On conformation and the connection with hoof form

We know that the conformational ideal for the equine athlete is for the two pairs of hooves to be evenly matched.  We also know the adage – ‘no hoof no horse’ is as true as it is ancient.  Logically, given the different ways the fore and rear hooves are loaded at rest and in movement, and the critical importance of straightness in the equine athlete, vets, farriers and other equestrian experts ought to be alert to any significant deviation in the size and/or orientation to the ground of the hooves, and be equipped to prevent, halt or reverse adverse changes.

If the dorsal angle of a left hoof is 5 degrees steeper than that of its partner, something is seriously amiss because when the dorsal angles of a pair of hooves are significantly different, the way the bones are loaded and the balance between the muscle systems which support and act across joints will be different.

Similarly, given the different primary roles of the two sets of hooves which is reflected in their shape, i.e. the fores carry upwards of 60% of body weight and are rounder, and the hinds are vital for propulsion and are narrower and more concave, the length of the toe in the hooves is critical.  

All too often, in addition to imbalances in the pairs of hooves, we see hinds with long, under run heels and long toes, and fores that have long but more vertical heels and shorter toes.

When you consider the roles of the hooves at rest and in movement, having hind hooves with long, shallow angled toe, and collapsed heel, and fores with a short toe and long vertical heel  is anatomical nonsense and is setting the horse up for all manner of musculoskeletal and metabolic problems.

The hoof is a ‘cast’ around P3, the distal phalanx or pedal bone. The hoof is a truncated, slanted cone in shape, open at the heel region where the hoof wall is attached to thick pads of cartilage which effectively extend the pedal bone rearward and allow the heel region to deform both laterally and vertically under load.

The front limb typically takes about 60% of the horse’s weight. In any given horse the dorsal angles of the pedal bones, when their distal margin is ground parallel, are between 2 and 5 degrees steeper in the hinds than the fore. The front pedal bone is typically 45-48  degrees and the hind around 48-50 degrees. 

The distal (ground most) margins of the pedal bone are fine and because bone yields to persistent pressure, the margins are susceptible to erosion when P3 is exposed to unphysiological pressure and/or impact. Too much load taken to one side or to the front can, and often does, lead to loss of bone. Incorrect loading of the coffin joint can and often does lead to damage to the coffin bone and the ligamentous structures of the joints.

Another conformational ideal that is widely spoken about but woefully misunderstood, is for the cannons to be vertical to the ground when the horse is in halt or,  more importantly for the horse’s long term health and wellbeing, when it is at rest.  

How often do we see horses standing completely square when they are sleeping / resting?  How many horses end up with short toes and high contracted heels in the front and long collapsed heels and long toes in the hinds because they habitually elevate the front heels by steepening the pastern, or pointing a toe, or by standing under in front or behind or both? 

How many farriers, vets and equine experts understand the stay apparatus, not just the mechanics of the patella locking of the hind limbs, but the way the front limb SA operates, and the consequences to the horse of a failure of that vital system?

My advice to ALL horse owners is to ask their farrier, vet and trainer, the people they pay a great deal of money to and in whose hands they place the well being of their horse, to explain the equine stay apparatus to them.

I will lay odds that many of them will not have a clue. Many will also not understand how vital the neutral position of the coffin joint is to the correct loading of the limb.

The adult horse sleeps mostly upright. It is unable to rest lying down without suffering severe system damage. Its ability to rest and repair its tissue optimally whilst upright depends on its ability to fully relax the bulk of its skeletal muscle which it can only do by means of a certain alignment of its skeleton and the passive weight of, and balance between certain muscle groups. A key element in this are the deep digital flexor muscles, and their check ligaments.

The reason the deep digital flexor tendon is so huge and has such a massive insertion point on the underside of the pedal bone is not because of its dynamic function of flexing the coffin joint, but because of its passive function of backing up the suspensory apparatus of the fetlock joint, without which the stay apparatus cannot function properly. And if the SA isn’t functioning, the horse is being set up for musculo-skeletal and metabolic harm. The progress of that harm and the speed with which it progresses is significantly increased in the equine athlete, for obvious reasons.

