Preoperative supine pelvic incidence predicts standing pelvic incidence following S2AI fixation in patients with adult spinal deformity: a prospective study.

STUDY DESIGN: A prospective study. OBJECTIVE: The aim of this study was to investigate the PI change in different postures and before and after S2­alar­iliac (S2AI) screw fixation, and to investigate whether pre-op supine PI could predict post-op standing PI. Previous studies have reported PI may change with various positions. Some authors postulated that the unexpected PI change in ASD patients could be due to sacroiliac joint laxity, S2-alar-iliac (S2AI) screw placement, or aggressive sagittal cantilever technique. However, there was a lack of investigation on how to predict post-op standing PI when making surgical strategy. METHODS: A prospective case series of ASD patients undergoing surgical correction with S2AI screw placement was conducted. Full-spine X-ray films were obtained at pre-op standing, pre-op supine, pre-op prone, as well as post-op standing postures. Pelvic parameters were measured. Spearman correlation analysis was used to determine relationships between each parameter. RESULTS: A total of 83 patients (22 males, 61females) with a mean age of 58.4 ± 9.5 years were included in this study. Pre-op standing PI was significantly lower than post-op standing PI (p = 0.004). Pre-op prone PI was significantly lower than post-op standing PI (p = 0.001). By contrast, no significant difference was observed between pre-op supine and post-op standing PI (p = 0.359) with a mean absolute difference of 2.2° ± 1.9°. Correlation analysis showed supine PI was significantly correlated with post-op standing PI (r = 0.951, p < 0.001). CONCLUSION: This study revealed the PI changed after S2AI screw fixation. The pre-op supine PI can predict post-op standing PI precisely, which facilitates to provide correction surgery strategy with a good reference for ideal sagittal alignment postoperatively.

Connecting the S2 alar-iliac screw head to the satellite rod for surgical correction of degenerative sagittal imbalance.

PURPOSE: The S2AI screw technique has several advantages over the conventional iliac screw fixation technique. However, connecting the S2AI screw head to the main rod is difficult due to its medial entry point. We introduce a new technique for connecting the S2AI screw head to a satellite rod and compare it with the conventional method of connecting the S2AI screw to the main rod. METHODS: Seventy-four patients who underwent S2AI fixation for degenerative sagittal imbalance and were followed up for ≥ 2 years were included. All the patients underwent long fusion from T9 or T10 to the pelvis. The S2AI screw head was connected to the satellite rod (SS group) in 43 patients and the main rod (SM group) in 31 patients. In the SS group, the satellite rod was placed medial to the main rod and connected by the S2AI screw and domino connectors. In the SM group, the main rod was connected directly to the S2AI screw head and supported by accessory rods. Radiographic and clinical outcomes were evaluated in both groups. RESULTS: There were no significant differences in postoperative complications, including proximal junctional failure, proximal junctional kyphosis, rod breakage, screw loosening, wound problems, and infection between the two groups. Furthermore, the correction power of sagittal deformity and clinical results in the SS group were comparable to those in the SM group. CONCLUSION: Connecting the S2AI screw to the satellite rod is a convenient method comparable to the conventional S2AI connection method in terms of radiological and clinical outcomes.

Anatomic trajectory for iliac screw placement adapts better to the morphological features of the pelvis of each individual than the S2 alar iliac screw: a radiological study.

