There is a general consensus in the hoof care community that, in order to be considered healthy and functional, a hoof must display the following characteristics
~ A ground parallel coffin bone (GPCB)
~ Unbroken hoof/pastern axis (HPA)
~ Wide, full bulbs and frog
~ Short, straight bars ramped up to the heel buttress (HB) and not longer than the wall
~ Flat heel platform and strong heel/bar turning point (HBTP)
~ Strong, low heels of adequate height and angle and a short toe
~ Balance both mediolaterally and craniocaudally
~ 2/3 of the hoof should be from widest part of the frog to the apex, with the remaining 1/3 from the apex to the point of breakover
~ Straight, strong walls
~ Solar concavity mirroring that of the coffin bone
~ Dorsal hoof wall angle matching the dorsal angle of the coffin bone
~ Relaxed coronary band, at approximately 30˚ to the ground when viewed laterally
~ Should not be reliant on a fixed device for ‘soundness’
~ An absence of pathological change to and within the hoof capsule
These are considered parameters for a healthy foot regardless of the horse’s age, breed or size, and will be expanded on individually. The overall picture of a healthy hoof should be one of strength, harmony and compactness. Physiological correctness and functionality go hand in hand, with the result being health. Unfortunately, most domestic horses’ feet are not healthy.
In a healthy hoof, the coffin bone will be ground parallel. This results in the even distribution of the weight of the horse across the solar floor and avoids pressure points caused by either rotated or (decidedly less common) counter rotated arrangements. Unnatural arrangements alter the bearing pressures the hoof can cope with as the projected area for the load is shifted rearward or decreased.
In a coffin bone that is rotated above ground parallel (rotation meaning being or having been rotated out of its physiologically correct alignment to the ground and bony column) and into a steeper alignment, the distal tip of the coffin bone points downwards. The weight of the horse is concentrated over the relatively insubstantial distal tip. Bone responds to stress, pressure and load, and this excess pressure can cause the coffin bone to remodel or erode (which may be relatively minor such as in the formation of a crena or “ski tip”, or more severe such as significant bone erosion as seen in chronic founder cases). Forward rotation as seen in a positive angulation to the coffin bone also causes unnatural stress to the coffin and pastern joints as the natural harmonic arrangement of the lower limb is disrupted. The horse will most often make up for this stress by overloading either the flexor or extensor systems. It is normal for the short and long pasterns to angulate backwards and forwards during normal locomotion, but prolonged fixation of the joints in an unnatural arrangement results in stress the limb is not able to comfortably cope with as the associated tendons, ligaments, tendon attachments and muscles are kept under constant unnatural strain.
Backward rotation of the coffin bone (negative plane coffin bone or NPCB) occurs when the coffin bone is tilted backwards, and is on a negative plane as compared to the ground surface. There are very different forces acting on the hoof capsule and skeleton in a NPCB compared to those occurring in forward rotation as certain structures are stressed beyond their elastic limit and internally applied forces are acted out internally. The coffin bone is not intended to bear weight in this counter rotated position; the weight bearing surface, instead of being centred through the bony column to the solar surface of the coffin bone, is shifted rearward onto the palmar processes and can result in palmar bone loss. This causes extreme stress to the rear of the hoof, as well as the coffin and pastern joints, ligaments, tendons, and muscles. Some horses compensate by acute oversteepening of the pastern and slackening the flexors, and thus overloading the extensor system.
The hoof/pastern axis (HPA) refers to the relationship of the hoof to the pasterns. The pastern angles should reflect the dorsal angle of the pedal bone. If the angles between the hoof wall and pastern do not match, the limb is out of correct alignment. The pasterns themselves may not even match each other; p1 may be steepened while p2 is lowered, or vice versa. The joint ligaments are being overworked in any such arrangement, and the coffin joint especially is kept under extreme strain. In the ideal arrangement, with the dorsal angles of p1, p2 and p3 all matching, the horse is able to load its limbs comfortably without compromising or overloading any of the soft or connective tissue systems. Large deflections of load by one member can cause dangerous strains and stresses to other members of a structure as the nature of the normally operating forces in the network are altered and stressed beyond their natural yield point. A system that is overloaded is more likely to reach its fatigue limit sooner as it can never be in equilibrium. If a structure as a whole is to be in equilibrium, then all joints of that structure must also be in equilibrium. A hoof under constant strain cannot be in equilibrium.
