The lumbar zygapophysial joint, otherwise known as facet joint, is a common generator of lower back pain. The facet joint is formed via the posterolateral articulation connecting the inferior articular process of a given vertebra with the superior articular process of the below adjacent vertebra. The facet joint is a true synovial joint, containing a synovial membrane, hyaline cartilage surfaces, and surrounded by a fibrous joint capsule. There is a meniscoid structure formed within the intra-articular folds. The facet joint is dually innervated by the medial branches arising from the posterior ramus at the same level and one level above the joint.
The facet joints play an important role in load transmission, assisting in posterior load-bearing, stabilizing the spine in flexion and extension, and restricting excessive axial rotation. Studies before and after facetectomy have shown that the facet joint may support up to 25% of axial compressive forces and 40% to 65% of rotational and shear forces on the lumbar spine.
Facet joint arthrosis is a pathological process involving the failure of the synovial facet joints. Degenerative changes begin with cartilage degradation, leading to the formation of erosions and joint space narrowing, and eventually sclerosis of subchondral bone. Risk factors include advanced age, a sagittal orientation of the facet joint, and concomitant intervertebral disk degeneration.
Facet joint arthrosis is a degenerative syndrome that typically occurs secondary to age, obesity, poor body mechanics, repetitive overuse and microtrauma. Numerous studies have linked facet joint degeneration to degeneration of intervertebral disks, showing that intervertebral disk degeneration likely occurs before facet joint arthrosis. One explanation for these findings is the increased mechanical changes in the loading of the facet joints following intervertebral disk degeneration. Other studies have demonstrated an increased propensity for facet joint degeneration with a more sagittal orientation of the facet joint.
Degenerative changes involving the facet joint begin with hyaline cartilage degradation, leading to the formation of erosions and joint space narrowing, and eventually sclerosis of subchondral bone. Studies have shown that over time the posterior capsule of the degenerative joint capsule becomes hypertrophied, with fibrocartilage proliferation and possibly synovial cyst formation. Osteophytes are likely to arise at the attachment sites (entheses) where the fibrocartilage extends beyond the original joint space. Facet mediated pain occurs secondary to these arthritic changes, as there is rich innervation of the entire joint complex. Other theories behind facet-mediated pain include, but are not limited to, facet intraarticular meniscoid entrapment and synovial impingement.
Estimates of the prevalence of lumbar facet-mediated pain have widely ranged in the literature, from under 5% to over 90% of patients reporting back pain. Many of these studies used a combination of history, physical exam, and radiologic findings, which have been shown to be unreliable in diagnosing facet pain. Studies following the criteria established by the International Association for the Study of Pain, involving controlled medial branch blocks, have implicated the lumbar facet joint as the source in 15% to 45% of the patients with chronic low back pain.
Age is strongly associated with the prevalence of lumbar facet arthropathy. According to one study, moderate to severe lumbar facet arthropathy was found in 36% of adults younger than age 45, 67% of adults age 45 to 64, and 89% of adults age 65 or older. Another study using lumbar CT and plain radiography found that women over the age of 50 are more likely to have facet joint osteoarthritis than men. The same study concluded that Caucasians are more likely to have facet joint osteoarthritis than African Americans. Body mass index (BMI) is another independent risk factor, with studies indicating increasing risk with higher BMI. BMI of 25 to 30 was found to increase the risk of facet joint osteoarthritis by 3 times normal, while a BMI of 30 to 35 was shown to have 5 times the risk of normal. Other independent risk factors noted are disk height narrowing, a sagittal orientation of the facet joint, and poor spinal extensors. Overall, lumbar facet osteoarthritis was found to be most prevalent at the L4-5 and L5-S1 levels. Less common are L3-L4 level, followed by the L1-2 and L2-3 levels.
Lumbar facet joint pain will typically present as unprovoked chronic lower back pain. The pain can have varying features, and it is impossible to diagnose based on history and physical exam alone. Clues that may point to the facet as the pain generator include pain localized over the back with a non-dermatomal radiation pattern. In the lumbar spine, the referred pain is typically around the buttock and thigh and is rarely felt below the knee. Numbness and weakness of the lower extremities are unlikely. However, patients with osteophytes, large synovial cysts, or facet hypertrophy may have accompanied lumbar radiculopathy via nerve root irritation. Other neurological signs, such as bowel and bladder dysfunction, should point the practitioner away from the facet as a pain generator.
Physical examination of the patient with facet pain may reveal tenderness to palpation over the lumbar paravertebral region over the transverse processes and paraspinal muscles. Pain may be exacerbated by spinal extension and rotation. Neurological findings, such as lower limb sensation, motor strength, and deep tendon reflexes will be normal. However, lower extremity strength may be diminished secondary to pain. The Kemp test (otherwise known as the quadrant test and extension-rotation test) is a provocative test that has been described as being potentially useful for diagnosing facet joint pain. In this maneuver, a patient performs combined extension and rotation of the spine, with a positive test defined as the reproduction of pain. However, studies have shown that the Kemp test has poor diagnostic accuracy, with a sensitivity under 50% and specificity under 67%.
Literature does not lend support to the routine use of radiologic imaging in the diagnosis of facet-mediated pain. The gold standard for diagnosis of facet-mediated pain is an anesthetic block of the facet joint. However, imaging remains a useful tool to rule out other causes of suspected lower back pain, including but not limited to disk herniation, spinal stenosis, spondylolisthesis, ankylosing spondylitis, diffuse idiopathic skeletal hyperostosis, infection, or neoplasm. Moreover, it is not uncommon to have concomitant facet joint degeneration along with the above alternative causes of lower back pain.
