Equine joint structures and problems arising from their failure

Conformation has long been used by horsemen and women to determine the suitability of a horse for certain disciplines, and also its potential ability to withstand the rigors of competition. Few of the criteria used have been established by scientific research, as studying the moving horse requires sophisticated and expensive equipment. Results of studies conducted in Sweden revealed that when dressage horses were compared with show jumpers, the dressage horses had a shorter neck and tibia and a larger elbow angle. Although these traits do not necessarily guarantee success, a positive correlation has been found between hock angles and competitive success in both dressage and show jumping horses. This reiterates what riders and trainers already know, that good joints are essential in the performance horse. The junction between any two bones is known as an articulation or a joint. There are several types of joints. Some are fixed or practically immovable, such as those found between the skull bones. Others are freely movable and have articular cartilage and joint capsules. Their function is to provide a join between bones that can withstand high impact forces and allow the leg to "swing" freely with little resistance. The movement of a joint is controlled and limited by the action of muscles and passive structures such as ligaments and tendons.

Joint conformation

The area of bone that the cartilage attaches to is called the sub-chondral bone. The cartilage protects the bone by absorbing some of the force placed on the legs. Cartilage is also called hyaline cartilage, which has a smooth surface to prevent friction from occurring between the two bones. The cartilage has no nerve or blood supply and so depends on the joint or synovial fluid for nutrition. Synovial fluid is secreted from cells in the synovial membrane that surrounds the joint. The membrane has a nerve and blood supply and so can pass nutrients from the blood into the joint via the synovial fluid. Should the joint capsule become inflamed the nerve supply to the membrane will produce a sensation of pain and may result in lameness.

The structure of cartilage

The thickness of cartilage varies within an individual joint and between different joints in the body. As you would expect, it is thickest in areas that have to absorb the greatest forces of impact such as the knees and hocks. Cartilage consists of chondrocytes and a matrix, which is predominantly water (about 70 to 80%). Of the dry matrix material 50% is collagen, 40% is proteoglycans and the remaining materials are proteins other than collagen. Collagen fibres give the cartilage its tensile properties and are arranged more densely nearer to the joint surface. Proteoglycans are molecules that have a protein core and glycosaminoglycan side chains. The side chains have an electrical charge, which means that they repel each other but attract water, which gives the cartilage its stiffness and compressibility properties. The other proteins link the different molecules together. The role of the chondrocytes is to synthesise the components that make up the matrix.

The effect of joint loading on cartilage structure

When pressure is placed on cartilage, the proteoglycans are squeezed and push water out of the cartilage. When the pressure stops, the cartilage expands allowing synovial fluid back in. Proteoglycans are more concentrated in areas where the pressures are greater to allow more synovial fluid to infiltrate the cartilage. As synovial fluid is essential for supplying nutrients to the cartilage, it is apparent that the joint requires repeated loading to maintain the health of the cartilage. The cartilage responds best to regular or intermittent compressions and relaxations such as when the horse is roaming as he grazes. If the horse is standing in his stable for long periods the joints have to cope with continuous compression, which can be quite detrimental to the health of the cartilage. If immobilization is enforced on the horse for long periods, such as box rest or constant stabling (common in racehorses), the proteoglycan content of the cartilage will decrease. As long as the horse is brought back into work slowly and the loads enforced on the joint are gradually increased, the proteoglycan content should return to normal.

The effects of exercise on joint structure

The frequency and level of exercise that the horse endures will influence the cartilage structure. Repeated moderate exercise increases the proteoglycan content and causes a slight stiffening of the cartilage, which is considered to make the cartilage stronger. Strenuous exercise can injure the cartilage and result in a decrease in the proteoglycan content. Cartilage has a limited ability to repair and re-build due to the relatively inefficient nutrient supply. If damage occurs the cartilage surface can become gradually rougher as stress and general wear and tear take their toll.

Degenerative joint disease

Repeated concussion and trauma can result in a progressive degeneration of the cartilage, which is termed degenerative joint disease (DJD) or osteoarthritis (OA). The actual process of deterioration can vary but the initiating injury is nearly always as a result of repetitive over-loading of a joint that is not able to withstand the pressure. The deterioration of the cartilage releases molecules into the synovial fluid that cause inflammation of the synovial membrane. This creates a vicious circle as the inflamed membrane causes the release of substances that contribute to the deterioration of cartilage material.

Osteochondrosis

Osteochondrosis (OCD) is one of the developmental problems grouped under the general heading of Developmental Orthopaedic Disease (DOD) and is most common in youngsters from 6 months to 2 years old. Osteochondrosis is recognised as the failure of cartilage to ossify to create bone. The retained cartilage then starts to die off and, if stressed can break up. If the disease progresses to this stage it is termed osteochondrosis dissecans. Fragments of cartilage can remain loosely attached or break off completely when they are then referred to as "joint mice". If the cartilage deteriorates and cracks in the centre of the joint it can allow synovial fluid to enter, creating a bone cyst. These problems can cause the horse considerable pain and often require surgical treatment. There are many reasons put forward as to why horses suffer from osteochondrosis. Many believe that a genetic predisposition exists. Various studies have identified individual stallions that produce a significantly higher number of offspring with OCD although they may show no signs of the disease themselves. Currently, there is no way of identifying whether mares or stallions will pass on OCD to their progeny. Research is being conducted on young horses to determine the risk of being affected by OCD from different feeding practices. Nutrition is inextricably linked with growth and development so it is no surprise that there are many ideas as to how the horse's diet may induce developmental problems. High protein diets have been attributed as the cause of many developmental problems but this has not been proven in scientific studies. However, research has shown that high-energy diets have, resulted in an increased incidence of OCD. There is increasing evidence that horses that have an incorrect nutrient to calorie ratio are more affected than those that have a balanced nutrient to calorie ratio. This is more evident in young horses that are "pushed" for growth at a young age such as halter or led horses and yearling sales horses. The importance of a correct balance of minerals is increasingly being recognised as necessary in trying to prevent developmental disorders. Too much or too little of a particular mineral can be equally as catastrophic to bone and cartilage formation and so it is important not to consider minerals in isolation as many can interact, affecting their availability to the horse. It is clear that the factors that result in OCD are not yet fully understood. If OCD or any other developmental problem occurs, the typical panic reaction is to take all the concentrates out of the horse's ration and leave him on hay and water. If the problem has been caused by excessive levels of energy and / or minerals, then it is certainly necessary to reduce the horse's caloric intake. However, it is most important to establish what imbalances were present in the diet. The necessary adjustments can then be made to the diet to provide optimum levels, in order for the repair process to occur effectively and to also try and prevent the formation of other tissues from being compromised.

Damage to joints can occur in horses of all ages for a variety of reasons. The health of all the tissues in the body, including those found in joints, depends on the raw materials provided in the diet and the level of stress and wear and tear enforced on them. The harder you expect your horse to work, the more attention you should give to making sure his diet is right.