Caroline Grant - Design of a Hip Screw for Injection of...

Design of a Hip Screw for

Injection of Bone Cement

Caroline Ann Grant, B.E. (Medical)

Submitted for the award of the degree of Master of

Engineering in The Centre for Built Environment and

Engineering Research, School of Engineering Systems,

Queensland University of Technology

Design of a Hip Screw for Delivery of Bone Cement

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Keywords

Bone Cement; Cement Augmentation; Compression Hip Screw; Fracture

Fixation; Head of Femur; Lag Screw; Modified Hip Screw; Modified Lag

Screw; Sliding Hip Screw


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Abstract

Project Title: Design of a hip screw for injection of bone cement

Author: Caroline Ann Grant

Supervisors: Prof. Mark Pearcy (Primary)

Prof. Ross Crawford (Secondary)

Fracture to the neck of femur is frequently stabilised with a hip screw

system, however the host bone is often weak or osteoporotic. This causes

premature failure of the system, commonly by cut-out of the lag screw

through the head of the femur. While augmentation of the fixation with

bone cement improves the holding power and decreases failure rate,

current methods of administering the cement are messy and inaccurate.

This project proposes a lag screw design which allows for direct injection

of the cement, via the lag screw itself, after the screw has been inserted

and correctly positioned in the femur. A method is also suggested to

reduce the risk of cement leakage into the joint space when the guide wire

has punctured the head of the femur.

The design uses a system of holes in the threaded section of a cannulated

screw to allow delivery of cement to the desired area; the modified screw

was also tested with and without the tip of the screw closed. These design


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and implantation techniques were compared to the standard design lag

screw both with and without bone cement augmentation by traditional

methods.

Initial testing in a synthetic bone analogue looked promising. The modified

screw with closed end performed better in push out tests than the standard

screw alone and comparably with the standard screw with cement

augmentation. A second phase of testing with the synthetic material was

then conducted to more closely represent physiological loading conditions.

In this case again the closed ended modified screw with cement

augmentation outperformed the original screw and was comparable with

the augmented original screw.

However, during this phase of testing problems were observed with the

synthetic testing material and it was decided to conduct further testing in

paired porcine cadaveric femurs. Several further problems occurred in this

phase of testing, including the bending of the test screws.

It was concluded that the modified screw showed potential in being a more

accurate and consistent method of cement augmentation, however neither

the synthetic bone analogue or the porcine material was an adequate

model of an osteoporotic human femur. If a suitable testing material could

be found, continued study of this prototype may prove beneficial.


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Table of Contents

1. Introduction 1

1.1. Injury – description, rates, causes, effects 1.2. Fixation 1.3. Bone Cement 1.4. Previous Work 1.5. Aims

2. Background and Literature review 7

2.1. Conclusions

3. Initial Testing 17

3.1. Background – Sliding Compression Hip Screw System

3.2. Screw Modifications 3.3. Test material – Bone or Analogue? 3.4. Bone Cement

3.4.1. Standard or low Viscosity Bone Cement 3.4.2. Curing requirements – 24hr @37oC

3.5. Apparatus 3.6. Screw Fixation Strength Studies

3.6.1. Rationale 3.6.2. Methodology 3.6.3. Cement Delivery to the Original Screw 3.6.4. Cement Delivery to the Modified Screw

3.7. Clinical risk of guide pin puncturing the head of the femur

3.7.1. Testing the modified screw with a temporary plug in the guide wire hole

3.8. Testing 3.9. Results


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3.10. Statistics 3.11. Discussion 3.12. Conclusion

4. Continued Testing 47

4.1. Rationale/Introduction

4.2. Method 4.2.1. Original screw with no cement 4.2.2. Original screw with alternative cement augmentation

method 4.2.3. Results 4.2.4. Discussion 4.2.5. Conclusions

4.3. Modified Plugged Screw with augmentation 4.3.1. Modified screw Method 1

4.3.1.1. Sample 1 4.3.1.2. Sample 2 4.3.1.3. Discussion

4.3.2. Modified screw method 2 4.3.2.1. Sample 3 4.3.2.2. Results

4.3.3. Modified screw method 3 4.3.3.1. Sample 4 4.3.3.2. Results

4.3.4. Modified screw method 4 4.3.4.1. Sample 5 4.3.4.2. Discussion 4.3.4.3. Sample 6 4.3.4.4. Discussion

4.4. New information from the manufacturer of the bone analogue – 95% closed cell

4.4.1. Bone Cement Penetration into Open and Closed cell foams

4.4.1.1. Method 4.4.2. Results 4.4.3. Discussion


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4.5. All Results

4.6. Discussion

4.7. Conclusions

5. Pig Testing 81

5.1. Porcine cadaveric material 5.1.1. Method Development 1

5.1.1.1. Discussion 5.1.2. Method Development 2

5.1.2.1. Discussion 5.1.3. Method Development 3

5.1.3.1. Discussion 5.1.4. Numerical Results 1

5.1.4.1. Results 5.1.4.2. Discussion

5.1.5. Numerical Results 2 5.1.5.1. Results 5.1.5.2. Discussion

5.1.6. Conclusions

5.2. New Modified Screw Pig Testing 5.2.1. Numerical Results 3

5.2.1.1. Results 5.2.1.2. Discussion

5.2.2. Numerical Results 4 5.2.2.1. Results 5.2.2.2. Discussion

5.3. Summary of All Pig Testing Results 5.3.1. Cement Distribution 5.3.2. Force – Displacement Data 5.3.3. Stiffness Data 5.3.4. Comparison to Sawbones samples

5.4. All Pig Discussion

5.5. Conclusion


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6. General Discussion 129

7. Conclusions 131

8. References 133

Appendix 1: Technical Drawings 139

1.1. Slotted Screw design 1.2. Final Screw design 1.3. Closed End Screw design 1.4. Hounsfield Adaptor 1.5. 45o Alignment Jig 1.6. Pig Test Rig 1.7. Pressure Transducer T-piece Adaptor

Appendix 2: Ringers Foam Compression Test 147

2.1. Toad Ringers Solution Formula

Appendix 3: Statistical analysis of Sawbones 90o Data 148

Appendix 4: Sawbones website details 149


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Figures and Tables

Figure 1.1. Stryker Howmedica Osteonics Omega+Plus Standard 85mm

lag screw and side plate, implanted in a Sawbones Femur

with cut-out sections to reveal fixation detail

Figure 3.1. A schematic diagram showing the modifications used by

Kramer et al. and Augat et al. The screw had set of three

rectangular slots placed axially through the threaded section

of the screw.

Figure 3.2. Photos of all screw designs, A: The slotted screw used in pilot

testing; B: A screw with holes similar to the final design, also

used in pilot testing; C: The Original unmodified screw; D: The

modified screw with holes for cement delivery.

Figure 3.3. A sample of Sawbones polyurethane foam with bone cement

penetration

Figure 3.4. Bone Cement Compression Data, samples with a mix ratio of

2:1 are shown in red when cured at 22oC and blue at 37oC,

samples with a mix ratio of 4:3 are shown in green when

cured at 22oC and yellow at 37oC

Figure 3.5. The yield load of bone cement samples with two different mix

ratios and curing temperatures, the mean value is shown in

column one in red with plus or minus one standard deviation

marked

Figure 3.6. Compression data of foam samples after soaking in ringers

solution (red) or in the standard dry state, the range of the

data is shown with error bars at selected data points

Figure 3.7. Diagrammatic representation of the Hounsfield Injection

Apparatus, the screw inserted in the foam block is supported

by the syringe, which in turn is supported in the metal tube on

the base plate


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Figure 3.8. A Photograph of the Hounsfield Testing Apparatus with

sample in place

Figure 3.9. Bone Cement distribution around the modified screw once

removed from the test block

Figure 3.10. Failure curves for all Sawbones 90o tests, the original screw

uncemented samples are shown in green, the original screw

augmented with cement in black, the modified augmented

samples in blue and the modified screw samples with closed

guide wire hole in red

Figure 3.11. Cracks visible in the base of the foam blocks after testing

Figure 4.1. A cut-away view of a hip screw implanted in a Sawbones

femur showing the force application angle.

Figure 4.2. Schematic diagram of screw placement in the Sawbones foam

blocks

Figure 4.3. Diagrammatic view of the testing procedure

Figure 4.4. Method of block sectioning, first cut in red, second in blue and

third in green, the screws placement in the block is marked by

the block oval.

Figure 4.5. Failure Curves for the original screw tested at 45o with and

without cement, the original uncemented screw samples are

shown in green and the original screw cemented samples are

in blue. The two samples in red were original cemented

samples that were removed from the analysis because of

material batch differences

Figure 4.6. Bone Cement distribution in Samples 1 and 2 of the modified

screw tested at 45o is shown on the left, on the right is a

schematic representation of the cement delivery holes in the

screw, the large centre hole is the guide wire hole, while the

three sets of holes for cement delivery are shown radially

representing their distance from the tip of the screw, shaded

holes have been closed to prevent cement flow


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Figure 4.7. The failure pattern of Sample 2, the foam was seen to

separate from the top of the screw, while crushing underneath

it

Figure 4.8. Cement distribution in Sample 3 of modified screw at 45o with

schematic view of the closed holes

Figure 4.9. Cement distribution and hole closure pattern in sample 4, with


Figure 4.10. Results of Samples 1-4 (methods 1, 2 and 3) for the

Sawbones 45o tests, Sample 1 is shown in dark blue, Sample

2 in pink, Sample 3 in yellow and Sample 4 in light blue

Figure 4.11. Cement distribution and hole closure pattern of sample 5, with


Figure 4.12. Cement distribution and hole arrangement for sample 6, with


Figure 4.13. Bone Cement Penetration, the foam is in the top 10mL of the

syringe with plasticine coating, with cement below it

Figure 4.14. Foam samples with plasticine edges to prevent leakage, the

left is the open cell foam prior to testing with the post testing

view on the right.

Figure 4.15. Cement Penetration Testing apparatus post testing, the

modified syringe is supported on a base plate with a hole in

the centre slightly smaller than the diameter of the syringe

Figure 4.16. Sectioned view of cement penetration into a closed cell foam

sample (top) and an open cell foam sample (bottom), in the

open cell foam the cement was seen to flow out of the foam

before curing leaving gaps in the foam

Figure 4.17. Pressure required to inject the bone cement into the foam, the

closed cell foam is shown in red and the open cell foam in

black

Figure 4.18. Failure loads of all sawbones samples, the mean and

standard deviation for each group are shown in red, each

sample within a group is shown in a different colour


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Figure 4.19. Stiffness of all sawbones samples, the mean and standard

deviation of each group are shown in red, each sample within

a group is shown in a different colour

Figure 5.1. Method Development 1, the holes for cement delivery were

seen to clog with bone material preventing cement flow out of

all but one hole, where the cement flowed back up the shaft of

the screw

Figure 5.2. Method Development 2, the cement delivery holes were seen

to clog with bone prior to cement injection, limiting the flow of

cement

Figure 5.3. Method Development 2, cement (white) pooled at the tip of the

screw as it was not inserted to the full depth of the hole

Figure 5.4. Schematic diagram of the testing method used in Method

Development 3, the femoral head was supported using a

dental acrylic ring and the force applied to push out the screw

Figure 5.5. Photograph of the stainless steel angled testing rig with

sample in place

Figure 5.6. The modified screw after testing of Numerical Results 2, The

shaft of the screw was bent at the edge of the angled test rig,

supporting the sample at 45o, this also occurred to the original

screw in Numerical Results 2

Figure 5.7. Bone Cement injection pressure as recorded in the head of

the femur from Numerical Results 2

Figure 5.8. Photograph of the cement delivery holes in the new modified

screw

Figure 5.9. Photograph of the new modified screw with sealed guide wire

hole in the tip of the screw

Figure 5.10. Photograph of the Injection pressure recording apparatus

used in Numerical Results 3 and 4. The barrel of the screw

(bottom left) and the pressure transducer (top) are attached to


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the brass T-piece adaptor, the cement injection gun attaches

to the right hand side of the T-piece.

Figure 5.11. Numerical Results 3, bone cement injection pressure as

measured inline with the delivery

Figure 5.12. Comparison of injection pressures measure in the head of the

femur (Numerical Results 2, shown in Dark Blue) and inline

with the injection (Numerical Results 3, shown in Pink).

