Back pain is one of our society’s most common medical problems—only headache is more common. In a three-month period, about one-fourth of U.S. adults experience at least one day of back pain (NINDS, 2011). Risk factors for low back pain (LBP) include age (more common after age 30); poor physical fitness; obesity; hereditary conditions (e.g., ankylosing spondylitis); other diseases (e.g., arthritis); occupational risks (e.g., heavy lifting); cigarette smoking (indirectly). The causes of LBP can include:
- Mechanical, e.g., intervertebral disc degeneration, muscle spasms or tension, ruptured or herniated intervertebral discs;
- Injury, e.g., strains, sprains or fractures;
- Acquired conditions, e.g., scoliosis, spondylolisthesis, spinal stenosis;
- Infections and tumors, e.g., osteomyelitis, discitis.
The physiologic basis for low back pain is highly complex, in large part because the lumbar spine itself is an unusually complex anatomic structure. The spine is the only organ comprised of bones, joints, ligaments, fatty tissue, multiple layers of muscles, and nerves (including peripheral nerves, nerve roots, sensory ganglia, autonomic ganglia, and the spinal cord). Furthermore, these structures are supplied by an intricate arterial and venous system and lie in close proximity to the skin with its sensory receptors. Diagnosis and treatment of low back pain requires an understanding of many different types of tissue, as well as knowledge of the biomechanics of complex spinal structures, the manner in which they can be injured, and the variety of biochemical manners in which each of these structures responds to trauma and to aging. In addition, consideration should be given to certain other psychosocial factors that affect the manner in which pain is processed centrally in the brain (BCBSA TEC 2007).
Spinal structures and tissues that possess either unmyelinated nerve innervation or documentable substance P or related peptides are assumed to have the capacity to cause pain. Such structures include the posterior facet joints, bones and periosteum, muscles, tendons, fascia, ligaments, nerve roots, dorsal root ganglia, dura mater and the intervertebral disc (BCBSA TEC 2007).
Intervertebral disc pain is a potential cause of low back pain. The intervertebral disc is a circular spongy pad of cartilage tissue that provides cushioning between each vertebrae of the spine. Each disc consists of a very firm outer ligamentous cover (annulus) and a soft jelly-like hydrated viscous “filling” (nucleus pulposus). In childhood, the nucleus is a hydrogel consisting of 80% water. The disc hydrogel desiccates gradually from childhood to old age, which causes the disc to lose height and resiliency, become less tense, and bulge anteriorly and posteriorly. These changes describe degenerative disc disease (DDD), which is commonly found in the population over age 40. As the disc narrows, osteophytes form on the margins, which is common in patients over age 60. DDD is considered to result from the inability of the disc’s reparative capacity to keep pace with the trauma that occurs with activities of daily living. There is no direct blood supply to the hydrogel in the nucleus pulposus; disc nutritive sources are dependent on diffusion across the vertebral endplate adjacent to the disc, which itself becomes less permeable and sclerotic, contributing to disc desiccation. As part of this degeneration process, the annulus becomes more densely innervated by pain fibers. As the disc narrows, increased load is placed on the facet joints. As part of the degeneration process, synovial fluid decreases, the joint space narrows, and osteophytes and sclerosis form at the margins. DDD can be accompanied by spinal stenosis and/or spondylolisthesis, which can cause compression of nerves and additional symptoms, such as radiculopathy and neurogenic claudication, in addition to back pain (AHRQ 2006).
There is a lack of consensus in the medical literature as to what extent the intervertebral disc is innervated. Once believed to be inert because nerve endings could not be demonstrated in the nucleus or inner annular fibers, the intervertebral disc is now known to contain fine nerve endings in the outer one-third of the annulus. These nerve endings are immunoreactive to a number of pain-related neuropeptides (substance P, calcitonin-gene-related peptide [CGRP], and vasoactive intestinal peptide [VIP]). Impulses from these free nerve endings in the outer third of the disc and the adjacent longitudinal ligaments reach the spinal cord through a number of sensory nerves in the following manner:
- the posterior and posterolateral annulus, together with the posterior longitudinal ligament and the ventral dura, is innervated by the sinu-vertebral nerves;
- the anterior and lateral aspect of the disc, together with the periosteum of the vertebral bodies, is innervated through the gray rami communicantes.
The sinu-vertebral nerves have been shown to innervate tissues one or two layers above or below their origin, a finding that may explain the poor localization of lumbar pain.
Evidence that the nerve endings observed in the outer one-third of the disc may be a source of low back pain is based upon several clinical observations. First, it has been demonstrated in human volunteers that injection of 11% sodium chloride into the intervertebral disc causes, after a few seconds, very severe pain with deep aching across the back and poor localization (Hirsch et al. 1963). There is an early case report describing a patient who had low back pain produced by pulling on a nylon suture that was looped through the intervertebral disc (Smyth and Wright 1958). More recently, Kuslich and Ahern (1994) observed that 33–40% of patients in their large back surgery series had significant pain when the affected central or lateral annulus was stimulated. Finally, other investigators have reported that examination of pathologic discs reveals unusually profuse innervation (Yoshizawa et al. 1980). Grigg and colleagues (1986) suggest from evidence in animal studies that the intervertebral disc contains a relatively rich supply of what they termed “silent nociceptors”— nerve endings that are not readily excited by mechanical stress, but which, when exposed to pain-inducing substances accompanying inflammatory, degenerative, or traumatic processes, become exquisitely responsive.
