So many times, a horse gets labeled as suffering from "navicular," and people shy away from him as if he were Typhoid Mary, not knowing how to treat or even visualize the problem. In recent decades, however, much has been learned about the area where the navicular bone lies. And many "set in stone" diagnostics (such as "lollipops" in the bone seen on radiographs) have gone by the wayside. Veterinarians and horse owners now know that there are many problems that can affect that area of the horse’s anatomy; some can be helped, but not all of them can be fixed. It also is known that in certain breeds of horses, the problem can become worse with age. In this era of horses living longer, it behooves owners to recognize the early signs of navicular syndrome, and know the options for your horse.
The navicular bone is a small bone that sits deep within the hoof at the back junction of the coffin bone and the short pastern bone. The navicular bone has the physical shape of a small canoe, which led to the name "navicular" bone; the prefix "navicu" means "small boat" in Latin. The navicular bone is also known as the distal sesamoid bone (the commonly known sesamoid bones behind the fetlock joint are the proximal sesamoid bones).
Associated with the navicular bone are several soft tissue structures. On the upper (proximal) aspect of the bone is the collateral sesamoidean ligament, which attaches the navicular bone to the distal end of the short pastern bone (remember that a ligament attaches bone to bone and a tendon attaches muscle to bone). On the lower (distal) aspect of the bone are the impar ligaments, which attach the navicular bone to the coffin bone. Cushioning the navicular bone from the pressure of the DDFT is a thin, soft serous sac called the navicular bursa.
The navicular bone is similar in structure to most bones -- there is a central marrow cavity, small channels (along the distal aspect of the bone) for blood vessels and nerves to enter, and a smooth surface on the back side (palmar or plantar) where the deep digital flexor tendon glides over the bone.
The navicular bone and its associated structures have complex anatomy, but just what is the function of the navicular bone?
The primary function of the navicular bone is to provide a gliding surface at the point where the deep digital flexor tendon changes angle; the tendon courses down the back of the cannon bone and bends around the back of the fetlock, between the proximal sesamoid bones, then makes a sharper bend over the navicular bone, and attaches on the bottom of the coffin bone.
Now that we have a better understanding of the navicular bone and its associated structures, it should be easier to understand, and believe, that the idea of "navicular disease" is more complex than one single pathology affecting the navicular bone. The term "disease" is defined as "a definite morbid process, often with a characteristic train of clinical signs."
The term "navicular disease" first was used to describe a lameness caused by pain associated with the navicular bone or surrounding area. Now, because the disease process leading to pain in this area has been determined to be caused by a variety of abnormalities, the label of "disease" is not really appropriate.
Next, the term "navicular syndrome" was used. The word syndrome means "the sum of signs of any specific morbid state," and really is not correct since a variety of specific "morbid states" concerning the navicular area can lead to the same set of clinical signs.
Currently, it is popular to call problems with the navicular area "caudal heel pain syndrome," which probably is the most correct. As we will see, lameness emanating from this area can be caused by a variety of problems affecting the caudal aspect of the horse’s foot, all leading to the same basic set of clinical signs.
What Is It, What Horses Get It, And What Does It Look Like?
Several different problems can lead to pain and lameness in the navicular/caudal heel area, including inflammation and injury of the supporting ligaments, problems in the flexor surface of the bone or flexor tendon itself in that area, and/or problems within the navicular bone itself (more on the specifics later). These problems are more common in the performance horse, and in general are the most common cause of "chronic" forelimb lameness in the horse.
Caudal heel pain has been diagnosed in most breeds of horses, but there does appear to be a breed predisposition, with Quarter Horses, Thoroughbreds, and warmbloods being diagnosed with the syndrome more frequently than others. Affected horses tend to be between the ages of seven and 14. Other common predisposing factors are conformation abnormalities of the hoof, including a broken forward or backward hoof axis, underrun heels, sheared heels, contracted heels, mismatched hoof angles, and disproportionally small feet.
Navicular syndrome typically affects both front feet to varied degrees. Typically one foot is more painful, so lameness of a single leg often is noticed first. Careful/further observation shows that the horse will be short-strided in both limbs and might swap lame legs when navigating tight circles or corners. Longeing in relatively tight circles on hard footing generally reveals a switching leg lameness, with the lame leg being on the inside.
Because the heel area of the foot is causing the lameness, the foot tends to land toe-to-heel, which is opposite of the normal heel-to-toe landing. When first learning to observe these subtle facts, it might help to capture your horse on video, focusing on the legs and feet and later observing the video in slow motion.
In addition, navicular syndrome pain often gets more severe with work and less severe with rest. Although there are other problems that could create this clinical picture, these are a typical collection of clinical signs associated with navicular syndrome.
