Facet (zygapophyseal) joints

Facet joints (also called zygapophyseal or apophyseal joints, Latin: articulationes zygapophysiales) are paired articulations between the articular processes (zygapophyses) of two adjacent vertebrae. These joints are found almost along the entire length of the spine. The first pair is located between the second and third cervical vertebra (C2/C3), while the last pair is situated between the fifth lumbar vertebra (L5) and sacrum.

Each vertebra has two paired facet joints. One pair is located at its superior part, while the other can be found at its inferior aspect. These joints provide various movements and additional stability for the entire spine. Together with the intervertebral discs, the facet joints distribute and bear the compressive load on the spine in a standing position.

Note: In anatomy, the term “facet” is usually referred to a flat and smooth articular surface of the bone. Some authors claim that the commonly used term “facet joint” is actually misused because facets are found not only within the zygapophyseal articulations but also within the costotransverse and costovertebral joints. In this article, we will mostly use the commonly used term “facet joints” while also applying its more correct synonym, “zygapophyseal joints.”

Classification of facet joints

Functionally (depending on the range of provided movement), facet joints are classified as diarthroses. Diarthroses are joints that are freely mobile.

Histologically (depending on the involved tissue), these articulations are synovial joints. Synovial joints can be further subdivided according to the type of movements they provide.

The zygapophyseal joints are plane-type joints that are characterized by movements of a sliding nature. Plane-type joints connect two flat bones that are similar in size.

Note: To read in detail about joint types and their functions, please visit our article dedicated to classification of joints.

Articulating structures of facet joints

The articulation within the facet joint happens between the superior and inferior articular facets located on the superior and inferior articular processes, respectively. Facets of the inferior articular processes of the vertebra above articulate with the facets of the superior articular processes of the vertebra below. The articular processes originate between the pedicles and the laminae of the vertebra.

The superior articular process has a more concave articular surface. It faces posteriorly and medially. In contrast, the inferior articular process appears to have a more convex facet. It faces anteriorly and laterally. The inferior articular process accounts for the posterior part of the whole articulation.

The orientation of the facet joints varies throughout the spine.

• The cervical facet joints appear oblique and are positioned in the coronal plane. Towards the skull, they lie more horizontally. The cervical facet joint plane slopes inferiorly in the anteroposterior direction.
• The thoracic facet joints are the most vertically oriented and are located in the coronal plane.
• The lumbar facet joints are also vertically oriented but positioned in the sagittal plane.

Characteristics of facet joints

The facet joints are simpler in the cervical and thoracic regions of the spine. The lumbar facet joints, on the other hand, appear more complex. Like any other synovial joint, the facet joints are also characterized by the following features:

• The articular surfaces are lined by hyaline cartilage.
• Both articular facets are enclosed by fibrous joint capsules, and between them is a joint cavity.
• The joint capsule attaches to the margins of the articular facets.
• The joint cavity is lined by the synovial membrane.
• Cells of the synovial membrane produce synovial fluid that acts as a lubricant.

Facet joint capsule

The capsule of the facet joint is thin and loose, and even looser in the cervical spine. Anteriorly to the joint, the capsule is absent, and it is entirely replaced by the ligamentum flavum.

The fibrous capsule has two recesses - superior and inferior. Both recesses contain fat pads that act as cushions and compensate for movements in the joint. Several ligaments are positioned outside the capsule. They stabilize and strengthen it.

Facet joint ligaments

The most important ligaments that stabilize the facet joints are ligaments that are included in the posterior ligamentous complex (PLC). They include the ligamenta flava, posterior longitudinal ligament, interspinous and supraspinous ligaments and the nuchal ligament in the neck.

The ligamenta flava (yellow ligaments) are paired fibrous bands that connect the laminae of the adjacent vertebrae and form the posterior wall of the vertebral canal. Laterally, the ligamenta flava merge with the fibrous capsules of the zygapophyseal joints.

The posterior longitudinal ligament (PLL) is a strong band that runs along the entire length of the spine. It is located within the vertebral canal. The PLL is attached to the posterior surfaces of the vertebral bodies and intervertebral discs.

The interspinous ligament is a thin and membranous band that connects the bases of the spinous processes of the adjacent vertebrae. In contrast, the supraspinous ligament is a strong cord-like band that stretches over the apices of the adjacent spinous processes.

The nuchal ligament is only found in the neck. It extends between the external occipital protuberance of the occipital bone and the tip of the spinous process of the seventh cervical vertebra (C7).

Facet joint movements

The range of motion in the facet joints is extremely limited by the adjacent structures. Nevertheless, the facet joints act simultaneously together with other joints of the spine to provide such movements as flexion, extension, lateral flexion and rotation of the spine.

The characteristic orientation of the zygapophyseal joints described above provides the following movements throughout the spine:

• In the cervical region, the facet joints facilitate flexion, extension, lateral flexion and rotation of the spine.
• In the thoracic spine, flexion and extension are restricted, but the joints permit rotational movements.
• In the lumbar part of the spine, the facet joints are responsible for limited flexion, extension and lateral flexion of the spine.

Related structures

Posteriorly and laterally to the zygapophyseal joints lie the deep muscles of the back. Anteriorly, the facet joints are fused with the yellow ligaments and form the posterior borders of the intervertebral foramina.

The intervertebral foramina transmit various neurovascular structures. Therefore, the facet joints are also related to these structures, and they include the dorsal rami of the spinal nerves and their medial branches that go along the lateral aspects of the joints.

Neurovascular supply of facet joints

The arterial blood supply of the zygapophyseal joints is provided by the posterior spinal branches of the segmental spinal arteries.

Venous blood from the facet joints is collected by the spinal and basivertebral veins that drain into the internal and external vertebral venous plexuses. The internal vertebral venous plexus drains into the external venous plexus via the intervertebral veins. The external venous plexus, in turn, empties into the vertebral veins of the neck and segmental veins of the trunk.

The facet joints are innervated by the medial branches of the dorsal rami of the spinal nerves. The medial branches give off small articular branches that innervate the articulations above and below them.

Facet joint issues

Clinically, various issues are associated with the facet joints, and all can be subdivided into acute or chronic damages. Acute damages to the zygapophyseal joints mainly happen due to different traumas, which can result in fracture or subluxation of the joints. Usually, injuries affecting the facet joints are accompanied by other damages to the vertebrae and their related structures and can be potentially harmful. The traumatic injury of the facet joints can also affect the contents of the vertebral canal - the spinal cord and its adjacent neurovascular structures.

The facet joints can also be damaged due to chronic causes, mainly because of osteoarthritis (OA). Osteoarthritis is an age-related “wear and tear” disease of the joints. Joint pain and tenderness, stiffness and limited joint movements are the most common symptoms of OA. If a concomitant degenerative intervertebral disc disease is present, the facet joints can develop OA earlier. Changes in the workload distribution on the spine due to intervertebral disc degeneration often result in a much higher load than the facet joints need to bear.

References:

1. Adam, A., Dixon, A., & Gillard, J. (2020). Grainger & Allison’s Diagnostic Radiology: 2-Volume Set (7th ed.). Elsevier.
2. Benzon, H. T., & Rathmell, J. P. (2013). Practical Management of Pain. Mosby.
3. Bogduk, N. (2012). Clinical and Radiological Anatomy of the Lumbar Spine (5th ed.). Churchill Livingstone.
4. Gray, H., & Carter, H. (2021). Gray’s Anatomy (Leatherbound Classics) (Leatherbound Classic Collection) by F.R.S. Henry Gray (2011) Leather Bound (2010th Edition). Barnes & Noble.