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July 18, 2008

The Caudal Heel Conundrum

Horses with heel pain benefit from new treatment approaches and specialized shoeing.

For as far back as anyone can recall, a diagnosis of navicular syndrome has been career-ending news for almost any horse.

Chronic, frustrating, and discouraging in the extreme, navicular is—or was—considered an incurable, degenerative condition, brought on by deterioration of the tiny navicular bone, which is tucked up behind the coffin bone in the middle of the foot.

Horses with navicular usually exhibit signs of heel pain in both front feet. They’re short-strided, tippy-toeing to avoid putting their full weight on their heels, with lameness increasing as work continues and subsiding, to some degree, with rest. When standing, they shift their weight from side to side to help relieve the pressure. Over time, because the circulation in the hoof is compromised, the heels tend to contract and the feet become upright and boxy.

That’s quite an influence for a structure so minute. Small as it is, the navicular bone has an important function: it acts as a fulcrum for the deep digital flexor tendon, which attaches to the coffin bone. As the coffin joint flexes, the deep digital flexor tendon glides against the navicular bone, a rubbing action that would cause tissue damage and pain, were it not for the navicular bursa (a small, fluid-filled sac) between the tendon and the bone. In the normal state, this bursa is filled with a lubricating substance, which eliminates pain and tissue damage as the navicular bone takes the brunt of the impact of each step.

When a horse exhibits signs of heel pain, pinpointing which of the many structures inside the hoof might be to blame can be a challenge.  The navicular bone itself can be explored via radiographs, but it’s difficult to examine the bursa, the deep digital flexor tendon, or the other soft structures inside the hoof by conventional diagnostic means.

We’ve had to rely on injecting short-acting anaesthetics to temporarily “block” the heel of the hoof.  A nerve block that alleviated heel soreness was said to be diagnostic for navicular syndrome, though recent studies now suggest that the traditional heel block anaesthetizes a far wider region of the lower leg than was previously believed—and so does little to pinpoint the heel.

X-rays of the navicular bone were a standard means of diagnosing navicular syndrome for many years. Trouble was, it’s almost impossible to correctly interpret the results of those radiographs.

Degenerative changes in the outline of the navicular bone, causing visible pitting, or even “lollipop”-shaped lesions on the X-rays, are common—but they’re not necessarily linked to lameness.

Every veterinarian who has ever done a navicular X-ray will likely tell you of horses who exhibited all the symptoms of navicular syndrome, yet had films as clean as a whistle and of other horses who were perfectly sound but had navicular bones that X-rayed like Swiss cheese. Over time, radiographs of the navicular bone came to be largely dismissed. But to what else could we attribute chronic heel pain?

Welcome to the 21st century, where magnetic resonance imaging has finally given us a way to peek inside the hoof and see the whole picture, soft and hard tissues alike. MRI has been a diagnostic revelation in many ways, but nowhere more so than in the case of so-called navicular syndrome.

Michael Schramme, DMV, Ph.D., Dipl. ECVS, the director of the Equine Orthopedic Research Laboratory at North Carolina State University, said, “The MRI has given us a completely new perspective on what’s going on inside the foot. Using MRI we now have a very good chance of diagnosing exactly what the problem is in the heel-sore horse, and it’s just as likely to be a soft-tissue problem as one involving the navicular bone. MRI has revolutionized the diagnosis of foot lameness.”

Mike Pownall, DVM, a farrier and veterinarian who focuses on sport and race horses at his McKee/Pownall Equine Services in Campbellville, Ont., concurred, saying, “The MRI has given us answers for all kinds of mystery lamenesses. What we’ve found with horses with heel pain, who have clean navicular X-rays, is that they frequently have some sort of soft-tissue problem, whether it be inflammation of the navicular bursa or a strain or rupture of the deep digital flexor tendon where it attaches to the coffin bone. In fact, we find the DDFT is involved about 60 percent of the time when the X-rays are normal. It’s taken the MRI to finally show us that.”

So What’s Really Going On?

MRI machines have also helped identify sources of lameness veterinarians have never detected before. Chief among those is the discovery of bone edema, which has been noted in other species but was previously unknown in horses.

