Raphaël Pietton

Houssam Bouloussa

Tristan Langlais

MD Jessica Taytard

MD Nicole Beydon

PhD, Head of Department C Wafa Skalli

Claudio Vergari

Raphaël Vialle

R Vialle Spine

PhD Vergari

Estimating pulmonary

Introduction

Adolescent idiopathic scoliosis (AIS) is defined as a 3D deformity of the trunk which develops during growth, causing abnormal curvature of the spine in the coronal and sagittal planes and changes in shape of the rib cage. Idiopathic scoliosis, which accounts for approximately 85% of cases, is defined as a structural scoliosis for which no cause has been established. [START_REF] Hwang | Evaluation and management of adolescent idiopathic scoliosis: a review[END_REF] The respiratory impact of scoliosis is multifactorial. [START_REF] Tsiligiannis | Pulmonary function in children with idiopathic scoliosis[END_REF] It is usually associated with the development of a restrictive ventilatory defect caused by dysfunction of the chest wall and diaphragm. Pulmonary function tests (PFTs) are currently the reference technique to evaluate this restrictive ventilatory impairment, but obtaining reliable static volumes relies heavily on patient compliance. [START_REF] Takahashi | Factors of thoracic cage deformity that affect pulmonary function in adolescent idiopathic thoracic scoliosis[END_REF][START_REF] Xue | An analysis of thoracic cage deformities and pulmonary function tests in congenital scoliosis[END_REF] In severe cases, pulmonary function impairment due to thoracic scoliosis can lead to an increased morbidity and early mortality. [START_REF] Akazawa | Pulmonary function and thoracic deformities in adolescent idiopathic scoliosis 27 years or longer after spinal fusion with Harrington instrument[END_REF] Several studies have shown the relationship between trunk deformity and respiratory impairment. Dreimann et al [START_REF] Dreimann | Scoliosis and chest cage deformity measures predicting impairments in pulmonary function: a cross-sectional study of 492 patients with scoliosis to improve the early identification of patients at risk[END_REF] identified curve magnitude and thoracic hypokyphosis as risk factors for a significant decrease in forced vital capacity (FVC). However, Newton et al [START_REF] Newton | Results of preoperative pulmonary function testing of adolescents with idiopathic scoliosis. A study of six hundred and thirty-one patients[END_REF] found a significant but very limited influence of these same factors on the magnitude of respiratory impairment in severe scoliosis. For Newton et al, [START_REF] Newton | Results of preoperative pulmonary function testing of adolescents with idiopathic scoliosis. A study of six hundred and thirty-one patients[END_REF] although spinal parameters are significantly correlated with lung function, their influence is less. Their variability would only explain 20% of the changes in lung function. These studies investigated different spinal parameters using standard radiographs. In practice, 2D analysis is insufficient to assess scoliotic deformities as it completely ignores the axial plane. CT or MRI are traditionally used to assess the scoliotic spine in three dimensions but have the disadvantage of being carried out with the patient supine, which modifies spinal curvatures in both planes. [START_REF] Chu | Dynamic magnetic resonance imaging in assessing lung function in adolescent idiopathic scoliosis: a pilot study of comparison before and after posterior spinal fusion[END_REF] Furthermore, increasing concerns about radiation safety and the use of CT scans in children limit their applicability in routine clinical practice. In the last decade, whole-body biplanar radiographs have become the new gold standard in the analysis of these deformities. [START_REF] Dubousset | EOS stereoradiography system: whole-body simultaneous anteroposterior and lateral radiographs with very low radiation dose[END_REF][START_REF] Ilharreborde | Use of EOS imaging for the assessment of scoliosis deformities: application to postoperative 3D quantitative analysis of the trunk[END_REF] In addition to reducing the exposure to radiation, they allow a semi-automated 3D reconstruction of the spine and rib cage in the standing position, even in severe cases. [START_REF] Ilharreborde | Angle measurement reproducibility using EOS three-dimensional reconstructions in adolescent idiopathic scoliosis treated by posterior instrumentation[END_REF][START_REF] Pietton | Rib cage measurement reproducibility using biplanar stereoradiographic 3D reconstructions in adolescent idiopathic scoliosis[END_REF] A recent study highlighted the relationship between 3D rib cage volume (RCV) using biplanar radiographs and total lung capacity (TLC) as assessed by PFT. [START_REF] Bouloussa | Biplanar stereoradiography predicts pulmonary function tests in adolescent idiopathic scoliosis: a cross-sectional study[END_REF] Unlike spinal fusion through an anterior approach, [START_REF] Yaszay | Progressive decline in pulmonary function 5 years post-operatively in patients who underwent anterior instrumentation for surgical correction of adolescent idiopathic scoliosis[END_REF] posterior spinal fusion (PSF) for AIS improves respiratory function and, above all, prevents long-term respiratory deterioration. [START_REF] Akazawa | Pulmonary function and thoracic deformities in adolescent idiopathic scoliosis 27 years or longer after spinal fusion with Harrington instrument[END_REF] However, no studies to date have evaluated the 3D morphological changes in the rib cage and spine after AIS surgery using biplanar radiographs, nor the resultant changes in lung function.

