The Perfect Hoof
- May 1, 2005
An interesting dichotomy exists when we turn our attention to the horse's foot. On the one hand, without a healthy foot, a modern-day horse really has no value. On the other hand, the foot often is the most neglected part of the horse's anatomy. Clichés concerning the equine foot have been around for years, the best known being: "No foot, no horse."
Through the years, this writer has at times referred to a father-mentor who compared the horse's feet and legs to a house's foundation. He declared that you could have the most beautiful house in the world, but if it didn't have a good foundation, it would eventually crumble. So it is with the horse's foot. Unless it is correctly formed and properly cared for, it can result in a horse's demise. That is literally true of horses running free in the wild. The horse's prime weapon against predators is flight. If its feet do not allow it to speed away when danger threatens, it becomes a predator's meal.
As a result, survival of the fittest played a key role in wild horses developing well-conformed, healthy feet. Once man became involved via domestication, things changed. Often poor foot conformation was overlooked when breeding decisions were made. In other instances, man lent his expertise in helping the horse develop an abnormal foot to facilitate certain show ring gaits.
As a result, it is difficult today to talk about what is normal and "good" when discussing the equine foot. What might be appropriate for one breed or discipline is inappropriate for another. However, when all is said and done, basic hoof construction remains the same for all horses. In other words, all equine feet contain the same components anatomically, even though their conformation can be altered through man's interference.
In this article, we'll take a look at just how a foot is constructed and the role or roles played by the various components. We'll be calling on a number of experts in equine foot anatomy for help in this discussion.
A key source will be David Hood, DVM, PhD, a guiding force in establishing and maintaining The Hoof Project at Texas A&M University. The Hoof Project has a subscription website at www.hoofproject.com that already contains a good deal of information, with much more to be added in the future. We will be drawing on some of that information as well as information from Hood himself.
To begin the discussion, however, we are going to provide, in layman's terms, a description of the components that make up the equine foot. To do that, we are going to call on two University of Florida extension officials, one of whom is still at the institution and another who was formerly on the staff. They are Edward L. Johnson, PhD, currently a Florida state equine extension specialist and university employee, the prime author of an informational paper titled Anatomy and Topography of the Equine Foot; and R.L. Asquith, DVM, MS.
What Is The Hoof?
They point out, first of all, that the hoof itself is a cornified epidermis, similar in makeup to man's fingernail and containing no blood vessels. Instead, it relies upon an inner layer called the corium to provide the circulation and sensitivity necessary to maintain a healthy foot.
Here's how the two Floridians present the nomenclature of the external parts of the foot.
Bulbs of the heel--These structures are at the back part of the ground surface of the foot, behind the angle of the hoof wall. Internally, they receive support from the digital palmar (front feet) or plantar (hind feet) cushion.
Frog--This is a triangular or wedge-shaped structure with the apex pointing toward the front or toe of the hoof. Two grooves, one on each side of this structure, are known as the collateral or frog sulci. The central ridge is named the frog stay or spine of the frog and contains a furrow called the central sulcus or the cleft.
Bars--Near the heel, the wall of the hoof turns back toward the front portion. This continuation forms a ridge bordering the collateral sulci on both sides of the frog; these ridges are known as the bars.
Wall--This structure surrounds the foot and all inner parts. It is tough, fibrous, and somewhat elastic in nature and continually grows downward from the coronet. It is divided into three general areas--toe, quarter, and heel.
Sole--The sole makes up a major portion of the surface area of the bottom of the hoof, but it is not designed to be the animal's primary weight-bearing structure. It provides support for the inner structures.
White line--This boundary serves as a junction between the wall and the sole and is clearly visible around the front three-fourths of the circumference of the sole in a freshly trimmed foot.
We switch now to a description of the inner foot. Our two sources point out that this discussion is complicated a bit by use of various names for the same component.
Third phalanx (P3)--It is also called the distal phalanx, os pedis, pedal bone, and coffin bone. It is the most distal (farthest out from the body) of the four bones comprising the digit (equivalent to man's finger or toe) and is completely enclosed by the hoof. Interaction between this bone and the surrounding hoof structures serves as a shock absorber for the horse in motion.
Second phalanx (P2)--This bone is also called the middle phalanx, os phalanx, and the short pastern bone. It rests on the third phalanx and articulates with it and the first phalanx, which is above P2.
Distal sesamoid--This structure is often called the navicular bone or shuttle bone and is located on the back surface of both the second and third phalanx. It is an integral part of the shock absorbing mechanism, along with its ligamentous attachments.
