Zygapophysial joint

A zygapophysial joint (zygapophyseal, or facet joint) is a synovial joint between the superior articular process of one (lower) vertebra and the inferior articular process of the adjacent (higher) vertebra. There are two facet joints in each vertebral motion segment.

The biomechanical function of each pair of facet joints is to prevent excessive torsion (twisting) of the spine, while allowing a small amount of lateral bending and flexion and extension. These functions can be disrupted by degeneration, dislocation, fracture, and/or instability of the facet joints from trauma, osteoarthritis, and/or surgery.