Navicular Disease is a soundness problem in horses, more accurately called "navicular syndrome" as opposed to "disease." It most commonly describes an inflammation or degeneration of the navicular bone and its surrounding tissues, usually on the front feet. It can lead to significant and even disabling lameness.
It usually causes rough edges on the navicular bone, which leads to tenderness and swelling within the hoof and hence lameness. Mild cases can be managed with bute (although, obviously, not if competing).
Navicular SYNDROME.. can not be classed technically as a disease as the symptoms and causes are so varied.
As said it is pain cause by inflammation of the surrounding structures of the distal sesimoid (the navicular bone) it can be cause by arthritic changes to the distal sesimoid which in turn roughens off and rubs the surround structures cause the pain.
causes vary such as flat feet, 'boxy' feet being cut flat', poor trimming, tendon damage.
Treatments are also varied, including bar shoes, bar shoes with wedges, and trimming can and does help some horses.
Navicular Syndrome is a very loose term used to describe pain in the caudal aspect of the foot with an undiagnosed origin - this is potentially very good news as often caused by weak digital cushion and low/under-run/contract heels which can all be fixed by regaining correct and health structures to the horses foot