The Zygapophyseal (Facet) Joints and the Capsular Ligaments of the Spine
An official anatomical term, "zygapophyseal joint," is difficult to pronounce and remember. Therefore it is universally substituted by the more convenient terms like "Z-joints" or "facet joints." We use the terms "zygapophyseal joints" and "facet joints" as synonyms on this website.
The zygapophyseal joints are the classical synovial joints (with the articular surfaces covered with cartilage, with articular gap and synovial cavity hermetically surrounded by the capsule) localized between the articulatory processes of adjacent vertebrae. The capsular ligaments of the facet joints consist of the fibers oriented more or less perpendicular to the joint line1 and look like thin ligaments. Their primary function is to seal the articulation itself. However, the strength of the capsular ligaments is on par with the other major spinal ligaments like the yellow and interspinal ligaments2.
The shape of the facet articulatory surfaces is almost flat at that level, and the orientation of the facet joint is oblique. This shape and orientation cause strong coupling of lateral bending with axial rotatory motions. I.e., the lateral flexion of the cervical spine (within the C2–C7) is invariantly associated with the axial rotation of involved vertebrae, and vice versa – the axial rotation of the neck is coupled with the lateral bending of cervical vertebrae*.
The shape of the articulatory surfaces of the facet joints at that level remains flat. However, the facets are oriented more vertically compared to the cervical level and approach the frontal plane. This shape and orientation restrict the motion in the sagittal plane that makes the thoracic the most rigid part of the spine in terms of the range of the flexion/extension motions*.
The lower thoracic spine's peculiarity is the facet joint plane orientation transition from the more or less frontal (above the Th11) to the predominantly sagittal (lumbar spine). This transition reflects the shift from the rotation-oriented (thoracic) to the more flexion/extension-oriented part of the spine (lumbar)*.
The shape of the articulatory surfaces of the lumbar spine is slightly rounded. Still, the facet's orientation is predominantly sagittal, facilitating the lumbar spine flexion & extension motion. On the other hand, this shape and orientation restricts the rotatory movements, making the lumbar spine the most rigid part of the spine in terms of the axial rotation1,2.