Tumors rarely affect joints, unless by direct extension of an adjacent bone or soft tissue tumor. However, 2 conditions—synovial chondromatosis and tenosynovial giant cell tumor (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.
There is a malignant soft tissue tumor called synovial sarcoma that arises in different types of soft tissue but is not of synovial origin and seldom occurs inside of a joint. It is a monomorphic blue spindle cell soft tissue cancer with variable degree of epithelial differentiation characterized by the specific SS18-SSX/1/2/4 fusion gene but is often biphasic with both spindle cell and glandular–like components. The term applied to the tumor as "synovial" many decades ago is a misnomer.
(See also Overview of Bone and Joint Tumors.)
Synovial chondromatosis
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.
Image courtesy of Michael J. Joyce, MD, and Hakan Ilaslan, MD.
Treatment of synovial chondromatosis may be symptomatic, but if mechanical symptoms are prominent, arthroscopic or open removal of the bodies or synovium is warranted.
Tenosynovial giant cell tumor
Tenosynovial giant cell tumor (previously called 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 nodular (focally localized) in smaller joints of the hands, feet, and knee, or, more commonly, diffuse in larger joints such as the knee and hip. 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. Tenosynovial giant cell tumor 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 tenosynovial giant cell tumor 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 is evolving (1). 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 in cancer centers through the manufacturer's Risk Evaluation and Mitigation Strategy Program.
Image courtesy of Michael J. Joyce, MD, and Hakan Ilaslan, MD.
Late management of tenosynovial giant cell tumor, especially after recurrence, may require total joint replacement. On rare occasions after several synovectomies, radiation therapy is sometimes used.
Reference
1. Gelderblom H, Wagner AJ, Tap WD, et al: Long-term outcomes of pexidartinib in tenosynovial giant cell tumors. Cancer 15;127(6):884-893, 2021. doi: 10.1002/cncr.33312. Epub 2020 Nov 16. PMID: 33197285; PMCID: PMC7946703.
