Calcium pyrophosphate dihydrate crystal deposition disease of the knee simulating spontaneous osteonecrosis

A retrospective review of the frontal and lateral knee radiographs of 200 p atients with calcium pyrophosphate dihydrate (CPPD) crystal deposition dise ase was performed. Of these 200 patients, nine patients (four male, five fe male, mean age 74 years, age range 63-87 years) had radiographic findings s imulating osteonecrosis of the knee. One patient also had magnetic resonanc e imaging of the involved knee. A total of 10 knee radiographs in nine pati ents showed articular and periarticular calcification diagnostic of CPPD cr ystal deposition of the knee. In addition, all 10 radiographs showed flatte ning of one femoral condyle. Four of the 10 cases demonstrated an area of r adiolucency in the subchondral bone surrounded by a halo of sclerosis. Eigh t of the 10 cases had narrowing of the involved joint compartment and osteo phytosis. These findings mimic the radiographic signs of spontaneous osteon ecrosis of the knee. In conclusion, flattening of the femoral condyles in C PPD crystal deposition disease simulates that of spontaneous osteonecrosis and probably relates to articular cartilage and meniscal. damage that subse quently leads to stress fracture of subchondral bone with bone collapse.