By: Christy Corp-Minamiji, DVM
Any discussion of therapeutic shoeing in the horse must begin with a discussion of what therapeutic shoeing is–and what it is not. According to Stephen O’Grady, DVM, MRCVS, professional farrier and owner of Northern Virginia Equine, in Marshall, therapeutic shoeing (or therapeutic farriery) is “the science and art of affecting/influencing the structures of the foot.” It is not a cookie-cutter, “apply shoe A to foot B” magic bullet in the war on equine lameness. To understand how the foot structures can be influenced, we should first have a clear picture of those structures and the forces that act on them.
“In an ideal world, you would begin from a diagnosis,” says Andrew Parks, MA, VetMB, MRCVS, Dipl. ACVS, professor of Large Animal Medicine at the University of Georgia’s College of Veterinary Medicine. “From that diagnosis you would get your treatment goals. By goals I mean the big picture concepts for treatment. You need to have principles (a range of techniques) that allow you to implement your goals.”
O’Grady concurs: “In order to be proficient at therapeutic farriery you must understand the forces that affect the foot. Excess forces or stresses on the hoof capsule lead to deformation, and excess forces or stresses on the internal structures of the foot lead to disease. Everything we are trying to do with therapeutic farriery is to change the forces on the foot.”
For Want of a Foot …
“There is no definition of a perfect foot,” says Parks. What’s considered a “normal” equine foot differs among breeds and disciplines. Thus, when evaluating trimming and shoeing options for a horse, owners and farriers must first consider foot conformation–what you see when you look at a horse standing at rest. Also evaluate limb conformation, which can affect how the horse’s foot bears weight and the way a horse wears a hoof or a shoe. Both limb and hoof conformation might alter the horse’s stride and landing, affecting the forces upon the foot structures. For instance, a club-footed horse’s high-set heels will alter weight distribution, placing more pressure on the toe and heel. While farriery might alter the distribution of the load on the foot, one misconception that O’Grady would like to correct is the idea that you can “change limb conformation by trimming. After 6-8 months (of age), conformation is conformation,” he says. Rather than attempting to make an atypically conformed foot or leg appear “normal,” O’Grady stresses the importance of trimming “for the cards the horse has been dealt.”
Figure 1: This illustration shows the forces around the center of rotation (the black dot) in a horse at rest. The ground reaction force is transmitted upward through the lamellae to the bony column. The extending force of the coffin joint is balanced by an equal flexing force generated by the deep digital flexor tendon.
The Foundation: An Appropriate Trim
According to O’Grady, therapeutic shoeing must begin with an appropriately trimmed foot. While much is made of hoof “balance,” O’Grady dislikes the term as he feels it is not clearly defined among practitioners and farriers. He prefers to work from a set of landmarks as clear guidelines to trim each individual foot appropriately:
1) Pick up the foot, and draw a line across the widest part of it. This line will fall just in front of the center of rotation (at the coffin joint). A well-trimmed foot should have approximately even proportions of hoof on either side of that line.
2) Look at the frog. The heels should lie approximately on the same line as the widest part of the frog.
3) Stand the horse on a hard, flat surface so his cannon bone on the leg you’re addressing is perpendicular to the ground. A line drawn down the front of the pastern should be parallel to a line drawn along the dorsal (toe) surface of the hoof wall. This is the hoof-pastern-axis (HPA). A broken-back HPA, in which the front of the foot is angled less steeply than the pastern, shows up in low heels; a broken-forward axis manifests in high heels (club foot).
According to O’Grady, anyone can use these three parameters to assess a horse’s trim. “Trimming is the mainstay of therapeutic shoeing,” he states. Thus, any benefit to be derived from therapeutic shoeing must have a basis in an appropriately trimmed foot. “The first function of a shoe is to protect that which is trimmed and also to complement it,” he says. “Then you can add mechanics … to that trim.”
Physics and the Equine Foot
Newton’s Third Law of Motion states that for every action there is an equal and opposite reaction. Similarly, the support and movement of the equine foot depends upon a system of opposing forces. Understanding these forces is key to understanding the principles of farriery.
In the standing horse:
1) The ground reaction force (GRF, as pictured in Figure 1) is the force transmitted from the ground to the ground surface of the foot, into the hoof wall, and from the wall through the lamellae (interlocking leaflike tissues attaching the hoof to the coffin bone) to the bony column.1 Think of the GRF as the ground pushing up as the foot lands. The GRF extends (i.e., an extensor motion rather than flexor) the coffin and pastern joints and is centered at the center of rotation.2 The GRF is counteracted by two forces.
2) The first of these forces, the weight of the horse, is transmitted down the cannon bone through the fetlock. Since the GRF and the weight of the horse through the cannon bone are both vertical but not aligned, they cause the pastern to rotate so the fetlock becomes closer to the ground.
3) At rest, the horse’s anatomy counters this effect by the upward and backward force of the deep digital flexor tendon (DDFT)–the flexing force illustrated in Figure 1. Picture a string attached to the back of the coffin bone and pulled upward over the navicular bone, over the fetlock, and along the back of the cannon toward the knee.