Herniated Discs Flashcards
Background
Nuclear material that is displaced into the spinal canal is associated with a significant inflammatory response, as has been demonstrated in animal studies. Disk injury results in an increase in the proinflammatory molecules interleukin (IL)-1, IL-8, and tumor necrosis factor (TNF)-α. Macrophages respond to this displaced foreign material and seek to clear the spinal canal. Subsequently, a significant scar is produced, even without surgery, and substance P, which is associated with pain, is detected.
Acute neural compression is responsible for dysfunction; compression of a motor nerve results in weakness, and compression of a sensory nerve results in numbness. Radicular pain is caused by inflammation of the nerve, which explains the lack of correlation between the actual size of an intervertebral disk herniation or even the consequent degree of neural compression and the associated clinical symptoms.
Furthermore, intervetebral disk degeneration may result in radial tears and leakage of the nuclear material, which leads to neural toxicity. The subsequent inflammatory response often results in neural irritation causing radiating pain without numbness, weakness, or loss of reflex, even when neural compression is absent.
The pathologic state of a weakened annulus is a necessary condition for herniation to occur. Many cases involve trivial trauma even in the presence of repetitive stress. An annular tear or weak spot has not been demonstrated to result from repetitive normal stress from customary activities or from physically stressful activities.
Several factors seem to influence the occurrence of herniated nucleus pulposus (HNP)
Smoking is a risk factor in the epidemiology of lumbar disk herniations and has been documented to decrease the oxygen tension in the avascular disk dramatically, presumably by vasoconstrictive and rheologic effects on blood.
Lumbar disk herniation may result from chronic coughing and other stresses on the disk. For example, sitting without lumbar support causes an increase in disk pressures, and driving is also a risk factor because of the resonant coupling of 5-Hz vibrations from the road to the spine. People who drive signifcant amounts have increased spinal problems; truck drivers have the additional risk of spinal problems from lifting during loading and unloading, which, unfortunately, is done after prolonged driving.
Studies have shown that peak stresses within a deteriorated intervertebral disk exceed those from average loads on a normal disk, which is consistent with a pain mechanism. Further repetitive stress at physiologic levels did not produce a herniation after prolonged testing, contradicting the concept of injury accumulation with customary work activities. However, after a simulated injury to the annulus (cutting), a lower mechanical stress did result in disk herniation, consistent with intervertebral disk degeneration and with clinical experience on discography.
Anatomy of Vertebral Disc
The intervertebral disc is the largest avascular structure in the body. It arises from notochordal cells between the cartilaginous endplates, which regress from about 50% of the disc space at birth to about 5% in the adult, with chondrocytes replacing the notochordal cells.
Intervertebral discs are located in the spinal column between successive vertebral bodies and are oval in cross-section. The height of the discs increases from the peripheral edges to the center, appearing as a biconvex shape that becomes successively larger by about 11% per segment from cephalad to caudal (ie, from the cervical spine to the lumbosacral articulation). A longitudinal ligament attaches to the vertebral bodies and to the intervertebral discs anteriorly and posteriorly; the cartilaginous endplate of each disc attaches to the bony endplate of the vertebral body.
Components of the Vertebral Disc
The disc’s annular structure is composed of an outer annulus fibrosus, which is a constraining ring that is composed primarily of type 1 collagen. This fibrous ring has alternating layers oriented at 60° from the horizontal to allow isovolumic rotation. That is, just as a shark swimming and turning in the water does not buckle its skin, the intervertebral disc has the ability to rotate or bend without a significant change in volume and, thus, does not affect the hydrostatic pressure of the inner portion of the disc, the nucleus pulposus.
The nucleus pulposus consists predominantly of type II collagen, proteoglycan, and hyaluronan long chains, which have regions with highly hydrophilic, branching side chains. These negatively charged regions have a strong avidity for water molecules and hydrate the nucleus or center of the disc by an osmotic swelling pressure effect. The major proteoglycan constituent is aggrecan, which is connected by link protein to the long hyaluronan. A fibril network, including a number of collagen types along with fibronectin, decorin, and lumican, contains the nucleus pulposus.
