Postoperative distal adding-on and related factors in Lenke type 1A curve

Study Design: A retrospective, multicenter study. Objective: To investigate the occurrence of and factors related to postoperative adding-on in Lenke type 1A curve. Summary of Background Data: Although several studies have investigated factors associated with adding-on in Lenke type 1A curve, these factors have not been elucidated in a large study population. Methods: This study included 112 patients who were followed more than 2 years after undergoing selective posterior thoracic fusion surgery for Lenke Type 1A curve (8 males, 104 females; mean age at surgery, 16.1 yr; mean follow-up, 3.6 yr). The lower instrumented vertebra (LIV) was T12 in 22 patients, L1 in 55, L2 in 32, and L3 in 3. Distal to the main thoracic curve, the end vertebra, neutral vertebra, stable vertebra (SV), and the last vertebra touching the central sacral vertical line (last touching vertebra, LTV) were determined. The occurrence and factors associated with distal adding-on were investigated. Results: The mean Cobb angle and apical translation of the main thoracic curve were 54.6 ° ± 9.6 ° and 53.1 ± 20.4 mm before surgery, and 14.2 ± 7.4 and 16.2 ± 12.7 at follow-up, respectively. Distal adding-on was observed in 21 patients (18.8%) at follow-up. Univariate analyses identifi ed several factors signifi cantly associated with adding-on, including the preoperative proximal thoracic curve, the apical translation of the main thoracic curve, Miyanji's subclassifi cation, the postsurgical proximal and main thoracic curves, the postsurgical apical translation of the main thoracic curve, the correction rate of the main thoracic curve and the clavicle angle immediately after surgery and at follow-up, and the difference in levels between the LIV and the end vertebra, neutral vertebra, LTV, and stable vertebra. Logistic regression analysis showed that the apical translation of the main thoracic curve immediately after surgery (apical translation > 25 mm, odds ratio: 10.7, 95% confi dence interval: 3.1-37.0, P = 0.001) and the difference in levels between LIV and LTV (LIV-LTV) (LIV-LTV < 0, odds ratio: 6.7, 95% confi dence interval: 1.9-23.9, P = 0.003) were signifi cantly associated with adding-on. Conclusion: Since the residual apical translation of the main thoracic curve and the lowest instrumented vertebra more cranial to the last touching vertebra were signifi cantly associated with addingon, surgeons may need to obtain the maximum reduction of the apical translation of the main thoracic curve and to extend the LIV at least to the LTV to avoid postoperative adding-on.