Exploring the use of biphasic quantitative thoracic computed tomography to evaluate halo-pelvic traction in managing severe adolescent idiopathic scoliosis

Introduction

Severe adolescent idiopathic scoliosis (AIS) is a complex 3-dimensional (3D) spinal deformity that often presents with pronounced impairment of cardiopulmonary function (1,2). The current method for treating spinal deformities involves osteotomy and internal fixation. However, managing severe rigid scoliosis is challenging due to the intricate nature of the required complex osteotomy and extensive correction. These procedures substantially elevate the risk of neurologic, vascular, and pulmonary complications (3,4). Preoperative halo-pelvic traction (HPT), as an effective and safe solution before surgery, increases the safety and correction rate in the surgical treatment of severe spinal deformities (5-9).

Computed tomography (CT) is a valuable tool that can not only assess morphology but also evaluate respiratory function (10). Previous studies (5,8,9) have examined the effects of HPT on pulmonary function tests (PFTs) and radiographic parameters. However, there is a scarcity of published research utilizing CT to investigate alterations in lung volume following HPT.

This study aimed to utilize biphasic quantitative CT scans to evaluate the alterations in lung volume and anatomical parameters in patients with severe AIS before and after HPT. Additionally, the study sought to determine whether biphasic thoracic CT offers advantages over monophasic CT in the treatment of severe AIS. The hypothesis was that the lung tidal volume, as determined by CT scans at end inspiration and expiration, would exhibit changes in conjunction with PFTs. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-24-568/rc).

Methods

Patients

The cohort for this study consisted of 5 male and 19 female patients, with an average age of 15.8±2.2 years before HPT. A priori sample size calculations indicated that a minimum of 15 patients were required. The medical records of patients diagnosed with AIS who underwent HPT and surgical correction were reviewed. All patients underwent staged treatment, initially receiving HPT until sufficient improvement in pulmonary function was achieved, followed by spinal corrective surgery. The inclusion criteria were as follows: (I) diagnosis of AIS and undergoing HPT before surgery; (II) presence of a major thoracic curve ≥80 degrees; (III) availability of pre- and post-traction radiographs, CT scans, and PFTs; and (IV) CT scans obtained using the same scanner model. The exclusion criteria were as follows: (I) prior spinal surgery; and (II) inability to obtain CT lung volumes through reconstruction. Figure 1 illustrates the patient enrollment flowchart. Clinical data, including apical vertebrae, HPT duration, and demographic information [sex, age, height, arm span, and body mass index (BMI)], were gathered for each patient. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). Approval for the study was obtained from the Ethics Committee of the Third People’s Hospital of Chengdu (No. 2024-S-105). The requirement for individual consent for this retrospective analysis was waived.

Figure 1 Patient enrollment flowchart. AIS, adolescent idiopathic scoliosis; HPT, halo-pelvic traction; CT, computed tomography; PFTs, pulmonary function tests.

HPT

The HPT device comprised halo and pelvic rings, which were affixed to the cranial and iliac bones, respectively. It featured 4 correcting rods that aligned the rings for spinal distraction. The fitting procedure required general anesthesia. The halo frame was positioned above the right skull equator and secured with 10 pins tightened to a torque of 6–8 inches. Then, 2 pelvic ring pins were inserted at the iliac crest and the posterior superior iliac spine. After the patient had adapted to the device and was confident in routine activities, the halo-pelvic apparatus was fitted. Traction commenced with the bars being distracted by approximately 4–5 mm weekly. Traction was temporarily halted if traction-related symptoms, such as neck pain or numbness, occurred. Respiratory exercises, including deep breathing, abdominal breathing, lip-constriction breathing, and balloon exercises, were performed during traction. Additionally, physical exercise was conducted to improve cardiopulmonary fitness. If pulmonary function had plateaued and the patient could tolerate complex surgery, surgery might have been considered. Figure 2 illustrates the process and appearance of HPT.

Figure 2 An 18-year-old female patient with severe AIS. (A,B) The patient’s appearance at admission. (C,D) The patient’s condition following 125 days of halo-pelvic traction. In comparison to (A,B), the patient’s height increased, and the degree of scoliosis decreased. (E) A 3D CT volume rendering reconstruction. The images are published with the consent of the patient. AIS, adolescent idiopathic scoliosis; 3D, 3-dimensional; CT, computed tomography.

