Navicular disease is an injury of the navicular bone of the front foot. Conformational faults or injury are the most common causes of navicular disease. A straight pastern and shoulder or small hoof will increase the concussion on the navicular bone, causing increased friction between it and the flexor tendon, resulting in injury.
Horses worked on hard surfaces are at greater risk to acquire navicular disease, which usually affects horses in their prime working years (ages 6 to 10). A horse with navicular disease will shorten its stride and tend to go more on its toes. When standing, the horse points the toes of the more seriously affected foot. The disease causes varying degrees of lameness, and there is no permanent cure.
Ashley Griffin, University of Kentucky