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.

Aims: The purpose of this study was to assess the effect of changes in peripheral attachment on stresses and displacements at the liner-shell interface. Methods: Three dimensional þnite element models were constructed of two acetabular cup designs for a liner with a 32 mm inner diameter, a liner thickness of 5 mm, and a shell thickness of 4 mm. An additional set of models was constructed with a 28 mm head diameter, corresponding to a liner thickness of 7 mm. 16 sequential quasistatic loading steps were used to describe the stance phase of a patientñs gait cycle. Results: Changes in the design had a larger inßuence on the backside relative motion during the gait cycle than load magnitude. However, changes in the design had a smaller inßuence on the backside contact stress, von Mises stress, or radial extrusion into screw holes. Reduction in head size from 32 to 28 mm diameter resulted in a slight decrease in screw hole extrusion. Conclusions: In this study, changes in the acetabular cup design, including screw hole placement and increased peripheral interlocking, were shown to decrease relative motion at the liner-shell interface, but the peak liner-shell contact stresses, backside von Mises stresses, and radial screw hole extrusion were less signiþcantly changed.

Aims: The purpose of this study was to assess the effect of gaps between the polyethylene liners and the metal acetabular shells used in two generations of acetabular component design. Methods: Finite element models were developed for two generations of acetabular component. The three variables assessed were: design (Mark I versus II); liner thickness (5, 8, and 11 mm); and gap size (0., 0.1, and 0.3 mm). 16 sequential quasistatic loading steps, coupled with ßexion/extension of the femoral head, were used to describe the stance phase of a patientñs gait cycle. Results: Gaps of less than 0.1 mm between the acetabular liner and the supporting metal shell will close under loading typical for gait, but only for smaller Ðsized acetabular cups. A gap of 0.1 mm seems to be at the edge of the range where rim loading, versus dome loading, occurs. Gaps of 0.1 to 0.3 mm between a polyethylene acetabular liner and the metal shell can produce ßuid pumping of approximately 100 to 250 microliters in each gait cycle. Conclusions: The changes from the 1st to the 2nd generation of this acetabular component led to important advantages. Indeed, due to an improvement of the liner conformity and the locking mechanism, backside micromotion, ßuid volume displaced, liner stresses and liner-shell contact stresses were strongly diminished.