Better diagnostic tools for imaging the horse's hoof are shedding new light on navicular disease and navicular syndrome in horses. There's a difference between navicular disease and navicular syndrome, although they overlap, and researchers are now uncovering several more causes for the symptoms in horses that have previously all been labeled "navicular."
The Navicular Bone
The navicular is a small, flattened bone, wide in the center, that resembles a flying saucer from an early sci-fi movie. It sits deep within the back portion of the foot, nestled between the coffin bone (P3 - third phalanx) and the short pastern bone (P2 - second phalanx).
If you were looking at the bottom of a normal horse's hoof, the navicular bone would be sitting inside the foot at roughly the level of the middle of the frog. Working your way out from the navicular toward the ground surface, the next structure is the navicular bursa, a fluid-filled cushion, the deep digital flexor tendon, the digital cushion, and finally, the frog. In other words, there's lots of padding.
The navicular bone doesn't just sit in place on its own. It is held there by several ligaments. The impar ligament is a short but thick and strong ligament that runs from the bottom surface of the navicular bone to the coffin bone. There are also ligaments running from either side of the navicular to the larger bones, and ligaments connecting it to collateral cartilages, large "wings" of cartilage extending back from the coffin bone that expand and contract upon weight-bearing. A major function of these ligaments is to hold the bone in place. But because all ligaments have some "give" to them, they also allow controlled movement in a variety of directions.
What Does It Do?
That's a very good question, without too many solid answers. Because of the way it projects out behind the coffin joint, the navicular bone changes the direction of the deep digital flexor tendon before the tendon inserts on the coffin bone. When the foot is off the ground, however, and the toe is back, the pull is in more of a straight line. Whether this is of any mechanical advantage to the horse is not clear.
Potential Risk Factors
- Horses with small feet in relation to their body size
- Extensive work over hard, uneven ground
- Poorly trimmed or shod hooves
- Jumping and activities requiring hard, fast turns
The navicular bone also gives some wiggle room to the coffin joint. The collateral ligaments that flank the coffin bone secure the short pastern bone to the coffin bone in tight alignment so that they function almost as one. This is an advantage in weight-bearing. That arrangement would work fine if all the ground the horse travels over were completely flat and smooth. But in reality, the horse has to deal with uneven ground surfaces all the time, so there needs to be a way for the foot to absorb uneven forces. The soft tissue cushioning and shock absorbing structures (frog, digital cushion, collateral cartilages) do just that, but it looks like the navicular is involved as well.
It is thought that the position of the bone and its attaching ligaments allows it to absorb and compensate for uneven ground surfaces. The navicular bone of horses that are exercised is also more dense than that of inactive horses.
Disease vs. Syndrome
There is some inconsistency in what researchers mean when they talk about navicular disease and navicular syndrome. Some limit navicular disease to abnormalities in the navicular bone and its cartilage. Others use the terms navicular disease and navicular syndrome interchangeably and include the navicular bone, navicular bursa, and the deep flexor tendon behind those structures. An even broader definition of navicular syndrome, sometimes called "heel pain syndrome" or "caudal heel syndrome," includes all horses that have pain in the back part of their foot.
As you might imagine, the tendency to label all horses with heel pain as navicular disease is too simplistic. Thanks to more sophisticated tools that allow us to actually see what's going on inside the foot, we now know there are several other possible causes of back-of-the-foot pain.
Heel Pain Syndrome
If your horse has heel pain syndrome, which includes navicular disease, he may display some or all of the following symptoms:
• Tendency to land toe first
• Stumbling or tripping
• Narrow, contracted heels and narrow frog
• Tendency to alternately "point" the front feet (leg resting on the toe)
• Excessive heel growth
• Shortened stride and stiff gait
• Exaggeration of pain going uphill or if jogged off after standing with the toe elevated on a block
• Worse pain on rough, uneven ground
• Involvement of both front feet, although one may be worse
• Pain with hoof testers applied across the heels and/or from center or sides of the frog over to the wall
• Pain that's improved or eliminated by a "heel nerve block"
These things tell you the horse has pain in the foot, but not where or why. Until the last five years or so, all these horses would be assumed to have navicular problems and treated accordingly, with results ranging from nearly miraculous to no response at all.
