The Influence of Radiographic Parameter on the S2 Alar-Iliac Screw Virtual Trajectory in Degenerative Lumbar Scoliosis Patients: A Computed Tomography Study.

OBJECTIVE: S2 alar-iliac (S2AI) screw had been widely used in the pelvic fusion for degenerative lumbar scoliosis (DLS) patients. However, whether S2AI screw trajectory was influenced by sagittal profile in DLS patients had not been comprehensively investigated. The objective of this study was to evaluate the associations between the optimal S2 alar-iliac (S2AI) screw trajectory and sagittal spinopelvic parameters in DLS patients. METHODS: Computed tomography (CT) scans of pelvis were performed in 47 DLS patients for three-dimensional reconstruction of S2AI screw trajectory from September 2019 to November 2021. Five S2AI screw trajectory parameters were measured in CT reconstruction images, including: 1) angle in the transverse plane (Tsv angle); 2) angle in the sagittal plane (Sag angle); 3) maximal screw length; 4) screw width; and 5) skin distance. The lumbar Cobb angle, lumbar apical vertebral translation (AVT); global kyphosis (GK); thoracic kyphosis (TK); lumbar lordosis (LL); sagittal vertical axis (SVA); sacral slope (SS); pelvic tilt (PT); and pelvic incidence (PI) were measured in standing X-ray films of the whole spine and pelvis. RESULTS: Both Tsv angle and Sag angle had significant positive associations with SS (p < 0.05) but negative associations with both PT (p < 0.05) and LL (p < 0.05) in all cases. Patients with SS less than 15° had both smaller Tsv angle and Sag angle than those with SS equal to or more than 15° (p < 0.05). The decreased LL would lead to the backward rotation of the pelvis, resulting in a more cephalic and less divergent trajectory of S2AI screw in DLS patients. CONCLUSIONS: For DLS patients with lumbar kyphosis, spine surgeons should avoid both excessive Tsv and Sag angles for S2AI screw insertion, especially when using free-hand technique.

S2 alar-iliac screws are superior to traditional iliac screws for spinopelvic fixation in adult spinal deformity: a systematic review and meta-analysis.

PURPOSE: Spinopelvic fixation (SPF) using traditional iliac screws has provided biomechanical advantages compared to previous constructs, but common complications include screw prominence and wound complications. The newer S2 alar-iliac (S2AI) screw may provide a lower profile option with lower rates of complications and revisions for adult spinal deformity (ASD). The purpose of this study was to compare rates of complications and revision following SPF between S2AI and traditional iliac screws in patients with ASD. METHODS: A PRISMA-compliant systematic literature review was conducted using Cochrane, Embase, and PubMed. Included studies reported primary data on adult patients undergoing S2AI screw fixation or traditional IS fixation for ASD. Primary outcomes of interest were rates of revision and complications, which included screw failure (fracture and loosening), symptomatic screw prominence, wound complications (dehiscence and infection), and L5-S1 pseudarthrosis. RESULTS: Fifteen retrospective studies with a total of 1502 patients (iliac screws: 889 [59.2%]; S2AI screws: 613 [40.8%]) were included. Pooled analysis indicated that iliac screws had significantly higher odds of revision (17.1% vs 9.1%, OR = 2.45 [1.25-4.77]), symptomatic screw prominence (9.9% vs 2.2%, OR = 6.26 [2.75-14.27]), and wound complications (20.1% vs 4.4%, OR = 5.94 [1.55-22.79]). S2AI screws also led to a larger preoperative to postoperative decrease in pain (SMD = - 0.26, 95% CI = -0.50, - 0.011). CONCLUSION: The findings from this review demonstrate higher rates of revision, symptomatic screw prominence, and wound complications with traditional iliac screws. Current data supports the use of S2AI screws specifically for ASD. PROSPERO ID: CRD42022336515. LEVEL OF EVIDENCE: III.

A New Approach to the Treatment of Sacroiliac Joint Pain and First Patient-Reported Outcomes Using a Novel Arthrodesis Technique for Sacroiliac Joint Fusion.

