No foot, no horse; these words are as true today as they were two hundred years ago. But during that time, the horse has gone from being a "beast of burden" as the major means of transportation to a leisure animal--one that we ride for pleasure whether it is in competition or on a weekend trail ride. One constant during this time is that the horse has single digits, encased in tough, keratinized hooves, on the end of relatively lightweight limbs.
Hoof Growth and Design
The hoof capsule has an amazing design. It is built not only for strength, but also to support the horse's skeletal column. Keratin is the main structural protein of the epidermis and is present in skin, hair, nail, claw, wool, horn, feather, and scale as well as hoof. The keratins can be loosely grouped into the "soft" keratins of skin and the "hard" keratins of horn and hair.
The strength, hardness, and insolubility of hard keratin is due to disulfide bonds between and within the molecules. The tubular hoof of the wall (more on this in a moment) is composed of hard keratin that is rich in these strong disulfide bonds and thus has great physical strength. The frog and the white zone are rich in sulfydryl groups, but poor in disulfide bonds--thus they have lower physical strength, but greater elasticity. The sulfur-
containing amino acids methionine and cysteine are incorporated into the keratinocytes (keratin-producing skin cells) in the final stages of their maturation; hence the requirement of these amino acids (or their sulfur-
containing precursors) in the diet.
New hoof is made continuously to compensate for the continual wearing away of hoof wall at the ground surface. Regeneration of the hoof wall occurs at the coronary band where germinal cells (epidermal basal cells) produce populations of daughter cells (keratinocytes) that mature and keratinize continually, adding to the hoof wall. It takes these cells six to 12 months to reach the ground surface.
The coronary band produces two types of horn. Tubular horn arises from cells surrounding papillae (fingerlike projections from the coronary band), which become organized into thin, elongated cylinders or tubules. In cross-section, the keratinocytes of individual hoof wall tubules are arranged around a central hollow medulla in non-pigmented concentric layers. Each hairlike tubule is continuous from its origin at the coronary band all the way to the ground surface (a distance of two to six inches depending on the breed). The keratinocytes generated between the tubules mature into the second type of horn--the intertubular hoof--which forms a keratin matrix in which tubules are embedded (see "Anatomy of the Growing Hoof Wall" above).
The area of the foot that nourishes the hoof wall cells is the corium. There are several distinct zones---the coronary corium, laminar corium, solar corium, and frog corium. The coronary corium fills the coronary groove and blends distally (further out toward the ground surface of the hoof) with the laminar corium. Its inner surface is attached to the extensor tendon and the cartilages of the coffin bone by the subcutaneous (under the skin) tissue of the coronary cushion.
Collectively, the coronary corium and the germinal epidermal cells that produce the hoof wall are known as the coronary band. A feature of the coronary corium is the large number of hairlike papillae projecting from its surface. Each tapering papilla fits into one of the holes on the surface of the epidermal coronary groove and is responsible for nurturing an individual hoof wall tubule.
The innermost layer of the hoof wall and bars is named the stratum internum, and includes 550-600 primary epidermal laminae that project from its surface in parallel rows. Close examination of the hoof capsule shows that the laminae of the dorsal hoof wall are shaped like long, thin rectangles, approximately 0.3 inches (7 mm) wide and two inches (50 mm) long. The rectangles are oriented vertically, with their short sides fanning out from the coffin bone (one edge of the rectangle is incorporated into the tough, heavily keratinized hoof wall, and the opposite edge faces the outer surface of the coffin bone). The proximal (nearer the center of the body; further from the end of the hoof) end of each lamina is curved and forms the shoulder of the coronary groove. The distal end merges with the sole and becomes part of the white line that's visible at the ground surface of the hoof.
Like all epidermal (skin or other outer layer) structures, the epidermal laminae that make up the inner hoof wall are avascular (have no blood vessels) and depend on capillaries (tiny blood vessels that join arteries and veins) in the microcirculation of the adjacent laminar corium (which covers the coffin bone) for nutrition.
