Hoof angles have always been considered an integral part of hoof balance. The angle of the hoof is formed at the junction of the hoof wall and the ground surface of the foot. A device called a hoof gauge is used to determine this measurement.
Until recently, vets and farriers said that angles of 45 to 50 degrees for the forefeet and 50 to 55 degrees for the hind feet were "normal." That is no longer given credence since it does not take into consideration individual conformation.
Ideal hoof angulation occurs when a line drawn down the front surface of the hoof wall and a line drawn along the surface of the heel are in alignment or parallel to a line drawn through the last three bones of the lower limb (i.e., the long pastern bone, the short pastern and the coffin bone – see figure 1).
This type of foot conformation allows normal physiology within the foot, it maintains hoof health and prevents lameness. Unfortunately, we are not always able to attain the ideal hoof angulation.
The foot is trimmed appropriately and the hoof angle is correct for the individual horse when the hoof wall and the pastern are parallel. In order to see this hoof-pastern axis visually, stand the horse squarely on a hard, level surface and view him from the side. The terms low hoof angle and high hoof angle can be used to describe a non-parallel relationship.
The correct hoof-pastern alignment may be hard to achieve when the toe of the foot is either too long or too short, and the heel is either too high or too low; trimming plays a very important role in achieving and maintaining a normal hoof-pastern axis.
Low hoof angles, where the angle of the hoof wall is less than the angle of the pastern, are commonly caused by trimming that encourages a long toe and underrun heel.
If the toe is allowed to grow excessively long, the heels grow forward and hence, lower. This causes the pastern to move forward, and leads to coffin joint extension and increased strain on the deep digital flexor tendon.
Long toes promote toe-first landing, which causes friction in the navicular bursa and delays the speed of breakover. There is experimental evidence that a low hoof angle will compromise circulation in the heel.
Obviously, these detrimental effects are proportional to the severity of the angle. For many years, racetrack trainers believed that lowering the heel would increase the length of stride. This has been discredited.
If excessive toe is present, the first step in correcting a low hoof angle is to "back up" the toe by rasping the dorsal hoof wall in an attempt to better align it with the pastern. Often, if the misalignment is mild or in the early stages, this adjustment will correct the problem.
A heel that remains underrun can be corrected by extending the heels of the shoe to the appropriate distance beyond the heel of the foot – see figure 2. If the hoof-pastern axis is not corrected by backing up the toe, then some form of heel elevation is necessary.
On the other hand, an extremely high hoof angle is created when the angle of the hoof wall is higher than the angle of the pastern.
A high-hoof angle causes coffin joint flexion, increases strain on the suspensory ligaments, promotes heel-first landing and increases pressure in the heel.
To correct a high hoof angle, the area from the point of the frog to the heel is lowered in a tapered fashion when trimming the foot. Removing excess heel may place excessive tension on the deep digital flexor tendon. If that happens, a small heel wedge can be placed between the shoe and the foot to decrease the tension.
Hoof trimming is the most important aspect of shoeing, with the goal of reducing unnecessary forces exerted on the toe, heel s or joints within or above the foot. Adjusting the hoof angle so that the surface of the hoof parallels the pastern will decrease the effects of high or low hoof angles and their contribution to lameness.
Stephen O'Grady, D.V.M., operates Northern Virginia Equine Practice.