Structures of the Equine 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.
Third phalanx (3P)--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).
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.
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.