Stress fracture of the vertebral arch in osteopetrosis.

Stress fracture of the fourth vertebral arch occurred in a 21-year-old woman with a benign osteopetrosis. The fracture healed in three months after application of a body cast. It seems likely that the osteopetrotic bone resists force, in parallel with the cortical layer, but is fragile to forces at right angles to the plane of the cortical layer.