Edema within the bone is the result of severe trauma or inflammatory processes. In essence, it’s a bone bruise. And tellingly, it often appears on MRI images in the lower part of the cannon bones, the long and short pastern bones, and the navicular bone, which is subject to considerable impact stresses.

Schramme explained, “The most common form of actual navicular disease that we see involves wear on the cartilage on the palmar surface of the navicular bone. It’s subtle, but that’s where it starts. As the cartilage wears away, the bone reacts by accumulating fluid, which is sometimes called bone edema.

“The second form of navicular disease is horses with chip fragments along the distal border of the navicular bone. When they’re big enough, they can trigger a reaction in adjacent bone and cause inflammation.

“And the third type,” he added, “which is rare, occurs when we see primary edema in the bone with no accompanying bony changes. It’s like an acute bone bruise.”

There are also a variety of soft-tissue problems, which can develop in the immediate vicinity of the navicular bone, causing the characteristic symptoms of caudal heel pain we once grouped under the category, “navicular syndrome.” Among them: navicular bursitis, lesions of the deep digital flexor tendon at or near its point of attachment to the coffin bone, lesions of the impar ligament (a short, thick ligament that runs from the bottom surface of the navicular bone to the coffin bone) or the collateral ligaments which attach the navicular bone to the long pastern bone, and coffin joint arthritis.

Schramme noted, “I still think navicular bone disease is the most common cause of heel lameness, but we have to take into account that by the time a horse is booked for an MRI, he’s already had a traditional exam and X-rays. So horses we put through the MRI unit are not the ones with visible navicular bone changes, which would show up on an X-ray. We’ve already excluded all the obvious cases.”

He has also noted breed and geographical differences in the causes of heel pain. “In Quarter Horses, for example, we see a lot more navicular bone disease, while with warmbloods the most common cause is deep flexor tendonitis,” he said. “In southern California, they’re seeing more collateral ligament desmitis. Here in North Carolina we get more flexor tendon and navicular changes. We’re not sure why.”

Treatment Techniques

What’s encouraging about being able to (sometimes) put the blame on soft tissue instead of the pesky navicular bone, is that many soft-tissue problems in the hoof are resolvable, with tincture of time being the most important therapeutic ingredient.

“Now that we’ve split navicular syndrome up into six or seven possibilities,” said Schramme, “we can target our treatments much more effectively. On the down side, the information is still new enough that we’re still trying to identify which treatments are going to be most effective for which problems. It’s too early to say whether we’ve improved our treatment outcomes.”

The use of MRI has certainly cut down on the number of horses who are treated by neurectomy (cutting the nerves that signal heel pain, leaving the horse sound but essentially unable to feel part of his foot). And it has helped veterinarians avoid a potentially dangerous situation, when a horse who actually has a deep flexor tendon injury is “nerved.”

“If you do a neurectomy on a horse with tendonitis,” Schramme explained, “you risk the tendon rupturing, which makes the horse a euthanasia case. At least now the MRI can tell us which horses are neurectomy candidates and which are not.”

Where the navicular bursa is inflamed, Pownall said he’s had good results with injecting the bursa with hyaluronic acid and/or corticosteroids.  A fluoroscope (3D, real-time X-ray) or digital X-ray machine
is used to help visualize the narrow bursal space and guide the needle in, rather than the MRI, because metal needles can’t be used in close proximity to the MRI magnet.

Schramme said his team at North Carolina State has had good success treating lesions of the deep flexor tendon with a two-pronged approach.

“We go in arthroscopically and debride the injured area of the tendon, and then we inject stem cells, harvested from the horse’s own bone marrow, into the bursa and/or the lesion,” he said. “We feel we can offer a better prognosis with the combination than with stem cell injections alone. The combination has a 50 to 60 percent success rate, and those numbers have been improving.”

Extra-corporeal shockwave therapy is also proving useful in the treatment of caudal heel pain, particularly when collateral ligament desmitis is to blame.

It can improve the prognosis of a horse with this issue, from about 28 percent (on six months’ stall rest alone) to 50 to 60 percent, according to Schramme, though he noted, “Rest is still a massive part of the cure.”

