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Vertebral derotation in adolescent idiopathic scoliosis causes hypokyphosis of the thoracic spine
© Watanabe et al. licensee BioMed Central Ltd. 2012
Received: 25 December 2011
Accepted: 12 June 2012
Published: 12 June 2012
The purpose of this study was to test the hypothesis that direct vertebral derotation by pedicle screws (PS) causes hypokyphosis of the thoracic spine in adolescent idiopathic scoliosis (AIS) patients, using computer simulation.
Twenty AIS patients with Lenke type 1 or 2 who underwent posterior correction surgeries using PS were included in this study. Simulated corrections of each patient’s scoliosis, as determined by the preoperative CT scan data, were performed on segmented 3D models of the whole spine. Two types of simulated extreme correction were performed: 1) complete coronal correction only (C method) and 2) complete coronal correction with complete derotation of vertebral bodies (C + D method). The kyphosis angle (T5-T12) and vertebral rotation angle at the apex were measured before and after the simulated corrections.
The mean kyphosis angle after the C + D method was significantly smaller than that after the C method (2.7 ± 10.0° vs. 15.0 ± 7.1°, p < 0.01). The mean preoperative apical rotation angle of 15.2 ± 5.5° was completely corrected after the C + D method (0°) and was unchanged after the C method (17.6 ± 4.2°).
In the 3D simulation study, kyphosis was reduced after complete correction of the coronal and rotational deformity, but it was maintained after the coronal-only correction. These results proved the hypothesis that the vertebral derotation obtained by PS causes hypokyphosis of the thoracic spine.
Posterior correction and fusion surgery with a segmental pedicle screw (PS) construct has been widely utilized for the surgical treatment of patients with adolescent idiopathic scoliosis (AIS), because it allows for better curve correction in the coronal plane [1–4] and the axial planes with direct vertebral rotation (DVR), compared with the use of hook or hybrid constructs [5, 6]. However, a postoperative decrease in thoracic kyphosis has been reported in association with posterior correction surgery using a PS construct [7, 8]. While slight increases in thoracic kyphosis have been obtained using a hook construct [9–14], PS constructs are reported to decrease the thoracic kyphosis by 3°-14° after surgery [2, 3, 13, 15, 16]. However, previous studies have also shown that PS constructs produce a significantly better vertebral derotation effect than hook-and-rod constructs [5, 6, 17]. The mean derotation angle obtained with a PS construct is reported to be 7.1-13.0° [5, 18, 19], while it is 0.4°-3.6° [20–26] with a hook-and-rod construct and 1.9°- 4.2° with the sublaminar wiring technique [27, 28]. We therefore hypothesized that the decrease in thoracic kyphosis after posterior correction surgery using PS constructs was associated with the correction of vertebral rotation. The purpose of this study was to test this hypothesis by analyzing computer-simulated corrections, using three-dimensional (3D) scoliosis models based on CT data from patients with AIS.
Twenty consecutive AIS patients (all female) who underwent posterior correction and fusion surgeries with a segmental PS construct between November 2008 and October 2009 were included in this study. All the patients had a major thoracic curve (Lenke type 1: 15 patients; type 2: 5 patients). The mean age at the time of surgery was 15.9 ± 3.2 years (range, 12–23 years). The mean preoperative Cobb angle of the main thoracic curve was 58 ± 13° (range, 41-81°), and the mean preoperative thoracic kyphosis (T5-12) was 18.9 ± 7.5° (range, 4.1-28.8°) on standing radiographs. The 3D computer simulation was conducted for all the patients.
To evaluate the relationship between thoracic kyphosis and vertebral derotation, the thoracic kyphosis angle (T5-12), the radius of the thoracic curvature, and the vertebral rotation angle at the apex were measured before and after the two different simulated corrections for each patient. The thoracic kyphosis angle and radius of thoracic curvature were measured on the mid-sagittal plane. The radius of curvature at a given point is the radius of a circle that mathematically best fits the spinal curve at that point. The radius of curvature was measured at each adjacent segment from T1-T12, and then a value for the whole thoracic spine was determined as the mean of the values for each segment. The vertebral rotation angle at the apex was measured using Aaro’s method  against a reference point set at the pelvis. The clinical relevance of these simulated corrections was evaluated by comparing the values obtained from the simulations with those measured on the postoperative CT taken for each patient.
Surgical procedures included the segmental placement of the PS, placement of the first rod on the concave side of the curve, rod rotation maneuver for the sagittal and coronal corrections, in-situ contouring for coronal correction, direct vertebral derotation for axial correction, and placement of the second rod, as described previously [18, 30, 31].
This study was approved by the medical ethics committee of Keio University Hospital (2009-203-2).
Segmented 3D scoliosis models have been used in previous studies to reproduce actual scoliotic spine deformities and correction maneuvers, including Cotrel-Dubousset surgical maneuvers, in situ contouring, and correction with a segmental PS construct using a personalized finite element model of AIS [34–37]. The objective of these simulations was to predict the corrected spinal geometry and to assist the pre-operative planning of the surgical instrumentation to be used. However, the segmented 3D scoliosis model created in the present study eliminates the constraining effects of soft tissues, to simulate the maximum corrections in the coronal plane independent of corrections in the axial plane, which cannot be accomplished in the actual surgery. Therefore, we also evaluated the clinical relevance of this simulation model. The mean rotation angle at the apex measured on patients’ post-surgical CT images was between the mean values obtained by the simulated C correction and the simulated C + D correction (Figure 6). The mean thoracic kyphosis angle and radius of curvature were also between the mean values of the two simulated corrections (Figures 4 and 5). These results suggest that the real surgery can achieve a limited correction in the axial plane that falls between the two extremes of the simulated corrections.
In conclusion, the present 3D simulation study demonstrated that the derotation of vertebrae caused a decrease in thoracic kyphosis during the correction of thoracic scoliosis.
Written informed consent was obtained from the parents of the patient for publication of this case report and any accompanying images. A copy of the written consent is available for review from the Editor-in-Chief of this journal.
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