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.