Interbody cages for spine fusion
In September of 1996, the FDA approved anterior interbody
cages for use in the disc space, providing a new technique
that allows the spine to be fused with less morbidity
(e.g. less post-operative discomfort) than in the past.
Anterior interbody cages are titanium cylinders that
are placed in the disc space. The cages are porous
and allow the bone graft to grow from the vertebral
body through the cage and into the next vertebral body.
The cages offer excellent fixation, so most patients
do not need additional instrumentation (e.g. pedicle
screws) or post-operative back braces for support.
Most of the cages are placed in the front of the spine (anterior
lumbar interbody fusion or ALIF) . The cages can
be inserted through a small incision (minilaparotomy)
or with an endoscope (a scope that allows the surgery
to be done through several one-inch incisions (see Figure
1).
By far the most popular approach for inserting
titanium cages is through a minilaparotomy as the
endoscopic approach is difficult and does not provide
as good exposure. Also, it does not seem as though
the endoscopic approach reduces the morbidity of
an ALIF surgery as was first believed it would.
While most cages are placed through an ALIF, they can
also be placed in the back of the spine (posterior
lumbar interbody fusion or PLIF) through a midline incision
in the back (see Figure
2).
Originally, anterior interbody fusions were all
done with a patient's own bone from their iliac
crest. Besides the bone graft site pain and problems,
there was a high nonunion rate with procedures.
Initially, the threaded cylindrical titanium cages
that became popular in the late 1990's helped the
success rate of the procedure by providing more
firm fixation of the disc space. Also, the amount
of bone that needed to be harvested from the iliac
crest was greatly reduced because only the soft
inner cancellous bone was needed for the fusion.
A structural cortical graft was not needed and
the outer table of the pelvis was not violated.
Currently, there are now several bone graft substitutes
that may even eliminate the need for bone graft
harvests.
After an initial surge in popularity, it was recognized
that the threaded cages did not fixate the spine
well enough in certain situations. Where they seemed
to work the best was in one-level severe degenerative
disc disease with disc space collapse. They are
particularly effective at L5-S1 where there is
not naturally a lot of motion.
At L4-L5 and above, especially in patients with
either a tall disc space or an associated isthmic
spondylolisthesis, cages do not provide as good
a fixation. They are not also all that effective
in multilevel lumbar fusions without supplemental
posterior fixation.
Another problem has been that the cages obscure
postoperative assessment with x-rays because of
all the metal in the disc space. There are various
radiolucent cages now (carbon fiber or PEEK) which
allow postoperative visualization of the bone graft
healing, but do not provide as good fixation. Generally,
posterior pedicle screw supplementation is also
necessary.
Another design that is now available is a metallic
cage that can firmly grip the disc space, but more
closely matches the anatomy of the disc space.
Because it is not a cylinder, it can be used in
disc spaces that are still tall. A cylinder needs
to be quite large in a tall disc space so a lot
of the strong subchondral bone at the end of the
vertebral body is reamed away. This can weaken
the disc space and lead to postoperative subsidence
of the cages into the vertebral bodies. If this
happens the fixation is lost.
Theoretically, a rectangular geometry should allow
for more surface area between the cage and the
vertebral endplate, which in turn should decrease
the force per unit area on the vertebral endplate.
Because anterior interbody fusions fail as a result
of the endplate not being able to support the stress
of the cage, decreasing the force per unit area
should lead to less endplate failures and cage
subsidence. However, there is not yet enough experience
with this type of implant to know if it will work
as a stand-alone device. If more support can be
gained with design, it will also be a more effective
way of distracting the disc space to relieve nerve
root compression in the foramen (indirect nerve
root decompression).
At this time there is not one best anterior device,
and some surgeons may use multiple different devices
in different situations.
Cages risks and potential complications
The risks and possible complications of inserting
an anterior lumbar cage would include all the possible
risks of an ALIF. Breakage of any of the cages
would be highly unlikely as they are stronger that
the vertebral body. Dislodgement is a possibility,
but especially for the titanium cylindrical threaded
cages, the risk is low. It is also possible that
the cages would impinge on a nerve root, but this
is uncommon.
By: Peter
F. Ullrich, Jr., MD
September 8, 1999 (Updated
January 20, 2004)
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