Laminectomy
A
laminectomy (etymology: -ectomy= removal: laminectomy= removal of the
lamina)
is a surgical procedure performed by orthopedic and neurosurgeons in
which the
laminae of a vertebra are removed. As you can see in the images below,
the
laminae form the posterior aspect of the vertebral bony arch that forms
the
spinal canal. The spinous process extends posteriorly from where the
two
laminae meet each other, and the transverse processes extend laterally
from
where the laminae meet the pedicles. A complete laminectomy, like the
one
performed by medical students in gross anatomy, requires the cutting of
the
laminae where they meet the transverse processes and removal of both
laminae
along with the spinous process.
Spinal
laminectomy is done to relieve back pain that is caused by impingement
or
compression of the spinal cord or a spinal nerve. The spinal nerves
exit the
vertebral column via the spinal foramina, which are created by the
pedicles and
laminae, and are held open by the intervertebral disks. Any spinal
injury or
degeneration that narrows or occludes the spinal foramina can also
compress the
spinal nerve as it is exiting the vertebral column, leading to back
pain and
symptoms of neuropathy along the affected dermatome. Common conditions
that
result in spinal nerve compression are:
- Herniated
disk - The intervertebral disk is made of a fibrous outer ring called
the anulus
fibrosus, which surrounds and contains a gelatinous inner core called
the
nucleus pulposus. With aging and overuse, the anulus fibrosus can
weaken,
allowing the nucleus pulposus to herniate through and enter the spinal
canal where it compresses the spinal cord or spinal nerves.
- Spinal
osteoarthritis - The intervertebral disks help to keep the spinal
foramina open
enough for the spinal nerves to pass through unimpeded. If the disks
begin to
degenerate and lose their size and suppleness, they allow the vertebrae
to rest
more closely to each other and so reduce the size of the spinal
foramina. This
can lead to compression of the spinal nerves.
- Osteophytosis -
Another complication of spinal arthritis is the formation of
osteophytes, or
bone spurs. As the intervertebral disks degenerate, they allow adjacent
vertebral bodies to rub against each other. This agitation can lead to
a compensatory
overgrowth of hard bone tissue, which forms an osteophyte. If these
osteophytes
grow into the spinal canal or foramina, they can impinge on the spinal
nerves
or the cord itself (see clinical case "Osteophytosis").
The
vertebrae and disks of the lumbar spine are particularly prone to
injury
because of their important role in weight bearing. The force exerted on
the
spine when one is bending is supported primarily by the intervertebral
disks
between L3-L4 and L4-L5. As an indicator of how prone the lower lumbar
vertebrae and disks are to overuse injury, approximately 95% of spinal
disk
operations are performed on L4 and L5.
An
open laminectomy begins with a single straight incision over the
affected
vertebra. The deep back muscles and connective tissue are moved aside
and the
laminae are exposed at their junction with the transverse processes.
The
supraspinous ligament is cut above and below the spinous process which
is to be
removed. The laminae are then both cut, and the posterior aspect of the
vertebral arch, including the laminae, spinous process, supraspinous
ligament,
and interspinous ligament are removed. This exposes the ligamenta
flava, which
is next cut to expose the spinal canal and the compressed nerve. At
this point
the surgeon is able to see what is causing the compression (typically a
herniated disk, disk fragment that has entered the spinal canal, an
osteophyte,
or a tumor). The laminectomy therefore helps to decompress a spinal
nerve via
two mechanisms:
It
enables the surgeon to directly see and fix whatever is causing the
compression.
By
removing the laminae, the spinal foramina are enlarged, giving the
spinal nerve
more room to leave the vertebral column unobstructed.
Removing
the entire posterior aspect of one or more vertebrae in a spine that is
likely
already weakened by overuse, injury, or arthritis can result in a
dangerously
unstable vertebral column. This is why many patients who receive a
laminectomy
also have several vertebrae fused after a procedure in order to help
stabilize
their spine. The classic spinal fusion involves taking a small piece of
bone
from the patient's hip and attaching it to the bodies of the affected
vertebrae
using metal plates and screws.
The
movie file below is the CT scan of cadaver 33492. Move the scan to
time=68, K12 and M12. At
this point you can see the 12th rib coming off of T12. Begin moving the
scan
downward (advancing the time forward) while keeping track of your level
on the
lumbar spine by counting the spinous processes as they come into view.
Notice
that just below L3 (time= 75, K13 and M13) two large screws come into
view. These
screws
can be seen entering the vertebra at the base of the transverse
processes and
extending through the pedicles into the vertebral body. Continue
scanning down
and you will notice that just below the 4th lumbar spinous process is a
metal
plate spanning the distance laterally across the posterior aspect of
the
vertebral column. Scan a little bit further (time= 79, L13) and you
will
see that
the laminae and spinous process of L5 are missing entirely, leaving the
spinal
canal open posteriorly. Also at this level are another set of screws
that can
be seen entering the superior articular facet of the sacrum.
This
laminectomy and spinal fusion were likely done for this patient to
alleviate
back pain that resulted from spinal cord or nerve compression in the
lower
lumbar spine. Examine the vertebral bodies of the lower lumbar spine
and notice
how bony outgrowths (osteophytosis) are visible on L3-L5 vertebral
bodies. This
is a sign of spinal osteoarthritis. See the clinical case
"Osteophytosis" for more information.
