Introduction and Aims: Retroacetabular osteolysis is a common cause of failure of hip replacements. Polyethylene wear particles from the joint are often present in osteolytic lesions. We investigate three theories that describe how fluid and debris could be pumped from the joint space through the holes in the shell to the retroacetabular bone.

Method: We report three experiments that investigate this question. We performed an in vivo study where we measured pressures in the hip joint and in the osteolytic lesion while cyclically loading the hip in 10 patients. We performed a series of biomechanical studies, where we model diaphragm pumping and piston pumping of the polyethylene liner within the metal shell in the laboratory. We also carried out a finite element analysis showing how loading of the hip affects the size of an osteolytic lesion and the pressure of fluid within an osteolytic lesion.

Results: In the in vivo study, loading of the hip produced a pressure increase in each of the four contained osteolytic lesions (mean 68mmHg), but not in the six uncontained osteolytic lesions. This pressure rise was independent of hip joint pressure, demonstrating that there is a pumping mechanism in the artificial hip joint that is independent of hip joint pressure. In the diaphragm pumping experiment, the pressure produced by the non-congruent liners (4030 ±1250mmHg) was six times the pressure produced by the congruent liners (670 ±240mmHg). In the piston pumping experiment, the pressure produced by the pistoning liners (5140 ±330mmHg) was eight times the pressure produced without pistoning (650 ±300mmHg). FEA demonstrates that loading of the hip may reduce the volume and, therefore, increase pressure in a contained osteolytic lesion.

Conclusion: The prosthetic hip contains a complex system of pumps transporting fluid and particles and generating pressures. The importance of each pumping mechanism varies with patient activity and with implant design features. These pumping mechanisms may contribute to the pathogenesis of osteolysis.