Formation of sidebones can be a red flag that more serious problems exist

SIDEBONE is the common term for a condition involving a part of the horse's foot scientifically referred to as the collateral cartilage.

These pieces of cartilage, one on the inside of the hoof and another on the outside, attach to the back border of the coffin bone and curve around toward the center of the foot to provide the base structure of the heels.

This specialized fibrocartilage functions in a number of different ways. Because it is elastic and yet somewhat rigid--like the cartilage that makes up the nose and ears--it allows the horse's heels to bend and flex, yet still maintains the shape of the hoof. This expansion and contraction of the back of the horse's hoof during weight-bearing is a crucial part of the shock-absorption mechanism that reduces stress on ligaments, tendons, and bones.

Compression and expansion of this area of the foot also is believed to help move blood through the hoof. The energy used to deform or "squash" the heel when the horse puts weight on a foot is returned to the leg during the next step as the horse moves off the foot and the cartilage in the heel helps the foot spring back to its original shape. This property is called distensibility, and a horse's collateral cartilages are designed to help maximize its effectiveness.

Normally, these structures remain as fibrocartilage. In radiographs, they appear as grayish extensions of the back of the wing of the coffin bone. They are firm but elastic to the touch and can be felt or palpated on the inside and outside of the back of the hoof just above the coronary band. But in a surprising number of horses, these cartilage structures are put under enough pressure to cause them to start to ossify or to become bonelike. At this point, they are referred to as sidebones. They can be enlarged only slightly or they sometimes can be massive. They can be symmetrical, or you can have a horse that has only one large sidebone on one side of the hoof.

Common on X-rays

Charles F. Reid, D.V.M., a distinguished professor of surgery who retired from the University of Pennsylvania School of Veterinary Medicine, where he created the first equine radiation suite, noted that ossification of the collateral cartilage is the most frequent finding in radiography of the feet of horses. This is exactly what makes the issue of sidebones so interesting. When are these bony structures important, and when are they merely incidental findings on a foot examination? Do they contribute to lameness, and what does their presence mean to the long-term athletic use and soundness of that horse?

Like Reid, who is regarded as the first equine radiologist, many veterinarians feel that ossified sidebones are not a problem.

"It has been my experience," Reid wrote, "that a horse with a good, wide foot and heel is usually not affected even by advanced ossification of the collateral cartilages."

Some anatomists feel that ossification begins at the junction of the cartilage and the coffin bone and is a normal consequence of aging. Fractures of the collateral cartilage are sometimes diagnosed but usually where the disorganized process of cartilage becoming bone leaves an island of bone within the collateral cartilage, which gives the false impression of a fracture. Even large sidebones in sound older horses are seen as having little to no importance by most clinicians. But bone production that is uneven or asymmetrical and bone production in an area where there normally is not bone in a young horse can be different cases.

In 1898, John A. W. Dollar, a member of the Royal College of Veterinary Surgeons, addressed the subject of sidebones in his text on the anatomy of the hoof and the principles of horse shoeing. Dollar cited Professor Anton Lungwitz's study of that era showing that 11.5% of 1,251 horses examined had significant and well-developed sidebones.

Dollar further stated that heavy, coarse-bred horses used for work on hard ground showed the most advanced development of sidebones, especially if they had small feet. Reid has echoed this observation in the modern era. He wrote, "Horses with good, wide feet and no contracture of the heels show little effect from collateral cartilage ossification."

Draft horses must carry their tremendous weight on their hooves and commonly work on hard surfaces. Consequently, they are at risk for the development of sidebones that might cause lameness. This condition is referred to as "hard at the heels" and is associated with small feet and narrow hooves.