Purpose: To report the development of a new sacroiliac joint (SIJ) arthrodesis system that can be used for isolated fusion of the SIJ and, unlike known implant systems, in combination with lumbar instrumentation or as an alternative to existing sacropelvic fixation (SPF) methods, and the patient-reported outcomes in two cases. Materials and Methods: After a comprehensive review of 207 pelvic computed tomography (CT) datasets, an implant body was designed. Its shape was modeled based on the SIJ recess. A screw anchored in the ilium secures the position of the implant and allows connection to lumbar instrumentation. Two patients with confirmed SIJ syndrome underwent surgery with the anatomically adapted implant. They were evaluated preoperatively, 6 months, and 12 months postoperatively. Visual Analogue Scale (VAS), Oswestry Disability Index (ODI), Million Visual Analogue Scale (MVAS), Roland Morris Score (RMS), reduction of SIJ/leg pain, and work status were assessed. Bony fusion of the SIJ was evaluated by radiographs and CT 12 months after the procedure. Results: Analysis of pelvic CT data revealed a wedge-shaped implant body in four different sizes. In the two patients, VAS decreased from 88 to 33 points, ODI improved from 67 to 35%, MVAS decreased from 80 to 36%, and RMS decreased from 18 to 9 points 12 months after surgery. SIJ pain reduction was 80% and 90%, respectively. Follow-up CT and radiographs showed solid bony integration. Conclusion: The implant used takes into account the unique anatomy of the SIJ and also meets the requirements of a true arthrodesis. Initial results in two patients are promising. Biomechanical and clinical studies will have to show whether the considerable theoretical advantages of the new implant system over existing SIJ implants - in particular the possibility of connection to a lumbar stabilization system - and SPFs can be put into practice.

[Feasibility analysis and nail planning ofS2 iliac crest screw placement in children].

OBJECTIVE: To evaluate the feasibility of S2 alar iliac screw insertion in Chinese children using computerized three-dimension reconstruction and simulated screw placement technique, and to optimize the measurement of screw parameters. METHODS: A total of 83 pelvic CT data of children who underwent pelvic CT scan December 2018 to December 2020 were retrospectively analyzed, excluding fractures, deformities, and tumors. There were 44 boys and 39 girls, with an average age of (10.66±3.52) years, and were divided into 4 groups based on age (group A:5 to 7 years old;group B:8 to 10 years old;group C:11-13 years old;group D:14 to 16 years old). The original CT data obtained were imported into Mimics software, and the bony structure of the pelvis was reconstructed, and the maximum and minimum cranial angles of the screws were simulated in the three-dimensional view with the placement of 6.5 mm diameter S2 alar iliac screws. Subsequently, the coronal angle, sagittal angle, transverse angle, total length of the screw, length of the screw in the sacrum, width of the iliac, and distance of the entry point from the skin were measured in 3-Matic software at the maximum and minimum head tilt angles, respectively. The differences among the screw parameters of S2 alar iliac screws in children of different ages and the differences between gender and side were compared and analyzed. RESULTS: In all 83 children, 6.5 mm diameter S2 iliac screws could be placed. There was no significant difference between the side of each screw placement parameter. The 5 to 7 years old children had a significantly smaller screw coronal angle than other age groups, but in the screw sagittal angle, the difference was more mixed. The 5 to 7 years old children could obtain a larger angle at the maximum head tilt angle of the screw, but at the minimum cranial angle, the larger angle was obtained in the age group of 11 to 13 years old. There were no significant differences among the age groups. The coronal angle and sagittal angle under maximum cephalic angle and minimum cranial angle of 5 to 7 years old male were (40.91±2.91)° and (51.85±3.75)° respectively, which were significantly greater than in female. The coronal angle under minimum cranial angle was significantly greater in girls aged 8-10 years old than in boys. For the remaining screw placement angle parameters, there were no significant differences between gender. The differences in the minimum iliac width, the screw length, and the length of the sacral screws showed an increasing trend with age in all age groups. The distance from the screw entry point to the skin in boys were significantly smaller than that of girls. The minimum width of the iliac in boys at 14 to 16 years of age were significantly wider than that in girls at the same stage. In contrast, in girls aged 5 to 7 years and 11 to 13 years, the screw length was significantly longer than that of boys at the same stage. CONCLUSION: The pelvis of children aged 5 to 16 years can safely accommodate the placement of 6.5 mm diameter S2 alar iliac screws, but the bony structures of the pelvis are developing and growing in children, precise assessment is needed to plan a reasonable screw trajectory and select the appropriate screw length.

A radiological analysis of pelvic fixation trajectories: patient series.

BACKGROUND: Three well-defined methods for pelvic fixation are used for biomechanical support in spine fusion constructs: iliac, recessed iliac, and S2-alar-iliac (S2AI) screws. The authors compared the maximum screw sizes that could be placed with these techniques by using image-guidance software and high-resolution computed tomography scans from 20 randomly selected patients. Six trajectories were plotted per side, beginning at recognized starting points (standard or recessed posterior superior iliac spine [PSIS] or S2AI screw) and ending at the anterior inferior iliac spine (AIIS) or supra-acetabular notch (SAN). OBSERVATIONS: The mean maximum screw length and width ranged from 80.0 ± 32.2 mm to 140.8 ± 22.6 mm and from 8.25 ± 1.2 mm to 13.0 ± 2.7 mm, respectively, depending on the trajectory. Statistically significant differences in length were found between the standard and recessed PSIS trajectories to the AIIS (p < 0.001) and between the standard PSIS-to-AIIS trajectory and the S2AI-to-AIIS (p = 0.007) or S2AI-to-SAN (p < 0.001) trajectories. The most successful trajectory was the PSIS to SAN (95%, 38/40). LESSONS: The traditional iliac screw trajectory enabled the longest and widest screw trajectories and highest rate of successful screw placement with the fewest theoretical breaches more reliably than recessed and S2AI trajectories. These findings may help surgeons plan for maximum screw purchase for pelvic fixation.

