Wear and bending strain characteristics of artificial hip joint with non-glued stem and UHMWPE femur

The aim of this study was to experimentally assess the effects of loosening between the stem of a cementless artificial hip joint and the femur on the wear and bending strain of the stem during the initial embedding term. The behavior of an ultrahigh-molecular-weight polyethylene (UHMWPE) femur model with this stem was then investigated. The interface between the stem and the UHMWPE femur model is either glued or not glued. Evaluation of differences between the two gluing conditions was conducted using testing equipment equipped with a reciprocating stage for an artificial hip joint. Total wear between the Co-Cr alloy head and UHMWPE socket was measured using a laser displacement meter installed in the axis direction on a main shaft of the tester. We measured the bending strain waveforms of the flexural direction, which was shown by three strain gauges on the upper, middle and lower part of the stem. The present results indicate that the rate of wear was markedly greater for non-glued stems than for the glued stems. In addition, the results indicate that the bending strain in the upper part of the non-glued stem tended to reach the maximum value of bending strain immediately after the femur model section stood upright, whereas the bending strain in the upper and middle of the glued stem tended to reach the maximum value of bending strain immediately before the femur model section stood upright. Furthermore, when the non-glued and glued stems had the same value of bending strain, the average amplitude was smaller for the non-glued stem than for the glued stem by 2-3 dB.