Impact of the Nuss procedure on spinal curvature across four time points: a longitudinal and subgroup analysis

Introduction

Pectus excavatum (PE) is the most common congenital chest wall deformity, defined by different grade of sternal depression. The thoracic cage, composed of the sternum, cartilage, joints, ribs, and the thoracic vertebrae, is structurally interconnected; thus, deformity of the sternum can affect surrounding structures, including the spine (1,2). A meta-analysis by van Es et al. found that all 19 studies included in the analysis reported a significantly higher prevalence of scoliosis in patients with pectus deformities compared to the general population, highlighting a notable association between these two conditions (3).

Several biomechanical theories have been proposed to explain the association between pectus deformity and scoliosis. Some suggest that abnormal configuration of the anterior chest wall in PE disrupts the balance of forces within the thoracic cage, redistributing mechanical loads and potentially influencing spinal alignment (4-7). In addition, the Nuss procedure, a widely utilized surgical technique for PE correction, introduces further mechanical adjustments, which may dynamically impact spinal curvature (2,8-10). While previous studies have explored the effects of the Nuss procedure on the spine, they have primarily focused on the comparison between pre-operative and post-bar removal stages (2,11-13).

This study aims to evaluate changes in spinal curvature at multiple stages of the Nuss procedure. By conducting a detailed longitudinal analysis, this research investigates the dynamic interactions between the thorax and spine and provide insights for optimizing perioperative management. We present this article in accordance with the STROBE reporting checklist (available at https://jtd.amegroups.com/article/view/10.21037/jtd-2025-357/rc).

Methods

Study design and population

A total of 211 patients diagnosed with PE underwent the Nuss procedure at the Department of Thoracic and Cardiovascular Surgery, Incheon St. Mary’s Hospital, between July 2011 and July 2024. Patients with known connective tissue disorders, a history of chest or spinal surgeries, reoperations after the Nuss procedure, loss to follow-up, unexpected early bar removal, or bars remaining in situ at the end of the study period were excluded. After applying these criteria, 177 patients were included in this retrospective analysis (Figure 1). This study was conducted in accordance with the principle of the Declaration of Helsinki and its subsequent amendments and was approved by the Institutional Review Board (IRB) of Incheon St. Mary’s Hospital (IRB approval No. OC25RISI0025). The requirement for informed consent was waived due to the retrospective nature of the study.

Figure 1 Flow chart of patient selection.

Surgical procedure

All procedures were performed by a single surgeon using the minimally invasive repair technique. The procedure involved preliminary elevation of the depressed sternum using a crane applied at the xiphoid area. Bilateral skin incisions were made on the lateral chest, and the mediastinum was dissected under video-assisted thoracoscopic guidance using a needlescope. Based on the patient’s type of PE, customized single or multiple bars were inserted along the pre-determined curvature, rotated 180° to elevate the chest, and stabilized with fixators or plates. Bar removal was performed typically 2–3 years post-operatively in children and 3–5 years in adolescents or adults.

Radiologic evaluation and parameters

Radiographic assessments were performed for all patients, including simple chest radiography (CXR) and computed tomography (CT), to evaluate thoracic morphology and spinal alignment. The severity of PE was assessed using the Haller index (HI), calculated as the ratio of the transverse chest diameter to the anterior-posterior chest diameter on CT (14). HI was further classified into three categories: mild (2≤ HI <3.2), moderate (3.2≤ HI <3.5) and severe (HI ≥3.5). Spinal curvature was assessed by measuring the Cobb angle (CA) on a standing CXR, calculated as the angle formed by two lines drawn parallel to the superior endplate of the upper vertebra and the inferior endplate of the lower vertebra (15). Scoliosis was diagnosed in patients with CA ≥10° in this study. The primary analysis of the study examined longitudinal changes in spinal curvature across four stages, with CA measurements taken at the following:

• Prior to pectus bar insertion (pre-operative);
• One week after pectus bar insertion (post-operative);
• Immediately before pectus bar removal (pre-removal);
• One week after pectus bar removal (post-removal).

