to uncemented in both primary THA and hip joint hemiarthroplasty. The
aetiology of revision THA and issues relating to cement removal from the femur during
revision are discussed.
2.1.1 PMMA cement in primary total hip arthroplasty
Polymethylmethacylate (PMMA) acrylic cements have been regularly used to
secure hip arthroplasty prostheses since the popularisation of joint replacement by Sir
John Charnley in the 1960’s (Charnley 1960; Ege et al. 2001; Smith 2005). The
functions of PMMA cement are to enable stable fixation of prosthetic components by the
anchoring of implant to bone, to provide an elastic buffer for the uniform transfer of load
from prosthesis to bone and as a delivery system for the local administration of
antibiotics (Kuehn et al. 2005).
Hip joint arthroplasty is one of the most successful and cost effective surgical
interventions in medicine, with approximately 27 000 procedures performed in Australia
per annum (Garrellick et al. 1998; Australian Orthopaedic Association National Joint
Replacement Registry 2004). Cemented conventional and hybrid total hip arthroplasty
(THA) prostheses constitute 49.6% of primary replacements performed in Australia,
with the Exeter cemented stem being the most frequently implanted primary femoral
component considering both cemented and uncemented fixation methods (Australian
Orthopaedic Association National Joint Replacement Registry 2004). Cemented and
hybrid implants constitute greater then 90% of primary THA procedures in both Sweden
and Norway (Department of Orthopaedic Surgery Haukeland University Hospital 2004;
The Swedish National Hip Arthroplasty Register 2004).
8
Cemented THA continues to demonstrate exceptional long term durability and
performance. The Swedish National Hip Arthroplasty Register has now recorded the
results of almost 230 000 primary THA procedures since 1979. With implant failure
defined as revision, cemented implants in the Swedish Hip Registry demonstrate 90.6%
12 year survival, which compares favourably to the outcomes observed with uncemented
THA implants, demonstrating only 74.5% survival at 12 years. Superior survivorship of
cemented THA implants compared to uncemented is similarly demonstrated in both the
Australian (Figure 2-1) and Norwegian joint arthroplasty registers.
While there is an increasing trend for use of uncemented THA in younger and
higher demand patients, improved prosthetic survivorship for uncemented implants in
these groups has not been demonstrated, and substantial evidence exists to suggest
otherwise. The Swedish Hip Arthroplasty Register shows that for male patients under
the age of 50 years, representing the youngest and highest demand patient cohort,
cemented THA has a 83.7% 12-year survivorship, compared to 71.5% for uncemented
prostheses (p<0.001; χ squared analysis). Indeed, the Swedish arthroplasty register
demonstrates superior prosthetic survivorship for cemented implants in all patients
regardless of age or gender (Table 2-1).
Despite strong evidence to support the use of PMMA cement as a method of
fixation in THA, the use of cemented implants is decreasing in favour of uncemented
technology (Jones et al. 1987; Australian Orthopaedic Association National Joint
Replacement Registry 2004; The Swedish National Hip Arthroplasty Register 2004).
Uncemented implants avoid the potential adverse intraoperative cardiovascular events
associated with pressurised cement implantation, although with appropriate surgical
technique and anaesthetic management, the significance of PMMA cement on
cardiovascular function has been questioned (Yasuda et al. 1975; d'Hollander et al.
1979; Dahl et al. 1992; Karlsson et al. 1995; Breusch 2001). Uncemented femoral
components characteristically have reduced inventory of instrumentation required for
implantation and are quicker to implant thereby reducing anaesthetic time. Uncemented
acetabular components also allow the use of alternative bearing surfaces to polyethylene,
9
Figure 2-1 Cumulative risk for revision of THA by cement status excluding
infection (Australian Orthopaedic Association National Joint Replacement Registry
2004).
Cemented (%) Uncemented(%) P
All Implants 90.6 74.5 <0.001Age < 50, Male 83.7 71.5 <0.001Age < 50, Female 74.8 67.6 <0.001Age 50 - 59, Male 82.4 81.5 0.52Age 50 - 59, Female 88.8 71.9 <0.001Age 60 - 75, Male 89.4 81.1 <0.001Age 60 - 75, Female 92.7 91.9 0.62Age >75, Male 93.5 * *Age >75, Female 96.4 * *
Table 2-1 Swedish National Hip Arthroplasty Register percentage survivorship for
cemented and uncemented THA implants after 12 years with failure defined as
revision for any cause. ^ = indicates 10 year follow up data. * = insufficient data.
Statistical analysis by two tailed χ2 test (The Swedish National Hip Arthroplasty
Register 2004).
Figure removed for digital reproduction.
10
the wear debris of which have been implicated in osteolysis about joint arthroplasty
implants. Furthermore, in revision situations with a solidly fixed uncemented acetabular
shell, modification of the bearing surface material or geometry is made possible by liner
exchange without disruption of the implant – bone interface (Australian Orthopaedic
Association National Joint Replacement Registry 2004). The exothermic reaction of
PMMA polymerisation has not been demonstrated to result in temperature elevations
significant enough to result in bone necrosis during THA (Jefferiss et al. 1975; Reckling
et al. 1976; Sih et al. 1980; Schultz et al. 1987; Kuehn et al. 2005).
2.1.2 PMMA cement in hip joint hemiarthroplasty
Hemiarthroplasty prostheses are principally used for the management of
displaced intracapsular hip fractures in the elderly, with greater than 95% of
hemiarthroplasty procedures in Australia conducted for this indication (Gebhard et al.
1992; Australian Orthopaedic Association National Joint Replacement Registry 2004).
The demographic features of patients requiring hip joint hemiarthroplasty are somewhat
different to THA, with patients on average being 14 years older (mean age 83 years) and
75% of patients being female (Figure 2-2). In Australia unipolar articulations are used in
72.1% of hemiarthroplasty procedures, with monoblock stems accounting for 83% of all
unipolar implants. Currently 45.8% of hip hemiarthroplasty prostheses recorded in the
Australian National Joint Registry are cemented, however the use of PMMA cement is
increasing due to the poor outcomes observed with uncemented hemiarthroplasty
implants (Australian Orthopaedic Association National Joint Replacement Registry
2003; Australian Orthopaedic Association National Joint Replacement Registry 2004;
Weinrauch et al. 2005; Weinrauch et al. 2006).
Elderly patients sustaining fractures of the proximal femur frequently have
significant concomitant medical conditions resulting in high mortality rates after
operative management (Parvizi et al. 2004). The 1 year mortality rate after the diagnosis
of femoral neck fracture across multiple studies is approximately 30%, with the majority
of deaths within the first 6 months of surgery (Kenzora et al. 1984; Lu - Yao et al. 1994;
McAuliffe et al. 2004). Patients surviving the first 6 – 12 months after surgical
management of hip fractures however have an equivalent mortality rate when compared
11
Figure 2-2 Age and sex distribution for hip joint hemiarthroplasty (Australian
Orthopaedic Association National Joint Replacement Registry 2004).
to matched controls (Fitts et al. 1959; Gordon 1971; Kenzora et al. 1984; Taine et al.
1985). Despite high mortality rates associated with the operative management of hip
fractures, hemiarthroplasty endoprosthesis implantation has been demonstrated to be a
highly cost effective treatment in terms of quality adjusted life years gained (Parker et al.
1992).
Excluding infection, common diagnoses necessitating the revision of
hemiarthroplasty prostheses are early aseptic loosening and secondary acetabular
arthrosis (D'Arcy et al. 1976; Devas et al. 1983; Dixon et al. 2004). Early revision due to
failure on the femoral side of hip hemiarthroplasty has a strong association with the use
of uncemented implants (Wrighton et al. 1971; Lausten et al. 1982; Sonne-Holm et al.
1982; Dorr et al. 1986; Gebhard et al. 1992; Warwick et al. 1998; Khan et al. 2002;
Australian Orthopaedic Association National Joint Replacement Registry 2004; Parker
et al. 2004). Multiple studies have demonstrated uncemented monoblock
Figure removed for digital reproduction.
12
hemiarthroplasty of the hip joint to be technically demanding, with a high rate of
intraoperative error contributing to early failure of the prosthesis (Kwok et al. 1982;
Sharif et al. 2002; Weinrauch 2004; Yau et al. 2004; Weinrauch et al. 2005; Weinrauch
et al. 2006). Parker and Gurusamy (2004) in a meta-analysis of fifteen trials involving
1670 patients concluded uncemented prostheses were associated with a higher risk of
failure to regain mobility (HR 0.60; 95% CI 0.44 to 0.82) and a higher incidence of
postoperative pain at one year (HR 0.51; 95% CI 0.31 to 0.81) (Parker et al. 2004). Khan
et al. (2002) reviewed the results of 244 patients managed with the Austin Moore
prosthesis, with and without the use of PMMA cement. At 32 – 36 month follow up,
patients who were managed without cement had greater pain (p=0.003), reduced walking
ability (p=0.002), increased use of walking aids (p=0.003) and reduced capacity for
activities of daily living (p=0.009). The use of cemented hemiarthroplasty in the elderly
was supported by these findings (Khan et al. 2002). Foster et al. (2005) in a review of
244 patients undergoing Uncemented Austin Moore (UAM) or Cemented Thompson
(CT) prosthesis implantation for femoral neck fractures found a 7% periprosthetic
fracture rate over a 2 year period in patients managed with the UAM (Foster et al. 2005).
There were no periprosthetic fractures observed in 174 patients managed with the CT
over the same time period (p=0.002) and the periprosthetic fracture rate was found to be
independent of age or gender. Sharif & Parker (2002) in a review of 12 month outcomes
after UAM implantation in 243 patients found 25.1 % of patients had residual hip pain
and 7% required revision within one year for aseptic loosening (Sharif et al. 2002).
Jadhav et al. (1996) reviewed the 12 – 48 month results of 40 patients managed with the
UAM prosthesis and demonstrated 70% had pain of non infective origin, with calcar
resorption detected in 85% of implantations and radiologic evidence of stem migration
in the majority of cases (Jadhav et al. 1996). The Australian National Joint Registry
demonstrates that the UAM has a significantly higher early revision rate compared to the
CT prosthesis, with over 6% of implantations requiring revision within 2.5 years (HR
2.89; 95% CI 1.8 – 4.6; p<0.0001) (Figure 2-3). The Australian registry data also
suggest that the Thompson prostheses when inserted without PMMA cement has a
similar failure rate to the uncemented Austin Moore, but if an Austin Moore prosthesis is
cemented then the incidence of early revision is similar to that seen with the Cemented
13
Figure 2-3 Cumulative risk for revision of UAM and CT prostheses (Australian
Orthopaedic Association National Joint Replacement Registry 2004).
Thompson (Australian Orthopaedic Association National Joint Replacement Registry
2004).The Australian Joint Registry concludes on the basis of these observations that
continued use of the UAM prosthesis cannot be justified and recommends the use of
PMMA cement as the critical factor influencing results after hemiarthroplasty of the hip
joint (Australian Orthopaedic Association National Joint Replacement Registry 2004).
On the basis of the extensive literature supporting the use of PMMA cement in hip joint
hemiarthroplasty it would therefore appear that the choice of which hemiarthroplasty
prosthesis to use is somewhat less important than the decision to use cement. In
conclusion, femoral side failure after hip joint hemiarthroplasty is a complication largely
avoidable with the utilisation of PMMA cement.
Despite concerns regarding the poor functional outcome and increased revision
rates associated with uncemented hemiarthroplasty prostheses, apprehension about the
systemic effects of PMMA acrylic cement implantation in the elderly patient continues
Figure removed for digital reproduction.
14
to influence prosthesis selection. Cement insertion has been demonstrated to adversely
effect pulmonary and cardiovascular function during the conduct of surgery and the
immediate postoperative period, which may be poorly tolerated in the elderly with pre-
existing co morbidity (Ries et al. 1993; Pitto et al. 1999). Parvizi et al. (1999) in a
review of 38488 hip arthroplasty procedures reported 23 intraoperative deaths due to
irreversible cardiorespiratory disturbances initiated during cementing. 12 of the 23
deaths were in patients undergoing cemented hemiarthroplasty, with the incidence of
intraoperative death contributed by the use of acrylic bone cement during
hemiarthroplasty found to be less than 0.2%. No intraoperative deaths were recorded in
any of the 15411 uncemented hip replacements during the 28 year period. Elderly
patients with pre-existing cardiovascular conditions undergoing arthroplasty for the
management of hip fractures were identified as patients at risk of intraoperative death
associated with the use of cement (Parvizi et al. 1999). These findings are consistent
with a review of 1118 hemiarthroplasty procedures conducted in Queensland public
hospitals, which demonstrated the intraoperative mortality rate attributable to the use of
PMMA cement to be 1/ 738 (Weinrauch et al. 2006). Patterson et al (1991) in a review
of 7 intraoperative cardiac arrests occurring during hip arthroplasty implicated advanced
age, osteoporotic bone and larger volumes of cement as risk factors during the
cementing process (Patterson et al. 1991). Offset against the quantitatively small risk
associated with the use of acrylic bone cement however are the previously outlined
advantages demonstrated for its use in hemiarthroplasty of the hip joint and the high
mortality risk associated with revision arthroplasty. The risk of intraoperative
complications related to the use of cement may be reduced by canal preparation using
pulsatile lavage, venting the femur, avoiding excessive pressurization during insertion
and adequate preoperative hydration (Patterson et al. 1991; Parvizi et al. 1999; Pitto et al.
1999; Breusch 2001).
For secure hemiarthroplasty prostheses, development of symptomatic acetabular
disease including acetabular erosion and protrusio represent common causes for
conversion to THA. Warwick et al. (1998) in a review of 56 patients requiring revision
of failed hemiarthroplasty to THA at an average of 33 months after primary implantation,
found the mode of hemiarthroplasty failure to be femoral loosening in 21 patients
15
(37.5%), acetabular erosion in 26 (46.5%) and combined femoral loosening and
acetabular disease in 5 (9%). Failure by femoral component loosening presented earlier
than acetabular erosion, and was associated with the use of uncemented
hemiarthroplasty prostheses (Warwick et al. 1998). It is important to note that over half
of hemiarthroplasty failures in this series were for the diagnosis of acetabular erosion,
and in over 80% of these cases the femoral component was secure. Revision arthroplasty
in this situation is made significantly more challenging due to the requirement to remove
a firmly secure femoral component from osteoporotic bone. Philips (1989) reviewed the
results of 72 subcapital fractures of the femoral neck managed with CT prosthesis 3 to
14 years after the procedure. Development of symptomatic acetabular erosion was
associated with high levels of functional activity as defined by the University of
California at Los Angeles hip rating system. 34 of 38 (89.5%) patients scoring 4 or
greater (capable of mild activity, including walking, limited housework and shopping)
were diagnosed with acetabular erosion, compared to 0/ 34 scoring 3 or less (p<0.001).
Radiographic evidence of acetabular erosion was also associated with pain and disability
during activity. Of the 57 hips displaying no radiographic evidence of loosening of the
hemiarthroplasty prosthesis, 21 displayed radiographic evidence of advanced acetabular
erosion, all of which were symptomatic. No patient with a radiographically well fixed
stem without evidence of acetabular erosion was symptomatic (Phillips 1989). Yamagata
(1987) demonstrated the incidence of acetabular wear after monopolar and bipolar
hemiarthroplasty increases over the first 5 years after surgery (Figure 2-4). Acetabular
Figure 2-4 Increasing incidence of acetabular disease over time with patients
managed with hip hemiarthroplasty (Yamagata et al. 1987)
Figure removed for digital reproduction.
16
wear was also associated with osteoporosis (p<0.05) and poor acetabular fit due to
inappropriately undersized prosthesis selection (p<0.01) (Yamagata et al. 1987).