Figure 5.13. Sectioned view of the modified cemented sample from

Numerical results 3, a bone void can be seen filled with

cement

Figure 5.14. Sectioned view of the modified cemented sample from

Numerical results 3, with the screw removed a bone void can

be seen filled with cement

Figure 5.15. X-ray of the Porcine femora used in Numerical Results 4 prior

to use

Figure 5.16. Numerical Results 4, bone cement injection pressure

measured inline with the injection

Figure 5.17. Comparison of the injection pressures recorded in Numerical

Results 2 (Blue), Numerical Results 3 (Pink) and Numerical

Results 4 (Yellow)

Figure 5.18. Sectioned view of Numerical Results 1, the white cement is

very hard to see

Figure 5.19. The modified screw once removed from Numerical Results

Figure 5.20. A sectioned view of the Numerical Results 2 modified

cemented sample, the bone cement is clearly visible in blue

Figure 5.21. The modified screw after removal from the bone in Numerical

Results 2

Figure 5.22. A sectioned view of Numerical Results 3, the bone void is

apparent by the large mass of cement

Figure 5.23. The same sample (Numerical Results 3) with the modified

screw removed, the size of the bone void is evident

Figure 5.24. The modified screw as removed from Numerical Results 3


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Figure 5.25. A sectioned view of Numerical Results 4

Figure 5.26. The Numerical Results 4 bone sample after removal of the

screw, the full extent of the bone cement penetration can be

seen

Figure 5.27. The Modified screw removed from Numerical Results 4

Figure 5.28. Force – Displacement data from the four sets of paired

cadaveric porcine femora, Numerical Results 1, Original screw

in pink and Modified screw in red, Numerical Results 2,

Original screw in light blue and Modified screw in dark blue,

Numerical Results 3, Original screw in light green and

Modified screw in dark green, Numerical Results 4, Original

screw in light orange and Modified screw in dark orange

Figure 5.29. Stiffness of the pig samples at 2, 3 and 4mm displacement,

Modified samples in blue and original samples in green, the

original samples in Numerical Results 1 failed after 3mm of

displacement

Figure 5.30. Failure or Peak loads of Sawbones foam and Porcine

Numerical Results, the mean and standard deviation of the

Sawbones samples is shown in red, the porcine Modified

screw samples are shown in blue and the Original screw

samples in green

Figure 5.31. Stiffness of all samples, mean values from the Sawbones data

are shown in red with standard deviations marked, the Porcine

Numerical results data are shown at displacements of 2, 3 and

4 mm with the Modified samples shown in blue and the

Original samples in green


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Table 3.1. Student t test p values comparing temperature and mix ratio

for bone cement samples

Table 3.2. Number of Samples of each type tested in initial testing

Table 3.3. Maximum Load Data for Sawbones 90o tests

Table 3.4. Stiffness data (N/mm) for Sawbones 90o tests, evaluated

along the linear section of the curve

Table 3.5. Simple Student t-test results using tables of p-values,

comparing the different implantation methods tested in the

Sawbones 90o testing

Table 4.1. Number of Samples of each technique in Sawbones 45o

testing as determined statistically from the data from

Sawbones 90o testing (Appendix 3)

Table 4.2. Failure load data for all Sawbones 45o samples

Table 4.3. Stiffness Data (N/mm) for all Sawbones 45o samples

Table 5.1. Stiffness (N/mm) data calculated at displacements of 2, 3

and 4mm


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Abbreviations used in the text

g/cc grams per cubic centimetre

mm Millimetres

N Newtons

Ø Diameter

PMMA Polymethylmethacrylate (Bone Cement)

PU Polyurethane (PU)


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Statement of Originality

“The work contained in this thesis has not been previously submitted for a

degree or diploma at any other higher education institution. To the best of

my knowledge and belief, the thesis contains no material previously

published or written by another person except where due reference is

made.”

Signature:

Date:


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Acknowledgments I would like to take this opportunity to thank all of the numerous people

who have helped me survive these past few years and produce what is

certainly my greatest achievement to date.

To Mark and Ross, my supervisors, for encouraging and helping me to do

this, without your help I wouldn’t have even started this. Thanks also for

all of your knowledge and insights along the long and winding road.

I would especially like to thank all of the lab staff, Greg, Kimble, Melissa,

for putting up with me and for your technical knowledge and ideas and for

helping to get my test methods actually working. The workshop staff

particularly Terry and John, for your technical knowledge and ideas, and

for modifying my hip screw, and then modifying it again after I broke it.

I’d also like to thank all the other Medical Engineering postgraduate

students and staff for you help and humour along the way. Especially to

Mr Ocean and his carbon rod, living next to you there is never a dull

moment.

I would especially like to thank my Mum and Dad for their financial support

over the last few years, it really means a lot to me to have been able to do

this. Dad, your constant supply of ideas and possible solutions to problems

and both of your continual love and support and encouragement, really


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helped get me through. And thankyou to my brothers, simply for being my

big brothers and for always loving me.

To my friends, particularly Robert and Kate and Elise, you’ve all been

there for me with love and support and jokes and distractions and tea and

coffee and shopping… I don’t know how I’d survive without you.


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Section 1: Introduction 1

1. Introduction

1.1. Injury – description, rates, causes, effects

Fractures to the neck or trochanteric region of the femur are frequent

occurrences in elderly populations of the world. In New South Wales,

Australia, the incidence of fracture in over 50s between 1990 and 2000

has increased from 4219 to 5648 per year (Boufous et al., 2004). This is

accentuated by an aging population. By the year 2025 the world-wide

occurrence of hip fractures will have increased to 2.6 million per year,

double the 1990 rate (Gullberg et al., 1997).

Mortality rates associated with this type of fracture are reported as being

as high as 35% in the first year post fracture (Goldacre et al., 2002). This

mortality is frequently the result of prolonged fracture healing time and

reduced mobility. The average time a patient spent in hospital with a hip

fracture was 14.2 days in 2000 (Boufous et al., 2004), during which time

the patient spends a large proportion of their day in bed. This reduction in

mobility and prolonged bed rest greatly increase the number of secondary

conditions and infections, such as pneumonia and chest infections.

Predominately this prolonged bed rest results in a general decrease in the

patients’ health and well being, as well as their standard of living.



1.2. Fixation

Fractures to the neck of the femur are most commonly corrected with

internal fixation. There are many different methods of fixation for these

types of fractures, including various screws, nails, hooks and pins. One of

the commonly used systems involves the use of a lag screw in the head of

the femur with a side plate attachment down the length of the femur with

several cortical screws (Figure 1.1). This system provides initial stability

and load bearing while allowing compression of the fracture fragments to

promote healing.

One of the most common problems with the fixation of this type of fracture

is Osteoporosis, which is often also the initial cause of the fracture.

Osteoporosis causes an often dramatic decrease in the strength and

quality of both the cortical and cancellous bone. In a severe case there

may be very little cancellous bone left in the head of the femur. This

creates a major obstacle to adequate fixation of the fracture as the host

material is too weak to hold the device, while increasing the chance of the

fixation failing prematurely.



1.3. Bone Cement

The use of a bone cement of either Acrylic or Calcium Phosphate base, is

a common method of increasing the strength of fixation. Current methods

of delivering the cement are inadequate. Some of the current methods

include placing either a runny or doughy mass of cement into the lag

screw hole prior to insertion of the lag screw (Bartucci et al., 1985, Elder et

al., 2000, Eriksson et al., 2002, Moore et al., 1997). When placing the

cement in the hole prior to screw insertion the low viscosity cement

frequently flows out of the desired placement area while the fast setting

nature of the cement makes using a doughy mass largely impractical. Low

viscosity cement runs the risk of leaking into the joint cavity if the guide pin

has punctured the head of the femur (Szpalski et al., 2004).

1.4. Previous Work

A small series of pilot tests were undertaken in order to test the feasibility

of a new design of screw to enable delivery of bone cement throughout the

threads (Grant, 2003). A comparison of the cut-out strength of the original

screw with and without bone cement and a modified screw with bone

cement augmentation was made. The addition of bone cement was found

to greatly increase the strength of fixation. A limitation of the test method

was noted in which the cracks caused by failure of one sample unduly

weakened the subsequent samples. However, the results suggested this

method had potential leading to the studies presented in this thesis.



1.5. Aims

This study aimed to address problems of fixation of a lag screw in an

osteoporotic femoral head by providing a convenient, timely and accurate

method of administering specific quantities of bone cement directly to the

area surrounding the threaded tip of the lag screw, without risk of

undesirable cement leakage.

A modified screw design is presented followed by comparative mechanical

tests of the existing screw, alone and with current cement augmentation

techniques, with the modified screw and augmentation method.



Figure 1.1 Stryker Howmedica Osteonics Omega+Plus Standard 85mm lag screw and side plate, implanted in a Sawbones Femur with cut-out sections to reveal fixation detail




Section 2: Background and Literature Review 7

2. Background and Literature review

Over the past century reported rates of hip fractures around the world

have been steadily increasing, but has this trend finally plateaued?

Boufous et al. (2004), suggest in their study of hip fracture incidence rates

in NSW, Australia, over the past 10 years, that in fact the age-specific and

age-adjusted rates remain virtually unchanged over this time and that

perhaps the trend of increasing incidence has finally passed. However the

number of hospital admissions for a fractured neck of femur has still

increased quite dramatically in the same period, with the number of

fractures reported increasing by 41.9% for men and by 31.2% for women.

The increased number of fractures is primarily caused by the overall aging

of the population world wide. Gullberg et al. (1997), suggest that even

with no change in the age- or sex-specific incidence, by the year 2025 the

world-wide occurrence of hip fractures will have increased to 2.6 million,

double the 1990 rate and that by the year 2050 it will have increased to

4.5 million cases each year.

With increases in the number of patients the amount of time spent in

hospital or secondary care and the cost of treatment becomes a critical

factor. Hollingworth et al. (1996), studied these costs in the UK and

estimated that the total cost of care would increase from a total of

approximately ₤280 million per year in 1991-2 to approximately ₤500

million per year in 2031. Graves (2004), reports that in Australia the



estimated cost of hip and knee replacements in 2002 was over $500

million.

Because of this it becomes increasingly important to reduce the hospital

and rehabilitation time of each patient. If an implant could be developed

that would increase the initial stability of the system the patient could be

rehabilitated sooner. This benefits not only the health care system but

also the quality of life of the patient.

A fracture to the neck of femur, or any other region of the hip, results in a

decline in physical function and often mortality in the elderly. Marottoli et

al. (1992) report on the decline in physical function seen in elderly hip

fracture patients. He found a substantial decline in the patients’ ability to

do everyday tasks such as dress themselves, walk across a room or

ascend a flight of stairs. There was a marked decrease in the number of

patients able to do each of the tasks 6 months post fracture, with the

number able to walk across a room independently falling from 75% pre-

fracture to only 15% at 6 months. He found that the only factors that

predicted this decline in function were pre-morbid physical and mental

function. He also reported a mortality rate of 18% within 6 months of

fracture.

The standardised mortality ratios associated with fractured neck of femur

are reported as being between 20% and 35% (Goldacre et al., 2002) and



Bannister et al. (1990) reports a mortality rate of 37% in the first year post-

fracture, in his study of fixation and prognosis. It is hoped that this could

be reduced with improved fixation and shorter recovery times.

Many different methods have been used to fix these fractures over the

years. The sliding compression hip screw system or Dynamic hip screw

(DHS) is one of the most common methods. Other devices include the

Jewit nail (Harrington, 1975), Gamma nail (Rosenblum et al., 1992,

Haynes, 1998, Haynes et al., 1997b, Haynes et al., 1997a, Bridle et al.,

1991)) and the Küntscher Y-nail (Davis et al., 1990), the use of multiple

screws or pins (Goodman et al., 1998, Goodman et al., 1992, Stankewich

et al., 1996, Kubiak et al., 2004), occasionally with additions such as

reinforced struts (Baixauli et al., 1999) and the Alta dome

plunger(Choueka et al., 1995, Choueka et al., 1996).

Many studies have been done comparing fixation methods. Eriksson et al.

(2002), studied five implants, including cannulated and solid lag screws as

well as a hybrid design with a barb and the LIH hook-pin. Each was tested

in a polyurethane bone analogue in its original state and when augmented

with a calcium phosphate cement and with a PMMA cement. The cement

was applied by injecting it into the pilot hole prior to insertion of the screw.

This is the method used in this study as the current clinical augmentation

method. In all cases those augmented with PMMA had the greatest pull

out load and extraction torque, and in most cases the calcium phosphate



cemented samples were stronger than the original uncemented implant

samples.

Sommers et al. (2004) created a cut-out model using a polyurethane foam

to compare a DHS and Gamma nail with two novel blade type implant

designs. They found that the DHS and gamma nail migrated further under

cyclic loading than the blade implants. A similar model to this was used in

the angled testing in Chapter 4 of this report with a static loading system.

The augmentation of fracture fixation devices with bone cement has been

done for many years. When augmenting fixation for a neck of femur

fracture two methods are common in the literature.

The first method of augmentation is to pack doughy cement into defects, in

particular the posteromedial defect. This method has been studied by

many people since the early 1980’s (Cheng et al., 1989, Chow et al., 1987,

Lau et al., 1983, Pun et al., 1987, Yetkinler et al., 1998, Yetkinler et al.,

2002). This method was not studied because of the risks associated with

this kind of augmentation preventing fracture healing through non-union as

well as preventing the sliding of sliding hip screw devices.

The second primary method of bone cement augmentation is by placing

the bone cement in the head of the femur prior to insertion of the screw or

fixation device (Bartucci et al., 1985, Eriksson et al., 2002, Elder et al.,



2000, Moore et al., 1997). This method proves effective in increasing the

holding power of the fixation but has several undesirable aspects. If a low

viscosity cement is used, it may flow out of the desired area before the

screw has been inserted and with a cement that is already doughy the

time required to put the screw in place may be such that the cement

completely hardens before the screw is fully inserted. Neither of these

methods allow for repositioning of the screw once it is in place.

The validity of bone cement augmentation around the threads of the screw

was also studied by Lee et al. (2001) who constructed an FE model of a

hip screw with and without a cement mantle. They found an 80%

reduction in the stresses observed in the cancellous bone, suggesting that

this makes it unlikely for further fractures or cut out to occur with a cement

mantle in place.

The dangers associated with bone cement injection are highlighted by

Bartucci et al. (1985), who point out the risk of cement leakage into the

joint space should the guide wire puncture the head of the femur.

A new method of delivering the bone cement to the desired location at the

right time is required. Some novel approaches have been developed to do

this.



Szpalski et al. (2004, 2001) conducted a review of cementing techniques

and gave preliminary results of a new method of delivering the cement.

They comment on the non-reproducible nature of injecting cement into the

predrilled hole prior to insertion of the screw. The method they have

suggested involves the use of a cannulated lag screw which is inserted to

the desired location. The screw is then retracted by the length of the

threaded section of the screw and a catheter inserted down the cannula.

The bone cement is then injected into the space in front of the screw. The

screw is then returned to its original position. This method also runs the

risk of cement leaking if the guide wire has punctured the head of the

femur. No mention is made as to whether the cement catheter is left in

place during tightening of the screw or if it has been removed. With the

catheter removed the cement will flow into the area where there is least

pressure – the cannula of the screw. This becomes a problem when such

a small volume of cement (2.5mL) is used.

Another method suggested to combat the difficulties of injection is the Alta

Dome plunger (Choueka et al., 1995, Choueka et al., 1996). This device

carries a small bolus of doughy cement at the tip, which is then squeezed

out into the surrounding bone once in place. It was noted that this device

did not always contain the cement in the head of the femur, with it

sometimes found at the fracture site or at the screw barrel junction. This

acted to transform the sliding screw into a rigid nail, eliminating all

mechanical advantages associated with the sliding nature of the device.



Two papers suggest a modification to a lag screw to allow the cement to

be injected to the area around the threads of the screw once positioned.

