Background:Distal adding-on (DA) in adolescent idiopathic scoliosis is a radiographic complication that can negatively affect clinical results. However, the risk factors for DA and the influences of DA on the residual lumbar curves have not been fully elucidated in Lenke type 1B and 1C curves. The objective of this study was to investigate risk factors for postoperative DA in Lenke type 1B and 1C curves, and the influence of DA on residual lumbar curves.Methods:We retrospectively evaluated 46 adolescent idiopathic scoliosis patients with Lenke type 1B or 1C curves who underwent posterior correction and fusion surgery with selective thoracic fusion. Patients were grouped according to the presence or absence of DA on radiographs at the 2-year follow-up. We compared coronal radiographic parameters between the 2 groups, including the Cobb angle, L4 tilt angle, apical translation, and relative positions of the end vertebra (EV), stable vertebra (SV), neutral vertebra (NV), and last touching vertebra (LTV) to the lower instrumented vertebra (LIV).Results:DA was present in 11 patients (24%) at the 2-year follow-up, and the mean LIV-EV, LIV-NV, LIV-SV, and LIV-LTV relative positions were significantly smaller in the DA than in the non-DA group. Preoperative radiographic parameters were similar between the 2 groups, including the mean L4 tilt angle (non-DA,-8±4 degrees; DA,-7±4 degrees). At the 2-year follow-up, the mean apical translation of the lumbar curve was smaller in the DA group (non-DA,-16±8 mm; DA,-7±11 mm) and the mean L4 tilt angle was significantly more horizontalized (non-DA,-8±4 degrees; DA,-1±5 degrees). Multivariate analysis showed that the number of levels between the LIV and LTV (LIV-LTV) was significantly associated with DA.Conclusions:A LIV at or cranial to the LTV was a significant risk factor for postoperative DA in Lenke type 1B and 1C curves. Spontaneous correction of the residual lumbar curve was superior in patients with DA.
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