The anatomy of the facet joints
The spinal column consists of 24 vertebrae. The cervical spine consists of seven, the dorsal spine of 12 and the lumbar spine of five vertebrae. With the exception of the two top cervical vertebrae, each vertebra consists of the vertebral body at the front and the vertebral arch at the back. These surround a large opening through which the spinal cord passes.
This opening is called the vertebral canal. Together, these vertebral canals form the spinal canal through which the spinal cord passes. On each side of the vertebral arch of each lumbar vertebra there is an inferior and a superior articular process.
The inferior (lower) articular processes of one vertebra form a zygapophyseal joint (facet joint) with the superior (upper) articular processes of the vertebra below it to connect the two vertebrae. Instead of the term zygapophyseal joint, we usually use the shorter term facet joint (or just facet for short).
The articular (joint) surfaces of the facet joints are about 1.5 cm wide and high and have a slightly oval shape. They are covered by a thin layer of cartilage and are surrounded by a connective tissue joint capsule. On the inside of this capsule, as in all other joints too, there is a mucous membrane that forms the inner lining of the joint, the synovial membrane. The function of this is to lubricate and supply nutrients to the articular cartilage.
The illustration on the left shows the lumbar spine seen from the side. The abdomen is to the right of picture, the back to the left. The joints between the vertebrae (facet joints) that connect one vertebra above with the one below are shown in red.
Function of the facet joints
The intervertebral discs lie between the vertebral bodies and connect them together. The combination of the soft, but tumescent elastic nucleus and the tough fibrous ring that surrounds it enables the disc to absorb and transmit the forces that act on it. Thanks to their box-like shape, the vertebral bodies can bear these forces. The facet joints bear up to 20% of the forces that act vertically on the spine.
Their primary function though is to limit forward gliding and rotation of the vertebrae with respect to one another. Depending on the position of the facet joints on the spine, different shapes and configurations of their articular surfaces limit the movements of the vertebral bodies with respect to one anotherto varying degrees.
Illustration on the left: the upper figure shows the facet joints seen from the side, the lower figure shows them from behind. The facet joints (shown in red) are formed by the superior and inferior articular processes on the vertebral arches. They are arranged in pairs on the left and right sides.