Friday, March 27, 2015

Steppin' Out: Hooves From An Artistic Perspective Part II: Structure

Welcome back to this twelve-part series discussing the equine foot as it relates to sculpture. There's a lot to digest, but the fact remains: in order to sculpt a correct foot, we first have to know what it looks like and why, right?

In Part I we were familiarized with the evolutionary effects that produced it, and now in this Part II and III, we'll get to know some of the biology behind it. So let's get to it!

General Biology

For starters, it’s important to know that “hoof” and “foot” aren't synonymous. Like our fingernails, the hoof is the cornified epidermal structure that encapsulates the inner anatomy. It lacks blood vessels and nerves, and therefore can be trimmed. The cornified sole and frog also are included in the term “hoof” because they’re calloused and can be trimmed, too. On the other hand, “foot” refers to the hoof plus all the internal structures within it. It's the whole enchilada.

How nature engineered the animal to move is what actually shapes his hooves. For instance, in the foal, the front and hind hooves are more or less identical, but begin to change shape with wear and force, sometimes in as little as two days. As a result, the front hooves of an adult are larger and rounder with more sloping walls while, in contrast, the rear hooves are narrower with more upright walls and a pointed toe. This divergence is created by the different mechanics of the forehand and hindquarter, which naturally shape the hooves over time. 

Specifically, the forehand bears approximately 60% of the animal’s mass and that burden makes them larger and broader to best facilitate weight-bearing, and with rounded toes to encourage rapid breakover. In contrast, the hindquarter is the source of momentum, and the pushing and pivoting effects on the hind hoof pull on the toe (and sometimes the quarters), creating a narrower hoof with a pointed toe (and sometimes broader quarters), maximizing its task.

Furthermore, the medial walls on all four hooves tend to be slightly steeper than the lateral walls since the animal’s center of gravity is focused down the inside of each limb a bit more. Although science is still uncertain as to exactly how much, the wall that erupts from the coronary band results in the youngest and most flexible horn occurring at the quarters and heels, precisely the areas needed to be most pliant for the necessary expansion and contraction cycle of the foot in motion. Moreover, science has routinely proven there’s no mechanical or structural difference between a pigmented hoof and an unpigmented hoof (Bertram and Gosline, 1986).

All this means it's a mistake to sculpt the hind hooves like the fore hooves, or to fudge their shapes. Each set has a distinct, characteristic shape which should be expressed in our clay. It also means, however, that newborns break this rule, and so should our sculptures of them.

Continuing on, movement is the basis of the equine foot, playing a pivotal role in its soundness and the animal’s overall health. Some statistics based on free roaming equines show the average individual travels to be about 20-35 miles a day (Ramey, 2005), mostly at 2-4 mph for grazing and over varied terrain, while other estimates place it at about 4,000-6,000 steps per day (Bowker). In contrast, a horse confined to a stall cannot roam, taking only about 800 estimated steps per day (Bowker). Research has found that increased activity translates into increased production and distribution of fluids and nutrients inside the foot, as blood and lymph flow is alternately pumped in and out with the expansion-contraction cycle created by each step. 

Proper nutrition is also an important component, but “proper nutrition” doesn’t necessarily mean “rich nutrition.” Equines evolved to fatten and prosper on foods that would otherwise starve a ruminant, such as low-nutrient, high fiber grasses, which are the primary and ideal food source for equines (Budiansky, 1997). They also evolved to be able to pick and choose what they ate, depending on what their bodies needed at the time. In contrast, domestic horses can eat only what we present to them, which often is supplemented with plentiful oils and rich high-carb accompaniments, even when the activity levels don’t warrant this extra energy. Leafy alfalfa is also a food of choice, but it’s a legume, not a grass, and is much higher in energy, protein and calcium/magnesium than what the equine system was designed to process. Alfalfa also doesn’t have the abrasive qualities of grasses, for which the equine tooth was specifically designed. Furthermore, the equine digestive system was designed for constant grazing. Indeed, the horse has no gall bladder and his stomach is comparatively small, and so his system isn’t equipped adequately to cope with the feast and famine feeding schedules typical of domestic management. Ultimately, all this affects the quality of the horn and lamellae, usually with negative results. Yet while these rich foods may be appropriate under certain conditions, it does beg the question of whether the average horse actually is benefited by these unnatural foodstuffs. Hopefully, science will further investigate equine nutrition and its effects on the foot so that we can gain a better perspective.

