Good synovial health is essential for proper joint function.

A horse's joints are subjected to stresses, pressures, and cyclic trauma every day of the animal's life. The greatest stress occurs when the horse is moving rapidly, jumping, or sliding to a stop, but even when he is just walking around the pasture or paddock, some stress is placed on the joints of the legs.

As time goes on, these stresses are responsible for wear and tear that can cause joint problems. Often the problem involves highly important membranes within the joint that help contribute to frictionless movement. They are called the synovial membranes, and when they become diseased or injured, the condition is referred to as synovitis.

Whether synovial membrane problems occur in the front or the hind legs depends on the horse's activity. One reason veterinarians see forelimb synovitis is 60-65% of a horse's weight is carried on his forehand. Stress on the joints from concussion and exaggerated movement increases when the horse travels at speed. A racehorse, for example, puts tremendous stress on his forelimbs when traveling at a high rate of speed. The same can be true of the Grand Prix jumper who soars over fences that might be six feet high.

Other disciplines place tremendous stress on the hocks and stifles. A prime example is the cutting horse as it lowers its haunches and whirls in one direction or another to head off a cow that is seeking to escape. Reining horses, team penning horses, jumpers, and roping horses also are subject to special stresses on the hind limbs.

While synovial membranes can provide up to 50% of the frictionless motion, the absorption of concussion is provided by principally the subchondral bone, as well as the fibrous joint capsule.

How Joints are Joined

Before one can understand the role of the synovial membranes, there must be a basic understanding of joint construction and function. First, there are three types of joints, but only one of the three is at risk for injury and disease as the result of ongoing activity. Information for a description of the joints comes from a variety of sources, with emphasis on information made available by C. Wayne McIlwraith, BVSc, MS, PhD, FRCVS, Dipl. ACVS, director of Orthopaedic Research at Colorado State University's veterinary school.

The three types of joints are fibrous, cartilaginous, and synovial.

Fibrous joints are the least likely to be afflicted with disease because they are basically immovable. They include joints in the skull and those between the shafts of some long bones.

Cartilaginous joints are in much the same category as far as their potential for disease is concerned because they, too, have limited movement. These primarily are joints of the pelvis and vertebrae.

The synovial joints are the ones at greatest risk because they are the most active joints in a horse's body. A synovial joint consists of two bone ends covered by articular cartilage. The cartilage is so smooth and resilient that when properly lubricated, it allows for frictionless movement of the joint.

The joint structure is designed to absorb concussion and permit leg movement. The two bone ends are encased in a fibrous capsule that helps provide stability. Collateral ligaments attach to the sides of each of the bones within the capsule, and this attchment plays a major stabilizing role. The ligaments are comprised of very tough fibers. There are other ligaments within the joint, such as the cruciate ligaments, that also help to stabilize some joints; the stifle is one example. Ligaments outside the joint capsule also lend support. Prime examples include the distal sesamoidean ligaments and suspensory ligaments that, together with the sesamoid bones, make up the apparatus that holds the fetlock in proper position.

The outer part of the joint capsule is the fibrous layer, and the inner part consists of synovial membranes that line the sides of the joint capsule. The synovial membranes secrete fluid that lubricates the joint.

The end of each of the bones within the fibrous capsule is comprised of hyaline cartilage, which is flexible and somewhat elastic. Beneath the cartilage in each bone end is subchondral bone.

The synovial membranes are integral to the joint because they produce "joint oil" so there can be frictionless movement of the joint. In addition to being a lubricant, synovial fluid also supplies nutrients and removes waste from hyaline articular cartilage. The fluid has the consistency of raw egg white.

An important component of synovial fluid is hyaluronic acid, also known as sodium hyaluronate or hyaluron. The lubrication of synovial membranes is provided by hyaluronic acid, but lubrication of the articular cartilage is usually attributed to a different product called lubricin.

There is a second method for lubrication of the cartilage within the joint. Fluid is stored within the cartilage itself and when weight-bearing occurs, it is squeezed out of the cartilage and onto the surface. When weight-bearing ceases, fluid is reabsorbed by the cartilage.

Joint Breakdown

It becomes obvious at the outset that, if the joint is to function normally, there must be the correct balance of synovial fluid for proper lubrication. When disease or injury afflict a joint, that fluid balance often goes askew and problems arise. Quite often the main problem involves the synovial membranes, and the result is synovitis.

When there is injury or disease, inflammation results. One of the first reactions of the synovial membranes when that transpires is an increase in fluid volume and dilution of the hyaluronic acid. This can make the joint warm and puffy. Testing of the synovial fluid by a veterinarian might reveal that the additional fluid is more watery than normal and lacking in hyaluronic acid.

Adding to the problem is the fact that when there is injury to a joint, the resulting inflammation can trigger the release of problem agents known as free radicals, prostaglandins, cytokines, metalloproteinases, and aggrecans. These agents attack the basic components of articular cartilage and set in motion a degenerative process that can quickly compromise a horse's ability to perform, if left untreated. This is often accompanied by a decrease in hyaluronic acid that might have to be replenished either via direct injection into the joint or intravenously.

As stated before, synovitis can strike front or rear legs; much depends on the discipline with which the horse is involved. The forelimb joints most frequently afflicted are the knee, fetlock, and coffin joints. The rear limb joint that appears to suffer most frequently is the hock joint.

