Joints Part 2: Joint Disease
Among the most highly engineered, finely tuned machines built by man are race cars that zip around the Indianapolis 500 track at speeds in excess of 200 miles per hour. However, even the most perfectly constructed car will develop problems from continued competition. It is much the same with the performance horse and his joints. Even with perfect construction or conformation (which is rare), there still is daily wear and tear that destroys parts or joints. The horse's joints are designed to effectively absorb shock, allow for frictionless movement, and effectively bear the weight of a body that often weighs 1,200-1,500 pounds or more.
When equine joint injury occurs, you can turn to sophisticated specialists who have devoted their careers to repairing the damage, just as race car drivers can turn to top mechanics. Unfortunately, the equine specialists do not have the luxury of spare parts.
Veterinary science has provided medical and surgical tools for helping repair equine joints, but there will always be the limitation of having to work with what's there.
In this article about joints and the forces that damage and destroy them, we will examine the types of joints so that we understand the terminology and anatomy. Next we will look at the forces generated by various forms of competition and diagnostics. Finally, we'll look at treatment.
A Look Inside
We begin our discussion with a description of joints. There are three different types or classifications of joints--fibrous, cartilaginous, and synovial.
Fibrous joints are the least likely to be afflicted with disease because they are pretty much immobile. They include joints in the skull and those between the shafts of some long bones.
Cartilaginous joints don't have a high propensity for disease because they, too, have limited movement. These are the joints of the pelvis and vertebrae as well as growth plates, which extend a bone's length during the horse's growing years.
That brings us to synovial joints, the ones most likely to suffer disease and injury because they are the most active joints in the horse's body. They consist of two bone ends covered by articular cartilage. It is this cartilage within the joint that is so smooth and resilient that, when properly lubricated, it allows for frictionless movement of the joint.
Of course, you can't just have two bones covered with smooth, resilient material. You need something to hold the whole thing together and lubricate it. The joint's stability is maintained by a fibrous joint capsule--which attaches to both bones--and collateral ligaments. The collateral ligaments are located on either side of most joints. They are key components of the fetlock, knee, elbow, hock, and stifle joints.
Other ligaments within joints--such as the cruciate ligaments in the stifle--help stabilize some joints.
Ligaments outside the joint capsule also lend support. A prime example is the distal sesamoidean ligaments and suspensory ligaments that, together with the sesamoid bones, make up the suspensory apparatus and hold the fetlock in its correct position.
An enemy of joint health is friction. What is needed to prevent friction? Lubrication. And where does this lubrication within a joint come from? The joint capsule contains an inner lining called the synovial membrane. This lining secretes the synovial fluid that lubricates the joint. A key ingredient in this fluid is hyaluronic acid, also known as sodium hyaluronate or hyaluronan, which lubricates the synovial membrane. Another substance in synovial fluid, a protein called lubricin, is the primary lubricant of the cartilage. In some cases of joint disease there is a depletion of this necessary fluid.
For a quick look at the synovial joints, we will start with the forelimbs. The forelimbs of the horse bear 60-65% of the animal's weight and are thus subjected to greater concussive effects than the rear legs when a horse is moving at speed. Of course, when some athletes such as cutting and reining horses are involved in competition, the heavy stress shifts to the rear limbs.
The knee (carpal joint) is composed of three main joints and numerous ligaments that keep everything in its proper place--when all is working well. There are normally seven individual bones in the knee that are arranged in two rows.
The knee rests on top of the cannon bone or third metacarpal, which is flanked on either side by splint bones, which are the second and fourth metacarpals. (For more on splint bones, see "What's a Splint?") Resting on top of the knee structure is the radius. Correct conformation is necessary for this joint to function at its peak and remain sound.
Just think of the stress on this complicated structure if the horse is over or back at the knee. Inappropriate conformation puts undue stress on the structures even at the walk. The stress is greatly magnified if the horse with poor conformation is traveling at speed, performing athletic maneuvers, or going over jumps.
