Association Between LIV+1 Bone Mineral Density Asymmetry and Distal Adding-On Phenomenon in Adolescent Idiopathic Scoliosis

Association Between LIV+1 Bone Mineral Density Asymmetry and Distal Adding-On Phenomenon in Adolescent Idiopathic Scoliosis | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Association Between LIV+1 Bone Mineral Density Asymmetry and Distal Adding-On Phenomenon in Adolescent Idiopathic Scoliosis Chaofan Han, Zihao Ding, Peng Yin, Yong Hai This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9158836/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 9 You are reading this latest preprint version Abstract Objective Distal adding-on phenomenon remains a common complication following corrective surgery for adolescent idiopathic scoliosis (AIS). This study aimed to characterize the spatial distribution of vertebral bone mineral density (BMD) across the scoliotic spine and to investigate whether preoperative BMD asymmetry at the vertebra immediately distal to the lowest instrumented vertebra (LIV + 1) predicts postoperative distal adding-on phenomenon. Methods This retrospective cohort study included 90 patients with AIS who underwent posterior spinal fusion and had preoperative full-spine computed tomography scans. Patient-specific three-dimensional finite element analysis (3D-FEA) models were constructed to quantify BMD along simulated pedicle screw trajectories throughout the spine. Bilateral BMD values were measured across the proximal thoracic curve (PTC), main thoracic curve (MTC), and thoracolumbar/lumbar curve (TLC). Patients were stratified according to curve severity and the presence or absence of postoperative adding-on phenomenon. Inter-side BMD asymmetry at the LIV and LIV + 1 levels was compared between groups. Receiver operating characteristic (ROC) analysis was performed to evaluate the predictive value of LIV + 1 BMD asymmetry. Results The mean whole-spine BMD was 0.85 ± 0.14 g/cm². No significant difference was observed between the overall concave and convex sides (p = 0.510), although region-specific asymmetry patterns were identified. Distal adding-on phenomenon occurred in 15 patients (16.7%). Patients with adding-on showed significantly greater LIV + 1 BMD asymmetry compared with those without (0.14 ± 0.06 vs. 0.11 ± 0.03 g/cm², p = 0.006). LIV + 1 BMD asymmetry demonstrated good predictive performance (AUC = 0.865, 95% CI 0.77–0.96), with an optimal cutoff value of 0.112 g/cm². Multivariable analysis identified LIV + 1 BMD asymmetry as an independent predictor of distal adding-on phenomenon. Conclusion Vertebral BMD in AIS exhibits a region-specific asymmetric distribution pattern. Increased preoperative LIV + 1 BMD asymmetry is independently associated with postoperative distal adding-on phenomenon and may serve as a useful parameter for risk stratification and surgical planning. adolescent idiopathic scoliosis bone mineral density three-dimensional finite element analysis adding-on phenomenon pedicle screw fixation Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Adolescent idiopathic scoliosis (AIS) is a three-dimensional spinal deformity characterized not only by abnormal curvature but also by substantial biomechanical remodeling of vertebral structures 1 . Chronic asymmetric loading along the scoliotic curve alters vertebral trabecular architecture and leads to regional variations in bone mineral density (BMD). These structural adaptations are clinically relevant because vertebral bone quality directly affects the mechanical competence of the spine and the stability of pedicle screw fixation during corrective surgery 2 . Understanding the spatial distribution of BMD within the scoliotic spine may therefore provide important insights into the biomechanical environment underlying deformity progression and surgical outcomes. Previous studies have demonstrated that patients with AIS often exhibit reduced overall BMD compared with healthy adolescents, and several investigations have reported associations between bone density and curve severity 3 – 5 . However, most prior work has relied on dual-energy X-ray absorptiometry (DEXA) or Hounsfield unit–based measurements at limited vertebral levels, primarily within the lumbar spine. These approaches cannot adequately characterize the three-dimensional spatial distribution of bone density across the entire scoliotic spine, particularly within the thoracic region. As a result, how vertebral BMD varies along different curve segments and between the concave and convex sides of the deformity remains incompletely understood. Beyond deformity progression, the mechanical properties of vertebrae adjacent to the fusion construct may also influence postoperative complications. One clinically important complication in AIS surgery is the distal adding-on phenomenon, defined as the postoperative extension of the curve beyond the lowest instrumented vertebra (LIV) 6 . Although several radiographic parameters—such as the stable vertebra and the last touched vertebra—have been proposed to guide LIV selection 7 , these indicators primarily reflect coronal alignment and do not account for the intrinsic biomechanical properties of the vertebrae distal to the fusion construct. The vertebra immediately distal to the fusion (LIV + 1) represents the first mobile segment subjected to concentrated postoperative mechanical stress. If substantial structural asymmetry exists within this vertebra, particularly in terms of bone density distribution, it may create an imbalanced load-bearing environment that predisposes the spine to distal adding-on phenomenon 8 , 9 . Therefore, the present study aimed to integrate three-dimensional quantitative assessment of vertebral BMD with clinical outcome analysis in AIS. First, we sought to systematically characterize the spatial distribution of vertebral BMD along the entire scoliotic spine using a CT-based three-dimensional finite element framework. Second, we investigated whether preoperative BMD asymmetry at the LIV + 1 vertebra is associated with the development of postoperative distal adding-on phenomenon. We hypothesized that greater bilateral BMD asymmetry at LIV + 1 reflects an imbalanced mechanical environment and may serve as a predictive marker for postoperative adding-on phenomenon. Materials and methods Inclusion and Exclusion Criteria This retrospective cohort study included patients with adolescent idiopathic scoliosis who underwent posterior spinal fusion at Beijing Chao-Yang Hospital between 2020 and 2025. The inclusion criteria were as follows: (1) diagnosis of AIS with a primary main thoracic curve (MTC); (2) availability of preoperative full-spine CT scans in DICOM format; (3) skeletal maturity or near maturity (Risser sign IV–V); (4) indication for posterior spinal fusion; (5) complete radiographic records; (6) high-quality CT data suitable for 3D modeling; and (7) absence of congenital vertebral anomalies or syndromes (e.g., neurofibromatosis). The exclusion criteria were: (1) Metabolic bone disorders (e.g., thyroid dysfunction) (2) Syndromic scoliosis associated with abnormal bone metabolism (e.g., Neurofibromatosis complicated with scoliosis). (3) Significant coronal or sagittal imbalance. A total of 90 patients were included in the final analysis. This study was conducted in accordance with the Declaration of Helsinki and was approved by the Institutional Review Board of Beijing Chao-Yang Hospital. Written informed consent was obtained from all participants. Clinical and Radiographic Assessment Demographic data (age, sex, height, weight, BMI) were collected. Preoperative radiographs were used to measure the Cobb angle, thoracic kyphosis, and apical vertebral translation (AVT). AVT was defined as the horizontal distance from the center of the apical vertebra to the C7 plumb line (for proximal thoracic curves) or to the central sacral vertical line (CSVL). Adding-on phenomenon 10 was diagnosed at the 1-year follow-up if either of the following was present: (1) angulation > 5° at the disc space immediately below the LIV, or (2) lateral deviation > 5 mm of the LIV + 1 vertebra from the CSVL. All patients underwent 64-slice spiral CT (1-mm slice thickness) for morphological assessment and DICOM file generation. Patients were stratified by Cobb angle into mild (40°–80°), moderate (80°–120°), and severe (> 120°) groups, and by adding-on status. 3D Finite Element Model Reconstruction DICOM files were imported into 3D Slicer (V5.10.0t, OPEN ACCESS) to generate initial surface models, which were then refined and exported as VTK files (Fig. 1 ). These were meshed into tetrahedral elements via ANSYS (V 15.0) to create 3D-FE models. Voxelwise HU values from CT were converted to BMD (g/cm²) through a three-step calibration process 11 :1. 𝜌 𝐻𝐴=0.0026𝐻𝑈−0.0829 (Converting HU values to hydroxyapatite density using a hydroxyapatite calibration model) 2. 𝜌 𝑎𝑠ℎ=(𝜌_𝐻𝐴+0.09)/1.14 (Ashdensity defined as mass divided by the true bone-tissue volume) 3. 