Kramer et al (2000) and Augat et al. (2002), both suggest similar

modifications. Both have started with a traditional cannulated sliding

compression lag screw and modified it to include open channels running

axially through the threaded section of their screws. The aim of this is to

deliver bone cement through the channels, once the screw has been

positioned. Both of these devices showed promise when compared with

standard devices and augmentation procedures. It was decided to use a

similar device in this testing. However it was felt that the use of axial

channels along the length of the threads was not an ideal method of

delivery. The act of screwing the thread into the bone will act to compress

or cut the bone, this material will then be forced into the open channels in

the same way that a thread tap works. In this case the material will not be

removed, but will cause the central cannula and delivery channels to

become clogged with material preventing cement delivery.

The use of bone cement to augment fixation raises questions about which

type of cement is best. Numerous people have conducted studies of the

various mechanical properties of different types of bone cements

subjected to different mixing and curing conditions (Krause and Mathis,

1988, Dalby et al., 2001, Hansen and Jensen, 1992, Jefferiss et al., 1975,

Knepper-Nicolai, 2002, Lee et al., 1978, Lee et al., 1977, Lewis, 1997,

Linden, 1991, Nzihou, 1998, Saha and Pal, 1984, Thompson et al., 2003,



Witschger et al., 1991, Yamamoto, 1998, Yetkinler and Litsky, 1998, Lee

and Ling, 1981, Khairoun, 1999, Older, 1990, Ooms, 2003). Despite all of

this it was decided to use a standard PMMA cement as a base line for

study.

A method of determining the permeability for cement infiltration into

osteoporotic bone was suggested by Baroud et al. (2003), this method

was subsequently used to determine the differences in cement penetration

into the two different foam products.

Two options were presented by the literature for a testing material,

cadaveric bone or polyurethane foam. The vast majority of studies

undertaken in this field use cadaveric material, however there are several

that use a polyurethane foam as a bone analogue. Eriksson et al. (2002)

used three densities of foam to study implant pull-out strength and

extraction torque. The three densities were said to represent severe,

medium and mild levels of osteoporosis. The density chosen for this study

is slightly less than that used by Eriksson as a medium level of

osteoporosis. Sommers et al. (2004) also used a polyurethane foam as a

bone substitute, though they do not report the density used. Sommers

study uses a very similar test method to that established here but with

cyclic loading rather than static.



Angled testing was conducted in most studies reported. Loading

conditions and angles were typically chosen to reflect a simulated one-leg

stance with the force being applied at an angle of 45o to the screw axis

(Elder et al., 2000).

Of the studies using cadaveric material Moore et al (1997) and Witschger

et al (1991) however, used a method of supporting and testing their

samples using only the head of the femur. In both cases the heads of

cadaveric femurs were removed and had lag screws implanted. The load

was then applied to the head of the femur using a curved or moulded

surface to give an even distribution of force, while the screw barrel was

supported at an angle of 45o to the force application. This method was

used in Chapter 5 of this report when cadaveric porcine material was used

for testing.

There were also several standards that were applicable to various aspects

of the materials and testing including (ISO, 2002, ASTM, 1999, ASTM,

2002, ASTM, 2001a, ASTM, 2000, ASTM, 2001b).



2.1. Conclusions

These papers have provided a lot of information on what is currently being

done in this field and an indication of where to go from here. The screw

modifications suggested by Augat and Kramer are the most relevant to this

project; however the testing methods and materials used by some of the

other papers are more appropriate at this stage of the study. It was

decided that testing would initially be carried out in polyurethane blocks

with a similar density to that used by Eriksson at the speed recommended

by the ASTM standard.


Section 3: Initial Testing 17

3. Initial Testing

In order to test any potential modifications to the hip screw several things

were required. Firstly a prototype screw was designed and manufactured,

then a testing material was chosen as well as a bone cement and test

methodology.

3.1. Background – Sliding Compression Hip Screw

System

The fixation system used in this study was the Omega + Plus sliding

compression hip screw system from Stryker Howmedica Osteonics

(Stryker (Worldwide Headquarters), 2725 Fairfield Road, Kalamazoo, MI

49002, U.S.A.). These were generously donated by Stryker Australia.

The Omega + Plus system consists of an 85mm Lag screw, a

compressing screw and a side plate with cortical bone screws. Both the

length of the lag screw and the length of the side plate can be changed to

suit the recipient and fracture characteristics.

For a femoral neck or intertrochanteric fracture, the lag screw and side

plate would normally be inserted then the compressing screw used to pull

the lag screw back, to compress the fracture. This assists in the healing of

the fracture while providing mechanical stability. The second mechanical

advantage of the system is the ability of the lag screw to slide within the

barrel of the side plate. This allows for further compression of the fracture,

while helping to prevent cut out of the screw through the femoral head.



3.2. Screw Modifications

In order to deliver the bone cement to the desired location in the head of

the femur, modifications to the lag screw design were required. The basic

principle behind the design was to use the lag screw as the delivery device

for the bone cement, allowing simple accurate and timely cement

augmentation after insertion.

Two possible designs are suggested by the literature (Augat et al., 2002,

Kramer et al., 2000). These are both very similar designs involving the

removal of three rectangular sections of material, running axially through

the threaded section of the screw, leaving slots for cement delivery (Figure

3.1).

Figure 3.1 A schematic diagram showing the modifications used by Kramer et al. (Kramer et al., 2000) and Augat et al. (Augat et al., 2002) The screw had set of three rectangular slots placed axially through the threaded section of a screw.

It was decided to avoid this style of design due to its similarity to a thread

tap – in which the debris produced during cutting is removed via the

channels along the side of the tap. In this situation, a ‘tap’ style of design



would cause the slots to fill with bony material and clog, reducing or

preventing the cement delivery.

For ease of manufacturing – both of the prototype and the final product, it

was decided to use an existing screw and make modifications to it, rather

than design a screw from scratch. As such the basic characteristics of the

screw, such as the thread shape, size and pitch, and the barrel diameter

were left unmodified. This also allows for the design modifications to be

easily transposed on to another screw design if required in the future.

The design then aimed to create an even mantle of cement around the

threaded section of the screw. Two designs were created; the first was

used only in pilot testing and involved a series of slots parallel to the

threads on opposite sides of the screw (Figure 3.2 and Appendix 1). This

design was abandoned due to a manufacturing difficulty creating weak

points in the screw. The second design was also used in the pilot testing;

it was then recreated for use in this first stage of testing. The design

consists of three sets of three holes, at 120o to each other and positioned

in alternate thread troughs. The holes were given a diameter of 1.6mm,

this gave an adequate cement flow without interfering with the thread

characteristics at all, reducing the likelihood of clogging or a significant

reduction in strength (Figure 3.2 and Appendix 1)



Figure 3.2 Photos of all screw designs, A: The slotted screw used in pilot testing; B: A screw with holes similar to the final design, also used in pilot testing; C: The Original unmodified screw; D: The modified screw with holes for cement delivery.



3.3. Test material – Bone or Analogue?

In order to undertake repeatable tests a Sawbones (Pacific Research

laboratories, 10221 SW 188th St, Vashon, Washington, 98070, USA)

material was used as a bone analogue for these tests.

This material was chosen over an animal or human cadaveric bone model

because of the uniformity and repeatability it offered for testing. A bone

model may be used at a later stage of testing.

The Sawbones material (Figure 3.3) is a cellular rigid polyurethane foam,

with a density of 0.16g/cc, which is equivalent to moderate osteoporosis

and is similar to the middle of three densities used by Eriksson, (2002) in

his study of hip screw fixation.

It was decided to use the cellular rigid foam rather than the solid rigid foam

to allow for cement penetration into the area surrounding the implant. The

solid foam conforms to (ASTM, F1839-01 Standard Specification for Rigid

Polyurethane Foam for Use as a Standard Material for Testing

Orthopaedic Devices and Instruments), and so gives much more

reproducible results than animal or cadaveric material. The cellular rigid

foam however, is slightly less consistent and uniform that the solid foam.



Figure 3.3 A sample of Sawbones polyurethane foam with bone cement penetration

3.4. Bone Cement

The cement used in this study was Howmedica Antibiotic Simplex ®

Radiopaque Bone Cement with Tobramycin (Howmedica International S.

de R.L., Raheen Business Park, Limerick, Ireland). It was decided to use

Polymethylmethacrylate (PMMA) cement rather than a calcium phosphate

cement as methacrylates are more commonly used clinically because of

their mechanical characteristics, though the modified screw could also be

used with a calcium phosphate cement in the future.

An antibiotic and radiopaque cement was chosen primarily because of its

availability. While cements with additives are mechanically weaker than

standard cements, this was not seen as a problem as the nature of the

study was comparative.



3.4.1. Standard or low Viscosity Bone Cement

In order to utilise the design features of the modified screw and allow for

injection, low viscosity cement was required. While the chosen cement

was relatively low in viscosity, it was decided to lower the viscosity further

by modifying the powder to liquid ratio from the standard 2:1 mix, to a less

viscous 4:3. This not only enabled injection but also increased the working

time of the cement in its liquid phase.

Compression tests were conducted on the cement to determine any

changes in mechanical properties caused by this change in mixing ratio.

These test were conducted in accordance with (ASTM, 1999). The testing

compared the standard 2:1 mix ratio with the lower viscosity 4:3 ratio. It

also compared two different curing conditions, room temperature 22oC or

body temperature 37oC simulated by an oven.

Using a mould described in the standard, between five and eleven

samples of each specification were made. The final number of samples

was dependant on how many were rejected from testing due to voids or

large bubbles; however it was ensured that there was a minimum of five

for each condition, in line with the standard’s recommendations. The

cement samples were then allowed to cure at the appropriate temperature

for 24hr. The samples then had their ends machined flat and the final

height of each was recorded. The samples were compressed between

two flat plates in the Hounsfield testing apparatus at a rate of 20mm/min



until failure, as recommended by the standard. The yield load was

calculated by taking a 2% offset (Figure 3.4 and 3.5). A student t test was

conducted to determine the significance of the results (Table 3.1).

Table 3.1. Student t test p values comparing the effect of temperature and mix ratio on yield strength for bone cement samples

Student t test results using tabled p values Ratio 4:3 37oC Vs 22oC

0.0001 Highly Significant Ratio 2:1 37oC Vs 22oC

0.005 Highly Significant

Temperature 37oC 4:3 Vs 2:1 0.0001 Highly Significant

Temperature 22oC 4:3 Vs 2:1

0.15 Not Significant



Figure 3.4 Bone Cement Compression Data, samples with a mix ratio of 2:1 are shown in red when cured at 22oC and blue at 37oC, samples with a mix ratio of 4:3 are shown in green when cured at 22oC and yellow at 37oC

-1000

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Displacement (mm)

Forc

e (N

)

2 to 1 @ 22deg2 to 1 @ 37 deg4 to 3 @ 22 deg4 to 3 @ 37 deg



Figure 3.5 The yield load of bone cement samples with two different mix ratios and curing temperatures, the mean value is shown in column one in red with plus or minus one standard deviation marked

6053.33

6915.75

6454.056038.00

0.00

1000.00

2000.00

3000.00

4000.00

5000.00

6000.00

7000.00

8000.00

9000.00

4 to 3 @ 37deg 4 to 3 @ 22 deg 2 to 1 @ 37 deg 2 to 1 @ 22 deg

Yeild

Loa

d (N

)



3.4.2. Curing requirements – 24hr @37oC

All samples containing bone cement were cured at 37oC for 24hr prior to

testing. This mimics the conditions the cement would experience in vivo.

It was originally thought that in order to maintain the samples at 37oC they

would have to be kept in a water bath. This raised questions about

whether the strength of the polyurethane bone analogue would be affected

by being submerged in saline or simulated body fluid for 24hr.

Because of this it was necessary to test for any changes that might occur

to the foam during the curing. The material properties of the Sawbones

polyurethane test blocks were tested in compression when dry (normal

usage) and after being soaked in Ringers solution for several days.

Small cubes of Sawbones (10mm side) were placed in sealed jars of

freshly prepared Toad Ringers Solution (Appendix 2). They were left to

soak for a period of four days. The samples were then compressed in the

Hounsfield testing machine at a rate of 5mm/min. This is the same rate at

which the final screw samples will be tested. Force displacement data

were recorded during testing.

There was a tendency for the soaking of the samples to lower the failure

load of the wet samples but no significant difference was seen (Figure 3.6).



However at this point an oven became available in which the samples

could be maintained accurately and consistently at 37oC in a dry

environment. This was considered to be the better option.



Figure 3.6 Compression data of foam samples after soaking in ringers solution (red) or in the standard dry state, the range of the data is shown with error bars at selected data points

0

50

100

150

200

250

300

350

400

450

500

550

600

0 1 2 3 4 5 6 7 8 9 10 11

Displacement (mm)

Forc

e (N

)



3.5. Apparatus

All testing was carried out on a Hounsfield 25kN universal testing machine

(model number H25KS, Hounsfield Test Equipment, 6 Perrywood

Business Park, Salfords, Red Hill, RH1 5DZ, UK), located in the

Biomedical Engineering Laboratory, School of Engineering Systems, QUT,

Brisbane.

Samples were cured in a Special liquid N2 Injection Chamber (Serial no

3863, Thermoline L+M Australia, Thermoline Scientific Equipment Pty Ltd,

9 Tarlington place, Smithfield, NSW 2164, Australia), which was

maintained at 37oC. All screws were implanted using a standard set of

Stryker Endoscopy Surgical Drills (Stryker (Worldwide Headquarters),

2725 Fairfield Road, Kalamazoo, MI 49002, U.S.A.).

3.6. Screw Fixation Strength Studies

3.6.1. Rationale

The Pilot testing demonstrated the possibility of greatly increased holding

strength when utilising this method. However, several problems with the

methods of augmentation and testing were identified. As all of the

samples were tested in the same block of testing foam, interference was

seen between tests. These issues were rectified in this initial phase of

testing.



In this phase three different implantation techniques were tested (Table

3.2). These were then compared with the data already collected on the

original un-cemented method. The original screw, augmented with bone

cement was re-tested as well as the modified screw with bone cement.

