MOST fractures associated with the racehorse involve the bones which form the fetlock joint, the lower cannon and the pastern bone, or the small bones in the knee.

Within the hoof there are two main bones, the pedal bone and the smaller navicular bone. The pedal bone is the large bone that attaches to the hoof wall and provides strength and stability. The navicular bone meanwhile is a small bone, which lies behind the pedal bone.

A large tendon called the deep digital flexor tendon runs over the surface of the navicular bone and attaches onto the pedal bone allowing for flexion of the lower leg.

The small navicular bone thereby acts as a fulcrum for the deep digital flexor tendon.

PEDAL BONE FRACTURES

The main causes of pedal bone fractures in the racehorse differ between the forelimb and the hind limb. In the forelimb, the main cause of pedal bone fracture is excessive forces applied unevenly to the foot.

Such conditions may be encountered when horses travel at high speeds on a hard uneven surface. In the hind limb, the main cause of fracture is direct trauma from a horse kicking a solid object such as the stable wall. The pedal bone will generally either fracture through the centre of the bone or in one of the outer “wings”, with the later being the most common.

Occasionally small chip fractures are seen at the edge of the bone. The severity and prognosis of the fracture depends on whether it extends into the coffin joint, which is formed by the pedal bone and the short pastern bone.

Fractures which extend into the coffin joint tend to be more serious, for two reasons. Firstly, as the horse bears weight through the coffin joint there will be some movement of the fractured bone therefore taking more time to heal. Also if the fracture involves the joint surface, it will lead to arthritis.

Horses with pedal bone fractures will be very lame on the affected leg. The horse will have a strong pulse to its foot, its hoof wall will be hot and there will be a response when hoof testers are applied. This can often make a correct diagnosis difficult initially as these symptoms are the same as those seen when a horse has an abscess in its foot.

Definitive diagnosis of a fracture in the pedal bone is achieved by obtaining an x-ray of the foot. The fracture line may not be visible straight away but after a few days as healing begins the bone around the fracture becomes less dense and hence the fracture will be visible on X-ray.

The most common treatment for a horse with a pedal bone fracture is to fit a bar shoe with side clips.

This prevents expansion of the hoof wall and heels and thereby compresses the pedal bone-facilitating repair of the fracture.

The horse will also remain on box rest for a significant period. Unlike fractures of other bones where their repair can be monitored by X-ray, this is not the case with the pedal bone. The fracture heals by fibrous tissue, which is less dense than bone and hence does not show up on X-ray.

Since veterinary surgeons cannot rely on X-rays to determine how successfully the fracture is healing, we instead rely on the horse to let us know. We achieve this by slowly reintroducing him to work and closely monitoring his soundness as the work increases.

The prognosis for return to racing following a pedal bone fracture is generally good.

NAVICULAR BONE FRACTURES

As mentioned above, the navicular bone is a small bone, which sits at the back of the pedal bone within the hoof. It acts as a fulcrum for the large deep digital flexor tendon and as the coffin joint flexes this tendon glides over the surface of the navicular bone.

Fractures of the navicular bone occur as a result of trauma, in the hind limb this is generally caused by the horse kicking a solid object. Horses with a fractured navicular bone will have a sudden onset of severe lameness and show the same symptoms as a horse with a fractured pedal bone. Fractures of the navicular bone however are a far less common injury in the racehorse.

Treatment is also similar to that described for a fractured pedal bone. The horse will require a long period of box rest. Bar shoes will also be fitted to give as much support as possible to the heel thereby relieving pressure on the navicular bone.

Healing of the navicular bone is poor and the prognosis for a return to racing is poor following a fracture. Here at Mark Johnston Racing, we have the facilities onsite to accurately diagnose these fractures should they occur to a horse in training.

The veterinary surgeons will then work alongside the team of farriers to treat the injury. These costs are included in the MJR daily training fees.

A graduate of University College Dublin, Laois native John Martin is in practice at Mark Johnston Racing. This article first appeared in the Kingsley Klarion.