BACKSIDE NONCONFORMITY AND LOCKING RESTRAINTS AFFECT LINER SHELL LOAD-TRANSFER MECHANISMS AND RELATIVE MOTION IN MODULAR ACETABULAR COMPONENTS FOR TOTAL HIP-REPLACEMENT

Nonconformity between the polyethylene liner and the metal shell may e xist in modular acetabular components by design, due to manufacturing tolerances, or from locking mechanisms that attach the polyethylene li ner to the metal shell. Relative motion at the liner/shell interface h as been associated with backside wear, which may contribute to osteoly sis which has been clinically observed near screw holes. The purpose o f this study was to investigate the effect of nonconformity and lockin g restraints on the liner/shell relative motion and load transfer mech anisms in a commercially available, metal-backed acetabular component with a polar fenestration. The finite element method was used to explo re the hypothesis that backside nonconformity and locking restraints p lay important roles in long-term surface damage mechanisms that are un ique to modular components, such as backside wear and liner extrusion through screw holes. The three-body quasi-static contact problem was s olved using a commercially available explicit finite element code, whi ch modeled contact between the femoral head, polyethylene liner, and t he metal shell. Four sets of liner boundary conditions were investigat ed: no restraints, rim restraints, equatorial restraints,and both rim and equatorial restraints. The finite element model with a conforming shell predicted between 8.5 and 12.8 mu m of incremental extrusion of the polyethylene through the polar fenestration, consistent with in vi tro experiments of the same design under identical loading conditions. Furthermore, idealized rim and/or equatorial liner restraints were fo und to share up to 71% of the load across the liner/shell interface. C onsequently, the results of this study demonstrate that backside nonco nformity and locking restraints substantially influence backside relat ive motion as well as load transfer at the liner/shell interface. (C) 1998 Elsevier Science Ltd. All rights reserved.