The aim of total hip arthroplasty is not only pain relief but also restoration of patient’s activity level. Walking is the highest load to which the hip joint is subjected, other than occasionally. It must, therefore, follow that in some situations fixation of the stem may prove inadequate and loosening may result. Single episode resulting in failure is certainly dramatic but fortunately relatively rare. Accumulation of damage over a period of time is insidious and may, and often does, pass unnoticed. Some of the clinical situations can be modelled experimentally. The RSA technique is most valuable in the measurement of the very minor changes of position of an implant within the skeleton. (The principle of the method known for millennia as “celestial parallax”).

The method aims to measure the earliest changes of the position of the implant within the skeleton. It is, therefore, reasonable to suggest that it should be used first to establish the validity of the method as applied to the concept under investigation. Next it should be used experimentally, to test the surgeon’s ability to consistently achieve the result. Only when this has been achieved should it be applied clinically. To miss the first two stages is not only to miss the earliest of changes that can and do occur under clinical conditions, but also to assume that the surgeon’s skill is not at fault.

Fatigue and wear testing of components are routine though the details of methods may vary. What cannot be reproduced adequately, experimentally, is the changing quality of the skeleton and the changes that the implant may impose on the skeleton. It is for those reasons that regular follow-up with good quality radiographs, and continuity of observer method, are essential. Combined with detailed documentation of surgical technique they will allow identification of both advantageous and problematic features of design, material and surgical technique.