Serial radiographs of a 40 year old male with ONFH bilaterally: panel a: early stages showing density changes, more pronounced in left hip; panel b: after 3 months, fragmentation changes have appeared in both femoral heads. Femoral head collapse is eminent at this point; panel c: 1 year follow-up shows partial collapse in both femoral heads, relatively preserved joint space, relative flattening at the lateral periphery of the femoral heads, and formation of impinging cam-type lesion.