by Vern Dryden, DVM, CJF
Navicular syndrome is a term used today when referring to palmar heel pain and pathology of the navicular apparatus in the horseâ€™s foot. Structures that comprise the navicular apparatus are the navicular bone, suspensory ligament of the navicular bone (connects the navicular bone to the bottom end of the pastern bone), impar ligament (connects the navicular bone to the back of the coffin bone), and the navicular bursa (sac of fluid protecting the flexor surface of the navicular bone from the deep digital flexor tendon). Commonly, horses affected have either very steep or very low
hoof-pastern angles and are usually bilaterally lame. These horses tend to land toe first and the lameness becomes more pronounced when being worked in a circle. Horses with histories of heavy work from an early age are at risk of developing navicular syndrome. In addition, breeds such as Quarter Horses, Thoroughbreds, and Warmbloods are commonly affected.
Years ago, the term "navicular diseaseâ€ was used to describe these horses and usually meant the horseâ€™s career was over. We now know that there are many causes of palmar heel pain that are not career ending and some of which do not involve the navicular apparatus. For example, coffin joint collateral ligament desmitis (inflammation) may present as palmar heel pain. This injury is not always considered career ending and does not involve the navicular apparatus. Most commonly, horses with palmar heel pain "block-outâ€ to a palmar digital (PD) nerve block. Unfortunately, localizing the lameness is often the easiest part of the process. A palmar digital nerve block can desensitize most of the structures within the hoof capsule therefore leaving a multitude of possibilities responsible for the lameness. For example, pedal osteitis of the coffin bone wings may present as heel pain and block to a PD nerve block. This is often seen in racehorses and may resolve with rest and therapeutic shoeing. Further localization of the lameness can be done by injection of local anesthetic into the navicular bursa or the coffin joint. Once the lameness is localized, the next step is to obtain radiographs. I recommend taking six standard views of the digit in order to completely assess the foot. Radiographs may often show evidence of chronic inflammation in the form of osseous changes. However, radiographs are not always conclusive since they donâ€™t give diagnostic information about soft tissue. This is a common problem when attempting to diagnose a lesion suspected to be involving the navicular apparatus.
In recent years, magnetic resonance imaging (MRI) has shown to be the gold standard for diagnosing lameness causing lesions within the equine foot. MRI uses strong magnetic fields and radiofrequency pulses to image with great detail both bone and soft tissue structures deep within the digit. The MRI unit at Rood and Riddle Equine Hospital is a 1.5 Tesla high-field magnet and one of less than 10 for equine use in the United States. On average, Rood & Riddle scans 200 equine patients per year with the MRI and the majority of those cases are feet. With the use of MRI we can tailor our treatment plan according to the pathologic findings. Some horses may benefit from coffin joint or navicular bursa injections while other horses may just need rest. In many cases therapeutic shoeing may help those horses diagnosed with navicular syndrome. The egg-bar shoe has long since been the choice of many veterinarians to apply in cases of palmar heel pain. Unfortunately, one shoeing application does not work for all navicular syndrome cases. The MRI is a very useful tool that allows us to develop a shoeing application specific to the horse. Some common lesions seen when horses with palmar heel pain are scanned with the MRI are desmitis of the suspensory ligament of the navicular bone, navicular bone degenerative changes, deep digital flexor tendonitis, adhesions of the deep digital flexor tendon to the flexor cortex of the navicular bone or to the suspensory ligament of the navicular bone, and impar ligament desmitis.
My main concern when shoeing a horse with an injury to the navicular apparatus is to reduce the stress being placed on these structures. A balanced hoof is very important and care should be taken to achieve this. The farrier should be able to view the radiographs prior to trimming/shoeing the horse. I often take radiographs before and after my trim to ensure I have balanced the hoof to the best of my abilities. One of the most important aspects of shoeing a horse with navicular syndrome is to provide ease of breakover. Breakover is a term used to describe the moment the heels lift off the ground and the hoof pivots over the toe. My goal is to move the breakover point at the toe toward the center of the foot to reduce the force being placed on the navicular apparatus. This can be accomplished by having a balanced trim with a rolled toe, rocker toe, or a half-round shoe applied. Other shoeing modifications may be required to provide comfort to these patients. Many cases present with palmar heel pain and chronic hoof capsule distortion such as sheered heels, under run heels, severe flares, and deep cracks. Often resolving the distortion through therapeutic shoeing can eliminate the lameness.
Navicular syndrome affects a large population of equine athletes. Fortunately we have tools at our disposal to help diagnose and treat these patients. However, when medical treatments and therapeutic shoeing fail to provide comfort to the patient other options should be considered. As a last resort, a palmar digital nerve neurectomy can be performed. This is a surgical procedure that removes a portion of the nerves at the level of the pastern. This treatment is palliative and does not prevent further degeneration of the navicular apparatus. Therefore, I recommend continuing therapeutic shoeing and medical management. This surgery is less commonly performed now due to all the medical advancements and therapeutic shoeing options that are currently available. There can be complications with the surgery such as neuroma formation and rupture of the deep digital flexor tendon. A neuroma is a painful tumor arising from nerve tissue at the surgery site. In severe cases of navicular apparatus degeneration the deep digital flexor tendon may also be damaged. In the case of a neurectomy, the horse may not protect the foot as it previously had and overuse may lead to the rupture of the tendon. There is a possibility of the nerve endings growing back and eventually providing sensation to the hoof again. However, when successful, a palmar digital neurectomy can provide soundness for months to years.
Dr. Vern Dryden is a graduate of the Oklahoma State Horseshoeing School and the Washington State College of Veterinary Medicine where he was also the resident farrier of the Veterinary Teaching Hospital. Dr. Dryden joined the Rood & Riddle Podiatry department in 2007 as an associate after completing a one year Podiatry internship with Dr. Morrison. Dr. Dryden is one of only 5 veterinarians in the U.S. to have earned Certified Journeyman Farrier status from the American Farriers Association.