The impact and deformation of press-fit metal acetabular components

Abstract

Early failure of some metal-on-metal (MoM) hip implants are extensively reported but not fully explained. These arthroplasties commonly utilise large-diameter, thin-walled acetabular cups that have the advantage of minimal removal of acetabular bone and a reduced chance of dislocation; however they may deform during insertion which involves impaction. The role of diametrical cup deformation as a factor to unsatisfactory implant performance has not been widely reported. The aim of this thesis was to investigate the extent to which deformations may occur in clinically relevant situations and to assess the significance of a range of variables on the deformation generated. 2D axisymmetric finite element (FE) models established a method of simulating impaction using different momentums. Experimentally validated 3D foam models showed that deformation is clearly influenced by the orientation of the cup, the support of the underlying bone and the geometry of the component itself. CT scans of the pelvis from 8 similarly sized female patients from two discrete age populations were used to develop clinically relevant FE models. Cup deformations were found to occur due to pinching between the iliac and ilial regions and were significant when compared to typical minimum diametrical clearances of 80-120 μm. In young pelvis models deformations of 34–63 μm were found to be significantly greater than in the older pelvis models, p<0.001. Surprisingly, small changes in the cup version increased deformations by up to 40% from the surgeon identified optimal position and were 30% greater when an eccentricity was introduced into the reamed acetabulum. The local deformations estimated in the acetabular cups may cause localised reductions in the fluid-film thickness, resulting in regions where boundary, rather than mixed lubrication takes place. This may help explain why failure and high wear rates are sometimes found in young patients with acetabular components positioned in clinically optimal positions.