Single position robot-assisted pedicle screw placement with S2-alar-iliac fixation in lateral decubitus: cadaveric feasibility study and early clinical experience.

OBJECTIVES: Single position lateral fusion with robotic assistance eliminates the need for surgical staging while harnessing the precision of robotic adjuncts. We expand on this technique by demonstrating the technical feasibility of placing bilateral pedicle screws with S2-alar-iliac (S2AI) fixation while in the lateral position. METHODS: A cadaveric study was performed using 12 human specimens. A retrospective clinical series was also performed for patients who had undergone robot-assisted placement of S2AI screws in lateral decubitus between June 2020 and June 2022. Case demographics, implant placement time, implant size, screw accuracy, and complications were recorded. Early postoperative radiographic outcomes were reported. RESULTS: In the cadaveric series, a total of 126 screws were placed with robotic assistance in 12 cadavers of which 24 screws were S2AI. There were four breaches from pedicle screws and none with S2AI screws for an overall accuracy rate of 96.8%. In the clinical series, four patients (all male, mean age 65.8 years) underwent single position lateral surgery with S2AI distal fixation. Mean BMI was 33.6 and mean follow-up was 20.5 months. Mean radiographic improvements were lumbar lordosis 12.3 ± 4.7°, sagittal vertical axis 1.5 ± 2.1 cm, pelvic tilt 8.5 ± 10.0°, and pelvic incidence-lumbar lordosis mismatch 12.3 ± 4.7°. A total of 42 screws were placed of which eight screws were S2AI. There were two breaches from pedicle screws and none from S2AI screws for an overall accuracy rate of 95.2%. No repositioning or salvage techniques were required for the S2AI screws. CONCLUSIONS: We demonstrate here the technical feasibility of single position robot-assisted placement of S2-alar-iliac screws in the lateral decubitus position for single position surgery.

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.

Innovative sacropelvic fixation using iliac screws and triangular titanium implants.

PURPOSE: Sacropelvic fixation is frequently used in combination with thoracolumbar instrumentation for the correction of severe spinal deformities. The purpose of this study was to explore the effects of the triangular titanium implants on the iliac screw fixation. Our hypothesis was that the use of triangular titanium implants can increase the stability of the iliac screw fixation. METHODS: Three T10-pelvis instrumented models were created: pedicle screws and rods in T10-S1, and bilateral iliac screws (IL); posterior fixation and bilateral iliac screws and triangular implants inserted bilaterally in a sacro-alar-iliac trajectory (IL-Tri-SAI); posterior fixation and bilateral iliac screws and two bilateral triangular titanium implants inserted in a lateral trajectory (IL-Tri-Lat). Outputs of these models, such as hardware stresses, were compared against a model with pedicle screws and rods in T10-S1 (PED). RESULTS: Sacropelvic fixation decreased the L5-S1 motion by 75-90%. The motion of the SIJ was reduced by 55-80% after iliac fixation; the addition of triangular titanium implants further reduced it. IL, IL-Tri-SAI and IL-Tri-Lat demonstrated lower S1 pedicle stresses with respect to PED. Triangular implants had a protective effect on the iliac screw stresses. CONCLUSION: Sacropelvic fixation decreased L5-S1 range of motion suggesting increased stability of the joint. The combination of triangular titanium implants and iliac screws reduced the residual flexibility of the sacroiliac joint, and resulted in a protective effect on the S1 pedicle screws and iliac screws themselves. Clinical studies may be performed to demonstrate applicability of these FEA results to patient outcomes.

Evaluation of iliac screw, S2 alar-iliac screw and laterally placed triangular titanium implants for sacropelvic fixation in combination with posterior lumbar instrumentation: a finite element study.

