Two tiny bones located at the back of the fetlock—the proximal sesamoid bones—can cause a world of trouble for horses (particularly racehorses) if they’re damaged. The consequences of sesamoid bones fractures can range from time off from training and racing for minor ones to death if a serious injury occurs.

Because sesamoid fractures are common in Thoroughbred racehorses, researchers are working to better understand them and their predisposing factors. Scott E. Palmer, VMD, Dipl. ABVP, the Equine Medical Director for the New York State Gaming Commission, and colleagues recently completed a preliminary study on the risk factors associated with biaxial sesamoid fractures. He presented the results at the 2016 American Association of Equine Practitioners’ Convention, held Dec. 3-7 in Orlando, Florida.

Biaxial proximal sesamoid bone (BPSB) fractures—when the horse breaks both sesamoid bones in the same leg—are severe and often associated with damage or rupture of the suspensory ligament and flexor tendons, which run behind the fetlock joint. In some cases, these fractures also break through the skin, leaving them open to environmental contamination. Surgical repair can be complicated or impossible. As a result, Palmer said, veterinarians’ only option with many of these fractures is euthanasia.

In their study, Palmer and colleagues evaluated the histories of 20 horses with BPSB fractures as well as 40 control horses, which were randomly selected from the fields of the races in which the case horses suffered their catastrophic injuries (the “incident race”). They examined 82 different potential risk factors for each horse to find associations with BPSB fractures.

Some key findings:

  • Case horses had fewer starts in their second and third years of racing compared to control horses;
  • They also had fewer high-speed workouts in the 12 weeks leading up to the incident race than did controls;
  • Case horses had more time off in the eight weeks leading up to the incident race than controls; and
  • Case horses were more likely to drop in race conditions by two classes (which are based on the participant’s skill level) between penultimate start and the incident race than controls.

“Previous studies have shown an association between increased exercise intensity and fatal musculoskeletal injury,” Palmer said. “The results of this study suggest that horses at increased risk for BPSB fracture may actually experience reduced exercise intensity in the months leading up to the incident race. … Horses can experience periods of decreased training during their career for many reasons, but the most common reason is unsoundness. Therefore, the inability of these horses to train and race at the same level as their peers (the control horses) in the months leading up to the incident race is likely an indication of subclinical or subtle unsoundness in the case horses.”

While there’s still more research to be done in this area, Palmer said trainers and regulatory veterinarians alike can use this data in their day-to-day practices.

“If (trainers) have horses in their stable that are not able to keep pace with the routine training programs of their peers, trainers should have a thorough soundness examination performed by a veterinarian,” he said.

The emphasis of this exam, he said, should be to rule out sclerosis (hardening) of subchondral bone (located beneath the cartilage) in the cannon and sesamoid bones, which are associated with increased risk of fracture.

Such examinations should include diagnostic imaging, he added.

Meanwhile, he said, “When veterinarians perform routine clinical examinations, the first step in that process is to gather a relevant history. Regulatory veterinarians can use this information by incorporating a review of exercise history (past-performances) into their pre-race examination protocols to identify horses at increased risk for this type of injury prior to performing the physical inspection. Trainers should be prepared to explain periods of extended gaps in training or changes in the training program to the examining veterinarian at the time of this examination as part of their due diligence.”

Moving forward, Palmer and colleagues are building on this preliminary study and examining data from an expanded study of 79 cases and 158 control horses.

“The increased number of horses in this study will provide much greater statistical power to investigate the significance of a wide range of risk fractures for association with BPSB fracture,” he said.

Additionally, he said, related studies are currently underway at Cornell University, in which researchers are investigating pre-race examination findings specific to the fetlock joint and use of diagnostic imaging to help identify horses at increased risk of BPSB injury.

“As stewards of the horse, we are obligated to do whatever we can to reduce injuries at the racetrack,” Palmer concluded. “Identifying horses at increased risk of injury is a core priority for this effort. The data from this preliminary study is a first step to help trainers and veterinarians identify horses that should undergo veterinary examination to rule out orthopedic conditions that can lead to catastrophic biaxial proximal sesamoid bone fractures.”