Hoof Anatomy


The sole:

The sole is the area inside the white line and doesn’t include the frog or bars. It should be smooth, shiny, callused, hard, robust and concaved from the tip of the frog all the way to the edge where it meets the hoof wall. The sole is an integral part of the hoof mechanism, it not only protects the sensitive inner structures it should carry the majority of a horse’s weight along with the heels & bars and not the walls which many in the equine industry believe or believed. The sole corium is covered with papillae only and unlike the hoof dermis layer has no laminae. These papillae connect the corium and sole horn providing nutrition and growing the sole of the hoof. The bars, however, have laminae and are part of the hoof wall.

 

Heels and bars:

Bars are an integral part of the hoof mechanism, a weight bearing structure and aid energy dissipation. They are an extension of the hoof wall, finishing half way along the frog and act like a spring in a suspension system. Research by Dr Bowker’s also revealed that the bars are the main impact bearing structure of the foot. The lateral cartilages are designed to absorb the initial impact and the bars are best placed to transfer this shock. The bars also stop the hoof from over expanding, so as the frog, heels and digital cushion give on the initial impact the actual jarring the stop is performed by the bars which then dissipates this energy into the lateral cartilages. The heels are designed to take the initial impact and along with the bars, sole and inner wall supports a horse’s weight. Bowker also discovered that the lamina of the bars was different than any other part, where previously it was assumed that structurally it was the same as the hoof wall. He also believes that the bar contributes both to the sole and hoof wall where tubules from the bars migrated forward towards the toe. Heels are mostly inner wall in construction which means it is better suited to take the impact, this is because of the higher moisture content and contain more intertubular horn. Heels also have proprioceptor sensory cells which transfer information to the central nervous system allowing the horse to feel the ground beneath them. Heel balance is important and made out to be a complicated subject, but in fact is quite simple. Balance is the same amount of material above the sole plain on each side.

 

Frogs:

Frogs are critical for a horse’s soundness and one of the most important but are often neglected. They need to be compacted, wide, callused, thick and thrush free, however, most domesticated horses frogs are unhealthy and vulnerable to infection. Also, their back of the foot is underdeveloped and will more often than not yield to thumb pressure. When a horse’s back of the foot gives them pain it will revert to a toe first landing. One of the main reasons why Natural Hoof Care works is it develops the back of the foot and returns the horse to a natural heel first impact and having a functional frog is a vital part of this process. The frog works in conjunction with the coronary band and allows the hoof to distort over uneven terrain. Also, the frog stay enables independent movement of each heel during this distortion.

The frog has several purposes:

It is a hinge which spreads and aids the hoof mechanism.

It is used for traction on all surfaces.

It also acts as a cushion and part of a sophisticated shock absorption system.

 

The central sulcus is in the centre back of the frog and a healthy sulcus is shallow and like a thumbprint in shape, but in a contracted hoof the frog is thin the sulcus is narrow and can be a haven for thrush and bacterial infection. In the wild, horses of midwestern United States frogs have passive contact with the ground and protected from excess wear by the solar dome, heels and the bars.

 

The corium:

The corium has a robust and thick connective tissue with a vast network of blood vessels. The corium is covered with papillae except for demis, these papillae produce the outer wall sole and frog. The corium provides nutrients to the basement membrane and also the outer wall via its attachment as it descends. The corium attachment holds the hoof wall to the coffin bone and helps to keep the bone in place, but it’s not designed to support the whole horse's weight which is a consequence of shoeing.

 

The frog and sole corium produce material that gives a horse its frog and sole and at the junction of the two are the collateral grooves. These grooves run either side of the frog from the frog apex to the heels and are an important landmark because it consistently gives the placement of the internal structures. These grooves are a barefoot trimmer's x-ray vision into the hoof and should be between 5/8 to ¾ of an inch below the outer edge of the sole plane in the front of the foot. The collateral groove depth is very accurate in the front half of the foot, but not quite as accurate in the back, so the sole plane is the main guide here and the collateral grooves secondary. This is because the lateral cartilages which form the back half of the foot are very flexible and when a horse is standing still or the hoof is in the air the lateral cartilages sit in a high position, but sink lower to the ground when the foot is fully loaded, so when looking at the collateral grooves for guidance this should be taken into account. Also, the horse could have contraction which will also make the lateral cartilages sit higher as well.

 

The coronet: 

The coronary band is a very tough structure which works in conjunction with the frog, allowing the hoof to distort and resides at the top of the hoof wall. The band is extremely strong and supports and adds to the strength of the hoof structure.

