Soft tissue and juxtaarticular tumors of the knee.

While the wide spectrum of soft tissue neoplasms found throughout the body may occur at the knee, this review focuses on the more commonly encountered intraarticular or juxtaarticular solid soft tissue lesions that may be encountered in this location. The benign intraarticular synovial-based tumors or tumor-like processes of tenosynovial giant cell tumor, synovial chondromatosis, and lipoma arborescens are reviewed, as are the solid juxtaarticular neoplasms of nerve sheath tumors, synovial sarcoma, and myxoid liposarcoma. Several uncommon lesions that occur at the knee while relatively rare elsewhere are also discussed and include synovial vascular malformations, the Hoffa fat pad chondroma, and popliteal arterial cystic adventitial disease. Finally, clear cell sarcoma, a rare malignancy which originates in ligaments and tendons, has the knee as the second most common site of occurrence and may be misinterpreted as a benign entity or sequelae of injury, is reviewed.