The mechanism of development of cam-type FAI in femoral heads after osteonecrosis: panel a: fragmentation in the femoral head with preservation of femoral head sphericity; panel b: it can be noticed from the radiograph that a large part of the femoral head is involved in the disease process; panel c: illustration of collapse of the femoral head. The entire diseased fragment undergoes collapse due to weakening of deeper metaphyseal bone. Subchondral bone and metaphyseal bone immediately deep to it remain intact; panel d: with remodelling of the step formed at anterolateral part of femoral head, a cam deformity develops.