 

The digital cushion:

The digital cushion is wedge shaped and sits above the frog filling the majority of the back of the foot while it also plays a vital part in shock absorption. This part of the shock absorption system arises with frog ground contact, it pushes up into the digital cushion pushing the lateral cartilages to the sides. Dr Bowker discovered that blood also helps with shock absorption by it being drawn in and around the digital cushion and working like the gel does in training shoes dissipating energy. The development of the digital cushion will greatly influence the angle of the of the coffin bone and hoof wall. The myth of the thoroughbred being flat footed is purely because they are shod too young and their digital cushions are underdeveloped. Recently I have started to think that the most important part of the digital cushion to develop in tender and flat footed horses is the digital cushion under the navicular bone and deep digital flexor tendon. This will help the horse to create a better hoof to pastern angle and support the internal structures. The digital cushion has a central depression and its lateral lumps are known as the bulbs. Young horses are born with a digital cushion of fat, which is enough to support their weight. In the wild, a young horse will develop this fat into fibrocartilage by movement, flexing and frog pressure on different terrain over many miles. Also, this movement and flexing develop the lateral cartilage and with the digital cushion form the foundation of the back of the foot. In domestication, young horses are left on soft pastures which don’t give enough flexing or frog pressure to develop the back of the foot. Commonly these horses are then shod and many go on to die without ever developing fibrocartilage or their lateral cartilage hardly at all. The lighter the horse the more common this lack of development is. The reason being is that the hoof wall will support the weight of lighter horses and keep the back of the foot off the ground, however with heavier horses such as draughts, the hooves will flare, split, crack and break away allowing the back of the foot to develop better.

 

The periople is a covering to protect the newly grown hoof wall just below the coronet. This new hoof horn is designed to be softer when first formed to cushion impact and protect the coronary band from damage.

 

The hoof wall:

The hoof is shaped like a cone and has a greater mass distally, so therefore, it makes sense that the hoof wall can’t possibly grow all from the coronet. This extra wall comes from the secondary epidermal laminae and it will respond to stress and stimulus. The hoof wall can be subdivided into three areas the toe, quarters and heels and has great strength this is because hard keratin is the main structural protein. Also tubular and the intertubular horn is formed at right angles to each other, forming a multi-directional reinforcement. The intertubular horn is much stronger than the tubular horn because of its higher moisture content, where the outer wall which contains tubular horn is harder and more brittle. The wall is made up of several parts there is a very thin outer layer called the stratum tectorium which gives the hoof it’s glossy shine. The stratum medium produces tubules and with the stratum externum are commonly known as the outer wall. The Inner wall is made up of stratum internum layer which is structured in a folding pattern, where each fold is separated by tissue stemming from the dermis. This snippet of information goes to prove what many barefoot trimmers found in the field that the dermis laminae produce a part of the inner wall. Where it may take 8 months for the outer wall to descend from top to bottom, but we were finding that we can stimulate thicker walls in a few short months. Also, part of the inner wall is epidermal laminae and the dermis laminae allow the wall to descend distally past the internal foot. The dermis is attached to the coffin bone and has hundreds of laminae organised in rows. Each of these rows has around 150 to 200 tiny secondary laminae and the tips of these lamellae all points towards the coffin bone. The dermis or corium forms into a thick band in the coronary region where it is covered with papillae and it’s this papillae germinative layer that produce the tough outer wall horn. The epidermal basal cells undergo mitosis, which means that they multiply and continually add to the hoof wall at the coronet. The coronary papillae insert into the hoof wall and from horn tubules running perpendicular to the surface of the wall. This outer wall will keratinised, harden and produces epidermal laminae that integrate with dermal laminae as the wall descends distally. These epidermal laminae do not have their own blood cells and they depend on the capillaries of the dermis for their nutrients. The Inner wall has a higher proportion of intertubular horn, which is produced by the valleys of the papillae, this acts like super glue and this keratin bond allows the inner wall to carry the outer wall to the ground. KC Lapierre states how an explanation of how fluid dynamics works also explains the distal flow of the wall. Dr Bowker’s says the inner wall is like peanut butter. What they mean is the hoof wall behaves and moves like a liquid where the flow of water at the surface will move at a different speed than the deeper water. Neither epidermal or dermal laminae produce pigmented horn and this is the white inner wall you will see when rasping the bottom of a hoof wall. This should not be confused with the white line which is golden in colour and joins the sole to the inner wall of the hoof. The terminal papillae come from the distal border of the coffin bone and produce this golden line which seals off the border protecting the coffin bone from bacterial infiltration and a shallow crease is normal, this and the dirt that packs here aids in traction. The inner wall is more pliable than the outer wall due to a higher proportion of intertubular horn which binds the tubules together. The inner wall has a higher moisture content and is better suited for shock absorption than the outer wall. The outer wall is pigmented and has a higher proportion of spiral tubules, It’s been suggested that these act like springs, but they are more likely there to add strength to the wall. The outer wall's main purpose is to protect the internal structures and a healthy wall will be smooth from top to bottom with no signs of flares or cracks. It's difficult to say what normal hoof wall growth is because hoof growth is a response to a stimulus, you stimulate the inner wall you get thicker walls. Also, if you believe KC Lapierre’s theory on hoof dynamics, the stimulus of growth of the inner wall will also increase the outer wall growth. If you peripherally weight the horse’s hoof as with shoeing then you get too much outer wall, also if you add lack of circulation or lack of attachment into the equation and you will get a poor quality horn. However, in saying this there are also other factors that can affect hoof growth and quality, such as diet, lack of movement, stable conditions, bacteria and fungal infections and injuries. So I can only say that in my experience a normal growth rate in a well conditioned barefooted horse is more like 1 centimetre, plus a month and not 5 to 6 mm which is commonly quoted.