Similarly other authors have demonstrated the risk of developing symptomatic
acetabular wear is higher in patients of younger age and higher functional demands
(D'Arcy et al. 1976; Kofoed et al. 1983).
2.1.3 Revision hip arthroplasty and cement removal
Diagnoses leading to the requirement of revision arthroplasty within the Swedish,
Australian and Canadian joint arthroplasty registers are presented in Table 2-2, Table
2-3 and Figure 2-5. Aseptic loosening of prosthetic components is consistently the
leading cause for revision of THA. Aseptic loosening clinically presents with increasing
pain, frequently of a mechanical or activity related nature, typically many years after the
primary surgery. A complete review of the radiographic assessment of THA implant
stability with cemented femoral components is presented in Chapter 2.2.2. Radiological
findings may include progressive radiolucency at the bone cement interface or between
the bone and implant for uncemented prostheses. Radionuclear studies may reveal
abnormal uptake of Tc99MDP consistent with increased bone turnover about the
prosthesis due to bone remodelling. Arthrography of the effected joint may show an
abnormal tracking of contrast medium between the bone–cement interfaces. Aspiration
of the joint for bacterial culture analysis is frequently required to assist in the exclusion
of prosthetic loosening secondary to infection. At time of revision, a loose implant is
unstable to surgical manipulation, but still may be difficult to revise due to sections of
cement or porous surface remaining well fixed to bone. Histological findings of aseptic
loosening include formation of a fibrous tissue interface between the bone and prosthesis,
allowing relative micro motion of the implant (Spinelli 1976; Linder et al. 1983).
Revision THA procedures in the Australian National Joint Registry are
classified as major or minor. Major revision is defined as removal or replacement of a
device which interfaces with bone, considering either the acetabular or femoral
components. Minor revision by comparison is defined as replacement of a prosthetic
device not interfacing with bone, for example exchange of the prosthetic head of a
17
Diagnosis Number PercentAseptic Loosening 13581 76.0Deep Infection 1292 7.2Dislocation 1176 6.6Fracture 966 5.4Technical Error 447 2.5Implant Fracture 276 1.5Miscellaneous 86 0.5Pain Only 57 0.3
Total 17881 100.0
Table 2-2 Diagnosis requiring revision hip replacement for primary THR
performed 1979 - 2003 (The Swedish National Hip Arthroplasty Register 2004).
Figure 2-5 Diagnosis requiring revision hip replacement (Canadian Institute for
Health Information 2004).
Figure removed for digital reproduction.
18
Diagnosis Number PercentAseptic Loosening 5575 49.5Dislocation 1688 15.0Lysis 1133 10.1Fracture 937 8.3Infection 855 7.6Wear Acetabulum 375 3.3Implant Breakage 291 2.7Pain 208 1.8
Total 11261 100.0
Table 2-3 Diagnosis requiring revision hip replacement (Australian Orthopaedic
Association National Joint Replacement Registry 2004).
modular femoral stem or liner exchange with an uncemented acetabular component
(Australian Orthopaedic Association National Joint Replacement Registry 2004).
Revision of cemented hip arthroplasty traditionally requires extraction of both
loose and secure cement after removal of the failed components. Removal of a cemented
femoral prosthesis may be considered in two steps, vis. extraction of the prosthesis and
disruption of the cement – bone interface. For taper design stems without porous coating,
removal of the component is easily completed by the selective removal of overhanging
cement from the shoulder of the stem, followed by application of a retrograde force in
line with the femoral component (Figure 2-6). In contrast however, removal of femoral
cement during revision THA is a time-consuming and hazardous procedure. Proximally
the cement is easily visualised, however the surrounding cortex may be thin and fragile
because of osteolysis. Distally the cement mantle becomes increasingly difficult to
visualise, particularly beyond the metaphyseal flare, sometimes requiring extensile
exposures such as extended trochanteric osteotomy or distal cortical perforation for
adequate access (Hozack 1998; Harkess 1999). Even within major arthroplasty centres,
cement removal results in intraoperative femoral fracture in approximately 8% of
revisions (Gardiner et al. 1993). Other risks associated with cement removal include
cortical perforation, loss of cortical bone stock, excessive blood loss and increased
duration of anaesthesia (Gray 1992; Hozack 1998).
19
Figure 2-6 Extraction of cemented femoral component in line of implant long axis
(Hozack 1998).
Multiple techniques and instrumentation systems have been developed to assist
in the removal of retained cement from the femoral canal, including the use of hand and
power tools, ultrasonic devices and lasers (Choy et al. 1987; Gray 1992; Schurman et al.
1992; Brooks et al. 1993; Gardiner et al. 1993; Cautilli et al. 1994; Hozack 1998).
Femoral cortical perforations occur in approximately 10% of cases using high speed
cement removal instruments (Turner et al. 1987). Such perforations often occur in an
uncontrolled manner producing large gaps in the cortex, rendering the femur at risk of
postoperative fracture (Larson et al. 1988). Cortical perforations commonly occur
distally and posteriorly within the femur, representing a difficult region to reconstruct
for anatomical and surgical exposure perspectives (Gray 1992; Harkess 1999).
Combined use of hand and ultrasonic devices reduces the risk of cortical perforation to
4%, however superficial bone burns are sustained in 9% of cases (Gardiner et al. 1993).
Sustained temperature elevation of magnitude high enough to result in clinically
significant thermal bone necrosis at the bone-cement interface is not produced with
Figure removed for digital reproduction.
20
appropriate use of ultrasonic cement removal tools however (Brooks et al. 1993). Fume
emissions resulting from ultrasonic removal of PMMA cement contain at most 1 part per
million MMA, well below the National Institute for Occupational Safety and Health
recommended exposure limit (Cautilli et al. 1994). Nd:YAG laser disintegration of
cement remains experimental, and is associated with the production of trace
concentrations of aromatic hydrocarbons (Choy et al. 1987).
Schurman and Maloney (1992) describe an interesting technique of femoral
cement removal taking advantage of the high cement-cement interfacial shear strengths
obtained with suitable preparation of the old PMMA cement mantle. After removal of
the femoral component by use of extractor, the retained cement is prepared by wire
brushing, irrigation and drying. A fresh layer of PMMA cement is introduced in a
retrograde manner with the use of a thin nozzle cement gun, and a threaded rod is placed
within the polymerising mass. After hardening of the new cement mantle has occurred,
the threaded rod is screwed out leaving a bilaminar cement mantle with an inner
threaded portion. Extraction rods are then screwed into the centre mantle in 2.5cm
increments, and cement is extracted by the use of a slap hammer. Segmental extraction
of the outer cement is thereby achieved. Using this technique Schuerman and Maloney
(1992) report the results of fifteen revisions, including cases in which the cement was
assessed as firmly secured at the bone–cement interface. No femoral fractures or
perforations were sustained, and complete removal of cement was obtained in 75% of
patients. It is interesting to note that for this technique to be successful, the shear
strength of the interface between new and old cement must be stronger than the bone
cement interface, and greater than the tensile strength of PMMA cement at the point of
controlled mantle fracture (Schurman et al. 1992).
The 2004 Australian Joint Registry demonstrates that 35.8% of major revision
THA procedures are for isolated acetabular revision, as compared to isolated femoral
component revision conducted in only 20.8 % of cases (Table 2-4). This observation
suggests that femoral reconstruction in primary THA in general maintains superior
survivorship to that of acetabular reconstruction. For cemented implants this conclusion
is also supported by the Swedish National Hip Arthroplasty Register and the published
long term survivorship series of cemented THA prostheses (Fowler et al. 1988; Barrack
21
et al. 1992; Mulroy et al. 1995; Williams et al. 2002; The Swedish National Hip
Arthroplasty Register 2004). Revision THA for isolated acetabular revision may
reasonably expected to increase over time, due increasing frequency of THA
implantation and primary cemented femoral implants providing reproducible long term
results (Australian Orthopaedic Association National Joint Replacement Registry 2004).
Component Used Number Percent Femoral and Acetabular 3178 38.2Acetabular Only 2974 35.8Femoral Only 1730 20.8Cement Spacer 168 2Removal Prosthesis 131 1.6Bipolar Head & Femoral Comp. 119 1.4Reinsertion of Components 9 0.1
Table 2-4 Components used in major THA revision (Australian Orthopaedic
Association National Joint Replacement Registry 2004).
During isolated acetabular revision, surgical access is made more difficult by the
position of the femoral neck and prosthetic head with respect to the socket within the
surgical field (Amstutz et al. 1982). Options to improve surgical exposure in these
situations usually include soft tissue and capsular releases, allowing recession of the
femoral head and neck into a position suitable to allow adequate access for
instrumentation to permit acetabular reconstruction. By performing additional capsular
releases a pocket is created, and with the use of retractors the femoral component may
be positioned and restrained into an appropriate location. Aggressive soft tissue
dissection may however predispose the patient to postoperative prothetic instability
(Goldstein et al. 2001). Furthermore, during complex acetabular reconstruction
significant osteolysis may be encountered, and forceful retraction by leverage upon the
acetabular rim may risk intraoperative periprosthetic fracture. Another alternative to
improve acetabular access with the use modular femoral components is for removal of
the prosthetic head and subsequent exchange, however this exposes the morse taper to
damage during the conduct of surgery and the retained femoral neck may still obscure
22
access to the acetabulum. Isolated acetabular revision with a cemented stem and secure
PMMA mantle in situ may alternatively be facilitated by the removal of the entire
femoral component, with retention of the well-fixed cement (Archibald et al. 1988).
Extraction of the femoral component while minimising disruption of the cement mantle
during revision is assisted by the use of polished taper stem designs during the primary
arthroplasty. In this situation the femoral component may be easily extracted by the
selective removal of cement overhanging the shoulder of the prosthesis in proximal and
lateral regions, followed by in-line withdrawal of the stem by mallet and punch or slap-
hammer device. In theory this technique should be classified as a minor revision
according to Australian National Arthroplasty Registry guidelines, as the cement – bone
interface has not been disrupted. Removal of the entire femoral component with
subsequent cement – within – cement (C-C) revision allows optimal exposure of the
acetabulum with minimisation of the soft tissue dissection and retraction required for
femoral positioning. After acetabular reconstruction has been completed femoral
revision may be conducted by replacing the original stem into the old mantle or selecting
a new femoral prosthesis (Archibald et al. 1988). Replacing the original stem has some
theoretical disadvantages however and is infrequently performed, as the original
prosthesis may have accumulated fatigue or become damaged during the process of
extraction therefore predisposing to delayed failure by stem fracture. New femoral
components should therefore be used when performing using C-C revision (Personal
communication, R. Ling 2005). Within the tolerances of the manufacturing process,
minor differences in geometry exist between matched stems of the same design, size and
offset (Stryker Howmedica Osteonics 2002). For this reason, selection of a smaller size
stem is recommended for C-C revision, with the relative reduction in stem size
determining the thickness of the new cement mantle.
23
2.2 Cement – Within - Cement Femoral Revision
Section 2.2 describes the clinical indications and requirements for the conduct of C-C
femoral prosthesis revision. Preoperative planning for C-C revision by radiographic
assessment of the bone-cement interface is discussed. Operative techniques and implants
facilitating the conduct of C-C femoral revision are presented.
2.2.1 Clinical application
Cement-within-cement (C-C) femoral revision refers to the technique of
extracting a well fixed stem with retention of a stable cement mantle, and subsequent
implantation of a cemented revision stem into the old cement mantle. The operative
techniques for C-C revision femoral arthroplasty are illustrated in Figure 2-7. Important
points to note in the revision technique are provision of a clean and dry cement mantle,
the early retrograde insertion of PMMA cement by gun and the use of a femoral
component with revision centraliser to allow normal stem subsidence with polished taper
stem designs (Gruen et al. 1976; Greenwald et al. 1978; McCallum et al. 1995; Li et al.
1996; Flivik et al. 1997; Park et al. 2001; Hubble et al. 2005; Stryker International 2005).
Indications and prerequisite requirements for the conduct of C-C femoral
revision arthroplasty are presented in Table 2-5. The primary cement mantle may be
retained with no modification, or alternatively portions of the cement mantle may be
removed as required either proximally or distally (Hozack 1998; Harkess 1999).
The rationale of cement-cement femoral revision is to avoid the complications
associated with unnecessary cement removal during revision hip arthroplasty as
previously described in Chapter 2.1.4. Advantages of C-C techniques for femoral
component revision are detailed in Table 2-6. C-C revision is assisted by the use of
polished taper design femoral components in primary arthroplasty, as the stem geometry
24
Figure 2-7 Surgical technique of C-C femoral revision (Stryker International).
Figure removed for digital reproduction.
25
and absence of firm adhesion to the cement mantle facilitates stem extraction without
damaging the cement-bone interface (Archibald et al. 1988; Hozack 1998).
Traditional cemented revision by extraction of the entire cement mantle
regardless of the stability of the bone – cement interface potentially compromises the
performance of the cemented revision implant. Halawa et al. (1978) demonstrated the
ultimate shear stress to failure at the interfacial region between bone and cement was
limited by the strength of cancellous bone. Extraction of secure cement requires the
simultaneous removal of cancellous bone, compromising the host surface available for
cement interdigitation during cemented revision THA. For this reason, the ultimate shear
strength of bone cement interface after revision arthroplasty is 30 per cent weaker than
primary interfaces (Rosenstein et al. 1992).
C-C revision is not an applicable technique in the majority of revision hip
arthroplasty procedures due to the presence of osteolysis, infection or femoral cement
loosening (Barrack et al. 1995; McCallum et al. 1995). Contraindications and
disadvantages for C-C femoral revision are presented in Table 2-7.
Only limited clinical studies are available describing the results obtained after C-
C revision. Liebermann et al. (1993) reviewed the results of 19 revision THA procedures
using this technique at an average of 64 months. Despite cement retention techniques,
femoral perforation was sustained in 2 cases in which the original stem was in varus and
limited cement extraction was attempted, thus highlighting the difficulties involved in
cement removal during femoral revision procedures. At final review no C-C femoral
component had required revision for loosening and all were stable on radiographic
evaluation. The authors recommended C-C revision for selected patients with good bone
stock and a secure cement mantle (Lieberman et al. 1993). McCallum and Hozak (1995)
reviewed the results of 15 patients with Barrack grade A or B femoral cement mantles
managed with C-C revision using ultrasonic tools to reshape the existing cement mantle
(Barrack et al. 1992). No femoral perforations or fractures were sustained, and no patient
required extensile approaches for cement removal. Radiographic assessment 4 years
after revision revealed no evidence of loosening in any femoral component or adverse
effect on the bone-cement interface. One patient required re-revision for dislocation at 1
26
Table 2-5 Indications and requirements for C-C Revision Hip Arthroplasty. Refer
to Figure 2-9 and text for explanation of Gruen modes of cemented femoral
component failure.
Indications for C-C Revision Hip Arthroplasty
1. Facilitate surgical exposure in complex acetabular revision
2. Fractured femoral component
3. Correction of leg length discrepancy after hip arthroplasty
4. Recurrent dislocation of THA where prosthetic femoral anteversion is
assessed as contributory
5. Revision of monoblock femoral components to modular systems to
enable adjustment of head size or bearing articulation, usually in
combination with acetabular revision
6. Adjustment of femoral offset
7. Conversion of cemented unipolar hemiarthroplasty to THA
8. Gruen Type 1a failure in composite beam femoral component
9. Significant complication associated with cement removal anticipated
Requirements for C-C Revision Hip Arthroplasty
1. Cemented primary and revision femoral components
2. Stable cement mantle and bone-cement interface
3. Technique facilitated by polished taper design primary stem and
specifically designed C-C revision femoral components
4. Ability to extract femoral component without jeopardising mechanical
integrity of bone – cement interface
27
Table 2-6 Advantages of C-C femoral revision.
year and was managed again with C-C revision. The authors recommended C-C revision
with limited cement mantle modification using ultrasonic instruments as a suitable
technique for management of fractured femoral components, obtaining surgical access
during isolated acetabular revision and correction of stem malposition or articular
bearing modification (McCallum et al. 1995).
