Horse hoof

  The horse hoof is the structure surrounding 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 each of the four limbs of Equus species, which is covered by complex soft tissue and keratinised (cornified) 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 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 extent 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".

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Anatomy

  The hoof is made up by an outer part, the hoof capsule (composed of various cornified specialised structures) and an inner, living part, containing soft tissues and bone. The cornified material of the hoof capsule is different in structure and properties in different parts. Anteriorly, it covers, protects and supports P3 (also known as the coffin bone, pedal bone, PIII). Posteriorly, it covers and protects specialised soft tissues (tendons, ligaments, fibro-fatty and/or fibrocartilaginous tissues and cartilage). The upper, almost circular limit of the hoof capsule is the coronet (coronary band), having an angle to the ground of roughly similar magnitude in each pair of feet (i.e. fronts and backs). These angles may differ slightly from one horse to the other but not markedly. The walls originate from the coronet band. 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, resilient structure named the 'frog'. In the posterior part of the foot, above the heels and the frog, there are two oval bulges named the '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 the triangular frog. Lateral to the frog, are two grooves, deeper in their posterior portion, named 'collateral grooves'. At the heels, the posterior portion of the walls bend inwards sharpy, 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 keratinised material, called the 'sole'.

Just below the coronet, the walls are covered for about an inch by a cornified, opaque material named the 'periople'. In the posterior part of the hoof, the periople is thicker and more rubbery over the heels and it merges with frog material. Not all horses have the same amount of periople. Dry feet tend to lack this substance, which can be substituted with Hoof Dressing.

Characters and functions of the external hoof structures

The walls

The walls are to be considered both as a protective shield, covering the sensitive internal hoof tissues, just like the exoskeleton of arthropods, as a structure devoted to dissipating the energy of concussion and to provide grip on different terrains. They are elastic and very tough, similar to a teflon layer; its thickness is approximately 6 mm - 12 mm. The walls are composed of three distinct layers: the pigmented layer, the water line and the white line.

The pigmented layer is generated by the coronet and its color is just like that of the coronet skin whence it is derived. If the coronet skin has any dark patch, the walls show a parallel pigmented line, from the coronet to the ground, showing the wall's growth direction. This layer has a prevalent protective role and is not so resistant to ground contact, where it can break and flake away.

The water line is built up by the coronet and by the wall's corium (the living tissue immediately beneath the walls). Its thickness increases proportional to the distance from the coronet and, in the lower third of the walls, is thicker than the pigmented layer. It is very resistant to contact to the ground and it serves a mainly support function.

The white line is the inner layer of the wall. It is softer and fibrous in structure. Its color is yellowish. You can see it, in the underside of the healthy hoof, as a thin line, joining the sole and the walls. The white line grows 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 bone. Since white line is softer than both the walls and the sole, it wears fast where it appears on the surface and it appears as a subtle groove between the sole and the walls, with some debris or sand inside.

The three layers of the wall merge in a single mass and they grow downwards together. If the wall doesn't wear naturally, from sufficient movement on abrasive terrains, then it will protrude from the solar surface. It then becomes prone to breakage and the healthy hoof will self-trim, by breaking or chipping off.

When a horseshoe is applied, it is fixed to the wall. Nails are driven in, oblique to the walls. They enter the wall at the outside edge of the white line and they emerge at the wall's surface, approximately 15-20 mm from the base of the wall.

The wall is anatomically analogous to the human finger or toe nail.

The frog

The frog is a triangular structure that extends forwards across about two-thirds of the sole. Its thickness grows from the front to the back and, at the back, it merges with the heel periople. In its midline, it has a groove, the central groove (sulcus), that extends up between the bulbs.

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

It is anatomically analogous to the human fingertip.

The sole

The sole has a whitish-yellowish, sometimes greyish colour. It covers the whole space from the perimeter of the wall to the bars and the frog, on the underside of the hoof. Its deep layer has a compact, waxy character and it is called 'live sole'. Its surface is variable in 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 consistency and it is easily abraded by scratching it with a hoofpick. Conversely, it has a very hard consistency, with a smooth, bright surface when there is a consistent, active contact with the ground. The front portion beneath the front of the pedal bone is called the 'sole callus'.