The lower end of the cannon bone connects with the long pastern bone (also known as the first phalanx or P1) at the fetlock joint. One of the key jobs of the fetlock joint is to absorb shock.
Next is the joint where the long pastern bone connects with the short pastern bone (second phalanx or P2). This is the pastern joint and it, too, is a shock absorber.
Then there is the coffin joint, which is composed of the second and third phalanges (P3 is also known as the coffin bone) and the navicular bone. There is a great deal of elasticity and motion in this joint, as well as shock absorbing capability.
Other integral parts of the front limbs are the shoulder and elbow joints. However, by the time concussion reaches them, its effects have been dissipated very effectively by the other joints, so the shoulder and elbow joints aren't as prone to concussive injury and disease.
The two key synovial joints in the rear limbs that are different from those in the front limbs are the hock and stifle joints. The hock or tarsal joint joins the tibia with the metatarsal bones. The horse's hock joint is a bit like its knee in that it is composed of a number of bones. There are four separate joints in the hock, with only the top one providing significant motion. Like the knee joint, the hock joint is held together by a complex set of ligaments.
The other joints from the hock down--pastern, fetlock, and coffin--function similar to their counterparts in front. Moving upward from the hock, we come to the stifle joint, which is the horse's largest synovial joint and is analogous to the human knee. However, unlike the human knee, there are three separate joint compartments.
Above the stifle joint is the hip joint, which is of ball and socket construction and is stabilized by strong bands of ligaments. Here the upper end of the femur fits into a socket on the pelvic bone.
Each joint is stabilized by a complex network of tendons, ligaments, and muscles. When all is well, this complex network enables a joint to function in a smooth, synchronized fashion. However, when any part of the network malfunctions because of injury or disease, repair via medical treatment might be necessary.
Causes of Joint Problems
Horses have joint problems because we often ask them to do things they weren't designed to do, says Jerry Black, DVM, senior partner and president of the Pioneer Equine Hospital in Oakdale, Calif., and president-elect of the American Association of Equine Practitioners.
After domesticating the horse, man designed competitions for him that put a great deal of additional stress on his joints. Think, for example, of the concussive force on joints of the front legs when a 1,200-pound horse sails over a six-foot jump and lands on his front feet.
Dressage seems like a fairly benign competition as far as placing stress on joints is concerned, but that isn't true. The advanced dressage horse is required to move his center of gravity more to the rear, putting more stress on the hind limbs. Some of the lateral movements, such as the shoulder-in and half-pass, cause high joint stress particularly on the hock. The types of disease and injury that can afflict dressage horses include degenerative joint disease of the hocks, inflammation and degenerative joint disease of the front pasterns, inflammation of the middle knee joint, and degenerative joint disease and inflammation of the fetlock.
Many English show horses also tend to shift their center of gravity to the rear, thus placing more stress on the hind limbs (especially the hock and pastern joints). The goal with some of these show horses is to travel with high front-end action. This is particularly true of the Tennessee Walking Horse in competition. Horses which load more weight on the rear are going to be prone to hock, rear fetlock, and stifle injuries and disease.
With the jumper, there is great stress on the hind limb joints on take-off and on the entire forelimb suspensory apparatus on landing. In addition, the show jumper is often asked to complete one jump, then make a sharp turn to line up for another. This places severe stress on the hocks. Sometimes the stress placed on the joint ligaments of the jumping horse causes inflammation and lameness.
Western horses also are stressed with competition. There is a lot of torque on the rear joints when a cutting horse drops its hindquarters toward the ground and spins a split second before accelerating to stop the movement of the calf it is seeking to hold away from the herd. Some cutting horses are susceptible to injuries and disease involving the hock and stifle joints.
The reining horse is asked to run down an arena at speed, slide to a stop, and spin in a circle, with the rear end anchored in place. This produces a great deal of torque on the hind limbs, especially the hocks.
Roping horses also put heavy pressure on their joints. The calf roping horse is asked to slide to a stop as the loop settles over the calf's neck. The sliding stop and the jerk from the calf hitting the end of the rope put stress on the hock and pastern joints.
