The Art and Science of

Introduction

Horses were domesticated as early as 6000 years ago on the central Asian steppe and by necessity hoof care most certainly came soon after. Throughout history horses played a significant role in the transfer of people, goods, and information, as well as the production and harvest of food. The people with quality horses had an upper hand and better hooves means better horses. The horses often benefited from hoof care, so people benefited from caring for their horse’s hoof, which included using some form of protection for the solar surface of the hoof. The early Egyptians, Persians, Romans and Mongolians used various types of horse slippers and horseshoes to protect their horses’ hooves. Iron horseshoes came into use by various groups, including the Chinese, Romans, and Celts around the middle or late years of the first millennium. The first machine designed to forge horseshoes was patented in 1834 in Troy, New York, and a nail manufacturing machine was patented soon after. Up until the introduction of machine made shoes all shoes were hand forged, either by the farrier applying the shoe to a specific hoof, or by a blacksmith who stocked horseshoes. Today many farriers continue to forge some of their own shoes, which is especially important where special types of support are required, and even machine made shoes must be shaped to fit the hoof of each individual horse.

Farrier practices have been developed through trial, error, and observation over the past thousand years. The knowledge that exists within the farrier community holds the solutions for many of the problems farriers have seen for centuries and continue to see today. By this, I do not mean that all old practices are correct simply because they have been practiced for many years. Rather, that there exists a wealth of horseshoeing and horsemanship knowledge which should be tapped. It is up to each individual farrier to discover this knowledge and develop it for their own use.

A farrier’s job is to direct the interactions between the horse’s hooves and the ground. The movement of each single limb within a stride can be broken down into four phases: landing, stance, break-over and flight. The properties of the hoof affect interaction with the ground which affects each of these four phases of a step. In turn, the interaction between the ground and the hoof affects the structures of the hoof through stress, wear, circulation and growth. The quality of this interaction can be defined by a single term - balance. When the interaction of the ground and the hoof creates mechanical forces on the structures of the limb that allow for a harmony of working parts, then balance has been achieved.

Advancements in science, medicine and technology are providing new methods of information analysis so that specific mechanical and physiological factors affecting the movement and health of the horse can be isolated and adapted. Researchers have begun to re-evaluate the movement of the horse and different shoeing methods in order to redefine the term ‘balance’ and apply the new found information to athletic performance, injury prevention and treatment.

Blacksmiths were forging and applying shoes long before radiography, digital gait analysis and machine made shoes. They relied on common sense and artistic vision. What it comes down to, in the end, is that the quality of farrier work, and the final product (the hoof), depend upon the farrier’s ability to apply theoretical knowledge to a horse’s hoof. The farrier doesn’t start with the basic building blocks of a hoof; hoof capsule, coriums, vascular system, digital nerves, bones, tendons, ligaments, etc. A farrier starts with a live horse, and works backwards to determine what needs to be done to maintain the system at its maximum potential, and then applies a trim or a shoe that will support the hoof capsule to allow for the best possible mechanics and physiology. An honest farrier will try to maintain hooves in as close to a natural state as the horse’s conformation, environment, and job allow. Whether this means barefoot, flat shoes, or bar shoes with frog support, the objective is for the horse and hooves to be functioning at their full potential.

This paper discusses theories of horses shoeing from a practical standpoint, covering all angles of farrier science so that they can be applied in the field. That said, there is no substitute for the knowledge gained from a master farrier, because each horse, hoof and situation is different. In the end the method in which the theories are applied matters just as much, if not more, than the theories themselves.

A tremendous thanks to the many equine professionals who have contributed to the information that is present here after. Any errors in the opinions or arrangement of the information that follows are the fault of the author only. This information is not intended as an anatomy or medical reference, or a horseshoeing textbook. It is intended solely to integrate hoof balance into the larger physiological and mechanical realm of equine studies because the hooves make the horse.

I. Evaluating Conformation

Evaluating the horse’s conformation is a farrier’s most important task. A correct evaluation of limb and hoof conformation, and how this conformation affects limb physiology and mechanics is necessary to apply the correct trim and shoe. A farrier first examining a horse must study the entire horse from the ground up. Background information such as the horse’s health, age, weight, discipline, job, behavior, environment, and medical history should contribute to decisions. Examine the horse from a distance of ten to fifteen feet for a clear perspective of conformation and hoof balance. From in front of the horse, examine the mediolateral balance of the hooves and the conformation of the limbs (fig. 2). The horse must be standing square and straight for this. Mediolateral balance occurs when the coronary band or hairline is an equal distance from the ground on both sides of the hoof. Ideally the coronary band should be symmetrical, both medially and laterally, because it responds to the forces placed on the hoof wall.

Fig. 2. This horse viewed from the front is balanced and the coronary band is level.

Next examine the hoof-pastern axis from a position beside the horse (fig. 3). Dorsopalmer balance occurs when the hoof pastern axis is parallel and the three bones of the digit are parallel to each other. When the horse is standing straight the angle of the shoulder should also match the pastern and hoof angles. A flared hoof appears dishy or crooked and the hoof wall does not form a straight line from the coronary band to the ground, but instead curves outwards as it approaches the ground. To accurately evaluate the hoof pastern axis, the hoof wall must be the same thickness at the coronary band as it is on the bearing surface. Even when the three phalanges are parallel, if the toe is flared, the hoof pastern axis will appear broken back.

Fig. 3. Three lines represent the hoof angle, pastern angle, and heel angle. This horse is an example of long toe- low heel syndrome, with a broken back hoof pastern axis and crushed under-run heels.

The next step is examining the hooves for symmetry, both within each hoof, and between pairs (front or hind pairs). Hooves should be near symmetrical, although they are never perfect because they are a product of limb conformation. Asymmetries between hooves should also be noticed (figs. 9a.b.c). Is one hoof more upright and narrower, the opposite more sloping and wider? This is common, and commonly referred to as high-low syndrome. The health of the inner structures depends greatly on the quality of the hoof capsule, so although the bones, tendons, ligament, etc are not directly altered by a farrier, correct maintenance of the hoof capsule is critical to their health.

The length of the hoof, from coronary band to the ground, determines the amount of protection given to the inner structures. Does the hoof appear large enough for the body above it? If the hoof appears too short when weight bearing, don’t trim hoof even if some can be trimmed from the bottom. On the other hand, if the hoof appears too long when weight bearing, but none can be taken from the bottom, the toe may need to be pulled back so that the hoof wall is a symmetrical thickness around the circumference of the hoof.

Examine the solar surface of the hoof. How is the shoe or hoof worn? Is the hoof symmetrical? Was the shoe the correct size for the hoof or was the duration between shoeings too long? Is the hoof excessively long or short? Is the sole cupped so that the walls bear the horses weight, or is the sole flat and bruised from contact with hard ground? Is the white line compact and healthy, with a consistent thickness around the perimeter of the hoof? Are the heels healthy or crushed? Is the toe solid or are the laminae damaged and the wall migrating forward? Are the frogs healthy and prominent or are they atrophied, narrow and infected? Is the hoof wet and spongy or dry and brittle? A healthy hoof is in between; moist enough to flex without cracking, but dry enough so it can maintain its rigidity. Fungal or bacterial infection such as thrush can be smelled very easily.

Fig. 4. This is a severely laminitic pony. The flare is very visible from the top. The yellow arrows point to a crack in the hoof wall caused by the lateral flare and weak laminae. The red arrow points to the stress rings visible across the whole hoof capsule.

The hoof capsule and the structures that it protects are directly affected by one another. Long-toe low-heel syndrome has attracted a lot of attention recently in the horse world. Mechanical problems arise when the center of the base of support, the bearing surface of the hoof capsule, migrates in front of the center of rotation of the coffin joint. Because the heels take more of the impact of landing and loading, they crush and become under-run heels. The long toe acts as a lever resisting break over. In doing so it weakens the hoof capsule, causes trauma to the toe laminae and strains the sensitive structures of the posterior aspect of the digit (see fig. 6).

