Facet joint (FJ)

A facet joint or zygapophysial joint (zygapophyseal, apophyseal, Z-joint, or facet joint) is a synovial joint between the superior articular process of one vertebra and the inferior articular process of the vertebra directly above it.

There are two facet joints in each spinal motion segment.

FJs are composed of an inferior articular process, facing anteriorly, and a superior articular process, facing posteriorly, of two adjacent vertebrae.

The pars interarticularis, or pars for short, is the part of vertebra located between the inferior and superior articular processes of the facet joint.

The facet joints bear ∼ 33% of the dynamic compressive load and 35% of the static load ^{1) 2)}

The first cervical vertebra has an inferior articulating surface but, as it does not restrict lateral or posterior translation, is not always considered a proper zygoma.

see Cervical facet joint.

see Thoracic facet joint.

see Lumbar facet joint.

FJ orientation planes differ at various levels, with a more sagittal and curved orientation for resistance against axial rotation in the upper compared to a more coronal and flat orientation for resistance against flexion and shearing forces in the lower lumbar segments.

Locked facets.

Facet joint syndrome.

It is important to note that the facet joints have a richly innervated capsule and the medial branches of the posterior lumbar rami (themselves branches of the lumbar spinal nerves) provide this innervation. In addition, the joints have been shown to hold up to a maximum of 1-2 mL of injected fluid by anatomical studies. Above this level, rupture of the joint capsule occurs and resultant extravasation into the back and epidural space leade to non-specific therapeutic effects ^{3) 4) 5)}.

Aggarwal A, Garg K. Additional Findings in Differential Diagnosis of Facet Joint Disorders. Radiographics. 2021 May-Jun;41(3):E94. doi: 10.1148/rg.2021210059. PMID: 33939550 ⁶⁾.