The header's team roping horse is asked to swing sideways, pulling a steer into position for the heeler to rope the hind feet. The header's horse places added stress on his lower forelimbs, especially the left, while turning the steer.
A barrel racing horse speeding through the cloverleaf course places severe stress on the joints of his front and rear limbs. There is often a compounding of problems with barrel racers, Black says, because in some cases the horses were retired from the racetrack and bring with them problematic front knees, front fetlocks, and front suspensory apparatus.
Western pleasure horses which travel sedately and slowly around the ring might also be prone to joint disease because of their conformation, Black explains. To accentuate a chosen way of going, he says, many Western pleasure horses have been bred and selected to have straighter shoulders and more upright pasterns than horses which perform at speed. This type of conformation can set the stage for poor shock absorption and thus joint disease.
Breeding practices also are implicated in some joint problems of horses which perform at speed. Cutting horse breeders, for example, often line breed to make certain that the horse has "cow sense." This, however, has the potential for compounding genetic joint problems when conformation isn't also taken into account.
And the Problems Arise
When joints suffer trauma, Black says, enzymes and other agents from the joint lining are released that destroy tissue inside the joint, especially articular cartilage (which covers the joint surface of the bone). The result is traumatic arthritis.
"Traumatic arthritis," Black says, "has been defined as the diverse collection of pathological and clinical states which develop after single or repetitive episodes of trauma. The components of traumatic arthritis may include inflammation of the joint lining such as synovitis (inflammation of the synovial membrane) and capsulitis (inflammation of the fibrous joint capsule); injury to the supporting ligaments of the joint (sprain); and fractures to the bones within the joint."
Traumatic arthritis comes in three forms:
Type 1--Synovitis and capsulitis without disturbance of articular cartilage or disruption of major supporting structures. This includes acute synovitis, capsulitis, and most sprains.
Type 2--This is caused by disruptive trauma damaging the articular cartilage or completely rupturing major supporting structures. This includes severe sprains, intra-articular fractures, and meniscal tears. (The meniscus is cartilage that lies between the weight-bearing surfaces of the joint.)
Type 3--Post-traumatic degenerative joint disease occurs when there is residual damage after initial trauma. Type 3 traumatic arthritis can lead to deformity, limited range of motion, or joint instability.
Equine joint disease and associated lamenesses are the most common athletic injuries seen in performance horses today, Black declares . "The pathological effect of trauma on joints is to cause synovitis and capsulitis, which, in turn, creates physical and biochemical damage to the articular cartilage," he says.
Wayne McIlwraith, BVSc, PhD, Dipl. ACVS, of Colorado State University (CSU), president of the American Association of Equine Practitioners adds, "Synovitis and capsulitis cause the release of (the enzymes) metalloproteinases and aggrecanase, prostaglandins, free radicals, and interleukin-1." The resulting tissue inflammation creates pain, effusion, and reduced range of motion.
With some horses, joint injury and disease occur after years of stress. These would be the horses with correct conformation whose joints were strong and supple enough to withstand competitive trauma for a long time before weakening. With others, joint injury and disease might occur after only a single traumatic incident. These might be the horses with conformation anomalies that predispose them to problems.
Clinical signs of joint disease include lameness, swelling, excessive synovial fluid, pain on flexion, and heat. However, these signs alone don't tell you what joint structures are affected or how badly they are damaged. A detailed examination is needed to determine a horse's exact problem, treatment regime, and prognosis for future athletic soundness.
"The clinical examination is always the starting point," says Mark J. Martinelli, DVM, PhD, Dipl. ACVS, Associate Surgeon (Orthopedics) at the San Luis Rey Equine Hospital in Bonsall, Calif. "Palpation of the musculoskeletal system will detect the aforementioned signs of joint disease, such as heat, swelling, and pain. The physical examination is usually followed by a lameness examination in order to identify which limb or limbs are affected. This phase may be carried out in hand, on a longe line, or even under saddle. In addition, flexion tests of joints may be carried out in an effort to localize the soreness further.
