Osteonecrosis

Ischaemic necrosis of bone occurs almost invariably within areas of predominantly fatty marrow, whereas necrosis in areas of normal active haematopoiesis is distinctly unusual, although it may occur in sickle cell disease and related haemoglobinopathies or after complete traumatic disruption of the arterial blood supply. Ischaemic necrosis of cortical bone is relatively rare and occurs only when the arterial blood supply is interrupted extensively, as in osteomyelitis.

Among the causes of osteonecrosis are trauma, haemoglobinopathies, hypercortisolism, renal transplantation, alcoholism, pancreatitis, caisson disease, Gaucher's disease, gout, irradiation and synovitis with elevation of intra-articular pressure.

On plain radiographs findings are characteristic. In an epiphyseal region, arc-like subchondral radiolucent lesions, patchy lucent areas and sclerosis, bone collapse, and preservation of the joint space are seen; in a diametaphyseal region, lucent shadows with a peripheral rim of sclerosis and periostitis are typical; and in a flat or irregular bone, patchy lucent areas and sclerosis with bone collapse are well recognized radiographic signs of osteonecrosis. A bone within bone appearance is diagnostic of osteonecrosis.

CT scanning allows early diagnosis of the condition and may help in defining the presence and extent of bone collapse. However, scintigraphy (sometimes combined with single photon emission computed tomography (SPECT) and MR imaging are far more sensitive in delineating early trabecular alterations of bone necrosis. In later stages, CT scanning with reformation of data yields information that may influence the choice of orthopaedic procedures that can be employed successfully.

MR imaging is a very sensitive method for the early diagnosis of ischaemic necrosis of bone. It appears to be more sensitive in this regard than standard bone scintigraphy. Evaluation of the femoral head has been particularly helpful. This imaging method may allow the identification of two changes that place a femoral head at risk for subsequent osteonecrosis: early conversion of haematopoietic to fatty marrow in the proximal portion of the femur and prominent physeal scars that serve to close off the femoral head.

Post-traumatic osteonecrosis occurs most frequently in the femoral head, talus, humeral head, scaphoid bone, capitate bone and vertebral body (Kummells disease). Bone death of variable extent occurs on either side of the fracture line, which may appear radiodense owing to compression of trabeculae or reactive eburnation.

Dysbaric osteonecrosis (also termed caisson disease) may result from exposure to high pressure environments, as in scuba diving, underwater and space exploration, and off-shore oil drilling. Gas bubbles present in the vascular tree probably cause the osteonecrosis.

Idiopathic (primary or spontaneous) osteonecrosis may appear in certain sites in the absence of any underlying disorder. Three main varieties are recognized: spontaneous osteonecrosis about the femoral head in adults or Chandler's disease; spontaneous osteonecrosis about the knee in adults or Ahlbacks disease and spontaneous osteonecrosis of the tarsal navicular bone in adults or Mueller Weiss syndrome.

Among the complications of osteonecrosis are cartilaginous abnormalities (fibrillation and erosion leading to loss of interosseous space), chondral or osteochondral intra-articular bodies, cyst formation, and malignant degeneration (fibrosarcoma, osteosarcoma, malignant fibrous histiocytoma).

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