The aim of this study was therefore to evaluate whether preand postoperative biplanar radiographs of the rib cage and spine with 3D reconstruction could be used to estimate respiratory function in patients with an AIS.

Methods

Patients. A total of 47 patients with severe thoracic AIS were included in a prospective study between January 2017 and December 2017. Written consent was obtained from parents or trustees. The study was approved by the institutional board (CPP-Paris VI-16-06). The cohort was composed of a consecutive series of patients followed by a single senior orthopaedic surgeon (RV). Patients with the following criteria were excluded: secondary scoliosis (neuromuscular, syndromic, congenital, or post-traumatic); a previous history of surgery; spinal infection; chronic respiratory failure; inability to perform PFT; and severe intellectual deficiency. All patients had PFTs and biplanar radiographs immediately before surgery and two years postoperatively. All patients were initially followed for thoracic AIS with or without a history of brace treatment. No patient underwent selective thoracic fusion or thoracoscopic anterior release. Facetectomies were performed on the concave side of the curve. Two polyaxial pedicle screws were placed at each vertebral level. Two 6 mm diameter chrome-cobalt rods were bent according to the surgical plan. The concave rod was first fixed at the upper and lower instrumented vertebrae. After ensuring sufficient fixation on at least two levels, upper and lower, the rod was orientated to the ideal sagittal plane. A minimum of five translation tools ("persuaders") were placed on the concave side. The posteromedial translation was made using persuaders for progressive rod to screws approximation. The thoracic deformity was then progressively reduced by the concave persuaders to bring each vertebra to the pre-contoured rod. The spine was translated medially and posteriorly. After applying the maximal correction using the concave rod, the convex rod was sited. The two rods were bent in the same manner and no overbending was used on the concave side.

Imaging protocol and 3D modelling. Patients were in freestanding position and were tidal breathing while radiographs were taken (Figure 1). The anteroposterior and lateral exposure parameters were 83 kV/ 200 mA and 102 kV/ 200 mA, respectively. A 3D reconstruction of the spine and rib cage was performed using a semiautomated process, which was validated in previous studies. [START_REF] Jolivet | Fast 3D reconstruction of the rib cage from biplanar radiographs[END_REF][START_REF] Mitton | 3D reconstruction of the ribs from lateral and frontal X-rays in comparison to 3D CT-scan reconstruction[END_REF] The operator was blinded from patients' pulmonary function during the reconstruction process.

Spinal and rib cage parameters. The following structural measurements of the rib cage and spine were obtained automatically using 3D reconstructions: [START_REF] Aubert | 3D reconstruction of rib cage geometry from biplanar radiographs using a statistical parametric model approach[END_REF][START_REF] Humbert | 3D reconstruction of the spine from biplanar X-rays using parametric models based on transversal and longitudinal inferences[END_REF][START_REF] Sabourin | Threedimensional stereoradiographic modeling of rib cage before and after spinal growing rod procedures in early-onset scoliosis[END_REF] Cobb angle (°); apical vertebral rotation (AVR, °); thoracic kyphosis (T4-T12, °); lumbar lordosis (L1-S1, °); rib hump (°); RCV (l); hypokyphosis index (°); maximum rib cage anteroposterior diameter (AP diameter, mm); and maximum medial-lateral diameter (ML diameter) (mm). [START_REF] Dubousset | Spinal penetration index: new three-dimensional quantified reference for lordoscoliosis and other spinal deformities[END_REF] Hypokyphosis index was defined as the difference between the local kyphosis (or lordosis) of the given patient at the apex and the mean value at the equivalent level for non-scoliotic subjects. [START_REF] Dickson | The pathogenesis of idiopathic scoliosis. Biplanar spinal asymmetry[END_REF] Pulmonary function test. All patients underwent PFT including plethysmography and standard spirometry (Body Box, MediSoft, Belgium) in the same centre according to European Respiratory Society (ERS) recommendations. [START_REF] Miller | Standardisation of spirometry[END_REF][START_REF] Wanger | Standardisation of the measurement of lung volumes[END_REF] The following measurements were obtained: TLC; slow vital capacity (SVC); FVC; Forced Expiratory Volume in 1 s (FEV 1 ) to FVC ratio, and functional residual capacity (FRC). Operators were blinded to patients' deformity characteristics during PFT.