Strong Support for the Horse
This intertubular horn is formed at right angles to the tubular horn and bestows on the hoof wall the unique property of a mechanically stable, multi-directional, fiber-reinforced composite. Interestingly, hoof wall is stiffer and stronger at right angles to the direction of the tubules. The tubules are three times more likely to fracture than intertubular horn. This finding is at odds with the usual assumption that the ground reaction force is transmitted proximally up the hoof wall parallel to the tubules. The hoof wall appears to be reinforced by the tubules, but it is the intertubular material that accounts for most of its mechanical strength, stiffness, and resistance to fracture. Thus, hoof wall is considered to have an anatomical design that confers strength in all directions. During normal locomotion, the hoof wall only experiences one-tenth of the compressive force required to cause its structural failure.
The fully keratinized cells of the tubular and intertubular hoof, cemented firmly to each other, form a continuum--the tough yet flexible stratum medium of the hoof wall. When mature, these cells form a tough protective barrier that prevents inward passage of water and water-soluble substances and outward loss of body fluids from the highly vascularized (supplied with blood vessels) dermis.
In addition to acting as a permeability barrier, the hoof wall ultimately is responsible for supporting the entire weight of the horse. The tubules of the equine hoof wall are not arranged randomly--they are arranged in four distinct zones based on the density of tubules in the intertubular horn. The zone of highest tubule density is the outermost layer. The density declines as you move in toward the internal laminar layer. Since the force of impact with the ground (the ground reaction force) is transmitted proximally (from the outer parts of the body towards its center) up the wall, the tubule density gradient across the wall appears to be a mechanism for smooth energy transfer. The impact energy transfers from the rigid (high tubule density) outer wall to the more plastic (lower tubule density) inner wall, and ultimately to the coffin bone (third phalanx). The gradient in tubule density is an inverse relationship to the gradient in water content across the hoof wall (the less tubules, the more water content, which helps increase elasticity inside the foot), and together these factors represent an optimum design for equine hoof wall durability.
The interface between zones absorbs energy and prevents the propagation of cracks toward sensitive inner structures. In addition, the hoof wall being stronger in one direction ensures that cracks propagate from the bearing surface upward parallel with the tubules (they progress along the weakest plane). They do not extend to the innermost layers of the hoof wall because in this region, the relatively high water content confers high crack resistance (the hoof wall is more pliable).
The hoof wall also has a powerful dampening function on vibrations from the hoof wall contacting the ground. It can reduce the frequency (rate of shockwaves passing a given point) and maximal amplitude (strength) of the vibrations. In fact, by the time the shockwave of impact with the ground reaches the coffin bone, about 90% of the energy has been dissipated.
Going a bit deeper, let's look at support of the coffin bone within the hoof. The primary function of the laminar hoof is to suspend the coffin bone within the hoof capsule (it reserves its ability to grow new tissue for the healing of injuries). This anatomical specialization has increased surface area for the attachment of the many collagenous fibers arising from the outer surface of the coffin bone. The secondary epidermal laminae (see "Microscopic Hoof Anatomy on page 40) add even more specialized support structures--an extra 150-200 secondary laminae project from the length of each of the 550-600 primary laminae.
The tips of the laminae (both primary and secondary) are directed toward the coffin bone, to create a locking mechanism with the hoof capsule (similar to the barbs on a fish hook). The surface area of the equine inner hoof wall (the laminae contained inside the hoof capsule) has been calculated to average 13.75 square feet--roughly equal to two-thirds the size of a standard door. This large surface area for suspension of the coffin bone and the great compliance of the interdigitating (interlocking) laminar architecture helps reduce stress on any one area and ensures even energy transfer during peak loading of the equine foot. This hoof wall/coffin bone interface is amazingly strong and can be separated only under extreme circumstances.
The Foot Professional
In the past 200 years, your horse's health care provider has changed quite a bit. Two hundred years ago your veterinarian and farrier were the same person; today, the professions have separated. The veterinarian goes through more than three years of pre-veterinary training followed by four years of veterinary school, then he/she must pass professional examinations before being allowed to practice. But he/she might have received little advanced education on foot care. And anyone can set up shop as a farrier, with or without training, testing, etc.
So how do you decide on a farrier? There are many reputable farrier schools throughout the country that you can contact, but these programs can vary from two weeks to two years. So how do you know who really knows what to do for your horse's feet?
