The facet joints (zygapophyseal or z-joints) are paired synovial joints at the posterior aspect of the spinal column (Figure 11.1). Each joint consists of the articulation between adjacent superior and inferior articular processes arising from adjacent vertebrae. Functionally, the joints are thought to play some role in weight bearing in support of the disc and are also felt to play a role in limitation of motion and prevention of damage to the intervertebral disc. The facet joints provide limitations for spinal flexion, extension, and rotation.3 The joint itself is a diarthrodial joint with a fibrous joint capsule and contains synovial fluid and a synovial membrane (Figure 11.2). Hyaline cartilage lines the articular surfaces of the superior and inferior articular processes. The joint capsule is attached to the bony articular processes and is slightly redundant at the superior and inferior margins of the joint (superior and inferior recesses). Each joint is bordered medially and anteriorly by the ligamentum flavum and posteriorly by the multifidus muscle. The articular processes provide a sliding surface for motion with roughly 5 to 7 mm of motion possible along the plane of the joint.
capsule inferior articular process superior articular process
nucleus ' pulposus hyaline cartilaginous endplate annulus fibrosus
Figure 11.1. Sagittal diagram depicting the components of the lumbar facet joint. The joint is a true encapsulated synovial joint formed from the articulation of adjacent inferior and superior articular processes.
The orientation of each joint varies considerably among individuals, but certain features of the obliquities are characteristic. The cervical facet joints are typically oriented in an oblique coronal plane, angled superior to inferior in a posterior direction. The thoracic facet joints are nearly vertical and coronal in orientation, rotating toward the sagittal plane near the thoracolumbar junction (Figure 11.3). The superior lumbar facet joints are oriented in a nearly sagittal plane, and the plane of orientation rotates outward toward the coronal plane with descent in the lumbar spine so that the joints are in a sagittal-coronal oblique plane at the lumbosacral junction. The articular surfaces of the joint have variable morphology as well and may be nearly linear or convex (Figure 11.4). Familiarity with the orientation of the joint is important in selecting an appropriate needle approach for injection into a joint.
Various ranges of fluid capacity for the joints have been reported, although a reasonable estimate would be roughly 0.5 to 1 mL in the cervical spine and 1.5 to 2 mL in the lumbar spine. In normal joints the capsule is a potential space containing a barely detectable quantity of fluid. The fluid content of the joint and the thickness of the hyaline cartilage typically decrease with age, although joint pathology may result in increases in the amount of joint fluid.
Figure 11.2. Axial anatomical diagram of the lumbar facet joint. The articular surfaces of the superior and inferior articular processes are capped with hyaline cartilage. The fibrous joint capsule contains a synovial membrane and a small amount of synovial fluid. The joint is bordered by the ligamentum flavum anteriorly, and by the multifidus muscle posteriorly.
Figure 11.3. Axial cross sections depicting the change in orientation of facet joints at the thoracolumbar junction. The orientation of the facet joints changes descending along the spinal column from a coronal an-gulation in the cervical spine to a sagittal oblique orientation in the lower lumbar spine. At the thoracolumbar junction, there is a gradual change in orientation of the joints from coronal to sagittal.
The synovial membrane and joint capsule are both innervated with sensory fibers, including unmyelinated C fibers.3-5 Each joint is innervated by a small nerve arising from the medial (median) branch of the primary posterior ramus, which passes over the transverse process and under the mammilloaccessory ligament. The median branch has a characteristic course. In the lumbar spine, the medial branch of the dorsal ramus courses from the neural foramen to the joint capsule directly over the medial aspect of the transverse process at the junction with the superior articular process (Figure 11.5). Each joint is innervated by two medial branches, one from above and one from the same level as the superior articular process of that joint. In the cervical spine, the medial branch courses from the foramen to the joint across a ridge in the middle aspect of the lateral mass of the vertebra (Figure 11.6). The course of the medial branch in the thoracic spine is less well established, though it is thought to be homologous to the course of the lumbar branches, extending over the medial aspect of the transverse processes.
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