Microendoscopic Assisted Transforaminal Lumbar Interbody
Fusion (METLIF)
INTRODUCTION
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| Intraoperative
fluoroscopic image depicting a multilevel decompression. |
Since their introduction by R.B. Cloward, lumbar interbody
fusions have gained popularity as a means to address
instability in the lumbar spine.(1) The result of the
combined anterior and posterior spinal fusion/360 fusion
is advocated by some surgeons as a means to increase
fusion rate, allow fusion under compression, allow for
an easier evaluation of the status of the fusion, as
a means to open the foramen by way of distraction, and
finally as a means to limit paraspinal muscle dissection
for those who perform an interbody fusion without posterolateral
fusion. Posterior Lumbar Interbody Fusion (PLIF) has
been advocated as a way to lessen the risks to the retroperitoneal
and peritoneal structures, to avoid the need for an
approach surgeon, and to be able to perform posterior
instrumentation to augment fusion without adding a second
surgical procedure. We have developed a novel means
of percutaneous fusion using a modification of the microendoscopic
assisted discectomy/decompressive laminotomy techniques
presently in use. We will describe this new Microendoscopic
Assisted Transforaminal Lumbar Interbody Fusion (METLIF)
technique. However, we will not discuss the preoperative
planning or set-up but instead will begin with the initial
incision and continue through to the end of the procedure.
SURGICAL PROCEDURE
Incision and Localization
The lumbar level to be approached is confirmed using
lateral fluoroscopy. For this, it is useful to use a
20-gauge needle inserted into the paraspinal musculature
two-to-three fingers’ breadth (3-4 cm) lateral
to the midline. The needle is repositioned until it
is directly over the disc space. Marcaine (0.25 –
0.5 percent) is injected into the muscle as the needle
is withdrawn, and again at each facet complex and along
the lamina at the level above. Finally, local is injected
along the intended skin incision.
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| Intraoperative
fluoroscopic image depicting a multilevel decompression |
The needle is removed and a vertical puncture is made
with a #11 blade knife. A Steinmann pin is placed perpendicularly
through the puncture site and directed toward the inferior
aspect of the superior lamina under lateral fluoroscopy.
Once confirmed, the incision is lengthened symmetrically
and should match the diameter of the respective tubular
retractor. To place an interbody device using the Tangent
system (Medtronic Sofamor Danek, Memphis, TN), at least
a 20mm working channel will need to be used. Therefore,
the incision must be this length or larger to accommodate
the retractor.
Serial Dilation
Utilizing a twisting motion, sequential soft tissue
dilators are inserted. Once the initial dilator is safely
docked onto the lamino-facet junction of the superior
lamina, the Steinmann pin is removed. A slight medial
angulation is desirable at this point to ensure optimal
visualization of the interlaminar space and lateral
recess during the opening. Confirmatory radiographs
are taken.
Subsequent dilators are sequentially placed over the
initial dilator down to the lamina. X-rays are taken
after each passage to confirm that the dilators remain
in position. Finally, once the desired working dimension
is obtained, the working channel/tubular retractor is
placed over the dilators and confirmed with fluoroscopy.
The flexible arm, which is secured to the table, is
attached to the tubular retractor to hold it firmly
in place. The dilators are removed, establishing an
operative corridor to the lamina and interlaminar space.
Positioning is finalized with fluoroscopy.
The endoscope is then inserted into the tubular retractor.
The locking arm on the ring attachment secures the endoscope
to the tubular retractor. Initial placement of the endoscope
should be in its most retracted position in order to
avoid endoscope contact with soft tissue and subsequent
smudging. Should this occur, remove the endoscope from
the tubular retractor and clean the lens using anti-fog
solution and gauze. The endoscope should be focused
and oriented to the standard position (on the video
screen, up is medial). A v-shaped indicator has been
placed on the video image to represent the position
of the endoscope within the tubular retractor. The v-shaped
indicator on the video screen should mirror the endoscope’s
position with respect to the tubular retractor. Whenever
the endoscope is turned, the orientation should be corrected.
Anatomic Localization
A Bovie electrocautery can be used to dissect any soft
tissue present in the operative field. It is best to
begin this dissection laterally where the bone is clearly
felt. Remaining tissue is removed with a pituitary rongeur.
It is essential to remove all residual soft tissue in
the operative corridor to maximize the working space
within the tubular retractor. Soft tissue removal helps
to identify the edge of the lamina and interlaminar
space, and the position of the retractor with reference
to the facet. It is this anatomy that aids in correctly
performing the laminotomy. The working channel should
be repositioned, if necessary, so that the medial facet,
lateral lamina, and small amount of interlaminar space
are present within the surgeon’s view.
