Recent decades have witnessed significant advances in the treatment of scoliosis. Critical to these advancements has been the progress made in determination of radiological parameters [9]. Scoliosis is a complex 3D anomaly of the spine including deviations in the frontal plane, modifications of the sagittal profile, rotations in the transverse plane, and alterations of the rib cage [10]. Owing to the introduction of digital radiography, important parts of the spine can be enlarged and seen more clearly by modifying the contrast, and the margins of the vertebrae can be enhanced by computerized options. Despite this technical support, an inaccuracy in angle measurement persists due to a disadvantage of the Cobb method itself. Inter- and intraobserver variabilities of approximately 3–5° have been published [4, 7].
Another systematic bias is that there is a high variability in end vertebra selection. We still measure a two-dimensional value of a three-dimensional structure. The end plate does not have a trajectory as a single clear line when the angle of its plane is other than perpendicular to the plane of the film. Even in the best conditions, an end plate that appears fusiform on the screen or on the film cannot be reduced to a single line [7]. Since each vertebra has a distinct rotational component, it is impossible to select a plane in X-rays in which the scoliotic region has a true anterior-posterior or posterior-anterior view of profile projection [11]. Especially, in combined deformities, like kyphoscoliosis or lordoscoliosis, the determination of the true Cobb angle is nearly impossible with conventional radiography [10] (Figs. 1 and 2). Stagnara stated that in cases of severe scoliosis the apical vertebral rotation was so appreciable that the upper vertebrae were seen only on the lateral view in the anteroposterior radiograph [12]. In some cases the kyphosis detected on the lateral radiograph is not real kyphosis, but the frontal curve of the scoliotic spine that has been displaced into the sagittal plane as a result of the appreciable vertebral rotation [12].
Thus, the radiographic image is just a simple, mere projection or a shadow of a three-dimensional structure. The widespread use of CT represents probably the single most important advance in diagnostic radiology. The continuing improvement in CT techniques and software has made 3D CT-reconstructed images a new tool for assessing 3D deformities. 3D reconstructed CT images are well-established diagnostic methods in vascular, transplantation and neuro-surgery [13–15].
Although CT radiation doses still limit their use for routine repeated scoliosis assessment, in very few cases a single preoperative CT scan for surgical planning may be justifiable. In some hospitals a single preoperative CT scan is performed in cases of patients undergoing complex scoliosis surgery (Cobb > 100°) [16]. In these cases appropriate decision-making is essential as to whether the pedicle diameters are large enough for screw placement. A preoperative CT scan allows for safer screw sizing and positioning because it helps to assess the pedicle diameter [17]. The remaining CT reconstructions measured in the present study were carried out for other medical reasons than surgical planning. Recently published data have shown that the estimated radiation dose for a 3D CT reconstruction can be reduced to 3.7 millisieverts (mSv) [18].
3D measurements can also be evaluated with the help of the EOS software imaging system [19]. This system is based on posteroanterior and lateral views of the spine. These authors also showed that routine 2D measurements of deformities underestimate the amount of Cobb angle because of the axial plane rotation. These techniques have the advantage that a software calculates a 3D reconstruction of the spine based on radiographs of the standing patient [20, 21].
We aimed to compare the intraobserver and interobserver reliability in the definition of end vertebrae, as well as in Cobb angle measurement in radiographs of scoliotic spines comparing digital radiographs and in 3D CT images. We found no significant difference in the intraobserver or interobserver reliability between conventional radiography and 3D CT images in Cobb angle measurement and determination of the end vertebrae. The intraobserver reliability for the Cobb angle found in this study was nearly the same for both methods (mean ICC radiographs, 0.978; mean ICC 3D CT images, 0.961). The variability and reliability of Cobb angle measurement in 3D CT images described in the present study are comparable to the results previous studies reported for manual or digital measurement of plain radiographs [7, 22]. We measured the Cobb angle with predefined end vertebrae because it has been shown that the reliability is higher when the error of end plate selection is limited [7].
A limitation of the present study is that Cobb angles measured from a supine CT cannot be directly compared with full-length standing radiographs due to changes in spinal geometry. Torell et al. reported an average deviation of 8.8° comparing standing and supine radiographs [23]. A decrease of the average Cobb angle from 56° to 39° regarding standing and supine position was shown by Yazici et al. [24]. Therefore, we compared the 3D CT images with supine full-length spine radiographs. Carman, using conventional radiographs, found that a difference of approximately 10° had to be present to be 95% confident that a true change in spinal geometry had occurred [3]. We found a mean difference of 9.2% between 3D CT images and radiographs (p < 0.005) (Table 2). The explanation for the difference during 3D measurements might be that the observer could rotate the 3D reconstruction of the spine 360° to measure the Cobb angle (Figs. 1 and 2). In daily clinical routine, conclusions about the severity of scoliosis and risk of progression are often drawn from the Cobb angle measurement of the major curve on the standing coronal image.
Another limitation of our study is that CT radiation doses currently preclude CT use in routine clinical practice and Cobb angle measurement in non-complex scoliosis. However, future advances in multislice CT will allow for lower dosages and faster acquisition times, thus enabling the increased use of CT for the assessment of scoliotic deformities. Currently the dosage can be reduced to 3.7 mSv [16].
Our data suggest that the Cobb angle of the scoliosis might be significantly larger than usually measured by plain X-rays. This should be especially considered in decision-making for conservative or surgical treatment in borderline scoliosis. We believe that, in comparison with conventional radiography, our 3D CT images of the spine feature a higher resolution and yield superior results regarding the “real” Cobb angle. Low inter- and intraobserver reliability and high variabilities in end vertebra definition are the reported major limitations of the Cobb angle. The difference of approximately 10° between 3D and 2D measurement found in the present study demonstrates another limitation of the Cobb angle.