Adult spinal deformity (ASD) alters sagittal alignment and trunk centre of mass location, increasing spinal loading. Current clinical assessment of ASD primarily focuses on alignment parameters disregarding patient’s body shape variability. This study aimed to evaluate the relative effects of spinal sagittal alignment and body mass distribution (i.e., body shape) on spinal loads and trunk centre of mass location in upright-posture and forward flexion. Sagittal alignments of 754 patients with ASD were obtained from clinical data. Trunk mass distributions were assessed in adult male volunteers (10 healthy weight, 9 obese). Using an established musculoskeletal model of the spine, inverse dynamics simulations were performed for each combination of sagittal alignment and trunk mass distribution profile. A factorial analysis using generalised linear models elucidated the relative effects of alignment vs. body shape. Sagittal alignment primarily determined the antero-posterior positioning of trunk centre of mass; body shape, its cranio-caudal location. In the healthy weight cohort, the relative effect of mass distribution on compressive loads was 14–98

STUDY DESIGN:Cross-sectional reliability study. OBJECTIVE:To propose and validate a novel biomechanical classification for high-grade spondylolisthesis (HGS) that incorporates compensatory mechanisms into the assessment of sagittal alignment patterns. BACKGROUND:Existing classifications of HGS are primarily descriptive, rely on parameters often distorted by sacral endplate dysplasia, offer limited guidance for surgical decision-making and do not completely capture the spectrum of compensation in sagittal alignment. The new classification system defines four alignment types: type 1: proportioned sagittal alignment (SVA ≤50 mm, LSA ≥90°, IP ≤T11, LL Apex ≤L3). Type 2: compensated lower lumbar hypolordosis, with cranialization of IP and/or LL Apex. Type 3: compensated lumbosacral kyphosis (LSA <90° despite SVA ≤50 mm). Type 4: sagittal malalignment with exhausted compensation (SVA >50 mm). METHODS:Fourteen spine surgeons from 12 countries evaluated 29 anonymized cases of HGS using the new classification. Each case was presented with full-spine and lumbosacral anteroposterior and lateral standing radiographs. Classifications were repeated in two separate rounds. Interobserver reliability was assessed with Fleiss' Kappa, while intraobserver reliability was evaluated with Cohen Kappa and simple percentage agreement. RESULTS:Interobserver reliability was almost perfect (Fleiss' Kappa=0.916; 95% CI: 0.888-0.945; overall concordance 82.8%). Intraobserver reliability was equally high (mean Cohen Kappa=0.958; 95% CI: 0.926-0.991; mean agreement 97.0%). CONCLUSION:This novel biomechanical classification provides a reproducible framework to describe HGS without reliance on approximated parameters. By integrating SVA, LSA, IP, and LL Apex, it captures compensated and decompensated states of malalignment with direct therapeutic implications. The system demonstrated excellent reliability and may serve as a foundation for future surgical guidelines.

Study Design. Retrospective cohort study based on a multicenter prospectively collected database. Objective. To determine whether postoperative anteverted pelvis (AP) in patients with preoperative normo- or retroverted pelvis (NRP) is associated with increased risk of mechanical or clinical complications after adult spinal deformity (ASD) surgery. Summary of Background Data. Pelvic anteversion is a rare spinopelvic morphology, typically considered physiological in young patients with low pelvic incidence. However, its occurrence after ASD correction—especially in patients with initially normo- or retroverted pelvic orientation—raises concerns about its potential impact on postoperative outcomes. Methods. From a database of 2043 surgically treated ASD patients, 84 patients with postoperative AP were identified. Based on preoperative pelvic version, patients were categorized into two groups: preoperative AP (n=38) and preoperative NRP converted to postoperative AP (n=46). Demographic, surgical, radiographic, and health-related quality of life (HRQoL) parameters were analyzed at baseline and at 2-year follow-up. Results. There were no significant differences in age, BMI, or baseline HRQoL between groups. Both groups underwent similar surgical procedures, although the NRP group required more frequent decompression and pelvic fixation. At 2 years, both groups showed significant improvement in ODI and SRS-22 scores. Mechanical complication rates were not significantly different (10.5% in AP vs. 23.9% in NRP, P =0.154). Radiographic analysis showed that postoperative AP patients maintained a lumbar lordosis >60°, despite low or normal pelvic incidence. NRP patients exhibited greater changes in spinopelvic parameters postoperatively. Conclusions. Postoperative AP does not appear to be associated with increased mechanical complications, even in patients who were normo- or retroverted preoperatively. These findings suggest that iatrogenic AP may represent a physiological adaptation rather than a pathologic outcome, particularly in younger patients without hip or neuromuscular comorbidities. Pelvic fixation is not necessary in cases of isolated AP when global sagittal balance is restored.

