By Lauren Lamb, DVM
Navicular ‘disease’ is a condition that affects the navicular bone, navicular bursa, deep digital flexor tendon and/or the associated ligaments attached to the navicular bone. Navicular disease, navicular syndrome, caudal heel pain syndrome in the horse, or insert some other name, are all terms used to describe the same condition, pain in the heel region of a horse. For the purposes of this article, navicular disease will be the term used to describe pain from the heel region of a horse. Navicular region will refer to the navicular bone, associated ligaments, navicular bursa and deep digital flexor tendon.
Before we can talk about navicular disease, we need to step back and review the normal anatomy. The navicular bone is located behind the coffin joint, within the hoof capsule. The navicular bone articulates (forms a joint) with the coffin bone and short pastern bone, which are located on the front side of the navicular bone.
The deep digital flexor tendon runs on the back side of the navicular bone. The navicular bursa lies between the navicular bone and the deep digital flexor tendon. The bursa is a fluid filled sac that functions as a shock absorber and a lubricant for the deep digital flexor as it passes around the navicular bone. Several other small ligaments help hold the navicular bone in place. These small ligaments are extremely important and frequently contribute to the horse’s pain.
The exact cause of navicular disease is unknown. Several potential causes for the disease have been proposed. Each theory revolves around some sort of trauma to the navicular bone and its associated ligaments, tendons or bursa. Another common theory is interference with the blood supply to the navicular bone. Navicular disease rarely develops in the hind limbs. It is predominantly seen in the front feet of a horse seven to 14 years of age. Thoroughbreds, Quarter Horses and Warmbloods are more commonly affected than other breeds. However, any breed of horse can develop navicular disease.
Lameness is the primary clinical sign seen in a horse with navicular disease. Navicular disease usually affects both front feet, but typically a horse will be more lame in the left or right leg. The lameness may only be seen with the leg on the inside of a small circle or when exercising on hard ground. The lameness will get worse with exercise and improve with rest. Typically Phenylbutazone (Bute) will improve the lameness to some degree.
Diagnosis can be challenging for a horse with navicular disease. The process of isolating the lameness to the heel region of a horse is easy and straight forward. This can be done with a temporary nerve block that will desensitize the horse’s heel. Once the lameness is localized to the heel, the real challenge of trying to identify the exact cause of heel pain begins.
Radiographs can be taken to evaluate the coffin joint, navicular bone and angles of the horse’s foot. The radiograph will provide little information regarding the soft tissue (ligaments and tendons) surrounding the navicular bone. Ultrasonography is typically used to image soft tissue structures in a horse; however, the location of the navicular bone within the hoof capsule makes ultrasound imaging challenging. The best way to image the navicular region is to use an MRI. The MRI provides the best quality imaging of both soft tissue and bone structures. The hoof wall does not influence the quality of image obtained with an MRI.
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