Optimal Selection of Lower Instrumented Vertebra Can Minimize Distal Junctional Kyphosis after Posterior Spinal Fusion for Thoracic Adolescent Idiopathic Scoliosis

Study Design. Retrospective cohort study of a prospectively collected multicenter database. Objective. To identify risk factors for developing distal junctional kyphosis (DJK) and elucidate optimal selection of the lowest instrumented vertebra (LIV) utilizing sagittal stable vertebra (SSV) and preoperative distal junctional angle (DJA) to prevent DJK. Summary of Background Data. While including the SSV may minimize DJK following posterior spinal fusion (PSF) for adolescent idiopathic scoliosis, relying solely on the SSV criteria can necessitate more extensive fusion. As LIV moves distally, a patient's motion, function, and chance of degeneration may all be negatively affected. Methods. This study included patients with Lenke 1/2 curves who underwent thoracic PSF (LIV≤L1); development of DJK (DJA≥10°) was evaluated 2 years postoperatively. Preoperative DJA was measured between LIV and LIV+1, consistent with postoperative measurements. Multiple logistic regression models identified risk factors for developing DJK. DeLong's test compared area under the curve (AUC) from different receiver operating characteristic curves to assess DJK predictive accuracy between models. Results. Of 1,034 patients, 86 (8%) developed DJK 2 years postoperatively. Identified risk factors included preoperative DJA, LIV at ≥SSV-2, an upper instrumented vertebra of ≥T2, lumbar modifiers B or C, and larger T5-12 kyphosis. Incorporating preoperative DJA and SSV-1 for LIV selection enhanced DJK prediction accuracy over solely considering SSV inclusion (AUC=0.81 vs. 0.72, P<0.001). Furthermore, a multivariate model with risk factors achieved the highest AUC (0.87). Patients with DJK experienced worsening of T10-L2 kyphosis and lumbar lordosis over time, without affecting the Scoliosis Research Society-22 quality of life score. Among those who developed DJK, five required an extension of fixation distally. Conclusion. To prevent DJK, PSF should end below preoperative kyphosis and no more proximal than SSV-1 in patients with thoracic adolescent idiopathic scoliosis, particularly for high-risk cases. DJK led to kyphotic regional thoracolumbar alignment at 2-year follow-up. Level of Evidence.