Racehorses with specific conformation are more likely to have certain musculoskeletal injuries, according to two recent studies completed at Colorado State University (CSU). At the 2003 American Association of Equine Practitioners' convention, C. Wayne McIlwraith, BVSc, PhD, DSc, FRCVS, Dr.medvet (hc), Dipl. ACVS, director of CSU's Gail Holmes Equine Orthopaedic Research Center, presented results from the studies in which objective measuring was used to determine which limb conformations predispose the racing Thoroughbred and Quarter Horse to musculoskeletal injuries ranging from minor to catastrophic.

Genetics, racing surface, number of starts, age of the horse, pre-existing disease, conformation, and trauma have all been implicated as potential factors in the cause of racing and training injuries. Racing surfaces, number of starts, and trauma have all been examined independently and objectively, but most reported relationships between injury and conformation are based on practical experience of the veterinarian and logical hypotheses. Tina Anderson (who worked on this project for her PhD) and McIlwraith developed a novel way to objectively determine what to look for structurally in racehorses that might predispose them to injury.

In the first portion of the study, 115 3-year-old Thoroughbreds (all bred and raised at one farm) were photographed from the front, rear, and side with special markers placed in specific places. All photographs included a ruler to ensure correct scale. McIlwraith and others measured lengths and angles of conformation with a software program, and they used an objective method of grading the degree of offset knees (bench knees).

Next, McIlwraith and colleagues recorded clinical observations, clinical conditions (including diagnoses from the radiographs), and made subjective evaluations of limb rotation in each horse. These parameters were collected every two months from birth to cessation of the study.

The researchers were able to associate several injuries with conformational variables, including effusion (the escape of fluid into a part or tissue) of the front fetlocks, carpus (specifically the right knee), and hind fetlock; fracture of the right or left carpus; problems with the right front fetlock; and problems with the hind fetlock.

Fetlock problems largely were attributed to offset knees, implying changes of stress in the fetlock joint in horses with this conformation. The most common finding was effusion of the front fetlock joints (28% and 31% for right and left, respectively), which wasn't surprising since inflammation, synovial effusion, and varying degrees of lameness are often seen in horses in training.

In looking at carpal angle (how straight the leg is while viewed from the front), the researchers found that as the angle increased (an increase in carpal valgus means an increased angle of more than 180 degrees), carpal effusion and fractures decreased.

"A straight leg would be 180 degrees," explained McIlwraith. "As we saw it become more valgus, we saw an increase in the carpal angle. The odds of injury were down by a factor of 0.68 for every one degree increase in carpal angle."
This is important to Thoroughbred owners because buyers generally seek a straight leg, and often owners request surgical manipulation of a horse with carpal valgus to make the forelimb straighter. "By having a really straight leg, we may be increasing carpal problems," he explained.

Researchers also found that long toes were associated with knee problems, and that having a longer scapula (shoulder blade) decreased the likelihood for forelimb fractures.

Quarter Horse Study
The second study was completed using 160 2-year-old Quarter Horses in training at Los Alamitos Race Course in California. The project again involved Anderson and also Dr. Nancy Goodman and Dr. Ricky Overly at Los Alamitos. All of the animals were unraced and had no known racing injury or lameness. McIlwraith and colleagues collected and analyzed the data as they did in the Thoroughbred study.

In the Quarter Horse study, the conformational findings that were statistically significant were less in number. However, it was found that the longer the humerus (the bone between the shoulder joint and the radius, which is the forelimb above the knee), the more likely a first phalanx (upper pastern bone) chip fragment would occur in the left foreleg or that the horse would develop synovitis/capsulitis in the knees. The horses were more likely to sustain carpal chip fragments as the length from their elbow to the ground increased. Also, for every degree that the shoulder was more upright, the horse was more likely to sustain coffin joint fracture, just as horses with more upright pasterns were more apt to develop capsulitis and synovitis. Finally, just as in the Thoroughbred study, offset knees increased incidence of fetlock synovitis and capsulitis.

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

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