𝜌 𝑎𝑝𝑝=𝜌 𝑎𝑠ℎ/0.6 (apparent density expressed as wet mass divided by the total volume occupied by bone tissue). BMD values were assigned to each finite element using Bonemat (open source, V3.2) yielding patient-specific, BMD-embedded 3D-FE models (Fig. 2 A). To evaluate measurement reproducibility, bone mineral density measurements were independently performed by two observers. Each observer repeated the measurements twice at a two-week interval. Region of interest (ROI) definition and BMD measurement The region of interest (ROI) was outlined on the selected cross-sectional plane to quantify local bone density. (figure 2 B) First, a line of arbitrary length was drawn adjacent to the plane to create a scale bar for calibration. Next, two elliptical ROIs were manually placed on either side of the vertebral midline. The ellipses were oriented to the pedicle’s longitudinal axis and directed toward the anterior vertebral body, thereby approximating the intended screw trajectory and enabling assessment of cancellous bone density surrounding the pedicle screw. Ellipse size and area were determined using the scale bar; grayscale values corresponding to the within-ellipse color variations were then extracted and converted to bone density measurements. Finally, the mean bone density (g/cm²) within each ROI was computed as the total bone density within the ROI divided by the elliptical area. The finite-element model was imported into the open-source visualization toolkit ParaView (v5.70-RC2) to perform selective planar rendering and virtual resection. (figure 3 )The resection plane was specified as a oblique axial-pedicular plane that intersected the pedicle midpoint and was parallel to the superior endplate. This orientation was selected to establish a standardized reference for subsequent biomechanical evaluation of pedicle-screw purchase. Following the plane definition, the cross-sectional region was mapped according to local BMD using a grayscale window of 0–225 HU and a median threshold of 125 HU. The resulting surface was then exported in VTK format and transferred to MATLAB (MathWorks, R2017a) for final postprocessing. We developed a custom MATLAB program (MathWorks, Natick, MA, USA) used to calculate the mean density in an elliptical ROI on the bone density slice. Whole Spine Bone Mineral Density Measurement BMD was measured at the apical vertebra (APEX) and averaged within the proximal thoracic curve (PTC), MTC, and thoracolumbar/lumbar curve (TLC). Concave and convex BMD values were compared within each curved region and across regions. The bone mineral density difference is the absolute value of the difference between the concave and convex sides. For the adding-on analysis, preoperative BMD values for the lowest instrumented vertebra (LIV) and distal to the lowest instrumented vertebra (LIV + 1) were extracted according to the actual surgical constructs. Statistical analysis Analyses were performed via SPSS v22.0. Continuous variables are presented as means ± SDs or medians (IQRs); and categorical variables are presented as n (%). Paired t tests were used to compare concave vs. convex BMD within regions. Independent t test was used to compare the BMD asymmetry of patients who suffered adding-on and those who did not. Pearson’s correlation was used to assess the relationships between BMD asymmetry and clinical/radiographic parameters. ROC curves were used to evaluate the predictive performance of LIV + 1 BMD asymmetry for adding-on phenomenon, with the Youden index used to determine the optimal cutoff. The AUC and 95% CIs were calculated via DeLong’s method. A p value < 0.05 was considered statistically significant. Multivariable logistic regression analysis was performed to identify independent predictors of distal adding-on. Variables with clinical relevance were selected a priori, including, Cobb angle, apical vertebral translation (AVT), body mass index (BMI), and LIV + 1 BMD asymmetry. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated. LIV + 1 BMD asymmetry was entered as a continuous variable, with ORs expressed per 0.01 g/cm² increase. Patients were further stratified into high and low BMD asymmetry groups according to the optimal cutoff value determined by ROC analysis. The incidence of distal adding-on phenomenon between groups was compared using the chi-square test or Fisher’s exact test when appropriate. All measurements were assessed for intra- and inter-observer reliability using intraclass correlation coefficients (ICCs). Results Patient characteristics A total of 90 patients with adolescent idiopathic scoliosis were included in this study. The mean age was 20.2 ± 6.7 years, and the mean Cobb angle was 87.2° ± 35.5°. The average thoracic kyphosis was 35.7° ± 21.8°, and the mean apical vertebral translation (AVT) was 62.9 ± 16.5 mm. Patients were stratified into three groups according to curve severity (40°–80°, 80°–120°, and > 120°). Postoperative distal adding-on phenomenon occurred in 15 patients (16.7%) at the one-year follow-up. Detailed demographic and radiographic characteristics are summarized in Table 1 . Table 1 General information and imaging parameters Age(yrs) All patients n = 90 40°-80° n = 41 80°-120° n = 27 >120° n = 22 20.2 ± 6.7 20.3 ± 7.1 18.3 ± 5.4 19.0 ± 4.8 Height (cm) 151.2 ± 13.5 155.2 ± 17.1 151.8 ± 13.2 134.7 ± 8.9 Weight (kg) 45.8 ± 13.6 47.8 ± 13.7 46.4 ± 9.7 35.4 ± 7.8 BMI (kg/m²) 18.7 ± 3.2 19.6 ± 3.5 20.2 ± 3.7 19.3 ± 2.4 Risser sign 4.8 ± 0.4 4.8 ± 0.5 4.8 ± 0.5 5.0 ± 0.0 Cobb angle (°) 87.2 ± 35.5 58.2 ± 10.1 93.3 ± 11.2 141.2 ± 17.6 kyphotic angle (°) 35.7 ± 21.8 18.6 ± 12.8 28.2 ± 16.2 80.5 ± 28.1 AVT (mm) 62.9 ± 16.5 46.2 ± 14.5 82.6 ± 15.2 93.1 ± 20.6 Mean BMD 0.85 ± 0.14 0.83 ± 0.10 0.81 ± 0.08 0.85 ± 0.08 Spatial Distribution of Vertebral BMD in AIS Mean BMD Values for all spinal segments in Table 2 . The mean BMD across the entire spine was 0.85 ± 0.14 g/cm 2 . Overall, vertebral BMD showed a gradual decline from the upper thoracic to the lumbar levels. No significant difference was observed between the mean BMD values of the concave and convex sides across the entire spine (p = 0.510). Table 2 Mean Bone Mineral Density Values for All Spinal Segments thoracic vertebrae T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 BMD (g/cm 2 ) 0.9884 0.9493 0.9069 0.8979 0.8745 0.8619 0.8293 0.8246 0.8644 0.8609 0.8026 0.7531 lumbar vertebrae L1 L2 L3 L4 L5 BMD (g/cm 2 ) 0.7370 0.7434 0.7312 0.7519 0.8189 However, clear regional differences were identified when the spine was analyzed by curve segments. In the proximal thoracic curve (PTC) and thoracolumbar/lumbar curve (TLC), BMD was significantly higher on the convex side than on the concave side (both p < 0.05). In contrast, within the main thoracic curve (MTC), BMD was significantly greater on the concave side than on the convex side (p < 0.001). These findings indicate a mirror-like spatial distribution pattern of BMD between the concave and convex sides across different curve regions (Table 3 ). Table 3 Average Bone Mineral Density Values at Different Locations (g/cm2) Entire spine Overall BMD (g/cm²) Concave BMD Convex BMD BMD Asymmetry P value 0.85 ± 0.14 0.85 ± 0.11 0.84 ± 0.15 0.01 ± 0.03 0.510 Apex vertebra 0.87 ± 0.12 0.98 ± 0.16 0.75 ± 0.14 0.27 ± 0.12 < 0.001 PTC 0.90 ± 0.14 0.87 ± 0.14 0.94 ± 0.14 0.08 ± 0.06 < 0.05 MTC 0.83 ± 0.15 0.89 ± 0.12 0.76 ± 0.12 0.18 ± 0.08 < 0.001 TLC 0.77 ± 0.13 0.73 ± 0.13 0.80 ± 0.13 0.06 ± 0.06 < 0.05 BMD asymmetry was defined as the absolute difference between concave and convex sides. PTC = proximal thoracic curve; MTC = main thoracic curve; TLC = thoracolumbar/lumbar curve. Association Between BMD Asymmetry and Curve Severity Correlation analysis demonstrated that Cobb angle was moderately associated with BMD asymmetry both at the apical vertebra and across the main thoracic curve region (r = 0.505 and r = 0.632, respectively; both p < 0.01). LIV + 1 BMD Asymmetry and Distal Adding-On Phenomenon Patients who developed postoperative distal adding-on phenomenon demonstrated significantly greater BMD asymmetry at the vertebra immediately distal to the lowest instrumented vertebra (LIV + 1) compared with those without adding-on (0.14 ± 0.06 vs. 0.11 ± 0.03 g/cm2, p = 0.006) (Table 4 ). In contrast, no significant differences were observed at the LIV or the last substantially touched vertebra (LSTV). Table 4 BMD asymmetry at LIV, LIV + 1, and LSTV (g/cm2) LIV + 1 Adding-on (n = 15) Non-Adding-on (n = 75) p 0.14 ± 0.06 0.11 ± 0.03 0.006 LIV 0.08 ± 0.05 0.10 ± 0.09 0.130 LSTV 0.08 ± 0.04 0.10 ± 0.08 0.191 Receiver operating characteristic (ROC) analysis yielded an area under the curve (AUC) of 0.865 (95% CI 0.77–0.96, p < 0.001) for LIV + 1 BMD asymmetry. The optimal cutoff value determined by the Youden index was 0.112 g/cm2, corresponding to a sensitivity of 77.3% and specificity of 86.7% (Fig. 4 ). Multivariate logistic regression analysis was performed to identify independent predictors of distal adding-on phenomenon. After adjusting for potential confounders including Cobb angle, AVT, and BMI, LIV + 1 BMD asymmetry remained independently associated with the occurrence of distal adding-on (OR = 1.638, 95% CI 1.26–2.14, p < 0.001) (Table 5 ). Table 5 Multivariate logistic regression analysis for distal adding-on phenomenon variable B OR 95% CI P value LIV + 1 BMD asymmetry 0.494 1.638 1.26–2.14 < 0.001 Cobb angle 0.021 1.021 0.97–1.08 0.452 AVT -0.024 0.977 0.92–1.03 0.412 BMI -0.069 0.933 0.73–1.20 0.586 Odds ratios for LIV + 1 BMD asymmetry are expressed per 0.01 g/cm² increase. Based on the optimal cutoff value (0.112 g/cm²), patients were categorized into high and low BMD asymmetry groups. The incidence of distal adding-on was significantly higher in the high asymmetry group compared with the low asymmetry group (44.8% vs 3.3%, χ² = 15.430, unadjusted OR ≈ 13.8, p 0.112 g/cm 2 13 16 44.8% χ² = 15.430, p < 0.001 Discussion The present study provides a comprehensive analysis of vertebral BMD distribution in adolescent idiopathic scoliosis using a three-dimensional finite element–based approach. Two principal findings emerged from this study. First, vertebral BMD demonstrated a region-specific mirror-pattern distribution along the scoliotic spine. Second, preoperative BMD asymmetry at the LIV + 1 vertebra was strongly associated with the development of postoperative distal adding-on phenomenon. Together, these findings suggest that side-to-side bone density imbalance may represent an important biomechanical feature of AIS and a potential determinant of postoperative junctional instability. Bone mineral density in the scoliotic spine is strongly influenced by long-standing asymmetric mechanical loading. Previous studies have reported reduced overall bone density in AIS patients and demonstrated correlations between BMD and curve severity 12 – 14 . Consistent with these observations, the present study found a significant association between Cobb angle and the magnitude of bilateral BMD asymmetry at the apical vertebra and along the main curve. This pattern may reflect adaptive remodeling of vertebral trabecular bone