Beginning in 1971, the American Farrier's Association (AFA) was founded on the concept of sharing skills and training and educating farriers. The AFA has developed a method of certification that ensures skill levels. It is the only program I know of in the United States in which all advancement is through testing. While no program or testing system is perfect, the AFA certification program does give horse owners the ability to choose a farrier who has passed testing by his peers as to his knowledge and skills. So my best advice in choosing a farrier is to choose an AFA-certified farrier.
The farrier's basic role in hoof care is to see to the routine trimming, and shoeing if necessary. He/she deals with the insensitive structures of the foot; this role could most easily be defined as keeping the foot in balance with the rest of the limb and horse.
The hoof is a living structure that grows continuously; the rate of growth depends on many factors, but the one most readily noted is temperature. In northern climates where winter temperatures can be quite extreme, we see that during winter hoof growth might almost completely stop, whereas in more temperate winters one might see no difference in growth rate. Generally speaking, the hoof grows approximately one-quarter inch per month. This means it takes a year to replace an entire hoof capsule. However, the foot does not wear evenly--hence it needs to be trimmed on a four- to six-week interval depending on the wear of the foot.
Does your horse need shoes? Not necessarily; many horses are perfectly fine going barefoot. After all, Native Americans did quite well without shoeing their horses. Generally speaking, your horse only needs shoes if:
- You need to prevent excessive hoof wear;
- Your horse needs additional traction; or
- You need to enhance or correct gait and lameness problems.
If your horse doesn't need shoes, don't use them.
How should your horse's feet be balanced? Should we use the "Natural Balance" or perhaps the "Strasser" method, or a different method perhaps? There are many balancing techniques--geometric, dynamic, four-point, or natural, among others. All are said to be the method to use and all, under the right circumstances, can be effective. But what must be understood is that each method of balancing is really more a method of observation and understanding how any particular horse varies from the "norm." The controversies and problems arise when you try to uniformly apply these methods of observation to all horses. Obviously, all horses are not the same, so one can't expect the same results on every horse. So with regard to hoof balance, trimming and shoeing remain an art more than a science.
Has science improved our ability to assess hoof balance? The answer is yes--two technological advances help immensely, in my opinion. The first is a computer program produced by EponaTech that allows you to keep digital records of your horse's feet. The software provides a way to measure photographs and radiographs of the horse's hoof. Utilizing these measurements, one can keep track of conformational changes in the lower leg and hoof. If your horse is having foot issues, this is an excellent way to keep a measured record of what is happening and what effect various trimming and/or shoeing methods have on the foot.
This software can also be used to aid the farrier. Precise measurements can be made and relayed to the farrier so that the guesswork can be taken out of corrections. Utilizing these measurements, a precise hoof health record can be maintained and any changes that occur can be more easily and quickly recognized.
The second advance is thermography, which measures temperature and makes a thermal picture, to assess hoof balance. Utilizing this modality, you can exercise the horse at his competitive gaits and examine the thermal patterns created by the interaction of the foot/shoe with the surface. In this manner, one can determine where the hotter, higher-friction points are on the horse's foot and thereby determine if the horse is landing on his foot correctly.
Space-Age Shoes and Glues
Shoe technology has changed along with everything else. Two hundred years ago you had one choice--steel. Today, you have steel, aluminum, titanium, a plethora of different alloys, and all the different plastics and rubbers. Shoe decisions are usually best left to the farrier, who is most familiar with all of the different options, and should be based on the horse's use and quality of his hooves.
Some of these materials offer some very interesting properties. For instance, some alloys are very light and yet have terrific wear properties. I have seen a number of horses wear the same set of these shoes through the entire season and barely show any wear. Some of the new plastics are designed with exceptional anti-concussive properties. These could have many applications for police service horses, coach horses, or any horses used on very hard ground.
Another very interesting advancement is the use of adhesives in hoof care. When hoof wall integrity is compromised by injury or disease, lameness results or existing lameness is exacerbated. Quarter cracks and toe cracks rank high among the more common problems, but crushed heels, and thin, brittle walls are also common problems. All of these conditions often are complicated further by the fact that hoof wall does not mend; instead, we must wait for new growth, which can take up to 12 months. Patiently waiting for new growth is difficult at best and can result in the loss of an entire show season. However, hoof repair materials can facilitate and speed recovery.
