Experimental and Finite Element Studies of Acetabular Cement Pressurisation and Socket Fixation in Total Hip Replacement

Abstract

With time, the rate of symptomatic acetabular component loosening accelerates and overtakes that of the femoral component as the principal reason for the revision of total hip replacement. In the femur extensive study has shown that cement pressurisation and good preparation of the bone bed improves the survival rate, but acetabular fixation requires further investigation. Production of cement pressure in the acetabulum is anatomically difficult. Pressurisation with conventional and novel designs of cement pressurisers has been compared to manual techniques and component insertion. The pressurisers increased peak and mean pressures and pressure duration. Finite element modethng of cup insertion showed that flanges and higher insertion rates increased cement penetration into cancellous bone. Per-operatively, one design of pressuriser produced cement pressures comparable to those found in the laboratory. Structural finite element modelling of the natural hip indicated that the subehondral plate and the relatively dense cancellous bone supporting it distribute the joint contact force into the medial and lateral pelvic cortices. A perfectly bonded cemented polyethylene cup stiffened the acetabulum so that more load was transferred directly to the cortices at the acetabular rim, with consequent interface stress concentrations. However, complimentary experimental studies using a dynamic joint simulator and a servo-hydraulic materials testing machine suggested that perfect fixation between cement and bone at the rim was not possible, even under laboratory conditions. Debonding of the cement bone interface at the rim, where dense bone prevents cement interdigitation, allowed micromotion. Since the clinical mechanism of failure of the acetabular component appears to be progressive debonding, from rim to apex, of the cement-bone interface, these studies support the initiation of the failure mechanism by mechanical factors, which may then allow the ingress of wear debris. The experimental studies suggested that the use of pressurisers reduces the amount of micromotion and thus may improve the long term stability of the interface.