Usefulness of modified S-line for upper instrumented vertebra selection in adolescent idiopathic scoliosis Lenke type 1C curve

Purpose Determine the effect of using the modified S-line vertebra (MSLV) as the upper instrumented vertebra (UIV) on postoperative trunk balance, L4 tilt, and clinical outcomes in selective thoracic fusion (STF) for adolescent idiopathic scoliosis (AIS) Lenke type 1C curve. Methods Twenty-eight consecutive patients (all female; mean age: 15.4±2.0 years) with AIS Lenke type 1C curve were retrospectively enrolled. Primary outcome measures were coronal balance (absolute distance between C7 and the center of the sacral vertical line), L4 tilt, and Scoliosis Research Society (SRS)-22r scores at 2 years postoperatively. The group with the MSLV at the UIV was designated as the MSLV group (18 patients), and the group with the MSLV proximal (12 patients) or distal (4 patients) to the UIV was defined as the non-MSLV group. Results We observed no significant differences between the groups regarding age, LIV and stable vertebra positioning, or preoperative X-ray parameters. Postoperative coronal balance was significantly better in the MSLV group (0.39±0.08 vs. 1.34±0.22 cm; P.