The Basics of Intervertebral
The Basics of Intervertebral
Brian Leonard, D.O.
There are a great number of conditions and a variety of states of illness that result in the symptom of “back/neck pain.” Back and neck pain can be related to conditions ranging from muscle strains, somatic dysfunction to nerve compression and anatomic anomalies.
The focus of this article is to discuss herniation of intervertebral discs as a cause of pain. We will examine the pathophysiology and biomechanics of disc degeneration and herniation as well as aspects of the epidemiologic data. Lastly, it is important to mention the role that manual/manipulative medicine plays with regard to this issue. While the general principles of herniated discs may be applied to any level of the spine, we will discuss each spinal level from cervical, thoracic, to lumbar.
An intervertebral disc is formed of two elements: the nucleus pulposis and the anulus fibrosis. The anulus fibrosis is composed of sequential layers of fibrocartilage that envelope the nucleus pulposis. The nucleus pulposis itself is formed of a proteoglycan and a water/gel substance that is held loosely in place by a network of collagen and elastin fibers. Together they form the intervertebral disc and serve to distribute weight and force equally throughout the spine, even during motions such as flexion and extension1. Blood vessels course along the outer edge of the anulus fibrosis and thereby force the disc to obtain its nutrient supply via osmosis. When the discs age, they are subject to gradual degeneration as the water content decreases and the ability to absorb impact diminishes. Degeneration begins on a microscopic level around the age of skeletal maturation, or fifteen years of age. At this time, cell densities begin to diminish, resulting in microstructural tears and clefts (2).
The microstructural defects accumulate over time as a person ages and the pulposis protrudes deeper into the anulus. These defects can result in frank tears of the anulus. There are three main tears that have been distinguished, these include:
— circumferential tears or delaminations
— peripheral rim tears
— radial fissures
The circumferential tears represent shearing forces acting on the interlaminar layers of the anulus fibrosis. The characteristic disc for this type of tear is an older disc that has an advanced amount of dessication and degeneration, retaining a limited ability to absorb these stressors (3). The second type of tear, the peripheral rim tears, are most frequently seen in the anterior portion of the disc and are associated with bony outgrowths. Histologic data suggest that the actual tears are a result of repeated microtrauma (4). Lastly, radial fissures represent a grouping of tears that typically occur in a posterior or posterolateral direction and are associated with degeneration of the nucleus pulposis. These tears have been simulated in cadavers with repeated cycles of sidebending and compression (5).
These variations of degeneration, dessication, and microstructural defects seem to be common among studies reported in the current base of literature. These tears, however, have not been shown to have a correlation with the actual prolapse, or herniation, of the disc. The tears and disc degeneration have been shown to be correlated only with repetitive mechanical loading and cigarette smoking6 (as this inhibits the body’s regulatory healing mechanisms in a vast number of ways). The prolapse of the disc has been shown to correlate with heavy lifting. That is to say, the degeneration of discs, and not the herniation, appears to be a normal process of aging (1).
For the discussion of rates of occurrence and particular mechanisms associated with disc herniation, we will begin at the cervical level and progress inferiorly to the thoracic and finish at the lumbar vertebrae.
Cervical Disc Herniation
Cervical radiculopathy, or pain in a pattern of the nerve root that is compressed, is estimated to occur in 85 per 100,000 people in the population. Most commonly affected regions include the seventh cervical vertebra, C7, and the sixth cervical vertebra, C6, at rates of 60% and 25%, respectively (7). These radiculopathies in the cervical region are commonly present in specific demographic groups. For instance, sudden weight load on the neck while in either flexion or extension can be the culprit. Also, in the elderly population, osteophyte formation can play a role as previously mentioned. Sport-related injury can be more insidious in nature, and can be attributed repetitive extension/rotation while actively using postural muscles, as in swimming (7).