Today, with the growing availability of better diagnostic tools, especially MRI and ultrasound, we're finding other causes of heel pain. In addition to inflammation in the area of the navicular bone, bursa, and flexor tendon, pain may also be caused by:
• Coffin joint collateral ligament injury
• Tears in the deep flexor tendon anywhere along its course in the foot
• Tearing at the insertion site of the deep flexor tendon on the coffin bone
• Inflammation and calcification of the collateral ligaments ("sidebone")
• Inflammation of the impar ligament or other ligaments of the navicular bone
• Coffin joint arthritis (which may be present at the same time as navicular disease)
• Inflammation of the sesamoidean ligaments where they attach to the short pastern bone
Causes of Heel Pain Syndrome
A study presented at the 1988 American Association of Equine Practitioners (AAEP) meeting showed that an impressive 90% of horses with navicular disease (or at least symptoms at that time that were felt to mean navicular disease) had hoof imbalances. Since the navicular bone bears the brunt of the uneven forces inside the foot, this should come as no surprise. In terms of strain to this area, attempting to work the horse with unbalanced feet is as bad as riding hard over lumpy, irregular surfaces. Hoof imbalances also put uneven forces on the coffin joint and the collateral ligaments of the coffin joint, potentially causing problems there.
Horses with small feet in proportion to their body size are widely accepted as being predisposed to navicular disease. This is described as a common-sense observation, since it means that more force per square inch is being loaded onto the feet than in a horse with a generously sized hoof.
Type of work likely plays a part in navicular disease as well. Dr. James Rooney, a pathologist and lameness expert, reports that navicular disease is virtually unknown in racing thoroughbreds and standardbreds. You would certainly think the feet of these horses take a beating, and they do-but navicular disease is not one of the consequences. The reason for this may well be that they are always worked over well-groomed, flat surfaces and do not make sharp turns.
Horses living and working in less controlled environments are dealing with uneven ground surfaces all the time. That alone produces some well-described aging changes in the navicular bone, but those are not necessarily associated with any lameness. Navicular disease shows up in horses in a wide variety of activities. Jumping, which involves landing forcefully on one foot after the jump, is a risk factor, but so is riding over uneven natural surfaces and performing sharp turns at speed. When you add the weight of a rider to the equation, it's easy to imagine how many stressors would add up over time, even if the horse is not being worked hard.
The same factors that can add up to produce navicular disease-poor hoof balance, conformation, type of work-may also result in the other possible causes of heel pain. Sharp turns at speed or turns upon landing from a jump also put the collateral ligaments under considerable strain. Collateral cartilage problems often result from hoof imbalances and narrow, high heels. Problems with the deep digital flexor are high risk for horses with overly long toes, especially when combined with underrun heels, speed work, and jumping.
Barefoot proponents contend that keeping horses in shoes can contribute to navicular disease in several ways, including increased vibratory forces in the foot, decrease in the surface area of the foot used for weight-bearing, and interference with normal circulation in the foot. One or more of these may be correct, but they are only theories and have never been actually proven.
Finally, some researchers have proposed that navicular disease is caused by clot formation in the blood vessels feeding the navicular bone. This led to a fad of treating horses with anticoagulants (blood thinners) like warfarin, brand name Coumadin. Some horses it seemed to help, others not, and this theory remains one of the most controversial and least widely accepted among veterinarians.
Navicular: It is well accepted now that response to a heel nerve block does not confirm a diagnosis of navicular disease. Radiographs have always been used, but radiologists are still arguing over what types of changes really diagnose navicular disease. Enlargements in the synovial fluid channels along the back edge of the navicular bone, where it contacts the bursa and flexor tendon, as seen in the so-called "skyline view," are generally accepted as at least indicating inflammation. The formation of large "holes" in the body of the navicular bone itself, believed to be enlarged vascular channels, is considered diagnostic by some, but others feel it is unreliable and can be seen in older horses that are perfectly sound.
