Femoral expansion in total hip arthroplasty.

With increasing age, the femoral medullary canal expands and cortical thickness decreases. Examination of the radiographs of 22 hips that had undergone total hip arthroplasty revealed that at a mean interval of 11.5 years the medullary canal had expanded at an average rate of +0.328 mm/year (P less than .0001). There was a concomitant mean cortical atrophy of -0.155 mm/year (P = .0006). These rates of expansion are similar to those observed in natural aging. The presence of a femoral prosthesis is associated with temporal changes in femoral geometry similar in magnitude to those occurring in the natural femur. Because of the fundamental changes in load transmission caused by the presence of a femoral prosthesis, these geometric changes may be in part secondary to the bone's adaptation to altered load transmission, as well as to natural aging phenomena. Recognition of the pattern of natural femoral expansion suggests certain strategies for design criteria that seek to maximize fit.