PURPOSE: This study aimed to implement laterally placed triangular titanium implants as a technique of sacropelvic fixation in long posterior lumbar instrumentation and to characterize the effects of iliac screws, S2 alar-iliac screws and of triangular implants on rod and S1 pedicle screw stresses. METHODS: Four female models of the lumbopelvic spine were created. For each of them, five finite element models replicating the following configurations were generated: intact, posterior fixation with pedicle screws to S1 (PED), with PED and iliac screws (IL), with PED and S2 alar-iliac (S2AI) screws, and with PED and bilateral triangular titanium implants (SI). Simulations were conducted in compression, flexion-extension, lateral bending and axial rotation. Rod stresses in the L5-S1 segment as well as in the S1 pedicle screws were compared. RESULTS: One anatomical model was not simulated due to dysmorphia of the sacroiliac joints. PED resulted in the highest implant stresses. Values up to 337 MPa in lateral bending were noted, which were more than double than the other configurations. When compared with IL, S2AI and SI resulted in lower stresses in both screws and rods (on average 33% and 41% for S2AI and 17% and 50% for SI). CONCLUSIONS: Implant stresses after S2AI and SI fixations were lower than those attributable to IL. Therefore, pedicle screws and rods may have a lower risk of mechanical failure when coupled with sacropelvic fixation via S2AI or triangular titanium implants, although the risk of clinical loosening remains an area of further investigation. These slides can be retrieved under Electronic Supplementary Material.

Spinal deformity in elderly patients: comparison of two distal termination sites of lumbar curve fusion.

PURPOSE: This report compares the clinical, radiographic, and functional outcomes of fusion from thoracolumbar region terminating at L5 or at sacrum and pelvis in elderly patients with spinal deformity. METHODS: Ninety-four elderly patients who underwent spinal deformity surgery at our institution were evaluated. Patients were divided into two groups. The group L included 43 patients who underwent fusion of lumbar curve from thoracolumbar region to L5. The group P consisted of 51 patients who were treated using fusion from lower thoracic region to S1 extending to the pelvis. Radiographic outcomes, health-related to the quality of life (HRQOL) parameters, and complications with a 3-year minimum follow-up were analyzed and compared between two groups. RESULTS: In the group L, the ideal sagittal balance was not achieved. The group P showed a better restoration of global spinal alignments compared with the group L. The HRQOL scores (VAS, ODI, SRS-24) of the patients after 3 years are slightly higher in the group P, but we did not get significant difference between groups. The total number of complications was higher in the group P. CONCLUSIONS: The research showed that fusion of lumbar curve extending to the pelvis provided good sagittal balance, global spinal alignments, and likely HRQOL parameters after 3-year follow-up. But, eventually, we obtained higher number of complications.

Modified iliac screw fixation: technique and clinical application.

BACKGROUND: A conventional iliac bolt and the S2 alar iliac screw fixation technique (S2AI) are commonly used sacropelvic fixation techniques. However, conventional iliac bolt technique requires a lateral connector and commonly has prominent screw head problems. S2AI reportedly has a high instrument failure rate. We aim to introduce a modified iliac screw fixation technique and to investigate its clinical application in adult patients. METHODS: The entrance site of the modified iliac screw fixation technique was 1 cm medial and 1 cm caudal from the posterosuperior iliac spine. From 2009 to 2015, ten adult patients underwent sacropelvic fixation with the modified iliac screw fixation technique in our spine clinic. A minimum 12-month clinical and radiographic follow-up was adopted. The mean follow-up period was 30.7 months (12-74 months). Mean number of fixation levels was 7.7 segments (5-10 segments). RESULTS: Postoperatively, the C7 plumb line (SVA) was significantly decreased (P = 0.04). Upon the last X-ray, SVA did not differ between postoperative and the last X-ray (P = 0.1). There was no breakage during our follow-up period. There was no prominent screw head. There were no cases requiring implant removal. CONCLUSIONS: The modified iliac screw fixation technique does not cause prominence in the sacral region, and does not require a lateral connector, both of which are necessary when using the classical iliac bolt technique. This technique also avoids the acute angle between the screw head and the shaft of the screw commonly seen in S2AI. The modified iliac screw fixation technique can be an effective alternative for sacropelvic fixation.

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.

Effect of Sacropelvic Hardware on Axis and Center of Rotation of the Sacroiliac Joint: A Finite Element Study.

