Severe biomechanical conditions in total hip replacement.

Hip simulators are designed to reproduce the forces and motion patterns of normal walking. In vivo demands on total hip replacements, however, are varied and often more severe than normal walking conditions. It is these severe conditions that often lead to implant failure.This is clinically based research aimed at understanding some of the more severe conditions in hips and theeffect that these have on the performance of the total hip replacement.The polyethylene liner can act as a pump in an acetabular component, forcing fluid and wear particlesthrough the holes to the retroacetabular bone causing osteolysis. Ten patients were studied at revisionsurgery. Pressures were measured in retroacetabular osteolytic lesions while performing pumpingmanouvers with the hip. Two laboratory experiments were then designed to study pumping mechanisms invitro. In patients with contained osteolytic lesions, fluid pressure fluctuations could be measured in thelesion in association with the pumping action. Patients with uncontained osteolytic lesions showed no suchpressure fluctuations. In the laboratory we identified 3 distinct mechanisms whereby fluid can be pumpedfrom the hip joint to the retroacetabular bone. These pumping effects could be mitigated by improvedimplant design.Loading of the femoral head against the edge of the acetabular component produces dramatically increasedcontact pressures particularly in hard-on-hard bearings. In an analysis of 16 retrieved ceramic-on-ceramicbearings we were able to characterise the mechanism of edge loading based on the pattern of edge loadingwear on the bearing surface.Finally in a radiographic study of patients with squeaking ceramic-on-ceramic hips. Squeaking was foundto be associated with acetabular component malposition. It seems that edge loading or impingement maybe an associated factor in these cases