Osteonecrosis of the knee: differences among idiopathic and secondary types

Objective. To describe the clinical and imaging features of patients with o steonecrosis 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 magne tic resonance imaging (MRI), and a comparison of idiopathic and secondary t ypes of osteonecrosis. Results. Twenty-four patients had idiopathic osteonecrosis, and in 13 patie nts one or more predisposing factors were identified (secondary osteonecros is). Idiogathic osteonecrosis of the knee was typically a disease of the el derly, characterized by severe knee pain of sudden onset, unilateral involv ement, and restriction of the lesions generally to one femoral condyle or t ibial plateau, with predilection for the medial compartment of the joint. S econdary osteonecrosis generally occurred in younger patients and frequentl y had an insidious onset with mild or vague pain, the lateral compartment o f the knee was often involved, and the lesions were generally larger than l esions arising spontaneously; in the great majority of cases they involved the femoral condyles and/or tibial plateaus. Bilateral distribution and mul tifocal involvement was also seen in these forms. Magnetic resonance imagin g was helpful in confirming the suspected diagnosis when conventional radio graphs were normal or equivocal, and demonstrated different patterns of abn ormalities in idiopathic and secondary types. Conclusion. There are significant differences between idiopathic and second ary osteonecrosis, especially in regard to clinical presentation and the lo cation, extent and MRI appearance of the lesions. These differences are pro bably due to a difference in the pathogenetic mechanism.