BACKGROUND: The sacroiliac joint (SIJ) transfers the load of the upper body to the lower extremities while allowing a variable physiological movement among individuals. The axis of rotation (AoR) and center of rotation (CoR) of the SIJ can be evaluated to analyze the stability of the SIJ, including when the sacrum is fixed. The purpose of this study was to determine how load intensity affects the SIJ for the intact model and to characterize how sacropelvic fixation performed with different techniques affects this joint. METHODS: Five T10-pelvis models were used: (1) intact model; (2) pedicle screws and rods in T10-S1; (3)pedicle screws and rods in T10-S1, and bilateral S2 alar-iliac screws (S2AI); (4) pedicle screws and rods in T10-S1, bilateral S2AI screws, and triangular implants inserted bilaterally in a sacral alar-iliac trajectory ; and (5) pedicle screws and rods in T10-S1, bilateral S2AI screws, and 2 bilateral triangular implants inserted in a lateral trajectory. Outputs of these models under flexion-extension were compared: AoR and CoR of the SIJ at incremental steps from 0 to 7.5 Nm for the intact model and AoR and CoR of the SIJ for the instrumented models at 7.5 Nm. RESULTS: The intact model was validated against an in vivo study by comparing range of motion and displacement of the sacrum. Increasing the load intensity for the intact model led to an increase of the rotation of the sacrum but did not change the CoR. Comparison among the instrumented models showed that sacropelvic fixation techniques reduced the rotation of the sacrum and stabilized the SIJ, in particular with triangular implants. CONCLUSION: The study outcomes suggest that increasing load intensity increases the rotation of the sacrum but does not influence the CoR, and use of sacropelvic fixation increases the stability of the SIJ, especially when triangular implants are employed. CLINICAL RELEVANCE: The choice of the instrumentation strategy for sacropelvic fixation affects the stability of the construct in terms of both range of motion and axes of rotation, with direct consequences on the risk of failure and mobilization. Clinical studies should be performed to confirm these biomechanical findings.

Short-Segment versus Long-Segment Spinal Fusion Constructs for the Treatment of Adult Degenerative Scoliosis: A Comparison of Clinical Outcomes.

OBJECTIVE: To compare clinical outcomes of patients diagnosed with degenerative scoliosis undergoing short-segment versus long-segment spinal fusion. METHODS: A retrospective cohort study was conducted of patients with degenerative thoracolumbar scoliosis undergoing elective spinal fusion at a single academic medical center. Cohorts were divided into short-segment (<3) or long-segment (≥3) groups. RESULTS: A total of 197 patients (122 short, 75 long) were included. Patients undergoing short-segment fusion more frequently presented with radiculopathy (P < 0.001) and had greater baseline visual analog scale (VAS) leg scores (P < 0.001). Patients with long-segment fusions had longer hospital length of stay (short, 3.82 ± 2.98 vs. long, 7.40 ± 6.85 days; P < 0.001), lower home discharge rates (short, 80.3% vs. long, 51.8; P = 0.003), higher revision surgery rates (short, 10.77% vs. long, 25.3%; P = 0.012), and greater percentage curve correction (short, 37.3% ± 25.9% vs. long, 45.1% ± 23.9%; P = 0.048). No significant differences were noted in postoperative complication rates (short, 1.64% vs. long, 5.33%; P = 0.143). At 1 year, patients with long fusions had worse ΔOswestry Disability Index (ODI) (P = 0.024), ΔVAS leg score (P = 0.002), and VAS leg minimum clinically important difference % (P = 0.003). Multivariate regression found that short-segment fusions were associated with greater improvements in ODI (P = 0.029), Physical Component Summary-12 (P = 0.024), and VAS leg score at 1 year (P = 0.002). CONCLUSIONS: Patients undergoing short-segment fusions more frequently presented with radiculopathy and had higher preoperative VAS leg scores compared with those receiving long constructs. Short-construct fusions in appropriately selected patients may provide satisfactory improvements in patient-reported outcome measures, particularly ΔODI and ΔVAS leg score, and mitigate hospital length of stay, revision surgery rates, and nonhome discharge.

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.

Stability and Instrumentation Stresses Among Sacropelvic Fixation Techniques With Novel Porous Fusion/Fixation Implants: A Finite Element Study.

