How translation of different parts of spine responds to selective thoracic fusion has not been well investigated. Furthermore, how posterior pedicle-screw-only constructs affect spontaneous lumbar curve correction (SLCC) remains unknown. In a retrospective study, we aimed to investigate the balance change after selective thoracic fusion in Lenke 1C type adolescent idiopathic scoliosis (AIS) treated with posterior pedicle-screw-only constructs. All AIS cases, surgically treated between 2002 and 2008 in our institute, were reviewed. Inclusion criteria were: patients with Lenke 1C scoliosis treated with posterior pedicle-screw-only constructs; the lowest instrumented vertebra (LIV) ended at L1 level or above; and a minimum 2-year radiographic follow-up. Standing anteroposterior (AP) and lateral digital radiographs from different timepoints (preoperative, immediately postoperative, 3 months postoperative, and final follow-up) were reviewed. In each standing AP radiograph, centre sacral vertical line (CSVL) was drawn first, followed by measurement of the translation (deviation from the CSVL) of some key vertebrae, such as the LIV, LIV+1 (the first vertebra below LIV), LIV+2 (the second vertebra below LIV), LIV+3 (the third vertebra below LIV), lumbar apical vertebra, thoracic apical vertebra, and T1. Additionally, the Cobb angles of major thoracic and lumbar curve were measured at different timepoints, and the correction rate was calculated. Furthermore, clinical photos of patients' back appearance were taken preoperatively and postoperatively.Introduction
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Distal adding-on is often accompanied by unsatisfactory clinical outcome and high risk of reoperation. However, very few studies have focused on distal adding-on and its attendant risk factors, and optimum treatment strategies remain controversial. In a retrospective study, we aimed to identify risk factors for the presence of distal adding-on in Lenke 1A scoliosis and to compare different treatment strategies. Data for all surgically treated patients with adolescent idiopathic scoliosis (AIS) were retrieved from one institutional database. Inclusion criteria included: patients with Lenke 1A scoliosis treated with posterior pedicle screw-only constructs; and a minimum 1-year radiographic follow-up. Distal adding-on was defined as a progressive increase in the number of vertebrae included distally within the primary curve combined with either an increase of more than 5 mm in deviation of the first vertebra below instrumentation from the centre sacral vertical line (CSVL), or an increase of more than 5° in the angulation of the first disc below the instrumentation at 1 year follow-up. Wilcoxon rank-sum test, Fisher's exact test, and Spearman's correlation test were used to identify the risk factors for adding-on. A multiple logistic regression model was built to identify independent predictive factors. Risk factors included: age at surgery; preoperative Cobb angle; correction rate; the gap difference of stable vertebra (SV) and lowest instrumented vertebra (LIV), neutral vertebra (NC) and LIV, and end vertebra (EV) and LIV (gap difference means, for example, if SV is at L2 and LIV is at Th12, then the difference of SV-LIV is 2); and the preoperative deviation of LIV+1 (the first vertebra below the instrumentation) from the CSVL (the vertical line that bisects proximal sacrum). Five methods for determining LIV were compared in both the adding-on group and the no adding-on group.Introduction
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