Bearing Options in Total Hip Replacement
Introduction
The traditional metal on polyethylene bearing has been used very successfully in total hip replacement for over 30 years. The problem of osteolysis caused by wear, particularly in young patients, has lead to the development of alternative bearing options.
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Tribiology
This is the study of surfaces acting under a load and in motion, including friction, lubrication and wear.
Wear is the removal of material and the generation of wear particles. This occurs by: adhesion, ie bonding of surfaces and material being pulled of the weaker surface; abrasion, ie irregularities in the harder surface or third bodies cut into the softer surface; and fatigue, ie the material fails after a certain number of loading cycles.
Lubrication reduces wear. The ideal is a high ratio of fluid-film thickness to surface roughness. This leads to fluid-film lubrication which is complete separation of the bearing surfaces, with the load being taken by the fluid and therefore minimal wear.
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Cross-Linked Polyethylene Acetabular Bearings
Ultra-high molecular weight polyethylene has been in use for more than thirty years, with a low probability of gross material failure and no systemic side effects.
Ethylene is a hydrocarbon (C2H4). Polyethylene is a long-chain polymer of ethylene molecules with the carbon atoms linked. High molecular weight polyethylene is a two phase viscoplastic solid with crystalline domains embedded in an amorphous matrix. Bridging tie molecules connect the crystalline domains and provide improved mechanical properties.
Cross-linking is a method to improve the wear resistance of polyethylene. Free radicals in the amorphous regions are induced to react and form covalent bonds between polyethylene molecules. This can be done with peroxide, ionising radiation or electron beam irradiation. Cross-linking reduces intermolecular mobility, making the polyethylene more resistant to deformation and wear.
Cross-linking also has a detrimental effect on other mechanical properties: yield strength; ultimate tensile strength and elongation to break. Despite this, cross-linking has been shown on laboratory tests to dramatically reduce wear in acetabular components by >95%.
The method of sterilisation also affects the mechanical properties of the polyethylene. Variables are the dose and type of irradiation (gamma or electron beam), thermal stabilisation (remelting or annealing), machining and final sterilisation.
To further complicate evaluation of cross-linked polyethylene, osteolysis and loosening of THR is related to the size and number of polyethylene wear particles that are generated. Thus a lower wear rate may not necessarily be better if it produces a higher number of smaller, biologically active wear particles. Cross-linked polyethylene results in less volumetric wear but a relatively high number of sub-micrometre and nanometre sized particles. These produce a greater inflammatory response in vitro than larger particles.
Although early results are encouraging, there are many many variables, and different commercially available polyethylene components can have different properties. Each must be evaluated separately in long term clinical studies, and these are not yet available.
Another important point to note in evaluating radiographs after THR is that penetration of the femoral head into the acetabular polyethylene in the first one or two years is due mainly to creep. After this it is due mainly to wear.
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Metal on Metal Bearings
Hip simulator studies have shown up to 200 fold reduction in volumetric wear rates for metal on metal bearings compared with metal on polyethylene bearings. Important factors are making the clearance as small as practically possible and the surface as smooth as possible. This is possible to achieve with modern machining technology.
Modern metal on metal bearings have been in use for over a decade with good results. Osteolysis and re-operation for a problem due to the bearing are extremely rare.
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Metal Wear Particles and Ion Release
Wear particles from metal on metal bearings are substantially smaller than polyethyelene wear particles, and more particles are produced despite lower volumetric wear. They also produce less inflammatory response locally. However, because metal wear particles are smaller, they have a greater aggregate surface area. This can lead to both local and systemic side effects.
Levels of metal ions in erythrocytes, serum and urine are elevated in patients with metal on metal bearings. These levels are higher in the short term and decrease over time, consistent with the running-in period of the bearing. The toxicity of these trace metal ions has not yet been established. It is possible that they are responsible for delayed type hypersensitivity reactions in susceptible patients. There is also a theoretical risk of cancer, but only 10 year follow up is available. Many known carcinogens have a latency period of several decades.
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Ceramic Femoral Heads
Alumina (Al203) and Zirconia (ZrO2) heads have high hardness and high strength, which makes them difficult to scratch. They are also more hydrophilic than metal and so have improved lubrication and lower friction. Hip simulator studies show up to 50% wear reduction with ceramic on polyethylene compared to metal on polyethylene bearings.
There is a small chance of fracture of the femoral head of approximately 0.004%. Zirconia heads from one manufacturer in 1998 had an increased rate of fracture following a change in the manufacturing process. This was an isolated problem that has now been corrected. Zirconia has a higher hardness and burst strength than alumina but is not heat stable, so the manufacturing process is of the utmost importance.
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Ceramic on Ceramic Bearings
Ceramic on ceramic bearings in current use are made of alumina. This material has low porosity, low grain size, high density and high purity. Hardness, fracture toughness and burst strength are increased.
Ceramic on ceramic bearings have demonstrated the lowest in vivo wear rates of any bearing combination. The hydrophilic property of ceramics enables a greater wettability of the surface. This ensures a uniform distribution of synovial fluid film. Ceramics also have a greater hardness than metal and can be polished to a much lower surface roughness. This all results in a lower coefficient of friction and greater likelihood of true fluid film lubrication.
Surgical technique is important, and rapid wear has been associated with suboptimal positioning of the implants. Chipping of the liner may also occur during insertion. Fracture is also a rare complication. Laboratory and mid term results have been encouraging, but long term results of modern ceramic on ceramic bearings are not yet available.
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Ceramic Wear Particles
Ceramics are more biocompatible than metal alloys. It appears that there is less inflammatory reaction than that found with either metal on metal or metal on polyethylene bearings. Osteolysis has been reported with ceramic bearings but it is rare. Ion toxicity is not a concern with ceramics as it is with metal on metal bearings.
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Summary
Alternative bearings in total hip replacement each have different risks and benefits. They show promising results in laboratory testing and early clinical trials in reducing wear. Long term studies demonstrating their impact on safety and revision rate are not yet available.
The choice of bearing in total hip replacement must be made based on a number of factors. These include the age, activity and preference of the patient, the cost of the implants, and the preference and experience of the surgeon. Choosing a bearing involves an analysis of the risks and benefits of each bearing on a patient by patient basis.
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Reference:
Heisel C, Silva M, Schmalzried TP. Bearing Surface Options for Total Hip
Replacement in Young Patients. JBJS (Am): 85-A (7); 1366- 1370; July 2003
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