Hoof form and function

When considering hoof form and function it is vital to look at the whole horse.  If a horse rider, trainer, professional is not already well acquainted with the principles of good conformation, they should become so or they should forget about horses and take up golf. Excellent conformation does not in itself lead to great athletic ability, and some great athletes do not have perfect conformation, but there are some non-negotiables. A key one is the ability to engage the fore and hind limb stay apparatus both of which depend on a certain set of hoof parameters - which centre around balance and comfort.

The equine hoof is far more than just a lump of horn around the foot bones (its metabolic activity according to Dr Strasser is equivalent to that of the kidneys); the horse spends most of its life with all four of its hooves in direct contact with the ground, and it has to combine all the postural and locomotor functions of a multi-toed foot in its single hoof.

Because the horse rests mostly upright, its overall health and well being is utterly dependent on the balance of its limbs. If a horse cannot load its coffin joints correctly, the suspensory apparatus of both coffin and the fetlock joints are under strain and the optimal balance between flexor and extensor muscle systems cannot be maintained. In that situation, the horse cannot properly engage its thoracic limb stay apparatus and has to tense muscles to remain upright, which leads to muscle tiredness and strain, and ultimately to the changing of its elbow and / or shoulder angle. This affects the way the all-important nuchal ligament functions in enabling the huge locomotor muscles of the back to relax. If the horse tries to alleviate hoof pain and /or muscle tension in the front limbs by standing under with its hinds, the hind limb stay apparatus will also be affected.

Relatively few horses have technically perfect hoof or body 'conformation' but they survive and may even thrive without it.  But, humans place demands on the horse that may take elements of the horse's conformation to breaking point. For a variety of reasons, the hoof is one of the horse's weak links.

In a very real sense, the horse with compromised hooves is not a truly healthy horse; the greater the degree of compromise, the longer it has been in situ, the greater the implications are for the animal's health.

Homeostasis

The ideal for any organism is homeostasis and for the horse, that relies on the maintenance of an essential balance in how the hoof is attached to the body, which affects how the joints are loaded and how the muscle systems operate, which affects how the hoof contacts and leaves the ground. Postural misalignments may result from hoof issues or they may create hoof issues, ie an imbalance in the hoof will result in imbalances in the rest of the body, which will perpetuate the imbalance in the hoof. 

Whatever the origins of a given problem, it can quickly create other problems and form into a series of overlapping vicious circles.  The longer that has been is situ, the harder it is for the horse to break out of it on its own. 

The best situation for the horse is to work its own feet and body back into an optimal balance but there are many occasions when that is simply not possible and the horse needs a helping hand - or two.

The Ideal 

Most horses will not have perfectly matched pairs of feet because the way their feet develop in their formative years will be conditioned by the amount and type of movement, their posture, the terrain they live on, and what they are fed.

BUT, the ideal is that each pair of hooves should be the same in terms of dorsal angle, concavity and size.

The front hooves are typically rounder than the hinds; the horse carries upwards of 60% of its body weight on its front limbs which the round shape of the hoof and the straight limb from fetlock to elbow reflects.

The hind hooves are more elliptical in shape and more concave which makes them better suited for propulsion, a factor that is also reflected in the way the hind limb is structured.

Just as the pairs of hooves ideally are the same in terms of size and shape, the two sides of a given hoof should be the same in terms of their basic parameters. This is especially important for the front hooves, but the closer all hooves are to an balanced shape, the healthier and more efficient they are likely to be.

Growth

Leaving aside the issue of diet, the growth of hoof horn is affected by circulation, abrasion and pressure. A more loaded area will abrade more and, depending on a number of factors, may grow in more steeply (which to a degree gives more efficient load bearing), or may compress or collapse. A less loaded area will grow more rapidly due to less abrasion and pressure and may either become pushed upwards by ground pressure or flare outwards, or break off.

A more divergent area of horn will be longer (in linear terms) than a steeper area and the ground most surface will contact the ground at a different angle. This affects the way the hoof responds to load. The greater the difference in terms of angles to the ground, the greater the potential for hoof and related musculo-skeletal issues.

Some people argue that a lateral or medial flare comes into being because the horse needs it and it is desirable for that horse at that point in time.  The hoof has to combine all the functions of a multi toed foot in a single digit, and that includes the lateral support which toes provide, but too much flare becomes pathological and affects the way the horse loads the hoof - and, depending on where the flare is, may affect the limb in flight as the horse tries to avoid striking itself.

Limb Alignment 

Hoof form will influence and be influenced by limb alignment, ie any deviation in limb alignment will be reflected in the hoof, which will in turn affect limb alignment and limb flight. Some deviations may be unchangeable, some may be offset only to a degree but a lot are correctable if caught early enough or given enough time and the correct management.

