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What is bone cement?
SHEWEIDIN AZIZ ST3 TRAUMA AND ORTHOPAEDICS FRIDAY 11/03/2016
Aim
Background Role of cement Composition Phases Polymerisation of cement
Sterilisation Mechanical properties Techniques Antibiotics Side effects
Background
Sir John Charnley, who is considered the founder of modern artificial joint replacement, developed the science and art of modern cementing technique in his laboratories in the late 1950s
Despite a variety of cement-less prostheses, cement fixation remains the gold standard, against which all forms of implant fixation techniques are assessed
However in 1870 ….
Themistokles Gluck a German surgeon, was the first surgeon to implant a total knee replacement made of ivory. He fixed the stems in both the tibia and the femur with cement: “...for a better fixation, I mixed plaster with colophony, which cures up to the hardness of glass.”
PMMA as a polymer is commonly used in daily life
Plexiglas, for example, is an acrylic resin that was invented at the beginning of the 20th century
In 1936, cold curing of MMA was developed and introduced into dentistry and craniofacial surgery
Today
Today, the use of PMMA bone cement is a widely used method of implant fixation
This fixation technique largely contributes to the success of modern joint replacement and in newer techniques such as percutaneous vertebroplasty and kyphoplasty
Role of cement
Allows secure fixation of implant and bone
Mechanical interlock and space filling
Load transferring
Maintenance/restoration of bone stock
Composition – Powder and Liquid
POWDER (bead-shaped particles – diameter 40microns)
LIQUID
POLYMER: PMMA +/- MMA copolymers – alters physical properties of the cement
MONOMER: MMA
INITIATOR: Benzoyl peroxide (BPO) - reacts with DMpt to catalyse polymerisation
ACCELERTOR: N, N-Dimethyl para para-toluidine (DMpt)
RADIO-OPACIFIER: Barium sulphate/Zirconia STABILISER: hydroquinone - Prevents premature polymerisation
ANTIBIOTICS: Aminoglycoside - Gentamicin
DYE: Chlorophyll
Phases
1. Mixing
2. Waiting: Temperature dependent
3. Working: Temperature dependent
4. Hardening/Setting: Temperature dependent
Mixing Phase 2-3 minutes
1. Liquid wets the surface of the pre-polymerised powder = homogenous dough
2. PMMA dissolves in its monomer and the pre-polymerised beads swell and some of them dissolve completely during mixing
3. Homogenous sticky mass
Waiting Phase
1. Further swelling of beads + polymerisation to proceed increase in the viscosity of the mixture
2. Cement turns to dough
3. The end of the waiting phase is when the cement is neither sticky nor hairy
Working phase 5-8 minutes
Low viscosity to allow application Polymerisation continues viscosity increases Heat of polymerisation causes thermal expansion
while there is a volumetric shrinkage Blood lamination of the cement causes weakness Prosthesis must be implanted before the end of the
working phase
Hardening/Setting Phase 8-10 minutes
Temperature continues to rise then slowly returns to body temperature
Volumetric and thermal shrinkage as it cools to body temperature
Ends when a hard consistency is reached Prolonged with environmental factors (low temp. high
humidity) Mixing too quickly can hasten the polymerisation
reaction leading to shorter setting time
Typical curing curve
Polymerization process (curing)
Carbon-to-carbon double bonds broken New carbon single bonds form Linear long-chain polymers Free of cross-linking Volume shrinkage (7%)
Initiator BPO + Activator DMpT = free radicals
Results in growing polymer chain
When two growing polymer chains meet the chains are terminated
Polymerisation
Exothermic reaction Can result in thermal bone damage Temp. can reach 70-120°C Heat is increased
Thicker cement mantle High ambient temp. (shorten dough and working
time by 5% per °C) Increased monomer to polymer ratio
Sterilisation
Ethylene Oxide does not affect many mechanical properties as does Gamma radiation
Mechanical Properties
Tensile strength 25MPa Shear strength 40MPa Compression 90MPa Young’s module of elasticity 2400MPa Viscoelastic Brittle
Viscoelastic
Material’s properties vary with rate of loading Behaves like both fluid and solid
e.g. ligaments/cartilage There are 3 main characteristics
Creep (Plastic deformation)
Time dependent deformation under constant load Rate reduces with time Caused by Load of daytime activities Is a mechanical problem that slowly and steadily can erode the long-
term performance of an implant Cements with higher porosity/viscosity are less resistant to creep
deformation
Stress relaxation
Is the change in stress with time under constant strain (deformation) caused by a change in the structure of the cement polymer
Reduced load at night allows stress relaxation
Hysteresis
The loading and unloading curves are not identical
Not all the energy applied to the specimen during loading is recovered on unloading
Cementing techniques
GENERATION
CEMENT MIXING
CANAL PREPERATION INSERTION CENTRALISATION
FIRST • Hand mix • Rasp only • Manual with finger packing
• No
SECOND • Hand mix • More aggressive rasping
• Cement gun• Distal canal plug
• No
THIRD • Vacuum mix
• More aggressive rasping
• Brushing • Pulsatile lavage
• Cement gun• Distal canal plug• Pressurisation
• Yes
Cementing techniques …
Clinical studies have shown that large voids, up to 5 mm in diameter, are often detrimental
Clinical studies have shown that a proximal-medial cement mantle >10 mm in thickness has been associated with a significant increase in cement fracture
An additional finite element analysis has demonstrated that extending the distal cement mantle >7.5 mm distal to the stem tip provides no significant additional decreases in distal cement strains
Gruen Zones
Antibiotics
Thermally stable Water soluble Bactericidal Gradual release into tissues Minimal local inflammation Does not affect mechanical integrity of cement Broad spectrum
Antibiotics cont…
Most common use of Gentamicin and Tobramycin however, Vancomycin and Ciprofloxacin were tried
Ciprofloxacin inhibits soft tissue healing
Penicillins and Cephalosporins exhibit stability and good elution properties but have potential allerginicity
Side Effects
1. Hypotension2. Cerebrovascular insults3. Pulmonary Embolism4. Hypersensitivity5. Cardiac Arrest6. Bone cement implantation syndrome
Bone cement implantation syndrome
1. Hypoxia2. Hypotension3. Cardiac Arrhythmias4. Increased Pulmonary Vascular Resistance5. Cardiac Arrest
Grading of BCIS
1. Moderate Hypoxia (94%) OR Hypotension (20% fall in SBP)
2. Severe Hypoxia (88%) OR Hypotension (40% fall in SBP) OR Unexpected loss of consciousness
3. Cardiovascular collapse requiring CPR