Instrumentation for Stability After Lumbar Interbody Fusion

Introduction

Posterior fixation (PF) after lumbar interbody fusion (LIF) is a standard practice in spine surgery. As stand-alone LIF devices and techniques continue to advance, there is debate around the necessity of PF. Data and literature substantiating fusion outcomes is lacking for stand-alone devices, and therefore the US Food and Drug Administration (FDA) has yet to approve stand-alone LIF as equivalent to LIF with PF.

As a surgeon, one of the first tenets of surgery is to accomplish the desired goal with the highest benefit-to-risk ratio. Stated another way, the surgeon wants to execute the surgical procedure that has the best opportunity to provide the greatest benefit to the patient while minimizing risks. In LIF, solid fusion allows the patient the best opportunity for relief and positive clinical outcomes. However PF, although understood to facilitate a solid fusion, is a greater surgical risk.

Requirements to achieve a fusion can be categorized into two groups. The first is to provide the biological environment that will be conducive to allow a fusion to occur (e.g., preparing bony surfaces and endplates until they are bleeding, maximizing graft and host interface by removing as much disk as possible and appropriately preparing the endplates). The second is to provide stability that will allow the fusion to occur postoperatively. There is very little to debate regarding the first principle, but the second principle is the focus for this chapter.

There are many ways to perform PF of the lumbar spine, and new techniques and implants are evolving. This chapter will discuss the history and rationale for the use of PF and the three most common techniques used today: facet screws (FS), pedicle screws (PS), and interspinous process (ISP) devices.

Facet Screws

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.

Pedicle Screws

History

Harrington and Tullous reported in 1969 on what most people credit as the first “true” pedicle screw. The screws were inserted into pedicles with modified Harrington instrumentation and were wired to Harrington distraction rods.

In 1970, Roy-Camille et al. reported the use of posterior plates and sagittal screws through the pedicle and articular processes. Although the trajectory was not the same as standard pedicle screw fixation today, this was a similar concept that showed significant success. Louis and Maresca reported success with a modified technique and instrumentation. The report on 455 cases with 31.6 months of average follow-up indicated solid arthrodesis was achieved in 97.4% of posterior only cases and 100% of the combined approach cases.

Roy-Camille then presented the pedicle screw technique in 1984 at the Academy of Orthopaedic Surgeons meeting in San Francisco. Steffee et al. was very interested in this technique and, in 1986, he published on a variable screw placement plate that could be used from the thoracic spine to the sacrum ( Fig. 20.3 ).

Pedicle screw fixation became the gold standard by the mid-1990s and, in 1993 and 1996, the North American Spine Society endorsed the use of pedicle screws by experienced surgeons.

Despite this, the 1990s were marred by unfortunate events for physicians, manufacturers, and medical societies supporting the pedicle screw. In August of 1993 the FDA sent a letter to six companies instructing them not to ‘advertise or promote’ the use of pedicle screws in the spine. As a class III device, the screws were approved as bone screws, but not specifically for the spine. By December of 1993, the infamous 20/20 episode aired and sparked litigation that continued throughout the United States. Acromed settled for $100 million in 1997, just before the FDA reclassified the devices from class III to class II in July 1998. The reclassification resulted in the dismissal of the majority of the remaining litigation cases.

Minimally Invasive Surgery (MIS) Pedicle Screws

Magerl was first to introduce MIS, the percutaneous placement of screws in 1977. The technique was cumbersome, and utilized an external spinal skeletal fixation (ESSF) system. Over time, this particular technique did not achieve good results primarily owing to wound issues from the external nature and was no longer used, but the concept of MIS screw placement remained ( Fig. 20.4 ).

Foley et al. were the first to publish a clinical series of a percutaneous technique in fusion of degenerative lumbar pathologies (Sextant; Medtronic, Minneapolis, MN, USA). A major step toward percutaneous screw placement, the Sextant system used fluoroscopic imaging to assist in the trajectory. The multiaxial screw was modified and placed percutaneously using an extension sleeve/tower. The engineering of Foley’s design was significant as it used the mechanics of the radius of an arc to deliver the rod percutaneously. Unfortunately, this required a separate incision to be made in order to pass the rod, and anatomic variations further made this a challenge. This original system could only be used with two screws, but a two-level system was subsequently developed ( Fig. 20.5 ).

The results with the early percutaneous pedicle screws showed good stability and fusion and the technology advanced to be used with multiple pathologies (trauma, tumors, deformity, and degenerative disease. ^. More recent studies have confirmed these results.

In 2012, Kruger et al. published a report on 51 prospective patients using Sextant II Rod insertion system. A total of 204 screws were used; 197 were correctly placed, with all fractures showing fusion after 6 weeks. “However, the authors reported that the multiaxial pedicle screws were not able to conserve the slight correction obtained perioperatively via positioning and longitudinal traction.”

In 2014, Wang et al. reported a retrospective review of 100 patients with kyphosis using the Sextant system. They concluded that Sextant was superior to conventional open posterior short segment four screw system.

Design Evolution of Minimally Invasive Surgery (MIS) Pedicle Screws

The evolution of minimally invasive systems flourished as designers and manufactures raced to develop better ways to place pedicle screws percutaneously.

The MIS pedicle screws required cannulation for the majority of the early systems. The technique first involved targeting the pedicle via C-arm guidance and then inserting a large gauge biopsy-style needle through the pedicle and into the vertebral body. The needle stylet is removed and a guidewire placed into the vertebral body and the needle removed. This would then allow taping, if indicated, and screw guidance through the pedicle into the vertebral body. There were biomechanical concerns as cannulation affected the overall strength of the screws, particularly in the smaller diameters. Guidewire diameter and screw designs were altered by many manufactures to account for these challenges.

Subsequent generations of MIS systems can be categorized as either rigid tower (Sextant II, Medtronic, Minneapolis, MN and Expedium, Depuy, Raynham, MA) or flexible tower systems (Serengeti, K2M, Leesburg, VA). Many manufactures had different designs of detachable towers that would be placed over conventional screws and then removed after the screws and rod were secure ( Fig. 20.5 ).

The concept of a flexible tower/retractor was developed that had certain advantages over the rigid tower designs. Notably, it was easier to pass rods for certain spinal deformities, including hyperlordosis, rotatory scoliosis, and spondylolisthesis.

More recent generations of the rigid tower systems incorporate the ‘tower’ into tabs that are manufactured with the screws and then ‘break away’ after the screw is placed and rod secured. Many of these systems have built-in screw reduction tabs that allow for easier placement and seating of the rod and reduction of spinal deformities ( Figs. 20.6 and 20.7 ). There are potential advantages of these systems, but at the time of publication no data on these new systems can be reported.