PURPOSE: The iliac fixation (IF) through the S2 ala permits the minimization of implant prominence and tissue dissection. An alternative to this technique is the anatomic iliac screw fixation (AI), which considers the perpendicular axis to the narrowest width of the ileum and the width of the screw. The morphological accuracy of the iliac screw insertion of two low profile iliac fixation (IF) techniques is investigated in this study. METHODS: Twenty-nine patients operated on via low profile IF technique were divided into two groups, those treated using 28 screws with the starting point at S2, and those treated with 30 AI entry point. Radiological parameters (Tsv-angle, Sag-Angle, Max-length, sacral-distance, iliac-width, S2-midline, skin-distance, iliac-wing, and PSIS distance) and clinical outcomes (early and clinic complications) were evaluated by two blinded expert radiologists, and the results were compared in both groups with the real trajectory of the screws placed. RESULTS: Differences between ideal and real trajectories were observed in 6 of the 9 evaluated parameters in the S2AI group. In the AI group, these trajectories were similar, except for TSV-Angle, Max-length, Iliac-width, and distance to iliac-wing parameters. Moreover, compared with S2AI, AI provided better adaptation to the pelvic morphology in all parameters, except for sagittal plane angulation, skin distance, and iliac width. CONCLUSIONS: AI ensures the advantages of low profile pelvic fixation like S2AI, with a starting point in line with S1 pedicle anchors and low implant prominence, and moreover adapts better to the morphological features of the pelvis of each individual.

Digital anatomical study and clinical application of the ideal S2 alar-lliac screw trajectory.

BACKGROUND: To investigate the ideal trajectory for the S2AI screw and to clinically validate its safety feasibility. METHODS: The 3D model was reconstructed from CT data of the pelvis of 30 selected adults, and the 3D coordinate system was established with the first sacral superior endplate as the horizontal plane. A set of cutting planes was made at 3 mm intervals in the coronal plane, and the cross-sectional internal tangent circles were divided in the target area. Using the linear fitting function, the axis of 90 mm length was calculated by the least squares method for each inner tangent circle center. The diameter of the axis is gradually increased until the first contact with the cortex, and the cylindrical model is the ideal screw trajectory. The intersection of the axis and the dorsal cortex is the screw placement point, which is located by Horizon Distance (HD) and Vertical Distance (VD); the diameter of the screw trajectory (d) is the diameter of the cylindrical model; the direction of the screw trajectory is determined by Sagittal Angle (SA) and Transverse Angle (TA). The screw trajectory orientation is determined by Sagittal Angle (SA) and Transverse Angle (TA). Based on the ideal screw trajectory, the 3D printed surgical guide and freehand techniques were used to verify its safety feasibility, respectively. RESULTS: The screw placement points [HD (4.7 ± 1.0) mm, VD (19.7 ± 1.9) mm], screw placement directions [SA (31.3°±2.3°), TA (42.4°±2.3°)], and screw dimensions for the ideal screw trajectory of the S2AI were combined for analysis. (L is 90 mm, d is 13.2 ± 1.4 mm). The S2AI screw superiority rate [96.6% (56/58)] and reasonable rate [100%] were higher in the guide group than in the freehand group [90.0% (63/70), 97.1% (68/70)], but the differences were not statistically significant (P > 0.05). Although screws invaded the cortex in both groups, there were no associated adverse events in either group. CONCLUSION: The S2AI screw-based ideal trajectory placement is a safe, feasible and accurate method of screw placement.

Radio-anatomical study of anterior-posterior placement sacroiliac screw channel.

PURPOSE: The outlet of the classic sacroiliac screw (SIS) cannot be precisely controlled by aiming devices, which may lead to malpositioned implants and neurovascular and visceral injury. This study aimed to radio-anatomically measure the parameters of the channel for anterior-posterior placement SIS (AP-SIS), which can be placed percutaneously with an aiming device. METHODS: Pelvic CT scan data of 80 healthy adults (40 males and 40 females) with an average age of 45 years (range 20-70 years) were collected. The length (L), width (W), height (H), cortical bone spacing (M), camber angle (E), anteversion angle (F), cross-sectional safety angle (P) and sagittal safety angle (Q) of the channel were measured by CT or Mimics software. RESULTS: The L, W, H, M, E, F, P and Q measures of S1 were 109.2 ± 8.0 mm, 18.5 ± 1.9 mm, 21.7 ± 1.7 mm, 8.1 ± 0.4 mm, 44.2 ± 3.2°, 42.4 ± 3.6°, 16.8 ± 1.1°, and 19.4 ± 2.0°, respectively, for S1, and 113.5 ± 9.4 mm, 18.2 ± 1.5 mm, 21.7 ± 1.7 mm, 7.7 ± 0.4 mm, 44.7 ± 3.2°, 31.2 ± 2.7°, 13.8 ± 1.0° and 15.4 ± 1.4°, respectively, for S2. Of the L measures, the intra-iliac segment was slightly longer than the intra-sacral segment. All parameters showed significant sex-related differences (p < 0.05). CONCLUSION: The AP-SIS channels of S1-2 have sufficient width and length to accommodate a cancellous screw with a Φ 7.0-8.0 mm and a length 90-130 mm. The intra-iliac segment is a long channel screw with better mechanical properties over classic SIS.

