When the vast majority of horse people hear that a horse has osteoarthritis, the first image that enters their brain is the old, stiff gaited, retiree horse that does nothing more than stand in a field and graze all day. After all, doesn’t osteoarthritis (OA) at minimum cause a dramatic reduction in a horse’s performance level, if not result in complete retirement? Unfortunately not only is osteoarthritis not just seen in the older horse, but it is an extremely complicated condition that both the horse professional and the veterinarian must deal with at some point in time.
Anatomy of a joint
To start to understand OA we must first think about the anatomy of a joint. It is important to remember that joints are more than just where the ends of two bones meet. There is the surrounding soft tissue to think about, there is the layer of bone under the cartilage, called subchondral bone, and there is the joint capsule. To make matters more complicated different joints have different shapes and these shapes can change throughout the full range of motion of a particular joint. All of these factors are important in starting to understand how OA may begin, and how it continues to progress. In all there are several different concepts of OA; these include the idea that the joint is flawed both in its mechanical properties and the development of the tissues in the joint, such as a horse with abnormal conformation and developmental joint disease like OCD. Another possible mechanism is that the tissue developed normally however there is abnormal mechanical properties applied to the joint, such as in the horse that develops an injury to the tissues supporting the joint. A third proposed mechanism is one where the mechanical properties of the joint are normal, yet the tissue develops abnormally. There are conditions in the human where the cartilage is not formed correctly during development which would demonstrate this proposed mechanism. Ultimately all of these mechanisms lead to the same overall theme, and that is the joint is not able to function properly and thus begins to sustain damage. In its most simplistic form OA is the result of damage (catabolic effects) exceeding the repair (anabolic effects) mechanisms of the joint. Something that is of note at this point is that nowhere in these different mechanisms have we mentioned the age of a horse! That is correct, OA is not only a disease of the older horse, but also in the young horse.
Instead of trying to help each of you gain a PhD in the pathophysiology of OA, let us more focus on the clinic aspect of this process. First and foremost something to remember about OA, in possibly your horse, is that OA is also known as degenerative joint disease. This is an important concept, because OA is a continual disorder that over time continues to cause degradation of the joint cartilage. Loss of hyaline (articular) cartilage is a major aspect of arthritis. Hyaline cartilage provides a very low friction system, thus allowing the joint to move in a smooth manner. As the cartilage degrades, then more friction builds up, which results in the production of inflammatory proteins which then further degrade the cartilage and the quality of the joint fluid. Is everyone starting to see the snowball getting bigger yet?
The next important clinical aspect of OA to remember is that accurate diagnosis can sometimes be difficult for a veterinarian. If you think back to the mention of anatomy earlier, there are more than just two bones involved in the joint. Typically when trying to diagnose the source of lameness, a veterinarian may recommend taking radiographs of a particular joint. Unfortunately radiographs do a poor job of examining the soft tissue structures involved in a joint. Therefore if the changes leading to the onset of OA are located within the soft tissue, meaning that no bone changes have occurred yet, and then the diagnosis may be missed, or diagnosing the horse with OA may cause confusion for the owner. The various types of tissues involved in the joint, and the varying degrees of joint degeneration also give rise to another source of confusion, and that is the number of different treatments available for a horse with OA. The treatments available for a horse with OA range from oral joint supplements, to intra-articular corticosteroids, to injectable drugs like Adequan and Legend, to regenerative therapies like IRAP, PRP, or stem cells. One of the difficult things about all of these treatments is that no single one of them is perfect for all cases of OA, and thus it is often recommended to use multiple types of therapy.
Progressive degenerative disease
As you can see OA is a very difficult disease process to understand and can be difficult to diagnose and treat. A horse can be predisposed to the development of OA for several reasons, such as poor conformation or trauma to the joint. The best way to evaluate your horse for its risk of developing OA, no matter its current age, is to have a good working relationship with your veterinarian. As a horse owner, you know your horse better than anyone else, and thus you are likely the first person to notice a subtle difference in their attitude or exercise efforts. When these things change in your horse, then that is a good time to involve your veterinarian in the situation. I have been asked many times, “Doc if I had brought my horse in earlier, then would things have been better?” Although the overall outcome may not change, the simplest answer to that question is always “yes”. Since OA is a progressive degenerative disease, then the earlier it is detected and treated, then the slower the degradation of the joint will occur, and the longer your horse will continue to remain comfortable.