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.

We have tested the porosity and fatigue life of five commonly used bone cements: Simplex P, LVC, Zimmer regular, CMW and Palacos R. Tests were conducted with and without centrifugation and with the monomer at room temperature and, except for LVC, at 0 degrees C. We found that the fatigue life of different specimens varied by a factor of nearly 100. It did not depend on porosity alone, but was more influenced by the basic composition of the cement. Simplex P when mixed with monomer at 0 degrees C and centrifuged for 60 seconds had the highest fatigue life and was still sufficiently liquid to use easily.