Disk herniation is thought to be a major cause of back pain and sciatica. Intervertebral disks consist of an outer ring and a central core. The outer ring, the anulus fibrosus (translated literally as a “fibrous ring”) can tear as a result of an acute event or attrition. This tear permits the central core of the disk, the nucleus pulposus, to bulge (herniate) from its normal location. The injured disk can compress the nerve roots (Figs. 1 and 2).
The nucleus pulposus consists of a gel that contains negatively charged proteins that are able to bind water. Normally, the water will be extruded with load bearing and then imbibed again when the load is lifted, allowing the disk to act as a cushion between adjacent vertebral bodies. The disk therefore acts as a hydraulic shock absorber. When there is a break in the anulus fibrosus, however, some of the nucleus pulposus may escape its central location, resulting in a bulge or herniation. Protruding pieces of nucleus pulposus can then compress a spinal nerve or cause local irritation.
The anulus fibrosus can tear at any point, but the direct anterior and posterior posi- tions are reinforced by the longitudinal ligaments of the spine; thus, the path of least resistance is posterolaterally, toward the neural foramen. A large herniation in the lumbar region can compress the cauda equina, the terminal nerve roots of the spinal cord that have not yet left via their respective foramina. Bilateral sciatic pain or saddle anesthesia (decreased sensation around the perineum) and loss of bowel or bladder control suggests such compression. This presentation demands urgent MRI and possibly emergency surgery.
Degeneration of the disk without herniation can occur after trauma but is also a function of aging. As people age, the disk dehydrates and loses its resilience. Thus, even without overt trauma, it is likely that the disk space of an elderly patient will be radio- graphically abnormal, although not necessarily symptomatic.
Osteoarthritis and Spinal Stenosis
Osteoarthritis is often a continuation of lumbar disk disease. As the disk is no longer able to cushion the joint, abnormal forces and motions are applied to the posterior facet joints and will tend to erode the normal architecture (Fig. 3).
This process is similar to the changes in the knee joint seen after the meniscus is severely damaged or completely removed.
Degenerative disk disease and arthritis can cause spinal stenosis, a condition characterized by narrowing of the canal housing the spinal cord (Fig. 4).
Spinal stenosis is commonly caused by overgrowth of the bone osteophytes or by enlargement of the ligamentum flavum. Such overgrowth can put pressure on the neural elements and produce pain in the legs, especially after activity.
The facet joints of the spine are true synovial joints; thus, they are susceptible to any of the inflammatory conditions affecting the synovium. Rheumatoid arthritis can affect the low back, but it is more common in the cervical spine. Ankylosing spondylitis is an inflammatory disorder that affects the low back and pelvis, almost always involving the sacroiliac joint. After a period of years, it may lead to autofusion of vertebral bodies. The characteristic finding of most rheumato- logic conditions is morning stiffness and discomfort that actually improve as the day progresses.
Spondylolysis and Spondylolisthesis
The term spondylolysis refers to a defect of the bone between the superior and inferior articular processes of the vertebra, or pars interarticularis (Fig. 5, A).
This is a common condition, occurring in 5% of the popula- tion.2 It is also seen in athletes, such as gym- nasts or football linemen, who routinely hyperextend their spines. This bony defect can progress to the point at which the vertebral body above can slip forward onto the one below, causing a condition called spondy- lolisthesis (Fig. 5, B). This progression can be avoided by early detection with bone scan- ning or MRI and activity modification.
Tumors and Infections
Tumors and infections can be described together because both are typically caused by hematogenous (bloodborne) seeding. Tumor masses and epidural abscesses can cause a cauda equina syndrome by direct impingement on the neural elements. They also can cause bone destruction and pathologic fractures (Fig. 6).
Almost all spine tumors are either metastatic or blood-cell malignancies. One exception is the osteoid osteoma, a benign tumor found typically in teenagers. This tumor can cause back pain severe enough to warrant excision, but it may resolve without treatment.