under asymmetric compressive forces. Increased compressive loading on the concave side promotes trabecular densification, whereas the convex side experiences relatively reduced mechanical stimulation, leading to lower bone density 15 , 16 . As curve magnitude increases, this imbalance in mechanical loading becomes more pronounced, resulting in greater concave–convex differences in vertebral BMD 17 . Beyond localized measurements, our three-dimensional analysis revealed a region-specific spatial distribution of vertebral BMD along the entire scoliotic spine. The concave side demonstrated a “low–high–low” pattern from the proximal thoracic to the thoracolumbar region, whereas the convex side exhibited the opposite “high–low–high” pattern. This mirror-symmetric distribution likely reflects the alternating mechanical environments created by the primary and compensatory curves. Importantly, these findings highlight that BMD variation in scoliosis is not uniform but rather follows predictable biomechanical patterns along the spinal axis. Mapping these spatial patterns may therefore provide new insights into vertebral load distribution and mechanical adaptation in AIS. A key finding of the present study is the strong association between BMD asymmetry at the LIV + 1 vertebra and postoperative distal adding-on phenomenon. The LIV + 1 segment represents the first unfused motion segment exposed to concentrated mechanical stresses after spinal instrumentation. When substantial differences in bone density exist between the concave and convex sides of this vertebra, the resulting asymmetry in structural stiffness and load-bearing capacity may disrupt the balance of coronal load transmission 18 , 19 . Under postoperative mechanical loading, such imbalance may lead to asymmetric disc degeneration 9 , altered load transfer, and progressive distal curve extension 20 .This biomechanical interpretation is supported by previous studies demonstrating that asymmetric trabecular microstructure and altered elastic modulus can significantly influence spinal load distribution 21 . In the context of rigid posterior instrumentation, stress concentration at the junctional segment may amplify these structural asymmetries, ultimately predisposing the spine to distal adding-on phenomenon. These findings have important implications for surgical planning in AIS. Current strategies for determining the lowest instrumented vertebra rely primarily on radiographic landmarks such as the stable vertebra or the last touched vertebra 7 , 21 , 22 . However, these parameters reflect only geometric alignment and do not account for the underlying mechanical competence of the vertebrae distal to the fusion construct. Our results suggest that quantitative assessment of vertebral BMD symmetry may provide complementary biomechanical information for optimizing LIV selection. Specifically, if substantial BMD asymmetry is identified at the vertebra immediately distal to the planned LIV, extending the fusion construct to a more structurally balanced vertebra may help reduce the risk of postoperative adding-on. Integrating advanced imaging-based bone quality assessment into preoperative planning may therefore improve the biomechanical stability of scoliosis correction constructs. This study has several limitations. First, the retrospective design and relatively limited sample size may restrict the generalizability of the findings. The relatively small number of events may limit the stability of the multivariable regression model and introduce a risk of overfitting. Second, the study population primarily consisted of patients with main thoracic curve patterns, and the observed BMD distribution may differ in other curve types. Third, although the CT-based three-dimensional finite element framework allows detailed estimation of vertebral BMD distribution, it represents a computational approximation rather than a direct mechanical measurement. Finally, other factors associated with distal adding-on phenomenon, including sagittal alignment and surgical technique, were not comprehensively evaluated. Conclusion This study demonstrates that vertebral BMD in AIS follows a region-specific asymmetric distribution pattern and identifies increased preoperative LIV + 1 BMD asymmetry as an independent predictor of distal adding-on phenomenon. Quantitative assessment of BMD asymmetry may provide clinically relevant biomechanical information and assist in optimizing distal fusion level selection in AIS surgery. Declarations Ethics approval and consent to participate: The study was conducted in accordance with the Declaration of Helsinki and approved by the Ethics Committee of Beijing Chao-Yang Hospital, Capital Medical University. Informed consent was obtained from all individual participants included in the study. Consent for publication: Not Applicable. Funding: None. Author Contribution Conceptualization, YH and CH; methodology, PY; software, CH; validation, PY, YH; formal analysis, CH; resources, YH; data curation, ZD; writing—original draft preparation, ZD; writing—review and editing, YH; visualization, PY; supervision, CH; project administration, PY. All authors have read and agreed to the final version of the manuscript Acknowledgements: We would like to thank Prof. Li Guan for his help in the methodology review. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9158836","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":629658244,"identity":"bde35df7-fc63-4b1c-afcc-b69443f1deab","order_by":0,"name":"Chaofan Han","email":"","orcid":"","institution":"Capital Medical University","correspondingAuthor":false,"prefix":"","firstName":"Chaofan","middleName":"","lastName":"Han","suffix":""},{"id":629658245,"identity":"d4cabc4e-fe16-4775-a6e9-aea8f34e450a","order_by":1,"name":"Zihao Ding","email":"","orcid":"","institution":"Capital Medical University","correspondingAuthor":false,"prefix":"","firstName":"Zihao","middleName":"","lastName":"Ding","suffix":""},{"id":629658246,"identity":"c3f2ba09-4b40-41e7-b799-5b5389faa21b","order_by":2,"name":"Peng Yin","email":"","orcid":"","institution":"Capital Medical University","correspondingAuthor":false,"prefix":"","firstName":"Peng","middleName":"","lastName":"Yin","suffix":""},{"id":629658247,"identity":"c6ce7c0b-3beb-415b-953d-cf6091df16ce","order_by":3,"name":"Yong Hai","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA1klEQVRIiWNgGAWjYLACxgYGHn4Em1gtkg2kamEwOECsFoPjvYdfMO6wkzE+fvjpZh4GG9kNB5ifPcCr5cy5NAvGM8k8ZmfSzG7zMKQZbzjAZm6AV8uNHDMDxjZmHrMbPGxALYcTNxzgYZMgQks9j/EMsJb/RGkxfsDYdpjHQAKs5QBhLZJnzpgxJLYd55EA+uXmHINk45mH2czwauE73mP84WNbtT1/++FnN95U2Mn2HW9+hleLwgEGNokEhDuBmBmfeiCQb2Bg/kBAzSgYBaNgFIx0AADv+kfTtHx9ZgAAAABJRU5ErkJggg==","orcid":"","institution":"Capital Medical University","correspondingAuthor":true,"prefix":"","firstName":"Yong","middleName":"","lastName":"Hai","suffix":""}],"badges":[],"createdAt":"2026-03-18 11:38:27","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9158836/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9158836/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":107949002,"identity":"d4f35e08-56f8-4c0a-80b2-4d8b98c4a9be","added_by":"auto","created_at":"2026-04-28 00:24:07","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":35687,"visible":true,"origin":"","legend":"\u003cp\u003eFollowing fine-tuned reconstruction, the model was segmented at each intervertebral disc space, resulting in individualized, anatomically isolated vertebral models.\u003c/p\u003e","description":"","filename":"groupimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-9158836/v1/b015c144badcd63e30c67099.jpeg"},{"id":107949003,"identity":"a6a03613-5a42-4235-8ac6-e38144f80776","added_by":"auto","created_at":"2026-04-28 00:24:07","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":423562,"visible":true,"origin":"","legend":"\u003cp\u003e2A. Material properties were assigned to the finite element model based on bone mineral density (BMD) values converted from Hounsfield units (HUs), resulting in distinct color mapping that reflects regional variations in vertebral density. 2B. The vertebrae were sectioned at the pedicle level, and regions of interest (ROIs) were delineated on the resulting cross-sectional plane.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-9158836/v1/971749bd9b64982851463ae4.png"},{"id":107949004,"identity":"74166772-e3c7-4646-b498-70c73cb138f0","added_by":"auto","created_at":"2026-04-28 00:24:07","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":34394,"visible":true,"origin":"","legend":"\u003cp\u003eGrayscale-based material assignment was performed using ParaView.\u003c/p\u003e","description":"","filename":"groupimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-9158836/v1/9065a18ce7fe402e23da3f74.jpeg"},{"id":107949016,"identity":"3f8999a7-b4c8-40b7-998f-14b36be6634f","added_by":"auto","created_at":"2026-04-28 00:24:12","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":44196,"visible":true,"origin":"","legend":"\u003cp\u003eReceiver operating characteristic (ROC) curve demonstrating the predictive performance of LIV+1 BMD asymmetry for distal adding-on phenomenon. AUC=0.865, p\u0026lt;0.001, 95%CI 0.77-0.96\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-9158836/v1/4a07ddccc320367374554e0e.png"},{"id":107949024,"identity":"2f9c860f-8030-4ec8-902c-abec81b8e62f","added_by":"auto","created_at":"2026-04-28 00:24:17","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":833272,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9158836/v1/33a8ad26-bd8f-4e16-98a6-beb48e16c63f.