First phalanx (P1)--This bone is also called the os compendale, os saffragenous, and long pastern bone. The first phalanx is the longest bone of the digit. It rests on the second phalanx and also articulates with the third metacarpal (in the foreleg) or metatarsal (in the hind leg), also called the cannon bone. It is closely attached to the paired proximal sesamoids by strong ligaments.
Johnson and Asquith then provide nomenclature for other structures within the foot.
Corium or pododerm--This part of the foot, they say, can be divided into five parts, but for discussion purposes, they considered only the general term and its function as a nutritional source to the hoof. Within this structure lies a massive supply of blood vessels that feed the hoof. These blood vessels--combined with nerves--form a sensitive layer intimately attached to the inside of the hoof wall and the third phalanx.
Digital (palmar or plantar) cushion--This is a wedge-shaped, modified subcutaneous tissue located within the back part of the hoof and composed of elastic fibers and some cartilage. As the name implies, it reduces concussion to the foot and puts pressure on blood vessels with weight bearing, which helps pump blood out of the foot.
Tendon of the common digital extensor muscle--It is considered in this discussion, the authors say, because of its insertion onto a process (protrusion) of the third phalanx and on the anterior (front) surfaces of the second and third phalanges. Its action is to extend the digit.
Deep flexor tendon--This is an extension of the muscle lying on the back part of the leg and which inserts on the posterior aspect of the third phalanx. It flexes the digit.
Superficial flexor tendon--This structure runs parallel to the deep flexor tendon and splits below the fetlock to insert on both the first and second phalanges. It also flexes the digit, but not the coffin joint (between P2 and P3).
Johnson and Asquith conclude their presentation of equine foot anatomy with this statement: "A horse that has healthy feet is the horse that can provide satisfaction and pleasure to its owner." It can't be said more succinctly or more accurately than that.
We now turn to Hood of Texas A&M. In addition to heading up the Hoof Project, Hood has written a number of papers and delivered numerous lectures in a variety of venues on the equine foot. He provides us with a description of proper equine foot conformation.
Most of the information that follows is taken from a paper he authored with Adrienne C. Jacobson, BA, called The Principles of Equine Hoof Wall Conformation. His opening remarks in the paper let us know right off that his task in describing hoof conformation is not an easy one:
"One of the most difficult and challenging subjects we meet in the study of the foot is defining and describing its proper shape or conformation," he states. "This has been discussed and argued for centuries, and even today there is little agreement regarding the ideal shape of the normal foot. Our purpose here is to summarize definitions, the past and current theories, and views on this important subject.
"We make a distinction between foot conformation (shape) and anatomy (its structure and relation of its parts)," Hood continues. "While these two are obviously related, important differences exist. One is that the foot conformation can be easily changed, whereas anatomy cannot. The various angles, widths, and lengths of the foot are commonly altered during shoeing, for either aesthetic or performance purposes. When these common changes are made, however, foot anatomy remains the same.
"Biomechanics is the application of mechanical laws to the living structures of the horse, especially those parts involving movement," Hood adds. "From a biomechanical view, the foot's conformation and anatomy make distinct contributions to its overall strength. For example, the attachment of the coffin bone to the hoof wall by the laminae is an anatomic strength. Wall curvature and thickness, on the other hand, are sources of strength coming from conformation. A simple way to study the foot is to consider conformation and anatomy as separate topics."
While man continues to alter foot conformation to meet his perceptions of the perfect equine foot for various disciplines, the true importance is the contribution that conformation makes to the foot's biomechanical function--its ability to accept, absorb, dissipate, and transmit the loads placed on it, Hood says. The basic geometric shape of the foot allows it to carry out this function.
Although millions of words have been spoken and written concerning good hoof conformation, it still is difficult to find a clear description of what actually is good conformation.
"Given its importance," Hood says, "one might first think it easy, after all the centuries the subject has been discussed and argued, to find a clear description of what constitutes good conformation. This is not the case, as is evident if you ask more than two people to describe what the normal foot should look like. Normal means different things to different people.
"Some of the extreme variation in what is considered normal is dictated by breed or use of the horse," Hood explains. "The foot shapes of a three-day event horse, the racing Thoroughbred, and the Tennessee Walking Horse are all considered normal, but their feet look so different that they cannot be considered to be the 'same' normal."
A second problem, Hood points out, has to do with the common practice of considering only the surface of the foot when evaluating conformation. This means that often only the highly visible hoof wall and sole are considered.
"But," he says, "we cannot ignore that the foot is a three-dimensional structure whose internal components do not function independently of the external components. They play just as significant roles in the biomechanics as do the external components."
