We looked for structural differences between normal patients and those with slipped capital femoral epiphysis to explain shear failure of the growth plate in terms of mechanics. Hip radiographs of 50 normal adolescents were examined, and it was found that lines drawn along the axis of the femoral neck and perpendicular to the growth plate intersected at an average angle of 23 degrees (neck shaft-plate shaft angle). Radiographs of 50 patients with slipped capital femoral epiphysis were examined, and the femoral neck shaft-plate shaft angle was found to be between 10 degrees and 20 degrees in patients with unilateral slips and less than 10 degrees in patients with bilateral slips. These abnormalities in the femoral neck shaft-plate shaft angle suggest there are mechanical differences between normal hips and those that develop a slip of the capital femoral epiphysis that may predispose to shear failure of the growth plate.