The Relationship Between Hoof Deformation and Underrun Heels

Horse owners, veterinarians, and farriers alike know that preventing hoof problems is better than having to fix them. But when a hoof problem, such as underrun heels, does develop, it’s important for your horse’s health care team to be up-to-date on the latest in research and treatment methods.

Peter Day, Dipl. WCF, farrier at the Royal Veterinary College, in Hertfordshire, United Kingdom, and colleagues conducted a series of studies looking at the relationship between hoof deformation (a shock-absorbing mechanism—the natural change in the hoof capsule’s shape every time it’s loaded with the horse’s weight) and underrun heels. They compared the degree of deformation in hooves with underrun heels versus without underrun heels as well as the effects of carbon composite hoof wall patches on underrun heels. Day presented his findings at the 2017 International Hoof-Care Summit, held Jan. 24-27 in Cincinnati, Ohio.

Regardless of the term used—underrun heels; long-toe, low-heel; slung heels; or collapsed heels—it appears to occur more frequently in the front feet than the hind. For this reason, Day’s research focused on only the front hooves. He said he considers heels to be underrun or collapsed when the heel angle is lower than the toe angle by about five degrees.

Underrun heels develop as the toe grows too long and the heels appear to grow forward rather than downward, becoming lower. Lower heels change the forces acting on the foot’s structures—especially the deep digital flexor tendon that inserts on the coffin bone—making the navicular bone more vulnerable to injury.

Horses can be genetically predisposed to a low-heeled foot, so breeders should take this into consideration to avoid perpetuating the problem. Wear and tear on the predisposed hoof over time causes the horn tubules to bend and the heels to lower. This affects hoof deformation and blood supply to the foot.

Hoof deformation is one of the important mechanisms involved in shock absorption; however, researchers hadn’t yet studied the relationship between hoof deformation and underrun heels. So Day’s team took on the job.

Day described two deformation theories—compression and depression:

• Compression of the frog and the sole puts pressure on the foot’s cartilages and pushes them upward, causing the hoof to expand.
• Forces going through the hoof wall’s laminar attachment are redirected as the middle phalanx lowers during the stance phase, pushing the hoof wall and the cartilage outward as foot deformation occurs.

Day’s team performed a series of four research projects using cadaver limbs as well as live horses. They tested different hypotheses revolving around carbon composite patches’ effects on hooves. The four studies’ results indicated that carbon composite patches applied to the hoof wall in horses with collapsed heels can improve horn tubule strength, affect circulation, and influence hoof deformation and shock absorption, offering farriers a practical application for preventing or treating collapsed heels.

Day performed his last trial in 12 front cadaver legs from Thoroughbreds to determine whether hoof deformation differed between horses with collapsed heels and without collapsed heels when artificially loaded at the equivalent of a fast trot. He found that overall deformation and foot spread was greater in feet without collapsed heels. In feet with collapsed heels, the medial heel showed significantly more proximodistal (from the inside outward) deformation. This confirmed their hypothesis that feet with collapsed heels have less heel deformation and, thus, reduced shock absorption, which can cause further foot problems.

Treating the Problem

Farriers’ support and treatment options for underrun heels are many, including:

• Buttressing the heels;
• Raising the heels;
• A natural balance or barefoot trim;
• Broad shoe;
• Heart bar shoe;
• Egg bar shoe;
• Packers and pads; and
• A cuff cast.

Treatment often depends on the horse’s environment, so he encouraged farriers to ask owners about the footing the horse is living and working on. “Sometimes a particular shoe on a particular terrain might be exactly what you want,” said Day.

“At the moment, there is a lot of (research) going on in pads and packers and the effects they’re having,” he added. “A lot of that early work was done in the laboratory on cadaver material. It’s quite exciting the results that are being got.”

Because this abnormal hoof conformation ultimately contributes to a range of foot problems, such as navicular syndrome, chronic heel pain, bruising, coffin joint pain, lameness, and reduced performance, Day encouraged farriers to take control of the way a horse is shod. He advised keeping the horse on a set trimming and shoeing schedule despite any owner resistance so they can try to prevent problems rather than fix what’s already broken.