Risk factors for distal junctional failure in long-construct instrumentation for adult spinal deformity.

PURPOSE: The aim of this study is to identify risk factors associated with postoperative DJF in long constructs for ASD. METHODS: A retrospective review was performed at a tertiary referral spine centre from 01/01/2007 to 31/12/2016. Demographic, clinical and radiographic parameters were collated for patients with DJF in the postoperative period and compared to those without DJF. Survival analyses were performed using univariate logistic regression to identify variables with a p value < 0.05 for inclusion in multivariate analysis. Spearman's correlations were performed where applicable. RESULTS: One hundred two patients were identified. 41 (40.2%) suffered DJF in the postoperative period, with rod fracture being the most common sign of DJF (13/65; 20.0%). Mean time to failure was 32.4 months. On univariate analysis, pedicle subtraction osteotomy (p = 0.03), transforaminal lumbar interbody fusion (p < 0.001), pre-op LL (p < 0.01), pre-op SVA (p < 0.01), pre-op SS (p = 0.02), postop LL (p = 0.03), postop SVA (p = 0.01), postop PI/LL (p < 0.001), LL correction (p < 0.001), SVA correction (p < 0.001), PT correction (p = 0.03), PI/LL correction (p < 0.001), SS correction (p = 0.03) all proved significant. On multivariate analysis, pedicle subtraction osteotomy (OR 27.3; p = 0.03), postop SVA (p < 0.01) and LL correction (p = 0.02) remained statistically significant as independent risk factors for DJF. CONCLUSION: Recently, DJF has received recognition as its own entity due to a notable postoperative incidence. Few studies to date have evaluated risk factors for DJF. The results of our study highlight that pedicle subtraction osteotomy, poor correction of lumbar lordosis, and sagittal vertical axis are significantly associated with postoperative occurrence of DJF.

Does the thoracolumbar kyphosis secondary to ankylosing spondylitis affect the iliac trajectory of S2AI screw?

BACKGROUND: The study aimed to evaluate the influence of thoracolumbar kyphosis secondary to ankylosing spondylitis (AS) on parameters of S2AI trajectory and to compare the ideal S2AI trajectory with those of the non-deformity patients with AS, sagittal deformity patients without AS, and the normal population reported in literatures. METHODS: Sagittal parameters including global kyphosis (GK), pelvic tilt (PT) and sacral slope (SS) were measured. Besides, according to the simulated ideal S2AI trajectory on the CT images, trajectory parameters were measured including Sag angle, Tsv angle, Max-length, Sacral distance and Iliac width. Starting-point parameters were also measured including PSIS distance, Skin distance, Iliac wing and S2 midline. RESULTS: Ninety-four AS-related thoracolumbar kyphosis patients were included. After adjusting the age and gender, twenty non-deformity patients with AS and 20 sagittal deformity patients without AS were selected to compare with patients with AS-related thoracolumbar kyphosis, respectively. Sag angle in deformity patients with AS was smaller than other two groups (P < 0.001). No difference was found in Tsv angle and Sacral distance between AS patients with and without deformity. However, these two parameters were shown significant differences between deformity patients with AS and without AS. In deformity patients with AS, no significant differences were found in all parameters between genders Furthermore, there were strong correlations between PT and the bilateral Sag angle (P < 0.001). CONCLUSIONS: The thoracolumbar kyphosis secondary to AS affects the Sag angle of the ideal S2AI trajectory which was approximately 20° smaller than that in non-deformity patients with AS, sagittal deformity patients without AS, and the normal population. Additionally, the Tsv angle and the Sacral distance in AS patients with thoracolumbar kyphosis were about 10° and 10 mm larger than those in sagittal deformity patients without AS, and the normal population reported in literatures.