Study Design. Retrospective comparative study using prospectively collected data from a multicenter adult spinal deformity (ASD) registry across six centers, with propensity score (PS) matching to compare surgical versus nonoperative management. Objective. To compare 2-year patient-reported outcomes and serious adverse events after surgical versus nonoperative management of adult spinal deformity (ASD). Summary of Background Data. Randomized trials comparing operative and nonoperative treatment for ASD are rarely feasible, and ASD presents heterogeneous clinical and radiographic features, complicating treatment decisions. Propensity score methods can reduce measured confounding and strengthen comparative effectiveness estimates from observational cohorts. Methods. Consecutive adults meeting radiographic criteria for ASD were identified from six participating centers. The primary endpoint was change in Oswestry Disability Index (ODI) at 2 years; secondary endpoints included SRS-22 outcomes. Propensity scores for surgery were estimated using logistic regression including age, sex, body mass index, major Cobb angle, pelvic incidence–lumbar lordosis mismatch, global tilt, pelvic tilt, baseline ODI, and SRS-22 total score. One-to-one nearest-neighbor matching (caliper 0.25) generated balanced pairs. Outcomes were compared within the matched cohort. Results. Among 764 screened patients, 580 were eligible (338 surgical; 242 nonoperative). Propensity score matching produced 160 well-balanced pairs (n=320). In the matched cohort, mean age was ~45 years in both groups. At 2 years, ODI improvement was greater after surgery than after nonoperative care (−19.4±14.2 vs. −4.2±12.3; P <0.001); 72% of surgical patients versus 29% of nonoperative patients achieved a clinically meaningful ODI improvement (≥15 points; P <0.001). SRS-22 outcomes favored surgery, including higher 2-year SRS-22 total score (3.95±0.67 vs. 3.46±0.75; P <0.0001) and a higher proportion achieving MCID for SRS-22 total score (81.3% vs. 36.9%; P <0.001). Conclusion. In a multicenter PS–matched ASD cohort largely representing younger patients with mild-to-moderate baseline impairment, surgery was associated with superior 2-year disability and HRQoL outcomes compared with nonoperative care.

Study Design Retrospective analysis of a multicenter prospective adult spinal deformity (ASD) database. Objective In ASD surgery, fusion to the sacropelvis improves stability but increases morbidity. Stopping at L5 preserves motion but predisposes to distal junctional failure (DJF). This study aims to identify risk factors for DJF in ASD patients undergoing posterior fusion with lowest instrumented vertebra (LIV) at L5. Methods Patients undergoing posterior fusion with LIV-L5 and minimum two-years follow-up were included. Demographic data, radiographic parameters, and patient-reported outcome measures were collected. The primary endpoint was surgical extension to the pelvis (EP) for DJF. Patients were divided into EP and NoEP. Survival analysis and multivariate regression were used to identify predictors of DJF and postoperative malalignment. Results Eighty-one patients met inclusion criteria, with a mean follow-up of 4.50 years. Fifteen patients (18.5%) required EP due to DJF. The EP group demonstrated significantly worse preoperative global alignment, including higher sagittal vertical axis (SVA) and global tilt. Postoperative SVA was a strong predictor of DJF: patients with postoperative SVA >45 mm had a 5-year survival free from EP of 48.6%, compared with 95.1% in well-aligned patients (p<0.001). Multivariate analysis identified older age and higher preoperative SVA as independent risk factors for postoperative malalignment. Conclusions Choosing L5 as LIV is safe in well-aligned ASD patients, with approximately 80% remaining free from pelvic extension at five years. Inadequate postoperative sagittal correction, significantly increases the risk of DJF. Older patients with preoperative malalignment may benefit from primary fusion to the sacrum.

