The Microdamage Before the Break

Identification of proximal sesamoid bone lesions contributes to 41% reduction in California racehorse fatalities

Decades of research at the UC Davis School of Veterinary Medicine have provided scientific understanding of equine musculoskeletal injuries. Longstanding collaborations between the university, the California Horse Racing Board (CHRB), and industry representatives have enabled the successful application of research results to enhance equine welfare. Combined with advances in diagnostic technology, this has contributed to a reduction in California racehorse fatalities.

The equine skeletal system includes bones, tendons, ligaments, and cartilage. The bones withstand the combination of a horse’s weight, speed, and the interactive forces with the ground surface during movement. Musculoskeletal injuries are common causes of decreased performance in equine and human athletes.

The most common cause of fatal injury among California Thoroughbred racehorses is proximal sesamoid bone (PSB) fracture. The PSBs are a pair of bones in the suspensory apparatus that supports the back of the fetlock joint. When they fracture, the fetlock loses support and the horse cannot bear weight on the limb. These fractures are related to stress fractures and occur because the bones are unable to repair accumulated microdamage caused by repetitive high loads. Normally, damage is continually repaired by removal and replacement of damaged bone. When bone removal is faster than replacement, temporary bone loss can create weakness and susceptibility to fracture. Until recently, there was no way to identify horses at risk for PSB fracture. A recent UC Davis study discovered and characterized changes that precede PSB fracture and put horses at risk for catastrophic fracture. Sarah Shaffer, a Ph.D. candidate in mechanical engineering who conducts research at the J.D. Wheat Veterinary Orthopedics Research Laboratory under the direction of Dr. Susan Stover, examined PSBs from racehorses that died due to PSB fracture during racing or training.

Microcomputed tomography, high-detail x-rays, tissue stains, and other techniques showed a bone bruise below the joint surface that was not present in unaffected horses. This lesion had a focal region of bone loss and evidence of high levels of microdamage.

These typical stress fracture characteristics provide guidance to veterinarians for injury prevention (via screening) and treatment. With the concurrent introduction of positron emission tomography (PET) scanning of PSBs— pioneered at UC Davis and now in use at Santa Anita Park and Golden Gate Fields—the discovered changes allow affected horses to be identified and rehabilitated before returning to training and competition.

“These research findings were key to developing injury prevention strategies that contributed to the 41% reduction in California racehorse fatalities over the previous year,” Dr. Stover said. “Knowledge of the warning signs of imminent catastrophic fetlock injury allow for detection of affected racehorses and their rehabilitation—saving horse lives and preventing jockey injuries associated with racehorse falls.”

The CHRB Postmortem Program, operating in partnership with the California Animal Health and Food Safety (CAHFS) Laboratory at UC Davis, identifies the pathology and provides scientific understanding of the cause for racehorse fatalities in California with the goal of improving health, safety, and welfare.

“Dr. Stover’s research has had an immeasurable positive impact on racehorse safety and welfare,” said Dr. Jeffrey Blea, Equine Medical Director for the CHRB.

Read the full report “Subchondral focal osteopenia associated with proximal sesamoid bone fracture in Thoroughbred racehorses” published in the Equine Veterinary Journal at https://beva.onlinelibrary.wiley.com/ doi/10.1111/evj.13291.