As an illustration of part of this complex issue,  think about what is happening during the stance phase of the stride.  

Taking the front limb only, the hoof comes to a rapid halt in early stance (initial touchdown) which should be fractionally heel first as the limb is being retracted, and with the coffin joint in or very close to neutral. 

Gravitational force means body weight keeps coming down but the ground reaction forces are dampened and spread by the lateral deformation of the heels which tightens the fibrocartlaginous bands in the digital cushion.  

The bars, frog and sole share load with the hoof wall, and critically, the suspensory apparatus of the fetlock joint allows a hyperextension of that joint which counterbalances the flexion of the coffin and pastern joints.   

The suspensory ligaments, which contain muscle fibres so are more extensible than normal ligament, are backed up by the digital flexor system. As the limb accepts load the direction of loading forces on the insertion point of the deep digital flexor tendon becomes horizontal. 

The toe region of the hoof is stablised by the tightening of the extensor branches of the suspensory ligaments. The check ligaments are ready to cut the muscle belly of the flexor muscles off should the strain on the digital flexor tendons become too great. 

The balance between the deep digital flexor muscle and the extensor system keeps the carpal joint stable; the deep digital flexor muscle, which is under tension, keeps the elbow in extension which in turn ensures the angle of the scapula is optimal as the horse's body weight passes over the planted limb prior to late stance when break over is initiated and the limb is flexed, i.e. the point at which the body has to overcome the resistance of the ground and when too long a toe can massively increase lever forces on the coffin joint. 

The coffin joint is the first joint to experience the forces of deceleration and concussion and also lever forces on break over.  In a very high heeled hoof and/or one with inadequate toe height, the horse starts with a coffin joint that is permanently out of neutral range, so it cannot land with the joint in neutral.  As a result, the critical balance between the various systems is thrown out: ground reaction forces are increased; lateral expansion of heels is reduced (worse case scenario, heels actually narrow under load), or in a splayed hoof, lateral expansion may be too great.  

In both a narrowed and a splayed hoof form, the frog, digital cushion and lateral cartilages are deformed and weak;  the balance between muscle systems – their ability to stretch or play out and to contract in synchrony with their opposites is reduced, leading to increased potential for soft tissue or bone injury.

Nor can such a horse stand with its joints in neutral when it rests which means that in order to remain upright, it  must be in a state of persistent muscle contraction.  Even the highly tendinous biceps and the superficial digital flexor muscle in the hind limb will break down if subjected to a persistent, unphysiological strain, i.e. too much and / or for too long.

Straightness is impossible to achieve or maintain for any length of time and a rider’s efforts to achieve it may lead to greater discomfort and fatigue which increases the risk of injury and anxiety.

I cannot comprehend how anyone could lecture people about the need for balance in diet, schooling, saddle fit, rein contact – whatever – and not comprehend the obvious fact that all of that is negated if the essential balance in the musculoskeletal system is out. As with all things, the problems manifest in different ways and at different times in different horses. 

It's why I bang on about the stay apparatus; given its fundamental importance to the horse, the fact that it remains the greatest area of ignorance is an indictment of the equestrian world. 

A horse with a long contracted heel and/or too short a toe has a persistently misaligned pedal bone and coffin joint. Because the coffin joint rotates back and forth (in the sagittal plane), it is in effect rotated out of its normal, neutral position, even if it is still tightly connected to the hoof wall.  Anatomical fact.  

The optimal balance between the coffin joint and the other joints of the limb exists within fairly narrow parameters and if those parameters are exceeded, harm will result. How much harm, when and where it occurs will vary but harm will be done.

The bars and other complicated stuff

All parts of the equine hoof grow groundward and, because the form of the hoof is a slanted, truncated cone, it also grows slightly forward.  The mysteries of the conveyor belt arrangement - how the hoof wall and sole stay attached as the horn grows, and importantly, why the system fails, are only just being unravelled.

