Navicular Syndrome/Heel Pain
There is probably no truer adage, "No hoof, no horse." Without this solid foundation, a horse cannot perform to his potential no matter how well trained, how fit, or how athletic he might be. A steady clip-clop rhythm at the trot gives you a sense of well-being as each hoof beat chimes aloud that your horse is sound. But when that cadence becomes even slightly irregular, it is time to take note of what might be wrong.
Subtle behavioral changes often tip you off to a problem before it becomes an obvious lameness. Refusal or dislike of work in a horse which normally likes to work might indicate pain or discomfort. Tail wringing, head tossing, bucking, refusing to pick up a lead, refusing to make sudden stops, or grumpy behavior when saddled are all tip-offs to an attitude problem that could stem from pain.
Your horse speaks volumes with his body posture even when standing idle. As you observe, does he stand camped out in front as if he's trying to get the weight off his front feet? Does he point one foot more than another? Does he shift his weight from foot to foot trying to get comfortable? Under saddle, does he insist on walking on grass or dirt rather than the gravel road? Does he shorten his steps as he goes down hill? Is the lameness accentuated as he turns on a bend or a circle? Is the trot choppy and jarring?
According to many veterinarians, more than 90% of lameness in the front legs of a horse results from a problem in one or both feet. The behaviors and expressions listed previously might be your horse's way of communicating that his feet hurt. One of the more common foot problems encountered is navicular syndrome.
What is Navicular Syndrome?
Navicular syndrome refers to a degenerative condition within the navicular apparatus, which includes multiple structures--the navicular bone (also known as the distal sesamoid bone), the navicular bursa, the deep digital flexor tendon (DDFT) as it courses beneath the navicular bone, and the supporting ligaments of the navicular bone.
In most cases of navicular disease, the condition affects both front feet with lameness as described previously. Horses most at risk are those with certain heritable conformation features such as upright pasterns and small foot size relative to body mass. Horses subjected to sports with front end impact such as cutting, reining, roping, or jumping are also at greater risk for developing navicular disease. Horses frequently worked on hard-packed surfaces form another group with increased risk.
Certain breed types tend to develop this syndrome more than other breeds. For example, while Arabian horses rarely develop navicular disease, Quarter Horses, Appaloosas, Paints, Thoroughbreds, and warmblood breeds are most often afflicted. This syndrome typically appears in horses aged seven to 14 years, but early athletic stress can bring this out in younger horses as well.
Historically, there have been multiple explanations for how this syndrome develops:
- Bursitis as a result of inflammation from concussion between the flexor tendons and the navicular bone;
- Thrombosis/ischemia due to local trauma and inflammatory mediators creating circulatory disturbance to the navicular bone with resulting pain; and
- Bone remodeling caused by abnormal pressures from the deep digital flexor tendon (DDFT) as it is pressed against the bone subsequent to increased loading in the heels.
Currently, a more complex theory based on recent anatomical discovery has provided plausible explanations for many of these consequences. Robert M. Bowker, VMD, PhD, a research scientist and associate professor in the Department of Veterinary Pathology of Michigan State University, has concluded that navicular syndrome is a problem within the entire foot, involving more structures in the foot than just the navicular bone. He noted that clinically healthy feet have a more substantially developed collateral cartilage complex (the collateral cartilages are on either side of the coffin bone and joint) within the hoof. It's thought that an increased vascular (blood vessel) system associated with thicker collateral cartilages might improve dissipation of stress forces within the foot. Sensory nerve endings associated closely with blood vessels are known to course through the collateral cartilages.
Bowker also notes that the sensory nerve supply to the navicular bone and its supporting ligaments courses through these ligaments. As the coffin joint moves in locomotion, the navicular bone is forced to assume some of the weight-bearing load with transfer of stress from the pastern bone to the suspensory ligaments of the navicular bone. Compression and upward pull on the DDFT creates shearing forces between the DDFT and the distal suspensory impar ligament of the navicular bone (which attaches the navicular bone to the coffin bone). Repeated and abnormal tensions ultimately lead to inflammation.
Adaptive responses within the navicular structures and ligaments are related to stress during the stance and impact phases of locomotion and not solely to age of the horse. In horses with navicular disease, Bowker has identified significant changes in the distal suspensory impar ligament and in its attachment to the coffin bone where it intersects the attachment of the DDFT. It is at this "bottleneck" that sensory nerves and blood vessels course on their way to other areas of the foot. The sensory nerves in this area serve to maintain blood flow within the soft tissues of the foot when it encounters loading during locomotion. The increased circulation that is stimulated by the sensory nerve signals serves as a hydraulic cushion to impact.
No matter the theory, it is recognized that an upright pastern exacerbates concussion within the heel region. Additionally, Bowker's theory lends an explanation as to why underrun heels and long-toed feet are afflicted with heel and foot pain due to reduced concussion dissipation. Bowker identified that the digital cushions of healthy feet might dissipate concussion better than feet with weak and collapsed heels.