Fig. 5. Notice the position of the heels in relation to the frog in these two photos. The heels of the left photo support the hoof all the way to the widest part of the healthy frog. Neither of the heels of the right hoof are back to the frog, which is atrophied and unhealthy. The arrowed lateral heel is severely under run because it is so much farther forward than the opposite even though the heels are the same length from coronary band to ground surface.

In a healthy hoof, only the top two thirds of the dorsal hoof wall is attached to the coffin bone. A long flared toe can cause the distal epidermal laminae to separate from the dermal laminae so that the toe takes on a dishy appearance. These dead laminae can only regenerate if the stress is relieved from the flare so that new growth at the coronary band can reestablish connection. The structures within the hoof capsule are extremely sensitive, and overloading of the heels can result in chronic heel pain and navicular syndrome. The structures supporting the fetlock and coffin joints are very efficient in their economical use of structre, but also very vulnerable. Damage can occur very quickly, such as with a bad step, or over a long period of time, such as through the degeneration of the specific components around the navicular region of the coffin joint due to long term imbalances and stress that increase friction and decrease circulation.

Fig. 6. This horse has chronic long toe-low heel syndrome, with a broken back hoof pastern axis and crushed under run heels. Correct trimming and shoeing hopefully can maintain a horse like this in a sound state, but this conformation predisposes a horse to acute and chronic strain-related injuries.

Fig. 7. View the limb by holding it above the fetlock so that the pastern and hoof hang naturally. Grasping the pastern or hoof will not allow you to see the true conformation of the limb. The left photo shows a relatively straight limb and digit. The right photo shows a deviation at the fetlock and coffin joints. In the author’s experience, deviations are common, especially in hind limbs. Joints of mature horses cannot be altered, so the hoof must be trimmed so that the horse can move as his conformation allows. The farrier compensates for imbalanced wear and growth, and resets the cycle that is pushed out of equilibrium by a conformation deviation.

Fig. 8. This is the same horse as the photo on the right side above. The photo on the right of the bottom of the hoof capsule shows that the lateral (left) side of the hoof is larger than the medial (right) side, which is caused by the digit’s lateral rotation and the way the horse bears weight on this hoof. When uneven pressure is applied to tissue, the vascular system distributes more nutrients to the unloaded tissue and less to the overloaded tissue. This process begins at birth, so a foal that bears weight unevenly on the hooves will develop an imbalanced conformation.

Fig. 9a. These are the opposite hooves of the same horse. The left hoof (left photo) is a severely clubbed hoof where the dorsal wall and heels are nearly vertical. The right hoof (right picture) is closer to a normal hoof conformation. Conformation abnormalities such as this are noticeable in the horse’s gait. The rhythm will be inconsistent when the point of break over relative to the center of rotation (CoR) of the coffin joint is different between pairs. Listen to the rhythm as the horse moves.

Fig. 9b. This is the same pair of hooves as the previous set. The yellow lines represent the length of the heels. The club foot (left) grows all heel, while the opposite foot grows mostly toe.

Fig. 9c. The left and right hooves possess no bilateral symmetry before trimming. Hooves should be trimmed and shod for each limb. An unnatural conformation should not be imposed upon a hoof simply to match the opposite hoof. However, when each limb is trimmed correctly, often they result in a much more closely matched pair. Because of this, frequent maintenance greatly benefits horses with this predisposition.

Every one of these issues must be examined before trimming, shoeing, or diagnosing lameness. Ask the owner questions. Don’t assume that they are aware of an issue or don’t have the answer to your question. A working relationship should be developed so that the owner is comfortable asking questions or discussing concerns. An educated owner will be much more helpful in maintaining healthy feet on their horse, which will make a farrier’s job easier and improve the quality of the horse’s life.

Examine the horse’s limb conformation. From the front and hind are the limbs straight? Ideal conformation is when a plum line can be dropped through the shoulder, knee, fetlock and center of the hoof. Is the horse base wide or narrow? Does the horse have offset knees or angular deformities such as a lateral cannon bone rotation or an angular deviation of a digit (see fig. 7)? Look for signs of injuries such as bone calcification, atrophied muscles or swollen joints, tendons or muscles. How is the horse bearing its weight? Are there old scars? Each of these deviations will affect the horse’s movement, circulation and hoof growth and wear.

Watch the horse move at both the walk and the trot. Does the horse appear comfortable when moving? Are there interference problems such as stumbling or forging, traveling close, winging in or out? How does the horse land, load and break over? When working with an experienced rider or trainer, ask them how the horse feels. Where does the drive or propulsion come from? What feels or looks like a front end problem could be a lack of support and propulsion from the hindquarters if the horse lacks muscling or stability in the hind. A farrier must know where their influence lies and not promise to fix problems that are caused by factors outside of this influence, such as conformation, environment, etc.

Evaluating the horse is the most important part of a farrier’s job. Before the hoof is trimmed, and a shoe applied, the farrier must know what the hoof needs in order to bring it into balance with itself, and as a component of the limb. The evaluation begins when the horse is first walked up the driveway or down the barn alley way, and does not end until it is out of sight again. The use of the information gained from the evaluation can either make or break any job.

II. Functional Anatomy

The Cycle of Balance, Support and Growth

(Redrawn based on Butler 1985. Permission requested.).

The equine hoof is a complex system consisting of sensitive and insensitive structures that up the horse’s base of support. Minute variations in hoof balance affect each component of the limb as the horse moves. On the other hand, the hoof can adapt to changes in the horse’s conformation, environment, and exercise. Because of this adaptive quality, hoof conformation both affects, and is affected by the horse’s movement. Balance is the critical factor. When a farrier balances a hoof, they are an artist applying a science to a living being. Not only should the hoof be shod for the present function and condition, but also for the future, when the hoof will grow, wear, and distort based on the forces placed upon it and the support provided to it. If the hoof is balanced correctly then the cycle of balance, support and growth will direct the mechanisms of hoof growth so that the hoof continues to support the horse. Unfortunately, no horse has perfect conformation. The parameters laid out above are ideals. By examining the hoof capsule to understand what is happening inside the hoof, a farrier can influence the hoof capsule, and work towards balanced wear, load distribution and circulation.

Limb Anatomy

The horse’s limb is a suspension system, first absorbing energy from impact and then releasing it in the form of upward and forward propulsion. The lower limb of the horse, from the tarsus (hock) joint and metacarpus (knee) joint down, resembles the wrist and ankle of a human, where the horse’s hoof is an overdeveloped nail of the middle (third) digit. The digit consists of three supporting bones: the distal phalanx (coffin bone), middle phalanx (short pastern), and proximal phalanx (long pastern). The dorsal surfaces of these three bones should be parallel so that the compressive forces are evenly distributed through the joints.

There are no muscles in the lower limb. Force is transmitted through tendons and ligaments that convey energy from muscles in the upper limb. The elastic property of tendons and ligaments converts the kinetic energy from impact into potential energy and releases it as propulsion. The system of tendons and ligaments is referred to as the suspensory apparatus, and consists of the deep digital flexor tendon (DDFT), the superficial digital flexor tendon (SDFT) and the suspensory ligament. The suspensory ligament runs from the metacarpus and tarsus down the palmer aspect of the limb around the fetlock joint where it divides into suspensory and extensor branches. The suspensory branch attaches to the palmer surface of the proximal phalanx and the collateral extensor branches wrap around the pastern and attaches to the dorsal surface of the distal phalanx.

Fig. 4. Sensitive structures of the posterior digit and the hoof capsule. (Redrawn based on Butler 1985. Permission requested.)

