The Equine Foot -- Form and Function
There is an adage that is as old as the modern-day horse. It goes something like this: No foot, no horse. My late father, who could pick out a minute leg or foot unsoundness at a glance, used to lecture his young son about the importance of good feet and legs in a horse. "The feet and legs," he would say, "are kind of like the foundation of a house. I don't care how pretty or how nice that house is, if it doesn't have a good foundation, it won't stand. It's the same with a horse. The feet and legs are the foundation. If you don't have good feet and legs, that horse won't hold up."
Sound wisdom then, and it remains so today.
There is nothing more disconcerting and disappointing than to be all set to go on a trail ride or enter a competition and have to cancel because your horse is lame. In the racing and competitive worlds, an unsound horse might also translate into serious financial loss to the owner.
When the problem is within the foot itself, it can be difficult to diagnose and treat. The reason is fundamental. An intricate and complicated host of tissues, blood vessels, tendons, ligaments, and bones comprise the foot, and the whole package is sealed within a hard, horny shell--the hoof wall.
The foot serves a variety of valuable functions for the equine. It supports the horse's weight, absorbs shock, provides traction, conducts moisture, and helps pump blood. In addition, it resists wear and has the capability of replenishing itself.
Just how nature has designed the foot and the way in which it functions is a complex and fascinating subject for discussion. The material that follows draws heavily on the research and reports of three experts in the field from Colorado State University. They are Dean A. Hendrickson, DVM, MS, Diplomate ????; Doug Butler, PhD, CJF, FWCF, and the late O. R. Adams, DVM, MS, whose book, Lameness in Horses, is as applicable today as when it was first published more than 30 years ago.
As a matter of fact, there is no better way to start a discussion of the foot and its functions than to use Adams' words:
"Good conformation of the foot is essential to normal activities of the horse. No matter how good the conformation of other areas, if the foot is weak, the horse is not a useful animal...To have good foot conformation, a horse must have reasonably good limb conformation, since the foot reflects poor conformation of the limbs.
"Much variation in quality of structure exists in the feet of horses. Ideally, the wall should be thick enough to bear the weight of the horse without excessive wear, resistant to drying, be pliable, and should have normal growth qualities. The sole should be thick enough to resist bruising and should shed normally. The bars should be well developed and the frog should be large, strong, and should divide the sole evenly, with its apex pointing directly to the toe of the hoof wall. A hoof wall that contains pigment is preferable to a white hoof wall, since a white hoof wall is subject to drying and cracking and is not as resistant to trauma as a pigmented hoof."
Laying The Foundation
So, just how is this foot constructed? you might ask.
Start with the hoof wall, which is a modified cornified epithelium. It is composed of keratinized epithelial cells that are solidly cemented with keratin. The keratinized cells are arranged in tubules that run perpendicularly from the coronary band to the ground surface of the wall and parallel to one another.
The hoof wall itself is composed of three layers. The outer layer is the periople and stratum tectorium. The periople, which functions much like the cuticle of a human fingernail, extends about three-fourths of an inch below the coronary band, except at the heels, where it caps the bulbs of the heels. The stratum tectorium is a thin layer of horny scales that gives the glossy appearance to the outside of the wall below the periople. One of its tasks is to protect the wall from evaporation. (The hoof wall is approximately 25% water.)
The middle layer composes the bulk of the hoof wall and is the most dense portion. The inner layer is the laminar layer that forms the epidermal laminae of the hoof. This layer is concave from side to side and bears about 600 primary laminae, each of which bears 100 or more secondary laminae on its surface. These laminae firmly attach the coffin bone to the hoof wall. These combined laminae bear much of the weight of the horse.
The ground surface of the hoof wall is divided into the toe, quarters, and heel. At the heel, the wall turns forward to form the bars that converge toward one another. The sole, comprising most of the ground surface of the hoof, conforms to the inner curvature of the wall and to the angles formed by the wall and the bars.
The structure of the sole is similar to that of the wall, with tubules running vertically as formed by the papillae of the sole corium. These tubules curl near the ground surface, which accounts for the self-limiting growth of the sole and causes shedding. The sole is not designed to bear weight from the ground surface, but it is designed to bear internal weight.
The frog is a wedge-shaped mass that occupies the angles bounded by the bars and the sole. The frog is divided into the apex, which is the forward most point; the base, which is the rear portion; and the frog stay, which is the central ridge. The frog is 50% water and quite soft.
