What causes sciatica?
Back problems and the sciatic nerve
The sciatic
nerve is the largest nerve in the body and is composed
of individual nerve roots that combine to form the “sciatic
nerve”. It starts
in the low back at lumbar segment 3 (L3). The
sciatic nerve roots run through the bony canal in the
spine, and at each level in the lower back a pair of
nerve roots exits from the spine and then comes together
to form the large sciatic nerve that runs all the
way down the back of each leg. Portions of the sciatic
nerve then branch out in each leg to innervate certain
parts of the leg (e.g. the calf, the foot, the toes).
The nerve roots that originate in the lower back are
named for the upper vertebral body that they run between
(for example, the nerve that exits at L4-L5 in the
spine is named L4). The nerve passing to the next level
runs over a weak spot in the disc space, which is the
reason discs tend to herniate (extrude) right under
the sciatic nerve root and can cause sciatica.
The sciatica symptoms (e.g., low back pain, leg pain,
numbness, tingling, weakness) are different depending
on where the pressure on the nerve occurs. For example,
a lumbar segment 5 (L5) nerve impingement can cause weakness
in extension of the big toe and potentially in the ankle
(foot drop) (See Figure
1).
Back problems that cause sciatica pain
Common low back problems and other spinal conditions
that can cause sciatica - pain along the sciatic
nerve - include:
- Lumbar herniated disc. A herniated disc occurs when the
soft inner core of the disc (nucleus pulposus) extrudes through the fibrous
outer core (annulus) of the disc and the bulge places pressure on the contiguous
nerve root as it exits the spine. In general, it is thought that
a sudden twisting motion or injury can lead to an eventual disc herniation
and sciatica. However, most discs weaken due to repetitive stress
and the final result is a herniation. A herniated disc is sometimes
referred to as a slipped disk, ruptured disk, bulging disc, protruding
disc, or a pinched nerve.
- Lumbar spinal stenosis. This condition commonly causes sciatica
due to a narrowing of the spinal canal. It is more common in adults over
age 60, and typically results from a combination of one or more of the
following: enlarged facet joints, overgrowth of soft tissue and a
bulging disc placing pressure on the nerve roots as they exit the spine.
- Degenerative disc disease. While disc
degeneration is a natural process that occurs with
aging, in some cases one or more degenerated discs
can also irritate a nerve root and cause sciatica.
Degenerative disc disease is diagnosed when a weakened
disc results in excessive micro-motion at the corresponding
vertebral level and inflammatory proteins from inside
the disc become exposed and irritate the area (including
the nerve roots). The term “degenerative
disc disease” is an unfortunate one as it is
a process, not a disease.
- Isthmic spondylolisthesis. This condition
is not that uncommon in adults (approximately 5%
to 7% of adults are thought to have it), but it only
rarely causes back pain or sciatica pain. Spondylolisthesis
occurs when a small stress fracture (most often at
the fifth segment) allows the L5 vertebral body to
slip forward on the S1 vertebral body. Caused by
a combination of disc space collapse, the fracture,
and the vertebral body slipping forward, the L5 nerve
can get pinched as it exits the spine.
- Piriformis syndrome. The sciatic nerve
can also get irritated as it runs under the piriformis
muscle in the rear. If the piriformis muscle
irritates or pinches a root that comprises the sciatic
nerve, it can cause sciatica-type pain. This
is not a true radiculopathy, but the pain can feel
the same as sciatica caused by a nerve irritation.
- Sacroiliac joint dysfunction. Irritation
of the sacroiliac joint at the bottom of the spine
can also irritate the L5 nerve, which lies on top
of it, and cause sciatica-type pain. This is not
a true radiculopathy, but the pain can feel the same
as sciatica caused by a nerve irritation.
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