The largest clinical series of C-C revisions has recently been reported by Hubble
et al. (2005). Since 1989, 354 cases of C-C femoral revision using the Exeter stem have
been conducted at the Princess Elizabeth Orthopaedic Centre, Exeter England. Clinical
and radiographic results of 175 cases beyond 5 years from revision were presented, with
no patients lost to review. Radiographic analysis revealed no cases of femoral loosening
or osteolysis, and on no occasion was femoral component re-revision required for
subsequent aseptic loosening. The authors conclude C-C revision demonstrates
acceptable outcomes at 5 years and should be considered a valuable approach to revision
hip surgery.
Advantages of C-C Femoral Revision
1. Extensile exposures for cement removal not required
2. Equipment inventory required for revision procedure reduced
3. Cancellous bone retained
4. Allows immediate patient mobility and postoperative rehabilitation
5. Decrease risks associated with cement removal
a. Cortical perforation
b. Blood loss
c. Thermal bone necrosis
d. Intraoperative fracture
6. High bone-cement interface strength maintained
7. Decrease operative time
28
Table 2-7 Contraindications and disadvantages of C-C femoral revision.
2.2.2 Radiographic assessment of the bone – cement interface
Radiographic evaluation provides valuable preoperative information to assist in
the planning of revision THA. Failure of the bone – cement interface may be predicted
by careful radiographic assessment, and is therefore essential prior to the conduct of C-C
femoral revision, as failure at this interface represents an absolute contraindication to
this technique (Archibald et al. 1988; Lieberman et al. 1993).
Contraindications for C-C Femoral Revision
(Relative and Absolute)
1. Prosthetic joint infection
2. Loose femoral component bone - cement interface
a. Preoperative imaging (see Chapter 2.2.2)
b. Intraoperative assessment
3. Revision using uncemented femoral prosthesis
4. Mechanical integrity of femur questionable, where femoral impaction
allografting or long stemmed revision prosthesis is planned
a. Previous femoral perforation with cement leak
b. Impending fracture
c. Osteolysis
Disadvantages of C-C Femoral Revision
1. Assessment of bone-cement interface integrity
2. Limits selection of revision femoral component
a. Cemented
b. Stem geometry and length
3. Interface between new and old cement - potential mechanism of failure
4. Accumulative fatigue in old cement mantle
29
Clinical loosening of prosthetic cemented implants in THA typically occurs at
the bone – cement interface. Stem migration at the prosthesis – cement interface is not
considered evidence of mechanical failure of modern polished taper design femoral
components, which are intended to debond and subside within a viscoelastic cement
mantle (Fowler et al. 1988; Williams et al. 2002). Radiographic assessment allows early
detection of subclinical prosthetic failure, as symptomatic loosening of THA
components is often preceded by evidence on plain radiographs (Gruen et al. 1979;
Weissman 1983). Preoperative radiographic assessment of the bone – cement interface
integrity is therefore universally conducted during the initial evaluation of the painful
THA (Evans et al. 1992).
Radiographic assessment of prosthetic stability after THA requires reproducible
and accurate radiographic technique and diagnostic criteria. Clinical and radiographic
criteria used to define loosening significantly influences the documented correlation
between radiographs and operative findings between published studies (Brand et al.
1986). Radiographic criteria defining loosening of cemented femoral and acetabular
components and the uncemented acetabulum are presently well established, however
assessment of the uncemented femoral stem remains problematic (Hodgkinson et al.
1988; Ranawat et al. 1995; Berger et al. 1996; Ritter et al. 1999; Udomkiat et al. 2001).
Radiographic features of cemented component loosening commonly used in clinical
practice are presented in Table 2-8.
Gruen et al. (1979) reviewed the radiographic appearances of 301 cemented
Charnley prostheses in 389 patients at 6 months to 6 years post implantation. Using a
system of delineated sections about the femoral component for zonal evaluation of
radiographic loosening (Figure 2-8), four predominant modes of cemented stem
loosening were described (Figure 2-9). Mode I failure refers to pistoning of the
prosthesis, either inside a fixed cement mantle (Mode Ia) or composite pistoning of the
cement and prosthesis (Mode Ib). Mode Ia failure was associated with distal cement
mantle fractures due to end bearing of the prosthesis. With regard to Mode Ia failure, the
differences in mechanical behavior between a taper-slip femoral component such as the
Exeter stem and composite beam prostheses require consideration. Composite beam
30
Table 2-8 Radiographic features of femoral component loosening in cemented THA
(Harkess 1999). See Figure 2-8 and text for explanation of Gruen zones.
Figure 2-8 Coronal plane Gruen Zones for the standardised assessment of the
femoral component bone – cement interface (Gruen et al. 1979).
Figure removed for digital reproduction.
31
cemented prostheses are not intended to subside within the surrounding cement mantle,
and debonding of the cement – prosthesis interface in this situation may be considered
evidence of mechanical failure of these stems (Gruen et al. 1979). Subsidence is
however the anticipated behavior for polished tapered collarless stem designs, and when
observed on postoperative radiographs should not be misinterpreted as failure (Fowler et
al. 1988; Williams et al. 2002; Yates et al. 2002). Subsidence of a taper – slip design
stem is readily observed at the prosthesis – cement interface in Zone 1 as demonstrated
in Figure 2-10 and is usually evident 12 months after initial implantation. The normal
subsidence of taper – slip design stems is approximately 2mm in the first two years after
implantation, after which minimal subsidence is observed (Figure 2-11).
Gruen Mode Ib failure (Figure 2-12) is characterized by disruption of the
mechanical integrity between cement and bone. This mode is characterized by a
radiolucent line around most or all of the cement mantle and is the most frequent mode
of failure observed with Charnley prostheses (Gruen et al. 1979).
Gruen failure modes II – IV are associated with varus migration of the prosthesis.
Mode II failure (Medial stem pivot - Figure 2-13) is characterized by medial
Figure 2-9 Gruen modes of radiographic failure of cemented femoral THA
components (for explanation see text) (Harkess 1999).
Figure removed for digital reproduction.
32
Figure 2-10 Radiograph demonstrating normal subsidence of an Exeter stem, with
cement-prosthesis debonding visible in Gruen Zone 1 (courtesy of R. Crawford).
Figure 2-11 Normal subsidence of a collarless cemented polished double taper
femoral component (Yates et al. 2002).
Figure removed for digital reproduction.
33
Figure 2-12 Gruen Mode IB failure (compound subsidence). Radiolucency in Zone
1 representing distal migration of the cement – prosthesis composite, with distal
radiolucency and pedestal formation (Harkess 1999).
migration of the proximal stem coupled with lateral migration at the tip. Mode III failure
(Calcar pivot - Figure 2-14) is due to insufficient distal support of the prosthesis, with
“windshield wiper” lateral migration of the distal stem despite adequate proximal
support. Type IV failure (Cantilever bending - Figure 2-15) is characterized by
insufficient proximal and medial support in the calcar region, with subsequent medial
migration of the proximal stem while the distal prosthesis remains rigidly fixed.
O’Neil & Harris (1984) examined the correlation between preoperative
radiographs and operative findings at revision THA in 61 patients. Radiographic
diagnosis of femoral loosening was made on the observation of prosthetic migration or
the presence of a continuous bone – cement radiolucent zone surrounding the prosthesis
measuring at least 2mm in thickness in any region. Operative criteria for the diagnosis of
stem loosing were defined as motion of the prosthesis with the manual application of
Zone 1 Lucency
Pedestal formation
Figure removed for digital reproduction.
34
Figure 2-13 Gruen Mode II failure (medial stem pivot). Note varus migration of the
stem with separation of the cement – prosthesis interface in zones 1 and 2 (Harkess
1999).
Figure 2-14 Gruen Mode III failure (calcar pivot). Initial postoperative radiograph
depicted on the left for comparison. Progressive varus migration of the distal stem
demonstrated and osteolysis of the greater trochanter however minimal migration
of the proximal prosthesis consistent with pivoting about a proximal (calcar)
location (Harkess 1999).
Separation of cement and
prosthesis
Varus migration
Figure removed for digital reproduction.
Figure removed for digital reproduction.
35
Figure 2-15 Gruen Mode IV failure (bending cantilever). Serial radiographs from
time of implantation from left to right. Left: Varus malposition of the stem at initial
surgery. Centre: Proximal cement – prosthesis lucency in zone 1 without
abnormality in distal cement fixation. Right: Fragmentation of proximal cement
mantle in Gruen Zone 7 with fatigue failure of prosthesis (Harkess 1999).
strong rotational and varus force without the use of a mallet. 47 of 61 femoral
components were assessed as loose at revision, of which 42 were predicted by
preoperative radiographs. The sensitivity and specificity for plain radiographs was found
to be 89% and 92% respectively. The authors concluded that plain radiographs are
accurate with respect to the assessment of cemented femoral components, however the
majority of patients in their series demonstrated prosthetic migration and the reliability
of bone – cement interface radiolucency assessment was not demonstrated (O'Neill et al.
1984).
Lyons et al. (1985) in a similar study to O’Neil & Harris (1984) examined the
correlation between femoral component loosening at revision THA and preoperative
plain radiography in 45 THA femoral components. Radiographic evaluation was
demonstrated to be 76% sensitive with 100% specificity for the diagnosis of femoral
loosening. Varus migration of the stem, fracture of the prosthesis or cement, endosteal
Figure removed for digital reproduction.
36
bone resorption, bone - cement radiolucency greater than 2mm and large cystic defects
in the PMMA mantle were strongly correlated with loosening at time of revision. The
authors noted that progressive widening of the bone cement interface beyond 2mm was
highly suggestive of clinical loosening, however extensive radiolucency involving
multiple Gruen zones is required for the radiographic diagnosis of a loose femoral
component (Lyons et al. 1985).
Hendrix et al. (1983) in a retrieval study at revision THA demonstrated 79%
sensitivity and 100% specificity for radiographic assessment of the cemented stem in 31
patients using bone – cement interface lucency 2mm or greater in thickness as the
criteria for radiographic loosening (Hendrix et al. 1983).
Arthrography provides another modality for the preoperative assessment of bone
– cement integrity. Arthrography is performed by the installation of radiographic liquid
contrast into the hip joint and inspection for penetration of the dye into the region of the
bone cement interface, indicative of loosening (Figure 2-16). Digital subtraction and
post-ambulation techniques can be applied to increase the sensitivity of arthrography
(Maus et al. 1987; Hardy et al. 1998). Aspiration of fluid for the hip joint can also be
undertaken during the conduct of arthrography for the assessment of deep prosthetic
infection (O'Neill et al. 1984). The diagnostic accuracy of contrast arthrography in
isolation is questionable for the diagnosis of loosening with uncemented prostheses and
should conducted in conjunction with other modalities (Swan et al. 1991).
Similar to plain radiography, various diagnostic criteria have been described for
the diagnosis of loosening of femoral components with arthrography. Arthrography
across multiple studies has been demonstrated to have approximately 90% sensitivity
and 85% specificity for the diagnosis of femoral component loosening. (Evans et al.
1992). Maus et al. (1987) reviewed the results of 178 hip arthrograms in 170 patients,
with 97 undergoing revision THA. Of the 62 cases where contrast was detected in the
bone – cement interface below the level of the intertrochanteric line, all except one were
loose at revision. Overall sensitivity was 96% with a specificity of 92%. The authors
recommended the use of digital subtraction and routine intra-articular infiltration of
bupivicaine to assist in the diagnostic assessment of loose THA implants. The diagnostic
37
value of intra-articular local anaesthetic injections for the diagnosis of hip pathology
prior to primary joint arthroplasty has been well established (Crawford et al. 1998).
Due to the high diagnostic accuracy of plain radiography in the assessment of
cemented femoral component loosening, O’Neill et al. (1984) support arthrography only
in the situation where prosthetic migration has not been demonstrated, particularly of the
acetabular component. Arthrography in the preoperative assessment of the painful THA
is recommended when inconclusive information is obtained from serial plain
radiographs (Ovesen et al. 2003).
Figure 2-16 Digital subtraction arthrogram demonstrating loosening of the femoral
implant with extensive extravasation of radio-opaque contrast about the cement –
bone interface and distal contrast leak to cortical perforation (Ovesen 2003).
Radionuclear imaging techniques are also frequently used in the preoperative
assessment of the painful THA. Standard bone scans involve the intravenous
administration of technetium-99m (Tc-99) isotope linked to phosphorus, which is taken
up by the skeleton in accordance with the rate of bone mineralisation (Weissman 1983).
The diagnostic sensitivity and specificity for femoral component loosening in THA is
lower for Tc-99 radionuclear imaging when compared to arthrography (Evans et al.
Figure removed for digital reproduction.
38
1992; Ovesen et al. 2003). Tc-99 scans normally demonstrate increased uptake in the
perioperative period, with up to 20% of cemented prostheses having false positive
results at 12 months, and almost all uncemented acetabular and femoral components
displaying abnormal uptake at 2 years (Evans et al. 1992).
Gallium – 67 (Ga-67) citrate isotope scanning is often used in combination with
Tc-99 bone scans to assist in the differentiation of septic and aseptic loosening. Ga-67
uptake in inflammatory conditions is predominantly by leukocytes, and incongruent or
increased uptake relative to Tc-99 scanning is suggestive of infection or less commonly
inflammatory disorders (Weissman 1983). Indium -111 diethylenetriaminepentacetic
acid (DTPA) radio-isotope scanning, infrequently used in Australia due to availability,
may be used as an alternative to Ga-67 scanning (Evans et al. 1992).
Due to the diagnostic limitations of Tc-99 bone scanning and other nuclear
radiology studies, their routine use in the assessment of the painful THA is not
recommended (Ovesen et al. 2003). Tc-99 bone scans in isolation do not usually provide
additional information over plain radiography. The value of Tc-99 scans remains in
combination with the use of Ga-67 imaging of the loose THA, to differentiate aseptic
and septic processes. In either of these situations however, the patient has a symptomatic
arthroplasty usually requiring surgical intervention, and the diagnosis of infection can be
made more effectively by other methods such as haematological inflammatory markers,
preoperative joint aspiration, intraoperative frozen section, tissue gram stain and
postoperative cultures. The role of nuclear radiography in the assessment of the painful
THA is therefore limited.
2.2.3 Prosthetic femoral components designed for C-C revision
C-C revision femoral arthroplasty is facilitated by particular design features of
both the primary and revision stems. Primary stems with a highly polished surface
minimise the interfacial bond between cement and prosthesis, and in combination with a
straight double taper geometry, prosthesis removal is facilitated with minimal disruption
of the cement mantle (Archibald et al. 1988; Stryker International 2005 B). Collar – free
stems are in general easier to revise, as the presence of a prosthetic collar may impede
39
access to the bone-cement interface should removal of portions of the cement mantle be
required (Hozack 1998).
Selection of suitable femoral components for C-C revision requires consideration
of both stem length and offset. Should the revision stem selected be of equal length to
the primary stem removed, limited cement mantle modification by distal reaming is
required to allow implantation of a centraliser to enable normal postoperative subsidence.
Distal cement reamers have recently been developed for this application, and the results
of a pilot study on the Exeter PMMA Cement Reamer are presented in Appendix 3
(Appendix 3 – Technical Report in the Exeter PMMA Cement Reamer).
