The diagnosis of navicular disease in horses has been redefined since the use of magnetic resonance imaging (MRI) to scan the feet of lame horses.
The navicular bone is the small bone at the back of the foot. The deep digital flexor tendon (DDFT) passes around this bone as it tracks down the back of the pastern and attaches to the pedal bone.
Until MRI was widely available, the two main diagnostic tools were the response to nerve blocking the back of the foot and apparent changes in the navicular bone as seen on X-rays. If a horse blocked sound to the low-volume nerve block of the heels or if X-rays showed blotches or irregularities of the navicular bone, the horse would be diagnosed as having navicular disease.