According to the spirometry guidelines on the severity of pulmonary impairment, patients with a TLC below 80% and with a FEV1/FVC ratio higher than 70% were considered to have a restrictive pattern. According to the guidelines for the severity of pulmonary impairment, [START_REF] Tzelepis | The Respiratory System and Chest Wall Diseases[END_REF] patients were classified as follows: patients with a FVC of more than 80% were considered to have no pulmonary impairment. Mild impairments were defined as FVC% values between 65% and 80%. Moderate and severe impairments were defined as FVC% values between Mean TLC (L)

Fig. 3 Agreement between measured and predicted total lung capacity (TLC)-Bland-Altman graph. The horizontal line is the mean bias while dashed lines represent twice the 95% confidence intervals of the difference between predicted and measured TLC.

Results

The surgical technique of choice was posterolateral translation with an all-pedicle-screw construct. There were no major perioperative complications. Figure 1 illustrates posterolateral translation with all-pedicle-screw constructs. Dashed rectangles illustrate the changes in intercostal distances of the hemithorax on the concave side. Figure 2 shows 3D final reconstructions as well as lateral and anteroposterior views. I). In particular, the mean RCV increased significantly from 4.9 l (SD 1.0) preoperatively to 5.3 l (SD 0.9) postoperatively (p < 0.001). Pulmonary function. All patients underwent PFT immediately before surgery and at a mean of 1.60 years (SD 0.44) after surgery. A total of 30 patients had a restrictive pattern. The mean FVC or TLC were not significantly different whether or not patients had a previous history of brace treatment (p = 0.821 (FVC), p = 0.742 (TLC), Mann-Whitney U test) or asthma. The II. According to the different FVC% groups, there were 12 patients in the "no impairment" and "mild impairment" groups, 16 patients in the "moderate impairment" group, and five patients in the "severe impairment" group. These different groups were combined into a pathological FVC% (FVC < 65%, 21 patients) group and a normal FVC% (FVC > 65%, 24 patients) group. The preoperative differences between both groups are summarized in Table III. Rib cage measurements such as RCV, anteroposterior diameter, and spinal penetration index (SPI) showed significant differences. Spinal measurements did not differ between FVC% groups. Nine of the 16 patients in the moderate impairment group had a FVC of more than 65% postoperatively (non-pathological), while three of the five patients in the severe impairment group had an increase in FVC beyond 50%. The effect of surgical correction on FVC% and the differences between groups are illustrated in Figure 3 while Table IV shows the main pre-and postoperative differences between each normal and pathological FVC group.

Preoperative correlation between trunk deformity and pulmonary function. The preoperative Cobb angle was weakly correlated with TLC (r = -0.38; p = 0.017, Spearman's rank test).

Hypokyphosis and apical vertebra rotation were also weakly correlated with pulmonary indices. No significant correlation was found between the other spinal and pelvic measurements and the preoperative pulmonary function. Preoperatively, RCV was correlated with TLC (r = 0.78; p < 0.001, Spearman's rank test), SVC (r = 0.78; p < 0.001, Spearman's rank test), and FVC (r = 0.76; p < 0.001, Spearman's rank test). The maximal AP diameter was also correlated with all pulmonary function indices.

Results are summarized in Table IV and Table V. Surgical correction and pulmonary changes. Postoperatively, TLC remained correlated with RCV (r = 0.81; p < 0.001). Table V and Table VI illustrate the postoperative correlations between radiological and functional indices. Finally, it was possible to predict the postoperative TLC from the postoperative RCV as shown in Figure 4; the root mean square error between the predicted and measured TLC was 0.38 l. Bland-Altman plots [START_REF] Bland | Statistical methods for assessing agreement between two methods of clinical measurement[END_REF] (Figure 3), analyzing differences between predicted TLC and postoperative TLC, illustrate this agreement without outliers. This showed that the differences between estimated and measured TLC are distributed around the mean TLC.

