S2 Alar Iliac Research Articles

Adjacent segment disease on hip joint as a complication of spinal fusion surgery including sacroiliac joint fixation.

Recently, the number of adult spinal deformity surgeries including sacroiliac joint fixation (SIJF) by using an S2 alar iliac screw or iliac screw has increased to avoid the distal junctional failure. However, we occasionally experienced patients who suffered from hip pain after a long instrumented spinal fusion. We hypothesized that long spinal fusion surgery including SIJF influenced the hip joint as an adjacent joint. The aim of this paper was to evaluate the association between spinal deformity surgery including SIJF and radiographic progression of hip osteoarthritis (OA). This study was retrospective cohort study. In total, 118 patients who underwent spinal fusion surgery at single center from January 2013 to August 2018 were included. We measured joint space width (JSW) at central space of the hip joint. We defined reduction of more than 0.5mm/year in JSW as hip OA progression. The patients were divided into two groups depending on either a progression of hip osteoarthritis (Group P), or no progression (Group N). The number of patients in Group P and Group N was 47 and 71, respectively. Factor that was statistically significant for hip OA was SIJF (p = 0.0065, odds ratio = 7.1, 95% confidence interval = 1.6-31.6). There were no other significant differences by the multiple logistic regression analysis. This study identified spinal fixation surgery that includes SIJF as a predictor for radiographic progression of hip OA over 12months. We should pay attention to hip joint lesions after adult spinal deformity surgery, including SIJF.

REPRODUCIBILITY OF S2-ALAR ILIAC SCREW MORPHOMETRIC ANALYSIS.

ABSTRACTObjective: To evaluate the reproducibility of a S2-alar iliac (S2AI) screw parameters measurement method by inter and intraobserver reliability.Methods: Cross-sectional study, considering computed tomography exams. Morphometric analysis was performed by multiplanar reconstructions. Screw length, diameter and trajectory angles were the studied variables. To analyze the measurements reproducibility, intraclass correlation coefficient (ICC) was used.Results: Interobserver reliability was classified as strong for screw shortest length (ICC: 0.742) and diameter (ICC: 0.699). Interobserver reliability was classified as moderate for screw longest length (ICC: 0.553) and for screw trajectory angles in the axial plane for the longest (ICC: 0.478) and for the shortest lengths (ICC: 0.591). Intraobserver reliability was interpreted as excellent for screw shortest (ICC: 0.932) and longest lengths (ICC: 0.962) and diameter (ICC: 0.770) and screw trajectory angles in the axial plane for the screw longest (ICC: 0.773) and shortest lengths (ICC: 0.862). There were weak interobserver and strong intraobserver reliabilities for trajectory angle in sagittal plane, but no statistical significance was found.Conclusion: Inter and intraobserver reliability of S2AI screw morphometric parameters were interpreted from moderate to excellent in almost all studied variables, except for the screw trajectory angle in the sagittal plane measurement. Level of Evidence IV, Diagnostic Studies - Investigating a Diagnostic Test.

Sacroiliac Joint Degeneration After Lumbopelvic Fixation.

Study Design:Retrospective Study.Objectives:Sacroiliac buttress screws (SBS) and S2 alar iliac screws (SAI) are used as distal screws in cases with long fusion to the pelvis. Distal fixation ends, whether exceeding the sacroiliac joint (SIJ), may affect postoperative degenerative changes in the SIJ. The aim of this study was to investigate SIJ degeneration after lumbosacral and lumbopelvic fixation, using SBS and SAI in degenerative spine diseases, respectively.Methods:This study included 70 patients aged ≥50 years with lumbosacral fusion (>3 levels). They were divided into 2 groups (SBS 20 and SAI 50 cases) based on the type of distal screws. Radiographical and clinical data were reviewed with a minimum 2-year follow-up. Radiographical SIJ degeneration was analyzed using computed tomography; clinical outcomes were evaluated using visual analog scale (VAS) and Japanese Orthopaedic Association Back Pain Questionnaire (JOABPEQ).Results:No significant differences were observed in patients’ preoperative characteristics between the 2 groups. The incidence of SIJ degeneration, including osteophyte formation (30.0% and 8.0%, p = 0.03), intraarticular air (75.0% and 16.0%, p < 0.001), and subchondral cyst (20.0% and 2.0%. p = 0.02) in SBS and SAI groups, respectively, at the follow-up, was significantly higher in the SBS group. Although SIJ degenerative changes were significantly different between the SBS and SAI groups, there was no significant difference in VAS and JOABPEQ scores between the groups at 2 years post-surgery.Conclusions:Lumbosacral fusion can cause SIJ degeneration, which is more frequent when SBS are used in fixation to the sacrum.

