PATHOGENESIS

The pathogenesis of osteonecrosis is commonly divided, artificially, into four different phases, 1) the avascular phase, 2) the revascularization phase, 3) the repair phase, and 4) the deformity phase. Along with histologic changes, radiographic changes correspond with each one of these phases. It is said that the phases are divided artificially because the process of osteonecrosis is on a continuum, with much overlap. This is important to keep in mind due to the fact that many times radiographic signs from two or more phases may be visualized at one time.

1) Avascular Phase

This first step in the process of osteonecrosis begins as an infarct to the bone from one of the many different causes discussed on the previous page. This infarction typically takes place within fatty marrow adjacent to the subchondral cortex of the bone. Infarction within the medullary portion of long bones can also happen and are termed intramedullary infarcts which are not discussed in terms of different phases of radiographic change. The infarction at the subchondral portion of the bone leads to bone loss leading to bone death. As the subchondral bone becomes necrotic, the overlying cartilage hypertrophies. This is an attempt to convert itself to bone.

The radiographic appearance that corresponds to this first phase is often, unfortunately, nothing at all. Minimal osteopenia or subtle soft tissue changes may be appreciated if there is a strong clinical suspicion. The reason why this is so unfortunate is because only in the first phase of osteonecrosis is treatment by far most effective. The osteopenia is not due to the infarction itself, but hyperemia surrounding the involved area as well as synovitis results in osteoclastic activity which leads to osteopenia. The subtle soft tissue changes are typically found around the proximal femur. One may observe hip joint fascial plane changes or an increase in the medial joint space between Kohler's teardrop and the femoral head. With a properly positioned AP pelvis view, a 2 mm difference is significant, and should be followed up. This increased medial joint space is termed Waldenstrom's sign.

Waldenstrom's sign is nonspecific also observed in hip trauma, a joint space infection and a slipped capital femoral epiphysis.

2) Revasularization Phase

The second phase, the revasularization phase, is a result of the area attempting to "clean up" the area via osteoclastic and osteoblastic activity. The corresponding radiographic appearance results in mixed areas of luceny and sclerosis. Because of the cell death that occurs beneath the articular surface of the bone, it is no longer able to withstand normal stresses put on it resulting in fragmentation and collapse of the overlying cortex. The first indication that collapse is near is the crescent sign.

The crescent sign is a thin curvilinear lucency which occurs in immediate subchondral bone and is located on the weight bearing portion. Further collapse may result in a "step sign" in which cortical offset is actually noted on the film. The "Snow Cap Sign" refers to diffuse sclerosis of femoral head or humeral head and is seen only if repair is sufficient in revascularization phase.

"Snow cap" sign of the left femoral head	"Snow Cap" sign of the humeral head

3) Repair Phase

The repair phase actually begins when revascularization begins. There is a variable degree of reconstitution and healing that is dependent upon 1) the degree of blood loss and cell death in the initial insult, 2) the patient's immune system and healing response, and 3) whether the joint is weight-bearing vs. non-weight bearing

4) Deformity Phase

The deformity phase is highly variable because it is dependent upon severity of other phases
and is site specific. It is also dependent upon stresses in the area (weight-bearing areas show increase in deformities).
The femoral head results in a classic "Mushroom deformity" seen in Legg Calve Perthes disease.

Early and severe degenerative joint disease will occur due to incongruent joint surfaces often necessitating a hip replacement by within the fifth decade of life.

The problem with osteonecrosis is that once enough cell death occurs, the process will cascade itself though all the stages of osteonecrosis with the final result being severe and often debilitating degenerative joint disease. The fortunate aspect of this process is that it is often very treatable if discovered early.