Distal Adding-On Phenomenon in Lenke 1A Scoliosis: Risk Factor Identification and Treatment Strategy Comparison

Study Design. Retrospective study. Objective. To identify risk factors for the presence of distal adding-on in Lenke 1A scoliosis and compare different treatment strategies. Summary of Background Data. 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 optimal treatment strategies remain controversial. Methods. All surgically treated patients with adolescent idiopathic scoliosis were retrieved from a single institutional database. Inclusion criteria included: (1) Lenke 1A scoliosis patients treated with posterior pedicle screw-only constructs, (2) 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 center 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 exact test, and Spearman correlation test were used to identify the risk factors for adding-on. A multiple logistic regression model was built to identify independent predictive factor(s). Risk factors included: (1) age at surgery; (2) preoperative Cobb angle; (3) correction rate; (4) the gap difference of stable vertebra-lowest instrumented vertebra (SV-LIV), neutral vertebra-lowest instrumented vertebra (NV-LIV), and end vertebra-lowest instrumented vertebra (EV-LIV). Gap difference means, for example, if SV is at L2 and LIV is at Th12, then the difference of SV-LIV is 2; (5) 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 no adding-on group. Results. Out of the 278 patients reviewed, 45 met the inclusion criteria; 23 of these met the definition for distal adding-on, and were included in the adding-on group. The remaining 22 patients were included in the no adding-on group. The average follow-up was 3.6 years. Age, SV-LIV difference, EV-LIV difference, and LIV + 1 deviation from CSVL were significantly different (P < 0.05) between the two groups, and were also found to be significantly correlated with the presence of adding-on (P < 0.05). Preoperative Cobb angle, correction rate, and NV-LIV difference were not found to be affiliated with the presence of adding-on. Multiple logistic regression results indicated that preoperative LIV + 1 deviation from CSVL was an independent predictive factor. Among the five methods, choosing EV as LIV was nearly unable to prevent distal adding-on; choosing EV + 1 as LIV resulted in fusing many more segments than necessary; only choosing DV as LIV showed satisfactory outcome from both perspectives. Conclusion. In Lenke 1A type scoliosis, the selection of LIV is highly correlated with the presence of adding-on; incidence increases dramatically when the preoperative LIV + 1 deviation from CSVL is more than 10 mm. Choosing DV (the first vertebra in cephalad direction from sacrum with deviation from CSVL of more than 10 mm) as LIV may provide the best outcome as it not only prevents adding-on but also conserves more lumbar motion.