Morphologic and biomechanical study of avascular necrosis of the femoral head.

Radiologic review and uniaxial compression tests of 1 cm3 cubes of avascular and subjacent portions of 64 femoral heads of 51 patients with idiopathic avascular necrosis of the femoral head were performed to determine the morphologic and biomechanical changes with progression of the disease. Cases were divided into three groups according to the simple radiographic findings: group I (precollapse), group II (collapse), and group III (advanced collapse with secondary degenerative arthritis). Fourteen group I, 36 group II, and 14 group III specimens were identified. In addition to the simple radiographs, 16 magnetic resonance images were reviewed. In group I, there was no significant difference of yield and maximum stress, and modulus of elasticity between avascular and subjacent portions below the reactive rim. In group II, the above parameters showed lower values in the avascular portion than in the subjacent portion (P less than .05). In group III, maximum stress of the avascular portion showed significantly higher value than that of the subjacent region (P less than .05). In groups II and III, a major crack was observed in the subchondral area in 14 specimens and in the deep avascular and reactive junction in 36 specimens. Bone densitometry data suggested higher mineral content indicative of more sclerosis in the deep crack group. These findings might suggest that different biomechanical properties in the subchondral crack group and interface crack group might contribute to varieties in clinical manifestations. Later sclerotic changes and aspherical collapse of the femoral head might play a large role in the development of secondary degenerative arthritis of the hip joint.