Abstract
Introduction
Sir John Charnley introduced his concept of low friction arthroplasty— though this did not necessarily mean low wear, as the initial experience with metal on teflon proved. Although other bearing surfaces had been tried in the past, the success of the Charnley THR meant that metal-on-polyethylene became the standard bearing couple for many years. However, concerns regarding the occurrence of peri-prosthetic lysis secondary to wear particles lead to consideration of other bearing surfaces and even to the avoidance of cement (although this has proven to be erroneous). Bearing combinations include polymers, ceramic and metallic materials and are generally categorised as hard/soft or hard/hard. In general, all newer bearing surface combinations have reduced wear but present with their own strengths and weaknesses, some of which are becoming more apparent with time.
Bearing surfaces must have the following characteristics: low wear rate, low friction, Biocompatibility and corrosion resistance in synovial fluid.
Hard/soft
Femoral head components are generally made of cobalt, chromium alloy, either cast or forged. Both alumina and zirconia ceramics have been used as femoral head materials and the hardness is thought to reduce the incidence of surface damage to the femoral head. The hard femoral heads have been traditionally matched with conventional ultra high molecular weight polyethylene
(UHMWPE) which has been produced by either ram extrusion or compression moulding. Over the past 10 years, most implant companies have moved to highly cross-linked UHMWP which in both laboratory and human RCTs have shown appreciably less wear.
Hard/hard bearings – Metal-on-metal (M-O-M)
The first generation of metal bearings were based on stainless steel couples but the metal on metal design by
McKee-Farrar was made from CoCrMo alloy with large head diameters. The second generation M-O-M bearing were introduced by Weber using wrought
CoCrMo alloy with low surface roughness and wear rates about 100 to 200 times less than traditional metal/UHMWPE. The re-introduction of resurfacing hip arthroplasty has been made possible by the improvement in metal technology.
Concerns however exist with the long term biologic effects of metal ions, the reported incidence of sensitivity reactions to metal and the more demanding techniques required for implantation.
Ceramic on Ceramic (C-O-C)
Alumina ceramic bearing surfaces are extremely hard, have high wear resistance and reported low concentration of wear particles in peri-prosthetic tissues. Unlike M-O-M there is no ion release. While the reported fracture rate for ceramic couplings is extremely low their proper implantation is important to minimise impingement. There is an incidence of squeaking not seen in other bearing couples and because of the hardness of the bearing, long term concerns with stress shielding of bone remain.
Clinical outcomes
Data will be presented from the Australian Orthopaedic
Association National Joint Replacement Registry on clinical outcomes of bearing surfaces. Overall metal on UHMWPE has the least revision of any bearing surface couple used with conventional hip replacement.
Future trends
Further research into hard/soft bearings will look at ways to reduce UHMWPE wear without compromise of clinical results based on over 40 years use. Hard-on-hard bearings may focus on combining the best features of both
M-O-M and C-O-C couplings without fracture risk or metal iron release.
When deciding which bearing surface is suitable for your patients it must be emphasised that wear reduction is only one of several considerations when taking into account the most appropriate implant.