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Association Between LIV+1 Bone Mineral Density Asymmetry and Distal Adding-On Phenomenon in Adolescent Idiopathic Scoliosis","fulltext":[{"header":"Introduction","content":"\u003cp\u003eAdolescent idiopathic scoliosis (AIS) is a three-dimensional spinal deformity characterized not only by abnormal curvature but also by substantial biomechanical remodeling of vertebral structures\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. Chronic asymmetric loading along the scoliotic curve alters vertebral trabecular architecture and leads to regional variations in bone mineral density (BMD). These structural adaptations are clinically relevant because vertebral bone quality directly affects the mechanical competence of the spine and the stability of pedicle screw fixation during corrective surgery\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. Understanding the spatial distribution of BMD within the scoliotic spine may therefore provide important insights into the biomechanical environment underlying deformity progression and surgical outcomes.\u003c/p\u003e \u003cp\u003ePrevious studies have demonstrated that patients with AIS often exhibit reduced overall BMD compared with healthy adolescents, and several investigations have reported associations between bone density and curve severity\u003csup\u003e\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. However, most prior work has relied on dual-energy X-ray absorptiometry (DEXA) or Hounsfield unit\u0026ndash;based measurements at limited vertebral levels, primarily within the lumbar spine. These approaches cannot adequately characterize the three-dimensional spatial distribution of bone density across the entire scoliotic spine, particularly within the thoracic region. As a result, how vertebral BMD varies along different curve segments and between the concave and convex sides of the deformity remains incompletely understood.\u003c/p\u003e \u003cp\u003eBeyond deformity progression, the mechanical properties of vertebrae adjacent to the fusion construct may also influence postoperative complications. One clinically important complication in AIS surgery is the distal adding-on phenomenon, defined as the postoperative extension of the curve beyond the lowest instrumented vertebra (LIV)\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. Although several radiographic parameters\u0026mdash;such as the stable vertebra and the last touched vertebra\u0026mdash;have been proposed to guide LIV selection\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e, these indicators primarily reflect coronal alignment and do not account for the intrinsic biomechanical properties of the vertebrae distal to the fusion construct. The vertebra immediately distal to the fusion (LIV\u0026thinsp;+\u0026thinsp;1) represents the first mobile segment subjected to concentrated postoperative mechanical stress. If substantial structural asymmetry exists within this vertebra, particularly in terms of bone density distribution, it may create an imbalanced load-bearing environment that predisposes the spine to distal adding-on phenomenon \u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eTherefore, the present study aimed to integrate three-dimensional quantitative assessment of vertebral BMD with clinical outcome analysis in AIS. First, we sought to systematically characterize the spatial distribution of vertebral BMD along the entire scoliotic spine using a CT-based three-dimensional finite element framework. Second, we investigated whether preoperative BMD asymmetry at the LIV\u0026thinsp;+\u0026thinsp;1 vertebra is associated with the development of postoperative distal adding-on phenomenon. We hypothesized that greater bilateral BMD asymmetry at LIV\u0026thinsp;+\u0026thinsp;1 reflects an imbalanced mechanical environment and may serve as a predictive marker for postoperative adding-on phenomenon.\u003c/p\u003e"},{"header":"Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eInclusion and Exclusion Criteria\u003c/h2\u003e \u003cp\u003eThis retrospective cohort study included patients with adolescent idiopathic scoliosis who underwent posterior spinal fusion at Beijing Chao-Yang Hospital between 2020 and 2025. The inclusion criteria were as follows: (1) diagnosis of AIS with a primary main thoracic curve (MTC); (2) availability of preoperative full-spine CT scans in DICOM format; (3) skeletal maturity or near maturity (Risser sign IV\u0026ndash;V); (4) indication for posterior spinal fusion; (5) complete radiographic records; (6) high-quality CT data suitable for 3D modeling; and (7) absence of congenital vertebral anomalies or syndromes (e.g., neurofibromatosis). The exclusion criteria were: (1) Metabolic bone disorders (e.g., thyroid dysfunction) (2) Syndromic scoliosis associated with abnormal bone metabolism (e.g., Neurofibromatosis complicated with scoliosis). (3) Significant coronal or sagittal imbalance. A total of 90 patients were included in the final analysis. This study was conducted in accordance with the Declaration of Helsinki and was approved by the Institutional Review Board of Beijing Chao-Yang Hospital. Written informed consent was obtained from all participants.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eClinical and Radiographic Assessment\u003c/h3\u003e\n\u003cp\u003eDemographic data (age, sex, height, weight, BMI) were collected. Preoperative radiographs were used to measure the Cobb angle, thoracic kyphosis, and apical vertebral translation (AVT). AVT was defined as the horizontal distance from the center of the apical vertebra to the C7 plumb line (for proximal thoracic curves) or to the central sacral vertical line (CSVL). Adding-on phenomenon\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e was diagnosed at the 1-year follow-up if either of the following was present: (1) angulation\u0026thinsp;\u0026gt;\u0026thinsp;5\u0026deg; at the disc space immediately below the LIV, or (2) lateral deviation\u0026thinsp;\u0026gt;\u0026thinsp;5 mm of the LIV\u0026thinsp;+\u0026thinsp;1 vertebra from the CSVL. All patients underwent 64-slice spiral CT (1-mm slice thickness) for morphological assessment and DICOM file generation. Patients were stratified by Cobb angle into mild (40\u0026deg;\u0026ndash;80\u0026deg;), moderate (80\u0026deg;\u0026ndash;120\u0026deg;), and severe (\u0026gt;\u0026thinsp;120\u0026deg;) groups, and by adding-on status.\u003c/p\u003e \u003cp\u003e \u003cb\u003e3D Finite Element Model Reconstruction\u003c/b\u003e \u003c/p\u003e \u003cp\u003eDICOM files were imported into 3D Slicer (V5.10.0t, OPEN ACCESS) to generate initial surface models, which were then refined and exported as VTK files (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). These were meshed into tetrahedral elements via ANSYS (V 15.0) to create 3D-FE models. Voxelwise HU values from CT were converted to BMD (g/cm\u0026sup2;) through a three-step calibration process \u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e:1. \u0026#120588; \u0026#119867;\u0026#119860;=0.0026\u0026#119867;\u0026#119880;\u0026minus;0.0829 (Converting HU values to hydroxyapatite density using a hydroxyapatite calibration model) 2. \u0026#120588; \u0026#119886;\u0026#119904;ℎ=(\u0026#120588;_\u0026#119867;\u0026#119860;+0.09)/1.14 (Ashdensity defined as mass divided by the true bone-tissue volume) 3. \u0026#120588; \u0026#119886;\u0026#119901;\u0026#119901;=\u0026#120588; \u0026#119886;\u0026#119904;ℎ/0.6 (apparent density expressed as wet mass divided by the total volume occupied by bone tissue). BMD values were assigned to each finite element using Bonemat (open source, V3.2) yielding patient-specific, BMD-embedded 3D-FE models (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA). To evaluate measurement reproducibility, bone mineral density measurements were independently performed by two observers. Each observer repeated the measurements twice at a two-week interval.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eRegion of interest (ROI) definition and BMD measurement\u003c/h3\u003e\n\u003cp\u003eThe region of interest (ROI) was outlined on the selected cross-sectional plane to quantify local bone density. (figure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB) First, a line of arbitrary length was drawn adjacent to the plane to create a scale bar for calibration. Next, two elliptical ROIs were manually placed on either side of the vertebral midline. The ellipses were oriented to the pedicle\u0026rsquo;s longitudinal axis and directed toward the anterior vertebral body, thereby approximating the intended screw trajectory and enabling assessment of cancellous bone density surrounding the pedicle screw. Ellipse size and area were determined using the scale bar; grayscale values corresponding to the within-ellipse color variations were then extracted and converted to bone density measurements. Finally, the mean bone density (g/cm\u0026sup2;) within each ROI was computed as the total bone density within the ROI divided by the elliptical area.\u003c/p\u003e \u003cp\u003eThe finite-element model was imported into the open-source visualization toolkit ParaView (v5.70-RC2) to perform selective planar rendering and virtual resection. (figure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e)The resection plane was specified as a oblique axial-pedicular plane that intersected the pedicle midpoint and was parallel to the superior endplate. This orientation was selected to establish a standardized reference for subsequent biomechanical evaluation of pedicle-screw purchase. Following the plane definition, the cross-sectional region was mapped according to local BMD using a grayscale window of 0\u0026ndash;225 HU and a median threshold of 125 HU. The resulting surface was then exported in VTK format and transferred to MATLAB (MathWorks, R2017a) for final postprocessing. We developed a custom MATLAB program (MathWorks, Natick, MA, USA) used to calculate the mean density in an elliptical ROI on the bone density slice.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eWhole Spine Bone Mineral Density Measurement\u003c/h3\u003e\n\u003cp\u003eBMD was measured at the apical vertebra (APEX) and averaged within the proximal thoracic curve (PTC), MTC, and thoracolumbar/lumbar curve (TLC). Concave and convex BMD values were compared within each curved region and across regions. The bone mineral density difference is the absolute value of the difference between the concave and convex sides. For the adding-on analysis, preoperative BMD values for the lowest instrumented vertebra (LIV) and distal to the lowest instrumented vertebra (LIV\u0026thinsp;+\u0026thinsp;1) were extracted according to the actual surgical constructs.