European Journal of Cardio-Thoracic Surgery 59 (2021) 375–381 ORIGINAL ARTICLE doi:10.1093/ejcts/ezaa328 Advance Access publication 30 October 2020

Cite this article as: Is¸can_ M, Kılıc¸ B, Turna A, Kaynak MK. The effect of minimally invasive repair on thoracic . Eur J Cardiothorac Surg 2021;59:375–81.

The effect of minimally invasive pectus excavatum repair on thoracic scoliosis

Mehlika Is¸can_ a,*, Burcu Kılıc¸b, Akif Turna b and Mehmet Kamil Kaynak b a Department of Thoracic Surgery, Gebze Fatih State Hospital, Kocaeli, Turkey b Department of Thoracic Surgery, Istanbul University-Cerrahpas¸a,Cerrahpas¸aSchool of Medicine, Istanbul, Turkey Downloaded from https://academic.oup.com/ejcts/article/59/2/375/5943430 by guest on 29 September 2021 * Corresponding author. Department of Thoracic Surgery, Gebze Fatih State Hospital, 41400 Gebze - Kocaeli, Turkey. Tel: +90-543-6609334; e-mail: [email protected] (M. Is¸can)._

Received 13 March 2020; received in revised form 17 July 2020; accepted 23 July 2020 THORACIC

Abstract OBJECTIVES: The Nuss technique comprises the placement of an intrathoracic bar behind the . However, besides improving the body posture through the correction of the pectus excavatum (PE), this procedure may cause or worsen thoracic scoliosis as a result of the considerable stress loaded on the chest wall and the . Our goal was to investigate the impact of the on the thoracic spinal curvature in patients with PE. METHODS: A total of 100 patients with PE who underwent the Nuss procedure were included in the study and evaluated retrospectively. The (HI), asymmetry index and sternal torsion angle were calculated from thoracic computed tomography images before the operation. To evaluate the scoliosis in the T2–T8 , Cobb angles were calculated on a plain chest X-ray before the Nuss operation and after the removal of the bar. Cobb angles were classified as normal (5), scoliotic posture (5–10) and scoliosis (>10). All angles before and after the Nuss operation were compared. The patients were followed up for a mean of 41 months. Substernal bars were removed after a mean of 33 months.

VC The Author(s) 2020. Published by Oxford University Press on behalf of the European Association for Cardio-Thoracic Surgery. All rights reserved. 376 M. Is¸can_ et al. / European Journal of Cardio-Thoracic Surgery

RESULTS: The mean age of the patients was 19.6 ± 6.7 years. The Cobb angle was statistically significantly increased in all patients (P = 0.01), male patients (P = 0.01) and children (P = 0.046) but not in adults (P = 0.11) and female patients (P = 0.54). The Cobb angle was increased in patients with severe (HI >_ 3.5) but not in patients with moderate (3.2 < HI < 3.5) or mild (2.0 < HI < 3.2) PE deformity. CONCLUSIONS: The present study shows that the Cobb angle indicates that the severity of thoracic scoliosis increases following the Nuss procedure, particularly in male patients, in patients with mild and moderate sternal torsion angle and in those with a high preoperative HI. This alteration might be due to correctional forces and torque applied by the bar. Patients undergoing the Nuss procedure for the correc- tion of PE should be followed up strictly for timely diagnosis and management of the scoliosis. Keywords: Nuss procedure • Pectus excavatum • Haller index • Scoliosis • Cobb angle

study was to investigate the impact of the Nuss procedure on ABBREVIATIONS thoracic spinal curvature in patients with PE and to assess whether it was associated with scoliosis. Downloaded from https://academic.oup.com/ejcts/article/59/2/375/5943430 by guest on 29 September 2021 AI Asymmetry index BMI Body mass index HI Haller index MATERIALS AND METHODS PE Pectus excavatum STA Sternal torsion angle Patient selection

