Biomechanics of the Lumbar Facet Joint

Abstract

Zygapophyseal, or facet, joints are complicated biomechanical structures in the spine, with a complex three-dimensional (3D) anatomy, variable mechanical functions in different spinal movements, and effects on the overall spine mechanical behavior. The 3D morphology of the facet joint is linked to its biomechanical function. Failure of the biomechanical function of the facet joint leads to osteoarthritic changes in it and is implicated in other spinal disorders such as degenerative spondylolisthesis. Facet joints and intervertebral disk are part of an entity called the spinal motion segment, the three-joint complex, or the articular triad. Functioning together, the structures in the spinal motion segments provide physiological spinal motion, while protecting the spine by preventing activities that can be injurious. Loss of intervertebral disk height associated with disk degeneration affects the mechanical behavior of facet joints. Axial compressive load transmission through the tip of the inferior articular process can occur in the extended position, especially with reduced disk height, which may cause capsular impingement and low back pain. The 3D curvature of the articular surfaces and capsular ligaments play important roles in different spinal positions. In this review article, we will summarize the anatomy of the lumbar facet joint relevant to its biomechanical function and biomechanical behavior under different loading conditions.

Anatomy of lumbar facet joint: a comprehensive review

“…Articular surfaces of SAP and IAP feature a consistently different morphological pattern. Hence, SAP are characterized by a more concave articular surface, whereas IAP by a more convex one [16,24]. In addition, orientation of SAP and IAP in sagittal and coronal planes features a noteworthy differentiation.…”

“…LFJs are comprised by the articulation of superior and underlying adjacent vertebra via the paired inferior and superior articular processes, respectively [10,16]. The major superior and the minor inferior articular processes (SAP and IAP) represent bony protuberances, emerging vertically from the coalescence of pedicles and laminae of respective vertebral arch, posteriorly to the ipsilateral transverse process [16,26]. SAP and IAP are lined with articular hyaline cartilage over the subchondral bone [26].…”

“…LFJs are, similarly to other synovial diarthrodial joints, completely encapsulated by a Capsular Ligament (LFCL). LFCL is histologically composed by two distinct layers; an outer layer of parallel with lateral-medial direction and densely organized collagen bundles, and an inner layer of elastic fibers with inconsistent orientation [5,16]. Furthermore, LFCL features rich innervation with autonomic and nociceptive nerve fibers, which may reproduce pain in cases of inflammatory or mechanical irritation [26,31].…”

“…The three-dimensional LFJs and the anteriorly located intervertebral disc at each lumbar spine level compose an anatomic entity described as spinal segment, "three-joint complex" or articular triad [15,16,21,23,26,32,38]. These articular structures are theorized to constitute a single unit since the emergence of degenerative alterations in one joint has a subsequent influence on the biomechanic behavior of the whole unit [26].…”

“…Facet joints, which are also classically described as apophyseal or zygapophyseal joints, represent the only synovial joints of spine [16,26]. They are paired diarthrodial joints, posterolaterally articulating the posterior arch between adjacent vertebral levels [26].…”