An in vivo and Numerical Analysis of the Effect of Femoral Stem Surface Finish on Cortical Strain in Cemented HIP Replacement

Abstract

The aim of this study was to assess the effect of femoral cemented stem surface finish on cortical strain. Eleven sheep underwent hemiarthroplasty of the left hip (study) with either a matt (M) or polished (P) finish, double tapered cemented stem and were subjected to controlled exercise for nine months. The sheep were then killed and the right (control) femur implanted with an identical stem type. The study and control femurs were then subject to strain gauge analysis. A 3D finite element (FE) model was constructed which assumed either a bonded or slipping stem-cement interface. The stem-cement friction coefficient was varied from 0 to 0.22 to 0.5. A viscoelastic cement mantle was included in the model. The in vivo model showed a significant decrease in the proximal strain of the study femurs compared to the controls, however no significant difference was noted between the M and P femurs. The FE model indicated that strain levels in the proximal femur increased significantly as the coefficient of friction decreased. It appeared in the present study that the stem-cement interface remained bonded in vivo, for both matt and polished stems. A polished stem surface finish may only be effectual in increasing proximal femur loading if complete stem-cement debonding occurs. FE models which consider stem-cement interface mechanics should include contact elements which disallow Coulomb friction effects until a failure capacity is exceeded.