Spinal rotation exists in structural scoliosis and is clearly observed by CT scans. Lamina , spinous process, accessory process , and vertebral body [2, 8] are used as the landmarks for rotational measurement on CT scans. However, because of the spinal body deformity it is sometimes difficult to define the anatomical structures. Ho's method, which is defined by bisecting the bilateral laminae angle, is reported to be useful. As the lamina is an anatomical landmark for Ho's method, it expresses the rotation of spinal posterior component. For measuring the spinal anterior component rotation such as the vertebral body itself, it is difficult to define the rotational angle because of the deformity [10, 13] and the lack of any anatomical points.
The apical vertebrae are most laterally deviated in the coronal plane and are usually not tilted. Horizontal vertebrae with the CT scanning of each pedicle can be chosen in all patients. In our series, both pedicles could not be seen in one CT slice in two patients. The differences of the measuring posture (standing for the AP radiograph and supine for the CT scan) are supposed to represent the discrepancy of horizontal vertebrae. Presently, we applied a new method that uses the posterior part of the vertebral body just beneath each pedicle. Compared with Ho's method, our new method was easier and more practical in daily medical practice. Furthermore, our new method was confirmed to be reliable by both intraobserver and interobserver analyses.
There are several important considerations that should be kept in mind when using our new measurement method. One is that our new method can only be applied to the vertebra where both pedicles can be seen in the CT slice. As the CT gantry is vertical to the floor, patient position critically influences the axial CT slice. In the measurement of scoliosis patients, our method can be applied only to the vertebrae around the apex. Another factor that should be kept in mind is that the rotation determined using our new method is a relative value. In this study, as we subtracted the value using two different measuring methods, we did not normalize the rotation angle. When the vertebral rotation is measured using our new method, and the values are compared for different patients, normalization should be done using any anatomical point, such as the sacral or iliac points.
The apical vertebral body adopts a complicated deformity in scoliosis. The spinal anterior component has been confirmed to be rotated more than posterior component by the observation of 3D images or by the specimen findings [9, 10]. The deformity of the apical vertebra in the transverse plane consists of a gradual torsion between the posterior complex and the vertebral body. The vertebral body is maximally rotated towards the convexity of the scoliotic curve, whereas the tip of the spinous process is pointed to the posterior. However, precisely why such a spinal torsional deformity exists and also why no quantitative measurement method has yet been established together remain unclear. Furthermore, the anterior vertebral body shape may change by the additional bone formation on the opposite side of the rotational deformity during growth [13, 15] in scoliosis. The complicated deformity makes it difficult to locate the anatomical landmarks of the anterior component. As a result, it is difficult to measure the anterior component rotation and to measure the spinal torsional deformity. We presently used the clear anatomical landmark of the posterior border of the vertebral body, which seemed to have less deformity by bony remodeling compared to more anterior vertebral body. By the combined use of our new method (expressing the spinal anterior component rotation) and Ho's method (expressing the spinal posterior component rotation), we could quantify the rotational discrepancy between the anterior and posterior, which is considered to represent the torsional deformity. The method described here is influenced by many factors, including the shape of the posterior part of the vertebral body, the pedicle length, and the lamina shape. Our method can measure the apical vertebral intrinsic torsion, but not the torsion of the vertebral body itself, nor the vertebral torsion in the global spinal system.
We clarified that the anterior component rotated more than posterior component. As the etiology of scoliosis is not known, the order at which such a deformity first occurs is not known. The existence of torsional deformity, the spinal anterior component rotated more than the posterior component, evokes the hypothesis that spinal anterior compartment deformities occur earlier than those of the posterior compartment in the worsening of scoliosis. Although not statistically significant, the present finding that torsional deformity seemed to correlate with anterior component rotation (Figure 3), but not with the posterior component (Figure 4), support the view that spinal anterior component is critical for the etiology of scoliosis deformity. According to our observations, the coronal curvature seemed to correlate with the spinal torsional deformity, but this correlation was not significant (Figure 5). To clarify the mechanisms of deformity, further investigation of the early change of such deformity in scoliosis is needed.
Right thoracic scoliosis, trunk asymmetry and thoracic vertebral right rotation are among the characteristics of adolescent idiopathic scoliosis. Even in the normal spine, trunk asymmetry [16–18], thoracic vertebral right rotation  and right thoracic curvature [19–21] have been reported. We were interested in whether torsional deformity exists in the normal spine, and we measured the vertebral rotation using both the new method and Ho's method at the T8 level. Interestingly, the discrepancy of the rotation was 2.6 ± 1.5 degrees (n = 25, adult females), which was considered to indicate the presence of spinal torsional deformity to the right side, even in the normal spine, at the T8 level (p = 0.0063). These deformities, which are the same features seen in scoliosis patients, support the possibility that the worsening of deformities existing in normal individuals may underlie the development and progression of scoliosis.
Clinically, paying attention to the difference between the spinal posterior and anterior components is important. During posterior scoliosis surgery, only the posterior component is visible. To avoid an overestimation of the rotational correction, the surgeon has to recognize the existence of such torsional deformity; in other words, understand that the anterior spinal component rotates more than the posterior component.