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If you've spent any time in the equine industry, you've probably heard of navicular disease. While it's perceived as a dreaded condition, what exactly is navicular disease and how does it occur?

Navicular disease is a complex of progressive degenerative changes involving many structures in the bottom-or distal-part of a horse's leg. These changes accumulate over time, resulting in lameness that often begins slowly and increases over time, though sometimes the lameness can occur suddenly.

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Navicular disease affects several structures in the lower part of a horse's leg. Degenerative changes of the navicular bone and its cartilage are central to the problem, but many other structures-ligaments, tendons, and the navicular bursa-can also be involved. The result of the changes is a slowly progressive disease resulting in inflammation, pain, and lameness.

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What causes navicular disease? Unfortunately, there is no easy answer to this question, and there are many things that can cause navicular disease. One thing most proposed causes have in common are physical forces (biomechanical forces) that the navicular bone and associated structures experience over time. On rare occasions, a sudden injury can damage the navicular bone, but most cases are the result of degenerative changes over time.

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As bones and other structures are exposed to forces, they adapt to better handle those forces-similar to how muscles will get bigger and stronger when workload is increased. To adapt to the forces, the navicular bone must "recreate" itself by going through a process called "remodeling." Remodeling is an intricate, highly regulated process where the bone uses two types of cells-osteoblasts and osteoclasts-to sequentially resorb and then reform the bone in a delicate balance to form a "new" bone that is better adapted to the forces it experiences as part of normal function. Osteoblasts build new bone after osteoclasts have resorbed old or damaged bone. Due to a variety of reasons, when this process gets out of balance, the osteoclasts remove bone faster than osteoblasts can create new bone. The result is a bone of lesser quality. When the navicular bone is involved, navicular disease can be the result.

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Typically, navicular disease affects mature riding horses, but can be seen in young horses early in their training careers. While any breed of horse, in any riding discipline, can be affected by navicular disease, there are certain breeds that are affected more often. It is common in Quarter Horses and has been associated with narrow, boxy, upright hooves that are small relative to the horse's body weight. Thoroughbreds and thoroughbred crosses with flat feet and low, collapsed heels are also commonly affected. Many European warmbloods also tend to have narrow, upright feet and can develop navicular disease.¹

The consistent feature is that all these hoof conformations lead to increased force being applied to the foot and the entire navicular apparatus. Interestingly, there has been recent work demonstrating certain lines of some warmblood breeds are prone to navicular disease, suggesting that these horses have a heritable or genetic tendency to develop navicular disease, while other breeds, like Arabians and the Friesian, have a lower tendency to develop the disease.

¹ Dyson S, Murray R, Schramme, et al. Current concepts of navicular disease. Equine Vet Educ 2011;23(1):27-19.

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How is navicular disease diagnosed? There are a variety of methods veterinarians can use to diagnose navicular disease. It begins when a horse's owner or trainer detects a lameness, usually a front leg lameness. A thorough history often reveals that lameness started as mild and intermittent and, over time, has increased in both severity and consistency. While variable and not 100% specific, horses with navicular disease often have some degree of lameness in both front legs with a gait described as "short and stabby"-especially on tight turns-and a tendency to land toe-first.

Various diagnostic methods can include:

• Physical examination-findings may show the horse has hoof conformation that fits into the type often associated with navicular disease
• Hoof testers- use will often find pain across the heel part of the hoof(ves)
• Nerve blocks- blocking the posterior digital nerves (heel nerves) should improve or eliminate the lameness in most cases. Often blocking one leg leads to the horse being lame in the opposite front leg.
• Radiographs- includes view to highlight the navicular bone and is the most common imaging technique used to diagnose navicular disease and can reveal a wide variation in change involving the navicular bone and the structures connected to the bone
• Ultrasound- not as commonly used as other imaging methods, it can provide information about changes in the deep digital flexor tendon that runs down the back of the pastern, underneath the navicular bone, and into the hoof.
• Magnetic Resonance Imaging (MRI)- has become the "gold standard in diagnosing navicular disease because of the ability to image both the soft tissue and bony structures of the navicular apparatus.
• Less commonly used procedures include nuclear scintigraphy, thermography, computed tomography (CT).

The diagnosis of navicular disease is not always straightforward. For example, there are horses that are lame on one or both front legs, go sound with the appropriate heel block, are painful across the heels with hoof testers, and have normal radiographs. Other horses have what most would consider abnormal looking navicular bones on radiographs and never take a lame step. Because there is not one diagnostic test that is always accurate for diagnosing navicular disease, in most instances several different diagnostic tests are performed, and the information is put together to reach a diagnosis.

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With a syndrome that has many potential causes, and potentially many different structures involved, there are many potential treatments for navicular disease. In most cases, the approach to treating a horse with navicular disease may include several different treatments used together and can vary between horses. In many instances, the goal of a treatment program is not to cure the disease but to manage the disease by making the horse more comfortable and able to return to some level of activity.

