Synovially-based soft tissue lesions of the knee: What is the rate of malignancy?

Malignant synovially-based lesions of the knee are rare and can present a diagnostic challenge. Accurate diagnosis is paramount as inadvertent, intralesional surgery of a malignant pathology can result in oncological contamination of the joint space, complicate local disease control and necessitate further major surgery. A case series of synovially-based lesions of the knee treated at a tertiary orthopaedic oncology centre is presented to better understand malignancy rates and characteristics.

All patients treated since the year 1996 with synovially-based lesions of the knee, including Hoffa's fat pad, were retrospectively identified from our prospectively maintained departmental database. Radiological and histological diagnoses of all lesions were reviewed with malignant presentations studied in detail.

Of 385 lesions, 254 (142:112 F:M) had a tissue diagnosis. Six primary malignancies were identified of which four were associated with Hoffa's fat pad. Three malignancies had undergone inadvertent excision prior to referral and required subsequent joint sacrificing surgery. Three malignancies were radiologically indeterminate at presentation and identified on biopsy as synovial sarcoma (n = 2) and myxoid liposarcoma (n = 1), with joint-sparing excision performed in two cases. The most common benign diagnoses were focal nodular synovitis (n = 189), vascular malformation (n = 11), synovial chondromatosis (n = 9), soft tissue chondromatosis (n = 8) and synovial haemangioma (n = 7).

Synovially-based lesions of the knee are rarely malignant, representing 2% of all cases in this series. Focal lesions should be heeded with caution and tissue diagnosis is recommended to avoid intralesional surgery. Slow-growing, symptomatic lesions, with a size 1-4 cm in young patients are suspicious for malignancy and tissue diagnosis is necessary.