Navicular syndrome

Last updated May 18, 2021

Navicular syndrome, often called navicular disease, is a syndrome of lameness problems in horses. It most commonly describes an inflammation or degeneration of the navicular bone and its surrounding tissues, usually on the front feet. It can lead to significant and even disabling lameness.^[1]

Description of the navicular area

Knowledge of equine forelimb anatomy is especially useful for understanding navicular syndrome. The navicular bone lies behind the coffin bone and under the small pastern bone. The deep digital flexor (DDF) tendon runs down the back of the cannon and soft tissue in that area and under the navicular bone before attaching to the back of the coffin bone. The DDF tendon flexes the coffin joint, and the navicular bone acts as a fulcrum that the DDF tendon runs over.^[2]

The navicular bone is supported by several ligaments above, below, and on the side. One of these ligaments is the impar ligament, which attaches the navicular bone to the coffin bone (distal phalanx). Cartilage lies between the navicular bone and the coffin joint, as well as between the navicular bone and the DDF tendon. The navicular bursa - a small sac that protects the DDF and navicular bone from abrasion as the tendon slides over the area - lies between the navicular bone and the DDF tendon.^[3]

Causes and contributing factors

There is no single known cause of navicular syndrome, although there are many theories, and several primary factors.

The first factor is compression of the navicular bone under the DDF tendon and the back of the small pastern bone. Repeated compression in this area can cause cartilage degeneration, with the cartilage flattening and gradually becoming less springy and shock-absorbing. It may also begin to erode. Cartilage degeneration is common in navicular horses, usually along the flexor surface. This finding, and the associated biochemical changes, have led some researchers to conclude that there are elements in navicular disease common to osteoarthritis, and to suggest similar therapeutic regimes.^[4]

Cartilage erosion may progress to the point that the bone underneath will become exposed. With the cartilage no longer present to protect it, the navicular bursa and DDF tendon may become damaged by the constant rubbing against the navicular bone. Navicular bursitis (inflammation of the navicular bursa) may occur, even if cartilage damage is not severe. This is probably due to the friction between the navicular bone and the DDF tendon from compression.

Constant compression can also increase the bone density directly under the cartilage surfaces, especially on the flexor side. This tends to make the bone more brittle, and thus more likely to break.

Another main factor is the tension placed on the ligaments that support the navicular bone. Some experts believe that the degenerative process begins with excess tension placed on these ligaments, causing strain and inflammation. Inflammation from strain of the impar ligament can decrease blood flow to and from the navicular bone, as the major blood vessels supplying the bone run up and down this area. If the ligament continues to be strained, it can thicken and permanently reduce blood flow to the navicular bone.

Because veins are more easily compressed than arteries, blood flow to the bone would be less obstructed than blood flow from the bone. This would cause a buildup of pressure within the navicular bone. The navicular bone, in response to both the increased pressure and overall decreased blood supply, would absorb mineral from its center.

Excess tension can also cause exostoses where the ligaments attach to the navicular bone, giving the bone a "canoe" shape. If tension is extreme, the ligaments may actually tear.

Recent research has found correlations between "toe-first landing" of the hooves and navicular problems, due to excessive strain put on the deep digital flexor tendon, as a consequence of misalignment of the lower joints.^{[ citation needed ]} Toe-first landing, usually seen as a consequence of navicular disease, may actually be a cause or at least a contributing factor to the onset of tendon inflammation and bone modifications.

Toe-first landing is often caused by frog and heel overtrimming, long toes, and/or poor shoeing.

Contributing factors

Conformation

Certain conformational defects may contribute to navicular syndrome, especially defects that promote concussion. These include upright pasterns, small feet, narrow and upright feet, significant downhill build (commonly seen in American Quarter Horses), and long toes with low heels (commonly seen in Thoroughbreds).