Some claim that it’s natural for horses to have mismatched foot pairs, since the horse is an asymmetrical quadruped, and it’s through this hoof asymmetry that he attains balance. Indeed, it’s true that horses often have a favored side, or “handedness,” which affects everything they do, from the wear of their feet to the wear of their teeth to their “better side” when moving. (Watch a horse chewing, for example, to get an idea of his favored side.) What’s more, there’s a direct link between the conformation of a limb and the shape of the foot. Yet there's been no formal comprehensive study across feral, wild and domestic horses to prove the validity of this assertion, or even to provide reliable percentages of asymmetries in any given population. This claim also fails to recognize the effects of horsemanship on hoof wear and foot function. In fact, on this point alone is where most (if not all) research into equine feet has been incomplete. There’s no differentiation within the data between good horsemanship and bad horsemanship upon the structure, mechanics, health and wear of the foot, not even an acknowledgment that there are pivotal differences in how different horsemanship influences equine motion. In other words, taking data from a horse habitually moving in false collection is considered equally valid as taking data from a horse moving in true collection, creating a possible skew in the interpretation and application of the data. It’s hoped this oversight will be mediated in the near future, as science begins to recognize and consider the performance influences on the domestic foot.

All this means that the equine foot isn’t a dead block of keratin at the end of the horse’s leg, but a living, extremely complex elastic mechanism that performs far more vital functions for the overall health of the horse than just locomotion. The hoof is also never static, but is highly adaptable and malleable through time, wear, environment and force, allowing it to accommodate many living conditions. In a very real sense, a horse develops a custom-made set of feet for his specific habitat and lifestyle, accounting for his ability to live in diverse ecological niches (and also probably accounting for the inconsistent data collected from field studies). This means we have to consider the characteristics of the foot in our sculpture in order to marry it properly to our chosen narrative. That is to say, one type of foot doesn't fit all horses! Or sculptures.

Skeletal Structure

For our purposes, the bones of the foot will include the second phalanx, the navicular bone, and the coffin bone.

The second phalanx (or os coronae, short pastern bone, or middle phalanx) is a cube-shaped bone between the first phalanx and the coffin bone, and articulates with the coffin bone and navicular bones at the coffin joint. Usually depicted in dissections and illustrations as partially encased in the hoof capsule, new data suggests the proper orientation of this bone is actually mostly above the wall. 

The navicular bone (or os naviculare, distal sesamoid bone, or shuttle bone) is a relatively thin, smooth and canoe-shaped bone lying within the cavity behind the second phalanx and coffin bone. Science has recently proven that its purpose is not to bear weight, as previously thought, but to act as part of the coffin joint as a fulcrum for the deep digital flexor tendon to maintain a constant angle of insertion.

The third phalanx (or os pediscoffin bone, third distal phalanx, pedal bone) lies within the hoof and is a miniaturized image of the hoof capsule. Often erroneously reported to be the only equine bone to lack a periosteum, this bone’s periosteum evolved to become the sensitive lamellae of the foot. It’s crescent-shaped when viewed from above and somewhat pyramid-shaped when viewed from the front or side, with a pair of wings at the rear for attachment of the lateral cartilages. Even so, the shape of the coffin bone exhibits variation between individuals, ages and breeds. It also reflects the hoof capsule, being rounder, flatter and wider in the forefoot and comparatively steeper, pointed and narrow in the hindfoot. However, in newborns and young foals, the coffin bones of the forefeet and hindfeet are identical (Bowker), only assuming the “front foot” and “hind foot” configurations with wear and force, just like with the hoof wall. The coffin bone itself is made of compact, dense bone, with a texture similar to pumice stone due to its foramina (openings for blood vessels and nerves) and attachments for the foot’s tissues. And although the coffin bone can withstand tremendous pressure, it’s designed more to resist tension rather than absorb compression, being suspended within the foot by the lamellae and supported by the underlying structures beneath it. While the coffin bone moves significantly within the hoof capsule during motion, the ideal nature of that motion still remains to be described. There’s also continuing debate regarding the proper orientation of this bone within the foot, due to a lack of comprehensive data and an incomplete understanding of foot mechanics (discussed later). 

Joints of the Foot

The pastern joint is located between the 2nd phalanx and 1st phalanx. It’s an “imperfect” hinge joint because it permits a slight degree of lateral movement, in addition to the hinge motion. Nevertheless, it's the least moveable of the phalangeal joints because its primary function is that of shock absorption.

The coffin joint lies within the foot, between the 3rd and 2nd phalanxes and the navicular bone, forming a complex series of three separate articulations, in a very condensed space—between the 2nd and 3rd phalanxes, between the 3rd phalanx and the navicular bone and between the 2nd phalanx and the navicular bone. However, the joint between the coffin bone and the navicular bone is rather immobile, so they essentially move together as a single articulating surface with the head of the 2nd phalanx. Consequently, this joint is a more flexible "imperfect" hinge, allowing a comparatively larger amount of lateral play and considerable rotation, and sometimes referred to as a saddle joint. Overall, its function is to facilitate footing on uneven ground and to a lesser degree, provide shock absorption. 

