A novel percutaneous sacral endplate penetrating screw technique: enhanced fixation strength with the M-probe: surgical technique

Introduction

Percutaneous sacral pedicle screw (PS) fixation is a minimally invasive technique that is used in spinal surgery to stabilize the lumbosacral region. This technique has the potential to reduce operative time, blood loss, and postoperative pain (1). However, this presents technical challenges due to the complex anatomy of the sacrum. The anatomical features of the sacrum include sacral morphology, which is a result of a small vertebral body and a lack of transverse processes that contain no true pedicle and are composed primarily of cancellous bone (2,3). Low bone mineral density (BMD) around the PS could indicate the risk of screw loosening (4). S1 screw tips are often in close proximity to neurovascular structures (2,3,5,6). Bicortical or tricortical screw placement while providing stronger fixation increases the risk of neurovascular injury, especially in the percutaneous pedicle screw (PPS) technique. The endplate penetrating screw (EPS) demonstrated higher pull-out forces and less rotation at the bone-screw interface during cyclic loading, indicating enhanced stability (7-12). This trajectory allows screw protrusion into the intervertebral disc space without compromising the neurovascular structure (11). Percutaneous sacral endplate penetrating screw (PSEPS) offer safety advantages by avoiding the anterior sacral cortex, thereby reducing the risk of neurovascular injury. Another issue associated with the PSEPS is that the hardness of the S1 endplate causes the hollow probe to slip, resulting in a perforation ahead of the target. To address this, we developed a hollow probe with a curved tip. Hence, we describe a novel technique for PSEPS that increases the screw insertion torque and reduces the risk of neurovascular complications. We present this article in accordance with the SUPER reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-25-15/rc).

Preoperative preparations and requirements

We developed the M-probe, a new, modified hollow probe, to facilitate the PSEPS technique (Figure 1). The tip of the M-probe is hollow and curved, designed to assist with trajectory correction and enhance penetration into the targeted area of the S1 endplate. This cross-sectional study analyzed prospectively collected data from patients at three hospitals and was conducted from April 2022 to March 2023. This study received approval from the Institutional Review Board by Hyogo Medical University (IRB No. 4172). The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. All patients provided informed consent. Twenty consecutive patients who underwent transforaminal lumbar interbody fusion (TLIF) at L5-S1 using the PSEPS technique for degenerative disease were included. Patients with metastatic bone tumors, spinal infections, trauma, hemodialysis, and rheumatoid arthritis were excluded from this study. The study population consisted of 6 males and 14 females, with an average age of 68±12.9 years. BMD was assessed by dual-energy X-ray absorptiometry scans. All patients underwent computed tomography (CT) preoperatively, and we measured the Hounsfield unit (HU) value of the screw trajectories of L5 and S1 using method by Ishikawa et al. (13). PS insertion torque was measured at the final phase using DTC-CN500REV; (Nakamura Co., Japan). Student’s t-test was used to compare the HU values and PS insertional torque. JMP^® 15 (SAS Institute Inc., Cary, NC, USA) was used for data analysis, with P<0.05 considered statistically significant.

Figure 1 Modified hollow probe (M probe). (A,B) A hollow curved tip was designed to facilitate the PSEPS. (C) The tip has a diameter of 2.5 mm, while the thickest part measures 3.5 mm in diameter. PSEPS, percutaneous sacral endplate penetrating screw.

Step-by-step description

Surgical technique

Initially, TLIF at L5-S1 was conducted using two cages, accessed either through a 4-cm longitudinal median incision or the Wiltse approach via a paramedian incision. Under fluoroscopic guidance, it is crucial to align the tilt of the S1 endplate with the fluoroscopic angle in the anteroposterior view. The entry point for S1 was located 1.5 cm caudal to the edge of the S1 upper endplate and 1.5 cm lateral to the medial border of the S1 pedicle, which is typically located in the lateral wall of L5 (Figure 2). The PSEPS trajectory was angled 20° inward toward the anterior third of the S1 endplate. A conventional straight probe was inserted and directed cephalad until it reached the cranial margin of the S1 end plate (Figure 3A,3B). The placement of the probe beyond the posterior wall of the vertebral body was confirmed in the lateral view (Figure 3C). The M-probe was replaced with a guidewire, and the guidewire was replaced with a cannula before advancing the M-probe toward the anterior third of the S1 vertebral endplate using a hummer. The cephalocaudal orientation of the trajectory was adjusted by rotating the tip of the M-probe by 180° (Figure 4A-4C). When the tip arrived at the S1 endplate, the M-probe was oriented cephalad and advanced forward to penetrate the endplate (Figure 4D-4F). Rotating the tip of the M-probe enlarges the bone hole in the S1 endplate and eliminates the need for tapping. Finally, a guidewire was used to verify the position of the tip in the intervertebral disc, followed by the insertion of the S1 PSs. The L5 PSs was placed using a conventional straight probe. Intraoperative C-arm images demonstrated the optimal positioning of the S1 screw (Figure 5). Precept spinal system screws (NUVASIVE) was used for all patients.

Figure 2 The entry point for S1 was located 1.5 cm caudal to the edge of the S1 upper endplate and 1.5 cm lateral to the medial border of the S1 pedicle, which is typically located lateral to the L5 wall.

Figure 3 Intraoperative radiographic findings. The probe is inserted inward at a 20-degree angle to reach the cranial edge of the S1 pedicle (A), and is directed slightly toward the midline and upward (B). Probe tip extension beyond the posterior wall of S1 is confirmed in the lateral view (C).

