By Dr. Crystal Lee, DVM, DACVS

A healthy joint in the limb of a horse provides a frictionless system and facilitates movement with ease. Each joint depends on the function of each of its components to serve this purpose in an equine athlete. Bone, articular cartilage, synovial fluid, synovial membrane, fibrous joint capsule, and ligamentous structures make up these components. 

Subchondral bone provides contour and stability to the articular cartilage, which constitutes the joint surface. In conjunction with the synovial fluid, it is the articular cartilage that allows for frictionless movement, and it is typically the state of the cartilage that is used to determine joint health. Articular cartilage does not have a blood supply, and as such it depends on the synovial fluid in the joint to provide its nutrition. About one to twelve percent of the cartilage is made up of chondrocyte cells, and the remainder is an extracellular matrix composed of collagens (which provide structure), proteoglycans (which provide resistance to compression), and water. Proteoglycans are a combination of protein and glycosaminoglycans such as chondroitin-4-sulfate.

The synovial fluid in a joint is produced by the synovial membrane, which lines the joint cavity. This synovial membrane also clears unwanted particles from the joint and secretes proteins that contribute to joint health such as hyaluronan and lubricin. The fibrous joint capsule surrounds the synovial membrane, and is supported by the periarticular ligaments.

The volume of the space in the joint varies with the location, and because of the elastic nature of the joint capsule it can change depending on level of exercise and disease. Horses working at a higher level often will have synovial effusion (an increased amount of fluid in the joint) in certain joints such as the fetlock and hock without signs of disease being present. When a joint moves and pressure is applied to the surface of the cartilage, fluid is squeezed from the surface of the cartilage and creates a wedge of fluid that separates the surfaces.

Osteoarthritis refers to a disorder of movable joints characterized by degeneration and loss of articular cartilage. It may result from a response of the joint components to exercise, to repetitive trauma, or to a single injury. In a young horse with normal, healthy cartilage, osteoarthritis is predominantly trauma related. It can result from overuse (repeated cycles of athletic trauma), fractures that involve the joint, ligamentous injury and loss of stability of the joint, or remodelling and microfractures of the bone underlying the articular cartilage. In contrast, degeneration can occur from normal forces on abnormal cartilage. This occurs in joints with the normal wear of articular cartilage that comes with aging, in joints affected by osteochondrosis (which is the failure of the bone that underlies the articular cartilage to mineralize properly), or on cartilage that has been damaged by synovitis (inflammation of the synovial lining).

Osteoarthritis can affect horses of any age and accounts for approximately 60 percent of lameness in horses. It may result from exercise, repetitive trauma, overuse, or a single injury. In a young horse with normal, healthy cartilage, osteoarthritis is predominantly trauma related. Photo: Dreamstime/CustomPosterDesigns

Osteoarthritis can be defined as the point at which the repair processes of the joint are overwhelmed by the destructive processes. It can affect horses of any age, and is thought to account for about 60 percent of lameness problems in horses. There is a continuum of disease severity that occurs in osteoarthritis, and it is likely that most of the components of the joint are involved. There is generally no central inflammatory role in the osteoarthritis that occurs in horses; this is similar to many types of osteoarthritis in humans, but is very different from the human rheumatoid arthritis, which does have a central inflammatory role.

As osteoarthritis takes hold in a joint, whether it be from a single incident of injury or chronic injury over time, inflammatory mediators are released from the synovial membrane. These inflammatory mediators may include prostaglandins, cytokines, and matrix metalloproteinases, and they contribute to the degradation of the cartilage. The synovial membrane becomes edematous and swollen and increases the amount of blood vessels present, which can increase the amount of joint fluid (referred to as effusion) and the amount of protein in the joint fluid. The inflammatory mediators contribute to the degeneration of the articular cartilage, making it less able to withstand normal loads. 

The joint capsule, synovial membrane, tendons, ligaments, and bone will all contribute to the pain the horse feels in osteoarthritis. Injury to the joint capsule causes it to form a fibrous repair tissue, which is not the same as the original tissue, and can lead to a decreased range of motion for the joint. Sclerosis (an increase in bone density), osteolysis (bone destruction), new bone production, a narrowing of the joint space, and bone fragments can be seen on radiographs in many cases. However, the level of pain and the degree of radiographic change do not always correlate - a significant number of horses will show pain despite relatively normal radiographs. This makes sense when you consider the number of soft tissue structures (which are not imaged well with radiographs) that can be involved. Clinically, an increase in synovial fluid (effusion) will often be seen, and the synovial fluid itself becomes less viscous (it is thinner and more watery).

In order for your veterinarian to best be able to treat your horse, an accurate and specific diagnosis is important. Arriving at this diagnosis may involve a lameness exam that includes diagnostic nerve blocks or joint blocks in which a local anesthetic is used to remove sensation from a region of the limb in order to gauge improvement and thereby localize the source of pain. Once the painful joint is determined, radiographs may be taken to determine the extent of change to the bones. The goals of treating osteoarthritis are to decrease pain and lameness and minimize the progression of the deterioration of the joint. Although osteoarthritis is not curable, it can be managed. Treatment is possible to manage pain and discomfort, control swelling, prolong athletic function (whether it be racing, jumping, or simply trail riding), and maximize the horse's lifespan.

