Rick C. Sasso MDTranslam Facet ScrewsPage 1

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 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. 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 further with the minimally invasive percutaneous delivery of translaminar facet screws under real-time image guided control using virtual fluoroscopy.

Image Guided PercutaneousTranslaminar 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.

0completely in bone

I < ½ screw out of bone

II >1/2 screw out of bone

III 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.

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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