The S2AI screw technique for spinopelvic fixation has been described in detail in the literature. This technique can be performed with an assist of a navigation system, robot, C-arm fluoroscopy or freehand placement [9, 11–17]. However, due to navigation or robotic system was not available in all centers, it was challenging to popularize and promote the technique. Furthermore, the requirement of intraoperative CT scan for the navigation system or robotic assist increases radiation exposure. The freehand S2AI technique, which was guided by anatomical landmarks, was presented and described in detail by Park et al  in 2015. In the following years, more scholars applied this technique to complete lumbosacral fixation [11, 16, 18, 19, 20, 21]. The entry point is 2 mm lateral to the midpoint between S1 and S2 dorsal foramen. The trajectory direction is approximately 20°-30° caudal angle in the sagittal plane and 30°-50° horizontal angle in the coronal plane connecting the posterior superior iliac spine (PSIS).
Previous anatomic and clinical studies have demonstrated that the freehand S2AI screw technique is as safe, accurate, and reliable as navigation and robotics. Park et al described a freehand S2AI screw technique in fresh-frozen human cadavers using pelvic anatomic landmarks . Eight screws were implanted with the direction of an approximately 20° caudal angle in the sagittal plane and 30° horizontal angle in the coronal plane connecting the PSIS. They reported an accuracy rate of 100% evaluated by fluoroscopy and naked eye examination. Their team had also reported a total of 45 S2AI screws in 23 patients, only five of which demonstrated a breach, with no visceral or neurovascular complications . Lombardi and colleagues preferred the freehand technique when spinopelvic fixation was required, which was thought to be a simple, safe, and effective method . Shillingford et al described the freehand S2AI screw technique in which the entry point is lateral to the midpoint of the S1-2 dorsal foramen, directed toward the AIIS by aiming to a point just cephalad to the posterior edge of the PSIS and perpendicular to the lateral sacral crest . The results showed that the average caudal angle was 24.2° ± 10.0° in the sagittal plane, and the mean horizontal angle was 39.3° ± 8.2° in the axial plane. The reported accuracy was 95% and only 5% of the screws were placed with cortical breaches. Their team then compared the accuracy of the freehand technique with that of the robot-guided insertion of S2AI screws, showing no difference in accuracy between the two methods (94.9% vs. 97.8%, p = 0.630) .
In our series, individualized protocols were performed to place S2AI screws. We excluded patients with tumors involving the bone of the dorsal foramina between S1 and S2 due to the compromised anatomic landmark of the entry point. The entry point was 2mm lateral to the midpoint of the dorsal foramen of S1-S2. The postoperative evaluation showed that only three (6.5%) screws were demonstrated with cortex breaches, and the accuracy rate of screw placement was 93.5%. There were no neurovascular and visceral injury complications related to S2AI screws during the operations, which was consistent with reports in the literature.
The advantages of S2AI screws for spinopelvic fixation make this technique more popular in recent decade. First, biomechanical studies have shown that S2AI screws have the same biomechanical strength as iliac screws and can be used as an alternative to iliac screws [11, 22–24]. Second, S2AI screw placement requires less dissection of the soft tissue. The rate of wound infection was significantly lower in patients with S2AI screws compared with those with iliac screws because the iliac screw technique requires dissection of the subcutaneous tissue off the lumbosacral fascia to the level of the PSIS [25–33]. In De la Garza Ramos’s meta-analysis, the infection rate in the iliac screw group was 25.4% compared with only 2.6% in the S2AI group . In our series, the wound infection was 8.7%, which was similar to the literature reports. Thirdly, the location of conventional iliac screws is not in line with proximal lumbar screws, requiring offset-connectors for the connection of rod-system and iliac screws. In contrast to iliac screws, S2AI screws are in line with the posterior rod-system, without requiring connectors or complex bends for the connection with proximal lumbar screws. Furthermore, due to the more extensive soft tissue dissection, iliac screw implantation causes more soft tissue damage than S2AI screws. Moreover, the deeper location of S2AI screws entry point and more extensive soft tissue covering than conventional iliac screws results in less risk of implant prominence, reducing associated complications.
However, there are also disadvantages to S2AI screw fixation. Some scholars believe that S2AI screw fixation has a higher rate of implant failure. Guler et al found a failure rate of 35% for S2AI screws and 12% for iliac screws (p > 0.05) in their retrospective study . All screw breakages were associated with the S2AI technique. Therefore, long-term follow-up results of S2AI screws need to be supported by large sample studies. There was no failure of internal fixation in our series during the follow-up. The reasons were as follows. First, no patients had a spinal deformity, and the balance between sagittal and coronal planes was not disturbed after the operation. Second, the follow-up time was not long enough to obtain extensive clinical data, which is also one of the shortcomings of the present study.
Current indications for spinopelvic fixation with S2AI screws mainly include high-grade spondylolisthesis, long-segment fusion constructs, flat back deformities, 3-column osteotomies, and correction of pelvic obliquity [2–4]. However, there is no consensus on the indications in the literature. It has been reported that S2AI screw fixation is also suitable for sacropelvic reconstruction after sacral tumor resection [2, 29, 35], but it is not widely used because lumbosacral spinal tumor is not common.
To our best knowledge, this study has the largest group of patients with lumbosacral tumors treated with the S2AI technique. Tumors in this region often require segmental resection or spondylectomy, which can cause spinal instability and require three-column reconstruction. In order to minimize surgical complications, S2AI screw fixation was selected as the preferred method. Therefore, we posit that the S2AI technique is suitable for spinopelvic reconstruction when no tumor is present in the bone between the S1-S2 dorsal foramina.