MRI to Identify Bone Changes in Racehorses (AAEP 2012)

Horses can't describe brewing musculoskeletal discomfort the way human athletes can, so trainers and veterinarians don't know which horses to put on the proverbial bench to prevent career- or even life-ending injuries. But an equine research team has been using MRI to detect bone changes that could indicate a horse is at risk for catastrophic fetlock fractures before an accident occurs, allowing such career- or even life-saving intervention.

John G. Peloso, DVM, MS, Dipl. ACVS, owner, partner, and surgeon at the Equine Medical Center of Ocala, in Florida described the team's research on fetlock and sesamoid fractures at the 2012 American Association of Equine Practitioners Convention, held Dec. 1-5 in Anaheim, Calif. Peloso relayed that the fetlock joint is the most common location of musculoskeletal disease leading to the euthanasia of Thoroughbred racehorses in the United States, United Kingdom, and Hong Kong. He cited several studies by Sue Stover, DVM, PhD, Dipl. ACVS, of the University of California, Davis, and colleagues that demonstrated catastrophic failure of the fetlock was responsible for more than 50% of cases submitted to the California Horse Racing Board (CHRB) postmortem examination program (which requires all horses that die within the racing enclosure at CHRB tracks to undergo a necropsy) over 15 years.

Additionally, he said, Stover et al. have found that pre-existing disease is present in almost every major bone that racing horses fracture.

Peloso said MRI is veterinarians' preferred imaging modality for bone disease, and evidence suggests it could be used prerace to identify bone abnormalities of the fetlock before injuries occur. To investigate this, he and colleagues evaluated standing MRI as a potential prerace tool for fracture-risk assessment by comparing differences in bone disease between horses with fractures (cases) and horses without fracture (controls).

The Study

Peloso and colleagues performed standing (low-field) MRI on 21 limbs of Thoroughbred racehorses euthanized within 48 to 72 hours of sustaining a catastrophic front fetlock fracture and 16 control limbs from racehorses euthanized for reasons other than front fetlock injury (such as colic).

Of the case horses, nine had condyle fractures (a fracture of the bottom or distal end of the cannon bone that fits into the fetlock joint) and 12 had proximal sesamoid bone fractures (fractures of the small, nodular bones located at the back of the fetlock).

Peloso then compared bone changes (e.g., increases in bone density or increases in bone marrow edema) detected in fractured limbs on MRI with those found in nonfractured control limbs. The team also considered horses' age, sex, and racing history in the assessment.

The Findings

Upon reviewing their results, the team found no significant differences between case (both condyle and sesamoid fractures) and control horses when considering age, sex, and affected limbs, and they noticed no significant differences in racing history, with the exception of horses with sesamoid fractures tending to make more starts than control horses.

Peloso also found that horses with catastrophic condyle fractures had significantly higher signal (which indicates bone marrow edema or fluid within the tissue space [i.e., edema from a fracture]) in STIR images (short tau inversion recovery, which is used to highlight fluid presence) when compared to control horses.

Interestingly, he said, this bone marrow edema often was present before the fracture occurred, providing further evidence of established prefracture disease.

No bone marrow edema was evident in sesamoid bone fractures, he said, though he noted that the presence of the fracture made it difficult to detect subtle bone changes.

Of the condyle fractures, Peloso explained that this study further supports what some researchers have been suggesting for years: MRI is useful for identifying racehorses at risk for sustaining catastrophic fetlock fractures. (Veterinarians in the U.K. have been successfully using this technology for this purpose for several years, he added.)

Veterinarians aren't sure exactly why sesamoid bone fractures occur, but Peloso has developed a hypothesis based on what researchers know about pre-existing disease and racehorse fractures. In evaluating the MRIs from the current study, he noticed that 18 of 20 horses with sesamoid fractures had pre-existing abnormalities in the opposite fetlock joint; the majority of those cases (13/18) had more damage in the opposite fetlock than the fetlock that sustained the fracture.

Based on this, Peloso believes the horse "protects" the limb with the pre-existing injury and, in turn, places extreme force on the limb that eventually sustains the sesamoid fractures.

He gave an example of one horse that had undergone two surgeries to repair a condyle fracture. Although the fracture had healed, he suffered a proximal sesamoid fracture in this same limb due to subchondral bone disease (bone on bone) in the opposite fetlock joint. Peloso believes this horse was protecting the pre-existing bone disease in the opposite fetlock and placed extreme forces on the healed condyle fracture leg, which caused the sesamoid bones to fracture.

Peloso and colleagues are conducting further research and analysis to test this hypothesis, he said.

The Recommendations

The challenge, Peloso said, is identifying horses for pre-race MRI, as current statistics indicate catastrophic racing injury rates at less than two horses out of 1,000 starters.

Peloso suggested that racing commission veterinarians, track practitioners, trainers, owners, and even grooms--essentially anyone that sees and works with the horse on a daily basis--could identify animals not quite working up to par for possible standing MRI examinations (other imaging modalities, such as radiographs, are unable to detect the bone changes studied, he noted.).

Identifying trouble spots early would allow veterinarians and trainers to implement proper therapeutics--ranging from stall rest to eliminating high-intensity exercise for a time--that would lessen the horse's chances of experiencing a catastrophic breakdown. Peloso noted that in these cases, standing, or low-field, MRI provides plenty of information for practitioners to work with. Unlike high-field MRI, this method doesn't require general anesthesia, which is an added benefit for trainers with horses preparing for races.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.