Although the causes of this condition are not fully understood, medical advances have led to better diagnosis and treatment.

For young horses with bright futures, the best bloodlines, training and attitude are nothing without good legs. Strong legs. Sound legs. A concert of bones, muscles, tendons and joints growing together in rehearsal for real work.

And inside these legs, which even on full-grown horses are no bigger in diameter than a nightclub bouncer’s biceps, any number of problems can arise before a horse enters serious training and strains a tendon on the cross-country field or stumbles in the dressage arena.

Osteochondritis dissecans, a developmental orthopedic disease better known as OCD, is a condition that has perplexed veterinarians for years, not because they can’t or don’t treat it, but because they can’t figure out why exactly it occurs and how exactly it could be prevented.

OCD is an interruption of the normal bone development process. Instead of soft cartilage degenerating and forming bone around joints as the horse grows, horses afflicted with OCD develop lesions. On X-rays, these lesions look like a dent, pit or unevenness in the cartilage or on the underlying bone. When this area of cartilage dies, it dissects itself from the main body in what looks like a flap, signifying OCD.

Those who work with older horses may not witness the development of OCD—if a horse is going to contract it, it will happen before the age of 2—but the aftermath of lameness can last a lifetime. In May, the disorder was the sole subject of a third international symposium in Sweden, where researchers and surgeons discussed questions about OCD that still lack comprehensive answers, such as why some horses essentially “grow out” of lesions while in others they develop into full-blown OCD and then require surgery.

“OCD is a huge problem in sport horses,” said Wayne McIlwraith, a veterinarian from New Zealand who is one of the leading researchers on equine joints and other leg issues. He’s also the director of the Orthopedic Research Center at Colorado State University.

McIlwraith said many people confuse OCD with the similar-sounding osteochondrosis, which relates to all problems of cartilage and bone development. Osteochon-drosis is simply the pathological condition that can lead to osteochondritis dissecans.

OCD is thought to develop after birth during bone development. It can affect any joint, and sometimes it affects the same joint in the opposite leg as well (both hocks, both stifles, etc.).

A Disruption In Natural Growth

In a normal maturation process, McIlwraith explained, joints develop from cartilage, and the bone grows in the center. But in horses with OCD, some of that cartilage that should have turned into bone fails to degenerate and instead stays thick as cartilage. This can cause the cartilage cells to be cut off from the blood supply and hence, from nutrients.

Now weak, the area of cartilage is susceptible to cracks or lesions. If these lesions progress to the point that they die and become flaps hanging off the cartilage or bone, debris from under the flap can float into the joint capsule. In severe cases, the entire flap can detach.

If debris floats into the joint from beneath the flap, joint inflammation, pain and lameness can develop. Surgery is often recommended to remove the flap. In some cases where lesions are detected early, horses have responded to more conservative treatments, such as long-term stall rest and a lean diet to keep extra weight from stressing the joints.

The process of removing the problem flap from a horse with OCD is a fairly common surgical procedure, said McIlwraith.

“Of all the orthopedic diseases we work with, OCD is not the biggest problem,” he added. “With the exception of the shoulder, our success rate [for surgery] is very high.”

Still, McIlwraith’s philosophy on surgery has changed over the years.

“If you had interviewed me 15 years ago, I would say that they all need surgery if they’re going to be athletes. But now, we see that some [lesions] go away in the first year of life. I’m always looking at radiographs and saying, ‘OK, this one can heal, leave it alone.…this one needs surgery if it’s going to be an athlete.’ ”

Based on his experience and research, McIlwraith offered how he typically handles OCD cases:

    •    Stifle
“Surgery results are good. Stifle OCD surgery generally gives the horse the same potential to
perform whether the discipline is dressage, racing or show jumping.”

    •    Hock
“Surgery results are similarly good. But hock problems can occur in four different places, and not every one of those places goes well. But overall, it’s good.”

    •    Fetlock
“If we don’t have a fragment, we don’t tend to do surgery. If it does have a fragment, we tend to go ahead with the surgery.”

    •    Shoulder
“It’s about 50 percent success rate; about half of the horses we operate on there go on to compete.” That may not be so encouraging until you think about it this way, McIlwraith said: without the surgery, the horse has zero chance of being sound.

A Fickle Condition

While the number of OCD cases is not necessarily increasing, Nan White, director of the Equine Medical Center in Leesburg, Va., said that veterinarians have gotten better at diagnosing the condition, which may make it seem as if there are more cases.

OCD can occur in all breeds, but the incidence rate seems to be higher in Thoroughbreds and Standardbreds. It is not generally thought of as a genetic disorder, but it does seem to occasionally follow “family lines,” said White. McIlwraith is working with researchers in the United Kingdom to more closely look for genetic biomarkers for OCD in blood samples of race horses.

If a young horse develops lesions, they aren’t usually detectable on radiographs until a horse is 6 months old. Yet it’s possible that even with the evidence of lesions on radiographs, the horse may not be showing any signs of discomfort or lameness. Sometimes lameness or swelling around the joint may not show up until later. The tough call here is to decide whether to do surgery anyway, or to employ more conservative treatments such as stall rest and limited hand-walking to see if the lesion goes away on its own.