Revision by S2-alar-iliac instrumentation reduces caudal screw loosening while improving sacroiliac joint pain-a group comparison study.

Lumbosacral instrumentation continues to be challenging due to complex biomechanical force distributions and poor sacral bone quality. Various techniques have therefore been established. The aim of this study was to investigate the outcome of patients treated with S2-alar-iliac (S2AI), S2-alar (S2A), and iliac (I) instrumentation as the most caudal level. Sixty patients underwent one of the 3 techniques between January 2012 and June 2017 (S2AI 18 patients, S2A 20 patients, I 22 patients). Mean age was 70.4 ± 8.5 years. Screw loosening (SL) and sacroiliac joint (SIJ) pain were evaluated during the course at 3-month and maximum follow-up (FU). All patients completed 3-month FU, the mean FU period was 2.5 ± 1.5 years (p = 0.38), and a median of 5 segments was operated on (p = 0.26), respectively. Bone mineral density (BMD), derived opportunistically from computed tomography (CT), did not significantly differ between the groups (p = 0.66), but cages were more frequently implanted in patients of the S2A group (p = 0.04). SL of sacral or iliac screws was more common in patients of the S2A and I groups compared with the S2AI group (S2AI 16.7%, S2A 55.0%, I 27.3% of patients; p = 0.03). SIJ pain was more often improved in the S2AI group not only after 3 months but also at maximum FU (S2AI 61.1%, S2A 25.0%, I 22.7% of patients showing improvement; p = 0.02). Even in shorter or mid-length lumbar or thoracolumbar constructs, S2AI might be considered superior to S2A and I instrumentation due to showing lower incidences of caudal SL and SIJ pain.

Second sacral sacralalar-iliac (S2AI) screw placement in adult degenerative scoliosis (ADS) patients: an imaging study.

BACKGROUND: The imaging characteristics of sacral sacralalar-iliac (S2AI) screw trajectory in adult degenerative scoliosis (ADS) patients will be determined. METHODS: S2AI screw trajectories were mapped on three-dimensional computed tomography (3DCT) reconstructions of 40 ADS patients. The starting point, placement plane, screw template, and a circle centered at the lowest point of the ilium inner cortex were set on these images. A tangent line from the starting point to the outer diameter of the circle was selected as the axis of the screw trajectory. The related parameters in different populations were analyzed and compared. RESULTS: The trajectory length of S2AI screws in ADS patients was 12.00 ± 0.99 cm, the lateral angle was 41.24 ± 3.92°, the caudal angle was 27.73 ± 6.45°, the distance from the axis of the screw trajectory to the iliosciatic notch was 1.05 ± 0.81 cm, the distance from the axis of the screw trajectory to the upper edge of the acetabulum was 1.85 ± 0.33 cm, and the iliac width was 2.12 ± 1.65 cm. Compared with females, the lateral angle of male ADS patients was decreased, but the trajectory length was increased (P < 0.05). Compared to patients without ADS in previous studies, the lateral angle of male patients was larger, the lateral angle of female patients was increased, and the caudal angle was decreased (P < 0.05). CONCLUSIONS: There is an ideal trajectory of S2AI screws in ADS patients. A different direction should be noticed in the placement of S2AI screws, especially in female patients.

Comparison of the 3D-printed operation guide template technique and the free-hand technique for S2-alar-iliac screw placement.

