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