The modified screw with bone cement augmentation and the guide pin

hole plugged was also tested. This last test was included at the end of

this series of tests.

Table 3.2. Number of Samples of each type tested in initial testing Original Screw Clinical Method Cemented Method

No. of samples 4 5 Modified Screw Cement Augmented Guide pin hole plugged

and Cement Augmented No. of samples 5 5

In order to test the modifications in a clinically relevant way it was decided

to model the tests on one of the common modes of failure of the screw.

Cut-out of the screw occurs by compression of the bone onto the tip of the

screw continuing until the screw cuts out of the femoral head. Because of

this mode of failure it was decided to test the push-through strength of the

screws rather than the more commonly tested pull-out strength. This was

also suggested by Eriksson, (2002), who noted that with the use of a

femoral plate the screw will not fail by pulling out.



3.6.2. Methodology

Large blocks of Sawbones PU foam (0.16g/cc) were cut into six (6) smaller

blocks each with dimensions 60 x 60 x 40 mm. Each of the small blocks

was then labelled as to which large block it had come from, and the six

individual blocks were given a number e.g. Large Block 1, Small Block 4.

The blocks were measured to determine their final size.

A central hole was created in each block. The diagonals were marked to

determine the centre, which was then drilled with a Ø9mm drill bit in a

manual drill press. The depth of drilling was preset to 2-3mm from the

base of the block, and was the same for all of the blocks.

The chosen screw (original or modified) was manually inserted into the

central hole of the block, ensuring that the shaft of the screw remained

vertical at all times (Subsequently referred to as Sawbones 90o Samples).

Due to the limited number of test screws available, tests were conducted

in pairs, with one original cemented sample and one modified cemented

sample being prepared and tested in each pair. The original screw,

uncemented samples were tested separately as they did not need to be

cured.



3.6.3. Cement Delivery to the Original Screw

The original screw was implanted and then removed from the block to tap

the hole and allow for easier insertion after cement had been added. The

guide pin hole at the tip of the screw was capped with plasticine to prevent

the bone cement from flowing back into the cannula of the screw as this

would have decreased the volume of cement left around the threads of the

screw.

The cement components were then measured, 4.7g of powder and 3.5mL

of liquid monomer. The cement was mixed and 2.5 millilitres of cement

was used to fill the pre-tapped hole. This method is consistent with that

used by Eriksson (2002) to augment hip fixation devices.

The volume of cement used was determined to be equal to the volume of

the central hole in the foam block, 2.5mL. This volume was chosen as it

was the maximum volume of cement able to be delivered to the original

cemented samples prior to re-insertion.

The original screw was then reinserted, and the sample was placed in the

oven to cure at 37oC for 24hr.



3.6.4. Cement Delivery to the Modified Screw

With the modified screw in the desired location, the Hounsfield injecting

apparatus was assembled (Figure 3.7). The cement was then measured

and mixed in the same proportions as in the original cementing procedure.

The volume of cement used for the modified screw samples was

determined to be equal to the volume of the initial hole plus the internal

volume of the screw, a total of 3mL. As the original screw had its guide

pin hole capped to prevent cement reflux, this resulted in the same

external volume of cement in each case.

The required volume of cement was placed in a 10mL syringe and a luer

lock fitting attached to the end. The syringe was placed inside an

aluminium tube on top of a base plate. The implanted screw was inserted

up through the base plate and attached to the end of the syringe via the

luer lock. This arrangement was used to keep the syringe body still and

attached to the screw while the Hounsfield crosshead applied force to the

plunger (Figure 3.7). A fume extraction system was used during the

cementing procedure.

The Hounsfield injection apparatus was used to deliver the cement at a

cross-head rate of 50mm/min. This rate was chosen as the cement is a

Non-Newtonian fluid which exhibits shear thickening, however the cement



needed to be delivered within 2-4mins of mixing. This rate allowed the

desired 3mL of cement to be delivered in 30sec.

The sample was returned to the oven to cure for 24hr at 37oC

Figure 3.7 Diagrammatic representation of the Hounsfield Injection Apparatus, the screw inserted in the foam block is supported by the syringe, which in turn is supported in the metal tube on the base plate



3.7. Clinical risk of guide pin puncturing the head of the

femur

At this point it was recognised that clinically there is a risk associated with

injecting cement into the head of the femur if the guide wire has punctured

the joint space. While care is taken by the surgeon to avoid this, it does

occasionally happen, and when combined with cement augmentation, runs

the risk of fusing the joint. A case such as this was reported by Bartucci et

al. (1985) in which cement was seen to leak into the joint space when the

guide wire had punctured the head of the femur. This method is risky and

because of this it was decided to also test an additional modification to the

implant that may reduce the risk of cement leaking in this situation.

The modification would ideally involve the permanent closure of the guide

wire hole in the modified screw, possibly even with an elongated tip to

block the hole left by the guide wire in front of the screw (Appendix 1:

Technical drawings).

To test this theory before any further modifications were made to the

already modified screw, a temporary plug of plasticine was used. This

was found to be sufficient to prevent any leakage of cement. Care was

also taken that the plug sealed the end of the screw without impeding the

cement flow out of any of the cementing holes.



3.7.1. Testing the modified screw with a temporary plug in the

guide wire hole

The procedure followed for the modified screw with a plugged guide wire

hole was essentially the same as for the standard modified screw.

However, a plasticine plug was used to block the guide wire hole prior to

the implantation of the screw. The bone cement was mixed and injected in

the same method as for the standard modified screws and cured at 37oC

for 24hrs.

3.8. Testing

A Hounsfield adaptor that could be screwed into the top of the lag screw

(Appendix 1) was attached to the crosshead. The construct was placed on

a suitable base, a flat doughnut shaped plate with a large internal diameter,

arranged to allow movement of the screw out of the base of the foam block.

The screw was attached to the adaptor and the crosshead lowered until

the foam was almost in contact with the base but without any load being

applied (Figure 3.8).

Testing was carried out at a rate of 5mm/min to a maximum displacement

of 10mm or until failure. Once removed from the blocks the screws were

photographed to show failure modes and cement distributions. (Figure 3.9)

The screws were then cleaned by soaking in acetone.



3.9. Results

The failure curves for all of the Sawbones 90o

samples are shown in Figure 3.10 and Table

3.3. Cracks were seen in the base of the

blocks after testing (Figure 3.11).

Figure 3.8 A Photograph of the Hounsfield Testing Apparatus with sample in place

Figure 3.9 Bone Cement distribution around the modified screw once removed from the test block



Figure 3.10 Failure curves for all Sawbones 90o tests, the original screw uncemented samples are shown in green, the original screw augmented with cement in black, the modified augmented samples in blue and the modified screw samples with closed guide wire hole in red

0

500

1000

1500

2000

2500

3000

3500

4000

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Displacement (mm)

Forc

e (N

)



Figure 3.11 Cracks visible in the base of the foam blocks after testing

3.10. Statistics

Table 3.3. Maximum Load Data for Sawbones 90o tests

Test No Original No Cement

Original Cemented

Modified Cemented

Modified Plugged Cemented

1 794.4 3476 2944 34122 933 2924 3028 34683 882 3180 2912 34324 871 3308 2708 33965 3224 3180 34006 2724 - 3156 Mean 870.1 3112.57 2954.40 3377.33Standard Deviation 57.2402 258.06 172.43 111.59Maximum 933 3476 3180 3468Minimum 794 2724 2708 3156Range 139 752 472 312

The stiffness of each sample was evaluated along the linear portion of the

failure curve. The results are shown in Table 3.4 and Figure 4.19.



Table 3.4. Stiffness data (N/mm) for Sawbones 90o tests, evaluated along the linear section of the curve

Original No Cement

Original Cemented

Modified Cemented

Modified Plugged Cemented

1 485.06 2734.10 2307.10 2576.202 1007.90 2264.80 2430.50 2506.503 879.47 2512.70 2336.30 3077.404 728.82 2623.20 2245.30 2554.605 2715.20 2587.90 2419.006 2414.10 - 2824.90

Mean 775.31 2544.02 2381.42 2659.77St Deviation 224.61 183.04 133.35 245.43Maximum 1007.90 2734.10 2587.90 3077.40Minimum 485.06 2264.80 2245.30 2419.00

A simple Student t-test was carried out on the data to determine the

apparent significance of the results. A comparison was made between the

mean failure loads of the original screw in its uncemented and cemented

form. This was found to be a highly significant difference (p ≤ 0.0001). A

comparison was also made of the differences between the different

cementing methods with the associated p-values displayed in table 3.5.

Table 3.5. Simple Student t-test results using tables of p-values, comparing the change in failure load from different implantation methods tested in the Sawbones 90o testing

Student t test results using tabled p values Original Cemented Vs Original Uncemented

p ≤ 0.0001 Highly Significant Blocked Modified Vs Modified

p ≤ 0.0005 Highly Significant

Original Cemented Vs Blocked Cemented p ≤ 0.025 Significant

Original Cemented Vs Modified Cemented

p ≤ 0.15 Not Significant



3.11. Discussion

The testing method used in this series of tests was chosen to mimic cut-

out of the lag screw through the femoral head. This is one of the common

modes of failure of lag screws with side plates. It is generally the result of

impaction of the femoral head down over the tip of the lag screw. For this

reason it was decided to test the push-through strength of the screw rather

than the pull-out strength. This method was also suggested as being

appropriate for fixation systems that use a lag screw and end plate by

Eriksson et al. (2002). While this is a very simplified method involving a

coaxial force, a more clinically relevant arrangement was tested in the next

phase of testing, described in Chapter 4.

It was decided that it was necessary to block the guide wire hole at the tip

of the screw because of the clinical risk associated with bone cement

injection with a punctured head of femur. In a clinical situation the guide

wire is inserted into the head of the femur first. As the bone is often very

weak from osteoporosis, it is not unheard of for the guide wire to puncture

a hole completely or partially through the head of the femur. Normally this

would not cause a great problem, however when a low viscosity bone

cement is being introduced to the area surrounding the screw after the

removal of the guide pin, there is the potential for the cement to flow down

the hole left by the guide wire, out of the head of the femur and into the

joint itself. This would be a disastrous outcome resulting in the cementing

of the joint and dramatic if not total reduction in range of motion. Because



of this it is desirable to prevent any cement from flowing past the tip of the

screw. Closing the guide wire hole at the tip of the screw will greatly

reduce the potential for cement penetration away from the tip of the screw.

During these experiments it was only possible to use a temporary cap to

close the guide wire hole; however it is desirable to use a more permanent

cap or plug clinically. Possible designs for the plug, as well as other

possible modifications to the screw and implantation method are

discussed later in this report. The temporary plug used was made of a thin

layer of plasticine; this was inserted carefully into the tip of the screw to

ensure the hole was completely plugged without causing any interference

to the cement flow out of the first of the cement holes. This method

proved adequate in preventing cement leakage out of the tip of the screw.

In each case the plasticine plug was found to be intact during cleaning of

the screw.

One problem identified with the technique used in these tests was in the

implantation of the screw. Insertion of the screws was done manually

without the use of a guide wire. This caused the screws to deviate slightly

from the ideal position, perpendicular to the surface of the foam. The

design of the testing apparatus then meant that the bottom of the foam

block was at an angle to the base plate. The slight variation in angle of

each of the screws may account for a small degree of error in the results

and the varying size of the toe region seen in Figure 3.10. This problem



was alleviated in subsequent tests by using an alignment jig to insert the

screw.

In this phase of the study the cement was injected via a 10mL syringe

either manually or using the Hounsfield injection apparatus. Clinically a

cement gun would be used to inject the cement via a catheter however it

was felt that this was not necessary at this stage of the testing. The use of

the Hounsfield injection apparatus allowed the cement to be injected at a

controlled rate.

The testing apparatus was designed such that the movement of the screw

out of the base of the block would not be impeded, while still providing

adequate support to the rest of the block. This proved to be an adequate

arrangement, with cracks visible in the base of the blocks after testing

demonstrating the movement of the screw.

All of the results gathered exhibited a small ‘bump’ in their force-

displacement graphs at around 600N. It was determined that this was due

to backlash on the screws driving the cross-head in the Hounsfield testing

machine. This means that the initial force is applied by the weight of the

cross-head, and then as this weight is reached the slack is taken up by the

screws – creating the ‘bump’, before any additional force is applied. It is

expected that this will be seen in all compression tests conducted on this

machine.



It was decided to test a minimum of five samples for each method as the

pilot testing suggested that this would be enough to determine any

significant differences between the methods.

The force – displacement data clearly show the benefits of bone cement

augmentation. The peak load prior to failure in the cemented samples

(minimum 2950N) was more than three times that of the un-cemented

samples (870N). The use of even such a small volume of bone cement to

augment the fixation greatly improved the holding power and stability of

the system. A simple Student t-test showed that this was a highly

significant difference (p ≤ 0.0001).

The original screw augmented with bone cement had a mean failure load

of 3112N; 200N more than that of the modified screw with augmentation.

However the range of values and the standard deviation of the original

cemented samples were much greater than the modified samples. So

while the modified samples were slightly weaker they were much more

predictable with the standard deviation and range of values being half that

of the original screw with cement.

The introduction of the plug to the guide wire hole in the modified screw

had a substantial effect on the results observed. Not only did it increase

the mean load to failure to 3377N but it also decreased the standard

deviation and range of values, making it not only the strongest fixation but



also the most reliable and predictable method of augmentation. This was

also found to be statistically significant (p ≤ 0.0005). This supports the

decision to only continue testing with the closed end screw and to

permanently close the guide wire hole in the future.

3.12. Conclusion

The results of this testing showed a highly significant (p ≤ 0.0001)

difference in mean failure load with the addition of bone cement. They

also showed a highly significant (p ≤ 0.0005) difference between the

closed end modified screw and the standard modified screw. These

findings support continued testing of this method of improvement with the

closed end modified screw.

Following these studies it was considered that a better model of the clinical

loading environment, with the primary load being applied to the screw at

an angle rather than coaxially, simulating one-leg stance, should be

investigated to more fully characterise the affect of cement augmenting on

the system, this is investigated in Chapter 4.


