Osteonecrosis of the knee:: differences among idiopathic and secondary types

Objective. To describe the clinical and imaging features of patients with osteonecrosis of the knee, emphasizing the differences among idiopathic and secondary types. Methods. A retrospective chart review of 37 consecutive patients (41 knees) with osteonecrosis of the knee confirmed by bone scintigraphy and/or magnetic resonance imaging (MRI), and a comparison of idiopathic and secondary types of osteonecrosis. Results. Twenty-four patients had idiopathic osteonecrosis, and in 13 patients one or more predisposing factors were identified (secondary osteonecrosis). Idiogathic osteonecrosis of the knee was typically a disease of the elderly, characterized by severe knee pain of sudden onset, unilateral involvement, and restriction of the lesions generally to one femoral condyle or tibial plateau, with predilection for the medial compartment of the joint. Secondary osteonecrosis generally occurred in younger patients and frequently had an insidious onset with mild or vague pain, the lateral compartment of the knee was often involved, and the lesions were generally larger than lesions arising spontaneously; in the great majority of cases they involved the femoral condyles and/or tibial plateaus. Bilateral distribution and multifocal involvement was also seen in these forms. Magnetic resonance imaging was helpful in confirming the suspected diagnosis when conventional radiographs were normal or equivocal, and demonstrated different patterns of abnormalities in idiopathic and secondary types. Conclusion. There are significant differences between idiopathic and secondary osteonecrosis, especially in regard to clinical presentation and the location, extent and MRI appearance of the lesions. These differences are probably due to a difference in the pathogenetic mechanism.