A New Method for Calculating the Exact Angle Required for Spinal Osteotomy

Study Design. Prospective study. Objective. To assess a new method for determining the exact angle required for spinal osteotomy in patients with ankylosing spondylitis (AS). Summary of Background Data. The ideal method for maintaining sagittal balance is to shift the center of gravity of the trunk over the hip axis when pelvic and lower extremity joints are in the neutral position. For patients with AS, various methods have been explored to calculate the required corrective angle. However, these methods carry some limitations. Methods. Twenty patients with AS who underwent spinal osteotomy for correcting kyphotic deformity were studied. Pre- and postoperative full-length freestanding radiographs of the whole spine and pelvis were obtained for all patients. We calculated the ideal postoperative pelvic tilt according to pelvic incidence, and chose the plumbline of the hilus pulmonis rather than C7 as gravity axis of the trunk. Then, the necessary angular correction at the level of osteotomy was calculated. Pre- and postoperative radiological parameters, including Cobb T1-S1, pelvic incidence, pelvic tilt, sacral slope, and sagittal vertical axis were measured. Health related quality of life, including Oswestry Disability Index and Scoliosis Research Society outcomes instrument-22, were administered before surgery and at 6-month follow-up. Results. The preoperative and postoperative Cobb T1-S1 were 52 degrees and 3 degrees, respectively (P < 0.001). All patients demonstrated changes in preoperative to postoperative radiographical parameters including decreased pelvic tilt (from 30 degrees to 11 degrees, P < 0.001) and sagittal vertical axis (from 18 cm to 7 cm), increased sacral slope (from 16 degrees to 35 degrees, P < 0.001), but no significant change in pelvic incidence. Health related quality of life scores at 6-month follow-up had significantly improved compared with those before surgery. Conclusion. This method provides an accurate and reproducible calculation for AS correction.