A joint is an area where two or more bones are joined but able to move. On each side of the spine the upper articular process of one vertebrae and the lower articular process of the adjacent vertebrae linked to form a facet joint.  There is a pair of facet joints at most levels of the spine. The facet joints are hinge-like and help link adjacent vertebrae together. The facet joints are synovial joints which mean that each facet joint is surrounded by a special membrane (synovial membrane) which lies inside a thicker lining of connective tissue referred to as the joint capsule.  The synovial lining of the joint produces a fluid (synovial fluid) which helps to nourish and lubricate the joint. The presence of synovial fluid also reduces weight bearing friction on the facet joint surfaces. Each of the articular processes has a relatively flat and smooth surface.  The surface is covered by a thin layer of cartilage.

The facet joints help to control the amount and direction of spinal movement. The relationship of the facet surfaces to one another is different in each part of the spine. For example, the low back facet joints are aligned in ways that promote flexion and extension while limiting the degree of side bending and rotation. The facet joints of the neck allow for more rotation because of their angle.

These smooth surfaces are configured somewhat like the facets of a diamond thus their name facet joints.  They are also referred to as zygapophyseal joints.  Each facet joint is aligned with cartilage and is lubricated with a special substance referred to as synovial fluid.  The synovial fluid helps reduce the friction between the adjacent articular (joint) surfaces.  Regular and repetitive movement is critical to the health of the cartilage and to maintain appropriate density of the bone. This includes the bone underlying the cartilaginous surface of the facet joint. Facet joints can be affected by wear and tear as well as arthritic disorders. Degenerative enlargement (osteoarthritis) of the facet joint can result in spinal nerve compression.