The coffin joint is the first joint in the body to experience ground reaction forces (GRF). The light weight pedal bone (P3) with its fine distal margins, is especially vulnerable to damage from both impact and unphysiological pressure. The palmar/plantar processes of P3 are finer than the main body of the bone. Everything about the way the hoof and limb should respond to load suggests the need for protection of the back of the foot from too much load; so why would or should the coffin joint extend further on touch down or as peak strain is experienced?
The common argument is that, to protect the coffin joint from extending below ground parallel under load (tilting backward), P3 must sit at a positive angle in rest stance. There is no consensus on what this angle should be but, because the front limb carries about 60% of the horse's body weight at rest and is most prone to damage, the assumption is the fore limb P3 should sit at anywhere between 3 and 10 degrees above ground parallel. This often results in the hind hooves having the same or an even shallower angle than the fores and in fact, long heeled, steep angled fores and low heeled shallow angled hinds are disturbingly common.
Because the horse gets most of its rest upright, and as a prey species its key evolutionary imperative is to conserve energy and avoid injury, its skeletal balance is critical to its overall health. The further away from ground parallel the pedal bone is, the greater the impacts are on this essential balancing act. High heels with a steepened P3 throw the balance out; heels that are too low and/or the presence of heel pain which the horse alleviates by for example elevating its pastern, will also throw the the balance out.
The evolutionary advantage of a long limb with one hoof attached to it and no weighty muscle below the knee/hock, has the potential disadvantage of the loss of the shock absorption that is conferred by the pads and muscles of the multi toed limb. This is why the horse has a range of impact dampening and energy absorbing mechanisms - including the suspensory apparatus - to dissipate the potentially harmful effects of concussion on bone and of excessive torque and lever forces on soft tissue.
When in neutral (rest stance) the fetlock joint (FJ) is already extended. It is held in permanent extension by the suspensory ligaments (SL) and by the passive action of the digital flexors which have check ligaments that run from tendon to bone to cut off the muscle belly from excessive strain experienced as the fetlock hyper-extends. When the FJ is at full extension - the pastern can be at, or even beyond parallel to the ground, but, like many other things, just because it can, some of the time doesn't mean it should, all of the time.
The leaf spring action of the pastern is facilitated by the loading surface of the FJ being increased by the proximal sesamoids, to which the SLs attach. The SLs contain muscle fibres which allows a far greater elasticity than normal ligament.
Ligaments in the coffin joint run from the distal sesamoid (navicular bone) to the back of the short pastern bone (P2) and the long pastern bone (P1). These are described as a suspensory apparatus also, BUT they are normal ligament and it seems to me that they do not facilitate extension of the coffin joint but limit over extension.
The coffin joint is not capable of anything remotely like the extension enabled by the suspensory apparatus of the FJ. It's greatest range of motion beyond neutral is flexural; the fetlock's greatest range of motion beyond neutral is extension. And the coffin joint's action is the first to be affected by the impact and resistance of the ground.
Taking the front limb only in order to simplify things, the stance phase of the stride starts when the hoof touches down - which it does as the limb is being retracted and is travelling forward and downward. The hoof touches down heel first but only fractionally. The faster the pace, the greater the load but the more fleeting any given part of the phase is.
There is a very rapid deceleration as the hoof is planted - it goes from movement to planted in milliseconds and huge concussive and torque forces are at work. The faster the pace the more rapid the deceleration is. Initial impact is dampened by several elements: the lateral expansion of the heels, the soft tissue of LCs and frog/DC complex, the blood in the hoof etc.
GRF peak is in the middle of the stance phase when the full body weight passes over the planted limb. Assuming all other things are equal - cannon and radius are vertical to the ground; the CJ and PJ are flexed (they should work together) and the FJ is hyper extended, with the SLs, backed up by the digital flexors, taking the load. This is the point of greatest strain on the suspensory apparatus of the fetlock joint.
The SLs have extensor branches which merge with the common extensor tendon at the extensor process of P3. As the SLs reach peak strain these extensions pull back on the extensor process and have the effect of making the toe more rigid -ie they help to stabilise the CJ.
Late stance is the initiation of break over where the body has to overcome the resistance of the ground on the toe of the hoof. The longer the toe and / or the deeper it has dug into the ground, the greater the effort required and the greater the lever force on the toe.
How the limb actually meets the ground depends on a number of factors. The fractionally heel first landing at speed is only visible under slow motion.
The angle of the scapula is determined in part by conformation but also by the angle of the elbow - which is highly influenced by both the passive and active functions of the digital flexors and the deep digital flexor (DDF) in particular. The weight of the DDF muscle, held in passive tension by the correctly tensioned DDF tendon, holds the elbow joint in extension. Change the insertion angle of the DDFT in the back of P3 or damage the DDF muscle and a domino effect is created.
For example, a long heel (or a too short toe) results in a steeper hoof-pastern angle. This alters the angle of insertion of the DDFT and with that the degree of elbow extension which results in a steeper angle of the scapula as the shoulder joint changes angle to accommodate. With a steepened scapula the horse cannot raise its forearm as far; in effect the protracted limb is straighter and lower and the toe flips up - something which some people think is a good thing because it is seen as necessary for the much desired and grossly misunderstood 'heel first landing'. A toe flip is NOT a good thing.
In other words, if there is too much extension in the knee and fetlock joints during protraction, the horse cannot swing the limb forward as far and the toe flips up, the coffin joint may be too extended when it comes into land. This is as damaging to the joints as the opposite where the CJ is flexed at touch down and the hoof lands toe first, and it is a situation in which the back of the CJ is highly likely to be stressed.
Everything I know - and I may be missing some vital piece of the puzzle - says to me that neutral position for the coffin joint is when P3 is at or very close to ground parallel when the horse is in rest mode - ie that point when the musclo-skeletal system is in an alignment that best permits the relaxation of all major muscle groups. The coffin joint needs to be in, or very close to neutral at initiation of the stance phase as either too flexed or too extended risks stressing the CJ which has the capacity to absorb some stress - but nothing like that which the fetlock is structured to absorb. Even that will break down if the peak GRFs are too much for it.
Dynamic action, however extreme, is fleeting compared to the persistent loading in rest stance but it can (and does) result in catastrophic injury if peak strain exceeds the limits of any given part of the system. The ligaments in the horse's joints are so strong that bone fractures are more likely than dislocations. In rest stance any damage done in movement is repaired. If the rest stance is compromised and as a result repair of damage is impaired or slowed, the animal is more at risk of traumatic injury when peak load is experienced. There are also a whole constellation of other consequences.
If the caudal hoof is deep and strong (ie vertical integrity of heels and bars, robustness of lateral cartilages, and of the frog/digital cushion complex); the suspensory apparatus is healthy, and there is a balance between strong healthy flexor and extensor muscle systems - I cannot see why P3 can or should tilt caudally either on initial impact or when the suspensory apparatus is under peak strain. If it does occasionally do so in a healthy hoof which is being subjected to an extreme load, the structures of the CJ should be able to absorb it. Problems arise when less than optimal structures are exposed to extreme load and this is made much worse when that load is repeated over and over.
All other things being equal, the horse will seek to conserve energy and avoid injury. WE make it do stuff that runs completely counter to that key imperative. We force it to run faster and further than it would ever choose or need to in nature; to leap obstacles it would prefer to go around, and to maintain display postures past the point even the most testosterone fuelled stallion would choose to - with the added burden of saddle and rider and the effects, on respiration, of a bit in its mouth.
Too often the demands of utility - our needs - ride rough shod (pun intended) over those of the horse. And too often healthy debate about these issues founders on the rocks of sectarianism and vested interest.
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