Abstract
Ceramic-on-ceramic (COC) bearing surfaces have consistently demonstrated reliable clinical results with when coupled with appropriated designed femoral stems and acetabular shells. Ceramic bearing surfaces are highly wettable and display both boundary and hydrodynamic fluid-film lubrication modes, which lead to extremely low wear rates. Furthermore, COC bearing couples have been shown to exhibit virtually no risk of adverse biologic reaction and have not been associated with corrosion-induced adverse tissue reactions that occur with metal taper junctions, particularly head-neck taper junctions. The relative brittleness of ceramics initially was thought to be a major disadvantage; however, four decades of improvement in the manufacture of ceramics and rigorous proof testing has led an extremely low risk of fracture, perhaps lower than that for cross-linked polyethylene. More recently it has become increasing appreciated that nearly all revisions for squeaking have been restricted to specific designs and materials, including the use of a titanium elevated metal rim on the acetabular side, and more flexible femoral components made of a beta-titanium alloy (TMZF) which had thin necks and relative small tapers. Multiple clinical studies document excellent long-term survival of COC bearing couples in young patients with revision for any reason as the primary endpoint. Our own experience with 341 hips with 2 to 15 and average 9.1 year f/u demonstrates a 95% overall survivorship (revision for any reason) at 13 years in patients under 50 years of age at the time of surgery.
By contrast, cross-linked polyethylene bearings have not been studied so carefully and have not been shown to be superior to ceramic-ceramic bearings in young patients. These bearing surfaces represent a very heterogeneous group of products, with varying degrees of cross-linking, post-irradiation processing methods, and additives. Cross-linked polyethylenes in general have a lower fatigue strength than conventional polyethylene and are prone to fracture, especially when thin material is subjected to high stress. Some types of cross-linked polyethylenes are prone to in vivo oxidation, leading to further mechanical compromise over time. Studies also demonstrate the absence of reduction in femoral head penetration or risk of osteolysis in heads 32mm and larger, which are commonly used today. The most recent cross-linked polyethylene products have the least clinical support for their use. The long-term biologic effects of the smaller HXLPE wear particles and newer additives, including the more recently added vitamin E compounds are unknown. Indeed, in vitro, the debris has been shown to be cytotoxic. There is a paucity of survivorship data for hips utilising HXLPE, especially in younger, more active individuals. For these reasons, we believe that the ceramic-ceramic bearings are the gold standard for THA in the young patient and that cross-linked polyethylene bearing are being continually changed and have little long term clinical outcomes data to support their use.
