The cause of osteonecrosis is not known, but may be related to abnormal bone resorption, resulting in excessive deposition of calcium salts in the bone with a marble or ivory-like appearance and increased brittleness. There is a family history of this disease, mostly in children of consanguineous marriages. It is thought to be a hereditary disease. There are two types of the disease, mild and severe, with lighter forms being dominantly inherited and heavier forms being recessive. Caffey believes that the basic pathological change in osteolysis is the malabsorption and retention of calcified cartilaginous matrix during the formation of bone in cartilage, resulting in a narrowing or even occlusion of the bone marrow cavity, the formation of sclerotic and brittle bone, and the thickening of the bone cortex and the thickening of the cancellous trabeculae so that there is no clear demarcation between the bone cortex and the cancellous bone. Microscopically, osteoclasts are seen to have abnormal loss of irregular margins indicating inactivity. In the skull, the skull base is mainly involved, and in severe cases, the skull cap can also be extensively involved.