The control of new prosthetic implants.

The recent well-publicised problem of early failure of the 3M Capital Hip has led both the public and orthopaedic surgeons to question the effectiveness of existing controls in the UK which govern the introduction of new pros-theses for joint replacement. It has also resulted in renewed demands for a National Hip Register, similar to those successfully developed by Sweden and Norway, 1,2 to provide better long-term surveillance of different designs of implant. The Scandinavian Registers have certainly been successful in drawing attention to poorly performing prostheses, such as the Christiansen Hip, 3 as well as identifying problems in the materials used for joint replacement, as with 'Boneloc' cement. 4,5 Their long-term data confirm that selection of the prosthesis is the factor which has the greatest influence on long-term survivor-ship. The problem of early failure of the implant is not new, nor is it confined to Europe, since major difficulties have also been reported recently from North America in a number of roughened femoral stems used with acrylic 'precoat' technology. 6,7 Should we be able to predict these types of failure? To do so requires a better understanding of the mechanism of early breakdown, which must be different from the well-recognised late failure resulting from the production of prosthetic wear particles characterised by the formation of excessive granulation tissue and osteolysis. The Capital Hip, which was introduced by the 3M Company in 1991, was marketed as a femoral stem with a shape similar, but not identical, to the current version of the Charnley stem. The emphasis at the time of its introduction was on the low cost of the prosthesis and the outstanding performance of the existing hip replacement based on the Charnley principles. The femoral implant was manufactured in two materials; a modular design with a titanium-alloy stem and either a chrome-cobalt or titanium-nitride head, and a monobloc in stainless steel. A number of stem shapes were available for each of these. Modularity allowed the use of heads of various sizes and different materials. The titanium alloy used for the stem was chosen to give the advantages of a lower modulus of elasticity, better biocompatibility, and reduced tissue toxicity. The stem design featured round corners proximally to ensure a more uniform load transfer to the supporting cement, a slightly roughened matt surface texture, and an acrylic centraliser at its tip. It was inserted using specially designed instruments with broaches and …