Laminectomy
In order to place an interbody device, the surgeon
must expose the nerve roots above and below the disc
space. Therefore, a fairly generous laminectomy and
facetectomy must be performed. The laminectomy preceeds
as it would with a standard MED. The ligamentum flavum
is first detached from the undersurface of the lamina
with a small curved curette. Endoscopic Kerrison rongeurs
are used to remove the lamina and medial aspect of the
facet. We tend to favor performing the facetectomy without
drilling significant bone away, and saving as much bone
as possible (for the posterolateral fusion). Using the
Kerrison rongeur, the hemilaminotomy, facetectomy and
foraminotomy over the nerve root below are completed.
The ligamentum flavum is eventually freed once enough
bone has been removed. The ligament is lifted then resected
with a Kerrison punch. In doing so, the lateral thecal
sac and root are exposed. Attention is then shifted
to the nerve root above. Working from the thecal sac
superiorly and laterally, the proximal aspect of the
root above is identified. The nerve is then traced laterally
exposing out this root as it exists its foramen. Once
there is complete visualization of both roots, it is
safe to perform the discectomy.
Discectomy
Given the angle of the approach, the lateral aspect
of the thecal sac should be well visualized upon removal
of the yellow ligament. If need be, utilizing a Penfield
dissector, the thecal sac can be freed of any attachments
so that it may be mobilized. The freed nerve root and
thecal sac are then retracted medially using a suction
retractor. Epidural veins may be cauterized with bipolar
cautery and divided with microscissors. A #10 blade
is used to open the disc space. Small Pituitary rongeurs
aid in evacuating the disc material.
Interbody Fusion
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| Fluoroscopic
image seen on the monitor as the endplates are prepared
for the bone graft. |
With the thecal sac mobilized and retracted medially,
the initial bone graft can be placed. The method for
this depends upon the nature of the interbody device
utilized. For the Tanget system, the remaining disc
material is removed with the aid of a disc space reamer
followed by an endplate scaper. This allows the disc
space to be prepared for the graft material. Especially
when the field of view is limited as it is in this case,
it is especially important that an assistant protect
the dura and adjacent nerve roots while the surgeon
controls the instruments. As well, liberal use of fluoroscopy
allows the surgeon to have knowledge of the depth of
the instrumentation placed into the disc space. A chisel
is used to prepare a corridor for the bone graft, then
the graft follows under fluoroscopic guidance. Autograft
collected locally from the laminectomy is added to the
interspace. The working channel is then angled back
towards the ipsilateral side, then the process is repeated
for the second bone graft. Once the interbody fusion
is complete, the working channel can be further angled
towards the surgeon, and a posterolateral fusion can
be performed.
Pedicle Screw Instrumentation
The working channel is removed and the fluoroscope
is brought in for an AP radiograph. Using the technique
similar to vertebroplasty, a Jamsheeti trocar is placed
through the prior incision. The fluoroscope is angled
to "owls-eye" the pedicle – so that the
X-ray beam is shot down the length of the pedicle. The
Jamsheeti, once in line to this angle, will safely guide
the K wire through the pedicle and into the body. All
four Steimanm pins are placed in one to two centimeters
while being watched and confirmed with AP flouro. Once
safely in position, the fluoroscope is brought into
position for a lateral X-ray, and they are advanced
to a total depth of about 2/3rds of the vertebral body.
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| After
the pedicle screws are placed percutaneously, they
are connected by a rod brought through the skin
through a separate puncture. |
Using canulated instruments, a drill and then a tap
is advanced over the K-wire. Finally, a canulated M-8
screw attached to a screw extender is placed over the
wire. Multiple X-rays must be taken to ensure that the
Stiemann pin is not advancing. The screw is brought
down until it is almost touching the facet. If advanced
too far, the surgeon is able to feel that the screw
has limited poly-axial movement, and the screw will
need to be withdrawn a turn or so. The second screw
on the same side is placed in the same manner, and the
extenders are connected to each other. Using the Sextant
arm, the rod is placed percutaneously and brought into
the gap between the screw head and the nut. This is
watched under lateral fluoroscopy, then confirmed with
a AP image. Once in place, the screws may be compressed
and the nuts tightened with a torque wrench. Attention
is then shifted to the contralateral side and the process
is repeated followed by closure.
Conclusions
The METLIF technique affords an option of percutaneous
interbody fusion comparable to that achieved by open
means while minimizing the destruction to normal tissues
not directly involved in the pathology.
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