Stover: Don’t Blame The Racing Surface And Other Lessons From Studying Musculoskeletal Injuries In Thoroughbreds

When it comes to racehorse injuries, there are a lot of popular misconceptions floating around the racetrack.

At the most recent annual convention of the American Association of Equine Practitioners, Dr. Sue Stover gave the event's centerpiece Frank Milne State of the Art Lecture and tackled a series of myth-busters about racehorse injuries.

Stover is widely recognized as one of the leading researchers on the topic of musculoskeletal injuries and biomechanics in racehorses. Stover completed an internship and surgical residency at University of California-Davis before embarking on private practice, and later returned to UC-Davis as a researcher and professor of surgical and radiological science. She has won numerous awards for her pioneering research, which has guided welfare regulation in racing and influenced everything from training practices to shoeing techniques to racetrack surface maintenance. She currently juggles her academic responsibilities with a position as chair of the Racetrack Safety Standing Committee of the Horseracing Integrity and Safety Authority and a member of the Authority's board of directors.

Here's a short sampling of the common misconceptions Stover addressed.

–Myth: Racehorses break down because they have weak bones.

As we've written about before, bone in most species (not just horses) responds to the force and concussion it experiences in a process called remodeling. Stress from strenuous work causes tiny microfractures in bone. Damaged bone is cleared away by cells called osteoclasts, and new bone is laid down in its place by osteoblasts. The new bone will be stronger and denser than the previous bone, because the cells will learn it needs to withstand more force.

Horses are constantly undergoing this damage/repair cycle, and we're still learning about its timeline and the many factors that can change that timeline.

Stover said that because the skeleton is dynamic, responding to the amount of work the horse is undergoing, a racehorse has denser, stronger bone than an unexercised horse.

–Myth: It's impossible to know whether a given horse in the shed row is predisposed to a catastrophic musculoskeletal injury.

Stover said a horse is at a higher risk of serious injury in one of three scenarios – the horse has had insufficient conditioning for the work they're being asked to do; they've undergone deconditioning during a layoff, or they've been overtrained.

All of these situations present increased risk because they're circumstances in which the remodeling cycle could get out of sync – either the bone hasn't undergone enough stress to stimulate stronger growth, or it's in the middle of a repair cycle and hasn't had time to put down the newer bone yet.

When grouping fractures by type, Stover sees patterns she thinks support this idea. She says more shoulder or tibial fractures occur in a horse who's early on in their career. One study of California injury data found that 56 percent of shoulder fractures happened in training, while 44 percent came in races. A third of the horses in that study hadn't yet made their first starts.

Different data showed that 90 percent of complete humeral fractures took place in training rather than racing, often at a slow gallop, and the majority of those horses were unraced. A horse had a 61 times higher risk of a humeral fracture coming immediately after a lay-up versus another time in the horse's career, which makes Stover believe the cause may be bone substance loss. The bone may have deconditioned while the skeleton wasn't undergoing the same stressors.

Overtraining injuries usually happen later in the training program and are seen in horses that have been doing high intensity work for a long period without a break, meaning they often happen in older horses. These overtraining injuries often include pelvic fractures near the sacroiliac joint, which Stover thinks is related to the repeated movement of a horse's pelvis with each stride. Overtraining is also associated with fetlock fractures. Fatal fractures of the proximal sesamoid bones, which form the back of a fetlock, are known to be associated with more works, races, and races per year versus horses who die from other causes.

“I'm concerned that while we've known about humeral fractures for coming on 30 years, we haven't reduced the incidence,” said Stover. “Similar with scapular fractures.

“I think we're most comfortable examining the distal (lower) part of the limb. The proximal (upper) part of the limb is more challenging.”

–Myth: Racetrack surface is THE cause of cluster breakdowns.

Stover's research has shown that of course the surface a horse is working over can significantly impact their movement. We've been able to quantify the differences certain types of surfaces (like dirt versus turf versus artificial) can make on safety and injury rates, but of course there's a lot of variability in surface components within each of those types.

Even within the same surface with consistent management, the experience of the surface for two different horses may not be the same. Stover points out that when one horse steps in the footprint of another, they're interacting differently with the surface than the first horse did. That surface is now a little more crushed, and this will go on through the day until the next renovation break. That doesn't mean the surface is dangerous, just that every horse's experience of the same track is unique.

In reality, fatal injuries are multi-factorial.

“The easiest thing to do is blame the racetrack [surface],” said Stover. “The easiest thing to do but potentially a really expensive thing to do is to do something different with the racetrack. The track likely influenced what was going on, but these horses were likely set up to have these injuries no matter where they were.”

–Myth: Injury detection and diagnosis are easy.

One of the trickiest things about identifying horses with chronic issues is the fact those issues may be bilateral. It's easier for trainers or veterinarians to see a horse shifting weight from one foreleg or hind leg to the other, but if the horse has some discomfort in both, there won't be the same clear swing in one direction.

Stover's examination of necropsies of racehorses who have suffered fatal injury frequently show damage in the opposite limb. This suggests that horses accumulate damage in two legs simultaneously (usually both fronts or both hinds), even though the fatal injury may only be seen in one.

We do know that statistically, previous lameness is associated with a higher likelihood of severe injury later. Stover's research indicates that diagnosed lameness up to three months earlier still elevates a horse's risk of fatal injury by 4.3 times.

“I don't know what to do about it other than to acknowledge that if you've had a horse that has had problems, to pay attention to them for a while,” she said.

This can mean observing not just a horse's way of moving but also their behavior. Horses who don't want to train, don't want to load in a trailer or a starting gate may not be contrary – they may be indicating that something about that particular activity is physically uncomfortable. Stover pointed to Dr. Sue Dyson's pain ethogram which sets out parameters like head carriage, attitude of eyes and ears, tail swishing, etc. to give clues about pain in horses under saddle.

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