"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