The long toe, low heel conformation places constant stress on the navicular bone, even as the horse is standing. Upright feet increase concussion, especially in the heel region of the hoof where the navicular bone is located. Excess concussion cannot be absorbed as well by the structures designed to do so (the frog, heels, and digital cushion), so more impact is transmitted to the structures within the foot.

Poor hoof shape is usually inherited, although poor shoeing and trimming can contribute to these shapes.

With the long toe, low heel conformation can come contracted heels (narrowing of the heel) which further compresses the navicular bone along with sheared heels adding more stress to the tendons and navicular bones.

Shoeing

Poor trimming, shoe selection, or inappropriate shoe attachment are well-known causes of lameness,^[5] and navicular disease is fairly common in the modern-day domesticated horse. However, it is also seen in wild horse populations.^[6] Those who advocate "barefoot trimming" of domestic horses propose that in nature, a horse's hoof is designed to expand and contract as the horse moves. This expanding and contracting acts as an auxiliary blood pump, and aids the circulation of blood to the lower extremities. When an inflexible metal shoe is improperly attached to the hoof, the hoof can no longer work as designed, and blood flow is inhibited.^[7] However, evidence of navicular degeneration has been established in the fossil record of early horses.^[8] Mary Thompson, a vertebrate paleontologist at Idaho Museum of Natural History, has found evidence in many species of early horses and concludes, "The results of this study strongly suggest that man’s intervention (whether by increased usage or improper breeding practices) may not be the sole cause of the syndrome", although she cautions that her results are preliminary.^[9]^[10]

Work

Working on steep hills, galloping, and jumping all contribute to navicular syndrome, as they place greater stress on the DDF tendons, and may cause overextension of the pastern and coffin joints.

Regular exercise on hard or irregular ground increases concussion on the hoof, thus increasing the risk of navicular syndrome.

It is possible that standing can also increase the chance of navicular disease (such as a horse that spends most of the day in a stall with little turnout, as with some racehorses and show horses). Blood flow to the hoof decreases when the horse is not in motion. The horse is also constantly applying pressure to the navicular bones (which is intermittent as the horse moves).

Body weight

Horses with a high weight-to-foot-size ratio may have an increased chance of exhibiting symptoms of navicular syndrome, since the relative load on the foot increases. This might explain why the syndrome is seen more frequently in Thoroughbreds, American Quarter Horses, and Warmbloods as opposed to ponies and Arabians.^[11]

Signs

Heel pain is very common in horses with navicular syndrome. Lameness may begin as mild and intermittent, and progress to severe. This may be due to strain and inflammation of the ligaments supporting the navicular bone, reduced blood flow and increased pressure within the hoof, damage to the navicular bursa or DDF tendon, or from cartilage erosion.

Affected horses display a "tiptoe" gait - trying to walk on the toes due to heel pain. They may stumble frequently. The lameness may switch from one leg to another, and may not be consistent. Lameness usually occurs in both front feet, although one foot may be more sore than the other.

Lameness is usually mild (1–2 on a scale of 5). It can be made worse when the horse is worked on a hard surface or on a circle.

After several months of pain, the feet may begin to change shape, especially the foot that has been experiencing the most pain, which tends to become more upright and narrow.

Treatment and prognosis

No single treatment works for all cases, probably because there is no single cause for all cases. The degenerative changes are usually quite advanced by the time the horse is consistently lame, and these changes are believed to be non-reversible. At this time, it is best to manage the condition and focus on alleviating pain and slowing the degeneration.

Trimming

Putting the foot into proper neurological and biomechanical balance is crucial. Often navicular horses have long toes and underrun heels with very little inner wall depth or strength. Exposing the horse to proper stimulus to improve hoof form and structure is also vital.

Hoof care

The issue of hoof care is a subject of great debate. Corrective shoeing can be beneficial to horses suffering from navicular disease, although sometimes the effects are only temporary. Others believe that removing the shoes altogether is the best way to manage this disease, as it allows increased circulation to the hoof.^[12] People on both sides agree that proper hoof shape and angle are an important long-term management plan for a horse with navicular disease. As with laminitis, different horses may respond in different ways to a given technique, so the farrier, owner, and veterinarian should work as a team to formulate a plan and to adapt if the initial plan is not effective.