About 70% of horses diagnosed with navicular disease have a long toe and low heel. Almost 60% of navicular horses have medial to lateral hoof imbalance.1 If trimmed and shod incorrectly, even horses with excellent feet can develop this syndrome. Prevention is everything--avoid a long-toe/low-heel foot configuration, a broken-back hoof-pastern axis, and foot imbalances.
What is a Broken-Back Hoof Axis?
With your horse standing on a level surface, examine the slope of each hoof and its relationship to the pastern axis. If this axis is "broken-back" (the lines' angle points toward the heel rather than being parallel), the pastern angle appears steeper (or more upright) than the hoof face. Ideally, the front of the hoof wall should be parallel to the slope of the pastern. This is a critical objective to have your farrier achieve with trimming. A broken-back hoof-pastern axis, like an upright pastern, amplifies stresses on the rear-most structures of the feet, particularly the navicular apparatus.
Long Toe/Low Heel Syndrome
In most instances, a horse with a broken-back hoof-pastern axis also has underrun heels and long toes. Both the low heel and long toe cause the hoof to assume a more acute angle, or an increased slope, relative to the pastern. This amplifies the steepness of the pastern and concentrates significantly greater stress on the navicular structures and the coffin joint while also stretching the DDFT. The heel hoof tubules are compressed with a long toe/low heel configuration, and over time they lose their shock-absorption ability much as would collapsed springs on your car. The long toe makes it more difficult for the horse to break over his foot with each stride, thereby increasing the muscular effort of each limb lift. More strain is applied to the tendons and ligaments in a foot with long toe/low heel configuration.
The gait that comes to be associated with a foot-sore horse is quite typical of many problems, ranging from heel soreness to laminitis. In general, a foot-sore horse moves with a short, choppy gait, is reluctant to negotiate downhill inclines, and doesn't want to walk or trot on hard or uneven terrain. On uneven or inclined surfaces, a moderately lame horse appears to be walking on eggs, mincing along with each step. A heel-sore horse, as one would be with navicular syndrome, tends to land toe-first in an effort to relieve loading of the heels. Ironically, this rapid unloading of the heels in response to heel pain increases the force exerted on the navicular bone by the DDFT (which is raising the heels). Each time the horse unloads the heels quickly, he exacerbates the progression of the disease by adding extra compressive stress on the navicular structures.
Before slapping a label of navicular syndrome onto any horse, it is important to identify if heel pain is diffuse or if it is associated specifically with navicular disease. In addition to gait analysis, other tools used to identify the source of pain include a hoof tester exam of the bottom of each foot. These plier-like devices are squeezed over the frog in an attempt to elicit a reaction due to pressure on painful navicular structures inside. However, this test has been found to be the "least sensitive manipulative test for navicular pain." A horse truly suffering from navicular syndrome is only positively responsive 48% of the time to a hoof tester exam.2 Response is dependent on the structural location of the degenerative disease, as well as the difference in how the limb is loaded by the horse's weight as compared to squeezing of the bottom of the foot with the limb suspended in the air.
Another common test for localizing navicular pain is a flexion test of the lower limb. This test yields accuracy of only 53% and has trouble differentiating between exclusively navicular pain versus other syndromes that create generalized heel pain.2
The frog wedge test places a wedge beneath the frog with the horse's full weight standing on the wedge for a minute, then he is trotted off and observed for signs of lameness. (Increased lameness indicates pain in the navicular area.) This gives an accurate response only 53% of the time. Similarly, in a hoof extension test, a wedge is placed beneath the toe in an effort to increase strain on the rearward structures of the foot. After holding the opposing limb in the air for a minute, the horse is trotted off; this is only 49% accurate.2
Nuclear scintigraphy is another tool for diagnosing navicular disease. Although it has been used in the United States for more than 20 years, it has only been used for the past seven years for this purpose. Mark Martinelli, DVM, Dipl. ACVS, a surgeon at San Luis Rey Equine Hospital in Southern California, has found it to be a valuable adjunct to the clinical examination for evaluating heel pain. Martinelli says, "Because scintigraphy is a metabolic imaging modality, it can help determine whether there is active bone remodeling at a specific site long before there are visible changes on the radiographs. In addition, it can help sort out whether there may be another region of the foot responsible for the lameness other than the navicular bone. For instance, it can determine if there is a problem with the coffin bone or the insertion of the deep digital flexor tendon, both of which may cause lameness that mimics navicular disease, but carry a better prognosis for return to soundness than does true navicular disease."