Study designComparative Cohort Study.ObjectivesThe aim of this study was to compare a biopsychosocial intervention for chronic low back pain to unimodal, biomedically focused care-as-usual in symptomatic, conservatively managed adult spinal deformity (ASD) patients.MethodsA comparative cohort study of a biopsychosocial intervention cohort (n=95) and a care-as-usual cohort (n=95). Inclusion criteria of both cohorts: age ≥18, no surgical indication, Oswestry Disability Index (ODI)>20, coronal Cobb angle ≥20, idiopathic (AdIS) or degenerative (de novo) scoliosis (DN), baseline and one-year follow-up ODI data. PRIMARY OUTCOME:functional status (ODIv2.1a). SECONDARY OUTCOMES:pain intensity (NPRS) and quality of life (SF36-PCS and SF36-MCS). CLINICAL RELEVANCE:minimal important clinical change (MCIC; improvement in ODI≥10 points), patient acceptable symptom state (PASS; ODI≤22).ResultsAt one-year follow-up the patients in the biopsychosocial intervention cohort showed significantly greater improvement in functional status (ODI) compared to the care-as-usual cohort (z=-3.97, p=<.001). Clinical relevance in terms of functional status: 49% of the patients in the biopsychosocial intervention cohort reached MCIC and 34% reached a PASS, compared to 23% and 15% respectively in the care-as-usual cohort. All secondary outcomes followed a similar pattern: greater improvement in the biopsychosocial intervention cohort.ConclusionThis study shows clinically relevant and significantly greater improvement in function, pain, and quality of life in the biopsychosocial intervention cohort, compared to the unimodal, biomedically focused care-as-usual cohort at one-year follow-up. Targeting biopsychosocial factors offers a promising option in terms of improving outcome in nonoperative care for ASD patients with predominant back pain symptoms.

Objective: To investigate feasibility and effectiveness of minimally invasive scoliosis surgery (MISS) via posterior approach in patients with adolescent idiopathic scoliosis (AIS) presenting with very severe curve exceeding 80°.Methods: A total of 257 flexible AIS patients who underwent deformity correction using posterior MISS were retrospectively categorized into 3 groups based on the curve magnitude: moderate (M group, main Cobb angle <60°, n=142), severe (S group, 60°–80°, n=91), and very severe (VS group, ≥80°, n=24). Radiological parameters, perioperative outcomes, and complication rates were compared among the 3 groups.Results: The VS group required significantly longer operative times (p<0.001), more fused segments (p<0.001), and a greater number of rib resections during thoracoplasty (p=0.032) compared to the other groups. Despite these surgical demands, there were no statistical differences in the correction rate (p=0.954) and last follow-up thoracic kyphosis (p=0.830) among the 3 groups. Regarding complications, while the overall complication rate showed no statistical difference, the VS group required significantly more invasive management for pleural effusions (p=0.012).Conclusion: Posterior MISS demonstrated acceptable radiological and perioperative outcomes even in flexible AIS with Cobb angles ≥80°. These findings suggest the posterior MISS can be a feasible surgical option for selected very severe AIS patients with sufficient flexibility.