Assessment of two entry points for S2 alar-iliac screw in a Japanese cohort using three-dimensional computed tomography scan.

BACKGROUND CONTEXT: Lumbosacral fusion supplemented with sacropelvic fusion has recently been increasingly employed for correcting spinal deformity and is associated with lower incidence of pseudarthrosis and implant failure. To date, few studies have evaluated anatomical parameters and technical feasibility between different entry points for S2 alar-iliac screws. PURPOSE: To compare anatomical parameters and technical feasibility of two entry points for the S2 alar-iliac screw (S2AIS) in a Japanese cohort using three-dimensional (3D) computed tomography (CT). STUDY DESIGN/SETTING: Retrospective cohort study. PATIENT SAMPLE: Fifteen men and 15 women aged 50-79 years who underwent pelvic CT at our hospital in 2013. OUTCOME MEASURES: Screw length, lateral angulation, caudal angulation, angle range, distance from the entry point to the sacroiliac joint, distance from the S2AIS to the acetabular roof, distance from the S2AIS to the sciatic notch, and insertion difficulty. METHODS: We used 30 pelvic CT images (15 men and 15 women). We selected two entry points from previous studies: one was 1 mm distal and 1 mm lateral to the S1 dorsal foramen (A group) and the other was the midpoint between the S1 and S2 dorsal foramen (B group). We resliced the plane in which the pelvis was sectioned obliquely from these entry points to the anterior inferior iliac spine in the sagittal plane. We placed the shortest and longest virtual S2AISs bilaterally in this plane using a 4-mm margin. We measured screw length, lateral angulation, caudal angulation, angle range, distance from the entry point to the sacroiliac joint, distance from the S2AIS to the acetabular roof, distance from the S2AIS to the sciatic notch, and insertion difficulty. These measurements were compared between Groups A and B. RESULTS: In group A, the angle in the sagittal plane was significantly smaller and the distance from the entry point to the sciatic notch was significantly longer than in group B. Group B demonstrated a significantly longer screw length, longer distance from the entry point to the sacroiliac joint, and longer distance from the entry point to the acetabular roof than group A. The rate of insertion difficulty of S2AIS was much higher in group A. CONCLUSIONS: Insertion of S2AIS from the midpoint between the S1 and S2 dorsal foramen compared with the entry at distal and lateral to S1 foramen enables insertion of longer screws with low insertion difficulty.

A Computed Tomography-Based Assessment of the Anatomical Parameters Concerning S2-Alar Iliac Screw Insertion Using "Safe Trajectory Method" in Indian Population.

STUDY DESIGN: A retrospective computed tomography (CT)-based radiological analysis. PURPOSE: To obtain CT-based morphometric data for the S2 alar iliac (S2AI) screw in the Indian population presenting to School of Medical Sciences and Research, Greater Noida, we used the concept of "safe trajectory" by Pontes and his colleagues in a recent study. OVERVIEW OF LITERATURE: Although previous CT-based morphometric studies on the S2AI screw have been published for a variety of ethnic groups, morphometric data specifically for the Indian population are scarce. METHODS: We used the three-dimensional multiplanar reformatting software to conduct a retrospective CT analysis of 112 consecutive patients who met our exclusion criteria for various abdominal and pelvic pathologies. CT imaging planes were rotated between the S1 and S2 foramen until they matched the ideal S2AI screw trajectory, which was represented by the longest and widest iliac osseous channel observed in the axial CT section. Following the concept of a safe trajectory, S2AI screw morphometric parameters were measured on both sides of the pelvis using corresponding axial and sagittal CT images. RESULTS: In the sagittal and transverse planes on both sides of the pelvis, females had significantly higher screw trajectory angulation than males (p<0.001). On both sides of the pelvis, males had significantly greater iliac width, maximum screw trajectory length, and intrascrotal length than females (p<0.001). On both sides of the pelvis, the S2AI screw entry point in females was significantly deeper than in males from the skin margin (p<0.001). CONCLUSIONS: Based on our methodology, we discovered that the S2AI screw trajectory is significantly more caudal and lateral in females, the maximum screw length is sufficient for use in clinical practice regardless of gender, and that 8.5 mm or even larger screw diameters are feasible in the majority of the Indian population.

