Underrun Heels--Not So Innocent

Underrun heels sounds like an innocent term. Certainly it doesn't strike fear into the hearts of horse owners in the same way as, say, navicular syndrome. It should. The disorder is so common today that many people fail to see it as an abnormality. And those who might recognize it as a problem often believe it's of minor importance. Yet, underrun heels, if not controlled, will steadily degrade the hoof's interior structure, leading to tremendous trouble for your horse--possibly including navicular syndrome.

To head off this serious problem, you need to understand more about it, including what causes underrun heels, how to spot them, why they lead to so much trouble, and what you can do to help your horse if he has them.

Definition of Underrun Heels

Underrun heels exist when the angle of the horse's heel is at least five degrees less than the angle of the toe. (In a healthy hoof structure, the toe and heel angles are virtually the same.) In severe cases, the horse might appear to stand on the bulbs of his heels. In addition, the slope of the hoof will be greater than the slope of the pastern, a condition known as a broken-back hoof-pastern axis, notes Stephen O'Grady, BVSc, MRCVS, a former professional farrier who is now an equine veterinarian with Northern Virginia Equine Practice, which does significant work in equine podiatry.

"On a lateral radiograph of an underrun-heeled horse, the coffin bone can be flat or lower at the heel than the toe," says Richard Mansmann, VMD, PhD, of Chapel Hill, N.C., a member of the International Equine Veterinarians Hall of Fame. "The lateral radiograph can prove the existence of--and monitor treatment success of--an underrun heel."

Contributing Factors

Numerous factors can lead to or exacerbate a case of underrun heels.

Conformation--A long toe/low heel conformation is one cause of underrun heels, says Derek Poupard, Certified Journeyman Farrier, who deals with the disorder on a near-daily basis. With normal hoof conformation, the hoof grows downward. But with long toe/low heel conformation, the hoof grows forward, he says. Since direction of heel growth follows direction of toe growth, as the toe becomes longer, the heel grows forward and thus drops lower.

Horses with conformation flaws, such as long pasterns, might be more likely to develop underrun heels because of increased weight-bearing in the heels, says O'Grady.

Shoeing and trimming--Improper shoeing and trimming is a common road to underrun heels, says O'Grady. (He estimates that 60-70% of foals are born with good heels and these feet are maintained until these youngsters go into training.) At the racetrack, trainers often ask farriers to lower a horse's heels and allow the toes to get long, incorrectly believing that this will extend the horse's stride, notes O'Grady. A too-small shoe, he adds, simulates the effect of a long toe because it places the weight-bearing surface of the foot in front of the vertical axis of the limb. Similarly, the use of a toe-grab on a hoof that already has a long toe and low heel can increase the severity of underrun heels by increasing pressure on them, he says.

Additionally, says Mansmann, not trimming to make feet symmetrical, irregular trimming or shoeing intervals, and allowing trimming or shoeing intervals to be too long are all causes of underrun heels.

Genetics--"Many offspring appear to be born with or acquire the same foot conformation as one or both parents," says O'Grady. "In recent years, it appears that an increasing number of foals are born with a low heel." He believes this could be due to a tendency for breeders to weigh other attributes more heavily than hoof conformation when making mating decisions.

A club foot (which can be hereditary) can cause an underrun heel in the opposite foot because of more weight bearing on the "normal" foot, adds Mansmann.

Breed--Although underrun heels seem more common in Thoroughbreds, they can occur in any breed, notes O'Grady. The prevalence of underrun heels in Thoroughbreds might stem from traditional racetrack shoeing practices that encourage long toe/low heel conformation and toe grabs.

Nutrition--Nutritional deficits can exacerbate underrun heels simply by leading to lower hoof quality, says Oregon farrier Michael Waldorf, a graduate of the Pacific Coast Horseshoeing School and an active American Farrier's Association member.

Environment--Hooves that are exposed to extreme changes from dry to wet and/or unsanitary conditions might have loss of laminar strength, says Waldorf. The wetter the environment, the weaker the hoof, says Mansmann. "And when the strength of the heels is undermined, they are more readily able to reshape or collapse as in the case of underrun heels," Mansmann notes.

