Navicular Disease in Horses

This radiograph shows the internal structures of the hoof: A) coffin bone, B) navicular bone, C) third phalanx (P3), and second phalanx (P2)

Better diagnostic tools for imaging the horse's hoof are shedding new light on navicular disease and navicular syndrome in horses. There's a difference between navicular disease and navicular syndrome, although they overlap, and researchers are now uncovering several more causes for the symptoms in horses that have previously all been labeled "navicular."

The Navicular Bone
The navicular is a small, flattened bone, wide in the center, that resembles a flying saucer from an early sci-fi movie. It sits deep within the back portion of the foot, nestled between the coffin bone (P3 - third phalanx) and the short pastern bone (P2 - second phalanx).

If you were looking at the bottom of a normal horse's hoof, the navicular bone would be sitting inside the foot at roughly the level of the middle of the frog. Working your way out from the navicular toward the ground surface, the next structure is the navicular bursa, a fluid-filled cushion, the deep digital flexor tendon, the digital cushion, and finally, the frog. In other words, there's lots of padding.

The navicular bone doesn't just sit in place on its own. It is held there by several ligaments. The impar ligament is a short but thick and strong ligament that runs from the bottom surface of the navicular bone to the coffin bone. There are also ligaments running from either side of the navicular to the larger bones, and ligaments connecting it to collateral cartilages, large "wings" of cartilage extending back from the coffin bone that expand and contract upon weight-bearing. A major function of these ligaments is to hold the bone in place. But because all ligaments have some "give" to them, they also allow controlled movement in a variety of directions.

What Does It Do?
That's a very good question, without too many solid answers. Because of the way it projects out behind the coffin joint, the navicular bone changes the direction of the deep digital flexor tendon before the tendon inserts on the coffin bone. When the foot is off the ground, however, and the toe is back, the pull is in more of a straight line. Whether this is of any mechanical advantage to the horse is not clear.

The navicular bone also gives some wiggle room to the coffin joint. The collateral ligaments that flank the coffin bone secure the short pastern bone to the coffin bone in tight alignment so that they function almost as one. This is an advantage in weight-bearing. That arrangement would work fine if all the ground the horse travels over were completely flat and smooth. But in reality, the horse has to deal with uneven ground surfaces all the time, so there needs to be a way for the foot to absorb uneven forces. The soft tissue cushioning and shock absorbing structures (frog, digital cushion, collateral cartilages) do just that, but it looks like the navicular is involved as well.

It is thought that the position of the bone and its attaching ligaments allows it to absorb and compensate for uneven ground surfaces. The navicular bone of horses that are exercised is also more dense than that of inactive horses.