McIlwraith is occasionally called in by race horse trainers or breeders to radiograph an entire crop of future  athletes. In many cases of horses that show lesions but that have not yet displayed clinical signs of lameness, he’ll  recommend that the flap be corrected with surgery before the horse goes to sale, in case it would become a problem in the future.

Other horses with lesions may not develop OCD, and the lesion may just sort of go away on its own. In one study  of Dutch Warmbloods in the Netherlands, foals from stallions that had sired a large number of horses with OCD were radiographed each month. Some foals developed lesions during their first eight months of life in their hocks, but then they healed. Other foals developed lesions in the first 12 months in their stifles, then healed.

But if the lesion turns into a flap, and the flap breaks loose, a horse will be lame. After that, McIlwraith said, surgery to repair the flap is typically necessary.

Size Matters

Experts have not pinpointed all the causes of OCD, but historically it’s been linked with poor nutrition and/or mineral imbalances. A balanced diet for mares and foals, which includes the essential minerals phosphorus, copper and zinc, is seen as particularly important.

Some research has shown higher rates of OCD in young horses who are fattened or who have experienced accelerated growth rates. The horse industry favors big horses, but nature seems to want to keep things small. Bigger horses usually experience more physical problems, as anyone with a mixed barn of hardy ponies and accident-prone Thoroughbreds can attest.

One study has shown that foals that grew faster than the average growth rate median were more likely to develop lesions that could lead to OCD, said Kevin Kline, a professor at the Department of Animal Sciences at the University of Illinois at Urbana-Champaign. Kline then studied the horses in the upper half of that fastest growing group and noticed that those young horses had far more lesions than any other group of youngsters.

For the past 20 years, Joe Pagan, president of Kentucky Equine Research, has studied how size has affected sales and racing performance in Thoroughbreds. Overwhelmingly, he said, the most successful athletes start out as overly large youngsters.

In the race records of 4,000 horses that Pagan had weighed and measured as yearlings, he found that stakes winners and graded stakes winners were significantly larger as yearlings than their peers. The horses who were smaller as yearlings had more starts, Pagan said, indicating they may have been more sound, but the bigger horses had more wins.

Pagan also studied the yearling prices and eventual careers of horses that had been sold at the Keeneland (Ky.) Thoroughbred sales. The smallest horses brought a median price of $20,000 each. Horses in the upper quartile in size brought a median price of $55,000.

“People will pay a lot more at sales for a yearling that’s big,” Pagan said. “It’s always been thought that you should grow horses slow and conservatively to reduce the risk of problems, but now we had this data that shows how performance seems to be linked to the size of yearlings. That makes us want to go against our instincts.”

Pagan said that some feed companies now are starting to change the makeup of their formulas from compositions with high starch and high levels of sugar to ones with higher levels of fiber. Fat and highly digestible components such as beet pulp and soy hulls are being increasingly used in horse feeds because they provide similar amounts of energy without coming from sugar or starch, which produce a high glycemic response.

“It’s like growing a skeleton,” Pagan said. “You want a steady growth of height without packing on the fat.”

Could Dietary Boron Help?

Kline doesn’t dispute the importance of proper nutrition for the growing horse, but as far as OCD is concerned, he thinks the problem starts far earlier than most people expect. Rather than occurring after birth, Kline has a theory that the problem may be starting in utero, with the mare.

Kline started by interviewing veterinarians who were seeing cases of OCD. Then they identified a group of youngsters with the disorder and manipulated their diets, taking regular radiographs along the way. Kline’s theory was that perhaps boron, which had proven successful in improving pigs with a condition similar to OCD, would in a dietary form be useful in horses. So they included boric acid in the feeds of about half the young horses studied.

“What we found was, it didn’t make a difference what we fed them, at 4 to 6 months of age, most of them hadn’t changed a whole heck of a lot,” he said.

That led Kline to believe that what the babies had was what they had and that no supplement was going to change that. Or, it could be that boron isn’t the answer.

However, boron seems like it should work, for a few reasons, Kline explained. First, studies have shown that OCD may be related to a lack of blood flow to the growth cartilage. And boron’s molecular makeup suggests that it could be transported by collagen fibers, which hold cartilage together.

Kline theorizes that boron, which is a cross-linking molecule according to studies in plants, may be able to keep blood vessels open so that enough blood gets to the cartilage, and the cartilage can then properly turn into bone. To further test his theory, Kline hopes to feed dietary boron to a wide study of mares to see if it has any effect on their offspring. He’s currently pulling together funding sources for the study.

As far as future treatments for OCD beyond surgery, McIlwraith at Colorado State is enthusiastic about the new research in gene therapy and stem cell therapy for horses. Just as such therapies are making inroads in the human medical field, McIlwraith sees them being equally beneficial in treating horses.

“We’ve already shown good effects with gene therapy in osteoarthritic horses,” he said. “We’re also doing stem cell therapy for soft tissue injuries in clinical cases.”

For the general public, such novel treatments are not far away.

McIlwraith added: “We’re really making good progress with these things.”