Radiographic characterization and predictive modeling of functional scoliosis secondary to leg length discrepancy in adolescents

Introduction

Leg length discrepancy (LLD) is commonly observed in children and adolescents, primarily reflecting unequal limb lengths and pelvic coronal imbalance. The main causes include bone shortening or excessive growth due to trauma, inflammation, or infection (1-3). Reid et al. are the first to classify LLD into categories: mild (under 30 mm), moderate (30–60 mm), and severe (over 60 mm) (4). Previous studies have reported that a limb length discrepancy of more than 5 mm can cause an imbalance in posture gait (5,6). In children and adolescents, LLD is primarily developmental and related to growth asymmetry (7). In contrast, adults may develop acquired LLD from causes such as malunion or joint replacement, and their reduced compensatory capacity increases the risk of osteoarthritis (8). This study focused on developmental LLD patients under 20 years old.

Previous studies have shown that LLD is associated with functional scoliosis. Among patients with an LLD, functional scoliosis can develop, with the curve convexing toward the shorter side (9-11). After correction of LLD, most cases of functional scoliosis are alleviated through methods such as surgery or by placing blocks under the shorter limb to restore pelvic balance (12-14). However, LLD and structural scoliosis may coexist, with their pathogenic mechanism independent or interrelated. For instance, in patients with Marfan syndrome, scoliosis and LLD can both occur, yet their mechanisms are independent and show no significant association (15). So in some cases, after correcting the LLD, the structural scoliosis may improve but not completely resolve (16). The LLD observed in structural scoliosis may represent one of the compensatory mechanisms to maintain coronal balance in spinal deformity (17,18). In fact, functional scoliosis may become structural scoliosis during long-term progression. Non-structural spinal curvature caused by postural imbalances due to pain, muscle spasms, or other factors may gradually develop into structural scoliosis over time (3,19,20).

In adolescent populations, the concurrent LLD and scoliosis presents a significant clinical challenge. When managing these patients, clinicians face a diagnostic dilemma that confuses therapeutic decision-making: does LLD drive compensatory scoliosis, do structural scoliosis and LLD occur concurrently? For instance, surgical intervention for structural scoliosis may require corrective surgery, whereas functional scoliosis secondary to LLD might resolve with limb correction or exercise treatment (21-23). Thus, elucidating whether LLD is the cause or consequence of scoliosis is pivotal for optimizing treatment strategies, including prioritizing LLD versus spinal deformity correction and selecting appropriate surgical or non-surgical approaches.

Functional scoliosis is often associated with reversible postural imbalances (such as LLD). In cases of LLD-related scoliosis, the spinal curvature typically shows partial or complete resolution after correction of the underlying LLD (12,22,24-26). Notably, while the lift test theoretically distinguishes functional from structural scoliosis by observing pelvic-spinal compensatory dynamics, three major limitations persist: First, repeated full-spine radiographic imaging exposes patients—particularly adolescents—to cumulative radiation doses with potential long-term health implications (27,28). Second, the test requires precise block height calibration matching the LLD magnitude, yet lacks standardized operational protocols (29). Third, part of combined-type scoliosis cases (with coexisting structural deformities and LLD) may yield false-negative outcomes. So currently no clinical guidelines have incorporated this method into routine diagnostic workflows (24).

Studies have shown that the discrepancy in LLD is significantly related to pelvic tilt and lumbar spine Cobb angle (25,30). However, the relationship between LLD-induced limb imbalance and pelvic balance, lumbar spine balance, and coronal balance remains unclear. Additionally, there is a lack of a clear approach to distinguish whether scoliosis is a functional scoliosis due to LLD or structural scoliosis causing the LLD. To address this gap, we analyzed and summarized the common features of functional scoliosis caused by LLD and developed a clinical diagnostic model to assist in diagnosis. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1275/rc).

Methods

Patients

The cross-sectional study included patients who underwent full-length lower limb X-rays and were diagnosed with LLD in outpatient clinic or radiology unit at Nanjing Drum Tower Hospital, China between January 2018 and October 2024. In our protocol, limb elevation tests were systematically employed to correct LLD, with pre- and post-correction full-spine anteroposterior radiographs acquired to evaluate scoliosis correction efficacy. Patients were recruited and diagnosed with LLD according to the following criteria: (I) age ≤20 years; (II) a LLD greater than 5 mm; (III) symptoms such as pelvic imbalance and unstable gait (pelvic imbalance was assessed by clinical observation of iliac crest asymmetry and confirmed radiographically measurements, while unstable gait was diagnosed clinically based on limping, compensatory trunk sway, or uneven step pattern). Exclusion criteria included: (I) history of spinal or lower limb surgery; (II) missing standing full spine or full-length lower limb radiographs; (III) improper imaging technique for lower limb full-length radiographs. According to the inclusion and exclusion criteria, this study enrolled a total of 236 patients (Figure 1).