All patients who underwent the minimally invasive Nuss proced- INTRODUCTION ure, later interventions (removal of the intrathoracic bar) and follow-up studies at our tertiary care university clinic between Pectus excavatum (PE) is a common congenital chest anomaly July 2011 and July 2015 were enrolled in this retrospective ana- with an incidence of 1 in 1000 live births and a male-to-female lysis. All study participants provided informed consent. The need ratio of 3-to-1 [1]. It is a musculoskeletal malformation resulting for institutional review board approval was waived according to from inadequate fusion between the costae and sternum during our country’s law because the study is a retrospective cohort embryologic development leading to the depression of the anter- study. Data concerning the demographic characteristics of ior chest wall [2]. Although the malformation itself seldom causes patients and their physical examination and their posterior–an- problems and most subjects are asymptomatic, the inward de- terior chest X-ray and thorax computed tomography results were formation may affect organs enclosed within the thoracic space retrieved from patient charts and the institutional digital data- and has been shown to result in various problems, including re- base. Offline measurements of the preoperative Haller index (HI), strictive pulmonary deficits, right-sided compression, mitral asymmetry index (AI) and the sternal torsion angle (STA) were valve prolapse and dysrhythmias. Furthermore, those with PE performed using the computed tomography images of the thorax may also experience intolerance and dyspnoea on exer- for each patient. Cobb angles were calculated from a plain chest tion in the presence of compression on the right ventricular out- X-ray. The Cobb angle measurement was repeated following the flow [3]. removal of the intrathoracic bar (Fig. 1). Measurement of the Conservative management of PE mainly focuses on the reduc- Cob angle was performed by an experienced orthopaedist tion of pain, which is the presenting complaint in many symp- blinded to patient data. tomatic patients. Anti-inflammatory agents, acupuncture, and osteopathic manipulative treatments have The Nuss procedure long been used in the management of pain [4]. A device known as the vacuum bell, which applies suction to the chest wall, may The indications for the operation that we used in the current also be used in younger patients [5]. However, surgical correction study are as follows: is recommended for subjects with a Haller score >3.25 or a cor- rection index score of >20% [6]. The optimal time for correction • Progressive and symptomatic PE, is currently accepted as mid-adolescence, because subjects • Restrictive breathing pattern, measured by pulmonary func- undergoing repair during this stage may complete their growth tion tests, with a low risk of recurrence [7, 8]. Nevertheless, successful repair • Cardiac compression, pulmonary , of PE with low morbidity and considerable improvement in • Heart disorders with or branch block, physiological function has also been demonstrated in the adult • HI index greater than 3.25, population [9]. • Psychosocial indications. Two decades ago, Nuss et al.[10] developed a procedure that has come to be known as the Nuss technique; it comprises the If the patient who was being considered for an operation had placement of a bar behind the sternum. However, although this an HI <3.25 and greater than 2.5 and had other indications, we technique improves body posture through the correction of the performed the operation. PE, it may also cause or worsen thoracic scoliosis as a result of The procedure was performed with the patient under general the considerable stress loaded on the chest wall and the thorax anaesthesia. Lateral incisions were performed on both sides of [11, 12]. the chest to insert the bar, which was curved according to indi- Data are limited concerning the effects sustained by the thor- vidual characteristics and requirements. The bar was initially acic spine after the correction of PE. The goal of the present inserted in line with the curvature of the sternum and M. Is¸can_ et al. / European Journal of Cardio-Thoracic Surgery 377 Downloaded from https://academic.oup.com/ejcts/article/59/2/375/5943430 by guest on 29 September 2021

Figure 2: Haller index = a/b.

