Selection of Lower Instrumented Vertebra in Treating Lenke Type 2A Adolescent Idiopathic Scoliosis

Study Design. A retrospective, observational, and multicenter study. Objective. To identify the ideal lower instrumented vertebra (LIV) to prevent distal Adding-on after surgical correction of Lenke type 2A curve. Summary of Background Data. LIV level may affect the risk of postsurgical Adding-on. The choice of the last touching vertebra (LTV)-the most caudal vertebra of the main thoracic curve that touches the central sacral vertical line when standing-as an appropriate LIV has been validated for Lenke type 1A but not type 2A curve. Methods. Radiographs obtained before, immediately after, and 2 years after surgery were evaluated for 116 consecutive patients who underwent posterior thoracic fusion surgery for Lenke type 2A curve. The LIV was proximal to the LTV in 18 patients (PLTV), distal in 43 (DLTV), and at the LTV in 55 (ALTV). Significant independent factors associated with Adding-on were analyzed first by univariate analysis, and then by stepwise logistic regression analysis. Results. Distal Adding-on was present in 16 patients (13.8%) at follow-up: 9 PLTV (50.0%), 3 DLTV (7.0%), and 4 ALTV (7.3%). Adding-on was significantly more common in the PLTV group. One PLTV-group patient required revision surgery to treat Adding-on. Univariate analysis identified the following significant factors associated with Adding-on: the T2-T5 kyphosis angle and shoulder height before, immediately after, and 2 years after surgery; the lumbar Cobb angle at the 2-year follow-up; the 2-year postoperative lumbar curve correction rate; and the difference between the LIV and the end vertebra, neutral vertebra, and LTV levels. Significant independent risk factors identified by stepwise logistic regression analysis included the clavicle angle at follow-up, the correction rate of the lumbar curve immediately after surgery, and the difference between the LIV and LTV levels. Conclusion. A LIV at or distal to the LTV may prevent postoperative Adding-on in Lenke type 2A curve.