More than 60% of people in developed countries will experience spinal pain at some time in their lives. Back pain is the most common complaint of patients referred to pain clinics. The pain is nonspecific in about 85% of the cases, and onset of symptoms is most often between the ages of 35 and 55 years. According to some sources, 15% to 45% of all adults experience lower back pain, and 1 in 20 people present with a new episode annually. In 2008, 3.7 million workplace injuries were reported, and of these, 65% were attributed to the low back. In 2012, state workers’ compensation programs provided almost $60 billion in cash and medical care benefits.

Risk factors for spinal pain include trauma, heavy physical labor, frequent twisting, bending, vibrations, pulling and pushing, and repetitive motion, especially that involving static postures. Psychological features such as anxiety, depression, job dissatisfaction, and stress also can play an important role.

The spine consists of 7 cervical, 12 thoracic, and 5 lumbar vertebrae in addition to the sacrum and coccyx. They articulate anteriorly through the disks and posteriorly through the left and right synovial facet joints. Anterior to the ligamentum flavum and covering the facet (apophyseal) joint is a variable amount of vascularized adipose tissue, which directly contacts the dural sleeve of the nerve root. The sleeve is located so close to the facet that it is possible, inadvertently, to inject medication directly into the cerebrospinal fluid. The articular surfaces of the facets are covered by cartilage. Joints are lined by synovium and contain variable amounts of fluid. The fibrous joint capsule forms superior and inferior joint recesses and blends anteromedially with the ligamentum flavum. It is located close to the neural foramen and the nerve root. Enlarged and osteophytic joints can contribute to significant narrowing of the neuroforaminal opening and can cause radicular symptoms.

Computed tomography (CT), magnetic resonance imaging (MRI), and intraarticular contrast medium can be used to demonstrate these anatomic features. The volume of injectate that can be accommodated by the facet joints varies as follows: cervical, up to 1.0 ml; thoracic, 1.0 ml to perhaps 1.5 ml; and lumbar, 1.0 ml to perhaps 1.5 ml.

In the upper lumbar spine, approximately 80% of the facet joints are curved and 20% are flat. This situation is reversed in the lower lumbar spine, where approximately 80% of the joints are flat. The upper lumbar facets are oriented more strongly in the sagittal plane, and, by the L5 to Sl level, they rotate obliquely. The lumbar facet joints are oriented 45 degrees from the sagittal plane, but because of the curvature of the joints, the posterior part of the joint is close to the sagittal plane. Lumbar facet syndrome has been considered to be a significant source of lower back pain, with a prevalence from 15% to 45% using strict diagnostic interventional criteria.

The thoracic facet joints are almost parallel to the coronal plane. They extend superiorly and inferiorly from the junctions of the laminae and pedicles and are oriented approximately 20 degrees from the coronal plane. Thoracic facet syndrome is less clearly established as a cause of spinal pain. However, recent limited, good-quality investigations have suggested a prevalence as high as 40%.

The anatomy of the cervical facets is significantly different from that of the lumbar ones. The cervical facets extend laterally from the junction of the laminae and pedicles and are oriented in the coronal plane to permit extension, flexion, and lateral bending. The atlanto-occipital and atlantoaxial joints are the C0–1 and C1–2 facet joints. Their structure, function, and innervation are unique. The C2–3 through C5–6 joints are angled 35 degrees from the coronal plane. The C6–7 apophyseal joint is tipped 22 degrees from the coronal plane. All of the cervical facet joints from C2–3 to C7–T1 are angled 110 degrees from the midline posterior sagittal plane, which makes their orientation similar to that of the thoracic facets. The cervical facets play a larger role, physically, in the spinal articular tripod structure and are commonly described as the superior and inferior ends of articular pillars. The vertebral artery, which passes through the transverse foramen of the transverse processes of the C1 to C6 vertebrae, is a landmark of the cervical spine. The prevalence of pain in the cervical spine may even be higher than that of the lumbar spine, using strict interventional diagnostic criteria.

The facet joints appear to function to protect the spine from excessive mobility and distribute axial loading over a broad area. The orientation and shape of the facets are specifically designed to accommodate the stresses and movements expected at each spinal level.