Selective removal of distally located cement risks femoral perforation and may
also result in the unnecessary removal of well fixed cement. Theoretically, during the
process of limited cement removal retained adjacent cement may also be damaged,
predisposing to cement mantle failure. Selecting a stem of decreased length minimises or
eliminates the requirement for distal cement mantle modification. Smaller length stems
however frequently have a corresponding decrease in femoral offset which will result in
poor soft tissue balancing and early impingement, predisposing to joint instability. The
Exeter Short Revision Stem (ESRS) (Stryker International; Rutherford, NJ) addresses
both length and offset issues in C-C revision femoral arthroplasty by providing a stem
25mm shorter than standard primary Exeter stems with a standard (+44mm) offset
(Figure 2-17). In the long axis of the implant, the ESRS measures 125mm from the distal
tip to shoulder, or approximately 140mm to the centre of hip rotation. The ESRS with
attached revision centraliser will fit within the secure cement mantle of a standard length
Exeter stem or other standard length cemented stem designs with minimal or no distal
mantle modification.
The Exeter Trauma Stem (ETS) (Stryker International; Rutherford, NJ) is a
polished double taper monoblock hemiarthroplasty for cemented use, with +40mm
femoral offset and stem size 0.5 (between sizes 0 and 1) relative to the existing Exeter
inventory (Figure 2-18). Consistent with other unipolar hemiarthroplasty prostheses, the
principal indication for implantation of the ETS is displaced intracapsular fractures of
the femoral neck, particularly in elderly patients. As described in Section 2.1.2,
common diagnoses
40
Figure 2-17 Standard length Exeter stem with winged centraliser (left) and Exeter
short revision stem (125mm, +44 offset) with revision wingless centraliser (right)
(Stryker International 2005).
Figure 2-18 Exeter Trauma Stem (ETS) monoblock hemiarthroplasty. Unipolar
polished double taper geometry without a prosthetic collar.
Figure removed for digital reproduction.
41
requiring revision of monopolar hemiarthroplasty prostheses are early aseptic loosening
and secondary acetabular disease (D'Arcy et al. 1976; Devas et al. 1983). As the ETS is
designed for cemented implantation, the risk of revision due to early aseptic loosening is
significantly lower than uncemented hemiarthroplasty prostheses (Section 2.1.2).
However should a stable ETS require revision due to symptomatic secondary acetabular
disease, the design features assist simplified revision by C-C techniques. Due to the
straight double taper geometry and polished surface finish, the ETS may be easily
removed by “tap out” with minimal or no disruption of the existing cement mantle
(Stryker International 2005 B). By comparison, the Cemented Thompson (CT)
prosthesis is much more difficult to revise in situations of symptomatic acetabular
disease, as the curved stem geometry requires significant cement removal and access to
the cement-bone interface is impeded by the presence of a prosthetic collar (Archibald et
al. 1988).
2.3 Composition and Material Properties of PMMA Cement
Section 2.3 provides an overview of the composition and material properties of PMMA
acrylic cements used in hip joint arthroplasty, with particular reference to Surgical
Simplex. Intraoperative factors influencing the material properties of PMMA cements
are discussed. The material properties of PMMA cements in shear are presented
separately in Section 2.4.
Surgical PMMA cement is presented for use in two parts, vis. powder and liquid
components (Figure 2-19). Typically the liquid is greater than 95% methylmethacrylate
(MMA) monomer, with the remaining ingredients consisting of the polymerisation
activator dimethyl-para-toluidine (DMpT) and a stabilising agent hydroquinone. MMA
monomer is an ester of methacrylic acid with polymerisable double bond. MMA is a
translucent colourless liquid with a boiling point of 100oC. It is only slightly water
soluble and has an intense odour, detectable when present greater than 0.2 ppm. MMA is
a mild skin irritant and has the potential to induce hypersensitivity reactions in humans
(Kuehn et al. 2005). The primary constituent of cement powder is
42
polymethylmethacrylate (PMMA), which may also include the addition of a copolymer
such as styrene. The powder component also includes the polymerisation initiator
dibenzoylperoxide (BPO) and a radio-opacifying agent such as zirconium dioxide or
barium sulphate. Some PMMA cement powder preparations contain antibiotic agents.
The powder and liquid compositions of four PMMA cements commonly used in
Australian orthopaedic practice are presented in Table 2-9. Variations in the composition
and manufacture of the powder and liquid constituents between surgical PMMA
preparations results in differences in the working behaviour and material properties of
the polymerised cements (Kuehn et al. 2005) (Figure 2-20).
Figure 2-19 Presentation of Antibiotic (Tobramycin) Simplex PMMA cement.
When the liquid and powder components are mixed during the conduct of
surgery, the PMMA polymer powder takes up the monomer liquid by physical processes
known as swelling and dissolution. Swelling and dissolution are responsible for the
initial viscosity and working characteristics of the cement, and is influenced by the
cement composition and powder to monomer ratio (Kuehn et al. 2005). PMMA cement
43
CMW-1 Palacos R Simplex P Zimmer Reg.Powder Benzoyl Peroxide 2.6 0.5-1.6 1.2 0.75BaSO4 9.1 10 10ZrO2 14.9PMMA 88.3 89.3PMMA/MA 83.5-84.7PMMA/PS 82.3
Liquid MMA 98.7 97.9 97.5 97.3Dimethyl toluidine 0.4 2.1 2.5 2.7Hydroquinone (ppm) 15-20 64 75 75
Table 2-9 Composition of four commonly used orthopaedic PMMA cements. MA =
Methacrylic acid; PS = Polystyrene; MMA = Methylmethacrylate (monomer).
Values expressed in wt% (Wright et al. 2001).
Figure 2-20 Differences in molecular weight of PMMA chains due to alternate
sterilisation methods. Variation in the constituents of different cement preparations
produces differences in the material properties of the polymerised products (Kuehn
et al. 2005).
Figure removed for digital reproduction.
44
cures by radical polymerisation, which begins with the interaction of DMpT activator
from the liquid component and BPO initiator from the powder component. This
initiation reaction between DMpT and BPO produces free radicals which react with the
double bond of MMA, resulting in the polymerisation of MMA monomer and polymer
chains in an exothermic reaction. PMMA polymer chains increase in length by the
addition of monomer at one end only. When the “growing end” of two polymer chains
connect, no addition of monomer is possible and the resultant polymer chain can no
longer increase in size (Kuehn et al. 2005). After the initial polymerisation reaction,
typically less than 5 percent of the monomer remains unpolymerised. Limited
polymerisation may still continue however, with a small detectible reduction in the
residual monomer present in the hardened cement over the first two weeks (Figure 2-21).
Continued polymerisation is partly responsible for variations in material properties of
PMMA cement related to duration of post-cure, particularly within the first 7 days of
preparation (Lee et al. 1973; Wilde et al. 1975) (Figure 2-22).
Figure 2-21 Residual monomer in Surgical Simplex PMMA Cement after initial
polymerisation (Kuhn 2000).
Figure removed for digital reproduction.
45
Figure 2-22 Increase in shear strength of Surgical Simplex P cement related to
duration of post-cure (Wilde et al. 1975).
Working properties of PMMA cements after mixing of powder and liquid
constituents may be described in four phases (Kuhn 2001; ISO 5833 2002). The
described phases characterising the early behaviour of PMMA cement are:
Phase I Mixing Phase
Time required to mix constituents into homogeneous consistency.
Phase II Waiting Phase
Dough sticky to touch with an unpowdered latex glove.
Phase III Working (Application) Phase
Cement no longer sticky. Suitable for implantation. ISO 5833
“dough time” refers to the onset of working phase.
Phase IV Hardening Phase
Dough can no longer be joined smoothly. Complete hardening
occurs shortly after (latter known as total setting time).
Figure removed for digital reproduction.
46
Commercial PMMA cement preparations have variable working characteristics,
and are influenced by the ambient temperature and humidity in which the cement is
prepared. The working curves characterising the behaviour of Surgical Simplex P
cement with respect to initial curing time and temperature are illustrated in Figure 2-23.
According to the working characteristics observed during initial preparation,
PMMA cements are classified as high, medium or low viscosity (Kuhn 2001). High
viscosity cements have a short wetting phase and quickly lose their stickiness, entering
the working phase quickly after mixing. The viscosity of “high viscosity” preparations
remains mostly unchanged during the working phase, and slowly increases after entering
the hardening phase. Examples of high viscosity cements include CMW 1, CMW 2,
Palacos R and Palamed (Kuhn 2000). By comparison, low viscosity cements remain
sticky for longer (3 minutes or greater), however when entering the working phase
viscosity rapidly increases, with a narrower transition time between working and
hardening phases. Examples of low viscosity cements include CMW 3, Palacos LV and
Zimmer LVC (Kuhn 2000). Low viscosity cements are generally used in clinical
practice for application by gun under pressure to increase interdigitation into cancellous
bone, thereby increasing the shear strength of the bone cement interface (MacDonald et
al. 1993). Medium viscosity PMMA cements have a low viscous wetting phase and as a
general rule are workable by 3 minutes. During the working phase medium viscosity
cements behave similarly to high viscosity preparations, with viscosity remaining mostly
unchanged or slowly increasing until the hardening phase. Examples of medium
viscosity cements include Antibiotic Simplex, Surgical Simplex P and Zimmer dough-
type radiolucent (Kuhn 2000).
During polymerisation, conversion of MMA monomer molecules to polymeric
form results in decreased intermolecular distance between MMA molecules.
Polymerisation volume reduction is therefore observed with surgical PMMA cement
preparations. Pure MMA has a theoretical volume reduction capacity of 21%. Surgical
cement preparations however contain pre-polymerised PMMA in powder form, which
accounts for approximately two-thirds of the total MMA. The maximum polymerisation
volume reduction of surgical PMMA cement preparations is therefore 6 – 7% (Kuehn et
al. 2005). Multiple factors have been demonstrated to influence the extent and
47
distribution of polymerisation volume changes within PMMA cements in clinical
practice. Polymerisation volume reduction is significantly limited or even reversed by
the presence of air inclusions and porosity (Charnley 1970; De Wijn et al. 1975; Muller
et al. 2002). Porosity within polymerised PMMA may be classified as macropores (pore
diameter greater than 1.0mm) or micropores (pore diameter 0.1 to 1.0mm diameter).
Reduction in porosity with the use of vacuum mixing increases both static and fatigue
mechanical strength of PMMA cements compared to hand mixing (Lidgren et al. 1984;
Alkire et al. 1987; Wixson et al. 1987; Wang et al. 2001; Kuehn et al. 2005). The
influence of vacuum mixing and porosity reduction on the fatigue resistance in cyclic
compression is illustrated in Figure 2-24. Numerous variables influence the effects of
vacuum mixing on porosity, including cement viscosity and volume, amount of suction
applied, stirrer geometry, mixing speed and method of transfer from mixing apparatus to
gun (Kuehn et al. 2005).
Figure 2-23 Working curves for Surgical Simplex P Cement (Kuhn 2000).
Figure removed for digital reproduction.
48
Figure 2-24 Cyclic compression fatigue life of PMMA cements are improved with
porosity reduction by vacuum preparation (Kuehn et al. 2005).
Table 2-10 Factors influencing the extent and distribution of porosity within
PMMA cements.
Factors influencing the extent and distribution of porosity within PMMA cements
Air inclusions produced during mixing of PMMA components
Air dissolved within liquid monomer component prior to mixing
Air inclusions created during cement transfer to application device or gun
Vacuum mixing (reduced porosity)
Prosthesis temperature (distribution of porosity effected)
Application of new cement onto a polymerising mass (increased porosity)
Figure removed for digital reproduction.
49
Factors influencing the extent and distribution of porosity within PMMA cements are
listed in Table 2-10. For hand mixed cement, the total dynamic volume change of the
polymerising mass has been determined as between 3.4% of shrinkage to 3 – 5%
expansion. Vacuum mixing reduces the porosity of the cement and therefore results in
greater polymerisation volume reduction when compared to hand mixed cements,
with up to 6% volume reduction observed after vacuum mixing (Alkire et al. 1987;
Muller et al. 2002). The greater volume reduction in vacuum mixed cement has been
suggested as an explanation for the higher rate of early prosthetic revision with the use
of vacuum mixing observed in the Swedish Hip Registry shortly after this technique
became commercially available (Muller et al. 2002). Latest results from the Swedish Hip
Registry demonstrate the risk ratio for early revision when using vacuum mixed cements
is reduced to 0.74 (Wang et al. 2001).
Porosity distribution and polymerisation volume reduction may not be uniform
within a cement mantle. Preheating the femoral component results in more rapid
polymerisation of the cement in contact with the prosthesis, with reduced porosity
demonstrated at the stem-cement interface and associated improvements in the material
properties of the cement in contact with the prosthesis (Bishop et al. 1996; Parks et al.
1998; Lesaka et al. 2003). This finding somewhat contradicts those of Black et al. (1982)
who demonstrated increased porosity in cements polymerised when in contact with hot
surfaces and considered this to be secondary to trapping of evaporated monomer. In vivo
water absorption into cement also results in a limited delayed volume expansion of the
polymerised mass (Kuehn et al. 2005).
Methods for the characterisation of the material properties of hardened surgical
PMMA cements are defined by ISO 5833 and ASTM standard F 451 – 99a. A useful
reference text comparing the material properties of commercially available PMMA
cements is provided by Kuhn (2000). A summary of typical material properties for
Surgical Simplex P Cement is provided in Table 2-11.
50
2.4 Properties of PMMA Cement and Bilaminar Mantles in Shear
Section 2.4 describes die-punch testing and other techniques for the investigation of
cements in shear. Previous research relating to the shear properties of PMMA cement
and bilaminar interfaces is presented.
Material failure at the interface between new and old cement mantles represents
a theoretical limitation of C-C revision techniques. Finite element analysis demonstrates
variability in the direction and magnitude of the stress vectors applied to the femoral
mantle related to the type of prosthesis, mechanism of loading and location within the
mantle (Bell 2005). Although no finite element analysis (FEA) or experimental models
are currently available to characterize cement mantle stress after C-C revision, it is likely
that the stress applied to interfacial region between layered cement mantles in vivo is a
combination of shear and compression. As compared to compressive loading, shearing
parallel to the interfacial region is more likely to result in dissociation between layered
cement mantles. Although cyclic fatigue testing in hip simulators would more closely
represent the interfacial loading conditions experienced after C-C revision in vivo, static
loading in pure shear represents the best method of providing reproducible data able to
be clearly interpreted independent of confounding factors prosthesis geometry, femoral
anatomy, mantle thickness, loading regime and anatomical position within the cement
mantle. It is for these reasons that pure shear testing is an appropriate and reproducible
laboratory based model for the investigation of C-C interfacial failure in vivo and has
been utilised by a number of authors (Gruen et al. 1976; Greenwald et al. 1978; Li et al.
1996). In order to interpret the results of C-C interfacial shear strength, an understanding
of the shear properties of the base material must first be understood.
Variable methods for the determination of shear properties of PMMA cement
and similar materials have been described (Greenwald et al. 1978; ASTM B 769 - 94
1994; Farshad et al. 1997; ASTM C 273-00 2000; ASTM D 732-02 2002; Guduru et al.
