Hoof Construction And Function In The Modern Horse
The hoof is evolving into a weaker less capable unit, probably as a direct result of mans intervention during the domestication and interbreeding of the modern horse.
Fundamentally, the feet and subsequently horses have become more prone to lameness as hoof strength has decreased.
Horn is derived from skin, effectively hard skin (keratinised) with each area of the foot modified the degree of hardness or horn texture suitable for function. The hoof wall is the hardest with the toe region being thickest and therefore strongest, wall thickness and height reduce gradually towards the heels.
The sole is less dense and built in layers, the depth of sole per individual hoof is dependent on the shape and position of the pedal bone, conversely the position of the bone is largely dependent on the conformation and in breed foot shape.
These two areas are vital to the well being of the foot, their ability to reproduce (grow) and maintain sufficient strength to resist the weight of the animal above is paramount. A good example of how feet are affected by conformation and shape, would be the horse with ‘odd feet’ the flatter foot has weaker shallower horn to all components whilst the more normal, possibly upright foot has denser stronger horn with a much deeper concavity to the sole. Odd feet occur in the forelimbs, rarely and only due to injury in the hind limbs. This observation deserves some explanation as the answers to the overall decline in hoof changes may be detected. The flatter foot is created by overload and excessive limb dwell, the more upright foot is produced by reduced load and reduced dwell. Dwell time is critical and relates to the overall time the limb spends under load during the vary phases of support.
In a modern sports horse for example, many factors may combine to produce excessive limb dwell, a poor fitting saddle, poor fitting rider, poor respiration, excessive lunging, inadequate shoeing and so on.
The quality of the hoof is dependent on so many factors from feed through environment, to type or breed, male or female. Stallions generally have strong feet with good growth of horn especially in the sole. Mare’s feet flatten and spread and often crack during pregnancy.
Horses feet can survive the extremes of frozen snow covered ground to hot sand in the desert, without freezing or boiling. The blood vascular system within the hoof is enhanced by an additional system known as Arterial Venous Anastomosis, these are vessels within the hoof coria including the sensitive laminae, which allow blood to shunt from artery to vein thus bypassing soft tissue. As each foot supports the weight of the animal the hoof is caused to change shape, at speed the compression of soft tissue (containing blood vessels) is so great against the horn that incoming arterial blood has to be shifted through the by-pass system. The vessels within the soft tissue are temporarily shut down and unable to receive supply. This system is seen to work to an excess in flat feet, the AVA’s overwork due to constant compression, horn growth is reduced through lack of nutrition and the hoof horn retained in a softened weakened state.
This article sets out to establish the fundamental natural requirements of the feet and their role in protecting and performing as part of the limb.
The hoof is he only contact with the ground and is designed with all terrain conditions. Imagine a mature horse standing unshod – all square on a level surface. All four feet are concerned with organising their respective limb and thereby organising the adjoining skeletal and muscular activity. Both pairs of limbs work together to arrange the supporting and suspending of body weight and delivering the power of movement and propulsion.
Feet determine posture in the individual limb and the part played by that individual limb in posturing the animal. Should components above the foot be misaligned as in a bone or joint deviation, the foot may be compressed unevenly and change shape accordingly. Since the hoof relies on compression to operate circulation, any shift may compromise blood flow locally. A misshapen, distorted foot addressing the ground causes adverse limb action with a subsequent reduction in limb stability and increased compensation in the remaining framework of the animal. In many cases these adverse uncontrolled movements create at very least reduced performance, muscle soreness and compression related lameness in the foot.
Front feet are naturally rounder than hind feet. Hind feet have a deeper concavity to the sole with straighter toe quarters. These variations are functional. Front feet offer stability at the base of a limb secured by muscle to the main chassis and help to distribute load over a larger surface area. Hind feet transmit power to ground, with a direct drive through pelvis, stifle and hock. The more ‘pointed’ toe of the hind foot cut into the ground on forgiving surfaces.
