Courtesy of UC Davis
Horses have been lame due to pain in their feet as long as people have been associated with them. Navicular disease and laminitis are two major diseases that have caused significant economic loss through layup, decreased sales value and veterinary expenses. Throughout history, horsemen have dreaded the diagnosis of both diseases. But now, due in large part to the introduction of new imaging techniques to the equine world, our understanding of navicular disease is rapidly changing.
As technology has improved over the years, significant leaps forward have been made in our general understanding of lameness in the horse. Many of these improvements have been associated with advances in imaging. In the case of navicular disease, the development of a simple radiographic projection—the navicular skyline view that enables visualization of the navicular bone without interference by other bones, has allowed researchers and clinicians to evaluate the bone in much greater detail.
Unfortunately, as visualization of the bone has become clearer, the disease has become more difficult to understand. For example, navicular bone degeneration occurs in horses with and without lameness, and horses that are lame may or may not have navicular bone degeneration. Some cases are clear-cut, but a significant number are not. Furthermore, radiographic changes in some breeds of horses seem to be more indicative of lameness than in other breeds.
The traditional thinking on navicular disease has changed over the past several years. Historically, it was thought of as a specific disease of the navicular bone itself, but now it is generally accepted that lameness arising from the heel of the horse may be due to injury of any number of structures within the hoof capsule. One major factor that can obscure an accurate diagnosis of the disease is the use of a local anesthetic (palmar digital or PD nerve block) as a primary tool to localize the painful structure. When a heel block is used, multiple structures within the foot can be anesthetized, rendering the horse sound. If no significant radiographic abnormalities involving the bones of the feet are identified, a veterinarian can be faced with a significant diagnostic challenge. They must assume that the lameness is arising from any one of a number of soft-tissue structures within the foot and/or the navicular bone. Typically in a horse with this problem, therapy is fairly general and would include shoeing changes, anti-inflammatory drugs administered orally, or anti-inflammatory drugs administered into the coffin joint, navicular bursa or tendon sheath. Other therapies could include substances to improve joint fluid composition and even shock wave therapy through the sole of the foot. However, a universal frustration with this syndrome is that the lameness will often recur, even with the best of care.
Most of the recent research involving navicular disease/syndrome has focused on imaging. Although navicular bone degeneration may indeed be the cause of lameness in many horses, one must not discount the role of the soft tissues within the hoof capsule. These include tendons, ligaments, the navicular bursa, the flexor tendon sheath and the digital cushion. We believe that all of these structures work together to allow the horse to propel itself forward. All of the soft tissue damage is not typically seen on standard X-rays unless the damage is very severe and chronic. Ultrasound evaluation is used for tendons and ligaments higher up on the horse’s limb but is seriously hampered in evaluating the foot. In humans, diagnosing injury to these types of structures is done with either MRI or CT. Today, because of the advances made in these imaging techniques, both are now available for horses at UC Davis.
MRI has come to the forefront of diagnosing lameness in the horse. Different manipulations of the images produced by MRI can be used to emphasize different characteristics of tissues. All of these features of MRI are used to develop a number of imaging sequences which together give a much clearer anatomic description of the abnormal tissue than previously possible. While MRI allows for better evaluation of the soft tissue structures, CT has better bone imaging capability. CT has long been accepted as the best imaging technique for identifying bone lesions. With the use of contrast-enhanced CT, soft tissue lesions have become much more visible, and with helical scanning, directed treatment such as tendon injection can be performed all at the same time. Each has its advantages, but together, MRI and CT are even better for diagnosing injuries involving both bone and soft tissue.