Bog spavin is the horseman's term for distension of the tibiotarsal joint capsule, the upper most joint of the hock and the joint most responsible for the flexing motion of the hock. Bog spavin represents an overproduction of joint fluid in the acute phase and a distended joint capsule that has lost its elasticity in the chronic phase. The initial cause of the joint capsule distension is inflammation of the joint capsule and called "synovitis". Though often just a cosmetic problem, it should be understood that bog spavin may also accompany joint pathology like osteoarthritis that may contribute to any lameness and influence treatment and the long term prognosis. This article discusses the different causes, diagnosis, treatment, and prognosis of this condition of horses.

Bog spavin is the horseman's term for distension of the tibiotarsal joint capsule, the upper most joint of the hock and the joint most responsible for the flexing motion of the hock. "Boggyness" appears in three distinct areas where the joint capsule is large and superficial. The swelling is concentrated in the front of the hock and just behind the tibia, the long bone that feeds into the hock from the top, on each side. Bog spavin makes the majority of the upper hock look big. Note that the swelling behind the tibia can resemble throughpin but throughpin will not have the anterior (front) swelling...more information and images.