Requirements for successful total knee replacement. Material considerations

Despite the overall clinical success of TKR arthroplasty, materials' limitations are becoming increasingly obvious as designs and surgical techniques improve. As in the case of THR components, the ultimate limiting factor now appears to be wear of the tibiofemoral articulation and its biologic consequences. Although some materials appear to hold out promise of improvement, no combination better than cobalt-base alloy/UHMWPE has yet appeared. In addition, the very long service times now expected routinely (greater than 10 years) for total joint replacement increase the problem of new materials' selection owing to the well-known disparities between in vitro wear tests and in vivo performance and the absence of any consensus on correlation between accelerated testing and real-time performance, either in vitro or in vivo. For these reasons, extreme caution is counseled in the early clinical trials of newer materials and combinations of materials. The clinical "rule of thumb" for publication of clinical results, 2-year minimum follow-up, is necessary but highly insufficient for materials' qualification. Good to excellent results, with no signs of progressive failure for 5 to 7 years of prospective study, combined with high-yield follow-up to 10 years, will probably be necessary for safe introduction of such materials into general clinical use. An excessively rapid introduction could predispose to clinical failure, through lack of knowledge of critical design and manufacturing parameters, and prove an impediment to improvement of TKR arthroplasty.