Fleshy Structures 
This is only a brief overview of the foot’s interior, so for more details, please refer to the Recommended Resources at the end of this series.

The interiors of the foot are lined with corium (also known as the "quick," or pododerm), a collagenous connective tissue rich in vascular and sensory systems that nourish the internal structures, moisturize the wall and serve the foot. The corium is also a highly metabolic tissue that creates the tissues it feeds. Being highly vascular, it’s an essential component for the foot’s vascular function. In fact, its metabolic activity has been compared to that of the liver, kidney, and skin and plays an important part in the removal of waste products and toxins from the foot. 

The corium is subcategorized according to the structures it produces and feeds, as follows:
  • Perioplic corium: Located at the coronary band, it produces the periople, the waxy coating that regulates the moisture content of the wall. It’s thickest at the coronary band and widens over the heels, blending with the heel bulbs and gradually diminishing approximately two-thirds down the wall. Visually, it’s the chalky-flakey substance under the coronet, most obvious on natural dark hooves. As it extends down the hoof, it becomes the tectorum, the thin keratin layer that gives a healthy hoof a burnished appearance.
  • Coronary corium: Right below the perioplic corium is the coronary corium, which inflects inwards at the heels to become part of the bars. Feeding the wall, it was thought to produce the wall, in entirety, with its villiform papillae (microscopic teat-like projections that each produce a "tube" of horn). However, new evidence suggests that at least one-third of the wall, and perhaps up to one half, also may be produced by the basal cells in the secondary epidermal lamellae (Bowker, 2003). Regardless, like the wall, the coronary corium is thickest at the toe and reduces towards the heels, and is quite vascular and helps to dissipate concussion and heat. Covered by the coronary corium is the elastic portion of the coronary band, called the coronary cushion, a fibro-fatty strip that's thickest at the toe and thinnest at the quarters, where it blends with the digital cushion internally and with the frog and sole externally. The coronary cushion houses the coronary vascular plexus that serves to dissipate heat and concussion. 
  • Laminar corium: Sheathing the coffin bone, the papillae of the laminar corium produce the sensitive lamellae (dermal lamellae) that interlock with the insensitive lamellae (epidermal lamellae) on the interior of the hoof capsule, through a velcro-like, interdigitating structure of primary and secondary lamellae. The laminar corium nourishes the sensitive and insensitive lamellae and the laminar horn of the white line while also helping to attach and stabilize the coffin bone to the hoof capsule; it’s microscopically inseparable from the coffin bone. The deep layer of the laminar corium blends with the outer surfaces of the lateral cartilages and has a full lamellar vascular system, including a rich network of microcirculation.
  • Corium of the sole: Covering the distal surface of the coffin bone, this corium (also known as the sensitive sole) nourishes and produces the horny sole that underlies it. It resembles velvet and is comprised of papillae similar to those of the coronary corium.
  • Corium of the frog: Also called the sensitive frog, it extends over a part of the digital cushion in the heel area, becoming a mirror image of the horny frog. It’s thicker than the corium of the sole and with its papillae, produces and nourishes the horny frog while also feeding the digital cushion. 
Let’s return to the laminar corium. While there’s still uncertainty as to the exact function of the lamellae (discussed later), it’s clear upon microscopic inspection that the hoof capsule connects to the coffin bone through the lamellae and that this connection is essential to the foot, unmistakably demonstrated when it fails. As previously mentioned, there are two kinds of lamellae, the sensitive (dermal) lamellae of the coffin bone and the insensitive (epidermal) lamellae of the hoof capsule’s interior. The visible lamellae folds, or primary lamellae, look like the gills on a mushroom and are visibly and microscopically angled towards the coffin bone, suggesting the direction of the mechanical tension produced by impact. They number about 550-600 folds per foot on an average equine. However, data reveals that feral hooves have only about 410 folds per foot, which are also much more massive than those found on domestic feet (the feral sole papillae were reported to be larger, as well) (Ramey, 2005). Nonetheless, each of these primary lamellae bears its own microscopic lamellae projecting from it, called secondary lamellae, like serrations on a shark’s tooth, numbering approximately 100-200 secondary lamellae along each edge of the primary lamellae (Bowker, 2003). Each primary and secondary fold of the sensitive lamellae interfold with that of the insensitive lamellae, like pressing the teeth of two combs together, with each prong having its own little teeth. In this way the coffin bone is stabilized and secured within the hoof capsule by a significant amount of surface area, creating a remarkably strong bond that can withstand considerable mechanical forces. In fact, this attachment has been estimated to create several square feet of contact, as much as 8-10 sq. ft. per foot, or 32-40 sq. ft. total per horse, a substantial increase over bovine hooves that lack secondary lamellae. 