Can pelvic incidence change after spinal deformity correction to the pelvis with S2-alar-iliac screws?

Pelvic incidence (PI) is assumed to be fixed, yet studies have reported PI changes after long fusions to the pelvis. In a cohort of ASD patients undergoing surgery with S2-alar-iliac (S2AI) screws, we sought to: (1) report the magnitude of PI changes, and (2) evaluate subsequent pelvic parameter changes. A retrospective case series of ASD patients undergoing surgical correction with S2AI screw placement and sagittal cantilever correction maneuvers was conducted. Patients were categorized based on preoperative PI: High-PI (H-PI) (PI≥60°); Normal-PI (N-PI) (60°>PI>40°); Low-PI (L-PI) (PI≤40°). PI was measured preoperatively and immediately postoperatively. A significant PI change was established a priori at ≥6.0. PI, pelvic tilt (PT), lumbar lordosis (LL), and PI-LL mismatch were analyzed. In 68 patients (82.3% female, ages 22-75years), the average change in PI was 4.6°±3.1, and 25 (36.8%) had a PI change ≥6.0° with breakdown as follows: H-PI 12 (66.7%) patients, 9 (25.87%) patients, and 4 (33.3%) patients. Of 25 patients with PI changes, 10 (14.7%) had a PI increase and 15 (22.1%) had a PI decrease. Significant improvements were seen in PT, LL, PI-LL mismatch in all patients with a PI change ≥6.0°, in addition to both subgroups with an increase or decrease in PI. PI changes of ≥6.0° occurred in 36.8% of patients, and H-PI patients most commonly experienced PI changes. Despite PI alterations, pelvic parameters significantly improved postoperatively. These results may be explained by sacroiliac joint laxity, S2AI screwplacement, or aggressive sagittal cantilever techniques.

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

BackgroundTo 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.MethodsWe 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.ResultsIn 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).ConclusionBoth 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.

Accuracy of S2 Alar-Iliac Screw Placement Under the Guidance of a 3D-Printed Surgical Guide Template

To evaluate the safety and accuracy of S2 alar-iliac (S2AI) screw placement guided by a 3-dimensional (3D)-printed surgical guide template. The data of 27 patients treated with S2AI screws were analyzed. S2AI surgical guide templates were designed and printed, and S2AI screw placement was completed intraoperatively with the guide templates. Postoperative computed tomography was performed to measure screw path parameters, namely the sagittal angle (SA), the transverse angle (TA), the horizontal distance (HD) between the entry point of the screw and the median sacral crest, and the vertical distance (VD) between the entry point of the screw and the lower edge of the first posterior sacral foramen. Screw placement was graded according to the Oh grading criteria. A total of 54 S2AI screws were placed. The screw grades were as follows: 52 screws were considered grade 0, 2 were grade 1, none were grade 2, and none were grade 3. Thus grade 0 accounted for 96.3% of the screws. When the preoperatively planned SA (32.3° ± 2.0°), TA (42.1° ± 3.9°), HD (5.1 ± 1.1) mm, and VD (19.0 ± 2.4) mm were compared with the corresponding postoperative SA (31.9° ± 3.8°), TA (42.5° ± 4.0°), HD (4.9 ± 1.1) mm, and VD (19.1 ± 2.3) mm, no significant differences were identified (P > 0.05). S2AI screw placement assisted by a 3D-printed surgical guide is safe and accurate.

Revision by S2-alar-iliac instrumentation reduces caudal screw loosening while improving sacroiliac joint pain\u2014a 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.

