The human spine is a miracle of engineering. A healthy spine allows a body to bend, twist, and stretch. The spine can twist in many different directions, bend forward and slightly backward, and elongate to help a person reach something on a top shelf. The spine is a column of vertebrae, stacked one on top of the other. They join together at points called facet joints. The facet joints are located posterior to the vertebral body, that is, they are on the side of the vertebral column nearest from the surface of the skin.
Facet joints allow the spine considerable variety of movement and flexibility, but they serve another purpose as well. Facet joints prevent excessive or extreme movement, which could be harmful. For example, the facet joints prevent hyperextension (stretching too much), hyperflexion (bending too far), and work against such injuries as whiplash.
Each pair of vertebrae is connected top and bottom with a pair of facet joints: on top is the superior articular facet and on the bottom is the inferior articular facet. Together, they allow the facet joint to work like the hinge of a door.
Like other joints in the body, each facet joint is surrounded by a capsule of connective tissue, which protects the joint. The actual joint surfaces are coated with cartilage so that they are very smooth and move easily against each other. Within the capsule, synovial fluid is present to nourish and lubricate the joint.