After evaluating a horse, the next step for your veterinarian is to perform some diagnostic tests to build evidence for a case of navicular syndrome. When trying to localize the area causing lameness, it is typical to perform some manipulations of the anatomy in an effort to make the lameness worse. The most important thing to remember about limb manipulations is that they are not very specific, as it is difficult to create pressure or tension on one specific structure without affecting another. For example, it is common to perform a hock flexion when suspecting hock pain, but it is impossible to flex the hock flexion without also flexing the stifle and hip joints as well.
In the case of navicular syndrome, a fetlock flexion test often is done for completeness, but a positive result does not rule out a specific problem. Two tests that do focus somewhat more specifically on the navicular area are the "wedge test" and the "frog pressure" test.
In the wedge test, the foot is placed on a wedge of wood, forcing the foot into a hyperextended position while the opposite leg is held in the air; after a period of time, the horse is trotted off in a straight line and watched for an increase in lameness. With the frog pressure test, the foot is placed over a small block of wood (often the handle of a hoof knife or hammer), placing pressure on the caudal aspect of the frog while the opposite leg is held in the air; after a period of time, the horse is trotted off in a straight line and watched for an increase in lameness.
Both of these tests apply pressure to other areas of the foot and are not 100% specific for the navicular, so their results are interpreted relative to other findings.
Hoof testers can be helpful when applied to the heel quarter of the hoof wall and the opposite side frog sulcus, applying pressure to the navicular bone sitting deep underneath. Again, this test applies pressure to other areas of the foot and is not 100% specific for the navicular, so the results have to be interpreted relative to other findings.
If the clinical examination fits and the aforementioned tests look more positive than negative, you are coming closer to a diagnosis of navicular syndrome.
The next step will be to confirm that the pain is coming from the caudal heel area using local anesthetic. When a local anesthetic is injected around the area of a peripheral nerve, the tissue below the injection site loses sensation, with the tissue above the injection site retaining sensation. For lameness, the anesthetic is started as low as possible, attempting to narrow the area affected in a predictable way. When the palmar digital nerve that innervates the back of the leg and hoof is "blocked" very low in the pastern area (a "low PD block"), the heel area loses sensation and the toe retains sensation.
After a low PD block is administered, the "block" is tested with something hard and pointed (typically a pen tip). Sensation in the skin should be gone in the lateral and medial heel and retained in the front of the foot when tested at the coronary band -- if the toe loses sensation, the block is not localized to the heels. If the pain is coming from the navicular area, the lameness should be improved, and if bilateral navicular syndrome is in fact the cause, the horse often will become overtly lame on the opposite limb. Nerve blocks often are not 100% effective, but you generally see a significant improvement (75% or better) if you are on the correct path.
If there is a positive result (reduced lameness) to the local anesthesia, the next step to confirming the diagnosis and determining a prognosis is obtaining radiographs of the caudal heel area. The variety of findings veterinarians see in radiographs illustrate the number of different pathologies that cause pain and subsequent lameness in the caudal heel area. Injury to this area could be a result of chronic, repetitive trauma resulting from the previously mentioned conformation abnormalities, changes in blood flow to the area as a result of conformation abnormalities, problems related to breed, and probably several as yet unknown factors.
Pathology known to be related to navicular syndrome includes:
enlargements of the vascular channels or synovial fossa within the navicular bone;
cyst-like lesions within the navicular bone;
mineralization or calcification of the ligaments associated with the navicular bone;
bone disease affecting the coffin joint in the area of the navicular bone;
degeneration of the flexor surface of the navicular bone;
degeneration of the flexor tendon in the area of its passage over the navicular bone;
fractures of the navicular bone
and any combination or degree of the aforementioned pathologies.
It typically takes three different radiographic views to evaluate the navicular area. Your veterinarian will want to look at the front, side, and flexor surface of the bone. One problem with the evaluation of navicular radiographs is the possible lack of correlation between the degree of radiographic pathology and the degree of clinical lameness. This is one factor that makes the prepurchase evaluation of navicular radiographs difficult; in some cases there can be substantial radiographic abnormalities on a horse that cause little or no clinical lameness. Unfortunately, there is no crystal ball for these cases and future soundness is uncertain.
There is no question that "bad" navicular radiographs, despite a very sound horse, will discourage most sales. It’s useful to have radiographs from a earlier time for comparison, but such cases still can be difficult.
Additional diagnostic tools include nuclear scintigraphy (the "bone scan") and thermography, but typically a good clinical examination, local anesthesia, and radiographs will lead to a diagnosis.
The treatments for navicular syndrome are varied and range from conservative to aggressive. They can involve therapeutic shoeing, various medications, and surgery. The response to therapy can be unpredictable and does not always correlate to the degree of lameness or radiographic abnormalities. Therefore, it generally is best to make gradual changes, working from conservative to more drastic. Many horses will respond favorably to shoeing changes and medication. If the horse’s foot has abnormal conformation, the first goal should be to get the foot back in balance; changes should be made gradually, and the response to changes carefully noted.
If the axis between the coffin bone and the short pastern bone is broken, the hoof angle should be changed gradually so that normal alignment is obtained. Underrun and contracted heels should be corrected.
