Mary Thompson, a vertebrate paleontologist at Idaho Museum of Natural History, has found evidence in many species of early horses and concludes, "The results of this study strongly suggest that man’s intervention (whether by increased usage or improper breeding practices) may not be the sole cause of the syndrome", although she cautions that her results are preliminary. Work
Working on steep hills, galloping, and jumping all contribute to navicular syndrome, as they place greater stress on the DDF tendons, and may cause overextension of the pastern and coffin joints. Regular exercise on hard or irregular ground increases concussion on the hoof, thus increasing the risk of navicular syndrome. It is possible that standing can also increase the chance of navicular disease (such as a horse that spends most of the day in a stall with little turnout, as with some racehorses and show horses). Blood flow to the hoof decreases when the horse is not in motion. The horse is also constantly applying pressure to the navicular bones (which is intermittent as the horse moves). Body weight
Horses with a high weight-to-foot-size ratio may have an increased chance of exhibiting symptoms of navicular syndrome, since the relative load on the foot increases. This might explain why the syndrome is seen more frequently in Thoroughbreds, American Quarter Horses, and Warmbloods as opposed to ponies and Arabians. Signs
Heel pain is very common in horses with navicular syndrome. Lameness may begin as mild and intermittent, and progress to severe.
The first factor is compression of the navicular bone under the DDF tendon and the back of the small pastern bone. Repeated compression in this area can cause cartilage degeneration, with the cartilage flattening and gradually becoming less springy and shock-absorbing. It may also begin to erode. Cartilage degeneration is common in navicular horses, usually along the flexor surface. This finding, and the associated biochemical changes, have led some researchers to conclude that there are elements in navicular disease common to osteoarthritis, and to suggest similar therapeutic regimes.Cartilage erosion may progress to the point that the bone underneath will become exposed. With the cartilage no longer present to protect it, the navicular bursa and DDF tendon may become damaged by the constant rubbing against the navicular bone. Navicular bursitis (inflammation of the navicular bursa) may occur, even if cartilage damage is not severe. This is probably due to the friction between the navicular bone and the DDF tendon from compression. Constant compression can also increase the bone density directly under the cartilage surfaces, especially on the flexor side. This tends to make the bone more brittle, and thus more likely to break. Another main factor is the tension placed on the ligaments that support the navicular bone. Some experts believe that the degenerative process begins with excess tension placed on these ligaments, causing strain and inflammation.