Hoof Anatomy

With its ability to withstand enormous amounts of stress, the hoof allows the horse to be an amazing athlete. The hoof wall, frog, digital cushion, bars, and laminae absorb the energy transferred down through the middle and distal phalanges from the limb to the ground. There are five forces placed on the hoof; the tension of the laminae holding the third phalanx to the hoof wall, tension from DDFT on the distal phalanx, compression form second phalanx, the extensor branches of the suspensory ligament on the extensor process of the distal phalanx, and the ground reaction force (7, 9).

Fig. 5. Sagittal section of the digit.

The three bones within the hoof capsule are the distal phalanx, distal aspect of the middle phalanx, and navicular bone. The dorsal surfaces of the phalanges are parallel to each other and to the forces exerted upon them, which results in even distribution of force through the joints. The hoof is constructed so that the whole system flexes in order to absorb shock as the forces applied down through the limb are resisted by the forces applied up from the ground. The hoof capsule of a healthy hoof follows the form of the inner structures. The proximal two thirds of the dorsal portion of the hoof capsule is attached to the distal phalanx and less flexible than the palmer portion, which is attached to the collateral cartilages that extend proximopalmarly from the distal phalanx. In young horses, the cartilages are highly elastic and are composed of hyaline cartilage tissue and fibrous cartilage tissue that flexes to allow for heel expansion. In some horses the cartilages can ossify, a condition known as side bone (2, 10). Each cartilage is stabilized by five ligaments attached to the middle and proximal phalanges, the distal phalanx, digital cushion, collateral ligament of the coffin joint and the extensor tendon, and the collateral sesamoidean ligament and navicular bone (10). The function of each cartilage is to provide rigidity to the palmer portion of the hoof while allowing expansion and contraction during weight bearing.

Fig. 6a. A lateral view of the hind digit of a young horse with the epiphyseal growth plates of the middle and proximal phalanges clearly visible and arrowed.

Fig. 6b. Dorsal (left) and palmer (right) views of the same digital bone structure as above.

Between the collateral cartilages lie the digital cushion, navicular bone, and the distal interphalangeal joint. The digital cushion functions to absorb the impact from the middle phalanx, which descends and compresses the cushion against the frog (5). The digital cushion is contained within the heels, beneath the navicular bone and DDFT, and palmer to the coffin bone. Distally the cushion follows the contours of the frog, and consists of collagen and elastic fibers, islands of cartilage, fat and modified skin glands with a poor vascular supply.

The navicular bone allows the attachment of the DDFT to maintain the same angle of insertion to the distal phalanx regardless of coffin joint movement. Proximally, the navicular bone is stabilized by the collateral sesamoidean ligament and the lateral palmer ligaments of the pastern joint. Distally it is supported by the distal impar ligament (10).

A horse with properly balanced hooves, moving over flat ground, lands either flat or heel first depending on conformation, footing, fatigue, and gait. As the hoof impacts the ground heel first, the toe rotates down to the ground. The horse’s mass forces the pastern to rotate downwards against the suspensory ligament, SDFT and DDFT. The latter two have insertions on the palmer aspect of the limb, pulling back on their distal attachment to the pastern and coffin bones. The suspensory ligament’s branches insert on the extensor process of the coffin bone countering the torque of the flexors. As the fetlock flexes, the middle phalanx pushes down on the digital cushion, forcing the digital cushion to expand horizontally against the collateral cartilages, and downwards against the frog. The plexus of vessels surrounding the cartilages is compressed at impact, pushing the blood through the cartilages and up the digital veins (9).

Fig. 7. Two vascular corrosion casts illustrating the digital blood supply. Nothing can illustrate better than these two casts the importance of balance on the health of the hoof. The majority of the vascular system is contained within structures that are immediately affected by the pressures applied to them. Imbalanced pressures limit the blood and nutrients to the compressed regions of the hoof (Pollitt 1995, permission requested).

The vascular system of the digit consists of a network of arteries, veins and capillaries supplied mainly by the palmer digital arteries that run down the rear of the pastern. These two arteries originate where the palmer artery of the limb bifurcates above the fetlock. They meet as the terminal arch in a canal inside the distal phalanx. The blood supply to the laminae and sole is protected by the porous distal phalanx (coffin bone). Both the proximal phalanx (long pastern), and middle phalanx (short pastern) are encircled by arterioles originating from the palmer digital arteries. Running parallel to the short pastern’s arterial circle is the coronary circumflex artery, which nourishes the coronary band with the help of arteries extending up from the terminal arch through the porous distal phalanx. At the heels, arteries branch off to the frog, digital cushion, laminae of the heels and bars, and the palmer coronary band. The palmer portion of the coronary circumflex artery also feeds arteries to the cartilages and dorsal middle phalanx. The circumflex artery of the sole is fed from the terminal arch and through a notch in the cartilages. The dermal laminae are fed through a network of small channels within the distal phalanx extending from the terminal arch (7).

Fig. 8. Left: Diagram of the digital blood supply. Right: Diagram of lamellar circulation (Pollitt 1995, permission requested).

With the exception of vessels within the distal phalanx or collateral cartilages, the vascular system is contained within flexible structures whose forms depend upon the applied forces. For instance, when imbalanced forces are applied to the hoof, the coronary circumflex artery, which supplies nutrients for growth of the hoof wall, is compressed within the soft structures that contain it. As a result it cannot provide a balanced supply of blood to the coronary band. This means that the coronary corium will stimulate unbalanced growth of the hoof wall.

The function of the sole is to protect the underlying sensitive structures and the solar surface of the distal phalanx. The sole is approximately 33% water with horn tubules curving near the ground surface, which results in the shedding of the sole. The higher moister content and the elastic structure allows the sole to flex at impact. The sole grows in the same way as the hoof wall and undergoes the same process of keratinization. Ideally the sole has a concave cupped shape so that it can flex downwards at impact without damage caused by contact with hard ground.

The function of the frog is one of the most highly debated of any hoof structure. It has been proposed that the frog aids in traction, concussion absorption, and circulation. In reality it probably aids with all three. But there is a question whether or not a healthy hoof requires a frog with ground contact, which all of these would require (2). It is possible that along with the lateral cartilages, the frog aids with an elastic function in holding the rear portion of the hoof together as the heels are forced outwards by the digital cushion. The frog aids in resisting the downward force transmitted through the digital cushion (10, 2).

The sensitive frog is composed of short papillae, similar to those of the periople ring, that nourish the horn producing cells of the stratum germinativum, that produces the horny frog. The horn produced is not completely keratinized or cornified, allowing it to remain flexible. Fat secreting apocrine glands keep the frog elastic. The frog has the highest water content of the hoof, approximately 50%, aiding in flexibility (10).

The hoof capsule is composed primarily of keratinized epidermal tissue with approximately 25% water content. The dermis provides nutrients, sensation, and attachment to the underlying hoof structures for the epidermis and is often called the corium when referring to the hoof. The five coriums of the hoof are the perioplic corium, coronary corium, laminar corium, solar corium, and corium of the frog (5, 10). Each corium is highly vascularized, and contains sensory nerve endings and sympathetic motor terminals to blood vessels. The two layers of the dermal structures are composed of collagen, elastic and reticular fibers, which are extremely tough and resilient. The thin outer papillary layer nourishes the epidermis through osmosis and diffusion. Each papilla protrudes into the epidermis and contains looping blood vessels and capillaries through which nutrients travel. Making up the majority of the dermis is the deep reticular layer of dense connective tissue. Where the corium is adjacent to the distal phalanx, it is continuous with the bone’s periosteum strongly connecting to the distal phalanx (4).

Fig. 9. Diagram illustrating hoof growth. The primary epidermal laminae (PEL) migrate by the secondary epidermal laminae while continuing to maintain an attachment. (Adapted form Pollitt, 1995, permission requested).