Wide, full bulbs and frogs are indicative of a strong and healthy development of the digital cushion/sling (DC), which should be made of firm, shock absorbing fibrocartilage, and make up the majority of the rear of the hoof. It is comprised of both the frog (cuneate) and bulbar (toric) cushions, and so forms the bulbs of the heels and the mass of the frog. The frog should have a shallow central sulcus if the cuneate cushion is well formed. The palmar foot is intended to cope with the majority of the load the hoof experiences under both static and dynamic weight bearing if properly formed, and so it is extremely important that these structures are well developed so that the hoof does not exceed its allowable stress limit under the shock of high impact situations. Underdeveloped digital cushions are unable to comfortably cope with the stresses the caudal hoof experiences during normal locomotion, so the horse will attempt to shift the load to alternate structures less suited to coping with the extreme forces associated with standing and ambulation.
The bars of the hoof must be short, straight, and strong. In a healthy hoof, they will rise from the solar floor up to the heel buttress, end halfway down the frog, be shorter than the hoof wall and will not be laying over any sole. The bars have a dual function; they assist in forming the heel purchase and prevent overexpansion of the hoof capsule upon weight bearing. The bars are the end connections of the hoof wall, allowing non-uniform stress distribution to take place within the hoof capsule by allowing the hoof to stretch and flex sufficiently to dissipate impact stress, while simultaneously retaining the integrity of the hoof capsule by resisting overexpansion under peak impact loading. Conversely, excess bar can inhibit expansion of the hoof capsule so much so that hoof mechanism is impaired and contraction can ensue. Almost all schools of thought agree that bars that are longer than the wall exert tremendous pressure on the structures above them, and can be levered into sensitive places that they shouldn’t be (since there is only sensitive tissue directly above them), such as into or above the frog (it is not uncommon for these type of bars to ‘drop’ after trimming, such is their extent). The bars can extend so far that they are obscured in part by the frog as the frog becomes compressed into the collateral groove. Bars of this nature are often as long if not longer than the walls, which causes bruising and pain, and are often bent into a semicircular shape, with which comes contraction of the heels. Semicircular bars are prone to cracks between the lamellar and any non-lamellar bar as lever forces are diverted to the junction between the two. These cracks can be quite deep and cause great discomfort to the horse if they extend into sensitive tissue. The preventative measure is to keep the bars short, straight and lower than the wall, with the ground surface of the bar trimmed flat.
The heel platform should be what impacts the ground first. It is imperative the platform be flat and level- if it is unlevel, one heel will impact the ground before the other, causing uneven loading of the joints and ligaments and strain on the hoof capsule as it rocks from side to side to stabilise itself as the opposite heel impacts the ground. This causes uneven stress distribution within the hoof capsule, and if prolonged overstresses the collateral ligaments outside of what they are normally able to cope with when travelling over uneven terrain. The platform should also be flat, on the same plane as the toe. If the heel platform is angulated backwards, it moves the bearing surface of the hoof forwards and disrupts the natural biomechanics of the hoof.
The formation of the heel/bar turning point is crucial. The bar needs to be smoothly ramped up to the heel, and the heel needs to flow harmonically down to the bar. If there is a great discrepancy between the horn of the heel buttress and that of the bar, it can weaken HBTP sufficiently so as to allow the heels to overexpand (though for this reason weakening the HBTP is sometimes beneficial in rehabilitating horses with contracted heels) and encourage weak and crooked growth of the bar and modify the natural parameters the hoof has for coping with energy absorption.
Perhaps the most important characteristics of a functional hoof are low heels and short toes- provided that the heels are not low because they are underrun. The horse is unable to load his limbs correctly if his heels are too high (or, conversely, too low). Heels that are longer than ideal experience considerable lever forces, be the heels excessively high or low; the longer the heel the greater the lever forces acting on them, which are quite destructive to the sensitive structures underneath. High heels cause the coffin bone to be rotated above ground parallel; this causes the dorsal lamellae to be overloaded (especially if the horse actively unloads the painful high heels) which could result in flare or separation, and can alter the wear pattern of the hoof, exacerbating the problem further. In the case of chronic high heels it is not uncommon to find associated toe cracks or, in some hoof forms, quarter cracks.