The walls, bars, and frog are the weight-bearing structures of the foot.
The white line is the junction between the laminae of the wall and the tubules of the sole.
The corium is a modified vascular tissue that furnishes nutrition to the hoof. It is divided into five parts. First is the perioplic corium, which is a narrow band lying in the perioplic groove above the cornary border of the wall. Its job is to distribute nourishment around the top of the hoof wall. Second is the coronary corium, which, with the perioplic corium, helps to form the coronary band. It furnishes the bulk of nutrition to the hoof wall and is responsible for growth of the wall. Third is the laminar corium, which is attached to the coffin bone and nourishes the dermal laminae, the epidermal laminae of the wall, and the interlaminar horn of the white line. Fourth is the sole corium, which is composed of the fine hairlike papillae over the entire inner surface of the sole. These papillae furnish nourishment and growth to the sole proper. Fifth is the frog corium, which is similar in structure to the sole corium and furnishes nourishment and growth to the frog.
The digital cushion is a fibroelastic, fatty, pale yellow pyramidal structure containing cartilage and is located in the posterior half of the foot. Its primary purpose is to reduce concussion to the foot.
The coronary cushion is the elastic portion of the coronary corium. It fits into the groove formed at the proximal part of the hoof wall. The cushion is widest at its center and narrows as it joins the heels.
Lateral cartilages are part fibrous tissue and part hyaline cartilage. They slope upward and backward from the wings of the coffin bone and reach above the margin of the coronary band.
The coronary band is the combined perioplic corium, coronary corium, and cornary cushion. It is the primary growth and nutritional source for the bulk of the hoof wall. Injuries to this structure are serious and usually leave a permanent defect in the growth of the hoof wall.
The blood supply to the foot is furnished by the medial and lateral digital arteries. The blood comes to the foot via the arteries with distribution of the nourishment it is carrying being handled by the capillaries. It is then returned by the veins.
Shunts or alternate pathways called arterio venous anastomoses (AVAs) exist between the arteries and the veins, bypassing the capillaries. The AVAs open during times of stress and when open, blood is shunted away from the capillaries that supply the tissues of the laminae and other sensitive foot structures. Some researchers believe this is what happens during laminitis--blood is shunted away from the sensitive structures for too long a period and serious damage is the result.
So there in a nutshell, with generous help from Adams and his book, is a description of the equine foot. (We'll get to the bones, ligaments, and tendons a bit later.)
When left to its own devices in the wild, the horse does a pretty good job of taking care of its feet. Because it often travels some distance to water and to find good grazing, it trims its own hooves through the wearing process.
Horses in the wild that do not have sound feet often become lame and are in danger of becoming a hearty meal for predators--survival of the fittest.
When humans domesticated the horse, they got in the way of nature. Today, horses' feet are trimmed and shod in myriad ways, depending on what is being asked of them.
In some breeds, such as the American Saddlebred and Tennessee Walking Horse, the foot is forced into an abnormal conformation to better enable the animal to preform the gaits being asked of it.
Adams provides us with a succinct explanation of what happens when a foot is forced into abnormal conformation: "Because of the artificial action that these horses are forced to use, the wall is allowed to grow excessively long. This removes frog pressure and contraction of the heel results. These horses are subject to tendon injuries, thrush, and contraction of the hoof wall around the third phalanx or 'hoof bound.' This artificial foot conformation also aggravates basic conformation weaknesses, such as base-wide, toe-out position of the feet and a high incidence of ringbone and sidebone results. It's very difficult to maintain such horses in a sound condition, since fundamental principles of foot health are violated."
Other breeds are not immune to human-induced foot violations. For years, it was fashionable for Quarter Horses shown at halter to have dainty, little feet. In extreme cases, these horses were like the nice house with a poor foundation that my father lectured about. They were pretty to the eye, but there wasn't enough foot there to provide adequate foundation. Many of those horses were useless, other than as art objects. Because the foot was small and the horse often large and bulky, the small foot simply couldn't handle the concussion load being placed upon it and lameness was often the end result.
Some Thoroughbreds also have poor feet. Again, the reason is pretty basic. The Thoroughbred for years has been bred for speed with other aspects of its conformation, including the foot, being sacrificed.