Work load--It is possible that demanding performance requirements can worsen the problem of underrun heels, particularly if the horse has other negative factors present, says Waldorf. Adds Poupard, "These horses do not have fully inflated tires. And the more you work them, the more limited their life spans will become."

A Chain Reaction

This poor hoof structure is a big deal, because in the long run it affects the hoof wall. "And the hoof wall, which includes the horny laminae of the heels, is essential in protecting the internal hoof structure from the harsh demands of the horse's environment," reminds Waldorf. Since the heel plays a vital role in the hoof's structure, when something goes wrong with the heel, larger problems can result.

In fact, the underrun heel structure starts a chain reaction in which the long-term effects can range from bruised heels to quarter and heel cracks, coffin joint synovitis, interference problems, and navicular syndrome.

Here's an overview of how the process snowballs when left to its own devices, according to O'Grady and Poupard:

As the toe grows forward, the heel lowers. As the heel lowers, the horn tubules at the heels bend. Eventually, they become parallel to the ground and lose all ability to support the horse's weight. The poor quality of hoof wall at the heels no longer allows the heels to transfer concussion to other soft tissue supporting structures above the heels, such as the frog, digital cushion, deep digital flexor tendon, navicular bursa, and suspensory ligaments. This can lead to bruising in the heels and associated soft tissues structures.

The hoof wall at the heels then begins to thin, separate, collapse, and roll under the horse's foot. This destroys the bars, which form the angle of the sole. Corns and quarter or heel cracks appear. With the heel out of commission, the frog, deep digital flexor tendon, and digital cushion take over more of the weight-bearing role than they are designed to handle.

The increased tension on the deep digital flexor tendon, caused by the underrun heel and broken-back hoof-pastern axis, increases pressure on the navicular bone and bursa, often leading to degeneration of those structures.

If the horse wears shoes, the heels are further damaged when they rub against the shoe during the expansion phase of each step. (The heel will actually wear a trough into the heel area of the shoe.) In contrast, the horse with a thick, solid heel base (hoof wall, angle of sole, and bar) will not wear into the branches of the shoe on the bearing surface, explains Mansmann.

This poor-fitting shoe also inhibits growth of the hoof wall in the overstressed heel area, but does not affect growth in the less stressed toe area. Over time, this exacerbates the long-toe problem, increasing the distance from the point of the frog to the point of breakover.

There now is delayed breakover with every stride, which creates more tension in the deep digital flexor tendon and can lead to interference problems such as overreaching, forging, or scalping. The overlong toe begins acting like a mechanical lever arm, applying outward force to the horn tubules with every step. The horn tubules in the toe area become bent, creating a "dish" (concavity) in the dorsal hoof wall, often accompanied by a toe crack.

"So now without protection from the heel and digital cushion, the bony column will receive more concussion," says Mansmann. "When the laminae inside the hoof wall stretch and tear, the bony column even lowers into the hoof, which thins the sole and puts additional pressure on the blood vessels internally. The wings of the coffin bone can become lower than the coffin bone at the toe, increasing bruising and pain. Then the toe will land first to protect the sore heel."

When the horse's toe lands first, more trouble follows. The horse is more likely to stumble, and, more importantly, the white line will widen; leading to white line disease and abscesses. It's easy to see how this process can contribute to numerous hoof problems, causing the horse pain and effectively limiting his useful life.

Taking Control

The good news about underrun heels is that you can control this process of deterioration. The earlier you start, the better chance you have of keeping your horse normal and sound. "If you take a horse with this (long toe/low heel) conformation as a youngster and address it from day one, you're going to be fine," says Poupard. "You don't let it get out of hand at all."

"In the adult horse, early recognition of the problem by the owner, farrier, or veterinarian is helpful," adds Mansmann. "Then their willingness to work together, be creative, and be patient will be necessary."

Poupard notes that such control includes more frequent farrier care than usual. "Four weeks is the longest you can go," he says of horses with the long toe/underrun heel conformation. "Even after three weeks, some horses look like they've gone a year without being shod."

