BONE CEMENT

• Polymethylmethacrylate remains one of the most enduring materials in orthopaedic surgery.

• It has a central role in the success of total joint replacement and is also used in newer techniques such as percutaneous vertebroplasty and kyphoplasty (tricalcium phosphate).

• The major breakthrough in the use of PMMA in total hip replacement (THR) was the work of Charnley

• In 1970 who used it to secure fixation of the acetabular and femoral component and to transfer loads to bone.

INDICATIONS

• Joint Replacement Surgery• Spinal Compression Fractures• Chronic Osteomyelitis• Tumours

IN ARTHROPLASTY

• Allows secure fixation of implant to bone• It’s not glue – has no adhesive properties• Mechanical interlock and space filling• Load transferring material (from the

component into bone) • Maintenance/restoration of bone stock

MECHANICAL PROPERTIES

• Poor tensile strength of 25 Mpa• Moderate shear strength of 40 Mpa• Strongest in compression of 90 Mpa• Brittle, notch sensitive• Low Young’s modulus of elasticity (E) =2400

Mpa• Viscoelastic

Viscoelasticity

• Materials properties vary with rate of loading

• Behaves like both fluid and solid• examples – ligaments and cartilage• Viscoelastic material has three main

characteristics viz. creep, relaxation and hysteresis

1. Creep

• Time-dependent deformation under constant load (also known as plastic deformation)

• Creep rate reduces with time• load of daytime activities causes creep• Creep, essentially is a mechanical problem that

slowly and steadily can erode the long-term performance of an implant.

• Cements with higher porosity and viscosity are less resistant to creep deformation.

2. Stress relaxation

• The change in stress / force with time under constant strain (deformation) caused by a change in the structure of the cement polymer

• at night reduced load allows stress relaxation

3. Hysteresis

• The loading and un loading curves are not identical.

• Not all the energy applied to the specimen during loading is recovered on unloading.

STERILISATION

• Gamma radiation shortens the polymer chains, probably affecting many mechanical properties,

• but this does not occur with ethylene oxide sterilisation

COMPOSITION

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

• Initiator BPO + Activator DMpT = free radicals• Results in growing polymer chain• When two growing polymer chains meet the

chains are terminated

PHASES

Mixing Phase• Starts with the addition of the liquid to the powder and

ends when the dough is homogenous and stirring becomes effortless.

• The liquid wets the surface of the prepolymerized powder. • Because PMMA is a polymer that dissolves in its monomer

(which is not the case for all polymers), the prepolymerized beads swell and some of them dissolve completely during mixing.

• At the end of the mixing phase, the mixture is a homogenous mass and the cement is sticky and has a consistency similar to toothpaste.

Waiting Phase

• Allows further swelling of the beads and to permit polymerization to proceed.

• This leads to an increase in the viscosity of the mixture. • The cement turns into sticky dough. • This dough is subsequently tested with gloved fingers

every 5 seconds, using a different part of the glove on another part of the cement surface on each testing occasion.

• This process provides an indication of the end of the waiting phase when the cement is neither “sticky” nor “hairy.”

Working Phase• The cement is no longer sticky, but is of sufficiently low

viscosity to enable the surgeon to apply the cement.• Polymerization continues and the viscosity continues to

increase;• Heat of polymerization causes thermal expansion of the

cement, while there is a competing volumetric shrinkage of the cement as the monomer converts to the denser polymer.

• With a very low viscosity, the cement would not be able to withstand bleeding pressure. This would result in blood lamination in the cement, which causes the cement to weaken.

• This phase is completed when the cement does not join without folds during continuous kneading by hand;

• Therefore, the prosthesis must be implanted before the end of the working phase.

Hardening or Setting phase

• The polymerization stops and the cement cures to a hard consistency.

• The temperature of the cement continues to be elevated, but then slowly decreases to body temperature.

• During this phase, the cement continues to undergo both volumetric and thermal shrinkage as it cools to body temperature.

