Cementless implant composition and femoral stress. A finite element analysis.

Proximal atrophy and thigh pain are recognized problems with some cementless femoral stems in total hip arthroplasty. It is thought that reduced femoral stress from alterations in load transfer caused by an intramedullary stem contributes to proximal femoral atrophy. An increase in flexural rigidity and bone stress near the stem tip is thought to contribute to thigh pain. A three-dimensional finite element analysis study was performed to calculate stresses in the proximal femur and bone near the stem tip before and after implantation of a collared, proximally coated, cementless femoral prosthesis. The influence of prosthetic material was examined by changing implant composition from cobalt chrome to titanium alloy and leaving all other parameters constant. Femoral stress was increased twofold immediately below the collar with the titanium implant compared with the cobalt chrome. However, the proximal femoral stress in the titanium implanted model was still 1/10 that in the corresponding region of the unimplanted femur model. At the stem tip, as much as a 30% reduction in femoral stress was seen with the titanium stem compared with the cobalt chrome. These findings suggest biomechanical evidence of an advantage for titanium as an implant material compared with cobalt chrome for cementless femoral stems.