In addition, the horse needs the right kind of trim to keep the condition in hand. That means making the hoof look as normal as possible. "It's our job to put the hoof capsule back into position to stop the degeneration inside the heels," says Poupard.

Unfortunately, it doesn't take much neglect for the chain reaction to start or to recur again following a previous problem. And once started, many variables--from the amount of riding, to the horse and rider's weight, to the degree of the hoof angle, and more--determine how quickly the process will progress. As soon as the digital cushion is compromised, your horse has reached a point of no return. At this stage, says Poupard, "You can stop it from getting worse, but it cannot recover. I have tried everything, and I wish there were more that I could do."

Shoeing Tactics

In order to halt the progress of underrun heels, the farrier must trim the foot so that it resembles, as closely as possible, the angles of a normal hoof. Often in order to do that, the farrier will need to use some special shoeing tactics, at least temporarily. Bar shoes are a common choice, but come with some caveats.

An egg bar shoe, agree Waldorf and Poupard, is the best of the bar shoes because it gives the most support. But your farrier must be careful not to go too long with it, they caution, or it can do more harm than good. Bar shoes fitted too long will act as a mechanical lever in the heel area and crush the heels further. Heart bar shoes, notes O'Grady, are also very useful in some horses. "The tongue of the heart bar fits over the frog and allows some weight bearing to be transferred to the frog and off the heels," he explains.

In the past, wedge pads were also a common method for "fixing" the different hoof angles of underrun heels. Unfortunately, notes Waldorf, "Wedge pads tend to perpetuate the problem by maintaining pressure on the collapsing heel structure." In addition, the horse will likely require larger and larger wedges over time to maintain a proper angle.

"Floating" the heels is a controversial technique used in some cases. Waldorf notes that the tactic involves leaving a small space--about one-eighth of an inch--between the heel and the shoe. This is supposed to free the heels from direct weight-bearing so that the new horn tubules growing down from the coronary band won't have direct ground forces pressing upon them, explains Waldorf. The idea is that the heels won't be forced into that low-angle growth and will have a chance to grow down toward the shoe in a normal manner. Mansmann states that this method often is not effective, and O'Grady has also not seen this process to be beneficial.

The most useful treatment for improving underrun heels, notes O'Grady and Poupard, is glue-on shoes. The composite between the shoe and the heel creates an interface so the damaged heels do not rub against the heel of the shoe. After one or two sessions in glue-on shoes, the horse will grow some heel along with a bar (heel base). The horse can now be transferred back into the appropriate shoe.

"Considering the necessary amount of sole support, composite products and modifying breakover can be helpful," adds Mansmann. "Being patient with some intelligent trial and error can also be helpful."

The End Game

Poupard notes that once the hoof has a reasonable heel, all that's necessary to maintain normal foot conformation is putting on "good shoes on a regular basis." However, he adds, if the condition progresses, the horse might have significant pain. In that case, you might need to return to a supportive shoe to control discomfort.

If you catch a case of underrun heels before it causes internal problems and take appropriate measures to treat your horse's feet, then a bright outlook is possible.

"All horses respond differently to treatment and they should be considered on a case-by-case basis," cautions Waldorf. "But depending on what the horse is asked to perform, it is quite common where the condition can be managed, and the horse can have a normal life."

5 TIPS: Underrun Heels

  1. When a horse's heel angle is at least five degrees less than the toe angle, he has underrun heels.
  2. Horses with long pasterns are more prone to underrun heels because of more weight-bearing in the heels.
  3. It is fairly common to see long-toe,
    low-heel conformation in racehorses, but this leads to underrun heels and can cause soft tissue injuries.
  4. Long-term effects of underrun heels can range from bruised heels to quarter and heel cracks, coffin joint synovitis to navicular syndrome.
  5. Underrun heels are much more easily prevented than treated; once internal hoof structures are damaged, they're not easily repaired.

About the Author

Sushil Dulai Wenholz

Sushil Dulai Wenholz is a free-lance writer based in Lakewood, Colo. Her work appears in a number of leading equine publications, and she has earned awards from the American Horse Publications and the Western Fairs Association.

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