Therapeutic Shoeing Part 1: Foot Fundamentals
Photo: Erica Larson, News Editor
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:
- 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.
- Look at the frog. The heels should lie approximately on the same line as the widest part of the frog.
- 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.
Foot—the part of the distal (lower) limb encased by the hoof
Hoof—the integument (skin) of the foot
Hoof capsule—walls and sole of the hoof (horny part of the "skin")
Frog—shock-absorbing inverted "V" structure extending from the heels at base to the mid-foot at apex; a softer, more pliable portion of the hoof capsule
Digital cushion—soft tissue structures at the palmar (back)/plantar (bottom) aspect of the foot, under the frog
Wall—cornified (hard) outer covering of foot, extending from coronary band to the ground
Sole—cornified surface of the bottom of the foot, excluding frog, wall, and bars
P3—(distal phalanx, coffin bone, third phalanx) the most distal bone of the limb, a triangular bone resting entirely within the hoof capsule
P2—(middle phalanx, short pastern, second phalanx) articulates (joins) P3 and P1; rests partly in the hoof and partly in the pastern
P1—(proximal phalanx, long pastern, first phalanx) articulates with MC3 (third metacarpal, or the cannon bone) at MCP (metacarpophalangeal, or fetlock joint) and with P2 distally
Deep digital flexor tendon (DDFT)—connects flexor muscles to bones of the lower limb; inserts into the palmar (back) aspect of P3; and provides upward force on the back of P3
Navicular bone—wedge-shaped bone that articulates with P2 and P3; acts as a fulcrum for the DDFT
Lamellae—also called laminae, these structures suspend P3 within the hoof capsule; these are the connective tissues that become inflamed during an episode of laminitis.
Center of rotation—the point of zero movement around which all actions in the hoof rotate
Christy Corp-Minamiji, DVM
In the standing horse:
- 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.
- 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.
- 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.
In the moving horse the forces on the foot and limb are dynamic. According to Parks, the stride has four phases.
- Impact/Landing: Horses land slightly heel-first or flat, with the entire foot hitting at once. As the horse's mass presses downward, the fetlock drops toward the ground and P3 (the coffin bone) rotates slightly down and backward. The hoof expands.
- Support: The foot is flat on the ground.
- Breakover: The heel has lifted but the toe remains in contact with the ground.
- Flight/Swing: The entire foot is off the ground.
Changes in trimming and shoeing alter these forces, and any alterations to these forces have consequences. "Any time you make a modification to a shoe to benefit one structure, you make changes that will be deleterious to another," says Parks, adding that shoeing changes create both immediate and long-term effects.
In impacting the mechanics of the foot, a shoe can, according to O'Grady:
- Change traction;
- Decrease concussion;
- Unload areas of the foot/change distribution of the load
- Affect breakover; and
- Alter heel elevation.
A Picture is Worth ...
To understand an individual foot's mechanics and internal forces at work--whether it be a lameness case or atypical conformation--imaging provides both veterinarian and farrier with visual cues.
"It's cheating. You get to see what's in there," says Al Moates, a Wilton, Calif., farrier who collaborates with veterinarians using diagnostic hoof radiographs (X rays). "(Otherwise) you get people who don't want to spend the money, and you've got to guess what's in there."
Moates compares treating a lame or atypical foot without diagnostic imaging to baking something without knowing the required ingredients.
O'Grady agrees with Moates that when pathology is suspected in the foot, practitioners need radiographs for "diagnosis and as a template for farriery," and he emphasizes collaboration between veterinarian and farrier when using these images. By law, the veterinarian taking the radiograph must be the one to interpret the images and to discuss the diagnosis with the farrier. The veterinarian should then also be involved in deriving the treatment plan.
Another innovation available for diagnosing and understanding hoof-based lameness is magnetic resonance imaging (MRI), which is becoming more accessible in equine medicine. With MRI veterinarians can visualize soft tissue structures of the foot unseen on radiographs. According to Parks, MRI has "opened up a whole range of potential diagnoses." For instance, Parks states that MRI has opened our eyes to concurrent changes with navicular disease--DDFT changes and impar ligament damage (that attaches the navicular bone to the coffin bone).
In the second therapeutic shoeing segment to come, we will examine the application of these principles to shoeing horses with selected foot pathologies and explore the ways in which various shoes and devices alter the forces upon the foot. However, don't hold your breath for a one-size-fits-all cure. Says O'Grady, "there's only so much you can actually do, regardless of the hype. Mechanics are mechanics."
1. Parks, A. "The Equine Foot: Form and Function." 2001 North American Veterinary Conference Proceedings.
2. Eliashar, E. "An evidence-based assessment of the biomechanical effect of the common shoeing and farriery techniques." Veterinary Clinics of North America - Equine Practice, 2007.
About the Author
Christy Corp-Minamiji, DVM, practices large animal medicine in Northern California, with particular interests in equine wound management and geriatric equine care. She and her husband have three children, and she writes fiction and creative nonfiction in her spare time.
POLL: Rehabbing the Injured Horse