Curing process time periods

• Dough time: mixing >> non sticky (approximately 2-3 minutes)

• Working time: difference between dough time and setting time (end of dough time until the cement is too stiff to manipulate, usually about 5-8 minutes)

• Setting time: mixing >> surface temperature is half maximum (usually about 8-10 minutes)

typical curing curve for acrylic bone cement where Tmax is the maximum temperature reached, Tset is the setting temperature and Tamb is the

ambient temperature.

• However, the rates of curing are very sensitive to environmental factors.

• Low ambient temperatures during storing and mixing, and high humidity both prolong setting time.

Heat production during polymerisation

• The polymerisation of PMMA is exothermic. • Can lead to thermal damage to bone.• Recorded temperatures range between 70°C

and 120°C.

• In vitro studies have shown that the production of heat is increased by-

1. thicker cement mantles, 2. higher ambient temperatures and 3. an increased ratio of monomer to polymer.• Increases in room temperature shorten both

the dough and setting times by 5% / degree centigrade

Factors that Affect Bone Cement Preparation

• The ambient temperature - higher the temperature, the shorter the phase and the colder the temperature, the longer the phases.

• The mixing process - Mixing cement too quickly or too aggressively can hasten the polymerization reaction resulting in a reduced setting time.

• In general, the lower the heat of polymerization, the longer the setting time, and the greater the heat of polymerization, the shorter the setting time.

• The powder to liquid ratio - If more liquid, or less powder, than required is used, setting time will be prolonged; - on the other hand, if less liquid, or more powder is used, setting time will be shortened

Cementing techniques

• First generation• Original technique of Charnley:• Hand mixing of the cement• Finger packing of cement in an unplugged and

uncleaned femoral canal and acetabulum• No cement restrictor, no cement gun and no

reduction in porosity

Second generation

• Femoral canal plug• Cement gun to allow retrograde filling• Pulsatile lavage• Cement restrictor

Third generation

• Pressurization of cement after insertion• Some form of cement porosity reduction

(vacuum or centrifugation)• Stem centraliser

Cartridge mixing and delivery

• Latest advancement in bone cement mixing technique

• It is a simple, universal power mixer that quickly mixes and then mechanically injects all types of bone cement.

• This type of device reduces mix times, as it requires fewer steps to load, mix, and transfer the cement.

• The rotary hand piece reduces variability, which results in consistent mix times and built-in charcoal filter reduces harmful fumes.

Antibiotics & Bone Cement

• Ideal antibiotic properties1. Preparation must be thermally stable2. Antibiotic properties not affected by heat3. Must be water soluble for diffusion into tisssues4. Bactericidal5. Must be released gradually over an appropriate time period6. Minimal local inflammatory response7. No resistance8. Must have action against common pathogens like s. aureus, s.

epidermidis ,coliforms and anaerobes.9. Must not significantly compromise mechanical integrity

• Gentamycin (most common) and tobramycin are commonly used

• Vancomycin and ciprofloxacin are also tried• Ciprofloxacin may inhibit soft tissue healing• Penicillins and cephalosporins exhibits stability

and good elution properties. But are avoided due to their potential allerginicity.

• Vancomycin p (ultrafine powder) is used as lyophilised vancomycin greatly reduces fatigue strength

• > 4.5 g gentamycin significantly reduces fatigue strength

• > 10% reduction in fatigue strength is considered inappropriate for use in arthroplasty fixation

• but weaker cement with higher antibiotic concentration may be used for spacers and antibiotic beads ( to deliver high antibiotic concentration)

• Vacuum mixing increases fatigue strength• ↑ed surface area leds to ↑ed drug elution

Antibiotic may be used in low doses or high doses

Low dose • Antibiotic content is 0.5 – 1 gm per 40 gm of

cement• Used in1. Prophylaxis in revision arthroplasty or 2. In high risk primary arthroplasty

HIGH DOSE

• Contains > 3.6 gm of antibiotic per 40 gm of cement

• Used in 1. Spacers and beads2. Second stage of two staged revision ( content

is 1-2 gm per 40 gm cement)• Cement spacer commonly contains 4 gm

vancomycin and 4.8 gm of tobramycin in 80 gm of cement

The Dangers

• Hypotension• cardiac arrest• cerebrovascular accident• pulmonary embolus• hypersensitivity reactions

BCIS(bone cement implantation syndrome)

• BCIS is characterized by hypoxia, hypotension or both and/or unexpected loss of consciousness occurring around the time of cementation, prosthesis insertion, reduction of the joint or, occasionally, limb tourniquet deflation in a patient undergoing cemented bone surgery.

