Avascular necrosis (AVN), better known as osteonecrosis, of the femoral head, is "a disease that causes death of bone."

It appears to be related to a disruption of the blood flow to the femoral head.

Four basic mechanisms are implicated: mechanical disruption (e.g., broken hip), external pressure on or damage to a vessel wall (e.g., vasculitis, radiation therapy), arterial thrombosis or embolism (e.g., sickle cell diseases, prednisone, alcohol), and venous or blood outflow occlusion (e.g., infection). Although the relationship between the inciting event and the onset of osteonecrosis is clear in some cases, such as vascular disruption from a femoral neck fracture or arterial thrombosis from sickled blood cells, the inciting event in most cases is poorly understood.

AVN may be present without any pain whatsoever. There may be early hip pain but unfortunately, pain often develops only once the osteonecrosis has progressed quite far. At that time the pain is caused by fragmentation and collapse of the femoral head.

Verification of the status of the opposite hip is very important as part of the evaluation of osteonecrosis, because studies have shown that often the contralateral hip is asymptomatic and has a normal x-ray, and even more importantly in cases of non-traumatic osteonecrosis, the incidence of bilaterality is up to 80%.

The natural history of osteonecrosis is linked to the size of the necrotic segment. Very small lesions (involvement of less than 15% of the femoral head) may resolve without any further treatment. On the other hand, lesions involving greater than 50% of the femoral head progress to collapse, and ultimately require in total hip arthroplasty.

Radiographic features of Spontaneous osteonecrosis in the femoral head :

1.     Changes seen in anterosuperior weight bearing portions of the head, in a wedged or semilunar configuration – bite sign.

2.     Articular cortical collapse ( step defect ) .

3.     Signs of degenerative joint disease.

4.     Fragmentation

5.     Periosteal bone apposition – buttressing of the medial surface of the femoral neck.

6.     Variable sclerosis and radiolucency. Occasionally the involved region appears homogenously sclerotic (snow cap sign).

7.     Subchondral fracture – an arc like radiolucency beneath the superior weight bearing cortex- crescent or rim sign.

8.     Atlered trabecular pattern.

MRI

MRI is the most sensitive study and is the imaging procedure of choice, with an overall sensitivity of more than 90%.

MRI findings include decreased signal intensity in the subchondral region on both T1- and T2-weighted images, suggesting edema (water signal) in early disease. This relatively nonspecific finding is often localized in the medial aspect of femoral head. This abnormality is observed in 96% of cases.

A curvilinear one of high signal intensity within the subchondral bone (MRI crescent sign) may be seen before it is apparent on plain films.

The next stage is characterized by a reparative process (reactive zone) and shows low signal intensity on T1-weighted scans and high signal intensity on T2-weighted scans. This finding is diagnostic for AVN.

At the interface of viable and necreotic bone, two opposing lines of high and low signal intensity may be seen- double line sign.

Advanced AVN is characterized by deformity of the articular surface and by calcification, which are easily detected by radiography and CT scan.

In early AVN, decreased uptake occurs in the area of necrosis.

The central area of decreased uptake is surrounded by an area of increased uptake. This phenomenon is known as the doughnut sign and indicates the reactive zone surrounding the necrotic area

CT scan shows sclerosis in the central part of femoral head as an asterisk sign.

There are several treatment choices, determined by the extent of involvement of the femoral head. Non-operative management consists of protective weight-bearing (e.g., partial weight-bearing with crutches) for six weeks then re-evaluation.

Core decompression is a procedure which entails drilling into the femoral neck, up into the necrotic area of the femoral head, to get the bone to heal faster.

Besides non-operative management and core decompression, there are several other treatment modalities, which include osteotomies, bone grafting, and total hip arthroplasty.