High heels reduce the shock absorbing capacity of the palmar hoof, as the excess of hard horn both resists expansion of the heels and lifts the fibrocartilagenous structures out of ground contact. The forces of impact must be absorbed somewhere, and so the shock is diverted to other structures unable to adequately cope with it. This is detrimental to the horse’s long term health for obvious reasons.
If the horse has high heels stressed beyond their elastic limit, then they will collapse. In the case of the heels that are too low, shock absorption is reduced for a different reason; the heels and bars have flattened the digital cushion so that it loses most of its shock absorbing properties and the cyclic stresses of impact will significantly damage it structure over time. The forces that act on a steep hoof form with steep heels and a low toe are very different to those acting on the reverse, especially upon weightbearing and breakover; although the hoof may be in correct alignment in relation to the ground surface, the coffin and pastern joints cannot be by virtue of the shallow heel disrupting the natural angles of the limb. The horse will also invariably endeavour to unweight the painful heel by shifting weight forwards and onto the toe (which then runs forwards in response…).
In either scenario, heels that are of an incorrect length and height detrimentally effect the whole hoof as a unit, and often the entire limb and horse.
Long toes exert excessive lever forces on the dorsal wall, lamellae and heels, particularly during breakover. The longer the toes, the greater the distance between the coffin bone and the point of breakover at ground level. Long toes interfere with correct protraction of the limbs during locomotion as the horse is unable to fully extend the limb mid stride and so must shorten his stride to compensate, necessitating landing on the toes. The lever forces of long toes pull the entire hoof capsule forwards, and can have a dramatic effect on the angulation of the heels if not rectified. Much hoof contraction and cases of heel collapse can be alleviated or at least improved by proper backing up of the toes. The toes should comprise no more then 1/3 of the total hoof length measured from the frog apex to point of breakover. This enables a correct relationship to be formed between the heels, bars, frogs and toes.
Everyone in the hoof care world agrees that the hoof needs to be balanced; but what is ‘balanced?’ Many generally consider a hoof to be balanced if it appears symmetrical mediolaterally and to have a certain relationship to other hoof or limb structures craniocaudally or proximal-distally. For the purposes of this article, ‘balance’ will refer to a hoof that is equal mediolaterally (heel heights not discrepant; equal amount of hoof medially as there is laterally with allowances for the asymmetry of the hinds) and that, craniocaudally, has a heel purchase at the widest part of the frog, with 2/3 of the hoof being occupied by the length from the widest part of the frog to the apex, with the remaining third from the apex of the frog to point of breakover. Static balance refers to a hoof that is ‘balanced’ when viewed as a static object, while dynamic balance pertains to the balance of the hoof in motion. The main loadings we are concerned about in the equine hoof are tension, bending, torsional and direct shear, and combined loading, which the hoof must be properly balanced to resolve. A static hoof that is not balanced can never find its true translational equilibrium and so will remain in a constant state of stress.
Breakover refers to the lapse in time between when the heel and the toe lift off the ground as the hoof fulcrums over. The breakover of the hoof is considered to be physiologically correct when the relationship of the length from the widest part of the frog to the apex and from the apex to the point of breakover equates to 2/3:1/3. This encourages a correct relationship between digital and mechanical breakover. The more hoof between digital and mechanical breakover, the greater the stress to the hoof capsule. It is the relationship between heel and toe that determines breakover. Too long, and the phalangeal lever action is too great; too short, and the pressures in the hoof are unable to be equalised under static stress.
Straight, strong walls are necessary for proper suspension and protection of the coffin bone, and to provide an adequate weightbearing and wearing surface.
Any time the hoof wall deviates from its normal straight growth, the hard horn is being pried away by the lamellar attachment from structures beneath, causing significant internal stress to the hoof capsule. The strength of the hoof capsule is derived from its shape; as a truncated cone it is able withstand great force, but with deformation of the hoof capsule comes a reduction in its structural integrity. Further, the greater the degree and extent of the flare, the more pressure is put on the remaining lamellar bond to help support the weight of the horse.