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Rest is an often-overlooked part of treating most causes of lameness in horses, including navicular disease. Time out of work decreases the stress placed on the navicular apparatus, allows soft tissue inflammation to resolve, and allows remodeling of the affected bones to take place. A period of rest as part of a treatment program can have a positive impact on the eventual outcome.

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The most common treatment for navicular disease involves changes in shoeing the horse. Many horses respond well to correction of the hoof abnormalities associated with navicular disease. Carefully evaluating the conformation and balance of the hoof is the first step in creating a shoeing plan designed to reduce the forces that are placed on the entire navicular apparatus. This shoeing goal is often started by correcting the balance of the hoof and correcting any abnormalities in the angle between the hoof and the pastern

Second, efforts are made to protect the back part of the foot (where the navicular bone sits) from excessive force, in part by maintaining good height and spread to the heel. The third part of most shoeing plans is to decrease the amount of work the foot must do by making it easier for the hoof to "breakover." This can be done by shortening the length of the toe and/or rolling the toe. In most cases, the horse with navicular disease benefits from a shoe that is set back to support the heels and wide enough to allow the hoof to expand. Veterinarians and farriers working together toward a common goal benefit both the horse and the horse owner.

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For a long time, horses with navicular disease have been treated with non-steroidal anti-inflammatory drugs (NSAIDs) such as phenylbutazone, flunixin meglumine, or firocoxib. These drugs are generally given to horses to improve their comfort level by decreasing inflammation but do not have an effect to help resolve the underlying problem. Despite that, periodic use of NSAIDs can benefit many horses diagnosed with navicular disease. However, long-term use of some NSAIDs can cause problems in horses such as ulcers and some kidney problems.

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Two other drugs that have been used to treat horses with navicular disease are isoxsuprine and pentoxifylline. Both drugs act to dilate blood vessels and may decrease the viscosity, or thickness, of the blood but the way they may potentially help a horse with navicular disease is not really known. A few studies have looked at the effects of each of these drugs on horses with navicular disease, and some have demonstrated some benefit to these horses. The effects are quite variable between horses.

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For many horses that have a specific site in the navicular apparatus that is identified as being a source of the problem- the navicular bursa, the coffin joint- direct injections of any of several medications with anti-inflammatory effects may be helpful for some horses. Corticosteroids were the original drugs used, and are still used today, but with the development of newer "orthobiologic" therapies many veterinarians are now choosing these products for these injections. Most of these products are derived from the horse's own blood and injected into the affected bursa or joint. Some examples of orthobiologics are interleukin receptor antagonist protein (IRAP), platelet rich plasma (PRP), other blood derived products, and products derived from the birth tissues of mares.

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Within the last ten years, a new class of drugs has been approved for use in horses with navicular disease, the bisphosphonates. As previously discussed, when the bone remodeling process gets out of balance, osteoclasts remove old and damaged bone faster than osteoblasts can lay down new, stronger bone. Bisphosphonates selectively target and disable osteoclasts, resulting in slowing the resorption of bone. This gives the osteoblasts a chance to "catch up" and get the remodeling process back into balance. The FDA has approved these products intended for treating horses with navicular disease, and they have been shown to be both safe and effective when used according to their label instructions for decreasing lameness in horses with navicular disease 4 years of age and older.

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For horses that have unsuccessfully been treated with multiple therapies to manage their navicular disease, surgical options may benefit them. Horses that become sound when local anesthetic is placed over the nerves in the back of the foot (posterior digital or "heel" nerves) can have those nerves cut (neurectomy) to remove sensation to the painful part of the foot. The results vary, but some studies report 74-77% of horses that had a neurectomy were sound one year after surgery, and 63% were sound after two years.^1,2

Another surgical technique involves cutting the ligaments that suspend the navicular bone, attempting to decrease the forces on the bone. In the study, looking at the largest group of horses that had this procedure, 76% were sound six months after surgery, but only 43% were sound after three years. As with all surgical procedures, there can be complications and some horses will not be sound after the surgery.

¹ Jackman B, Baxter G, Doran R, et al. Palmar digital neurectomy in horses 57 cases (1984-1990). Vet Surg 1993;22(4):285-288.

² Matthews S, Dart A, Dowling B. Palmar digital neurectomy in 24 horses using the guillotine technique. Aust Vet J 2003;81:402-405.

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Another treatment method that has been advocated for navicular disease is extracorporeal shockwave therapy (ESWT). ESWT uses high-energy sound waves delivered to the targeted tissue to stimulate blood flow and promote healing at the site. For horses with navicular disease, shockwaves are usually administered through the frog in a series of treatments. As with many therapies for navicular disease, the reported results have been quite variable- some horses respond well, while others do not improve at all.