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The horseshoe nail from a veterinarian’s perspective

Roland Perrin, Veterinarian, ECVS Graduate

Introduction
Horseshoe nails are used to secure a horseshoe to a horse’s hoof.  They are one of the key elements of the shoeing process and have still not been replaced, despite numerous attempts to glue horseshoes to the hoof. In this presentation, we will first provide some general information on hooves and the shoeing process. We will then proceed to important information on the horseshoe nail itself and to a description of veterinary problems related to horseshoe nails.

The hoof:
The hoof wall and the coffin bone are closely related, since the horn and the corium insert into each other. This can be considered as an enthesis of the hoof wall and the coffin bone; an enthesis is the point at which a tendon inserts into the bone. Thanks to this enthesis, the horse is suspended in its four hooves. The hoof wall is elastic and is deformed when it hits the ground. As a result, the resonance and vibrations produced upon impact are reduced and more evenly divided. This relieves the horse’s bones and joints. The enthesis between the hoof wall and the coffin bone and the elasticity of this wall are two important elements of the impact-absorbing mechanism. The hoof’s innervation depends on mechanoreceptors and nocireceptors. Mechanoreceptors are sensitive to slow pressure differences (baroreceptors) when the horse is standing upright as well as when he is moving. They give the horse a very pleasant sensation of the ground. This led our ancestors to say “that the horse could see through its hooves”. Nocireceptors are spread over the hoof and it can be assumed that a substantial number of them are present in the junction between the horn and the corium, since such entheses in other body parts also contain many pain receptors. Depending on the pressure, the horse will have a sense of comfort or pain while resting or moving. As the horse is unable to control the positioning of its hooves efficiently during trot or canter, this sensation will be experienced even more strongly by the horse from an emotional perspective.
In case of oedema, the hoof wall is stiff and does not allow the soft tissue to expand. This can lead to the extremely painful compartment syndrome. Since the corium tissue is put under heavy pressure upon impact, an inflammation may occur with secondary oedema, which results in pain.

Shoeing:
The hoof wall gradually grows thicker in the direction of the distal edge. This progression is called hoof growth.  The growth of this wall depends on numerous parameters, such as the forces exercised by the ground, which can have a stimulating or suppressing function based on the physical activity. As the hoof wall grows, the sole of the non-shoed foot will present wear as a result of contact and friction of the wall with the ground. This is called natural wear if it occurs as a result of the horse’s conduct. It is called artificial if it is the result of human intervention. In that case, it is called trimming. The shoe is intended to protect the hoof from wear and destructive forces to which it is exposed, to increase the grip on slippery ground, to improve locomotive performance and to increase the horse’s sense of comfort. One of the most important aspects of the farrier’s art is the proper distribution of pressure by means of trimming and shoeing. The horseshoe nails keep the horseshoe in place on the horse’s foot and help the horse maintain his stability.

Horseshoe nails
Definition
Horseshoe nails consist of a head, a neck, a shank and a point. French horseshoe nails have a square head in the form of two frusta whose bases are positioned against each other. The lower frustum forms the neck, which is as long as the horseshoe is thick and thus defines the size of the horseshoe nail. The crown is the upper part of the head and is perpendicular to the shank. The shank follows the neck. It has a flat and a hollow side with smoothed edges in the form of the hoof. The edges are smoothed to prevent the hoof wall from crumbling when the nail is driven into the hoof. The nail is thicker at the edge and the point is bevelled. It is interesting to compare these nails with surgical suture needles. They have a special point for each anatomic structure and a profile that allows the nail to be driven into the structure without the use of force and in a direction determined by the surgeon. The farrier and the surgeon thus have the same objectives: driving the nail in with great precision and with the least possible damage to the anatomic structure.

