DJD, also known as osteoarthritis, or literally, inflammation of the joint, is a painful, debilitating, chronic progressive joint condition. Estimates suggest that it is the single most destructive disease of the equine economy. Equine joints are highly susceptible to DJD and in this article I will explain how joints work, and what goes wrong.
The Normal Joint
The joint is a complex environment, all the components relying on each other to remain healthy. The joints are physically held in place by fibrous bands known as ligaments, and the joint capsule, a fibrous case that holds the joint contents. The joint capsule is lined with specialized cells known as synoviocytes, special cells that secrete synovial (joint) fluid. This is rich in complex sugar-protein molecules such as hyaluronic acid, which gives the fluid a thick, oily lubricant action. Joint fluid also contains simple ‘building blocks’ such as glucose, amino acids and oxygen, to provide nutrition and repair for the cartilage cells, which do not receive blood supply, and rely entirely on the synovial fluid for nutrition.
The pressure in healthy joints is slightly below atmospheric pressure, and this causes air bubbles to form which causes the joints to click. So, if a horses joints click occasionally, their joints are probably fairly healthy. Joints clicking excessively, however, can also be a symptom of other problems. The subchondral base plate, is the surface of the end of every bone, and forms a hard, and strong plate for cartilage to grow on. It is always just right in the normal joint, but in the osteoarthritic joint, it’s thickness varies, and this has a negative effect on the cartilage.
Each joint component is critical to the success of the entire system; Which joint component fails first becomes of secondary importance, as when one fails, so do all the others. Therefore, the primary concern is stopping, or attempting to reverse the damage. Even better, prevent it in the first place! When something becomes inflamed, white blood cells are leaked into the joint capsule, and while clearing invaders, they also begin to damage the joint components, such as the synoviocytes. These then cannot produce normal joint fluid, and this causes the joint structures to suffer malnutrition, and the cartilage fails to repair itself, causing wear and tear and pain, with the inflammation chronically worsening.
Now, a vicious cycle has started, and the joint is irrevocably damaged. Lameness is usually the first sign and positive pressure in the joint can result in effusion (windgalls). Some horses actually get effusion on the back of the fetlock for a reason unrelated to DJD, however, if lameness and effusion are noted, you should call your vet. The subchondral base plate, as said before, is the surface of the end of every bone, and forms a hard, and strong plate for cartilage to grow on. If the bone is too thin, the cartilage collapses, but if it is too thick, the cartilage cracks, and is damaged by concussion. In the normal, undamaged joint, this is just right, but when osteoarthritis sets in, this can thicken, or become too thin. As the cartilage wears, the joint grows bone chips in unsuitable places, usually the edge of the joint, in an attempt to stabilize the joint, and prevent wear and tear. Ironically, however, this increases stiffness, and indeed, cartilage damage!
Diagnosis can be by X-ray – but if DJD can be seen on x-ray, the condition is well advanced, and the long-term prognosis becomes poorer. Assessment of the symptoms is also a form of diagnosis.
To prevent the joint degenerating, osteoarthritis needs to be diagnosed, and treated quickly and aggressively. Some cases require anti-inflammatory medication and rest, while others require surgical treatment, for example, the removal of joint chips, or joint support, using screws.
Some horses can be treated with phenylbutazone (‘bute) or suxibuzone (a non steroidal anti-inflammatory drug (NSAID) that turns into ‘bute inside the horse’s body) and rest.
If the case is more severe, talk to your vet to establish an alternative treatment method.
Prevention This is definitely a case of prevention is better than cure – using a recommended and tested joint supplement helps to provide the joints with necessary nutrition – this is useful for all horses, especially older horses, or those in competition.