Section 4: Continued Testing 47

4. Continued Testing

4.1. Rationale/Introduction

After completion of the first stage of testing, it was decided to progress to a

more physiologically accurate model. As such the second stage of testing

was designed with the force being applied to the screw as it would be

clinically. This method is also a better simulation of the cut-out mode of

failure.

In a clinical situation the force applied to the implanted screw is applied at

an angle, rather than the coaxial force used in the initial testing. For

example in a simulated one-legged stance the force is applied at an angle

of 45o to the axis of the screw (Figure 4.1), this angle was often used in

similar testing scenarios in the literature (Moore et al., 1997, Sommers et

al., 2004, Witschger et al., 1991).

The necessity of plugging the guide pin hole was shown in the initial

testing. This negated the need to test the open ended modified screw.

Only three techniques were tested, the original clinical method, the original

screw with bone cement augmentation and the modified screw with

plugged guide pin hole and augmented with cement. A statistical analysis

of the data gathered in phase one of testing was used to determine the

number of test required in this stage (Table 4.1 and Appendix 3).



Figure 4.1 A cut-away view of a hip screw implanted in a Sawbones femur showing the force application angle.

Table 4.1. Number of Samples of each technique in Sawbones 45o testing as determined statistically from the data from Sawbones 90o testing (Appendix 3)

Original Screw Clinical Method Cemented Method No. of samples 9 9 Modified Screw Guide pin hole plugged and Cement Augmented No. of samples 9



4.2. Method

Blocks of Sawbones bone analogue foam, of the same density (0.16g/cc)

and strength as those used previously, were cut into smaller cubes of

dimension 60mm x 60mm x 40mm. Pilot holes of 9mm diameter were

then drilled in the blocks at an angle of 45o and to a depth of 25mm such

that the tip of the screw, once implanted, was in the centre of the block

(Figure 4.2). An alignment jig was used to ensure the screws were

implanted in the blocks at the correct angle (Appendix 1).

Figure 4.2 Schematic diagram of screw placement in the

Sawbones foam blocks

4.2.1. Original screw with no cement

The original screw was implanted in the chosen block using the alignment

jig, and oriented such that the flat sides of the screw were vertical during

testing. This was done to ensure the screw was in the same orientation

for each of the tests. The sample was then placed in the Hounsfield

testing apparatus (Figure 4.3). The screws were implanted to a depth

such that there was a 5-6mm gap between the foam block and the sample



holder. If necessary the screw was retracted a half rotation to ensure this

clearance.

Figure 4.3 Diagrammatic view of the testing procedure

Using a flat plate attachment, testing was then carried out at a speed of

5mm/min until failure or for a maximum displacement of 5mm, at which

point the foam block would then be in contact with the sample holder. This

is consistent with the testing parameters used in phase one, with the

maximum displacement reduced to reflect the distance between the

bottom of the foam block and the top surface of the sample holder, a

distance of 5mm. If the displacement exceeded this amount the sample

holder would interfere with the loading of the block.



Load-displacement data were recorded for each sample. Photographs of

each sample were also taken post testing showing any failure mode

characteristics visible. The screw was then removed from the block.



4.2.2. Original screw with alternative cement augmentation

method

For the technique involving the original screw with bone cement

augmentation, the screw was inserted in the foam using the alignment jig,

and correctly oriented. The screw was removed in preparation for cement

augmentation.

The bone cement was prepared in the same way as in the initial phase of

testing, using a 4:3 ratio of 4g powder to 3ml liquid. The cement was

injected to fill the hole in the foam (~3mL) and the screw re-inserted. The

samples were left to cure for 24hr at 37oC.

The blocks from sample four onwards were placed on their sides in the

oven while curing to simulate the angle at which they would be cured

clinically i.e. with the patient lying flat on their back. This is discussed

further in the modified cemented methods and discussion.

After 24hr the samples were removed from the oven and placed in the

Hounsfield testing apparatus. Testing was then conducted in the same

manner as for the un-cemented samples.

Load-displacement data were recorded for each sample. Photographs of

each sample were also taken post testing showing any failure mode

characteristics visible. Samples were sectioned to show cement



penetration and failure characteristics. The blocks were sectioned such

that one side of the block was removed parallel with the screw, then the

base of the block and finally the opposite side of the block, each time

removing material till the first sign of bone cement (Figure 4.4).

Photographs were taken at all stages including the totally removed screw

with cement in place.

The screws were cleaned of cement by soaking in acetone.

Figure 4.4 Method of block sectioning, first cut in red, second in blue and third in green, the screws placement in the block is marked by the black oval.



4.2.3. Results

The results of the original screw with and without cement are shown in

Figure 4.5. The first two samples tested with cement augmentation are

shown in red, these were found to have been from an earlier batch of

Sawbones material and have been removed from further analysis. Further

results and statistics are shown in section 4.5.



Figure 4.5 Failure Curves for the original screw tested at 45o with and without cement, the original uncemented screw samples are shown in green and the original screw cemented samples are in blue. The two samples in red were original cemented samples that were removed from the analysis because of material batch differences

0

500

1000

1500

2000

2500

3000

0 1 2 3 4 5Displacement (mm)

Forc

e (N

)



4.2.4. Discussion

When reviewing the results of the original cemented samples it can be

seen that the first two samples failed at a much lower load than the rest of

the samples, a failure load difference of 17%. While this may have just

been random chance, closer examination of the blocks of sawbones

material that were used, indicated that the first two samples had been

taken from a previous batch to all of the others. Consultation with the

manufacturer suggested that one of the batches was of a different density

to the other, as the difference in strength was greater than what they

considered possible from batch differences. Samples of the two batches

were tested and both were found to have a density of 0.16g/cc or 10pcf.

The only explanation that could be found was the degradation of the

samples over time. The first two samples came from a batch that was

ordered eight months before the second batch of material. This was a

large difference in strength to be attributed to long term storage and it was

decided that there was sufficient uncertainty about the material that data

from the first two tests should be discarded. Two extra specimens were

tested using the new material.

As with the original 90o testing a ‘bump’ can be seen in the compression

data at around 600N. Its presence confirms that it is an external factor,

the mass of the Hounsfield cross-head. A series of small failures can be

seen in several of the samples of both the original cemented and original



uncemented samples. These failures can most likely be attributed to

minor failures in the PU foam matrix.

The first three samples were cured in the same orientation as they were

inserted, with the screw coming out of the top of the block. Due to

difficulties encountered in the Modified plugged 45o samples that were

being tested simultaneously with the Original samples, the curing

orientation of the samples was changed to reflect clinical conditions. This

meant that as soon as the bone cement was sufficiently thick not to flow

out of the hole, the blocks were placed on their sides with the screw

parallel to the floor. This reflects the clinical orientation of the patient lying

flat on their back during and after surgery. This was done for the

remaining samples, including the two replacement samples. This is

discussed further in the modified plugged 45o testing discussion (Section

4.3.4).

4.2.5. Conclusions

The benefit of bone cement augmentation can again be clearly seen in this

test. The Original Cemented samples withstand a load of two and a half

times as much as the Original uncemented samples.



4.3. Modified Plugged Screw with augmentation

Initially the modified plugged screw was to be implanted and tested in a

similar manner to the original cemented 45o samples. However as this

method was employed, problems developed and had to be corrected for.

The evolution of the method is given below sequentially, with discussion of

the problems encountered, their results and the solutions implemented.

4.3.1. Modified screw Method 1

4.3.1.1. Sample 1

In the first sample, a temporary plug of plasticine was used to close the

guide wire hole, as was done in the 90o push out tests. The screw was

then inserted and correctly oriented in the block using the alignment jig.

With the screw in place the bone cement was then mixed and injected,

using the same ratio and volume of cement used in the original cemented

samples. For this series of tests the injection procedure involved using a

10mL syringe with luer lock fitting to attach to the end of the screw. The

desired volume of bone cement was then injected manually, pressure was

not recorded.

The sample was left to cure at 37oC for 24hr and testing was carried out

as for the original cemented samples. The sample was sectioned and

photographed, again in the same manner as for the original cemented

samples.



The force – displacement curves from samples one through four are

shown in Figure 4.10. Sample 1 did not fail when tested. It was

determined that this was because the screw was implanted too deeply in

the block restricting movement.

Sectioning showed that the cement was not evenly distributed around the

screw. Figure 4.6 shows a large bolus of cement around the tip and

underside of the screw with very little around the top threads. This is

consistent with the large amount of force observed and non-failure of the

sample. Figure 4.6 also shows a representative diagram of the modified

screws’ hole pattern and orientation, holes that are shaded in grey have

been closed. The directions of forces applied to the sample are also

marked, with GC being the orientation of gravity while curing and the

testing force F.

Figure 4.6 Bone Cement distribution in Samples 1 and 2 of the modified screw tested at 45o is shown on the left, on the right is a schematic representation of the cement delivery holes in the screw, the large centre hole is the guide wire hole, while the three sets of holes for cement delivery are shown radially representing their distance from the tip of the screw, shaded holes have been closed to prevent cement flow



4.3.1.2. Sample 2

The method used for sample one was repeated for sample two; the depth

of implantation was checked prior to cement augmentation to ensure there

was adequate room for failure.

This sample failed under load as expected. Sectioning of the sample

showed a similar cement distribution to that seen in sample one. The

sectioning also showed that the sample failed by the separating of the

foam and cement/screw along the top surface of the threads and had

crushed the material under the screw (Figure 4.7).

Figure 4.7 The failure pattern of Sample 2, the foam was seen to separate from the top of the screw, while crushing underneath it

4.3.1.3. Discussion

The bone cement that was injected was seen to pool around the bottom

surface and tip of the screw. This is not an ideal cement distribution. To

attempt to correct this it was decided to minimise the cement flow out of

the area around the tip of the screw.



4.3.2. Modified screw method 2

4.3.2.1. Sample 3

It was then attempted to fix this problem of cement pooling in sample three

by closing some of the cement holes. In this sample the hole closest to

the tip of the screw in each of the three sets of holes was plugged as well

as the guide wire hole, this is shown in Figure 4.8.

4.3.2.2. Results

This sample also failed as expected and by the same mode as sample 2.

Sectioning however showed that the distribution problems had not been

corrected. The cement had still pooled slightly at the tip of the screw and

showed an even distribution down the back of the threads however the top

surface of the screw was still not covered, the cement had also leaked out

onto the surface of the block as shown in Figure 4.8.

Figure 4.8 Cement distribution in Sample 3 of modified screw at 45o with schematic view of the closed holes




4.3.3.1. Sample 4

To reduce the pooling of the cement and increase distribution around the

top threads of the screw the plug in the first hole of the top set of holes

was removed leaving only the first hole in each of the bottom sets plugged,

Figure 4.9.

4.3.3.2. Results

Failure was by the same method as for the previous samples. Sectioning

showed the cement was evenly distributed around the threads; however

there was still significant pooling at the tip of the screw (Figure 4.9).

Figure 4.9 Cement distribution and hole closure pattern in sample 4, with schematic view of the closed holes



Figure 4.10 Results of Samples 1-4 (methods 1, 2 and 3) for the Sawbones 45o tests, Sample 1 is shown in dark blue, Sample 2 in pink, Sample 3 in yellow and Sample 4 in light blue

0

500

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1500

2000

2500

3000

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Displacement (mm)

Forc

e (N

)




4.3.4.1. Sample 5

It was determined that the pooling of the cement at the tip, and under the

bottom threads of the screw was due to the effects of gravity during curing.

Because of this it was necessary to cure the samples in the manner and

orientation that they would be cured clinically. This would normally be with

the patient lying flat on their back, making the screw barrel parallel to the

floor. This change in procedure was also instigated in the Original

Cemented samples at sample number four.

It was also decided to try a new arrangement of open and closed holes.

With the screw now in an upright Y position, the entire bottom row of holes

were plugged, Figure 4.11. The screw was then oriented such that the

closed holes would be pointing vertically down when the screw was in its

curing position.

It was decided not to mechanically test any more of the trial methods until

the final method had been decided upon, and so the sample was then left

to cure for only 1 hr at 37oC. After this time the cement had hardened

sufficiently to allow sectioning and observation of the cement penetration

pattern.

Sectioning of the sample showed very even coverage around the threads

of the screw, however it was evident that the screw had not been



implanted to the full depth of the pilot hole, as there was a section of

cement at the tip, that had pooled in an open area in front of the screw,

this is shown in Figure 4.11.

Figure 4.11 Cement distribution and hole closure pattern of sample 5, with schematic view of the closed holes

4.3.4.2. Discussion

It could be seen in the sectioning of the screw that it had not been

implanted to the correct depth. This is an easily avoidable mistake caused

by human error. Marking the depth of implantation of the side of the screw

prior to the final implantation will prevent this.

4.3.4.3. Sample 6

The results from sample 5 were promising however it was evident that the

screw had not been implanted correctly. Because of this sample 6 was

done as a repeat of the method used for sample 5.



This sample was also sectioned after 1 hr of curing without testing. The

cement distribution was very even and showed no pooling whatsoever as

shown in Figure 4.12 Cement distribution.

Figure 4.12 Cement distribution and hole arrangement for sample 6, with schematic view of the closed holes

4.3.4.4. Discussion

The distribution of cement around the threads of the screw using this

method was excellent and it was decided to use this method for further

testing.

Experience of surgical colleagues working in this area suggested that

there would be no pooling of the cement clinically. So it was decided to

first test the standard method in porcine cadaveric femurs. It was thought

that the density of the porcine bone may act to prevent any pooling of



cement. This would eliminate the need to block any of the cementing

holes. It would also raise questions about the validity of the Sawbones

material as a bone analogue.

4.4. New information from the manufacturer of the bone

analogue – 95% closed cell

At this point further information from the manufacturer of the Sawbones

bone analogue foam came to light. A new product, 95% Open cell foam,

was released with the further information that the current cellular rigid

foam is 95% closed cell. Prior to this it was not known that the foam was

closed cell. This made it an inappropriate material for use in mechanical

studies involving bone cement. The new open cell foam also proved

inadequate for this style of testing as it did not have the necessary

mechanical properties to be considered a bone analogue. A copy of the

updated website is included in Appendix 4.