Is There a Difference in Screw Accuracy, Robot Time Per Screw, Robot Abandonment, and Radiation Exposure Between the Mazor X and the Renaissance? A Propensity-Matched Analysis of 1179 Robot-Assisted Screws.

STUDY DESIGN: Prospective single-cohort analysis. OBJECTIVES: To compare the outcomes/complications of 2 robotic systems for spine surgery. METHODS: Adult patients (≥18-years-old) who underwent robot-assisted spine surgery from 2016-2019 were assessed. A propensity score matching (PSM) algorithm was used to match Mazor X to Renaissance cases. Preoperative CT scan for planning and an intraoperative O-arm for screw evaluation were preformed. Outcomes included screw accuracy, robot time/screw, robot abandonment, and radiation. Screw accuracy was measured using Vitrea Core software by 2 orthopedic surgeons. Screw breach was measured according to the Gertzbein/Robbins classification. RESULTS: After PSA, a total of 65 patients (Renaissance: 22 vs. X: 43) were included. Patient/operative factors were similar between robot systems (P > .05). The pedicle screw accuracy was similar between robots (Renaissance: 1.1%% vs. X: 1.3%, P = .786); however, the S2AI screw breach rate was significantly lower for the X (Renaissance: 9.5% vs. X: 1.2%, P = .025). Robot time per screw was not statistically different (Renaissance: 4.6 minutes vs. X: 3.9 minutes, P = .246). The X was more reliable with an abandonment rate of 2.3% vs. Renaissance:22.7%, P = .007. Radiation exposure were not different between robot systems. Non-robot related complications including dural tear, loss of motor/sensory function, and blood transfusion were similar between robot systems. CONCLUSION: This is the first comparative analyses of screw accuracy, robot time/screw, robot abandonment, and radiation exposure between the Mazor X and Renaissance systems. There are substantial improvements in the X robot, particularly in the perioperative planning processes, which likely contribute to the X's superiority in S2AI screw accuracy by nearly 8-fold and robot reliability by nearly 10-fold.

Safe Zones for Spinopelvic Screws in Patients With Lumbosacral Transitional Vertebra.

STUDY DESIGN: Retrospective matched-pair analysis. OBJECTIVES: Lumbosacral transitional vertebrae (LSTV) have a reported prevalence of 4-36% in the population. The safe zones for screw placement for spinopelvic fusion in adult spinal deformity surgery for patients with LSTV have not been described in the literature. Our study aimed to assess the safety of S1-pedicle screw (S1PS), S2-alar screw (S2AS), S2-alar-iliac screw (S2AIS), and iliac screw (IS) placement in patients with LSTV. METHODS: Out of the 819 examined patients, 49 patients with LSTV were included in our retrospective analysis with a matched pair control group. We used the 3-dimensional planning tool mediCAD for screw placement of S1PS, S2AS, S2AIS, IS with different angles, length and diameters. RESULTS: We evaluated a total of 10 192 screw trajectories. No serious complications occurred due to the trajectories used for S1PS. LSTV increased the risk of vessel injury for S2AS trajectories (P = .001) but not for S2AIS (P = .526). Besides the presence of an LSTV, the screw trajectory had a major influence on the frequency of serious complications. CONCLUSIONS: Sacral anchoring of long spinal constructions using S1PS, S2AS, S2AIS and IS is also possible in the presence of LSTV. For S2AS the trajectory with 30° lateral and caudal angulation of 10° showed the least vascular injuries and the least sacro-iliac-joint violations in patients with LSTV. S2AIS trajectories with 40° lateral and 0° sagittal angulation reduced the risk of serious complications in our patients collective with LSTV.

Biomechanical investigation of S2 alar-iliac screw and S1 pedicle screw fixation in the treatment of Denis type II sacral fractures.

Although S2 alar-iliac screw technique has been widely used in spinal surgery, its applicability to pelvic fractures is largely unknown. This study aimed to evaluate the biomechanical stability of S2 alar-iliac screw and S1 pedicle screw fixation in the treatment of Denis II sacral fractures. Twenty-eight artificial pelvic fracture models were treated with unilateral lumbopelvic fixation, sacroiliac screw fixation, S2 alar-iliac screw and S1 pedicle screw fixation, and S2 alar-iliac screw and contralateral S1 pedicle screw fixation (Groups 1-4, respectively; N = 7 per group). Each model was cyclically tested under increasing axial compression. Optical motion-tracking was used to assess relative displacement and gap angle, and the number of failure cycles. Relative displacement was significantly smaller in Group 3 than in Groups 1 (p = 0.004) and 4 (p < 0.001) but not significantly different between Groups 3 and 2 (p = 0.290). The gap angle in Group 3 was significantly smaller than that in Group 1 (p = 0.009) on the sagittal plane but significantly larger than that in Group 4 (p = 0.006) on the horizontal plane. A number of failure cycles was significantly higher in Group 3 than in Groups 1 (p = 0.002) and 4 (p = 0.004) but not significantly different between Groups 3 and 2 (p = 0.910). From a biomechanical perspective, S2 alar-iliac screw and S1 pedicle screw fixation can provide good stability in the treatment of Denis II sacral fractures.

