Hoof - Anatomy & Physiology

Introduction

The hoof is defined from a physiologic perspective as the modified skin (epidermis) covering the tip of the digit and all enclosed structures. The hoof provides protection to the distal limb and is formed by keratinisation of the epithelial layer and modification of the underlying dermis. The keratin in the epidermis, when thickened and cornified, is referred to as horn. Horn makes up the outer surface if the hoof and is particularly resistant to mechanical and chemical damage.

The origin of the hoof is as a form of protection to the distal phalanx and stems from local modifications of the epidermis, dermis and subcutaneous layers. There is great variation in the shape and types of hooves across different species and this is often due to the multi-role nature of the hoof. In some species the hoof may play an important role in non-locomotion roles such as digging or being used as a weapon. The hoof also acts as a shock-absorber in that the hoof acts to reduce the concussion of foot impact on the structures of the distal limb. It has also been suggested in some species that the elastic nature of components of the hoof may aid the return of blood to the heart. The following information relates primarily to the horse hoof.

Five Segments of the Hoof

Wall

The wall of the hoof is the element of the hoof that is visible. The wall forms the medial, lateral and dorsal aspect of the hoof and it can be further divided into the toe, quarters and heels. At the heel the walls reflect back on themselves at a point called the angles and in doing so forms the bars. The bars, although moving cranially, gradually fade along the edge of the frog and never actually meet. The wall of the hoof is widest at the distal aspect of the hoof, i.e. at the toe. The wall of the hoof decreases in width laterally and medially (around the quarters of the hoof). In most domestic species the wall of the hoof is between 5 - 10mm thick and consists of three layers. There is an outer layer of thin but dense horn that is shiny and acts to reduce evaporation from the horn and therefore prevent dehydration of the other layers of the horn – the periople. There is an intermediate layer which represents the main structure of the wall and is composed of amorphous horn reinforced with many tubular shaped horn rods. Finally, there is an inner laminar layer where there are interdigitating laminae of horn and dermal laminae which ensure the hoof itself is firmly anchored to the distal phalanx. The bond between the horn and the underlying dermis is a living bond that gradually allows the wall of the hoof to slide in a distal direction towards the ground where the distal border is worn away via contact with the ground.

Periople

The periople is a small band of soft tissue found over the proximal surface of the wall of the hoof. The periople represents the junction between the wall and the skin of the limb. The periople is responsible for the production of the outer layer of the wall (above). The periople segment extends around the proximal edge of the wall and in horses covers the bulb of the heel and part of the frog. In ruminants is forms part of the join between both hooves of the foot.

Coronary Dermis Segment

The wall of the hoof is created at the coronary dermis and grows in a distal direction from the coronary dermis. The coronary dermis is studded with many papillae which are directed towards the ground in the direction of growth. The epidermis covering these papillae produce horn tubules which are embedded into amorphous inter-tubular horn. This inter-tubular horn is created by the spaces between the papillae within the coronary dermis. The combination of both of these horn types ensures the horn has sufficient strength.

The pigmentation of the hoof is derived from melanocytes found in the coronary epidermis. Any pigmentation in the hoof will be most pronounced in the outer part of the hoof wall as the deeper layers of the hoof usually contain fewer melanocytes. It is this unpigmented element of the hoof that forms the 'white line' in the sole of hoofs and is particularly important in horses as a landmark for shoeing.

Sole Segment

The sole is the area distal to the bars and apex of the frog enclosed by the hoof wall. The area where the bars and wall enclose it is known as the angle of the sole. Since the sole is slightly concave, the majority of the horse's weight is transferred through the margin of the sole.

The sole represents the part of the foot in contact with the ground and its composition differs between species. The keratin found in the sole is formed from the epidermis on the underside of the third phalanx and can grow to a thickness of around 10mm in domestic species. The keratin found on the sole is much more easily worn down or abraded than that of the wall of the hoof. The equine sole has a central frog structure whilst ruminants and pigs have a bulb structure to the sole.

Frog-Bulb Segment

The frog is a wedge-shaped structure which sits between the bars and has an apex facing distally, with 2 crura flanking a central sulcus. Between the crus and bar of each half of the sole lies the collateral sulcus. Opposite the apex, the frog expands forming the bulbs of the heel. The frog is a mass of keratinized stratified squamous epithelium, which is softer than other parts of the hoof due to its increased water content. Usually, the frog contributes to the weightbearing surface where it functions as a shock absorber. Apocrine glands within the corium of the frog produce secretions on the surface. The frog ensures that the wall of the hoof is forced outwards when weight is put on the limb thus ensuring that the 'hoof mechanism' functions correctly and ensuring circulatory flow around the hoof and back towards the heart. The ruminant/pig 'bulb' provides the hoof with the caudal and mid-hoof contact area with the ground and is chiefly involved in weight bearing. The bulb inserts into the V-shaped sole. The bulb is made of relatively soft material, mainly inter-tubular horn and is of a considerable thickness.

