Severe Varus Deformity of the Knee due to Synovial Osteochondromatosis Treated With Megaprosthesis

Synovial osteochondromatosis is a rare disease of the synovial membrane of joints that can cause severe disability and dysfunction of the joints. In this report, we present the case of a patient with severe joint destruction due to synovial osteochondromatosis treated with a megaprosthesis. The patient was a 58‐year‐old man who had received conservative treatment for a knee fracture caused by a traffic accident at 16 years of age. His activities of daily living were preserved until 40 years of age, when limitation of range of motion and varus deformity began to gradually progress. He was referred to our hospital for surgical treatment due to progression of joint destruction. Physical examination of the left knee revealed varus deformity and swelling, and range of motion was limited to −15° extension and 50° flexion. Because a high‐volume femoral osteotomy was required in order to remove the tumor on the posterior femur and also because of the lack of bone stock on the medial side of the tibia, we performed surgery using a megaprosthesis. The pathology was synovial osteochondromatosis. Postoperatively, the patient developed periprosthetic joint infection, which was treated with debridement and antimicrobial therapy. Six months postoperatively, a periprosthetic fracture occurred and osteosynthesis was performed. Two years after surgery, the patient is able to walk unassisted, and the range of motion of the left knee is about 0° extension and 125° flexion. Bone fusion was achieved and the patient continues to be followed up as an outpatient. Although megaprosthesis is a well‐established treatment for periarticular tumors, complications are varied, so the timing and indications for surgery must be carefully determined. In addition, careful long‐term follow‐up is necessary after surgery.