Restoration of thoracic kyphosis by posterior instrumentation in adolescent idiopathic scoliosis - Comparative radiographic analysis of two methods of reduction

Study Design. A retrospective comparison of radiographic results for 2 consecutive series of patients treated for adolescent idiopathic scoliosis (AIS) by posterior instrumentations with thoracic screws using 2 methods of reduction: sequential approximation by cantilever reduction (CR) and simultaneous translation technique on 2 rods (ST2R). Objective. To compare correction of thoracic hypokyphosis and coronal radiographic results between the 2 methods of reduction. Summary of Background Data. Publications concerning AIS confirm the moderate correction of thoracic hypokyphosis by posterior instrumentation with hooks and also with pedicle screws. Methods. Forty-four patients with AIS (Lenke type 1, 2, 3) underwent a posterior spinal fusion and instrumentation (CR series: 21 patients - ST2R series: 23 patients). Three groups of preoperative kyphosis were generated: 12 patients with severe hypokyphosis (<= 10 degrees) (5 in CR series and 7 in ST2R series); 12 patients with mild hypokyphosis (10-20 degrees) (5 and 7 patients, respectively) and 20 with normal kyphosis (>20 degrees) (11 and 9 patients, respectively). Thoracic kyphosis (T4-T12) and Cobb angle measurements of major and minor curves were evaluated by an independent observer. The minimum follow-up was 2 years. Results. At final follow-up, regarding patients with a severe preoperative hypokyphosis, the mean gain was 14 degrees in the CR series (8 degrees preoperative-22 degrees postoperative) and 27 degrees in the ST2R series (3-30 degrees) (P = 0.018). Concerning patients with mild hypokyphosis, the mean gains were, respectively, 8 degrees (17-25 degrees) and 18 degrees (16-34 degrees) (P = 0.052). After surgery, 3 patients of CR series had hypokyphosis whereas the patients of the ST2R series all had normal kyphosis. In coronal plane, the mean correction of scoliosis was similar for both groups (75% vs. 69%; P = 0.177). Conclusion. Simultaneous translation on 2 rods provides a better correction of thoracic kyphosis than the sequential approximation by CR on patients with preoperative hypokyphosis. This surgical technique restores normal thoracic kyphosis in all cases.