Background Data: The selection of the lower instrumented vertebra (LIV) in single thoracic (Lenke1A and B) Adolescent Idiopathic Scoliosis (AIS) remains controversial. Purpose: To report a new method of selecting the LIV and report the results of surgery based upon this method. Study Design: Retrospective radiological analysis of Lenke 1A and B AIS patients. Patients and Methods: Fifty six patients met the inclusion criteria, 4 (7.1%) male and 52 (92.9%) females. Mean age at time of surgery was 14.7 years. The following data were recorded on preoperative, immediate postoperative and
latest follow up x-rays: the lower end vertebra (EV), the stable vertebra (SV), the neutral vertebra (NV), the vertebra most closely bisected by a central sacral perpendicular line that was designated the left bending stable vertebra (LBSV)
and the lower instrumented vertebra (LIV). Patients were checked for coronal balance and the development of adding on the latest follow up X-rays. Results: Patients were followed up for a mean of 28.5 months. Mean thoracic
curve correction was 69%. Five patients (9%) developed adding on at their latest follow up. All patients instrumented at or distal to the LBSV were balanced without adding on. Five out of the six patients (83%) instrumented proximal to
the LBSV developed adding on at their latest follow up. Conclusion: The LBSV is a useful and reliable way of determining the LIV in Lenke 1A and B AIS. Both the LBSV and Suk et al method predicted adding on; however, the LBSV can allow a more proximal LIV and therefore a shorter fusion in up to 46% of this case series. (2012ESJ022)