Effect of Implant Material Properties on Remodelling of a Composite Hip Stem

INTRODUCTION Stress-shielding following uncemented hip replacement causes bone remodelling due to alteration of the load transfer pathways. The degree of bone resorption is related to the geometry and bending stiffness of the stem. Large diameter, cobalt chrome stems are attributed with producing the most pronounced bone loss. The Epoch hip prosthesis (Zimmer Inc., Warsaw, IN) is a low bending stiffness, noncemented stem. It consists of a CoCr core with a polymer (Ultrapek) outer surrounded by a porous coated layer. The design couples the concepts of the low modulus cemented hip with the canal filling fully coated hip. The bending stiffness of the prosthesis is similar to that of a normal femur [1]. Traditional clinical appraisal of femoral stem performance requires clinical follow-up for many years to fully assess the efficacy of the device. Numerical bone remodelling simulations allow for theoretical preclinical prediction of implant success. Periprosthetic bone remodelling after reconstruction with the Epoch hip prosthesis was investigated by finite element modelling. To demonstrate the influence of the implant material properties, the results were compared with the same stem made entirely of cobalt chrome, and a stem with the same stiffness as cortical bone (isoelastic).