Then there are the relatively new medications, Tildren (tiludronate) and IRAP (interleukin-1 receptor antagonist protein).  Tildren, an intravenous drug, inhibits bone resorption and remodeling, resulting in an increase in bone density without impairing the bone’s capacity to adapt to mechanical loads, so it can actually help repair a damaged navicular bone, particularly when used soon after the onset of symptoms. Still fairly experimental, Tildren may also prove useful in treating coffin joint arthritis and other osteoarthritic lesions.

IRAP is derived from the horse’s own blood, which is drawn and incubated in a syringe containing glass beads coated with a substance that enhances production of anti-inflammatory proteins. The serum collected from this incubation is then injected back into the joint. It’s not recommended for use in tendon sheaths or bursae, but it has an encouraging success rate in horses who have damaged cartilage and have not responded well to conventional anti-inflammatory joint medications.

“Medical intervention is really important with these cases,” said Pownall, “but so is substantial stall rest, along with shoeing for support.”

Shoeing Options

Therapeutic shoeing has always been the cornerstone of the traditional treatment for heel pain, and that’s one thing that hasn’t changed. Often, a shoeing change that improves breakover of the foot, decreases the strain in the deep digital flexor tendon, and provides better heel support results in a significant improvement in soundness.

Said Pownall, “Easing the breakover, and raising the heels in some cases, are still the aims when you’re shoeing a horse with caudal heel pain.”

Before you make any radical shoeing changes, of course, the feet must be properly trimmed and balanced. Nailing a shoe onto a foot with underslung heels, long toes, or medial/lateral balance issues won’t accomplish very much. Often, just addressing these common problems with some expert trimming will bring about considerable relief for the horse.

Stephen O’Grady DVM, MRCVS, a specialist in podiatry whose Northern Virginia Equine clinic sees dozens of cases of caudal heel pain each year, notes that correcting hoof imbalances is, by necessity, a gradual process. He can often alleviate the discomfort of a horse with caudal heel pain more quickly, however.

“There are four things you can do [for a horse with caudal heel pain] with therapeutic shoeing,” he said. “You can raise the heels up, change the breakover, set the shoe in a certain place, and protect the foot.

“Heel elevation is helpful for horses who have long toes and low heels, but it’s not a great idea for the opposite conformation, the club foot,” he said. “What you’re trying to do is normalize the shape.”

Another common approach, O’Grady said, is to provide the horse with more support by increasing the amount of ground surface his foot covers, with wide-web shoes, extended heels, or bar shoes. “Bar shoes help increase the ground surface, but egg bar shoes have fallen out of favor for heel pain,” he said. “We found they were actually destroying the foot, because horses land heel first (with egg bar shoes on) and crush the heels further.”

Pownall finds that a rocker-toe shoe, built like the runners on a rocking chair, can often help to ease breakover and reduce the strain on soft tissues.  Another approach is the French heel or “onion shoe,” which adds a small, bulbous projection at the heels, bulging in toward the frog, “like a pearl onion,” according to O’Grady.

“You forge a little onion shape over the damaged heels and make it concave on the inside [against the foot],” he said. “It creates more ground surface as well as covering and protecting the damaged heels.”

The shoeing approach for horses with caudal heel pain must be tailored to the individual, and arrived at—ideally, at least—with farrier and veterinarian working in concert based on MRI results. There is no cookie-cutter method, no single solution. Some horses, for example, do best without shoes at all.

O’Grady and Pownall agree that while MRI is a vast improvement over previous diagnostic modalities when it comes to heel pain, it should not be considered the be-all and end-all. As Pownall pointed out, “We’ve only been doing MRIs the last three or four years, really, so we’re still in the data-collecting stage.”

O’Grady agreed: “Suddenly there are an awful lot of MRIs out there, but it’s a whole new ball game learning to interpret the images. There’s a pretty steep learning curve, and not everyone is an expert at it yet.

“What MRI allows you to do is get a better diagnosis and a prognosis.  Most of these things need time to heal. Given enough rest, many of these horses can be brought back to work.”

So while you’re not guaranteed a good prognosis when your horse’s heel pain is diagnosed via MRI, your odds of getting that horse back on the track are significantly greater than they were in the days of the “navicular syndrome” rubber stamp.

“So often these issues are now proving to be reversible,” said Pownall.

 
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