Horse hoof

The horse hoof is the distal phalanx of the 3rd digit (digit III of the basic pentadactyl limb of vertebrates, evolved into a single weight-bearing digit in equids) of the four limbs of Equus species, which is covered by complex soft tissue and keratinised (horny) structures. Since a single digit must bear the full proportion of the animal's weight that is borne by that limb, the hoof is of vital importance to the horse. The well-known phrase "no hoof, no horse" underlines how much the health and the strength of the hoof is crucial for the horse soundness.

Both wild and feral equid hooves have enormous strength and resilience, allowing any gait on any ground. A common example of the feral horse type is the Mustang. The Mustang is not a particular breed but rather a combination of many horse breeds. Therefore, the famous Mustang hoof strength is likely to be a potential common trait of many or even all domesticated horse breeds.

The recent Barefoot movement claims that such a strength can be almost completely restored to domesticated horses, when appropriate trimming and living conditions are applied, to such an extend that horseshoes are no longer necessary in almost any horse. This is not an unopposed view but, if true, would undermine the belief that "the horseshoe is a necessary evil".

Outline of horse hoof anatomy

The hoof is made up by an outer part, the hoof capsule, made up by various horny specialized structures, and an inner, living part, containing soft tissues and bone. The corneous material of the hoof capsule is different in structure and properties in different parts; it covers and supports P3 (the coffin bone) anteriorly, and specialyzed soft tissues (tendons, legaments, fibro-fatty and/or fibrocartilaginous tissuen, cartilages) posteriorly. The upper, circular limit of the hoof capsule is the coronet (coronary band), obliquous in its plane with an angle of approx 30° to the ground. The walls originate from the coronet band; they are modified nails. Walls are longer in the anterior portion of the hoof (toe), intermediate in length in the lateral portion (quarter) and very short in posterior portion (heel). Heels are separated by an elastic, gummous structure named frog. In the posterior part of the foot, above the heels and the frog, there are two oval bulges named bulbs. When you pick up the hoof and you look at its lower surface, you can see the wall's free margin, encircling most of the hoof. In the central area, you can see a triangular gummy structure; lateral to the frog, you can see two grooves, deeper in their posterior portion, named collateral grooves. At the heels, the walls bend suddenly, following the external surface of collateral grooves; here they are named bars. The lower surface of the hoof, from the outer walls and the inner frog and bars, is covered by an exfoliating horny material, called sole.

Just below the coronet, the walls are covered for about an inch by a horny, opaque material named perioplium. In the posterior part of the hoof, the perioplium is thicker and gummous over the heels, and it merges with frog material.

Characters and functions of the external hoof structures

The walls

The walls are to be considered both as a protective shield, covering sensible internal hoof tissues, just like exoskeleton of arthropods, and as a structure devoted to concussion energy dissipation and to grip into different terrains. They are elastic and very tough, similar to a teflon layer; its thikness is approx mm 6 to mm 12. The walls are built up by three different layers, the pigmented layer, the water line, and the white line.

The pigmented layer is made up by the coronet, and its color is just like that of the coronet skin where it comes from. If the coronet skin has any dark patch, the walls show a parallel pigmented line, form the coronet to the ground, showing the walls growth direction. It has a prevalent protective role, and not so resistant to the contact to the ground, where it breacks and chips away.

The water line if built up by the coronet and by the walls corium, the living tissue deep to the walls; its thickness grows from the coronet to the ground, and in lower third of the walls is thicker than pigmented layer. It is very resistant to contact to the groung, and it has mainly a support function.

The white line is the inner layer of the walls; it is softer and fibrous in structure. Its color is yellowish. You can see it in the healty hoof as a thin line separating the sole and the walls. The white line comes out from the laminar connections; any visible derangement of the white line indicates some important derangement of laminar connections, that fix the walls to the underlying P3. Since white line is softer than both the walls and the sole, it wears fast where it appears on the surface, and it looks like a subtle groove between the sole and the walls, with some debris or sand inside.

The three layers of the walls merge in a single mass, and they grow down together. If they don't wear from sufficient movement on abrasive terrains, then they stick out from the solar surface; then, in the best case, they self-trim breaking or chipping out.

Walls are used too to apply horseshoes; the nails are applied oblique to the walls; they enter the wall at white line and they come out to walls surface approx mm 15-20 from the hoof bottom.

Their anatomical analogue into the human finger are the nails.

The frog

The frog is a triangular structure, about 2/3 of the sole in its length. Its thickness grows from the front to the back, and in the back it merges with heel perioplium. In the middle, it has a groove, the central groove, that extend up separating the bulbs.

It is dark gray-blackish in color, and a gummous consistence, suggesting its role as shock absorber and grip tool on hard, smooth ground. In the stabled horse, it doesn't wear, but it degrades for bacterial and fungal activity with an irregular, soft, slashed surface. In the free roaming horse, it hardens into a callous consistence with a rather smooth surface.

Its anatomical analogue in the human finger is the fingertip.

The sole

The sole has a whitish-yellowish color, sometimes a grayish one; it fills the whole space from the walls edge to the bars and the frog into the bottom of the hoof. Its deep layer has a compact, waxy character and it is called "live sole". Its lower surface is variable in its character, as a result of ground contact. If there is no contact, as in shod hooves, or when the walls are too long or the movement poor, the lower surface of the sole has a crumbly consistence and it is easily removed scratching it with a hoofpick. On the contrary, it has a very hard consistence with a smooth, bright surface when there is a consistent, active contact with the ground; the resulting configuration is called "sole callus".

