Articular surfaces and capsule
Each facet joint links the articular processes of two adjacent vertebrae, forming, with the intervertebral disc in front, a three-joint complex at every spinal segment.
The inferior articular process of the upper vertebra rests against the superior articular process of the vertebra below. Both surfaces are covered by hyaline cartilage, and the joint is enclosed in a fibrous capsule lined by synovial membrane. The capsule is thin and lax in the cervical spine, allowing a large range of movement, and thicker in the lumbar spine. Small fibro-fatty synovial folds (meniscoids) project between the joint surfaces at the poles of the joint.
The ligamentum flavum, the elastic ligament joining adjacent laminae, forms the anteromedial part of the capsule and separates the joint from the vertebral canal. The joints between the occiput, atlas and axis are not called facet joints; the zygapophyseal series begins at C2/C3.
Orientation and movement by region
The plane of the facet surfaces changes down the spine, and this is the main reason cervical, thoracic and lumbar segments move differently.
| Region | Facet orientation | Movements favoured | Movements limited |
|---|---|---|---|
| Cervical | Inclined about 45 degrees to the horizontal; superior facets face up and back | Flexion, extension, lateral flexion, rotation | Little limitation; flexibility at the cost of stability |
| Thoracic | Near the coronal plane; superior facets face back and slightly laterally | Rotation and lateral flexion | Flexion and extension, further limited by the rib cage |
| Lumbar | Near the sagittal plane, curved; superior facets face medially and back, inferior facets laterally | Flexion and extension | Rotation |
| Lumbosacral (L5/S1) | More coronal than other lumbar levels | Flexion and extension | Forward sliding of L5 on S1 |
The facets carry part of the axial load, especially in extension and when the intervertebral disc has lost height. They also resist shear and torsion that would otherwise strain the disc.
Innervation and blood supply
Each facet joint is supplied by the medial branches of two posterior rami: the one from its own level and the one from the level above.
In the lumbar spine the medial branch runs across the root of the transverse process in the groove where it meets the superior articular process, passes beneath the mamillo-accessory ligament, and supplies the joint above and the joint below, as well as multifidus. For example, the L4/L5 facet joint is supplied by the L3 and L4 medial branches. At L5/S1 the corresponding nerves are the L4 medial branch and the L5 posterior ramus, which crosses the sacral ala.
In the cervical spine the medial branches curve around the waist of each articular pillar. The C3 posterior ramus gives a large superficial medial branch, the third occipital nerve, which supplies the C2/C3 joint and is the target for treating headache arising there.
Blood comes from dorsal branches of the segmental arteries: the vertebral arteries in the neck, the posterior intercostal arteries in the thorax, and the lumbar arteries in the lumbar spine.
Relations
The facet joint forms the back wall of the intervertebral foramen and the lateral recess, so enlargement of the joint crowds the spinal nerves.
The exiting spinal nerve and its dorsal root ganglion pass through the intervertebral foramen immediately in front of the facet joint. The traversing nerve root, heading for the next foramen down, runs in the lateral recess just medial to the superior articular process. The ligamentum flavum lies anteromedially, and multifidus and the other deep back muscles lie behind.
Clinical relevance
Facet joints are a common source of axial neck and back pain and a key contributor to spinal stenosis.
- Facet arthropathy: osteoarthritis of the facets causes axial pain that may refer to the buttock, thigh or shoulder girdle without true radicular signs.
- Lumbar spinal stenosis: facet hypertrophy, thickened ligamentum flavum and disc bulging together narrow the canal and lateral recesses, causing neurogenic claudication.
- Degenerative spondylolisthesis: forward slip of one vertebra on another with an intact pars interarticularis, most often at L4/L5, where sagittally oriented facets resist slipping poorly.
- Facet synovial cyst: may bulge into the lateral recess and compress a nerve root.
- Cervical facet dislocation: flexion and rotation injuries can lock one or both facets forward; bilateral dislocation produces greater translation and a high rate of cord injury.
- Diagnostic and therapeutic procedures: intra-articular injections, medial branch blocks and radiofrequency denervation of the medial branches are used to diagnose and treat facet-mediated pain.