87. Rates of loosening, failure, and revision of iliac fixation in adult deformity surgery

BACKGROUND CONTEXT Traditional iliac (IS) and S2-alar-iliac (S2AI) pelvic fixation methods have unique technical characteristics in their application, and result in varied biomechanical and anatomic impact. These differences may lead to variance in lumbopelvic fixation failure types and rates. PURPOSE We intended to evaluate the influence of type of iliac fixation utilized for adult spinal deformity (ASD) correction on the frequency and manner of lumbopelvic fixation failure. STUDY DESIGN/SETTING Retrospective review of prospective, multicenter ASD database. PATIENT SAMPLE This study included 410 patients. OUTCOME MEASURES HRQL (ODI, SF-36, NRS), rod fracture, screw fracture, screw loosening, revision surgery, pseudarthrosis METHODS Inclusion criteria included ASD (coronal Cobb≥20°, sagittal vertical axis (SVA) ≥5cm, pelvic tilt ≥25° and/or thoracic kyphosis >60°) ≥ 18 years old, 2yr follow-up, and >5 level fusion with pelvic fixation. Cohort subdivided by type of pelvic fixation (IS vs S2AI). Loosening was defined by lucency around the screw shank on radiographs. Univariate testing was performed using t-tests and chi-squared tests. Multivariate logistic regression, accounting for significant confounders, was used to examine differences between the two groups for screw loosening/fracture, rod fracture, and revision surgery. Level of significance = p RESULTS Of the total of 1422 patients, 410 met inclusion criteria (IS=287, S2AI=131). The two groups had similar age, BMI, baseline co-morbidities, and number of levels fused (p>0.05), as well as similar baseline HRQLs (SF-36, ODI, SRS-22, NRS leg and back, p>0.05) and deformity (PT, PI-LL, SVA, p>0.05). Patients in the IS group had a higher proportion of unilateral fixation compared to S2AI group (12.9% vs 5.6%; p=0.001). The overall pelvic fixation failure rate (screw loosening/screw fracture/rod fracture) was 29.4%. Loosening of pelvic fixation occurred in 13.4% of patients, and was more prevalent in the S2AI fixation group (OR 2.74, p=0.001). The rate of S1 screw loosening was 2.9%, and more likely to occur in the S2AI group (OR 4.17, p=0.045). The rate of pelvic fixation fracture in the overall cohort was 2.3%, with no difference between groups (p=0.37). Rod fracture occurred in 14.1% below L4, with a trend toward less occurrence in the S2AI group (OR 0.47, p=0.06). Revision surgery was required in 22.7% of our cohort, with no difference between groups (p=0.449). Patients with failure of the pelvic fixation had less improvement in their HRQL at 2years (PCS 7.69 vs 10.46 p=0.028; SRS 0.83 vs 1.03 p=0.019; ODI 12.91 vs 19.77 p=0.0016). CONCLUSIONS Pelvic fixation is commonly used in long segment adult spinal deformity surgery to improve the rate of L5-S1 fusion and protect from construct failure at the lumbosacral junction. Our results demonstrate a substantial rate of pelvic fixation hardware issues following ASD correction. Lumbopelvic fixation failure occurred at a rate of 29.4% following surgical correction of ASD that involved iliac fixation and was associated with diminished clinical outcomes. Additionally, S2AI screws were more likely to demonstrate loosening, but less commonly associated with rod fractures. Patients who experienced failure of their pelvic fixation had less improvement in their HRQL two years postoperatively. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.

P24. Acute failure of S2-alar-iliac pelvic fixation following adult deformity correction

