The hooves of a horse endure significant forces. Not only does the body weight of the horse need to be carried; it is primarily biomechanical forces that burden the hooves. The hooves absorb the impact when the horse puts its hoof on the ground. Subsequently, they come under great pressure during the rolling motion. The faster the gait, the greater the load on the hooves.
When will things go wrong?
In a healthy, well-developed and correctly trimmed horse’s hoof, every anatomical part is perfectly capable of absorbing these forces. Something goes wrong when it is mainly the hoof wall and the lamellar connection that have to do this job. This happens in hooves with an excessively long hoof wall and in shod hooves. We call this ‘peripheral loading’ (peripheral=located on the outside or circumference). The parts of the hoof that are ideally suited to serve as shock absorbers are, so to speak, sidelined. These are the sole and the tissues in the back of the hoof (hoof bulbs, hoof cartilage, digital cushion and frog).
Hydraulic shock absorber
Normally, the blood mass in the hoof also catches much of the force. One could compare this to the action of a hydraulic shock absorber. According to Dr Robert Bowker, the blood circulation in a peripherally loaded hoof comes to a complete standstill with every step. The shock-absorbing effect of blood inertia is therefore much lower than it can and should be.
The greater the peripheral loading, the worse the blood flow to all of the hoof’s tissues. Among other things, the sole suffers greatly from this. The blood supply to the sole is via arteries that wrap around the edge of the coffin bone. Under peripheral pressure, these arteries are pinched off at every step. The solar dermis now lacks sufficient oxygen and nutrients. This has an inhibiting effect on the growth of the sole. A farrier or vet who recommends shoeing until the sole has grown a bit thicker apparently does not take this into account or even knows of its existence.
Dr Bowker spoke at the ‘NO Laminitis! Conference’ of 2017 about a link is between demineralisation (decalcification) of the coffin bone and peripheral loading. Fibres in the hoof dermis, he said, try to cope with constant overload by clinging convulsively to the hoof bone. They penetrate this bone, making tiny holes in it. Each hole means less calcium in the bone. Furthermore, peripheral loading causes overloading of the edge of the coffin bone. Bone that is structurally overloaded also demineralises. The risk of coffin bone fractures and inflammation of the periosteum and the bone itself increases the more the bone demineralises.
Risk of laminitis
It is not only the coffin bone that suffers. All tissues in the hoof are subjected to greater forces than they can handle when subjected to peripheral pressure. The lamellar connection and the wall dermis, because they lie directly under the hoof wall, are in the firing line here. They are hit first and worst. They will weaken and damage earlier when laminitis sets in, regardless of what kind of laminitis is involved (endocrinopathic, sepsis-related or traumatic). So a healthy horse with peripherally loaded hooves is more likely to get laminitis than one where all parts of the hoof are ‘carrying’.
If the horse is already laminitic, the lamellar connection is damaged and sore. Peripheral loading not only causes pain here but also causes painful pressure on the coronary band. By removing the peripheral load during trimming, we often see a great improvement in the horse’s state right away. It has less pain and can move better. This is good for blood flow and thus for recovery.
Under normal conditions, the digital cushion undergoes alternating pressure and pressure relief. It needs this to develop and stay in good health. Under peripheral loading, this alternation is very much less. A poorly developed digital cushion causes toe-first landing; toe-first landing causes or exacerbates traumatic laminitis. If the horse is already laminitic – regardless of the type of laminitis – toe-first landing leads to greater damage and pain to the lamellar connection in the toe. Also, the stretched white line found in horses with laminitis will become more stretched in a peripherally loaded hoof. The white line is simply pulled apart with every step.
Even though tradition in farriery, many vets and our gut feeling tell us otherwise: even horses suffering from a sinker or sole penetration benefit from removing peripheral load. The hoof wall is not made to absorb that much force. With a sinker or sole penetration, the connection between the hoof wall and the internal foot is so damaged that the whole thing collapses. With every step, the hoof wall is pushed up along the internal foot. The problem gets worse and worse. Only when we allow the entire underside of the hoof to carry the weight again, we create a situation in which improvement can occur. Of course, this requires a lot of professional care. The underside of the hoof must be well supported with soft material. After all, the pain will move from the wall dermis to the solar dermis. A sole penetration requires specialised veterinary care to prevent infections.
Thrush and white line disease
As the frog has too little contact with the ground, the risk of thrush increases. White line disease can strike because the horny material of the hoof wall is damaged by overloading and provides access to fungi and bacteria.
Cause: hoof care
You immediately get peripheral strain if the hoof wall is left too long when trimming. Unfortunately, this is a practice that is deeply ingrained in many farriers. Many people still think that the horse walks on its nails (read: hooves) and hangs with its full weight on the dermal connection between the bone and that nail. While the hoof wall should carry some of the weight, it should certainly not be the sole supporting part of the hoof.
Drastic trimming of the frog, cutting the bars too short and hollow trimming of the sole contribute to peripheral loading. Too much time in between trims obviously also causes the hoof wall to become too long. Besides the length of the hoof wall, one can often recognise peripheral loading by the fact that the growth rings take on a wavy shape.
With a shod hoof, peripheral loading is logically inevitable. All the force with which the hooves hit the ground and with which the horse propulses is transferred to the hoof wall and underlying tissues via the shoe. The vibration of the shoe certainly does the hoof no favours either. There are therapeutic shoes that should distribute the force better to the underside of the hoof, but it is still carrying water to the sea.
The use of hoof boots is also not blissful with regard to peripheral load, by the way. Normally, a soft surface distributes pressure evenly over the entire underside of the hoof. When using hoof shoes, this is less the case. Incidentally, this can be remedied by using soft pads.
The surface that the horse moves over and is housed on also plays a role. The harder the ground, the greater the peripheral load. Since a firm surface is good for the hoof mechanism and therefore good blood flow, we must ensure that the hoof wall is kept short and the horse is possibly ridden with shoes and pads. Pea gravel and sand appear to be the best surface to minimise peripheral loading. This surface distributes the force evenly over the entire underside of the hoof. The speed at which blood flows through the hoof goes down, ensuring that all blood vessels are well filled. In an ideal situation, the horse has the choice of both a hard surface and one made of sand or gravel. In a paddock paradise, this can be achieved.
If peripheral loading is present, its effects are more severe if the horse spends its days stabled and thus not moving enough. Not only do the hooves then wear down too little; the problem of lack of alternating pressure and pressure relief of the digital cushions and pinched arteries described above are a bigger problem.
Horses that are overweight have the additional problem of the hooves carrying all that weight, making the peripheral strain even worse for them. Weight loss is already important for almost all horses suffering from laminitis. Now you have an additional reason to fix this problem.
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