Figure 1: Cobb angle measurement. THORACIC placement was performed under the guidance of a thoraco- scope. When the bar was in the target position behind the ster- Figure 3: Asymmetry index = c/d. num, it was flipped 180 so that the curvature would elevate thedepression.Followingthefixationofthebarstotheribs,the primary closure of the incision was performed. A steel plate was used to facilitate the fixation of the bar to the . Following a The STA was measured from evaluations of the axial plane and was defined as the angle of the axis of the mediolateral length to median treatment time of 33 (20–60) months, the bar was  removed. Our policy was to leave the bar in place for 2 years. the horizontal length. An STA value higher than 30 indicates se- Patients were called for removal at 2 years. However, some vere torsion of the sternum (Fig. 4)[16]. The Cobb angle was patients did not return for removal for as long as 60 months. used to quantify the magnitude of the scoliosis. We located the We followed all patients for at least 2 years after the Nuss oper- apex, which is at the deepest part of the scoliosis curve, and then ation. The bars were removed as early as 20months after the determined the most-tilted vertebrae above and below the apex. operation for reasons related to the patients’ educational activ- The angle between the 2 lines extending from the most-tilted ities (i.e. patients tend to be admitted to the hospital during the vertebrae above the apex and the most-tilted below the apex was determined as the Cobb angle [17] (Fig. 1). It is summer or winter break from school). accepted that angles <10 represent minor spinal asymmetry; any angles exceeding 70 indicate severe scoliosis. The fourth Parameters from chest computed tomography thoracic vertebra (T4) is at the same level as the sternal angle, images and chest radiographs and the begins at the T8–T9 vertebrae. The deep- est point of the PE deformity was between the sternal angle and The HI was calculated by dividing the maximum transverse intra- the xiphoid process; therefore, the preoperative Cobb angles thoracic diameter by the minimum anterior–posterior diameter were calculated between the T2 and the T8 thoracic vertebrae, on the same slice (Fig. 2)[13]. The following HI values were used which were visualized on the plain chest X-rays, and the meas- to define the severity of the PE: <2.0 normal, 2.0–3.2 mild PE, urement was repeated on the plain chest X-rays taken 10 days 3.2–3.5 moderate PE and >_3.5 severe PE [14]. The AI was calcu- after the removal of the intrathoracic bar. The presence of scoli- lated by dividing the largest anteroposterior diameter of the left osis between the T2 and the T8 vertebrae corresponding to the hemithorax by the largest anteroposterior diameter of the right lateral curvature of the spine, which was defined as being more hemithorax and subtracting the result from 1. The AI ranges be- than 10 in the coronal plane, was recorded. The difference in tween 0.05 and +0.05 in healthy individuals. An AI below or the Cobb angle before the Nuss procedure (precorrection) and above this range indicates asymmetry of the thorax (Fig. 3)[15]. on the 10th day after removal of the bar (post-correction) was 378 M. Is¸can_ et al. / European Journal of Cardio-Thoracic Surgery

Table 1: Summary of preoperative patient characteristics

Patient characteristics Variables

Age (years) 19.6 ± 6.7 Child (<18) 45 (45.0) Adult (>_18) 55 (55.0) Gender Male 88 (88.0) Female 12 (12.0) Height (cm) 172 (130–190) Weight (kg) 58 (23–89) Body mass index Underweight 15 (15.0)