"Once a gait deficit has been identified in a specific limb, regional anesthesia is often performed to pinpoint the exact location of the lameness. Regional anesthesia involves injecting a local anesthetic agent into a joint or surrounding nerves that supply a specific joint. If the gait deficit or lameness disappears, then the definitive location of the problem is determined. For instance, if lameness in the left forelimb disappears after injecting local anesthetic into the fetlock joint of that limb, then the fetlock joint is the location of the problem."
Next, Martinelli radiographs the affected joint. "Radiographs highlight the bone abnormalities seen with joint disease, such as osteophytes (bone spurs), sclerosis (denser than normal bone), lysis (softer than normal bone), and joint space narrowing," Martinelli continues. "These changes usually take time to develop, and therefore may not be present in the early stages of joint disease. When they are visible, however, radiographic changes clearly indicate that the joint is not normal. Conversely, nuclear scintigraphy (bone scan) is a sensitive imaging modality that will detect early changes in the bones around a joint. Because it is a metabolic imaging tool, it relies on living and changing properties of bone to produce an image. Therefore, even the most subtle bone abnormalities can be detected. Numerous studies in humans and other species have implicated hardening of the subchondral bone (bone beneath the cartilage of a joint) as the earliest stage of joint disease. If this is also true in the horse, then nuclear scintigraphy is clearly the imaging modality of choice to identify these incipient signs."
Although routinely employed in the diagnosis of joint disease, neither of these imaging modalities can help visualize the articular cartilage. "The challenge with traumatic arthritis is to determine the degree of cartilage damage," states McIlwraith. "Currently, the only way of completely assessing the cartilage in early osteoarthritis is with diagnostic arthroscopy (inserting a small instrument to visually inspect the joint). However, at CSU investigators have recently developed markers that are antibodies specifically made against breakdown products in the articular cartilage. Their usefulness in diagnosing early cartilage damage (which means early specific treatment) looks very promising."
One of the most successful approaches in treating joint disease, says Black, is with intra-articular medication--the direct administration of a drug into a joint. If done correctly, he maintains, intra-articular injection is a safe way to treat joint disease.
Black underlines the safety aspect by saying, "In a recent six-year period, 6,465 injections have been conducted at Pioneer Equine Hospital with no infections or significant joint reactions."
That doesn't mean that other approaches, such as intravenous and intramuscular injections, are not used. They can be combined with intra-articular injections.
The drugs used most often for the treatment of non-infectious joint conditions, says Black, include polysulfated glycosaminoglycans (PSGAGs), sodium hyaluronate, and corticosteroids.
Glycosaminoglycans--PSGAGs can be administered either intra-articularly or by intramuscular injection. "The most widely used PSGAG today is Adequan," Black says. He uses intra-articular injections weekly for three to five weeks. If the drug is administered intramuscularly, he gives it every three to five days for a minimum of four weeks.
The PSGAGs, says Black, can combat elements within the joint that cause inflammation. Studies have shown that PSGAGs stimulate the production of natural hyaluronic acid.
There are advantages and disadvantages to the PSGAG approach. The advantages, according to Black, include beneficial anti-inflammatory effects and chondroprotection (protecting the ends of the bones). These positive effects, Black feels, make this the drug of choice for Type 2 or 3 traumatic joints with damage to the articular cartilage.
Black says disadvantages include a risk of intra-articular reactions unless one also administers appropriate antibiotics, such as amikacin. The antibiotics would be administered at the same time via intra-articular injection. Another disadvantage of PSGAGs is cost. Maintaining a horse on the recommended therapeutic levels, Black says, can result in "significant cost to the owners."
Sodium Hyaluronate--Sodium hyaluronate (also called hyaluronic acid, or HA) is the most recent class of anti-inflammatory medication to be used in the equine joint, says Black. This group of therapeutics, he says, provides significant lubrication to the synovial membrane that is responsible for dissipating more than 50% of the friction within the joint. Sodium hyaluronate counteracts metalloproteinases, prostaglandin E2 (involved in several inflammatory processes including the perception of pain), and free radicals.
