BIOMECHANICAL ASSESSMENT OF A NOVEL DEFORMABLE ACETABULAR CUP DESIGN

Abstract

One potential limitation with uncemented, hemispherical metal-backed acetabular components is stress shielding of bony structures due to the mismatch in elastic modulus between the metal backing and the peri-prosthetic bone. A proposed substitute is a horseshoe-shaped acetabular component, which replicates the bony anatomy. One such device, the Cambridge cup, has shown successful clinical and radiological outcomes at five years follow-up (Brooks 2004, Field 2005). We conducted a study of the Cambridge cup from a biomechanical perspective, using validated, high-resolution computational models of the bilateral hip. Peri-prosthetic stress and strain fields associated with the Cambridge cup were compared to those for the natural hip and a reconstructed hip with a conventional metal-backed hemispherical cup during peak gait loading. We found that the hemispherical cup caused an unphysiologic distribution of bone stresses in the superior roof and unphysiologic strain transfer around the acetabular fossa. These stress distributions are consistent with bone remodelling. In contrast, the peri-acetabular stresses and strains produced by the Cambridge cup differed from the natural hip but were more physiologic than the conventional hemispherical design. With the Cambridge cup, stresses in the superior acetabular roof, directly underneath the central bearing region, were greater than with the conventional design. Despite the thin bearing, the peak liner stresses in the Cambridge cup (max. tensile stress: 1.2 MPa; yield stress: 4.5 MPa) were much lower than the reported material strengths. Fossa loading by the hemispherical cup has been suggested as a possible mechanism for decreased implant stability (Widmer 2002). Conversely, the Cambridge cup produced semi-lunar peri-prosthetic stress fields, consistent with contact regions measured in natural hips (Widmer 2002). These analyses provide a better understanding of the biomechanics of the reconstructed acetabulum and suggest that a change in component geometry may promote long-term fixation in the pelvis.