Different types of disc injuries can potentially lead to pain. These include annular tears, disc protrusions with extrusion of nucleus pulposus into radial tears in the annular fiber of the disc, and disc herniation, in which nucleus pulposus tissue escapes the confines of the annulus. These events cause pain by stretching or tearing peripheral innervated disc fibers or by generating an irritating inflammatory reaction in adjacent spinal tissues (Swenson 1999). Degenerative changes in the collagen fibers of the intervertebral disc may also lead to increased focal segment instability. As the intervertebral disc ages, nuclear hydrostatic pressure is lost, leading to buckling of the annular lamellae, increased shear stress across the annular wall, and eventually annular delamination and fissuring of the annular wall. All of these changes have been shown to alter disc mechanics, making annular disruption, a precursor of disc herniation, more likely.
Alternative Treatments for Low Back Pain Due to Degenerative Disc Disease
Acute LBP is short term, lasting a few days to a few weeks, is usually mechanical in nature, and usually requires no major treatment. Chronic LBP is pain that persists for three months or more. It is often progressive and the cause can be difficult to determine. There are two categories of treatment for chronic LBP: surgical and conservative non-surgical. The vast majority of cases of chronic LBP do not require surgery, and conservative non-surgical treatment will nearly always be tried first. Conservative non-surgical treatment primarily includes prescription strength analgesics (including anti-inflammatory medications, if not contraindicated); participation in physical therapy (including active exercise); and evaluation and appropriate management of associated cognitive, behavioral or addiction issues when present. Some controlled trials have shown these modalities to be effective (Cherkin et al. 2003). However, many patients do not respond to such treatments.
Surgical arthrodesis, or fusion, has been a surgical treatment for DDD that is not responsive to other treatments. Elimination of motion across the disc space and reduction of loads on disc tissues theoretically result in pain relief. Spinal fusion is an increasingly common procedure that is performed as an adjunct in the surgical management of patients with degenerative lumbar disease and instability (Resnick et al. 2005). National survey data indicate that the number of spinal fusion operations rose 77% between 1996 and 2001, in contrast with hip replacement and knee arthroplasty, which increased 13-14% during the same period (UW Med Report 2004). Other conditions for which spinal fusion is performed include neurologically dangerous segmental instability after trauma; unstable spondylolisthesis; chronic or complicated spinal infection; and cases of progressive neurological deficit due to a structural disorder, such as herniated disc, neoplasm, fracture, or severe stenosis. However, treatment for lumbar disc disorders is controversial. The relationship between an abnormal disc and neural dysfunction does not correlate statistically with the imaged pathology, and biochemical and inflammatory factors are thought to play primary roles. Therefore, the biological influence of a disc herniation would be expected to change over time and to be altered by passive and active non-surgical interventions (Wheeler et al. 2011).
Usually, the outcome of fusion is much more predictable in people with sciatica than in those with predominant LBP (NINDS 2011). The AHRQ 2006 Technology Assessment states that lumbar fusion has significant short term risks, particularly in the elderly in whom mortality rates of 1-1.6% have been reported. In addition, long term lumbar fusion reoperation rate is up to 3.7% annually, but this rate is only slightly higher than the reoperation rate for non-fusion lumbar spine operations, suggesting that the progression of degenerative disease in the spine is the major factor leading to reoperation. Common complications of fusion include instrument failure (7%), complications at the bone donor site (11%), neural injuries (3%), and failure to achieve a solid fusion or pseudarthrosis (15%) (Deyo et al. 2004). Fusion is thought to cause increased rate of disc degeneration in spinal segments adjacent to the fusion.
- Acute pain The most common type of back pain. Acute pain often begins suddenly—after a fall or injury, for example—and typically lasts up to 6-12 weeks.
- Ankylosing spondylitis A form of arthritis that affects the spine, the sacroiliac joints, and sometimes the hips and shoulders. In severe cases, the joints of the spine fuse and the spine becomes rigid.
- Axial back pain Back pain that is confined to the low back, and does not radiate to other parts of the body (e.g., buttocks, legs, etc). It is usually associated with activities, posture and/or position, may be relieved with rest, and is usually self-limiting and resolves.
- Cauda equina syndrome A condition in which the nerves that control the bowels and bladder are pinched as they leave the spine. Unless treated promptly, the condition can lead to the loss of bowel or bladder function.
- Cervical spine The upper portion of the spine closest to the skull. The cervical spine comprises seven vertebrae.
- Chronic pain The least common type of back pain. Chronic pain may come about suddenly or gradually; it generally lasts for three months or longer.
