Osteonecrosis of the knee and related conditions.

Osteonecrosis (ON) of the knee is a progressive disease that often leads to subchondral collapse and disabling arthritis. Recent studies have identified three distinct pathologic entities, all of which were previously described as knee ON: secondary ON, spontaneous ON of the knee, and postarthroscopic ON. Radiographic and clinical assessment is useful for differentiating these conditions, predicting disease progression, and distinguishing these conditions from other knee pathologies. The etiology, pathology, and pathogenesis of secondary ON of the knee are similar to those found at other sites (eg, hip, shoulder). Spontaneous ON is a disorder of unknown etiology. Postarthroscopic ON has been described as an infrequent but potentially destructive complication. Various treatment modalities (eg, core decompression, bone grafting, high tibial osteotomy, arthroplasty), have been used with varying degrees of success for each type of ON. Secondary ON frequently progresses to end-stage disease, and early surgical intervention is recommended. Initial management of spontaneous ON of the knee and postarthroscopic ON is typically nonsurgical, with observation for clinical or radiographic progression.