Overview
The (adult) spine consists of 25 separate bones called vertebrae (7 in the neck, 12 in the thorax (chest), 5 in the low back, plus the sacrum (infants may have multiple bones in the sacrum, which by adulthood normally fuse together to become one bone).
Except for the sacrum, each of these 25 vertebra has a left-right pair of half-joints on the top that connect with the bottom left-right pair of half-joints of the vertebra above, and a left-right pair of half-joints on the bottom that connect with the top left-right pair of half-joints of the vertebra below. Thus, every vertebra has 4 facet joints, two connecting with the vertebra above, and two connecting with the vertebra below (or the sacrum).
These joints are similar to the joints in your fingers - they have a smooth cartilage surface above and below that allow the upper and lower parts of the joint to slide to-and-fro, a fibrous "joint capsule" that holds the parts of the joint together, and a slippery fluid inside called synovial fluid, that moistens the joint and helps it move easily. These are called synovial joints.
These joints allow the spine to bend forward, backwards, left and right, and to twist.
These joints can be damaged due to the normal wear and tear of aging, through overuse, through traumatic events such as auto accidents, and through inflammation such as rheumatoid arthritis.
Damage to facet joints can cause back pain that can either be localized to the damaged area, or in some cases radiate to other parts of the body.
Facet Syndrome