Modification of a Simplified Hip Joint Simulator Into an ISO 14242-1 Compliant Design and a Comparison of Wear Test Results

The ISO 14242-1 standard for hip joint simulator wear testing specifies a set of test conditions for the simulation of normal level walking in optimal conditions. Since some of the established simulators, such as the 12-station HUT-4, are not ISO 14242-1 compliant, the present study was carried out to answer the following question. Does wear produced in ISO 14242-1 conditions differ from that obtained earlier with the simplified HUT-4 hip joint simulator for similar specimens in normal level walking, optimal conditions? The established HUT-4 hip joint simulator was made ISO compliant by an implementation of a number of modifications. One of the modifications was the design and construction of a novel servo-electric load actuator with a proven dynamic response. The other modifications were related to the Euler sequence of motions, acetabular abduction angle, enclosure of the lubricant chamber, and temperature control. A 5 million-cycle wear test with thin, largediameter VEXLPE liners resulted in a wear rate close to that obtained earlier with the HUT-4. The burnished bearing surface in both tests was in agreement with clinical retrieval studies. It appeared that a more simple, inexpensive hip joint simulator can reproduce clinical wear mechanisms. However, the simulator must meet certain basic requirements. The most important of these is likely to be the correct type of multidirectional relative motion, for which biaxial motion is sufficient. It is emphasized that the present study was not intended to show a similarity in wear produced by the ISO 14242-1 and HUT-4 wear test systems.