Diagnostic nerve blocks are invaluable in localizing pain to a specific area. A common nerve block used to anesthetize the heel region of the foot is the palmar digital nerve (PDN) block. Nerves that innervate the back third of the foot are anesthetized with a small bleb of local anesthetic over these nerve branches low down in the pastern. However, there are problems with this block in its specificity:
- A PDN block can numb the sole, giving a false positive if the horse becomes sound despite having a bruised sole or a problem associated with pain in the solar region.
- In some horses, accessory branches of this nerve course across the front of the pastern in addition to supplying the navicular area; these are not anesthetized by a couple of blebs on the back of the pastern, hence giving a false negative result if the horse does not go sound.
- Fibrous adhesions can create a mechanical restriction to limb movement, so although pain is numbed, the horse could still appear lame.
- There might be a concurrent lameness issue such as coffin joint arthritis or a problem elsewhere, again yielding a false negative response to the nerve block.
Perhaps the most valuable diagnostic procedure to isolate navicular pain from other causes of heel pain is that of anesthesia injected directly into the coffin joint. This procedure is about 90% accurate for navicular disease,2 although instances of sole pain can also be blocked out with anesthesia into the coffin joint.
Coffin joint arthritis often develops as a result of navicular disease; in fact, more than half of horses with navicular disease also have pain associated with the coffin joint. Also keep in mind that some cases of coffin joint arthritis occur due to a joint-specific injury unrelated to the navicular structures. Ultrasound of the navicular structures and coffin joint is also useful to identify structural changes.
No single diagnostic test gives the exact answer. It is important to couple a thorough clinical exam with the findings of multiple tests as described above, then corroborate all the information with the clinical picture of the horse. In this way, an accurate diagnosis can be made.
Although it's tempting to read a lot into navicular X ray films, a radiographic exam isn't always conclusive. Much discussion among veterinary practitioners hinges on the significance of increased number, size, shape, and location of synovial invaginations (hollows) in the bone; the presence of cysts or sclerosis (hardening) within the navicular bone; flattening or erosion of the central ridge of the bone; or the presence of enthesiophytes (bone spurs) on the edges of the navicular bone. All of these are signs of bone remodeling and changes in the navicular bone. However, they might or might not be related to a clinical problem.
In one study presented at the International Symposium on Podotrochleosis in Germany in 1993, only 34% of horses with navicular disease showed radiographic lesions of three of the "changes" mentioned above. Radiographic changes most commonly associated with navicular disease include loss of corticomedullary distinction (increased density in the medullary canal of the bone), remodeling of the proximal or distal borders of the navicular bone (enthesiophytes), lysis of the flexor cortex (the surface in contact with the DDFT), and more controversial as to association with disease is the increase in size and alteration of the shape of the distal border foramina (small openings in the bone surface). Many of these changes are also found in clinically normal horses.
Note that absence of radiographic changes does not rule out a diagnosis of navicular syndrome. Many painful issues that stem from disease of the navicular apparatus have to do with soft tissue inflammation of the bursa, tendon, or supporting ligaments. Such soft tissue injuries will not show up on an X ray.
It is generally accepted by the veterinary community that radiographs of the navicular bone correlate less than 40% of the time with clinical disease. In other words, horses with navicular "changes" on X ray exam do not always have clinical disease, while many horses with severe clinical disease will not show radiographic "changes" within the navicular bone. Caution should be taken in over-interpretation of navicular radiographic findings in a lame horse or in a horse being examined for purchase.
Farrier Care--The most important element of easing a foot-sore horse's pain is to trim and shoe him correctly to relieve abnormal stresses within the foot. This custom hoof care needs to be individualized to each horse. This includes balancing hooves side-to-side (medial to lateral) for landing as flat as possible on all aspects of the hoof. Landing on one side of the hoof wall even infinitesimally sooner than the other side elicits abnormal stresses. The foot will feel asymmetrical impact concussion along with excess tension in supporting tendons and ligaments, while the uneven loading creates damaging shearing forces in the foot.
Almost any horse with navicular pain, low laminitic pain, and/or crushed heels will find some degree of clinical relief from a well-made, well-placed egg bar. However, some prefer not to use egg bar shoes because of possible long-term crushing of the heels. Past studies have indicated that 57% of horses with heel pain respond favorably to application of egg bar shoes; however, some have experienced this benefit for only two to three resets.
Synthetic horseshoes play an important role in both treatment and prevention of foot-sore and joint-sore horses. Anti-concussion properties of polyurethane-impregnated aluminum shoes have been well recognized in sport horses for years. The varieties and brands are plentiful; experiment and find which style of synthetic shoe works best for your horse. The ideal strategy is one of prevention, and these shoes are a great adjunct in a preventive strategy.
Medical Therapy--Over the years, oral supplementation with isoxsuprine hydrochloride has been a mainstay in treating navicular horses. Another product that works similarly for the same purpose is pentoxifylline. Currently, these products are held in debate as to how effective either one is for this application.