People who choose to treat navicular disease through shoeing may use a shoe designed to lift and support the heels.^[13] This can sometimes be accomplished with a flat shoe and trimming alone. Wedge pads or wedged shoes are often used, but can amplify heel-related problems if present.^[14] Another strategy is to use a bar type shoe. Often, an egg-bar shoe,^[15] or straight bar shoe.^[13] Some horses benefit from shoes that change the breakover of their foot (like a rolled toe). With or without shoes, the hoof must be trimmed in such a way as to restore the balance and angle that may have been lost. Horses with long toe-low heel conformation need careful trimming to counter this. Horses with upright feet may need their heels lowered and a shoe that will allow their heels to spread. Early intervention is key; in one study, shoeing was successful in 97% of horses treated within ten months of the onset of signs, while only 54% of horses lame for over a year responded.^[16]

Advocates of barefoot trimming cite recent studies which show that removing the shoes can help alleviate the symptoms of navicular disease, and in some cases, reverse some of the damage done to the hoof.^[17] Because navicular disease can be caused, or at least exacerbated by shoeing, removing the cause is the first step in this strategy toward the management of pain in the animal.^[12] Successive carefully applied trims help to restore the natural angle and shape of the hoof, while walking helps to stimulate circulation to the hoof.^[17] It is not uncommon to find horses whose navicular disease is completely manageable through corrective barefoot trimming.^[12] However, this may require a transition period lasting from weeks to years where the horse may remain lame,^[6] or may never become sound.^[18]^[19]^[20]

If there is significant degeneration in the bone, a flexor cortex cyst, adhesions to the deep digital flexor tendon, or avulsion fractures, relief is typically incomplete no matter what foot care technique is used.^[21]

Exercise

Horses with navicular syndrome need a less intense work schedule. Their fitness can be maintained through slow long-distance work or swimming, as opposed to being worked at high speeds, up steep hills, or on hard surfaces, irregular terrain, or deep footing. Reducing the frequency of jumping is also important. Some veterinarians and hoof care practitioners recommend exercising the horse on varied terrain to stimulate and strengthen the caudal hoof structures.^[22]

Medication

Vasodilators improve the blood flow into the vessels of the hoof. Examples include isoxsuprine (currently unavailable in the UK) and pentoxifylline.

Anticoagulants can also improve blood flow. The use of warfarin has been proposed, but the extensive monitoring required makes it unsuitable in most cases.

Anti-inflammatory drugs are used to treat the pain, and can help the lameness resolve sometimes if shoeing and training changes are made. Include Nonsteroidal anti-inflammatory drugs (NSAIDs), corticosteroids, and other joint medications. The use of intramuscular glycosaminoglycans has been shown to decrease pain in horses with navicular disease, but this effect wanes after discontinuation of therapy.^[23] Oral glycosaminoglycans may have a similar effect.^[24]

Bisphosphonates can be useful in cases where bone remodeling is causing pain.^[25]

Gallium nitrate (GaN) has been hypothesized as a possible treatment for navicular disease, but its benefits have not been confirmed by formal clinical studies. One pilot study examined horses given gallium nitrate in their feed rations. While it was absorbed slowly, it did stay in the animals' system, providing a baseline dosage for future studies.^[26]

Surgery

Palmar digital neurectomy (or "nerving" or "denerving") is not without adverse side effects and should therefore be used as a last resort. In this procedure, the palmar digital nerves are severed, so the horse loses sensation in the back of the foot. This procedure should only be performed if it will eliminate the lameness associated with navicular syndrome, and only after all other options have been explored. The procedure is usually performed on both front feet. Complications can include infection of the wound, continuation of the lameness (if the nerves regrow or if small branches of the nerves are not removed), neuromas, and rupture of the deep digital flexor tendon. After the neurectomy, if the horse becomes injured in the area the injury may go undetected for a long period of time, which risks the animal's health. Due to this, the feet should be cleaned and inspected regularly. Neurectomy tends to lower the market value of a horse, and may even make the horse ineligible for competition. Neurectomy is controversial. The most common misconception about "nerving" a horse is that it will permanently solve the lameness/pain issue. In fact, though the time periods vary based on the individual horse and surgical method utilized, these nerves often regenerate and return sensation to the afflicted region within two to three years.