Basic guidelines to foot geometry relate to conformation and their functional relationship to the system, and are often drawn in straight lines embossed upon a static animal. Hoof and pastern axes are seen to be similar to shoulder axes in the forelimbs, in reality these axes change with hoof growth, should an animal be seen to have a perfectly aligned axes on day one, with correctly trimmed feet, on day thirty five (5 weeks) this alignment will have undergone insidious axes change as inevitably toe growth is greater than heel growth in the vast majority of horses. This observation would mean that the system is more efficient on day one than in the later stages of trimming/shoeing. Alternately having developed the assumed axes over a period of weeks, then the growth is removed in a matter of minutes to assume the 'new' posture. Often horses are seen to be ‘short' of stride following these immediate changes, more as a result of muscular reaction in the upper limb.
The coronary trajectory should be used to relate fore and hind limb synchronisation and check the ‘correct' posture of the specimen. With the animal stood ‘square' the hind foot trajectory, determined by the coronary band where hair meets horn, should arrive at the forelimb within the upper to lower limits of the knee. Should the line of trajectory be above the knee, towards the elbow, or into the ribs, then the hind toes are too long, this often means the hind heels are too low and weak. Limited power can be delivered by the hind limb in these circumstances with the sacroiliac area of the spine prone to stress, effectively creating a power blockbetween hind and fore limb. The fore foot trajectory should arrive at a point immediately beneath the centre of gravity. At any point closer to the heel, the limb will be retarded by the additional effort required to lift the limb over the foot. Often the toe is long and again the heels are low and weak
All sports horses could be ‘set up' using this method unshod or shod, although by the very action of attaching shoes the equation outlined above is more prone to abuse. In shod feet the toe grows reasonably unhindered whilst the heels are compressed and subsequently worn away on the shoe as they are trapped and sandwiched between mobile body weight and uncompromising steel.
Walking a horse in hand is commonly requested by veterinarians and farriers and is hugely affected by the person leading the animal. Should the head be held or pulled off centre the manor in which the front feet meet the ground may be altered. Ideally, as the animal is walked towards you, the feet would come to earth in a horizontal plane, not landing too one side or the other. In instances where a foot or feet land on one side and then ‘slap' down on the other side, mediolateral alignment of the limb, not just the foot, should be examined. e.g. Horses that rotate a limb outwards as they walk will land on the outside first, then ‘slap' down on the inside.
Some evidence of posture with subsequent compensations may be observed in foals where more angular detours are apparent in forelimbs than in the corresponding hind limbs, probably as a direct result of the body being ‘slung' between the forelimbs. These functional observations may be carried forward into the mature animal where hind limb posture is closely affected by trimming and shoeing. Here a direct skeletal link from feet to spine can be ‘set up' using the geometry outlined. The animals ability to drive from behind may therefore be organised from hind foot angle and limb axes, determined via the coronary trajectory.
Thoroughbred feet have a strong influence on today's horses, their finer coats and thinner skin reproduce weaker shallower horn, many thoroughbred foals appear to have flat soles and very weak heels from birth. A direct comparison may be made with the warm bloods; here feet are often stronger at the heel with a more robust wall and deeper concave sole. Frequently the horn of the sole is retained, this may reduce hoof physiology and cause the hoof to tighten or contract slightly. Skilful trimming is required to retain natural hoof activity when under load; this means that the right amount of sole should be removed at each trimming.
Too many horse today suffer early lameness as a direct result of weak feet. Should feet have weak overloaded heels with flat thin soles, then these are the outward signs of collapsed feet. This means that there has been a downward shift in the pedal bone. The process may be slow and take months or even years to manifest as lameness detectable to the foot. Alternately, with excess toe levering the pedal bone backwards onto already weal heel the shift may be in weeks. Either way recovery takes a long time even with the modern farrier/ vet aids to hand. Weak feet are not capable of sustaining the weight of the animal. Load is thereby increased to all other limb structures, joints, ligaments, tendons, muscles etc.. As feet collapse, effectively the pedal bone is now nearer the ground, laminae around the toe have been at least inflamed (low grade laminitis) or even torn, in either case the abused laminae produce a scar horn in an attempt to protect from further damage, an aberrant horn is produced in an horizontal plane between the laminae and the wall, effectively pushing the wall forward away from the bone. Laminae are now concerned with production not bonding; this allows the pedal bone to shift more easily. Once activated this process is difficult to control. In all cases hind feet have collapsed ahead of front feet, this relates immediately back to the fact that the whole of the animals framework is involved and that feet are either the product of the biomechanics of the individual or the victim of the manner in which we breed and manage the modern horse.
Elizabeth Launder