Background:Lumbar spine disorders are among the most prevalent and disabling conditions worldwide. Patient selection for surgery remains highly complex, and the benefits of surgical interventions remain uncertain, potentially depending on patients' baseline health characteristics. Patient-related outcome measurements represent a standard method for assessing treatment success in lumbar surgery. The aim of this study is to prospectively validate the accuracy of a deep learning algorithm in predicting the clinical outcomes of patients undergoing lumbar surgery [minimal clinically important difference (MCID)/no-MCID]. Material and methods:This study is multicentric, longitudinal, and prospective study was conducted over a 16-month period (September 2021 to December 2022). Patients with a surgical indication for lumbar decompression were included preoperatively and enrolled in the Surgery Medical Outcomes (SuMO©) mobile application to fill in the preoperative and postoperative data. Patients were classified into two categories according to their postoperative outcomes. The MCID was defined using the Oswestry Disability Index (ODI), combined with the intake of opioids and the presence of motor loss in patients. These results were then compared to the prediction of the algorithm based on preoperative data to determine the accuracy of the algorithm. Results:A total of 119 patients were enrolled preoperatively, and postoperative follow-up data were obtained for 103 patients. The mean preoperative ODI was 0.43 (SD 0.17). The postoperative mean ODI was 0.28 (SD 0.18) at 1 month and 0.14 (SD 0.16) at 3 months. At 8 months, the mean postoperative ODI in the MCID group was 0.12, while it was 0.26 in the no-MCID group. The algorithm predicted the outcome with an accuracy of 81.6% (receiver operating characteristic score). Conclusion:This study confirms the validity and accuracy of the algorithm in prospectively predicting postoperative outcomes, as well as the sensitivity of the MCID definition, especially when coupled with remote, patient-centered follow-up. Artificial intelligence-based algorithms may help physicians in their future daily practice by addressing personalized patient care.

Surgical treatment for Scheuermann's Kyphosis (SK) in adolescents is well-established, but its role in adults remains controversial. This study challenges the traditional non-operative stance by evaluating the clinical outcomes of surgery in adult SK patients, using a matched cohort of adult idiopathic scoliosis (Adult IS) patients as a clinical benchmark. A retrospective, 1:3 matched-cohort study from a multicenter database with a minimum 2-year follow-up. Thirty-three SK patients were matched with 99 Adult IS patients by age and number of instrumented levels. Demographics, radiographic parameters, surgical data, complications, and patient-reported outcome measures (PROMs) were analyzed. 33 SK patients and 99 patients from the Adult IS comparative cohort were included. Mean age was 32.1 years; 82.8

STUDY DESIGN:Retrospective comparative study using prospectively collected data from a multicenter adult spinal deformity (ASD) registry across six centers, with propensity score (PS) matching to compare surgical versus nonoperative management. OBJECTIVE:To compare two-year patient-reported outcomes and serious adverse events after surgical versus nonoperative management of adult spinal deformity (ASD). SUMMARY OF BACKGROUND DATA:Randomized trials comparing operative and nonoperative treatment for ASD are rarely feasible, and ASD presents heterogeneous clinical and radiographic features, complicating treatment decisions. Propensity score methods can reduce measured confounding and strengthen comparative effectiveness estimates from observational cohorts. MATERIALS AND METHODS:Consecutive adults meeting radiographic criteria for ASD were identified from six participating centers. The primary endpoint was the change in Oswestry Disability Index (ODI) at two years; secondary endpoints included SRS-22 outcomes. Propensity scores for surgery were estimated using logistic regression, including age, sex, body mass index, major Cobb angle, pelvic incidence-lumbar lordosis mismatch, global tilt, pelvic tilt, baseline ODI, and SRS-22 total score. One-to-one nearest-neighbor matching (caliper 0.25) generated balanced pairs. Outcomes were compared within the matched cohort. RESULTS:Among 764 screened patients, 580 were eligible (338 surgical; 242 nonoperative). Propensity score matching produced 160 well-balanced pairs (n=320). In the matched cohort, the mean age was ~45 years in both groups. At two years, ODI improvement was greater after surgery than after nonoperative care (-19.4±14.2 vs. -4.2±12.3; P <0.001); 72% of surgical patients versus 29% of nonoperative patients achieved a clinically meaningful ODI improvement (≥15 points; P <0.001). SRS-22 outcomes favored surgery, including a higher two-year SRS-22 total score (3.95±0.67 vs . 3.46±0.75; P <0.0001) and a higher proportion achieving MCID for SRS-22 total score (81.3% vs . 36.9%; P <0.001). CONCLUSION:In a multicenter PS-matched ASD cohort largely representing younger patients with mild-to-moderate baseline impairment, surgery was associated with superior two-year disability and HRQoL outcomes compared with nonoperative care.