Basement Membrane
At the interface between the epidermal and dermal secondary lamellae lies an uninterrupted, thin sheet of tough tissue called the basement membrane. It lines the entire inner wall, bars, sole and frog and every tiny nook and cranny within the foot. Though nearly identical to the basement membranes of other animals, the equine basement membrane is unique owing to the novel specialization of being the glue sheet, or double-sided tape, that fuses the lamellae together on a molecular level. This is assisted by specialized sites called hemidesmosomes that act as reinforcing spot-welds to attach the basal cells of both kinds of lamellae directly to the basement membrane, along with multitudes of anchoring filaments that lash these hemidesmosome welds to the basement membrane to further stabilize these areas. 

These molecular bonding characteristics also make the membrane fundamental in equine hoof growth. The lamellae react to the stresses of motion and the environment by releasing MMPs (matrix metalloproteinase enzymes) and TIMPs (tissue inhibitors of metalloproteinases, to “turn off” the MMPs) in a highly choreographed mechanism for cellular reorganization and growth of more horn (Pollitt). Specifically, the MMP enzymes target the basement membrane and its hemidesmosomes to create regulated, tiny breaks in strategic attachments between them, allowing the insensitive lamellae to slide past the sensitive lamellae in hoof growth, with the MMPs then promptly neutralized by TIMPs. It’s this carefully synchronized operation that allows growing horn to slide past the sensitive lamellae while always maintaining constant molecular adhesion between the lamellae. Interestingly, recent research implies that when an imbalance of these enzymes occurs, the result is the partial or total breakdown of the basement membrane, causing the lamellae to separate and the blood supply in the foot to be disrupted, resulting in laminitis or founder (interestingly, imbalances in MMPs/TIMPs has been implicated in certain cardiovascular diseases and several cancers in people). It may be that laminitis is a normal process gone horribly wrong through a failure of the MMPs to be properly inhibited. And because laminitis severely damages or destroys the basement membrane altogether, this may be one of the reasons why laminitic horses have permanent problems with lamellae and horn quality since the template has been compromised or destroyed. 

This brings us to another of the basement membrane’s essential functions, that of a horn-making template for both normal growth and repair. Even if the wall is torn away, this tough membrane remains intact to provide a template upon which to build more wall. Indeed, horses with damaged membranes have permanent problems re-growing proper hoof wall. Even though research is ongoing, it’s clear the equine basement membrane is unique in the animal kingdom and may require far more consideration in the animal’s management than previously thought.

Digital Cushion
The digital cushion (or plantar cushion) is a wedge-shaped, fibro-cartilage mass of hard, dense tissue with few blood vessels and nerves, making it essentially insensitive, with two large arteries passing through it to supply the foot. It lies under the coffin and navicular bones, between the lateral cartilages, extending towards the toe and back to the heel bulbs, and sits below the distal attachment of the deep digital flexor tendon and above the sensitive frog, frog and bars. 

It’s an integral part of foot mechanics, including the expansion-contraction cycle and for energy management, and perhaps also functions as a tough shield of protection for the coffin join and its vascular and sensory bundles against the upward forces of impact.

In the foal and until the age of 4-5 years, the digital cushion is made of soft fibro-fatty tissue rich in proteoglycans (essential building blocks for cartilage), which is appropriate for the weight of a foal or young horse. But with regular stimulation from ample exercise (especially that which impacts the back part of the foot), it hardens into dense fibro-cartilage in maturity, with hardening starting around the apex of the frog and spreading towards the heel. A fully developed, mature digital cushion also connects to the distal projection of each lateral cartilage to create a solid supportive sling for the internal foot (Bowker, 2003). This fibro-cartilage apparently can be grown only through rigorous exercise that stimulates the sole, frog and bars, and once grown, appears to remain permanent. However, when exercise is insufficient in these formative years, the fibro-cartilage fails to develop and the digital cushion remains a mass of soft tissue, creating a foot that tends to suffer long-term lameness problems rather quickly (Bowker, 2003). In fact, current data implicates an undeveloped digital cushion, one made mostly of collagen and fatty tissue, in several long-term hoof pathologies, including navicular. The horse was made to move, and without adequate movement, especially in the formative years, the foot may be permanently compromised.

On the other hand, the ideal dimensions of a sufficient digital cushion are still unknown, largely because the variation of individual hooves makes it difficult to standardize. But two factors presently believed to influence its dimensions are hoof size (particularly on the cushion’s wideness) and how far the frog is off the ground (which seems to affect the height of the cushion). However, the data suggesting these correlations came from a very small sample size and the various means to measure the digital cushions failed to be consistent. Hopefully, future study will shed better light on this essential component of the equine foot.