Butler puts it this way: "A hoof proportional to the horse's body size allows the ideal distribution of body weight over the foot's laminar surface area. It prevents the over-compression of the sensitive and bony structures and allows the hoof to expand normally during movement. There has been a trend, especially in a few breeds, to select for small feet for beauty or aesthetic purposes. Hoof size in horses is apparently highly heritable, since it correlates with bone growth. Hoof size is also influenced by nutrition. Horses that are fed an optimum diet have an 80% increase in hoof-sole-border size compared to those fed a limited diet. Optimum nutrition encourages maximum bone and hoof size development. Importantly, hoof size proportionate to body size encourages soundness."
What are the forces acting on the foot that require it to be sound and sturdy? Hendrickson gives this explanation: "The normal forces acting on the equine foot include tension directed toward the laminae of the wall, tension from the deep digital flexor tendon, downward compression from the middle phalanx (short pastern bone), upper compression from the sole, and forces acting on the extensor process, including extensor branches of the suspensory ligament and the common (or long) digital extensor tendon. The normal forces exhibited on impact are quite complex. The angles of the heels strike first followed by the ground surfaces of the bars, quarters, and toe. Most of the concussive force during impact is transferred from the distal phalanx (coffin bone) to the stratum internum. The sole is slightly depressed as it counters the downward force and the frog acts with the sole to support the inner structures of the foot and to dissipate the force on the distal phalanx. The venous plexi of the foot are compressed, forcing the blood up into the digital veins, acting as hydraulic shock absorbers."
The ideal hoof will support the primary weight of the animal on the hoof wall. A casual examination of the bottom of a horse's foot will reveal that the hoof wall is thicker at the toe than at the quarters. Nature had a good reason for designing the hoof that way. The horse breaks over at the toe in a normal stride, and that means the toe must be able to tolerate more wear and tear. In addition, the wall at the toe is farther from the coronary band. As a result, it is more keratinized and harder than the hoof in the quarters, which is closer to the coronary band.
Now for the bones, tendons, and ligaments of the foot and the role they play in absorbing concussion and facilitating movement.
The foot contains three bones. They are the distal or lower end of the short pastern bone (second phalanx or middle phalanx), the coffin bone (third phalanx or distal phalanx), and the navicular bone (distal sesamoid bone).
Two key tendons are found within the foot. The extensor tendon attaches to the front of the coffin bone and the deep digital flexor tendon attaches to the bottom of the coffin bone.
The navicular bursa is positioned between the deep digital flexor tendon and the flexor surface of the navicular bone and functions to reduce the shock of concussion.
Between the coffin bone, the short pastern bone, and the navicular bone is the coffin joint. The collateral cartilages are positioned on either side of the coffin bone. The neuro-vascular supply comes from the palmar/plantar arteries, veins, and nerves. The digital cushion is found between the bottom of the coffin bone and the sole. The digital cushion, as has been mentioned, is a wedge-shaped structure with a fibro-fatty composition. It is very elastic and has very few blood vessels and nerves. When it is compressed by the pastern bones and frog, it absorbs shock, cushions the bones, and is divided by the frog's exterior spine so it is forced outward and obliquely upward against the lateral cartilages.
Butler provides us with this description of the way in which the equine foot acts as a shock absorber:
"The foot helps to reduce concussion in the horse as a part of an overall shock-absorbing mechanism. The angle or slope of the shoulder and pastern is especially important in determining the amount of concussion that can be absorbed by the limb. A sloping shoulder and pastern not only create less ???? and absorb more concussion than straighter ones, but they also make a smoother riding horse.
"Elasticity of the suspensory and sesamoid ligaments also relates to the concussion-absorption qualities of the fetlock joint. Further, the elasticity and movement of the hoof wall absorbs some degree of concussion. When the horse takes a step, the sole descends and flattens slightly to absorb concussion, while the white line absorbs the impact as the wall moves out. The hoof laminae diminish concussion of the coffin bone as they transfer weight and re-direct forces between hoof and skeleton.
"In its normal state, the frog absorbs concussion from two directions. It acts as a rubber shock absorber, absorbing concussive force from the ground and acting as a stop which redirects concussive force coming down from the pastern through the lateral cartilages and hoof.
"Nearly all structures of the horse's foot are elastic or springy to some degree. The lateral cartilages, the digital cushion, and the coronary cushion are all classified as elastic structures. The primary purpose of these highly elastic structures is to reduce concussion. They cause the hoof to expand and contract at the heels, not only acting as shock absorbers, but also assisting in the circulation of blood."
