Effect of femoral stiffness on bone remodeling after uncemented arthroplasty

The current study examined the relationships among femoral stiffness, implant stiffness, and bone remodeling in 40 femurs retrieved at autopsy from 20 patients with unilateral uncemented hip replacements. The purpose of the study was to determine if the magnitude of periprosthetic bone loss after arthroplasty was correlated with, and could be predicted from, stem and femoral stiffness terms. For analysis, the contralateral normal femur was used as a control to represent the unremodeled condition of the in vivo implanted femur. Bone loss attributable to remodeling was quantified by video-densitometric analysis. Stiffness terms were calculated as the product of the elastic modulus and geometric properties digitized from cross-sectional slab radiographs. Femoral stiffness calculations accounted for variations in modulus attributable to patient differences in bone mineral density and geometric properties attributable to differences in the shape of individual femurs. Similarly, calculations of implant stiffness accounted for variations in implant shape. Results showed axial bone stiffness was the variable most strongly correlated with bone loss. Individual stem stiffness terms were not significantly correlated with bone loss. Multiple linear regression analysis, using stem-to-bone stiffness ratios as independent variables, accounted for 46% of the variance in bone loss data. In the regression analysis, the axial stem-to-bone stiffness ratio was the strongest correlate with bone loss. Although these results show the influence of mechanical stiffness factors on bone remodeling, other factors (hormonal status, drugs, disease, activity level) could represent the variance in bone loss data not accounted for in the study.