The greater the misalignment and the longer it has been in situ, the greater the implications for the horse's utility. Note I do not say the horse's survival; the horse can survive with grossly deformed feet but it will not thrive, its utility will be severely compromised and its longevity will be reduced.

The principles of optimal limb alignment are well established. Different sets of muscles, via their tendons, act across joints. One muscle - the agonist - contracts and its antagonist, 'plays out'. Ligaments hold the joint together, cartilage allows the bones to glide across each other etc etc. Each joint has an optimal range of movement, too little or too much causes problems. The muscles that act across a joint need to be in balance to ensure the optimal range of movement. Muscles, such as the digital flexors, cannot maintain optimal strength if they are constantly compensating for hoof imbalances.

Dorsal Angles

In a healthy hoof, the pedal bone sits high in the hoof capsule, with the extensor or pyramidal process (the point where the common extensor tendon attaches) sitting level with or fractionally below the coronet. The angle of the growth of the hoof wall and the angle of the pedal bone should be the same at all points around the hoof.

Although there is a range of dorsal angles (DA) in the pedal bones of horses, all horses will have some degree of difference between the DAs of their fore and hind pedal bones.  Typically, the front pedal bone has a DA of between 45 and 50 degrees, and the hinds 50 to 60 degrees. These DAs are derived from the pedal bone if it was sitting ground parallel.

Many horses end up with a more elevated or positive palmar (front hoof) angle than their plantar (hind hoof) angle. Assuming a tight laminar bond, a hoof with a 45 degree front pedal bone that sits at at 5 degrees above ground parallel will have an hoof dorsal angle (HDA) of 50 degrees. If its hind pedal bone has a 50 degree DA but it sits ground parallel, the hind HDA will be the same as the front.

Given the different primary roles of front and hind feet, this arrangement seems counter-intuitive. Yet, not only is this common, it is disturbingly common for the hinds to be shallower in angle than the fronts. 

Hoof-Pastern Angle

There is a long-standing debate about what the ideal or optimal hoof-pastern angle (HPA) is but everyone agrees that the HPA must not be broken - either forward or back. As Dr Strasser puts it, it must be harmonic, in other words, a line from the ground up the front of the hoof and pastern should be unbroken AND that line must intersect with a cannon bone that is aligned vertically to the ground.

Strasser  argues that the foundation for the HPA is a ground parallel coffin bone and that, given the way the hoof accepts full load, and the importance of a correct limb alignment to the all-important stay apparatus, it is illogical for the load-bearing front limbs in particular to have a positive palmar angle.

But many others point to a common finding from cadaver feet and radiographs of live horses that the front feet typically have a steeper hoof dorsal angle than would be the case if P3 was ground parallel. It is argued that, as long as the HPA is not broken forward or broken back, this is not a problem for the horse. The argument is that the pedal bone has to sit at a slightly elevated angle at rest, in order for the back of the coffin joint not to be stressed when the hoof is fully loaded.

I agree that a slightly positive angle in the front hooves is more easily accommodated than a negative angle and this would certainly be the case where the rear part of the hoof is poorly developed, but we need to ask why so many horses end up with high degrees of positive hoof angles in the load bearing front hooves, and ground parallel or negatively angled hind hoof angles, and what are the long term consequences for this.

Positive Palmar / Plantar Angle

The positive palmar / plantar angle comes about when the heels of the hooves grow longer and /or  the toe growth reduces (more persistent pressure and/or abrasion) resulting in a steepened hoof form which, even if the HPA is still harmonic, alters the way the digit and limb function.

Its key impacts are on the laminar and solar coria in the hoof, especially in the toe region; the suspensory apparatus of the coffin and fetlock joints, the circulation of blood through the hoof, and the angles of the elbow and shoulder joints.

The greater the angle, the greater the long term consequences for the horse.

Negative Palmar / Plantar Angle

A too shallow angle occurs when the heels on the hooves do not grow sufficiently or have collapsed ie they are long but are under-run or compressed, which leaves the back of the hoof too low relative to the toe. The toe runs forward which pulls the whole hoof under and forward, creates a lever force on the laminar structure and importantly, makes an energy efficient break over impossible.

If the back of the hoof is weak (thin lateral cartilages, weak frog and digital cushion, shallow angle to heel horn or lack of heel growth etc), the back of the coffin joint may be stressed under full load,  or even at rest. When this occurs the horse tries to reduce load on the heel by various measures.