Lateral Cartilages
Unique to solipeds (i.e. equids), the lateral cartilages (ungul or ungular cartilages) are two flexible, wing-like plates of fibro-cartilage on either side of the coffin bone that ascend vertically to the second phalanx where they thin and taper off, and are attached by ligaments to the coffin bone, second phalanx and first phalanx. They are more convex and cup-like at the heels, forming those familiar bulbs at the back of the foot. Each descends about halfway into the hoof capsule, to curve around the internal structures at the back of the foot. The portion outside of the wall is subcutaneous and yields to thumb pressure. When seen from the front in dissection, they have an “L” or “C” shape, with the lower projection merging with the fibro-cartilage of the digital cushion with multiple fibrous bands extending from it, to unite with those of the opposite cartilage.

They’re comprised of hyaline cartilage in equines under 4-5 years and are about 1/8 inch thick, but with proper stimulation, change into masses of dense, thick fibro-cartilage. Also, the “L” projection in foals is only a thin sheet of fibrous tissue, but with continued exercise, thickens into fibrous tissue at about 4-5 years. With continued proper stimulation, it changes into thick dense fibro-cartilage in adulthood (Bowker, 2003). While the thickness of the lateral cartilages varies with individuals, it ranges from about 4/16 inch to 1 inch at the navicular bone and about 3/16 inch to 4/8 inch at the heels in a horse weighing about 950-1200 pounds (since dimensions vary among individuals)(Bowker, 2003). Also, the cartilages of the forefeet usually are considerably thicker and more extensive than those of the hindfeet because they tend to contain significantly more fibro-cartilage. 

Each cartilage also contains a rich network of vascular foramina, which becomes more profuse as the cartilage becomes thicker (Bowker, 2003). For example, the average number of these vascular channels near the navicular bone may exceed 25-30 channels between both cartilages. And inside and around each lateral cartilage exists an extensive venus plexus originating from the large central vein inside its tubal core, from which emanates a thick network of mircovessels (usually referred to as the coronary plexus). These mircovessels lace inside and outside the cartilage and return to the vein without actually seeming to feed the cartilage itself, or surrounding structures, but which coalesce to form the medial and lateral palmar digital veins. 

Functionally, the lateral cartilages act as flexible extensions of the coffin bone, helping to support the limb and the foot, and also work to manage energy within the foot by absorbing and dissipating shock and heat in many ways, as follows:
  • They elastically attach the coffin bone to the wall, allowing it to move within the hoof capsule during impact or when the animal stands on different kinds of terrain, as recent data from transducers, radiographs, and fluoroscopic images have revealed. 
  • They allow the posterior foot to expand and contract independently of the coffin bone, which is now believed necessary for proper foot function.
  • The sling they create with the digital cushion creates a strong, flexible support for the posterior foot. 
  • Their rich plexus of veins and capillaries dissipate the high frequency vibrations and heat energy generated by impact and compression.
  • They actively participate in the blood pumping action of the foot. 
  • They aid the transfer of compressive and impact forces away from the bony column.
  • They’re an integral energy management component of the new hemodynamic flow theory, and the new Suspension Theory of Hoof Dynamics™ (discussed later).
  • They allow the foot to twist and distort to accommodate varying terrain or the angle at which limbs impact the ground. 
This last aspect is of particular interest since medial-lateral balance has been considered of imminent importance in conventional management. However, observation and biomechanical studies show that when a horse moves on uneven terrain or turns, his foot impacts the ground at an angle, not straight down (Ramey, 2006). In fact, even the limbs’ plane of motion plays a role since horses track along the median line, causing the legs to be angled inwards at impact. And if the horse’s lifestyle requires him to maintain this habitual motion, his lateral cartilages can actually reorient themselves so that they (and the heel) match that angle so they impact level with the ground surface (the lateral cartilages also adjust themselves to limb deformity so that the heels impact the ground evenly despite the abnormalities).

The overall effect of this, though, is a foot in stance that appears to have “crooked” or “sheared” lateral cartilages and a “crooked” coronet, with the outside more elevated (with a shorter wall) and the inside lower (with a longer wall)(Ramey, 2006). However, this latter effect is a direct consequence of domestication (and typically seen in trotters and pacers), since feral or wild equines rarely assume the same gait for the majority of their lives, begging the question about how domestic feet should actually be maintained for optimum sport performance. The interesting thing is this isn’t a permanent situation since when the habitual impact changes, so do the lateral cartilages (and the heel). Nevertheless, all of this is now challenging the concept of a balanced foot, particularly medial-lateral balance, as it appears nature intended the foot to adapt more to habitual motion than to stance, and that balance during stance or motion may have very little in common when considering foot function.