2005; Lutton et al. 2005). ISO and ASTM standards for the testing of PMMA cement do
not define a uniform method for shear strength evaluation (ASTM F 451 - 99a 1999;
ISO 5833 2002). Greenwald (1978) describes the casting of PMMA cylinders which are
placed within a testing rig which guillotines the material in a transverse direction relative
51
Table 2-11 Properties of a typical surgical PMMA cement preparation (Greenwald
et al. 1978; Harper et al. 2000; Kuhn 2000; Kuhn et al. 2001; Kuehn et al. 2005).
to the cylinder long axis, allowing shear stress determination as the cylinder cross-
sectional area is known (Figure 2-25). Moran et al. (1978) and Black et al. (1982)
employed similar methods for the shear testing of PMMA cement. More recently
however, Demian et al. (1998) and Kuehn et al. (2005) in review papers on the
regulatory perspectives and characterisation of material properties of orthopaedic bone
cements, cite ASTM D 732 as the appropriate standard for the testing of PMMA in shear.
Characteristics of Surgical Simplex P Cement
Medium viscosity
Barium sulphate radio-opacifier (10%)
Powder contains styrene copolymer (29.4g per 40.0g powder)
Sterilised by γ irradiation
Recommended mixing sequence: powder then liquid
Long working time
Molecular weight less than 350 000 Da
Relatively low impact strength
Material Properties of a Surgical Simplex P PMMA Cement
Ultimate Compressive Strength 80.1 MPa
Bending Strength (ISO 5833) 67.1 MPa
Bending Modulus (ISO 5833) 2643 MPa
Tensile Strength 50.1MPa
Shear Strength 41.2 MPa
Glass Transition Temperature 97oC (dry post-cure 37oC 24hrs)
Glass Transition Temperature 70oC (wet post-cure 37oC 8 weeks)
Impact Strength 3.9 kJ/m2
52
This is supported by the United States Food and Drug Administration recommendations
for industry guidance into PMMA cement control (Centre for Devices and Radiological
Health 2002). ASTM D 732 – 02 (2002) describes the standard test method for shear
strength testing of plastics by punch tool. The test method describes shear punch testing
(SPT) for the evaluation of shear strength of organic plastics in sheets and moulded discs
between 1.27 and 12.7mm in thickness. Design of the ASTM D 732 – 02 punch tool is
illustrated in Figure 2-26. A similar standardised test method has been described for the
shear testing of aluminium alloys (ASTM B 769 - 94 1994).
Guduru et al. (2005) provides a detailed analysis of factors influencing the
application of punch testing in the evaluation of material shear properties, including
machine – punch compliance, die – punch clearance and specimen thickness. For the
testing of relatively soft materials such as PMMA, Guduru et al.(2005) demonstrated
Figure 2-25 Schematic diagram of the PMMA shear testing apparatus used by
Greenwald et al. (1978). Cast PMMA cylinders with a known cross-sectional area
are subjected to shear between two plates similar to a guillotine.
Figure removed for digital reproduction.
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Figure 2-26 ASTM D732 - 02 Shear punch tool
that correction for compliance of the testing apparatus was not required, however the use
of a retaining ring to constrain the test specimens is recommended. When performing
SPT the tolerance between the die and punch is critical in the interpretation of the data
obtained. Deep drawing (forming) occurs when the clearance is excessive, and if
inadequate clearance is provided then true shear conditions are not achieved (Guduru et
al. 2005). Furthermore, the calculation of shear stress from force data obtained within
the laboratory is dependent on the geometry of both punch and die (Young et al. 2002;
Guduru et al. 2005). This relationship is presented in Figure 2-27. Gurudu et al. (2005)
Figure removed for digital reproduction.
54
Figure 2-27 Variables influencing the calculation of material properties by shear
punch tool (Guduru et al. 2005).
recommends a die punch clearance between 5μm and 20μm as suitable for materials
which demonstrate an ultimate shear stress to failure less than 100MPa.
A summary of the available literature concerning the ultimate shear strength of
PMMA cement is presented in Table 2-12. Greenwald & Wilde (1973) examined the
effect of the addition of the radio-opacifier barium on the shear properties of Surgical
Simplex P. Addition of 10% barium sulphate did not significantly alter the ultimate
shear strength of PMMA. Wilde et al. (1975) determined the ultimate shear stress of
Surgical Simplex P to be 5 762 psi (39.72 MPa). Moran et al. (1979) using a similar
testing technique examined the effect of antibiotic Gentamicin addition on the ultimate
shear stress of Palacos R, with comparison to other readily available PMMA cements.
Increasing concentrations of gentamicin added to the mixture were associated with
statistically significant reductions in the ultimate shear stress of Palacos R. The addition
of 2.0g of gentamicin per 40g cement mix reduced the ultimate shear strength from
41.9Mpa to 37.5Mpa. The ultimate shear stress of gentamicin impregnated Palacos R
greatly exceeded that demonstrated for the bone–cement interface in all situations (<
Figure removed for digital reproduction.
55
4Mpa), with the authors concluding that failure of cancellous bone is the limiting factor
relevant to shear failure in a joint arthroplasty model (Moran et al. 1979) (Figure 2-28).
The shear strength of PMMA Simplex P cement can be increased by 59% with the
addition of 6.6% w/w graphite fibre without significantly influencing the ultimate
compressive stress, however carbon reinforced PMMA cements are not available for
clinical use (Saha et al. 1979).
Figure 2-28 Shear strength of four orthopaedic PMMA cements with variable
concentrations of added gentamicin (Moran et al. 1979).
Black & Greenwald (1982) examined the weakening of Simplex P when applied
to a cement mantle undergoing concurrent polymerisation. Bilaminar cement mantles
were created by the initial production of a PMMA base, with the sequential application
of new cement at differing time periods while the original PMMA base was still
undergoing polymerisation. The second layer of cement was then machined into
cylinders to enable shear testing in a manner similar to that previously employed by
Wilde & Greenwald, and compared to the material properties of the PMMA base as the
Figure removed for digital reproduction.
56
Table 2-12 Summary of literature: PMMA ultimate shear strength. Refer to text
for details.
control (Figure 2-29). Reduction in the ultimate shear strength of Simplex P was
demonstrated when it was applied to a polymerising PMMA cement base, with time
periods of 2 minutes, 5 minutes and 7 minutes between mixing the base and
experimental PMMA mantles. No weakening of the secondary cement mantle was
observed however when applied 20 minutes after polymerisation of the PMMA cement
base. This structural weakening correlated with increased porosity observed within the
secondary cement mantle. The authors suggest the application of new cement onto a
Author Cement MPa Comment
Astleford (1975) Simplex – P 24.7 Strain rate 10-3 rad/ sec
Simplex – P 32.0 Strain rate 10-1 rad/ sec
Black (1982) Simplex 44.8 Exptl 20 min; Density 1.055g/ml
Simplex 15.2 Exptl 7 min; Density 0.554 g/ml
Simplex 8.6 Exptl 5 min; Density 0.542 g/ml
Simplex 15.5 Exptl 2 min; Density 0.708 g/ml
Greenwald (1973) Simplex 33.4 Without radio-opacifier
Simplex 35.1 10% Barium Sulphate added
Greenwald (1978) Simplex 41.2 Uniform cylinders
Li (1996) Palacos LV 40.0 Uniform cylinders
Moran (1979) Simplex 43.5 Without antibiotic
CMW 43.0 Without radio-opacifier
CMW 42.5 10% Barium Sulphate
Sulfix 6 42.5
Palacos R 41.9 Without addition of Gentamicin
Saha (1979) Simplex P 30.0
Simplex P 48.0 6.6% w/w 6mm graphite fibres
Wilde (1975) Simplex 39.7
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polymerising PMMA base adversely affects the material properties of the secondary
cement mantle because of the exothermic environment to which the secondary mantle is
exposed, creating cement voids due to trapped air or vaporising monomer, therefore
increasing the porosity and reducing the material density (Figure 2-30). When applied to
a PMMA base 20 minutes old, more complete polymerisation of the initial mantle had
occurred with a relative reduction in temperature, and the ultimate shear strength of the
newly applied cement mantle in shear was found to be unaffected (44.9 MPa) (Black et
al. 1982).
Figure 2-29 PMMA cement shear properties determination conducted by Black et
al. (1982). Interfacial shear strength was not assessed with this experiment.
Accurate determination of the shear properties at the interface between new and
old cement requires the development of a testing method and instrumentation with
precision greater than that required for the testing of uniform cement mantles.
Greenwald et al. (1978) created bilaminar cement mantles by first pouring PMMA into
cylindrical moulds. Old cement mantles were post-cured for 7 days at 21oC prior to
reinsertion into the casting mould and subsequent casting of new cement to create
bilaminar specimens (Figure 2-31). The bilaminar cylinders were then placed within a
blunt guillotine shear testing device as previously described (Figure 2-25). This process
of test bilaminar specimen preparation and shear stress determination is presented in
Figure 2-31. As the surface area of contact at the interfacial region was known, the
ultimate shear stress for the interface could be calculated. In addition, the influence of
different methods in surface preparation of the old mantle was investigated, including
the effect of surface roughening and the inclusion of blood at the interface.
Figure removed for digital reproduction.
58
Figure 2-30 Porosity observed in the experimental PMMA cement mantle applied
onto concurrently polymerising cement. Heat is transferred to the experimental
cement mass during early polymerisation and porosity is induced by the trapping
of vaporised monomer (Black et al. 1982).
Figure 2-31 Method of interfacial shear stress evaluation (Greenwald et al. 1978).
ASTM C 273-00 describes a standardised test method for the determination of
the shear properties of sandwich core materials. The testing apparatus requires the
application of the test material onto opposing rigid mounting plates which are attached
to swivels to adjust the line of force application normal to the interface between the test
materials (Figure 2-32). Limitations of the test method prevent the useful application of
ASTM C 273-00 to the evaluation of bilaminar PMMA cement mantles. Reproducibility
of the test specimen geometry, particularly of the second mantle applied, and the
relatively complex specimen geometry required is not conducive to the preparation of
Figure removed for digital reproduction.
Figure removed for digital reproduction.
59
multiple standardised test specimens for repeated evaluation. The testing apparatus also
requires the rigid application of the test materials onto a metal backing, which creates
two additional failure regions. The ASTM C 273 - 00 testing method has not been
validated and experimental bias has not been determined (ASTM C 273-00 2000). For
these reasons ASTM C 273 – 00 is unsuitable for the evaluation of interfacial shear
stress of bilaminar PMMA cement mantles.
Figure 2-32 ASTM C 273 – 00 Standard test method for shear properties of
sandwich core materials.
A summary of the available literature regarding the ultimate shear strength of
PMMA cement is presented in Table 2-13 (Gruen et al. 1976; Greenwald et al. 1978; Li
et al. 1996). Greenwald et al. (1978) demonstrated the interfacial strength of bilaminar
Figure removed for digital reproduction.
60
cement mantles approximated that of uniform mantles when the second mantle was
applied within 3 minutes of initiating mixture, the surfaces were clean and rasping was
undertaken to improve mechanical interlock of the surfaces (Figure 2-33). Weakening of
the C-C interface was demonstrated with the late application of cement, which correlated
with monomer loss by evaporation from the polymerising cement mass (Lee et al. 1973;
Greenwald et al. 1978). The authors concluded that high concentrations of PMMA
monomer within the freshly applied cement dough are associated with improved
adhesion to the pre-polymerised PMMA cement mantle. As monomer loss by
evaporation increases with time to application of the fresh cement, this explained the
observed increase in interfacial shear stress values when cement was applied early
(Figure 2-34, Figure 2-35).
Figure 2-33 Interfacial ultimate shear strength shows marked weakening with
blood present at the interface prior to application of the second cement mantle,
however approaches the strength of a uniform mantle with suitable preparation
(Greenwald et al. 1978).
Figure removed for digital reproduction.
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Table 2-13 Summary of literature: PMMA C-C interfacial ultimate shear strength
Author MPa Comment
Greenwald (1978)1 34.8 Clean/ dry interface. New cement applied at 3 min.
38.5 Rasped interface. New cement applied at 3 min.
25.8 Blood at interface. New cement applied at 3 min.
32.3 Clean and dry interface. New cement applied at 4 min.
22.4 Clean and dry interface. New cement applied at 9 min.
Gruen (1976)2 21.9 New cement applied at 2 minutes post mixture.
19.5 New cement applied at 3.5 minutes post mixture.
Li (1996)3 39.0 Clean/ dry interface.
1.2 Blood/ debris at interface.
1 Old cement mantle post-cured for 7 days at 21oC prior to application of fresh
PMMA cement. Bilaminar samples post-cured for 7 days at 21oC prior to testing.
Surgical Simplex P.
2 Old cement mantle post-cured 24 hours prior to fresh PMMA application. Post-
cure temperature not specified. Duration of post-cure of bilaminar specimens not
specified. Surgical Simplex P.
3 Old cement mantle post-cured for 4 days at 37oC prior to application of fresh
PMMA cement. New cement applied when “doughy”. Bilaminar samples post-cured
for 4 days at 37oC prior to testing. Palacos LV.
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Figure 2-34 Interfacial ultimate shear stress between PMMA mantles decreases
when the second mantle is applied late. Late application is associated with a
decreased mixture mass due to the evaporation of monomer from the cement
surface prior to polymerisation (Greenwald et al. 1978).
Figure 2-35 Interfacial ultimate shear strength between PMMA cement mantles
decreases in proportion to monomer loss from the second mix of cement applied
(Greenwald et al. 1978).
Figure removed for digital reproduction.
Figure removed for digital reproduction.
63
Li et al. (1996) found the ultimate shear stress of cleaned bilaminar Palacos LV
cement mantles to be 39 MPa, approximating that of uniform mantles (40MPa). The
inclusion of a thin layer of blood with clots and marrow debris significantly reduced the
ultimate interfacial shear strength (6.2MPa). The authors argue that in clinical practice
the removal of all contaminants from the interface cannot be reliably achieved, and
therefore all cement should be removed.
2.5 Summary of Literature Review and Relevance to Thesis
Section 2.5 discusses the rationale of the selected thesis objectives and hypotheses
presented in Chapter 1 with respect to the available literature.
Cemented THA demonstrates exceptional durability and reliable long term
clinical results in numerous series. Compared to uncemented prostheses, the use of
PMMA cement also provides superior outcomes for patients managed with unipolar
hemiarthroplasty devices for the treatment of femoral neck fractures. Failure of
cemented femoral prostheses in hip joint arthroplasty is predominantly by aseptic
loosening, with symptomatic acetabular wear being an additional significant cause of
failure of hip joint hemiarthroplasty. Revision of cemented hip joint arthroplasty
traditionally involves the extraction of both loose and well fixed PMMA cement. The
removal of secure cement is associated with significant intraoperative complications and
may also jeopardise the stability of the revision implant due to the associated extraction
of cancellous bone attached to the primary cement mantle. In the presence of a secure
cement-bone interface, C-C femoral revision by retention of well fixed cement has been
demonstrated to reduce the complications associated with preparation of the femoral
canal. The suitability of the bone-cement interface for the conduct of C-C revision can
be reliably predicted preoperatively by careful evaluation of plain radiographs and the
adjunctive use of arthrography in selected cases. Limited clinical studies of short to
medium term duration support the technique of C-C revision, and this has lead to the
design of prostheses specific to this application such as the Exeter Short Revision Stem
and the Exeter Trauma Stem.
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Static loading in pure shear represents an appropriate method of providing
reproducible data on C-C interfacial properties which is able to be interpreted
independent of confounding factors such as prosthesis geometry, femoral anatomy,
mantle thickness, loading regime and position related stress variations within the cement
mantle. It is for these reasons that static shear testing has been utilised by a number of
authors for the investigation of C-C interfacial failure in vitro. High interfacial shear
strengths between layered PMMA cements have been demonstrated, however critical to
the technique is adequate preparation of the old cement mantle by provision of a clean,
dry surface which is free from debris such as blood and fat. Mechanical roughening of
the old cement mantle has been demonstrated to result in a small increase in the
interfacial shear strength between layered PMMA cements. C-C adhesion is enhanced
by the application of fresh cement dough with a high monomer concentration shortly
after mixture of powder and liquid components.
