HIP

The most common location in the body for osteonecrosis occurs of the hip. It is most commonly idiopathic and more often seen in children than adults. When idiopathic osteonecrosis is observed in children it is termed Legg Calve Perthes disease (AKA Perthes or Legg Perthes disease). If a child gets osteonecrosis of the hip from a known source it should not be termed Legg Calve Perthes disease.

Legg Calve Perthes disease is the classic example of idiopathic osteonecrosis and has well known and published radiographic signs that may be observed at the hip. Early signs, as discussed in the previous section, would include soft tissue swelling and an increased Kohler's teardrop distance (Waldenstrom's sign).

The "crescent" sign is one of the earliest bony changes seen radiographically. This crescent sign is best visualized on the Frog-leg lateral view, therefore, if AVN is clinically suspected, this view should definitely be taken.

It must be stressed that while a crescent sign is regarded as an "early" sign of avascular necrosis, the disease process itself is relatively advanced and the prognosis at this point is relatively poor when compared to the prognosis if discovered before any bony changes.

Other signs would include a small femoral head (Fig. 4) presumably due to lack of blood flow resulting in cessation of growth, and enlarged greater trochanter. The enlarged greater trochanter is due to decreased femoral neck growth and continued greater trochanter growth. The elevation of the trochanter alters the efficiency of the attached gluteal musculature and may result in a positive Trendelenburg test, clinically. Later features would include, as in all osteochondroses, fragmentation, mixed areas of lucency and sclerosis, and collapse.

Finally the femoral head will remodel into the "mushroom deformity."

The signs of the mushroom deformity of the femoral head include coxa magna (enlargement of the femoral head) and coxa plana (flattening of the femoral head). Other associated radiographic features to look for include remodeling of the acetabulum, coxa vara (femoral neck angle of less than 120 degrees) and the sagging rope sign (representing the edge of the flattened femoral head).

The average onset of Legg Calve Perthes disease is between 4 and 8 years of age and affects males more commonly, about 5:1. The patient typically will present with a limp due to pain with insidious onset of symptoms. There may be a positive Trendelenburg test as noted above related to an enlarged greater trochanter. 90% of cases are unilateral.

The prognosis is important for doctors to know so that they may implement proper treatment and accurately inform their patients. The prognostic variables include gender, age of onset, and most importantly which stage that the osteonecrosis was diagnosed. Males, an early age of onset, and early diagnosis all have a favorable prognosis.

Treatment for Legg Calve Perthes disease is surgical and may include a correctional osteotomy stabilized with plate and screws or an intramedullary rod to correct lateral displacement of the femoral head and/or "core decompression" that relieves the pressure inside of the medullary cavity of the femoral neck.

In the adult, the most common known cause is due to corticosteroids. As previously discussed, however, there are many other causes. (Remember ASEPTIC?) Idiopathic osteonecrosis in an adult is termed Chandler's disease.

AVN of the femoral head in the adult is often times much harder to detect, radiographically, than in a child. This is because the subtle radiographic changes observed in a child are not applicable in an adult, such as a difference in femoral head size and fragmentation. Waldenstrom's sign is also harder to appreciate in an adult. The early radiographic signs in an adult are a patchy increase in density.

This may be very difficult to appreciate due to possible suboptimal technical factors and if it is appreciated it may be misinterpreted as degenerative subchondral sclerosis. An important finding that is often times overlooked is lack of joint space narrowing. It is this lack of joint space narrowing that should aid in this differential. Not until there is secondary degenerative joint disease will there be narrowing of the joint. Flattening of the weight bearing portion of the femoral head is another early sign. As the disease progresses, a crescent sign, then subarticular collapse, resulting in a "step defect" may be observed. Also due to altered stresses put on the medial portion of the femoral neck, buttressing is often a result.

As the disease progresses, more sclerosis, collapse and fragmentation of the weight bearing portion of the femoral head may be appreciated.

After the femoral head continues through the revascularization and deformity phases, severe secondary degenerative joint disease in inevitable.

As stated numerous times already, if clinically suspected, insidious onset of progressive hip pain that does not respond to conservative therapy should be further investigated. The next imaging of choice would be magnetic resonance imaging (MRI). There are different appearances of avascular necrosis on MRI depending on the stage of the disease. A classic appearance that has been described, however, in the early stages is termed the "double line" sign. This describes a focal area of high or intermediate signal intensity (white) that is surrounded by a rim of low signal intensity (black) on both T1 and T2 images.

MRI - Osteonecrosis of the left hip. Double line sign

MRI - Double line sign

The stages and radiographic appearance of avascular necrosis of the hip are very similar to those seen in other body regions as will be discussed next.