Within a normal hoof capsule and a properly oriented coronet, the lateral cartilages have plenty of room to shift, expand and contract to facilitate foot function. However, if the hoof capsule is deformed it can orient the coronet too high up on the bony column, thus confining the lateral cartilages and pushing them upward into a cramped position that impairs their function and places unnatural stresses on the wall (sometimes resulting in bleeding quarter cracks and inflammation of the lamellae). This is often seen with contracted heels or with a "mechanical sinker" (discussed later).

This new information shouldn’t be all that surprising, since the significance of the lateral cartilages has been largely dismissed in the past. Indeed, just trying to find anything more than a terse and indifferent mention of them in the anatomy, lameness and farrier references in my entire library was quite a chore! In fact, most of the references didn’t bother to mention, describe, or even depict them at all, even in texts about lameness and biomechanics. What’s more, historic descriptions and illustrations of the lateral cartilages that have found their way into today’s common understanding may be flawed by not accounting for where the information was gathered—perhaps most likely from dissections of chronically lame horses taken from slaughter houses (Bowker, 2003). For example, historical depictions usually illustrate underdeveloped cartilages that tend to be small and uniform, whereas modern data taken from sound horses reveal lateral cartilages that are far more robust and extensive, with a great deal of individual variation in thickness, shape and vascularity content within and surrounding them. Research consistently seems to be implying that there’s a direct correlation between well-conformed cartilages and improved foot soundness due to a maximization of their implied contributions in how impact energy is managed within the foot. Initial data also strongly suggests that positive environmental factors have a greater influence on the quality of the lateral cartilages than genetics, since variation is still quite high even within the same gene pool (Bowker, 2003). These positive factors include ample exercise that stimulates the sole, frog and bars (the more exercise, the better), living in an environment of firm depressible textures such as dirt, gravel, grass, rubber, etc., and varied terrain to cause distortion and twisting of the hoof capsule. In other words, living a lifestyle that closely matches a biologically normal state for the genus maximizes the health and efficiency of the lateral cartilages. Interestingly, in feral horses the digital cushion is smaller and denser while the lateral cartilages are larger and thicker, often in direct contradiction to most domestic horses (Bowker).

It’s hoped that continued research will give the lateral cartilages their due credit as a key component in foot mechanics and soundness. Clearly, there’s a very good biological reason why the lateral cartilages are unique to solipeds (equines)! 

Blending with the lateral cartilages, the coronet (or coronary band) is the visible junction between the wall and the leg. It should be smooth, symmetrical, slightly rounded in front and flat on the heels. 

Recent dissections also reveal the coronary band is extremely elastic; in fact, if the hoof capsule is removed entirely, the coronet can yield to thumb pressure by as much as an inch (Ramey, 2006). This means that the coronet not only changes shape when loaded, but also that the entire hoof capsule can be shifted upwards or downwards on the bony column in response to impact or routine foot management. The coronet also may play a significant role in foot circulation and in energy management during impact, as suggested by the Suspension Theory of Hoof Dynamics™ (La Pierre, 2001)(discussed later).

Consequently, new data suggests the coronet should be oriented properly on the bony column, either encircling only the top of the coffin bone or just below it, and not oriented about halfway up the 2nd phalanx, as usually is described or illustrated (Ramey, 2006). (These "artificial sinkers" adopt a characteristic foot structure—one typically found in sculpturewhich we'll explore later.) In other words, the hoof capsule should encapsulate only the coffin bone so that the coffin joint can be free to work properly without the coffin joint being locked inside the hoof capsule. This also may be the optimum location of the coronet for the Suspension Theory of Hoof Dynamics™ to work correctly. 

However, there’s ongoing debate regarding the proper angle of the coronet (or “hairline”) to the ground, with some asserting this angle must be 30˚ even if the foot is aggressively trimmed to match this alignment (Strasser). However, the evidence supporting this claim largely is circumstantial and inconsistent and needs to be correlated directly to proper foot function. 

Comprised of cornified epidermis much like our fingernail and lacking nerves or vessels, the wall (or hoof capsule) ideally contains about 25% water, either drawn from body fluids or the external environment. Despite appearances, the wall is flexible, not rigid, and is readily malleable and adaptable, especially in the rear portion of the hoof. At the back of the foot, the wall bends sharply inwards, doubling back towards the toe, at angles referred to as the buttress of the heel (or angles of the wall) which are comprised of walls twice thicker than neighboring walls. At the top, the hoof capsule thins at the coronary band and blends with the coronary corium and lateral cartilages. The wall should be firmly attached to the coffin bone for its entire length, creating a discrete and distinct white line around the perimeter.

