Reduced bone stress as predicted by composite beam theory correlates with cortical bone loss following cemented total hip arthroplasty

Clinical and experimental evidence suggest that periprosthetic bone loss fo llowing total hip arthroplasty is caused in part by stress-shielding. Chang es in bone stress in the proximal femur following implantation can be estim ated with use of composite beam theory. We hypothesized that the degree of stress-shielding predicted by beam theory correlates with the magnitude of bone loss following cemented total hip arthroplasty. We analyzed cross sect ions from the proximal femur of 13 patients who had undergone unilateral ce mented total hip arthroplasty. A matching implant was inserted contralatera lly, and the cross-sectional properties of the implant and bone and the bon e density were determined. Bone loss was calculated on the basis of differe nces between contralateral (control) and ipsilateral (remodeled) sections a nd correlated to several beam-theory parameters calculated from the control sections: implant rigidity, bone rigidity, ratio of implant to bone rigidi ty, and predicted decrease in bone stress. All parameters except implant ri gidity were significantly correlated with bone loss (p < 0.05). Parameters that included implant and bone properties were more strongly correlated wit h bone loss than were those based on bone properties alone. The predicted d ecrease in bone stress explained 50-60% of the variance in bone loss. The d ata also indicated that patients were not likely to lose substantial amount s of bone unless the reduction in bone stress exceeded a threshold value. A lthough limited by a small and heterogeneous sample, these results indicate that beam-theory predictions correlate with the degree of femoral resorpti on and should be investigated further as a means to identify patients at hi gh risk for bone loss.