Injection of local anesthetic directly into the navicular bursa is a bit more specific than a heel nerve block but could end up anesthetizing nearby structures, such as the flexor tendon and impar ligament, so it's not really 100% reliable. Similarly, injection of anesthetic into the coffin joint often improves navicular-related lameness but would improve coffin joint arthritis pain as well.
By far the best diagnostic technique is MRI imaging of the foot. MRI can detect a variety of changes that x-rays, and even bone scans, cannot. This includes increased fluid in the navicular bursa and adhesions between the bursa and the tendon. Considered most diagnostic of all is increased fluid within the body of the navicular bone itself.
Other causes of heel pain: Ultrasound is of some use in determining additional causes of pain. It can pick up damage to the collateral ligaments of the coffin joint, find some lesions in the deep flexor tendon if not located too deeply inside the foot, and may be able to see a very enlarged navicular bursa.
However, again, MRI is the premier diagnostic tool. All of the other mentioned causes of heel pain can be easily diagnosed by MRI.
Unfortunately, MRI isn't widely available yet and is expensive. But if you have a horse plagued by chronic foot pain that isn't responding to therapies or shoeing changes, you may want an MRI examination to find out exactly what you are dealing with so that a reasonable treatment plan can be made.
Pain medications such as phenylbutazone are almost always part of the treatment approach to navicular, but they don't fix or cure anything. The cornerstone of traditional treatment for navicular/heel pain has been shoeing changes. The usual approach is a rocker-toed shoe, usually padding, and heel elevation either by shoes that are thicker in the heels or wedges. This relieves some stretch on the deep flexor tendon, which can bring relief in horses with both true navicular syndrome or lesions in the deep flexor tendon.
It is critically important to realize that even before considering a shoeing change, you must make sure the foot is properly trimmed and balanced. Slapping a shoe onto a foot in which the toe is too long, the heels are underrun, and/or there is an imbalance side to side will do you little, if any, good. The hoof itself must be properly trimmed first. For many horses, this alone provides considerable relief. If you don't take that step, everything else you might do is doomed from the start.
The shoe adjustments we described earlier often provide at least temporary relief and improvements. A major drawback is that over time, these shoeing arrangements can take the natural shock-absorbing structures of the foot-the frog and digital cushion-out of the picture. Soft packing under a sole pad helps provide stimulation to those areas the way dirt does in a barefoot horse. But if the heels are continually raised, as is often done if the horse begins to show increased discomfort, the frog and digital cushion will eventually wither and the navicular bone will become more locked in place. Some feel this is detrimental to the horse's circulation. These long-term complications of navicular shoeing may be at least part of the reason why navicular is considered to be incurable.
Is it incurable? The world of navicular disease research and the prognosis for it has only recently been turned on its head by the knowledge being gained from MRI imaging. We still don't know the answer to the curable-or-not question, and it's going to take several years of following horses with various navicular area changes to see how they respond.
In the final analysis, it may well turn out that horses with the classical picture of big, heavy bodies and tiny feet are doomed to navicular disease and lameness no matter what we do. Researchers have already determined that horses with adhesions between the navicular bursa and the flexor tendon also have a very poor outlook for soundness. Beyond that, the prognosis for horses with fluid in their navicular bone or excess fluid in the navicular bursa is largely unknown.
Precise diagnosis of the problem may improve the outcome for horses with heel pain syndrome by changing the way they are managed. For example, while a horse with a diagnosis of presumed navicular disease might not be worked as hard as usual, actual rest usually isn't part of the treatment plan. However, for some soft tissue lamenesses in the foot, rest could be critical. One study found that horses with damage to the collateral ligaments of the coffin joint responded very well to rest and casting of the feet. Tearing of the flexor tendon at its attachment to the coffin bone might also respond well to a similar approach. And with any ligament damage, strictly controlled exercise is always important to healing-as is providing a lot of time for the tissue to heal and following its progress with periodic imaging exams.
While navicular disease/syndrome is a very real thing, recent advances in diagnosis have revealed other causes of heel pain. Furthermore, different problems may call for different treatment approaches. Researchers are following horses with various changes in their feet to determine the best treatment options. In the meantime, all this new knowledge is leaving us even more confused about what to do. However, it will ultimately result in finding better ways to help horses be pain free.