Spondylolisthesis
Introduction 
There are 33 vertebrae in the human spine: 7 in the neck area (cervical),
12 in the chest area (thoracic), 5 in the lumbar (lower back), 5
fused vertebrae in the pelvic area (sacrum) and 4 fused vertebrae
forming the tailbone (coccyx).
The cervical, thoracic and lumbar vertebrae are held in place,
one above the next, by projections on each vertebra called superior
and inferior processes. The inferior (lower) process of the top
vertebra fit into the superior (upper) process of the lower vertebra,
forming a joint that holds the vertebrae in place. Between each
vertebra (except in the sacrum and coccyx) intervertebral (between
the vertebrae) discs cushion and separate the vertebrae.
What is spondylolisthesis?
 Spondylolisthesis
is a Latin term meaning improper forward movement of a vertebra
over the vertebra below it. Most often, this forward slip of
the vertebra occurs in the lumbar area of the spine. This slippage
and herniation (deformity) of the disc places pressure on the
nerve roots associated with the affected vertebrae, causing
pain and dysfunction. While the herniation of the disk causes
pain, discectomy alone is unable to provide relief . The reduction
in disk space height and abnormal amount of movement allowed
by the joint also causes pressure on the nerves. This intervertebral
space must be restored in order to provide adequate space for
the nerves.
What causes spondylolisthesis?
Spondylolisthesis occurs only in people who are able to stand upright
and walk, so is virtually nonexistent among newborns. The upright
position of human walking seems to have a direct effect on the development.
It is more common in persons who participate is sports such as diving,
weight lifting, wrestling and gymnastics . All these activities
require repetitive hyperextension, which can contribute to instability
of the spine.
Can spondylolisthesis be prevented?
Good spinal care, both in developing good musculature and in preventing
overuse or injuries, is key into reducing the chance of developing
spondylolisthesis. Athletes, especially, need to be knowledgeable
about body mechanics and the importance of both strengthening and
resting the muscles of the back.
What treatment options are there for spondylolisthesis?
Anterior or Posterior Decompression
with fusion cages
 The
goals of surgery are to remove pressure on spinal nerves (decompression),
and to provide stability to the lumbar spine. Decompression
involves removing the damaged structures that are causing the
spondylolisthesis. In most cases of spondylolisthesis, lumbar
decompression is accompanied by the uniting of one spinal vertebra
to the next (spinal fusion) with spinal instrumentation (implants
that are used to assist the healing process). Surgery can be
performed from the back of the spine (posterior) or from the
front of the spine (anterior). A structural graft is inserted
into the place previously occupied by the removed structure.
The purpose of this graft is to hold the disc space open until
the fusion is complete. The graft is often held in place by
a "cage" device, such as the BAK cage.
Laminectomy decompression with graft
In the laminectomy procedure, the spine is approached through a
two-inch to five-inch incision in the midline of the back, and the
left and right back muscles are detached from the lamina on both
sides. The lamina are flat bone projections on each side of the
vertebra. After this is accomplished, the lamina is removed (laminectomy),
allowing the doctor to see the nerve roots. The facet joints, which
are directly over the nerve roots, may then be trimmed to give the
nerve roots more room. Once the nerve roots have adequate space
made by the removed lamina and facet joint trimmings, pressure is
eliminated, thereby alleviating pain. Bone graft chips may be placed
between the vertebrae to create a solid section of bone, preventing
motion that may detract from healing.
Posterolateral fusion
The posterolateral fusion involves placing bone graft in the posterolateral
portion of the spine (behind and to one side of the spine).The surgical
approach to the spine is from the back through a midline incision
that is approximately three inches to six inches long. First, bone
graft is obtained from the pelvis (the iliac crest). Most surgeons
work through the same incision to obtain the bone graft and perform
the spinal fusion.
Next, the harvested bone graft applied to the posterolateral portion
of the spine. This region lies on the outside of the spine and is
rich in blood to supply the nutrients for it to grow. A small extension
of the vertebral body in this area (transverse process) is a bone
that serves as a muscle attachment site. The large back muscles
that attach to the transverse processes are elevated to create a
bed to lay the bone graft on. The back muscles are then laid back
over the bone graft, creating tension to hold the bone graft in
place.
After surgery, the body uses a natural process to repair itself,
which usually means growing bone. As the harvested bone graft grows
and adheres to the transverse processes, the spinal fusion is achieved
and motion at that segment is stopped. Spine surgery instrumentation
(medical devices) is sometimes used as an adjunct to obtain a solid
fusion.
Spinal instrumentation with
pedicle screws
For spine operations to be successful, solid healing of bone across
the spine must be achieved. The use of metal devices, also called
instrumentation (screws, rods, plates, cables, wires) can help correct
a deformed spine and will also increase the probability of obtaining
a solid spinal fusion.
Spinal instrumentation can be placed in the front or in the back
portion of the spine. The devices are usually made of metal,
commonly stainless steel or titanium. In order to place this
instrumentation into the spine, the spine is at first exposed
by making a skin incision, and then gently clearing the muscles,
ligaments and other soft tissues from the levels of the vertebrae
to be fused. Specific tools are used to carefully prepare the
bone in such a way to obtain good seating of the implants (screw,
rod, wire, cable or other). When these devices are in the proper
position, a rod (or plate) is positioned to link the implants
together. Screws are inserted into the pedicles, which are part
of the arch of the vertebra. This essentially forms a rigid
scaffolding to hold the spine in the desired position. The bone
graft which has been placed into the area of fusion gradually
solidifies over several months. The spinal instrumentation is
gradually covered by scar tissue and sometimes bone which the
body lays down.
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John
J. Regan, M.D.