The dermal (sensitive) laminae are attached to the coffin bone and interlock with the epidermal (horny) laminae lining the hoof wall. The laminae attaching the coffin bone to the hoof wall can be compared with vertical interlocking grooves where the hoof wall, which grows downwards, remains attached to the coffin bone as the primary epidermal laminae migrate by the secondary epidermal laminae which remain attached to the dermal laminae. Each hoof contains approximately six hundred primary laminae (although this number can vary greatly depending on size and structure) and each primary lamina has approximately one hundred secondary laminae interlocking with the corresponding laminae (7, 2).

Between the dermis and epidermis is the basement membrane, a continuous connective sheet of tissue which is covered by the stratum germinativum. Hemidesmosomes are bonds between the basal cells of the epidermal laminae and the basement membrane, and desmosomes are the bonds between the cells of the basement membrane. The basement membrane attachments can fail as a result of chemical reactions causing laminitis (8, 6). The white line is the visible component of the dermal epidermal junction on the solar surface of the hoof capsule. The terminal papillae are the extension of the laminae from which the white line grows. The health of the laminae can be judged from the health of the white line.

Fig. 10. Diagram of the coronary dermal epidermal junction (Pollitt 1995, permission requested).

The epidermal hoof wall grows down from the coronary band and consists of keratinized epithelial cells cemented with keratin and arranged in tubules. The horn is generated from the stratum basal, or the stratum germinativum, the deepest layer of the epidermis. The stratum germinativum is a single layer of live cells at the dermal epidermal junction that divide and push their daughter cells distally. The epidermal stratum germinativum receives the dermal papillae. The papillae-receiving stratum germinativum produces the horny tubules and the inter-papillary stratum germinativum produces the inter-tubule horn. Both the cells of the hoof wall and the sole undergo a full process of keratinization. As the daughter cells migrate down, their organelles die and the cells are filled with keratin, making them dense and rigid. Holding the horn cells together are glycoproteins made up of intercellular lipids and corneosomes, modified desmosomes. In addition to attachment, these lipid layers limit the movement of water and water soluble substances through the hoof capsule.

Fig. 11. Diagram of the terminal papillae and white line (Pollitt 1995, permission requested).

The inner epidermal layer, the stratum lamellatum, consists of the primary and secondary epidermal laminae. Nutrients can diffuse through the basement membrane from the dermal laminae and into the stratum lamellatum. Making up the bulk of the hoof and providing the density and structure is the second layer, the stratum medium, consisting of horn tubules and intertubular horn. The outer layer, the stratum tectorium, is the thin glossy layer of horny scales that help to prevent loss of moisture through evaporation from the hoof capsule (10).

It is the flexibility of the coronary band and laminae that allows distortion to occur as forces are applied during the support phase of the stride and as support is provided to the hoof, either by a ground surface or a shoe. The coronary band is not rigidly attached to the bony structures beneath it. Instead it is attached to the coronary cushion, a thick layer of elastic stratum germinativum, which joins the coronary corium to the common digital extensor tendons and the collateral cartilages (7).

The periople protects the coronary band and the young hoof wall, and the heels where it merges with the horny frog. The periople is a tough skin originating from the periople ring just above the coronary band. Its main function is to protect the coronary band and young top portion of the hoof wall, as well as covering the bulbs of the heels. This allows the hoof wall to grow as a flexible tissue and harden as it grows down (2).

Balance, Support and Growth

Conformation, environment, exercise, shoeing and trimming all affect hoof growth. The portion of the hoof bearing more weight grows less than the portion bearing less weight. This is easily visible with medial lateral imbalances. The hooves of most horses are not symmetrical because neither the structures within the hoof and limb, nor the movement of the horse, are totally symmetrical.

Correct dorsopalmer balance and hoof pastern axis are essential in achieving full support so that the horse moves and grows balanced. The dorsal surfaces of the phalanges, the hoof pastern angle, must be parallel. When the toe is allowed to grow too long the hoof pastern angle is broken back, the leverage of the elongated toe pulls the whole hoof capsule forward and stretches the lamellar attachment. Weight becomes unevenly distributed around the hoof capsule with the heels bearing more, so that they collapse and grow forward under the hoof. Ultimately the problem can be explained by examining the tubules in the hoof. They are meant to be parallel to the applied forces. If the forces are not parallel to the tubules then the hoof capsule deforms. As soon as the hoof capsule deforms, it will continue to grow deformed. The longer hoof increases the leverage acting against the DDFT so that it must sustain more force in order to overcome the increased moment of the applied force from the ground. This affects the movement of the whole horse and can lead to degeneration of the overloaded structures.

An upright hoof with a broken forward hoof pastern axis has the exact opposite problem. The toe bears significantly more weight, shutting of its blood supply, causing degeneration of the hoof wall and laminae. Heel growth in upright hooves is excessive because the heels are loaded less, causing the hoof to become more and more upright. Medial lateral imbalances cause similar circumstances. The unloaded side receives less wear and more nutrients, while the loaded side receives more wear and fewer nutrients.

The hoof is a sensitive structure that responds to the forces upon it. The inner structures contain the blood supply which nourishes the hoof. When imbalanced force is placed on the structures of the hoof, the blood supply to the overloaded areas is constricted. The constricted blood supply limits the growth to the overloaded area, so that as the time passes the overloaded structures become increasingly overloaded. If the load is more than the hoof can support it will collapse unless the cycle of hoof growth, support and balance is corrected.

References:

Back, Willem and Hilary Clayton. Equine Locomotion; Hardcourt Publishers, UK 2001
Butler, Doug. The Principles of Horseshoeing; Butler Publishing 1985
Clayton, Hilary. The Dynamic Horse. Sport Horse Publications 2004
Colville, Thomas and Joanna M. Bassert. Clinical Anatomy and Physiology for Veterinary Technicians; Mosby 2002
Curtis, Simon. Corrective Farriery; Volume 1; R&W Publications, Great Britain 2002
Curtis, Simon. Corrective Farriery; Volume 2; R&W Publications, Great Britain 2006
Pollitt, Christopher C. Color Atlas of the Horse’s Foot; Mosby-Wolfe 1995
Pollitt, Christopher C. Equine Laminitis. RIRDC 2001
Rooney, James R. The Lame Horse; The Russell Meerdink Company, Ltd. 1998
Stashak, Ted S. Adams’ Lameness in Horses; Lippincott Williams and Wilkins 2002

III. Balance and Motion

Balance is the most difficult term to define in farriery, but at the same time it is the most important concept to apply to a horse’s hoof. Not only does the horse’s athletic ability depend upon balance, but also his health and well being. The interaction between the hoof and the ground sends a chain reaction up the limb and through the body, affecting the movement of the whole horse. The quality of this movement affects the form of the hoof. Following is an examination of a major biomechanical question relating to horseshoeing, “How does the balance of the hoof affect the musculoskeletal system and the horse’s movement?”

The hoof is the base of support for the horse and it has been known by horsemen for centuries that small alterations in the form of the hoof through trimming or shoeing can drastically alter the manner in which the horse moves. New research is re-evaluating the movement of the horse to define the term ‘balance’ and apply the new found information to injury prevention and treatment, and the improvement of gait and conformation faults. Most research is evaluating old horseshoeing methods. This research is evaluating equine lameness problems and their solutions to define the link between form and function.

Balance can be defined in many different ways. Perhaps the truest definition of balance is a harmony of parts. If we look at the horse as a system made up of separate components or smaller systems we can gain a better understanding of the larger system through understandings of its individual parts. The function of the limb is to support the horse in motion. Considering that the limb is made up of individual components, it is extremely important that each of these components is in a state of equilibrium and functioning properly. Balance is a static and dynamic equilibrium of force distribution over the weight bearing structures of the hoof and the joints of the limb, so that the interaction between the horse and the ground is uniform and movement is fluid. The horse’s hoof accepts the concepts of both form following function, and function following form, creating a cycle between the two. The farrier’s job is to direct this cycle.

In this paper I will attempt to make the concept of balance practically applicable to the world of farriery by looking at how the horse moves and what constrains this movement.

