"We have recognized since we've had the ability to use magnetic resonance imaging (MRI) that deep digital flexor tendonitis is a common cause of lameness, but the etiology (cause) is poorly understood," said Sue J. Dyson, VetMB, PhD, FRCVS, of the Center for Equine Studies at the Animal Health Trust, at the 50th annual American Association of Equine Practitioners (AAEP) Convention in Denver, Colo., Dec. 4-8, 2004. "In our hospital population, horses that jump are at increased risk of injuring the deep digital flexor tendon (DDFT), especially elite show jumpers. We believe, based on histopathological studies, that there may be pre-existing degenerative changes that predispose some horses to injury."

Dyson and her colleagues discovered in a recent study that there were no significant differences in the angles of the distal phalanx (P3, or the coffin bone) between horses of mixed breeds, with and without DDFT injury. However, they found that when Thoroughbreds were considered separately from the other breeds, horses with DDFT lesions were more likely to have a more acute P3 angle (less upright of an angle between the concave solar border of P3 and the ground), which might increase strain in the DDFT and cause injury. Differences in the angle of P3 to the ground between breeds and different shapes of P3 among these horses might account for the masking of differences between horses with DDFT injury and normal horses.

"We know there's a very close relationship between the DDFT, the navicular bone, and the distal interphalangeal joint," said Dyson. In anin vivo study, the force exerted by the DDFT on the navicular bone in horses with navicular disease was compared with control horses. In the early stance of each horse's stride, there was almost double the peak force and stress on the DDFT in the horses with navicular disease. That increase can result in a potential vicious circle and exacerbation of pain.

It was concluded in another study that for every one degree increase in the angle of the solar margin, there was a 4% decrease in the peak force of the DDFT on the navicular bone.

Drawing information from these past studies, Dyson and her colleagues thought that horses with DDFT injury  would have a more acute angle of P3 to the horizontal compared with sound horses. So they determined the repeatability of measurements of various angles of P3 and the hoof capsule and tried to assess whether these measurements were related to DDFT injury. Additionally, they looked at the relationships between age, height, body weight, prevalence of injury and, in a separate study,  DDFT cross-sectional area.

The first study comprised two groups: Sound horses and lame horses with MRI-confirmed DDFT injury (but excluding horses with concurrent laminitis). They adopted strict inclusion or exclusion standards for radiographs used in the study--more so than any past studies that have been performed--and many radiographs had to be rejected or repeated.

The measurements showed four angles:

  • P�That of the dorsal aspect of P3 to the horizontal;
  • W-- That of the dorsal hoof wall to the horizontal;
  • C-- That of the concave, solar surface of P3 to the horizontal; and
  • S-- That of the solar border of P3 to the horizontal.

These measurements were performed by a single trained person, and initially each radiograph was analyzed 10 times to assess repeatability.

There were no significant differences in age and height between the groups, but the lame horses were significantly heavier, and therefore had a significantly greater weight to height ratio. There was some variation in the shape of P3 among horses. There were similar numbers of Thoroughbreds and Warmbloods in each group, but there was an overrepresentation of ponies in the sound horses and Thoroughbred crosses in the lame horses. "So perhaps there is some sort of genetic predilection to DDFT injury in Thoroughbred cross horses," suggested Dyson.

She concluded, "The hypothesis was unproven; however, there was a strong trend towards a more acute angle C (that of the concave solar border of the P3 and the ground) in the horses with DDFT injury compared with the sound horses.

"There were a number of complicating factors," she added. "There was the overrepresentation of ponies in the sound group. Also, it's well known that chronic lameness may result in changes in the hoof capsule, with it becoming more narrow and upright. This certainly could have confounded the results. However, we have limited knowledge to what extent changes in the shape of the hoof capsule are mirrored by changes in shape or angle of P3."

In a separate study comparing different groups of sound horses and horses with DDFT injury confirmed using MRI, the size of the DDFT was measured from MR images. There was a correlation between DDFT cross-sectional area and weight, but no correlation between cross sectional area and height of the horse. In the first study, the size of the DDFT was not measured, and although the lame and sound horses were of similar height, the lame horses were heavier and had a greater weight to height ratio. If the cross-sectional area of the tendon does not correlate with the weight/height ratio, then forces per unit area on the DDFT would have been greater in horses with DDFT injury in the first study than in the sound horses.

Dyson suggested that there might be yet other factors that could predispose some horses to injury of the DDFT--degenerative change and perhaps genetic susceptibility.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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