Posterior spinal fusion for Lenke 1 C and 2 C AIS: can the lowest instrumented vertebra stop at the lumbar apex?

Abstract

OBJECTIVE: To identify risk factors for distal adding-on (AO) in Lenke 1 C/2 C AIS patients with the lowest instrumented vertebra (LIV) at the lumbar apex vertebra (LAV). METHODS: This study included 60 Lenke 1 C/2 C AIS patients undergoing posterior spinal fusion with LIV at LAV and > 2 years follow-up. Patients were categorized into AO (n = 17) and non-AO (n = 43) groups. Radiographic analysis assessed thoracic/lumbar curve flexibility, apical vertebral translation (AVT), LAV rotation/tilt, coronal balance, Harrington stable zone on anteroposterior(AP)and concave-side bending films, and LAV/AV + 1 disc opening/closing status. Clinical outcomes used SRS-22. Statistical comparison was performed. RESULTS: The AO group exhibited significantly poorer preoperative thoracic curve flexibility, greater coronal imbalance toward the lumbar convex side, larger lumbar AVT, smaller Harrington stable zones, and fewer patients with favorable LAV/AV + 1 disc status. Logistic regression identified thoracic flexibility, Harrington stable zone on concave-side bending, and disc status as significant AO predictors. Optimal thresholds for selecting LAV as LIV were thoracic flexibility > 47.4% ,concave-side bending Harrington zone > 84.9% ,and favorable opening/closing status of the LAV/AV + 1 disc .At final follow-up, the AO group had larger lumbar Cobb angle and AVT with lower correction rates, but SRS-22 scores showed no significant difference. CONCLUSION: For Lenke 1 C/2 C AIS, terminating LIV at LAV is feasible when thoracic flexibility is good (> 47.4%), the LAV/AV + 1 disc status is favorable, and the Harrington stable zone on concave-side bending is large (> 84.9%).