By Miranda Noseck, DVM, 

Petroglyph Animal Hospital

First let’s begin by locating the navicular bone in the horse. Each of your horse’s hooves contains two bones: the distal phalanx (coffin bone or P3) and the distal sesamoid bone (navicular bone). The navicular bone is a small, boat-shaped bone that is bordered by the coffin bone, middle phalanx (P2), and deep digital flexor tendon (DDFT). It is approximately six centimetres in length and two centimetres in width in the average 1200 pound horse.

The main purpose of the navicular bone is to act as a fulcrum for the DDFT as it changes direction and attaches to the bottom of the coffin bone. It is at this level in the foot where the DDFT naturally exerts major compressive forces against the distal one third of the navicular bone. The navicular bone also acts as a shock absorber for the coffin joint. Because of its two purposes, the tiny navicular bone is required to withstand constant concussive and compressive forces. Between the navicular bone and the DDFT is a small space called the navicular bursa. This small space contains synovial fluid, which provides frictionless movement for the DDFT as it passes over the navicular bone. Beneath the DDFT lies the digital cushion, which acts to provide further shock absorption for the foot. The collateral ligaments suspend the navicular bone proximally, while distally the bone is suspended by the impar ligament.

Illustration by Elisa Crees

Navicular disease is the chronic degeneration of the navicular bone and its surrounding structures. The reason for this degeneration is not yet fully understood. There are two theories. The vascular theory suggests a profound decrease in blood supply to the navicular bone causes thrombosis (formation of blood clots) and ischemia (deficient blood supply) and subsequently pain and degeneration. However, this theory has largely been rejected due to researchers’ inability to identify ischemic bone or thrombosis. Researchers have also been unable to reproduce the clinical signs of navicular disease by occluding the blood supply to the bone.

The second and more widely accepted biomechanical theory suggests that pain and degeneration are due to an imbalance between the horse’s anatomical conformation and the mechanical load on the foot. Because of this imbalance, stress is placed on the navicular bone and its surrounding structures including the DDFT, collateral ligaments, impar ligament, and navicular bursa. Concussion, compression, friction, and tension from the ligaments are forces every foot is required to withstand. The biomechanical theory suggests the size, shape, and balance of the foot along with the size and shape of the navicular bone can drastically affect the surrounding tissues when these daily forces are applied.

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Contact studies have found that the maximum amount of force is applied to the navicular region during the propulsion phase of the stride, which is when the coffin joint is extended. Several things happen at this particular moment during the stride:

• The DDFT increases pressure against the navicular bone;
• There is an increase in contact of the navicular bone with P2; and
• Finally, tension of the collateral and impar ligaments also increases.  

With a poorly balanced foot or poor foot conformation, the navicular bone is subjected to abnormal forces causing inflammation and degeneration. It is also theorized that there is a hereditary component to this disease as well.  

The resulting lameness is most commonly bilateral (affecting both feet) in the forelimbs, but it is occasionally seen in the hind limbs. It can be slowly progressive and insidious in its onset as a bilateral lameness; however, it can also arise as acute, severe, unilateral limb lameness.

Researchers have found that the most commonly affected breeds include Quarter Horses, Thoroughbreds, and European Warmbloods. Quarter Horses tend to have narrow, upright feet that are small relative to their body size. Many European Warmbloods also have tall, narrow feet. Thoroughbreds tend to have feet that are flat with low, collapsed heels. Navicular disease is least commonly seen in Arabians, ponies, and draft breeds.

Researchers have found that Quarter Horses are one of the breeds most commonly affected by navicular. Photo: River Bend Designs

Diagnosis

Diagnosis of navicular disease can be challenging due to the location of the navicular region within the hoof capsule as well as the numerous structures that could be the source of the resultant pain. To begin diagnosis, your veterinarian will start by performing a lameness exam to locate where the pain is emanating from. This is done by watching the horse during a moving evaluation on hard and soft surfaces, as well as in straight lines and in circles. This moving evaluation is most commonly done in-hand; however, your veterinarian may ask to see the horse under saddle. A series of flexion tests will be performed. Hoof testers will be placed on each foot in search of sensitive areas.

