Defining risk factors for adding-on in Lenke 1 and 2 AR curves

Purpose To identify curve/correction characteristics associated with adding-on at 2 years after posterior spinal fusion (PSF) in Lenke 1/2 AR curves. Methods A prospective multicenter registry was queried to identify patients with adolescent idiopathic scoliosis (AIS), Lenke 1/2 AR curves, who underwent PSF with a minimum of 2-year follow-up. Patient characteristics as well as pre- and post- operative radiographic variables were evaluated using Classification and Regression Tree (CART) analysis to determine which factors contribute to adding-on. Results 253 patients treated with PSF were assessed for adding-on, of which 50 met our adding-on criteria. Univariate analysis revealed adding-on was associated several factors including: post-operative thoracic and lumbar curve magnitude ( p > 0.001), first erect (FE) end instrumented vertebra (EIV) angulation ( p = 0.009) and EIV translation ( p = 0.001), younger age ( p = 0.027), Risser stage ( p = 0.024), and a more proximal lowest instrumented vertebra (LIV) ( p 0.5°). Conclusion In Lenke 1 and 2 AR curves, pre-operative LIV translation appears to be an independent risk factor for adding-on; and in those with