[By Douglas Novick, DVM]
The first and most important point of this entire article is that Navicular Disease is NOT TERMINAL. With treatment and care the large majority of horses that have a diagnosis of navicular disease can be managed and returned to their previous level of performance. A horse diagnosed with navicular disease is similar to that of a human track athlete with bad feet. It does not mean that the track star can never run again. What it means is that he or she has to be aware of the problem and take special care of his or her feet. The same is true for a horse with navicular disease.
In this article I will review how a veterinarian makes the diagnosis of navicular disease, a little bit about the anatomy of the navicular bone, and theories on the causes of navicular disease. The options for treatment will also be covered. These include corrective shoeing and trimming, physical therapy, therapeutic drugs, and surgery. Many of you may want to skip directly to the treatment section of this article and then come back and read the rest.
Diagnosis of navicular disease is based on both clinical and radiographic signs. What that means in plain English is that a veterinarian bases the diagnosis on what he or she sees during a physical examination and by looking at x-rays of a horse's hoof area. The x-rays are used to support a diagnosis of navicular disease and to rule out other causes of lameness.
A horse with navicular disease typically presents to the veterinarian with some or all of the following signs. The horse may have a history of front leg lameness. The lameness may be of gradual onset. Many times lameness is present in both front feet. One leg may be worse than the other. The horse may have a history of stumbling or developing a choppy uncomfortable gait. Although any horse can develop navicular disease, the two most commonly affected breeds are quarterhorses and thoroughbreds. The reason these two breeds are most predisposed to navicular disease is that both are very large horses resting on relatively small feet. Although onset of clinical signs can occur at any age, the highest incidence is between the ages of 7 and 14.
When examining the horse, the veterinarian will look for the following signs. The horse will only bring the legs partly forward in an effort to land more on the toe than the heel. This gives the choppy appearance to the horse's trot. The lameness may increase when the horse is turned in the direction of the affected leg. When hoof testers are applied, the horse will respond with pain to their application across the middle third of the frog. This corresponds to applying pressure directly to the navicular bone. On the most affected side, the hoof will often be smaller than the opposite front hoof. This comes from bearing less weight on that foot over time. A common finding among affected horses will be long toes and underslung heels. This concept of long toes and underslung heels is important to understand because it has been my experience that this single problem has led to more horses developing signs of navicular disease than any other.
Horses with navicular disease will respond to further tests. A flexion test of the lower limb will elicit a positive response in these horses. The fetlock and pastern are flexed tightly for 90 seconds and the horse is then trotted off. A positive response is an increased lameness in the first few strides. One of the most distinguishing tests in determining a case of navicular disease involves the use of nerve blocks. A small amount of a local anesthetic (similar to Novocain used by dentists) is used to desensitize an area of the horse's leg. If the lameness is being caused by pain in that area and local anesthesia removes that pain, the horse will stop limping. In the case of navicular disease a small amount of anesthetic is placed about 1 inch below the fetlock on the inside and outside of the back third of the pastern. This will block sensitivity to the back one-half to two-thirds of the hoof. Because this area contains the navicular bone, a horse with navicular disease will no longer feel any pain, and will trot off without limping.
In fact, many horses with navicular disease are lame in both front legs. When first examined they will appear to be lame on only one leg. However, when that leg is nerve-blocked the horse begins to limp on the opposite leg. This pattern of a horse responding to the nerve block on one leg only to start limping on the opposite leg is almost a red flag waving at the veterinarian saying this horse has a problem in the navicular bone.
Finally to support the diagnosis of navicular disease and to rule out other causes of lameness, x-rays (radiographs) are taken of both front feet. Let me emphasize at this point. If changes are found on x-rays in the navicular bone, this does not mean the horse has navicular disease. ONLY if the horse has clinical signs consistent with navicular disease and changes in the navicular bone on x-rays should a horse be diagnosed with navicular disease.
This is a constant dilemma in pre-purchase examinations. Many horses will have changes in the their navicular bone on x-rays which are consistent with navicular disease. However, they will have no clinical signs or lameness. Therefore, they do not have navicular disease. In one study of several hundred horses almost one-third of the horses which were perfectly sound had changes on x-rays in their navicular bone consistent with navicular disease. What this means is that x-rays cannot be taken out of context. Without seeing a horse work, condemning them with the label of navicular disease solely on the basis of x-rays is premature and short sighted.
The classic signs on x-rays for navicular disease occur along the bottom border of the navicular bone. Your veterinarian will look for several lollipop shaped areas. The more lollipop shaped figures, the worse the prognosis of the disease. Although, there are several other changes a veterinarian would look for on this and other x-ray views, this article will not go deeper into interpreting x-rays.
