Indications and Results of Translaminar Screw Fixation
Rick C. Sasso MD
Assistant Professor
Clinical Orthopaedic Surgery
Indiana University School of Medicine
Indiana Spine Group
Indianapolis, IN
A less invasive alternative for posterior stabilization is translaminar facet
screw fixation. Devised by Magerl, 1 this technique requires a small incision with
dissection only out to the facet joints. The transverse processes and cephalad
juxtalevel facet joints are not exposed. Clinical studies have reported a high
success rate with minimal complications2, 3, 4, 5 Magerl’s technique is a
modification of Boucher, which is a modification of King’s description of facet joint
screws. King6 in 1948 reported his operation whereby short screws are placed
horizontally directly across the facet joint. The screw enters the inferior articular
process just medial to the joint and crosses the joint into the ipsilateral superior
articular process. In 1959, Boucher7 described his method that uses the same
starting point as King, but the screw is directed more vertical into the pedicle
thereby increasing the length of the screw in the caudal vertebrae. Magerl’s
screw is significantly longer because the entry point is at the base of the
contralateral spinous process. This increases the effective working length of the
screw on both sides of the facet joint thus increasing strength of the fixation. The
anatomic angle of screw insertion and screw length8 at the various levels in the
lumbar spine has been studied for this technique and translaminar facet screw
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stabilization has been successfully used after selective decompression for spinal
stenosis and disc protrusion9. Biomechanical studies have demonstrated
significant stability in flexion, extension, and rotation10. Translaminar facet screws
significantly increase the stiffness of spinal motion segments11. When coupled
with threaded cylindrical interbody fusion devices, translaminar facet screws
provide substantial stability in the weakest loading directions, extension and axial
rotation12, 13. Interbody cages separate the facet surfaces with distraction, which
reduces the role of the facets in extension and axial rotation14. Translaminar facet
screws stabilize this facet uncoupling caused by the interbody distraction.
Translaminar facet screw technique has also been evaluated in a
biomechanical model of PLIF. Zhao15 compared the segmental stiffness of three
different PLIF constructs: two posterior cages, a single long diagonally placed
threaded cylindrical cage from a posterolateral position, and the single long
posterolateral cage with simultaneous facet joint fixation. The two, standard PLIF
cages construct was the weakest due to the need for bilateral facetectomy and
posterior element destruction, which is detrimental to segmental stiffness. The
single posterolateral cage technique requires only a unilateral facetectomy and
conserves more of the posterior elements. As expected, this model was more
stable than the two-cage construct. The addition of translaminar joint fixation to
the remaining facet provided significantly more stability in compression,
extension, flexion, bending, and torsion. This study clearly proves the advantage
of even unilateral facet stabilization, and the disadvantage of the standard PLIF
approach, which results in a profound decrease in biomechanical stiffness.
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Extensive removal of the posterior elements is required to insert the cylindrical
cages of appropriate size and kyphosis may occur when larger cages are used.
Also, cauda equina retraction is necessary during insertion of these cages and
may be severe with potential neurologic damage when appropriate, larger cages
are employed.
In conclusion, technical and biomechanical advantages support the
combination of interbody cages and least invasive posterior translaminar facet
screw fixation. An ALIF approach is less damaging to the soft tissues and
supporting structures of the spine than a PLIF technique for interbody fusion.
Clinically, Vamvanij16 found simultaneous ALIF with BAK cages and posterior
facet fusion offered the highest fusion rate, pain relief, and clinical success
compared to three other lumbar fusion techniques. Limited, posterior soft tissue
dissection only to the facet joints appears to be important. Interbody fusion cages
are least able to resist extension due to distraction and restoration of disc height,
which uncouples the posterior facet joints. Insertion of transfacet screws
significantly increases the stiffness in an interbody cage model, especially in
extension.12, 13 Extension moments on a stand-alone interbody cage without
posterior stabilization tends to separate the vertebral endplates from the
interbody cage, potentially resulting in nonunion, loosening, or migration of the
cage. Stiffness of a cage model loaded in compression is also significantly
greater with the addition of facet screws13. Thus, translaminar facet screws
should help resist collapse and subsidence of the cage as well as loss of lordosis
and foraminal narrowing. In the future, this concept may be developed even
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further with the minimally invasive percutaneous delivery of translaminar facet
screws under real-time image guided control using virtual fluoroscopy.