Discussion

This study reports data from a homogeneous cohort of prospectively enrolled AIS patients who were recruited to explore the level of agreement between 3D rib cage measurements obtained from biplanar radiographs and pulmonary function indices obtained from PFT. The use of CT does not allow the acquisition of trunk images in the standing position. The cumulative radiation dose may also increase the risk of cancer in adulthood, [START_REF] Law | Cumulative radiation exposure and associated cancer risk estimates for scoliosis patients: Impact of repetitive full spine radiography[END_REF] and 3D reconstructions from CT scans are time-consuming in routine clinical practice. Some studies have suggested the use of dynamic MRIs. [START_REF] Chu | Dynamic magnetic resonance imaging in assessing lung function in adolescent idiopathic scoliosis: a pilot study of comparison before and after posterior spinal fusion[END_REF] Recently, a significant relationship has been shown between chest measurements, such as the hump or the apical vertebral rotation and clinical parameters, using MRI. This is an interesting approach but, for the time being, it is reserved for research activities. The 3D rib cage reconstruction method from biplanar radiographs compared to CT scans has been validated in several studies. [START_REF] Jolivet | Fast 3D reconstruction of the rib cage from biplanar radiographs[END_REF][START_REF] Mitton | 3D reconstruction of the ribs from lateral and frontal X-rays in comparison to 3D CT-scan reconstruction[END_REF] An accuracy and reproducibility study of this method in mild idiopathic scoliosis confirmed the effectiveness of this technique. [START_REF] Courvoisier | Evaluation of a three-dimensional reconstruction method of the rib cage of mild scoliotic patients[END_REF] Finally, a reproducibility study of rib cage reconstruction in severe cases of idiopathic scoliosis confirmed a very good inter-and intraoperator reproducibility of this technique. [START_REF] Pietton | Rib cage measurement reproducibility using biplanar stereoradiographic 3D reconstructions in adolescent idiopathic scoliosis[END_REF] The correlation between Cobb angle and lung function is still controversial. Redding et al [START_REF] Redding | Lung function asymmetry in children with congenital and infantile scoliosis[END_REF] found weak correlations between Cobb angle and PFT results, while Johnston et al [START_REF] Johnston | Correlation of preoperative deformity magnitude and pulmonary function tests in adolescent idiopathic scoliosis[END_REF] reported a stronger correlation. However, the different measurements were made on standard radiographs without any 3D analysis. Wang et al 32 recently showed an association between FVC and spinal and thoracic measurements. These authors insisted on the link between thoracic measurements, such as translation and rotation of the apical vertebra, and the severity of the Cobb angle with respiratory impairment. Buckland et al 33 in a similar study concluded that there was no significant increase in RCV and TLC after surgery. The difference between our results and this study may be explained by a higher severity of Cobb angle in our study population (68° vs 48°). Even though Cobb angle measurement is at the centre of the decision-making process and surgical planning, it is not certain that it should still play a fundamental role in the risk-benefit ratio explained to patients Difference between predicted and measured total lung capacity (TLC). RMSE, root mean square error.

and their parents concerning their respiratory condition before surgery. Bouloussa et al, [START_REF] Bouloussa | Biplanar stereoradiography predicts pulmonary function tests in adolescent idiopathic scoliosis: a cross-sectional study[END_REF] in a recent study assessing 3D rib cage reconstructions and the impairment of pulmonary function in preoperative AIS, showed that RCV was the most important structural factor affecting lung function in patients with AIS and a severe thoracic curve (Cobb angle > 50°). The best correlation was found with TLC, suggesting that poor RCV was a predictor for the diagnosis of a restrictive ventilatory defect on PFT. This study confirms these results. Spinal measurements are poor predictors of impairment of pulmonary function. This was further confirmed by the predictive model, which was developed in this study and illustrated in Figure 5. Postoperative TLC could be predicted from postoperative RCV. Therefore, RCV is a key 3D measurement in the preoperative and postoperative assessment of pulmonary function in AIS.

In a study on AIS of variable severity, Ilharreborde et al 10 also showed a relationship between RCV measured from the same procedure and TLC. The present study shows the effectiveness of surgical correction by posteromedial screw translation. This technique, initially described with the use of sublaminar bands, can therefore be used with pedicle screws. Spinal measurements were all improved by surgery, including restoration of the thoracic kyphosis. The assessment of the 3D correction of the different spinal and thoracic measurements in this study tends to show that the increase in the RCV is probably multifactorial.

