Mechanical factors in slipped capital femoral epiphysis.

We performed three-dimensional force analyses on the hips of 50 normal patients and 50 patients with slipped capital femoral epiphysis to determine if mechanical factors alone could account for slips. We found that slipped epiphysis patients have reduced resistance to shear because of increased body weight and a decreased neck shaft-plate shaft angle. Slipped epiphysis patients have relative retroversion, and this generates increased sagittal plane shear stress at the proximal femoral growth plate. We suggest that, during running, these mechanical factors cause shear failure of the growth plate in obese adolescents.