BACKGROUND CONTEXT Pelvic fixation with S2-alar-iliac (S2AI) screws is a widely accepted technique in adult deformity surgery, with few descriptions of complications. PURPOSE To report novel acute failure mechanisms in S2AI instrumentation. STUDY DESIGN/SETTING Retrospective chart review. PATIENT SAMPLE N/A OUTCOME MEASURES N/A METHODS We performed a retrospective review of all fusion surgeries extending from L2 or higher with instrumentation to the pelvis in which S2AI screws were placed with at least 6 months follow-up between 3/2017 and 6/2019. We excluded patients under 18 years of age and non-ambulatory patients. Acute instrumentation failure was defined as any case requiring revision of the pelvic fixation within 6 months from the index surgery. We reviewed the demographic and surgical characteristics of each patient, and reviewed preoperative and 6-week postoperative radiographs to document the change in their deformity parameters as well as for the presence of a transitional lumbo-sacral segment. RESULTS A total of 134 cases were identified, of which seven had acute S2AI instrumentation failure requiring revision (5%). Failure cases averaged age 56.6 years, BMI 32.5, 57% female, 12 levels of fusion, and average deformity correction was LL increase 32°, T1PA decrease 19°, and CSVL decrease 3.5cm. Failure mechanism was either set plug dislodgement from S2AI screw heads, sacral fracture/kyphosis with rod slippage out of S2AI set plugs, or both, and involved two different implant manufacturers. Three of seven (43%) failure patients had a nonmobile transitional segment at the lumbosacral junction and fractured through the transitional segment with kyphotic angulation. Salvage strategy involved removal and revision of the original S2AI screws with placement of additional stacked pelvic fixation and a multi-rod construct. CONCLUSIONS We have identified a novel mechanism of pelvic fixation failure involving set screw dislodgement and rod slippage, which affected 5% of the deformity patients in our series across two implant manufacturers. Patients with large angular deformity correction and/or transitional lumbosacral anatomy may be particularly at risk, and may benefit from supplemental pelvic fixation. FDA DEVICE/DRUG STATUS S2AI Screws (Approved for this indication), thoracolumbosacral pedicle screw system (Approved for this indication).

1. Use of ALIF at the lumbosacral junction results in less lumbopelvic fixation failure than TLIF or no interbody fusion following correction of adult spinal deformity

BACKGROUND CONTEXT Based on biomechanical studies, the use and type of interbody device at the lumbosacral junction creates variable stress/strain on the lumbopelvic fixation. This variability may lead to differential failure mechanisms and rates within following correction of adult spinal deformity (ASD). PURPOSE We aimed to evaluate the relationship of lumbosacral interbody presence and type with lumbopelvic fixation failure after ASD correction utilizing iliac (IS) or S2 alar iliac (S2AI) screws. STUDY DESIGN/SETTING Retrospective review of prospective, multicenter ASD database. PATIENT SAMPLE A total of 342 patients. OUTCOME MEASURES Pelvic fixation loosening, IS/S2AI screw fracture, S1 screw fracture, rod fracture (below L4), revision surgery, pseudarthrosis, HRQLs. METHODS Inclusion criteria consisted of patients with ASD (coronal Cobb ≥20°, sagittal vertical axis (SVA) ≥5cm, pelvic tilt ≥25° and/or thoracic kyphosis >60°) ≥18 years old and 2yr f/u who underwent >5 level fusion with pelvic fixation. Patients with prior L5-S1 fusion were excluded. Patients were divided into three groups based on the lumbosacral interbody fusion strategy (none, ALIF, TLIF/PLIF). Univariate testing via one-way ANOVA and chi-square tests, and multivariate logistic regression, accounting for significant confounders, were used to examine the difference between the three groups in regard to pseudarthosis, pelvic fixation loosening, rod fracture and revision surgery. Level of significance=p RESULTS A total of 342 patients met inclusion criteria (L5-S1 interbody groups: none=164, ALIF=87, TLIF=91). The three groups were similar in terms of baseline comorbidities, gender, BMI, pelvic fixation type (iliac screws vs S2AI), and use of BMP2 posteriorly. There were differences in rate of interbody BMP2 use (ALIF 22.9%, TLIF 48.35%, p=0.0001), use of unilateral pelvic fixation (none 7.36%, ALIF 19.8%, TLIF 3.29%, p=0.0001) and age (none 65.41, ALIF 60.47, TLIF 63.81, p=0.0005). Multivariate logistic regression revealed ALIF at L5-S1 was associated with lower odds of pseudarthrosis (OR 0.16, p-0.026) and rod fracture (OR 0.19, p=0.012) compared to no interbody fusion, while TLIF was no different than no interbody. There were no differences between groups in regard to pelvic fixation loosening or revision surgery. CONCLUSIONS The use of ALIF at L5-S1 during correction of ASD protects against fixation failure at the lumbopelvic junction compared to the use of TLIF or no interbody. This more protective effect of ALIF at the lumbosacral junction may allow for selective unilateral pelvic fixation for ASD surgery. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.

