Avascular Necrosis of the Femoral Head: Background, Diagnosis and Classification
Avascular necrosis (AVN), also known as osteonecrosis, atraumatic necrosis or ischemic necrosis, is a debilitating disease that affects a wide range of individuals including children, but is most common in the middle aged population. AVN of the femoral head (AVNFH) is associated with impaired blood supply to the femoral head resulting in bone necrosis and collapse.
AVN of the femoral head is characterized by apoptosis of bone cells (bone marrow, bone forming, and bone destroying cells) that results in bone collapse with a subsequent involvement of the overlying cartilage, provoking flattening of the head surface with, eventually, development of secondary osteoarthritis . The denomination “avascular” is not entirely appropriate as the femoral head blood supplying vessels do not disappear; rather they suffer a pathological process which results in blood flow interruption.
The most common etiology for altered metabolism in adults is alcohol intake. This is dose dependent with the relative risk increasing as intake increased, with 9.8 relative risk with 1 liter per week and 17.9 relative risk if more than 1 liter per week . Alcohol disturbs the phospholipid and cytokine metabolism and this interrupts blood flow. Steroid administration has also been shown to be dose dependent with prolonged treatment more than 20-40 mg per day for more than 3 months having a greater chance of developing AVN of the femoral head . Alcohol intake and steroid usage accounts for more than 80 percent of atraumatic cases .
Figure 1. Crescent sign (adopted from )
The initial imaging modality should be plain film radiography and include AP views and frog leg lateral views. Close attention should be paid to the superior portion of the femoral head for subchondral abnormalities. It is possible that these can be normal for months and the earliest findings are mild density changes. The crescent sign refers to a linear area of subchondral lucency seen most frequently in the anterolateral aspect of the proximal femoral head and is a sign of imminent collapse. Later stages reveal loss of sphericity or collapse of femoral head.
Radionuclide testing (Technetium-99m) bone scanning can be used in patients with suspected disease with negative radiographs and has been shown to be slightly more sensitive in early stages. Increased burn turnover increases uptake in the femoral head and surrounds a cold area. This is sometimes called the doughnut sign .
Image 2. “Doughnut” sign on the left hip, adopted from )
Figure 3. “Double Line” sign, adopted from )
Figure 4. Calculation of necrotic angle, adopted from )
Table 1. ARCO classification system (adopted from )
Figure 5. Progression of AVNFH (adopted from )