Although we have observed a strong relationship between spinal and thoracic measurements, the improvement of pulmonary function secondary to surgery cannot be explained simply by the correction of the Cobb angle. Only the RCV and the anteroposterior diameter were significantly related to improvement in the pulmonary function indices. One of the hypotheses that could explain the increase in RCV after surgery is the restitution of the volume of the concave hemithorax, as shown in the dashed rectangle in Figure 1.

The prognosis for pulmonary function is known to be of particular importance in ensuring that AIS patients remain healthy. As part of the natural history of AIS, patients with a severe thoracic curve have been reported to be at high risk of developing a restrictive ventilatory pattern. Long-term pulmonary function results after spinal fusion are becoming crucial. Akazawa et al, 5 in a study of long-term respiratory impairment in patients undergoing surgery for AIS, found more severe restrictive ventilatory defects in patients with greater rib humps and apical vertebral rotation. This illustrates the importance of performing vertebral derotation to correct the rib cage deformity. Kato et al 34 concluded in a recent review of the literature that there was no evidence of improvement in lung function after surgery for adolescent scoliosis. However, the authors themselves emphasized the heterogeneity of the different studies and the variability of the surgical techniques used. A prospective study is needed to evaluate the long-term respiratory result after surgical correction using modern techniques. Implants and modern reduction techniques will hopefully improve the longterm results.

The fact that we did not see a significant difference between pre-and postoperative FRC suggests that the increase in TLC is indeed due to an increase in RCV and not to alveolar development. Surgical correction and forced vital capacity (FVC%) changes between groups.

In total, 18 patients out of 45 (31.4%) had a history of asthma with the typical findings of obstructive lung disease. However, in all cases, the asthma was controlled, without long-term medication or symptoms reported four weeks prior to PFT. One study reported the increased incidence of obstructive lung disease in patients with AIS, but its pathophysiology remains uncertain. 35 Extrinsic mechanisms such as compression (indirectly caused by rib humps or apical rotation), gas trapping, and intrinsic mechanisms (including hypersecretion) have been suggested. 36 However, excluding these patients would have compromised the external validity of the study. Moreover, TLC was measured using plethysmography and not by the dilution technique, which gives a reliable result even in case of bronchial obstruction. The sample of patients is relatively small, however, and this is the first study on this topic. A larger number of participants would permit a better investigation of the influence of certain measurements, particularly of thoracic kyphosis. [START_REF] Dreimann | Scoliosis and chest cage deformity measures predicting impairments in pulmonary function: a cross-sectional study of 492 patients with scoliosis to improve the early identification of patients at risk[END_REF][START_REF] Newton | Results of preoperative pulmonary function testing of adolescents with idiopathic scoliosis. A study of six hundred and thirty-one patients[END_REF] Some authors have been able to highlight the role of hypokyphosis in the severity of respiratory impairment. It was not possible to corroborate these results in this study, nor was it possible to verify the relationship between hypokyphosis and respiratory failure because the mean kyphosis of the study population was within normal values.

In conclusion, this study describes a innovative way to approach pulmonary function in AIS. It gives a better understanding of the relationship between rib cage correction and the improvement in pulmonary function. In addition, this natural posture, weightbearing analysis of both spine and rib cage, through paired low-dose biplanar radiographs, explains the postoperative improvement in respiratory function in AIS patients with PSF. The 3D RCV appears to reflect virtual chest capacity.

Take home message

-This prospective study provides an explanation for the postoperative improvement of respiratory function in adolescent idiopathic scoliosis patients with posterior spinal fusion.

-The 3D rib cage volume appears to be a virtual chest capacity reflecting respiratory function.

Fig. 1 PreoperativeFig. 2 Rib

 12 Fig. 1 Preoperative and postoperative biplanar radiographs of a 15-year-old female. The dashed rectangles illustrate the restoration of the intercostal spaces on the concave side.

  Fig.4

  Fig.5

Table I .

 I Changes in spinal and rib cage indexes.