Surgical Technique and Accuracy of S2 Alar-Iliac Screw Insertion Using Intraoperative O-Arm Navigation: An Analysis of 120 Screws

To evaluate the surgical technique and accuracy of S2 alar-iliac (SAI) screw placement using intraoperative O-arm-based 3-dimensional navigation (O-arm). This study involved 60 patients who underwent SAI screw placement using the O-arm system between September 2013 and September 2019. These surgeries were performed by 5 different surgeons. For O-arm-based SAI screw insertion, a reference frame was attached to the spinous process of the lower lumbar spine (usually L4) so as not to interfere with SAI screw insertion and to facilitate simultaneous L5-S posterior interbody fusion with navigation. The navigated probe, iliac tap, and screwdriver were used for SAI screw insertion. Screw placement accuracy and screw length were assessed using postoperative computed tomography. Perioperative complications were also evaluated. Mean age at surgery was 68.1 (range, 30-83) years. In total, 120 screws were inserted. Rate of accurate screw placement was 98.3% (118/120 screws). The 2 misplaced screws breached the anterior cortex of the sacrum. Mean screw length was 85.6 (range, 70-90) mm. There were no significant neurovascular or visceral complications perioperatively. SAI screw placement can be safely performed using the intraoperative O-arm system and our surgical technique.

Risk Factors for Loosening of S2 Alar Iliac Screw: Surgical Outcomes of Adult Spinal Deformity.

Study DesignRetrospective study.PurposeTo determine the risk factors for S2 alar iliac (S2AI) screw loosening and its association with lumbosacral fusion in patients with adult spinal deformity (ASD).Overview of LiteratureS2AI screws have been widely used for ASD surgery in recent years. However, no studies have analyzed the risk factors for loosening of S2AI screws and its association with lumbosacral fusion.MethodsCases of 50 patients with ASD who underwent long spinal fusion (>9 levels) with S2AI screws were retrospectively reviewed. Loosening of S2AI screws and S1 pedicle screws and bone fusion at the level of L5–S1 at 2 years after surgery were investigated using computed tomography. In addition, risk factors for loosening of S2AI screws were determined in patients with ASD.ResultsAt 2 years after surgery, 33 cases (66%) of S2AI screw loosening and six cases (12%) of S1 pedicle screw loosening were observed. In 40 of 47 cases (85%), bone fusion at L5–S1 was found. Pseudarthrosis at L5–S1 was not significantly associated with S2AI screw loosening (19.3% vs. 6.3%, p=0.23), but significantly higher in patients with S1 screw loosening (83.3% vs. 4.9%, p<0.001). On multivariate logistic regression analyses, high upper instrumented vertebra (UIV) level (T5 or above) (odds ratio [OR], 4.4; 95% confidence interval [CI], 1.0–18.6; p=0.045) and obesity (OR, 11.4; 95% CI, 1.2–107.2; p=0.033) were independent risk factors for S2AI screw loosening.ConclusionsHigh UIV level (T5 or above) and obesity were independent risk factors for S2AI screw loosening in patients with lumbosacral fixation in surgery for ASD. The incidence of lumbosacral fusion is associated with S1 screw loosening, but not S2AI screw loosening.

Pelvic parameters directly influence ideal S2 alar-iliac (S2AI) screw trajectory

BackgroundThe utilization of the S2 Alar-Iliac (S2AI) screw provides an optimal method of spinopelvic fixation. The free-hand placement of these screws obviates the use of intra-operative fluoroscopy and relies heavily on sacropelvic anatomy; variations of this anatomy could alter the ideal screw trajectory. The S2AI corridor is near several neurovascular structures, thus an accurate trajectory is critical. The reported angles of trajectory vary within the literature and a paucity of data exists on how patient morphometry influences ideal screw trajectory. We sought to examine the relationship between ideal screw trajectory and pelvic parameters. MethodsThe records of 99 consecutive patients with degenerative thoracolumbar pathology were reviewed and pelvic parameters including sacral slope, pelvic tilt, and pelvic incidence were measured with preoperative standing radiographs. Using 3-dimensional computed tomography (CT) reconstructions, an ideal S2AI trajectory was defined and anteroposterior (horizontal) and cephalocaudal (sagittal) angles were recorded. ResultsPelvic tilt was found to have a moderate inverse correlation with cephalocaudal screw trajectory (r=-0.467, p-value=0.006). Pelvic incidence and sacral slope had weaker correlations with cephalocaudal screw angle. In subgroup analysis, patients with high pelvic tilt (>20°) had a significantly lower cephalocaudal screw trajectory (24.9 ± 3.7° versus 29.8 ± 2.8°, p-value=<0.001) compared to those with a normal pelvic tilt (≤20°). ConclusionsThis study found an inverse relationship between pelvic tilt and cephalocaudal S2AI screw trajectory. Therefore, the sagittal angle of insertion becomes increasingly more perpendicular to the floor (less caudally orientated) as pelvic tilt increases in reference to a patient positioned prone on an operating table parallel to the floor. This may bolster safety and efficacy when utilizing the free-hand technique for placement of the S2AI screw as it allows the surgeon to plan a more ideal trajectory by accounting for pelvic parameters.

