Instrumentation for Stability After Lumbar Interbody Fusion

History

In 1944, King was first to publish on vertebral screw fixation. These screws were placed parallel to the inferior border of the lamina and perpendicular to the lumbar vertebrae facets joints. Recovery time was long, required bed rest, and the rate of pseudoarthrosis was extremely high— 55.1% in a study of 49 patients.

Difficulties in the surgical technique and poor results continued until 1959 when Boucher reported a new material and method of screw insertion that improved fixation. The screws were placed through the inferior facet and into the inferior pedicle. He utilized stainless steel, machined screws that were 1.5 to 2 inches in length. In 1964, Pennel used a similar fixation technique to that of Boucher. These screws would be considered ‘unilateral’ or transfacet screws as they were directed from each ipsilateral facet ( Fig. 20.1 ).

In the 1980s, Magerl developed the technique of ‘translaminar’ facet screw fixation, a variation of the transfacet method. These screws started on the contralateral side of the spinous process, traversed through the ipsilateral lamina, and then crossed the joint and exited laterally at the posterolateral inferior aspect of the pedicle. In 1989, Jacobs reported 91% solid arthrodesis with 93% clinical success in 88 consecutive patients with a prospective average follow up of 16 months using Magerl’s technique.

Although translaminar and transfacet fixation are both less costly than pedicle screws, transfacet fixation is believed to have greater incidence of incomplete fusion.

Translaminar fixation has remained the more popular technique, although it does not come without complications. It requires a larger surgical area, has been associated with a 10% incidence of laminar wall violation and improper screw position, and higher neurologic complications.

In recent years, there have been some novel ideas for facet fixation to include the facet gun that was last marketed by US Spine (Javelin MIS Locking Facet System, Amedica/US Spine, Salt Lake City, UT). Not all ideas or techniques are successful as this product is no longer available and no papers could be found to report on outcomes. It remains to be seen if other devices or techniques will prove to be safe and efficacious.

Several companies have developed specific screws to be utilized in transfacet and translaminar facet fixation; however, the most frequently used screw to date is a simple 4.5 mm titanium cortical bone screw found in all orthopedic trauma sets. These screws are extremely inexpensive in comparison to pedicle screws. From an industry point of view, it is unlikely that there will be significant marketing for this technique. However, when clinical results have been excellent and cost-effectiveness is vitally important, we may likely see a resurgence of this technique in the future ( Fig. 20.2 ).

Indications

The clinical indications for facet screws are broad in nature, but the most common indication from the surgeon’s perspective is posterior fixation with or without interbody support on patients with an intact lamina (no previous laminectomy or pars intra-articularis defect).

Technique

The technique for placing facet or Magerl screws is straightforward, but requires a keen sense of three-dimensional vertebral anatomy. The typical screw is 4.5 mm titanium or stainless steel with cortical threads. A few screws have been developed specifically for this purpose, but no data have been reviewed by this author that demonstrate superiority over basic screws.

Surgically, a midline incision is made to expose the lamina of the superior vertebrae in the segment to be fused (e.g., L4 for and L4-5 fusion). This dissection allows for clear visual identification of the entire lamina. Further dissection should be done to reveal the caudal edge of the inferior vertebral transverse process at the attachment to the pedicle (e.g., L5 transverse process for an L4-5 fusion). The trajectory is a line that is coplanar with the lamina where the starting point is from the contralateral side of the spinous process at the junction of the lamina aiming toward the intersection of the caudal junction of the transverse process of the caudal vertebrae at its attachment to the pedicle.

In order for the midline incision to remain small with minimally invasive exposure, a separate stab wound has to be made cranial and laterally on each side that will be collinear with the facet screw.

A 3.0-mm drill bit is advanced through the stab wound and docked on the lamina and advanced in this trajectory and exits the cortical bone of the inferior lateral pedicle at the junction of transverse process. A 4.5-mm tap is then advanced just beyond the joint. The purpose of this is so that the screw will engage the caudal vertebrae immediately, so there is no distraction of the facet joint before the screw engages distally. The screw is then inserted, with typical screw lengths between 44 and 54 mm. The second screw is placed on the contralateral side using the same technique. Decortication of the lamina and facet joints is accomplished with a high-speed bone dissector and graft is placed to facilitate a posterior fusion.

Limitations

Obviously the patient must have intact pars intra-articularis and lamina to perform the technique.

Biomechanically the technique is not as robust as pedicle screw fixation; however, with anterior column support (e.g., anterior lumbar interbody fusion, lateral lumbar interbody fusion, posterior lumbar interbody fusion, or transforaminal lumbar interbody fusion), the fixation is essentially clinically equivalent.

Facet screw technique requires a keen understanding of the 3D anatomy of the spine, whether unilateral or translaminar. The technique currently requires open exposure, but may be successfully and reliably performed percutaneously in the future with the use of navigation and/or robotics.

Tips and Tricks

In the past, there have been guide devices available to assist with facet screw implantation, but their availability at this time is unknown. Ideally, the drill bits and taps are calibrated to a sleeve that passes through the soft tissue to provide protection of the paraspinous processes during instrumentation. If such a system is not available, this technique can be done with the instrumentation from most large fragment trauma sets.

Always start with the most cranial screw first and stay as cranial as possible to allow for ample room for the second screw. If the first screw is placed in the middle, then there will not be enough room for the caudal screw.

There are no reliable and repeatable fluoroscopic landmarks to place these screws with 2D fluoroscopy. This technique requires understanding 3D anatomy, specifically the start point and exit point with the ability to drill in that specific trajectory.

A patient may have a previous one-side hemilaminotomy, and the cranial screw needs to be placed on the side of the previous decompression.

Complications

Many complications can occur using facet screw fixation. Intraoperative complications include screw misdirection that could lead to dural tear, nerve compression, and/or nerve injury. Lamina and facet fractures may also occur if screws are not placed in the correct trajectory. Postoperative complications are rare, but would include loosening or failure of the hardware, typically in the presence of a pseudoarthrosis. If the screws fracture, it is almost always at the facet joint. Reoperation for pseudoarthrosis can be accomplished with pedicle screws. It is important to note that the caudal segment may have a screw pass through the pedicle so that the facet screw will need to be removed before placing the caudal pedicle screw. Careful examination of the preoperative x-rays and computed tomography scan will help with planning.