The hydraulic effect of the vertebral disc
The hydraulic effect of the contained, hydrated nucleus within the annulus acts as a shock absorber to cushion the spinal column from forces that are applied to the musculoskeletal system. Each vertebra of the spinal column has an anterior centrum or body. The centra are stacked in a weightbearing column and are supported by the intervertebral discs. A corresponding posterior bony arch encloses and protects the neural elements, and each side of the posterior elements has a facet joint or articulation to allow motion.
Stability of the Vertebral Disc
Clinical stability has been defined as the ability of the spine under physiologic load to limit patterns of displacement so as to avoid damage or irritation to the spinal cord or nerve roots and to prevent incapacitating deformity or pain caused by structural changes.
Any disruption of the components holding the spine together (ie, ligaments, intervertebral discs, facets) decreases the clinical stability of the spine. When the spine loses enough of these components to prevent it from adequately providing the mechanical function of protection, surgery may be necessary to reestablish stability.
Degeneration: process and models
Low back pain (LBP) is ubiquitous, with 60-80% of people having an activity-limiting episode at least transiently in their lifetime. Genetic factors appear to have a dominant role, with LBP starting at an earlier age than previously suspected on the basis of subsequent structural changes; men begin having LBP about a decade earlier than women.
The water-retaining ability of the nucleus pulposus, or the inner portion of the intervertebral disk, declines progressively with age. The decline in the mechanical properties of the nucleus pulposus is associated with the degree of proteoglycan deterioration and the decrease in hydration, which lead to excessive regional peak pressures within the disk. As the hyaluronan long chains shorten and swelling pressure decreases as a result of this deterioration, the mechanical stiffness of the disk decreases, which causes the annulus to bulge, with a corresponding loss of disk and foramen height
The etiology of back pain
The etiology of back pain for a particular individual cannot be determined, because of the multiplicity of potential sources. Although periosteal disruption causes pain with fractures, bone itself is devoid of pain receptors (eg, asymptomatic compression fractures commonly are seen in the thoracic spine of elderly individuals with osteoporosis). However, the degenerating intervertebral disk is known to have neurovascular elements at the periphery, including pain fibers.
Disk deterioration and loss of disk height may shift the balance of weightbearing to the facet joint; this mechanism has been hypothesized as a cause of LBP through the facet joint capsule, as well as through other tissues attached to and between the posterior bony elements.
When the annulus in animals is incised, a degenerative cascade is initiated that mimics the natural aging process observed in humans, thus providing a model of disk deterioration. As the use of discography has increased for various clinical applications, similar annular tears are seen routinely that are associated with the degeneration of the intervertebral disk, even in patients who are asymptomatic. Annular tears may simply be the result of aging and the degenerative cascade
Pathophysiology- Deterioration of the Disc
Whether the deterioration of the disk or that of the facet comes first has not been determined; however, deterioration is known to occur in both.
Dehydration results from shortening of the hyaluronic chains, deterioration of the state of aggregation, and decreases in the ratio of chondroitin sulfate to keratan sulfate, leading to the disk bulging and disk height loss. The consistency of the nuclear material undergoes a change from a homogeneous material to clumps, which leads to the altered distribution of pressures within the disk and resistance to the flow of nuclear material; the nuclear material thereby becomes mechanically unstable. The clumping of the degenerating nuclear material can be likened to a marble held between two books—that is, it is difficult to contain.
These clumps may be lateral to the posterior longitudinal ligament and, therefore, may have the least resistance to herniating through the corner of the intervertebral disk and into the spinal canal or foramen. Surgical removal of the herniated fragments is achieved by grasping them with a pituitary rongeur.
This method of surgical removal is not possible with normal, homogeneous material, which is encountered when healthy interverterbral disks are excised anteriorly in patients having surgery because of deformity or trauma. Using the pituitary rongeur technique to perform a microdiscectomy on a herniated fragment necessitates a preexisting state of deterioration; the weakened areas in the annulus provide a path of least resistance for the nuclear material to egress.