4.4.1. Bone Cement Penetration into Open and Closed cell

foams

A sample of the open cell material was obtained and used to quantify the

difference in cement penetration between the open cell and the closed cell

foams. As the open cell foam is of a lower density (0.12g/cc) than the

material used in this study it was decided to test the open cell (0.12g/cc)

and compare it with a closed cell 0.12g/cc foam.

The pressure required to push a known volume of cement into the foam

was tested in a similar apparatus to that used by Baroud et al. (2003).

4.4.1.1. Method

A 50mL Syringe was modified for use as

the testing apparatus. The tip of the

syringe was removed to leave a cylinder

with a plunger.

Figure 4.13 Bone Cement Penetration, the foam is in the top 10mL of the

syringe with plasticine coating, with cement below it

Samples of the foam were then cut and shaped to fit tightly in the bottom

of the syringe. The full height of the open cell foam was used (19mm)

whereas the closed cell blocks were cut down to create samples of

approximately 20mm in height. Care was taken during this process to



minimise damage done to the edges of the foam, particularly in the open

cell foam where the filaments were weak and easily crushed or bent. Due

to the cellular nature of the foams even when the cylinders were as closely

fitting as possible there was still sufficient space between the inner wall of

the syringe and the foam to allow cement leakage. To prevent this

leakage the edges of the foam cylinders were sealed with a thin coating of

plasticine, this prevented any leakage without interfering with the

movement of cement through the rest of the foam (Figure 4.14).

Figure 4.14 Foam samples with plasticine edges to prevent leakage, the left is the open cell foam prior to testing with the post testing view on the right.

To test the sample the syringe was loaded with bone cement while held

upside down (Figure 4.13). The foam sample was then placed in the top

and any air removed. The testing apparatus was then inverted and placed

in the Hounsfield testing machine. A flat plate attachment was used to

compress the syringe plunger while measuring the force and displacement



data. The base of the syringe was supported on a flat doughnut to allow

cement flow out of the base of the foam sample (Figure 4.15). Testing

was conducted at a speed of 120mm/min to a maximum of 20mm as used

by Baroud et al. (2003).

Samples were removed from the testing apparatus immediately and

allowed to cure. The curing orientation

of the samples was the same as the

testing orientation.

Once the cement had hardened the

samples were cut in half and

photographed to show the cement

distribution (Figure 4.16).

Figure 4.15 Cement Penetration Testing apparatus post testing, the modified syringe is supported on a base plate with a hole in the centre slightly smaller than the diameter of the syringe



4.4.2. Results

The recorded injection pressures are shown in Figure 4.17.

Figure 4.16 Sectioned view of cement penetration into a closed cell foam sample (top) and an open cell foam sample (bottom), in the open cell foam the cement was seen to flow out of the foam before curing leaving gaps in the foam



Figure 4.17 Pressure required to inject the bone cement into the foam, the closed cell foam is shown in red and the open cell foam in black

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0 2 4 6 8 10 12 14 16 18 20Displacement (mm)

Pres

sure

(Mpa

)



4.4.3. Discussion

The force required to push the cement into the closed cell foam was much

greater than that required for the open cell foam. It was noted that, if

merely placed on the surface, the cement would flow down through the

open cell foam. The force required for the closed cell foam is greater than

that which would be applied manually with a syringe. It is expected that

the force required to push the cement into the 0.16g/cc foam would be

much higher still, however this has not been tested.

This result suggests, as expected, that in the previous testing the cement

flowed only into those cells that had been ruptured by the drilling or

implantation procedure, giving a consistent thickness to the cement mantle.

This confirmed the unsuitability of the Sawbones material for use in the

mechanical testing of implants with bone cement augmentation.

It has been noted that the appearance of the screw samples with cement

augmentation created in the closed cell foam are very similar to those

shown by Eriksson (2002) in his results using a polyurethane foam. No

mention is made is his paper as to wether the foam is open or closed cell,

however his cement distributions suggest that in fact the foam he was

using was also of a closed cell nature.



From this testing it is apparent that the cement distribution that would be

seen around a screw augmented in an open cell environment would be

much greater than that seen in the closed cell foam.

4.5. All Results

A summary of the failure loads of all Sawbones samples is shown in table

4.2. The stiffness of the samples was also evaluated (Table 4.3)

Table 4.2. Failure load data for all Sawbones 45o samples Original No Cement Original Cement Modified Cemented

1 1053.75 2282.5 1554 2 914 2162.5 2077.5 3 1002.5 2475 1882 4 917 2482.5 5 903 2470 6 978 2365 7 915 2620 8 1038.75 2177.5 9 957

Mean 964.33 2379.38 1837.83 St Dev 57.23 161.81 264.53 Maximum 1053.75 2620.00 2077.50 Minimum 903.00 2162.50 1554.00 n 9 8 3

A simple Student t-test was carried out on the data to determine the

significance of the results. A comparison was made between the mean

failure loads of the Original screw in its uncemented and cemented form.

This was found to be a highly significant difference (p ≤ 0.0001). A

comparison was also made of the differences between the Original

cemented screw samples and the modified screw with plugged end

samples, this difference was also found to be highly significant (p ≤ 0.005).



Table 4.3. Stiffness Data (N/mm) for all Sawbones 45o samples Original No Cement Original Cement Modified Cemented

1 506.43 813.11 605.65 2 353.34 613.25 1130.40 3 421.62 839.66 709.41 4 518.03 820.72 5 485.74 743.48 6 393.45 800.41 7 288.44 766.32 8 418.87 603.62 9 396.62

Mean 420.28 750.07 815.15 St Dev 74.51 92.58 277.90 Maximum 518.03 839.66 1130.40 Minimum 288.44 603.62 605.65 n 9 8 3

The failure loads and stiffness of all of the Sawbones samples tested at

90o and at 45o are shown in Figures 4.18 and 4.19.



Figure 4.18 Failure loads of all sawbones samples, the mean and standard deviation for each group are shown in red, each sample within a group is shown in a different colour

870.10964.33

3112.572954.40

1837.83

2379.38

3377.33

0

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2000

2500

3000

3500

4000

Mod Plug 90 Mod 90 Orig Cem 90 Original 90 Mod Plug 45 Orig Cem 45 Original 45

Forc

e (N

)



Figure 4.19 Stiffness of all sawbones samples, the mean and standard deviation of each group are shown in red, each sample within a group is shown in a different colour

2659.77

2381.42

2544.02

775.31

815.15

750.07

420.28

0

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1000

1500

2000

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3000

3500

Mod Plug 90 Mod 90 Orig Cem 90 Original 90 Mod Plug 45 Orig Cem 45 Original 45

Stiff

ness

(N/m

m)



4.6. Discussion

The angled load testing of the implant was intended to be a better model

of physiological loading conditions than the original coaxial load. This

testing however proved mainly to highlight problems with the testing

materials and methods.

The closed cell nature of the Sawbones foams combined with the batch

degradation problems experienced combine to make an unreliable and

inappropriate testing material for orthopaedic implants, particularly those

involving the use of bone cements.

This became more of a problem with the modified screw because of the

method of cement augmentation. In the original screw samples that were

augmented with bone cement, the cement was placed in the pilot hole

prior to reinsertion of the screw. As the screw was then inserted into the

hole the cement was forced to flow around the threads and pushed into

the surrounding foam, creating an even coverage. With the modified

screw samples, the screw was already in place when the cement was

injected and gravity played a much greater role in determining the flow

pattern of the cement. The cement was seen to flow down the central

cannula and out of the lower holes with no reason to flow out of the upper

holes or coat the threads of the screw.



Despite the problems experienced with the test material, the results of this

angled phase of testing appear promising. The cemented samples of the

original screw were still consistently stronger than their uncemented

counterparts. The stiffness of the samples was also seen to almost double

with the addition of a cement mantle, though this is a much smaller

increase than that seen in the initial testing.

4.7. Conclusions

The Sawbones polyurethane foam was shown not to be suitable as a

testing material due to its predominantly closed cell structure. However

despite this, the method of injection with the modified screw and the use of

bone cement to augment the implant showed significant improvement in

load to failure. To further test the hypothesis a new testing material was

required. The most appropriate material to switch to at this point was

Porcine Cadaveric femora, maintained in pairs from individual animals. It

was predicted that there would not be problems with cement pooling in

bone samples, allowing a return to the original design modifications, with

the only addition being the closure of the guide wire hole in the tip of the

screw.




Section 5: Pig Testing 81

5. Pig Testing

Owing to the problem of cement pooling and the fact that the Sawbones

material was a closed cell foam it was decided to undertake testing with

Porcine Cadaveric material. This material is more variable than the

Sawbones foam, but by conducting tests with paired femora, the effect of

this variability is reduced. Despite this variability the porcine material is a

more appropriate material for studying bone cement due to the open cell

trabeculae and presence of bone marrow and body fluids, giving a better

representation of the clinical environment.

As the bone samples to be used were taken from young animals the bone

was much denser and stronger than that of a typical hip screw recipient.

This will not only result in higher than expected yield strengths but may

also act to reduce the penetration of the bone cement into the bone. It is

predicted that in osteoporotic bone the cement penetration would be

greatly increased and despite weak bone, create a much stronger implant

construct.



5.1. Porcine cadaveric material

Owing to problems encountered in previous methods it was decided not to

test any of the samples until a suitable method of cement injection was

determined. It was also decided not to create any samples using the

original uncemented screw until the method was established as this would

be a waste of resources.

5.1.1. Method Development 1

A porcine femur was acquired and sectioned leaving only the head and

proximal end of the shaft intact. Care was taken to ensure that the

remaining length of the femur was sufficient for correct insertion of the lag

screw.

The implantation procedure then closely followed the standard clinical

procedure. A set of Stryker Endoscopy Surgical drills (Stryker (Worldwide

Headquarters), 2725 Fairfield Road, Kalamazoo, MI 49002, U.S.A.) were

used to first create a guide wire hole through the head of the femur, it was

necessary to have the guide wire puncture the head of the femur to ensure

correct alignment and position without the use of x-rays.

A 9mm diameter hole was created over the guide wire to a depth of 55mm,

such that the threaded section of the screw would be located centrally in

the head of the femur.



The guide wire was removed prior to insertion of the lag screw such that

the end hole of the screw could be plugged to prevent cement leakage.

The modified lag screw, with temporary end plug, was inserted to the

desired depth. The area around the screw barrel and the guide wire hole

were both plugged with plasticine to prevent any cement leakage, cement

restrictors would be used clinically. Bone cement was mixed as per

previous samples and injected using a syringe.

Upon injection it was evident that there was a problem with the method as

a large force was required to start the injection, whereupon the cement

immediately flowed back up the barrel of the screw and out around the

plasticine barrier. The cement was left to cure for one hour before the

sample was sectioned.

Removal of the screw showed that all of the cement holes in the modified

screw had been clogged with bony material, and that the cement had only

been delivered though one of the holes. The clogging of the holes and the

density of the bone was such that the cement had flowed directly up the

barrel of the screw with very minimal penetration into the surrounding bone

(Figure 5.1).



Figure 5.1 Method Development 1, the holes for cement delivery were seen to clog with bone material preventing cement flow out of all but one hole, where the cement flowed back up the shaft of the screw

5.1.1.1. Discussion

The sectioning of the sample showed that all of the holes had been

clogged with debris, most likely created from the drilling of the base hole.

In this sample the base hole was not tapped prior to insertion of the screw

as this is standard clinical procedure.


The first test in the porcine material demonstrated the need to tap the base

hole prior to insertion of the modified lag screw. This technique was

expected to reduce the clogging of the cement holes with bony debris.

The procedure used for this sample was predominantly the same as for

the previous sample. After drilling of the base hole, with the guide wire still

in place, the modified screw was used to tap the hole. The modified screw

itself was used as no other suitable tap was available. After tapping the

modified screw was removed and any material was cleaned from the

cement holes. Removal of the screw after tapping showed that all of the



cement holes had been thoroughly clogged with bony material, as was

seen to be the problem in the first pig bone sample. The guide wire was

removed and the temporary plug inserted into the tip of the screw. The

modified screw was reinserted and cemented as per the previous sample.

This sample was not mechanically tested. Sectioning occurred after the

cement had been allowed to cure.

Sectioning of the sample showed that a large number of the cement holes

had become clogged with bony material. Some of the cement had

managed to form a thin coating around the threads, however most of the

cement was found to have flowed back up the shaft of the screw, or

pooled around the tip (Figure 5.2 and 5.3).

Figure 5.2 Method Development 2, the cement delivery holes were seen to clog with bone prior to cement injection, limiting the flow of cement



Figure 5.3 Method Development 2, cement (white) pooled at the tip of the screw as it was not inserted to the full depth of the hole

5.1.2.1. Discussion

The tapping of the base hole prior to insertion was not sufficient to prevent

the holes from becoming clogged with material. A better method of

cleaning the hole or another method of preventing clogging was required.

The sectioning of the sample showed pooling at the tip of the screw

(Figure 5.3) where it was evident that the hole had been tapped to a

greater depth than the screw was finally inserted. This can easily be

avoided by marking the screw with the depth of the hole prior to insertion.




At this point a new method for inserting the lag screw needed to be

developed to counteract the clogging of the holes. It was decided to drill a

larger base hole and to tap it prior to insertion. This technique was

expected to create a large enough gap between the screw barrel and the

bone to allow for compression of the material during the tapping without

impinging on the cement holes.

As it was expected that this method would work, a pair of porcine femora

were obtained and the original screw implanted in one without cement.

This was done using standard clinical implantation procedure.

In the contralateral femur the modified screw was implanted. In this case

after the guide wire was inserted a base hole of Ø10mm was drilled rather

than the standard Ø9mm. This left a gap of 0.5mm around the barrel of

the screw. With the guide wire still in place, the modified screw was again

used to tap the hole. The depth of the hole was marked on the side of the

screw to ensure correct placement. It was removed and cleaned and the

guide wire removed. With the end plugged the screw was reinserted to

the marked depth.

The modified screw was cemented in the same manner as for the previous

samples. Both samples were then cured at 4oC for 24hr. This was done

to ensure the samples did not decay during curing.