Feasibility analysis and nail planning ofS2 iliac crest screw placement in children / 中国骨伤

OBJECTIVE@#To evaluate the feasibility of S2 alar iliac screw insertion in Chinese children using computerized three-dimension reconstruction and simulated screw placement technique, and to optimize the measurement of screw parameters.@*METHODS@#A total of 83 pelvic CT data of children who underwent pelvic CT scan December 2018 to December 2020 were retrospectively analyzed, excluding fractures, deformities, and tumors. There were 44 boys and 39 girls, with an average age of (10.66±3.52) years, and were divided into 4 groups based on age (group A:5 to 7 years old;group B:8 to 10 years old;group C:11-13 years old;group D:14 to 16 years old). The original CT data obtained were imported into Mimics software, and the bony structure of the pelvis was reconstructed, and the maximum and minimum cranial angles of the screws were simulated in the three-dimensional view with the placement of 6.5 mm diameter S2 alar iliac screws. Subsequently, the coronal angle, sagittal angle, transverse angle, total length of the screw, length of the screw in the sacrum, width of the iliac, and distance of the entry point from the skin were measured in 3-Matic software at the maximum and minimum head tilt angles, respectively. The differences among the screw parameters of S2 alar iliac screws in children of different ages and the differences between gender and side were compared and analyzed.@*RESULTS@#In all 83 children, 6.5 mm diameter S2 iliac screws could be placed. There was no significant difference between the side of each screw placement parameter. The 5 to 7 years old children had a significantly smaller screw coronal angle than other age groups, but in the screw sagittal angle, the difference was more mixed. The 5 to 7 years old children could obtain a larger angle at the maximum head tilt angle of the screw, but at the minimum cranial angle, the larger angle was obtained in the age group of 11 to 13 years old. There were no significant differences among the age groups. The coronal angle and sagittal angle under maximum cephalic angle and minimum cranial angle of 5 to 7 years old male were (40.91±2.91)° and (51.85±3.75)° respectively, which were significantly greater than in female. The coronal angle under minimum cranial angle was significantly greater in girls aged 8-10 years old than in boys. For the remaining screw placement angle parameters, there were no significant differences between gender. The differences in the minimum iliac width, the screw length, and the length of the sacral screws showed an increasing trend with age in all age groups. The distance from the screw entry point to the skin in boys were significantly smaller than that of girls. The minimum width of the iliac in boys at 14 to 16 years of age were significantly wider than that in girls at the same stage. In contrast, in girls aged 5 to 7 years and 11 to 13 years, the screw length was significantly longer than that of boys at the same stage.@*CONCLUSION@#The pelvis of children aged 5 to 16 years can safely accommodate the placement of 6.5 mm diameter S2 alar iliac screws, but the bony structures of the pelvis are developing and growing in children, precise assessment is needed to plan a reasonable screw trajectory and select the appropriate screw length.

Metastatic renal cell carcinoma of the lumbar spine with long posterior instrumented fusion and repetitive dislodgement of the set screws of the S2 alar-iliac screw.

Background: S2 alar-iliac screws (S2AIS) are widely used to anchor the pelvis to a lumbar fusion. Here, we report a patient who experienced repetitive dislodgement of the set screws of the S2AIS following a posterior instrumented fusion. Case Description: A 68-year-old male presented with an L3 metastasis and L2-L3 subluxation attributed to renal cell cancer. Following an L2-L5 posterior decompression and T9-pelvic fusion utilizing bilateral S2AIS fixation, the set screws of the S2AIS repeatedly dislodged, requiring two additional operations. The final surgery required multiple anchors to the ilium and provided adequate fixation until the patient's expiration. Conclusion: The set screw fixing force was weaker than the fixing force of the S2AIS; multiple iliac anchors effectively salvaged this condition.

Sacropelvic Fixation for Adult Deformity Surgery Comparing Iliac Screw and Sacral 2 Alar-Iliac Screw Fixation: Systematic Review and Updated Meta-Analysis.

