Conditions of the Hock
Dr. Patricia J. Provost
Metalloproteinases: MMP- 1, 2, 3, and 9 actively degrade components of joint cartilage.
Arthritis of The Hock
Arthritis can occur in any one of the multiple joints that horses have but the lower joints of the hock, the tarsometatarsal and the distal intertarsal joints, are common sites. Arthritis develops in horses usually secondary to "repeated, cyclic trauma," or the wear and tear of everyday riding, racing, and training exercise, that inflames the joint capsule and synovial membrane lining. Arthritis also develops secondary to any single major external or internal trauma, such as a fracture of the joint, a sprain of the soft tissue surrounding the joint, a joint infection or osteochondrosis.
Inflammation of the joint is thought to be the first step in the development of arthritis in most horses. If not treated appropriately, the inflammation leads to early degeneration of the joint through degradation of joint cartilage.
The synovial membrane releases enzymes that are mediators of joint disease, causing breakdown of joint cartilage components:
Prostaglandins increase blood flow, enhance pain perception, cause bone demineralization and deplete cartilage proteoglycans.
Oxygen free radicals cleave proteoglycans, collagen, and hyaluronic acid.
Cytokines [interleukin 1 (IL-1)] function to stimulate the release of metalloproteinases.
The end result is a vicious circle of joint inflammation and cartilage degradation. Changes in the cartilage ultimately affect the ability of the cartilage to withstand compressive and tensile forces placed upon it during exercise. Cartilage develops fibrillation (fine cracks in the smooth surface), then partial and full thickness areas of cartilage loss.
The tarsometatarsal and the distal intertarsal joints are considered high load, low motion joints, meaning that they transfer and absorb concussive forces of locomotion but do so with very little movement. As cartilage is lost, bony changes occur and over a period of time the joint surfaces fuse (ankylose) together. Until they do, not only is the "shock absorbing" function of the joint less effective, but the horse experiences pain as bone impinges on bone.
In addition to arthritis of the hock, other conditions that can cause a hind limb lameness include but are not limited to arthritides of other joints, such as the hip, stifle, fetlock, pastern, and coffin joints; impingement of the dorsal spinous processes of the thoracolumbar spine; and polysaccharide storage disease.