Normal 83 (83.0) Downloaded from https://academic.oup.com/ejcts/article/59/2/375/5943430 by guest on 29 September 2021 Overweight 2 (2.0) Comorbid diseases 42 (42.0) Figure 4: Sternal torsion angle = a. Family history 3 (3.0) Cardiac features 31 (31.0) Mitral valve prolapse 10 (10.0) the primary outcome of the present study. Body mass index Mitral/aortic insufficiency 10 (10.0) (BMI) was calculated as body weight in kilograms divided by Tricuspid insufficiency 8 (8.0) squared height in metres. Patients were divided into 6 groups on Pulmonary hypertension 3 (3.0) 2 2 Asymmetry index the basis of their BMI (<18.5 kg/m underweight; 18.5–24.9 kg/m Normal 38 (38.0) 2 normal; 25–29.9 kg/m overweight; 30–34.9 class I obesity; 35– Mildly asymmetrical 0 39.9 class II obesity; >_40 class III obesity) according to the World Severely asymmetrical <-0.05 or >0.05 62 (62.0) Health Organization. Sternal torsion angle Symmetrical (<5) 25 (25.0) Mild torsion (5–14.99) 9 (9.0) Statistical analyses Moderate torsion (15–24.99) 24 (24.0) Severe torsion (>_25) 42 (42.0) Haller index All analyses were performed on SPSS v21 (SPSS Inc., Chicago, IL, Normal (<2) 0 USA). The Shapiro–Wilk test was used for the normality check. Mild (2–3.19) 19 (19.0) Data are given as mean ± the standard deviation median (min- Moderate (3.2–3.49) 28 (28.0) Severe (>_3.5) 53 (53.0) imum–maximum) for continuous variables regarding normality Bar length (inches) 11 (8–14) of distribution for quantitative variables and frequency (percent- Bar count age) for categorical variables. Normally distributed variables 1 93 (93.0) (BMI) were analysed with the two-way repeated measures ana- 2 7 (7.0) Bar removal time (months) 33 (20–60) lysis of variance. Non-normally distributed variables (Cobb angle) were analysed with the Wilcoxon signed ranks test for repeated Data are given as mean ± standard deviation or median (minimum–max- imum) for continuous variables regarding normality and as frequency (per- measurements. The groups were compared concerning these centage) for categorical variables. variables by analysing differences between the measurements with the Mann–Whitney U-test or the Kruskal–Wallis test, de- pending on the number of groups compared. Categorical varia- Cobb angle from the initial measurement to the measurement bles were evaluated using the generalized estimating equations performed after the removal of the bar (precorrection to post- for between-group comparisons, whereas the McNemar’s test correction) was compared in several subgroups (Table 2). The was utilized for repeated measurements. Statistical significance change in the Cobb angle was significant in male subjects, in sub- was defined as a P-value <0.05. jects with mild and moderate STA and in patients with severe PE according to the preoperative HI. Table 3 compares the fre- quency of underweight subjects in several subgroups before and RESULTS after correction. The Nuss procedure led to a significant decrease in the frequency of underweight subjects in those with a normal A total of 100 patients (mean age 19.6 ± 6.7 years, 88% male) who AI and in those with symmetrical STA. However, there were no had undergone the Nuss procedure and bar removal following a significant changes in scoliosis frequency from the preoperative median treatment time of 33 (20–60) months were enrolled in to the postoperative period (Table 4). Postoperative complica- this retrospective study. Cardiovascular involvement was tions were 4 wound complications (4%), 2 minimal pneumo- detected in 31% of the subjects. The Nuss procedure was per- thoraces (2%) and 1 reactive pleural effusion (%1). formed by placing a single bar in 93 patients (93%) and 2 bars in the remaining 7 cases. Demographic characteristics, cardiac in- volvement and radiological measurements of the study popula- DISCUSSION tion are presented in Table 1. A significant increase occurred in the median Cobb angle The goal of the present study was to investigate the impact of the measurements from the preoperative period to the removal of Nuss procedure on the spinal curvature. We hypothesized that the bars (4.3 ± 7.0 vs 5.0 ± 7.0; P = 0.010). The change in the the Nuss procedure would cause considerable stress on the spine M. Is¸can_ et al. / European Journal of Cardio-Thoracic Surgery 379

Table 2: Changes in Cobb angles in patients before and after Table 3: Frequency of underweight (body mass index <18.5) intervention patients before and after ıntervention

Cobb angle Underweight P-value Precorrection Post- P-value Precorrection Post- (within correction (within correction groups) groups) Total 15 (15.0) 9 (9.0) 0.180 Total () 4.3 ± 7.0 5.0 ± 7.0 0.010 Age (years) Age (years) Child 5 (11.1) 4 (8.9) 1.000 Child 3.7 ± 3.5 4.7 ± 4.3 0.046 Adult 10 (18.2) 5 (9.1) 0.180 Adult 4.8 ± 8.9 5.2 ± 8.6 0.113 P-value (between groups) 0.452 P-value (between groups) 0.227 Gender Gender Male 12 (13.6) 7(8.0) 0.267 Downloaded from https://academic.oup.com/ejcts/article/59/2/375/5943430 by guest on 29 September 2021 Male 4.5 ± 7.3 5.2 ± 7.3 0.011 Female 3 (25.0) 2 (16.7) 1.000 Female 2.6 ± 3.1 3.7 ± 3.3 0.538 P-value (between groups) 0.889 P-value (between groups) 0.546 Asymmetry index Asymmetry index Normal 10 (26.3) 3 (7.9) 0.016 Normal 3.1 ± 3.6 3.9 ± 4.2 0.065 Severely asymmetrical 5 (8.1) 6 (9.7) 1.000 Severe 5.1 ± 8.3 5.7 ± 8.2 0.070 P-value (between groups) 0.030 P-value (between groups) 0.590 Sternal torsion angle Sternal torsion angle Symmetrical 8 (32.0) 1 (4.0) 0.016 Symmetrical 2.8 ± 3.2 3.5 ± 3.8 0.067 Mild and moderate 6 (18.2) 6 (18.2) 1.000 Mild and moderate 3.6 ± 4.5 4.3 ± 5.0 0.022 Severe 1 (2.4) 2 (4.8) 1.000 Severe 5.8 ± 9.6 6.4 ± 9.2 0.317 P-value (between groups) 0.032 P-value (between groups) 0.865 Haller index Haller index Mild 3 (15.8) 3 (15.8) 1.000 Mild 3.3 ± 3.5 3.8 ± 3.9 0.282 Moderate 2 (7.1) 2 (7.1) 1.000 Moderate 3.8 ± 4.1 4.4 ± 4.2 0.182 Severe 10 (18.9) 4 (7.6) 0.109 Severe 4.9 ± 8.9 5.7 ± 8.8 0.049 P-value (between groups) 0.344 P-value (between groups) 0.916 Data are given as frequency (percentage). Data are given as mean ± standard deviation.

