Facet joints connect the vertebrae, the bones of the spine. They help guide your spine when you move. The low back area of the spine is called the lumbar region. It contains five vertebrae.
Facet joints are found on both sides of the spine. Each is about the size of a thumbnail. Lumbar facet joints are named for the vertebrae they connect and the side of the spine where they are found. The left L4-5 facet joint, for example, joins the 4th and 5th lumbar vertebrae on the left side.
Medial branch nerves are found near facet joints. They communicate pain from the facet joint. They tell the brain when facet joints have been injured.