Susan M. Stover, DVM, PhD, Dipl. ACVS, presented the Frank J. Milne State-of-the-Art Lecture at the 2022 American Association of Equine Practitioners Convention, held Nov. 18-22 in San Antonio, Texas on skeletal injuries in equine athletes and training concepts to prevent them.

Stover, professor of surgical and radiological sciences at the University of California, Davis, School of Veterinary Medicine, began by highlighting the importance of the topic, stating, “We are all looking to improve our rates of catastrophic musculoskeletal injuries. Injury prevention, not just catastrophic ones, will help enhance equine health and welfare, minimizing economic losses and attrition in competitive horses.”

Repetitive Injury and Adaptation

Just as in marathoners, baseball players, or any elite athlete, repetitive “occupational”  activities in horses can contribute to injuries.

“We didn’t recognize until the 1990s that catastrophic (or career-ending) injuries were occurring where there were stress fractures,” Stover said.

“Bone is a living structure, capable of repair and regeneration. This is why a fracture is not appreciable after repair in the future. Skeleton is dynamic. It gets rid of mineral when it doesn’t need it, and vice versa. The nature of training will determine if the bone gets strong or weaker.”

When loaded, the cannon bone, for example, compresses from the proximal to distal end (i.e., gets shorter/changes length)—a process Stover referred to as strain. In response, bone can increase in diameter, decreasing its risk of damage.

As an example, Stover described the changes in the cannon bone of a Thoroughbred racehorse.

“Newly imposed speeds and increased exercise intensity during training increases strains on bone,” she explained. “There is then a little bit of swelling on dorsal shins (the front of the shins), and cross-section of bone in that region reveals linear cracks near the cortex/periosteum (the outer lining of the bone). The cracks are not destroying anything. Instead, new layers of bone are produced. This is adaptation. In trained horses the dorsal cortex is thicker on cross-section and the medullary (inner) aspect of the bone is no longer in the middle.”

That adaptation, however, must be work-specific. In the racehorse example, the cannon bone adapted in a specific location.

“If you take that horse and suddenly put it in eventing, he may get injured because the adaptation was not matched to the new athletic event,” Stover explained.

Repair, however, takes time. While osteoclasts (cells that break bone down) do their jobs very quickly, osteoblasts (cells that create new bone) are slow. Stover compared the adaptation process to construction—it takes no time at all to tear down an old house, but it takes time to build the one replacing the original.

“But once the repair process by the osteoblasts is completed, the bone has increased resistance to crack propagation and an increased fatigue life, which increases the number of cycles that bone can sustain without injury,” she said.

Insufficient Bone Adaptation and Catastrophic Injuries

What happens if you continue to train and compete before the adaptation process is complete?

“The result would be more damage and potentially fracture,” said Stover. “Catastrophic injuries are not due to a bad step.”

Instead, preexisting injury can be found in more than 85% of catastrophic injuries, which means we have the opportunity to intervene before catastrophe occurs.

Similar processes happen in the bone supporting cartilage in joints. In these locations the bone can adapt to new or increased activities but is also susceptible to damage and repetitive overuse injuries.

Managing the Bone Adaptation Period

Typically, elite equine athletes receive high levels of care, said Stover, with optimal nutrition to support healing rates (repair and removal of microfractures). Thus, injury will be a result of the magnitude of the load on the bone as well as the number of times the leg is loaded (i.e., the number of strides the horse takes).

“The entire musculoskeletal system is just a series of levers connected by joints,” she said. “The pastern and hooves are levers for the fetlock. When the length of the lever is extended, injury ensues. “Horses with a long-toe, low-heel conformation, for example, have 3.5 times increased odds of suspensory apparatus injury.”

For this reason, shoeing and conformation are important in injury prevention because hoof conformation affects load magnitude on the limb with each stride.

With increased exercise intensity, the forces between the hoof and ground (ground reaction forces) increase.

Researchers can assess exercise intensity based on the distance covered during both works and races. If we graph those distances over time, the slope shows the intensity of exercise. A higher slope represents higher-intensity exercise, and we can compare these rates between horses.