Radiographic parameters

Full-length standing anteroposterior and lateral radiographs were obtained before and after HPT. Full spine radiographs were obtained using 2 distinct X-ray systems: a GE Definium 6000 (GE Healthcare, Chicago, IL, USA) for conventional radiography and a Shimadzu Sonialvsion C200 (Shimadzu, Kyoto, Japan) for slot radiography. Patients were positioned upright with minimal movement, ensuring imaging coverage from the foramen magnum to the femoral head. The imaging technique required precise patient positioning and alignment of the central ray with the spinal axis. The machine’s current and exposure time were automatically controlled. The focal distance was set at 150 cm. The major thoracic Cobb angle and thoracic height (T1–T12) were measured on anteroposterior radiographs, whereas thoracic kyphosis (T5–T12) was measured on lateral radiographs. The thoracic height was defined as the vertical distance between the T1 and T12 vertebral bodies, as illustrated in Figure 3A,3B.

Figure 3 Radiographic and CT images with 3D volumetric reconstructions in a 12-year-old female patient with AIS. The anteroposterior radiographs captured before (A) and after (B) traction displayed the changes in radiographic parameters. Following 128 days of halo-pelvic traction, the major Cobb angle (blue lines) decreased from 123.6° to 91.4° and the thoracic height (red line) increased from 14.0 to 20.5 cm. Axial CT image (C) was used to measure hemithoracic symmetry ratio, thoracic rotation, and spinal rotation at the apical vertebral level. The hemithoracic symmetry ratio was calculated by dividing the larger distance (represented by the red line) by the smaller distance (represented by the blue line). The calculation provided a value of 6.1 (16.6/2.7). The measured values for thoracic rotation (purple angle) and spinal rotation (green angle) were 67.2° and 6.8°, respectively. 3D volumetric reconstructions of the lungs were obtained at end inspiration (D) and end expiration (E) after traction. The total lung tidal volume was calculated as 1,221.0 mL by subtracting TLV at end expiration from TLV at end inspiration. H, head; R, right; A, anterior; L, left; CT, computed tomography; 3D, 3-dimensional; AIS, adolescent idiopathic scoliosis; TLV, total lung volume.

CT scanning and quantitative image analysis

All patients enrolled in the study underwent unenhanced and enhanced scanning using a Philips 256-slice Brilliance iCT scanner (Philips, Amsterdam, The Netherlands) both before and after HPT. To maintain consistent lung volume, patients held their breath at end inspiration during the enhanced scanning and at end expiration during the unenhanced scanning. Low-dose CT scans were acquired with a collimation of 0.625 mm and autoexposure control, adjusting the exposure based on the patient’s size. The hemithoracic symmetry ratio, thoracic rotation, and spinal rotation at the apical vertebral level were calculated using the method described by Campbell et al. (11), as follows (Figure 3C).

• Hemithoracic symmetry ratio: the posterior aspect of the thorax was used as a reference point, specifically the anterior tip of the rib heads where they connected with the spine. The distance from the spine at each rib head to the inner border of the hemithorax was measured. This measurement was then used to calculate the ratio by dividing the larger distance by the smaller distance. In a normal thorax, this ratio is equal to 1. However, as a rib hump develops, the ratio increases, indicating asymmetry in the thorax.
• Thoracic rotation: the angle between a line dividing the thorax and the sagittal plane of the vertebra was measured. An increase in rotation suggested progressive asymmetry in the thorax.
• Spinal rotation: the angle between the sagittal plane of the apical vertebra, defined as a line perpendicular to a line drawn between the rib heads, and a line perpendicular to the inferior edge of the ribs, was measured.

CT lung volumes

3D volumetric reconstructions of the lung parenchyma were created using Lung Density Analysis software on the Philips Extended Brilliance Workspace (R4.5; Philips). Figure 3D,3E illustrate a representative volumetric reconstruction of the lungs and trachea. The trachea and main bronchi were excluded from the 3D reconstruction to generate separate volumes for the right and left lungs. A total of 9 volumetric parameters were computed: the left lung volume at end inspiration, right lung volume at end inspiration, total lung volume (TLV) at end inspiration, convex-to-concave lung volume ratio at end inspiration, left lung volume at end expiration, right lung volume at end expiration, TLV at end expiration, convex-to-concave lung volume ratio at end expiration, and total lung tidal volume. The calculation for total lung tidal volume involved subtracting TLV at end expiration from TLV at end inspiration.

