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Background  Previous studies have reported that rod composition and diameter, as well as the correction technique are key factors associated with thoracic kyphosis (TK) restoration. However, few study has analyzed the correlation between screw density and TK restoration in hypokyphotic adolescent idiopathic scoliosis (AIS).    Methods  Fifty-seven thoracic AIS patients with preoperative TK < 10° treated with all pedicle screw fixation with a minimum 2-year follow-up were recruited. Preoperative and postoperative radiographic measurements, and information of posterior instrumentation were reviewed. Pearson and Spearman correlation coefficient analysis were used to assess relationships between change in TK and number of variables. Then, the included patients were classified into two groups (Group 1: postoperative TK ≥ 20°; Group 2: postoperative TK < 20°) to evaluate the influence factors of TK restoration.    Results  The average preoperative TK was 4.75°, which was significantly restored to 17.30° ( P  < 0.001). Significant correlations were found between change in TK and flexibility of major thoracic curve ( r  = 0.357,  P  = 0.006), preoperative TK ( r  = −0.408,  P  = 0.002), and screw density of concave side ( r  = 0.306,  P  = 0.021), respectively. In the subgroup comparison, 17 patients (29.8%) maintain the postoperative TK ≥ 20°, increased flexibility of major thoracic curve ( P  < 0.001), screw number of concave side ( P  = 0. 029), and cobalt chromium rods ( P  = 0.041) were found in the group of postoperative TK ≥ 20°.    Conclusions  TK restoration remains a challenge for AIS patients with hypokyphosis, especially for the poor flexibility ones. Except for thicker and cobalt chromium rods, screw density of concave side might be another positive predictor of restoring normal kyphosis, which provides a stronger corrective force on the sagittal plane with more pedicle screws.

Influence of screw density on thoracic kyphosis restoration in hypokyphotic adolescent idiopathic scoliosis