What to look for

Veterinarians at Liphook Equine Hospital in Hampshire, England, have addressed the problem of ossification of the collateral cartilage in horses and have separated the development of this condition into four main causes:

• Poor foot conformation. Veterinarians and horsemen have long known that small feet, imbalanced hooves, uneven or sheared heels, and contracted heels all contribute to excessive loading forces on the back portion of the foot and heels when the horse moves. This concussive force, even after only short periods of time, causes the body to respond, and flexible, elastic cartilage will become bony in an attempt to help support the hoof. Sidebones in young horses generally indicate poor foot conformation.

  The conformational imbalance that causes sidebones can be genetic or can be the result of the second major cause of sidebone formation, poor trimming or shoeing, or both.

• Improper trimming. Trimming of the hoof that causes imbalance upon landing and loading and any shaping of the hoof that restricts normal heel and frog pressure will just as easily predispose a horse to sidebone formation. Proper trimming is the prime preventive measure for early sidebones.

  When having a horse trimmed or shod, owners should ask the farrier to watch the horse at a walk and a trot on flat ground so the shoer can evaluate the landing and loading forces before trimming the hooves. This means owners should make sure even young horses are handled to the point where this type of evaluation is possible. Too often, young horses are allowed to go without the handling and care that would lessen problems that must be dealt with later.

• Uneven limb conformation. Young horses with mild conformational challenges, such as rotated coffin bones (turned in or turned out); angular limb deformities, such as carpal varus (bow-legged) or valgus (knock-kneed); offset or bench knees; or any other structural problems in the leg can be at risk for sidebone formation.

  If these conditions make them carry their weight unevenly, they should be monitored closely from an early age and perhaps trimmed earlier and more frequently than usual. Splints and casts, specialty shoes, and surgical procedures, if necessary, should be used to help lessen the effects of these conformationally based imbalances. Attention to balancing the loading forces on a young horse's feet will pay later dividends and could help prevent sidebones.

• Direct trauma. Being kicked, stepped on, or otherwise injured in the area of the hoof where the collateral cartilage is vulnerable might lead to sidebone formation.

Take a closer look

Diagnosis of sidebones is generally not that difficult. Thickened tissue with decreased flexibility can be felt above the coronet at the sides of the foot. In severe cases, the coronary band might bulge or be lifted up in this area.

Most clinicians agree that lameness is rarely seen due to sidebones alone, though some horses could have slightly shortened, stiffer gaits and show some shortness of stride when turning toward the side of the hoof with sidebone, especially on hard surfaces. Most lameness in these horses is related to other structures in the foot, such as the navicular bone and its ligaments.

The imbalanced forces that cause sidebone formation undoubtedly cause increased wear and tear on other structures in the foot. These slightly more crucial parts of the hoof are involved in most lamenesses.

Prevention and treatment of sidebones entails trimming and shoeing to balance the loading forces on the hoof. Recommendations for shoeing horses with sidebones have not changed since the late 1800s. Dollar and the veterinarians at Liphook Equine Hospital support the use of flat, wide-webbed shoes with a rolled toe. The extra width at the quarters and heels help support the hoof and help encourage as much expansion as possible. The use of an "onion" shoe is favored by some farriers to provide the required heel support.

Formation of sidebones is the horse's way of responding to a problem, so it can be a useful signal to horse owners and trainers to take a closer look.

Overall, in most horses, especially sound older horses, sidebones are not significant. Younger horses with sidebones--usually found on radiographs taken for a prepurchase examination--should be evaluated carefully for conformation of both the feet and the rest of the legs. The presence of sidebones is a telltale sign that some source of imbalance exists or might have existed at one time but is now corrected.

If the horse is sound, working well, and has good, wide, balanced feet and correct legs, sidebones are less of an issue, regardless of age, and good farrier care will help ensure an athletic career for that horse.

If the horse has sidebones and is unsound, ask your veterinarian to take a close look at related structures in the foot. If the horse is currently sound but shows some conformational challenge that affects balance, however minor, it would be wise to evaluate the problem closely and determine if proper trimming and shoeing can help or if the conformation problem most likely will worsen with time and use.