This article describes osteoarthritis of the knee and a giant intraosseous cyst of the medial femoral condyle in an 83-year-old man that was treated successfully with total knee arthroplasty and autologous bone grafts. The patient presented to the authors' hospital with a 2-year history of right knee pain. A plain radiograph of the right knee revealed grade 3 medial compartment osteoarthritis with an oval area of radiolucency in the medial femoral condyle. The area of radiolucency seen on the radiograph was larger than that on the radiograph obtained 2 years earlier, with no progression of osteoarthritis. Computed tomography showed a well-defined giant intraosseous cyst adjacent to the medial femoral cortex measuring 3.5x3.5x1.6 cm. A medial femoral cortical defect and a small bone break on the articular surface of the medial femoral condyle, both of which allowed communication between the cyst and the joint, were also detected, along with gases in the cyst. Total knee arthroplasty with bone grafts was performed. Resection of the distal femur revealed that the fatty synovia intruded into the cyst through the medial cortical defect. Two years after the surgery, the grafted bone was well incorporated, without loosening of the prosthesis. Two theories, the synovial intrusion theory and the bone contusion theory, have been proposed for the etiology of cyst formation in an osteoarthritic knee. The current case strongly supports the synovial intrusion theory as the mechanism underlying the development of an intraosseous giant cyst in the osteoarthritic knee.
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