Navicular Disease

Navicular Disease is a soundness problem in horses, more accurately called "navicular syndrome" as opposed to "disease." It is 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.

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Description of the Navicular Area

See also: Equine forelimb anatomy and Skeletal system of the horse

Knowledge of equine forelimb anatomy is especially useful for understanding navicular syndrome. The navicular bone (or distal sesamoid) 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.

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.^[1]

Causes and Contributing Factors of Navicular Syndrome

There is no single known cause of Navicular Syndrome, although many theories. The two most important factors in Navicular Syndrome are:

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. The cartilage 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.^[2]

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 actually tends to make the bone more brittle, and thus more likely to break.

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. Excess tension causes 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 build up 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.

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), and 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 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.

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 increase the chance of navicular disease (such as a horse that spends most of the day in a stall with little turnout, usually seen in racehorses and some show horses). This is because 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 Navicular Syndrome is seen more frequently in Thoroughbreds, American Quarter Horses, and Warmbloods as opposed to ponies and Arabians.

Symptoms

Heel pain is very common in horses with Navicular Syndrome. Lameness may begin as mild and intermittent, and progress to severe lameness. 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.

Symptomatic 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 for Navicular Syndrome

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

Shoeing

Proper support is an important long-term management plan for a horse with Navicular Syndrome. The farrier designs a shoe that will not only provide support for the heels - typically, a bar shoe, e.g. an egg-bar shoe - but trims the hoof to restore the balance and angle that may have been lost. Some horses will also benefit from shoes that change the breakover of their foot (like a rolled toe). Horses with long toe-low heel syndrome 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.

Exercise

Horses with Navicular Syndrome need a work schedule that is less intense. Their fitness can be maintained by slow long-distance work or swimming, as opposed to being worked at high speeds, up steep hills, on hard surfaces, irregular terrain, or deep footing. Reducing the frequency of jumping is also important.

Medication

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

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

Anti-Inflammatory Drugs: used to treat the pain, can help the lameness resolve sometimes if shoeing and training changes are made. Include NSAIDs, corticosteriods, and other joint medications. Some vets report success with glycosaminoglycans, but this is still experimental.^[3]

Surgery

Palmar Digital Neurectomy (or "nerving" or "denerving"): 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), and neuromas. 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. It is best to speak to your vet if you consider it as an option. 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 will often regenerate and return sensation to the afflicted heel region within two to three years.

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 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 Navicular 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.

References

Sources

• King, Christine & Mansmann, Richard, VMD, PhD. Equine Lameness. Copyright Equine Research (1997). (p. 610-626).
• PT Colahan, IG Mayhew, AM Merrit & JN Moore Manual of Equine Medicine and Surgery Copyright Mosby Inc (1999). (p. 402-407).
• RJ Rose & DR Hodgson Manual of Equine Practice Copyright WB Saunders (2000). (p. 126-128).
• Viitanen M, Bird J, Smith R, Tulamo RM, May SA, "Biochemical characterisation of navicular hyaline cartilage, navicular fibrocartilage and the deep digital flexor tendon in horses with navicular disease.", Res Vet Sci. 2003 Oct;75(2):113-20
• Dr Reid Hanson, Auburn University, cited in Proceedings of the World Equine Veterinary Association Congress 1997. See [2]

See also

• Horse hoof
• Lameness (equine)
• Equine forelimb anatomy