Navicular Mystery

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Your client’s horse appears uncomfortable at best, lame in the worst-case scenario. You suspect navicular or chronic heel pain syndrome, a.k.a, navicular disease.

The navicular bone is actually a distal sesamoid bone, located behind the coffin or distal phalanx bone and “directs” tension of the deep digital flexor tendon into the horse’s foot. It’s a little bone with a big job, and yet this disease is still quite mysterious.

“Navicular syndrome is a mechanical disease resulting in progressive and chronic degeneration,” says Martin Vidal, BVSc, PhD, DACVS, assistant professor of equine surgery at the University of California, Davis. Navicular, as it is often called, is believed to result from inappropriate foot balance, shoeing and long-term management of the foot. It also stems from what we ask of our horses: jumping fences and riding on hard surfaces with us on their backs, for instance.

Diagnosis can be tricky. Your first clues to navicular: Horses presenting with it often have a stilted gait or shortened stride, not moving as freely—or perhaps not turning as fluidly. This incurable, relatively common condition also tends to be bilateral, or on both sides. “Telltale signs are that the horse may walk or land on the toe first rather than the heel—normal gait occurs over the heel rather than toe,” Vidal notes.

Who Suffers Most?

“Certain horses are more predisposed because of shorter pasterns or a narrower and upright foot conformation and position of the feet, which impose abnormal pressure on the navicular bone. This results in pain and bone remodeling, which eventually become visible in the form of radiographic (x-ray) changes.”

Quarter Horses, Warmbloods, and also Thoroughbreds may suffer from navicular more than many other breeds, Dr. Vidal observes. “There may be a hereditary component to the shape of the navicular bone surface as it has been shown for the Dutch Warmblood horse,” he adds. Both size and conformation of the foot in horses in general are also thought to be implicated.

“Besides conformation, the degree of athletic activity and use of the animal is also a determinant,” says Dr. Vidal. “Even a younger animal, depending on career requirements or breed, may be at risk.”

The most important factors in preventing navicular disease are good footing and appropriate hoof care. “If the ground is too hard,” says Dr. Vidal, “you may run into soundness problems.”

Diagnosis

An initial clinical exam will determine whether the horse is rolling over its heel or not, or pointing its toes excessively. Hoof testers, a thorough lameness examination, and diagnostic techniques, including radiology, will usually expose serious degenerative changes—those telltale bony abnormalities, which include “cones” or “lollipops” on the radiographs of the navicular bone surface.

“Skyline” views of the navicular bone, with a normal radiographic series of the foot, “will tell us a great deal about the health of the bone,” says Dr. Vidal.

Magnetic resonance imaging (MRI) may now be used to collect more subtle information. It can illustrate, for example, edema formation within the navicular bone that would not normally show on a radiograph.

Pictures in hand, it’s time for a plan. “We want to always treat the horse, not just the radiographs,” says Dr. Vidal, who may incorporate additional diagnostics such as nerve blocks to assess the horse’s response.

Digging deeper, persistent pain can also be due to soft-tissue lesions in the foot on top of navicular disease, best seen with an MRI or contrast computed tomography (CT) scan.

What To Do Now

Veterinarians are increasingly teaming up with farriers for successful therapeutic interventions. “It is a disservice to underestimate the value of approaching treatment adequately through shoeing,” Dr. Vidal says.

But healthy feet depend upon a lifetime of quality trimming and shoeing rather than constant medications or on-the-spot treatments. So instead of focusing on specific foot areas, it’s best to evaluate the overall health of the foot. Even going barefoot is making a comeback, supported by studies from top navicular researchers that find that trimming the foot to become weight-bearing over the frog, sole and bars, to distribute weight evenly, results in a more mature development of fibrous tissue within the digital cushion.

“Shod horses are fairly underdeveloped in that way. A horse moving in nature freely will have a stronger, more developed foot,” Dr. Vidal says.

Then there is the surgery option. Perhaps you’re familiar with digital neurectomy, or “nerving,” a salvage procedure often recommended for long-term problems.

Among surgical techniques employed, neurectomy is most common and involves resecting or removing nerves within the pastern area, taking away sensation or feeling to the foot. It’s life-changing and makes horse management even more crucial—this may involve stable confinement or minimal pasture time and more intense management of the horse to monitor for possible foot injuries. It’s a Catch-22 since regular, controlled turnout, versus confinement, on acceptable footing stimulates circulation and is beneficial to the overall health of the foot.

“Put simply, the horse is not going to feel as much any more. Turn him out to pasture, he could step on a nail or seriously bruise his foot, and you wouldn’t know,” says Dr. Vidal.

After this surgery, “which doesn’t last forever and isn’t a permanent solution,” the remnant nerve ends may re-establish sensation or may develop painful tumors called neuromas. Signs of some nerve sensation may re-occur in 1.5 to 2 years.

Relief of clinical symptoms is realized in 65 percent of horses for between 1 to 1.5 years in duration, and yes, you can “do it again but it may not be as effective,” Vidal says.

As to medications, the use of Isoxsuprine is popular, which Dr. Vidal agrees “may” help increase peripheral vasodilation or circulation. “Nobody really knows the mode of action, but it affects the way red blood cells travel through a horse’s vascular system,” he says.

Nonsteroidal anti-inflammatory medications such as phenylbutazone, or flunixin meglumine, and intra-articular medications with corticosteroids such as triamcinolone are other categories of drugs that are often called into battle against navicular. Complications with neuromas may respond to local treatment with corticosteroids.

Mixed results are reported in veterinary literature about the “natural” analgesic and chemical neurotoxin Sarapin, derived from the Pitcher Plant and injected around the palmar digital nerves. Results last approximately a month. Alcohol has also been used to numb these nerves, with so-called “chemical neurectomies,” often used to assess the horse’s ultimate response to a surgical neurectomy.

“A horse with navicular syndrome that is managed appropriately can have a productive life,” Dr. Vidal says, “but if you have a choice between a horse that has it and one that doesn’t, consider going with the one that does not.” That’s good advice in this economy, a buyer’s market.

Farriers to the Fore

Dr. Vernon C. Dryden, DVM, is an in-demand veterinarian, podiatrist and AFA-certified farrier currently practicing at the respected Rood & Riddle Equine Hospital in Lexington, Ky. When treating navicular disease in his practice, he says it frequently involves either the upright or low-angle foot.

“Given the diagnosis and area of the lesion, we can tailor-make our shoeing to fit that horse,” says Dr. Dryden. “If the horse has, for example, an adhesion (adherence or “sticking to”) of the deep digital flexor surface of the navicular bone, the horse would benefit from having a wedged shoe to elevate the heel. Also these horses benefit from having a rocker toe or rolled toe—that’s where we put a very gradual bend in the shoe so it has a gradual slope to the ground surface and reduces the breakover point of the foot.”

The art of farriery as a real science is apparent, as it is becoming one of the leading treatment modalities for navicular disease, and is, he says, emerging as a specialty within equine medicine. Although there currently is no residency program or board certification yet within this specialized discipline, veterinary medicine is visibly moving in that direction, no doubt because of the effectiveness of shoeing as a major assist to horses with not only navicular disease, but many other debilitating problems of the foot.