At each level of the spine between two vertebrae and the disc, the posterior bony elements meet on each side of the spine and a joint is formed, known as a facet joint. Each facet joint is composed of two articular processes, one from the upper vertebra and one from the lower. Facet joints have smooth cartilaginous surfaces similar to the hip, knee, and shoulder. Each facet joint is enclosed in a thickened ligament-like covering called the joint capsule. The facet joints connect all of the vertebrae, facilitating coordinated movement of the vertebrae. Because the surfaces of the vertebral facet joints can slide and shift across each other, the spine can bend forward, backward, and to the side as a whole unit. Facet joints may become worn down or affected by disease, causing stiffness and ultimately becoming arthritic and causing dysfunction or back pain. [Figure 1 ] The thoracic vertebrae have additional specialized joints that articulate with the ribs.
Figure 1: Parts of Vertebrae, lumbar example