Low back pain (LBP) and neck pain represent an epidemic throughout the industrialized world. More than 75% of patients reporting spine pain are between 18 and 65 years old, thereby adding cost through lost productivity and wages. Although the prevalence of LBP varies greatly throughout the literature, some lifetime estimates are as high as 84% to 90%, and the 5-year recurrence rate of LBP may be as high as 69%. The lifetime prevalence of neck pain has been estimated to be about 67%. The estimated cost of treatment and lost wages for spine pain in the United States each year is estimated to be more than $140 billion. As the population ages, the impact of LBP and neck pain will continue to grow.
The zygapophyseal (facet) joint (z-joint) is a potential source of neck, shoulder, midback, low back, and leg pain. In addition, cervical facet disease can cause headaches. Interventions on the z-joints are second only to epidural steroid injections as the most common type of procedure performed in pain management centers in the United States. Despite a great deal of research on the diagnosis and treatment of facet pain, the issue still remains controversial. This chapter discusses the relevant anatomy, mechanisms of injury, prevalence, pain referral patterns, diagnosis, and treatment of facet arthropathy.
Anatomy and Function
The spine is normally composed of 7 cervical, 12 thoracic, and 5 lumbar vertebrae (see Fig. 61.1 ). The z-joints are paired structures situated posterolateral to the vertebral body. In conjunction with the intervertebral disk, they make up what is commonly known as “the three-joint complex.” Together, these joints function to support and stabilize the spine and prevent injury by limiting motion in all planes of movement. The lumbar z-joints are true synovial joints formed from the superior articular process of one vertebra and the inferior articular process of the vertebra above. The volume capacity of these joints is approximately 1 to 1.5 mL in the lumbar region and 0.5 to 1.0 mL in the cervical region. The articular surfaces are covered by hyaline cartilage and contain a fibrous capsule. The fibrous capsule is about 1 mm thick and is formed mostly of collagenous tissue arranged in a transverse fashion to provide resistance to forward flexion. The superior and inferior joint borders are formed by the fibrous capsule. In the lumbar spine, the multifidus muscle serves as the posterior joint border, and the ligamentum flavum replaces the fibrous capsule at the anterior border. The position of the joint relative to the sagittal and coronal planes helps determine the role that the joint plays in protecting the spine against excessive motion. Joints oriented parallel to the sagittal plane provide little resistance to backward and forward shearing forces but allow a greater degree of rotation, flexion, and extension. Joints oriented closer to the coronal plane will allow less rotation, flexion, and extension but serve as excellent protection against shearing forces. The cervical z-joints are inclined at roughly 45 degrees from the horizontal plane and angled 85 degrees from the sagittal plane. This alignment functions to prevent excessive anterior translation and to assist the disks in weight bearing.
The medial branch of the posterior rami supplies sensory innervation to the facet joint. Each exiting spinal nerve splits into an anterior and posterior primary ramus ( Fig. 61.2 ). The anterior ramus is the largest of the two branches and the main source of motor and sensory fibers. The posterior ramus divides into lateral, intermediate, and medial branches. In the lumbar region, the lateral branch provides innervation to the paraspinous muscles, thoracolumbar fascia, and sacroiliac joint and variable sensory fibers to the skin overlying the spinous processes. The small intermediate branch supplies the longissimus muscle. The medial branch is the largest branch of the posterior primary ramus and innervates not only the lumbar z-joint but also the multifidus muscle, interspinal muscle and ligament, and periosteum of the neural arch. To block sensory input from one facet joint, two adjacent medial branches must be anesthetized. In some people, facet joint innervation may be derived from other sources.
Facet joints are imbued with a rich innervation containing encapsulated (Ruffini-type endings, pacinian corpuscles), unencapsulated, and free nerve endings. In addition to being a potential pain generator, the z-joint capsule is thought to serve in a proprioceptive capacity as well, as evidenced by the presence of low-threshold, rapidly adapting mechanosensitive neurons. Kallakuri and colleagues used immunocytochemistry to characterize the presence of substance P and calcitonin gene–related peptide–reactive nerve fibers in the cervical facets of 12 human cadavers. In addition to substance P and calcitonin gene–related peptide, a substantial percentage of nerve endings in the facet capsules contain neuropeptide Y, which is indicative of the presence of sympathetic efferent fibers. Nerve fibers have been found in subchondral bone and intra-articular inclusions of facet joints, thus signifying that facet-mediated pain may originate in structures besides the joint capsule. Inflammatory mediators such as prostaglandins and the inflammatory cytokines interleukin-6 and tumor necrosis factor-α have been found in facet joint cartilage and synovial tissue in degenerative lumbar spinal disorders.
Lumbar Facet Joints
The lumbar facet joints are aligned lateral to the sagittal plane and vary in angle ( Fig. 61.3 ). The inferior articular process faces anterolaterally, and the superior articular process faces posteromedially. In an anatomic study published in 1940 by Horwitz and Smith, the authors found that the L4-5 z-joints tended to be more coronally positioned (almost 70 degrees with respect to the sagittal plane), whereas the L2-3 and L3-4 joints were likely to be oriented more parallel (<40 degrees) to the sagittal plane. In more recent studies by Masharawi and coworkers and Punjabi and associates, the investigators found that the upper lumbar facet joints (T12-L2) were oriented closer to the midsagittal plane of the vertebral body (mean range, 26 to 34 degrees), whereas the lower facet joints tended to be oriented away from that plane (40 to 56 degrees). In the upper lumbar spine, approximately 80% of the facet joints are curved and 20% are flat. In the lower lumbar spine, these numbers are reversed. Studies by Grobler and colleagues and Boden and associates found a positive association between degenerative spondylolisthesis and more sagittally oriented lower lumbar facet joints. The inferior articular processes of L5 combine with the superior articular processes of the sacrum to form the L5-S1 facet joints. The dip in the sacrum immediately lateral to the superior articular process is termed the sacral ala.
Sensory innervation of the facet joints is derived from the medial branches arising from the posterior primary rami at the same level and the level above the facet joint. For example, the L4-5 medial branch receives its innervation from the L3 and L4 medial branch nerves. The medial branches of L1 to L4 run across their respective transverse processes one level below the named spinal nerve (e.g., L4 crosses the transverse process of L5) and traverse the dorsal leaf of the intertransverse ligament at the base of the transverse process. The nerve then proceeds along the junction of the superior articular process and the transverse process, courses underneath the mamilloaccessory ligament, and splits into multiple branches as it crosses the vertebral lamina ( Fig. 61.4 ). Calcification of the mamilloaccessory ligament can be a source of nerve entrapment. This is most common at L5 (20%) but also occurs at L4 (10%) and L3 (4%). The nerve at L5 that is amenable to blockade is actually the dorsal ramus itself, which runs along the junction of the superior articular process of the sacrum and the sacral ala. Some authors have claimed that a branch from the S1 nerve root can run cephalad to supply a portion of the L5-S1 facet joint, although this point remains controversial.