In theory, circulation should increase in the foot with either of these medications; if the theory of thrombosis-ischemia is partially to blame for pain associated with navicular syndrome, then such medication might play an important role.
A 1996 study on six horses given isoxsuprine or pentoxifylline showed that neither medication elicited an increase in blood flow to the feet of healthy horses. However, it was also mentioned in that study that it is possible that these medications could be beneficial in diseased feet while having no effect on healthy feet. Also, the study only administered the medication for 10 days, yet it has been recognized that at least three to four weeks of treatment has been necessary to notice clinical improvement.
It is the author's experience that many horses with navicular syndrome have benefited from isoxsuprine treatment. Attempts to discontinue this medication in horses which have responded favorably have resulted in lameness relapses. If a horse is placed on isoxsuprine therapy for six weeks with no signs of improvement, then this medication probably has no role in treatment of that individual. If a horse is treated with isoxsuprine, check with medication control at horse shows for withdrawal time of this medication prior to competition.
Other systemic medications used to manage navicular pain include poylsulfated glycosaminoglycans (PSGAGs) given intramuscularly, or hyaluronic acid given intravenously. PSGAGs have been found to be most useful in navicular lameness that has been present for less than a year. It is thought that such systemic medication might work by inhibiting enzymes involved in the breakdown of the cartilage matrix.
Direct treatment of the navicular region involves injecting anti-inflammatory medications such as corticosteroids and hyaluronic acid directly into the coffin joints. Such intra-articular injections have provided excellent success in managing the chronic lameness of navicular disease. Generally, these horses require intra-articular injections one or two times a year to maintain athletic comfort. In refractory (chronic) cases, it might be necessary to keep horses comfortable by using a low dose of non-steroidal anti-inflammatory medications such as phenylbutazone, flunixin meglumine, or ketoprofen.
Surgical Options--Palmar digital neurectomy is a last-ditch option when medical and mechanical strategies fail to eliminate pain and lameness. A neurectomy involves cutting the palmar digital nerves that feed the navicular region, and eliminates sensation of pain from the back third of the foot. It is only palliative in its relief from pain; the degenerative process continues within the foot. Favorable results from this surgery might extend for as little as a few months to as long as many years. Adverse consequences can occur such as development of a painful neuroma (tumor arising from a nerve) or rupture of the DDFT (due to devitalization of the blood supply related to surgical trauma). Although an uncommon event, progressive degeneration of the flexor tendon in the area of the navicular structures could potentiate rupture of the DDFT with too much athletic stress over time in a horse that has undergone a neurectomy and can no longer feel the back third of the foot. Often, failure of this surgery to relieve pain is due to either incomplete removal of all the nerve branches at the time of surgery or regeneration of the severed nerves. In one report, 74% of horses undergoing a neurectomy remained sound for one year, while 63% remained sound for two years.3
Desmotomy (severing) of navicular suspensory ligaments is another surgical option with mixed reviews. In some reports, 50% of cases had improvement of clinical signs.
In all cases of medical and/or surgical intervention for navicular syndrome, appropriate shoeing techniques are still critical to success.
Managing the Heel-Sore Horse
Many sore-footed horses have spent time moving guardedly. Muscles in the neck and shoulders are often tight and restricted since the horse tries to land more softly on sore feet, and so he holds his body relatively rigid in front. Over time, the myofascial tissues of the upper body tighten, causing muscles to contract and shorten. Although shoeing management and foot therapy relieve immediate foot pain, these horses might continue to move with a short, choppy stride despite alleviation of the original source of the problem. Along with treating the feet, it is helpful to gather a support team of skilled people to address soft tissue restriction in your horse's shoulders and back. Find a capable acupuncturist who can improve circulation and energy flow through these tissues, and have massage or myofascial release techniques done as necessary to further free up your horse's movement.
Exercise and turnout help maintain lubrication and nutrition of cartilage in joints and navicular structures. Movement improves blood flow within the hooves, and regular movement minimizes adhesions between the navicular bone and flexor tendon.
As with most equine lameness problems, prevention is much better than treatment. But if you have a horse with heel pain, there are ways to make him more comfortable and allow you both to continue to enjoy equine athletic activities.
1 Anderson, B.; Turner, T. Diagnostic Findings Using a Navicular Syndrome Protocol. AAEP Proceedings, Vol. 39, Lexington, Ky., 209-211, 1993.
2 Turner, T. Predictive Value of Diagnostic Tests for Navicular Pain. AAEP Proceedings, Vol. 42, Lexington, Ky., 201-204, 1996.
3 Watkins, J. Navicular Suspensory Desmotomy in the Management of Navicular Syndrome: A Retrospective Analysis. AAEP Proceedings, Vol. 39, Lexington, Ky., 261-262, 1993.
About the Author
Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her recent book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care (available at Shop.TheHorse.com or by calling 800/582-5604). She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.
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