<Table> <Tr> <Td> </Td> <Td> This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations . Please help to improve this article by introducing more precise citations . (March 2016) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations . Please help to improve this article by introducing more precise citations . (March 2016) (Learn how and when to remove this template message) </Td> </Tr> <P> The facet joints, (or zygapophysial joints, zygapophyseal, apophyseal, or Z - joints) are a set of synovial, plane joints between the articular processes of two adjacent vertebrae . There are two facet joints in each spinal motion segment and each facet joint is innervated by the recurrent meningeal nerves . </P> <P> The biomechanical function of each pair of facet joints is to guide and limit movement of the spinal motion segment . In the lumbar spine, for example, the facet joints function to protect the motion segment from anterior shear forces, excessive rotation and flexion . Facet joints appear to have little influence on the range of side bending (lateral flexion). These functions can be disrupted by degeneration, dislocation, fracture, injury, instability from trauma, osteoarthritis, and surgery . In the thoracic spine the facet joints function to restrain the amount of flexion and anterior translation of the corresponding vertebral segment and function to facilitate rotation . Cavitation of the synovial fluid within the facet joints is responsible for the popping sound (crepitus) associated with manual spinal manipulation, commonly referred to as "cracking the back ." </P>

The zygapophyseal joints of the vertebral column are classified as