Windt im Wald A Wind in the Woods Geauga County, Northeast Ohio since 1995

If The Shoe Fits #9

© Randy Sublett

(Randy, a practicing farrier, writes articles oriented to horse owners.)

Navicular disease . Everyone has heard of it. No one wants it to happen to their horse, but it does happen and it happens to Quarter horses often. Over the past few articles I have been talking about the bones, tendons and ligaments of the lower leg. I purposely left out a bone of the lower leg until we arrived at this point.

The navicular bone. This is a most curious bone and how it evolved, I cannot say. It is one of the smallest bones in the lower leg and it serves a purpose that allows horses to travel the way they do. The navicular bone is shaped like a boat and is located at the rear of the leg between the short pastern bone and the coffin bone. It is held in place by the suspensory ligament of the navicular bone. This is not a mystery novel. A suspensory ligament does just that - it suspends a bone in place. The coffin joint, which is the joint between the short pastern bone and the coffin bone, is not what is called a perfect joint. Indeed if it were, it probably would not work nearly as well. Being imperfect, the coffin joint allows for a great deal of side-to-side movement of the joint. This makes it possible for the horse to stand or move on uneven ground with considerable ease. The position of the navicular bone is not, however, by chance. The position of the bone at the coffin joint allows for the majority of concussion to be channeled away from the very fragile coffin bone. Concussion is transferred to the short pastern bone via the navicular bone.

Another equally important function of the navicular bone is that of the fulcrum. Think of a seesaw; the center pivot point would correspond to the navicular bone, the plank on which you sit would be the deep flexor tendon. In the horse, however, the fulcrum is off center - by a long shot. The deep flexor tendon, as you may recall, connects muscle in the top portion of the leg with the bottom rear portion of the coffin bone.

The deep flexor tendon is the main tendon that propels the horse on the power stroke of its stride. Let's say that the horse weighs 1200 pounds, along with a 165-pound rider and the deep flexor tendon is transferring muscle power to the coffin bone over a distance of maybe 12 to 18 inches before passing over the navicular bone. It then connects to the coffin bone mere inches away. This means that the navicular bone is taking a beating. Take a fairly good-sized rubber band and place it around the top of a chair back near the edge. Place a pen or a pencil under the rubber band near the top and stretch the rubber band taut. The short part of the rubber band between the pencil and chair top represents the deep flexor tendon between the navicular bone and the coffin bone. The long portion of the rubber band represents the remainder of the deep flexor tendon. The more you stretch the rubber band, the harder it will be to remove the pencil from under it because of the increased pressure on it. Get the picture?

I have mentioned that long tendons are encased in a sheath that contains synovial fluid, which is the lubricant of tendons. The navicular bone has what is called a bursa - a small sack filled with synovial fluid. There are, however, basically two schools of thought. One says that there is a navicular bursa with synovial fluid; the other says it does not exist. I cannot tell you one way or the other because the dissections I did in farrier school were done on the feet of horses that had been destroyed because they had that very problem - navicular disease. It is thought that navicular disease starts out with bursitis of the navicular bursa. In other words, this little sack of fluid bursts and all of the fluid dissipates. There is then no lubricant left to allow the deep flexor tendon to slide easily over the navicular bone, causing irritation in the area which causes exostosis, or bony growth.

So what does all of this have to do with Quarter horses? They are the most predisposed breed for navicular disease. There are probably many factors; however, one factor in particular stands out - high hoof angle. It seems that high hoof angle was desirable in the show ring for Quarter horses. This high angle puts an abnormal amount of concussion on the navicular bone thus predisposing many Quarter horses to navicular disease. To illustrate this, picture getting on the floor on all fours with the palms of your hands flat. Feels comfortable, right? Now raise yourself up onto the tips of your fingers; not so comfortable now, is it? If you really want to know how a horse with a high hoof angle feels, stay on your fingertips and have some of your friends put your pickup truck on top of your back. Now you've got it!

Randy welcomes comments or questions about his articles.

Randy Sublett
P.O. Box 9
Telluride, CO 81435

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