BACKGROUND: To compare the safety and accuracy of the 3D-printed operation guide template technique and the free-hand technique in the placement of the S2-alar-iliac (S2AI) screw. METHODS: We conducted a retrospective analysis of 47 patients undergoing S2AI screw placement in our hospital, divided into the 3D-printed operation guide template group and the free-hand screw placement group. The duration of single S2AI screw placement was documented in all patients. A postoperative CT scan was performed to assess screw placement effectiveness according to the distance from the screw tip to the breach of the cortical bone wall. RESULTS: In total, 42 screws were placed in the guide template group, with an average screw placement duration of 151.6 ± 44.8 s. Screw placement grading was as follows: 40 screws in grade 0, two in grade 1, and none in grades 2 and 3. This grading resulted in excellent and good rates of 95.2% and 100%, respectively. In total, 52 screws were placed in the free-hand group, with an average screw placement duration of 138.3 ± 45.9 s. Screw placement grading was as follows: 42 screws in grade 0, seven in grade 1, three in grade 2, and none in grade 3. This grading resulted in excellent, good and acceptable rates 80.8%, 94.2% and 100%, respectively. Screw placement duration did not significantly differ between the groups (p > 0.05). The excellent rate of screw placement was greater in the guide template group than in the free-hand group (p < 0.05), but the good and acceptable rates did not significantly differ between the two groups (p > 0.05). CONCLUSION: Both techniques can be applied to S2AI screw placement. The 3D-printed guide technique is superior to the free-hand technique in terms of safety and accuracy.

[Free-hand technique of S(2) alar iliac screw placement for sacropelvic fusion in lumbar degenerative scoliosis].

Objective: To evaluate the feasibility of placement of S(2) alar iliac screw (S(2)AI) using free-hand technique for sacrapelvic fusion in lumbar degenerative scoliosis. Methods: Eighteen patients with Lumbar Degenerative Scoliosis treated by S(2)AI screw fixation at Department of Orthopedics, General Hospital of Southern Theater Command of People's Liberation Army and Department of Orthopedics, 89th hospital of People's Liberation Army from August 2014 to October 2018 were analyzed retrospectively. There were 5 males and 13 females, aged 63.2 years old (range:55 to 71 years old).Parameters of spine including: Cobb Angle, C(7) plumb line -center sacral vertical line (C(7)PL-CSVL), lumbar lordosis(LL), sagittal vertical axis(SVA), pelvic incidence(PI), pelvic tilt(PT), sacral slope (SS) and pI-LL were measured on the whole spine X-ray before operation and at final follow-up. Pelvic CT scan was performed postoperatively to assess the accuracy of S(2)AI placement. Oswestry disability Index (ODI) was also recorded. The data were compared by paired t test or Wilcoxon tests. Results: All patients were followed up for 23.7 months (range: 12~62 months).At the last follow up, Cobb Angle decreased from (32.28±4.97) °preoperative to (6.56±3.20) ° (t=41.142, P<0.01) and C(7)PL-CSVL deceased from (1.11±2.07) cm preoperative to (0.18±1.08) cm (t=41.142, P=0.06) .LL improved from (-22.39±13.07) °preoperative to (-36.39±4.29) ° (t=4.470, P<0.01) , PI-LL decreased from (26.83±14.83)°preoperative to (13.72±8.3)° (t=4.396, P<0.01) , PT decreased from (27.94±4.26) °to (23.39±6.08) ° (t=2.680, P=0.02) , and SS increased from (22.22±6.36) °to (26.28±7.24) ° (t=-2.178, P=0.04) .SVA decreased from (6.54±4.51) cm preoperative to (2.62±1.29) cm (t=3.052, P=0.01) .ODI decreased from 0.58(0.40) (M(Q(R))) to 0.18 (0.15) (Z=-4.567, P<0.01) .No complications such as nerve and blood vessel injury occurred during the operation. A total of 32 S(2)AI screws were placed, 3 screws were placed with mild to moderate cortical breaches, 2 were perforated the pelvis ventrally, 1 was perforated posteriorly, with no clinically notable neurovascular or visceral complications. Eight patients finished the SRS-22 questionnaire, with mean score of 4.4 in terms of satisfaction with management. Conclusions: Free-hand technique of S(2)AI screw placement for sacrapelvic fusion in degenerative lumbar scoliosis is safe and feasible.S(2)AI fixation in DLS can provide great correction of deformity, maintain the stability of lumbo-pelvic area and improve the clinical symptoms.

