Although the ligamentous structures are quite strong, neither they nor the vertebral body–disc complexes have sufficient integral strength to resist some of the enormous forces that may act on the spinal column. Consequently, the stability of the lower back is also dependent on the voluntary and reflex activity of the sacrospinalis, abdominal, gluteus maximus, and hamstring muscles, and on the integrity of the ligamentum flavum and posterior longitudinal ligament.
The vertebral and paravertebral structures derive their innervation from the meningeal branches of the spinal nerves (also known as recurrent meningeal or sinuvertebral nerves). These meningeal branches spring from the posterior divisions of the spinal nerves just distal to the dorsal root ganglia, reenter the spinal canal through the intervertebral foramina, and supply pain fibers to the intraspinal ligaments, periosteum of bone, outer layers of the annulus fibrosus (which enclose the disc), and capsule of the articular facets. Coppes and associates have found A-δ and C pain fibers extending into the inner layers of the annulus, and even into the nucleus pulposus. Although the spinal cord itself is insensitive, many of the conditions that affect it produce pain by involving these adjacent structures. For example, the sensory fibers from the lumbosacral and sacroiliac joints enter the spinal cord via the fifth lumbar and first sacral roots. Motor fibers exit through the corresponding anterior roots and form the efferent limb of segmental reflexes. This explains some of the patterns of pain referral from these joints.
The spinal roots in the lumbar region (cauda equina) angulate sharply to exit horizontally through the intervertebral foramina. Prior to entering the short foraminal canal, the lumbar spinal root lies in a shallow furrow along the inner surface of the pedicle, the lateral recess. This is a common site of root entrapment by disc fragments and bony overgrowth. Because the thoracic and cervical discs do not have to travel downward and laterally to their points of exit at the foramina, they exit horizontally from their points of formation in the spinal subarachnoid space.
Degeneration in the intervertebral discs and ligaments is a consequence of aging and the succession of inevitable minor traumas to the spine. Deposition of collagen and elastin and alterations of glycosaminoglycans combine to decrease the water content of the nucleus pulposus; concomitantly, the cartilaginous endplate becomes less vascular (Hassler). The dehydrated disc thins out and becomes more fragile. Similar changes occur in the annulus of the disc, which frays to an increasing degree with the passage of time, permitting the nucleus pulposus to bulge and, sometimes with injury, to extrude. This process can be observed by MRI, which shows a gradual reduction in the high signal of the nucleus pulposus with the passage of time. For example, among women who had MRI for gynecologic reasons, Powell and coworkers found an increasing frequency of lumbar disc degeneration and bulging, approaching 70 percent by the fiftieth year of life.
The problem of degenerative spinal disease has been conceptualized as having its genesis in shrinkage of the disc that subsequently alters the alignment of the articular facets and vertebral bodies, leading to facet arthropathy and bony spur formation across the disc space (a spondylitic “bar”). These reactive changes create stenosis of the spinal canal and compromise the lateral recesses and the intervertebral foramina, where they impinge on lumbar nerve roots and, at higher levels, on the spinal cord. Osteoporosis, especially in older women, is a further important cause of vertebral flattening or collapse, additionally narrowing the spinal canal.
The lower parts of the spine and pelvis, with their massive muscular attachments, are relatively inaccessible to palpation and inspection. Although some physical signs and imaging studies are helpful, diagnosis often depends on the patient’s description of the pain and his behavior during the execution of certain maneuvers. Seasoned clinicians appreciate the need for a systematic inquiry and method of examination.