The navicular bone rests at the junction of the coffin bone and the lower pastern bone. The navicular bone acts as a fulcrum. Muscles in the back of the upper leg contract causing tension in the deep flexor tendon. As the coffin joint flexes, the deep flexor tendon slides over the navicular bone pressing it upward.
This relates to the first theory of the cause of navicular disease. As was just explained, the navicular bone acts as a fulcrum for the deep flexor tendon. The deep flexor tendon pushes on the navicular bone. Over years, it is believed that the constant pressure causes damage to the navicular bone resulting in pain and lameness. The pressure on this bone will be concentrated into a smaller area in thoroughbreds and quarter horses with large bodies and small hooves. If the toe of the hoof is allowed to grow too long, this would increase the fulcrum effect. More pressure would be needed on the deep flexor tendon to flex the leg. This means more force applied to the navicular bone. By allowing the heels to become underslung, the weight bearing surface the heels is shifted. The result is that each time the hoof lands, the brunt of the force is taken directly over the navicular bone area.
The second theory explains navicular disease as an ischemic or circulatory problem. The navicular bone has tiny blood vessels that supply it. Investigators have shown these vessels to be blocked in horses with navicular disease. To do this they took the navicular bones of affected horses and made razor thin slices. They then examined them under high power microscopes. By designing treatments which help restore circulation to the navicular bone, remarkable success has been achieved.
A byproduct of the research dispelled some earlier concepts of navicular disease. It was believed that the lollipop shapes seen on x-rays of navicular bones were expanded canals for arteries. This research showed that these shapes were caused by the bone remodeling itself to allow the joint capsule to invaginate into the bone.
Now we can talk about treatments. Let me say again, the majority of horses affected with navicular disease can be helped and returned to previous function. It is a problem that does not go away but with management becomes a problem readily managed.
THE foundation for treating any horse showing signs of navicular disease is correct shoeing. I did not say corrective shoeing; it was not a typo. What I meant to say is correct shoeing. This means following the essential tenants of balancing the hooves front to back and side to side. The front of the hoof should be parallel to the line of the pastern and the back of the hoof should be parallel to the pastern.
The most common problem I see in horses suspected of navicular disease is long toes and underrun heels. It has been shown that you can take a normal horse and trim and shoe them in this way and create the signs of navicular disease. To correct the problem the long toe is trimmed away until the front of the hoof is once again parallel to the line of the pastern.
Correcting the heel is a more difficult problem. To address this the shoe is extended backward from the horse's heel to the place where the heel should have been if the heels were not underslung. In some horses this can be up to one-half inch. Over several shoeings the heels will expand backwards to the correct position. Once the front and back of the hoof are aligned, the heel is lowered 2 to 3 degrees and this is replaced with a 2 to 3 degree rubber pad between the shoe and the hoof. The heels on this pad are slippered to inside the white line (angled in at the sides). This forces the hoof wall to expand every time the hoof hits the ground. This aids in the blood pumping action of the foot and helps to correct contracted feet common in horses with navicular disease. By adding the pad, the foot is cushioned. By extending the shoe backwards we increase the weight bearing area of the foot, minimizing the concentration of force on the navicular bone.
Balancing the foot side to side is equally important. A horse's hoof should land heel then toe. If either the inside hoof wall or the outside hoof wall is longer than the other problems can develop quickly. It is like trying to run a sneaker that does not have an even sole. If, for example, the outside hoof wall is longer than the inside hoof wall, each time the horse lands the outside hits the ground first. As the horse moves forward the leg is twisted to the outside causing strain on ligaments and tendons throughout the leg. To check your horse for side to side imbalance, simply walk your horse on a flat paved surface. If the either the outside heel or the inside heel lands slightly before the other, your horse's hooves are imbalanced. It is important to note that this condition causes many more problems in the front hooves than in hind hooves. And, while many horses have imbalanced feet without lameness, it is my experience that a horse that is limping is frequently doing so on the same leg on which a hoof is imbalanced.
The second aspect of treatment is drug therapy. The most successful drug therapy thus far is the use of the drug Isoxsuprine. This drug dilates blood vessels. Presumably this will help increase circulation of blood to the navicular bone. Reports have shown that 80% of horses treated with Isoxsuprine will respond to it. The response is of variable duration and repeated courses of treatment may be needed.
The third aspect of treatment is exercise. As stated earlier, theories concerning the causes of navicular disease involve decreased circulation and trauma to the bone. The best way to increase circulation in the foot is through exercise. I have horses treated in my practice exercised 30 - 60 minutes 6 times per week. This riding is very structured. The horse is trained to bear more weight on the hind legs. In that effort riders are trained to use the dressage exercise call "long and low". The horse is worked at a trot or canter. The neck is extended and lowered in an effort to reach for the bit. At the same time, the horse is pushed forward in an effort to work off the hind legs. As the horse becomes more and more proficient, the weight bearing will move from the front legs, which are painful, to the back legs.