. Recently, several reports were published concerning THORACIC and change the anatomical characteristics of the thorax. The pre- the impact of treatment with minimally invasive procedures for sent study shows that the Cobb angle, as a measure of the sever- thoracic scoliosis in patient with PE. The study conducted by ity of thoracic scoliosis, significantly increases following the Nuss Ghionzoli et al. [24] included a cohort of 67 patients with PE, in procedure, particularly in male subjects, in patients with mild which 34 subjects had concurrent adolescent idiopathic scoliosis and moderate STA and in those with a high preoperative HI with a Cobb angle greater than 10. The authors reported that value. In other words, treatment of PE with the Nuss procedure 95% of the subjects undergoing minimally invasive procedures may lead to an increase in the Cobb angle. experienced a significant regression of the degree of scoliosis. The Nuss procedure, which was developed by Nuss et al., However, patients with severe thoracic scoliosis were not replaced the traditional Ravitch technique in the management of recruited in this study, and the majority of the subjects had con- patients with PE, mainly due to the minimally invasive nature of current scoliosis with PE and the initial degree of the scoliosis the Nuss procedure [18], which could be performed using thora- was mild. In addition, 69% of the subjects in this cohort were coscopy and a stabilizer for the fixation of the bar [19]. However, younger than 18 years of age. Park et al. [25] also recently investi- the procedure is not without complications: Pneumothorax is the gated the influence of the Nuss procedure on thoracic scoliosis most common early complication, whereas late complications in- in 468 patients with PE. They reported no difference between clude bar displacement and over-correction [20]. Despite these pre-corrective and post-corrective Cobb angle values. However, problems, early experience with this procedure provided promis- subgroup analysis demonstrated that the Cobb angle had ing results, such as fast recovery, relief of symptoms, reduced car- increased in those who had undergone late correction, whereas, diac compression and better safety [21]. interestingly, a significant decrease was observed with early cor- In the follow-up of all patients, there were no complications rection with the Nuss procedure. Female gender was also associ- other than wound infection, which regressed with antibiotic ther- ated with an increase in the post-corrective Cobb angle apy in 4 patients, with drainage of a reactive pleural effusion in 1 measurements. Also, the severity of PE was a significant predictor patient and oxygen therapy for a minimal pneumothorax in 2 of precorrection scoliosis. However, PE severity was not a pre- patients. We found a significantly lower rate of complications dictor of post-corrective scoliosis. compared to that reported in the literature. In the present study, we found that the post-corrective mean However, previous reports of acquired thoracic scoliosis in 3 Cobb angle was significantly higher than the mean initial angle. It cases following minimally invasive repair of severe PE deformity is plausible to suggest that the Cobb angle increased after repair; led to the consideration that the Nuss procedure might trigger however, the classification of scoliosis did not change after the the development of scoliosis, particularly in patients with severe operation. In other words, minimally invasive surgery did not PE [22, 23]. Authors reported that thoracic scoliosis had improved lead to clinically worse scoliosis other than the change in the in these cases with physical therapy and range-of-motion angle. 380 M. Is¸can_ et al. / European Journal of Cardio-Thoracic Surgery