The Stride

A horse’s movement can be broken down into strides and steps. A stride is the full cycles of movement of all four limbs. A step is the full movement of an individual limb within one stride. The pattern of limb movement within a stride is the gait of a horse.

When the horse is standing, roughly sixty percent of its body weight is supported by the front end and forty percent by the hind end. In motion the front end retains its supporting function while the hindquarters become the horse’s engine, generating propulsion. This is why front and hind hooves differ in their conformation. Healthy front feet generally have broad and round structure, because they carry the weight of the horse. The hind hooves are generally longer and with a more defined shape with different radii for the toe, quarters, and heels. The support they provide is in line to the horse’s movement.

The movement of each limb and the pattern of limb movement during each stride affects the role that each hoof plays. Different gaits have distinct patterns (see table 1). The walk is a diagonal four beat gait. As the horse moves at a walk an alternation between two and three supporting limbs occurs. The trot is a two beat diagonal support gait where the diagonal pairs alternate support, with an airborne phase in between each diagonal pair’s support. The canter is a three beat gait where the horse is first supported by just the leading hind, then the opposite diagonals, and finally the leading front, followed by an airborne phase. The gallop is a four beat gait similar to the canter, except that the canter has a support phase where three limbs bear weigh, while, at most, two limbs bear weight at any time during a gallop stride. These gaits are described to illustrate the forces present during locomotion, when the support of a thousand pound running animal must be provided by one, two or three limbs. Each limb is supporting much more than its static support role requires (3).

Fig. 1. Edward Muybridge was the first to capture the stride of a horse with a series of twelve cameras in rapid succession, proving that there is indeed an air borne phase during the gallop stride.

Gait support patterns:

Walk: RH, LF, LH—LF, LH—LF, LH, RF—LH, RF—LH, RF, RH—RF, RH—

RF, RH, LF—RH, LF—RH, LF, LH—

Trot: RH, LF—Air borne—LH, RF—Air borne—RH, LF—

Canter: RH—RH, LH—RH, LH, RF—LH, RF—RF, LF—LF—Air borne— RH—

Gallop: RH—RH, LH—LH, RF—RF, LF—LF—Air borne—RH—

Table 1. RH= right hind, RF= right front, LH= left hind, LF= left front.

This table describes the pattern of limb support during a stride. The dashes indicate the movement of a limb. The walk support pattern begins with the RH, LF and LH limbs supporting the horse. As the horse’s body moves forward the RH comes off the ground, and the RF comes down to the ground. The typical way of describing movement of the walk as a four beat diagonal gait is also embedded in this pattern – RH, LF, LH, RF.

Each step of the stride can be split into a support and flight phase (see fig. 2). The support phase includes impact when the hoof contacts the ground, support when the limb is loaded, and break over when the heel leaves the ground and the hoof rotates around the toe before the whole hoof leaves the ground and begins its forward swing. The flight phase includes protraction after break over, when the leg is pulled forward, and then retraction once the limb reaches its maximum forward extension and pulled backwards relative to the movement of the horse’s body, just before impact.

Fig. 2. The full step of the right forelimb, beginning with a&b: impact. c: mid-support. d&e: break over. f&g: flight. h&i impact. (Permission requested, Equine Practice, Veterinary Clinics of North America, Dalin. “Locomotion and Gait Analysis,” 1985, V1, No 3).

The major muscles that support and propel the horse are in the upper limb and body. In the forelimb the serratus ventralis muscles, composed of a cervical and thoracic component, supports the body of the horse by suspending it between the scapulae in a sling like fashion. The center of rotation of the forelimb is not at the proximal end of the limb, but instead at the shoulder joint, between the scapula and humerus. The pectoral muscles support the center of rotation of the limb by holding the shoulder joint to the chest.

In flight the lower leg acts as a pendulum, swinging forward, and then retracting at impact. At impact the leg has already begun retraction. With an insertion above the center of rotation of the scapula, the serratus ventralis cervisis muscle contracts, pulling the distal end of the scapula forward. At the same time the latissimus dorsi muscle contracts, pulling the proximal end of the scapula back. This begins the retraction of the upper limb, which snaps the lower leg into alignment so that the metacarpus is in line with the radius at impact. The carpus remains immobile during weight bearing phase.

At impact, the fetlock and coffin joints extend as the fetlock drops towards the ground under load, supported mainly by the deep digital flexor tendon (DDFT), superficial digital flexor tendon (SDFT), and the suspensory ligament. As momentum carries the horse over mid-stance, where the leg is perpendicular to the ground, the serratus ventralis cervicis and serratus ventralis thoracis contract to support the scapula while the brachial biceps supports the shoulder joint, holding the upper limb in an extended position. The triceps extend the elbow joint by pulling forward on the olecranon process above the center of rotation of the radius. The DDF muscle aids in flexing the extended fetlock and coffin joints.

Fig. 3. Forelimb musculoskeletal system.

As the foot leaves the ground and the upper limb begins protraction, the carpus bends, and the distal end of the humerus is pulled forward by the brachiocephalus muscle. At the same time the serratus ventralis thoracis pulls the proximal end of the scapula back and down, rotating the distal end forward. The radius and digit follow the upper limb and snap forward into extension with inertial energy. Contraction of the main extensor muscle aids in extension of the lower limb and digit at the end of protraction. By moving the distal limb closer to the center of rotation during the flight phase, the center of mass is moved closer to the center of rotation and less energy is required to swing the limb forward.

Fig. 4. Hind limb musculoskeletal system.

The stride of the hind limb is similar to the fore. The limb begins to retract just before impact. The large gluteus medius contracts, pulling on the femur above the center of rotation, so that the distal end moves backward and extends the hip joint. The hamstring muscles pull the stifle joint to the rear. The quadriceps support the stifle and resist the hamstrings by holding it in an extended position. The hamstrings also extend the hock joint by pulling on the tuber calcis, the most proximal bone in the hock above its center of rotation. The extension of the stifle and hock joints add tension to the suspensory ligaments and tendons of the fetlock joint, flexing the fetlock and coffin joints, propelling the horse forward.

When the hoof leaves the ground the hip, stifle, and hock joints flex. The iliopsoas muscles rotate the femur forward. The digit of the hind limb is extended with the common digital extensor tendon just before impact at the beginning of retraction. Because the hip, stifle, and hock joints are closed, the whole leg is closer to its center of rotation, which reduces the energy needed to pull the leg forward and allows the hind quarters to move parallel to the ground instead of wasting energy with vertical motion (12).

Research

Over the past two decades many different methods have been used to assess theories on hoof balance and how the function of the horse is controlled by its conformation. Through examining hoof balance and how it affects the forces placed on individual structures of the limb, the effects that balance has on the overall movement of the horse can be determined.

Fig. 5a. The photo on the left shows a balanced, shod, left front hoof. The photo on the right shows the same hoof before the shoe was applied.

It has been argued that a flat shoe applied to a balanced hoof only affects the mechanics of movement during landing and break over (12). This is conceivably true if an unshod hoof provides the same amount of friction as a shod hoof, a shod hoof weighs the same as an unshod hoof, and a shod hoof gets the same amount of solar, frog and heel support as an unshod hoof. It is unlikely that these conditions are met on almost any surface or with any shoe. However, it is also arguable that the horse can, in most cases, compensate for these changes so that they have very little affect on the overall movement of the horse.

Fig. 5b. The top is a balanced trimmed hoof with a parallel hoof pastern axis. The bottom is the same hoof after the application of a flat shoe.

Force plates, plates equipped with force measuring transducers that the horse steps upon, were used to determine that the application of a standard flat shoe has a minimal effect on the movement of the point of force, or center of pressure (CoP), relative to a balanced unshod hoof. Hooves balanced to a parallel hoof pastern axis with the solar surface of the hoof perpendicular to the metacarpal bone were then shod with 3.7 degree medial and lateral wedges, 5 degree heel and toe wedges, and a test group with flat plates. The medial laterally imbalanced hooves showed a CoP significantly shifted towards the wedged aspect of the hoof with the landing phase affected twice as much as the stance phase. It has also been demonstrated that heel wedges delay the time at which the heels are lifted off the ground during break over, and toe wedges advance heel off (13).