\u003c/p\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eStatistical analysis\u003c/h2\u003e \u003cp\u003eAnalyses were performed via SPSS v22.0. Continuous variables are presented as means\u0026thinsp;\u0026plusmn;\u0026thinsp;SDs or medians (IQRs); and categorical variables are presented as n (%). Paired t tests were used to compare concave vs. convex BMD within regions. Independent t test was used to compare the BMD asymmetry of patients who suffered adding-on and those who did not. Pearson\u0026rsquo;s correlation was used to assess the relationships between BMD asymmetry and clinical/radiographic parameters. ROC curves were used to evaluate the predictive performance of LIV\u0026thinsp;+\u0026thinsp;1 BMD asymmetry for adding-on phenomenon, with the Youden index used to determine the optimal cutoff. The AUC and 95% CIs were calculated via DeLong\u0026rsquo;s method. A p value\u0026thinsp;\u0026lt;\u0026thinsp;0.05 was considered statistically significant. Multivariable logistic regression analysis was performed to identify independent predictors of distal adding-on. Variables with clinical relevance were selected a priori, including, Cobb angle, apical vertebral translation (AVT), body mass index (BMI), and LIV\u0026thinsp;+\u0026thinsp;1 BMD asymmetry. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated. LIV\u0026thinsp;+\u0026thinsp;1 BMD asymmetry was entered as a continuous variable, with ORs expressed per 0.01 g/cm\u0026sup2; increase. Patients were further stratified into high and low BMD asymmetry groups according to the optimal cutoff value determined by ROC analysis. The incidence of distal adding-on phenomenon between groups was compared using the chi-square test or Fisher\u0026rsquo;s exact test when appropriate. All measurements were assessed for intra- and inter-observer reliability using intraclass correlation coefficients (ICCs).\u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003ePatient characteristics\u003c/h2\u003e \u003cp\u003eA total of 90 patients with adolescent idiopathic scoliosis were included in this study. The mean age was 20.2\u0026thinsp;\u0026plusmn;\u0026thinsp;6.7 years, and the mean Cobb angle was 87.2\u0026deg; \u0026plusmn; 35.5\u0026deg;. The average thoracic kyphosis was 35.7\u0026deg; \u0026plusmn; 21.8\u0026deg;, and the mean apical vertebral translation (AVT) was 62.9\u0026thinsp;\u0026plusmn;\u0026thinsp;16.5 mm. Patients were stratified into three groups according to curve severity (40\u0026deg;\u0026ndash;80\u0026deg;, 80\u0026deg;\u0026ndash;120\u0026deg;, and \u0026gt;\u0026thinsp;120\u0026deg;). Postoperative distal adding-on phenomenon occurred in 15 patients (16.7%) at the one-year follow-up. Detailed demographic and radiographic characteristics are summarized in Table \u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eGeneral information and imaging parameters\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eAge(yrs)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAll patients\u003c/p\u003e \u003cp\u003en\u0026thinsp;=\u0026thinsp;90\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e40\u0026deg;-80\u0026deg;\u003c/p\u003e \u003cp\u003en\u0026thinsp;=\u0026thinsp;41\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e80\u0026deg;-120\u0026deg;\u003c/p\u003e \u003cp\u003en\u0026thinsp;=\u0026thinsp;27\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026gt;120\u0026deg;\u003c/p\u003e \u003cp\u003en\u0026thinsp;=\u0026thinsp;22\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e20.2\u0026thinsp;\u0026plusmn;\u0026thinsp;6.7\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e20.3\u0026thinsp;\u0026plusmn;\u0026thinsp;7.1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e18.3\u0026thinsp;\u0026plusmn;\u0026thinsp;5.4\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e19.0\u0026thinsp;\u0026plusmn;\u0026thinsp;4.8\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHeight (cm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e151.2\u0026thinsp;\u0026plusmn;\u0026thinsp;13.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e155.2\u0026thinsp;\u0026plusmn;\u0026thinsp;17.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e151.8\u0026thinsp;\u0026plusmn;\u0026thinsp;13.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e134.7\u0026thinsp;\u0026plusmn;\u0026thinsp;8.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eWeight (kg)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e45.8\u0026thinsp;\u0026plusmn;\u0026thinsp;13.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e47.8\u0026thinsp;\u0026plusmn;\u0026thinsp;13.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e46.4\u0026thinsp;\u0026plusmn;\u0026thinsp;9.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e35.4\u0026thinsp;\u0026plusmn;\u0026thinsp;7.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBMI (kg/m\u0026sup2;)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e18.7\u0026thinsp;\u0026plusmn;\u0026thinsp;3.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e19.6\u0026thinsp;\u0026plusmn;\u0026thinsp;3.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e20.2\u0026thinsp;\u0026plusmn;\u0026thinsp;3.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e19.3\u0026thinsp;\u0026plusmn;\u0026thinsp;2.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRisser sign\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e4.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e4.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e4.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e5.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCobb angle (\u0026deg;)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e87.2\u0026thinsp;\u0026plusmn;\u0026thinsp;35.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e58.2\u0026thinsp;\u0026plusmn;\u0026thinsp;10.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e93.3\u0026thinsp;\u0026plusmn;\u0026thinsp;11.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e141.2\u0026thinsp;\u0026plusmn;\u0026thinsp;17.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ekyphotic angle (\u0026deg;)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e35.7\u0026thinsp;\u0026plusmn;\u0026thinsp;21.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e18.6\u0026thinsp;\u0026plusmn;\u0026thinsp;12.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e28.2\u0026thinsp;\u0026plusmn;\u0026thinsp;16.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e80.5\u0026thinsp;\u0026plusmn;\u0026thinsp;28.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAVT (mm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e62.9\u0026thinsp;\u0026plusmn;\u0026thinsp;16.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e46.2\u0026thinsp;\u0026plusmn;\u0026thinsp;14.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e82.6\u0026thinsp;\u0026plusmn;\u0026thinsp;15.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e93.1\u0026thinsp;\u0026plusmn;\u0026thinsp;20.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMean BMD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e0.85\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e0.83\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e0.81\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e0.85\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eSpatial Distribution of Vertebral BMD in AIS\u003c/h3\u003e\n\u003cp\u003eMean BMD Values for all spinal segments in Table \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The mean BMD across the entire spine was 0.85\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14 g/cm\u003csup\u003e2\u003c/sup\u003e. Overall, vertebral BMD showed a gradual decline from the upper thoracic to the lumbar levels. No significant difference was observed between the mean BMD values of the concave and convex sides across the entire spine (p\u0026thinsp;=\u0026thinsp;0.510).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eMean Bone Mineral Density Values for All Spinal Segments\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"13\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c11\" colnum=\"11\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c12\" colnum=\"12\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c13\" colnum=\"13\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003ethoracic vertebrae\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eT1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eT2\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eT3\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eT4\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eT5\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eT6\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eT7\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003eT8\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c10\"\u003e \u003cp\u003eT9\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c11\"\u003e \u003cp\u003eT10\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c12\"\u003e \u003cp\u003eT11\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c13\"\u003e \u003cp\u003eT12\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBMD (g/cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.9884\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.9493\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.9069\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.8979\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.8745\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.8619\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.8293\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.8246\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e \u003cp\u003e0.8644\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c11\"\u003e \u003cp\u003e0.8609\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c12\"\u003e \u003cp\u003e0.8026\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c13\"\u003e \u003cp\u003e0.7531\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003elumbar vertebrae\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eL1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eL2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eL3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eL4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eL5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c13\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBMD (g/cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.7370\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.7434\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.7312\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.7519\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.8189\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c13\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eHowever, clear regional differences were identified when the spine was analyzed by curve segments. In the proximal thoracic curve (PTC) and thoracolumbar/lumbar curve (TLC), BMD was significantly higher on the convex side than on the concave side (both p\u0026thinsp;\u0026lt;\u0026thinsp;0.05). In contrast, within the main thoracic curve (MTC), BMD was significantly greater on the concave side than on the convex side (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001). These findings indicate a mirror-like spatial distribution pattern of BMD between the concave and convex sides across different curve regions (Table \u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eAverage Bone Mineral Density Values at Different Locations (g/cm2)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eEntire spine\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eOverall BMD (g/cm\u0026sup2;)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eConcave BMD\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eConvex BMD\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eBMD Asymmetry\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eP value\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.85\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.85\u0026thinsp;\u0026plusmn;\u0026thinsp;0.11\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.84\u0026thinsp;\u0026plusmn;\u0026thinsp;0.15\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.01\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.510\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eApex vertebra\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.87\u0026thinsp;\u0026plusmn;\u0026thinsp;0.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.98\u0026thinsp;\u0026plusmn;\u0026thinsp;0.16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.75\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.27\u0026thinsp;\u0026plusmn;\u0026thinsp;0.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePTC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.90\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.87\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.94\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.08\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.05\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMTC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.83\u0026thinsp;\u0026plusmn;\u0026thinsp;0.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.89\u0026thinsp;\u0026plusmn;\u0026thinsp;0.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.76\u0026thinsp;\u0026plusmn;\u0026thinsp;0.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.18\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTLC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.77\u0026thinsp;\u0026plusmn;\u0026thinsp;0.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.73\u0026thinsp;\u0026plusmn;\u0026thinsp;0.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.80\u0026thinsp;\u0026plusmn;\u0026thinsp;0.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.06\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.05\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"6\" nameend=\"c6\" namest=\"c1\"\u003e \u003cp\u003eBMD asymmetry was defined as the absolute difference between concave and convex sides.\u003c/p\u003e \u003cp\u003ePTC\u0026thinsp;=\u0026thinsp;proximal thoracic curve; MTC\u0026thinsp;=\u0026thinsp;main thoracic curve; TLC\u0026thinsp;=\u0026thinsp;thoracolumbar/lumbar curve.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eAssociation Between BMD Asymmetry and Curve Severity\u003c/h2\u003e \u003cp\u003eCorrelation analysis demonstrated that Cobb angle was moderately associated with BMD asymmetry both at the apical vertebra and across the main thoracic curve region (r\u0026thinsp;=\u0026thinsp;0.505 and r\u0026thinsp;=\u0026thinsp;0.632, respectively; both p\u0026thinsp;\u0026lt;\u0026thinsp;0.01).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eLIV\u0026thinsp;+\u0026thinsp;1 BMD Asymmetry and Distal Adding-On Phenomenon\u003c/h2\u003e \u003cp\u003ePatients who developed postoperative distal adding-on phenomenon demonstrated significantly greater BMD asymmetry at the vertebra immediately distal to the lowest instrumented vertebra (LIV\u0026thinsp;+\u0026thinsp;1) compared with those without adding-on (0.14\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06 vs. 0.11\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 g/cm2, p\u0026thinsp;=\u0026thinsp;0.006) (Table \u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). In contrast, no significant differences were observed at the LIV or the last substantially touched vertebra (LSTV).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eBMD asymmetry at LIV, LIV\u0026thinsp;+\u0026thinsp;1, and LSTV (g/cm2)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eLIV\u0026thinsp;+\u0026thinsp;1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAdding-on (n\u0026thinsp;=\u0026thinsp;15)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNon-Adding-on (n\u0026thinsp;=\u0026thinsp;75)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ep\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.14\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.11\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.006\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLIV\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.08\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.10\u0026thinsp;\u0026plusmn;\u0026thinsp;0.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.130\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLSTV\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.08\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.10\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.191\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eReceiver operating characteristic (ROC) analysis yielded an area under the curve (AUC) of 0.865 (95% CI 0.77\u0026ndash;0.96, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001) for LIV\u0026thinsp;+\u0026thinsp;1 BMD asymmetry. The optimal cutoff value determined by the Youden index was 0.112 g/cm2, corresponding to a sensitivity of 77.3% and specificity of 86.7% (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eMultivariate logistic regression analysis was performed to identify independent predictors of distal adding-on phenomenon. After adjusting for potential confounders including Cobb angle, AVT, and BMI, LIV\u0026thinsp;+\u0026thinsp;1 BMD asymmetry remained independently associated with the occurrence of distal adding-on (OR\u0026thinsp;=\u0026thinsp;1.638, 95% CI 1.26\u0026ndash;2.14, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001) (Table \u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab5\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eMultivariate logistic regression analysis for distal adding-on phenomenon\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003evariable\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eB\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eOR\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e95% CI\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eP value\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLIV\u0026thinsp;+\u0026thinsp;1 BMD asymmetry\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.494\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.638\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.26\u0026ndash;2.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCobb angle\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.021\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.021\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.97\u0026ndash;1.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.452\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAVT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-0.024\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.977\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.92\u0026ndash;1.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.412\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBMI\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-0.069\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.933\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.73\u0026ndash;1.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.586\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"5\" nameend=\"c5\" namest=\"c1\"\u003e \u003cp\u003eOdds ratios for LIV\u0026thinsp;+\u0026thinsp;1 BMD asymmetry are expressed per 0.01 g/cm\u0026sup2; increase.