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.
1Bogduk N. Clinical Anatotmy of the Lumbar Spine and Sacrum. 4th ed, Elsevier Churchill Livingstone, 2005, p.33
2White, A. A. & Panjabi, M. M. Clinical Biomechanics of the Spine. Lippincott Williams & Wilkins, 1990; p.22