Treadmills in conjunction with instrumented shoes have proved useful because they allow continual recording both of force readings and video analysis. Horses measured on a treadmill at the walk, trot and canter with shoes containing force measuring transducers, first in a balanced state with a parallel hoof pastern axis and then with four degree wedge pads elevating the heels and the toes of the front feet showed that elevating the heels shortens the time of break over and increases the peak forces on the medial side of the hoof and elevating the toe increased the peak forces on the toe and the time until heel off. Studies such as this suggest that a parallel hoof pastern axis provides the most balanced loading of the hoof, supporting the theory that a parallel hoof pastern axis is a good reference for caudiopalmer hoof balance (4).

Fig. 6. The hoof angle is between the bearing and dorsal surfaces of the hoof wall. The hoof wall should be parallel to the pastern bones (PI and PII). The photo on the left shows a chronic broken-back hoof pastern axis with a long toe and under run heels, while the photo on the right shows a parallel hoof pastern axis.

The hoof pastern axis is very important to consider in balancing a hoof. It is sometimes argued that a long-toed, low-angled hoof will increase the length of a stride, because it increases the length of the digit and limb. However it has been shown with video analysis that long-toed, low-angled horses do not take longer strides. The total times of the stance and swing phase do not increase, but the break over time within the step does increase. The broken-back hoof axis that this long-toe, low-heel creates, requires more force from the DDFT to lift the heel off the ground and begin break over. Moving the point of break over towards the center of rotation (CoR) of the distal interphalangeal joint decreases the force on the structures of the limb during break over. Hoof pastern axis also effects the impact phase of a stride, determining whether horses land heel first, flat footed, or toe first. Horses with a parallel hoof pastern axis most often land either flat footed or heel first. Longer toed horses had a much larger chance of landing toe first, increasing the strain on limb structures and the chance of injury (6).

Fig. 7. The black dot shows the center of rotation of the coffin joint. This is the hind digit and hoof capsule of a young horse. The physeal growth plates are visible below the proximal interphalangeal joint and the metacarpal phalangeal (fetlock) joint.

Analysis of horse performance can certainly tell us about the effects that conformation plays in performance. A study of ninety five thoroughbred race horses examined both the success these horses had on the track in terms of their placement in races, and the lameness problems that these horses had. By looking at both the health and performance of these ninety five horses next to the conformation of their hooves it was determined that the horses with more success racing had a larger hoof angle (more upright hooves). It was also determined that horses with injuries statistically possessed lower hoof angles. Simply by looking at the hoof, digit and limb of the horse, one can see the mechanical effect of different conformations. A lower angled hoof has a longer toe creating a lever that the structures of the limb have to overcome in order for the weight of the horse to roll over the toe during break over. A basic mechanical understanding of motion is extremely important to provide an arena for technical research (11).

Force plates have been used for many studies to determine the location, direction, and magnitude of forces between the hoof and the ground. Force measuring transducers give a force reading when the horse steps on the force plate. By trotting a horse over a force plate, the changing forces during a step form landing through break over can be measured. Using force plates, research on eighteen horses demonstrated that lateral asymmetrical landings in both front and hind feet were the preferred type of landing, meaning that the horses consistently landed on the lateral aspect of their hooves (see fig. 8 below). The eighteen horses studied were measured before and after trimming at a four week interval. Trimming was standardized to a straight hoof pastern axis, with the solar surface of the hoof perpendicular to the metacarpal bone. This lateral asymmetrical landing was not always visible to the eye. Landing is directed by the conformation of the hoof and limb. The conformation of the limb of a mature horse can not be altered by a farrier. A balanced hoof will achieve the best possible mechanics of motion for each horse, so that a horse whose conformation dictates a lateral asymmetrical landing should have the motion dictated by its conformation (10).

Fig. 8. This is an exaggerated lateral asymmetrical heel landing of the left fore foot. The first part of the hoof to impact the ground here is the lateral heel. (Permission requested, Equine Practice, Veterinary Clinics of North America, Dalin. “Locomotion and Gait Analysis,” 1985, V1, No 3).

Force plates have shown that the CoP moves significantly throughout landing, loading and break over. Studies have indicated that many horses land asymmetrically, but it has also been demonstrated that the preferred side of landing is not necessarily the side of loading or break over. Conformation of the musculoskeletal system, as well as any injuries, affect movement so that a horse can land laterally, load medially and break over laterally, or land, load, and break over medially. The distribution of the forces that the CoP represents can be seen in the growth, wear and distortion visible in the hoof (4).

The forces placed on the structures within the limb are tremendous during movement. Studies have shown that the measured peak vertical forces on the hoof at impact for the walk, trot and gallop are 50-70%, 90-130%, and 175% of body weight respectively (9). This means that each limb must be able to support almost twice the horse’s body weight. The horse’s athletic ability is derived from the fact that most of its muscular mass is in the body. The limbs are primarily a system of levers and pulleys transferring muscular energy to the ground. Because of the forces involved, these cannot be rigid structures, or they would snap. The joints within the limb absorb the energy from impact and release some as elastic energy. The limb acts as a system to support the horse, so that each bone, joint, muscle, tendon, and ligament affect the whole system.

Force plates and 3-D motion analysis systems used in tandem can be used to analyze forces place on the limb and determine where the forces from impact are absorbed and where the forces of propulsion are generated. Using these systems to analyze the horse’s gait, it was determined that the fetlock joint components of the suspensory apparatus (the suspensory ligament, DDFT, and SDFT) act as an elastic spring. Ligament attachments between the tendon and bone, called check ligaments, give the SDFT and DDFT properties of both ligaments and tendons. In this way they can resist forces without muscular energy. The muscles supporting the elbow joint seem to absorb as much energy as they generate so that the fetlock, carpus, and elbow joints show no net energy absorption or generation. Because the carpus is fully extended it moves very little during the support phase. The shoulder has a net energy generating function and the coffin joint has a net energy absorption function. The DDF and SDF muscles restrict joint movement more than generate propulsive energy. Graphical representation of forelimb kinematics has also shown that the upper limb protracts and inertia carries the lower limb and digit forward for extension of the carpal joint at the end of the flight phase. The significance of this research is that the lower limb functions mostly as a passive supporting system as opposed to an energy generating system (1, 2, 7).

Fig. 9a. Forelimb joint movement. (Reprinted with the permission of Equine Veterinary Journal from W. Back, “How the horse moves,” 1995, V27)

Fig. 9b. Hind limb joint movement. (Reprinted with the permission of Equine Veterinary Journal from W. Back, “How the horse moves,” 1995, V27)

Further research of limb mechanics examines the balance of the hoof and its affects on the movement of each of the digital joints. Using a joint coordinate system (JCS) it was possible to document the movement of the coffin, pastern and fetlock joints with both a flat shoe and a wedge pad. A JCS is an ultrasonic triangulation system where the joints of the limb are fixed with markers and the horse is moved past three ultrasonic emitters so that the coordinates of each joint can be measured and movement calculated. Six degree plastic heel wedges applied with flat shoes increased the maximum flexion of the pastern and coffin joint and the maximum extension of the fetlock joint in comparison to a flat shod hoof. The wedges caused extension of the pastern and coffin joint to decrease at heel off and landings to occur more laterally. The landing and bearing phase times were increased and the time for break over was shorter with wedge pads. The extended times of stance can be associated with increased strain on the supporting structures of the limb, and the shorter times of break over can be associated with decreased strain (5).

Fig. 10. These are photos of a horse with chronic heel pain from long toe, low heel syndrome. An egg bar shoe with a slightly wedged frog support pad was applied to support the palmer region of the hoof and allow the frog to bear some of the horse’s weight to reduce crushing forces on the heels.

