Abstract
Introduction: Acetabular osteolysis is common behind cups with holes (the reported incidence is 9% to 36%). Fluid pressure has been implicated in the pathogenesis of osteolysis.
Aim: To test the hypothesis that a polyethylene liner in a metal cup can act as a pump in vivo.
Methods: This study was performed during revision surgery in six cases. The components were from several manufacturers. All were ingrown uncemented cups that had osteolytic lesions associated with holes in the cup.
A cannula was placed through the capsule into the hip joint and another was placed through the periosteum and bone of the ilium into the osteolytic lesion above the ingrown cup. The continuity of these two spaces through the holes in the cup was confirmed by the injection of methylene blue. Pressure transducers were then connected to both cannulae. Measurements were taken while applying compression and distraction forces across the artificial hip joint.
Results: Compression and distraction loads produced a rise (48mmHg) and a fall (35mmHg) respectively in the pressure in the osteolytic lesion but no change in the hip joint pressure, thereby, demonstrating a pumping action. After exposing the prosthesis we were able to demonstrate 1mm to 2mm of in-out excursion of the polyethylene liner in the metal cup, which may explain the mechanism of the pumping effect.
Discussion: The polyethylene liner in the metal shell can act as a pump. Compression and distraction forces, such as occur in normal gait, produce changes in fluid pressure, which are transmitted through the holes and may cause osteolysis behind the cup.
The abstracts were prepared by Professor A. J. Thurston. Correspondence should be addressed to him at the Department of Surgery, Wellington School of Medicine, PO Box 7343, Wellington South, New Zealand
