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Calcified plate and flattening of the femoral condyle with spontaneous osteonecrosis.

Four cases with spontaneous osteonecrosis of the knee are reported to highlight mainly the features of the cartilage flap over the necrotic lesion. Case 1 and Case 2 showed late Stage 2 and Stage 3 spontaneous osteonecrosis in the medial femoral condyle, respectively, but featured a convex calcified plate with no flattening in the lateral view of the radiographic knee. Case 3 showed multiple small fractures in the subchondral necrotic bone of the cartilage flap. In Case 4, the cartilage flap became detached and was found in the synovium of the supra-patellar fossa; moreover, the necrotic bone of the flap seemed to be revascularized, and the small fragments were found to be united. Flattening is not always found in the early stage of the necrotic process. As spontaneous osteonecrosis progresses in the femoral condyle, it is understood that a cartilage flap with a layer of necrotic bone sustains multiple fractures and thus forms a calcified plate. Therefore, a subchondral radiolucency may appear first, followed by a calcified plate with flattening.