This last point brings us to the critical divergence of the two fields of balance. One field views the hoof, the other views the hoof as a component of a larger system. The hoof must possess balance within itself and as part of the larger system, so that it is balanced to the limb. Most biomechanical research now agrees that correct dorsopalmer balance is when the dorsal surface of the hoof is parallel to the pastern bones so that the three phalanges are in line with one another and force is equally distributed through the joints. In addition to uniform load distribution through the joints, the optimal parallel hoof pastern axis can be explained by the anatomical function of the suspensory apparatus. The fetlock joint is supported primarily by the DDFT, SDFT and the suspensory ligament. Both the DDFT and the SDFT insert on the rear aspect of the digit, the DDFT to the semilunar crest of the distal phalanx and the SDFT to the proximal interphalangeal joint. The suspensory ligament bifurcates distal to the fetlock joint and courses dorsally to the extensor process of the third phalanx. Raising the hoof angle increases the stress on the suspensory ligament and lowering the hoof angle increases the strain on the DDFT. As more tension is place on the DDFT it begins to take the force of the SDFT as well as the suspensory ligament. When the three phalanges are parallel, then the forces of load bearing are distributed correctly over these three supporting components.

Correct mediolateral balance is when the plane of the weight bearing surface of the hoof is perpendicular to a sagittal plane through the third metacarpal bone. When a horse has good limb conformation, the radius or tibia, metacarpal bone, and three phalanges are on the same sagittal plane. The hoof will be balanced to the limb when it is symmetrical. However, when the limb has poor conformation, even a slight deviation makes the application of balance very difficult. Applying a balanced trim and a shoe with the base of support under the limb will allow the horse to move in a balanced manner. Not only does deviation from balance create the possibility of injuries from asymmetrical loading on the limb structures, but the unbalanced loading causes constriction of the vessels and capillaries that supply nutrients to tissues for their repair, maintenance and growth.

A balanced hoof allows for an equality of forces so that the hoof is loaded uniformly, which transfers uniform forces up the limb. Uniform forces allow a harmony between the working components for a maximum athletic ability and optimal health of the whole horse.

Bibliography

Back, W. How the horse moves: Significance of graphical representations of equine forelimb kinematics. Equine Veterinary Journal; V 27, 31-38, 1995

Back, W. How the horse moves: Significance of graphical representations of equine hind limb kinematics. Equine Veterinary Journal; V 27, 39-45, 1995

Back, W, and Hilary Clayton. Equine Locomotion; Hardcourt Publishers, UK 2001

Balch, Olin K. Locomotor effects of hoof angle and mediolateral balance of horses exercising on a high-speed treadmill: preliminary results. American Association of Equine Practitioners; 1992

Chateau, H. Effects of six degree elevation of the heels on 3D kinematics of the distal portion of the forelimb in the walking horse. Equine Veterinary Journal; volume 36, 2004

Clayton, Hilary M. Comparison of the stride of trotting horses trimmed with a normal and broken-back hoof axis. American Association of Equine Practitioners; 1988

Clayton, Hilary M. Net joint moments and powers in the equine forelimb during the stance phase of the trot. Equine Veterinary Journal; volume 30, 1998

Clayton, Hilary M. The Dynamic Horse; Sport Horse Publications 2001

Dalin, Goran. Locomotion and Gait Analysis. Veterinary Clinics of North America Association Equine Practice; 1985

Heel, M.C.V. Van. Dynamic pressure measurements for the detailed study of hoof balance: the effect of trimming. Equine Veterinary Journal; volume 36, 2004

Kobluck, Calvin N. The effect of conformation and shoeing; a cohort study of 95 thoroughbred racehorses. American Association of Equine Practitioners; 1989

Rooney, James. The Lame Horse. The Russell Meerdink Company 1998

Wilson, A. M. The effect of foot imbalance on point of force application in the horse. Equine Veterinary Journal; volume 30, 1998

IV. Trimming for Balance

Each hoof has its own balance that it should be trimmed to so that the components of the limb above work in harmony. There are two types of balance; static and dynamic. The rules of static balance are based on mechanical principles and anatomical symmetry. The rules of dynamic balance are based on symmetrical fluid motion. When the correct static balance is achieved then dynamic balance will also have been achieved. Correctly balancing hooves requires evaluating the conformation of the whole horse. Each component of the horse’s locomotor system affects the whole system. Basic parameters exist for trimming hooves and can be adjusted for individual horses. They require addressing both the solar surface of the hoof and the outer surface of the hoof wall.

There is no mathematical formula for trimming hooves. Instead, the maintenance a hoof requires depends upon the quality and conformation of the limb and hoof. The most basic principles of trimming hooves aim towards helping the limb function physiologically and mechanically as efficiently as possible. Achieving this requires establishing a parallel hoof pastern axis so that the three phalanges are parallel and a mediolateral balance where the coronary band is a symmetrical distance from the ground collaterally. Generally the heels are trimmed back to the widest part of a healthy frog. Under run heels of a hoof with an atrophied frog should be pulled back to where a healthy frog would be.

To find the toe parameters, the sole is pared away at the point of the toe until it just barely gives to thumb pressure. As much protection should be left for the coffin bone and sensitive structures as is possible, especially with horses left barefoot, ridden over rough terrain, or with thin soles. Some horses will have hooves where no sole should be removed, while others will have soles where exfoliation has not occurred, requiring more sole removal. Before any tissue removal takes place, the hoof should be examined when bearing weight. The hoof should appear proportional in length to the size of the horse. Sometimes this requires leaving more hoof than looks necessary from the bottom. Care should always be taken to leave protection on the bottom of the horse’s hoof. Always make sure that the wall and not the sole, is bearing weight.

Fig. 2. These two photos show a hoof that has been trimmed balanced. The right shows the hoof ready for shoe application where the heels are the same length, neither heel is jammed and the surface is flat. The left photo shows a lateral view of the same hoof. It is always important to assess the coronary band for symmetry. The visible damage to the hoof wall is from an abscess that blew out at the coronary band four months previously.

Achieving mediolateral balance also requires viewing the horse from different angles. The medial and lateral aspects of the coronary band should appear a symmetrical distance from the ground, toe to heel. View the hoof from the front and rear, making sure the heels are trimmed so that they are level. Then pick the hoof up and hold the limb so that the digit hangs naturally (see fig. 2). The solar surface of the hoof should be perpendicular to the limb, and the heels should be equal length from coronary band to ground surface. The heels can easily become jammed if they are trimmed unevenly. When this happens, the corium of the wall actually is pushed upwards due to the ground reaction force on the hoof. When the hoof is trimmed balanced the jammed heels will eventually drop back down to level.

The wall should be the same thickness all the way around the perimeter of the hoof (see fig. 1. chapter page cover), and there should be no dish in the hoof wall, meaning it is a straight line from the coronary band to the bearing edge of the wall (see fig. 4). Flares should be removed from the hoof with a rasp. It is impossible to achieve balance when the hoof is flared because the hoof wall must be parallel to the coffin bone. Flare acts as a lever on new hoof, weakening the laminae and causing more flare.

Fig. 3. Long toes, causing excessive concussion and stress on the heels, constrict the frog. Only toe was trimmed from this hoof except for a small amount of rasping of the heels to bring them back to the widest part of the frog. A significant amount of flare was rasped off the dorsal surface of the hoof wall. Notice how much shorter this hoof is after it was trimmed (left photo) and how much thinner the hoof wall at the toe is. If the hooves are maintained at frequent intervals, the frogs should become healthier and larger.

Fig. 4. Before trimming, (left photo) the toes were long and flared, and the hoof pastern axis was severely broken back. This is the same hoof as the above photos where only toe and toe flare was removed to achieve a more mechanically functional hoof (right photo). The yellow lines represent hoof angle, pastern angle and heel angle. The red line represents the approximate angle of the coffin bone and the amount of flare to be removed. The green line represents the trim.