• it is characterized by a number of clinical features that may include-

• hypoxia,• hypotension, • cardiac arrhythmias,• increased pulmonary vascular resistance

(PVR),• and cardiac arrest.

Proposed severity classification of bonecement implantation syndrome

• Grade 1: moderate hypoxia (SpO2-94%) or hypotension [fall in systolic blood pressure (SBP) 20%].

• Grade 2: severe hypoxia (SpO2-88%) or hypotension (fall in SBP 40%) or unexpected loss of consciousness.

• Grade 3: cardiovascular collapse requiring CPR.

ETIOLOGY

• Several hypothesis1. Monomer absorption into circulation – but

very low levels of MMA in circulation are found during cementation.

2. Embolisation

• Embolization occurs as a result of high intramedullary pressures developing during cementation and prosthesis insertion.

• The cement undergoes an exothermic reaction and expands in the space between the prosthesis and bone, trapping air and medullary contents under pressure so that they are forced into the circulation.

• Embolic showers have been detected using echocardiography in the right atrium, RV, and pulmonary artery during surgery.

• Post-mortem studies have demonstrated pulmonary embolization in animals and man.

• The physiological consequences of embolization are considered to be the result of both a mechanical effect and mediator release, which provokes increased pulmonary vascular tone.

Pulmonary vessel with embolus comprising fat, platelets, fibrin, and marrow debris. (1) Reflex vasoconstriction and endothelial production of endothelin 1. (2) Release of vasoconstriction mediators; platelet derived growth factor (PDGF), serotonin (5-HT), thromboxane A2 (Tx-A2), platelet activating factor (PAF), adenosine diphosphate (ADP). (3) Vasoconstriction attributable to non-cellular components of embolus including thrombin.

the increased Pulmonary Vascular Resistance ↓ reduced right-ventricular ejection fraction ↓ the compliant right ventricle (RV) distends ↓ the interventricular septumto bulge into the left ventricle (LV) ↓ further reducing LV filling, and therefore CO

• When cement is inserted into the femur using a cement gun, the pressures generated are almost double those seen when manual packing is used.

• It has been demonstrated that this debris includes-

• fat, • marrow, • cement particles, • air, • bone particles, • and aggregates of platelets and fibrin.

3.Histamine release and hypersensitivity

• Anaphylaxis (Type 1 hypersensitivity) was implicated as a potential cause for a fatal case of BCIS in 1972.

4.Complement activation

• The anaphylatoxins C3a and C5a are potent mediators of vasoconstriction and bronchoconstriction.

• An increase in C3a and C5a levels, suggesting activation of the complement pathway, has been demonstrated in cemented hemiarthroplasty.

Multimodal model

• It is likely that a combination of the above processes is present in any individual patient who develops BCIS.

Patient risk factors

Numerous patient-related risk factors have been implicated in the genesis of BCIS, which are :-

1. old age,2. poor preexisting physical reserve,3. impaired cardiopulmonary function,4. pre-existing pulmonary hypertension5. osteoporosis,6. bony metastases,7. and concomitant hip fractures, particularly

pathological or intertrochanteric fractures.

• These latter three factors are associated with increased or abnormal vascular channels through which marrow contents can migrate into the circulation.

Surgical risk factors

• Patients with a previously un-instrumented femoral canal may be at higher risk of developing the syndrome than those undergoing revision surgery.

• There are two possible mechanisms. 1. First, there is more potentially embolic material present in

an un-instrumented femur.2. Second, once the canal has been instrumented and

cemented, the inner surface of the femur becomes smooth and sclerotic and offers a less permeable surface.