The samples were tested by supporting the femoral head in a dental

acrylic ring, with the force applied to the screw to push it out of the head

(Figure 5.4). However, on testing, the femoral heads collapsed pushing

through the acrylic ring. A new method was devised for subsequent

testing.

Figure 5.4 Schematic diagram of the testing method used in Method Development 3, the femoral head was supported using a dental acrylic ring and the force applied to push out the screw

5.1.3.1. Discussion

The insertion method was quite successful in preventing the holes from

becoming clogged with debris. This method was used as the standard

method for subsequent tests.

It was also seen that the cementing of samples using a syringe was not

sufficient. In order to more accurately represent clinical practice a cement



gun should be used. This would also allow the cement to be pressurised

in the femur, giving a better distribution.

The testing method used did not work and a new method was developed

for subsequent tests.

5.1.4. Numerical Results 1

With a successful implantation method now developed an adequate

testing procedure was required. The method used by both Witschger

(1991) and Moore (1997) in their studies was adopted. To facilitate this, a

stainless steel test rig was manufactured (Figure 5.5 and Appendix 1).

Two fresh femora were then implanted, one with the original screw and the

other with the modified screw augmented with bone cement. After

placement of the modified screw, the guide wire hole in the head of the

femur was closed with a self tapping screw and washer. A cement gun

was loaded with bone cement and attached to the end of the modified

screw using a luer lock fitting. Ten millilitres of bone cement was injected

into the screw. The luer lock and cement gun were removed and cleaned

immediately. The self tapping screw in the guide wire hole was left in

place for a few minutes to ensure there was no leakage of cement, it was

then also removed and cleaned.



Both samples were allowed to cure at 4oC for 24hr. The new angled

testing rig was then assembled (Figure 5.5). A Hounsfield adaptor

originally designed for use with an acetabular cup was used to apply an

even load to the head of the femur. The samples were tested at a cross-

head speed of 5mm/min until failure or to a maximum displacement of

10mm. They were subsequently sectioned and photographed.

Figure 5.5 Photograph of the stainless steel angled testing rig with sample in place

5.1.4.1. Results

The modified cemented screw (2732N) proved stronger than the original

uncemented screw (1815N). Full results for all pig tests are given in

section 5.3: Summary of all Pig testing results.



5.1.4.2. Discussion

The cement injection procedure using the cement gun worked well and

gave a better cement penetration than was achieved previously.

Recording the pressure at which the cement is applied would be a useful

addition to the procedure.

The sectioning of the samples highlighted another problem with the bone

cement, that of visibility in the bone. As the cement is effectively the same

colour as the bone, it becomes difficult to differentiate between the two,

preventing an adequate determination of cement penetration. A solution

to this problem is to dye the monomer of the cement prior to mixing. The

addition of a few crystals of Crystal Violet allowed to dissolve in the

monomer will create blue cement while only very marginally affecting the

mechanical properties.

The testing procedure worked very well, and was used for subsequent

testing.


In this pair of samples the method followed was very similar to that of the

previous pair. However, the cementing procedure for the modified screw

was altered slightly.



Once the modified screw was correctly positioned a pressure transducer

(RS 256 – 736, 10 Bar G, 0-100mV) was screwed into the guide wire hole

in the head of the femur. This was set up to record the pressure in the

head of the femur during bone cement injection. A few crystals of Crystal

Violet were added to the bone cement monomer and allowed to dissolve

completely prior to mixing. The injection then continued as normal.

The samples were cured and tested in the same manner as for the

previous samples. They were then sectioned and photographed as usual.

5.1.5.1. Results

The modified cemented screw (4340N) again proved stronger than the

original uncemented screw (3152N). This pair of samples proved much

stronger than the previous pair of samples (All results shown in section

5.3).

The pressure observed in the head of the femur during injection was much

higher than anticipated with peak pressures exceeding 30Bar or 3.5MPa,

the saturation point of the transducer. The injection pressure was applied

using an injection gun with a trigger system; this caused the pressure

applied to appear cyclic in nature (Figure 5.7).



5.1.5.2. Discussion

Sectioning of the samples showed immediately that both of the screws had

been bent during testing (Figure 5.6). The bend had occurred at the edge

of the sample supporting apparatus. This tells us that in this case the

strength of the bone/screw or bone/cement interface was greater than the

bending strength of the stainless steel screw.

Figure 5.6 The modified screw after testing of Numerical Results 2, The shaft of the screw was bent at the edge of the angled test rig, supporting the sample at 45o, this also occurred to the original screw in Numerical Results 2.

The coloured cement was much easier to see than the original white

cement, giving a very definite cement distribution. However this also

made it evident that the large pressures applied by the cement gun had

removed the temporary plug in the tip of the screw.



Figure 5.7 Bone Cement injection pressure as recorded in the head of the femur from Numerical Results 2

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0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0 10 20 30 40 50 60

Time (Sec)

Pres

sure

(Mpa

)



5.1.6. Conclusions

Both the modified and the original screw were deformed and could not be

reused for testing. Two further screws were obtained and one was

modified. It was decided to take this opportunity to review the design of

the modified screw and permanently close the tip of the screw. This

modification would prevent the leakage of bone cement out of the guide

wire hole.

As a precaution, to prevent screws bending it was decided that further

tests would be limited to a maximum load of 3000N or to a maximum

displacement of 5mm.

At this point the modified cemented samples were consistently stronger

than the original uncemented samples. This is consistent with results

obtained with the Sawbones material despite its unsuitability to this kind of

testing. It was thought that this new screw with a permanently closed tip

will continue this trend, and possibly improve on the results as seen

previously.



5.2. New Modified Screw Pig Testing


The new modified screw was created from a standard Stryker Howmedica

Omega + Plus sliding compression hip screw. It was noted that the new

screw used for modification was of a different batch number to the one that

was originally modified. Some slight differences were seen in the thread

characteristics of the two screws; however these could be attributed to

standard wear of cutting tools and manufacturing equipment and would be

well within the tolerance of the design. The effect of these differences on

the mechanical characteristics of the screw would be negligible.

The new screw was modified in house to include the nine cementing holes

from the original design (Figure 5.8). The guide wire hole at the tip of the

screw was permanently closed (Figure 5.9). This will prevent the flow of

cement out of the tip and along the guide wire hole, while increasing the

flow of cement out of the side cementing holes and so increase the radial

penetration of cement into the bone.



Figure 5.8 Photograph of the cement delivery holes in the new modified screw

Figure 5.9 Photograph of the new modified screw with sealed guide wire hole in the tip of the screw



The standard implantation procedure was followed for both screws;

however the cementing procedure for the modified screw was altered

slightly. In this instance the pressure of the cement delivery was

measured in-line with the delivery by way of a T piece adaptor (Figure 5.10

and Appendix 1). This was designed to give a more accurate measure of

the pressure of delivery rather than the final pressure experienced in the

head. Because of this arrangement the guide wire hole in the head of the

femur was closed using a self tapping screw and washer as was done

previously.

The samples were cured at 4oC for 24hr and tested in the same manner

as the previous sample. They were then sectioned and photographed.

Figure 5.10 Photograph of the Injection pressure recording apparatus used in Numerical Results 3 and 4. The barrel of the screw (bottom left) and the pressure transducer (top) are attached to the brass T-piece adaptor, the cement injection gun attaches to the right hand side of the T-piece.



5.2.1.1. Results

The stiffness and final load (3000N or at a displacement of 5mm) of the

modified screw sample (1447.5N) was much lower than that of the Original

uncemented sample (3060N) (Results in section 5.3).

The pressures observed during the cement injection were also much lower

than were previously seen in the head of the femur during injection.

Figure 5.11 shows the pressure recorded in-line with the injection, and

then compared with the first pressure measurement in the head of the

femur in Numerical Results 2 (Figure 5.12).



Figure 5.11 Numerical Results 3, bone cement injection pressure as measured inline with the delivery

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.00 5.00 10.00 15.00 20.00 25.00 30.00

Time (sec)

Pres

sure

(Mpa

)



Figure 5.12 Comparison of injection pressures measure in the head of the femur (Numerical Results 2, shown in Dark Blue) and inline with the injection (Numerical Results 3, shown in Pink).

-0.50

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0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0 10 20 30 40 50 60

Time (Sec)

Pres

sure

(Mpa

) Injection Pressure in Head of FemurInjection Pressure in line 1



5.2.1.2. Discussion

Sectioning of the modified cemented sample showed immediately why the

cementing pressure had been much lower than previously and why the

final load was much lower than that of the uncemented sample. A large

void in the bone had caused most of the cement to flow away from the

head of the femur and down towards the shaft of the femur (Figures 5.13

and 5.14). This void not only acted to channel the cement away from the

desired area but suggests that the remaining bone in the head of the

femur would have been diseased and weakened. The only way to avoid

this happening in the future is to x-ray the samples prior to testing to check

for any pathology. It does however highlight what might happen clinically if

the patient has any pre-existing pathology.

As anticipated neither of the bone samples failed before the safety cut-offs.

Instead comparisons of the maximum load reached and the stiffness could

be made (Section 5.3).



Figure 5.13 Sectioned view of the modified cemented sample from Numerical results 3, a bone void can be seen filled with cement

Figure 5.14 Sectioned view of the modified cemented sample from Numerical results 3, with the screw removed a bone void can be seen filled with cement.




Due to the possibility of voids or other pathology in the porcine cadaveric

material the samples were X-rayed prior to use (Figure 5.15).

Figure 5.15 X-ray of the Porcine femora used in Numerical Results 4 prior to use

After any major pathology of the specimens was excluded the implantation

and cementing of the screws proceeded by the same methods used for

the previous samples.

Samples were cured for 24hr at 4oC and tested as for previous samples.

Specimens were then sectioned and photographed.



5.2.2.1. Results

While the modified cemented sample was much stronger than in the

previous pair it was still dramatically lower than the Original uncemented

sample (Full results in Section 5.3).

The pressures measured during cement injection were very similar to

those recorded when the pressure was measured in the head of the femur,

Numerical Results 2 (Figures 5.16 and 5.17).



Figure 5.16 Numerical Results 4, bone cement injection pressure measured inline with the injection

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0.50

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1.50

2.00

2.50

3.00

3.50

4.00

0 10 20 30 40 50 60

Time (s)

Pres

sure

(MPa

)



Figure 5.17 Comparison of the injection pressures recorded in Numerical Results 2 (Blue), Numerical Results 3 (Pink) and Numerical Results 4 (Orange)

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3.00

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0 10 20 30 40 50 60

Time (Sec)

Pres

sure

(Mpa

)

Injection Pressure in Head of FemurInjection Pressure in line 1Injection Pressure in line 2



5.2.2.2. Discussion

The testing of this sample went completely according to plan. The bones

were x-rayed prior to implantation and were found to be of good quality.

The Injection pressures recorded in line with the injection were the same

as when measured in the head of the femur. Sectioning showed that the

cement was evenly distributed (Figures 5.25 and 5.26). In spite of this the

failure load of the modified cemented screw was again much lower than

that of the uncemented original fixation. The difference between this pair

is much less than for the previous pair, however there is no apparent

reason for this as there was in the previous samples.

The stiffness of the modified cemented sample was also consistently lower

than that of the original sample at 2mm, 3mm and 4mm of displacement.

This result appears so unusual because of the results of samples one and

two. In both of these pairs the modified screw was consistently stronger

and stiffer than the original screw. The only difference between pairs one

and two, and pair four, is the closed end of the screw. The cement

patterns seen in pair two and four are quite similar, with a large proportion

of the cement around the threads and no reflux down the barrel.



5.3. Summary of All Pig Testing Results

5.3.1. Cement Distribution

Pictures of the bone cement distribution of each of the modified samples

are shown in figures 5.18 – 5.27. The limit of bone cement penetration is

marked with a black line. It is difficult to see the bone cement in sample

one as it is white. Crystal violet was added to subsequent samples for

better visibility. The approximate area of penetration has been marked on

each of the photographs. The void present in sample three can clearly be

seen filled with bone cement; this is believed to be responsible for the

greatly reduced strength in this sample.



Figure 5.18 Sectioned view of Numerical Results 1, the white cement is very hard to see

Figure 5.19 The modified screw once removed from Numerical Results 1



Figure 5.20 A sectioned view of the Numerical Results 2 modified cemented sample, the bone cement is clearly visible in blue

Figure 5.21 The modified screw after removal from the bone in Numerical Results 2



Figure 5.22 A sectioned view of Numerical Results 3, the bone void is apparent by the large mass of cement

Figure 5.23 The same sample (Numerical Results 3) with the modified screw removed, the size of the bone void is evident



Figure 5.24 The modified screw as removed from Numerical Results 3



Figure 5.25 A sectioned view of Numerical Results 4

Figure 5.26 The Numerical Results 4 bone sample after removal of the screw, the full extent of the bone cement penetration can be seen



Figure 5.27 The Modified screw removed from Numerical Results 4

5.3.2. Force – Displacement Data

The force – displacement curves for all of the samples tested in porcine

material are shown in Figure 5.28. The pairs of bones have been grouped

by colour with the darker of the colour representing the modified screw

sample and the lighter representing the original screw sample.

In pairs one and two the modified screw samples are much stronger than

the original screw samples, however the reverse is true for samples three

and four.



Figure 5.28 Force – Displacement data from the four sets of paired cadaveric porcine femora, Numerical Results 1, Original screw in pink and Modified screw in red, Numerical Results 2, Original screw in light blue and Modified screw in dark blue, Numerical Results 3, Original screw in green and Modified screw in black, Numerical Results 4, Original screw in light orange and Modified screw in dark orange

2732

1815

4340

3152

1447.5

3060

2330

3012

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 1 2 3 4 5 6 7 8 9 10

Displacement (mm)

Forc

e (N

)

Numerical Results 1 Modified ScrewNumerical Results 1 Original ScrewNumerical Results 2 Modified ScrewNumerical Results 2 Original ScrewNumerical Results 3 Modified ScrewNumerical Results 3 Original ScrewNumerical Results 4 Modified ScrewNumerical Results 4 Original Screw



5.3.3. Stiffness Data

The stiffness of each of the samples was calculated at three

displacements (Table 5.1 and Figure 5.29). This was done because the

limitations placed on the testing to prevent the bending of the screws also

prevented failure of the samples and as such there are no true failure

loads to compare.