During a lameness exam, the veterinarian is performing a flexion test of the distal limb. Inflammation in the coffin, pastern, or fetlock joint would increase the lameness visualized after this flexion. Photo: Burwash Equine

One of the mainstays of the medical treatment of osteoarthritis for many years has been nonsteroidal anti-inflammatory drugs (NSAIDs) such as phenylbutazone (Bute). These medications inhibit the production of prostaglandins, leading to pain relief and decreased inflammation. If these medications are used intermittently - for example, if the horse is ridden only occasionally - the more traditional non-specific medications such as phenylbutazone are used quite frequently. If more chronic use is desired, newer medications such as firocoxib are available that are more specific and therefore less likely to cause side effects such as stomach ulcers, kidney damage, and colitis that are of concern with more prolonged NSAID use.

The specific joint that is affected with osteoarthritis can also be injected with various medications. Corticosteroids are potent anti-inflammatory medications, and are the most traditional component of joint injections. They have been used for many years to effectively treat osteoarthritis in horses. There has been some concern over their use due to several research studies showing potentially detrimental effects on cartilage through the use of corticosteroids. However, this research was performed using normal cartilage, and more current research studies suggest that joints inflamed with osteoarthritis do not exhibit these detrimental effects. The most commonly used corticosteroids are triamcinolone and methylprednisolone. Horses are typically rested for three to five days after these injections, and then return to work.

Corticosteroids and hyaluronic acid are being injected to treat mild lameness due to osteoarthritis of the stifle. Photo: Burwash Equine

Hyaluronan is another medication that is often included in joint injections, either alone or in combination with corticosteroids. It is a normal component of synovial fluid that increases its viscosity and helps with lubrication. Administration into a joint is thought to have an anti-inflammatory effect, and it may also increase the production of hyaluronate by the joint itself. Hyaluronan may also be administered intravenously to have an effect on the whole horse rather than one specific joint (marketed as Legend®). Although the mechanism of action of this method of administration is unclear, improvements have been documented in horses.

Polysulfated glycosaminoglycan (marketed as Adequan®) is another medication that may be included in joint injections or injected intramuscularly. It is thought to inhibit many of the inflammatory mediators that are released into the joint in osteoarthritis. Pentosan polysulfate is an intramuscular injectable product that is thought to reduce damage to articular cartilage (marketed as Cartrophen®). Both of these medications are not readily available in Canada at this time. Oral supplements are also commonly used. Most of the supplements marketed to combat osteoarthritis contain chondroitin, glucosamine, or hyaluronan. Although there is limited concrete research supporting the use of these supplements, they do promote improvement in some horses. There is also some research to support the use of green-lipped mussel extracts in horses with osteoarthritis, and these are included in some supplements.

More recent research is focusing on the use of biological therapies to treat osteoarthritis. Interleukin-1 receptor antagonist protein (IRAP) has been shown to both reduce symptoms and modify the disease course of osteoarthritis. It involves drawing approximately 50mL of the horse’s blood into a specialized syringe that stimulates the production of the antagonist protein as it is incubated for 24 hours. The plasma containing the protein is then separated from the blood cells in a centrifuge, and is injected into the joint. It works by preventing the binding of the interleukin-1 (which is one of the cytokines that lead to cartilage damage) to its receptors in the joint, thereby blocking this damage and inflammation. There is also ongoing research into using platelet-rich plasma or mesenchymal stem cell therapy in the treatment of osteoarthritis.

Arthroscopic surgery provides a way to introduce a camera into the joint and visualize the damage that is present. It can be used as a diagnostic tool, to visualize problems that could not be seen on radiographs or ultrasound, and it can be used for therapeutic purposes, such as the surgical removal of small bone and cartilage fragments from the joint. In general, arthroscopy is of more benefit early on in the course of disease. Removing a fragment before osteoarthritis has a chance to develop has a far better outcome than waiting until after the fragment has been present and causing damage in the joint for some time.

These are intraoperative arthroscopic images showing smooth articular cartilage without any evidence of inflammation. Photo: Burwash Equine

Surgical joint resurfacing is also possible to treat a localized area of cartilage loss in a horse without ongoing joint degeneration. One method of joint resurfacing is to debride the lesion down to the level of bone so that it is stimulated to repair the cartilage. The other method, which is used in people, is to use osteochondral grafting to transplant tissues into the cartilage defect; this technique is still being actively researched in horses.

A fetlock chip (osteochondral fragment) indicated with arrow in the attached image. These fragments arise from hyperextension of the fetlock joint. They cause synovitis, cartilage degeneration, and the development of osteoarthritis. Lameness may be mild to moderate. Photo: Burwash Equine

In cases where destruction of a joint is beyond any other treatment, it is possible to assist the joint to fuse. This is termed arthrodesis of a joint. In low motion joints such as the pastern and the distal hock joints, the goal is for horses to return to their athletic career after this procedure. In these joints, arthrodesis may be performed using chemical techniques such as injection of alcohol, laser-based techniques, or surgical techniques. In high motion joints such as the carpus, fetlock, or coffin joints, surgery is required to perform arthrodesis. The goal of the procedure in these joints is to remove the pain associated with movement of the joint and allow the horse to be comfortable in the pasture.

Overall, it is important to recognize that any horse of any age can develop osteoarthritis. Although it invariably is a progressive disease that will worsen over time, there are some viable options for slowing that progression. Working with your veterinarian to develop an individual treatment plan for your horse will allow the best possible outcome for the animal’s long term athletic soundness and comfort. 

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