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eBased on the optimal cutoff value (0.112 g/cm\u0026sup2;), patients were categorized into high and low BMD asymmetry groups. The incidence of distal adding-on was significantly higher in the high asymmetry group compared with the low asymmetry group (44.8% vs 3.3%, χ\u0026sup2; = 15.430, unadjusted OR\u0026thinsp;\u0026asymp;\u0026thinsp;13.8, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001) (Table \u003cspan refid=\"Tab6\" class=\"InternalRef\"\u003e6\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab6\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 6\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eIncidence of distal adding-on phenomenon according to BMD asymmetry group\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGroup\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAdding-on\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNon-Adding-on\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eIncidence\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u0026le;\u0026thinsp;0.112 g/cm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e59\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3.3%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u0026gt;0.112 g/cm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e44.8%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003eχ\u0026sup2; = 15.430, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe present study provides a comprehensive analysis of vertebral BMD distribution in adolescent idiopathic scoliosis using a three-dimensional finite element\u0026ndash;based approach. Two principal findings emerged from this study. First, vertebral BMD demonstrated a region-specific mirror-pattern distribution along the scoliotic spine. Second, preoperative BMD asymmetry at the LIV\u0026thinsp;+\u0026thinsp;1 vertebra was strongly associated with the development of postoperative distal adding-on phenomenon. Together, these findings suggest that side-to-side bone density imbalance may represent an important biomechanical feature of AIS and a potential determinant of postoperative junctional instability.\u003c/p\u003e \u003cp\u003eBone mineral density in the scoliotic spine is strongly influenced by long-standing asymmetric mechanical loading. Previous studies have reported reduced overall bone density in AIS patients and demonstrated correlations between BMD and curve severity\u003csup\u003e\u003cspan additionalcitationids=\"CR13\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. Consistent with these observations, the present study found a significant association between Cobb angle and the magnitude of bilateral BMD asymmetry at the apical vertebra and along the main curve. This pattern may reflect adaptive remodeling of vertebral trabecular bone under asymmetric compressive forces. Increased compressive loading on the concave side promotes trabecular densification, whereas the convex side experiences relatively reduced mechanical stimulation, leading to lower bone density\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. As curve magnitude increases, this imbalance in mechanical loading becomes more pronounced, resulting in greater concave\u0026ndash;convex differences in vertebral BMD\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eBeyond localized measurements, our three-dimensional analysis revealed a region-specific spatial distribution of vertebral BMD along the entire scoliotic spine. The concave side demonstrated a \u0026ldquo;low\u0026ndash;high\u0026ndash;low\u0026rdquo; pattern from the proximal thoracic to the thoracolumbar region, whereas the convex side exhibited the opposite \u0026ldquo;high\u0026ndash;low\u0026ndash;high\u0026rdquo; pattern. This mirror-symmetric distribution likely reflects the alternating mechanical environments created by the primary and compensatory curves. Importantly, these findings highlight that BMD variation in scoliosis is not uniform but rather follows predictable biomechanical patterns along the spinal axis. Mapping these spatial patterns may therefore provide new insights into vertebral load distribution and mechanical adaptation in AIS.\u003c/p\u003e \u003cp\u003eA key finding of the present study is the strong association between BMD asymmetry at the LIV\u0026thinsp;+\u0026thinsp;1 vertebra and postoperative distal adding-on phenomenon. The LIV\u0026thinsp;+\u0026thinsp;1 segment represents the first unfused motion segment exposed to concentrated mechanical stresses after spinal instrumentation. When substantial differences in bone density exist between the concave and convex sides of this vertebra, the resulting asymmetry in structural stiffness and load-bearing capacity may disrupt the balance of coronal load transmission\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. Under postoperative mechanical loading, such imbalance may lead to asymmetric disc degeneration\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e, altered load transfer, and progressive distal curve extension\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e.This biomechanical interpretation is supported by previous studies demonstrating that asymmetric trabecular microstructure and altered elastic modulus can significantly influence spinal load distribution\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. In the context of rigid posterior instrumentation, stress concentration at the junctional segment may amplify these structural asymmetries, ultimately predisposing the spine to distal adding-on phenomenon.\u003c/p\u003e \u003cp\u003eThese findings have important implications for surgical planning in AIS. Current strategies for determining the lowest instrumented vertebra rely primarily on radiographic landmarks such as the stable vertebra or the last touched vertebra\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. However, these parameters reflect only geometric alignment and do not account for the underlying mechanical competence of the vertebrae distal to the fusion construct. Our results suggest that quantitative assessment of vertebral BMD symmetry may provide complementary biomechanical information for optimizing LIV selection.\u003c/p\u003e \u003cp\u003eSpecifically, if substantial BMD asymmetry is identified at the vertebra immediately distal to the planned LIV, extending the fusion construct to a more structurally balanced vertebra may help reduce the risk of postoperative adding-on. Integrating advanced imaging-based bone quality assessment into preoperative planning may therefore improve the biomechanical stability of scoliosis correction constructs.\u003c/p\u003e \u003cp\u003eThis study has several limitations. First, the retrospective design and relatively limited sample size may restrict the generalizability of the findings. The relatively small number of events may limit the stability of the multivariable regression model and introduce a risk of overfitting. Second, the study population primarily consisted of patients with main thoracic curve patterns, and the observed BMD distribution may differ in other curve types. Third, although the CT-based three-dimensional finite element framework allows detailed estimation of vertebral BMD distribution, it represents a computational approximation rather than a direct mechanical measurement. Finally, other factors associated with distal adding-on phenomenon, including sagittal alignment and surgical technique, were not comprehensively evaluated.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThis study demonstrates that vertebral BMD in AIS follows a region-specific asymmetric distribution pattern and identifies increased preoperative LIV\u0026thinsp;+\u0026thinsp;1 BMD asymmetry as an independent predictor of distal adding-on phenomenon. Quantitative assessment of BMD asymmetry may provide clinically relevant biomechanical information and assist in optimizing distal fusion level selection in AIS surgery.\u003c/p\u003e"},{"header":"Declarations","content":" \u003cp\u003e \u003cstrong\u003eEthics approval and consent to participate:\u003c/strong\u003e \u003cp\u003e The study was conducted in accordance with the Declaration of Helsinki and approved by the Ethics Committee of Beijing Chao-Yang Hospital, Capital Medical University. Informed consent was obtained from all individual participants included in the study.\u003c/p\u003e \u003cp\u003e \u003cstrong\u003eConsent for publication:\u003c/strong\u003e \u003cp\u003eNot Applicable.\u003c/p\u003e \u003ch2\u003eFunding:\u003c/h2\u003e \u003cp\u003eNone.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eConceptualization, YH and CH; methodology, PY; software, CH; validation, PY, YH; formal analysis, CH; resources, YH; data curation, ZD; writing\u0026mdash;original draft preparation, ZD; writing\u0026mdash;review and editing, YH; visualization, PY; supervision, CH; project administration, PY. All authors have read and agreed to the final version of the manuscript\u003c/p\u003e\u003ch2\u003eAcknowledgements:\u003c/h2\u003e \u003cp\u003eWe would like to thank Prof. Li Guan for his help in the methodology review.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eYu P, Tang Z, Chang W, et al. Degenerative scoliosis may trigger higher incidence of adjacent vertebral fractures following percutaneous vertebroplasty: a clinical evidence-based biomechanical research. Osteoporos Int. 2026;37(1):133\u0026ndash;43.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCastro FP. Jr. Adolescent idiopathic scoliosis, bracing, and the Hueter-Volkmann principle. Spine J. 2003;3(3):180\u0026ndash;5.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi XF, Li H, Liu ZD, Dai LY. Low bone mineral status in adolescent idiopathic scoliosis. Eur Spine J. 2008;17(11):1431\u0026ndash;40.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDiarbakerli E, Savvides P, Wihlborg A, Abbott A, Bergstr\u0026ouml;m I, Gerdhem P. Bone health in adolescents with idiopathic scoliosis. Bone Joint J. 2020;102\u0026ndash;b(2):268\u0026ndash;72.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYang KG, Lee WY, Hung AL, et al. Distinguishing risk of curve progression in adolescent idiopathic scoliosis with bone microarchitecture phenotyping: a 6-year longitudinal study. J Bone Min Res. 2024;39(7):956\u0026ndash;66.