Interventional pain and spine specialist
The facet joints are a pair of joints in the posterior aspect of the spine. Although these joints are most commonly called the facet joints, they are more properly termed the zygapophyseal joints (abbreviated as Z-joints), a term derived from the Greek roots zygos, meaning yoke or bridge, and physis, meaning outgrowth. The term facet joint is a misnomer because the joint occurs between adjoining zygapophyseal processes, rather than facets, which are the articular cartilage lining small joints in the body (eg, phalanges, costotransverse and costovertebral joints). This joint is also sometimes referred to as the apophyseal joint or the posterior intervertebral joint.
As is true of any synovial joint, the facet-joint is a potential source of pain. In fact, the facet-joint is one of the most common sources of low back pain (LBP). The first discussion of the facet-joint as a source of LBP was by Goldwaith in 1911. (1) In 1927, Putti (2) illustrated osteoarthritic changes of facet-joints in 75 cadavers of persons older than 40 years. In 1933, Ghormley(3) coined the term facet syndrome, suggesting that hypertrophic changes secondary to osteoarthritis of the zygapophyseal processes led to lumbar nerve root entrapment, which caused LBP. In the 1950s, Harris and Mcnaz (4) and McRae (5) determined that the etiology of facet-joint degeneration was secondary to intervertebral disc degeneration. Hirsch et al were later able to reproduce LBP with injections of hypertonic saline solution into the facet-joints, thus affirming the role of the facet-joints as a source of LBP (6)
The spine is composed of a series of functional units. Each unit consists of an anterior segment, which is made up of 2 adjacent vertebral bodies and the intervertebral disc between them, and the posterior segment, which consists of the laminae and their processes. One joint is formed between the 2 vertebral bodies, whereas the other 2 joints, known as the facet-joints, are formed by the articulation of the superior articular processes of one vertebra with the inferior articular processes of the vertebra above. Thus, the facet-joints are part of an interdependent functional spinal unit consisting of the disc-vertebral body joint and the 2 facet-joints, with the facet-joints paired along the entire posterolateral vertebral column.(7)
Facet joints are well innervated by the medial branches of the dorsal rami. In the thoracic and lumbar spine, the facet joints are innervated by medial branches of the dorsal rami of the spinal nerves except at L5 level (8). After the medial branch splits off from the dorsal ramus, it courses caudally around the base of the superior articular process of the level below toward that level's Z-joint (e.g., the L2 medial branch wraps around the L3 superior articular process to approach the L2-L3 facet-joint). The medial branch then continues in a groove between the superior articular process and transverse process (or, in the case of the L5 medial branch, between the superior articular process of S1 and the sacral ala of S1, which is the homologous structure to the transverse processes of the lumbar vertebrae). As it makes this course, the medial branch is held in place by a ligament joining the superior articular process and the transverse process, termed the mamillo-accessory ligament (MAL).
The MAL is so named because it adjoins the mamillary process of the superior articular process to the accessory process of the transverse process. The MAL is clinically important because it allows precise location of the medial branch of the dorsal ramus using only bony landmarks, which is essential for fluoroscopically guided procedures.
After passing underneath the MAL, the medial branch of the dorsal ramus gives off 2 branches to the nearby facet-joints. One branch innervates the facet-joint of that level, and the second branch descends caudally to the level below. Therefore, each medial branch of the dorsal ramus innervates 2 joints—that level and the level below (e.g., the L3 medial branch innervates the L3-L4 and L4-L5 facet-joints). Similarly, each facet-joint is innervated by the 2 most cephalad medial branches (e.g., the L3-L4 facet-joint is innervated by the L2 and L3 medial branches). Medial branch also innervates the multifidus, interspinales, and intertransversarii mediales muscles, the interspinous ligament, and, possibly, the ligamentum flavum. (9)
This has several important clinical implications. First, pain relief from anesthetizing the medial branch does not necessarily implicate the facet-joints as the primary pain generator, because one of the other structures innervated by the medial branch may have been the pain generator. Second, denervation of the medial branch by RFA may affect the nerve supply to the multifidus muscle. This is important because lumbosacral radiculopathy is often another consideration in the differential diagnosis of LBP.
The L5 dorsal ramus divides into medial and lateral branches, with the medial branch continuing medially, innervating the lumbosacral joint.
