Lameness caused by foot problems is common in the horse, and it can significantly impact how well a horse can perform. Hoof bruising, heel soreness, hoof cracks all create discomfort that alter a horse’s gait and prevent him from giving his utmost to an athletic task. Nearly all equine foot diseases have their root in biomechanics, noted a University of Georgia veterinarian at a recent in-depth podiatry seminar, and veterinarians and farriers must take a biomechanical approach to treating these problems.
Andrew Parks, MA, VetMB, MRCVS, Dipl. ACVS, professor of Large Animal Medicine at the University of Georgia’s School of Veterinary Medicine reviewed important elements of equine foot anatomy during the session, which was held during the 2012 American Association of Equine Practitioners’ Convention, held Dec. 1-5 in Anaheim, Calif.
Parks started with a bit of biomechanical anatomy review: While the long bones of the skeletal system, such as the radius (forearm) or the cannon bone, effectively transmit force from one end to the other, the distal phalanx (coffin bone, a short bone) , acts as a shock absorber, transferring weight-bearing forces from the hoof to the skeletal system. This bone is also well-adapted for attachment to soft tissues (tendons and ligaments) that aid or resist movement. “The principle forces acting on the foot are the weight of the horse, the ground reaction force (GRF), and the tension in the deep digital flexor tendon (DDFT, which runs from the underside of the coffin bone to the flexor muscles higher in the leg),” he explained.
The GRF matches the weight the limb bears, but it is exerted in the opposite direction. When a horse’s foot stands on a flat, firm surface, the GRF distributes around the perimeter of the hoof capsule. But when standing on a conformable surface such as sand, the GRF distributes broadly across the bottom of the horse’s foot. In both cases GRF pressure is greatest approximately in the center of the foot, just in front of the coffin joint.
The hoof is unique in that it is comprised of many different types of integument that continually grow, yet it functions as an extension of the musculoskeletal system. Parks said the hoof wall responds differently to forces depending on the rate at which they’re applied. “For example,” he says, “a force applied rapidly and immediately removed, such as the foot landing on the ground at speed, causes elastic change of foot shape that then immediately returns to its prior shape. In contrast, a prolonged and slow force applied to the foot deforms the tissue but when this force is removed, it takes much longer to return to its normal shape.”
When Biomechanics Go Wrong
Prolonged abnormal loading or force on the foot, as occurs with improper hoof growth, trimming or shoeing, has consequences–it might deform the hoof wall, causing flaring and the coronary band to move proximally (upward). Hoof growth slows as the body attempts to restore the hoof to a normal shape, resulting in growth ring spacing irregularities.
Parks commented, “The coffin bone is suspended in the hoof by the lamellae on three sides with the deep digital flexor tendon taking up tension on the fourth side. Interestingly, if the horse is lacking a functional hoof wall, he can’t walk because of painful pressure between the sole and coffin bone. However, if lacking a functional sole, he walks tolerably well if sensitive tissues are protected from pressure because the lamellae and DDFT support the coffin bone off the ground.”
Biomechanics and Treatment
As the horse begins each stride, associated shock waves can cause foot injuries. “Normally,” Parks reports, “There is natural damping of concussion by many structures such as the inner lamellae of the hoof wall, the digital cushion, collateral cartilages, the vascular plexus, and thick articular cartilage.” Applying a plain steel shoe to the hoof increases frequency of impact vibrations and maximum acceleration of the foot. In addition, he said, a steel shoe increases pressure on the navicular bone (which acts as a fulcrum around which the DDFT passes, restricts hoof expansion, and causes the heels to wear more rapidly than the toes.
To reduce impact shock waves, Parks recommended that veterinarians and farriers, “change the concussion of impact via a plastic shoe or a viscoelastic pad.” He suggested other biomechanical modifications for improving foot function: Use a pad to distribute the force evenly, move the GRF’s center of pressure, and move the point of breakover back. In the latter case, rolling the toe shortens the moment arm around which the coffin joint rotates and eases breakover.
In all cases, Parks urged, “A proper diagnosis of abnormal forces on the foot must be achieved in order to apply appropriate therapeutic shoeing strategies. This doesn’t mean that horses shouldn’t be shod, just that clinicians should be aware that adverse effects occur (with certain shoeing practices) and there may be a need to mitigate these effects.”