Table Rotation Technique to Access the L4-5 Disc Space in the Prone Transpsoas Approach: A Technical Note

Table Rotation Technique

Patient positioning and setup are performed as previously described with the standard PTP approach. ⁵ Before inserting the dilator at the L4-5 level, emphasis should be placed on obtaining accurate anteroposterior and lateral fluoroscopy images, indicated by parallel endplates and spinous processes located at the midline between the pedicles. The skin is marked accordingly using these lateral images. The table is then rotated away from the surgeon using the anteroposterior images. If the width of the vertebral body on anteroposterior fluoroscopy is visually divided into thirds, the table should be rotated until the spinous process moves from the midline upward toward the contralateral pedicle at the two-thirds mark on repeated fluoroscopy images (Figure 1). The initial dilator is then inserted under fluoroscopic guidance, using lateral images, until it is positioned on top of the psoas muscle. Because the table is rotated, the dilator will not be perfectly orthogonal to the disc space but at a slight angle. Directional EMG is then performed. If the EMG thresholds are within the risk zone (<6 mA), the table can be further rotated and the dilator reinserted. If the EMG thresholds are within the warning zone (6-11 mA) or the safety zone (>11 mA), the dilator is advanced through the psoas on the disc space, and directional EMG is performed again. If EMG thresholds remain in the warning or safe zones, the surgeon inserts the Kirshner wire into the disc space. Sequential dilators are then inserted under fluoroscopic guidance, and repeated directional EMG is performed after each dilation. The retractor is then inserted. The table is rotated back to the neutral position while maintaining pressure on the dilator to hold its position on the disc space. Once the table is back in a neutral position, the dilator should be positioned orthogonally to the disc space and affixed to the table via the articulating arm. Lateral fluoroscopy can be used to ensure adequate positioning in the anteroposterior dimensions. The spinal surgeon performs the remainder of the procedure using the standard PTP technique, as described elsewhere. ⁵ The described technique is performed without direct visualization and instead relies on palpation of anatomical structures such as the psoas muscle, transverse process, and quadratus lumborum confirmed with fluoroscopy.

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Figure 1.

(A) Anteroposterior radiograph before table rotation, showing that the spinous processes at the operative level are directly aligned (black line) with the midline between the pedicles. (B) Anteroposterior radiograph showing the spinous processes at the operative level at the “30% line” (black line) between the pedicles, indicating that the table has been rotated enough. Used with permission from Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, Arizona.

The mechanism by which this technique places the dilator farther from the lumbar plexus while allowing a more posterior docking position is due to the anterior-to-posterior insertion angle of the initial dilators. The dilators enter the psoas more anteriorly but can dock closer to the “50-yard line” on the disc space because of the oblique angle of entry. When the table is rotated back to neutral, the retractor is then orthogonal to the disc space. The tip of the dilator on the disc space remains around the 50-yard line, but there is more muscle between the posterior blade of the retractor and the lumbar plexus because the dilators initially went through the psoas more anteriorly and farther away from the plexus. The concept behind the table rotation technique is illustrated in Figures 2 and 3.

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Figure 2.

Illustration of how the table rotation maneuver improves access to the L4-5 disc space (purple shading), farther from the femoral nerve during the PTP approach (green arrow). The top panel illustrates the standard PTP approach with the table in neutral position and the retractor docked close to the femoral nerve (red circle). The middle panel shows the approach to the disc space with the table rotated, which allows the initial docking to occur farther from the femoral nerve, with more muscle between the retractor and the nerve (blue circle). The bottom panel shows the table returned to the neutral position while maintaining pressure on the retractor to hold its position on the disc space (blue circle). Abbreviation: PTP, prone transpsoas. Used with permission from Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, Arizona.

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Figure 3.

Operative view down the retractor after rotating the table back to the neutral position and final positioning on the L4-5 disc space. Used with permission from Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, Arizona.

Feasibility of Performing LLIF at L4-5

Several studies have explored the feasibility and safety of LLIF at L4-5. Uribe et al ¹⁶ were among the first to investigate the safe working zone (SWZ) for the LLIF technique, showing that the zone was more anterior at the L4-5 level. Additionally, a morphometric study by Guerin et al ¹⁷ showed that the SWZ at L4-5 was significantly narrower than the upper levels of the lumbar spine. Their study found that the SWZ at L4-5 included only 38.7% of the endplate, compared to 75.3% at L1-2 or 51.9% at L3-4. ¹⁷ Additionally, Garg et al ¹⁸ analyzed several factors at the L4-5 level. They showed that the plexus resides in Moro zone III in most cases (60%), with the vena cava or the right common iliac vein residing in Moro zone I in 80% of cases, further reducing the SWZ. When considering other aspects of LLIF at L4-5, such as iliac crest height, the necessary angulation to reach the disc, and the SWZs proposed by Uribe et al, ¹⁶ A study suggested that LLIF may be contraindicated in up to 27% of cases. ¹⁹ The unique anatomy and challenges associated with LLIF at L4-5 necessitate a careful review of preoperative magnetic resonance and computed tomographic images to assess the vascular and retroperitoneal structures related to the psoas and to avoid injury to these structures during the approach. This careful imaging review is imperative when using the table rotation technique, due to the more anterior-to-posterior angle of approach with the dilator and retractor when the table is rotated (Figure 2).