The nonspecific localization of facet joint pain is explained by profuse overlapping of sensory innervation. The medial branch nerve supplies the lower facet at its own level as well as the upper part of the joint below (the L4–5 joint is supplied by the L3 and L4 medial branch nerves). Therefore, each of the facet joints receives innervation from a medial branch nerve of two posterior primary rami. These branches also innervate the paraspinal muscles, ligaments, and periosteum, with significant dermatomal sensory overlapping.

In the lumbar region (Fig. 39-1), the medial branch nerve lies in a groove on the base of the superior articular process and passes under the mammilo-accessory ligament. It then runs in a posterior and inferior direction, first sending fibers to innervate the adjacent joint capsule before sending fibers to the next lower level. The course of the L5 medial branch is different because the transverse process is replaced by the ala of the sacrum. The L5 medial branch is actually the L5 dorsal ramus proper. The L5–S1 facet joint may have additional innervation from the S1 nerve root branch. Because of the dual nerve innervation, each joint must be blocked at two segments, both at and above the level of the involved joint. The lumbosacral facet joint should be blocked at two, and perhaps even three, levels.

There is evidence of multilevel innervation of the lumbar facet joints, which includes not only the posterior primary rami, but also the sympathetic and parasympathetic ganglia. The sympathetic fibers have been reported to regulate the activity of sensory neurons and may contribute to the experience of lower back pain.

Innervation of the thoracic facets is similar to that of the lumbar spine. Medial branches from two segmental levels innervate each joint (i.e., the T4–5 joint is supplied by the T3 and T4 medial branch nerves). However, unlike in the lumbar spine, the nerve crosses the superolateral corner of the transverse process (at least 12 mm lateral to the root of the transverse process) and then passes medially and inferiorly across the posterio surface of the transverse process. The exception to this description occurs at the midthoracic level (T5–T8). Here, the nerves assume a course parallel to the typical levels but does not reliably make bony contact with the superolateral corner of the transverse process and, as they turn medialy, remain separated from the surface of the transverse process. The T11 branch also has different anatomic features and runs across the lateral surface of the root of the relatively smaller T12 transverse process. At the T12 level, the medial branch localization is analogous to that of the lumbar spine.

The cervical medial branches (Fig. 39-2) mainly supply the facet joints, with minimal innervations of the following posterior neck muscles: multifidus, interspinalis, semispinalis cervicis, and semispinalis capitis. The C3 dorsal branch is the only cervical dorsal ramus below C2 that has a cutaneous distribution. Therefore, if neck pain or headache is caused by cervical facet disease, cervical facet joint blocks can relieve it.

The upper cervical synovial joints—the atlanto-occipital and lateral atlantoaxial joints—are innervated by cervical ventral rami (Fig. 39-3). The only suitable procedure to relieve pain at these joints is an intraarticular injection. The C2–3 facet joint is innervated mainly by the third occipital nerve and sometimes by the C2 dorsal rami (the greater occipital nerve). There are eight cervical nerves and seven cervical vertebrae. The first seven cervical nerve roots exit the spine above the vertebral body, and they are numbered according to the vertebral body below them. The C3–4 through C7–T1 facet joints are supplied by the medial branches at the same level as the joint and from the segmental level above (i.e., C5 and C6 medial branch nerves supply the C5–6 facet joint). These nerves branch off from the cervical posterior primary rami and wrap around the waists of the articular pillars (viewed as “centroid” of the articular pillar on the lateral projection). They are attached to the periosteum by fascia and tendons of the semispinalis capitis.

Ghormley was the first to define facet syndrome, describing it as lumbosacral pain with or without sciatic pain and associated with sprain or violent twisting of the facet joint. Since then, much research has been carried out to identify the pathologic processes involved and to find a specific therapeutic approach to this disease.

Neurophysiologic studies have shown that the medial branch nerves transmit nociceptive and proprioceptive signals from the facet joints, which are triggered by inflammatory and mechanical factors. Researchers have identified multiple mechanosensitive somatosensory receptors and neuromodulators of nociception within the facet capsule, including calcitonin gene-related peptide, substance P, and vasoactive intestinal peptide. Chronic inflammation—with consecutive joint hypertrophy, degeneration, and osteophyte formation—may contribute to neuroforaminal narrowing and compression of the nerve roots. On occasion, this causes referred pain to the extremities, as well as abnormal joint stress with possible subluxation and muscle spasm (Fig. 39-4). Pain originating from the apophyseal joints can be attributed to a synovitis, degenerative arthritis, and segmental instability.