The cause of this inflammation of the synovial membrane of the dorsoproximal aspect of the forelimb fetlock joints is unknown. It is suspected to be caused by repeated trauma of the thickened synovial plica in the dorsal region of the joint. Affected horses can be of any age. Bilateral involvement has been reported. The dorsal plicae are usually attached by a broad stalk to the dorsal portion of the dorsal proximal pouch of the fetlock joint, are firm and grayish white, and may be circumscribed or lobulated. Erosive bone lesions are typically associated with the mass and, in some cases, may extend to erosion of the articular surface. Microscopically, the lesions consist of dense, well-collagenized stroma lined by synovial cells. Vascularization is prominent, and hyaline change in the stroma and osseous metaplasia are occasionally seen.
Diagnosis can be suspected by palpation. Radiography can be used to identify associated osteolysis of the adjacent underlying condylar bone, and definitive diagnosis of the thickened dorsal soft tissues is made through ultrasonography and arthroscopy. Treatment is by surgical excision via arthroscopy.
Last full review/revision March 2012 by Stephen B. Adams, DVM, MS, DACVS; Andrew L. Crawford, BVetMed, CertES (Orth), MRCVS; James K. Belknap, DVM, PhD, DACVS; Jane C. Boswell, MA, VetMB, CertVA, CertES (Orth), DECVS, MRCVS; Peter Clegg, MA, Vet MB, PhD, CertEO, DECVS, MRCVS; Marcus J. Head, BVetMed, MRCVS; C. Wayne McIlwraith, BVSc, PhD, DSc, FRCVS, DACVS; James Schumacher, DVM, MS, DACVS, MRCVS; John Schumacher, DVM, MS, DACVS, MRCVS; Roger K. W. Smith, MA, VetMB, PhD, DEO, DECVS, MRCVS; Chris Whitton, BVSc, FACVSc, PhD; Jean-Marie Denoix, DVM, PhD, Agregé; Joerg A. Auer, DrMedVet, Dr h c, MS, DACVS, DECVS