Description of the material properties between layered cement mantles remains
incomplete. To date the literature characterising the shear properties of C-C interfaces
has been limited to old cement mantles with a post-cure duration of 7 days or less prior
to the application of fresh cement. These specimens still contain relatively high
concentrations of unpolymerised monomer which may potentially enhance interfacial
adhesion with the freshly applied PMMA cement dough. Furthermore, PMMA cements
demonstrate variability in material properties with respect to duration of post-cure
predominantly within the first 14 days of initial polymerisation. In clinical practice
however aseptic loosening of prosthetic components is a process which takes many
years to occur, and typically the primary cement mantle has post-cured in situ for many
years prior to revision. It is of clinical relevance therefore to characterise the interfacial
adhesion of fresh PMMA cement to an old cement mantle with material properties
similar to that seen in the retained primary cement mantle during C-C revision. For
experimentation of this nature old cement mantles should be post-cured for a minimum
of 14 days. Furthermore, the literature to date has limited the testing of C-C interfaces to
bilaminar specimens which have been post-cured for 7 days or less after the application
of fresh cement. As the monomer content and material properties within the new cement
mantle are expected to change beyond this time period, what effect will this have on the
65
C-C adhesion? Will the shear properties between layered PMMA cements be altered
with maturation of the freshly applied cement? Does the interfacial region display
variations in shear strength behaviour which could impact on recommendations for
postoperative rehabilitation? Characterisation of the material properties and bilaminar
PMMA cement mantles in shear with respect to duration of post-cure is a clinically
important subject which remains largely unanswered.
Cemented revision hip arthroplasty in modern practice is usually conducted with
the use of antibiotic PMMA cement preparations. The literature to date has not yet
assessed the influence of the commercial addition of antibiotics on cement- cement
adhesion. Finally, a description of the mechanisms of adhesion between layered PMMA
cements has not been provided in the literature. These issues form the final subjects for
evaluation within this thesis.
66
67
Chapter 3
Experimental Evaluation of the Shear Properties
of PMMA cements
3.1 Introduction
Chapter 3 describes the experimental evaluation of the shear properties of
PMMA cement, with specific reference to the variation in shear properties with respect
to duration of post-cure. These experiments provide data to validate the testing method
and to supply reference data for the interpretation of results obtained for bilaminar
PMMA cement test samples.
3.2 Hypotheses
1. The shear properties of Surgical Simplex and Tobramycin Antibiotic Simplex
PMMA cements (Stryker, Rutherford, NJ) have variability with respect to
duration of post-cure.
2. Antibiotic Simplex (Tobramycin) PMMA cement demonstrates equivalent shear
properties to Surgical Simplex P.
3.3 Method
3.3.1 Equipment design considerations and objectives
Objectives of the equipment design for this experimentation are to:
1. Enable testing of PMMA cements in shear;
2. Enable the accurate manufacture of test specimens with reproducible geometry;
3. Facilitate the accurate production of multiple specimens;
4. Minimise the wastage of consumables;
68
5. Enable future testing of the interfacial shear properties between bilaminar cement
mantles;
6. Allow modification of test specimen characteristics in accordance with
experimental objectives (cement type, duration of post-cure); and
7. Enable future testing of the shear fatigue properties PMMA cement with minimal
or no modification to testing apparatus.
3.3.2 Overview of experimental equipment and outline of applications
Design of the shear punch testing (SPT) device developed for this research is
illustrated in Appendix 1 and Figure 3-1. The SPT device has two modes of application,
vis. “Casting” and “Testing.” Casting mode refers to utilisation of the device as a mould,
into which PMMA is poured after mixing of liquid and powder constituents shortly after
mixing the cement constituents. This casting process enables the production of PMMA
cylinders from which individual test specimens are manufactured. The process of test
specimen manufacture is discussed further in Section 3.3.3.
Figure 3-1 SPT demonstrating casting and testing modes of application. The
central casting rod is removed for the manufacture of solid PMMA cast cylinders.
Figure removed for digital reproduction.
69
“Testing mode” refers to the utilisation of the SPT device as a die-punch for the
application of shear force onto individual test specimens. Opposite faces of the base
plate and top plates have design features specific to the application of either testing or
casting mode (Appendix 1). For testing mode the surfaces of both top plate and base
plate display 37.9mm diameter recesses which correspond to the outer geometry of the
test specimens. These recesses centrally locate the specimens relative to the zone of
punch. Two 3mm diameter stainless steel locating pins align the base and top plates so
that the centre holes through which the punch passes during testing are aligned. The
process of specimen testing is discussed further in Section 3.3.4.
3.3.3 Test specimen manufacture
Test specimens were manufactured by initially casting PMMA cement cylinders
with a standardised geometry from which individual test specimens were subsequently
produced. Cement preparation was undertaken at 1 atmosphere with a room temperature
of 22 ± 1oC. PMMA cement powder and liquid were mixed in an open bowl by hand
with proportions according to manufacturer recommendations. Cement was mixed at
frequency of 1Hz for 60 seconds, and immediately transferred into the casting mould.
Early pouring of the cement into mould ensured suitably low enough viscosity to enable
the cement to completely adopt the desired geometry prior to polymerisation. The
casting mould and cement were then transferred to a warming cabinet, maintaining
temperature at 37.0 ± 0.5oC. After the cement had hardened, the cylinder was extracted
from the casting mould and stored in a dry environment at 37.0 ± 0.5oC. The
temperature of the casting mould prior to application of cement was measured as 22 ±
1oC using a QM7222 Digitech Infrared Thermometer.
Individual test specimens were manufactured from the cast cylinders by parting
and facing the material on lathes. Heating of PMMA during parting and facing
procedures, particularly above the glass transition temperature (70oC), was considered
undesirable as this would potentially alter the geometry and material properties of the
test specimens. A method of individual test specimen production was therefore
70
developed to minimise heating of the PMMA cylinders during specimen manufacture.
This technique included the selection of suitable cutting tools with appropriate cross
feed rate and lathe speed. Parting of individual specimens was undertaken using a lathe
fitted with four jaw self centring chuck and 3mm Iscar double ended parting tip (product
number: DGN 3102J; IC 354; P20-P40) (Figure 3-2). Parting was undertaken at 650rpm
with a cross feed rate of 300mm per minute. Temperatures at the cutting face during the
parting of specimens were measured perpendicular to the test specimen using a QM7222
Digitech Infrared Thermometer typically ranged from 35.0 – 37.0 oC. The maximum
temperature recorded during the parting of any specimen was 42.1 oC. Inspection of the
PMMA swarf did not show evidence of material melting. Facing was then conducted on
both sides of every specimen using a lathe fitted with a three jaw high precision self
centring chuck and linear scale positioning system with precision of 0.01mm (Figure
3-3). The cutting tool employed for facing was a Seco ground and polished tip usually
used for aluminium product fabrication (product number: TPUR110304R-95, HX).
Facing was undertaken at 1100rpm with a cross feed rate of 300mm per minute.
Temperatures recorded during the facing of specimens were measured by QM7222
Digitech Infrared Thermometer, and typically ranged from 25.0 – 26.5 oC. The
maximum temperature recorded during the facing of any specimen was 27.0 oC, and
swarf inspection did not reveal evidence of PMMA melting during facing procedures.
Parting and facing of the cylinder on lathes resulted in the creation of flat cement discs
of 5mm thickness. The thickness of the discs was confirmed with the use of a digital
verniers calliper with a precision of ± 0.01mm (Figure 3-4). The outer surface of the
cylinder was not reduced during the process of turning, and therefore the cement discs
created retained the same radial geometry as when removed from the original casting
mould. In this manner it was possible to produce a large number of test specimens with
highly reproducible geometry. A small number of test specimens were found to have
macroscopic porosity, defined as a cement mantle surface defect greater than 2mm in
diameter approximating the region of shear testing, and these were discarded due to
concerns regarding potential influence on the shear properties. Test specimens were
stored at 37oC in a dry environment (Figure 3-5).
71
Figure 3-2 Parting of PMMA discs from solid cylinder. With the use of suitable
lathe tip allowing adequate swarf clearance, temperature of the cutting tool
typically measured at 35.0 – 37.0oC, well below the glass transition temperature of
polymerised PMMA (see text).
Figure 3-3 Facing of the test specimens was performed on both sides. With the use
of suitable lathe tip allowing adequate swarf clearance, maximum temperature of
the cutting tool typically measured 25.0-26.5oC, well below the glass transition
temperature of polymerised PMMA (see text).
72
Figure 3-4 Confirmatory measurement of specimen thickness using digital verniers
calliper with precision of 10μm.
Figure 3-5 Dry post-curing of PMMA specimens at 37oC.
73
3.3.4 Evaluation of PMMA shear properties
Determination of the ultimate shear strength of individual specimens was
undertaken using the experimental apparatus described above in “testing” mode (Figure
3-6, Appendix 1). PMMA test discs were placed into the base plate and top plate
37.9mm retaining rings, corresponding to the outer geometry of the test specimens. This
enabled the test specimens to be centrally located relative to the zone of punch testing
within the SPT apparatus. The 37.9mm test specimen recesses on top and base plates
were each 1.5mm in depth, so that when a 5mm thickness specimen was placed into the
device a 2mm gap between the plates is created (Figure 3-7). This enabled a
compressive preload to be to the outer section of the disc with the use of four lag bolts
passing through gliding holes drilled in the top plate and screwing into the base plate.
Deformation (flexing) of the test specimens during the application of shear force by the
punch device was therefore minimised.
Figure 3-6 Shear punch testing using Hounsfield unit.
74
The central SPT punch was advanced using a Hounsfield H25K-S UTM
(Hounsfield Test Equipment, Salfords Redhill, UK) with cross head speed of 1mm per
minute until the ultimate shear strength of the PMMA disc was exceeded. The load cell
employed had a maximum load range of 25 000N, and typically failure of the 5mm
thickness PMMA test specimens occurred at 40% – 60% of this range. Calibration
details of the load cell employed are presented in Appendix 2. The radial tolerance
between SPT punch was measured to be 20 μm with the use of internal and external
micrometers. Shear testing was conducted at 37.0 ± 2.0oC.
Figure 3-7 Sectioned isometric view of SPT device in testing mode. 2mm gap
between top plate and base plate are demonstrated.
3.3.5 Testing schedule and data analysis
Testing was conducted on uniform PMMA cement mantle specimens using
Antibiotic Simplex (Tobramycin) and Surgical Simplex P (Stryker Inc., Rutherford NJ)
cements. Specimens were post-cured prior to SPT evaluation according to the schedule
presented in Table 3-1.
Figure removed for digital reproduction.
75
Cement Type Age (Days) Number Specimens
Surgical Simplex P 0 (1 Hour) 9Surgical Simplex P 1 9Surgical Simplex P 7 8Surgical Simplex P 14 14Surgical Simplex P 30 9Surgical Simplex P 90 13
Antibiotic Simplex 0 (1 Hour) 8Antibiotic Simplex 1 8Antibiotic Simplex 7 8Antibiotic Simplex 14 15Antibiotic Simplex 30 9Antibiotic Simplex 90 5
Table 3-1 Testing schedule for determination of PMMA cement shear properties.
Force required to exceed the ultimate shear strength of the test specimens was
recorded using QMat software (Hounsfield Test Equipment, Salfords Redhill, UK) and
exported into Microsoft Excel (Version 2003; Microsoft Corporation) for further
analysis. The ultimate shear strength of the PMMA material was determined using
Equations 3.1 to 3.3. Equation 3.1 demonstrates the calculation of test specimen ultimate
shear strength, where A is the surface area of the test specimen subjected to shearing
force within the SPT device. Equation 3.2 calculates the test specimen shear area (A),
where Rave is the average radius of the SPT die (R1) and punch (R2), and T is the
thickness of the test specimen (5.00mm). Equation 3.3 demonstrates the calculation of
Rave using R1 and R2 values demonstrated in Figure 3-8.
Calculation of the force/ displacement gradient within the elastic deformation
region of the test specimens was determined using a software specific to this application
written using Matlab 7.0.4 (The MathWorks, Massachusetts, US). Determination of the
shear modulus using the force/ displacement gradient is discussed in Section 3.3.6 and
Section 3.5.
76
Statistical analysis was conducted using Minitab 14 and SPSS for Windows 11.5
statistical software. Parametric and non parametric methods were utilised depending on
the data set and are specified with presentation of results in Section 3.4.
ultFA
τ =
2 aveA R Tπ=
1 2
2ave
R RR +=
Figure 3-8 Illustration of R1 and R2 measurements used in Equation 3.3, where R1
is the radius of the SPT die and R2 is the punch radius.
Equation 3.1
Equation 3.2
Equation 3.3
77
3.3.6 Testing method validation and shear modulus calibration
Testing of 20 pcf (0.32 g/cc)and 40 pcf (0.64 g/cc) solid rigid polyurethane foam
specimens (Sawbones, Pacific Research Laboratories Inc, Washington USA) with
known shear properties was conducted at 22oC in order to validate the experimental
technique and to calibrate the SPT device to allow the evaluation of shear modulus. The
individual foam specimens were manufactured from solid block material using the
turning and facing methods described in Section 3.3.3, resulting in specimens of
identical geometry to the PMMA test specimens (37.9mm diameter x 5mm thickness
discs). Foam reference specimens were tested using the SPT device as described in
Section 3.3.4. The ratio of the experimental force/ displacement gradient to the known
shear modulus for the sawbones specimens was calculated in order to provide a
calibration ratio (x) for the calculation of shear modulus of the PMMA test specimens
(Equation 3.4 and Equation 3.5). Equation 3.4 demonstrates calculation of the shear
modulus calibration ratio (x), where ΔF/ΔD is the gradient of the force/ displacement
plot and G is the shear modulus. Equation 3.5 calculates the shear modulus of test
specimens using calibration ratio (x) obtained in Equation 3.4. Rationale for this method
of test specimen shear modulus calculation is further discussed in Section 3.5.
foam
foam foam
FxD GΔ
=Δ
specimenspecimen
specimen
x FGD
=ΔΔ
Equation 3.5
Equation 3.4
78
3.4 Results
3.4.1 Sawbones reference and shear modulus calibration
Comparison of expected and experimental shear properties of 20 pcf and 40 pcf
Sawbones solid rigid polyurethane foam are presented in Table 3-2. Ultimate shear
strength of the sawbones material is quoted as having ±10% variability according to the
manufacturer (James 2005). Experimental shear strength measurements obtained for the
reference sawbones material closely correlated with the expected values. The shear
modulus calibration ratios (x) calculated for both 20 pcf and 40 pcf foams were similar,
demonstrating the reproducibility of the method of shear properties evaluation. The
shear modulus calibration ratio obtained for 40 pcf foam was selected as the reference
standard for calculation of shear modulus of the PMMA test specimens as 40 pcf foam
had a higher shear modulus which more closely represented that of PMMA and the
force/ displacement gradient measurements obtained for 40 pcf foam were more
reproducible as demonstrated by a lower percentage variation between samples.
20 pcf Foam 40 pcf Foam
Expected shear strength (MPa)* 4.50 14.00Experimental shear strength (MPa) 5.33 17.84Shear strength 95% C.I. (MPa) 4.91 - 5.37 17.52 - 18.16
Expected shear modulus (MPa)* 67.00 187.00Experimental force displacement (N/mm) 2503.55 7992.42Force/ displacement 95% C.I. (N/mm) 2294.76 - 2712.34 7845.01 - 8139.83Shear modulus calibration ratio (X) 0.02676 0.0234
Table 3-2 Results of Sawbones reference sample testing. Expected shear strength
and modulus values obtained from manufacturer. * Values +/- 10%. (James 2005;
Pacific Research Laboratories 2005).
