The hock is a complex joint of the rear limb situated between the stifle and the ankle that consists of six bones and four joints. Similar to the human ankle–but elevated and bending backward–the hock works in concert with the stifle, flexing and extending together with the stifle to achieve rear end propulsion. As the pivotal hind limb joint, the hock receives considerable forces. It’s no surprise, then, that this hard-working joint is the most common site for rear limb stress injuries, performance-limiting problems, and pain.

As inflammatory changes develop, normal hock structure and function are interrupted. The joint capsule can swell or a bony enlargement can develop on the inside of the hock, signaling inflammation of the joint capsule. Eventually, degeneration of the joint cartilage and remodeling of the underlying bone can occur.

The two most common hock disorders are distal tarsal osteoarthritis (bone spavin) and tarsocrural effusion (bog spavin).

Bone Spavin

Distal tarsal osteoarthritis–also known as spavin, bone spavin, jack spavin, blind spavin, juvenile spavin, and occult spavin–is a common cause of hind limb lameness in performance horses and is probably the most common cause of hock lameness or pain in all horses, says Julie Dechant, DVM, MS, Dipl. ACVS, an equine emergency clinician at Oklahoma State University. “Spavin typically refers to problems in the narrow lower joints in the hocks,” she explains. “These joints do not appear to contribute to much motion in the hock joint–the majority of motion originates from the large upper joint–but they can be a significant source of pain when inflamed or arthritic