In sample one it can be seen that both of the samples have failed, with the

original sample failing before a displacement of 3mm had been reached.

In the third pair the effect of the bone void on the stiffness can clearly be

seen by the marked differences in stiffness between the two samples at

each point.



Table 5.1. Stiffness (N/mm) data calculated at displacements of 2, 3 and 4mm. * Specimen failed before this displacement

2mm 3mm 4mm Pair 1 Mod 747.06 657.74 490.75 Orig 698.53 40.428 * 2mm 3mm 4mm Pair 2 Mod 856.15 911.83 754.88 Orig 746.67 611.21 418.8 2mm 3mm 4mm Pair 3 Mod 247.66 390.11 540.51 Orig 863.22 930.35 808.19 2mm 3mm 4mm Pair 4 Mod 522.35 697.99 512.87 Orig 603.26 784.42 694.36



Figure 5.29 Stiffness of the pig samples at 2, 3 and 4mm displacement, Modified samples in blue and original samples in green, the original samples in Numerical Results 1 failed after 3mm of displacement

0

100

200

300

400

500

600

700

800

900

1000

1 2 3 4

Numerical Results

Stiff

ness

(N/m

m)

Modified @ 2mm Original @ 2mm





5.3.4. Comparison to Sawbones samples

A comparison of the stiffness and failure loads of all Sawbones and

Porcine samples tested are shown in figures 5.30 and 5.31. The means of

the Sawbones data are represented by the red bar at the start of each set,

with the error bars showing one standard deviation in each direction. For

the porcine samples the peak or failure load was used as appropriate.

It is interesting to note the stiffness values of the Pig bone and the

Sawbones 45o samples as well as the Original uncemented 90o Sawbones

samples, which are all very close to one another (Figure 5.31).



Figure 5.30 Failure or Peak loads of Sawbones foam and Porcine Numerical Results, the mean and standard deviation of the Sawbones samples is shown in red, the porcine Modified screw samples are shown in blue and the Original screw samples in green

870.10

3377.33

2379.38

1837.83

2954.40

3112.57

964.33

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

Mod Plug 90 Mod 90 Orig Cem 90 Original 90 Mod Plug 45 Orig Cem 45 Original 45 Pig 1 Pig 2 Pig 3 Pig 4

Forc

e (N

)



Figure 5.31 Stiffness of all samples, mean values from the Sawbones data are shown in red with standard deviations marked, the Porcine Numerical results data are shown at displacements of 2, 3 and 4 mm with the Modified samples shown in blue and the Original samples in green

2659.77

2381.42

2544.02

775.31

815.15

750.07

420.28

0

500

1000

1500

2000

2500

3000

3500

Mod Plug 90 Mod 90 Orig Cem 90 Original 90 Mod Plug 45 Orig Cem 45 Original 45 Pig 1 Pig 2 Pig 3 Pig 4

Stifn

ess

(N/m

m)



5.4. All Pig Discussion

The use of porcine cadaveric material was not as successful for testing as

was hoped. Several problems were encountered with implantation,

cementing and testing methodology.

After several trials a method was developed in which the cement delivery

holes did not become clogged with material upon insertion. This method

involved the creation of a base hole 1mm larger than the barrel of the

screw, leaving a 0.5mm gap in each direction. This creates a space for

the bone to be compressed into from the tapping process, with a reduced

chance of blocking any of the delivery holes. Questions were raised as to

whether this would weaken the fixation by decreasing the purchase on the

surrounding bone, however as the gap is filled with solid cement, the

cement then penetrates further into the surrounding bone.

The second pair of pig bone samples then created further problems.

While the initial failure curves looked good, with the modified sample

having a failure load 27% greater than the original sample, it soon became

apparent that the failure had in fact been in the screws rather than the

bone. Both of the screws had bent at the edge of the supporting part of

the testing apparatus. While this does happen clinically, it was not

expected in this trial and so safe guards were not in place. However it

does indicate that the weakest part of the construct was the screw itself.

This is not unreasonable as the bone samples were taken from young



healthy pigs. To prevent this from happening again limitations were put

into the testing, the maximum displacement was reduced to 5mm and the

maximum applied force to 3000N. This was sufficient to prevent this from

happening again in subsequent testing. However the secondary effect of

these limitations was to prevent any of the subsequent samples from

failing under load. For comparison the stiffness of the samples was

compared at three displacements.

The use of a cement injection gun greatly increased the penetration of the

cement into the area around the threads. As the gun pressurises the

cement it is useful to know what pressure is actually being delivered by the

gun. Initially the pressure transducer was set up to measure the pressure

experienced in the head of the femur, the pressure recorded was much

larger than expected, exceeding the maximum of the transducer. However

it was then decided that knowing the pressure of delivery was more

relevant than the final pressure experienced. In the second pressure

recording the transducer was inserted inline with the delivery, between the

cement gun and the screw, using a T-piece adaptor (Appendix 1:

Technical Drawings). This gave an accurate measure of the pressure the

cement was being applied at. However the value was much lower than

the previous measurement. Upon sectioning it became evident that this

was caused by the cement flowing into a large void in the bone, which

prevented the cement gun from pressurising the cement to any great

degree. The third attempt to record the injection pressure was more



successful. Again the transducer was placed inline with the injection, this

time the pressure measured was very large, exceeding the maximum of

the transducer. As both the first and third recordings exceeded the

maximum value recorded by the transducer it is impossible to say what the

actual pressure of delivery was, however it is evident that in intact bone

the pressure easily exceeds 3.5MPa. This could create a problem in weak

bone, however as this bone is normally also of a very low density, the

pressures experienced would be much lower than recorded here.

While the radiopaque nature of the bone cement makes it highly visible

under x-rays, the white colour of the cement makes it very hard to see in

reality. This is evident in the first pair of pig bones tested, where it is very

difficult to determine the extent of cement penetration into the bone. This

problem was corrected by the addition of Crystal Violet to the monomer of

the cement prior to mixing. The larger amount of Crystal Violet used, the

brighter blue the cement became, and the easier it was to see.

The presence of the bone void in sample 3 was unexpected because of

the age of the bone samples. However in order to prevent this in the

future samples, all remaining pairs of cadaveric femurs were X-rayed. The

films showed no obvious signs of pathology and all pairs of bones were

cleared for use.



As the initial modified screw and the original screw were both bent beyond

further use in sample 2, a new pair of screws were required. A pair of lag

screws had already been acquired in case they were needed. These new

screws were of the same type as the initial screws but were of a different

batch number. The bending of the old screws provided an opportunity to

review the current design of the screw before another was modified. From

looking at the results up to this point the current design was looking very

good, but one adjustment was made to the design. Because of the clinical

risk of the guide wire puncturing the head of the femur and subsequent

cement leakage, and the improvement of the results in the Sawbones

material with the use of a temporary plug, it was decided to permanently

close the guide wire hole in the tip of the screw. The new modified screw

was then manufactured and used for subsequent testing. However,

because of the closed tip it was no longer possible to tap the base hole

with the modified screw and still have the guide wire in place. Instead the

remaining original screw was used to tap the hole with the guide wire in

place, both were then removed and the modified screw inserted to the

correct depth. This created only a minor inconvenience and it is expected

that should this design be adopted clinically a specific tapping device

would be created for this purpose.

The results of the pig testing - the bending of the screws and the better

results without cement, raise questions about the validity of using young

porcine material as an analogue for Osteoporotic human bone. In



Osteoporotic bone it is rare for the implant to be the weakest component

and fail through bending. In samples three and four, the final load of the

original uncemented samples was considerably higher than that of the

modified cemented sample. While in sample three this can be attributed to

the bone pathology, in sample four there were no extenuating

circumstances. Whereas the literature consistently shows an increase in

failure load with the addition of bone cement in human cadaveric material.

This suggests that the porcine material is both too strong and too dense

for this style of testing to be an accurate analogy for Osteoporotic bone.

5.5. Conclusion

While the porcine material is a more accurate model than the Sawbones

foam, problems still exist in using it as an osteoporotic bone analogue.

And while it is thought that the modified screw with the permanently closed

tip, would be at least as good as the initial modified screw, it was unable to

be shown in the porcine material.


Section 6: General Discussion 128



6. General Discussion

Many problems have been encountered and overcome over the course of

this study. Most of the problems centre on the appropriateness of testing

materials.

The Sawbones polyurethane foam was found to be 95% closed cell. This

made it completely unsuitable for use in testing implants with bone cement

augmentation. As well as having degradation problems resulting in a 17%

decrease in strength over a period of eight months.

While the Porcine cadaveric material offered a better model of human

bone, it did not represent the Osteoporotic nature of most patients who

require fixation. The porcine bone had a far greater strength and density

than would be seen clinically in an osteoporotic patient. What this did

demonstrate was that there is no point in augmenting fixations in healthy

bone as it gives no mechanical advantage.

Testing the modified screw in a better analogue of osteoporotic bone may

be beneficial. Currently an Osteoporotic Sheep is being developed in

Adelaide; if this is successful then suitable cadaveric test material may be

able to be obtained.



Once a suitable testing material becomes available the modified screw

could be tested in a cyclic manner to better represent the clinical failure

mechanisms.


Section 7: Conclusions 131

7. Conclusions

The Sawbones Polyurethane foam was found to be completely

inappropriate for use as a testing material for implants augmented with

bone cement. Care should also be taken when using this product for other

mechanical testing due to its propensity to degrade over time.

The testing methodology was suitable for preliminary testing of the

modifications, however in order to more fully model the cut-out behaviour

of the lag screw, cyclic testing would need to be carried out.

In terms of the modifications made to the lag screw it is difficult to make

any definitive conclusions. In the Sawbones material the failure load was

increased significantly with the addition of bone cement, this finding is

supported by the literature where cement augmentation consistently

increased holding power in cadaveric and synthetic materials. However

the results of the porcine testing do not agree with this finding, with the

augmentation of the fixation being no better, and in some cases worse,

than the original screw alone. This finding tends to indicate more that

cement augmentation is inappropriate in healthy bone; rather than

commenting on its usefulness in osteoporotic bone.

If a better model of osteoporotic human bone could be found for a testing

material, then the modified screw with bone cement augmentation may yet


Section 7: Conclusions 132

prove better, or at least more convenient, than standard augmentation

method.


Section 8: References 133

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ASTM (2001a) F1839-97 - Standard Specification for Rigid Polyurethane Foam for Use as a Standard Material for Testing Orthopaedic Devices and Instruments, ASTM, West Conshohocken.

ASTM (2001b) F2118-01a - Test Method for Constant amplitude of force controlled fatigue testing of Acrylic Bone cement materials, ASTM, West Conshohocken.

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Appendices 139

Appendix 1: Technical Drawings

1.1. Slotted Screw design

1.2. Final Screw design

1.3. Closed End Screw Design

1.4. Hounsfield Adaptor

1.5. 45o Alignment Jig

1.6. Pig Test Rig

1.7. Pressure Transducer T-piece Adaptor


Appendices 140

1.1. Slotted Screw design


Appendices 141

1.2. Final Screw design


Appendices 142

1.3. Closed End Screw Design


Appendices 143

1.4. Hounsfield Adaptor


Appendices 144

1.5. 45o Alignment Jig


Appendices 145

1.6. Pig Test Rig


Appendices 146

1.7. Pressure Transducer T-piece Adaptor


Appendices 147

2. Appendix 2: Ringers Foam Compression Test

2.1. Toad Ringers Solution Formula

Toad Ringers Solution mol 1 litre 4 litres 20 litres mmol One litre Weight Grams mmol Grams Grams Na 144.54NaCl 58.44 7.00 119.77 28 140 K 1.88 KCl 74.55 0.14 1.88 0.56 2.8 Ca 1.80 CaCl2 110.98 0.222 1.80 Mg ≈ 1 MgCl2 95.21 0.095 Cl 129.46NaHCO3 84.01 2.00 23.81 8 40 PO4 0.96 NaH2PO4 156.01 0.15 0.96 0.6 3 HCO3 23.81 Glucose 180.20 2.00 11.10 8 40 Glucose 11.10 Note: CaCl2 and MgCl2 will kill the solution over time Make up 1M solutions of each – add as required 1 ml of 1 M solution into 1 litre = 1mMol Final concentration of: CaCl2 = 2 mMol MgCl2 = 1 mMol


Appendices 148

3. Appendix 3: Statistical analysis of Sawbones 90o Data

A power analysis of the Sawbones 90o data was used to determine the number of samples required in further testing. A slight overestimate of the average standard deviation for the groups being compared was used, with results shown in Table 1. Power is 0.8 in each instance and the significance level is 0.05. Table 1: The approximate no of subjects required in each group are:

Difference in Means 100 200 400

Modified plugged vs. Modified 37 10 4

Mod plugged vs. Original Cemented 64 17 5

Modified vs. Original Cemented 85 22 7

Results were also calculated using the maximum standard deviation for the groups being compared in each case (this is a more conservative estimate) in Table 2. Table 2: approximate no of subjects required in each group are:

Difference in Means 100 200 400

Modified plugged vs. Modified 52 14 5

Mod plugged vs. Original Cemented 125 32 9

Modified vs. Original Cemented 125 32 9

It was decided to use the results of the power analysis using the maximum standard deviations, with a difference of 400. As such it was decided to test nine samples of each method in the subsequent angled testing in the Sawbones material.


Appendices 149

4. Appendix 4: Sawbones website details

The foam material used as a bone analogue in the initial stages of testing was purchased from Sawbones (Pacific Research laboratories, 10221 SW 188th St, Vashon, Washington, 98070, USA). At the time of initial purchase the manufacturers’ website stated that the material was a cellular rigid polyurethane foam, and gave basic material properties. Unfortunately a copy of the website was not made at this time. Several months later the company released a new Open cell, cellular rigid polyurethane foam. At this time the page relating to the original Cellular foam was updated to include the information that it is 95% Closed Cell.

“The appearance of cellular rigid polyurethane foam resembles that of cadaveric cancellous bone, however, the cell structure is 95% closed as compared to the open cell structure of cancellous bone. “

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