79
3.4.2 Uniform PMMA test specimen shear properties
The ultimate shear strength of Antibiotic Simplex and Surgical Simplex P
cements were comparable (p=0.13; two-way ANOVA), and both variations in ultimate
shear strength with respect to duration of post-cure (p<0.001) (Figure 3-9, Table 3-3). A
quantitatively small reduction in ultimate shear strength of approximately 5 MPa was
observed for both cements at days 7 (p<0.05; LSD post hoc analysis) and 14 (p<0.001),
which recovered by day 30 to initial values (Figure 3-9, Table 3-3). Variability in the
force displacement gradient and derived shear modulus was observed to be less than five
percent for both cement types over the 90 day time period (Figure 3-10, Table 3-3). The
average shear modulus of Surgical Simplex over the 90 day time period was 442 MPa.
Antibiotic Simplex cement did not demonstrate a statistically significant difference in
the shear modulus compared to Surgical Simplex (446 MPa; p=0.10 Mann Whitney U
test).
Ultimate Shear Strength.
0
10
20
30
40
50
60
0 1 7 14 30 90Days
MPa
Antibiotic Simplex Simplex
Figure 3-9 Ultimate shear stress to failure of Antibiotic Simplex and Surgical
Simplex P cements with 95% confidence intervals marked. Similar variations in
shear properties related to duration of post-cure are observed for both cement
preparations.
80
PMMA Shear Modulus.
0
100
200
300
400
500
0 1 7 14 30 90Days
MPa
Antibiotic Simplex Simplex
Figure 3-10 Calculated shear modulus of Antibiotic Simplex and Surgical Simplex
P cements with 95% confidence intervals marked.
Cement Type Age (Days) Strength (Mpa) 95% CI Modulus (Mpa) 95% CI
Ab Simplex 0 50.51 1.86 444.30 11.33Ab Simplex 1 49.77 1.80 444.93 7.97Ab Simplex 7 48.75 1.58 461.66 10.00Ab Simplex 14 44.98 1.27 448.58 8.92Ab Simplex 30 48.83 1.73 459.10 6.60Ab Simplex 90 46.62 1.69 424.12 9.26
Simplex 0 47.45 0.93 438.96 7.13Simplex 1 49.29 0.77 442.85 6.42Simplex 7 44.85 1.53 435.13 12.67Simplex 14 43.00 1.40 414.90 9.36Simplex 30 47.09 1.99 445.08 7.14Simplex 90 50.59 1.01 451.50 7.48
Table 3-3 Tabulated data from Figures 3-9 and 3-10.
81
3.5 Discussion
This experiment demonstrates the application of punch testing for the materials
evaluation of uniform cement mantles in shear. The method of shear properties
evaluation described within this research is modelled on ASTM D 732 and observes
recommendations made by Guduru et al. (2005). Determination of the ultimate shear
stress and modulus of PMMA cements using the described testing technique provides
highly reproducible results which are comparable to that previously described in the
literature.
Shear modulus was approximated by indirect methods using materials of known
shear modulus for a calibration standard as this was considered the most accurate
method of shear modulus calculation using data obtained from punch testing. Although
direct calculation of shear modulus is possible by measuring the punch displacement and
die punch clearance (Figure 3-11), this would provide only a gross approximation at best
as this technique is limited by the accuracy of measurements taken to determine the
clearance of the SPT device. Radial clearance of the SPT device is estimated to be 20μm
by subtraction of the punch and die radii obtained by the use of internal and external
micrometers. The concern with using this measurement for the direct calculation of
shear modulus relates to the accuracy of the clearance measurement and precision of the
micrometers used. Quantitatively small errors in the measurement of either the punch or
die would result in proportionally large errors in the calculated shear modulus due to the
small clearance margin of the SPT device. By using a reference material with known
material properties to correlate force/ displacement data to shear modulus, precise
calculation of the die-punch clearance is not required. This indirect method of shear
modulus determination is therefore preferable when using a die-punch apparatus with
small radial tolerances. Validation of the precision of shear modulus calculation by this
indirect method is provided by the high correlation between shear modulus calibration
ratios obtained for both 20 pcf and 40 pcf reference specimens.
This experiment demonstrates variability in the ultimate shear strength of
Surgical Simplex P and Antibiotic Simplex PMMA cements with duration of post-cure.
Although the variability in shear strength observed was quantitatively small, due to the
precision of the testing method employed, changes in shear strength related to duration
82
Figure 3-11 Above: Schematic representation of SPT device in testing mode. R1 =
radius of die. R2 = radius of punch. Below: Magnification of the zone of shearing.
The portion of material undergoing shear is represented in light blue. Die-Punch
clearance was estimated to be 20μm by use of micrometers, however inaccuracy in
this clearance measurement would potentially create large errors in the assessment
of shear modulus by direct calculation. Indirect methods for shear modulus
determination were therefore employed.
83
of post-cure were found to be statistically significant. As relative weakening of the
PMMA cement preparations was only observed at the 7 and 14 day time intervals, and
quantitatively the reduction was small (approximately 5MPa or 10% of initial values), it
is unlikely that variations in shear strength of PMMA cements related to duration of
post-cure are clinically relevant. The characterisation of the shear properties of uniform
PMMA cement mantles with respect to duration of post-cure serves as a useful reference
for investigation of the interfacial shear strength of bilaminar cement mantles.
3.6 Conclusions
Surgical Simplex P and Antibiotic Simplex (Tobramycin) PMMA cements
demonstrate variability in ultimate shear strength with respect to duration of post cure,
however the magnitude of these variations are small and are unlikely to be of clinical
relevance. The manufacturer’s addition of Tobramycin antibiotic does not significantly
alter the shear properties of Simplex PMMA cement. The developed method of material
properties evaluation using a SPT apparatus modelled on ASTM D732 provides an
accurate and reproducible means of calculation of ultimate shear strength and shear
modulus estimation for surgical PMMA cements.
84
85
Chapter 4
Experimental Evaluation of the Shear Properties
of Bilaminar Cement Mantles
4.1 Introduction
Chapter 4 describes experimental investigations relating to the ultimate shear
strength of cement – cement interfaces. Three main objectives were selected for this
experimentation based on issues relevant to the clinical practice of cement – cement
revision arthroplasty:
1. How does the shear strength of bilaminar cement mantles compare to uniform
mantles?
2. Does the interfacial region display time dependant variations in shear strength
behaviour which could influence recommendations for postoperative weight
bearing?
3. Given that antibiotic loaded cements are preferentially used in revision hip
arthroplasty procedures, does the use of antibiotic PMMA cement in primary or
revision hip arthroplasty result in weakening of the C-C interfacial region?
4.2 Hypotheses
1. The ultimate shear strength of the interfacial region between bilaminar cement
mantles approximates that of uniform mantles.
2. Bilaminar PMMA cement mantles demonstrate variation in the interfacial shear
properties with duration of post-cure.
3. Antibiotic Simplex when used in bilaminar cement mantles results in weakening
at the interfacial region.
4. Interfacial adhesion by mechanisms other than mechanical interlock play a
significant role in the bond formed between new and old cement mantles.
86
4.3 Method
4.3.1 Bilaminar test specimen manufacture and testing schedule
Bilaminar PMMA cylinders containing layered cement were manufactured by
inserting a smooth 316L stainless steel casting rod into the centre of the casting mould
prior to the pouring of cement. The casting rod had a diameter of 19.1mm and surface
roughness approximating that of a polished femoral stem used in primary hip
arthroplasty (<0.2 μm Ra). Both ends of the outer casting mould and central casting rod
were located and constrained by recesses in the casting mould base plate and top plate.
In this way the spatial location of the casting rod was accurately and reproducibly
centralised throughout the long axis casting cylinder. Cement constituents for the outer
PMMA mantle were mixed in an open bowl by hand at 1Hz for 60 seconds at 1ATM
and immediately poured into the casting mould including the centrally located casting
rod. The temperature of the casting mould and rod prior to application of cement was
measured as 22 ± 1oC using a QM7222 Digitech Infrared Thermometer. Early pouring
of the cement into the mould ensured suitably low viscosity and enabled the cement to
completely adopt the desired geometry prior to polymerisation. After the cement had
cured for a minimum duration of 30 minutes, the casting rod was extracted from the
PMMA cylinder using a Hounsfield testing device with a cross head speed of 500mm
per minute. This process enabled the production of cast PMMA cylinders with a central
hole of diameter and central location corresponding to the casting rod (Figure 4-1).
PMMA cylinders were removed from the casting mould and stored in a dry environment
at 37.0 ± 0.5oC for a variable duration according to the testing schedule presented in
Table 4-1. Surface roughness of the old cement mantle interfacial region after casting
rod extraction was measured in representative samples using a surface profilometer
(Taylor Hobson Surtronic 3+; Leicester England). Light microscopy (LM) and scanning
electron microscopy (SEM) were used to visually assess the interfacial surface of the
outer cement mantle in representative samples.
After appropriate post-curing of the outer cement mantle (Table 4-1), the fresh
PMMA cement was prepared as previously described and poured into the central portion
87
Figure 4-1 Simplex PMMA cement cylinder with central hole after removal of
casting rod. After post-curing of the PMMA cylinder, fresh PMMA cement was
poured into the central hole, therefore creating a solid cement cylinder with outer
“old” cement and inner “new” cement.
of the hollow cement cylinder, creating a bilaminar PMMA cylinder with the freshly
applied cement surrounded by an old cement mantle. Pouring of the second PMMA
mantle was undertaken 90 seconds after initiating mixture of the liquid and powder
constituents, as this was judged to be representative of the earliest time in which cement
could be applied in a clinical setting during C-C revision. The bilaminar PMMA
cylinders were then stored in a dry environment at 37.0 ± 0.5oC for post-cure of variable
duration in accordance with the testing schedule (Table 4-1).
Individual test specimens were manufactured from the cast cylinders by parting
and facing the material on lathes as described in Section 3.3.3. This resulted in
individual bilaminar test specimens of 5mm thickness (Figure 3-4, Figure 4-2). LM and
SEM examination of the bilaminar PMMA test specimens was undertaken in
representative samples to evaluate the interfacial region between the new and old cement
mantles.
88
Group PMMA Cement (Old) Age (Days) α PMMA Cement (New) Age (Days) β Number Specimens
1 Surgical Simplex P 14 Surgical Simplex P 0 (1 Hour) 162 Surgical Simplex P 14 Surgical Simplex P 1 173 Surgical Simplex P 14 Surgical Simplex P 7 124 Surgical Simplex P 14 Surgical Simplex P 90 145 Surgical Simplex P 90 Surgical Simplex P 1 156 Antibiotic Simplex 14 Antibiotic Simplex 1 18
α Age of outer (old) cement mantle prior to new cement applicationβ Age of bilaminar specimens prior to shear testing
Table 4-1 Testing schedule of bilaminar test specimens for the determination of
PMMA interfacial shear properties.
Figure 4-2 Completed 5mm thickness bilaminar PMMA test specimen. For
illustration purposes, coloured cement is used to differentiate the outer (old)
cement mantle from the inner (new) cement mantle.
89
4.3.2 Bilaminar test specimen evaluation of shear properties
Determination of the ultimate shear strength of individual specimens was
undertaken using the SPT apparatus as described in Chapter 3 in “testing” mode.
For LM and SEM examination of the fracture surface in two planes, sectioning of
the bilaminar discs was conducted using a water irrigated microtome (Microslice 2,
Malvern Instruments Ltd, Worcestershire UK) fitted with a 280 grit diamond annular
blade (Van Moppes Super Abrasives; Gloucestershire UK). Detection of mantle cracks
under LM was assisted with the use of fluorescent dye penetrant (Ardrox 970P25;
SureChem, Girraween Australia) and an ultraviolet light source (Magnaflux ZB24A220/
240; Swindon Wiltshire UK).
Shear testing of PMMA cement against polished 316L stainless steel with an
equivalent surface roughness to the interfacial region of the outer cement mantle
(0.22μm Ra) was conducted in order to provide a reference for further assessment of the
mechanisms of C-C adhesion. For this test a steel mould with a 19.1mm central hole was
filled with PMMA cement and post-cured for 24 hours. Cement preparation and shear
testing at the cement-metal interface was conducted as described previously. This test
was conducted 5 times utilising the same metal mould.
4.4 Results
4.4.1 Surface characteristics & fracture evaluation by LM and SEM
Roughness of the interfacial surface of the outer (old) cement mantle after
casting rod extraction prior to the application of fresh cement was 0.22 μm Ra (95% CI
0.20 – 0.24 μm Ra). LM and SEM of the interfacial surface of the old cement mantle
prior to the application of new cement showed a smooth surface with minimal porosity
(Figure 4-3 and Figure 4-4).
Examination of the bilaminar PMMA specimens using LM and SEM prior to
punch testing was unable to detect a crack or easily identifiable interface plane between
the new and old cement mantles. After punch testing both LM and SEM revealed clean
separation between the new and old cement mantles with a regular linear fracture at the
interfacial region, and is illustrated in Figure 4-5 to Figure 4-9.
90
Figure 4-3 SEM of the interfacial surface of the old cement mantle prior to the
application of new cement. Smooth surface with minimal porosity is demonstrated.
Surface roughness of this interface measured by profilometry was 0.22μm Ra.
(Original magnification x27).
Figure 4-4 SEM of the interfacial surface of the old cement mantle prior to the
application of new cement. At high power magnification the surface profile of the
cement secondary to defects on the casting rod and irregularities created during
the process of rod extraction are appreciated on careful inspection (Original
magnification x1500).
91
Figure 4-5 LM of interfacial region between inner PMMA and outer PMMA
cement mantles after SPT testing. Clean separation between the cement mantles is
observed (Original magnification x10).
Figure 4-6 SEM of interfacial region between Surgical Simplex P cements after
punch testing. Brittle fracture at the interface is demonstrated (Original
magnification x80).
Figure removed for digital reproduction.
92
Figure 4-7 Transverse section SEM of a bilaminar Simplex – Simplex test specimen
at the interfacial region after SPT evaluation. Linear separation between new and
old cement mantles is demonstrated. Outer (old) cement mantle is located upper
left and inner (new) cement mantle is located lower right. (Original magnification
x110).
Outer Mantle (old cement)
Inner Mantle (new cement)
Figure removed for digital reproduction.
93
Figure 4-8 LM florescent dye penetrant enhanced transverse section examination
of a bilaminar PMMA test specimen after SPT testing demonstrating linear cement
fracture through the interfacial region.
Figure 4-9 LM florescent dye penetrant enhanced examination plan view of a
bilaminar test specimen after SPT testing demonstrating cement fracture through
the interfacial region.
94
4.4.2 Bilaminar PMMA test specimen shear properties
Bilaminar cement mantles demonstrated a 15 to 20 percent decrease in shear
strength over all time periods evaluated when compared to uniform PMMA cement
mantles (p<0.001; two-way ANOVA) (Figure 4-10, Table 4-2). The ultimate shear
strength of bilaminar cement specimens was observed to temporarily decrease by 8.2
MPa (19.4%) seven days after initial cure (p<0.001; two-way ANOVA). Specimens
with an outer cement mantle aged for 90 days prior to the application of fresh cement
had equivalent shear properties to specimens with an outer mantle aged for only 14 days
(ultimate shear strength 43.75 MPa; 95% CI 41.47 - 46.03 MPa; p=0.17). Variability in
shear modulus
Simplex Ultimate Shear Strength. Uniform v Bilaminar Mantles
0
10
20
30
40
50
60
0 1 7 90Days
MPa
.
