The three-dimensional printed template guided technique for S2AI screw placement and a comparison with freehand technique

Background Sacropelvic fixation continues to present challenges when involved in the adult spinal deformity correction. The S2 alar iliac (S2AI) fixation is commonly used in sacropelvic fixation. Several techniques, including intraoperative navigation and freehand technique, were used for S2AI screws placement. The aim of this study is to analyze the anatomic parameters for S2AI screw trajectory in Asian population and introduce a novel technique described as three-dimensional printed template guided technique (TGT). Meanwhile, the accuracy and safety of this technique were compared with conventional freehand technique. Methods The S2AI trajectory parameters were measured in100 Asian adult volunteers. Parameters were compared between different genders. Forty-eight adult patients who underwent S2AI screw placement were reviewed, 28 patients received freehand technique and 20 patients received TGT technique. Postoperative CT was used to assess the accuracy of screw trajectory and cortex violation related complications were recorded. Results The cephalocaudal angles (CA), maximal length of screw pathway (ML), narrowest width of pathway within the iliar teardrop (NW), distance from the center of teardrop to sciatic notch(SD) and distance of the start point distal to S1 dorsal foramen(DD) showed significant gender-related difference (p<0.05). All 48 patients were placed S2AI screws bilaterally (40 screws in TGT vs 56 screws in freehand). One screw penetrated iliac cortex in TGT group but 10 screws penetrated iliac cortex in freehand group (3% vs 17.9%) (p<0.05). Conclusion Approximately 30-35° of cephalocaudal angle and 39°mediolateral angle are appropriate for S2AI screw placement in Asian patients. Either freehand or TGT technique is safe for S2AI screw placement. TGT technique is more accurate compared with conventional freehand technique. Trial registration This is a retrospective study. Key words Sacropelvic fixation, Second sacral alar iliac (S2AI) screw, three-dimensional printed template,


Background
Spinopelvic fixation plays an integral role in achieving solid osseous fusion across the lumbosacral junction, especially in deformity procedures requiring substantial correction or long-segment constructs. [1,2]There are several techniques for spinopelvic fixation that include Galveston technique [3], iliac screws (iliac bolts), iliosacral screws, and S2 alar iliac screws (S2AI) technique [1,4] Compared with other pelvic fixation techniques, S2AI screws has been developed to be a prevailing alternative spinopelvic fixation technique due to the advantages of keeping screw tail away from the skin, no incision overexposure, more excellent pull-out strength and standing on the same longitudinal line of lumbar pedicle screws [5,6]. However, S2AI technique is still challenging to less experienced spine surgeons who have no navigation system available. Inadequate accuracy of screw implantation is potential to result in surgery-related complications [7].
The incidental cortical breach of an S2AI screw may not only result in decreased fixation strength but also injury to the major vessels, particularly the internal iliac artery and the superior gluteal artery when breaching anteriorly and caudally. Injuries of these major vessels could lead to major intrapelvic hemorrhage, which is a life-threatening complication [8]. Therefore, surgeons should avoid anterior or caudal deviation during screw insertion to achieve safe insertion of S2AI screws. The anatomic parameters in American population have been previously analyzed [9], but in Asian population remains unclear. The aim of this study is to analyze the anatomic parameters for S2AI screw placement in Asian population and introduce a novel technique depends on these parameters described as three-dimensional printed template guided technique (TGT). The accuracy and safety of this technique compared with conventional freehand technique were also evaluated.

Methods
From Oct 2014 to March 2015, 100 Chinese adult volunteers were recruited to measure the S2AI trajectory. They were 56 males and 44 females with a mean age of 51.6±7.1 years old. We defined the optimal trajectory of the screw to be determined by the line connected the start point and the center of cross section of teardrop above sciatic notch.
The start point 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. Then we used UG software to mimic the optimal trajectory and measured the following parameters were described as following ( Figure 1). All the parameters were compared between different genders.

3.
Maximal length of screw pathway (ML), defined as maximal length of the screw trajectory in pelvis.

4.
Narrowest width of screw pathway (NW), defined as the narrowest canal width along the pathway.

5.
Distance from the center of cross section of teardrop above sciatic notch to sciatic notch (sciatic notch distance, SD).

6.
Distance of the start point to midline (midline distance MD), defined as the distance of the start point away from the midline.

7.
Distance of the start point inferior to S1 dorsal foramen (dorsal foramen distance, DD Surgical procedure Screw placement began with meticulous subperiosteal dissection of the posterior elements and exposed anatomic landmarks including the S2AI start point. In freehand group, the start point 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, and the following procedures were described by Kebaish and Sponseller previously [9][10][11]. In TGT group, the parameters of the optimal trajectory mentioned above were analyzed by the post-processing software and individualized 3D template was designed and printed( Figure   2). In the surgical procedure, the start point is as same as the abovementioned. All soft tissues were thoroughly dissected and dorsal foramen of S1 and S2 were clearly exposed, the printed 3D guide template was closely matched to the sacral dorsal bone surface. It is crucial to closely match the guide template with the bone surface, which could guide the 2.5mm K-wire to be drilled forward along the optimal trajectory. Otherwise, the TGT technique should be given up if the template could not closely match the bone surface.
After checking the position of the K-wire, tapped the track along K-wire, then removed Kwire and placed the screw along the track( Figure 3).

Postoperative accuracy evaluation of screw placement
Fine-cut (width of 1mm) CT is used to assess the accuracy of screw placement after surgery. We define that the screw contained between the lateral and medial iliac cortex as being accurately placed. By contrast, screw that violated or broke medial or lateral cortex, -or the sciatic notch was defined as being inaccurately placed. The screw breach were divided into 4 grades according to the previous description by Oh CH [12]. No screw breach was defined as grade 0; The screw breach distance away from the cortex less than 3mm was defined as grade 1 (mild), 3-6mm as grade 2 (moderate), more than 6mm as grade 3 (severe). The screws breaches are considered unqualified screws placement. All measurements were assessed bilaterally.

Statistical analysis
Data are presented as the mean ± standard deviation. For the optimal trajectory, the  Table 2). There is no cortex violation related complications occurred in all patients. One screw were placed with penetrated iliac cortex in TGT group (3%, 1/39) but ten screws were placed with penetrated iliac cortex in freehand group (17.9%, 10/56).
There is a significant different between two groups. (P < 0.05) ( Table 3) (Fig. 4).   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 [8]. 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 [15]. But a study from Shillingford showed there is no significant differences in the CA or MA between females and males [2]. 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 [16]. 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 [18]. 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 [19]. 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 [15], 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 [20], 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].

Consent for publication
Written informed consent was acquired from each of volunteers and patients to authorize treatment, imageology findings, and photographic documentation. The patients consented to the publication of their pictures as well as their anonymous and clustered data.

Availability of data and materials
The datasets analyzed during the current study are not publicly available because a showed that the start point of S2AI 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. MD is defined as the distance of the start point away from the midline and DD is defined as the distance of the start point inferior to S1