Preventing Nerve Injury

The most prominent risk of performing LLIF at L4-5 is postoperative quadriceps weakness from retraction injury to the femoral nerve. Injury to the femoral nerve can be caused by either direct or indirect injury. Direct injury occurs with blunt injury or laceration of the nerve during the dissection, retraction, discectomy, or placement of the interbody cage. These injuries are usually more severe and have poorer prognoses than indirect injuries. ²⁰ Intraoperative monitoring with directional EMG can assist in locating the relative position of the femoral nerve to avoid direct injury.^20,21 Indirect injury can occur through a combination of compression against the nerve and deformation along the long axis of the nerve with retraction. Multiple studies have indicated that total retraction time during LLIF can predict postoperative weakness.^21,22 In vivo animal studies suggest that nerve impairment is a product of both the magnitude and duration of nerve compression. ²³ Because of this phenomenon, the proximity of the posterior retractor blade to the femoral nerve likely plays a role in postoperative femoral nerve injury, with increased nerve proximity and retraction time leading to nerve injury.

The introduction of the table rotation technique we describe can help surgeons work around the inherent limitations of the PTP approach at the L4-5 disc space and reduce the overall risk of nerve injury. First, higher EMG thresholds demonstrated with this technique compared to the standard approach indicate that this technique can access the disc space at a safer distance from the plexus. The risk for direct nerve injury during docking of the retractor, discectomy, and interbody cage placement is reduced when the nerves are farther from the posterior blade. Second, the table rotation technique can also help circumvent some of the risk factors for indirect nerve injury. Because this technique allows access to the L4-5 disc space more posteriorly than traditional docking of the retractor, the posterior blade does not need to be retracted as far to ensure access to the optimal position within the disc space. Without the table rotation technique, the retractor is usually docked more anteriorly to avoid the plexus, which requires the posterior blade to be opened more in the posterior direction before placing the posterior shim. This anterior docking position increases the pressure applied to the femoral nerve while it is retracted. Although retraction time was not evaluated in the current study, it was expected to be similar to that of the standard approaches. However, by positioning the retractor in a more favorable location relative to the plexus, with improved EMG thresholds, the technique allows for safer retraction while working in the disc space.

Optimal Positioning of the Interbody Cage

The anterior location of the lumbar plexus and femoral nerve at L4-5, compared to higher lumbar levels, often results in a more anterior placement of the interbody cage at this level. Directional EMG is commonly used as a guide for the safe docking of the retractor and to ensure the retractor blades remain in safe proximity to the traversing nerves. Lower EMG values that fall within the risk zone can bias the placement of the retractor more anteriorly to avoid possible postoperative neuropraxia. This situation can increase the risk of anterior longitudinal ligament (ALL) rupture or vascular injury to the great vessels. Inadvertent rupture of the ALL is a concern when placing the interbody in a more ventral position. ²⁴ Intraoperative rupture may necessitate the placement of a lateral-plating system that can increase the risk of vertebral body fractures or lead to postoperative cage migration if the rupture goes unnoticed intraoperatively. ²⁵ The risk of major vascular complications with the transpsoas approach is remarkably low. ²⁶ Nonetheless, when the surgical corridor is more anterior on the vertebral body, it introduces a greater risk of injury to the great vessels. In addition to the advantage of farther separation of the posterior blade from the lumbar plexus, the table rotation technique can also prevent the need to dock more anteriorly on the vertebral body, given the more favorable EMG value obtained at the initial docking of the retractor.

Table Rotation

This study is one of the first to describe the table rotation technique used to approach the L4-5 disc space in detail. Safely accessing the L4-5 disc space through the transpsoas approach is challenging in the lateral decubitus position and can be exacerbated in the prone position due to anatomical changes in the psoas muscle and lumbar plexus. As seen in the present work, 7 out of 10 patients had risk zone EMG thresholds before table rotation; however, all thresholds significantly improved to the warning zone or safety zone after table rotation.

Limitations

The study’s limitations include its small sample size and retrospective nature. Future studies with larger cohorts and prospective data are needed to assess the effects of the table rotation technique on patient outcomes. Additionally, the small sample size limits the ability to perform statistical testing with subgroup analysis to identify confounding variables. Finally, all procedures were performed by the senior author, which may limit the generalizability of the study.