Computer Simulation of Two-level Pedicle Subtraction Osteotomy for Severe Thoracolumbar Kyphosis in Ankylosing Spondylitis.

Background: Advanced ankylosing spondylitis is often associated with thoracolumbar kyphosis, resulting in an abnormal spinopelvic balance and pelvic morphology. Different osteotomy techniques have been used to correct AS deformities, unfortunnaly, not all AS patients can gain spinal sagittal balance and good horizontal vision after osteotomy.

Materials and Methods: Fourteen consecutive AS patients with severe thoracolumbar kyphosis who were treated with two-level PSO were studied retrospectively. All were male with a mean age of 34.9 ± 9.6 years. The followup ranged from 1-5 years. Preoperative computer simulations using the Surgimap Spinal software were performed for all patients, and the osteotomy level and angle determined from the computer simulation were used surgically. Spinal sagittal parameters were measured preoperatively, after the computer simulation, and postoperatively and included thoracic kyphosis (TK), lumbar lordosis (LL), sagittal vertical axis (SVA), pelvic incidence, pelvic tilt (PT), and sacral slope (SS). The level of correlation between the computer simulation and postoperative parameters was evaluated, and the differences between preoperative and postoperative parameters were compared. The visual analog scale (VAS) for back pain and clinical outcome was also assessed.

Results: Six cases underwent PSO at L1 and L3, five cases at L2 and T12, and three cases at L3 and T12. TK was corrected from 57.8 ± 15.2° preoperatively to 45.3 ± 7.7° postoperatively ( P < 0.05), LL from 9.3 ± 17.5° to -52.3 ± 3.9° ( P < 0.001), SVA from 154.5 ± 36.7 to 37.8 ± 8.4 mm ( P < 0.001), PT from 43.3 ± 6.1° to 18.0 ± 0.9° ( P < 0.001), and SS from 0.8 ± 7.0° to 26.5 ± 10.6° ( P < 0.001). The LL, VAS, and PT of the simulated two-level PSO were highly consistent with, or almost the same as, the postoperative parameters. The correlations between the computer simulations and postoperative parameters were significant. The VAS decreased significantly from 6.1 ± 1.9 to 2.0 ± 1.1 ( P < 0.001). In terms of clinical outcome, 10 cases were graded "excellent" and 4 cases were graded "good."

Conclusion: Two-level PSO using a preoperative computer simulation is a feasible, safe, and effective technique for the treatment of severe thoracolumbar kyphosis in AS patients with normal cervical motion.