This is a common cause of hindlimb lameness.
The horse’s hock is equivalent to our ankle. It consists of several small bones and 4 joints. The largest joint is called the tibio-tarsal joint and this is a high motion joint and it allows the leg to bend at that hock. The 3 other joints (proximal intertarsal, distal intertarsal, and tarso-metatarsal) in the horse’s hock do not move. These joints are very narrow and act as shock absorbers. They absorb the impact as the horse moves. As the horse ages these joints are subject to degeneration and the cartilage lining the joints can wear away. This results in bone on bone contact which is painful. Degeneration typically involves lysis or breakdown of bone and the stimulation of new bone growth in a haphazard fashion (resulting in osteophyte or spur formation) . This therefore results in changes that can be seen on X rays. However, it is important to remember that horses can be very painful with relatively mild radiograph changes, or alternately sound with relatively severe radiograph changes.