Statistical analyses

Continuous variables were expressed as means and standard deviations, and categorical variables as frequencies and percentages. For descriptive analysis, one-way analysis of variance (ANOVA) was used for continuous variables and the Chi-squared test of independence for categorical variables. The Friedman test was used to assess longitudinal CA changes across the four intervals, with Wilcoxon signed-rank tests for post-hoc pairwise comparisons. Subgroup analyses for parameters such as sex, age group, thoracic morphology (symmetry), pre-operative HI, and pre-operative CA were each performed using the Friedman test and post-hoc comparisons. Lastly, Kruskal-Wallis test and Mann-Whitney U test were used to compare CA changes between the pre-operative and pre-removal time points across subgroups. Statistical analyses were conducted using Python (version 3.11) with the SciPy module and visualizations with Matplotlib. A P value <0.05 was considered statistically significant.

Results

Clinical and procedural characteristics

In this study, scoliosis (CA ≥10°) was present in 30 (16.9%) out of 177 patients. The cohort was predominantly male (80.2%), with adolescents aged 10–19 years, comprising 52.0% of participants. Asymmetric thoracic morphology was observed in 54.2% of cases. The mean pre-operative CA was 5.91°±5.72°, while the post-removal CA was 6.57°±6.13°. Table 1 summarizes the clinical and procedural characteristics of the PE patients. When patients were stratified into three age groups (<10, 10–19, >19 years), comparisons of baseline characteristics revealed significant differences in post-operative HI, post-operative CA, pre-removal CA, number of bars inserted, and the duration of bar placement across the age groups (Table 2).

Changes in CA across four phases of the Nuss procedure

The effects of bar insertion, maintenance, and removal on spinal alignment were assessed by comparing CA across four distinct time points. Longitudinal analysis revealed significant changes in CA (P=0.03) (Table 3). Compared to the pre-operative status, CA increased significantly immediately after bar insertion and before removal with mean changes of 1.09° (P=0.003) and 1.12° (P=0.03) respectively (Figure 2, Table 4). However, no statistical changes in CA were observed during the maintenance phase (P=0.68) and between the pre-operative and post-removal phase (P=0.15).

Figure 2 Cobb angle measurements across four time points. (A) Box-plot diagram of CA in each time point. (B) Mean CA across four time points. CA, Cobb angle.

Factors influencing CA dynamics across time points

To determine factors influencing variations in CA over the four time points, subgroup analyses were conducted based on sex, age group, thoracic morphology, pre-operative HI, and baseline CA (Table 5). Significant changes in spinal curvature were identified in males (P=0.001), adolescents aged 10–19 years (P=0.047), individuals with asymmetric thoracic morphology (P=0.04), patients with severe HI values (HI ≥3.5, P=0.01), and in both groups stratified by CA severity (CA <10°, P<0.001; CA ≥10°, P<0.001). Conversely, no substantial differences were found in females, younger children, adults, patients with symmetric thoracic morphology, or those with mild to moderate HI values. Figure 3 demonstrates visualization of the post-hoc analysis of the subgroups with notable dynamic changes in CA.

Figure 3 Subgroup analyses of Cobb angle dynamics across four time points (male sex, age of 10–19 years, asymmetric pectus excavatum, severe HI, pre-op CA <10°, pre-op CA ≥10°). ^†, outliers with CA above 20° were removed. CA, Cobb angle; HI, Haller index; PE, pectus excavatum; Pre-op, pre-operative.

Factors influencing CA dynamics in specific time intervals

Since both the “pre-operative to post-operative” and “pre-operative to pre-removal” intervals showed significant changes in spinal curvature, further subgroup analyses were conducted for each interval separately. For the first interval, a significant difference was observed only in the severity of baseline CA. Patients with CA <10° exhibited a mean increase of 2.05, whereas those with CA ≥10° experienced a mean decrease of 3.60° in CA (P<0.001). In the second interval, significant differences were found based on sex (P=0.004) and pre-operative CA (P<0.001). Male patients showed a mean increase of 1.6° in CA, while female patients exhibited a mean reduction of 0.83°. Patients with a pre-operative CA <10° demonstrated a mean increase of 2.1°, whereas those with CA ≥10° experienced a mean decrease of 3.7°. Tables 6,7 provide a detailed summary of the data for each interval, respectively.

Discussion

PE, the most common congenital chest wall deformity, is attributed to costal cartilage overgrowth, causing depression of the sternum (16). Given the structural connection between the sternum, rib cage and vertebrae, multiple studies have reported an association between PE and scoliosis (3,4,6,16,17). The Nuss procedure, a minimally invasive technique for PE repair, corrects the deformity by placing a retrosternal bar or multiple bars to elevate the sternum (18). While effective, it may also impose significant mechanical stress on the thoracic cage, potentially altering spinal alignment (8). However, its impact on spinal curvature remains inconclusive and no comprehensive analysis has assessed each stage of the process, including bar insertion, maintenance and removal. This study is the first to evaluate CA changes across four distinct stages, providing a dynamic analysis of the procedure’s influence on spinal alignment.