This means, he explains, that dimensions, shapes, and angles of the coffin bone, for example, along with the bone's relationship to the external hoof wall, become important considerations in evaluating foot conformation.
One of the key differences between the external and internal components, Hood says, is the time involved in responding to or changing with stresses placed on them. The exterior is capable of changing much more rapidly than is the coffin bone, for example. The coffin bone can change in response to stresses, but more time is required for that change to occur.
While the exterior can change more rapidly, there often can be consequences. Hood puts it this way: "Such 'remodeling' of the external foot is not cost-free and cannot be done without affecting the internal structures and function."
The basic shape of the hoof wall, Hood tells us, is that of a cone. This shape allows a relatively larger area where the loading forces are first applied and also allows for efficient distribution of the load inside the foot. (Loading refers to the horse's weight being placed on a foot.).
"Anything that changes this basic cone," Hood declares, "necessarily affects the loading pattern. Too short or too steep walls decrease the desired cone shape and concentrate the loads both on the bearing surfaces of the wall and sole as well as inside the foot. If excessive, this can result in injury."
The basic cone structure, Hood explains, is lopsided or asymmetrical in two directions: "Imagine leaving the bottom of the cone flat on the ground surface and moving its tip, first toward the heels and then toward what will be the medial (inner) side of the foot. The first, the rear asymmetry, is the major of the two and governs the angle of the front (dorsal) wall. In the front feet, the angle is usually in the range of 50-54 degrees; in the rear feet this angle is approximately 53-57 degrees."
In explaining the role played by the cone-shaped hoof, Hood points out that the tilted shapes makes the hoof wall into an arch or series of arches, reaching from the ground on one side of the foot across the dorsal wall to the ground on the other side.
"Like the arches of bridges," Hood explains, "the arch of the hoof wall allows it to bear substantial weight. Instead of the sole and solar portions of the wall having to bear the entire load, the wall spreads a portion of the load over a shape that is better designed to bear loads. It is a slanted arch, which allows it to act in several different directions. When the horse is standing, the loads are placed vertically on the foot and the arch shape is shortened, expanding from the coronet to the toe at 90º to the ground. When the horse is moving and its foot acts in propulsion, the arch extends from heel to heel across the dorsal wall, increasing the size of the arch as the loads increase. Factors that affect the placement of the arch relative to the foot, such as long toes and underslung heels, affect the supporting ability of the hoof."
The other asymmetry, Hood says, is produced by moving the tip of the cone slightly toward the inner side of the foot. This means that the inner wall is at a steeper angle than the outer wall.
There is some debate concerning the importance of the difference in the angle of inner and outer walls. Hood theorizes that the reason for the adaptation might occur because most standing horses place more load on the inner hoof wall than the outer.
Hood says that in addition to the rear and medial asymmetries, the basic cone shape has also been modified (asymmetrically truncated). This means that the front wall is longer than the heels.
"This asymmetrical truncation is important," Hood states, "because it permits the foot's circumference at the coronet to more closely match the circumference at the solar surface. The bulk of the hoof wall is produced by, and grows down from, the coronary band. Because of its cone shape (i.e., smaller at the top than bottom), the amount of wall produced by the coronary band could not completely cover the foot at the solar surface if the coronary surface were parallel to the ground like the bearing surface."
The asymmetrical truncation also plays a role in determining the size of the foot and the length of the dorsal toe.
"The proper toe length," according to Hood, "is related to the size of the horse, with the larger horse having a longer foot. The length of the toe is partly dictated by the length of its upper coronary surface. The longer the distance from heel to heel across the coronary surface, the larger the bearing surface can be. Conformations that attempt to force a foot to a larger solar area, such as lengthening the toes, make the foot mechanically unstable because there is not enough wall produced at the coronary band to join the surface area being created at the sole." This can lead to the overly long walls spreading outward and chipping or cracking away from the sole.
Hood also describes the cone shape as being "incomplete." Instead of forming a complete oval, the wall at the heels turns forward, forming the bars of the wall.
"It is the combination of the cone being incomplete at the heels and the angle of the bars to the ground that allows the heels of the foot to be relatively free to spread apart during loading," Hood says. "In summary, the geometric shape of the foot can be described as an asymmetrical, truncated, incomplete cone. It is little wonder that such a complex shape makes it difficult to accurately describe what the normal foot should look like."
Hood then switches to a discussion of conformation of the sole. The normal sole should be dome-shaped or concave, with the concavity being more noticeable on the hind foot than the forefoot.
"Similar to the arches of the hoof wall," Hood says, "the sole's domed shape increases its weight-bearing capacity. Flattened soles should always be suspected to have a pathological cause until proven otherwise."
