Hoof Size, Shape and Balance

As humans we tend to like things to be symmetrical – balanced.  Things should match; order from chaos.

But, when trimming or shoeing our racehorses, making those feet look the same all the way around could be a step toward breakdown.  So says a study from researchers at the University of California-Davis (UC-Davis).

“We’re not dealing with a structure that’s engineered to exacting specifications,” explained Albert J. Kane, DVM, MPVM, PhD.  “When you’re balancing tires on a car, you have specifications as to what the angle should be.”  However, when dealing with the equine, those specs can be as individual as each horse’s markings.

The research team, lead by Kane and Susan M. Stover, DVM, PhD, were the same ones involved in the study attributing a higher risk of catastrophic injury to the use of toe grabs.

Like the toe grab study, the animals were examined through the California Horse Racing Board Post Mortem Program.  The front hooves of 95 Thoroughbred racehorses that died between 1994 and 1996 were used for the study.  The objective was to identify hoof size, shape and balance measurements associated with the risk of forelimb catastrophic musculoskeletal injury.  As a control, 25 animals that died for reasons unrelated to the musculoskeletal system were compared to those that had suffered fatal suspensory apparatus failure or cannon bone lateral condylar fractures.

“We evaluated overall hoof size with about a dozen measurements,” said Kane.  Specifically, they looked at mediolateral hoof balance – how the bottom, inside portion of the hoof compared to the bottom, outer portion of the hoof.

“What we found was actually the opposite of what we expected.  The traditional goal for trimming feet has always been symmetry – but it was the injured horses that had more symmetry” to their hooves.

“Horses’ joint surfaces are not perfectly symmetrical from medial to lateral (inner to outer).  The medial side of the limb tends to carry more weight because of the location of the center of gravity.”

Because of this, the inside of the hoof of the control animals was steeper from the coronet band to the ground; whereas, the outer side of the hoof tended to have a more sloping angle.

This doesn’t mean that the hoof should be flared explained Kane.

“If you look at the foot from the front, there should be a straight line with no breaks” from the coronet band to the ground.  “If there’s a straight (hoof) wall, but the lateral side has more angle to it, then that foot is probably okay.”

“It could be that the horse’s foot is not supposed to be perfectly symmetrical from side to side,” Kane said.

“For some horses, it’s better if they toe out a bit and for others, it’s better that they stand straight.  To think that those two horses should have the exact same shape to the foot is not correct.  There’s not just one picture that’s accurate.”

The difference between the angles of the inside of the hoof vs. the outside were minor, but enough to have an impact on the incidence of injury.  “Typically, the medial hoof wall is at an angle of 77 degrees and the lateral is 73 degrees.”

Other findings by the team correlated toe angle to heel angle.  “It’s a progression,” he said.  “Heel angle should approximate toe angle, toe angle approximate pastern angle and, in some breeds, pastern angle should approximate shoulder angle.”

Hilary M. Clayton, BVMS, PhD, MRCVS, researched hoof angle and its impact on trotting horses over twenty years ago.  Her findings were much the same as the UC-Davis study.

“The ‘normal’ hoof angle will depend on the slope of the pastern,” she said.  “It is important that each horse be evaluated as an individual and trimmed accordingly.”

When Clayton was looking at trimming techniques, the beliefs of the time were that a horse trimmed for a long toe/low heel would improve performance by delaying break over, producing a long sweeping swing phase and result in increased stride length.   What she found instead was that, as the horse increases speed, the stance phase of the stride (when the foot is on the ground) is shortened while the swing phase (airtime) remains the same.  Using an acute hoof angle to prolong break over may limit the horse’s ability to shorten the stance phase.  This would lead to restricting the stride rate and ultimately, restricting speed.

Although Kane feels farriers of today do not intentionally trim for a long toe/low heel effect, it is still quite evident in the racehorse population.

“I think environment plays a huge role” in hoof shape.  “Horses are generally on ground that is too soft and too moist.  There is so much water on the racetrack.  Then, we stand them in ice, we hose them down, we wash them, and we stand them in a stall that may or may not be wet.”

All this moisture leads to under run heels, he said.  “Correcting under run heels is a very difficult thing to do.  If I had a sure fix for it, I would be a millionaire.”

These under run heels may seem to be only a nuisance, but the UC-Davis study found evidence to suggest that if the toe/heel angle is different, the risk of the animal suffering suspensory apparatus failure was increased.

When trimming the feet, Kane stresses looking at the toe first.  With under run heels “it doesn’t mean you want to decrease the toe angle – instead you want to bring the heel in line.”

Ideally, researchers found that the toe angle should be a bit steeper than originally thought (50 – 52 degrees at a minimum).  But we should remember that the toe should mimic the pastern angle.

“We did two things in our study,” said Kane.  “We re-confirmed what we traditionally thought of as a proper toe and hoof angle and we questioned whether or not the ideal hoof has to be perfectly symmetrical.

“This was just the tip of the iceberg.  You have to remember that the horses’ foot is a finely tuned structure.  Because he is operating at his peak when he’s racing, he’s working at his design capacity and very close to his limits.

“Anything that affects his balance, no matter how small, can push him over his limit.”