Pedicle subtraction osteotomies (PSOs) are well-established for rigid sagittal deformities, but the efficacy and safety of biplanar PSOs (BiPSOs) remain uncertain. This study aims to evaluate whether BiPSOs compromise sagittal correction or elevate surgical morbidity or complication rates. A retrospective analysis of a prospective multicentric adult spinal deformity database included patients who underwent a single Schwab-3/4 PSO with at least two years of follow-up. BiPSOs were defined by segmental angular changes (SAD) greater than 5° in both the sagittal and coronal planes, while uniplanar (UPSOs) had SAD greater than 5° in one plane. Demographic, surgical, and outcome variables were compared. The study included 33 BiPSOs and 123 UPSOs. No differences were found for baseline demographic characteristics. UPSOs were more common in patients with prior spinal surgery (57.6

Study design.Retrospective analysis of prospectively collected data.Objective.To investigate whether two clustering approaches applied to the same database would lead to differences in the minimal clinically important difference (MCID) for health-related quality of life parameters (HRQoL).Summary of Background Data.Machine learning approaches are being increasingly employed for the analysis of complex and heterogeneous settings such as that of adult spine deformity (ASD). However, it is not yet clear whether and how the choice of number and type of variables impacts the outcomes of a study.Methods.Two previously published clustering approaches (C12 and C16) were applied to a multicentric database of ASD patients who underwent surgery and had a minimum follow-up of one year. After clustering, the MCID for the Oswestry Disability Index, SRS-22, and SF-36 PCS were calculated for all clusters using the ROC method.Results.Data from 516 patients were available. Both algorithms led to a division of the database into three clusters, which presented similar characteristics both for C12 and C16. In particular, patients in clusters 1 to 3 presented an increasing level of imbalance and disability. The MCID for ODI, SRS-22, and SF-36 for each cluster differed between C12 and C16, but a similar pattern of increase of the MCID from cluster 1 to cluster 3 was observed for all HRQoL parameters and in both C12 and C16. The error rate, however, was smaller for C16.Conclusion.Different clustering algorithms applied to the same database allowed for the obtaining of similar clusters of ASD patients. However, the obtained MCIDs for the evaluated HRQoL parameters were different, highlighting the relevance of the choice of variables for the investigation of these parameters. The results suggest that clinically driven clusters should be used when investigating clinical outcomes, as they allow for a smaller error rate.

STUDY DESIGN:Retrospective cohort study based on a multicenter prospectively collected database. OBJECTIVE:To determine whether postoperative anteverted pelvis (AP) in patients with preoperative normo- or retroverted pelvis (NRP) is associated with increased risk of mechanical or clinical complications after adult spinal deformity (ASD) surgery. SUMMARY OF BACKGROUND DATA:Pelvic anteversion is a rare spinopelvic morphology, typically considered physiological in young patients with low pelvic incidence. However, its occurrence after ASD correction-especially in patients with initially normo- or retroverted pelvic orientation-raises concerns about its potential impact on postoperative outcomes. METHODS:From a database of 2043 surgically treated ASD patients, 84 patients with postoperative AP were identified. Based on preoperative pelvic version, patients were categorized into two groups: preoperative AP (n=38) and preoperative NRP converted to postoperative AP (n=46). Demographic, surgical, radiographic, and health-related quality of life (HRQoL) parameters were analyzed at baseline and at 2-year follow-up. RESULTS:There were no significant differences in age, BMI, or baseline HRQoL between groups. Both groups underwent similar surgical procedures, although the NRP group required more frequent decompression and pelvic fixation. At 2 years, both groups showed significant improvement in ODI and SRS-22 scores. Mechanical complication rates were not significantly different (10.5% in AP vs. 23.9% in NRP, P=0.154). Radiographic analysis showed that postoperative AP patients maintained a lumbar lordosis >60°, despite low or normal pelvic incidence. NRP patients exhibited greater changes in spinopelvic parameters postoperatively. CONCLUSIONS:Postoperative AP does not appear to be associated with increased mechanical complications, even in patients who were normo- or retroverted preoperatively. These findings suggest that iatrogenic AP may represent a physiological adaptation rather than a pathologic outcome, particularly in younger patients without hip or neuromuscular comorbidities. Pelvic fixation is not necessary in cases of isolated AP when global sagittal balance is restored.

