by Julie Cary, DVM, MS
Diplomate, American College of Veterinary Surgeons
Associate, Cave Creek Equine Surgical Center
Navicular disease is a term that induces horror and panic in many horse owners. Fortunately, with advancing technology, the condition is not one that should be feared as much. Most veterinarians now refer to the condition as palmar or heel pain. This more accurately reflects the nature of the condition.
Heel pain is defined as a lameness originating in the back half of the horse’s foot which blocks to a palmar digital nerve block. This is the area where the navicular bone resides, as well as several other soft tissue structures. There are certain types of hoof conformation or shoeing imbalances that can predispose an individual to developing pain in the back of the foot. The first line of diagnostics include taking radiographs of the foot. The classic navicular disease will have bone changes that can be seen on radiographs, such as bone spurs, holes in the marrow cavity, roughening of the flexor surface of the navicular bone, or thickening of the marrow cavity.
Unfortunately, radiographs do not reveal what is going on with the soft tissue structures. We now know that many times, the soft tissue injury precipitates the changes to the navicular bone. If we can see the soft tissue damage and manage it appropriately, we can better manage the horse and its lameness. Ultrasound can be used to image many of the structures within the foot including the deep digital flexor tendon, navicular bursa, and the small supporting structures of the navicular bone. The problem is that ultrasound of the foot can only be performed through a couple of windows (the frog and through the back of the heel bulbs) and requires considerable practice on the part of the veterinarian to diagnose the condition.
Recently, MRI has been used to document more conclusively the problems in the foot. Washington State University has been studying horses with palmar heel pain for the past seven years using MRI. Several practices have recently added MRI to their diagnostic capabilities. We have learned a wealth of knowledge from this modality. Interestingly, the majority of cases seen at one institution, which have been worked up by several veterinarians prior to referral, have turned out to be damage to the deep digital flexor tendon as it curves around the navicular bone and inserts on the coffin bone. These cases need to be rested and rehabilitated much the same was as a bowed tendon.
Treatment of heel pain includes corrective shoeing to ensure the foot is balanced and to minimize the stress on the soft tissue structures. Other treatments include pain modulation and anti-inflammatory medication, which is usually in the form of phenylbutazone. Isoxsuprine is a medication that is often used in cases of heel pain. The purpose of the medication is to help improve blood flow to the area. Also, treating the coffin joint or navicular bursa with hyaluronic acid with or without a cartilage friendly steroid can help with certain types of pathologic changes.
Some people advocate the use of “nerving” these horses. Nerving entails removing a chunk of nerve from the palmar digital nerve so that the horse can not feel the back 1/2 of the foot. This does not stop the disease process, and can contribute to its worsening in some circumstances. The nerve will eventually grow back in one to five years. It is the author’s opinion that nerving should only be performed on horses that do not respond to any other treatments and who have a finite career time required.
End stage navicular disease is a complex of problems associated with both the soft tissue and bone in the heel region of the foot. The most common is that the deep digital flexor tendon becomes stuck to the navicular bone which then interferes with the pulley function of the DDFT. Every time the horse steps it pulls on those soft tissue attachments. This is why many of the horses with this condition take extremely short steps with their front feet.
The key to palmar foot pain is early and accurate diagnosis. Lameness that has been noticed for a short period of time is much more likely to be resolved than lameness present for a long time. Getting an accurate diagnosis of the offending structure or structures helps more effectively treat the problem. Veterinarians with a special interest and understanding of feet are extremely helpful in diagnosing and treating these types of cases.