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Chronic Proliferative Synovitis in Horses

(Villonodular synovitis)

By Matthew T. Brokken, DVM, The Ohio State University ; James K. Belknap, DVM, PhD, DACVS, Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Ohio State University ; Tracy A. Turner, DVM, MS, Anoka Equine Veterinary Services ; Jane C. Boswell, MA, VetMB, CertVA, CertES (Orth), DECVS, MRCVS, The Liphook Equine Hospital ; Peter Clegg, MA, Vet MB, DipECVS, PhD, MRCVS, Veterinary Teaching Hospital, School of Veterinary Sciences, University of Liverpool ; Marcus J. Head, BVetMed, Rossdales Equine Hospital and Diagnostic Centre ; James Schumacher, DVM, MS, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee ; John Schumacher, DVM, MS, Department of Clinical Sciences, College of Veterinary Medicine, Auburn University ; Chris Whitton, BVSc, FANZCVS, PhD, Equine Centre, University of Melbourne

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.