Abstract
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 the
effect 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 particles
through the holes to the retroacetabular bone causing osteolysis. Ten patients were studied at revision
surgery. Pressures were measured in retroacetabular osteolytic lesions while performing pumping
manouvers with the hip. Two laboratory experiments were then designed to study pumping mechanisms in
vitro. In patients with contained osteolytic lesions, fluid pressure fluctuations could be measured in the
lesion in association with the pumping action. Patients with uncontained osteolytic lesions showed no such
pressure fluctuations. In the laboratory we identified 3 distinct mechanisms whereby fluid can be pumped
from the hip joint to the retroacetabular bone. These pumping effects could be mitigated by improved
implant design.
Loading of the femoral head against the edge of the acetabular component produces dramatically increased
contact pressures particularly in hard-on-hard bearings. In an analysis of 16 retrieved ceramic-on-ceramic
bearings we were able to characterise the mechanism of edge loading based on the pattern of edge loading
wear on the bearing surface.
Finally in a radiographic study of patients with squeaking ceramic-on-ceramic hips. Squeaking was found
to be associated with acetabular component malposition. It seems that edge loading or impingement may
be an associated factor in these cases.