DESIGN AND EVALUATION OF A DEVICE FOR MEASURING 3-DIMENSIONAL MICROMOTIONS OF PRESS-FIT FEMORAL STEM PROSTHESES

Implant micromotion is considered to De a major factor in the loosenin g of cementless total hip replacements. Translational micromotion at t he bone-implant interface generally occurs in all three spatial direct ions. Under physiological loading, the interfacial micromotion consist s of a cyclic amplitude and changes in the mean, which, in the cranioc audal direction, represents subsidence of the prosthesis. Existing mea surement strategies, which are based on dial gauges, extensometers, LV DTs, hall-effect transducers or strain gauge techniques provide inform ation about only one component of the general three-dimensional microm ovement. Moreover, in the majority of the studies, the data are diffic ult to interpret due to the measured motions being composed of interfa cial micromotion and femoral strains. A new transducer was designed th at allows the accurate measurement of all three isolated components of micromotion. An optoelectronic approach, based on silicon position-se nsitive detectors (PSD) in combination with high precision mechanical karts, was chosen. To exclude thermodrifts during long-term testing a thermistor was integrated in the sensor. Validation experiments on a p recision positioning table indicated the high precision and resolution of the developed sensors. Furthermore, in-vitro tests on a standard p ress-fit prosthesis demonstrated the easy handling and reliability of tile system. (C) 1997 Elsevier Science Ltd for IPEM.