BACKGROUND: Sacropelvic fixation is frequently combined with thoracolumbar instrumentation for correcting spinal deformities. This study aimed to characterize sacropelvic fixation techniques using novel porous fusion/fixation implants (PFFI). METHODS: Three T10-pelvis finite element models were created: (1) pedicle screws and rods in T10-S1, PFFI bilaterally in S2 alar-iliac (S2AI) trajectory; (2) fixation in T10-S1, PFFI bilaterally in S2AI trajectory, triangular implants bilaterally above the PFFI in a sacro-alar-iliac trajectory (PFFI-IFSAI); and (3) fixation in T10-S1, PFFI bilaterally in S2AI trajectory, PFFI in sacro-alar-iliac trajectory stacked cephalad to those in S2AI position (2-PFFI). Models were loaded with pure moments of 7.5 Nm in flexion-extension, lateral bending, and axial rotation. Outputs were compared against 2 baseline models: (1) pedicle screws and rods in T10-S1 (PED), and (2) pedicle screws and rods in T10-S1, and S2AI screws. RESULTS: PFFI and S2AI resulted in similar L5-S1 motion; adding another PFFI per side (2-PFFI) further reduced this motion. Sacroiliac joint (SIJ) motion was also similar between PFFI and S2AI; PFFI-IFSAI and 2-PFFI demonstrated a further reduction in SIJ motion. Additionally, PFFI reduced max stresses on S1 pedicle screws and on implants in the S2AI position. CONCLUSION: The study shows that supplementing a long construct with PFFI increases the stability of the L5-S1 and SIJ and reduces stresses on the S1 pedicle screws and implants in the S2AI position. CLINICAL RELEVANCE: The findings suggest a reduced risk of pseudarthrosis at L5-S1 and screw breakage. Clinical studies may be performed to demonstrate applicability to patient outcomes. LEVEL OF EVIDENCE: Not applicable (basic science study).

Biomechanical evaluation of multiple pelvic screws and multirod construct for the augmentation of lumbosacral junction in long spinal fusion surgery.

Background: Posterior long spinal fusion was the common procedure for adult spinal deformity (ASD). Although the application of sacropelvic fixation (SPF), the incidence of pseudoarthrosis and implant failure is still high in long spinal fusion extending to lumbosacral junction (LSJ). To address these mechanical complications, advanced SPF technique by multiple pelvic screws or multirod construct has been recommended. This was the first study to compare the biomechanical performance of combining multiple pelvic screws and multirod construct to other advanced SPF constructs for the augmentation of LSJ in long spinal fusion surgery through finite element (FE) analysis. Methods: An intact lumbopelvic FE model based on computed tomography images of a healthy adult male volunteer was constructed and validated. The intact model was modified to develop five instrumented models, all of which had bilateral pedicle screw (PS) fixation from L1 to S1 with posterior lumbar interbody fusion and different SPF constructs, including No-SPF, bilateral single S2-alar-iliac (S2AI) screw and single rod (SS-SR), bilateral multiple S2AI screws and single rod (MS-SR), bilateral single S2AI screw and multiple rods (SS-MR), and bilateral multiple S2AI screws and multiple rods (MS-MR). The range of motion (ROM) and stress on instrumentation, cages, sacrum, and S1 superior endplate (SEP) in flexion (FL), extension (EX), lateral bending (LB), and axial rotation (AR) were compared among models. Results: Compared with intact model and No-SPF, the ROM of global lumbopelvis, LSJ, and sacroiliac joint (SIJ) was decreased in SS-SR, MS-SR, SS-MR, and MS-MR in all directions. Compared with SS-SR, the ROM of global lumbopelvis and LSJ of MS-SR, SS-MR, and MS-MR further decreased, while the ROM of SIJ was only decreased in MS-SR and MS-MR. The stress on instrumentation, cages, S1-SEP, and sacrum decreased in SS-SR, compared with no-SPF. Compared with SS-SR, the stress in EX and AR further decreased in SS-MR and MS-SR. The most significantly decreased ROM and stress were observed in MS-MR. Conclusion: Both multiple pelvic screws and multirod construct could increase the mechanical stability of LSJ and reduce stress on instrumentation, cages, S1-SEP, and sacrum. The MS-MR construct was the most adequate to reduce the risk of lumbosacral pseudarthrosis, implant failure, and sacrum fracture. This study may provide surgeons with important evidence for the application of MS-MR construct in the clinical settings.

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.

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.

Is L5/S1 interbody fusion necessary in long-segment surgery for adult degenerative scoliosis? A systematic review and meta-analysis.

