Equine Navicular Disease
The navicular bone is a small, boat-shaped bone nestled deep in the protective womb of the hoof, cushioned by the digital bursa (a small, fluid-filled sac), shrouded by the deep digital flexor tendon, and bathed in synovial fluid. It only measures approximately 6 cm wide and 2 cm deep (top to bottom) in an average 1,200-pound horse, so how can such a small bone be such a nuisance?
Part of the problem is that, despite its cushy abode and small stature, the navicular bone has a big job.
"The function of the navicular bone is to act as a fulcrum around which passes the deep digital flexor tendon (DDFT) before it inserts on the distal phalanx (pedal bone)," explains Sue Dyson, MA, VetMB, PhD, DEO, FRCVS, head of clinical orthopaedics at the Animal Health Trust's Centre for Equine Studies in Newmarket, U.K. "The navicular bone is also an integral part of the distal interphalangeal (coffin) joint, which is one of the major shock absorbing joints in the lower part of the limb. The orientation of the DDFT depends on the position of the coffin joint (flexed or extended), and the position of the coffin joint also influences the size of the forces applied to the navicular bone by the DDFT, which are maximum in the propulsion phase of the stride just before liftoff."
Navicular disease is a common cause of equine lameness, primarily in the forelimb. Current estimates suggest that up to one-third of all horses with chronic forelimb lameness have navicular disease.
Here we'll review prevailing theories regarding the cause of navicular disease and describe diagnostic and treatment options, both traditional and modern.
The Face of Navicular Disease
Researchers have noted that Quarter Horses, Thoroughbreds, and European Warmblood horses (e.g., Dutch Warmbloods) between the ages of four and 15 years are most commonly affected by navicular disease. Geldings are more frequently diagnosed than mares or stallions; ponies and Arabians are rarely affected. The lameness is often bilateral (affects both feet) in the forelimbs rather than hind limbs. Horses with navicular disease often appear sore after working, but they can appear sound when not working.
Navicular horses' gaits are quite characteristic. Severely affected horses place their weight on the toe first when walking, supposedly to minimize pressure in the heel area where the associated pain originates. While moving the gait is rough, and when standing horses often shift their weight from foot to foot or point their foot (again, to relieve pressure in the heel-region).
"This latter sign is not necessarily specific for navicular disease. It is also seen in horses with primary DDFT injuries," notes Dyson.
What Causes Navicular Disease?
Navicular disease is a catchall phrase that describes horses with forelimb pain originating from the navicular bone and/or its supporting soft tissues, including the:
- Collateral ligaments of the navicular bone;
- Distal sesamoidean impar ligament; and
- Deep digital flexor tendon.
"Historically, navicular disease was considered a single disease," notes Dyson. "Considering the variety of clinical manifestations of navicular disease, however, it is likely that there are probably a number of clinical conditions that give rise to palmar foot pain (that found in the rear of the foot) characteristic of navicular disease. In fact, magnetic resonance imaging (MRI) and postmortem studies have shown us that there are a number of different disease processes that can affect the navicular bone."
This explains why some horses have a slow, insidious onset of progressive bilateral forelimb lameness, while others suffer acute, sudden onset of severe, unilateral lameness. Also, it explains why some horses have obvious and classic change to the navicular bones, whereas other horses might not ever have radiographic evidence of changes.
"Since navicular disease has never successfully been reproduced experimentally, the exact mechanisms of the disease remain speculative," relays Dyson.
The vascular and biomechanical theories are two popular explanations of how navicular disease develops:
The vascular theory A profound decrease in blood flow to the foot causes pain and degeneration of the navicular bone. For example, thickening of the inner lining of the blood vessels that supply the foot can cut off the blood supply, causing tissue damage and death due to lack of blood flow.
The biomechanical theory Chronic, sustained pressure on the navicular bone by the deep digital flexor tendon secondary to abnormal forces placed on the tendon results in abnormal remodeling of the navicular bone. In response, the bone begins to degenerate and becomes edematous (fluid-filled) and painful.
According to Dyson, "There is little evidence to support the vascular theory for navicular disease (defined as lesions involving the bone), but we do have evidence that blood vessel occlusion does occur within the DDFT and may predispose to degenerative lesions of the DDFT."
Dyson notes that there is likely some genetic predisposition involved in the development of the disease, as well.
Diagnostic Hurdles and Advances
Due to the surreptitious location of the navicular bone, together with the dearth of knowledge about the underlying cause(s) of disease, diagnosis remains challenging.