In Summary

• The angles of the hoof wall must not differ from the angles of the pedal bone.
• The dorsal angle of the hind hooves must not be shallower than that of the forefeet in the same horse.
• The ideal is for the horse to have matching pairs of hooves.
• Too great a difference between a pair of hooves, or between the two sides of the same hoof, always signals an underlying pathology (in the hoof and/or elsewhere in the body) which always has negative consequences for the horse.
• The horse can accommodate imbalances in the hoof and limb to a degree, but they will have wider consequences for the animal's health and athleticism. 
• The responsibility of the owner and any professional working with the horse is to understand this, to correct imbalances as far as possible (which means looking at the animal holistically), and devise a lifestyle and a work programme which is appropriate to that animal, at a given point in time. 

GP or not GP, that is the question...

This was posted a couple of years ago on EC Hoof in response to a question about ground parallel pedal bones.


I trim to allow the horse to stand, land and move as naturally as it is able to at any given point in time. 'Naturally' is mainly what looks right to me but that relates back to the standard of 'good' conformation and movement I was taught decades ago - and which I see no reason to discard. 

I prefer to do the least possible and let the horse sort itself out - where it can - although sometimes I might trim quite intensively. 

It depends. 

I do take into account a horse's age and physical condition - especially joint adaptation / arthritis. There's no point in lowering a heel if the horse really can't or won't weight it. But I try to identify the reasons why it can't/ won't weight correctly and, if they are resolvable, address them. If they are not resolvable then the trim must aim for maximum comfort but (hopefully) not at the expense of vital functions. 

It's sometimes a tight rope walk.

The critical thing is to break the vicious circle of chronic pain, skeletal imbalance, muscle tension etc as quickly and as naturally as possible. The time frame, methods involved will all vary depending on the situation obviously.

I often don't get xrays - for practical reasons (cost, distance to clincs etc) - unless I really need to see inside the hoof. I work mostly on what I can see, feel and logically deduce from the hoof and from the way the horse stands and moves. But, sometimes I won't trim without them. And sometimes I wish I hadn't bothered because of the points John (Stewart) makes about the importance of angle etc.  

The points I am making about the ground parallel pedal bone are about whether it is just a theoretical plane from which to measure degrees of deviation. 

What is the acceptable deviation - John's 5-ish degrees, Pete Ramey's 1-3ish degrees, some farriers' 5-10+ degrees - or Strasser's 0-ish degrees? 

Why is it, as John says he observes in the UK, that it's ok to trim to GP on hind feet, but trim to as much as 10 degrees above that on the main weightbearing limbs? 

What are the implications, to the skeleton, of fore and hind pedal bones that are sitting at differential angles to the ground? 

Is this something which differs according to terrain? 

We know that hard terrain /high movement horses tend to have pedal bones carried higher in a more concave hoof capsule than soft terrain horses - and have thicker but more abraded (rolled) toe walls which look steeper than the thinner 'sharper' edged walls typical of soft terrain horses. 

Estimates of pedal bone orientation in these hard terrain hooves tend to come from populations of feral horses - as I understand it from x-rays of cadaver hooves or recumbent sedated horses - as it is very hard to x-ray a feral horse whilst upright.

What difference does a completely unweighted capsule make to how P3 appears to sit in the capsule in an x-ray? Can an artificial load on a cadaver hoof replicate natural loading forces? 

Strasser says that the cadaver feet of US mustangs she's seen have highly abraded toes but GP pedal bones. Current studies of Australian brumbies show a wide variation in dorsal wall angles - all steeper than 45 degrees but I don't know how these were measured or if x-rays were done and if so - how. Other feral populations like the ones here in NZ have more typical soft terrain hoof form. I have no idea about their angles.

I do agree that some horses can cope very well with a pedal bone that is quite forward of ground parallel - and that any pain and degeneration from that situation may be less problematic than the reverse situation of a too low heel with the back of the coffin joint being persistently stressed. 

The points I made about how stable P3 SHOULD be in a healthy hoof that is attached to a healthy properly functioning body - are not to suggest that I have a magic formula for restoring feet and bodies to that state. No-one does - and some horses will never have optimal feet because the opportunity to develop them has been lost forever. Some may well need a bit of extra height at the heel, ie the deal for them is that the potential for harm of an elevated palmar angle is less than the potential for harm of a GP pedal bone which is at risk of becoming negatively inclined on impact or persistently.

Which is pretty much the argument for shoes - the harm they do is outweighed by the harm they prevent.

BUT that doesn't mean a 5-10+ palmar angle is or should be the norm for all horses - and, if it is the case that a horse has such a weak heel that it needs extra heel height to avoid over-stressing the coffin joint and tissues at the back of the foot, aren't we ethically obligated to ask whether we should be exposing such a horse to high impact work - with or without that elevation of the palmar processes?

If Dr Strasser's calculations on load on the coffin joint are correct - the steeper the front feet are, the greater the load on the front of P3 and the greater the potential for laminar strain, solar horn damage and damage to the distal edge of P3. And that's aside from the question of how much persistently elevated palmar/plantar processes affect circulation.