OBJECTIVE: There is no consensus regarding the best surgical strategy at the lumbosacral junction (LSJ) in long constructs for adult spinal deformity (ASD). The use of interbody fusion (IF) has been advocated to increase fusion rates, with additional pelvic fixation (PF) typically recommended. The actual benefit of IF even when extending to the pelvis, however, has not been vigorously analyzed. The goal of this work was to better understand the role of IF, specifically with respect to arthrodesis, when extending long constructs to the ilium. METHODS: A systematic review of the PubMed and Cochrane databases was performed to identify the relevant studies in English, addressing the management of LSJ in long constructs (defined as ≥ 5 levels) in ASD. The search terms used were as follows: "Lumbosacral Junction," "Long Constructs," "Long Fusion to the Sacrum," "Sacropelvic Fixation," "Interbody Fusion," and "Iliac Screw." The authors excluded technical notes, case reports, literature reviews, and cadaveric studies; pediatric populations; pathologies different from ASD; studies not using conventional techniques; and studies focused only on alignment of different levels. RESULTS: The PRISMA protocol was used. The authors found 12 retrospective clinical studies with a total of 1216 patients who were sorted into 3 different categories: group 1, using PF or not (n = 6); group 2, using PF with or without IF (n = 5); and group 3, from 1 study comparing anterior lumbar interbody fusion versus transforaminal lumbar interbody fusion. Five studies in group 1 and 4 in group 2 had pseudarthrosis rate as primary outcome and were selected for a quantitative analysis. Forest plots were used to display the risk ratio, and funnel plots were used to look at the risk of publication bias. The summary risk ratios were 0.36 (0.23-0.57, p < 0.001) and 1.03 (0.54-1.96, p = 0.94) for the PF and IF, respectively; there is a protective effect of overall pseudarthrosis for using PF in long constructs for ASD surgeries, but not for using IF. CONCLUSIONS: The long-held contention that L5/S1 IF is always advantageous in long-construct deformity surgery is not supported by the current literature. Based on the findings from this systematic review and meta-analysis, PF with or without additional L5/S1 interbody grafting demonstrates similar overall construct pseudarthrosis rates. The added risk and costs associated with IF, therefore, should be more closely considered on a case-by-case basis.

Treatment of Unstable Posterior Pelvic Ring Injury with S2-Alar-Iliac Screw and S1 Pedicle Screw Fixation.

OBJECTIVE: The S2-alar-iliac (S2AI) screws have been described as an alternative method for lumbosacropelvic fixation in place of iliac screws. However, the clinical effect of the short-segment S2AI screw fixation technique in the treatment of unstable posterior pelvic ring injuries remains unclear. In this study, we report the preliminary clinical results of the internal fixation connecting a S2-alar-iliac screw and a S1 pedicle screw (i.e., S2AI-S1 fixation) in the treatment of unstable pelvic posterior ring injuries. METHODS: Twenty-five patients with unstable posterior pelvic ring injury were treated with S2AI-S1 fixation from February 2019 to June 2020. The incision length, surgical time, blood loss, frequency of intraoperative fluoroscopy, quality of reduction, complications, and functional outcome were analyzed. RESULTS: A total of 29 groups of S2AI-S1 fixation were used in 25 patients. The mean incision length was 8.3 (6.2 - 10.3) cm, mean operative time was 86.4 (60 - 142) minutes, mean frequency of intraoperative fluoroscopy was 7.9 (4 - 12) times, and mean blood loss was 148 (50 - 500) mL. The mean postoperative follow-up time was 17.8 (10 - 26) months. The satisfaction rate of pelvic reduction quality was 25/29, and the satisfactory rate of functional outcome was 23/25. There were no obvious signs of screw prominence, screw loosening, or implant failure. CONCLUSIONS: The case series presented in this study show the successful use of S2AI-S1 fixation to treat unstable posterior pelvic ring injuries. The S2AI-S1 fixation, not including the lumbar spine in the fixation range, is a simple, safe, and effective fixation method.

Ball Tip Technique for S2AI Screw Placement in Sacropelvic Fixation: A Comparative Study with Conventional Freehand Technique.

