Differences of the Mechanical Setup of Hip Simulators and Their Consequences on the Outcome of Hip Wear Testing

The search for improved wear resistant materials for hip endoprostheses has lead to an international standard for wear testing of total hip joint prosthesis (ISO 14242). The displacement and load curves in this standard are derived from clinical gait analysis. However the standard leaves the interpretation of its displacement curves open since no coordinate system is defined in the document. By reviewing the development of the standard, the apparent coordinate system is identified and its application from both, an anatomical point of view and a technical perspective for designing hip simulators is explained. The study examines the coordinate transformations necessary for using common gait analysis data with the standard. On the basis of these transformations, the relative movements of the articulating surfaces are compared with existing hip simulators, the situation in vivo and the guidelines of the ISO standard. Thus possible consequences on the outcome of hip wear testing are discussed.