Osteonecrosis or Avascular Necrosis of the hip is death of a segment of bone in the femoral head due to disruption of the blood supply. The etiology of this condition is not fully understood. There are several risk factors associated with osteonecrosis of the hip. 
The condition occurs bilaterally in about 80% of patients. It is important for the physician to check both hips, even if one is asymptomatic. An early diagnosis is important because during the early stages of osteonecrosis, a femoral head preserving procedure can be done. In the late stages of osteonecrosis, the femoral head collapses and cannot be saved. The femoral head may need to be replaced. Obtain AP and Frog Leg Lateral views of the hip. The Frog Leg Lateral view will show the crescent sign.
An MRI is the imaging of choice, especially when the patient has persistent hip pain, the radiographs are negative, and the diagnosis of osteonecrosis is suspected.
The Ficat Classification is a commonly used system to stage osteonecrosis of the hip. Stage I classifications will have a normal appearing x-ray and an MRI will detect the lesion; changes in the marrow. Stage II classifications are identified by sclerosis and cyst formation. Characteristics of the Stage III classifications are a subchondral fracture and the appearance of a crescent sign and flattening of the femoral head. Stage IV classifications will show advanced lesions with arthritis, osteophyte formation, and a loss of the joint space.
Treatment
For early stages of osteonecrosis of the hip, initial trial of nonsurgical treatment is usually done. Surgery may be needed if nonsurgical methods are not successful. Nonoperative treatment typically consists of bisphosphonates.
These drugs may be used before the femoral head collapses and are still in the experimental stages. In regards to traditional surgical treatment, when the lesion is small a head preserving procedure can be done. For stages I & II, a core decompression is used. The surgeon can make a single large hole or multiple small holes in the femoral head. It decompresses the head and stimulates a healing response. The lesion is done anteriorly and superiorly.
If the surgeon chooses to perform a core decompression with bone graft, they will debride the necrotic area and insert the bone graft into the open space. Vascularized Fibular Grafts are done in younger patients. Complications include: donor site pain and leg dysfunction as well as tibial stress fractures on the side the graft was taken. Stages III and IV will require a total hip arthroplasty (cementless cup and stem) or total hip resurfacing; however, resurfacing is not commonly used. A total hip replacement is considered to be the traditional procedure for advanced stages of osteonecrosis of the hip. Total hip resurfacing is considered to be controversial because the patient will need adequate bone stock to support the femoral component. The result is not as good when compared with a patient with osteoarthritis.
Dr. Nabil Ebraheim's Blog 