Navicular Syndrome Treatment: The Brave New World

In spite of the best care given to horses in the history of their domesticated lives, record numbers of carefully bred, reared, and trained saddle horses are prevented from fully athletic lives by the crippling disease known as "navicular syndrome." More a condition than a disease, navicular syndrome mystifies the veterinary profession by existing on several levels, and by defying the rules for both diagnosis and prognosis as set down by medical guidelines.

On the most basic level, navicular syndrome might almost be any lameness that is traced to the posterior part of the horse's foot. In most horses, one or both front feet are affected. Veterinarians use a variety of diagnostic tools--flexion tests, hoof testers, radiographs and nerve blocks--to look for signs of lameness in the foot. A poster-ior digital nerve block "numbs" this part of the foot, and many horses will walk soundly when the nerves are blocked.

Radiographs, usually a key diagnostic tool for a veterinarian, often can be misleading if other tests do not confirm the location of the lameness. Examination of the navicular bones of hundreds of dead horses has shown that many sound horses have competed to old age without ever showing any signs of lameness or weakness, even though their navicular bones are riddled with "lollipops" and "cones," which are tiny indentations in the edge of the bone. Still other horses have spotlessly clean radiographs, but have problems with the way that the deep digital flexor tendon passes under the navicular bone, where it actually might "stick" to the bone and form blistery adhesions, making each step a painful experience for the horse.

In most horses, radiographs of the navicular bones are difficult--and sometimes even dangerous--for the veterinarian to shoot, since several angles require the veterinarian to stand in close under the horse while focusing the radiographic beam.

The best way to utilize radiographs in the diagnosis of lameness is to have a set taken when the horse is purchased, or early in its career. Subsequent radiographs taken after the onset of lameness, sometimes years later, will show whether the tiny patterns at the edge of the bone have deepened or whether other abnormalities have appeared around the bone.

Plenty of controversy surrounds the diagnosis of navicular syndrome, since many horses "recover" with rest or changes in shoeing, while others are crippled for life. The treatment of navicular disease often is a series of experiments, trying to find a comfortable range of motion for the horse.

Changes in tack, riders, arena surfaces, and most of all shoes or foot trimming, are areas where most efforts begin. Veterinarians often will suggest a vasodilating drug such as isoxsuprine, which is very helpful to some horses. Other potentially helpful protocols include injecting the coffin joint and supplementing diets with glucosamine or chondroitin sulfate, "joint lubricant" products. Anti-inflammatory drugs are commonly prescribed. Still, horses react differently and on different time intervals.

Therapeutic Shoeing

Shoeing is controversial for navicular horses only in that the owner might be shocked at the cost of special shoeing and at the fact that shorter intervals between shoeings will be necessary to keep the foot sound. The standard shoe of choice is the egg bar, which is popularly made of aluminum in America. However, the problem of navicular-type lameness has become so common here that many horses are shod with egg bar shoes from an early point in their careers.

Egg bars can be made or purchased in steel or aluminum, plus they can be made of varying thicknesses of steel. This can allow the shoe to have a high-heeled wedge effect to support the heels and hopefully remove stress from the navicular zone. Some horses respond well to these shoes, while others "fall through" the egg bars and worsen. Often a "full support shoe," or egg bar/heart bar combination, is required to give the horse plenty of bearing surface and create alternate weight-bearing areas.

Foot trimming is an essential art in the treatment of navicular lame-ness. Many proponents of the "natural" or "four point" trimming and/or shoeing ideas find that it will help navicular horses find comfort. It goes without saying that correcting anterior-posterior balance problems in the foot will help alleviate pain. Many horses "carry too much toe," so that breakover is made more difficult on each stride than might be necessary.

Pads can be a helpful aid to navicular horses, simply by protecting the sole and, when wedge pads are used, by elevating the heels, at least temporarily, to see if the horse will respond. Plastic wedge pads require short-term replacement and the heels must be watched for crushing and collapsing.

Associated Problems

When the problem really is centered in the navicular area, the veterinarian will have eliminated heel lameness, frog soreness, and digital cushion malfunction as causes of pain. Instead, these areas are sore or deformed because pain in the navicular bone zone is causing the horse to break over, bear weight, or stride in an artificial manner to avoid more pain. Navicular horses commonly shorten stride, stand in unusual postures, and "point" one foot. Before labeling a horse as having navicular syndrome, it is necessary to correct problems such as sheared heels or a broken back axis, as these problems may simply be putting pressure on the navicular area and causing it to be painful, rather than the navicular area itself being the source of pain.

Navicular fractures are not common in horses, but they do occur, and will usually be visible on radiographs. Lesions in the bone also appear on radiographs, often as dark shadows in the middle part of the bone.

Several theories exist as to what navicular syndrome really "is." The biomechanical theory blames pressure of the deep digital flexor tendon against the navicular bone. The vibration theory suggests that the tendon vibrates against the bone. Several vascular (blood supply) ischemia theories have been proposed, blaming a shift in blood supply from the navicular area for changes and pain.

The real problem areas for navicular disease are those that do not show up on radiographs, specifically the navicular bursa, the tendon surface, and the "intersection" where the tendon, bone, and distal sesamoid impar ligament (DSIL) join. Recent research from Robert Bowker, DVM, at Michigan State University suggests that nerve fibers found in this area might be responsible for transmitting different stimuli to the nervous system. A malfunction in that area would have a devastating effect on the horse's athletic ability. If pain response receptors are present as well, this area could be a message center in the foot for transmission of pain messages.

The navicular bursa is a sac of fluid that helps the bone to pivot and the tendon to slide. Post-mortem studies of many horses with navicular pain have revealed that the bursa is often damaged or deformed. Careful injection of the navicular bursa with local anesthetic will "block" the bursa and determine if it is the site of pain; however, this is a delicate procedure that is often inconclusive because the tendon has created adhesions to the bone and obscured the bursa.

A new technique pioneered at the University of Minnesota by navicular disease expert Tracy Turner, DVM, uses contrast medium injected into the navicular bursa. Subsequent radiographs will show the contour of the sac and any deformities or "leaks." This procedure is necessarily conducted in a sterile setting. Turner reported that he was able to see results through this technique in 20%
of cases that showed no other signs of radiographic changes.

The most drastic treatments of navicular disease are surgical methods. Navicular suspensory desmotomy is simply the severing of the small ligaments that suspend the navicular bone in the foot. Cutting in the area of the bulbs of the heels allows the bone to relax and to have increased flexibility. The technique was developed at the University of Cambridge in England by Ian Wright, MRCVS, and has been successful in some horses.

Neurectomy is a last-resort procedure for an unsound horse, and it is one of the most controversial procedures in the industry. "Nerving" is simply the cutting of the palmar digital nerves so that the horse cannot feel the posterior part of its foot, a procedure that is similar to the posterior nerve block used in diagnosis. However, nerving is irreversible. Surgeons use lasers to cap the ends of the nerves. The procedure has grown more sophisticated in recent years, but the horse's inability to feel its foot, along with the potential danger to the rider, makes this treatment a last resort.

 


FURTHER READING

Color Atlas of the Horse's Foot, by Chris Pollitt

Concise Guide to Navicular Syndrome in the Horse, by David Ramey

About the Author

Fran Jurga

Fran Jurga is the publisher of Hoofcare & Lameness, The Journal of Equine Foot Science, based in Gloucester, Mass., and Hoofcare Online, an electronic newsletter accessible at www.hoofcare.com. Her work also includes promoting lameness-related research and information for practical use by farriers, veterinarians, and horse owners. Jurga authored Understanding The Equine Foot, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

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