STUDY DESIGN:Retrospective register study. OBJECTIVE:Determine when preoperative quality of life (QoL) declines in adult thoracolumbar/lumbar adolescent idiopathic scoliosis (TL/L-AIS) patients, analyze postoperative age-related changes, and assess the influence of preoperative sagittal alignment and the lowest instrumented vertebra (LIV) on QoL. SUMMARY OF BACKGROUND DATA:The effect of surgery on self-image, function, and pain may vary depending on age, alignment, and fusion length. MATERIALS AND METHODS:A total of 310 patients were analyzed preoperatively and postoperatively until two-year follow-up. Scoliosis Research Society-22 (SRS-22) and Short Form-36 (SF-36) scores were collected. Relative spinopelvic alignment (RSA), T4 and L1 pelvic angles (T4PA-L1PA), and LIV were determined on radiographs. Linear mixed-effect regression models were used, including group, time, and interaction. RESULTS:The phase 30 to 42 years represented an inflexion point in preoperative SRS-22 total scores, after which scores were unlikely to reach postoperative levels of younger patients. All age groups improved postoperatively. The SRS-22 total score difference at two years was -0.48 between the 40 and 60-year groups and younger-than-40-year groups ( P <0.001), -0.50 between older-than-60 and younger-than-40-year groups ( P <0.001). Self-image improved in all groups: +1.16 in those younger than 40 years, +1.08 in those aged 40 to 60 years, and +1.14 in those older than 60 years. Pain improved in patients aged 40 to 60 years, +0.75, and older than 60 years, +1.07. The SF-36 Physical Component Summary improvement was larger in patients aged 40 to 60 years, +6.76 (P<0.001), and older than 60 years, +8.74 (P<0.001), versus those younger than 40 years. Preoperatively, severely malaligned patients had the lowest SRS-22 and the largest postoperative improvement. Patients with LIV S1/iliac had lower SRS-22 than those with LIV L2/L3 or L4/L5 at two-year follow-up. CONCLUSION:In adult TL/L-AIS patients, QoL declined around the fourth life decade; postoperative recovery patterns differed. All age groups improved QoL postoperatively. Self-image improved the most. Pain and the physical component improved most in patients older than 40 years. Preoperative sagittal malalignment and the LIV influenced QoL.

Technical case report. Severe thoracic hyperkyphosis caused by multiple osteoporotic fractures can lead to significant disability. When deformities are rigid, surgical correction may be required. This report describes a rare case of four-level pedicle subtraction osteotomy (PSO) to address a fixed deformity, with focus on technique, outcomes, and complications. A 65-year-old female with Scheuermann’s disease developed progressive thoracic hyperkyphosis due to osteoporotic vertebral fractures. Radiographs showed a rigid kyphosis of 130.3°. The patient underwent a two-stage surgery: cement-augmented instrumentation (T2–L3), followed by contiguous PSOs from T6 to T9. Neurophysiological monitoring was used throughout. Kyphosis was corrected from 130.3° to 48°. Postoperatively, the patient developed progressive lower limb weakness due to spinal cord kinking. Revision surgery allowed neurological recovery. At two years, radiological parameters remained stable, and the patient reported reduced pain, improved mobility, and restored quality of life. Four-level PSO can be an effective treatment for severe rigid thoracic hyperkyphosis in osteoporotic patients. Although technically demanding and associated with potential complications, careful planning and staged correction can result in successful outcomes. IV (Case Report).