The bars

They are inward folds of the walls originating from the heels with an abrupt angle; the strong structure built up by the extremity of the heel and of the bar is named "heel buttress". The sole between the heel walls and the bars is named "seat of the corn", and it is a very important landmark used by natural hoof trimmers to evaluate the correct heel height. The bars have a three layer structure, just like the walls. When overgrown, they bent outwards and cover the lower surface of the sole.

Internal structures

The third phalanx (coffin bone; pedal bone; P3) is completely, or almost completely, covered by the hoof capsule; it as a crescent shape, and a lower cup-like concavity. Its external surface mirrors the walls shape. The corium, a dermoepidemal, highly vascularized layer between the walls and the coffin bone, here has a parallel, laminar shape and is named "laminae". Laminar connection has a key role in the strength and the health of the hoof. Into the back of the hoof, there is the digital cushion, that separates the frog and the bulb from underlyng tendons, joints and bones. In the foal and in the colt, the digital cushion is made by fibrofatty, soft tissue; in the adult horse if hardens into a fibrocartilagineous tissue, when sufficient, consistent concussion stimulates the back of the hoof. Normal transformation of the digital cushion into fibrocartilagineous tissue is now considered a key goal both to navicular syndrome prevention and rehabilitation ^*. The flexor tendon lays deeper, just along the posterior surface of small pastern bone and navicular bone, and it connects with posterior surface of P3, the navicular working as a pulley.

The hoof mechanism

The horse hoof is not at all a rigid structure; it is elastic and flexible. Just pull and push the heels one to the other one, and you'll feel it. When loaded, the hoof phisiologically changes its shape. In part, it is a result of solar concavity; it has a variable depth, about cm 1 - 1.5. In part, it is a result of an arched shape of the lateral lower profile of the walls and sole, so that an unloaded hoof touches a concrete, regular ground only at toe and heels (active contact), where a loaded one has a wide area of ground contact (passive contact) covering lower walls edge, most of the sole, bars, frog. Active contact areas can be seen as slightly protruding spots in the walls or/and in the calloused sole.

The shape change in a loaded hoof is a rather complex one. The plantar arch flattens, the solar concavity decreases in depth, and heels wander away one from the other. The hoof diameters increase to a "dilated" configuration; P3 slightly drops into the hoof capsule. There is some recent evidence that a depression takes place in this phase, with blood pooling ("diastolic phase") mainly into the walls corium. When unloaded, the hoof restores its "contracted" configuration, the pressure raises and the blood is squeezed off ("systolic phase"). This is just the opposite of what was believed (a "systolic" phase under load, with the frog acting as a blood pump).

The hoof mechanism ensures an effective blood circulation into the hoof, and it helps general circulation too.

Time-related changes of the hoof

Hooves have to be considered a plastic structure, and their time-related, very complex changes can be considered in the short period (days/weeks), in the medium period (the horse's lifespan), and in the long period (the evolution of equids).

Hoof changes in the short period

Just like the horny layer of epidermis and of any mammal nail, the hoof capsule is built up only by epidermis covering the skin. From a microscopic point of view, epidermis is a multi-layered, specialyzed cornifying epithelium. It lays on the dermis, and it it separated from it by a basal lamina; it has no blood vessel inside, and living cells get oxygen and nutrients by fluid exchanges and molecular diffusion from underlying dermis flowing into microscopical spaces among individual cells. Epidermis growth take place by mitotical activity in its deepest layer, mainly into the basal layer, and slow outward migration and maturation of cells; when approaching to the surface, peculiar proteins accumulate into their cytoplasm, then the cells die and "dry off" into microscopic, tightly connected individual laminae, made up mainly by cheratin; the resulting "dead" superficial layer has a protective function, saving underlying living tissues from injuries, from drying off and from fungal and bacterial attack. The constant thickness of the corneous layer results most commonly from regular superficial exfoliation; when a specialyzed corneous structure has a very peculiar toughness, as in nails and hair, little/no exfoliation occurs, and corneous structures have to slowly migrate away from their original position.

So, specialyzed horny structures of the hoof are the walls, the sole, the frog and perioplium. The first one doesn't exfoliate at all; it is constantly growing downward (about cm 1 a month) and self-trims by ground contact by wearing, sometimes by chipping, in wild and feral horses. Solar, frog and perioplium matherial grow outwards, and exfoliate at surface by ground contact and wearing. In the domesticated horses, movement and usual ground hardness are not sufficient to allow self-trimming, so humans have to care for them, trimming the walls and the frog and scratching the dead sole.

Hoof changes in the medium period

Front and hint hooves are perfecly identical in the foal, but differ visibly in the adult horse, a good evidence of longterm plasticity of the whole hoof shape as a result of variation in its use. Slow changes in hoof shape occur under any consistent change in the horse's movement pattern and under a wide variety of pathological conditions. They could be seen now as a bright example of a complex adaptive system, a frequent feature of living beings and structures.

Self-adapting capabilities of the hooves show their maximal effectiveness in the wild equids, but is preserved in domesticated horses too, as shown by the examples of perfect soundness of feral horses, such as mustangs, in a wide variety of environments.

Hoof changes in the long period

Equid hooves are the present, amazing result of a 50-million-year long evolution of the horse. Wild and domesticated equus species share a very similar hoof shape and function; they result from a slow loss of I, II, IV and V rays of the limbs, with deep changes in bones, joints, and hoof capsules. The resulting design allows a heavy, strong body to move with a high speed and resistance on any ground, with top effectiveness on open, hard, flat areas like prairies and deserts.

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