Comparative tomographic study of the S2-alar-iliac screw versus the iliac screw.

AIMS: Iliac screws and S2-alar-iliac screws provide adequate mechanical stability for the fixation of lumbosacral spine pathologies, which has led to a significant increase in the use of these techniques in the routine practice of spine surgeons. However, studies on the ideal technical positioning for both techniques are limited. STUDY DESIGN: This is an observational, retrospective, analytical descriptive study. OBJECTIVE: To analyze, describe and compare the insertion and positioning parameters of the S2-alar-iliac and iliac screw techniques in adult patients without spinal deformities. METHODS: The present study comprises a retrospective analysis of lumbosacral computed tomography images selected continuously in 2016 from 25 patients at a university hospital. Mann-Whitney-Shapiro-Wilk tests were performed. Data reliability was assessed using intraclass correlation. RESULTS: The mean length of the iliac screw was greater than that of the S2-alar-iliac screw, and the S2-alar-iliac screw sat 20.5 mm deeper than the iliac screw. The mean of the greatest bone thickness for the iliac screw was 20.72 mm; that of the S2-alar-iliac screw was 23.24 mm. The mean distance from the iliac screw entry point to the skin was 32.46 mm, and the mean distance from the S2-alar-iliac screw entry point to the skin was 52.87 mm. CONCLUSION: The trajectory of the S2-alar-iliac screws studied via computed tomography was greater in terms of bone thickness and deeper relative to the skin compared with the iliac screws. The S2-alar-iliac technique may have desirable clinical advantages in terms of the diameter of the screws and reduced protrusion when used in adults. These slides can be retrieved from Electronic supplementary material.

S2-AI screw placement with the aide of electronic conductivity device monitoring: a retrospective analysis.

STUDY DESIGN: A retrospective analysis of two consecutive patients who underwent a novel surgical technique. OBJECTIVE: A report of a novel surgical technique utilizing an electronic conductivity device guidance to aide placement of S2-Alar-Iliac (S2-AI) instrumentation. Electronic conductivity guidance for instrumentation of the thoracolumbar spine is an accepted means of improving intraoperative accuracy. Although commercially available for percutaneous techniques, there is a paucity of literature regarding its use. Percutaneous implantation of S2-AI screws has been previously described as another technique surgeons can avail, primarily employing fluoroscopy as a means of intraoperative feedback. We describe a novel technique that utilizes electronic conductivity as an added feedback measure to increase accuracy of percutaneous S2-AI fixation. METHODS: Two patients were treated by the senior author (FAS) who underwent surgery employing S2-AI fixation utilizing an electronic conductivity device (Pediguard cannulated probe, Spineguard, Paris, France). The surgical technique, case illustrations, and radiographic outcomes are discussed. RESULTS: Stable and accurate fixation was attained in both patients. There were no peri-operative complications related to hardware placement. CONCLUSION: To the authors' knowledge, this is the first reported literature combining S2-AI screws with electronic conductivity for immediate intraoperative feedback. This technique has the opportunity to provide surgeons with increased accuracy for placement of S2-AI screws while improving overall radiation safety. This feedback can be particularly helpful when surgeons are learning new techniques such as placement of S2AI screws.

S-1 and S-2-alar-iliac screw fixation via intraoperative navigation.

Adult deformity patients often require fixation to the sacrum and pelvis for construct stability and improved fusion rates. Although certain sacropelvic fixation techniques can be challenging, the availability of intraoperative navigation has made many of these techniques more feasible. In this video case presentation, the authors demonstrate the techniques of S-1 bicortical screw and S-2-alar-iliac screw fixation under intraoperative navigation in a 67-year-old female. This instrumentation placement was part of an overall T-10-pelvis construct for the correction of adult spinal deformity. The video can be found here: https://youtu.be/3HZo-80jQr8 .