The use of triangular implants to enhance sacropelvic fixation: a finite element investigation

Background contextLong thoracolumbar fixation and fusion have become a consolidated treatment for severe spinal disorders. Concomitant sacropelvic fixation with S2 alar-iliac (S2AI) screws is frequently performed to limit instrumentation failure and pseudarthrosis at the lumbosacral junction. PurposeThis study explored the use of triangular titanium implants in different configurations in which the implants supplemented standard sacropelvic fixation with S2AI screws in order to further increase the stability of S2AI fixation. Study designFinite element study. MethodsFour T10-pelvis instrumented models were built: pedicle screws and rods in T10–S1 (PED); pedicle screws and rods in T10–S1, and bilateral S2 alar-iliac screws (S2AI); pedicle screws and rods in T10–S1, bilateral S2AI screws, and triangular implants inserted bilaterally in a sacral alar-iliac trajectory (Tri-SAI); pedicle screws and rods in T10–S1, bilateral S2AI screws and two bilateral triangular titanium implants inserted in a lateral trajectory (Tri-Lat). The models were tested under pure moments of 7.5 Nm in flexion-extension, lateral bending and axial rotation. RESULTSSIJ motion was reduced by 50% to 66% after S2AI fixation; the addition of triangular titanium implants in either a SAI or a lateral trajectory further reduced it. S2AI, Tri-SAI, and Tri-Lat resulted in significantly lower stresses in S1 pedicle screws when compared to PED. Triangular implants had a protective effect on the maximal stresses in S2AI screws, especially when placed in the SAI trajectory. Sacropelvic fixation did not have any protective effect on the posterior rods. ConclusionsSupplementing S2AI screws with triangular implants had a protective effect on the S2AI screws themselves, as well as the S1 pedicle screws, in the tested model. Clinical significanceTriangular implants can substantially reduce the residual flexibility of the SIJ with respect to S2AI fixation alone, suggesting a possible role in patients needing reinforced fixation. In vivo investigation is needed to determine if these in vitro effects translate into clinically important differences.

Strategy for obtaining solid fusion at L5-S1 in adult spinal deformity: risk factor analysis for nonunion at L5-S1.

Maintaining lumbosacral (LS) arthrodesis and global sagittal balance after long fusion to the sacrum remains an important issue in the surgical treatment for adult spinal deformity (ASD). The importance and usefulness of LS fixation have been documented, but the optimal surgical long fusion to the sacrum remains a matter for debate. Therefore, the authors performed a retrospective study to evaluate fusion on CT scans and the risk factors for LS pseudarthrosis (nonunion) after long fusion to the sacrum in ASD. The authors performed a retrospective study of 59 patients with lumbar degenerative kyphosis (mean age 69.6 years) who underwent surgical correction, including an interbody fusion of the L5-S1, with a minimum 2-year follow-up. Achievement of LS fusion was evaluated by analyzing 3D-CT scans at 3 months, 6 months, 9 months, 1 year, and 2 years after surgery. Patients were classified into a union group (n = 36) and nonunion group (n = 23). Risk factors for nonunion were analyzed, including patient and surgical factors. The overall fusion rate was 61% (36/59). Regarding radiological factors, optimal sagittal balance at the final follow-up significantly differed between two groups. There were no significant differences in terms of patient factors, and no significant differences with respect to the use of pedicle subtraction osteotomy, the number of fused segments, the proportion of anterior versus posterior interbody fusion, S2 alar iliac fixation versus conventional iliac fixation, or loosening of sacral or iliac screws. However, the proportion of metal cages to polyetheretherketone cages and the proportion of sacropelvic fixation were significantly higher in the union group (p = 0.022 and p < 0.05, respectively). LS junction fusion is crucial for global sagittal balance, and the use of iliac screws in addition to LS interbody fusion using a metal cage improves the outcomes of long fusion surgery for ASD patients.

