10. Navicular bone
A horse has four navicular bones: one in each hoof. It is a small bone that is positioned crosswise at the bottom of the hoof. A tendon runs over it to the bottom of the coffin bone. This tendon applies pressure to the navicular bone and that pressure can be quite high during, for example, the landing following a jump. But in horse jargon, the term ‘navicular bone’ refers to a certain disorder: if the quality of the navicular bone is insufficient and may give cause to lameness. That bone quality is an hereditary factor in horses. Which is why stud-horses are radiologically tested for the quality of their navicular bones.
Navicular bones of an insufficient bone quality become painful when subjected to pressure. A horse with such navicular bones will eventually become lame. But because the deviation usually occurs in both front hooves, the lameness is not always obvious. It often is not noticeable until the animal is required to jump. A horse that refuses to jump an obstacle may do so due to ‘navicular bone disease’. Years have been invested in the breeding and training of the horse, only to discover later that it is not suitable for equestrian sports. If the animal has meanwhile been sold, this will undoubtedly lead to major problems: purchase disputes and legal proceedings are not uncommon in such cases. Which is why it is presently customary that a veterinary inspection of a horse prior to its sale/purchase includes examining X-rays of the hooves and joints of the animal concerned. The inspection itself and making and assessing X-rays is not an easy task; this has led to a specialization within the profession.
The inspection should preferably take place before investing in the animal’s training. But too young is not advisable either, as sufficient bone maturity is required for a sound assessment concerning the future performance of an animal in equestrian sports.
the navicular bones are subjected to great pressure during a landing
Horses suffering from ‘navicular bone disease’ are useless in sports. And you cannot use them to breed good quality foals as the disorder is hereditary. Which is why these lame animals were administered the painkiller phenylbutazone on a large scale in the past. That made it possible to ride them at least. But once the medication wore off, the pain was more intense because of the unrestrained pressure in the foregoing hours. It is currently prohibited to use or dispose of phenylbutazone in the Netherlands.
It is possible to permanently eliminate the pain in part of the hoof. The rear half will become numb by removing a small piece from two nerves in the pastern cavity. And the navicular bone is located in that rear part. The feeling in the front section of the hoof will remain intact following such neurectomy. And so the animal will have a normal walk after the operation. The disorder will still exist after that surgery as the procedure does not improve the bone quality of the navicular bone. The procedure only removes the horse’s discomfort forever because the ends of the nerves do not grow together again. The procedure was carried out on hundreds of horses in the seventies and eighties. Since the nineties however, X-rays of the navicular bones are taken into account in the selection of breeding stallions. As a result, the surgery has become unnecessary in the course of time.
It is a Monday morning when a call comes in concerning a horse that has not been eating properly for a few days. Horse dentists do not yet exist and so teeth problems make up one of the tasks of the veterinarian. There is a horse walker behind the renovated farm of the owner. That is a large carousel-like structure that can be used to walk a maximum of six horses in circles. Three horses are being walked when I arrive: two are in high spirits. But it seems the third is being dragged along by the lead rope of its halter. He is not very into the exercise this Monday morning. “That is Tarzan. Our Rien finished first with him in the Z-class only yesterday. The wood measured 1.30 metres in the jump-off!”
We walk to the stable to see the horse that the farmer had called about that morning. The poor eater has meanwhile finished all of his feed. “Have I ever, his trough is empty! I found yesterday’s feed in his trough this morning.” But the feeding-trough is not entirely empty: something resembling a small stone lies at the bottom. It is a milk tooth. The young animal is shedding its teeth. The permanent tooth had pushed the milk tooth upward out of the jaw. It remained on the new tooth as a ‘cap’ of sorts and caused the animal problems when chewing. But the problem has been solved: the cap let go all by itself. And so we have time to chat: “I bred Tarzan myself.” I ask if he still has the mother. “No, I disposed of her shortly after. She was constantly lame.”
Looking at the horses being walked, I take a chance: “If the painkilling powders no longer help to reduce Tarzan’s pain, you might consider a neurectomy. That can be very beneficial to some horses.” We make an appointment to examine the gelding that same week.
During that examination, Tarzan is walking carefully, a bit stiff actually. He seems slightly lame in the turns. Bending and flexing the front hooves is painful and if I keep that up for a bit he is lame afterwards. After administering an anaesthetic in the left front hoof, he then becomes lame in his right front leg: the pain on his left has been eliminated, enabling him to apply full pressure to that hoof while sparing the other painful hoof. That explains the lameness on the right side. When the pain in the right hoof is eliminated as well, Tarzan seems five years younger: he has a lively trot and cheerfully lifts his tail. The same result can be expected of a neurectomy.
The operation takes place three weeks later. A regional block anaesthetic is applied above the fetlock of both front legs and I inject a sedative to ensure the horse is groggy. Not too groggy, as he needs to remain standing during the procedure. He is then placed in a stocks. This structure was used in the past by the farrier when shoeing horses. I secure the left lower leg to a side beam. The skin in the pastern cavity is shaven and disinfected. I can feel the nerves beneath the skin. After making an incision, I cut away two centimetres from each nerve. The wounds are closed with a few sutures. Done. The leg is released and set back down. Time for the right side. I apply pressure bandages once both hooves have been placed back on the ground. Tarzan must now rest in his box for three weeks, followed by three weeks of grazing in the meadow. No riding until then.
in the stocks with a tied up front leg
How it ends
Afterwards, Rien and Tarzan continue competing and they are winning again. They are even more successful than before. But then things change. Tarzan becomes lame again a year later.
“It’s back.” This time it is Rien who makes the call. “I suppose the nerves grew together again?”
But the rear half of the hoof is found to be still numb. This time, the cause of the lameness is located a bit higher, in the fetlock joint. Once this joint is numbed, Tarzan starts trotting around again. X-rays show the bone quality of the proximal sesamoid bones to be moderate. An operation is not an option. Tarzan will therefore have to be slaughtered. Still, the owner and his son do not regret the surgery: it has been a great year. Not only for them, but for Tarzan as well.
© Leo Rogier Verberne