Causes and Contributing Factors of Navicular Syndrome
There is no single known cause of Navicular Syndrome, although many theories. The two most important factors in Navicular Syndrome are:
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. The cartilage 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 actually tends to make the bone more brittle, and thus more likely to break.
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. Excess tension causes strain and inflammation. Inflammation from strain of the impar ligament can decrease blood flow to and from the navicular bone, as the major blood vessels supplying the bone run up and down this area. If the ligament continues to be strained, it can thicken and permanently reduce blood flow to the navicular bone.
Because veins are more easily compressed than arteries, blood flow to the bone would be less obstructed than blood flow from the bone. This would cause a build up of pressure within the navicular bone. The navicular bone, in response to both the increased pressure and overall decreased blood supply, would absorb mineral from its center.
Excess tension can also cause exostoses where the ligaments attach to the navicular bone, giving the bone a "canoe" shape. If tension is extreme, the ligaments may actually tear.
Toe-First Landing
Recent research has found correlations between "toe-first landing" of the hooves and navicular problems, due to excessive strain put on the deep digital flexor tendon, as a consequence of misalignment of the lower joints. Toe-first landing, usually seen as a consequence of navicular disease, may actually be a cause or at least a contributing factor to the onset of tendon inflammation and bone modifications.
Toe-first landing is often caused by frog and heel overtrimming, long toes, and/or poor shoeing.
Read more about this topic: Navicular Disease
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