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The literature is contradictory regarding the effect of metal density on correction of thoracic AIS. Given the rising cost of surgery for AIS, instrumentation should be used rationally. We aim to assess the impact of metal density on coronal and sagittal correction of AIS and assess if larger, less flexible curves necessitate high metal density.Material and Methods   It is a retrospective multicenter case series. Inclusion criteria: Lenke 1–2 AIS, curve flexibility assessed by fulcrum-bending technique, single stage posterior only surgery, constructs using &gt; 80% screws, variable metal density, and &gt; 2-year follow-up. Outcome measures: Coronal main thoracic (MT) and lumbar curve correction, fulcrum-bending correction index (FBCI), thoracic kyphosis, lumbosacral lordosis, metal density (number of instrumented pedicles vs. total available), and fusion length. Surgical technique: Standard technique used in all centers employed extra hard titanium–aluminum–niobium alloy 6 mm diameter rods (DePuy Synthes, Raynham, Pennsylvania, United States) with reduction of the curvature via cantilever-segmental translation maneuvers. Implant location and density was according to curve stiffness and intraoperative bone density. The majority of the implants were on the thoracic curve convexity. Analysis: Bivariate analysis of outcome measures. Given the numerous correlations performed, a Bonferroni corrected p value was used, p &lt; 0.005. Two groups of pairs matched by curve type and exact fusion levels but differing MT curve magnitude were compared using student t-test. Outcomes for the quartiles of preoperative MT flexibility were compared using an ANOVA.Results   A total of 106 patients, 94 were female patients. A total of 78 patients had Lenke 1 and 28 had Lenke 2. Mean age was 14 years. Bivariate analysis: No significant correlations were present between metal density and coronal curve correction rates of the MT (r = 0.13, p = 0.19) or lumbar curves (r =  ␂ 0.15, p = 0.12) or FBCI; MT (r =  ␂ 0.1, p = 0.31), lumbar (r = 0.00, p = 0.97). Metal density had no effect on change in thoracic kyphosis (r = 0.22, p = 0.04) or lumbosacral lordosis (r = 0.27 p = 0.01). Longer fusion length was associated with greater loss of thoracic kyphosis (r =  ␂ 0.31, p = 0.003) and less postoperative thoracic kyphosis (r =  ␂ 0.31, p = 0.003) but did not affect lumbosacral lordosis. Curve magnitude: Two groups of 21 matched cases from preoperative MT curve magnitude (&lt; 70 vs. ≥ 70 degrees) treated with comparable metal density (54 vs. 57%, p = 0.4) had equivalent MT and lumbar curve correction rates and FBCI. Sagittal profile changes did not differ significantly. Flexibility: No significant differences in MT, lumbar, and sagittal curve changes were present among the four MT flexibility quartiles (&lt; 32, 33–43, 43–51, 52–87%) despite all groups receiving comparable metal density (58, 58, 60, 62%, p = 0.2).Conclusion   Metal density does not affect coronal and sagittal plane correction in surgery for thoracic AIS. Longer fusion length adversely affects sagittal profile. Larger and less flexible curves do not necessarily require high metal density.

The Effect of Metal Density on Coronal and Sagittal Curve Correction in Thoracic Adolescent Idiopathic Scoliosis