In navicular suspensory desmotomy, the ligaments supporting the navicular bone are severed. This makes the navicular bone more mobile, and thus reduces the tension of the other ligaments. It is successful about half of the time.

Prognosis

The prognosis for a horse with navicular syndrome is guarded. Many times the horse does not return to its former level of competition. Others are retired. Eventually all horses with the syndrome will need to lessen the strenuousness of their work, but with proper management, a horse with navicular syndrome can remain useful for some time.

Related Research Articles

The foot is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is a separate organ at the terminal part of the leg made up of one or more segments or bones, generally including claws or nails.

The heel is the prominence at the posterior end of the foot. It is based on the projection of one bone, the calcaneus or heel bone, behind the articulation of the bones of the lower leg.

A hoof, plural hooves or hoofs, is the tip of a toe of an ungulate mammal, strengthened by a thick and horny keratin covering.

The navicular bone is a small bone found in the feet of most mammals.

Diseases of the foot generally are not limited, that is they are related to or manifest elsewhere in the body. However, the foot is often the first place some of these diseases or a sign or symptom of others appear. This is because of the foot's distance from the central circulation, the heart and its constant exposure to pressures from the ground and the weight of the body.

Laminitis is a disease that affects the feet of ungulates and is found mostly in horses and cattle. Clinical signs include foot tenderness progressing to inability to walk, increased digital pulses, and increased temperature in the hooves. Severe cases with outwardly visible clinical signs are known by the colloquial term founder, and progression of the disease will lead to perforation of the coffin bone through the sole of the hoof or being unable to stand up, requiring euthanasia.

Bone spavin is a bony growth within the lower hock joint of horse or cattle. It is caused by osteoarthritis, and the degree of lameness that results can be serious enough to end a horse's competitive career.

Tendinitis/tendonitis is inflammation of a tendon. Many times, the tendon tissue is torn. A bowed tendon is a horseman's term for a tendon after a horse has sustained an injury that caused the tendon fibers to be torn, and then healed with "bowed" appearance.

Equine conformation evaluates a horse's bone structure, musculature, and its body proportions in relation to each other. Undesirable conformation can limit the ability to perform a specific task. Although there are several faults with universal disadvantages, a horse's conformation is usually judged by what its intended use may be. Thus "form to function" is one of the first set of traits considered in judging conformation. A horse with poor form for a Grand Prix show jumper could have excellent conformation for a World Champion cutting horse, or to be a champion draft horse. Every horse has good and bad points of its conformation and many horses excel even with conformation faults.

Ringbone is exostosis in the pastern or coffin joint of a horse. In severe cases, the growth can encircle the bones, giving ringbone its name. It has been suggested by some authors that such a colloquial term, whilst commonly used, might be misleading and that it would be better to refer to this condition as osteoarthritis of the inter-phalangeal joints in ungulates.

Sidebone is a common condition of horses, characterized by the ossification of the collateral cartilages of the coffin bone. These are found on either side of the foot protruding above the level of the coronary band. The lateral cartilages support the hoof wall and provide an important role in the support and cushioning provided to the heel. The front feet are most commonly affected.

The frog is a part of a horse's hoof, located on the underside, which should touch the ground if the horse is standing on soft footing. The frog is triangular in shape, and extends mid way from the heels toward the toe, covering around 25% of the bottom of the hoof.

