Measurement of vertebral rotation in adolescent idiopathic scoliosis with low-dose CT in prone position - method description and reliability analysis

BACKGROUND To our knowledge there is no report in the literature on measurements of vertebral rotation with low-dose computed tomography (CT) in prone position. AIMS To describe and test the reliability of this new method, compare it with other methods in use and evaluate the influence of body position on the degree of vertebral rotation measured by different radiological methods. STUDY DESIGN Retrospective study. METHODS 25 consecutive patients with adolescent idiopathic scoliosis scheduled for surgery (17 girls, 8 boys) aged 15 +/- 2 years (mean +/- SD) were included in the analysis of this study. The degree of the vertebral rotation was in all patients measured according to the method of Perdriolle on standing plain radiographs and on supine CT scanogram, and according to the method of Aaro and Dahlborn on axial CT images in prone position and on magnetic resonance imaging (MRI) in supine position. The measurements were done by one neuroradiologist at two different occasions. Bland and Altman statistical approach was used in the reliability assessment. RESULTS The reliability of measuring vertebral rotation by axial CT images in prone position was almost perfect with an intraclass correlation coefficient of 0.95, a random error of the intraobserver differences of 2.3 degrees , a repeatability coefficient of 3.2 degrees and a coefficient of variation of 18.4%. Corresponding values for measurements on CT scanogram were 0.83, 5.1 degrees , 7.2 degrees , and 32.8%, respectively, indicating lower reliability of the latter modality and method. The degree of vertebral rotation measured on standing plain radiographs, prone CT scanogram, axial images on CT in prone position and on MRI in supine position were 25.7 +/- 9.8 degrees , 21.9 +/- 8.3 degrees , 17.4 +/- 7.1 degrees , and 16.1 +/- 6.5 degrees , respectively. The vertebral rotation measured on axial CT images in prone position was in average 7.5% larger than that measured on axial MRI in supine position. CONCLUSIONS This study has shown that measurements of vertebral rotation in prone position were more reliable on axial CT images than on CT scanogram. The measurement of vertebral rotation on CT (corrected to the pelvic tilt) in prone position imposes lower impact of the recumbent position on the vertebral rotation than did MRI in supine position. However, the magnitude of differences is of doubtful clinical significance.