More on the caudal hoof - and the digital cushion

Everything about the way the hoof and the limb is structured is about dissipating ground reaction forces, ensuring traction, optimizing circulation and conserving energy.

The evolutionary advantages of the tough but lightweight single hoof are offset by the facts that:

• it has to fulfil all the loading and balance functions of a multi toed foot; 
• there is no contractile muscle tissue below the knee to buffer the bones and to aid venous return; 
• the joints of the digit are at a considerable distance from the muscles which act across them. 

To understand why the hoof and its limb have to function in a certain way, we have to understand the suspensory apparatus of the lower limb and its role in the stay apparatus; to understand that we need to understand the anatomy of the whole limb, and to properly understand that, we need some understanding of the anatomy and physiology of the whole horse.

It has become a barefoot mantra that the 'horse lands heel first', and as a result, the health of the soft horn/soft tissue structures of the heel is paramount. I agree with that but the heel first landing is important not just because the back part of the hoof is more flexible, it is also important because of bone alignment at the point the heel comes into land, which is important because of blood flow and muscle health.

At rest, the cannon should be vertical to the ground, and the pastern bones and pedal bone are offset to the vertical axis of the limb, ie the fetlock joint  is held in permanent extension and is able to achieve hyperextension by its suspensory apparatus. 

The tendinous interosseous muscles or suspensory ligaments  are strong enough to passively hold the fetlock joint in permanent extension, but elastic enough to allow it to hyperextend under load. At rest, the joint should be about 25% extended; at full load, it can extend to 180 degrees and even beyond.

The SLs are backed up by the digital flexor tendons, both of which have a check ligament that come into play to protect the contractile tissue of the muscles of the deep and superficial flexors tendons.

The deep digital flexor tendon (DDFT) is massive and has a fan shaped insertion point in the back  of the pedal bone. It is so over-dimensionalised not because it is needed to flex the lightweight pedal bone, or even because of its distance from its muscle, but because of its role in backing up the suspensory apparatus of the fetlock joint.

The coffin joint, between P2 and 3, has its own suspensory apparatus which gives the joint a greater mobility than the pastern joint (P1 and 2) but much less than the fetlock joint. Like the fetlock joint, the coffin joint has a sesamoid bone (navicular) which extends the bearing surface of the joint.

Both joints have an optimal alignment at rest, deviations from which can create a myriad of problems.

The hoof comes into land as the limb is being retracted, at which point the coffin joint should be fully extended with the digital arterial pathways fully open.

As the hoof engages the ground:

• the heels expand laterally and deform vertically; 
• the suspensory apparatus of the fetlock allows it to descend; 
• the suspensory apparatus of the coffin joint allows the two pastern bones to also descend, as a unit, with the fetlock;
• as the pastern bones descend, the angle of the back of the coffin joint begins to close and the digital arteries are compressed between bone and DDFT until, at full pastern extension, the blood flow into the hoof is completely closed off;
• after body weight has passed over the limb, the coffin joint is flexed and the limb is lifted off the ground;
• as the limb is extended, arterial flow is resumed.

Maximum fetlock extension is reached as the horse’s body weight passes over the vertical axis of the limb at which point the hoof should be fully and evenly planted with load dispersed across its volar surface and as a consequence, the whole bearing surface of the coffin joint. The extensor branches of the SLs are fully tensioned to stablise the coffin joint.

The venous plexuses are expressed by the reversible deformation of the hoof capsule and the mechanical regulation of arterial flow when the heart is pumping hard is a vital part of balancing hydraulic pressure in the hoof. 

Problems occur when the persistent alignment of the coffin joint mimics the degree of pastern extension that occurs at high impact. This can happen with a grossly steepened pedal bone where the palmar/plantar processes are elevated, and in a ground parallel pedal bone where the hoof-pastern angle is severely broken forward.

The role of the Digital Cushion

As the pastern bones descend, the digital cushion (DC) is engaged. This is a structure that is both misnamed and misunderstood.

It is widely argued that the DC aids shock absorption and also contributes to venous return by helping to mechanically express the palmar venous plexus.

Strasser argues that the DC’s effectiveness, both as a shock-absorbing cushion, and in transmitting ground reaction forces to the lateral cartilages, is negated by its ability to displace caudally because there is only thin skin at the back of the pastern above the heel bulbs. Because of its location and its structure, she argues that it is more akin to a sling, the role of which is to assist in arresting the downward pressure of the pastern bones under load and with the bars, to assist in controlling the lateral expansion of the heels.

The DC is made up of horizontally arranged bands of fibrocartilage (FC) that  attach to the lateral cartilages and run above the frog corium and below the distal fibrous sheath of the deep digital flexor tendon (DDFT). There are tubular glands over the spine of the frog, ducts from which pass through the frog dermis to the frog surface. Strasser argues these are vital to frog health as they affect the pH of the pad and help make it resistant to pathogens.

In between the fibrocartilaginous bands is loose connective tissue (LCT).  The proportions of FC to LCT varies between hooves and Bowker argues that lots of fibrocartilage is evidence of a healthy hoof. He also argues that in a healthy hoof, the lateral cartilages are thicker and denser. it seems logical that this would be the case but there are many other factors at play.

