Utilizing stable vertebra on push-prone traction radiographs for the determination of the lowest instrumented vertebra: a novel approach for AIS patients with Lenke type 3C and 6C.

Objective: To assess whether using stable vertebra on push-prone traction radiographs for selecting the lowest instrumented vertebra (LIV) in adolescent idiopathic scoliosis (AIS) patients with Lenke types 3C and 6C undergoing posterior spinal surgery can preserve more lumbar motion segments while still achieving satisfactory surgical outcomes.

Background: AIS patients requiring surgical treatment typically present with progressive curves exceeding 40° and are skeletally immature. This study specifically focuses on AIS patients exhibiting Lenke curve types 3C and 6C, which include structural thoracic as well as thoracolumbar and lumbar curves. The selection of the LIV remains a contentious issue due to concerns about distal adding-on and the potential for spinal imbalance postoperatively. Existing literature suggests that longer fusion constructs and positioning the LIV below L3 can lead to significant functional limitations and accelerated disc degeneration. While Lenke advocated for identifying the stable vertebra (SV) as the LIV, our recent study indicates that push-prone traction radiographs provide superior predictability for correcting postoperative spinal alignment. This study aims to evaluate the effectiveness of using the stable vertebra identified through push-prone traction radiographs as the LIV in preserving segmental motion during posterior spinal surgery for managing Lenke type 3C and 6C curves.

Methods: AIS patients with Lenke type 3C and 6C who underwent posterior spinal surgery between 2021 and 2024 were enrolled in the study. Preoperative 36-inch whole spine radiographs, including push-prone traction view, were obtained for curve flexibility assessment. The lowest instrumented vertebra (LIV) was determined by identifying the stable vertebra (SV) on push-prone traction radiographs. Demographic data, including sex, age, BMI, Lenke's curve type, and pre- and postoperative major coronal Cobb angle, thoracic kyphosis, lumbar lordosis, and C7 to central sacral vertical line (C7-CSVL), were collected. Statistical analysis was conducted to assess the differences in curve magnitudes between pre- and postoperative measurements.

Results: Thirty-six AIS patients (33 female and 3 male) with a mean age of 13.9 ± 2.2 years were included in this study, with a mean follow-up period of 28.4 months. Preoperatively, the cohort presented with Lenke type 3C (24 out of 36) and Lenke type 6C (12 out of 36). The preoperative thoracic curve was corrected to an average of 5.7°, demonstrating an average correction rate of 89%. Similarly, the preoperative lumbar curve was corrected to an average of 5° with a correction rate of 90%.

Conclusion: Push-prone traction radiographs may serve as an alternative method for determining the optimal LIV level in patients with Lenke type 3C and 6C. Identifying stable vertebra on push-prone traction radiographs as LIV can potentially preserve more lumbar motion segments while achieving favorable surgical outcomes.