Proximal suspensory desmopathy (PSD) causes debilitating, possibly career-ending, hind-limb lameness, particularly in sport horses. But new study results could help owners and veterinarians prevent it—or at least select to avoid it. British researchers have learned that PSD development could be associated with straight hock conformation.

For every degree of increase in hock angle (leading to straighter hocks), the researchers found a 12% greater chance one of their study horses would have PSD, said Sue Dyson, MA, Vet MB, PhD, DEO, FRCVS, head of clinical orthopedics at the Animal Health Trust Centre for Equine Studies, in Newmarket, United Kingdom.

Horses with PSD have inflammation and tissue damage in the upper part of the suspensory ligament, a structure that connects to the top back of the cannon bone, divides into two branches that attach to the proximal sesamoid bones, and lies under the superficial and deep digital flexor tendons and the check ligament. Traditional treatment approaches have generally involved an extended, expensive period of confinement or inactivity, although results are often poor, and Dyson’s group usually recommends surgery.

In this study, Dyson and her fellow researchers investigated 194 horses with confirmed hind-limb lameness caused by PSD or control horses that were sound or had unrelated lameness. They used markers on standardized anatomical sites on the horses’ legs, photography, and image technology to accurately measure hock angles. Jenny Routh, an intern under Dyson’s direction, presented their study at the 2017 British Equine Veterinary Association Congress, held Sept. 13-16, in Liverpool.

They found that horses with PSD had larger hock angles than horses with other hind-limb lamenesses and horses that were sound, Dyson said. They also noted that Warmblood horses had a higher risk of PSD than other breeds (“which may suggest an inherent predisposition to injury,” Dyson said), although they did not have larger hock angles. Furthermore, they observed that dressage horses were more likely to have PSD than horses involved in other disciplines.

The reason for the dressage effect, however, is currently unclear, Dyson said.

“This may be multifactorial,” she said. “It could be related to genetic selection for advanced diagonal placement, a repetitive strain injury superimposed on a degenerative condition, continued work on artificial surfaces that are not always ideally maintained, the work surface characteristics (e.g., cushioning, slide), an incorrect interpretation of how to achieve collection, or more.”

This isn’t the first time scientists have noted the connection between straight hocks and PSD, she added. A study Dyson carried out in 1994 revealed some basic associations, and in 2007 she and her colleagues found links between PSD, large hock angles, and increased radiopharmaceutical uptake in the third metatarsal (cannon) bone at the site of attachment of the suspensory ligament in a scintigraphy-based study. Then in 2012 Dyson published a study showing that horses with hock angles of at least 165 degrees did not respond well to surgical intervention (neurectomy of the deep branch of the lateral plantar nerve and plantar fasciotomy); they stayed lame and even deteriorated, she said.

This new study, though, focused specifically on the association of clear, objective hock angle measurements and the diagnosis of any level of PSD. It helps confirm what practitioners have long suspected and brings the connection to light for breeders and owners, as well.

“We think that the association between large tarsal (hock) angles and PSD is well-recognized in the veterinary profession, but not in the general horse-owning population, among trainers, breeders, etc.,” said Dyson.

Why straight hocks are linked to PSD, however, remains uncertain. “Knowledge about the biomechanics of the function of the hind-limb is limited,” Dyson said. “We basically do not know.”

They also cannot make any conclusions about the sequence of events, she added. “To prove cause and effect would require a very long-term longitudinal study, following horses from a very young age,” she said.

Based on this newly confirmed association between PSD and straight hocks, Dyson offers sound advice for buyers and breeders: “Do not select horses with previous PSD to be used for breeding,” she said. “Avoid selection of horses with large tarsal angles (≥ 165⁰) as potential long-term athletes. Make sure the horse is fit enough for the work intended. Do not over-train. Avoid excessive work in either collected or extended paces. Ensure that arena surfaces are well-maintained. And consider cross-training (training in multiple disciplines).”

The study, “Is There An Association Between Straight Tarsus (Hock) Conformation and Hindlimb Proximal Suspensory Desmopathy?” was published in the Equine Veterinary Journal.