• The use of a long-stem femoral component increases the likelihood of developing BCIS.

Anaesthetic risk reduction

• In high risk cases discussion should occur between the surgeon and anaesthetist regarding the most appropriate anaesthetic and surgical technique, including the potential risk-benefit of uncemented compared with cemented arthroplasty.

• The avoidance of nitrous oxide should be considered in high risk patients to avoid exacerbating air embolism.

• Increasing the inspired oxygen concentration should be considered in all patients at the time of cementation.

• Avoiding intravascular volume depletion may reduce the extent of the haemodynamic changes in BCIS.

• The use of an intraoperative pulmonary artery catheter or trans oesophageal echocardiography has been suggested in high risk patients.

Surgical risk reduction

• Medullary lavage,• good haemostasis before cement insertion, • minimizing the length of the prosthesis,• using non-cemented prosthesis (especially if

using a long-stem implant),• and venting the medulla.

• Venting the bone permits the air to escape from the end of the cement plug and reduces the risk of an air embolus.

• Can increase the risk of femoral fracture.

• If cement is used, insertion with a cement gun and retrograde insertion have been suggested as ways of reducing the incidence of BCIS.

• Cement guns result in more even pressure distribution in the medullary cavity, and less reduction in oxygen saturation .

• Paradoxically, it has been demonstrated that intramedullary pressures are higher when cementation is performed with a cement gun rather than finger packing.

• Vacuum mixing of cement reduces mortality due to BCIS.

• reduction of the prosthetic femoral head is also a time of increased risk because previously occluded vessels are re-opened and accumulated debris may be allowed into the circulation.

• During knee arthroplasty, significant venous emboli are released at the time of tourniquet deflation and this may also be a high risk period

Management

• Early signs of BCIS in the awake patient undergoing regional anaesthesia include dyspnoea and altered sensorium.

• If BCIS is suspected, the inspired oxygen concentration should be increased to 100% and supplementary oxygen should be continued into the postoperative period.

• It has been suggested that cardiovascular collapse in the context of BCIS be treated as RV failure.

• Aggressive resuscitation with i.v. fluids has been recommended

• Haemodynamic instability should be treated with the potential aetiology in mind.

• Sympathetic a1 agonists should be first-line agent in the context of right heart dysfunction and vasodilatation.

• Fluid resuscitation should then be commenced if there is insufficient pre-load.

Calcium Phosphate Cement• Mainly used as bone graft substitute• Is capable of hardening into calcium deficient hydroxyapatite

and remodeling at a similar rate to bone (1-2 years).• Different forms: monocalcium phosphate monohydrate,

dicalcium phosphate dihydrate, hydroxyapatite, and alpha and beta tricalcium phosphate (TCP).

• Although beta phase tricalcium phosphate is very biocompatible, it does not form calcium deficient hydroxyapatite (CDHA) in the body.

• This is the reason why alpha TCP is preferred to be used as bone void fillers.

USES

• Bone replacement ( instead of bone grafts) e. g. osteoporotic bone, bone loss due to trauma or tumor surgery, kyphoplasty.

Calcium Phosphate

• Injectable• Very high compressive strength once

hardens• Some studies of its use have allowed

earlier weightbearing and range of motion

Tricalcium Phosphate

• Wet compressive strength slightly less than cancellous bone

• Available as blocks, wedges, and granules• Numerous tradenames– Vitoss (Orthovita)– ChronOS (Synthes)– Conduit (DePuy)– Cellplex TCP (Wright Medical)– Various Theri__ names (Therics)

Fracture Fixation

Bioactivity analysis

• The liquid and powder (ratio of 0.34ml/g) undergoes a solid-state reaction at 370 C forming silicone induced calcium deficient hydroxyapatite (CDHA+Si).

• New bone formation in silicone doped cement.• Osteoclasts are also seen in silicon doped cement (bone

resorption). • Silicone has excellent osteoblastic activity, enhanced reactivity

with collagen and apatite increasing the rate of bone remodeling.

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