Navicular Disease

If you are not quite sure what navicular disease is, join the crowd. Veterinarians don't agree as to what it really is. They prefer to call it "navicular syndrome," which signifies that a variety of problems can cause the disease. It is not always possible to pinpoint the exact cause of the navicular syndrome. Because there are a variety of causes, no one treatment is successful.
The navicular bone is a small bone in the foot of the horse. It is a part of the bony skeleton of the leg, sitting at the posterior aspect of the joint where the third phalanx bone articulates with the second phalanx bone. The third phalanx is often called the coffin bone in the horse because it is encased in the hoof. The second phalanx is often called the short pastern bone.
The navicular bone is held in place by ligaments which arise from the dense connective tissue of the bone surface and stretch to the adjacent bones. Along the posterior aspect of the navicular bone and curling underneath it, the deep digital flexor tendon stretches to attach firmly to the under surface of the coffin bone.
As the coffin joint flexes, the deep digital flexor tendon glides against the navicular bone. This rubbing action would cause tissue damage and pain if it were not for the bursa which is located between the tendon and the bone. In the normal state, this bursa is filled with a lubricative substance which eliminates pain and tissue damage.
Definition and causes
If it were possible to define navicular disease, it would be a simple matter to describe the cause. Navicular disease has never been adequately defined, thus the term navicular syndrome is used by veterinarians to denote this fact. From time to time various theories of development have been proposed. None of them answer all of the questions about the syndrome.
Probably the oldest idea of cause is that the navicular bone undergoes changes which bring about pathology and pain in the area. While it is true that the bone often undergoes changes which can be seen in radiographs, many cases of navicular disease have no evident bony change. A dozen years ago veterinarians generally believed that the condition could be diagnosed by careful study of radiographs of the navicular bone, and certain "tell tale" changes were "proof" of navicular disease. Now most veterinarians realize diagnosis is not that simple.
Changes in the navicular bone do occur from horse to horse, just as changes always occur in bone. Certain types of foot conformation causes more pressure to come to bear on the navicular bones, from the flexor tendon which presses hard against it. Continual heavy pressure on small bones cause them to harden up, or become more dense. As this occurs the blood supply to the inner part of the bone is shut off. A popular theory of cause of navicular disease is that ischemia (reduced blood supply) is produced and this causes more damage.
The ischemic theory was very popular a decade ago. It was thought that blood clots in the capillaries supplying the navicular area brought about the ischemia. The improvement many horses showed following treatment with drugs which improve capillary blood flow was proof enough for many. Over time, however, most veterinarians have come to believe that navicular ischemia is not the entire answer to the problem.
Another early theory of cause was that the navicular bursa was at fault. Like the bursa which causes "tennis elbow," loss of lubricating fluid and inflammation causes pain and can lead to permanent damage to the bursa. Of course this may be the only cause of the syndrome in some cases, but it is not the entire answer by any means.
Some cases of navicular syndrome are caused by pathology in the coffin joint. With today's sophisticated diagnostics, however, this situation can usually be differentiated from navicular bone pathology. Sometimes navicular bone degeneration can occur along with degenerative joint disease in the coffin joint.
There is no doubt that fracture of the coffin bone will result in navicular disease, and it does sometimes happen- but not often. A fracture is easily diagnosed with x-ray. Usually, however, there are other causes of the syndrome.
Diagnosis
When a horse is lame, you or the veterinarian begin a process of diagnosis to determine the cause of lameness. When it is suspected that the lameness is in the foot, usually the front foot, navicular disease is the first thing that comes to the mind of most people. There are other causes of foot lameness, however, and these must be eliminated before navicular disease can be pronounced as the cause. A crack in the coffin bone could be the cause. Radiographs will generally determine this. A simple sole bruise, or heal bruise, might be the cause.
Selective nerve blocks are the most important technique for differentiating various causes of foot lameness, but they have their limitations. Veterinarians who are adept at injecting the navicular bursa can eliminate that as a source of the problem if after injecting a local anesthetic in it there is still lameness. Generally speaking, however, the best veterinarians can do is to pinpoint the source of the pain to the central portion of the foot, surrounding the navicular bone.