Several studies have investigated the impact of the Nuss procedure on spinal curvature, but the results remain controversial. Some case reports have documented acquired thoracic scoliosis after Nuss operation, which was managed with brace therapy or bar removal (19,20). Similarly, İşcan et al. analyzed 100 PE patients and found a significant increase in CA (4.3±7.0 vs. 5.0±7.0, P=0.01) after the Nuss procedure (11). On the other hand, Ghionzoli et al. followed 67 PE patients with mild-to-moderate adolescent idiopathic scoliosis for three post-operative years, observing a mean CA reduction from 10.9° to 9.4° (P=0.001) (2). In our study, while no significant difference was observed between pre-operative and post-bar removal CA, a significant increase in CA was noted after bar insertion and maintenance. Similarly, Park et al.’s retrospective study of 468 PE patients reported no significant difference between pre-operative and post-bar removal CA (5.0°±4.2° vs. 5.0°±4.8°, P=0.77); however, different results showed when stratified by age (12). The inconsistent results of previous studies are likely due to methodological differences; thus, further well-controlled research is needed to better understand the effects of the Nuss procedure on spinal curvature.

Our study, which analyzed CA at four distinct stages of the Nuss procedure, found that bar insertion and maintenance exert a notable influence on spinal curvature. While the underlying biomechanical mechanisms remain unclear, several studies utilizing mathematical models have attempted to explain these effects. Boia et al. measured the force-displacement relationship of the thoracic cage in 40 healthy children and noted that immediate correction of PE may impose high mechanical stress, potentially leading to pain, complications or unintended spinal effects (21). Further supporting this, Chang et al. and Nagasao et al. utilized 3-dimensional computer-assisted design models employing the finite element method (FEM) to simulate and analyze mechanical stresses on the thoracic cage after the Nuss procedure. Chang et al., in their analysis of three PE patients, observed that high stress was concentrated on the 3rd–7th costal cartilages, with bilateral stress affecting the ribs near the spine (9). Nagasao et al., in a study of 25 patients, found that different thoracic morphology could influence improving or worsening of the spinal curvature, depending on the interplay between thoracic concavity and spinal bowing direction (10). These findings suggest that the effect of bar insertion on spinal curvature is influenced by the complex morphological interactions between the thorax and spine, leading to variable outcomes.

On the other hand, our findings showed that after an initial post-operative increase in CA, a subsequent decrease was observed following bar removal. We hypothesize that the spine may naturally seek equilibrium once external forces are removed, though further research is needed. Meng et al. reported a case of a 14-year-old boy who developed acquired scoliosis (CA >50°) after bar insertion, which resolved following bar removal and brace therapy (19). Additionally, other studies have suggested that age-related skeletal flexibility and the adaptive response of paraspinal muscles may contribute to these complex biomechanical changes (7).

Scoliosis progresses most rapidly during puberty, peaking during periods of maximal height growth, and gradually slowing after skeletal maturity (22,23). Given this, we hypothesized that dynamic CA changes would vary by age group. Following WHO’s definition of adolescence (10–19 years), we classified patients into child (<10 years), adolescent (10–19 years), and adult (>19 years) group. Our findings confirmed that only adolescents exhibited dynamic CA changes across the various time points. Similarly, Park et al. reported age-related differences, where early correction (<10 years) led to CA improvement (P=0.08), while late correction (≥10 years) resulted in CA augmentation (P=0.002) (12). However, a recent meta-analysis by van Es et al. suggested it remains unclear whether post-Nuss CA changes result from natural growth-related scoliosis progression or the procedure itself, warranting cautious interpretation (3).

Spinal surgery for scoliosis is generally considered in skeletally immature patients with a CA ≥45°, in cases of progressive curvature despite conservative measures such as bracing, or in the presence of significant symptoms such as pain (15). In this study, two patients were referred to orthopedic specialists for evaluation, but neither proceeded with surgical intervention. One patient, a 21-year-old female, remained asymptomatic with preserved trunk balance and was advised against surgery. The second patient, a 16-year-old male, had not yet made a decision regarding surgery. The female patient’s initial CA of 44° remained relatively stable throughout the period of bar insertion and removal. However, the male patient presented with a CA of 31°, which progressed to 45° during the bar maintenance phase but subsequently improved to 39° after bar removal. Although based on only two cases, these findings highlight the dynamic nature of CA progression during adolescence.

