Main thoracic curve adolescent idiopathic scoliosis: association of higher rod stiffness and concave-side pedicle screw density with improvement in sagittal thoracic kyphosis restoration

The aim of this study was to evaluate the effects of rod stiffness and implant density on coronal and sagittal plane correction in patients with main thoracic curve adolescent idiopathic scoliosis (AIS).The authors conducted a retrospective study of 77 consecutive cases involving 56 female and 21 male patients with Lenke Type 1 main thoracic curve AIS who underwent single-stage posterior correction and instrumented spinal fusion with pedicle screw fixation between July 2009 and July 2012. The patients' mean age at surgery was 15.79 ± 3.21 years. All patients had at least 1 year of follow-up. Radiological parameters in the coronal and sagittal planes, including Cobb angle of the major curve, side-bending Cobb angle of the major curve, thoracic kyphosis (TK), correction rates, and screw density, were measured and analyzed. Screw densities (calculated as number of screws per fusion segment × 2) of < 0.60 and ≥ 0.60 were defined as low and high density, respectively. Titanium rods of 5.5 mm and 6.35 mm diameter were defined as low and high stiffness, respectively. Patients were divided into 4 groups based on the type of rod and density of screw placement that had been used: Group A, low-stiffness rod with low density of screw placement; Group B, low-stiffness rod with high density of screw placement; Group C, high-stiffness rod with low density of screw placement; Group D, high-stiffness rod with high density of screw placement.The mean coronal correction rate of the major curve, for all 77 patients, was (81.45% ± 7.51%), and no significant difference was found among the 4 groups (p > 0.05). Regarding sagittal plane correction, Group A showed a significant decrease in TK after surgery (p < 0.05), while Group D showed a significant increase (p < 0.05); Group B and C showed no significant postoperative changes in TK (p > 0.05). The TK restoration rate was highest in Group D and lowest in Group A (A, -39.32% ± 7.65%; B, -0.37% ± 8.25%; C, -4.04% ± 6.77%; D, 37.59% ± 8.53%). Screw density on the concave side was significantly higher than that on the convex side in all the groups (p < 0.05).For flexible main thoracic curve AIS, both rods with high stiffness and those with low stiffness combined with high or low screw density could provide effective correction in the coronal plane; rods with high stiffness along with high screw density on the concave side could provide better outcome with respect to sagittal TK restoration.