Figure 1 Flow diagram illustrating the process of inclusion and exclusion of study cohort.

This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments and approved by the Institutional Review Board of Nanjing Drum Tower Hospital, Affiliated Hospital of Medical School, Nanjing University (IRB No.2025-0233-02). The Ethics Committee waived the need for informed consent for this retrospective study.

Limb elevation test

Following the method described by Raczkowski et al., we performed the limb elevation test (12). The height of the block placed under the shorter limb was determined based on the measured LLD obtained from full-length standing lower limb radiographs. Specifically, the block height was set to match the measured LLD value for each patient, using 5-mm increments. The block was positioned under the entire plantar surface of the shorter limb to ensure uniform elevation and prevent ankle inversion/eversion. Imaging before and after elevation followed identical radiological protocols to ensure consistency.

Radiological measurements

Radiological parameter measurements

All patients underwent full-length radiography of the lower limbs and spine, and the images were assessed and measured by three pediatric orthopedic or spine surgeons according to the Spinal Deformity Study Group Radiographic Measurement Manual. Interobserver reliability was quantified using a two-way random intraclass correlation (ICC) model for measurements by three blinded pediatric orthopedic or spine surgeons. Results demonstrated good agreement for all radiographic parameters (95% confidence interval: 0.73–0.82). Discrepancies >5° (angles) or >5 mm (linear measurements) underwent consensus review. Cases exhibiting rotation, motion artifacts, or inadequate inclusion of anatomical landmarks (e.g., hip-knee-ankle joints) were flagged as ‘improper technique’ and excluded from analysis. The average values were used (31).

Lower limb length measurement

The whole lower limb length (WL) was measured from the center of the femoral head to the center of the superior contour of the talar trochlea (32,33). LLD was defined as the difference between the left- and right-side WLs. Femoral length (FL) was measured from the femoral head to the center of the medial femoral condyle, and tibial length (TL) was measured from the center of the medial tibial plateau to the talar trochlea (34). Structural LLD (SLLD) was defined as the difference in FL + TL between the two sides (Figure 2A).

Figure 2 A 17-year-old male patient diagnosed with LLD and associated functional scoliosis. (A) Representative images of WL, FL, and TL measurements. LLD is defined as the difference between bilateral WLs; in this patient, LLD is 27.3 mm, and SLLD is 23.6 mm. (B) Representative images for POA and FHHD measurements. In this patient, POA is 17.7°, and FHHD is 24.2 mm. (C) A representative image for SOA, lumbar Cobb, and CBD measurements. In this patient, SOA is 13.4°, lumbar Cobb is 23.9°, and CBD is 3.1mm. (D) After a 3-cm lift on the shorter limb, significant improvement was observed in the patient’s POA and lumbar Cobb. CBD, coronal balance distance; FHHD, femoral head height difference; FL, femoral length; LLD, leg length discrepancy; POA, pelvic obliquity angle; R, right; SLLD, structural LLD; SOA, sacral obliquity angle; TL, tibial length; WL, whole lower limb length.

Pelvic tilt measurement

Pelvic obliquity angle (POA) was measured as the angle between the line connecting the top of both iliac crests and the horizontal line (35). The femoral head’s highest point was used to create a horizontal line, and the distance between the left and right horizontal lines was recorded as the femoral head height difference (FHHD) (Figure 2B) (36).

Lumbar spine balance and coronal balance measurement

Sacral obliquity angle (SOA) was measured as the angle between the S1 endplate and the horizontal line. Coronal balance distance (CBD) was defined as the distance from the C7 plumb line to the sacral midline (37). Lumbar Cobb angle was measured in both normal and shortened limb elevation states on coronal spine radiographs (Figure 2C).

Identification of structural and functional scoliosis

According to the 2016 SOSORT guidelines, functional scoliosis denotes a reversible spinal curvature secondary to extra-spinal causes (e.g., limb length discrepancy), while structural scoliosis involves permanent alterations in spinal morphology (24). Consistent with these definitions and the scoliosis diagnostic threshold (Cobb angle ≥10°), we implemented the following protocol:

Based on the lower limb length measurement results, a block was placed under the shorter limb, and a standing full spine radiograph was taken again. If pelvic tilt and lumbar Cobb angles showed significant improvement (Cobb angle after correction <10°), the scoliosis was classified as functional; otherwise, it was classified as structural (Figure 2D). Figures 2,3 present representative cases of structural and functional scoliosis associated with LLD.

