The effect of axial and torsional loading on strain distribution in the proximal femur as related to cementless total hip arthroplasty.

This study investigated the normal load transfer pattern in the intact proximal femur. Both longitudinal and circumferential strains at eight strain-gauge locations were measured in response to both axial loads (1,000 and 2,000 N) and torsional loads (10 and 20 N . m). Under axial loads, strain magnitudes were large in the proximal regions and decreased distally. Very high compressive longitudinal strain was generated at the surfaces of the proximal medial cortical region, yet circumferential strain at this region was relatively low. Singh osteoporosis classification and femoral head diameter significantly affected the strains in the proximal part of the femur, and femoral head offset significantly affected strain levels in the diaphysis. Under torsional loads, the magnitudes of strain were much smaller than those generated under axial loads, but they increased distally. This study suggests that the proximal part of the femur is equipped for high longitudinal strain but is not equipped to handle high circumferential strain. The diaphysis of the femur is exposed to both axial and torsional loads and is equipped to handle high longitudinal and circumferential strains. Femoral implants that generate high circumferential strain in the proximal femur and high bending stress in the diaphysis are likely to fail mechanically or cause significant bone remodeling. Femoral implants and surgical procedures should be designed to apply load to the femur in a manner in which the femur can handle it best.