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi T, Li Y, Wang X, Long Y. Risk factors for the progression of distal adding-on phenomenon after surgery in patients with Lenke type 1 and 2 adolescent idiopathic scoliosis. Sci Rep. 2025;15(1):30237.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKim DH, Hyun SJ, Lee CH, Kim KJ. The Last Touched Vertebra on Supine Radiographs Can Be the Optimal Lower Instrumented Vertebra in Adolescent Idiopathic Scoliosis Patients. Neurospine. 2022;19(1):236\u0026ndash;43.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRayudu NM, Subburaj K, Mohan RE et al. Patient-Specific Finite Element Modeling of the Whole Lumbar Spine Using Clinical Routine Multi-Detector Computed Tomography (MDCT) Data-A Pilot Study. Biomedicines 2022;10(7).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eNanjo Y, Morio Y, Nagashima H, Hagino H, Teshima R. Correlation between bone mineral density and intervertebral disk degeneration in pre- and postmenopausal women. J Bone Min Metab. 2003;21(1):22\u0026ndash;7.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang Y, Hansen ES, H\u0026oslash;y K, Wu C, B\u0026uuml;nger CE. Distal adding-on phenomenon in Lenke 1A scoliosis: risk factor identification and treatment strategy comparison. Spine (Phila Pa 1976). 2011;36(14):1113\u0026ndash;22.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePauchard Y, Fitze T, Browarnik D, et al. Interactive graph-cut segmentation for fast creation of finite element models from clinical ct data for hip fracture prediction. Comput Methods Biomech Biomed Engin. 2016;19(16):1693\u0026ndash;703.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHan C, Zhou C, Zhang H, et al. Evaluation of bone mineral density in adolescent idiopathic scoliosis using a three-dimensional finite element model: a retrospective study. J Orthop Surg Res. 2023;18(1):938.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDede O, Akel I, Demirkiran G, Yalcin N, Marcucio R, Acaroglu E. Is decreased bone mineral density associated with development of scoliosis? A bipedal osteopenic rat model. Scoliosis. 2011;6(1):24.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYip BHK, Yu FWP, Wang Z, et al. Prognostic Value of Bone Mineral Density on Curve Progression: A Longitudinal Cohort Study of 513 Girls with Adolescent Idiopathic Scoliosis. Sci Rep. 2016;6:39220.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCheng Y, Yang H, Hai Y, Pan A, Zhang Y, Zhou L. Hounsfield unit for assessing asymmetrical loss of vertebral bone mineral density and its correlation with curve severity in adolescent idiopathic scoliosis. Front Surg. 2022;9:1000031.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJin LY, Su XJ, Xu S, Liu HY, Li XF. Reliability of Hounsfield Unit for Assessing Asymmetrical Vertebral Bone Mass in Adult Degenerative Scoliosis. Int J Gen Med. 2022;15:5869\u0026ndash;77.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSchlager B, Krump F, Boettinger J, et al. Characteristic morphological patterns within adolescent idiopathic scoliosis may be explained by mechanical loading. Eur Spine J. 2018;27(9):2184\u0026ndash;91.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJi C, Wang X, Weng Y, et al. Asymmetric distribution of vertebral bone microstructure in coronal adult spinal deformity: a cross-sectional study based on quantitative CT. Eur Spine J. 2026;35(1):256\u0026ndash;66.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDriscoll M, Aubin CE, Moreau A, Villemure I, Parent S. The role of spinal concave-convex biases in the progression of idiopathic scoliosis. Eur Spine J. 2009;18(2):180\u0026ndash;7.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDieckmeyer M, Rayudu NM, Yeung LY, et al. Prediction of incident vertebral fractures in routine MDCT: Comparison of global texture features, 3D finite element parameters and volumetric BMD. Eur J Radiol. 2021;141:109827.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSollmann N, Rayudu NM, Lim JJS, et al. Multi-detector computed tomography (MDCT) imaging: association of bone texture parameters with finite element analysis (FEA)-based failure load of single vertebrae and functional spinal units. Quant Imaging Med Surg. 2021;11(7):2955\u0026ndash;67.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYang S, Yaszay B, Bauer J. The Clinical Significance of the Lowest Instrumented Vertebra in Adolescent Idiopathic Scoliosis. J Am Acad Orthop Surg. 2024;32(18):e889\u0026ndash;98.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"journal-of-orthopaedic-surgery-and-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"josr","sideBox":"Learn more about [Journal of Orthopaedic Surgery and Research](http://josr-online.biomedcentral.com)","snPcode":"13018","submissionUrl":"https://submission.nature.com/new-submission/13018/3","title":"Journal of Orthopaedic Surgery and Research","twitterHandle":"@MSKmedBMC","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"adolescent idiopathic scoliosis, bone mineral density, three-dimensional finite element analysis, adding-on phenomenon, pedicle screw fixation","lastPublishedDoi":"10.21203/rs.3.rs-9158836/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9158836/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003eObjective\u003c/h2\u003e \u003cp\u003eDistal adding-on phenomenon remains a common complication following corrective surgery for adolescent idiopathic scoliosis (AIS). This study aimed to characterize the spatial distribution of vertebral bone mineral density (BMD) across the scoliotic spine and to investigate whether preoperative BMD asymmetry at the vertebra immediately distal to the lowest instrumented vertebra (LIV\u0026thinsp;+\u0026thinsp;1) predicts postoperative distal adding-on phenomenon.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e \u003cp\u003eThis retrospective cohort study included 90 patients with AIS who underwent posterior spinal fusion and had preoperative full-spine computed tomography scans. Patient-specific three-dimensional finite element analysis (3D-FEA) models were constructed to quantify BMD along simulated pedicle screw trajectories throughout the spine. Bilateral BMD values were measured across the proximal thoracic curve (PTC), main thoracic curve (MTC), and thoracolumbar/lumbar curve (TLC). Patients were stratified according to curve severity and the presence or absence of postoperative adding-on phenomenon. Inter-side BMD asymmetry at the LIV and LIV\u0026thinsp;+\u0026thinsp;1 levels was compared between groups. Receiver operating characteristic (ROC) analysis was performed to evaluate the predictive value of LIV\u0026thinsp;+\u0026thinsp;1 BMD asymmetry.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e \u003cp\u003eThe mean whole-spine BMD was 0.85\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14 g/cm\u0026sup2;. No significant difference was observed between the overall concave and convex sides (p\u0026thinsp;=\u0026thinsp;0.510), although region-specific asymmetry patterns were identified. Distal adding-on phenomenon occurred in 15 patients (16.7%). Patients with adding-on showed significantly greater LIV\u0026thinsp;+\u0026thinsp;1 BMD asymmetry compared with those without (0.14\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06 vs. 0.11\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 g/cm\u0026sup2;, p\u0026thinsp;=\u0026thinsp;0.006). LIV\u0026thinsp;+\u0026thinsp;1 BMD asymmetry demonstrated good predictive performance (AUC\u0026thinsp;=\u0026thinsp;0.865, 95% CI 0.77\u0026ndash;0.96), with an optimal cutoff value of 0.112 g/cm\u0026sup2;. Multivariable analysis identified LIV\u0026thinsp;+\u0026thinsp;1 BMD asymmetry as an independent predictor of distal adding-on phenomenon.\u003c/p\u003e\u003ch2\u003eConclusion\u003c/h2\u003e \u003cp\u003eVertebral BMD in AIS exhibits a region-specific asymmetric distribution pattern. Increased preoperative LIV\u0026thinsp;+\u0026thinsp;1 BMD asymmetry is independently associated with postoperative distal adding-on phenomenon and may serve as a useful parameter for risk stratification and surgical planning.\u003c/p\u003e","manuscriptTitle":"Association Between LIV+1 Bone Mineral Density Asymmetry and Distal Adding-On Phenomenon in Adolescent Idiopathic Scoliosis","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-28 00:23:42","doi":"10.21203/rs.3.rs-9158836/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"editorInvitedReview","content":"","date":"2026-04-26T06:45:31+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-24T16:12:35+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"152597450444143657344372261826655634853","date":"2026-04-24T00:32:15+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"170534889567619971931586651663765347260","date":"2026-04-22T00:36:46+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"303695077084254841741523004952039367089","date":"2026-04-19T23:33:13+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-04-19T22:51:42+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-03-21T09:39:24+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-03-21T09:38:49+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Orthopaedic Surgery and Research","date":"2026-03-18T11:26:43+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-orthopaedic-surgery-and-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"josr","sideBox":"Learn more about [Journal of Orthopaedic Surgery and Research](http://josr-online.biomedcentral.com)","snPcode":"13018","submissionUrl":"https://submission.nature.com/new-submission/13018/3","title":"Journal of Orthopaedic Surgery and Research","twitterHandle":"@MSKmedBMC","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"3b8ccab3-36f7-4710-8876-5556ff7c5258","owner":[],"postedDate":"April 28th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-04-28T00:23:44+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-28 00:23:42","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9158836","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9158836","identity":"rs-9158836","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}