This area has grown with the goals of finding a better hoof wall repair material and a better method for attaching horseshoes. Essentially there are two types of adhesives--acrylics and polyurethanes. When applied properly, these materials have the texture, strength, and flexibility of natural hoof wall, allowing the farrier to rasp and nail to and through the bonded material, which "grows down" with the hoof. Using available hoof repair composites, we can address capsular maladies such as hoof wall cracks, avulsions (tearing away of part of the hoof), underrun heels, and thin walls. In effect, rather than having simple cosmetic repair materials, we now have materials suited for structural bonding and repair.
Applied properly, the composite material alone generally provides great structural support; when combined with a reinforcing cloth such as a fiberglass or Spectra cloth, the strength of the material can be increased up to five-fold. Thus, horses with conditions that would have resulted in lay-up situations several years ago are actively competing today.
Successful application of these composite materials depends on numerous things, but manufacturers and farriers agree that the most important concern is proper preparation. The farrier must thoroughly debride the hoof wall where the composite will be applied and follow this with a solvent rinse, ensuring that the hoof wall is clean, dry, and smooth. Any loose, flaky hoof wall, moisture, or oil weakens, and sometimes negates, the bond. Indeed, improper application can result in greater disaster than a simple failed bond. Using these materials to seal moist, infected areas simply provides a protected breeding ground for anaerobic activity, allowing bacteria and fungi to actively proliferate under the patch, undermining the hoof and destroying more hoof wall.
Undoubtedly, these materials have proven themselves as a highly effective tool in the hands of the skilled farrier. Never before have we been able to get horses back "on track" as effectively. However, it is important to recognize that there is no acrylic magic wand--no composite panacea. Even with the spectacular materials available to us, we are still replacing form more than function, especially in chronic conditions.
Assessing Hoof Structure Injury
Radiography is the most common diagnostic tool used to assess hoof injury. Unfortunately, these X ray images only give information about bone, and there is much more soft tissue (such as ligaments, tendons, and connective tissue) in the foot than bone. Several new imaging modalities offer unique ways to further assess injury in the foot.
Navicular bursography is a simple technique that can be used to confirm injection into the navicular bursa and can also give valuable information regarding pathology in the region of the navicular bone. Changes seen via contrast navicular bursography represent stages of pathologic damage. Visualizing this damage allows more timely therapeutic intervention and more accurate prognostication. Bursography has improved the ability to identify pathology such as flexor cartilage erosions and to utilize therapy such as chondroprotective agents. Identification of tendon injuries causes concern for tendonitis; strict rest to allow tendon healing can be instituted. The identification of adhesions has been a grave prognostic indicator for conservative management.
Recently it has become possible to
examine the portions of the foot sonographically. In order to examine the podotrochlea (navicular region), the superficial horn must be pared from the frog to expose soft, spongy frog tissue (dirt and an uneven frog surface can create irregularities or "artifacts" on the image that can be mistaken for deeper problems). Next, sonographic gel is liberally applied to the frog. The ultrasound transducer is then applied to the frog. Images of the podotrochlea are apparent from the center of the frog to the apex; the insertion of the deep digital flexor tendon (DDFT) on the coffin bone, the navicular bursa, the impar (navicular) ligament, and the distal (nearer the ground surface of the foot) DDFT can all be clearly seen. By utilizing ultrasonography around the coronary band, the collateral ligaments of the coffin joint can be clearly identified. This has helped practitioners identify collateral desmitis (inflammation of these ligaments) as a cause of foot lameness.
Scintigraphy measures gamma ray emission from a radioactive nuclide injected into the animal. The technique provides information on relative vascularity (level of blood supply) and rate of tissue metabolism. This is particularly useful in studying bone pathology and can help differentiate sites of injury in the foot.
Thermography provides information regarding surface temperature. It has been shown to be useful in assessing the relative blood flow to a region. This information is of particular interest when pre- and post-exercise temperatures are determined. Exercise will normally cause a 0.9ºF (0.5ºC) rise in surface temperature. When the temperature doesn't rise to this degree following exercise, poor blood flow should be considered a factor in the disease being assessed. Alternately, areas that are much warmer than adjacent areas should be investigated as possible sites of inflammation.