	Parameter	Preoperative	Postoperative	p-value*
	Mean Cobb angle, ° (SD) 68.2 (17.0)	19.7 (11.9)	< 0.001
	Mean T4-T12 kyphosis,	20.8 (17.6)	34.5 (11.4)	0.001
	° (SD)
	Mean hypokyphosis	-4.8 (4.5)	-1.2 (3.4)	< 0.001
	index, ° (SD)
	Mean rib hump, ° (SD)	12.0 (7.8)	8.7 (4.5)	0.005
	Mean rib cage volume,	4.9 (1.0)	5.3 (0.9)	< 0.001
	l (SD)
	Mean rib cage ML	230.9 (18.7)	233.0 (17.1)	0.010
	diameter, mm (SD)
	Mean rib cage AP	143.6 (15.4)	137.0 (13.6)	0.009
	diameter, mm (SD)
	Mean apical vertebral	-12.3 (19.3)	-6.3 (9.44)	0.036
	rotation, ° (SD)
	*Paired Wilcoxon test.
	AP, anteroposterior; ML, medial-lateral; SD, standard deviation.

Table II .

 II Pre-and postoperative lung function.

Indexes, litres Preoperative Postoperative p-value*

	Mean TLC (SD)	4.1 (0.9)	4.3 (0.8)	0.001
	Mean SVC (SD)	2.9 (0.7)	3.3 (0.6)	< 0.001
	Mean FVC (SD)	3.0 (0.7)	3.2 (0.6)	< 0.001
	Mean FRC (SD)	2.0 (0.5)	2.1 (0.4)	0.628
	Mean FEV1/FVC ratio (SD) 0.82 (0.077)	0.84 (0.079)	0.021
	*Paired Wilcoxon test.
	FRC, functional residual capacity; FVC, forced vital capacity; SD,
	standard deviation; SVC, slow vital capacity; TLC, total lung capacity.

Table III .

 III Differences between spinal and rib cage indexes in both preoperative forced vital capacity % groups.

	Spinal and rib cage	Pathological	Normal FVC%	p-value*
	parameters	FVC%
	Total, n	21	24
	Mean Cobb angle, ° (SD)	73.2 (25)	63 (11.2)	0.058
	Mean T4-T12 kyphosis, °	21.6 (21)	19.9 (14.8)	0.782
	(SD)
	Mean hypokyphosis index,	-5.4 (4.8)	-4.4 (4.7)	0.234
	° (SD)
	Mean rib hump, ° (SD)	12.5 (8)	9.9 (4.4)	0.484
	Mean rib cage volume, l	4.6 (0.9)	5.2 (0.8)	0.023
	(SD)
	Mean rib cage width, mm	226.9 (17.0)	234.9 (16.8)	0.039
	(SD)
	Mean rib cage AP diameter,	138.7 (15.5)	145 (14.1)	0.138
	mm (SD)
	Mean apical vertebral	-16.7 (19.5)	-5.5 (19.6)	0.057
	rotation, ° (SD)

*Paired Wilcoxon test. AP, anteroposterior; FVC, forced vital capacity; SD, standard deviation.

Table IV .

 IV Main pre-and postoperative differences between each normal and pathological forced vital capacity group. All patients with asthma were asymptomatic and did not require additional treatment for at least four weeks prior to PFT, in accordance with current guidelines.26 Of the 45 patients, 26 had previously undergone brace treatment. In terms of ethnicity, 23 were European Caucasian, ten were Middle Eastern, seven were Black African, and five were Southeast Asian. Statistical analysis. Volumes were expressed as litres or as a percentage of predicted volume, but expression in litres was used for statistical analyses. Differences were analyzed using

	Variable	Normal FVC group	Pathological FVC group
		Preop	Postop	Preop	Postop
	n	24	33	21	12
	FVC, %	81	79	56	57
	Total lung capacity, l 4.3	4.3	3.8	4.3
	Cobb angle, °63	17	73	27
	Rib cage volume, l	5.1	5.3	4.6	5.3
	FVC, forced vital capacity.
	50% and 65% and below 50%, respectively. A FVC cut-off
	value of 65% was used to define pulmonary function patho-
	logical status.

Demographic data. Overall, 45 patients (38 female) with a thoracic AIS were included in the study. According to the Lenke classification,

[START_REF] Lenke | Adolescent idiopathic scoliosis: a new classification to determine extent of spinal arthrodesis[END_REF]

33 patients had a Lenke type 1 curve and 12 a Lenke type 2. Mean age at surgery was 14.7 years (12 to 17). The mean Risser stage was 3.48 (standard deviation (SD) 1.1). A total of 18 patients had a previous history of asthma. paired Wilcoxon tests and Mann-Whitney U tests and correlations using Spearman's rank test. Significance was set at α = 0.05. A predictive model was then developed to estimate postoperative TLC from the postoperative radiological indexes, using stepwise linear regression. The limits of agreement of the model were expressed as Bland-Altman graphs. Statistical analysis was conducted using SPSS v20 (IBM, USA).

