Objective:To investigate the clinical and imaging outcomes of Lenke 5 idiopathic scoliosis posterior selective fusion with "Last Touching Vertebra-1" as the lower instrumented vertebra (LIV).Methods:A total of 103 patients with Lenke 5 idiopathic scoliosis who underwent posterior selective fusion orthopedic surgery from April 2009 to March 2020 were analyzed retrospectively. The LIV was the last touching vertebra (LTV) in 45 cases (LTV group) and the LTV-1 in 58 cases (LTV-1 group). The follow-up duration was more than 2 years. SRS- 22 questionnaire was used to evaluate the clinical effects at 2 years after operation. The anterior and lateral radiographs of the whole spine were measured preoperatively, postoperatively and at 2 years after operation to obtain all the following imaging parameters, including scoliosis Cobb angle, apical vertebral translation (AVT), coronal balance, LIV tilt, LIV lower intervertebral disc angle, LIV translation, LIV lower vertebral translation, LTV/LIV rotation degree, lumbar lordosis angle, pelvic incidence angle, sagittal balance. The complications were summarized and were analyzed for investigating potential risk factors.Results:At 2 years after operation, the correction rates of main Cobb in LTV group and the LTV-1 group were 60.2%±11.1% and 55.3%±14.1%, respectively. The coronal balance was 3.5±9.8 mm and 4.9±10.6 mm respectively. The sagittal balance was -15.5±18.1 mm and -19.6±22.6 mm respectively. There was no significant difference between the two groups ( t=2.305, P=0.085; t=-0.695, P=0.489; t=0.992, P=0.324). The incidence of proximal junction kyphosis in the two groups was 2.2% (1/45) and 8.6% (5/58), respectively. The incidence of significant loss of main Cobb correction and distal adding-on was 13.3% (6/45) and 25.9% (15/58) respectively without significant difference (χ 2=1.891, P=0.169; χ 2=2.451, P=0.117). Compared with non-complication patients (39 cases), 19 patients with complications in LTV-1 group had a greater degree of coronal balance to the convex side (23.9±9.5 mm vs. 14.6±11.5 mm, t=3.06, P=0.003), a greater LIV tilt (29.2°±3.7° vs. 25.3°± 5.3°, t=2.85, P=0.006), and a greater degree of LTV rotation (1.0(1, 1) vs. 0.6(0, 1), Z=-2.97, P=0.003). Logistic regression analysis showed that large preoperative LIV tilt and large preoperative coronal balance were the risk factors of complications during follow-up. Conclusion:The selection of LTV and LTV-1 as LIV in patients with Lenke 5 adolescent idiopathic scoliosis could obtain satisfied coronal, sagittal balance and low incidence of mechanical related complications during follow-up. For patients with preoperative coronal balance >17.0 mm or LIV tilt >25.3°, the risk of mechanical related complications might be higher than that when "LTV-1" was selected as LIV.