A 3D-Printed Simulator and Teaching Module for Placing S2-Alar-Iliac Screws.

The concept of the S2-alar-iliac (S2AI) screw was developed approximately one decade ago and has rapidly become an important component of spinal arthrodesis. Two challenges to placing S2AI screws are gaining an intuition for free-hand screw placement trajectory and acquisition of the appropriate radiograph to both guide screw placement and diagnose misplacement. To present the design and manufacture of an S2AI screw placement simulator and teaching module that addresses both challenges. This simulator involves using a 3D printer to create a life-sized pelvis. Participants first used this print to practice placing free-hand S2AI screws. Then participants used another print to practice taking radiographs showing the posterior superior iliac spine-anterior superior iliac spine corridor (teardrop) view. The accuracy of screw placement increased from 17 to 80% on the left side and 7 to 100% on the right side. The number of radiographs taken by each participant to obtain the teardrop view decreased after practice with the simulator compared to baseline. Practice with the S2AI simulator led to an improved intuition of an appropriate free-hand S2AI screw trajectory and a decrease in the number of radiographs needed for obtaining the correct diagnostic view.

Extra-articular Portion of the Sacroiliac Joint-Between the Sacral and Pelvic Tuberosities: An Anatomical Guide for the S2-Alar-Iliac Screw Trajectory.

Study Design:A novel technique for S2-alar-iliac (S2AI) screw placement was analyzed.Objectives:Accurate confirmation of the S2AI screw trajectory with free-hand techniques is not simple, although some anatomical landmarks have been reported. To overcome the drawback, we aimed to introduce our technique for S2AI screw placement assisted with a guidewire using a new anatomical landmark.Methods:A total of 104 S2AI screws of 52 patients who underwent S2AI screw placement were investigated. Navigation software was used to simulate S2AI screw placement preoperatively. Screw placement was performed with the nonfluoroscopic free-hand technique. In this technique, a guidewire is inserted into the ilium from the extra-articular portion of the sacroiliac joint just lateral to the ideal screw entry point toward the tip of the greater trochanter and guides the screw trajectory. If the direction of the guidewire is satisfactory, all procedures of screw insertion are performed accordingly. The screw accuracy was assessed with computed tomography.Results:The modal size of the screw was 9.5 mm × 90 mm. The average horizontal angle was 42.0° (SD = 5.1°) on the right and 40.7° (SD = 4.7°) on the left. Of the 104 screws, 4 screws (3.9%) breached dorsally. No screw-related complication was observed.Conclusions:Because the guidewire can be inserted at an angle according to the individual morphology of the sacroiliac joint, it will be a reliable guide for the screw trajectory. This technique with a guidewire would help improve the accuracy of S2AI screw placement.

Clinical Effectiveness of S2-Alar Iliac Screws in Spinopelvic Fixation in Pediatric Neuromuscular Scoliosis: Systematic Literature Review.

Study Design:Systematic literature review.Objectives:To comprehensively review the S2-alar iliac (S2-AI) screw technique for pelvic fixation in pediatric neuromuscular scoliosis.Methods:Articles identified from the PubMed and EMBASE databases were reviewed for relevance and applicability, and the studies were summarized.Results:Eight articles met the inclusion criteria. A total of 277 pediatric patients underwent spinopelvic fixation using S2-AI fixation for neuromuscular scoliosis; the mean follow-up was 3 years (range = 0.75-6 years). Six articles had level III evidence (5 retrospective cohort studies, 1 observational study), and 2 articles had level IV evidence (case series). Wound complications occurred in 34 (12.2%) patients. Instrumentation complications occurred in 36 patients (13.0%), including lucency around the screw (6.5%), screw fracture (3.6%), disengaging of the set/screw or rod from the tulip head (2.8%), and screw displacement (0.7%). Three patients (1.1%) required reoperation for instrumentation failures. The overall reoperation rate—including 3 hardware replacements and 3 cases of L5-S1 pseudarthrosis—was 2.1%. The mean Cobb angle correction was 51.4°, and the mean pelvic obliquity correction was 14.8°; deformity correction was maintained at 3- and 5-year follow-ups. There were 10 (3.6%) cases of implant prominence/implant-related pain, 1 case of sacroiliac joint pain (resolved with longer screw placement), and no major neurological or vascular complications secondary to S2-AI screw placement.Conclusions:This review suggests that the use of S2-AI screws in pediatric neuromuscular scoliosis is efficacious with a reasonable safety profile and provides a useful technique for pelvic fixation in children with scoliosis.