Remember that blood flow through the foot is largely dependent on the pumping action of the normal foot. As pressure is applied to the heel of the foot, the heels expand, compressing the digital cushion under the sole and pumping blood throughout the foot. The blood flow through the foot thus can be negatively affected by the application of tight-fitting shoes and the placement of nails in the heel area. Poor foot conformation also can contribute to poor blood flow. All horses should have a rim of extra shoe onto which the heels can expand. The nails should be kept as far forward as possible to maintain normal foot health.
Other changes to help the horse suffering from caudal heel pain include trimming the toe as short as possible and utilizing round (rocker) or squared-toe shoes. This encourages easy breakover and reduces stress forces up the front of the limb. Farriers often will set the shoe back slightly from the toe, again to encourage breakover. They will also frequently shoe with a full-fitting heel, allowing normal heel expansion.
Some practitioners and farriers feel that applying a small pad to raise the angle of the foot will decrease the tension on the flexor tendon and will benefit these horses. This might be necessary to establish a normal pastern-foot axis. The correct angle of rise will vary from horse to horse, so the rise should be gradual, watching for the best effect. The response to increased hoof angle with pads varies greatly depending on the specific problem affecting the horse, but too steep an angle can make the horse more lame.
Some horses respond well to an egg bar shoe that provides additional support to the caudal heel structures. Again, it is important to remember that each case has varied pathology, so each case must be treated individually when applying therapeutic shoeing.
Medication generally involves the use of an anti-inflammatory drug such as phenylbutazone (Bute), and works better in some cases than others. Depending on the horse’s work schedule and the cause of his pain, you might be able to only dose him on days surrounding work, thereby minimizing total dosing. The specific medication program should be worked out by your veterinarian.
Many cases of navicular syndrome respond to therapeutic shoeing and varied degrees of anti-inflammatory therapy. If you are showing, remember that you must stay within the limits for the non-steroidal anti-inflammatory drug (NSAID) if showing under American Horse Shows Association (AHSA) rules. Many horses with navicular syndrome remain sound enough for AHSA competition through the use of therapeutic shoeing and allowable NSAID dosing programs. This becomes a different issue when the horse is showing at Federation Equestre Internationale (FEI) competitions due to their "zero tolerance" drug rules.
Another popular drug used in the treatment of navicular syndrome is isoxsuprine, which is a vasodilator and addresses the decreased blood flow aspects of these disease processes. The effectiveness of isoxsuprine in the treatment of navicular syndrome is somewhat controversial, with some research showing a positive response and some not supporting its use. (In my experience, the therapeutic response to isoxsuprine has not been very impressive, but as we have learned in this article, navicular syndrome has some factors that might respond to this therapy and others that won’t.)
There have been cases of positive drug tests for isoxsuprine under AHSA rules. (Isoxsuprine is not allowed and requires a medication report and 24 hours off the drug before competition.) Some of these positive tests were on horses which were not intentionally given the drug, but were exposed to it by eating from feed tubs that once contained it from another horse. (Wash those feed buckets!)
Two other drugs being evaluated for their ability to improve circulation to this area of the foot are pentoxifylline and metrenperone, but much more research needs to be performed before their clinical usefulness in the horse can be established.
The most reliable surgical approach to navicular syndrome, and one that has been performed for many decades, is digital neurectomy (also known as "nerving" or "having the horse nerved"). The suffix "ectomy" means "to cut or transect," and a digital neurectomy is just that -- cutting the digital nerve in the low pastern area. There have been a variety of surgical techniques developed for the digital neurectomy, ranging from simply cutting the nerve with a scalpel blade to performing the transection with a high-tech surgical laser. The reason for exploring different techniques is to minimize complications and to prolong the inevitable regrowth of the nerve. Even with removal of a one-inch section of the nerve and using a variety of techniques to seal or "cap" the nerve endings left behind, the nerve regenerates, causing the foot to regain sensation and the lameness to return.
Studies demonstrate that approximately 60% of horses with navicular syndrome which have had a neurectomy are sound one year after the surgery; however, the procedure is not a permanent cure and probably will need to be repeated to maintain soundness (see "Neurectomy" in the October 1997 issue of The Horse or go to http://www.thehorse.com/ViewArticle.aspx?ID=642). There are occasional complications of this procedure, the most common of which is the formation of neuromas. These are benign, but very painful tumors that grow at the end of cut nerves. Some nerved horses can show under AHSA rules (the procedure is allowed, but only at a lower place on the leg as a higher cutting of the nerves could create a dangerous situation).
The neurectomy is a viable option for many horses when all else fails. One thing to keep in mind is that this is not a treatment; it only eliminates the pain. The disease process potentially is continuing inside the hoof. The neurectomy should be considered only after all other therapies have been exhausted and after a thorough consultation with your veterinarian.
About the Author
Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.
Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.