Supine thoracolumbar sagittal spine alignment: Comparing computerized tomography and plain radiographs

Study Design: Imaging supine sagittal alignment study. Objective: Our goal was to determine differences between plain radiographs and computed tomography (CT) scans in supine sagittal alignment and to establish supine reference Cobb angles for both. Summary of Background Data: Few studies have compared plain radiographs versus CT scans with regard to sagittal spine alignment. None have utilized supine patients. Methods: Fifty sequential blunt trauma individuals who had routine clearance of the thoracolumbar spine had their plain radiographs and CT scans reviewed independently by 2 spine surgeons. The Cobb method was utilized to determine angles at each spine level from T4 to L5. All imaging was obtained in the supine position, and no patient had acute ligamentous or bony pathology. Intraclass correlation coefficients (ICCs) were utilized to determine intraobserver, interobserver, and method reliability. Results: Every level within the thoracic region was kyphotic, with a peak at T6. The lumbar region demonstrated a fairly linear progression from a near-neutral alignment at L1 to approximately 50° of lordosis at L5. The intraobserver reliability was consistent, with ICCs for Observer 1 plain radiographs at 0.653, Observer 2 plain radiographs at 0.891, Observer 1 CT scans at 0.677, and Observer 2 CT scans at 0.648. The interobserver reliability was very high, with ICCs for plain radiographs at 0.902 and 0.895 for CT scans. Finally, method reliability (between plain radiographs and CT scans) was excellent as well, with ICCs of 0.808 for Observer 1, 0.781 for Observer 2, and 0.817 after averaging the 2 observers. Conclusion: The results from this study provide a supine reference for sagittal spine alignment using the Cobb method for both CT scans and plain radiographs. It also demonstrates the high degree of reliability between measurements from 2 imaging sources and various observers as shown with the ICC values.