Uniform Mantle Cement - Cement
Figure 4-10 Ultimate shear strength of Surgical Simplex P compared to bilaminar
test specimens manufactured from the same material with 95% confidence
intervals marked. Outer mantle of the bilaminar test specimens was post-cured for
14 days prior to application of the new mantle and tested 0 – 90 days after new
mantle application. Bilaminar PMMA mantles demonstrate time dependant shear
properties similar to that of uniform mantles, however a comparative reduction in
ultimate strength by 15 to 20 percent was observed.
95
of bilaminar Simplex test specimens was observed to be less than five percent over the
90 day time period (Figure 4-11). The average shear modulus of bilaminar surgical
simplex was 443.6 MPa (95% CI 438.9 – 448.3 MPa) over the time period. No
difference in the shear modulus was observed between bilaminar and uniform cement
mantles (p>0.1; two-way ANOVA).
Bilaminar PMMA test specimens manufactured using Antibiotic Simplex for
both cement mantles demonstrated equivalent ultimate shear strength (p=0.52; Kruskall-
Wallis test) and modulus to specimens using Surgical Simplex for both cement mantles
(Figure 4-12, Figure 4-13, Table 4-2).
The ultimate shear strength of Surgical Simplex P cement against 316L stainless
steel (0.22μm Ra) was 1.2 MPa (95% CI 0.6 – 1.8 MPa).
Simplex Shear Modulus. Uniform v Bilaminar Mantles.
0
50
100
150
200
250
300
350
400
450
500
0 1 7 90Days
MPa
.
Uniform Mantle Cement - Cement
Figure 4-11 Calculated shear modulus for uniform mantle and bilaminar Surgical
Simplex P PMMA samples. 95% confidence intervals marked.
96
Interfacial Shear Strength. Plain v Antibiotic Cements.
0
10
20
30
40
50
60
Simplex - Simplex Ab Simplex - Ab Simplex
Cement Type
MPa
.
Figure 4-12 Interfacial shear strength bilaminar test specimens comparing cement
preparations with and without the commercial inclusion of Tobramicin antibiotic,
95% confidence intervals marked.
Interfacial Shear Modulus. Plain v Antibiotic Cements.
0
100
200
300
400
500
Simplex - Simplex Ab Simplex - Ab Simplex
Cement Type
MPa
.
Figure 4-13 Interfacial shear modulus of bilaminar test specimens comparing
cement preparations with and without the commercial inclusion of Tobramicin
antibiotic. 95% confidence intervals marked.
97
Group Outer Age (Days) Inner Age (Days) Strength (Mpa) 95% CI Modulus (Mpa) 95% CI
1 Simplex 14 Simplex 0 36.27 1.34 427.84 8.352 Simplex 14 Simplex 1 42.67 0.99 448.36 5.933 Simplex 14 Simplex 7 34.40 1.30 438.40 6.904 Simplex 14 Simplex 90 43.36 1.22 459.93 8.265 Simplex 90 Simplex 1 43.75 1.15 457.96 7.286 Ab Simplex 14 Ab Simplex 1 42.19 1.16 451.25 6.68
Table 4-2 Tabulated data for C-C interfacial shear properties displayed in Figures
4-10 to 4-13.
4.5 Discussion
Mechanisms of adhesion between polymer solids include surface mechanical
interlocking, polymer chain inter-diffusion, chemical bonding and electrostatic
interactions (Garbassi et al. 1998) (Figure 4-14). It is generally accepted that the
different mechanisms of adhesion make a variable contribution to the strength of the
interface between bonded polymers. In this study the interfacial surface roughness of the
outer (old) cement mantle after casting rod extraction was 0.22 μm Ra (95% CI 0.20 –
0.24 μm Ra). This surface roughness is of particular clinical relevance as it approximates
Figure 4-14 Contributions to adhesion between polymer solids: (a) mechanical
interlocking; (b) interdiffusion of chains; (c) electrical interactions; (d) chemical
interactions (Garbassi et al. 1998).
Figure removed for digital reproduction.
98
that of a PMMA cement mantle after removal of a polished cemented stem without
further intraoperative modification such as rasping (<0.25 μm Ra; Weinrauch 2005:
Unpublished data, Queensland University of Technology). Despite the smooth surface
finish reducing the opportunity for mechanical interlock between the cement mantles
(Bundy et al. 1987; Chen et al. 1999), high C-C interfacial shear stress values were
observed in this study and also by previous authors (Gruen et al. 1976; Greenwald et al.
1978; Li et al. 1996). The observed C-C interfacial shear stress (approximately 40MPa)
greatly exceeded the cement-metal interfacial shear stress (1.2 MPa) despite similar
surface roughness. The limited role of adhesion by mechanical interlock between
PMMA and metals of roughness similar to that used in this study has been well
established in the literature (Beaumont et al. 1977; Raab et al. 1981; Bundy et al. 1987;
Chen et al. 1999). Interfacial adhesion by mechanisms other than mechanical interlock
must therefore play a significant role in the bond formed between new and old PMMA
cement mantles. Bundy et al. (1987) noted the adhesion of PMMA cement to metals
paradoxically increased with decreasing surface roughness beyond a threshold limit
(Figure 4-15). On the basis of these observations, Bundy et al. (1987) hypothesised that
with decreasing surface roughness, adhesion by mechanical interlock decreases however
there is increased adhesion by atomic (or chemical) interactions such as Van der Waals
forces (Bundy et al. 1987). At the interfacial region between C-C mantles only limited
chemical bonding is possible as the residual monomer content within aged orthopaedic
cements is characteristically less than 1 percent, and terminated PMMA chains within
the old mantle possess few reactive sites for further polymerisation with the monomer of
the freshly applied cement (Kuhn 2000; Kuehn et al. 2005). This would therefore
suggest that the predominant adhesion mechanisms resulting in firm bonding between
new and old cement are interfacial inter-diffusion and possibly electrostatic adhesion.
Adhesion by inter-diffusion or “molecular interdigitation” is obtained when the addition
of fresh cement dough containing relatively high concentrations of monomer causes
swelling and separation of the PMMA chains at the interfacial surface of the old cement
mantle. This allows limited diffusion of the monomer from the freshly applied cement
between the PMMA chains of the old cement mantle, and subsequent polymerisation
results in the formation of PMMA chains which bridge the interfacial region. This
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mechanism would explain the findings of previous authors correlating monomer
concentration with interfacial shear strength between bilaminar cement mantles (Gruen
et al. 1976; Greenwald et al. 1978). Molecular interdigitation between the cement
mantles mechanism also explains the absence of a clearly defined interface between
cement mantles with LM and SEM examination prior to shear testing.
This experiment demonstrates C-C interfacial strength results similar to that
obtained by previous authors (Gruen et al. 1976; Greenwald et al. 1978; Li et al. 1996).
Bilaminar mantles demonstrated a reduction in ultimate shear strength of only 15 – 20 %
compared to of uniform mantles across all time periods, with interfacial stress to failure
34.4 – 43.7 MPa depending on the duration of post-cure. Variation in interfacial shear
strength with respect to the duration of specimen post-cure was observed for bilaminar
test samples. After initial maturation and strengthening of the interfacial region within
the first 24 hours of the application of fresh cement, a temporary weakening of
Figure 4-15 Hypothetical components of the adhesion force between PMMA bone
cement and metal surfaces (Bundy et al. 1987).
Figure removed for digital reproduction.
100
8.2 MPa (19.4%) was observed at day seven. For bilaminar test specimens with an old
mantle with at least 2 weeks of age, variability in the interfacial shear strength was
demonstrated to be most dependent on the duration of post-cure of the freshly applied
cement (new mantle). Similar patterns of shear strength variability were observed for
uniform mantle test specimens and that observed for bilaminar mantle test specimens
with an equivalent duration post-cure of the new cement mantle. Duration of outer (old)
cement mantle post-cure prior to application of new cement (2 or 12 weeks) was not
demonstrated to influence the interfacial shear strength between cement mantles. These
observations provide additional support for the importance of adhesion by molecular
interdigitation between layered PMMA cements, as the PMMA chains bridging the
interfacial region are polymerisation products of the freshly applied cement dough.
Commercial addition of Tobramycin to Simplex cement in this study was not
associated with a reduction in interfacial shear strength between the new and old cement
mantles. As infection risk is significantly higher for THA revision than primary surgery
(Poss et al. 1984), and the use of antibiotic loaded cement has been demonstrated to
reduce infection rates resulting in superior prosthetic survivorship in primary THA
(Havelin et al. 2000), the use of antibiotic PMMA cement in C-C revision is
recommended.
In this experiment the outer (old) cement mantle was clean, dry and free from
debris prior to the application of fresh cement, resulting in a high interfacial shear
strength consistent with previous investigations (Gruen et al. 1976; Greenwald et al.
1978; Li et al. 1996). Li et al. (1996) found the ultimate shear stress of cleaned bilaminar
Palacos LV cement mantles to be 39 MPa, however the inclusion of a thin layer of blood
with clots and marrow debris was associated with a significant reduction in ultimate
interfacial shear strength to 6.2MPa. Li et al. (1996) argues that in clinical practice the
removal of all contaminants from the interface cannot be reliably achieved, and therefore
all cement should be removed. The suggestion that adequately clean and dry mantles
cannot be achieved is challenged by the extensive clinical experience with this technique
as reported by Hubble et al. (2005) and the favourable clinical results described by
multiple authors (Lieberman et al. 1993; McCallum et al. 1995; Hubble et al. 2005). The
101
results obtained in this study are clinically applicable only after appropriate surgical
technique with attention paid to the preparation of the old cement mantle (Greenwald et
al. 1978; Li et al. 1996; Stryker International 2005).
When considering the overall clinical significance of the shear strength of
bilaminar cement interfaces in the context of revision THA, the relative strength at bone-
cement interface must be considered. Moran et al. (1979) using SPT methods
determined the bone – cement interfacial shear strength to be less than 5 MPa (Moran et
al. 1979). Utilisation of modern cementing techniques with distal plugging, canal lavage
and retrograde fill of low viscosity cement using syringe pressurisation results in
improved cement interdigitation, however higher bone-cement interfacial shear strength
consistently remains less than 10 MPa in all regions of the femur (Halawa et al. 1978;
MacDonald et al. 1993). Rosenstein et al. (1992) found the bone-cement interfacial shear
strength after revision was reduced 30 per cent compared to primary implantation due to
compromise in cancellous bone stock sustained during cement removal. The importance
of these findings is that even though bilaminar cement mantles demonstrate lower
ultimate shear strength compared to uniform mantles, the strength still greatly exceeds
that of the bone – cement interface, particularly in revision THA where the bone stock is
compromised. Conversely it may be argued that while the bone-cement interface has
some biological potential for repair, fatigue at the C-C interface is accumulative, and
therefore the standard by which C-C interfaces should be compared is to a uniform
cement mantle. An ideal bilaminar cement mantle in this regard should therefore have
the same material properties as a uniform mantle. Attention to appropriate surgical
technique to maximise the strength of the C-C interfacial region therefore remains
important, even though the C-C interfacial shear strength significantly exceeds that of
the bone-cement interface.
4.6 Conclusions
Bilaminar cement mantles demonstrate minor variations in interfacial shear
strength related to the duration of post cure of the inner (new) cement mantle. Even
without surface roughening of the old cement mantle, the ultimate shear strength at the
102
interfacial region of bilaminar cement mantles is approximately 80 - 85 percent of
uniform mantles when the new cement is applied within 90 seconds after mixture of the
liquid and powder constituents. High C-C interfacial strengths are demonstrated as early
as one hour after cement application, therefore not precluding early postoperative
rehabilitation and weight bearing. Interfacial adhesion by mechanisms other than
mechanical interlock play a significant role in the bond formed between new and old
cement mantles, and the contribution of diffusion based molecular interdigitation is
emphasised.
C-C femoral revision is recommended as a viable technique on the basis of the
strong interfacial bond formed between new and old cement mantles. The use of
Antibiotic Simplex in C-C revision is recommended as detrimental effects on the
interfacial shear properties have not been demonstrated with the commercial addition of
Tobramycin.
103
104
105
Chapter 5
Conclusions and Recommendations
5.1 Application of Research Findings to Clinical Practice
Surgical Simplex P and Antibiotic Simplex (Tobramycin) PMMA cements
demonstrate variability in ultimate shear strength with reference to duration of post cure,
however the magnitude of these variations are small and probably of no clinical
relevance. The manufacturer’s addition of Tobramycin antibiotic does not significantly
alter the shear properties of Simplex PMMA cement. Bilaminar cement mantles
demonstrate variability in interfacial shear properties related to the duration of post cure
of the freshly applied cement mantle. Without surface roughening of the old cement
mantle, when the new cement is applied early after mixture of the liquid and powder
constituents the ultimate shear strength at the interfacial region between cement mantles
is approximately 80 - 85 percent of uniform mantles. High C-C interfacial strengths are
demonstrated as early as one hour after cement application, therefore not precluding
early postoperative rehabilitation and weight bearing. Interfacial adhesion by
mechanisms other than mechanical interlock play a significant role in the bond formed
between new and old cement PMMA cement mantles, with an important contribution by
diffusion based molecular interdigitation.
C-C femoral revision is recommended as a viable technique on the basis of the
strong interfacial bond formed between new and old cement mantles. The use of
Antibiotic Simplex in C-C revision is recommended as detrimental effects on the
interfacial shear properties have not been demonstrated with the commercial addition of
Tobramycin.
5.2 Further Research
Various aspects of C-C interfacial shear properties have been evaluated within
this thesis and within the literature. To date the longest clinical follow up of any series of
106
C-C femoral revisions is 5 years, and the results thus far are highly favourable for the
technique. In vitro testing conducted within this study and reported within the literature
however demonstrates that the material properties of bilaminar cement mantles are
inferior to uniform mantles. What is not known presently is if the reduction in material
properties observed in bilaminar cement mantles in vitro will become clinically
significant beyond 5 years in vivo. Ultimately this will not be determined adequately
until the current cohort of patients in the Exeter series is assessed at 10 years or greater
post C-C revision. Laboratory based tests can however be utilised to predict the long
term clinical outcome, and in particular fatigue testing of C-C interfaces in a simulated
THA revision would be of value. Within a suitably controlled experimental design,
fatigue limit testing of bilaminar cement mantles using SPT techniques would also
provide useful information of clinical relevance. Valuable insight will be obtained by
finite element modelling of the stress distribution within the cement mantle and at the
interfacial region after C-C femoral revision.
Presently the majority of research relevant to C-C femoral revision has been
conducted using Surgical Simplex P and Antibiotic Simplex cements. Variation in the
types of cement used and preparation techniques, and the effect of dissimilar C-C
combinations has not been evaluated. Interfacial shear testing of bilaminar cement
mantles using various combinations of cement preparations, with and without vacuum
mixing, would provide useful clinical reassurance that C-C revision may be considered
regardless of what cement preparation was used for the primary arthroplasty.
107
108
109
Appendix 1 Design Drawings of Shear Testing Device
.
110
111
112
113
114
115
116
117
Appendix 2 Calibration of Hounsfield Load Cell
Load cell serial number: 705421
Calibration date: 9 Sep 2004
Next calibration due: 9 Sep 2006
Compression range: 25 000 N
Grade A from: 1 000 N
Grade B from: 300 N
Grade C from: 160 N
Tension range: 25 000 N
Grade A from: 160 N
Australian Calibrating Services Pty Ltd
Report number: QAC/401 B-A-1
118
119
Appendix 3 Technical Report on Exeter PMMA Cement Reamer
Weinrauch P, Beach T, Goss B, Lutton C, Bell C & Crawford R
Queensland University of Technology
School of Engineering Systems, Brisbane Australia
131
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