The bars

They are inward folds of the wall, originating from the heels at an abrupt angle. The strong structure built up by the extremity of the heel and of the bar is named the 'heel buttress'. The sole between the heel walls and the bars is named the 'seat of 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 (see above). When overgrown, they bend 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 has a crescent shape and a lower cup-like concavity. Its external surface mirrors the wall's shape. The corium, a dermo-epidermal, highly vascularized layer between the wall and the coffin bone, has a parallel, laminar shape and is named the 'laminae'. Laminar connection has a key role in the strength and the health of the hoof. Beneath the rear part of the sole, there is the 'digital cushion', which separates the frog and the bulb from underlying tendons, joints and bones, providing cushioning protection. In the foal and in the colt, the digital cushion is composed of fibro-fatty, soft tissue. In the adult horse it 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 for prevention of navicular syndrome and for rehabilitation of recovering cases [1]. The flexor tendon lays deeper, just along the posterior surface of the small pastern bone (PII) and navicular bone and it connects with posterior surface of P3, the navicular functioning as a pulley.

The hoof mechanism

  The horse hoof is not at all a rigid structure. It is elastic and flexible. Just squeezing the heels by hand will demonstrate that. When loaded, the hoof physiologically changes its shape. In part, this is a result of solar concavity, which has a variable depth, in the region of 1 - 1.5 cm. In part, it is a result of the arched shape of the lateral lower profile of the walls and sole, so that when an unloaded hoof touches a firm ground surface, there is only contact at toe and heels (active contact). A loaded hoof has a much greater area of ground contact (passive contact), covering the lower wall edge, most of the sole, bars and frog. Active contact areas can be seen as slightly protruding spots in the walls and in the callused sole.

The shape changes in a loaded hoof are complex. The plantar arch flattens, the solar concavity decreases in depth and heels spread. The hoof diameter increases to a 'dilated' configuration and P3 drops marginally 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 wall's corium. When unloaded, the hoof restores its 'contracted' configuration, the pressure raises and the blood is squeezed out ('systolic phase'). There is a secondary pumping action, with the flexion of the foot, as it is raised.

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

Time-related changes of the hoof

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

Hoof changes in the short term

Just like the cornified layer of epidermis and of any mammal nail, the hoof capsule is created only from epidermis, the outer living layer of the skin. From a microscopic point of view, epidermis is a multi-layered, specialised cornifying epithelium. It overlays the dermis and it is separated from it by a basal lamina. It has no blood vessels and living cells acquire their oxygen and nutrients by fluid exchanges and molecular diffusion, from underlying dermis, flowing into microscopical spaces among individual cells. Products of metabolism are cleared by a reverse of this process. Epidermis growth take place by mitotic activity in its deepest layer, into the basal layer, with slow outward migration and maturation of cells. As these cells approach the surface, special proteins accumulate into their cytoplasm, then the cells die and 'dry', into microscopic, tightly-connected individual layers, composed mainly of keratin. The resulting 'dead' superficial layer serves a protective function, saving underlying living tissues from injury, from dehydration and from fungal and bacterial attack. The constant thickness of the cornified layer results most commonly from regular superficial exfoliation. When a specialised cornified structure has a particular toughness, as in nails and hair, little or no exfoliation occurs and the cornified structures must slowly migrate away from their original position.

Thus, the specialised cornified structures of the hoof are the wall, the sole, the frog and periople. The wall doesn't exfoliate at all, It is constantly growing downward (about 1 cm per month) and self-trims by ground contact by wearing or by chipping, in wild and feral horses. Solar, frog and periople material grow outwards and exfoliate at the surface by ground contact and wearing. In the domesticated horse, movement and usual ground hardness are insufficient to allow self-trimming, so humans have to care for them, trimming the walls and the frog and scraping off the dead sole.

Hoof changes in the medium term

Front and hind hooves are identical in the foal but differ visibly in the adult horse. This is good evidence of medium-term 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 can be seen now as a clear 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 wild equids (but domesticated horses show this too, to a lesser extent), as shown by the perfect soundness of feral horses, such as mustangs, in a wide variety of environments.

Hoof changes in the long term

Equid hooves are the result of the 50-million-year evolution of the horse. Wild and domesticated equus species share a very similar hoof shape and function. The present-day conformation of the hoof is a result of a progressive evolutionary loss of digits I, II, IV and V of the basal pentadactyl limb, with changes in bones, joints and hoof capsule. The resulting conformation allows a heavy, strong body to move with high speed on any ground, most efficiently on open, hard, flat areas like prairies and deserts (i.e. 'cursorial specialisation').

See also

• Horseshoe
• Farrier
• Barefoot horse
• Laminitis
• Horse evolution
• Navicular syndrome
• Equine forelimb anatomy