To identify postoperative radiographic parameters independently associated with clinically meaningful pain improvement one year after surgery in a selected cohort of adolescents with idiopathic scoliosis (AIS) reporting substantial preoperative pain. This was a retrospective analysis of a prospectively collected multicenter database including 378 AIS patients treated with posterior spinal fusion. Patients with substantial preoperative pain (SRS-22 pain ≤ 4.0) and ≥ 1-year follow-up were included. A 0.6-point MCID in SRS-22 pain defined meaningful improvement. Groups were compared and significant postoperative radiographic variables plus preoperative pain entered a multivariate logistic regression model. Nagelkerke R² and ROC-AUC assessed performance. Eighty-eight patients met the inclusion criteria (mean age 14.7 ± 1.8 years; 77 females). Sixty-four patients (72.7

Introduction:Proximal junctional kyphosis/failure (PJK/PJF) remains a frequent and severe complication following adult spinal deformity (ASD) surgery. While alignment risk factors are known, the specific mechanical influence of anterior malalignment and pelvic retroversion across different fusion levels remains poorly understood. Research question:How do postoperative anterior malalignment and pelvic retroversion influence PJK/PJF risk and biomechanical forces at the proximal junction based on upper instrumented vertebra (UIV) selection? Material and methods:We retrospectively analyzed 351 ASD patients fused to the pelvis, stratified by UIV: lower thoracic (LT, T9-T11; n = 206) or upper lumbar (UL, T12-L2; n = 145). Radiographic spinopelvic alignment was evaluated. Additionally, a validated finite element model (FEM) of T10-pelvis and L2-pelvis constructs simulated progressive anterior offsets and pelvic retroversion to quantify UIV endplate compressive and shear forces. Results:PJK/PJF incidence was comparable between groups (LT: 22.4%, UL: 22.2%). In both cohorts, PJK patients exhibited greater 6-week postoperative global sagittal malalignment and pelvic retroversion. LT failures were driven by higher SVA and thoracic kyphosis, whereas UL failures associated with increased segmental T10-L2 kyphosis. FEM showed LT constructs experienced predominant compressive forces scaling with anterior offset, while UL constructs experienced predominant posterior shear forces. Pelvic retroversion offered negligible mitigation against compression and limited shear reduction. Discussion and conclusion:UIV selection dictates the biomechanical failure mechanism, not the overall PJK/PJF risk. LT instrumentation exposes the proximal junction to compression, whereas the UL spine is susceptible to shear-driven failure. Pelvic retroversion cannot compensate for residual anterior malalignment. Therefore, UIV choice must account for regional alignment and predictable force vectors.

STUDY DESIGN:Retrospective analysis. OBJECTIVE:This study will compare post-operative pelvic incidence (PI) modification by types of pelvic-fixation. BACKGROUND:In adult spinal deformity (ASD), restoring sagittal spinal alignment which is usually based on PI is essential to improve the patient's quality of life. While PI was postulated to be a constant value, recent literature reported that the latter can change with pelvic fixation after ASD surgery. METHODS:This is a retrospective multicenter study of 423 patients who have undergone ASD surgery between 2012 and 2022. These patients were divided into four groups, based on the type of pelvic fixation they received being either S2-Alar-Iliac screws (S2AI), Iliac screws (IS), Iliosacral screws (ISS), and no pelvic-fixation. Post-operative PI change was defined by an absolute difference of ≥6° between pre- and post-operative values. RESULTS:Patients with S2AI had the higher rate of post-operative PI change (80%) followed by patients with ISS (39.6%) and patients with IS (15.3%).). In addition, patients with a low pre-operative PI had an increased post-operative PI (33.3° pre-operatively to 42.1° post-operatively) while patients with a high pre-operative PI had a lower post-operative PI (69.9° pre-operatively to 66.5° post-operatively). A logistic-regression model controlling for gender, pre-operative PI groups, the change in lumbar lordosis, pelvic tilt, sacral slope, and pelvic fixation showed that only the latter predicted the post-operative change in PI with an adjusted odd-ratio of 6.4. However, pelvic fixation was not a predictor of PI modification when we selected only patients with IS. CONCLUSION:Pelvic fixation was found to be an independent risk factor for PI change with an adjusted Odds-ratio of 6.4. When stratifying by pelvic fixation type, S2AI screws had the greatest impact on post-operative PI change (Odds-ratio=25.3) followed by ISS (Odds-ratio=5.9) with IS having no impact on post-operative PI change.