"Traditional diagnostic techniques include diagnostic anesthesia (joint and nerve blocks), radiography, nuclear scintigraphy (bone scans), and diagnostic ultrasonography," explains Chris Bell, DVM, from the department of large animal clinical sciences at the Western College of Veterinary Medicine in Canada.
According to Bell, "Despite having and using this arsenal of diagnostics, these tests can generate inconclusive results because they are not specific or sensitive for assessing the subtle soft and bony structures of the foot."
Radiography remains a mainstay in the diagnosis of orthopedic problems, but veterinarians might also consider MRI in addition to the traditional techniques.
"MRI provides detailed information about both the soft and bony tissues of the foot that may not be evident when using traditional diagnostic techniques," summarizes Bell. "When used as an adjunctive diagnostic tool, application of MRI can aid in achieving a precise diagnosis and devising targeted treatment plans."
In a study published in the Journal of the American Veterinary Medical Association in April 2009, Bell and colleagues used both radiography and MRI in 23 horses (36 feet) with localized foot pain suggestive of navicular disease.
"MRI detected a larger number of abnormalities," relays Bell. These included:
- Enlarged synovial evaginations--areas of inflammation and resulting small pockets of fluid in the bottom of the navicular bone (18/35 feet);
- Navicular bone flexor surface erosion (8/35 feet);
- Focal cortical bone loss or thinning of the distal (outer) margins of the navicular bone (7/35 feet);
- Fluid within the navicular bone (16/35 feet);
- Adhesions between the navicular bone and the DDFT (22/35 feet);
- Adhesions between the navicular bone and the navicular bursa (17/35 feet);
- Adhesions between the DDFT and the impar ligament (8/35 feet);
- Adhesions between the DDFT and suspensory ligament of the navicular bone (12/35 feet);
- Synovial proliferation and excessive fluid in the navicular bursa (13/35 feet);
- Fiber disruption or degeneration in the DDFT (21/35 feet); and
- Desmopathy (ligament disease) of the collateral ligament of the distal ¬interphalangeal joint (coffin joint) in 19/35 feet.
Downsides to using MRI are that it's a very expensive procedure (upwards of $2,000), and there are a limited number of units in the United States.
Treatment Options Old and New
The process of addressing navicular disease is a great example of the "kitchen sink" approach to treatment. That is, there are many treatment options available and little evidence-based data regarding the efficacy of these various treatments. Some of the more popular treatment options for navicular disease include rest, controlled exercise, and corrective shoeing.
In his article "Navicular Disease: Research Needed to Better Understand" (TheHorse.com/14069) Tracy A. Turner, DVM, MS, Dipl. ACVS, from Anoka Equine Veterinary Services in Elk River, Minn., writes, "The first steps in developing a logical approach to the treatment of this hoof lameness are an accurate assessment of the pain and careful evaluation of hoof structure that may predispose it to the condition or cause the pain. Treatment should be based on the type and location of the disease. Shoeing should be the basis of all treatment, and any medicinal or surgical therapy should be as an adjunct to shoeing."
A treating veterinarian might consider additional medical and surgical therapies including administration of non-steroidal anti-inflammatory drugs, oral administration of isoxsuprine hydrochloride to dilate the blood vessels to the foot, acupuncture, and oral joint health supplements.
Despite the variety and number of different treatment options for navicular disease, some horses do not improve following the use of one or more of the above-described techniques.
Bell notes, "When response to treatment is inadequate, the reason for failure is often unknown because of a lack of information regarding the specific lesion or abnormality causing the lameness."
When faced with this issue, MRI can help identify the specific lesion and provide the veterinarian with a prognostication tool to help guide the client's decision regarding the future athletic performance of the horse. In the event the horse is unresponsive to medical treatment and has a poor prognosis to return to athletic use, surgical treatments--such as palmar digital neurectomy (cutting the palmar digital nerve) to desensitize the foot, navicular suspensory desmotomy (cutting the ligament), and carpal (inferior) check ligament desmotomy--can be discussed to desensitize the foot and relieve the pain associated with the foot pathology.
"Appropriate management strategies and accurate prognosis require an accurate diagnosis," advises Dyson.
Radiography and ultrasound are valuable tools; however, MRI is the imaging modality of choice in the absence of radiological abnormalities. "MRI has taught us a huge amount about the large number of different injuries within the foot that can cause pain and lameness, alone or in combination," says Dyson. "More research is needed to better understand the different pathological processes occurring in the navicular bone, such as the influence of biomechanics, conformation, and genetics on injury risk."
About the Author
Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.
POLL: University Equine Hospitals