Anatomy

Zygapophyseal (apophyseal) or facet joints are found paired throughout the spine between the third cervical level and the first sacral level. Facet joints are diarthrodial synovial joints between the posterior elements of the spine with articular surfaces lined by hyaline cartilage, with marked variability in the size, shape, and position of the joint capsule (19,20).

The facet joint is comprised of the superior articular facet originating from the caudad vertebral body and inferior articular facet from the craniad vertebral body. In the lumbar spine the superior articular facets are directed anterolaterally, with the inferior articular facets facing posteromedially. The orientation of these structures is further such that the anterior portions of the joint are directed closer to the midline than the posterior portions (Figs. 1 and 2). This allows for flexion and extension, with minimal opportunity for rotation. In the cervical spine, the facet joints are oriented in an oblique axial plane, allowing for maximal flexion and extension and rotation, with lesser opportunity for lateral flexion. In the thoracic spine, flexion, extension, and lateral flexion are much more available than rotation secondary to the oblique coronal orientation of the facet joints (19,21,22).

The source of innervation ofthe facetjoints of the spine includes the median branch of the dorsal ramus from the level of the joint as well as from the level above. In addition to this bisegmental innervation, autonomic nerves have been reported to exist in the joint as well, reportedly contributing to back pain (23,24).

Zygapophysial Joint

Fig. 1. Model showing the relationship of the facet joints in the lumbar spine. Notice how the orientation of the facet joints (arrowheads) changes as you move down the lumbar spine. A facet block at L4-5 would be at a steeper oblique than the facet joint above. SAP, superior articular process; IAP, inferior articular process; PARS, pars interarticularis; ILS, interlaminar space.

Lumbar Spine Anatomy

Fig. 2. Axial CT image using bone window algorithm revealing normal appearing facet joints at the L4 level in an asymptomatic volunteer (arrowheads).

Fig. 1. Model showing the relationship of the facet joints in the lumbar spine. Notice how the orientation of the facet joints (arrowheads) changes as you move down the lumbar spine. A facet block at L4-5 would be at a steeper oblique than the facet joint above. SAP, superior articular process; IAP, inferior articular process; PARS, pars interarticularis; ILS, interlaminar space.

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