The patient is a 41-year old woman who presented with pain and swelling of her right knee. An arthroscopy was performed and several intraarticular nodules were seen and shaved.
Histological sections show synovium with an underlying proliferation of variably-sized thin-walled blood vessels. The intervening stroma contains numerous spindle cells. Calcifications are present.
DIAGNOSIS: MULTINODULAR SYNOVIAL HEMANGIOMA WITH PROMINENT SPINDLE CELL COMPONENT
Synovial hemangioma (SH) is an exceedingly rare type of vascular malformation with only about 200 cases reported in the literature. These lesions occur primarily in children or adolescents with a predilection for males. The knee is the most commonly affected site, followed by the elbow and finger. Patients notice a slowly growing bump that often causes pain, recurrent knee swelling, and restricted range of motion due to effusion. The lesion is usually confined to the intra-articular synovium, but sometimes it is located in a bursa adjacent to a joint. The imaging findings are non-specific.
Histologic sections show proliferation of cavernous and capillary vascular channels located underneath the synovial membrane. Numerous congested, variably dilated vessels of different calibers are often seen. The vessels are often embedded in a myxoid or fibrotic stroma. Possible reactive changes include hemosiderin deposition and villous hyperplasia of the synovium.
Several microscopic patterns can be seen, reflecting the architecture of the blood vessels. The most common one is cavernous hemangioma, followed by lobular capillary hemangioma, arteriovenous hemangioma, and venous hemangioma. A spindle cell component seen in this case is highly unusual and has not been described in the literature.
Since synovial hemangioma is a very rare lesion, other conditions must be considered both clinically and histologically in a patient with the above presentation. Pigmented villonodular synovitis is a much more commonly encountered entity in the clinical practice that also occurs in young adults and involves the unilateral knee. However, the histopathology reveals a mixed inflammatory cell infiltrate composed of mononuclear cells, multinucleated giant cells, foam cells, and hemosiderin-laden macrophages. Synovial lipomatosis (lipoma arborescens) is another benign synovial lesion with a similar clinical presentation that on the histology is recognized by infiltration of mature adipocytes into the synovium. Histologic differential diagnosis includes hemangiopericytoma arising inside the joint and intracapsular chondroma. Nodular fasciitis can also be seen in an intra-articular location. It has features similar to those seen in the soft tissue lesions, although it is more likely to develop stromal hyalinization and hemosiderin deposition. Most malignancies involving the joint space represent extension from tumors located in the adjacent bones. Rare cases of primary malignancy involving the joint include synovial sarcoma, epithelioid sarcoma, arthroplasty-associated malignant fibrous histiocytoma, and malignant lymphoma that may arise in a joint that sustained a long-term damage by rheumatoid arthritis.
Synovial hemangiomas are typically small lesions that can be cured by arthroscopic removal with no risk of local recurrence. However, since these lesions are exceedingly rare, and the imaging findings are non-specific, the diagnosis may be delayed by months to years, resulting in delay in treatment and complications that may be irreversible in nature.
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