- Cobb angle The measurement in degrees of side-to-side angle of the spine in scoliosis. A Cobb angle of 10 degrees is the minimum to define scoliosis.
- Degenerative disc disease (DDD) Changes of disc desiccation and disc narrowing.
- Disc A circular spongy pad of cartilage tissue that provides cushioning, situated between each vertebrae of the spine.
- Discectomy The surgical removal of a herniated disk. A diskectomy can be performed in a number of different ways, such as through a large incision in the spine or through newer, less invasive procedures using magnifying microscopes, x rays, small tools, and lasers.
- Discitis An inflammation of the vertebral disk space often related to infection. Although an uncommon condition, discitis is often the cause of debilitating neurologic injury.
- Facet joints The zygapophysial joints where the vertebrae of the spine connect to one another; these joints are also involved in the degeneration process. Facet joint degeneration can lead to degenerative spondylolisthesis.
- Facet syndrome Degeneration of the zygapophysial joints, and associated muscle spasms, which cause pain.
- Fibromyalgia A condition of widespread muscle pain, fatigue, and tender points on the body, including low back pain.
- Flatback syndrome Loss or flattening of the normal lumbar curve of the spine after surgery to correct scoliosis.
- Foraminal stenosis Narrowing of the intervertebral foramen, which are small holes through which nerves exit the spine. The foraminal narrowing, or stenosis, can cause compression of the nerves, resulting in pain.
- Herniated disc A potentially painful problem in which the hard outer coating of the disk is damaged, allowing the disk’s jelly-like center to leak and cause irritation to adjacent nerves.
- Kyphosis Exaggerated outward curvature of the thoracic spine, resulting in a rounded upper back.
- Laminectomy The surgical removal of the lamina (the back of the spinal canal) and spurs inside the canal that are pressing on nerves within the canal. The procedure is a major surgery requiring a large incision and a hospital stay.
- Lumbar spine The lower portion of the spine. The lumbar spine comprises five vertebrae.
- Lumbar degenerative stenosis Degenerative lumbar spinal stenosis describes a condition in which there is diminished space available for the neural and vascular elements in the lumbar spine secondary to degenerative changes in the spinal canal. When symptomatic, this causes a variable clinical syndrome of gluteal and/or lower extremity pain and/or fatigue which may occur with or without back pain. Symptomatic lumbar spinal stenosis has certain characteristic provocative and palliative features. Provocative features include upright exercise such as walking or positionally-induced neurogenic claudication. Palliative features commonly include symptomatic relief with forward flexion, sitting and/or recumbency (NASS 2010).
- Neurogenic claudication A common symptom of lumbar spinal stenosis, or inflammation of the nerves emanating from the spinal cord. Neurogenic (originates with a problem at a nerve) claudication (Latin for limp) is exhibited by a painful cramping or weakness in the legs that may be precipitated by walking and prolonged standing, may be relieved by a change in position or flexion of the waist, or may be painfully persistent, as in severe compression of the nerve roots.
- Osteoarthritis A disease in which the cartilage that cushions the ends of the bones at the joints wears away, leading to pain, stiffness, and bony overgrowths, called spurs. It is the most common form of arthritis and becomes more likely with age.
- Osteomyelitis An infectious usually painful inflammatory disease of bone often of bacterial origin that may result in the death of bone tissue.
- Osteoporosis A condition in which the bones become porous and brittle and break easily.
- Pseudoarthrosis Literally mean “false joint” and refers to a nonunion, i.e., a fusion that has failed to unite.
- Radicular pain Neuralgia due to irritation of the sensory root or the dorsal root ganglion of a spinal nerve.
- Radiculopathy Objective loss of sensory and/or motor function as a result of nerve conduction blockage. Symptoms include numbness, motor loss, wasting, weakness, and loss of reflexes. Radiculopathy and radicular pain can occur simultaneously or independently.
- Rheumatoid arthritis A disease that occurs when the body’s immune system attacks the tissue that lines the joints, leading to joint pain, inflammation, instability, and misshapen joints.
- Sacroiliac joints The joints where the spine and pelvis attach. The sacroiliac joints are often affected by types of arthritis referred to as spondyloarthropathies.
- Sciatic neuralgia Pain felt in the distribution of the sciatic nerve due to pathology or the nerve itself.
- Sciatica Pain felt down the back and outer side of the thigh. The usual cause is a herniated disk, which is pressing on a nerve root.
- Scoliosis A condition in which the spine curves to one side as a result of congenital malformations, neuromuscular disorders, injury, infection, or tumors.
- Spinal osteotomy A procedure in which bone, usually a wedge shape, is removed from the spine to correct a spinal misalignment.
- Spinal stenosis The narrowing of the spinal canal (through which the spinal cord runs), often by the overgrowth of bone caused by osteoarthritis of the spine.
- Spondyloarthropathy A form of arthritis that primarily affects the spine and sacroiliac joints.
- Spondylolisthesis A condition in which a vertebra of the lumbar (lower) spine slips out of place.
- Vertebrae The individual bones that make up the spinal column.