Finite element analysis of the resurfaced femoral head.

Failure of the resurfaced femoral head may occur in the short term owing to femoral neck fracture or in the long term owing to aseptic loosening as a result of strain shielding. Resurfacing arthroplasties are not all the same. In particular, there is considerable debate regarding the role of the metaphyseal stem and cementing technique. This study examines the influence of various metaphyseal stem configurations (diameter, percentage length in contact with bone, and bonded versus debonded) and cement mantle thickness on the load transfer within the femoral head. Resurfacing resulted in significant strain shielding in the superior femoral head and elevated strain in the superior femoral neck. Although the increase in strain in the femoral neck was significant, the mean strains were below the yield strain for cancellous bone. Peak strains were observed above the yield strain, but they accounted for less than 1 per cent of the total head-neck bone volume and therefore were unlikely to result in femoral neck fracture. Increasing the stem diameter and increasing the percentage stem length in contact with bone both increased the degree of strain shielding. Bonding the metaphyseal stem produced the most dramatic strain shielding, which also extended into the head-neck junction. In contrast, varying the cement mantle thickness had a negligible effect on the load transfer.