"A synthetic form of sodium hyaluronate--Legend--has been developed that can be administered intravenously or intra-articularly," Black says. "When given intravenously, Legend has proven to have positive anti-inflammatory effects on the synovial membrane of the traumatized joint, and it has the advantage of offering the veterinarian a method for treating multiple joints with a single injection." Intravenous sodium hyaluronate is given weekly for three to four injections, usually followed by a decreasing number of injections for maintenance.
The intra-articular injection of sodium hyaluronate can be repeated in two to three weeks, depending on the severity of the joint inflammation. This would be followed by repeat injections as needed for maintenance of joints that have degenerative arthritis.
"Many practitioners," Black says, "will begin the sodium hyaluronate series with an intra-articular injection followed by a series of two to three weekly intravenous injections. The intravenous form of HA can then be used for maintenance by administering Legend bi-weekly, monthly, or before major competitions."
"It should be recognized that HA is only effective against mild to moderate synovitis, and consequently corticosteroids are often used in conjunction with HA," says McIlwraith. The drug has been proven safe, with few side effects or injection reactions.
Corticosteroids--For some time, corticosteroids were the bad boys on the block. Many thought that injecting a joint with corticosteroids was giving the joint a death sentence. Yes, it would almost immediately reduce the inflammation, but in the long run it would have such a deleterious effect that the horse could wind up a cripple.
Research at CSU has challenged that theory, and some corticosteroids have once again taken their place as part of the treatment regime for joint disease.
"Corticosteroid preparations," Black says, "are often categorized by duration of action, with the medium- or long-acting products usually favored over the short-acting due to the decreased treatment frequency. The three preparations favored at our clinic all have different duration of action and are used accordingly.
"Methylprednisolone acetate (MA, a corticosteroid) has a long duration of action and tends to be used in lower motion joints, such as the distal tarsal joints or the pastern joints for degenerative joint disease or soft tissue arthropathies (joint disease)," says Black.
McIlwraith adds, "CSU research has shown that MA did have deleterious effects on the articular cartilage. This medicine should be used with caution."
"Triamcinolone acetate (TA) is a potent medium-duration steroid favored in higher motion joints at low dosages," says Black.
McIlwraith continues, "In contrast to the negative effects of MA, work at the CSU Orthopedic Research Laboratory has shown that TA not only had no negative effects, but that it promoted synthesis of essential elements of the articular cartilage. It also appears to be chondroprotective (protecting the cartilage)."
"Betamethasone phosphate is used routinely at our clinic to reduce synovitis and joint inflammation in the young, immature equine athlete," adds Black. "It, too, is a potent anti-inflammatory, but of short duration."
McIlwraith adds, "A study with betamethasone esters showed that they had no deleterious side effects."
Included in the advantage category for corticosteroids, says Black, is the fact that they are the most potent and cost-effective joint therapy available. Black doesn't shy away from the fact that there is potential danger when corticosteroids are used. One side effect of corticosteroid use outside the joint is laminitis if it is used incorrectly at improper doses, Black adds.
An effective approach, Black says, involves combining corticosteroids and hyaluronic acid and administering this combination intra-articularly. The combination, he believes, provides maximum anti-inflammatory response as well as providing additional lubrication to decrease friction.
Other treatment methods include physical therapy and conditioning programs to slowly adapt the joints to stress rather than going straight from no work to heavy training. Phenylbutazone, ice, and cold water therapy are often used first on a joint problem before owners call the veterinarian.
"Nutraceuticals and other oral joint supplements are commonly used, but since they have been much less well studied than joint injections, it's hard to draw conclusions about them at this stage," says McIlwraith.
The message to horsemen is that athletic events of all kinds take a toll on a horse's joints, and it is the owner's responsibility to constantly monitor the horse's well-being. When that monitoring indicates joint trouble is brewing, a veterinarian should be consulted immediately. And if joint disease has already set in, it behooves the horse owner to allow the administration of appropriate therapy to combat the problem.
About the Author
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 www.exclusivelyequine.com or by calling 800/582-5604.
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