Comparative tomographic study of the iliac screw and the S2-alar-iliac screw in children

OBJECTIVES:The recent advancements in spine fixation aid in the treatment of complex spinal pathologies. Both the iliac screw (IS) and the S2-alar-iliac (S2AI) screw provide adequate stability in the fixation of complex lumbosacral spine pathologies, leading to a significant increased rate of using these techniques in the daily practice of the spine surgeons. This study aims to analyze, describe, and compare the insertion and positioning parameters of the S2AI screw and IS techniques in children without spinal deformities.METHODS:An observational retrospective study was conducted at a university hospital in 2018, with 25 computed tomography (CT) images selected continuously. Mann-Whitney-Shapiro-Wilk tests were performed. The reliability of the data was assessed using the intraclass correlation. The data were stratified by age group only for Pearson’s correlation analysis.RESULTS:The mean age was 11.7 years (4.5 SD). The mean IS length was 106.63 mm (4.59 SD). The mean length of the S2AI screw was 104.13 mm (4.22 SD). The mean skin distance from the IS entry point was 28.13 mm (4.27 SD) and that for the S2AI screw was 39.96 mm (4.54 SD).CONCLUSIONS:Through CT, the S2AI screw trajectory was observed to have a greater bone thickness and skin distance than the IS. There was a linear correlation between age and screw length for both techniques. A similar relationship was observed between skin distance and age for the S2AI screw technique. In children, the S2AI screw technique presents advantages such as greater cutaneous coverage and implant thickness than the IS technique.

The Prevalence and Risk Factors for S2 Alar-Iliac Screw Loosening with a Minimum 2-Year Follow-up.

Study DesignA retrospective cohort study.PurposeThe purpose of this study was to investigate the prevalence and risk factors for S2 alar-iliac (SAI) screw loosening following lumbosacral fixation, with a minimum 2-year follow-up.Overview of LiteratureAlthough SAI screws allow surgeons to perform lumbosacral fixation with a low profile and enhanced biomechanical strength, screw loosening following surgery can occur in some cases. However, few studies have investigated the prevalence and risk factors for SAI screw loosening.MethodsThis retrospective study included 35 patients (mean age, 72.8±8.0 years; male, 10; female, 25) who underwent lumbosacral fixation using SAI screws with at least 2 years of follow-up. SAI screw loosening and L5–S bony fusion were assessed using computed tomography. The period for which the screws appeared loose and the risk factors for SAI screw loosening were investigated 2 years after surgery.ResultsA total of 70 SAI screws and 70 S1 pedicle screws were inserted. Loosening was observed 0.5, 1, and 2 years after surgery in 17 (24.3%), 35 (50.0%), and 35 (50.0%) SAI screws, respectively. Bony fusion rate at L5–S was significantly lower in patients with SAI screw loosening than in those without screw loosening (65.0% vs. 93.3%, p =0.048). The score for SAI screw contact with the iliac cortical bone and the bony fusion rate at L5–S were significantly lower in the loosening group than in the non-loosening group (1.8±0.5 vs. 2.2±0.3, p <0.001, respectively). Postoperative pelvic incidence–lumbar lordosis was significantly higher in the loosening group than in the non-loosening group (7.9°±15.4° vs. 0.5°±8.7°, p =0.02, respectively).ConclusionsSAI screw loosening is closely correlated with pseudoarthrosis at L5–S. Appropriate screw insertion and optimal lumbar lordosis restoration are important to prevent postoperative complications related to SAI screws.