Achieving pelvic fusion across the lumbosacral junction with S2AI screw can be challenging for less-experienced surgeons in management of spinal deformity. Compared to other sacropelvic fixation techniques, S2AI technique has several theoretical merits including lower rate of implant failure and less surgical revisions. Additionally, the S2AI screw technique precludes the need for cross-connectors[9, 4], which can significantly reduce the incidence of screw loosening. With lower screw prominence and deeper subcutaneous locations, S2AI screws are covered by full-thickness skin and subcutaneous tissue, which can significantly reduce the incidence of local skin ulceration and deep infection[5, 6]. Furthermore, the direction and the length of the S2AI screw sacropelvic fixation provides more reliable stability compared to traditional iliac screw fixation[13, 14]. However, in spite of emerging clinical evidence demonstrating the advantages of S2AI screws, the accuracy of S2AI screw placement remains concerns. The accuracy of S2AI screw placement depends on pelvic anatomic landmarks and trajectory.
Two start point were recommended for S2AI screw placement. One is the midpoint between the S1 dorsal foramen and the S2 dorsal foramen where they meet the lateral sacral crest. The other is 1 mm inferior and 1 mm lateral from the S1 dorsal foramen. Two start points have different safety margins. However, in most cases, the difference between the two start points was considered negligibly in terms of safe screw insertion. In this study, the point which was located lateral to the midpoint between S1 and S2 dorsal foramina and standing on the extending line from L5 and S1 pedicle screw anchor points was adopted as a start point and the optimal trajectory of the screw to be determined by the line connected the start point and the canal center of teardrop. The result showed that the CA of optimal trajectory is on average of 29.21°±6.52° in males and 35.21°±6.86° in females. CA presented significant differences between males and females. This result is consistent with a previous study from Zhu et al, a radiographic study assessing optimal S2AI screw placement and presented that CA in females have 4 to 5 degrees more caudal trajectory compared with males. But a study from Shillingford showed there is no significant differences in the CA or MA between females and males. This study showed there is no difference in MA between males and females with an average 40° (39.75°±2.40° vs 40.38 ± 3.58°). This result is also consistent with previously studies. Based on this, the author recommended that S2AI screws of females should be placed 5 degree more caudally than males.
Whether the S2AI screws can penetrate the ideal iliac plane smoothly mainly depends on the iliac width, which is described as the narrowest width of pathway within the iliar teardrop (NW). Previous studies defined the standard S2AI screws ranges from 70 to 100 mm in length and 5.0 to 7.5 mm in diameter[17, 10]. A study from Wang showed that the iliac canal width ranged from 17.4 to 32.4 mm in males and 13.5 to 20.3 mm in females in Chinese population. They suggested that that screws ranging from 5.0 to 7.5 mm in diameter can be appropriate. This study showed that the NW was 16.60 ± 2.51 mm in males and 13.86 ± 2.65 mm in females. Although in females, the canal showed narrower with an average difference of about 3 mm the frequently-used screws in the clinical could go through the iliac canal without difficulty.
In the present study, the ML, SD and DD showed significant sex-related difference. These parameters depend on the morphology of the pelvis, varied from 50 to 75 mm in practice . In our study, the ML were 110.3 ± 7.22 mm in males and 102.64 ± 14.28 mm in females. The average max-length of trajectory in females was approximately 8 mm shorter than that in males. Compared with previous study from Zhu et al, ML is different from their investigation in which the average max-length of trajectory in females was approximately 5 mm shorter than that in males. In clinical practice, the optional S2AI trajectory exceeds more the length of usually-used screw. Although O’Brien et al consider that 65 mm length S2AI screw were equivalent to 80 mm length S2AI screw in respect of providing biomechanical purchase , using longer screw was much more in surgery may consider the long term stability of sacropelvic fixation particularly in long range fusion. And if the stress of longer screw fixation can be distributed as far anteriorly and laterally to the spine as possible, more stability could be achieved[14, 13].
Based on the optimal S2AI screw trajectory, we used software to design 3D printed template to guide S2AI screw placement individually to avoid the screw penetration which might arise screw-related complications. Compared with freehand group, in which ten screws penetrated iliac cortex, only one screw penetrated iliac cortex in TGT group. The accuracy rate of the freehand and TGT group are 82.1% and 97.5%, respectively. The TGT technique is quite accurate. Additionally, other merits can also be obtained via TGT technique. There is no need to probe to identify the integration of the track canal repeatedly, which could save surgery time. Second, CA and MA more rely on the surgeon's subjective estimation during surgery because of without objective measurement tool when placing S2AI screw in freehand technique, and there still has mal-positioned screws even under repeated fluoroscopy. So, TGT technique could surely decrease radiation exposure. Finally, the TGT technique is easier to study for less-experienced surgeons who are not familiar with the complexomorphic pelvis, and easier to place the screw even for experienced surgeons when facing anatomically abnormal pelvis which is not rare in congenital deformity.
There are some limitations in this study. The parameters of optimal trajectory analyzed in our study were based on Chinese population. It is crucial to closely match the guide template with the bone surface, otherwise, the technique has to be abandoned because of the potential wrong direction guided by the template. The material for printing template and the precision of printing machine would influence the quality of template. Another limitation is that practitioner need extra-time to design the individual template if plan to perform TGT technique.