Distal adding-on in Lenke 1–2 adolescent idiopathic scoliosis: A focused analysis of the Lenke 1a subgroup

Vollständiger Abstract

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Aim: Distal adding-on (DOA) is an important postoperative concern after thoracolumbar fusion in adolescent idiopathic scoliosis (AIS); however, its mechanical drivers, particularly the role of sagittal spinopelvic compensation, remain poorly understood.Materials and Methods: We reviewed Lenke type 1 and 2 AIS patients with a minimum 2-year follow-up. To isolate biomechanical failure patterns and minimize confounding variables, a targeted, exploratory analysis was performed on the Lenke 1A subgroup (n=21). Firth penalized logistic regression was utilized as an exploratory method to identify preliminary "radiographic signals" rather than definitive associated factors, given the low-event nature of the cohort.Results: We analyzed 72 patients and found that 6.9% (n=5) experienced distal adding-on (DOA). A preliminary analysis of the Lenke 1A subgroup (n=21) revealed a potential failure pattern: all 3 DOA events occurred when the lowest instrumented vertebra (LIV) was placed at the last touched vertebra (LTV) level. Conversely, extending the fusion beyond this point was associated with a reduced risk of this outcome (p=0.030). Additionally, we identified a potential 'warning sign': patients who developed DOA showed a more anteverted pelvic tilt after surgery compared to those who did not (-3.67° ± 12.50° vs. 8.39° ± 6.41°, p=0.016).Conclusions: DOA remains a rare but relevant clinical challenge. Our exploratory findings suggest that the LTV may be an insufficient standalone landmark in Lenke 1A subtypes, and that pelvic anteversion may serve as a mechanical warning sign for potential junctional failure. These observations underscore the necessity of a more comprehensive, three-dimensional approach to LIV selection and require further prospective validation studies.