Classic radiographic findings in facet joint arthrosis include both degenerative and proliferative features. Imaging may reveal narrowing of the facet joint space, subarticular bone erosions, subchondral cysts, osteophyte formation, and hypertrophy of the articular process. Standard lumbar radiographs (x-ray) have limited value and should contain oblique views, as the facet joints are in an oblique position. However, oblique x-ray has only 55% sensitivity and 69% specificity in distinguishing the presence of facet joint disease. While some studies have reported MRI as more than 90% sensitive and specific in visualizing facet degeneration, others have shown that MRI is less sensitive than CT in depicting the bony cortex margin. Thus, CT remains the preferred evaluation method for imaging of facet arthropathy due to a more precise demonstration of bony details and relatively lower cost than MRI. However, MRI remains a superior diagnostic tool to rule out non-facet mediated pain. Other imaging techniques, such as bone scintigraphy with SPECT may be useful to depict bone areas with synovial changes and degenerative remodeling.
The diagnostic block of the facet joint is the most reliable means for diagnosing facet-mediated pain, with level I or level II evidence based on the United States Preventive Services Task Force criteria. Both intra-articular injections of the facet joint and medial branch blocks are equally effective; however, both have their drawbacks. Studies have also shown comparable accuracy between fluoroscopic guided and ultrasound-guided injection, although ultrasound is less accurate in obese patients. Intra-articular injection involves the injection of local anesthetic directly into the facet joint capsule. The joint capsule is so small that after injection of 1 to 2 mL of fluid the capsule is likely to rupture, spreading anesthetic to other surrounding possible pain-generating structures. In comparison, the medial branch block involves the injection of local anesthetic at the medial branch divisions of the dorsal rami at the level of and one above a given facet joint. A successful diagnostic block is considered if there is greater than or equal to 80% pain relief post-injection. Since the medial branch innervates many other possible local pain generating structures, including paraspinal muscle, fascia, ligaments, sacroiliac joint, there is a high false-positive rate (reported up to 25% to 40%). It has therefore been suggested to perform a double diagnostic block. However, this is rarely practiced due to the risk of patient drop-out, cost-effectiveness, and an increased complication rate.
Treatment of chronic lower back pain begins with conservative management. This is true for facet mediated pain, even in the absence of confirmation via a diagnostic block. Physical therapy is a cornerstone of the treatment of chronic lower back pain and should include postural education, stretching, and exercises tailored to strengthen the core musculature. Pain medications, including nonsteroidal anti-inflammatory drugs (NSAIDs) and acetaminophen, are widely considered the first line for treatment of lower back pain. Adjuvant medications, such as antidepressants in chronic back pain and muscle relaxants in acute back pain have also demonstrated effectiveness.
Patients who fail trial conservative management are considered candidates for a diagnostic block. If the diagnostic block is successful, more invasive treatment can be considered. Intra-articular steroid injections are a controversial treatment, with level III evidence to support their use. Radiofrequency neurotomy, via continuous, high-temperature medial branch radiofrequency ablation, carries level II evidence. Due to axonal regeneration, pain may return requiring repetition of the procedure, typically between 6 months to 1 year. Other reported interventions include pulsed radiofrequency ablation, cryo-denervation, and phenol neurolysis; however, these studies are uncontrolled and require further evaluation.
Facet mediated pain is rarely an indication for surgical intervention in itself. Studies have not demonstrated evidence for surgical intervention except in cases of traumatic dislocation. However, due to facet arthropathy, osteophytes or large synovial cysts may impinge upon other surrounding structures and lead to stenosis, nerve root impingement, and radiculopathy. In such cases, lumbar facetectomy may be performed. Medial facetectomy is the most commonly performed form of facetectomy and is often performed in conjunction with laminectomy. Complete facetectomy may be performed in the setting of lumbar fusion.
Differential diagnoses for lumbar facet arthropathy include, but are not limited to:
Facet arthropathy will increase with age. Conservative management, such as physical therapy is the first line in management. Patients who fail to respond to a trial of physical therapy may undergo diagnostic block of the facet joint. Successful radiofrequency neurotomy has been demonstrated to relieve pain for 6 months to up to 1 year, at which point repeat procedure may be indicated.
Serious complications of facet intervention are rare. Intraarticular steroid injections carry the risk of metabolic and endocrine side effects related to elevated glucose levels and suppression of the hypothalamic-pituitary-adrenal access; however, there have been no studies reported to date. There have been case reports of infection following intraarticular steroid injection, including septic arthritis, epidural abscess, and meningitis. Other complications include dural puncture and spinal anesthesia.
The most common complication of radiofrequency neurotomy is neuritis, with a reported incidence of 5%. There have also been reports of transient numbness and/or dysesthesias. Other rare complications include burns, which may result from electrical faults.
Deterrence of facet arthropathy includes patient education of proper posture. Given that obesity has a strong correlation with the development of facet arthropathy, weight loss is another good strategy to employ. Patients are advised to seek a professional opinion for chronic back pain as the potential differential diagnoses are numerous.
Chronic back pain is one of the leading causes of disability worldwide. Modifiable risk factors for facet arthropathy are best targeted via a multidisciplinary approach. Physical therapy is an invaluable tool both for relief and prevention of back pain. Weight loss strategies, including exercise, proper nutrition, and if all else fails bariatric surgery, may improve and deter the progression of symptoms. A psychosocial evaluation may be warranted in certain patients, given the high association with back pain. Alternative and complementary medicine, while unproven, may have a role in pain relief as well. Interventional procedures for facet mediated pain, including radiofrequency neurotomy and intraarticular block, carry level I and II evidence respectively. The overall outlook for patients with low back pain is poor. Relapse of pain is common after all treatments. The overall quality of life of these patients is poor. (Level V)
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