PFTs

Pre- and post-HPT PFTs were conducted on all patients in the study. The collected pulmonary function parameters included forced vital capacity (FVC), percent predicted forced vital capacity (FVC%), forced expiratory volume in the first second (FEV1), percent predicted forced expiratory volume in the first second (FEV1%), FEV1/FVC ratio, total lung capacity (TLC), and percent predicted total lung capacity (TLC%). The percent-predicted volumes were calculated based on normative values adjusted for age and arm span. Pulmonary impairment severity was categorized according to the American Thoracic Society guidelines: ‘no’ impairment for FVC% >80% of predicted, ‘mild’ for FVC% ≤80% but >65%, ‘moderate’ for FVC% ≤65% but >50%, and ’severe’ for FVC% ≤50% (12). Obstructive lung disease was defined as an FEV1/FVC ratio below the 5th percentile (13).

Statistical analysis

Statistical analysis was performed using SPSS software version 27.0 (IBM Corp., Armonk, NY, USA). The data were presented as the mean ± standard deviation. The differences between variables before and after HPT were assessed using matched-pair t-tests. The correlation coefficients were calculated to evaluate the associations between traction duration, alterations in lung volume, thoracic anatomical parameters, and changes in PFTs. Pearson’s correlation coefficient was used if the data conformed to a normal distribution, whereas Spearman’s correlation coefficient was used for the data that failed to conform to a normal distribution. A P value <0.05 indicated a statistically significant difference.

Results

In the study, all 24 patients demonstrated right thoracic curves, with the apex between the T7 and T9 vertebrae. The HPT duration varied from 41 to 182 days, with a mean duration of 91.7±37.1 days. Following HPT, there were statistically significant increases in the patient’s arm span, height, and body weight, accompanied by a decrease in BMI. Detailed patient information is presented in Table 1.

The primary thoracic Cobb angle, kyphosis angle, thoracic height, hemithoracic symmetry ratio, thoracic rotation, and spinal rotation data for pre- and post-HPT are presented in Table 2. Following traction, reductions in the thoracic Cobb angle, thoracic kyphosis, and hemithoracic symmetry ratio were observed, with average decreases of 32.9%, 33.0%, and 18.8%, respectively. Additionally, thoracic height showed an increase of 36.1% after HPT. However, there were no significant changes noted in thoracic rotation and spinal rotation.

Following HPT, the mean values of total, left and right lung volumes at end inspiration, total and left lung volumes at end expiration showed significant increases ranging from 18.1% to 25.7% (refer to Table 3). Although right lung volume at end expiration and total lung tidal volume increased, the differences were not statistically significant. Changes in TLV at end inspiration and expiration, and total lung tidal volume before and after HPT are depicted in Figure 4. Furthermore, we employed the convex-to-concave (right to left) lung volume ratio to evaluate lung volume asymmetry. Our findings revealed no significant alterations in either the convex-to-concave lung volume ratios at end inspiration or expiration.

Figure 4 TLVEI, TLVEE, and TLTV before and after HPT. TLVEI, total lung volume at end inspiration; TLVEE, total lung volume at end expiration; TLTV, total lung tidal volume; HPT, halo-pelvic traction; CI, confidence interval.

In terms of the results of PFTs, all pulmonary function values, including FVC, FVC%, FEV1, FEV1%, FEV1/FVC, TLC, and TLC% showed significant increases ranging from 6.3% to 24.4% after HPT (refer to Table 4 and Figure 5). Among the patients, 91.7% (22/24) exhibited pulmonary impairment, with 16.7% (4/24) classified as having moderate impairment and 62.5% (15/24) as having severe impairment. Additionally, 6 patients had concomitant obstructive pulmonary abnormalities, whereas 22 patients presented with segmental or lobar bronchial stenosis and atelectasis.

Figure 5 PFTs before and after HPT. (A) FVC, FEV1, and TLC for pulmonary function tests before and after HPT. (B) Percent predicted values (FVC%, FEV1%, TLC%) and FEV1/FVC for pulmonary function tests before and after HPT. FVC, forced vital capacity; FEV1, forced expiratory volume in the first second; TLC, total lung capacity; FVC%, percent predicted forced vital capacity; FEV1%, percent predicted forced expiratory volume in the first second; TLC%, percent predicted total lung capacity; CI, confidence interval; PFTs, pulmonary function tests; HPT, halo-pelvic traction.