Image Guided Percutaneous Translaminar Facet Screws:
Cadaveric study to determine the accuracy of inserting translaminar facet screws
via a closed, percutaneous, image guided approach using navigable instruments.
4.0 mm screws placed through percutaneous portal without exposing the facet
joint under image guidance. A total of ten translaminar facet screws were placed
bilaterally at five levels: After all 10 screws placed in each cadaver obtain CT.
(Create a grading scale for optimal position of screw based on mechanical
factors-is screw centered in facet joint or only partially fixed.) Also assess safety
issues. Measure distance from screw to nerve root and distance from dura in the
canal.
The results will focus on the accuracy of image-guided technique in a
percutaneous setting (% of screws “well placed”) and safety (% of screws with
the neurologic structures “at risk”).
AP and lateral images were used to navigate 4.0 mm screws through a
percutaneous portal under virtual fluoroscopy. Optimal entry point was gauged
at the spinous process-laminar junction of the cephalad vertebrae. Screw target
was the contralateral pars-pedicle junction of the caudal vertebrae. Surgeon
reporting of screw purchase was recorded. After all screws were placed, an axial
CT scan through these levels was obtained. These images were analyzed with
stealth station software and reconstructed orthogonal views.
Results
Surgeon Assessment
Ten screws were successfully placed. All screws had good purchase, based on
surgeon feed back.
CT Results
We grade screws on entry, course through lamina and terminus. The entry and
termination points were graded as either optimal or nonoptimal. The following
grading system was utilized to grade the course though the lamina.
0
I
II
III
completely in bone
< ½ screw out of bone
>1/2 screw out of bone
screw completely out of bone
Entry Point
All ten-screw entry points were judged optimal at spinous processs-laminar
junction.
Laminar Course
There were five Grade I breeches with less than ½ the screw through the lamina;
five Grade 0 screw placements with the screw contained completely within the
lamina. No screws placed the spinal canal at risk.
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Terminus
The termination point was acceptable in five screws. All screws that began on
the left and terminated on the right were found to engage the base of the pedicle
in the superior articular process at the pars/pedicle junction. The screws that
began on the right and terminated on the left were all found to have grade II
breakouts. The entry points for the screw starting on the left were in the
cephalad and ventral aspect of the spinous process, while the screws starting on
the right entered caudad and dorsally. Although clinically these screws were felt
to have adequate purchase, the CT images demonstrated all to be dorsal with
>50% of the screw not completely engaging the superior articular process of the
caudal segment.
Discussion
This study is the first experience with complete percutaneous placement of
translaminar facet screws. All screws were successfully placed. All screws were
safe with respect to neural structures. This study validates the entry points, as
picked from AP and lateral images to be adequate. Trajectory of screws was
adequate in ½ of the screws. The screw situated dorsally with respect to the
other screw, always had problems with adequate engagement of the caudal
segment. This difficulty is directly linked with the more dorsal position of the
screw. In future studies close attention will have to be paid to changing the exit
of the screw to a more ventral position.
The course of the screws through the lamina was acceptable in all cases. Grade
I breakouts, of which we had five, are not clinically significant. These screws will
come in contact with ligamentum flavum but not place any neural structures at
jeopardy at this location. Those perforating dorsal in the lamina are also not
problematic. Virtual fluoroscopy makes this procedure much easier. It reduces
radiation exposure to both the patient and the surgeon. Additionally it allows
planning of appropriate screw length. When the drill is at the entry point, a virtual
screw can be projected from this point. This is an important procedure to avoid
inappropriate length screws. Screws that are too short risk not engaging the
caudal segment, while screws that are too long risk penetration into the foramen
and risk nerve root injury.
Conclusion
Percutaneous placement of translaminar facet screws is possible using virtual
fluoroscopy. This is a safe technique to provide posterior fixation in the lumbar
spine. Caution needs to be exercised when placing the more dorsal of the two
screws. Further studies are underway to modify the trajectory to result in more
acceptable placement of this dorsal screw. This is a promising technique in the
rapidly evolving realm of minimally invasive surgery.
References
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skeletal fixation. Clin Orthop 189:125-141. 1984.
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2. Montesano PX, Magerl F, Jacobs RR, Jackson RP, Rauschning W: Translaminar facet joint
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15. Zhao J, Hai Y, Ordway N, Park C, Yuan H: Posterior lumbar interbody fusion using
posterolateral placement of a single cylindrical threaded cage. Spine 25:425-430, 2000.
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