Abstract
Introduction
Metal-on-metal (MoM) hip resurfacing arthroplasty is a popular choice for young and active patients. However, there are concerns recently regarding soft tissue masses or pseudotumours. The appearance of these complications is thought to be related blood metal ion levels. The level of metal ions in blood is thought to be the result of MoM wear. In the present study the contribution of acetabulum orientation to stress distribution was investigated.
Methods
Four subjects with MoM resurfacings and with known blood metal ion levels underwent motion analysis followed by CT scans. The positions of the acetabular (cup) and femoral components were determined the CT data relative to local coordinate systems in the pelvis (PCS) and the femur (FCS). Transformations, calculated from the motion analysis data, between the PCS and FCS gave the position of the cup relative to the femoral component for each frame of captured motion data.
Hip reaction forces were taken from published data1. The intersection of hip reaction force with each subject's cup and the increase in inclination required to move the force to the edge of the cup was calculated for 2% intervals during the stance phase of gait. Finite element models representing each subject's cup and femoral components were created and contact stresses were determined for the native cup inclination angle. For each model, the effect of increasing the inclination of the cup, by up to 10°, in 1° increments, was determined.
Results and Discussion
The two subjects with high metal ion levels had inclination angles of 60.2° and 53.7° whereas the two with low metal ion levels had inclination angles of 45.6° and 46.5°. The subjects with high metal ion levels required very little increase to their inclination angle to cause the hip reaction force vector to intersect at the edge. The contact stress on the cup increased dramatically when the inclination angle was such that the hip reaction force intersected with the edge. The average increase in contact stress under edge-loading conditions was 57% for the two subjects with high metal ions. In contrast, the subjects with low metal ions exhibited no change in contact stress when the inclination angle of their cups was increased by 10°. The inter-subject variability in the measured hip reaction forces was greater than the amount of increase in cup inclination required to induce edge-loading for the subjects with high metal ion levels. These results suggest that poor positioning of the cup during surgery may result in edge-loading, a greater rate of wear and adverse biological reactions associated with metal ion release.