“If you look at large numbers of horses, you can see high-intensity exercise increases risk of injury,” Stover explained. “For catastrophic injury of the fetlock and proximal sesamoid bone fractures, there is a difference in the intensity of exercise with affected horses having been trained at a higher intensity.

“Hoof conformation, shoeing, and training and competition schedules are all manageable when we have good discussions with the trainers,” she added.

Myths about Catastrophic Racing Injuries

Myth No. #1: Racetrack surface causes clusters of catastrophic injuries.

“Surface does play a big role in injury,” said Stover. “If you jog on the beach, the character of the surface changes as you approach the water. You tailor where you run for optimal support without it being too hard or inefficient. For horses, we’re just struggling to find the best place for the horse on the beach, so to speak.”

Dirt surfaces cause greater fetlock hyperextension, she said, potentially resulting in sesamoid fractures and soft tissue ruptures. Dirt also has higher ground reaction forces- those exerted by the ground on a body in contact with it. Synthetic appears better than turf and dirt, but some synthetics have shorter slide, which might also affect injury risk.

To complicate things, not all synthetics are alike, and how the race surfaces are managed also matters.

“If a horse steps on a freshly harrowed surface, it is different than stepping into another horse’s previous footprint,” said Stover. “Harrowing, water content, additives, etc. can all play a role in ‘what the horse sees.’”

While the public often blames the racetrack for breakdowns, Stover said she believes surface is often the scapegoat.

“In reality, these injuries are the result of multiple factors acting simultaneously,” she said. However, a high-intensity exercise likely predisposed the horse to injury no matter what the surface.”

Myth No. #2: It is impossible to know when a horse is at risk for injury.

“This is false,” emphasized Stover, noting that injuries are related to three things:

1. Insufficient conditioning. This is a common cause of scapular (shoulder blade), humeral (the bone between the shoulder and elbow joints), and tibial (the long bone that stretches from stifle to hock) fractures, which occur early in training with high increases in training load, she explained, even if horses aren’t going at their highest speeds.
2. Injuries due to deconditioning after rehabilitation. These occur soon after return to training after a lay-up when there are high increases in load. “The horse already knows what to do and seems keen, but the bone is no longer adapted to those loads,” explained Stover.
3. Injuries due to overtraining. Such injuries occur later in training with high-intensity training for a long period. The bone never has the opportunity for recovery between bouts of exercise. An example of an overtraining injury is a pelvic fracture in an older horse.

Myth No. #3: It’s easy to determine injury in individual horses.

“If we pay more attention to a horse’s behavior using tools such as Sue Dyson’s ridden horse pain ethogram (RHpE), we may have more chances to pick up earlier stages of disease,” said Stover. “Ideally, we should be backing off of training earlier rather than waiting and having to lay the horse up later and then deal with deconditioning and reconditioning.”

In addition to the RHpE, Stover recommended considering all available tools from the physical exam to advanced imaging—bone scan (scintigraphy), magnetic resonance imaging (MRI), computed tomography (CT scan), and positron emission tomography (PET).

“The PET scan, which is a 3D version of a bone scan, can now be done standing and is a very exciting tool we now have available at some racetracks,” she said.

Myth No. #4: Tradition should determine training schedules.

Training induces cycles of damage and repair to increase strength in a step-wise manner and, ideally, result in skeletal adaptation. The timing of the next increase in intensity level relative to the repair stage will dictate a horse’s ability to adapt or incur risk of injury.

For fitness, relatively little exercise is required to maintain bone mass. But the work must be relevant to what the horse is doing.

“Athletes aren’t born with racehorse musculoskeletal systems,” said Stover. “They need time to adapt. They’re built with training,” Stover said.

Summary: Racehorse Injuries are Preventable

In Stover’s opinion, musculoskeletal injuries in athletes are preventable because injuries are simply acute manifestations of a chronic process. We can influence the balance between two competing rates of damage accumulation and repair, she said.

“Over the past few years we have seen a 40-50% reduction in catastrophic injuries in racehorses in several major racing jurisdictions,” said Stover. “I am extremely optimistic that we can further decrease this rate, and the industry is already demonstrating that we are capable.”