The slanted, truncated cone of the hoof capsule should reduce evenly in its length (linear distance from hair bearing skin to distal margin) and its height (vertical distance from the ground to hair bearing skin) from the toe region to the heel buttress.

You can mark the lines of growth by drawing a line in a contrasting pen down a tubule from coronet to distal margin. Do that all round the hoof at regular intervals and you get a clear picture of the way the hoof is growing and whether there are areas that are growing at significantly different angles to the ground.

The hoof wall is open at the heels.  At the heel buttress, the wall turns inwards and becomes the bars - so the bars are, in effect, a continuation of the wall and share many of the same characteristics eg there is corium at the top of the bar which produces the bar horn and a laminar corium which produces interdigitating laminae that bind bar and sole horn together.

The bars'  outer zone, like the wall, is pigmented (most obvious in a dark hoof) and more tubule dense than the bright white and moister inner zone of horn which interfaces with the sole. At the top of the bars (i.e. at the top of the collateral groove) the corium that produces the bar horn merges with the frog corium. 

The bars both allow and control lateral expansion of the heels; they share load, and they help in deceleration if the wall and bar sit slightly proud of the sole. As well as expanding laterally under load, the heels can also deform vertically when the hoof loads on uneven ground.

This flexibility is achieved because the rearward third or so of the hoof grows from dense cartilage - not bone. The wall in the heel region is a little thinner than the toe region (or should be but often isn't because the toe is too weak) but the wall and bar work together to add substance and stability to the heel region.

Just as the wall proper reduces in length/height and width from the toe region to the heel buttress, the bars - also should reduce in length/height and width from the heel to the bar apex where bar horn merges seamlessly with sole horn.

This region, forward of the apex of the bar and about half way along the frog, where bar horn merges seamlessly with sole horn is a zone unlike any other in the hoof.  The two types of bar corium (coronary and laminar) merge with each other and merge with the sole corium.

In the hoof several different types of horn and other tissue meet: hair bearing skin and capsular horn at the coronet; hair bearing skin and the bulbs and frog; sole and wall horn; sole and frog; frog and bar…. and each junction has properties which ensure it's not subjected to too much strain. 

Eg. the wall and sole junction is buffered both by the white line - which is formed by terminal papillae at the distal margins of the coffin bone - AND by the change in the structure of the wall to a less tubule dense, more  moist horn that makes the inner zone of the wall more like  sole horn.   The frog/bar junction is buffered by the interdigitation of laminae and by the elastic and dampening properties of the frog (the region of the bar corium is aligned with the twin folds of the frog in its rearward third). The sole/frog junction is buffered by the elastic frog and by solar concavity - and so on.

We don't actually know much about how these coalescing zones function - what is obvious is that they can become problematic if they are exposed to severe or long term unphysiological stresses.

As a slanted cone, the hoof should diverge towards the ground, i.e. have a smaller circumference at the coronet than at the distal margin. In order to be able to deform laterally under load, the heels have to diverge to some degree. 

If the heels are vertical, ground reaction forces will result in them being deformed upwards. 

If they are beyond the vertical i.e. they slant towards the midline of the hoof, they will tend to narrow under load which compresses the bulbs and the frog. 

If they are too shallow (under-run), impact forces will result in too much lateral or forward spread and the heel horn may completely collapse and lie over the sole.

Each region of the hoof has a certain optimal angle of growth.  This may be a range and it can change according to environment etc, but it is a fairly narrow one. If hoof form is too steep or too shallow, or there is too great a difference between sections of the same hoof, problems will arise.

For example, it's often not understood that, in a splayed or 'forward running'  hoof, where wall and bar sit at too shallow an angle, the sole also will have too shallow an angle of growth.  If the hoof is too steep and convex, the angle of growth of the sole will be too steep.

The angle of growth of the bar is always matched by the angle of growth of the heel. The steeper the heel is, the steeper the bar; the more sloping the heel is, the more shallow the growth angle of the bar is.  If a heel is bent inwards beyond the vertical, the bars will also be bent beyond the vertical and / or it will form a curve. If the heel has completely compressed and is lying flat, so is the bar. 