Figure 3 An 18-year-old female patient diagnosed with LLD and associated structural scoliosis. (A) Full-spine and full-lower limb X-rays of the patient in a standing position without a lift. In this patient, LLD is 20.4 mm, and SLLD is 16.9 mm, POA is 14.6°, and FHHD is 26.9 mm, SOA is 15.9°, lumbar Cobb is 40.6°, Cobb angle of structural curve is 57.8° and CBD is 92.3 mm. (B) After a 3-cm lift on the shorter limb, lumbar Cobb is 39.3 and Cobb angle of structural curve is 52.2°. CBD, coronal balance distance; FHHD, femoral head height difference; LLD, leg length discrepancy; POA, pelvic obliquity angle; SLLD, structural LLD; SOA, sacral obliquity angle.

Statistical analysis

Data analysis was performed using GraphPad Prism 9.0 (Boston, MA, USA). Data distribution was first assessed for normality using the Shapiro-Wilk test as the primary method. For comparisons involving multiple variables, P values were adjusted using the Holm-Bonferroni correction. An unpaired t-test was used to assess quantitative correlations, while the Chi-squared test or Fisher’s exact test was used for qualitative variables. Pearson correlation or nonparametric Spearman correlation was used to assess correlations. Using univariate and multivariate logistic regression analyses, independent predictive factors for the occurrence of LLD were evaluated, and a clinical prediction model was constructed. The score of the prediction model was calculated based on the coefficients from the logistic regression model. A receiver operating characteristic (ROC) curve was plotted, and the area under the ROC curve (AUC) was calculated to assess the accuracy of the prediction model. To evaluate the model’s performance and ensure generalizability, the dataset was divided into a training set and a test set. Specifically, 70% of the data was randomly allocated to the test set for model development, while the remaining 30% was reserved as the validation set for performance evaluation. For classification tasks, stratified sampling was employed to maintain the same distribution of target labels in both the training and validation sets, thereby preventing bias due to imbalanced class distributions. R software was used to construct the clinical diagnostic model, with packages such as corrplot, tidyverse, and dplyr used for analysis and graphic generation.

Because this was a retrospective study including all eligible patients, no a priori sample size calculation was performed. A post hoc power analysis of the primary outcome (lumbar Cobb angle: 43° vs. 15°, pooled SD =26.9°) indicated a large effect size (Cohen’s d=1.04) and an achieved power >0.99 at α=0.05 (two-sided).

Results

Basic characteristics of patients

As mentioned earlier, this study retrospectively included 236 patients with LLD. Based on the changes in pelvic tilt and lumbar scoliosis after elevating the shorter limb, all patients were divided into two groups: 181 cases of functional scoliosis and 55 cases of structural scoliosis. The basic information of the patients and relevant imaging results are shown in Table 1.

Characteristics of functional scoliosis caused by LLD

We further characterized the feature of functional scoliosis caused by LLD. As shown in Table 2, in all patients with functional scoliosis, the side of the longer leg corresponded to the side of the pelvic tilt, but was opposite to the direction of the compensatory scoliosis. Additionally, we examined the distribution of apical vertebrae in the functional scoliosis group, and found that the majority of patients had their apical vertebrae located in the lumbar spine, with L3 being the most frequently involved level (Figure 4).

Figure 4 Apical vertebra distribution in the functional scoliosis group.

Comparison of functional scoliosis and structural scoliosis

Intergroup comparisons revealed that SLLD, PDA, lumbar Cobb angle, and CBD were significantly increased in the structural scoliosis group. However, there were no significant differences between the two groups in terms of longer leg, sex, age, LLD, POA, and FHHD (Table 3). The quantitative correlational indicators are shown in Figure 5.

Figure 5 Differences and individual data in LLD (A), SLLD (B), POA (C), lumbar Cobb (D), FHHD (E), SOA (F), and CBD (G) between structural scoliosis and functional scoliosis. *, P<0.05; **, P<0.01; ****, P<0.0001. CBD, coronal balance distance; FHHD, femoral head height difference; LLD, leg length discrepancy; POA, pelvic obliquity angle; SLLD, structural LLD; SOA, sacral obliquity angle.

Correlation analysis

We further analyzed the correlations between various indicators in functional scoliosis group (Figure 6). The LLD indicators, LLD and SLLD, were correlated with pelvic balance (POA), lumbar spine balance (lumbar Cobb), and CBD. The results showed that LLD indicators were significantly correlated with pelvic balance and lumbar spine balance, but no significant correlation was found with coronal balance (Figure 7).