Common Questions and Answers
Hood, his colleagues involved with the Hoof Project, and other foot care professionals put forth frequently asked questions on equine feet for which they have given answers, and the logic behind the answers:
Q: Are white hooves weaker than dark hooves?
A: No. This is a statement that has been around for quite some time and is still commonly believed by many. Several studies have critically examined the question and all have produced the same results. There is no difference in strength or elasticity between white and dark hooves. It is likely that the impression that white feet are weaker came about because it is much easier to see defects--such as wall bruising and small cracks--in them than in dark hooves.
Q: Which is better, barefoot or shod?
A: There is no accurate answer to that question. To shoe or not to shoe a horse depends on many factors. The intended use of the horse, the previous shoeing history, the potential for problems in the feet, and the availability of a qualified farrier all impact the appropriate answer to this question. Because of the number of variables, it is necessary to answer the question for each horse as an individual rather than an absolute statement that applies to all horses.
One important part of this question is the ability of the foot--both internally and externally--to adapt to its environment. The foot of a horse that has never been shod and the foot of a horse that has always been shod will be different, as each has structurally adapted to the non-shod or shod state. It is for this reason that the occurrence of lameness is not uncommon when horses are shod for the first time as well as when those that have always worn shoes are left unshod. (See "To Shoe or Not To Shoe" on page 47.)
Q: What causes white line disease?
A: The first theory as to the underlying cause of white line disease is that it is due to a fungus that invades and destroys the non-pigmented regions of the inner hoof wall. It is usually assumed the route of entry is the foot's solar surface. The second most popular theory is that fungi (or bacteria) are present, but that some other insult, such as trauma, is required before they can invade and damage the hoof wall. A third school of thought is that white line disease is a consequence or form of chronic laminitis. More recently, it has been proposed that white line disease is a metabolic disease in which the cells of the wall's inner layers of the hoof are prematurely aging.
Q: What causes a frog to fall off?
A: Most frogs have the tendency to shed or come off periodically. This can occur so that it looks like the entire frog is being lost at one time. The frog of the horse's foot--like the hoof wall, sole, bulbs, and periople--are composed of highly modified skin. Compared to soft skin, these hoof structures are biochemically unique in that the individual cells that make them up are cemented together so that loss of individual cells occurs rarely or very slowly. This allows the hoof components to grow outward or downward until it is either cut away by the farrier or worn away by contact with the ground. Some frogs have a tendency to shed or come off periodically. When this happens, it is only the outer layers that are shed so that there is ample frog left with the foot. This appears to be a normal way for the frog to control its thickness.
Q: What causes the white, flaky appearance sometimes seen on the upper part of the hoof wall (coronary band)?
A: The outer-most layer of the hoof wall is technically known as the stratum externum and is often called hoof varnish. Like the rest of the hoof wall, it is produced at the coronary band and grows down the wall. Except for its thinness, it is similar in structure to the rest of the hoof wall, but biochemical differences do exist between this outer layer and the rest of the hoof wall. When this layer gets thicker than normal, it wears away. As it dries out, it tends to turn white and flake off. By itself, the condition poses little problem to the horse, but its cause should always be investigated.
Q: What's normal hoof size?
A: This one is answered by Doug Butler, PhD, American Farrier's Association Certified Journeyman Farrier, Fellow of the Worshipful Company of Farriers, who has written books on equine feet and hoof care and has been quoted in this magazine and others in the past. A hoof that is proportional to the horse's body size, he has declared, allows for the ideal distribution of body weight over the foot's laminar surface. When the foot is proportional to body size, he explains, it prevents over-compression of the sensitive and bony structures and allows the hoof to expand normally during movement.
Hoof size, he says, is influenced both by heredity, management, and nutrition. Horses fed an optimum diet, he avers, have an 80% increase in hoof-sole-border area size compared to those fed a limited diet.
Q: What influences a hoof's growth rate?
A: Butler says first that the rapidly growing hoof is often the healthier one, and that young horses have a higher hoof growth rate than older horses. Warm temperatures produce a higher hoof growth rate than cold temperatures. Exercise can increase growth rate, and front hooves generally grow faster than rear hooves. Generally speaking, the average rate of growth is about three-eighths of an inch or one centimeter per month. Growth rate can also be affected by hoof trauma and injury. Vitamin A is essential for proper hoof growth and health. Also essential is moisture, although an excessive amount of moisture in the hoof can be unhealthy.
In summary, it is obvious that ongoing hoof care is essential in maintaining good feet in our horses. It also is obvious that not all feet are created the same and that foot care should be carried out on an individual basis, based on each individual's normal foot conformation and anatomy.
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.