Maintaining a healthy foot is a prime concern of all horsemen because, as has been noted, a lame horse is not capable of performing.
Nutrition For Hoof Health
One of the key elements in maintaining healthy feet is good nutrition. As already stated, horses receiving appropriate nourishment will have faster growing hooves than will those that don't. Rapidly growing hooves, generally speaking, are of higher quality and are easier to maintain than are slower growing hooves.
There are, of course, other factors involved in hoof growth. For example, the hooves of young horses will grow faster than older horses. Some researchers believe that heart rate can affect hoof growth and point to the fact that young horses have a heart rate almost twice that of older equines and, as a result, have faster growing hooves.
Weather can also be a factor. Hooves grow faster in warm weather than in cold.
Water for the Foot
Also of significance in maintaining a healthy hoof is moisture. Butler explains it like this: "Hoof moisture has been shown to have a direct effect on hoof quality. There is constant evaporation taking place from the hoof. Moisture must be replaced to compensate for this loss. Systemic water is transferred from the extensive blood and lymph supply of the sensitive structures to adjacent horn cells and they, in turn, transfer it to other horn cells.
"Environmental water from ground sources is also conducted throughout the hoof in similar manner. Balance of the two sources of moisture is probably maintained through the principle of osmosis. When one source is insufficient, the animal depends more heavily on the other.
"Hoof quality may relate more to the hoof's ability to regulate the moisture content than anything else, because as the moisture content of the hoof wall decreases, the hoof becomes harder and tougher. Commercial hoof dressings have not been shown to be helpful in increasing hoof moisture and improving hoof strength. Younger horses usually have softer hooves than older horses. There is also a variation of moisture content between the young and old wall within the same hoof. This contributes to its biomechanics, since the young wall of the heel and top is more flexible than the old wall of the toe and bottom."
The key is for the hoof to be provided with the correct amount of moisture--not too much and not too little. Ric Redden, DVM, a lameness specialist from Versailles, Ky., feels strongly that one of the reasons many racing Thoroughbreds have deteriorating feet is because the hooves are kept too moist. He is of the firm opinion that excessive moisture causes the hoof walls to lose their strength. Excessive moisture also causes deterioration of the matrix that holds the tubules together. Soon, Redden says, the heels are compressed and the hoof is out of balance, with the breakover point constantly moving forward.
This brings us to that highly important factor in determining overall hoof health and soundness--balance.
What Is a Balanced Foot?
Here, too, Redden is emphatic in his views. When the equine foot is out of balance, Redden says, a tremendous amount of torque is applied to the deep flexor tendon that runs beneath the navicular bone as well as putting stress on the ligaments that hold the bone in place. Lack of balance also causes stress to be applied to the other sensitive structures within the hoof, including the anterior laminae.
When the breakover point is moved forward beyond its normal point of rotation, the increased leverage from the extra toe length disrupts equilibrium between the short and long segments of the deep flexor tendon and a vicious cycle begins. Forces are no longer distributed evenly to the bursa, to the ligaments, to the digital cushion, to the sensitive frog, to the coronary band, and on up the leg.
In the process, the digital cushion and the heel are compressed due to increased load and this, in turn, reduces heel growth and further enhances toe growth, all of which causes the foot to become even more out of balance.
Proper hoof maintenance has much to do with balance. A key aspect of that maintenance is trimming. It is here, in Redden's opinion, that many equine hoof problems are born.
"Proper trimming and care from 'little' on is critical," Redden says of foot care starting from the time a horse is a foal.
The problem in many cases, he says, is that too much is pared from the hoof, eliminating vital hoof mass. He admits that in his early days as a farrier and veterinarian, he was guilty of the mindset that still prevails in many quarters today: "Like everybody else, I carved everything extra out of that foot that grew and threw it on the ground. I carved out the sole, over-trimmed the frog, and cut the weak walls back to good walls, regardless of mass, so that the foot looked smooth, shiny, and glistening."
Later, he says, came the realization that such an approach solved no problems, but created a host of them. At that point, he ceased the practice and concentrated on leaving everything the horse needed in the way of hoof mass and removing only what it didn't need.
Maintaining good feet that help keep a horse sound is no simple matter. Proper trimming, good nutrition, the correct amount of moisture...the list goes on. The good horse owner, however, will pay this price of diligence and attention to detail because the other option results in, "No foot, no horse."
About the Author
Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.