The
spinal column, or backbone, consists of 33 bones (vertebrae)
and can be divided into five segments (Fig. 1A).
The uppermost 24 vertebrae are separated from one another
by fibrous cartilage pads, called intervertebral discs (Fig.
1B), which provide flexibility to the spine and
act as shock absorbers during activity. In the lowest
part of the spine, the vertebrae are naturally fused
to form the sacrum and the coccyx (tail bone).
Protruding
from the back of each vertebral body is an arch of
bone that forms the large, vertical opening (the
spinal canal) through which runs the spinal cord
and nerve bundles. A fluid-filled protective membrane,
the dura, covers the contents of the spinal canal
from where the cord begins at the base of the skull
to where it ends (in a bundle of nerve fibers known
as the cauda equina).
A
pair of spinal nerves branches at each vertebral
level (one to the left and one to the right), providing
sensation and movement to all parts of the body.
Three
large, bony projections, or processes, arise from
the vertebra's arch - one to each side (transverse)
and one straight toward the back of the body (spinous).
Strong ligaments and muscles attached to the vertebra's
body and processes support the spine and further
protect the delicate spinal cord and nerves encased
within.
Back
and leg pain, among other symptoms, may occur when
an intervertebral disc herniates (Fig. 1B, above).
This happens when some of the disc's jelly-like center
(the nucleus pulposus) bulges or ruptures through
its tough, fibrous outer ring (the annulus fibrosis)
to press upon a nerve. (Fig. 1C, above). |
Pressure on a nerve root in the
lower spine, often called nerve root compression, causes
back and leg pain. In this operation the surgeon reaches
the lumbar spine through a small incision in the lower
back. After the muscles of the spine are spread, a
portion of the lamina is removed (Fig. 2) to
expose the compressed nerve root(s). 