In all cases, correct conformation is highly important if a horse is to remain sound. Poor conformation exacerbates the negative effects on the joint from concussion. The joints are designed for straight-line movement and normally hold up quite well, but when a horse is over or under at the knee, for example, there is added torque and stress with each stride. The same is true if the knee is offset to one side.

When one examines the construction of the knee, it becomes obvious why it often is a part of the anatomy at risk for injury. There are eight bones in the horse's knee and three main joints to keep them all aligned and functioning properly. The eight bones are arranged in two neat rows. One of the three joints of the knee is located between the radius and the proximal (upper) row of four carpal bones; one is between the two rows of carpal bones (normally four bones in each row), and the third is between the distal (lower) row of carpal bones and the cannon bone and splint bones. There also are tiny joints between the individual carpal bones.

Any deviation from good conformation in this complicated structure can have a negative impact on its good health.

The other synovial joints are a bit less complicated, but good conformation is also a must if they are to remain healthy in an active horse.

Diagnosis and Treatment

Unfortunately, even well-constructed joints can be afflicted with synovitis and other problems due to ongoing wear and tear from vigorous activity over a period of time. As mentioned earlier, synovial membranes are key shock absorbers in each joint, and damage can result in synovitis.

The first indication of something awry is when the caretaker observes that a joint is puffy, warm, and tender to the touch. Often the first-line treatment of choice involves the application of cold water to the injured joint in an effort to reduce inflammation.

The next step is to consult a veterinarian. There are a variety of treatment protocols out there, and they often work best when administered in the early stages of joint disease or injury. Some of the treatment approaches have been around for a while, and others are in the "cutting-edge" stage.

Jerry Black, DVM, an associate veterinarian at Pioneer Equine Hospital in Oakdale, Calif., will guide us through these treatments. The approaches range from extracorporeal shock wave therapy (ESWT), to interleukin-1 receptor antagonist protein (IRAP), to injections of healing drugs directly into the joint. The former two approaches are the "new kids on the block," with joint injections having been around for a while.

McIlwraith reported on research in 2004 involving ESWT. It revealed there might be a place for the modality in treating joint problems, specifically synovitis.

ESWT is a noninvasive procedure that utilizes high-intensity acoustic sound waves to produce high-pressure waves that are focused at a small point of tissue or bone to stimulate healing.

Here is what McIlwraith and his co-workers had to say in part about the CSU research in the proceedings: "Treatment with ESWT reduced the clinical signs of pain measured by lameness evaluations. With injections of healing drugs directly into the joint, pain was even reduced 42 days after the last treatment, the last timepoint measured. There was, however, no significant improvement in response to flexion of the carpus. This suggests that the improvement in lameness was not caused by local desensitization of the region, or more specifically, the joint capsule. Concurrently, a parameter of synovitis --synovial fluid total protein--was significantly reduced, suggesting a possible mechanism for the treatment effect of ESWT. The results of this study suggest that ESWT is an effective method of reducing clinical lameness and synovitis, but does not significantly improve gross or histologic progression of arthritis. Therefore, it would be best considered in combination with a chondroprotective agent. Further work evaluating ESWT in clinical cases of joint disease is definitively warranted."

Black says that ESWT sometimes is used on hocks that haven't responded appropriately to intra-articular medication.

The other "new kid on the block," IRAP, has shown strong promise. IRAP, Black explains, competes with interleukin-1 (IL-1)--an enzyme that is considered to be a major joint inflammatory mediator--for the IL-1 receptor. In this fashion IRAP blocks the effect of IL-1, producing a significant anti- inflammatory effect. IRAP is harvested from the patient's own blood, says Black, and it is incubated for 24 hours in a syringe filled with chromium-coated beads. The centrifuged and treated product is a concentrated mix of IL-1 antagonist, protein growth factors, and other anti- inflammatory agents described as an "anti- inflammatory soup." Black says affected joints are usually treated three to five times at one to two week intervals.

Also a part of the new treatment wave for joint problems is regenerative stem cell therapy. There are products on the market, with more research under way.

Injecting a healing drug directly into the joint remains a treatment of choice in many cases when synovitis and other problems are involved, Black says. The drugs recommended are designed to reduce inflammation and to increase the production of lubricants by the synovial membrane. The drugs most often used for treatment of noninfectious joint problems via injection, says Black, include sodium hyaluronate, polysulfated glycosaminoglycans (PSGAGs), and corticosteroids.

A synthetic form of sodium hyaluronate is Legend, which can be injected directly into the joints or given intravenously. Sodium hyaluronate provides significant boundary lubrication to the synovial membrane, Black says, and it also combats inflammatory agents.

PSGAGs combat elements within the joint that cause inflammation and also stimulate the production of hyaluronic acid. Adequan is a popular PSGAG that can be injected directly into the joint or intramuscularly.

Research has shown that certain corticosteroids such as betamethasone esters and triamcinolone acetonide cause no harm in the joint and are beneficial. On the other hand, methylprednisolone acetate (Depo-Medrol) is harmful.

Take-Home Message

The synovial membranes are key elements in a horse's joints, and when they are compromised, immediate steps should be taken to return them to good health.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at or by calling 800/582-5604.

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