Facet Joint Disease
The facet joint is the structure where the facets of the superior and inferior processes of the vertebral bones articulate, and are held together by a strong, fibrous capsule. There is also a layer of smooth cartilage located between the faces of the two facets, where they touch, to keep the surfaces of the facets from becoming worn out. Facet joint disease may occur when the smooth cartilage between the facets becomes worn down, or when the fibrous capsule that holds the structure together becomes worn, as a result of wear and tear, or forces trying to pull the joint apart.
The worn facet joint. The facets joints are designed to be one of the puzzle pieces holding the spine together, so that it can move and work as a single unit. The facet joint is meant to stabilize the posterior (back of the) spine, but it is not meant to be a weight bearing joint, nor is it designed to resist forces on it, caused by strain or movement beyond a healthy range of motion. When the rest of the spine is healthy, there is very little strain on these facet joints, and their rate of degeneration is very slow. When spinal structures such as the ligaments of the spine become loose, or the intervertebral discs begin to dry out, losing their height, the facet joints may suffer. While the drying out of a disc's nucleus results in less pressure being exerted on the annulus (outer portion of the disc), it can also result on more pressure being exerted on the facet joints.
Discs that have changed as a result of age related changes, or herniated discs, may be as much as a half of an inch thinner. Since the disc separates the vertebrae, a significant loss of height in the disc, brings the bones closer together. The result is that the facet joints at the back of the vertebrae move closer together as well.
The ligaments of the spine also have a role in stabilizing the spine, and maintaining a separation of the facets. If the ligaments have been chronically strained, as a result of bad posture, they may eventually stretch. This stretching of the ligaments may have the same effect as degenerated discs - pushing the facets closer together. Without the support of the ligaments, the facet joints, that were not designed to be weight bearing joints, must absorb added forces and weight. A third cause of added strain to the facet joints is the stretching of the annulus at the front of the disc. This may occur when the annulus bulges at the front of the disc, lowering the height of the disc, and pressing the facets together.
What happens when added pressure on the facet joints moves the facets closer to each other. The added pressures to the smooth cartilage between the facets causes the cartilage to wear out, and the facets may eventually touch each other. If the facets are in contact with one another as they move, their surfaces may eventually become worn down themselves or become irritated. This friction between the facets may either wear out that surface of the bone, or cause osteophytes (bone spurs) to develop between them.