Horse PET scan

The racing industry is leading the charge in identifying at-risk athletes before catastrophe occurs.

What do ice cream shop employees and racehorses have in common? Both populations are at risk for developing injuries based largely on how they perform their jobs.

Chris Riggs, BVSc, PhD, DEO, Dipl. ECVS, FRCVS, says the most memorable stress fracture he’s ever come across in scientific literature is “ice cream ­scooper’s hand.” As Riggs, director of the Hong Kong Jockey Club’s Equine Welfare Research Foundation and chief advisor of veterinary mainland engagement, notes, “That simply reflects that someone is serving out lots of lovely gelato using the same old scooping motion on a regular basis and loading the bones repetitively.

“Stress fractures are a manifestation of mechanical fatigue,” he says. “Take a paperclip and bend it once, it’s fine. Bend it 50 times, it’s probably okay. But on the 56th time, it suddenly snaps. The same thing happens if you take a bone and bend it.”

As a result of repetitive motion and loading patterns, bone suffers fatigue. The resulting microscopic internal damage can suddenly coalesce into a fracture. For ice cream servers, this means time off work with a broken hand. For racehorses, the consequences are often fatal.

Therefore, detecting stress-related wear is a critical part of keeping racehorses safe. While researchers are making strides in understanding which horses are at risk and identifying stress fractures before catastrophe occurs, they have much to learn.

Crucial Early Detection

“Stress fractures are small fissures in bone that can be painful and cause decreased performance,” says Mathieu Spriet, DVM, MS, Dipl. ACVR, ECVDI, ACVR-EDI, professor of diagnostic imaging at the University of California, Davis, School of Veterinary ­Medicine.

Veterinarians commonly identify them at specific sites within bones at the top of or above the limbs, including the humerus, scapula, tibia, pelvis, and mid-cannon bone. Stress remodeling—a process in which a bone remodels when exercise demands exceed what it can handle—is commonly seen in the fetlocks, particularly the sesamoids, Spriet says. The fetlock region, he adds, is the most common site of fatal fracture in racehorses worldwide.

In and of themselves, stress fractures and remodeling aren’t necessarily serious injuries and are even normal to an extent. They often heal well with conservative treatment, which can include rest, controlled exercise at a reduced intensity, and/or anti-inflammatory drugs. But if these changes aren’t identified, catastrophic consequences are a real concern.

“A stress fracture represents a progressive process that, if it’s allowed to go to its final conclusion, becomes a complete fracture, which may well be irreparable,” Riggs says. “If we diagnose it before that point, the chances of a successful outcome are above 90%.”

Identifying them can be easier said than done; while complete fractures and many soft tissue injuries produce consistent and sometimes marked lameness, stress fractures and remodeling can produce subtle or inconsistent clinical signs. Affected horses might develop a short or choppy gait. They might experience mild swelling or show pain on palpation of specific areas, Spriet says, or they might develop transient lameness.

“They’re severely lame typically a few hours after speed work or racing, and that lameness might be completely resolved in 24 to 48 hours,” he says. Some horses don’t display any signs at all.

While veterinarians typically examine and radiograph lame and underperforming horses, our sources say stress fractures and remodeling aren’t always visible on X rays, and X rays can’t penetrate the muscle at the top of the legs to reveal bone damage. Ultrasound can reveal some stress fractures, particularly in the humerus and pelvis, but it isn’t helpful in most other areas.

Current Diagnostic Options

The good news is advanced imaging techniques are more effective at revealing stress fractures and remodeling.

Nuclear scintigraphy

Full-body bone scans can help vets identify potential injury sites. A radioisotope injected into a sedated horse gathers where bone is changing (healing or remodeling) rapidly, and a gamma camera reveals those sites.

“If you want to screen for stress fractures through the whole body, then bone scan is the way to go,” Spriet says. “Bone scans allow imaging of the pelvis, tibia, and humerus, which are quite challenging to image with other modalities.”

MRI and CT

Standing magnetic resonance imaging (MRI) and computed tomography (CT) scanning both produce highly detailed 3D views of damage in the lower limb—specifically the fetlocks and surrounding structures—noninvasively and without much risk, as only a small amount of sedation is required. Vets can use traditional MRI and CT to image higher up a horse’s limb, but the patient must be placed under general anesthesia. This, of course, carries its own risks, making it a less conducive option for diagnosing an actively competing athlete.

PET scan

One of the newest imaging modalities being used in horses is also one of the most effective at identifying stress remodeling, particularly in the fetlocks. In 2016 Spriet and his colleagues were the first to perform a positron emission tomography (PET) scan on a horse. Instead of showing changes in bone size, shape, and density like most imaging modalities, PET scans can detect microscopic changes at the molecular level. This can give veterinarians a leg up in identifying stress-related bony changes before fractures develop. These scans can also reveal whether changes are actively occurring or represent old scars. Veterinarians administer a small amount of radioactive dye and sedate horses before imaging.

These imaging modalities, however, still come with challenges: “They’re medical procedures that require catheterization and sedations, and sedation close to a race can conflict with medication regulations,” Spriet says. They can also be expensive. Further, researchers, veterinarians, and racing regulators haven’t settled on thresholds or criteria that could mandate rest for a horse or prohibit him from starting in a race.

PET scan image

Areas of Advancement

“I feel so helpless every time a horse does suffer a fracture on the racecourse,” says Riggs. “However, the very nature of stress fractures makes it very hopeful in the sense that they’re a progressive process. We just need to develop a better way to screen for them and identify the horses that are at greater risk.”