The relationships between the alterations in PFTs and CT, as well as radiographic parameters, were examined. The findings revealed negative correlations between the change in TLC% and hemithoracic symmetry ratio (r=−0.410, P=0.047). Furthermore, a positive correlation was observed between the change in FEV1 and TLV at end expiration (r=0.423, P=0.039). However, no statistically significant associations were found between the changes in PFTs and other CT and radiographic parameters. This study further explored the relationship between traction duration and alterations in radiographic and CT parameters, CT lung volumes, and PFTs. The findings revealed a negative correlation between traction duration and Cobb angle changes (r=−0.688, P<0.001), as well as positive correlations between traction duration and changes in left lung volumes at end inspiration (r=0.518, P=0.010) and expiration (r=0.452, P=0.027) (refer to Figure 6). No correlations were identified for other parameters. Furthermore, the study investigated the association between the primary Cobb angle and changes in PFTs, revealing a positive correlation between the primary Cobb angle and the increase in TLC (r=0.413, P=0.045), with no association observed with other pulmonary function parameters.

Figure 6 Scatter plot for correlations between HPT duration and the changes in Cobb angle and left lung volumes. A negative correlation is evident between HPT duration and changes in Cobb angle (A). Additionally, positive correlations are evident between traction duration and changes in left lung volumes at end inspiration (B) and end expiration (C). HPT, halo-pelvic traction; LLVEI, left lung volume at end inspiration; LLVEE, left lung volume at end expiration.

Discussion

Severe AIS often presents with respiratory function impairment, which is associated with thoracic Cobb angles (14,15). Studies (15,16) have shown that the prevalence of moderate to severe pulmonary impairment in preoperative AIS cases ranges from 14% to 28%, significantly increasing to 74% in cases with thoracic curves exceeding 80 degrees. Our study findings align closely with existing literature, revealing that 91.7% (22/24) of patients exhibit pulmonary impairment, with 79.2% (19/24) experiencing moderate to severe pulmonary dysfunction.

In the majority of AIS cases, the primary respiratory dysfunction is restrictive, characterized by equally reduced FEV1 and FVC, while maintaining a normal FEV1/FVC ratio (17). This restriction is primarily linked to chest wall deformity, decreased chest wall compliance, and respiratory muscle weakness (18-20). However, among thoracic AIS patients undergoing surgery, the prevalence of obstructive lung disease ranges from 21% to 39% (21-23). Our study identified that 25% (6/24) of patients exhibited combined obstructive pulmonary abnormalities. Furthermore, 22 patients were diagnosed with segmental or lobar bronchial stenosis and atelectasis. These findings support that ventilatory dysfunction in thoracic AIS is not solely restrictive but often includes an obstructive component, consistent with previous research (24). Moreover, 27.3% (6/22) of patients exhibited improvements in segmental or lobar bronchial stenosis and atelectasis as observed in CT scans (Figure 7). Following HPT, there was an increase in FEV1/FVC ratio. These observed improvements are likely associated with the correction of deformities, particularly the reduction of the hemithoracic symmetry ratio and the primary Cobb angle as shown in the results, leading to the enlargement of the right hemithorax.

Figure 7 A 13-year-old girl with severe adolescent idiopathic scoliosis presenting with atelectasis. Pre-HPT CT imaging (A) showed bronchial stenosis and atelectasis in the lower lobe of the right lung. After 132 days of traction, a notable improvement in bronchial stenosis and atelectasis was noted (B). HPT, halo-pelvic traction; CT, computed tomography.

Preoperative HPT has been shown to significantly improve both spinal deformity and pulmonary function, with minor and manageable complications (5,8,9), which aligns with the results of our study. Following traction, there was a reduction in the thoracic Cobb angle, thoracic kyphosis, and hemithoracic symmetry ratio. Additionally, an average increase of 6.5 cm in thoracic height was observed. These outcomes suggest that this intervention effectively addresses deformities in the coronal, sagittal, and axial planes, reduces scoliotic curvature, and elongates the spine. The PFT results demonstrated significant improvements in all parameters, including FVC, FVC%, FEV1, FEV1%, TLC, and TLC%, with enhancements ranging from 8.3% to 24.4% post-HPT. These results are consistent with previous research (8,9). Notably, the observed increase in the FEV1/FVC ratio has not been reported in prior studies.

This study is the first to utilize CT for evaluating changes in lung volume following HPT. Accurate measurement of lung volume is essential for assessing restrictive pulmonary dysfunction (25). CT-based lung volume measurement provides more precise results than conventional PFTs (25,26). The results demonstrated an improvement in CT TLV following traction. These findings were further supported by PFTs, which showed an increase in TLC and TLC% after HPT. A previous study (27) showed that patients diagnosed with AIS exhibit lower right-to-left lung volume ratios compared to age-matched individuals without AIS. Therefore, we assessed lung volume asymmetry by analyzing the convex-to-concave lung volume ratio. Our results indicated no significant changes in the convex-to-concave lung volume ratios at both end inspiration and expiration. Previous studies (28-30) indicated that the ratio of convex-to-concave lung volume remained relatively stable following corrective surgery, consistent with our results, despite the utilization of diverse intervention approaches.

