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FINITE ELEMENT ANALYSIS OF A CEMENTED HIP RESURFACING

University Dept of Orthopaedic Surgery, Western Infirmary, Glasgow

During the last few years there has been renewed interest in hip resurfacing. The advantages of such prostheses include minimal bone resection and more physiological loading of the proximal femur. The purpose of this study was to investigate the stress distribution to the upper femur following a metal-on-metal hip resurfacing and the influence of a short stem on femoral bone loading.

An accurate and validated finite element (FE) model of the proximal femur was utilised. This was created from CT data of cadaveric femurs. The validation process included weighing, modal analysis, strain gauging and ultrasound material testing of the bone. The maximum elastic modulus in the principal direction was 22.9ÊGPa. The elastic moduli of the cement and implant were 1.8 and 200 GPa respectively. The joint force and 4 muscle loads were applied accordingly and adapted to the specific geometry of the bone. The load case represented the 45% position in the gait cycle, corresponding to toe-off. The hip joint force of 2.2kN, approximately 30° superior to the pole of the implant, was applied as a pressure distribution over a 60° spherical segment, modelling the large contact area of the metal-on-metal articulation. Various scenarios with and without an implant were compared.

The distribution of the von Mises stresses in the normal femur without an implant reflected the distribution of the bone’s mechanical properties: the joint load was transferred from the superior surface of the femoral head, through its centre to the dense cortical bone of the calcar and diaphysis. The presence of the resurfacing prosthesis did not significantly affect the stress distribution in the proximal femur, except for a reduction of stresses in the superior region of the femoral head. Varying the length of the stem and its fixation did not significantly affect this stress distribution. A resurfacing prosthesis without a stem resulted in more normal stresses in the superior region of the femoral head.

Compared to the normal femur without an implant the FE analysis of the resurfacing prosthesis demonstrated stress shielding in the superior region of the femoral head. This stress shielding was reduced when a resurfacing component without a stem was used. This advantage must be weighed against the disadvantage that without a stem it is more difficult to accurately position the implant and achieve a uniform cement mantle.

Abstracts prepared by Dr P E Watkins, Hodgkin Building, Guys Campus, King’s College London.