Natural Histroy of Lower Back
Much has been written concerning the process of spinal deterioration or spondylosis, which occurs over a lifetime. Intervertebral disk deterioration leads to decreased stiffness of the disk, as well as diminished stability, resulting in episodic pain that is common and may be temporarily severe. However, continued deterioration ultimately leads to restabilization of the spine by collagenization, which stiffens the disk.
Patients in their 50s and 60s customarily have stiffer spines but less pain than patients in their 30s and 40s who are undergoing initiation of the degenerative cascade. Patients who ask if they have to live with this pain “for the rest of their lives” can be reassured to some extent by this natural history. Furthermore, spontaneous recovery from an acute pain episode routinely occurs, so any treatment must be demonstrated as effective by positively altering the expected course without treatment.
In general practice, the overall incidence of HNP in patients who have new LBP onset is lower than 2%. Therefore, most of these patients have deterioration of the intervertebral disk and dysfunction of the functional segmental unit. They will have LBP, and some will have associated leg pain but without sciatica (an intractable, radiating pain, below the knee) or radiculopathy. A disk fragment that is no longer contained within the annulus but is displaced into the spinal canal has decreased hydration and deteriorated proteoglycan that can be expected to undergo further deterioration and consequent annular desiccation, essentially like a grape being transformed into a raisin.
Prognosis
Patients with “broad-based” intervertebral disk herniations generally have a deterioration of the disk or a failure of clinical stability with associated back pain, rather than isolated sciatica. These patients are not appropriate candidates for microdiscectomy alone.
Lumbar fusion is being used increasingly in these cases, and arthroplasty is also being considered; however, this treatment remains controversial because it is, again, based inevitably on subjective patient pain and clinical judgment without objective determination. Many reports in the literature have described specific cytokines elevated, but not comprehensively; endplate changes are observed but no clear correlation identified to this point. Various nuclear replacements that reduce postoperative loss of disc height restoring compressive loading are being studied.
With a discectomy, patients with dominant leg pain have excellent results, with 85-90% returning to full function. However, as many as 15% of patients have continued back pain that may limit their return to full function, despite the absence of radiculopathy. Patients who undergo surgery do not necessarily show better results than patients who defer surgery.
The remaining concern of recurrent herniation is small, though it is correlated with obesity. Efforts to minimize this complication have included annulus repair and injecting hemostatic materials or bioactive molecules.
Deterioration and Low Back Pain (LBP)
Intervertebral disk degeneration that causes clumping of the nuclear material and relative mechanical instability is the necessary preceding condition for HNP. However, it is impossible to tell which patients will do well after microdiscectomy for a herniation and which will have continued problems, of varying severity, from the disk degeneration. Studies have shown that degenerated discs have different growth factors and other molecules; thus, even introducing mesenchymal stem cells requires significant further research and development.
Significant deterioration and accompanying LBP increasingly are being treated with stabilization, via either an anterior lumbar interbody fusion (ALIF) or a posterior lumbar interbody fusion (PLIF) in association with posterior decompression (when necessary) and instrumentation. Results are not yet available, as techniques are still evolving, but experience is accumulating.
Physical Exam
Physical examination classically involves range-of-motion (ROM) testing of the lumbar and cervical spine, but these findings may be more reflective of aging or deterioration in the intervertebral disks and joints than any quantifiable assessment of impairment. The remainder of the examination is essentially a neurologic assessment of weakness, dermatomal numbness, reflex change, and, most important, sciatic or femoral nerve root tension in the lumbar spine.
Numerous examination maneuvers (eg, Lasegue classic test, Lasegue rebound sign, Lasegue differential sign, Braggard sign, flip sign, Deyerle sign, Mendel-Bechterew sign, well leg test or Fajersztajn sign, both-legs or Milgram test) are available but cloud the issue, because the sciatic nerve root tension or straight-leg raising test is the basis for nearly all of them. They are essentially modifications for subtle differences, but the provocation of radiating pain down the leg is of a neural compressive lesion and compression of the sciatic nerve root, if it goes below the knee. Furthermore, the provocation of radiating pain down the leg is the most sensitive test for a lumbar disk herniation.
For a higher lumbar lesion, reverse straight-leg raising or hip extension that stretches the femoral nerve is analogous to a straight-leg raising test.
