Stephen E. O’Grady DVM, MRCVS

Hind limb interference is a gait abnormality that occurs when the medial (inside) portion of a hind foot strikes the opposite hind limb anywhere from the coronet to just below the hock. The inside of the pastern or fetlock is the area most often struck by the offending foot. Causes for hind limb interference can be divided into temporary or permanent. Temporary causes would include age and stage of training, degree of fitness, fatigue and inappropriate trimming and/or shoeing. Permanent causes of hind limb interference are linked to faulty conformation. Horses with base narrow conformation that turn out are usually candidates for this type of abnormal gait. This type of conformation will cause the arc of flight of the hind foot to deviate inwards toward the opposite limb instead of moving straight ahead. Winging inward may cause the foot to travel close enough to the opposite limb to interfere. Interference generally occurs at the trot. Horses with cow-hocked conformation will also move in such a way that the flight of the foot moves inward toward the opposing limb but because of the distance between the hind limbs, these horses seldom interfere. The base narrow horse will stand with its hind feet together. If you stand behind the horse you can note the severity of the inward flight pattern of the limb. Often when the hind foot contacts the ground, the hock will move outwards which denotes instability of the hind limbs with a further tendency to interference. Most base narrow horses, and for that matter, horses with cow-hocked conformation will have a narrow rolled-under heel on the outside of both hind feet. As the hind limbs bear weight, the foot rolls off on the outside heel which allows the inside of the foot to move closer to the opposite limb often compounding the problem.

Temporary causes of interference are often treated with patience. As the horse matures, advances in its training level, builds up the musculature in the hind limbs and attains a higher degree of fitness, the intermittent signs of interference will disappear. Often the horse is shod behind with shoes that are too small. This reduces overall ground surface and hind limb support causing instability which in turn can lead to the horse occasionally striking the opposite limb. This can be corrected simply by using a shoe that is the appropriate size. Protective boots can be placed on the fetlocks until any cause of temporary interference is resolved.

Interference caused by abnormal conformation is corrected through proper farriery. Your farrier should observe the horse at a walk and trot to determine the flight pattern of the limb and the severity of the interference. If you can’t see the interference, carpenter’s chalk can be placed on the inside of the hoof walls to determine where the interference is occurring and if one or both feet are involved. When trimming the base narrow horse, the medial (inside) hoof wall should be lowered. This will normally provide more clearance when the horse moves. A square toe shoe (angled toward the outside of the toe) is useful as it helps to straighten the path of flight. The shoe should be long enough to extend behind the heels of the foot. A trailer on the outside with the shoe fit full is beneficial if the outside heel is narrow and rolled under. This is generally all that is needed to remedy the interference.

If the horse has a cow-hocked conformation, whether or not it interferes, it should be trimmed in such a manner that the lateral (outside) hoof wall is trimmed lower. This will change the position of the foot, allowing the toes to be more in line with a straight flight path. Again a square toe shoe will help the foot breakover straighter and a lateral trailer should be used. The trailer will contact the ground first and rotate the position of the foot straighter when landing.

In summary, to rectify hind limb interference, your farrier is attempting to change the flight pattern of the limb, correct any instability present and provide adequate support.

Any discussion of gait abnormalities of the hind limbs would not be complete without discussing forging. Forging is noted at a trot (sometimes at a walk) and happens when the toe of the hind foot strikes the heel or bottom of the front foot on the same side just as the front foot is starting to leave the ground. When the toe of the hind shoe strikes the heel of the front shoe, a constant metallic sound is heard as the horse trots. What happens is the front foot is too slow in leaving the ground to avoid the forward advancing hind foot. Overreaching is a more severe form of forging where the advancing hind limb approaches the fore limb even faster and the toe of the hind foot lands on the heel bulb of the front foot on the same side before the foot leaves the ground. This will result in the shoe being pulled and the heel bulb being lacerated (known as a heel grab).

Forging can again be caused by age, fatigue, improper riding, degree of fitness and faulty conformation. Faults in conformation that lead to forging are seen in a young horse that is experiencing a surge in growth, short backed horses with long legs, front or hind feet that are set too far under the body, long hind limbs, and “sickle” hock conformation. Another area that is often overlooked is forelimb lameness. Any horse with forelimb discomfort that causes a short “stilted” gait or lack of extension can develop forging. If an abnormal gait is noted in the forelimb accompanied by forging, local anesthesia can be used to block the front feet, and if the gait improves and the forging disappears, a thorough lameness examination should be initiated.

The treatment of forging can be thought of as a function of timing. The intent of treatment is to speed up the breakover of the forefeet and slow down the advancing hind feet. Again, the horse is observed by the farrier at a walk and trot to determine the extent of the gait abnormality. To speed up breakover in the front limbs, any excessive toe length is trimmed accordingly. Then a square, rolled or rocker toe shoe is used to enhance breakover. Some feel a lighter shoe such as aluminum is beneficial in the front feet. It’s imperative that the appropriate size shoe is used to provide the proper ground surface and support. Often a shoe that is too small is mistakenly used with the thought that the horse will not pull the shoe but in fact it compounds the problem The hind feet are trimmed in a manner that the heels are moved back to the widest part of the frog to create as much ground surface as possible. A square toe shoe is used and the toe of the shoe is set back ¼ to 3/8 inch from the outline of the hoof wall at the toe. The heels of the hind shoes are fitted well beyond the buttress of the foot. This type of the shoe coupled with the trimming will tend to keep the foot on the ground longer, i.e., decrease breakover of the hind limb.

Hind limb abnormalities often require trial and error to correct. If a horse continues to forge despite various shoeing modifications, then it may be necessary to determine whether the horse is suitable for the athletic endeavor being pursued. The one necessary requirement to eliminate forging, is a good working relationship with a knowledgeable, competent farrier.

Dr. Steve O’Grady is a veterinarian and farrier who runs Northern Virginia Equine in The Plains, VA. Derek Poupard a farrier also located in The Plains, VA contributed to this article.

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