Failures and successes in total hip replacement--why good ideas may not work.

The course of development of total hip replacement (THR) is neither harmonious nor linear. Progress and set-backs alternate. Progress in THR manifests itself through reductions in the number and the severity of complications (infection, aseptic loosening, prematurely worn components, etc.). Innovation is the motor of progress. However, today's innovation may well be tomorrow's revision! Progress has been achieved in part through new implant materials and designs that provide improvements in such things as stress distribution in surrounding bone, tissue compatibility and osseointegration, and resistance to both wear and cyclic fatigue. Of at least equal importance, however, are improvements achieved in operative procedures (e.g., cementing technique) and finally, in clinical quality control: more complete documentation of implant and patient variables, establishment of implant registers, and utilization of outcome studies to guide the course of further development. Causes of failure in THR are numerous. However, unexpected side effects of innovations are the most frequent cause. An innovation may solve one problem, but also creates new ones. Problems in innovation which can lead to failures include: over-generalization of expected patient responses, ignoring past experience or assigning wrong causes to encountered problems, and finally, ignoring the dynamic nature of the living system (which can be described as using "necro-" instead of biomechanical thinking). Quality control in both manufacturing and clinical practice has to be improved. The pioneer times have come to an end. Today's patients should have the right to be operated on by a well trained surgeon and to be provided with well tested, well-understood implant materials and devices. Progress in endoprosthetics has led to the present high level of clinical success. Paradoxically, however, success is the greatest obstacle to further progress. This is because the curve of progress as a result of effort expended has turned asymptotic in endoprosthetics, as it does in many endeavors. In such situations the more a product (e.g., surgical implant and procedure, automobile design, computer program) becomes successful, the more efforts (and finances) are needed for further progress. On the other hand, the "scissors"--created by crossing what might be feasible with what resources are available--open widely and can cut sharply. In fact, financial restrictions may force orthopaedic surgeons and the medical device and technology industry to turn to lower technologies in the future. However, whatever new developments in endoprosthetics may bring (be they sophistications or simplifications), we must remain open-minded and not assume things to be facts until there is evidence to support them.