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© Diatom Graphics

Extinction by NAIS

Hoof Structure

© Ray Miller

While from the surface the hoof may appear to be a fairly simple structure, it is actually a very complex one made up of many different tissues, each with a specific function. Adding to the confusion regarding the foot are the many conflicting theories concerning its care and maintenance. This essay is designed to describe the structures of the foot, interior and exterior, and their functions, along with some common-sense ideas to keep the foot healthy and your horse sound.

The hoof wall is the outermost structure, a horny tissue secreted by the coronary band. It is composed of keratinized epithelial cells arranged into tubules running perpendicular to the ground surface of the wall. The outer surface of the wall is covered by the periople which extends from the coronary band down about three-quarters of an inch, except at the heels, where it covers the heels and blends into the frog. This material protects the junction between the hoof wall and the coronary band. Below the periople is the stratum tectorium that continues to the bottom of the wall. This is just a thin layer of cells, so it is often rubbed off along the bottom by friction with dirt, grass and other ground materials. The stratum tectorium helps control the movement of moisture into and out of the wall. It takes approximately one year for the hoof wall to grow from coronary band to the ground; therefore, any changes in hoof quality will be noticeable at the top of the foot. The wall grows evenly all the way around, so the heel (which is the shortest area) is the youngest portion. Because the heels are the youngest, they are also the most elastic, aiding in their expansion. The hoof wall is the major weight-bearing surface for the horse, so should be encouraged and allowed to grow as thick as possible.

The sole covers the bottom surface of the hoof and is about one-third water. It is similar to the wall because it is made up of tubules; the tubules curl near the ground surface, which explains its self-limiting growth and causes shedding. On a healthy foot the sole will be slightly concave, elevated off the ground. If the sole bears pressure it will often bruise, causing lameness, and may develop into an abscess. The bars are also on the bottom of the foot, arising from the heels where the walls turn inward and grow along the frog. Along with the outer wall, the bars are a weight-bearing surface. The bars need to be pared out by the farrier periodically to prevent their overgrowing the sole. This is called an impacted or double sole. When this occurs, the bars grow longer and longer, folding over and eventually covering the whole bottom of the foot, creating a great deal of pressure on the true sole. At its extreme, the false sole connects to the hoof wall and develops a nerve and blood supply. At this point the farrier must work very carefully and slowly to remove the excess tissue.

The frog is the triangular mass that lies between the bars of the foot at the heels. It is almost 50% water, and therefore is quite soft and pliable. The point of the frog is called the apex, the grooves along the sides are the sulci and the depression along the bottom surface is called the central sulcus or cleft. Since it, too, is a weight-bearing surface, the frog should be allowed to grow thick and full, even touching the ground. Another function of the frog is to expand and contract the heels of the foot as it moves. If the frog is not sufficient to perform this job, contracted heels may develop. It should only be pared out if the tissue is diseased or infected - for example, when thrush occurs. The frog will shed itself twice a year; other than those times, the healthy tissue should be left natural.

The corium is the vascular tissue of the foot that provides nutrition to the other parts of the hoof. There are actually five coriums that feed the periople, hoof wall, laminae, sole and frog. The coronary corium lies within the coronary band and is responsible for hoof wall growth. Injury at this point is serious and will usually lead to a defect to the structure and future wall growth.

Inside the hoof wall is the laminae. The insensitive laminae is closest to the wall and intermesh with the sensitive laminae. This is the support structure for the interior of the hoof, holding the hoof wall to the coffin bone. The laminae also help to arrange the weight to be borne by the hoof wall, distributed over a large surface area. Where the two layers of laminae mesh is called the white line, and can be seen on the bottom surface of the foot. It should not be confused with the unpigmented (white) area of the hoof wall adjacent to the white line.

There are three bones within the foot: The second phalanx (short pastern), third phalanx (coffin bone) and the navicular bone. The coffin bone sits mainly at the front of the foot and slightly to the outside. It is the largest bone in the foot and appears hoof-shaped. Just above it and connecting at the coffin joint is the short pastern, a bone that is partly inside the hoof and partly above the coronary band. The navicular bone is the smallest bone in the foot, lying behind the coffin joint, where the deep digital flexor tendon runs.

Healthy feet have three requirements. The first is proper nutrition. A balanced ration that provides all the necessary nutrients in the correct proportions goes a long way toward promoting healthy growth. Unless the ration is deficient in some way or the region's hay and pasture grass lacks a needed nutrient, supplementation is usually unnecessary. Hoof growth reflects the quality of the diet; changes in the wall will be seen when the ration improves or falls. These changes are first seen at the top of the hoof and then grow down. A second requirement of healthy feet is proper farrier care. Having the feet trimmed on a regular schedule by a qualified farrier will ensure that the hoof is kept balanced to the horse's body, properly shaped according to his wear pattern. The farrier also provides a second pair of objective eyes that will often notice changes the owner misses. When problems arise, either from use, age or disease, the farrier can work with the veterinarian to provide trimming and shoeing therapies that may keep the horse more comfortable and possibly extend his use. Finally, it is crucial that the owner exhibits conscientious maintenance of the feet. This includes frequent cleaning of the horse's feet, maintaining healthy living conditions and scheduling the necessary professional care.

A strong foot is essential to the total health and usefulness of the horse. Too much cannot be done to keep it in condition and performing properly. Considering the size and weight of the animal it supports, it is an amazing structure. Many of the lamenesses that arise from the hoof are not necessarily a function of the foot's weakness, but of a fault in the care it receives. When the owner provides the maintenance, nutrition and farrier care the horse needs, keeping the feet healthy and strong for a long, useful life is made easier.