Several lameness problems can exist in the rear (palmar) half of a horse's foot; most bear the very general title of "heel pain." The real problem is that without very careful diagnosis, lameness in this general area might be attributed to the wrong structures within the foot, and treating the wrong structure is ineffective at best. Additionally, understanding what structures are damaged is essential to understanding just how that foot's function is affected.

At the recent American Association of Equine Practitioners convention, held Dec. 2-6, 2006, in San Antonio Texas, one practitioner sought to clarify veterinarians' understanding of this complex area and its function. Andrew Parks, MA, VetMB, MRCVS, Dipl. ACVS, professor of large animal surgery and head of the department of large animal medicine at the University of Georgia, presented a very detailed anatomical review of the palmar foot with state-of-the-art computerized imagery from the Glass Horse project (, and discussed functional anatomy of the palmar foot in no less detail.

Beginning to Understand Heel Pain

Parks noted that "heel pain" is not a very specific diagnosis because there is no clearly demarcated division between the heel and the dorsal (front) aspect of the foot, and many structures span this division. Additionally, most practitioners use the term "heel pain" to refer to any lameness that is dispelled with a palmar digital nerve block. The trick is to identify which structure in that area is causing the pain--is it the navicular bone? The ligaments supporting the navicular bone? The deep digital flexor tendon? The ungual cartilages? Advances in diagnostic imaging, such as magnetic resonance imaging (MRI), have allowed veterinarians to begin making such distinctions in living horses for the first time, he commented.

"Paralleling our increased ability to diagnose diseases of the heel has been an increase in the understanding of the function of the foot," he went on. "As such, the mechanics of weight bearing and associated stresses are now beginning to be understood. Because the most likely cause of most lameness occurring in the heels is recurring excessive stresses, the dual development of increased diagnostic capabilities and increased understanding of function are likely to lead to more effective treatment strategies."

The Role of the Heels

After a thorough review of anatomy in the palmar region of the foot, Parks moved on to a discussion of palmar foot function and functional characteristics.

"The evidence from several studies suggests that the principal function of the heels is to dissipate energy during the impact phase of the stride," he explained. Effective energy dissipation means that forces on the outside of the hoof are translated to internal structures, which in turn shift, stretch, or compress to absorb the shock so it doesn't damage any part of the hoof or limb. If any internal structure can't handle the stress, damage and pain result.

"Any conformation or imbalance that impairs this natural function is likely to result in disease, because the majority of lameness associated with the heels is related to stress-induced injuries," he stated. "Stress-induced injury is both a function of the structure of the heel and the severity of work performed. That is, injury may occur in any horse if the insult is severe enough, but if there is an anatomical predisposition because of abnormal balance or conformation, or a farriery manipulation that focuses stress, then injuries will be more likely to occur at submaximal work."

The majority of energy dissipation associated with impact is absorbed between the hoof and the distal phalanx, through a variety of mechanisms including hoof expansion, said Parks. "Hoof expansion maximizes about 30% into the stride, a little longer than vibrations from impact last," he reported. "At around 75% of the stride, expansion is stable, and at about 85% of stride, the heel comes off the ground and the quarters come in a bit."

How does hoof expansion work? "There are two traditional explanations. One is that frog pressure pushes on the digital cushion, which expands outward," Parks said. "The second theory says that middle phalanx descent (with weight bearing) compresses the digital cushion, also expanding the hoof. But three functional studies show somewhat conflicting results. One suggests that there is no direct relation between frog pressure and foot expansion. Another study suggests that pressure within the frog may actually decrease with foot expansion. The third study indicates that there is a relationship between increased frog pressure and hoof expansion. Additionally, an anatomic study, which examined the structure of the frog, digital cushion, and associated venous plexuses provides a hypothesis for a hemodynamic mechanism for dissipating shock (blood pushed out of the foot during loading allows room for structures to shift and dissipate energy)."