Nailing:
Horseshoe nails are driven into the hoof vertically on the white line in the direction of the hoof wall. The nail first runs straight and then curves. This is caused by two forces: a straight force which follows the nail’s axis when the nail is driven into the hoof and a curved force arising from the hoof wall’s resistance against the bevelled point. These two forces determine how the nail is driven into the hoof. The resistance and the sound are good indicators for this. When the point is inserted into the deeper layers, the hoof wall’s resistance is weak and the sound is muffled. Once it enters the superficial layers, the resistance become heavier and the sound becomes clear.
The farrier uses light taps to drive the nail into the hoof until he hears a clear sound. After that, he taps both the nail and the horseshoe. The nail is then wrung off and clinched. The nails are clinched to form a square surface at the outer edge of the hoof wall. The nails should not be weakened when the clinches are smoothed off. They are driven into the front of the foot. We can see another similarity with the surgeon’s practice here, particularly with an orthopaedic surgeon reconstructing fractures by means of ostheosynthesis. The principles of fixation by means of screws and pads are the same. De head of the screw is positioned in a hole of the pad and is then embedded in the hard part of the bone to keep it in place and sustain the fracture.

Problems related to horseshoe nails:
Street nail:
Foreign objects that hurt and irritate the horse can enter the sole at different points. This condition is called «Street nail». Depending on the penetration point, the inclination and the length of this object, different anatomic structures may be affected: the coffin bone, the digital cushion, the deep digital flexor tendon, the navicular bursa, and the distal interphalangeal joints. The most serious problems include a disease of the navicular busta, which can lead to stenosing tenosynovitis, or a disease of the distal interphalangeal joints, which can lead to septic arthritis. The treatment is surgical and is based on flushing with physiological saline and antibiotics using tenoscopy or arthroscopy.

Superficial or deep hot nails:
These are injuries of the internal organs of the foot caused by nails used to fix the horseshoe.
In case of deep hot nails, the nail directly injures the soft tissue of the hoof and causes purulent inflammation. In case of superficial hot nails, the corium tissue is affected. This leads to aseptic pododermatitis. In case of a deep hot nail, the horse reacts immediately. It is recommended to remove the nail and disinfect the wound. If the nail is clinched, this may lead to purulent pododermatitis. The horse starts limping, the lameness becomes worse and purulence emerges.
If the foot has been clinched by a nail, this nail will compress the corium, without injuring it directly. If the nail is close to a young and wet hoof wall (laminar or tubular), the sulphur in the keratinized cells reacts with the iron of the nail, which leads to the formation of iron sulphide and hydrated iron oxide. The hoof wall at the supporting edge colours black and appears “burnt”. The nail must be removed.

Position of the nail:
The position of the nail is very important. If it is placed too low, the horn is damaged. If it is placed too high, this may lead to a deep or superficial hot nail. If the nails are driven too deep into the hoof wall, it grows insufficiently during the shoeing interval and the traces of the nail cannot be removed by trimming. The horseshoe nails end up in the wrong position. This can result in a deep hot nail if the hoof wall has been trimmed too much. If, in case a pad is inserted during the shoeing process, the pads move or change in thickness, the clinches may release and the resulting movement of the horseshoe nails can lead to pododermatitis. In certain cases, the corium tissue may be inflamed and oedema may appear. This soft tissue can then make contact with the nails and lead to a hot nail. If the nails are positioned too much to the rear, the hoof capsule may lose elasticity, no longer fulfil its role as an impact absorber and offer no more relief to the bone tissue and joints. If the horse is in pain due to a nail, the horse’s biomechanics will change, which may lead to other problems. If the horse feels pain in its foot, he may try to compensate for it by placing his foot in such a way that the deep digital flexor tendon is tended just before it impacts the ground. This compression may result in overload of the  navicular bone system. The horse’s movements will be less precise due to a loss in proprioception and the interphalangeal joints, the ligaments, will be overloaded. A simple pain resulting from a nail can thus have far more serious repercussions than a simple hot nail.

Conclusions:
The specific anatomy of a horse’s foot and the principles of shoeing and, in particular, of the fixation of the shoe using nails can strongly influence the horse’s comfort and thus his locomotion. The position of the nails is therefore just as important as the prescription of a certain type of horseshoe. During his discussion with the farrier, the veterinarian will have to take this into account in his prescription of a horseshoe for horses that are clinically or subclinically lame.

 

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