Variability in Cobb Angle Measurements Using Reformatted Computed Tomography scans

Study Design: Survey of intra and inter-observer measurement variability. Objectives: To assess the use of reformatted computed tomography (CT) images for manual measurement of coronal Cobb angles in idiopathic scoliosis. Summary of Background Data: Cobb angle measurements in idiopathic scoliosis are traditionally made from standing radiographs, whereas CT scans are often used for assessment of vertebral rotation. Correlating Cobb angles from standing radiographs with vertebral rotations from supine CT is problematic since the geometry of the spine changes significantly from standing to supine positions, and two different imaging modalities are involved. Methods: We assessed the use of reformatted thoracolumbar CT images for Cobb angle measurement. Pre-operative computed tomography scans of twelve idiopathic scoliosis patients were used to generate reformatted coronal images. Five observers measured coronal Cobb angles on three occasions from each of the images. Intra and inter-observer variability associated with Cobb measurement from reformatted CT scans was assessed and compared with previous studies of measurement variability using plain radiographs. Results: For major curves, 95% confidence intervals for intra and inter-observer variability were ±4.3 degrees and ±7.5 degrees respectively. For minor curves, the intervals were ±5.1 degrees and ±7.4 degrees. Intra and inter-observer TEMs were 2.4 degrees and 2.7 degrees, with reliability coefficients of 88% and 84%. There was no correlation between measurement variability and curve severity. Conclusions: Reformatted CT images may be used for manual measurement of coronal Cobb angles in idiopathic scoliosis with similar variability to manual measurement of plain radiographs