Other drug therapies have also been tried with success. Several studies suggest navicular disease is similar to degenerative joint disease. This led to research using Adaquan*, a drug used in joint disease, for cases of navicular disease. After several intramuscular injections a strong response was shown in these horses. Palosein*, another drug used in joint disease, has been shown to be effective when injected into the heel of affected horses.
Management of unresponsive case can also be done through chemical blocking agents. If you remember diagnosis is based on the horse going sound after injecting local anesthetic over the nerve in the back of the pastern. Below I will discuss a surgery that involves cutting that nerve to relieve the pain for unresponsive cases. However, other horses can be managed by getting the same results with a chemical neurotoxin. Serapin* is such a drug. A derivative of the Pitcher plant, Serapin will deaden a nerve for a period of 2 to 4 months. It is injected in the same place as the local anesthetic was given. By periodic injections of this blocking agent, horses that were otherwise non-responsive to other drug therapies can be managed without resorting to surgery.
The last option of treating horses with navicular disease is surgery. Here again major advances have been made in recent years. In the majority of cases that I treat, the horse responds to the combination of proper and therapeutic shoeing, drugs, and physical therapy. However, in cases that do not respond, surgery is a very viable option. There are two surgeries currently used to treat navicular disease. The first is a desmotomy of the suspensory ligament of the navicular bone. The second is a neurectomy.
The first surgery one can try, short of a neurectomy, is a desmotomy of the suspensory ligaments of the navicular bone. A desmotomy of a ligament means simply cutting the ligament. On either side of the navicular bone is a ligament that wraps around the pastern and anchors itself in the first pastern bone. In this surgery these ligaments are cut. The results of two studies have shown an improvement in 80% of the horses treated in this way. Why this surgery works remains a mystery.
The last resort is a neurectomy. Here, advances in surgical techniques have been made in the last decade. The basic surgery involves cutting the posterior digital nerve. This is the nerve that detects pain in the navicular bone. It lies just below the fetlock on both sides of the back of the pastern. It is the same nerve we block to make the diagnosis of navicular disease. This surgery will desensitize the back two-thirds of the hoof. Long term results have show that 68% of cases treated in this way are sound for at least 1 year.
I am frequently asked if a de-nerved horse is safe to jump. The answer is yes. A de-nerved horse will have feeling in the front third of the hoof so that he can still tell where the hoof is in relation to the ground. Also, muscular control of the hoof is much higher up the leg near the elbow. Therefore, the fact that a horse could not feel the back two-thirds of the foot would not affect its ability to control the hoof.
Complications of the neurectomy fall into two categories. First, some regrowth of nerve endings is to be expected. If this regrowth is extensive, the horse may regain feeling in the navicular area and become lame again. Second, a certain small percentage of patients given a neurectomy will develop a neuroma. A neuroma is a large bundle of nerve fibers that cause pain at the site where the nerve cut took place.
To combat the two complications of neurectomy several strategies have been developed. The simplest is the use of epineurial capping. The technique here is to tie the epineurium (a sheath that surrounds the nerve) over the top of the cut end of the nerve. By creating this physical block, it is theorized that re-enervation will be decreased. The use of lasers to cut the nerves has recently being implemented. Cutting the end of a nerve with a laser causes the end of the nerve to melt. This seems to prevent re-growth of the nerve and prevent the formation of neuromas. Certainly this has been shown to be the case in human medicine. Work is now in progress to show the same effect in horses. Finally, experimental work in chemical (i.e. not surgical denerving) is currently underway but results have not yet been published.
To conclude navicular disease is NOT TERMINAL. Its cause is thought to be a combination of trauma and a decreased blood supply to the navicular bone. Diagnosis must be made on BOTH clinical and radiographic signs. X-ray changes alone without symptoms do not make a diagnosis. The vast majority of cases can be helped with a combination of proper and therapeutic shoeing, drugs and physical therapy. For those horses that do not respond to physical therapy, two surgeries that are effective include the cutting of the suspensory ligament of the navicular bone and neurectomy of the posterior digital nerve.
Dr. Douglas Novick is an equine veterinarian practicing in the San Francisco Bay Area. His practice is limited to the treatment of horses with special interests in equine lameness and reproduction. He is also the first veterinarian in Northern California to implant horses with ID Microchips with optional freeze brands as a method of preventing horse theft.
See more at www.novickdvm.com
(C) 2004 Douglas Novick, DVM- This article is copyrighted. It is licensed for personal use only. Any re-use, duplication, re-transmission via electronic or other means without the expressed written permission of the author, Douglas Novick, is strictly forbidden.