Figure 4 Surgical procedure of PSEPS. The M-probe is placed using the guidewire. Point the tip of the M-probe toward the caudal side (A,C-1) and slide it forward the anterior third of the S1 vertebral body endplate (B,C-2). The cephalocaudal orientation of the trajectory can be adjusted by rotating the tip of the M-probe 180 degrees (D,F-3). Upon the tip's arrival at the S1 endplate, the M-probe should be oriented cephalad to facilitate penetration of the S1 endplate (E,F-4). PSEPS, percutaneous sacral endplate penetrating screw.

Figure 5 Intraoperative C-arm images (left, anteroposterior view; right, lateral view) demonstrate the optimal positioning of the S1 screw.

Results

The patient and surgical demographic data are presented in Table 1. The S1 screw was 7.5 mm in all cases. All patients underwent successful S1 endplate penetration using the PSEPS. No neurovascular complications were observed with S1 PS. The HU value of the screw trajectory was significantly lower at S1 than at L5, whereas the screw insertion torque was significantly higher at S1 than at L5 (Table 2).

Postoperative considerations and tasks

The perioperative and postoperative procedures and considerations for lumbosacral posterior fusion using the PSEPS method were the same as those for other lumbar fixation procedures. All patients wore a hard lumbar corset for three months.

Tips and pearls

There are notable sex differences in the morphology of the iliac crest, particularly in males, where the iliac crest may protrude medially. If the entry point for the S1 is positioned laterally, insufficient medial angulation may occur. Therefore, it is crucial to assess the positional relationship between the entry point of S1 and the iliac crest using preoperative CT imaging. Additionally, care should be taken to avoid placing the entry point for L5 excessively laterally, to ensure seamless alignment with the lower lumbar spine screw.

Discussion

Lumbosacral interbody fusion is the gold standard for managing lumbosacral disorders, but it remains challenging due to sacral anatomy and may cause issues such as screw failure due to PS loosening, cage subsidence, and nonunion (14,15). In our study, SEPS demonstrated superior screw insertion torque without inducing neurovascular injury, even though the HU value was lower than that at L5. The BMD of the sacral ala and conventional S1 PS trajectory are lower than that of the superior sacral endplate (3,16). Biomechanical studies have shown that bicortical fixation using the EPS technique has superior pull-out strength compared with a traditional trajectory (12). Matsukawa et al. reported that the average insertion torque obtained using the EPS technique was 296±133 cNm. This technique achieved a 141% higher PS insertion torque than monocortical purchase. In our study, the PSEPS technique resulted in a higher mean torque of 394.0±104.8 cNm compared to that reported by Matsukawa et al. (17). One possible reason for this is that our study employed traditional screw trajectories and thicker and longer screws. The M-probe simplifies the endplate penetration and allows safe screw insertion without requiring a tap. Therefore, the PSEPS technique is expected to provide a high torque and strong fixation.

The unique anatomy of the sacrum makes conventional radiographic views less reliable for determining screw depth and penetration. Techniques such as the pelvic inlet and outlet views have been developed to improve visualization and accuracy during screw placement (18,19). Fluoroscopy-guided S1 PPS fixation presents additional challenges as it requires screw insertion through the anterior sacral cortex via a guidewire, which increases the risk of neurovascular injury (2,5,16). The EPS technique has a safety advantage because the screw protrudes into the L5-S1 disc space (17). Thus, it can be verified that the tip of the guidewire resides within the intervertebral disc, even during percutaneous techniques. With the PSEPS, screw insertion can be performed easily using only lateral fluoroscopic guidance.

The PSEPS trajectory was angled 20 °inward toward the anterior third of the S1 endplate, whereas the EPS trajectory moved straight toward the center of the S1 endplate (Figure 6). The PSEPS trajectory has several advantages: (I) the inward trajectory decreases the risk of neurovascular injury; (II) the guidewire can be safely placed within the intervertebral disc; and (III) longer screws can be used for increased stabilization. Due to the thickness and hardness of the sacral endplate, patients with a small sacral slope are more prone to anterior slippage when a straight probe is used to penetrate the endplate. This indicates an increased likelihood of anterior slippage when using a straight probe, potentially resulting in unintended bicortical or tricortical purchases. Orienting the curvature of the M-probe toward the S1 endplate made it easier to angle the approach relative to the S1 endplate. Additionally, a shape-memory alloy (Nitinol) guidewire was used to reduce the risk of guidewire breakage when a curved hollow probe was used.

Figure 6 Illustrations of the traditional and modified trajectories of the penetrating S1 endplate screw. The PSEPS trajectory is angled 20 degrees inward toward the anterior third of the S1 endplate (A), while the traditional trajectory is aligned directly toward the center of the S1 endplate, directing toward the anterior vertebral vessels (B). PSEPS, percutaneous sacral endplate penetrating screw.

This study has several limitations. First, long-term radiographic outcomes such as screw loosening, cage subsidence, and fusion rates were not evaluated. However, M-probes are not commercially available. Due to the small sample size, no comparison of the PS insertion torque between osteoporotic and non-osteoporotic patients was made. We have only started to measure torque values using the PSEPS method and have no data on torque values for bicortical or tricortical. We were not able to compare the torque values for bicortical, tricortical and PSEPS. Further research involving a larger patient population and long-term outcomes or biomechanical evaluations are required to confirm the effectiveness of the PSEPS technique.

Conclusions

This study demonstrates the enhanced torque of the S1 PS and the benefits of the PSEPS technique using an M-probe. The PSEPS technique may be a safer option for lumbosacral interbody fusion, especially in patients with osteoporosis.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the SUPER reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-25-15/rc

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-25-15/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jss.amegroups.com/article/view/10.21037/jss-25-15/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study received approval from the Institutional Review Board by Hyogo Medical University (IRB No. 4172). The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. All patients provided informed consent.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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