OBJECTIVE: Two commonly used techniques for spinopelvic fixation in adult deformity surgery are iliac screw (IS) and sacral 2 alar-iliac screw (S2AI) fixations. In this article, we systematically meta-analyzed the complications of sacropelvic fixation for adult deformity surgery comparing IS and S2AI. METHODS: The PubMed, Embase, Web of Science, and Cochrane clinical trial databases were systematically searched until March 29, 2023. The proportion of postoperative complications, including implant failure, revision, screw prominence, and wound complications after sacropelvic fixation, were pooled with a random-effects model. Subgroup analyses for the method of sacropelvic fixation were conducted. RESULTS: Ten studies with a total of 1,931 patients (IS, 925 patients; S2AI, 1,006 patients) were included. The pooled proportion of implant failure was not statistically different between the IS and S2AI groups (21.9% and 18.9%, respectively) (p = 0.59). However, revision was higher in the IS group (21.0%) than that in the S2AI group (8.5%) (p = 0.02). Additionally, screw prominence was higher in the IS group (9.6%) than that in the S2AI group (0.0%) (p < 0.01), and wound complication was also higher in the IS group (31.7%) than that in the S2AI group (3.9%) (p < 0.01). CONCLUSION: IS and S2AI fixations showed that both techniques had similar outcomes in terms of implant failure. However, S2AI was revealed to have better outcomes than IS in terms of revision, screw prominence, and wound complications.

Radiological Evaluation of Pelvic Morphology for S2 Alar-Iliac Screw Insertion in the Japanese Samples: A Retrospective Cohort Study.

Introduction: S2 alar-iliac screw (S2AIS) insertion for lumbosacral fixation is becoming a common procedure for deformity surgeries. However, studies that have reported the anatomy and morphometric features of the pelvis for S2AIS insertion in the Japanese samples are scarce. This study aimed to elucidate the morphometric features of the pelvis regarding S2AIS insertion in the Japanese samples. Methods: We used 60 computed tomography scans of the pelvis (30 men and 30 women). The entry point for the S2AIS was determined as 1-mm lateral and 1-mm distal to the S1 dorsal sacral foramen. We resliced the plane in which the pelvis was sectioned obliquely from this entry point to the anterior inferior iliac spine in the sagittal plane. We bilaterally placed the shortest and longest virtual S2AISs in this plane using a 4-mm margin. We analyzed the length, angle, and safety of the determined trajectory and compared these measurements according to sex and age. Results: The median longest and shortest screw lengths were 108.1 and 103.3 mm, respectively. The median longest and shortest distances from the entry point to the sacroiliac joint were 31.2 and 28.2 mm, respectively. The median smallest and largest lateral angulations were 40.7° and 47.3°, respectively. The median angle range was 4.2°. The median caudal angulation was -2.8°. The median shortest and longest distances from the S2AISs to the acetabular roof were 23.5 and 27.4 mm, respectively. The median distance from the S2AISs to the sciatic notch was 23.1 mm. Assuming the insertion of screw with a diameter of 8 mm, S2AIS insertion was difficult in 32 of 120 (27%) screws because the dorsal cortex of the sacrum was damaged. Conclusions: Screw length and lateral angulation were similar to those in previous studies. Insertion difficulty occurred in 27% of screws.

Dynamic change of pelvic incidence after long fusion to pelvis with S2-alar-iliac screw: a 2-year follow-up study.

INTRODUCTION: Pelvic incidence (PI) is a key morphological parameter that reflects the relation between the sacrum and iliac wings. It is well accepted that PI remains constant after reaching maturity. However, recent studies indicated that PI might be altered after lumbosacral fusion. Additionally, it remains uncertain on the long-term influence of long fusion to pelvis with S2-alar-iliac screw on PI in patients with adult spinal deformity (ASD). STUDY DESIGN: A retrospective study. OBJECTIVE: To investigate whether and how PI would change during the follow-up in ASD patients who underwent S2AI fixation and to identify factors associated with the change in PI. METHODS: We retrospectively reviewed all ASD patients who underwent spinal surgery using S2AI screws between November 2014 and January 2017 at our institution. Patients with minimum follow-up of two years were included. The following sagittal radiographic parameters were measured: PI, Lumbar lordosis (LL), pelvic tilt (PT), PI-LL, sagittal vertical axis (SVA) at pre-op, post-op and 2-year follow-up. According to the changes in PI at immediate post-operation, patients were classified into two groups; Group A: Changes of PI less than or equal 5° and Group B: Changes of PI greater than 5°. RESULTS: A total of 82 ASD patients (Group A: 32, Group B: 50; mean age of 53.5 ± 12.6 years) with a mean follow-up period of 30.2 ± 9.2 months were included in this study. At immediate post-operation, Group A showed no significant change in PI (45.7° ± 11.4° to 45.3° ± 11.2°, p = 0.749); while Group B had a significant decrease in PI (51.6° ± 14.5° to 40.9° ± 14.0°, p < 0.001). At the last follow-up, 48% patients (24/50) in Group B had a significant increase in PI (32.8° ± 6.4° to 45.8° ± 11.2°, p < 0.001). Intergroup analysis showed that ΔPI, post-op PI, post-op PT and age were significantly different between both groups. In addition, pre-op PI, post-op PI, post-op PT, post-op PI-LL were significantly correlated with ΔPI at last follow-up. Also, logistic regression analysis showed that post-op PI was the associated risk factor (OR = 0.865, p = 0.024) for PI-LL mismatch. CONCLUSION: Our study showed that PI decreased in more than half of ASD patients immediately after spinal surgery using S2AI screws. Approximately 48% of them were able to recover during the 2-year follow-up. Lower pre-op PI, post-op PI and PT were found to be strongly associated with the return of PI. Thus, these current findings indicated that patients with a high PI at pre-operation should not be over-corrected to avoid PI-LL mismatch postoperatively.