Table V .

 V Preoperative correlations between functional and radiological indexes.

	Variable	T4-T12 kyphosis L1-S1 lordosis	HK index	AVR	Cobb angle	Width	AP diameter	Rib hump	RCV
	p-values*
	TLC	0.991	0.591	0.006	0.052	0.017	0.003	0.001	0.671	< 0.001
	FVC	0.548	0.677	0.017	0.022	0.066	0.001	< 0.001	0.658	< 0.001
	SVC	0.708	0.541	0.020	0.004	0.012	0.004	< 0.001	0.096	< 0.001
	FEV1 ratio	0.271	0.322	0.267	0.203	0.386	0.071	0.016	0.063	0.035
	R coefficients
	TLC	0.00	-0.09	0.43	0.31	-0.38	0.46	0.53	-0.07	0.78
	FVC	0.09	-0.06	0.36	0.34	-0.28	0.50	0.58	-0.07	0.78
	SVC	0.06	-0.10	0.36	0.44	-0.39	0.44	0.53	-0.26	0.77
	FEV1 ratio	0.17	-0.15	-0.17	0.20	-0.13	-0.27	-0.36	-0.28	-0.32
	*Spearman's rank test.
	AP, anteroposterior; AVR, apical vertebral rotation; FEV1, forced expiratory volume in one second; FVC, forced vital capacity; HK, hypokyphosis;
	RCV, rib cage volume; SVC, slow vital capacity; TLC, total lung capacity.

Table VI .

 VI Postoperative correlations between functional and radiological indexes.

	Variable	T4-T12 kyphosis L1-S1 lordosis	HK index	AVR	Cobb	Width	AP diameter	Rib hump	RCV
	p-values*
	TLC	0.647	0.657	0.116	0.002	0.800	0.097	< 0.001	0.875	< 0.001
	FVC	0.349	0.367	0.028	0.016	0.526	0.117	< 0.001	0.216	< 0.001
	SVC	0.753	0.568	0.023	0.001	0.870	0.176	< 0.001	0.362	< 0.001
	FEV1 ratio	0.961	0.881	0.981	0.311	0.955	0.028	0.127	0.310	0.011
	R coefficients
	TLC	0.07	-0.07	0.25	0.48	-0.04	0.26	0.65	-0.02	0.81
	FVC	0.14	-0.14	0.33	0.36	-0.10	0.24	0.58	-0.19	0.72
	SVC	0.05	-0.09	0.35	0.50	-0.03	0.21	0.63	-0.14	0.69
	FEV1 ratio mean TLC was 4.1 l (SD 0.87) preoperatively and 4.3 l (SD 0.01 0.02 0.00 0.16	-0.01	-0.34	-0.24	-0.16	-0.38
	0.76) postoperatively (p = 0.001, paired Wilcoxon test). The
	mean FVC increased significantly from 3.0 l (SD 0.7) preop-
	eratively to 3.2 l (SD 0.6) postoperatively (p < 0.001, paired
	Wilcoxon test). FRC did not vary significantly between the pre-
	operative measurement (2.05 l) and final follow-up (2.07 l) (p =
	0.628, paired Wilcoxon test). Lung function results are summa-
	rized in Table

*Spearman's rank test. AP, anteroposterior; AVR, apical vertebral rotation; FEV1, forced expiratory volume in one second; FVC, forced vital capacity; HK, hypokyphosis; RCV, rib cage volume; SVC, slow vital capacity; TLC, total lung capacity.

Funding statement:

No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this article.

ICMJE COI statement:

R. Vialle has a consulting agreement with Stryker spine and Nuvasive. J. Taytard reports consultancy fees and payment for development of educational presentations from Roche, and expenses from ADEP Assistance, all unrelated to the study. C. Vergari reports an institutional grant from the BiomecAM chair program for the study. W. Skalli reports a grant from the Paris Tech Foundation, the BiomecAM chair program on subject-specific modeling, and financial support from the Cotrel Foundation, Proteor, Covea, and Societe Generale, related to the study. W. Skalli also reports patents and royalties related to the EOS system and associated 3D reconstruction methods.