Merck Manual

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Professional Version

Chronic Proliferative Synovitis in Horses

(Villonodular synovitis)


Matthew T. Brokken

, DVM, DACVS, DACVSMR, Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University

Reviewed/Revised Sep 2015 | Modified Oct 2022

Proliferative synovitis is the enlargement of the fibrocartilaginous pad on the dorsoproximal aspect of the joint capsule attachment of the fetlock joint. The cause of this inflammation is thought to be from repetitive trauma from exercise. Typically, this condition is found most frequently in racing Thoroughbreds, but it may also develop in Standardbreds and nonracing breeds. Clinical signs include fetlock joint effusion, firm swelling over the dorsoproximal aspect of the fetlock joint, lameness, and decreased range of motion and a positive response to firm flexion of the fetlock.

Diagnosis can be suspected by palpation. Radiography can be used to identify associated osteolysis at the proximal aspect of the dorsal mid-sagittal ridge of the distal third metacarpal bone on the lateromedial projection. The radiolucency is a result of the damage to the cortical bone from the overlying fibrous mass. Ultrasound examination can also be performed, and the synovial pad is considered abnormal if it is >4 mm thick, has rounded distal margins, or if hyperechoic regions are found within the pad. Treatment is surgical excision via arthroscopy.

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