Lumbosacral fixation brought a high rate of complications including pedicle screw loosening and pseudarthrosis, whose rate was up to 20%–60%, and it was the frequently cited reason for reoperation (25%). The reason being that the instrumentation at L5–S1 is under more stress, S1 pedicle is shorter and has larger diameter than lumbar pedicle, leading to the screw lacking holding power [1,2]. When choosing S1 as the lowest level of instrumentation complications like S1 screw loosening occur. Facing this problem, there have been various techniques for the prevention of S1 screw failure introduced, including sacropelvic fixation, bicortical or tricortical insertion of S1 screw [3-5]. Extension of the instrumentation to the pelvis or iliac wings has gained increasing interest. The risk factors of the S1 screw loosening are still in dispute. Evidence is still lacking that inserting the iliac screws simply for preventing S1 screw loosening can contribute to a better clinical outcome for patients. On the other hand, iliac screws require extensive subfascial dissection, increasing the rate of complications such as implant prominence, deep infection and poor wound-healing. Meanwhile, several studies have shown increased rigidity of lumbosacral fixation techniques contributing to late sacroiliac joint arthritis and pain [4-6]. The S1 pedicle anatomy is characterized by a short length, high cancellous bone content, lacking a cortical ring and the L5–S1 is a transitional motion segment with an oblique disc space. These factors lead to an increased shear forces and increase the risk of S1 screw loosening by 41% [7]. Known trajectories: Anteromedial trajectory which suggests an entry point 5 mm caudal and 10 mm lateral to superior articular facet 20°medial angulation and parallel to superior end-plate screw length (35–40 mm). Anterior trajectory which has a shorter screw (30 mm) [8,9] (Figure 1).

This study aims to define the new Vishal Peshattiwar (VP) trajectory for the placement of S1 pedicle screw to eliminate the complications encountered with the conventional technique and compare its advantages and disadvantages to the pre-exisiting techniques.

This study was conducted following due approval from the Scientifc and Ethics committee. A detailed informed consent was taken from all the patients included in the study.

This prospective analysis was conducted between June 2019 to August 2022 at the Kokilaben Dhirubhai Ambani Hospital, Andheri West, Mumbai. We included 117 patients who underwent L5–S1 MIS-TLIF and multilevel MIS-TLIF including the L5–S1 level, during the given period, at the same center, operated by the same surgeon. In this study, we placed 234 S1 pedicle screws using the newly described trajectory in our study under O-arm guided 3D CT Navigation. The efficacy of this method was assessed with the help of the following parameters: (1) length of screw, (2) entry point for S1 pedicle screw placement, (3) caudocranial and lateromedial angulation of the screw, and (4) finite element analysis of the construct and the benefits of the newer trajectory in countering the forces acted upon the construct.

Our study included a total 117 patients out of which 61 (52.14%) were females and 56 (47.86%) were males. The average age of male and female patients was found to be 57.8±8.4 and 52.3±10.5 years respectively, out of which 25 males (44.64%) and 22 females (36.06%) were older individuals (>60 years of age). We placed a total of 234 S1 pedicle screws using the newly described VP trajectory in our study under O-arm guided 3D CT Navigation with the mean length of screws in the male patients was found to be 45.90 mm and that in females was found to be 44.88 mm. the caudocranial angulation of the screw ranged from 50°–65° and the lateromedial angulation ranged from 18° to 25° based on minor anatomical variations amongst the patients belonging to our random sample (Table 1).

We used the Altair OptiStruct software (Altair, USA) which is an Optimization-enabled Structural Analysis software built on more than 2 decades of innovation [10]. It is a modern structural problem solver with accurate and scalable solutions for linear and nonlinear analyses across static and dynamic constructs using topology optimization. In our study, we used finite element analysis to understand the forces acting upon the interconnecting rods and how would the newer trajectory of the S1 pedicle screw improve the strength and stability of the construct. The analysis gave us 2 main forces acting on the lordotic rods: displacement forces and elemental stresses. OptiStruct software described these forces acting upon the rod in the form of a contour plot. The contour plot for displacement analysis system showed that the displacement forces acted mainly upon the cranial end of the rod contributing to the pull-out forces acting upon the construct. The caudocranial angulation of approximately 60° in this new trajectory helps in countering these forces by angulating the lever arm so acutely that it increases the pull-out strength of the construct by several folds (Figure 6). The contour plot for the elemental stress analysis system talks about the stress acting onto the rod due to the mere lordotic shape of the rod, which is found to be the highest at the caudal end of the interconnecting rod. The 45-mm-long screw, which made possible only with the help of this trajectory helps in overcoming these stresses by increasing the strength and stability to the construct, especially with help of the tricortical purchase taken by these screws at the sacral promontory (Figure 7).

