Are more screws better? A systematic review of anchor density and curve correction in adolescent idiopathic scoliosis

Study Design: Systematic review of clinical studies. Objectives: To critically evaluate existing literature to determine whether increased anchor or implant density (screws, wires, and hooks per level fused) results in improved curve correction for adolescent idiopathic scoliosis (AIS) surgery. Summary of Background Data: Wide variability exists in the number of screws used for AIS surgery. High numbers of pedicle screws are increasingly used, but there is limited evidence to support this as best practice. Methods: Online English-language databases were searched to identify articles addressing anchor density. Articles were reviewed for anchor type/number, radiographic measures, and patient-reported outcomes. Results: Of 196 references identified, 10 studies totaling 929 patients met the inclusion criteria. Reported mean anchor density varied from 1.06 to 2.0 implants per level fused. Mean percent coronal curve correction varied from 64% to 70%. Two studies (463 patients) analyzed hook, hybrid (combined hooks and screws), and screw constructs as a single cohort. Both found increased correction with high-density constructs (p =.01, p <.001), perhaps as a result of the hooks and hybrid constructs. Eight retrospective studies and 1 prospective randomized, controlled trial had predominantly screw constructs (466 patients). Increased anchor density was not associated with improved curve correction. The studies evaluating screw density are significantly underpowered to detect a difference in curve correction. Conclusions: Wide heterogeneity in anchor density exists in the surgical treatment of AIS. Reports evaluating the effects of increased anchor density are mostly retrospective and significantly underpowered to detect a difference in curve correction and patient outcomes. At this time, there is insufficient evidence to show that anchor density affects clinical outcomes in AIS.