Radiologic case study. Spontaneous osteonecrosis of the knee reclassified as insufficiency fracture.

Three questions need to be answered. First, does spontaneous osteonecrosis of the knee in fact exist? The answer is "probably," but much more rarely than initially believed. In five of the authors' most recent MRIs in patients with clinically diagnosed or radiographically suggested spontaneous osteonecrosis of the knee, only one patient's images fulfilled the criteria for spontaneous osteonecrosis (Figure 3). Three of the five had subchondral changes of either insufficiency fractures or true fractures, while one patient had equivocal findings. Second, will the actual diagnosis influence the treatment? The answer appears to be "no" because early subchondral fractures and early spontaneous osteonecrosis of the knee are treated conservatively and only the later changes of subchondral surface collapse are treated more aggressively with joint replacement. Third, is there any method of halting the progression of early subchondral changes before they progress into collapse, flattening, and loose-body formation? The answer appears to be "maybe" as non-weight-bearing conservative therapy has not consistently prevented progression of the disease. Whether new arthroscopic techniques can provide the necessary intervention to halt osteoarthritic sequelae remains unanswered. Thus, it appears most patients with what has been called spontaneous osteonecrosis of the knee actually have subchondral insufficiency fractures. The histologic findings of avascular areas in biopsy specimens appear to be a secondary phenomenon and not the primary cause. If diagnosed early enough, conservative therapy may lead to healing without subsequent collapse and fragmentation of the subchondral bone.