At the end of 2 to 4 years of follow-up (mean follow-up period, 2.79 years), there was 1 case of bilateral S1 screw breakage at 6 months postsurgery, however the CT scan done at that time showed union at L5–S1 intervertebral space hence it was not revised. This patient gave history of repeated forward bending and lifting heavy weights from 1 month postoperative which probably lead to the failure of S1 screws. One case had an intraoperative dural tear which was managed with microscopic dural repair and augmentation with the help of Duragen patch (Integra LifeSciences Corp., USA) and sealed off with dura-seal [11]. Out of the 117 cases included in the study, none of the cases showed any other complications like screw loosening, cage migration or subsidence, postoperative neurodeficit, deep infections and wound gaping, transient neuralgic pain, conversion to open surgery from MIS, etc. (Figure 8, Table 2).

The lumbosacral fixation is needed in patients with symptomatic disc degeneration, foraminal stenosis, spondylolisthesis and oblique takeoff at L5–S1. The results in our study are compared to the pre-existing literature on the older techniques to place the S1 pedicle screw, some studies reported that fixation to the sacrum demonstrated better correction of lumbar lordosis than fixation stopping at L5. Besides, fixed to S1 could prevent subsequent development of pre-existed L5–S1 disc degeneration [12]. A common misconception about this trajectory of the S1 pedicle screw is that it is the same as the tricortical S1 pedicle screw, however, the following studies describe the traditional tricortical S1 pedicle and hence we conclude how different is our trajectory to it. Carlson et al. [13], in their in vitro study in 1992 described the S1 tricortical screw entry point at 2 mm lateral to the most inferior portion of the articular surface of S1 facet. The drill was oriented through the S1 pedicle towards the sacral promontory, 5 mm off the midline (approximately 30° in the anteromedial direction and 20° in the antero-caudal direction) [13].

Ronald Lehman et al. [14], in their biomechanical analysis, described the entry point for the sacral screw insertion as approximately 1cm proximal and slightly lateral to the S1 foramen. First, an awl was used to breech the dorsal cortex of the sacrum. Then a 4.5-mm Kirschner wire (K-wire) was directed approximately 25° medially and inserted into the sacrum. With the bicortical technique, the endplate was paralleled approximately 1 cm distally to its proximal margin. With the tricortical technique (insertion into the apex of the sacral promontory), the wire was placed directly into the sacral promontory under direct visualization. The screw was then inserted into the dorsal cortex and the maximum insertional torque was measured after initial purchase and with each revolution of the digital torque wrench [14].

Sargut et al. [15] in 2022, described the criteria for correct placement of S1 screw were an entry point laterocaudal to the S1 pedicle and converging screw trajectory directed into the superior and anterior cortices of the sacral promontory. Importantly, failure to achieve tricortical S1 purchase with anterior and superior breach due to an insufficient convergence of the screw trajectory was not per se considered a reason for revision [15].

García-Fantini and De Casas [16] in 2018 quoted that regardless of the imaging system used the pedicle screws in S1 at the bicortical or tricortical levels were biomechanically superior to monocortical fixation and the pedicle screws that were implanted under navigation guidance had a margin of error of less than 0.5 mm (Figure 9).

Our trajectory is different, right from the fact that the entry point is 5 mm superior to the inferior margin and 5 mm lateral to the medial margin of the S1 pedicle, targeting the inferomedial quadrant of the pedicle and angulating 60° (50°–65°) caudocranially and 20° (18°–25°) lateromedially. This allows us to achieve the angle of the tricortical or delta screw without having to breach the S1 superior end plate and avoid the major pitfalls of the screw itself preventing adequate bony fusion at the intervertebral space [17]. The longer S1 screw with a tricortical purchase increases the length of the lever arm and neutralizes the forces acting on the screw in functional positions like forward bending and sitting. The acute caudo-cranial angle facilitated by an inferior entry point strengthens the fulcrum during movements across the fused segment, to prevent implant failure at the S1 screw-rod-junction (mainly the neck of the S1 screw). Targeting the inferomedial quadrant of the pedicle increases the risk of medial breach, however, in our study, all the screws were placed with 3D computed tomographic O-arm guided navigation reducing the risk of medial cortical breach.

The VP trajectory of the S1 screw with entry point caudal to the standard point and caudocranial angulation increased the scope for longer screw placement (45 mm and more) which provides a more stable construct at this level. Given that 99.15% of screws showed no complications, the low complications rate suggests the reliability of the VP trajectory for S1 pediclescrew placement.

This trajectory of S1 screw placement is possible only with the MIS technique and is easier with navigation. It could prove to be challenging with open surgery. The scope of this study was limited to the description and results provided by this trajectory. However, a follow-up comparative analysis with conventional techniques would help to expand on these findings.