Fibrocartilage is a specialist type of cartilage that contains Type 1 collagen as well as the normal Type 2 which is the main component of cartilage.  Type 1 is found in skin, tendon, vascular ligature, organs and is the main component of the organic part of bone.

The fibrous element in fibrocartilage confers flexibility and toughness, and the cartilaginous element confers elasticity.

The proportions of each element in any given digital cushion may vary according to such factors as age, breed and importantly, the terrain the horse lives on and how much movement it gets. The relative amounts of the two types of collagen may vary as well.

Cartilage has no blood vessels or nerves of its own. The cells that secrete the matrix of cartilage and become embedded in it obtain all their nourishment from blood vessels in the perichondrium (the connective tissue that envelops cartilage where it is not at a joint) and surrounding connective tissues – and it does so by diffusion of oxygen and nutrients through the matrix. It is because of this lack of a direct blood supply that cartilage heals so poorly and the lack of nerves is why a weak DC in itself will not cause discomfort.

With age, normal cartilage may calcify which inhibits the diffusion of nutrients through the matrix and may result in degeneration of the cartilage. I do not know if this happens to the cartilaginous components of the DC.

For diffusion to occur, cartilage (and tendon & ligament) need movement - preferably under load.The movement the DC gets is from the direct expansion and contraction of the heels, and to a lesser extent, from the upward pressure of the ground on the frog pad.

The horse evolved from a multi toed ancestor and it contains remnants of those toes in the splint bones, the ergots and the suspensory ligaments which, as their anatomical name indicates, are not true ligament. It is suggested that the SLs of certain breeds (eg standardbreds) contain a higher proportion of muscle fibres than other breeds; perhaps the same is true of the relative amounts of the different types of tissue and/or collagen in the digital cushion.

The truncated cone shape of the hoof, the attachment of the caudal (rear) part of the hoof to cartilage, the inward turning of the walls, and twin folds of the frog in its rearward third  - all enable the hoof to reversibly deform (expand) laterally under load, to compress, and for the heels to displace vertically.

The hoof has to have a balance between the load bearing properties of a vertical structure and the load spreading properties of a diverging one. 

Too vertical a cone concentrates ground reaction forces on a smaller area and is less capable of dissipating energy, ie it is more prone to concussive damage both of hoof and the structures it protects.

A too diverging cone will spread load over a wider area but be incapable of retaining vertical integrity and will expose the sole and pedal bone to the effects of both concussive and persistent loading.

In either extreme, the soft horn/soft tissue structures of the caudal hoof are permanently deformed, incapable of the desirable degree of reversible deformation that is the basis of healthy hoof mechanism.

In the narrow hoof form which cannot deform sufficiently under load, and especially if combined with a steepened bone alignment: 

• blood flow will be compromised; 
• the DC in particular will not get the movement it needs for diffusion to occur and atrophies; 
• the bars will deform and very likely become high pushed, compressing veins and arteries and causing discomfort; 
• the LCs will be deformed by the inward turning heels; 
• the laminar corium in the heels will be under a state of persistent pressure; 
• the frog corium will be starved of nutrients and the glands that maintain its protective acidic pH will not be able to function; 
• shock absorption will be reduced with consequent effects on the joints; 
• a steepened bone alignment will throw out the balance necessary for the optimal operation of the stay apparatus and may impair arterial flow to the frontal regions of the hoof in particular; 
• the abnormal angle of the pedal bone to the ground, combined with the mechanical effects of high heels or short toe, will force the horse onto its forehand, overloading the toe reducing toe growth, stressing the solar corium, allowing greater heel growth ......

In the splayed hoof form:

• vertical integrity cannot be maintained; 
• the heels may expand too much under load; 
• the DC will be unable to get the stimulation it needs for nutrition and, as a result, will atrophy; 
• the pastern bones have nothing to dampen their downward movement; 
• the frog will be exposed to too much direct pressure; 
• the back of the coffin joint will be exposed to too much movement overstressing the suspensory apparatus; 
• the same applies to the SA of the fetlock joint and as a result the SLs may be damaged;
• the horse may try to  steepen its pastern angle by contracting its flexor muscles and, because that cannot be maintained for any length of time, by changing its shoulder/elbow angles which will impact on the muscles of the upper limb, shoulder, neck and back; 
• the entire volar surface of the hoof will be exposed to too much concussive impact and persistent pressure and of course bone yields to persistent pressure ......

This is not an exhaustive list of the worst case scenarios but it gives a flavour of the sorts of things that can - and do - occur. 

Some horses have one hoof of each kind; some have steep high heeled hooves on their load bearing front limbs and shallow low heeled long toed hooves on their hinds. 

What we must all agree on is that every part of the hoof has a role, and all are important.  The role of some parts of the hoof and the ill effects of them not functioning optimally are immediate and obvious; that of other parts may be less obvious and more gradual and subtle. 

The bottom line is  :

• the hoof is a unit ;
• the whole hoof is more than a sum of its parts, and
• the whole hoof is never truly healthy if any of its parts is unhealthy, and
• the whole body is never truly healthy if the hooves are unhealthy.