Figure 6 Results of correlation analysis between various diagnostic indicators. *, P<0.05; **, P<0.01; ***, P<0.001. CBD, coronal balance distance; FHHD, femoral head height difference; FLI, femoral length inequality; LLD, leg length discrepancy; LLI, limb length inequality; POA, pelvic obliquity angle; SLLD, structural LLD; SOA, sacral obliquity angle.

Figure 7 Correlation analysis of indicators in entire group, functional scoliosis and structural scoliosis. (A,B) Correlation analysis of LLD/SLLD with POA in the entire group, functional scoliosis, and structural scoliosis groups. (C,D) Correlation analysis of LLD/SLLD with lumbar Cobb in the entire group, functional scoliosis, and structural scoliosis groups. (E,F) Correlation analysis of LLD/SLLD with CBD in the entire group, functional scoliosis, and structural scoliosis groups. CBD, coronal balance distance; LLD, leg length discrepancy; POA, pelvic obliquity angle; SLLD, structural LLD.

Construction of the clinical prediction model

We further validated the clinical model by developing a related model to distinguish whether LLD is associated with functional scoliosis or structural scoliosis. LASSO regression was used for automatic feature selection, followed by cross-validation (Figure 8A-8C). We then extracted 70% of the patients as the training group (n=165), with the remaining 30% as the test group (n=71). In the training set, we selected and screened lumbar Cobb, FHHD, SOA, FLI, and CBD as predictive indicators to construct a nomogram (Figure 8D). Predicted incidence curve of the train set and test set were shown in Figure 8E,8F. The training set was evaluated with an AUC of 0.9383 (Figure 8G), while the test set yielded an AUC of 0.8657 (Figure 8H). The predicted incidence curve is shown in Figure 8E, and the decision curve analysis is shown in Figure 8I.

Figure 8 Construction of the clinical prediction model. (A) ROC curve for each diagnostic indicator. (B) Coefficient paths for LASSO regression. (C) Cross-validation plot for LASSO regression. (D) Nomogram for prediction. (E) Predicted incidence curve of the train set. (F) Predicted incidence curve of the test set. (G) AUC for the training set. (H) AUC for the test set. (I) Decision curve analysis. AUC, area under the curve; CBD, coronal balance distance; CI, confidence interval; FHHD, femoral head height difference; FLI, femoral length inequality; LASSO, least absolute shrinkage and selection operator; LLD, leg length discrepancy; LLI, limb length inequality; POA, pelvic obliquity angle; ROC, receiver operating characteristic; SLLD, structural LLD; SOA, sacral obliquity angle.

Discussion

This study provides a systematic characterization of functional scoliosis caused by LLD and, through comprehensive analysis, systematically distinguishes between functional scoliosis resulting from LLD and structural scoliosis associated with LLD. Our findings demonstrate that all patients in the functional scoliosis group exhibited pelvic obliquity, with the direction of the longer leg aligning with the direction of pelvic tilt but opposing the direction of the spinal curves. Furthermore, we delineated the distribution of apex vertebrae in functional scoliosis patients, identifying L3 as the most frequently apex vertebrae. These results suggest that functional scoliosis associated with LLD is primarily restricted to the lumbar spine. We also found that patients with structural scoliosis exhibited significantly higher values (P<0.05) in SLLD, lumbar Cobb angle, and CBD compared to those with functional scoliosis. Furthermore, the LLD indicators, LLD and SLLD, were significantly correlated with POA, lumbar spine balance (lumbar Cobb), and CBD. A clinical prediction model incorporating lumbar Cobb, FHHD, SOA, FLI, and CBD demonstrated high diagnostic accuracy, with AUC values of 0.9383 in the training set and 0.8657 in the test set. These findings provide valuable insights into the diagnostics of scoliosis in LLD patients.

It is generally believed that functional scoliosis is an asymmetry in the coronal plane, potentially accompanied by vertebral rotation (20,26,38). Functional scoliosis caused by LLD is compensatory and non-progressive, and can be corrected by pelvic leveling through methods such as sole lifts (12,13,26,39). This study similarly differentiated functional scoliosis from structural scoliosis by elevating the shorter limb and re-taking standing full-spine X-rays. We further reviewed the medical histories of patients whose lumbar scoliosis could not be corrected, and found that 70.91% of these patients had one or more conditions such as hemivertebra, neurofibromatosis, or congenital vertebral synostosis, which is consistent with the characteristics of structural scoliosis.