Pelvic Fixation Construct Trends in Spinal Deformity Surgery.

Purpose: Although many techniques exist, spinopelvic fixation continues to present challenges in the management of adult spinal deformity. Shear forces, complex anatomy, and bone quality are common reasons why spine surgeons continue to explore options for fixation. Methods: A retrospective chart reviewed of patients receiving pelvic fixation for adult spinal deformity over a 12-year period was conducted. Patients were divided into 3 cohorts based on date of surgery: (1) 2010 to 2013, (2) 2014 to 2017, and (3) 2018 to 2021. Pelvic fixation constructs in the study included traditional iliac screws, stacked S2-alar-iliac (S2AI screws), and triangular titanium implants. Results: Of the 494 patients with multiple implant constructs who met the inclusion criteria for this study, patients undergoing pelvic fixation surgery who received at least 2 implants increased by approximately 5% every 4 years (90.2%, 94.6%, 99.1% respectively). Over the 12-year span, the implementation of the S2AI screw grew 120%. Conclusion: At our institution, there is a trend toward using multiple bilateral implant constructs for pelvic fixation, with nearly a tenfold percentage increase between the most recent cohorts. These include iliac screws with S2AI screws, multiple stacked S2AI screws, and S2AI screws used in conjunction with triangular titanium implants in hopes to decrease implant failure.

3D CT modeling demonstrates the anatomic feasibility of S1AI screw trajectory for spinopelvic fixation in neuromuscular scoliosis.

PURPOSE: In patients with neuromuscular scoliosis undergoing posterior spinal fusion, the S2 alar iliac (S2AI) screw trajectory is a safe and effective method of lumbopelvic fixation but can lead to implant prominence. Here we use 3D CT modeling to demonstrate the anatomic feasibility of the S1 alar iliac screw (S1AI) compared to the S2AI trajectory in patients with neuromuscular scoliosis. METHODS: This retrospective study used CT scans of 14 patients with spinal deformity to create 3D spinal reconstructions and model the insertional anatomy, max length, screw diameter, and potential for implant prominence between 28 S2AI and 28 S1AI screw trajectories. RESULTS: Patients had a mean age of 14.42 (range 8-21), coronal cobb angle of 85° (range 54-141), and pelvic obliquity of 28° (range 4-51). The maximum length and diameter of both screw trajectories were similar. S1AI screws were, on average, 6.3 ± 5 mm less prominent than S2AI screws relative to the iliac crests. S2AI screws were feasible in all patients, while in two patients, posterior elements of the lumbar spine would interfere with S1AI screw insertion. CONCLUSION: In this cohort of patients with neuromuscular scoliosis, we demonstrate that the S1AI trajectory offers comparable screw length and diameter to an S2AI screw with less implant prominence. An S1AI screw, however, may not be feasible in some patients due to interference from the posterior elements of the lumbar spine.

Optimization of S2-alar-iliac screw (S2AI) fixation in adult spine deformity using a comprehensive genetic algorithm and finite element model personalized to patient geometry and bone mechanical properties.

PURPOSE: To optimize the biomechanical performance of S2AI screw fixation using a genetic algorithm (GA) and patient-specific finite element analysis integrating bone mechanical properties. METHODS: Patient-specific pelvic finite element models (FEM), including one normal and one osteoporotic model, were created from bi-planar multi-energy X-rays (BMEXs). The genetic algorithm (GA) optimized screw parameters based on bone mass quality (BM method) while a comparative optimization method maximized the screw corridor radius (GEO method). Biomechanical performance was evaluated through simulations, comparing both methods using pullout and toggle tests. RESULTS: The optimal screw trajectory using the BM method was more lateral and caudal with insertion angles ranging from 49° to 66° (sagittal plane) and 29° to 35° (transverse plane). In comparison, the GEO method had ranges of 44° to 54° and 24° to 30° respectively. Pullout forces (PF) using the BM method ranged from 5 to 18.4 kN, which were 2.4 times higher than the GEO method (2.1-7.7 kN). Toggle loading generated failure forces between 0.8 and 10.1 kN (BM method) and 0.9-2.9 kN (GEO method). The bone mass surrounding the screw representing the fitness score and PF of the osteoporotic case were correlated (R2 > 0.8). CONCLUSION: Our study proposed a patient-specific FEM to optimize the S2AI screw size and trajectory using a robust BM approach with GA. This approach considers surgical constraints and consistently improves fixation performance.