Thoracic Disc Herniation
Thoracic disc herniations appear to be less common than lumbar and cervical herniations for a number of reasons. While they peak at the third to fifth decade of life, similar to other herniations, estimates place thoracic disc herniations only between 0.25% to 1% of all disc herniations (10,11). One reason for decreased incidence, it is thought, is the lesser degree of mobility in the thoracic spine due to the presence of the rib cage. The articulation of the rib head with the vertebral body naturally limits the amount of flexion, extension, and sidebending. The majority of thoracic herniations occur below the level of T7. Rib pairs 8-10 maintain a cartilaginous attachment to the sternum, thus allowing more motion than vertebrae at higher levels. Rib pairs 11 and 12 are known as “floating ribs” and do not maintain any attachment to the sternum. This supports the theory that part of the pathophysiology of herniated thoracic discs is directly related to the ability of the segment to maintain a certain degree of flexability (12).
Lumbar Disc Hernation
Herniation of the nucleus pulposis of the lumbar disc is present more commonly than the former two types. It is estimated that 95% of herniated lumbar discs occur at the L4-L5 or L5-S1 level (13). Typical presentation includes radicular pain that patients often describe as shooting or stabbing pain that courses down the leg. There may also be paresthesias present in the same distribution pattern. Often, the pain is exacerbated by coughing, sneezing, straining, or standing for long periods of time (14), as this increases the pressure on the disc and therefore on the impinged nerve root. Pain is usually relieved by rest and taking weight off of the prolapsed disc.
Manual/Manipulative Medicine and Cervical Disc Herniation
Considering the implications of nerve root impingement (including pain, paresthesia, and decreased motor function) secondary to a herniated disc, there is a natural concern regarding the safety of manual manipulation of such an anomalous disc.
With regard to manipulation, a 2006 study was done to evaluate the efficacy and safety of cervical manipulation in patients with spinal cord compression and radiculopathy. The study incorporated a variety of chiropractic techniques, including high-velocity, low-amplitude methods. The conclusions drawn by the authors states, “The finding of cervical spinal cord encroachment on magnetic resonance imaging, in and of itself, should not necessarily be considered an absolute contraindication to manipulation.” (8) The authors are specific in mentioning exclusion criteria such as acute myelopathy or changes indicating myelomalacia and make clear the message that special care and astute clinical judgement need be exercised in cases of cervical radiculopathy and pathologic segments.
A separate study suggests othewise, stating, “Cervical spinal manipulation therapy may worsen preexisting cervical disc herniation or cause disc herniation resulting in radiculopathy, myelopathy, or vertebral artery compression.” (9) This study describes 22 case studies and states in its conclusion a list of absolute contraindications including patients with rheumatoid arthritis, acute fractures and dislocations, os odontoideum, infection of bone, osseous malignancies, or cervical myelopathy. These case studies included reports from patients previously treated by chiropractors as well as osteopathic physicians. The article puts forth the modality of surgical intervention as the best treatment for certain cases of disc herniation and radiculopathy.
With regard to the necessity of surgical intervention, let us consider a 2007 article from the Massachusetts Medical Society (15). The study examines the outcomes of two groups of patients with herniated lumbar discs who were randomly assigned to either a surgical intervention or observation and symptom management. The study was inconclusive statistically due to the high rate of crossover. That is, 40% of patients assigned to the surgical intervention declined surgery because their symptoms improved before any intervention could take place (with observation alone). Conversely, 45% of patients referred to the observation therapy, opted for surgical intervention due to worsening of symptoms (15). Even though the study is scholastically inconclusive and statistically insignificant, it does highlight the need for individualized care.
As with any topic at the forefront of medicine, especially issues which can be treated via different modalities and by different specialists, there will be controversy, bias, and ever-emerging new evidence to consider. This article demonstrates the basic science behind disc degeneration leading to pathologic herniation. It also shows two sides of a clinical debate to which there is no defined rule for treatment. Patients, therefore, need to be evaluated and treated appropriately on clinical grounds of their individual situation by a physician well-versed in neuromusculoskeletal medicine to determine which specific modality best suits the individual.
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