Canine synovial lipomatosis: Clinicopathologic findings and HGMA2 immunohistochemistry in 4 cases.

Synovial lipomatosis is an uncommon, intra-articular, fat-containing, proliferative lesion with unknown etiology that is rarely reported in dogs. A retrospective study spanning 13 years was conducted to search for cases of canine synovial lipomatosis. Among 188 synovial biopsies of major diarthrodial joints (ie, shoulder, elbow, carpus, hip, stifle, and tarsus) from 186 dogs, 4 cases (2.1%) of synovial lipomatosis were identified. One case occurred in a stifle with chronic lateral patellar luxation. The other 3 cases had microscopic evidence of synovitis (eg, synovial hyperplasia, lymphoplasmacytic infiltrates, hemosiderin-laden macrophages, myxomatous changes, fibrosis, and increased vascularity) but lacked a clinical history of arthritis. Immunohistochemistry for HMGA2 was negative, suggesting canine synovial lipomatosis is a non-neoplastic proliferative lesion, yet the cause remains unknown.