Correcting Hoof Problems
Underrun heels--One chronic problem is that of underrun or "low" heels. Underrun heels have been defined as occurring when the angle at the heels is at least 5º lower than the toe angle. This is the most commonly encountered hoof abnormality. One study of foot-related lameness found underrun heels in 77% of the horses, and another study of normal performance horses found this condition in 52% of them. The necessity of correcting underrun heels has been well documented. If left uncorrected, they can cause alterations in hoof wall growth that can be very difficult to correct, and can predispose the horse to lameness problems ranging from bruised heels to navicular syndrome.
In dealing with underrun heels, it's important to assess several factors--the first is palmar hoof support. This is most easily assessed by radiography and seeing where heel-ground contact is relative to the widest part of the hoof and to the navicular bone. It is generally accepted that at least half of the weight-bearing area of the foot should be palmar to (closer to the heels than) the widest part of the foot. If this is not the case, the second assessment is of the orientation of the horn tubules in the heel region. With chronic abuse, these tubules can begin to grow more horizontally than vertically towards the ground. This leads to more problems in that some correction methods can actually lead to further crushing of the heel structure.
Proper heel position can be determined by either drawing a bisecting line through the cannon bone to the ground and the heel or by measuring the appropriate position on the radiograph. Where these lines contact the ground is the point where the heels should be, and the heel-ground contact should be even with the base of the frog. Shoeing to achieve adequate heel support can frequently restore the foot health. In some cases, however, you must rebuild the heels with acrylics or urethanes and thus alter the stresses on the coronary band to get heel growth to improve. If stresses remain uncorrected for too long, some cases simply cannot be corrected.
Club foot--The upright hoof is another common problem. There are two variations of this malady; the first is the club foot. Most veterinarians consider this to be present when the hoof angle exceeds 60º. However, I believe that a more important consideration is the hoof-pastern axis, which should form a straight line when the horse is standing square. When the pastern angle (angle with the ground) is less than the hoof angle, this causes what is called a "broken-forward" hoof axis. The problem with this conformation is that in order to correct it, one must lower the hoof angle. However, if the deviation between the two angles is more than 5º difference, then the DDFT will not allow further correction. Trying to force correction of these cases only by lowering the heels can lead to several problems such as hoof capsule separation, toe bruising, and coffin joint inflammation.
Many correction possibilities exist; for example, in horses less than two years old, shoe extensions or acrylic/urethane extensions are placed on the toe. The theory is that this will cause stretching of the DDFT and muscle and slowly allow the hoof to assume normal conformation. However, a problem with this method is that the stretching can lead to tendon injury and worsen the problem if the toe extension is too long.
A unique approach that has merit--especially in acute cases--is heel wedges, in which the conformational deformity is actually made worse by raising the hoof angle and stall-resting the horse for a time. The theory is that raising the heel reduces DDFT stress, and the rest will allow the muscle attached to the tendon to relax--which will then allow the hoof to be shod two weeks later in a normal position. If shoeing fails, shoeing correction can always be aided by an inferior check ligament desmotomy (cutting the check ligament that anchors the DDFT upward to the cannon bone just below the knee). This surgery releases strain on the DDFT, and the key to success is the immediate correction of the hoof problem at the time of surgery.
Mismatched feet--The second form of upright malady is mismatched feet. This is sometimes referred to as the "high foot, low foot" syndrome. Whatever you call it, this is the horse with one foot that's more upright than the opposite one--about 25% of the horse population has this problem to some degree. No specific predisposition to lameness has been identified, but the condition does create a problem when trying to make a horse look balanced.
There have been many theories on how to correct this problem, and different advocates have advised a plethora of solutions. The best solution, in my opinion, is to trim each foot individually and according to the conformation of that limb. Secondly, because one leg will tend to be straighter than the other, this can cause a functional limb length disparity. Once the horse's hooves have been trimmed, then both legs need to be assessed together; the fetlocks, knees, and shoulders should be at the same level. If not, pads can lift the "shorter" leg to balance the horse.
The twenty-first century is going to be exciting for the horse world. We will see technology continually improve our ability to study, diagnose, correct, and hopefully prevent more of the hoof problems that have plagued horses for centuries.
About the Author
Tracy A. Turner, DVM, MS, Dipl. ACVS, is a veterinarian with Anoka Equine Veterinary Services in Elk River, Minn. He was inducted into the International Equine Veterinarians Hall of Fame in 2004.
POLL: University Equine Hospitals