Structure and Function

Equine Hoof

A X-section through a typical hoof. ©Rachael Wallace2008

A view of the solar surface of an equine hoof. The wall has been removed on the right to show the underlying dermis. ©Rachael Wallace2008

As outlined above, the equine hoof can be divided into three topographical regions; the wall, the frog and the sole. The wall forms the medial, lateral and dorsal aspect of the hoof and it can be further divided into the toe, quarters and heels. At the heel the walls reflect back on themselves at a point called the angles and in doing so forms the bars. The bars, although moving cranially, gradually fade along the edge of the frog and never actually meet. The frog sits between the bars and has an apex facing distally, with 2 crura flanking a central sulcus. Between the crus and bar of each half of the sole lies the collateral sulcus. Opposite the apex, the frog expands forming the bulbs of the heel. The sole is the area distal to the bars and apex of the frog enclosed by the hoof wall. The area where the bars and wall enclose it is known as the angle of the sole.

The dermis of the distal phalanx is arranged in hundreds of leaves or laminae, each of which has microscopic secondary laminae. The coronary region has a germinative layer associated with papillae that is responsible for producing the horn tubules that make up the hoof wall. This wall glides distally at a rate of 5-6mm a month and by forming epidermal laminae itself it interdigitates with the underlying dermal laminae. Neither of these laminae are pigmented so when the epidermal laminae appear on the solar surface, a non-pigmented region known as the white line appears. The white line is used as important landmark in farriery as structures central to the line will be dermal and so vascular and sensitive.

The dermis in the frog is also arranged in papillae and produces incompletely keratinised flexuous horn tubules resulting in a soft, elastic horn. The hypodermis of the region of the frog forms the digital cushion. This lies between the ungual cartilages and is collagenous, elastic tissue infiltrated by adipose tissue. At the bulbs of the heel, it is subcutaneous and is soft and loose in texture. The sole area also has papillae that produces superficially flakey horn. The coronary part of the wall is surrounded by a bony prominence called the periople. This soft, lightly coloured area is restricted to this proximal area and is produced by the germative layer covering the papillae. The rest of the hoof is covered by the tectorial layer, this is a very thin layer of horn that is covered distally by the growth of the horn.

A well-trimmed foot should weight bear on its walls, bars and frog. This occurs as the weight applied to the distal phalanx is then transferred across the interdigitating laminae to the hoof wall. Thus an injury resulting in damage to the laminae is of extreme importance to the horse.

See equine hoof for further detail.

Ruminant Hoof

The ruminant hoof, although resembling the equine hoof in some characteristics, differs from the equine hoof in several ways. In the ruminant hoof there are two separate main digits and the wall of the hoof is bent to form a border. Also the bulb of the heel covers the entire caudal surface of the hoof and most of the plantar surface, leaving only a small area of sole visible. In ruminants the interdigitating lamellae are smaller and less well developed than in equids.

The hooves of the main digits curve medially towards each other. The lateral digit carries more weight than the medial digit, and is larger. On the abaxial wall, the distal border makes contact with the ground along its entire length, whereas, on the axial wall, only does so toward the toe. The thickness of the wall increases towards the apex and the plantar surface. The horn of the hoof generally grows at a rate of 5 mm per month, and in cattle allowed to move freely, growth should equal wear. In intensively kept cattle, growth exceeds wear, and foot trimming is required to maintain optimal shape and angle. The optimal angle of the toe from the ground is 50 degrees. Where horn overgrowth occurs, the coffin joint is gradually overextended and the deep flexor tendon tensed. This results in greater weight being placed over the caudal part of the hoof and can cause pain and lameness.

Dewclaws are present in most ruminants but do not make contact with the ground. They consist of wall and bulb and have no practical importance.

See the bovine lower limb for further detail.

Porcine Hoof

Pig Hooves and a cracked hoof wall

The hooves of pigs are principally similar to those of ruminants, however the wall is straight, not bent medially at the toe, and they have a soft bulb that is well distanced from the wall and sole. The hooves of the accessory digits are of the same structure as the principal digits, but only bear weight on soft ground.

Hoof trimming in pigs is rarely required due to the short lifespan of the farmed pig.

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