Finite element analysis of a cementless proximal femoral stem in relation to early stem stability and interface bone strain

The adequacy of assessing stems stability using only one femoral bone property was tested by varying the bone properties systematically and examining the resultant stem stability and interface strain. Unlike previous published studies which looked at the micromotion values only, the interface strains were also examined in this thesis, because recent studies suggest that interface strain is a predictor of implant migration. Stem micromotion and interface strains were found to increase nonlinearly with the overall stiffness of the femur, which suggest that risk of fixation failure and implant migration increase with decrease of overall bone stiffness. This suggests that more predictive preclinical finite element analyses of new stem designs should be performed with multiple femurs, which span the range of the bone quality likely to be expected in vivo. The effect of varying the degree of interference-fit on stem stability and interface strains was examined. Interference-fit was found to reduce significantly the micromotions of the stem, but with a diminishing reduction of micromotions with further increase in the degree of interference-fit. Strains at the interface bone and surface of the femur increase rapidly with increasing interference-fit, which increase the interface bone damage and risk of femoral fracture with decreasing benefit. The effect of bone creep on the residual stresses of bone induced by the interference-fit was also examined. The results suggest that it could be pointless to increase interference-fit beyond certain level as the creep will reduce the residual stresses to value similar to that of a lower degree of interference-fit. Stem stability was found to be influenced by the residual stresses of the femur, in particular the residual stresses generated within the cortex. Poorer quality bone was found to increase stem micromotion due to lower residual stress and poorer resistance to deformation.