 

The Coffin bone:

The distal phalanx, the third phalanx, P3, the pedal bone is encased in a box of horn, so it is also known as the coffin bone. The coffin bone is often viewed as a cross section and looks very odd, especially the first few times. The coffin bone provides the structure and foundation of the front half of the hoof and the digital cushion and lateral cartilage the back. As the horse ages, the environment and outside influence can manipulate the shape of the hoof capsule and the bone. The palmar processes start out as cartilage and forms the wings of the coffin bone and this isn’t fully developed until a horse is 5 years old. So as you can imagine, shoeing especially at this stage will deform the hoof by pulling the wings of the coffin bone inward and alter the hoof into an unnatural shape. This can also happen to foals if they don’t get enough movement in different terrain and their hooves aren’t trimmed properly. These contracted hooves will have too much concavity in the foot, making it difficult for the sole to flatten on weight bearing. It is interesting that Strasser says that the pinching of the corium on these contracted horses causes inflammation and is often diagnosed as navicular. Light bulb moment! I thought the cause of navicular was the lack of development of the back of the foot, but the back of the foot development is the cure!  The coffin bone is covered in a corium of sensitive laminar and this produces the intertubular horn of the hoof wall. The coronary corium lies across many soft tissues such as the extensor tendon and lateral cartilages. The sole of the coffin bone is covered by solar corium and this produces the sole horn and these are fed by a network of blood vessels surround the coffin bone and supplying the corium with nutrients. The coffin bone is the anchor for many important structures, such as the extensor tendon, which run down the dorsal of the limb and attaches beneath the coronary band to the extensor process. The deep digital flexor tendon which attaches to the underside of the coffin bone known as the semilunar line.

 

Blood vessels:

A network of blood vessels run through and around the coffin bone. There are five venous plexuses the Solar, Digital cushion, Lateral cartilage, Lamellae and Coronary. These plexuses supply their relevant area with their own blood and nutrients. Producing tubules, corium, hoof wall and also helping with shock absorption. Most of the veins of the hoof are situated in the back of the foot and nearly all veins are valveless in the hoof and only the superficial caudal hoof, coronary and subcoronary veins have valves. There are three interconnected valveless venous plexuses which drain into the medial and lateral digital veins the pressure of weight bearing and the change in shape squashed the veins which help the blood shoot back up the leg when the foot is lifted. This is a vital part of the hoof mechanism and shock absorption system. Also unique to horses vein actually pulsate, veins in the distal limbs have smooth muscle around them and this muscle seems to be controlled by the nervous system.

 

A neurosensory organ:

The equine foot has a wealth of sensory nerves allowing the horse to feel its environment. This affects all gaits and blood flow in and out of the hoof. The sensory nerves of the equine foot, work in a similar fashion to our own fingertips and use receptors to transmit information like proprioception, pressure, feel and touch. Elephants have a similar setup and it’s been proven they actually hear through their feet by interpreting vibrations. This may sound crazy but our own hearing is just vibration interpretation. Pacinian corpuscle receptors detect vibration like the rumbling of an earthquake days before it happens. These receptors along with others are found in the nerve centre of the foot in heel bulbs and frog stay. The equine foot is a neurosensory organ which controls blood flow and changes in one have an effect on others. Experiments have proven that one foot in cold water or on a different surface will affect the blood flow in the other feet. It difficult to comprehend the problems nerving of a foot will cause and is totally unnecessary.