A horse hoof is a structure surrounding the distal phalanx of the 3rd digit of each of the four limbs of Equus species, which is covered by complex soft tissue and keratinised (cornified) structures. Since a single digit must bear the full proportion of the animal's weight that is borne by that limb, the hoof is of vital importance to the horse. The phrase "no hoof, no horse" underlines how much the health and the strength of the hoof is crucial for horse soundness. A 2018 study has found that it may possibly contain remnants of the horse's other digits.

Equine anatomy refers to the gross and microscopic anatomy of horses and other equids, including donkeys, and zebras. While all anatomical features of equids are described in the same terms as for other animals by the International Committee on Veterinary Gross Anatomical Nomenclature in the book Nomina Anatomica Veterinaria, there are many horse-specific colloquial terms used by equestrians.

A flexion test is a preliminary veterinary procedure performed on a horse, generally during a prepurchase or a lameness exam. The purpose is to accentuate any pain that may be associated with a joint or soft-tissue structure, allowing the practitioner to localize a lameness to a specific area, or to alert a practitioner to the presence of sub-clinical disease that may be present during a pre-purchase exam.

The skeletal system of the horse has three major functions in the body. It protects vital organs, provides framework, and supports soft parts of the body. Horses typically have 205 bones. The pelvic limb typically contains 19 bones, while the thoracic limb contains 20 bones.

Lameness is an abnormal gait or stance of an animal that is the result of dysfunction of the locomotor system. In the horse, it is most commonly caused by pain, but can be due to neurologic or mechanical dysfunction. Lameness is a common veterinary problem in racehorses, sport horses, and pleasure horses. It is one of the most costly health problems for the equine industry, both monetarily for the cost of diagnosis and treatment, and for the cost of time off resulting in loss-of-use.

Natural hoof care is the practice of keeping horses so that their hooves are worn down naturally and so do not suffer overgrowth, splitting and other disorders. Horseshoes are not used, but domesticated horses may still require trimming, exercise and other measures to maintain a natural shape and degree of wear.

The limbs of the horse are structures made of dozens of bones, joints, muscles, tendons, and ligaments that support the weight of the equine body. They include two apparatuses: the suspensory apparatus, which carries much of the weight, prevents overextension of the joint and absorbs shock, and the stay apparatus, which locks major joints in the limbs, allowing horses to remain standing while relaxed or asleep. The limbs play a major part in the movement of the horse, with the legs performing the functions of absorbing impact, bearing weight, and providing thrust. In general, the majority of the weight is borne by the front legs, while the rear legs provide propulsion. The hooves are also important structures, providing support, traction and shock absorption, and containing structures that provide blood flow through the lower leg. As the horse developed as a cursorial animal, with a primary defense mechanism of running over hard ground, its legs evolved to the long, sturdy, light-weight, one-toed form seen today.

The treatment of equine lameness is a complex subject. Lameness in horses has a variety of causes, and treatment must be tailored to the type and degree of injury, as well as the financial capabilities of the owner. Treatment may be applied locally, systemically, or intralesionally, and the strategy for treatment may change as healing progresses. The end goal is to reduce the pain and inflammation associated with injury, to encourage the injured tissue to heal with normal structure and function, and to ultimately return the horse to the highest level of performance possible following recovery.

References

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1 2 Natural Hoof - Articles
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↑ Equine Podiatry | Dr. Stephen O'Grady, veterinarians, farriers, books, articles
↑ Dyson S (2003) "Treatment and Prognosis of Horses with Navicular Disease" In Ross M, Dyson S, Diagnosis and Management of Lameness in the Horse 299-304, Saunders ISBN 0-7216-8342-8
↑ Bringing the Sparkle Back in to Crystal's Life | Dr. Tomas Teskey, DVM
↑ Crissman et al. (1993) "Evaluation of polysulfated glycosaminoglycan for treatment of navicular disease", Proceedings of the American Association of Equine Practitioners Convention pp.219-220
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