The effect of metal density in thoracic adolescent idiopathic scoliosis

Purpose Determine impact of metal density on curve correction and costs in thoracic adolescent idiopathic scoliosis (AIS). Ascertain if increased metal density is required for larger or stiffer curves. Methods Multicentre retrospective case series of patients with Lenke 1-2 AIS treated with single-stage posterior only surgery using a standardized surgical technique; constructs using >80 % screws with variable metal density. All cases had >2-year follow up. Outcomes measures included coronal and sagittal radiographic outcomes, metal density (number of instrumented pedicles vs total available), fusion length and cost. Results 106 cases included 94 female. 78 Lenke 1. Mean age 14 years (9–26). Mean main thoracic (MT) Cobb angle 63° corrected to 22° (66 %). No significant correlations were present between metal density and: (a) coronal curve correction rates of the MT ( r = 0.13, p = 0.19); (b) lumbar curve frontal correction ( r = −0.15, p = 0.12); (c) correction index in MT curve ( r = −0.10, p = 0.32); and (d) correction index in lumbar curve ( r = 0.11, p = 0.28). Metal density was not correlated with change in thoracic kyphosis ( r = 0.22, p = 0.04) or lumbosacral lordosis ( r = 0.27, p = 0.01). Longer fusions were associated with greater loss of thoracic kyphosis ( r = −0.31, p = 0.003). Groups differing by preoperative curve size and stiffness had comparable corrections with similar metal density. The pedicle screw cost represented 21–29 % of overall cost of inpatient treatment depending on metal density. Conclusions Metal density affects cost but not the coronal and sagittal correction of thoracic AIS. Neither larger nor stiffer curves necessitate high metal density.