As with the wall, the angle of growth of the bar determines how the different zones of the bar meet the ground and how loading and abrasive forces affect it.

A feral horse can cope with all manner of pathology in its hooves if it lives in a benign environment but if its environment is harsh and it cannot adapt its hooves to cope with the demands on it, the horse will not survive. 

In domesticity, people often ask horses with severe hoof pathology and all manner of related conformational issues to cope with being ridden or driven and they use shoes and boots to allow them to push the horse past the point it would be able to go to if it had bare hooves. Sometimes the demands people make are too much and the domestic horse also does not survive - because it is put it down. 

Some specific issues:

'The bars help form the sole' 

In a hoof with an 'white' wall and sole, it's not always easy to distinguish the inner zones of the wall from the sole but in a dark hoof, the unpigmented region of the bar is very obvious. If the bar does contribute to the production of the sole in the way that Bowker suggests,  given the bar has an inner bright white zone, all hooves with a dark sole should have areas of bright white in them running forward from the bar apex towards the toe. They don't. 

I don't think it's as simple as 'bars help form the sole' - and there is the question of which bit of the bar, given the complexity of the junction at a cellular level. We just don't know enough about the bar-sole junction to be able to say for certain what is going on in any given hoof. 

I have here at the moment, one horse with all black hooves, two with one white hoof and one with one black hoof. The one with all black hooves has no white horn forward of the bar apex - although when I first started trimming him he did have pools of white horn that spread out from the bar apex. The horse with one black hoof has areas of white sole in his black hoof but their location never alters and it forms discrete patches towards the toe.  In the two with one white hoof, the black hooves have no white at all.  In the white hooves of course the outer horn is not pigmented or not heavily so. (See post on pigmentation)  I have seen bicolored hooves with white soles that have dark bars and dark horn running from the bar apex all the way round the tip of the frog. Is this bar? Some would say yes, some no. 

The effects of ground reaction forces

The hoof and the structures it protects have to be able to deal with massive deceleration and impact forces - and the persistent pressure of rest stance. 

When a horse stands on a level, hard surface, the toeward region of the anatomically normal bar in a normal hoof is passive when the horse is in rest stance. The buttress region of the bar shares load with the wall, which shares load with the rearward third (folded) region of the frog and the sole adjacent to the wall.  The concave nature of the hoof means the sole under the sharp distal margin of the pedal bone is well protected. 

But the world is not usually completely level and smooth and on a conforming surface the load is more evenly shared, and under maximum impact on any surface the entire volar surface shares load. 

However, the outer regions always impact first and carry most load; maximum impact is fleeting on any given hoof, and concussion should be damped/dissipated by many different mechanisms.

Problems for the horse arise when any of those mechanisms is impaired and any given part of the hoof receives too much load at a given point or over time.

For example, in a severely contracted hoof the bars may not touch the ground on a level surface, or a shoe will lift them out of ground contact even further  but - the internal structures of the digit - specifically the region of the navicular bone - can be impacted by the bars even if they never actually touch the ground, even on a flat surface where they are sitting above the ground because gravity ensures that body weight pushes downwards onto them.  If the digital cushion is weak and suspensory apparatus is compromised,  the situation is worsened.

Photographs of a dissection of a foundered hoof with ingrown bars are shown in this blog post. I have seen far worse impacted bars in foundered hooves. The worst I have seen were in a pony, whose bars  were arched up into the hoof past the level of the coronet and curved inwards under the frog. The pony had spent years denied grazing and movement because she was prone to laminitis. No-one responsible for her care understood the connection between her metabolic and endocrine issues and her persistent attempts to avoid loading her increasingly painful long and contracted heels and bars.

The bottom line is that the bars are a critically important part of the hoof - in horses which cannot grow, load and wear their hooves optimally or have them trimmed optimally, the bars can grow in ways and into places which impact on sensitive structures and affect the correct functioning of the hooves and associated muscle-skeletal structures. The long term effects can be profound.