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.

Direct Iliac Screw vs Sacral-2-Alar-Iliac Screws Technique for Sacropelvic Fixation: Technical Nuances and a Review of the Literature.

Sacropelvic (SP) fixation is the immobilization of the sacroiliac joint to attain lumbosacral fusion and prevent distal spinal junctional failure. SP fixation is indicated in numerous spinal conditions (eg, scoliosis, multilevel spondylolisthesis, spinal/sacral trauma, tumors, or infections). Many SP fixation techniques have been described in the literature. Currently, the most used surgical techniques for SP fixation are direct iliac screws and sacral-2-alar-iliac screws. There is currently no consensus in the literature on which technique carries more favorable clinical outcomes. In this review, we aim to assess the available data on each technique and discuss their respective advantages and disadvantages. We will also present our experience with a modification of direct iliac screws using a subcrestal approach and outline the future prospects of SP fixation.

How common is acute pelvic fixation failure after adult spine surgery? A single-center study of 358 patients.

OBJECTIVE: There is a paucity of literature on pelvic fixation failure after adult spine surgery in the early postoperative period. The purpose of this study was to determine the incidence of acute pelvic fixation failure in a large single-center study and to describe the lessons learned. METHODS: The authors performed a retrospective review of adult (≥ 18 years old) patients who underwent spinal fusion with pelvic fixation (iliac, S2-alar-iliac [S2AI] screws) at a single academic medical center between 2015 and 2020. All patients had a minimum of 3 instrumented levels. The minimum follow-up was 6 months after the index spine surgery. Patients with prior pelvic fixation were excluded. Acute pelvic fixation failure was defined as revision of the pelvic screws within 6 months of the primary surgery. Patient demographics and operative, radiographic, and rod/screw parameters were collected. All rods were cobalt-chrome. All iliac and S2AI screws were closed-headed screws. RESULTS: In 358 patients, the mean age was 59.5 ± 13.6 years, and 64.0% (n = 229) were female. The mean number of instrumented levels was 11.5 ± 5.5, and 79.1% (n = 283) had ≥ 6 levels fused. Three-column osteotomies were performed in 14.2% (n = 51) of patients, and 74.6% (n = 267) had an L5-S1 interbody fusion. The mean diameter/length of pelvic screws was 8.5/86.6 mm. The mean number of pelvic screws was 2.2 ± 0.5, the mean rod diameter was 6.0 ± 0 mm, and 78.5% (n = 281) had > 2 rods crossing the lumbopelvic junction. Accessory rods extended to S1 (32.7%, n = 117) or S2/ilium (45.8%, n = 164). Acute pelvic fixation failure occurred in 1 patient (0.3%); this individual had a broken S2AI screw near the head-neck junction. This 76-year-old woman with degenerative lumbar scoliosis and chronic lumbosacral zone 1 fracture nonunion had undergone posterior instrumented fusion from T10 to pelvis with bilateral S2AI screws (8.5 × 90 mm); i.e., transforaminal lumbar interbody fusion L4-S1. The patient had persistent left buttock pain postoperatively, with radiographically confirmed breakage of the left S2AI screw 68 days after surgery. Revision included instrumentation removal at L2-pelvis and a total of 4 pelvic screws. CONCLUSIONS: The acute pelvic fixation failure rate was exceedingly low in adult spine surgery. This rate may be the result of multiple factors including the preference for multirod (> 2), closed-headed pelvic screw constructs in which large-diameter long screws are used. Increasing the number of rods and screws at the lumbopelvic junction may be important factors to consider, especially for patients with high risk for nonunion.