Functionally, the wall preserves moisture within the foot, resists wear, permits expansion, absorbs concussion, and protects and insulates the internal structures of the foot. It not only accomplishes all this through its conical shape, but also through its laminated multi-layered structure, constructed of different zones for different functions, listed from external to internal:
  • Stratum externum: The periople/tectorum (discussed)
  • Stratum medium: Forming the outer wall, tube-shaped fibers are created by the papillae on the coronary corium that undergo mitosis to produce daughter cells that mature and compress tightly together and harden through keratinization, to continually produce a tough, fibrous tube that reaches from coronet to the ground, which can be seen close-up as fine longitudinal lines (Pollitt, 2004). The tubules are spherical in cross-section when new but flatten as they grown down, hardening and thickening to overlap each other like Venetian Blinds. Tough glutinous horn tissue, called intertubular horn, forms perpendicular to these tubules, perhaps from the valleys between the papillae or perhaps from the secondary dermal lamellae, creating a plasticized lattice that strongly bonds the tubules together for strength and stabilization (some tubules also are filled with broken-down horn cells called intratubular horn). However, this outer wall has a very high ratio between the tubules and intertubular horn, making it the densest part of the hoof and creating a barrier that protects the inner foot from the outside elements and forces. Ultimately, it may be that the outer wall is intended more for protection rather than for primary support, which contradicts conventional theory. Nevertheless, these tubules microscopically are designed for compression and flexing through their leaf-spring construction, making the wall strong not through unyielding rigidity, but through flexibility, while perhaps also acting as millions of little springs to store and release energy. These tubules should be parallel to each other and match the angle of the wall, which is easily recognizable since this layer holds the wall’s pigment. Thus the direction of the tubules can be discerned by the striations in the wall, or by any present striping (pigmented by melanin). 
  • Stratum internum (lamellatum): The insensitive epidermal layer on the inside of the hoof capsule is composed of about 600 primary lamellae made of horn that run from the coronary groove to the ground, and each insensitive primary lamellae has 100-200 secondary lamellae that haven’t been turned into horn. This layer continues around the entire inner circumference of the hoof capsule and down each bar, to interlock with the sensitive dermal lamellae of the laminar corium (Pollitt). Newly made insensitive lamellae can take up to 6 or 8 months to make the journey to the ground, sliding past the stationary secondary epidermal lamellae which are anchored to the basement membrane. The stratum internum ultimately produces a thicker, unpigmented layer high in moisture content, and is often referred to as the “water line.” In contrast to the outer layer, the water line has a very low ratio of tubules and intertubular horn (which is produced by the secondary dermal lamellae), making it far more flexible and fluid than the outer wall. Since this is the layer that allows the stratum medium to slide past the sensitive lamellae, it acts as a conveyor belt for the wall to migrate from the coronet to the ground (La Pierre, 2004). In this way, it’s theorized that this layer acts like a fluid, insofar as the thicker it is, the better and faster the outer wall flows to the ground (La Pierre, 2004). Indeed, it has been observed that when stimuli are applied through motion, the inner wall increases in thickness, accelerating the downwards growth and flow of the outer wall. This implies that the conventionally held belief that it takes a horse a year to replace the hoof capsule may not be a consistent measure (La Pierre, 2004). Observation also implies that growth or stress rings actually may be an imbalance between the growth of the outer wall and the inner wall, as the faster growing outer wall “buckles” if the inner wall is unable to keep up. This imbalance is thought to be caused by an outer wall that is made to bear weight in entirety, such as through a shoe or a particular trim that would cause peripheral loading, which increases the stimuli on the coronary band almost exclusively thereby increasing the growth of the outer wall, yet denying this same stimuli to the water line and the rest of the foot (La Pierre, 2004). Incidentally, some reports have claimed that when the outer wall is trimmed to relieve it of ground contact, such as on a “feral trim,” growth or stress rings eventually disappear because impact energies are allowed to stimulate the foot evenly, as nature designed, allowing the inner and outer wall to find equilibrium and stabilizing the coffin bone within the hoof capsule (La Pierre, 2004). The water line also may act as an additional, and perhaps primary, buffer zone owing to its pliable structure which makes it a far better mediator between the rigid outer wall and the vulnerable sensitive corium and vascular systems. In support of this view, observational accounts claim that in those hooves with thin water lines, the occurrence of bruising around the white line is more common, yet nearly nonexistent in those hooves with thick water lines (La Pierre, 2004). Interestingly, in the feral or wild foot, the water line develops into what the horse actually walks on, as seen in the “mustang roll” pattern of natural wear, and not the hoof wall, contradicting conventional thought which loads the hoof wall almost exclusively via targeted trims and the application of a shoe. 
The wall is also divided into three sections, the toe, quarters and heel. Wall thickness is greatest at the toe (about 3/8 inch to 7/16 inch on an average horse), thinner at the quarters (where expansion starts to occur) and thinnest at the heels (about 3/16 inch). However, in most fore feet the medial wall is thinner than the lateral wall.