Manipulating the Foot

In the quest to dispel palmar foot pain in horses, several foot manipulations have been tried. Parks discussed the effects of basic shoeing and the effects of three common manipulations--raising the heels (with wedges), applying wedges to raise the lateral or medial aspect of the foot, and using extended-heel shoes (such as egg-bars).

"Steel shoes attached with nails are known to diminish the natural (shock) dampening mechanisms of the distal limb compared with unshod feet," reported Parks. "Specifically, shoeing with plain steel shoes increases the frequency and amplitude (degree) of the oscillations in the ground-reaction force (exerted on the foot by the ground) during the impact phase of the stride and decreases the expansion of the quarters and heels during the stance phase. But certain plastic shoes used with synthetic viscoelastic pads reduce both the frequency and magnitude of the vibrations associated with the foot's impact with the ground.

"Additionally, finite element analysis suggests that the nails, particularly those placed most palmarly (rearward) in the foot, cause concentration of stress in the wall," he noted. "This would seem to be an inescapable consequence of nailing on shoes, and is a good reason why the nails should not be put in the rear half of the foot."

Raising the heels "This practice changes joint angles, places more pressure on the dorsal (forward) aspect of joints between the second and third phalanges (the distal interphalangeal joint), decreases heel growth, and decreases deep digital flexor tendon tension," said Parks. "In motion, elevating the heels increases the likelihood of heel-first contact and causes the distal phalanx to roll forward more after first contact before the foot becomes in stable contact with the ground. During the second half of the stance phase of the stride, elevating the heels delays the movement of the center of pressure to the toe at breakover and thus, their unloading."

Medial/lateral wedges "The static effects of medial or lateral elevation (raising the medial or inside aspect of the foot with a wedge) include compression of medial or lateral joint spaces respectively and some rotation of phalangeal joints," Parks noted. "The dynamic effects of medial or lateral wedges are that they move the landing pattern of the hoof and its center of pressure to the higher side." The same would be true of lateral wedges.

Research suggests that this is because the way the horse's foot lands (flat or on one side first) is governed by proprioceptors (sensory mechanisms that tell the horse where his feet are). They send nervous system messages that tell the horse if his coffin bone is flat or tilted; he will try to land with the bone flat (parallel to the ground), but if he's wearing a medial wedge, the high wedged side of the foot strikes the ground first because his coffin bone is "flat." The horse doesn't realize that the hoof and wedge make that side of his foot longer. So perhaps we should think of a wedge not as not only raising one side of the foot when it is on the ground, but also as extending or lengthening that side of the hoof wall when the foot is in the flight phase of the stride.

Extended-heel shoes "Egg-bar shoes, which function as a short palmar extension of the shoe, are probably the next most frequently used shoe for treating heel pain," he continued. "Studies have primarily examined the effect of egg-bar shoes when the horse is standing or moving on a firm, flat surface. At rest, they move the center of pressure palmarly in relation to the toe; this is considered beneficial for the deep digital flexor tendon and navicular bone but is potentially harmful to the hoof capsule at the heels."


"The equine foot is an intricately designed structure that accommodates the weight-bearing and propulsive functions of the foot at rest and during locomotion," concluded Parks. "It seems that the function of the various structures of the heels varies with the phase of the stride. At impact, they dampen the damaging high frequency of the rapidly accelerating and decelerating forces. During the stance phase of the stride, they facilitate the expansion of the hoof as increasing weight is borne by the foot and the flexion of the interphalangeal joint. At breakover, they participate in the extension/dorsiflexion of the distal interphalangeal joint.

"Consequently, it follows that if the heels are unduly loaded during the stance phase of the stride because of either poor conformation or improper farriery practices, then the additional load is likely to cause injury, damage these structures, and impair their function," he added. "During the second half of the stride, particularly at breakover, the navicular apparatus is under the greatest amount of stress. Consequently, it follows that either poor conformation or farriery practices that increase these stresses are injurious to the navicular bone, its associated ligaments, and the deep digital flexor tendon."

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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