 

Arterio-venous anastomoses AVA connect an artery and a vein, they are controlled by autonomic vasomotor nerve and their associated peptidergic nerve. This setup has a major role in controlling microcirculation of the equine foot. AVA ‘s divert the blood flow away from the laminae on the onset of laminitis It was thought that AVA’s caused laminitis, but in light of recent research the diversion of blood is much more likely a response to the damage and not the cause.

 

Hemidesmosomes are discrete cells that attach the basement membrane to the base of the epidermal basal. These attachments are likened to spot welds on sheet metal and firmly attach the base membrane to the lamellar of the hoof. Lamellar basal cells only reproduce rapidly when the hoof wall is injured and healing is needed. Bridging the gap between the hemidesmosome and base membrane are submicroscopic anchoring filaments and each of these filaments has a unique glycoprotein molecule called laminin-5. It stands to common sense if the hemidesmosomes or their anchoring filaments are damaged or destroyed, then this will affect the attachment of the hoof wall.

 

Hoof wall attachment:

Hoof wall attachment is extremely strong this is because the tiny secondary laminar of the inner and outer wall intertwine together becoming extremely strong. Between the dermal and epidermal laminae is a microscopic skin called the basement membrane and this is the actual point of attachment. The epidermal laminae grow past the stationary dermal laminae while still being firmly attached and there are approximately 1.3 square metres of attachment in one hoof which is a substantial increase over other animals hooves that do not have many secondary lamellae. The movement from the coronet down means the attachment is constantly being modified and this is achieved with a zinc containing remodelling enzymes called matrix metalloproteinases that remove anchoring filaments that hold the epidermal laminae to the basement membrane. Dr Chris Pollitt has proven that at the outset of laminitis, the remodelling enzymes go through a population explosion. Which then go on to destroy all of the anchoring filaments, in my imagination this is like a lot of little Pacman eating away the wall connection. Pollitt has only found one trigger that causes laminae separation and that is hindgut bacteria streptococcus bovis and if these bacteria get into the bloodstream they will trigger a population explosion of the remodelling enzymes. Horses have no enzymes to digest non-structural carbohydrates, which are frictionless, sugars and starches, these ferments into lactic acid which in turn promote the proliferation of streptococcus bovis. Pollitt has also proven that the laminae can be separated by glucose starvation, this starvation causes the destruction of the hemidesmosomes between the basement membrane and epidermis laminae. It is thought that a reduction of glucose supply to the hoof is done to preserve vital organs. The secondary laminae are made up of keratinocytes and so, therefore, the hoof wall production will be affected by this reduction of glucose. A well formed barefoot hoof will be able to cope with laminitis much better because they are supported by a well callused sole and better back of the foot development. Another phenomenon of these amazing adaptive structures is that laminae are created in response to stress. So extra laminae in the area around where a toe clip of a shoe, is a sign of stress. With stress such as constantly flared walls, the laminae will become thinner and longer making them more susceptible to laminitis.

 

The basement membrane is a thin unbroken layer of skin like material that covers the entire dermis layer, sole and frog corium. This connective tissue membrane enables the attachment of the dermis and epidermis layers of the hoof. The basement membrane is made up of fibrillar collagen called type IV collagen and it is woven into a mat-like structure that gives the membrane its strength.


The navicular bone:

The navicular bone or digital sesamoid is shaped like a small disc and sits in the back junction of the coffin bone. It is held in place by the navicular suspensory ligament and Its main purpose is to allow the deep digital flexor tendon to glide where it changes the angle. This bone pivots on and is supported by the suspensory navicular ligament, while the deep digital flexor tendon runs around the back and underneath of this bone. The navicular bursa has a smooth surface that lubricates and reduces friction. The navicular bone and deep digital flexor tendon do their job of absorbing pressure not at impacted, but as the weight of the horse passes over the foot during a stride. The navicular bone central ridge takes most of the pressure and also is supported by the digital cushion which sits directly below this structure and is thick fibrocartilage in a well-developed foot.

 

The Lateral cartilages:

Ungual / lateral cartilages form the foundation of the back of the foot and are designed to absorb the initial impact which is transferred by the bars. They attach to the palmar processes of the of the coffin bone and are held in place by a series of 6 small paired ligaments either originating or inserting onto lateral cartilages. These ligaments attach to the long and short pastern, the navicular and coffin bones. It protrudes above the coronary band and can felt and checked them for thickness and improvement. Lateral cartilages are covered with the dermis and with the coffin bone dictates the shape of the hoof. These cartilages allow sideways expansion of the digital cushion and if these cartilage have stiffened or ossified, which is known as sidebone then this will stop the hoof mechanism from working correctly and have a harmful effect.
 

© Copyright 2015 Chris Simpson

 

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