Correlation analysis between changes in PFTs, lung volumes, and thoracic anatomical parameters revealed that only alterations in TLV at end expiration and hemithoracic symmetry ratio were significantly correlated with PFTs. Contrary to previous findings (31), although TLVs increased, these changes showed no correlation with changes in TLC and TLC%. Moreover, variations in thoracic Cobb angle, thoracic kyphosis, and thoracic height did not exhibit direct correlations with changes in PFTs. Additionally, we found no correlation between the primary Cobb angle and the improvement in most PFT parameters. These results demonstrate that despite increases in TLV and spinal deformity, most parameters do not directly impact pulmonary function. We propose that this outcome may be partially attributed to the respiratory training and physical exercises, which enhanced chest wall compliance and respiratory muscle strength. Therefore, the enhancement in PFTs may not solely be influenced by alterations in lung volume and spinal deformity but also by these interventions.

Our study further investigated the influence of traction duration on radiographic and CT parameters, CT lung volumes, and PFTs. The results indicated that extended traction duration was associated with a more substantial improvement in the Cobb angle and an increase in left lung volume. In contrast, no significant association was observed between TLV, right lung volume, and traction duration. These findings imply that as traction duration is prolonged, the correction of scoliotic deformity becomes more pronounced, with a more direct effect on the enhancement of concave (left) lung volume rather than convex (right) lung volume.

Biphasic quantitative thoracic CT has been extensively employed in chronic obstructive pulmonary disease and small airway disease, showcasing its capability to assess pulmonary function (32,33). In this study, lung tidal volume was computed using CT scans at both end inspiration and expiration, assuming it mirrors respiratory movement akin to FVC. Patients were instructed to maintain a consistent lung volume by breath-holding during the scans, and pre-scan breath-holding training was conducted to enhance the precision of CT lung volume measurements. Although there was a rise in lung tidal volume, this difference was not statistically significant, in contrast to the significant increases seen in FVC and FVC%. This finding suggests that lung tidal volume cannot be considered a reliable predictor of pulmonary function, as originally hypothesized.

This study has some limitations. First, biphasic CT scans were used to assess lung volume at both end inspiration and expiration for calculating lung tidal volume. Radiation exposure is a critical concern that must be addressed. In this research, both low-dose unenhanced and enhanced CT scans were employed to evaluate spine and vessel conditions, ensuring precision and safety in surgical procedures rather than for the primary study objective. To minimize radiation exposure, lung volume at end inspiration was measured from enhanced scans, whereas lung volume at end expiration was measured from unenhanced scans. In addition, we utilized low voltage (80–100 keV), automated exposure control, and iterative reconstruction algorithms (iDose⁴) to minimize patient radiation exposure. Table S1 provides details of the radiation doses for both unenhanced and enhanced CT scans. The total dose is within the recommended American College of Radiology 2022 guidelines for only the unenhanced chest CT (34). However, the routine use of CT scans for lung function assessment is not recommended. Another limitation of this study is that CT lung volume results may be influenced by breathing. Despite conducting pre-scan breath-holding training, lung volume measurements were still susceptible to factors such as breathing patterns and patient compliance. The lack of correlation between the alteration in CT lung tidal volume and PFTs may be partly attributed to this. Quantitative dynamic thoracic magnetic resonance imaging (MRI) could serve as a potential alternative for evaluating dynamic thoracic function and lung volume, particularly in pediatric and adolescent patients. One notable advantage of this method is its capability to capture MRI scans while the patient breathes naturally, without radiation exposure (35,36).

Conclusions

Preoperative HPT significantly improved spinal deformity, TLV, and pulmonary functions. CT lung tidal volume, as the key parameter in biphasic CT, was found to be an unreliable predictor of pulmonary function. The use of biphasic thoracic CT did not demonstrate substantial benefits over monophasic CT and is not recommended for severe AIS.

Acknowledgments

We thank Xiaodi Zhang for editing the manuscript and for editorial assistance.

Funding: This work was supported by the Research Project of Chengdu Medical Association (No. 2021117).

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-24-568/rc

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-24-568/coif). All authors report that this work was supported by the Research Project of Chengdu Medical Association (grant No. 2021117). The authors have no other 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. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by the Ethics Committee of the Third People’s Hospital of Chengdu (No. 2024-S-105) and the requirement for individual consent for this retrospective analysis was waived.

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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