pectus deformity using the bar applies sheer forces to the ster- Table 4: Frequency of thoracic scoliosis in patients before num. Shear forces are unaligned forces that cause a part of the and after intervention skeleton to deviate in 1 direction and another part of the skel- eton to deviate in the opposite direction. It is physically conceiv- Thoracic scoliosis able that those forces caused by bar compression could have rotated the spine. Therefore, mechanistic studies are needed to Precorrection Post- P-value correction (within explain the modus operandi of these forces. groups) The increase in the post-corrective Cobb angle in subjects with high preoperative HI compared to those with mild or moderate Total 6 (6.0) 11 (11.0) 0.063 Age (years) HI values supports this suggestion. Considering that the majority Child 1 (2.2) 4 (8.9) 0.250 of patients with post-corrective scoliosis may benefit from early Adult 5 (9.1) 7 (12.7) 0.500 physical therapy and exercise, we believe that patients under- P-value (between groups) 0.243 going the Nuss procedure should be followed closely and moni- Gender Downloaded from https://academic.oup.com/ejcts/article/59/2/375/5943430 by guest on 29 September 2021 Male 6 (6.8) 10 (11.4) 0.125 tored strictly in order to diagnose and manage the scoliosis that Female 0 1 (8.3) NA (1) may develop following this procedure. P-value (between groups) NA (1) Asymmetry index Normal 1 (2.6) 2 (5.3) 1.000 Limitations Severely asymmetrical 5 (8.1) 9 (14.5) 0.125 P-value (between groups) 0.940 Some limitations concerning our findings must be mentioned. Sternal torsion angle Firstly, this study is a retrospective analysis of the available data Symmetrical 1 (4.0) 1 (4.0) 1.000 Mild and moderate 1 (3.0) 3 (9.1) 0.500 and thus is vulnerable to selection bias. The exact timing of imag- Severe 4 (9.5) 7 (16.7) 0.250 ing measurements in the respiratory cycle was not available be- P-value (between groups) 0.081 cause respiratory monitoring was not performed during image Haller index a acquisition. This fact might have influenced the accuracy of the Mild 0 (0.00) 1 (5.3) NA measurements. However, standard procedures were followed in Moderate 2 (7.1) 3 (10.7) 1.000 Severe 4 (7.6) 7 (13.2) 0.250 all imaging studies, which would limit variations in this context. P-value (between groups) 0.753b Nevertheless, these results need to be interpreted with caution. Data given as frequency (percentage). aCannot be calculated because 1 row or column has observed count <1. bOnly calculated for the groups classified as moderate and severe. CONCLUSIONS NA: not applicable. The present study shows that the Cobb angle, as a measure of the severity of thoracic scoliosis, significantly increases following In contrast to the findings of the previous studies, male gender the Nuss procedure, particularly in those with a high preopera- rather than female gender was associated with a post-corrective tive HI value. Consequently, patients undergoing the Nuss pro- increase in the Cobb angle. Significant increases in the post- cedure for the correction of PE should be strictly monitored for corrective Cobb angle were also noted in patients with mild and timely diagnosis and . moderate STA values and in subjects with severe preoperative HI. However, there were no significant changes in the degree of Conflict of interest: none declared. scoliosis from the preoperative to the postoperative period. The types of scolioses were idiopathic and acquired. However, we Author contributions were not able to document the reasons for acquired scoliosis. Our results suggested that there could be a reciprocal causative Mehlika Is¸can_ : Conceptualization; Data curation; Formal analysis; Methodology; relationship between scoliosis and PE. However, the modus oper- Writing—original draft. Burcu Kılıc¸: Conceptualization. Akif Turna: Supervision; andi of this possible relationship is yet to be identified. We rec- Writing—review & editing. Mehmet Kamil Kaynak: Conceptualization; Project ommended physiotherapy to the patients who had clinically administration; Supervision; Writing—review & editing. evident scoliosis after surgery. The patients did not have any clin- ical symptoms or pain. Reviewer information The post-corrective increase in the Cobb angle following the Nuss procedure may be explained by the fact that the placement European Journal of Cardio-Thoracic Surgery thanks Yusuf Bayrak, Morris of the bar during the Nuss procedure leads to a significant Beshay, Abdullah Erdogan, Jozsef Furak and the other, anonymous reviewer(s) change in thoracic anatomy. Therefore, the integrity and continu- for their contribution to the peer review process of this article. ity between the anterior and the posterior thorax [26] are ad- versely affected. We consider that the Nuss procedure corrects REFERENCES the depression of the anterior thorax while leading to deterior- ation in the curvature of the spine [27]. Because the Cobb angle [1] Fokin AA, Steuerwald NM, Ahrens WA, Allen KE. Anatomical, histologic, is a metric of the curvature of spine, our results showed an in- and genetic characteristics of congenital chest wall deformities. Semin crease in the Cobb angle. 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