A joint is where two or more bones are joined. Joints allow motion (articulation). The joints in the spine are commonly called Facet Joints. Other names for these joints are Zygapophyseal or Apophyseal Joints.
Each vertebra has two sets of facet joints. One pair faces upward (superior articular facet) and one downward (inferior articular facet). There is one joint on each side (right and left). Facet joints are hinge–like and link vertebrae together. They are located at the back of the spine (posterior).
Facet joints are synovial joints. This means each joint is surrounded by a capsule of connective tissue and produces a fluid to nourish and lubricate the joint. The joint surfaces are coated with cartilage allowing joints to move or glide smoothly (articulate) against each other.
These joints allow flexion (bend forward), extension (bend backward), and twisting motion. Certain types of movement are restricted. The spine is made more stable due to the interlocking nature to adjacent vertebrae.
Although both the annulus fibrosus and nucleus pulposus are composed of water, collagen, and proteoglycans (PGs), the amount of fluid (water and PGs) is greatest in the nucleus pulposus. PG molecules are important because they attract and retain water. The nucleus pulposus contains a hydrated gel–like matter that resists compression. The amount of water in the nucleus varies throughout the day depending on activity.