Introduction
Adolescent idiopathic scoliosis (AIS) is a complex three-dimensional spinal deformity which occurs in adolescence with unknown etiology [
1]. The overall incidence of AIS can reach 2–3% in adolescents, while the incidence in females is 1.5–3 times higher than that in males [
2,
3]. About two-thirds of patients will experience continued progression of spinal deformity during rapid growth period, which not only affects appearance perception and causes psychological disorders, but may also leads to spinal degeneration and even cardiopulmonary dysfunction [
3]. Therefore, a standard treatment strategy should be performed to protect mental and physical health of patients with AIS.
With its satisfactory therapeutic effects, bracing stands as one of the commonly used interventions for conservative treatments of AIS [
4]. Numerous studies have demonstrated that bracing can arrest the progression of spinal curvature and correct coronal curve by external force [
4,
5]. However, very limited studies focus on vertebral derotation effects of bracing [
6,
7]. Vertebral rotation leads to cosmetic appearance abnormalities including razorback and uneven waist, which inevitably impairs patients’ appearance perception. Furthermore, insufficient derotation effects also increase failure rate of brace treatment [
8,
9]. Thus, more attention should be paid on the effects of bracing on vertebral derotation.
The EOS imaging system has been widely used to evaluate spinal morphology recently [
10,
11]. It allows three-dimensional modeling of spine in a standing position with a low-does X-ray system. The application of EOS imaging system enables quantitative evaluation of vertebral rotation in the axial plane in a standing position [
11‐
13]. Thus, the current study was performed to investigate the effects of bracing on apical vertebral derotation and explore the factors that influence in-brace derotation effects in AIS. The hypotheses of this study were that brace treatment could contribute to apical vertebral derotation, and there existed several meaningful parameters to predict the derotaion effects of bracing.
Discussion
The current study investigated the effects of bracing on apical vertebral derotation by applying EOS imaging system. Factors which influenced in-brace derotation effects were also explored. A satisfactory apical vertebral derotation rate (approximately 50%) could be obtained immediately after bracing. It was identified that pre-brace Cobb angle of curve, pre-brace apical vertebral rotation, apical vertebral level and coronal balance were significantly correlated with in-brace derotation effects of apical vertebra.
AIS is a complex three-dimensional spinal deformity characterized by a Cobb angle greater than 10° [
15]. Vertebral rotation causes cosmetic appearance abnormalities including thoracic cage asymmetry and uneven waist. In addition, it serves as an indicator for curve progression, and insufficient derotation effect also contributes to failure of brace treatment [
8,
9]. Thus, it is of great importance to investigate the effects of bracing on axial plane, as well as the factors that influence in-brace derotation effects. However, correction of vertebral rotation in the axial plane is not always the primary focus of brace treatment. To the best of our knowledge, only Baymurat et al. investigated the effects of bracing on vertebral rotation [
6]. Using Nash and Moe classification, they found that Boston brace could significantly reduce apical vertebral rotation in AIS patients [
6]. However, the evaluation of vertebral rotation by Nash and Moe was not a quantitative measurement method.
Computed tomography (CT) images were typically used to quantitatively analyze the vertebral rotation [
11]. However, obtaining images in the standing position is not feasible with CT since it is performed in the supine position, leading to a significant underestimation of vertebral rotation. In addition, the determination of vertebral rotation angle using the reference of the radiographic sagittal plane ignores the pelvic rotational position [
16]. The high radiation exposure also impeded the wide application of CT for vertebral rotation evaluation. Thus, other techniques are required to precisely evaluate the vertebral rotation in the axial plane in a standing position. The introduction of EOS imaging system to clinic makes quantitative measurement of vertebral rotation more feasible [
11‐
13]. EOS is a relative new technique which can simultaneously acquire biplanar anteroposterior and lateral images in a standing position, with radiation exposure only one-tenth of the conventional radiography [
17]. Moreover, the high measurement reliability of rotational deformity by EOS has been demonstrated by several investigations [
11], which is comparable to CT measurement [
18]. Thus, the application of EOS imaging system enhances the credibility of the conclusions drawn from the current study.
The current study found that approximately 50% vertebral derotation rate could be achieved immediately after bracing. Mechanistically, brace corrects spinal deformities by distracting concave side and compressing convex side, thereby reinstating the normal alignment of the spine in coronal planes [
19]. The effectiveness of brace in correcting coronal deformity of AIS has been well demonstrated [
4,
6]. As for sagittal plane, there was significant correction loss of thoracic kyphosis and lumbar lordosis after bracing in current study, which was similar with previous finding that brace treatment could flatten these two parameters [
20,
21]. Although progressive modifications of spine may occur during treatment period [
21], long-term effects of bracing on sagittal alignment for patients in current study should be further investigated. In addition, only Baymurat et al. investigated detailed derotation effects of brace treatment and they found that the apical vertebral rotation improved from 2.1 ± 0.6 before bracing to 1.1 ± 0.5 after bracing by using Nash and Moe classification evaluation [
6]. To the best of our knowledge, the current study is the first investigation to illustrate the detailed quantitative derotation rate of apical vertebra.
The current study also demonstrated that pre-brace Cobb angle of curve, pre-brace apical vertebral rotation, apical vertebral level and coronal balance were significantly correlated with in-brace derotation effects of apical vertebra. Since Cobb angle of curve and apical vertebral rotation were closely related [
22], it was reasonable to find that both pre-brace Cobb angle and apical vertebral rotation were positively correlated with derotation effects. However, it is worth noting that in-brace derotation effects were not significantly correlated with thoracic kyphosis or lumbar lordosis. Since the patients in currents study showed no abnormal sagittal alignment, we supposed that there was limited association between vertebral rotation and sagittal profile. In addition, apical vertebra of major curve located in thoracic area showed worse derotation effects in brace, which could be resulted from resistance of rib cage and related tissues attached in thoracic vertebra. This finding indicated that more treatment strategies should be focused on correcting thoracic apical vertebra other than brace treatment. Furthermore, patients with coronal balance shower better in-brace derotation effects. Given that AIS is a complex three-dimensional deformity of spine, decompensated scoliosis may also influence axial plane and increase difficulty of vertebra derotation.
The present study was not without limitations. Firstly, this was a retrospective study which would be affected by some inherent biases. Secondly, we only analyzed the data before bracing and immediately after bracing. Since the in-brace torsional correction may be lost with time, the conclusion in current study should be applied exclusively to this time interval. Thirdly, EOS system is not routinely used in clinical setting in the majority of hospitals, which may limit the generalizability of current study’s finding in daily practice decision-making. In addition, the current study failed to include curve flexibility when considering additional radiation exposure. Curve flexibility is closely associated with Cobb angle correction, so it may also be a crucial factor in determining in-brace vertebral derotation effects. Thus, further study should be performed to include information of curve flexibility and investigate the results of long-term follow-up of in-brace derotation effects.
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