An 11-year-old girl was evaluated for right knee pain following a twisting injury. She belonged to a volleyball club and practiced almost every day. However, no history of antecedent trauma and no relevant medical history were noted. She had suffered mild knee pain in activities of daily living for three days before the injury and been managed by her primary care physician. Radiographic examination was performed by the initial doctor, and she was referred to our clinic for investigation of an abnormal lesion found in the radiograph.
Physical examination on initial presentation at our clinic showed full range of motion of the affected knee without limping, and effusion, swelling, local heat, instability or tenderness were not detected. Plain radiographs of the knee showed a small circumscribed radiolucency with a thin sclerotic margin in the subchondral region of the lateral femoral condyle. The lesion was located near the contact area at maximum knee extension (Fig 1). The size of the cyst was about seven millimeters in diameter. MRI examination showed a fluid signal in this structure with low signal intensity on T1-weighted images and high signal intensity on T2-weighted images (Fig 2). Complete blood count, erythrocyte sedimentation rate, and serum chemistry studies were normal. In the differential diagnosis, osteochondritis dissecans and tumorous lesions such as chondroblastoma, chondromixoid fibroma, giant cell tumor, and fibrous dysplasia were considered. Based on the patient's age and the radiologic finding of subchondral radiolucency, the lesion was tentatively diagnosed as osteochodritis dissecans. However, its location was atypical and the MRI findings were not coincident with this preoperative diagnosis. Since accompanying clinical symptoms and signs were minimal, we instructed the patient to stop sports activities and wear a hinged brace limiting full extension during the daytime. Six months after starting the treatment, although she remained symptom free, the size of the bony lesion had not decreased, and we decided to perform arthroscopy to make a definitive diagnosis.
On arthroscopic examination, an area with a dimple and irregularity was seen at the weight bearing region of the lateral femoral condyle, where it contacted the tibial articular surface at full extension (Fig 3). The remaining structure within the knee joint was normal. On excision of the overlying tissue, the lesion was cystic containing brown mucous fluid. Removal of the surrounding lining tissue and curettage were performed. The biopsy included the wall of the cyst and the overlying cartilage. No association between the cyst and the articular structures such as the joint capsule, cruciate ligaments and meniscusi was observed. Histologically, the lining tissue consisted of dense fibrous tissue with spotty areas of calcification without a continuous synovial layer (Fig 4). These findings were consistent with those reported in previous studies as ganglion [2, 3].
The postoperative course was uneventful and the patient was permitted to resume sports activity at 4 months. At the nine-month postoperative follow-up, she remained asymptomatic and the radiographic examination showed the bony lesion was barely identifiable with apparent healing (Fig 1).