As mentioned previously, there’s debate as to how the wall is actually grown. Traditional thought has maintained that only the coronary corium at the coronet is responsible for wall growth. However, some research suggests that the basal cells in the secondary epidermal lamellae may contribute 1/3 of total wall production, or as much as 50% (Bowker), though these are early findings. Similarly, other research and observational veterinary notes suggest that wall growth from these basal cells occurs only for repair purposes (Pollitt), whereas other data suggests that the intertubular horn actually is grown by the epidermal lamellae, and not by the coronary corium.

How ever the hoof is grown, the average domestic hoof grows about 1/4 inch to 3/8 inch per month and usually produces an entirely new hoof capsule every year; growth rates are unknown for feral or wild horses. However, growth varies under different conditions, with strongest influences being circulation (or heart rate, i.e. movement) and the abrasion of the habitual footing. In fact, it has been reported that barefoot horses heavily worked on hard, abrasive surfaces, such as asphalt roads, can grow a whopping 1/2 inch of horn a week to keep up with the rate of wear (Ramey, 2005). This wouldn’t have been noticeable had these animals not been pastured on grass on their off-days, which removed days of abrasion and allowed the wall to “get ahead." This has caused some to hypothesize that the inconsistent footing domestic horses live and work on never provides the constant abrasive qualities feral and wild horses experience, denying domesticated feet a self-maintaining equilibrium and thereby necessitating the need to trim and/or shoe. 

Equilibrium of wall growth seems also to be influenced by the balance of the foot, i.e. the more symmetrical and balanced the foot, the more likely the wall growth will equalize with abrasive forces. However, there’s debate as to whether the wall grows or slows growth with pressure, with some studies suggesting pressure may promote growth and others suggesting that pressure stunts growth. However, these claims don’t seem to differentiate between the areas of the wall, the toe, quarters and heel, so it’s unclear if different parts of the hoof react to pressure differently or whether these ideas apply to the entire wall.

Another side effect of circulation on hoof growth is that foals and yearlings have faster hoof growth than adults due to their higher heart rates. Young horses can also have different growth rates between their hind hooves and fore hooves, with the former tending to grow about 12% faster in foals, to diminish to about 7% faster in yearlings and to find equilibrium in adulthood (Butler, 1992). This does mean the front hooves and rear hooves of young horses develop and mature at different rates, which is an important point to consider for equine management (and brings into question the practice of shoeing young feet). Additionally, overall foot growth continues until the horse is about 5-6 years old while the coffin bone undergoes significant changes during this time as well, forming its palmar processes (or “wings”). The hoof grows fastest in spring and slowest in winter and, when kept in a manner with how nature intended, horn production tends to reach equilibrium with the rate of wear to create a customized-to-lifestyle, self-maintaining foot.  

In wild conditions, hoof lengths tend to be about 3 inches at the toe (Ramey, 2006), which is short by domestic standards where 3 1/4 inches to 3 3/4 inches is more common. It’s thought this difference is largely due to the different orientations of the bony column within the hoof capsule, with wild feet having coffin bones suspended high inside the hoof (with a short hoof capsule), whereas domestic feet having coffin bones unfortunately sunk into the hoof to varying degrees (with a long hoof capsule) (Ramey, 2006); otherwise known as a "mechanical sinker" which is typical of the domestic population. There's still uncertainty as to the proper length of the heel in a functional foot since lifestyle and terrain seem to influence this measurement in natural conditions. For instance, feral or wild feet sometimes can have heels that appear nonexistent (but this actually is due to the frog bulbs, or the heel bulbs, being pulled into a lower position), or sometimes have heels about 1 inch to 1.5 inches high, or sometimes heel height can be much longer. Nevertheless, many farriers gauge heel length by using the “seat of the corn,” the area of sole between the heel walls and the bars that’s particularly hard and dense, and rasping away heel until they begin to approach 1/8 inch above, or just touching, the solar surface in this area. There’s observational evidence that a pony’s coffin bone may be as tall as that of a horse, but not as wide, implying that the heel length on a pony actually is about the same as a horse, causing a pony foot to look more upright due to the comparatively longer heel. 

Hopefully, our understanding of wall growth and heel length will improve as studies advance on the subject of foot mechanics in relation to terrain and anatomy. 

Sum Up

Now that we're thick into structure, we'll continue our exploration of the foot's construction in Part III. There's a lot more to go, so keep steppin' out and learning about equine feet!

"It is important that students bring a certain ragamuffin, barefoot irreverence to their studies; they are not here to worship what is known, but to question it." ~ Jacob Bronowski

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