A combination of nerve blocks and radiographs is the most commonly used diagnostic techniques. Even then, diagnosis of navicular disease is not certain. The best that can usually be hoped for is a pronouncement that the cause is the navicular syndrome, which could be one or more of a variety of pathologies in and/or around the navicular bone.
As difficult as it is to diagnose the navicular syndrome, it is even more difficult to determine if a horse is going to get navicular disease. This is why veterinarians do not generally prognosticate about a possible navicular problem when doing a pre-purchase veterinary exam. There is simply no way to tell for sure that a horse is going to have a problem.
There are predisposing factors for navicular disease. Younger horses tend to be effected more. Certain breeds of horses are more effected. This relates more to size of the feet in relation to the size of the body, than to breed. Quarter horses do have a higher incidence of the problem than other breeds. The smaller the foot on a large horse, the more likely the horse is to develop navicular disease. The activity of the horse may be an important factor. Barrel racing, for example, puts a lot of stress on the horse's foot, and can tend to lead to navicular disease, some people believe. Contrary to the activity theory of predisposition is the genetics theory. There is at least one study which shows a genetic predisposition to navicular syndrome. This may relate to the conformation of the foot and the natural angle of the pastern. Some believe that the more upright the pastern is, the more likely the horse to develop navicular syndrome.
Treatment
Bute is probably the most commonly used drug for not only navicular disease, but other common lamenesses in the horse. Bute tends to reduce the pain and inflammation in early stages, and is therefore effective.
When it is suspected that the pain originates in the navicular bursa or the coffin joint, injection with steroids is a common treatment. This is not really a cure, since steroids only reduce inflammation and do nothing to correct the underlying pathology.
Various techniques for increasing the circulation to the navicular area are effective in reducing the symptoms. The most commonly used is administration of isoxsuprine hydrochloride, a drug which causes dilation of the small blood vessels. This is a long course of regular drug therapy which can get expensive and time-consuming. Recently, a magnetic hoof pad has come on the market which is said to help relieve navicular pain. The theory behind its use is that magnetic therapy increases local circulation.
The anticoagulant warfarin is sometimes used for the same reason, to increase local circulation. It also is a rather permanent course of therapy. The horse must be watched carefully for bleeding problems, since the drug reduces clotting in the entire body.
Corrective shoeing is generally used, especially in the more chronic cases of navicular disease. Each horse should be trimmed according to the particular conformation that needs correct. A variety of shoe types have been used, including the egg bar shoe, the rolled toe shoe, the slippered heel shoe, the Tennessee navicular shoe, the wide web shoe, and others. All have been effective on specific horses, reflecting the fact that the navicular syndrome has multiple causes.
Neurectomy is a drastic treatment, but is often done in long-standing chronic cases of navicular disease. The nerve to the foot is cut above the fetlock, and the entire foot is permanently deadened. This causes the horse to stumble often because it cannot tell exactly where the feet are. Needless to say this is not a solution for a riding horse.
No other lameness in the horse invokes such a wide variety of opinions about definition, diagnosis, and treatment than does navicular disease. Ask a dozen veterinarians and you will get nearly a dozen opinions. The consensus, however, is moving towards a broad definition and the use of navicular syndrome for the problem.
For a detailed modern description of this problem, read the book Navicular Syndrome in the Horse, by David W. Ramey, DVM, published by MacMillan. Dr. Ramey writes, "Most likely no condition of the horse's limbs is as feared by horse owners as is navicular syndrome. Probably no condition is as frequently suspected as a cause of lameness in the horse as disease of the horse's navicular bone. Certainly, few conditions that cause lameness in the horse are as frequently diagnosed as is navicular syndrome."
By William E. Jones, DVM
Equine Foot Anatomy. The navicular area is shown, with its relationship to the navicular bursa, the coffin joint, and the deep digital flexor. Note the ligaments attaching the navicular bone to the adjacent bones.