As with any synovial joint, degeneration, inflammation and injury of facet joints can lead to pain upon joint motion. Pain leads to restriction of motion, which eventually leads to overall physical deconditioning. Irritation of the facet joint innervation in itself also leads to secondary muscle spasm. It has been assumed that degeneration of the disc would lead to associated facet joint degeneration and subsequent spinal pain. These assumptions were based on the pathogenesis of degenerative cascade in the context of a three joint complex that involves the articulation between two vertebrae consisting of the intervertebral disc and adjacent facet joints, as changes within each member of this joint complex will result in changes in others (10, 11). It was also the view of Vernon-Roberts and Pirie (12) that disc degeneration causes osteophyte formation and facet joint changes, because facet joints at relatively normal disc levels are either normal or only slightly degenerate.
The Facet joint is a common pain generator in the lower back. The 2 common mechanisms for this generation of pain are either (1) direct, from an arthritic process within the joint itself, or (2) indirect, in which overgrowth of the joint (e.g., facet joint hypertrophy or a synovial cyst) impinges on nearby structures. (13)
The Facet-joints are diarthrodial joints with a synovial lining, the surfaces of which are covered with hyaline cartilage, which is susceptible to arthritic changes and arthropathies. Repetitive stress and osteoarthritic changes to the facet joint can lead to zygapophyseal hypertrophy. Like any synovial joint, degeneration, inflammation, and injury can lead to pain with joint motion, causing restriction of motion secondary to pain and, thus, deconditioning. In addition, facet-joint arthrosis, particularly trophic changes of the superior articular process, can progress to narrowing of the neural foramen. In addition, as is the case for any synovial joint, the synovial membrane can form an outpouching and, thus, a cyst. Facet-joint cysts are most commonly seen at the L4-L5 level (65%), but they are also seen at the L5-S1 (31%) and L3-L4 (4%) levels. These synovial cysts can be clinically significant, particularly if they impinge on nearby structures (e.g., the exiting nerve root).
Facet-joint hypertrophy or a synovial cyst can also contribute to lateral and central lumbar stenosis, which can lead to impingement on the exiting nerve root. Thus, facet-joint pain can occasionally produce a pain referral pattern that is indistinguishable from disc herniation.
Numerous other causes, including rheumatoid arthritis, ankylosing spondylitis and capsular tears, etc., also have been described as sources of facet joint pain (14).
Facet joints have been implicated as responsible for spinal pain in 15% to 45% of patients with low back pain (15), 54% to 67% of patients with neck pain and 48% of patients with thoracic pain in controlled studies. These figures were based on responses to controlled diagnostic blocks of these joints, in accordance with the criteria established by the International Association for the Study of Pain
Establishing a diagnosis of lumbosacral facet syndrome is difficult because the findings are nonspecific and correlation between the history and physical examination findings is poor. However, obtaining a detailed history and performing a physical examination help rule out other entities and assist with guiding the examiner in establishing the diagnosis of facet-joint–mediated LBP.
Although no single sign or symptom is diagnostic, Jackson et al demonstrated that the combination of the following 7 factors was significantly correlated with pain relief from an intra-articular facet-joint injection (16):
1. Older age
2. Previous history of LBP
3. Normal gait
4. Maximal pain with extension from a fully flexed position
5. The absence of leg pain
6. The absence of muscle spasm
7. The absence of exacerbation with a Valsalva maneuver
Facet-joint pathology should be considered if the patient describes nonspecific LBP with a deep and achy quality that is usually localized to a unilateral or bilateral Paravertebral area.
The common referral areas for facet-joint–mediated pain are flank pain, buttock pain (often extending into the posterior thigh, but rarely below the knee), pain overlying the iliac crests, and pain radiating into the groin.
The pain is often exacerbated by twisting the back, by stretching, by lateral bending, and in the presence of a torsional load. Some patients describe their pain as worse in the morning, aggravated by rest and hyperextension, and relieved by repeated motion. Often, this lumbosacral facet syndrome may occur after an acute injury (e.g., extension and rotation of the spine), or it may be chronic in nature.
Unlike other lumbar spine pathologies such as disc herniation, facet-joint–mediated pain likely will not worsen with an increase in intra-abdominal and thoracic pressure. Therefore, worsening of pain with coughing, laughing, or a Valsalva maneuver is suggestive that the facet-joint is not the primary pain generator.