Fig. 5. Both photos show medial lateral balance with a level coronary band. The photo on the right shows a small amount of lateral flare (left), which was not rasped in order to leave some integrity for the heel quarter.

Fig 6a.

Fig 6b.

Fig. 6c.

Fig. 6d.

6a.The left and right front feet of a 14 year old Morgan mare. The left front is a club foot, while the right is a more normal hoof. Notice here the difference in hoof angles.

6b. The left club foot before, and after trimming. 0nly heel was removed from this hoof.

6c. The right front, before and after trimming. Only toe was removed from this hoof.

6d. The left and right feet after trimming. The trim almost resulted in a matched pair of hooves even though the conformation is quite different. They were trimmed to their individual parameters, not to match the opposite limb, but still resulted in a pair that will function mechanically similarly.

Hooves are trimmed for balance and shod for support. Trimming hooves to balance is not an easy task, because every horse, limb and hoof moves, wears and grows differently. Sculpting a living, growing hoof to symmetry and balance is an art and a science, requiring the application of mechanical principles to understand the function of the limb and the physiology of the hoof. If a hoof is not trimmed correctly, no shoe will correct the problem. The hoof is part of a system. Every bone, joint, tendon, ligament and muscle of the limb is affected by the natural conformation of the limb that cannot be altered, and the one imposed by a farrier upon the hoof. A farrier must view and balance the whole horse.

V. Shoeing for Support

A horse is trimmed for balance and shod for support. A trim is applied based on conformation, and a shoe is applied as a base of support for the horse’s conformation and discipline. A correctly applied shoe provides rigid support for the hoof, decreasing wear and distortion, and transferring force from the limb to the ground. The hoof is the base of support for the horse, and the shoe is the base of support for the hoof. The main functions of a shoe are to protect and support the hoof capsule, provide traction, and/or alter motion. Shoes alter the movement of a horse through their influence over the landing, support, break over and flight phases of a stride. Understanding how a shoe will affect the horse in order to properly forge and fit a shoe is both an art and a science. Many different shoes can be applied to the same hoof, and each one will affect how the horse moves and how the hoof grows because of how it functions as a platform of support.

The standard shoe is fit a dime’s width outside the perimeter of the hoof wall. The shoe stock width should be approximately two times the width of the hoof wall including the white line, and extend half the width of the stock behind the heels. The shoe should be fit to the hoof and the hoof should mirror the coffin bone (i.e. no flare). Fit the shoe to the same shape as the white line because it follows the coffin bone (see fig. 4). If the nail holes are punched in the center of this standard shoe then they will be correctly placed right over the middle of the white line. The purpose of a shoe is to support the hoof. With this in mind, any alteration from this standard should have a definable purpose, either to provide more support or to relieve stress from a specific component of the limb or hoof, such as setting the toe of the shoe back to ease break over and reduce the forces on the DDFT and related structures, or supporting the frog to reduce the load on the heels and support the bony column.

As discussed in Chapter III (Balance and Motion), during a stride each limb takes a step, which is composed of the flight, landing, loading, and break over phases. The shoe is the median through which the hoof interacts with the ground. A shoe affects landing, loading and break over differently, so when a shoe is applied it must be considered how each phase may be affected.

Fig. 1. This shoe is perimeter fit. It is the same shape as the white line, which outlines the perimeter of the coffin bone.

Fig. 2. This shoe is set back slightly from the toe to move the point of break over back. Notice ample heel support.

Fig. 3. This horse has large flared hooves with long toes and under run heels. The heart bar and pour-in pad allow the frog to take a significant load off the heels, so that the new growth of the heels grows down straight. Relieving pressure from the heels increases the blood supply and promotes healthy hoof growth.

In the above case (fig 3 & 4), a large Clydesdale Thoroughbred cross had long toe low heel syndrome. The front hooves were more of a concern than the hind. The frogs were prominent, wide, and healthy. When the foot was trimmed, the frogs were below the ground bearing surface of the walls, meaning that barefoot, there was significant contact between the frog and ground. The heels were weak, crushed and folded under the hoof. When trimmed, they were significantly ahead of, rather than aligned with, the widest portion of the frog as they would be if they grew straight and healthy. The goal here was to shoe the horse so that the frog shared as much of the load as possible, relieving some of the stress on the heels. A heart bar shoe has a bar extending from the heels, supporting the frog. The heels were “floated” slightly, so that when the foot is not weight bearing, there is a thin air space between heel and shoe created by leveling the foot, and then rasping an eighth of an inch off the heel which will not contact the shoe.

The hoof is not static. When the hoof is weight bearing, especially at higher speeds when more than the horse’s weight can be placed on one single limb, at each step the hoof flexes tremendously and the heels and frog drop towards the ground. Because this horse had such a prominent frog, it nearly bears weight while he is standing with a regular shoe and no frog support. When his hoof was absorbing impact, the heels were crushed as the frog descended to the ground to support the limb above it. When viewed from the side for caudiopalmer balance and hoof pastern axis, the crushing of the heels was visible. Where the tubules of the hoof originated and grew out of the coronary band, they were parallel to each other and to the dorsal surface of the distal phalanx. However, instead of continuing at this angle to the ground, the tubules bent and were crushed forward under the hoof. Once they started to crush, they acted as a lever continuing to degenerate. The heart bar shoe should break the cycle by allowing the frog to share the load. When this happens the new growth will continue straight down to the ground and the crushed portion will grow out.

A condition such as this usually begins because of one reason; the center of the hoof’s base of support is in front of the center of the applied pressure. This happens when the toe grows too long and becomes dished, acting as a lever that pulls the hoof forward. When the center of the hoof is in front of the center of the applied pressure, the heels take the brunt of the landing, and the long toe acts as a lever at break over, straining the flexor tendons, suspensory ligaments, and hoof capsule.

Fig. 4. This is the horse from fig. 3 with the shoes applied.

Fig. 5a This horse had symptoms of chronic heel pain with undetermined causes. The lines represent, left to right, most posterior point of heel support, center of rotation (CoR) of the coffin joint, and point of break over. The shoe on the right and below was applied to support the soft tissue (tendons and ligaments) of the posterior limb, and to ease the break over (by moving back the point of break over). The heel support is provided by the egg bar and the break over is eased by setting back the shoe and rolling the toe of the shoe. The difference between the left and right photos is significant when mechanics are considered. The photo on the right shows that the CoR of the coffin joint is much closer to the center of the support provided by the shoe.

Fig. 5b. The egg bar shoe and wedged frog-support pad allows the frog to take some of the load off the heels and brings the hoof pastern axis closer to alignment.

Fig. 6a. This front left lateral heel was damaged and torn from the hoof. The arrow points to the missing wall.

Fig. 6b. This egg bar, heart bar will allow the frog to take some of the load off the heel until the damage has grown out.

Fig. 6c. The left photo shows that the shoe was placed where the hoof wall should be so that the base of support for the limb is symmetrical. Then the gap was filled with a hoof repair material similar to equilox which allows the heel to continue to bear weight

Conclusion

A farrier’s job is to correct or maintain this cycle of balance and support, matching the hoof to the horse’s conformation, job and environment. There is an appropriate saying, “No hoof, No horse.” The hoof is a living structure, and therefore the shoe is not being applied to a static mass. Without proper hoof care and maintenance, today’s equine athletes could not sustain the level of activity demanded of them. A properly maintained hoof should be trimmed or shod every six to eight weeks on average. For good or bad, education in the farrier profession in the United States is entirely self-disciplined. There are no lawful requirements, so it is up to each individual to take their education into their own hands and gain an understanding of the implications of each task that they perform. The profession of hoof care cannot progress without the progress of individual farriers who are willing to strive towards an educated generation of horseshoers.