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Proliferative tenosynovitis (PT) is an inflammatory and proliferative disorder of the synovial membrane of the tendon sheath that is rare in animals. The histological alterations are characterized by multinodular neovascularization, with infiltration of histiocytic and multinucleated giant cells and haemosiderin deposition. We reviewed necropsy and biopsy records of horses submitted to the Setor de Anatomia Patológica of the Universidade Federal Rural do Rio de Janeiro from January 2017 to December 2020 to select cases of PT. We identified PT in three adult Brazilian Mangalarga Marchador horses with nodular lesions on the metacarpophalangeal, metatarsophalangeal or carpal joints. The three horses were under 6 years of age and presented with lameness and pain on palpation. There were recurrences in two horses after surgical removal. Radiographic and ultrasound examinations detected masses in the flexor or extensor tendons and subtendinous bursa. Histological study of synovial membrane and tendon sheath revealed an increased number of vessels, fibroplasia, osseous metaplasia and infiltration of lymphocytes, plasma cells and siderophages. This is the first description of PT in horses, which should be included as an orthopaedic differential diagnosis, especially in Mangalarga Marchador horses with lameness., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

A 7-year-old neutered male domestic shorthair cat was presented with chronic lameness in the right forelimb. A cystic bony lesion was identified in the distal right humerus and amputation was performed. The epiphyseal trabecular bones of the capitulum and trochlea was replaced by a tan to pink, expansile mass that was surrounded by a thin rim of cortical bone. Microscopically, the tumour was composed of a bland, osteoid producing spindle cell population within a well-vascularized fibrous stroma. Radiographical and histological features were consistent with osteoblastoma. Osteoblastoma and the related osteoid osteoma are uncommon, benign osteoblastic tumours that are reported rarely in animals. These tumours should be considered as differential diagnoses for slow growing, cystic bony lesions in cats., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

A 5-year-old male domestic rabbit had severe swelling of the left hindlimb. Radiographs demonstrated a proliferative, infiltrative lesion involving the stifle joint, femur and soft tissues of the thigh. Osteomyelitis or neoplasia was suspected and the limb was amputated. Grossly, there was a multilobular mass comprised of cystic spaces containing yellow mucinous material. Microscopically, the mass formed coalescing lobules of stellate to rounded cells embedded in varying amounts of myxoid to collagenous matrix, and some rimmed by narrow walls of metaplastic bone and/or cartilage, and some infiltrated by plasma cells, lymphocytes, heterophils and histiocytes. Immunohistochemically, neoplastic cells expressed vimentin but not cytokeratin, sarcomeric actin, Mac387 or BLA36. Cytokeratin was not detected in normal synovial cells. The radiographic, gross and histological findings were most consistent with synovial myxoma; however, because of the extensive involvement of the limb in the absence of confirmed metastatic disease, the term infiltrative synovial myxoma was applied., (Copyright © 2012 Elsevier Ltd. All rights reserved.)