Another key pre-operative factor noted in prior studies is the degree of spinal curvature. The results of our subgroup analyses for factors influencing CA dynamics across four time points and within each significant interval revealed distinct trends based on pre-operative CA. Patients with CA <10° exhibited a notable increase in CA following bar insertion and during the maintenance period (before removal), whereas those with CA ≥10° experienced a decrease. Among patients with pre-operative CA <10°, 38 experienced progression to CA ≥10° at one or more subsequent phases. Specifically, 19 patients (50%) developed CA ≥10° immediately after bar insertion, and 16 patients reached this threshold before bar removal. Notably, of the 19 patients whose CA exceeded 10° after bar insertion, 13 returned to CA <10° following bar removal. However, two patients showed continued progression, with CA increasing even after bar removal.

Conversely, among 30 patients with a pre-operative CA ≥10°, a reduction to CA <10° was observed in 16 cases (53%), indicating a potential alleviating effect of the Nuss procedure on pre-existing scoliosis in a subset of patients. In a retrospective study by Park et al. involving 468 PE patients, CA decreased from 5.0°±3.0° to 2.8°±0.6° (P=0.09) after PE correction in patients with pre-existing scoliosis (CA ≥10°). The study also identified pre-operative CA as an independent predictor of scoliosis progression (P<0.001) (12). Conversely, Chung et al. set the CA threshold as 15, reporting that patients with CA <15° showed improvement (−2.88°), while those with CA >15° exhibited worsening curvature (+3.86°, P<0.001) after pectus bar removal (13). Although findings vary across studies, pre-operative CA consistently emerges as a significant determinant of post-procedural CA outcomes following the Nuss procedure.

This study has several limitations that warrant consideration. First, its retrospective design and relatively small sample size limit the ability to establish causal relationships and generalize findings to broader populations. Second, scoliosis is a three-dimensional deformity, yet this study relied solely on the CA as a parameter, which assesses only coronal plane deformities. Incorporating sagittal and axial views or three-dimensional imaging would provide a more comprehensive evaluation of spinal alignment. Third, measurement variability due to patient posture or pain could introduce errors, despite efforts to minimize bias. Among the four time points, post-operative CA measurements are likely to be most influenced by post-operative pain, necessitating careful interpretation. Fourth, while statistically significant CA changes were observed, the clinical relevance of small absolute changes remains unclear and requires further investigation to establish meaningful thresholds for patient outcomes. Finally, the lack of long-term follow-up limits insights into the potential evaluation of spinal curvature after bar removal, highlighting the need for longitudinal studies to assess these effects. Future studies should investigate long-term outcomes using larger sample sizes, well-defined parameters, and prospective study designs to better elucidate the underlying mechanisms.

Conclusions

In conclusion, this study analyzed spinal curvature changes across four key stages of the Nuss procedure: pre-operative, post-operative, pre-removal, and post-removal. Significant CA changes were primarily observed during bar insertion and the maintenance phase, suggesting that high mechanical stress on the thoracic cavity may contribute to spinal curvature alterations. However, pre-operative CA and post-removal CA presented no statistical difference. Sex, age and pre-operative CA emerged as key factors in influencing changes in CA during the Nuss procedure. These findings highlight the importance of routine spinal examinations, particularly for patients susceptible to dynamic spinal alignment shifts. Future research should further investigate the biomechanical mechanisms underlying these changes and assess the long-term effects of PE correction on spinal alignment.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://jtd.amegroups.com/article/view/10.21037/jtd-2025-357/rc

Data Sharing Statement: Available at https://jtd.amegroups.com/article/view/10.21037/jtd-2025-357/dss

Peer Review File: Available at https://jtd.amegroups.com/article/view/10.21037/jtd-2025-357/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jtd.amegroups.com/article/view/10.21037/jtd-2025-357/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 principle of the Declaration of Helsinki and its subsequent amendments and was approved by the Institutional Review Board (IRB) of Incheon St. Mary’s Hospital (IRB approval No. OC25RISI0025). The requirement for informed consent was waived due to the retrospective nature of the 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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