Hoof walls should be of adequate thickness and strength with a strong, undisrupted lamellar attachment and short enough so as to be able to share weightbearing responsibilities with the sole. Well formed hoof walls are malleable to a degree and have an appropriate moisture content; tension tests indicate that brittle materials break by direct stress while ductile materials break at a 45 degree inclination to the line of loading. Therefore, the greater the ductility of the hoof wall, the stronger and more resistant to stress it will be.
Stress raising factors can occur in the hoof wall, and reduce its strength in response to load. Stress raisers include sharp corners, cavities, cracks, holes and notches, flaws, abrupt changes in consistency, and small areas supporting concentrated loads. It is therefore preferable that the hoof wall exhibit none of these factors, however the seriousness of the stress concentration depends on both the properties of the material (strength, thickness, ductility of the hoof wall) and the nature of the load (whether static or cyclic). It is required to be strong enough to ably cope with the thrust loads associated with breakover and forward movement.
In order for the hoof wall to provide the optimum degree of resistance to abrasion, it must be thick enough to perform a supportive role at the ground surface, and healthy enough to cope with the abrasion associated with normal locomotion.
A healthy hoof with have a solar surface exhibiting concavity mirroring that of the coffin bone. Solar concavity is indicative of a strong lamellar attachment and is necessary for healthy biomechanics; on weightbearing, as the heels and walls spread the sole must be able to also spread and the solar vault bottom out. If there is inadequate concavity, there is the risk of bruising to the corium as the full force of impact shocks the sole whilst the weight of the horse descends upon a sole that is unable to structurally support it. Flat soles are therefore more prone to stone bruising, tenderness and lameness issues, problems which vary directly in severity with the thinness of the sole. Soles filled in with unexfoliated material can cause the same problems, as they are also resistant to expansion (thus reducing proper hoof mechanism) and prevent the normal descent of the coffin bone by being unable to draw flat.
The dorsal hoof wall angle should match the dorsal angle of the coffin bone; this ensures that there is a constant width to the horn/lamellar zone (H/L zone) and indicates that the while line is not stretched nor the coffin bone displaced relative to the remainder of the hoof capsule (except in the case of non-rotational founder, in which the lamellar attachment fails and the entire bony column sinks down but does not rotate). Studies indicate that the angles of coffin bones are found to be in a Gaussian distribution centred around 45˚ for the front coffin bones and 55˚ for the back. Therefore for the dorsal hoof wall to match the angle of the coffin bone, it too should be distributed around 45 and 55˚ respectively.
The coronary band can be used as a guide as to the orientation of the internal structures of the hoof. A coronary band that is at approximately 30˚ to the ground when viewed laterally indicates a ground parallel coffin bone. An angle steeper than 30˚ indicates that the coffin bone may be on a negative plane, and a coronary band that approaches ground parallel in indicative of forwards rotation.
The coronary band should not be displaced or ‘jammed’ up in places; if it is, that is a reflection of the displacement of the soft tissue structures below and beside it, or uneven pressures acting on the hoof wall. A curved coronary band can indicate that the lateral cartilages have been vaulted up and out of their natural position in the hoof capsule, and can be caused by a number of incorrect hoof forms. It is symptomatic of a greater problem with imbalance and often involves physical trauma to hoof tissues.
A hoof cannot be classed as sound and healthy if its soundness is dependent on the affixation of any external device such as horseshoes. If the hoof is not sound without the device, then it is just not sound. If it is not sound bare, there is a reason; that should not change purely due to the addition of an appliance. This is not an article about shoeing vs. not shoeing; it is simply an article about health and functionality- but suffice to say, a hoof that relies on an external device to be ‘functional’ cannot truthfully be considered to be sound.
It should now be apparent that the absence of pathological change to and within the hoof capsule is paramount to the hoof’s health and functionality. Pathology of any kind varies the internal morphology and causes stress within the hoof capsule, and alters the loading, lever and shear forces outside of the parameters that a healthy hoof is able to tolerate.