Spriet agrees that researchers now have additional, more technologically advanced imaging tools with which to gather more and better information that ultimately could help pinpoint horses at greater risk of catastrophic injury. But it takes time to gather and analyze the data. Here’s what we know and what we’re learning:

Epidemiology

Riggs says researchers have identified risk factors associated with developing complete fractures. For example, “horses that have had a period of enforced rest, maybe after an injury,” he says. “When they return to work, those horses are at higher risk of sustaining a humeral (the bone between the shoulder and elbow joints) stress fracture.”

A potential complicating factor with these studies is horse numbers: “We’re dealing with a relatively small number of horses,” Riggs says. “We all get very upset, quite rightly, when we see a horse suffer a fracture when racing, but it’s still a small proportion of the overall population.”

Applying statistical tests to small numbers can yield unreliable results, he says.

Physical exams

Spriet says the physical exam is the “first line” for detecting issues. When horses are scanned following this exam, the imaging team has the benefit of knowing whether the vet noted potential problems or the horse appeared sound.

Veterinarians can certainly identify an unsound horse and recommend he not race based on a physical exam. But exams are best used in conjunction with other methods when assessing horses that appear sound but might be at risk of fracture. For instance, Spriet says, one research team showed if you only considered physical exam findings, “in order to prevent one horse from breaking down, you’d have to scratch over 100 horses.”

Biomarkers

Researchers have also looked at whether biomarkers in blood and/or urine could help identify horses with stress fractures and/or remodeling. Riggs says it’s unclear whether these tests would be specific enough to be the only tool used in screening, “but they could still have an important role in identifying subsets of the population that should be given greater scrutiny.”

Imaging

“I think that some form of imaging is the most likely way that we’re going to identify horses at risk with sufficient specificity to be able to say to someone, ‘Your horse is entered into a big race with a lot of money and prestige, but I’m sorry, even though it appears fine, we’ve identified an issue that means that it is not fit to race,’ ” Riggs says. “There needs to be a high level of confidence that you’re not withdrawing their horse without very good reason.”

As we’ve noted, researchers still don’t know exactly which imaging findings suggest a horse is likely to experience catastrophic injury. To achieve the most scientifically sound study, Spriet says, “we would need to image a large number of horses and let them all run, regardless of the imaging findings, to then assess which ones did well and which ones develop further injuries. Obviously, from an ethical standpoint, this is not okay.”

So, researchers are gathering as much data as they can from scans and studies, while putting horse health and welfare first. For example, Spriet and colleagues are gathering data on fetlock PET scans.

“A specific spot on the sesamoid bone will light up on PET scans in 5-10% of (unsound or underperforming) horses we scanned,” he says. “And we’ve learned from necropsies that this spot is commonly involved in breakdowns. So we can confidently say that horses with this finding on PET are at greater risk of breakdown.”

On the other hand, he adds, “we see some level of uptake in roughly 60% of the condyles of the cannon bones of the horses we scan. Condylar fractures are a potentially serious issue, too, but we need to refine the knowledge and reach more specifics; only a small portion of horses with condylar uptake will develop a complete fracture. We are working on figuring out if a more specific location in the condyle or a cutoff value of uptake can improve the prediction of horses at higher risk for fracture.”

Spriet says he sees common patterns in the more than 400 horses he’s scanned in the past couple of years, but new findings keep popping up.

“Just yesterday, I had one I hadn’t seen before,” he says. “That’s when I like to discuss it with the pathologists who do the racetrack necropsies. ‘Do the imaging findings fit the lesions observed on the post-mortems?’ If we see changes on imaging at a site where fractures are commonly seen on necropsies, we know we have a horse at risk for breakdown.”

More studies and scans could also help veterinarians, researchers, and regulators develop thresholds and cutoffs for not allowing horses to run.

Ideally, costs and other logistics notwithstanding, Spriet says he’d have horses imaged about four to six months into race training, when they reach maximal intensity, to try to predict trouble spots going forward. The patterns observed could help trainers adapt horses’ training and help veterinarians decide if and when to recommend further imaging. Subsequently, he’d screen horses with scans regularly, rather than just before big races.

“The stress fracture did not happen the day before the race,” he says. “It takes some time to develop. Scanning healthy horses every three to six months would likely be ideal to monitor the health of bones throughout the year and not miss developing issues.”

One challenge with this program is relaying results: “It’s tough when you have a talented horse training and running well that seems to be unsafe based on the scans,” he says. “It’s harder to sideline a top-performing horse than a horse with obvious clinical issues, but it is definitely the right thing to do.”

Take-Home Message

Identifying stress fractures and remodeling to prevent catastrophic injury can be a complicated process, but it’s slowly getting easier as researchers progress in their ability to pinpoint at-risk horses.

“We really have to make all the efforts we can to prevent breakdowns at the racetrack, and identifying stress fractures is a big part of that,” Spriet says.

Riggs agrees. “It really needs a commitment by everyone. … Relevant stakeholders need to be well-informed so that they understand the reasons behind what might seem like a harsh decision resulting in what appears loss to them.

“It’s not just the science,” he adds. “It must also involve education and a preparedness of people to embrace and apply good science and a willingness to put welfare in front of everything else.”



This article is from the June 2022 issue of The Horse: Your Guide to Equine Health Care. We at The Horse work to provide you with the latest and most reliable news and information on equine health, care, management, and welfare through our magazine and TheHorse.com. Your subscription helps The Horse continue to offer this vital resource to horse owners of all breeds, disciplines, and experience levels. Click here to access the digital archives of previous issues.