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Unwin et al. (1991) determined that the probability o f avoiding loosening in 10 years was 92% for proximal femoral replacements but only 72% for distal femoral replacements. Considering only patients under 20 years of age the probability of avoiding loosening o f distal femoral replacements in 10 years reduced to 47%. This represents a serious problem considering that the average age o f all patients included in the study was only 27 years and that having survived tumours these patients would have good life expectancy.

Similar results have been found with other implants incorporating an artificial knee joint (total knee and proximal tibial as well as distal femoral replacements). Inglis & Walker (1991) studied the failure of hinged knees used for cases of serious instability or for revision o f failed condylar knees and reported that the failure rate and resulting disability was a serious problem. In all cases o f large implant failure, whether for tumour or revision knees, the revision involves further bone resection and the eventual prospect of complete disability or amputation.

Cabb et al. (1991) found that there was a steady deterioration o f the cement-bone interface over time and that this deterioration correlated with a diniinishirig clinical performance. This meant that even before loosening reached the point where revision was essential, the patients experienced pain and diminished function.

The problem of treating bone tumours (which occur mostly in the young) and revising failed knee and hip implants, where a part of the bone shaft is involved as well as the joint, is a complex one. Several hundred people in the UK require such treatment each year. Apart from amputation, the two major methods o f treatment are using combined allografts and joint replacements, or large implants with joint replacements. The former method, popular in the USA, is becoming more and more impractical for reasons o f disease, availability and cost, while there are still many serious unresolved problems including non-union, infection and fracture. Artificial components on the other hand are highly successful in the early years, in terms of both function and fixation. However, the two long term problems are: stem loosening and wear of the metallic shaft causing metal uptake by the tissues. Wear in the joint itself has also been a problem, but the use o f cobalt-chrome alloy bearing upon UHMWPE has been shown to reduce this, although doubts remain about the performance o f this combination o f materials in the long term.

By combining biological, biomaterials and biomechanical technologies, solutions to both the problems of wear and loosening may be possible. Recent wear testing has indicated that various types o f coating on the titanium alloy surfaces, e.g. titanium nitride, greatly reduces the wear in the soft tissues. If cobalt-chrome was used for the body o f the prosthesis, as well as for the femoral head, the soft tissue wear would be greatly reduced. However, titanium alloy is used for three main reasons: it is more lightweight, and more easily machinable than stainless steel or cobalt-chrome, and biocompatible. New designs and coatings such as hydroxyapatite may achieve the rigid bone-implant fixation necessary for indefinite fixation. Achievement o f these goals will have a dramatic effect on the lives o f the individuals requiring such prostheses. Furthermore, the technologies can be applied not only to these specialised implants but to total joints in general.

Direct telemetric measurement o f the forces acting on the prosthesis, and the distribution of those forces throughout it, enables quantitative assessment o f the fixation in vivo, in contrast to radiographic observations from which no forces can be derived. Measurements can be made at regular intervals without risk to the patient and the loading patterns logged for a variety o f activities, thus enabling a history o f the load distribution to be produced. Such data, together with complementary radiographic analysis, are valuable in the advancement o f our understanding o f the mechanism o f loosening, and for pointing towards methods of improvement in design to extend the life of the fixation. Measurement o f the forces and moments within implants incorporating such improved design features will provide direct evidence o f the efficacy or otherwise of those features, particularly in the area o f fixation. This data might also highlight other effects, such as the linearity of the fixation [#5.3.4], providing fiirther insight into implant design for the long term.

The ultimate step might be to routinely incorporate telemetric measurements into orthopaedic implants, to provide constant feedback of the stress distribution in the prosthesis and to warn of potential failure conditions. Instrumenting the bone itself would also yield usefiil data, e.g. on stress shielding. Since routine telemetry would increase the cost o f implants and require regular monitoring, such major advances are unlikely to occur within the foreseeable future.

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