The effects of the method of fixation and interface conditions on the biomechanics of the femoral component of the Birmingham hip resurfacing arthroplasty were examined using a highly detailed three-dimensional computer model of the hip. Stresses and strains in the proximal femur were compared for the natural femur and for the femur resurfaced with the Birmingham hip resurfacing. A comparison of cemented versus uncemented fixation showed no advantage of either with regard to bone loading. When the Birmingham hip resurfacing femoral component was fixed to bone, proximal femoral stresses and strains were non-physiological. Bone resorption was predicted in the inferomedial and superolateral bone within the Birmingham hip resurfacing shell. Resorption was limited to the superolateral region when the stem was not fixed. The increased bone strain observed adjacent to the distal stem should stimulate an increase in bone density at that location. The remodelling of bone seen during revision of failed Birmingham hip resurfacing implants appears to be consistent with the predictions of our finite element analysis.

We studied the pattern of . 99m. Tc-methylene diphosphonate uptake around uncemented femoral components in 44 asymptomatic hip arthroplasties, performing isotope scans at intervals from 4 to 48 months after operation. We used phase-III images obtained with a high-resolution gamma camera and measured the activity in various zones using a specially designed computer program. The components studied at 4, 6, 9 and 12 months were coated with hydroxyapatite (HA) and those studied at 18, 24, 36 and 48 months were not coated. We found a statistically significant fall in activity between four and six months around HA-coated prostheses in all five femoral periprosthetic zones. After six months activity was relatively uniform, but remained higher than that in normal femoral bone at 48 months in non-coated prostheses. We discuss the application of these patterns in the evaluation of painful cementless hip arthroplasties