Multiple studies have shown that in adolescent patients, LLD causes pelvic tilt, which in turn affects lumbar spine balance, with a clear positive correlation between LLD and lumbar Cobb angle (25,26,30,40). Our study further confirms this finding, with the largest patient cohort reported to date. Furthermore, analysis of apical vertebrae distribution suggests that functional scoliosis predominantly involves the lumbar spine, with limited extension into the thoracic region. However, a study by Hoikka et al. found no clear correlation between LLD and lumbar Cobb in leg-length inequality patients aged 47±7 years, which seems inconsistent with recent studies. We speculate that this may be related to factors such as patient age, skeletal development, and lumbar degeneration (41). Additionally, our study found no significant correlation between LLD and CBD. While literature suggests no significant correlation between LLD and thoracic Cobb, we believe that CBD, when assessing overall coronal balance, better reflects the relationship compared to thoracic Cobb (34).

Studies by Raczkowski et al. highlight that functional scoliosis, when accurately identified, can often avoid unnecessary spinal surgery through targeted interventions such as limb lengthening or heel lifts (12). Our study characterized the common features of functional scoliosis caused by LLD, identifying the following key delineations: (I) LLD is consistently associated with ipsilateral pelvic tilt and contralateral spinal curvature; (II) functional curves are primarily localized in the lumbar region, with the apex vertebra most frequently located at L3; (III) both LLD and SLLD positively correlate with pelvic tilt angle and lumbar Cobb angle; (IV) a significant reduction in lumbar Cobb angle (to <10°) is observed after limb elevation tests.

Recent research by Manocchio et al. demonstrates that mild LLD is common in adolescents with spinal dysfunction and may influence compensatory postural curves (42). Although multiple radiographic evaluations can differentiate between structural and functional scoliosis, they also pose issues such as repeated radiation exposure and financial costs. Our predictive model demonstrates substantial potential in decreasing dependence on repeated radiographic evaluations. These methods may serve as complementary rather than replacement role relative to the established limb elevation test as the reference standard for clinical diagnosis. For patients stratified into the intermediate risk category (predictive scores 0.3–0.7), we advocate for a multimodal diagnostic approach incorporating supplemental clinical gait evaluation. This study introduces a novel clinical decision-making framework that integrates key radiographic parameters: lumbar Cobb angle, FHHD, SOA, FLI, and CBD. This comprehensive assessment protocol enhances diagnostic precision in scoliosis classification and provides an evidence-based foundation for therapeutic strategy formulation.

There are several limitations in this study. First, missing data from some patients may have influenced the results, as these cases were excluded from the final analysis. Second, as a retrospective single-center study with imaging-based inclusion criteria, our findings lack external validation in independent cohorts. Third, although we differentiated between functional and structural scoliosis, potential confounding factors were not fully modeled. For example, many structural scoliosis cases were associated with congenital anomalies or neurofibromatosis, which may have affected both radiographic parameters and clinical outcomes. Fourth, the study cohort primarily consisted of pediatric and adolescent patients. Consequently, these findings may not be directly generalizable to adult populations, particularly those with degenerative scoliosis. Further studies should therefore adopt a multicenter design, include a broader age range, and implement more comprehensive modeling of confounding factors to validate and extend these results.

Conclusions

This study delineates key characteristics distinguishing functional scoliosis caused by LLD from structural scoliosis concomitant with LLD. Functional scoliosis predominantly localizes to the lumbar spine (apex at L3), correlates ipsilaterally with pelvic tilt and contralaterally with spinal curvature, and resolves (Cobb angle <10°) following limb elevation. Structural scoliosis, in contrast, demonstrates significantly higher lumbar Cobb angles, SLLD and CBD, persisting post-correction. The developed clinical prediction model, integrating lumbar Cobb, FHHD, SOA, FLI, and CBD, achieved robust diagnostic accuracy (AUC: 0.938 training, 0.866 testing), offering a radiation-sparing tool to guide therapeutic decisions. These findings underscore the importance of differentiating scoliosis subtypes in LLD patients, as functional cases may benefit from limb correction, while structural cases necessitate surgical intervention.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1275/rc

Data Sharing Statement: Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1275/dss

Funding: This study was supported by fundings for Clinical Trials from Nanjing Drum Tower Hospital, Affiliated Hospital of Medical School, Nanjing University, the Fondation Yves Cotrel pour la Recherche en Pathologie Rachidienne—Institut de France and Nanjing Medical Science and Technology Development Foundation (No. ZKX22017).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1275/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments and approved by the Institutional Review Board of Nanjing Drum Tower Hospital, Affiliated Hospital of Medical School, Nanjing University (IRB No. 2025-0233-02). The Ethics Committee waived the need for informed consent for this retrospective study.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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