Tumors rarely affect joints, unless by direct extension of an adjacent bone or soft-tissue tumor. However, 2 conditions—synovial chondromatosis and pigmented villonodular synovitis—occur in the lining (synovium) of joints. These conditions are benign but locally aggressive. Both usually affect one joint, most often the knee and second most often the hip, and can cause pain and effusion. Both are treated by open synovectomy and, for synovial chondromatosis, removal of any intra-articular bodies and metaplastic synovium.
(See also Overview of Bone and Joint Tumors.)
Synovial chondromatosis (previously called synovial osteochondromatosis) is considered metaplastic synovium. It is characterized by numerous calcified cartilaginous bodies in the synovium, which often become loose. Each body may be no larger than a grain of rice, in a swollen, painful joint. Malignant change is very rare. Recurrence is common.
Diagnosis of synovial chondromatosis is by imaging, usually CT or MRI.
Treatment of synovial chondromatosis may be symptomatic, but if mechanical symptoms are prominent, arthroscopic or open removal of the bodies or synovium is warranted.
Pigmented villonodular synovitis is considered a benign neoplastic tumor of the synovium that can occur around as well as in a joint. The tumor may be localized (focal) in smaller joints of the hands and feet or, more commonly, diffuse in larger joints. When the tumor involves a tendon, it is called a giant cell tumor of tendon sheath. The synovium becomes thickened and contains hemosiderin, which gives the tissue its blood-stained appearance and characteristic appearance on MRI. This tissue tends to invade adjacent bone, causing cystic destruction and damage to the cartilage. Pigmented villonodular synovitis is usually monarticular but may be polyarticular.
The tumors arise from neoplastic synovial cells that overexpress a growth factor CSF-1 (colony stimulating factor-1). The tumors are typically composed of a small number of these cells and a high percentage of myeloid precursors (monocytes and macrophages) that have CSF-1 receptors (CSF-1R). The CSF-1 stimulates the growth of these myeloid precursor cells.
Diffuse pigmented villonodular synovitis has a high local recurrence rate that often leads to further surgery and morbidity. The standard treatment is complete removal by synovectomy. Smaller lesions of an accessible joint may be treated with arthroscopic resection, although there is some risk of seeding the entire joint. Open arthrotomy is usually necessary for a more complete resection. The tumor may lie both within and outside the capsule of the joint, especially when involving the popliteal space.
Pexidartinib, an oral drug, is used to treat symptomatic tenosynovial giant cell tumor causing severe morbidity or functional limitations that is not amenable to improvement by surgery. By binding to the CSF-1R expressed on monocytes, macrophages, and osteoclasts, the drug may prevent tumor proliferation. Adverse effects include hepatitis and liver failure. The role of pexidartinib is evolving. The US prescribing information includes a boxed warning about the risk of serious and potentially fatal liver injury. Pexidartinib is available in the US only through the manufacturer's Risk Evaluation and Mitigation Strategy Program.
Late management of pigmented villonodular synovitis, especially after recurrence, may require total joint replacement. On rare occasions after several synovectomies, radiation therapy is sometimes used.