Clinical utility of minimally invasive posterior internal fixation within the pelvic ring using S2 alar iliac screws for unstable pelvic ring fracture.

INTRODUCTION: Posterior internal fixation for unstable pelvic ring fractures is often associated with complications, including pelvic hemorrhage and gluteal necrosis. Pelvic ring fixation using the S2 alar iliac screw (SAIS) without fixation of the lumbosacral vertebrae may have potential as a novel, minimally invasive technique for treating unstable pelvic ring fractures. The present study compared clinical outcomes in patients who underwent SAIS fixation within the pelvic ring with a historical control group of patients who underwent conventional trans-iliac plate fixation for the treatment of unstable pelvic ring fractures. MATERIALS AND METHODS: Thirty-two patients diagnosed with unstable pelvic fractures with sacral fracture or sacroiliac joint fracture dislocation were retrospectively evaluated. Eight consecutive patients underwent trans-iliac plate fixation from April 2012 to March 2015, and 24 consecutive patients underwent SAIS fixation from April 2015 to February 2020. Rates of soft tissue complications, intraoperative blood loss, and intraoperative blood transfusion volume were compared in these two groups. RESULTS: Mean intraoperative blood loss was significantly lower in patients who underwent SAIS fixation than in those who underwent trans-iliac plate fixation (141.0 ml vs 315.0 ml; P = 0.027), although there were no between-group differences in intraoperative blood transfusion volume (0.0 ml vs 140 ml; P = 0.105), incidence rate of soft tissue complications (4.2% vs 0%; P = 1.000), and operation time (88.5 min vs 93.0 min; P = 0.862). Bone healing was confirmed in all patients who underwent SAIS fixation without dislocation of the fracture site, whereas one patient who underwent trans-iliac plate fixation experienced a dislocation of the fracture site during follow-up (0% vs 12.5%; P = 0.250). CONCLUSIONS: SAIS fixation reduces intraoperative blood loss and ensures bone healing without major complications, including dislocation of the fracture site. SAIS fixation may therefore be an alternative, minimally invasive method of treating unstable pelvic fractures.

S2 alar-iliac screw loosening as a preventive factor for hip joint osteoarthritis after adult spinal deformity surgery: a case-control study.

PURPOSE: This study aimed to evaluate the progression of hip pathology and risk factors after ASD surgery. METHODS: This case-control study enrolled 123 patients (246 hips); seven hips underwent hip arthroplasty were excluded. We measured the center-edge (CE) angle, joint space width (JSW), and Kellgren-Lawrence (KL) grade. We defined a CE angle˂25° as developmental dysplasia of the hip (DDH). We evaluated S2 alar-iliac (AI) screw loosening at final follow-up. RESULTS: The annual decrease in the JSW was 0.31 mm up to 1 year, and 0.13 mm after 1 year (p = 0.001). KL grade progression occurred in 24 hips (10.0%; group P), while no progression occurred in 215 (90.0%; group N) hips. Nonparametric analysis between groups P and N revealed that significant differences were observed in sex, DDH, KL grade, ratio of S2AI screw fixation at baseline, and ratio of S2AI screw loosening at final follow-up. Multiple logistic regression analysis revealed that DDH (p = 0.018, odds ratio (OR) = 3.0, 95%CI = 1.2-7.3), baseline KL grade (p < 0.0001, OR = 37.7, 95%CI = 7.0-203.2), and S2AI screw fixation (p = 0.035, OR = 3.4, 95%CI = 1.1-10.4) were significant factors. We performed sub-analysis to elucidate the relationship between screw loosening and hip osteoarthritis in 131 hips that underwent S2AI screw fixation. Non-loosening of the S2AI screw was a significant factor for KL grade progression (p < 0.0001, OR = 8.9, 95%CI = 3.0-26.4). CONCLUSION: This study identified the prevalence and risk factors for the progression of hip osteoarthritis after ASD surgery. Physicians need to pay attention to the hip joint pathology after ASD surgery.