Diagnosing navicular disease can be challenging. The veterinarian will start by performing a lameness exam to determine where the location of the horse’s pain. Photo: Pam MacKenzie

Related: Understanding Laminitis

When the area of concern is identified, your veterinarian will perform one or more nerve blocks to confirm. A nerve block is performed by instilling a local anesthetic, such as carbocaine, alongside a nerve, temporarily desensitizing it. After the nerve block is performed, the horse continues the moving evaluation to verify whether or not the nerve block affected the lameness. If the horse is no longer showing lameness, your veterinarian has confirmed the area of the limb where the pain is originating. When navicular disease is suspected, a palmar digital nerve block will resolve the pain. This nerve block desensitizes many structures within the hoof capsule including the navicular bone and its surrounding structures.

Hoof testers are used to find sensitive areas of the hoof. Photo: Pam MacKenzie

The next diagnostic step is to take a full series of radiographs of the suspected foot or feet. There are various different radiographic changes seen in association with navicular disease, though some horses never develop radiographic changes. This presents another challenge in diagnosis. While radiography remains the main route for diagnosis, it is not always reliable. An advanced diagnostic technique called magnetic resonance imaging (MRI) is becoming increasingly widespread in the equine industry. Radiography provides an image that only focuses on bony changes, whereas MRI can focus on both bony and soft tissue changes. This difference opens a wide window to aid in specifically identifying the affected structures. Some of the abnormalities that MRI can decipher include: DDFT tendonitis, fibre disruption of the DDFT, DDFT adhesions to the navicular bone or navicular bursa, navicular bursitis, desmopathy or adhesions of the impar or collateral ligaments, erosion of the flexor surface of the navicular bone, and fluid within the navicular bone. The advantage of using MRI is that it provides a very precise diagnosis of what structures are involved in the navicular disease process of your particular horse and subsequently a specific treatment plan can be instituted. The disadvantages to using MRI to obtain the diagnosis are cost, the need for general anesthesia, and the limited number of MRI units in the country.

A moving evaluation is an important part of the lameness exam. Photo: Pam MacKenzie

Treatment

Treatment options for navicular disease depend on the structures involved. For a case of bilateral lameness that shows a slow, progressive onset, management of the disease is key. For acute cases, rest, anti-inflammatories, and a proper rehabilitation program are important. The mainstay to treatment is proper shoeing and trimming. Therefore, it is important that you have an open relationship with your farrier in order to develop a proper shoeing plan for your horse.

The most conservative approach to pain control is through use of non-steroidal anti-inflammatories, such as phenylbutazone (bute). For horses not responsive to phenylbutazone more advanced techniques may be necessary. Intra-articular injections of hyaluronic acid (synthetic joint fluid) and steroid can be used to quiet inflammation occurring in the coffin joint, which may or may not alleviate pain originating from the navicular region. The navicular bursa can be injected by a radiograph-guided injection technique to help quiet inflammation. The DDFT sheath can also be injected in hopes of reducing inflammation. The amount of time that these types of injections provide pain relief varies drastically among horses and depends on the severity of the disease. Other pharmacologic approaches include joint supplements of intravenous, intramuscular, and oral forms, and oral isoxsuprine hydrochloride, which is theorized to dilate blood vessels and secondarily increase blood flow. Acupuncture can also be performed to help control pain.

If the horse is no longer responsive to the medical therapies mentioned above, surgical options are available. A palmar digital neurectomy can be performed, which is a procedure that removes a portion of the palmar digital nerve. The result is a desensitized, non-painful foot. This procedure is used as a last effort to allow the horse to remain comfortable. Horses that have had a neurectomy are not recommended to continue in a high-level, athletic, competitive fashion due the safety risks of riding these horses.

Navicular disease is a complex disease process. Specific diagnosis and appropriate treatment continue to be the main challenges.

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Main Photo: A full series of radiographs will be required to determine if any bony changes have taken place in the hoof. Credit: Nottingham Vet School