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.