OBJECTIVE: To evaluate the efficiency of the ball tip technique for S2AI screw placement and introduce this technique. METHODS: Sixty-three patients who underwent pelvic fixation with S2AI screws were retrospectively reviewed. They were 29 males and 34 females with an average age of 59.6 ± 12.5 years. Among these patients, 35 patients (14 males and 21 females with an average age of 58.8 ± 11.3 years) received ball tip technique and 28 patients (15 males and 13 females with an average age of 63.7 ± 12.6 years) received conventional freehand technique. Ball tip technique was used in ball tip technique group. After a pedicle probe just penetrated the sacroiliac joint, a ball-tipped probe consisting of a ball shaped metal tip with a flexible shaft was malleted to make a guide track within ilium. This ball-tipped probe could bend automatically away from the cortex and forward through the cancellous bone when the tip met the cortical lamina of ilium, which can avoid penetration. After repeating the procedures, a guide hole was gradually formed. S2AI screw was inserted along the guide hole after tapping. In the conventional freehand group, S2AI screw was placed according to the conventional method. Postoperative computed tomography (CT) was used to assess the accuracy of screws. The time cost of screw insertion and screw-related complications were recorded. Independent t-test was used to compare the time cost between ball tip group and conventional freehand group. A chi-square test was used to compare the accuracies of the ball tip group with the conventional group. RESULTS: There were 35 patients (70 S2AI screws) in ball tip group and 28 patients (56 S2AI screws) in conventional freehand group. No screw-related complication occurred in all patients. Time costs were 9.8 ± 4.5 mins in ball tip group and 20.2.0 + 8.6 mins in conventional freehand group, respectively (P < 0.05). Four screws penetrated iliac cortex in the ball tip group vs 10 screws in conventional freehand group (5.7% vs 17.9%) (P < 0.05). CONCLUSIONS: The ball tip technique enhances the accuracy of screw placement and has less time cost compared with conventional freehand technique.

Biomechanics of a laterally placed sacroiliac joint fusion device supplemental to S2 alar-iliac fixation in a long-segment adult spinal deformity construct: a cadaveric study of stability and strain distribution.

OBJECTIVE: S2 alar-iliac (S2AI) screw fixation effectively enhances stability in long-segment constructs. Although S2AI fixation provides a single transarticular sacroiliac joint fixation (SIJF) point, additional fixation points may provide greater stability and attenuate screw and rod strain. The objectives of this study were to evaluate changes in stability and pedicle screw and rod strain with extended distal S2AI fixation and with supplemental bilateral integration of two sacroiliac joint fusion devices implanted using a traditional minimally invasive surgical approach. METHODS: Eight L1-pelvis human cadaveric specimens underwent pure moment (7.5 Nm) and compression (400 N) tests under 4 conditions: 1) intact (pure moment loading only); 2) L2-S1 pedicle screw and rod with L5-S1 interbody fusion; 3) added S2AI screws; and 4) added bilateral laterally placed SIJF. Range of motion (ROM), rod strain, and screw-bending moment (S1 and S2AI) were analyzed. RESULTS: Compared with S1 fixation, S2AI fixation significantly reduced L5-S1 ROM in right lateral bending by 50% (0.11°, p = 0.049) and in compression by 39% (0.22°, p = 0.003). Compared with fixation ending at S1, extending fixation with S2AI significantly decreased sacroiliac joint ROM by 52% (0.28°, p = 0.02) in flexion, by 65% (0.48°, p = 0.04) in extension, by 59% (0.76°, p = 0.02) in combined flexion-extension, and by 36% (0.09°, p = 0.02) in left axial rotation. The addition of S2AI screws reduced S1 screw-bending moment during flexion (0.106 Nm [43%], p = 0.046). With S2AI fixation, posterior L5-S1 primary rod strain increased by 124% (159 µE, p = 0.002) in flexion, by 149% (285 µE, p = 0.02) in left axial rotation, and by 99% (254 µE, p = 0.04) in right axial rotation. Compared with S2AI fixation, the addition of SIJF reduced L5-S1 strain during right axial rotation by 6% (28 µE, p = 0.04) and increased L5-S1 strain in extension by 6% (28 µE, p = 0.02). CONCLUSIONS: Long-segment constructs ending with S2AI screws created a more stable construct than those ending with S1 screws, reducing lumbosacral and sacroiliac joint motion and S1 screw-bending moment in flexion. These benefits, however, were paired with increased rod strain at the lumbosacral junction. The addition of SIJF to constructs ending at S2AI did not significantly change SI joint ROM or S1 screw bending and reduced S2AI screw bending in compression. SIJF further decreased L5-S1 rod strain in axial rotation and increased it in extension.

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.

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.