Abstract
An 83-year-old woman presented with a 1-year history of a growing mass on the lateral surface of the right knee. Magnetic resonance imaging revealed a large soft tissue tumor in the subcutis of the right knee. The mass in the right knee rapidly increased, due to hemorrhage from the tumor. A needle biopsy revealed that the diagnosis was synovial sarcoma. The patient underwent wide excision and lateral collateral ligament reconstruction using the plantaris tendon. The patient had a Musculoskeletal Tumor Society Score of 86% at the lateset follow-up. In conclusion, reconstruction of the lateral collateral ligament using the plantaris tendon may be useful for preserving the function of the knee joint after resection of the soft tissue due to sarcoma of the knee.