Freehand S2-Alar-Iliac Screw Placement Technique in Lumbosacral Spinal Tumors: A Preliminary Study.

OBJECTIVE: S2-alar-iliac (S2AI) screw technique is widely used in spinal surgery, but it is rarely seen in the field of spinal tumors. The aim of the study is to report the preliminary outcomes of the freehand S2AI screw fixation after lumbosaral tumor resection. METHODS: The records of patients with lumbosacral tumor who underwent S2AI screw fixation between November 2016 to November 2020 at our center were reviewed retrospectively. Outcome measures included operative time, blood loss, complications, accuracy of screws, screw breach, and overall survival. Mean ± standard deviation or range was used to present continuous variables. Kaplan-Meier curve was used to present postoperative survival. RESULTS: A total of 23 patients were identified in this study, including 12 males and 11 females, with an average age of 47.3 ± 14.5 (range,15-73). The mean operation time was 224.6 ± 54.1 (range, 155-370 min). The average estimated blood loss was 1560.9 ± 887.0 (600-4000 ml). A total of 46 S2AI screws were implanted by freehand technique. CT scans showed three (6.5%) screws had penetrated the iliac cortex, indicating 93.5% implantation accuracy rate. No complications of iatrogenic neurovascular or visceral structure were observed. The average follow-up time was 31.6 ± 15.3 months (range, 13-60 months). Two patients' postoperative plain radiography showed lucent zone around the screw. One patient underwent reoperation for wound delayed infection. At the latest follow-up, eight patients had tumor-free survival, 11 had survival with tumor, and four died of disease. CONCLUSION: The freehand S2AI screw technique is reproducible, safe, and reliable in the management of lumbosacral spinal tumors.

Intraoperative imaging and navigated spinopelvic instrumentation: S2-alar-iliac screws combined with tricortical S1 pedicle screw fixation.

PURPOSE: The present study aimed to assess the feasibility, safety and accuracy of navigated spinopelvic fixation with focus on S2-alar-iliac screws (S2AIS) and tricortical S1 pedicle screw implantation with the use of high-resolution three-dimensional intraoperative imaging and real-time spinal navigation. METHODS: Patients undergoing navigated intraoperative CT-based spinopelvic stabilization between January 2016 and September 2019 were included. Pelvic fixation was achieved by implantation of S2AIS or iliac screws (IS). S1 screws were implanted with the goal of achieving tricortical purchase. In all cases, instrumentation was performed with real-time spinal navigation and intraoperative screw positioning was assessed using intraoperative computed tomography (iCT), cone-beam CT (CBCT) and robotic cone-beam CT (rCBCT). Screw accuracy was evaluated based on radiographic criteria. To identify predictors of complications, univariate analysis was performed. RESULTS: Overall, 52 patients (85%) received S2AIS and nine patients (15%) received IS instrumentation. Intraoperative imaging and spinal navigation were performed with iCT in 34 patients, CBCT in 21 patients and rCBCT in six patients. A total number of 10/128 (7.8%) iliac screws underwent successful intraoperative correction due to misalignment. Tricortical purchase was successfully accomplished in 58/110 (53%) of the S1 screws with a clear learning curve in the course of time. S2AIS implantation was associated with significantly fewer surgical side infection-associated surgeries. CONCLUSIONS: Real-time navigation facilitated spinopelvic instrumentation with increasing accuracy of S2AIS and tricortical S1 screws. Intraoperative imaging by iCT, CBCT or rCBCT permitted screw assessment with the chance of direct navigated revision of misplaced iliac screws to avoid secondary screw revision surgery.

Comparative Study of S2-Alar-Iliac Screw Trajectories between Males and Females Using Three-Dimensional Computed Tomography Analysis: The True Lateral Angulation of the S2-Alar-Iliac Screw in the Axial Plane.

The S2 alar-iliac screw (S2AIS) is commonly used for long spinal fusion as a rigid distal foundation in spinal deformity surgeries, and it is also used in percutaneous sacropelvic fixation for providing an in-line connection to the proximal spinal constructs without using offset connectors. Although the pelvic shape is different between males and females, reports on S2AIS trajectories according to gender have been scarce in the literature. In this paper, S2AIS trajectories are compared between males and females using pelvic three-dimensional computed tomography (3D-CT) in a normal Japanese population. After resetting the caudal angulation in CT-imaging plane manipulation, the angulation of S2AIS was more lateral in the axial plane and more horizontal in the coronal plane in females. Mean distances from the midline to starting points of S2AIS tended to be shorter in females, whereas mean distances from the midline to the posterior superior iliac spine was significantly longer in females. We also found that there were positive correlations between the patients' height and the maximal lengths of S2AISs, and the patients' height and minimal areas of S2AIS pathways. Our results are useful not only for conventional open spinal surgery, but also for minimally invasive spine surgery.