Junctional Failures Following Long-Level Fusion to L5 in Elderly Patients: Impact of Spinopelvic Alignment and L5–S1 Disc Degeneration

Background and Objectives: Long spinal fusion terminating at L5 remains controversial because of the risk of postoperative junctional failure. Although degeneration of the residual L5–S1 disc has been suggested as a contributing factor, the relative impact of disc degeneration versus sagittal spinopelvic alignment on different junctional failure patterns has not been fully clarified. Materials and Methods: This retrospective cohort study included 47 patients aged ≥60 years who underwent ≥5-level thoracolumbar fusion ending at L5 with a minimum follow-up of 2 years. Junctional failures were classified as proximal junctional failure (PJF) or distal junctional failure (DJF). Preoperative L5–S1 disc degeneration was assessed using modified Weiner and Pfirrmann classifications. Spinopelvic parameters were measured preoperatively, postoperatively, and at final follow-up. Junctional failure–free survival was analyzed using the Kaplan–Meier method, and risk factors were explored using Cox proportional hazards models. Results: Junctional failures occurred in 28 patients (59.6%), including 16 PJFs (34.0%) and 10 DJFs (21.3%). Lower grades of L5–S1 disc degeneration (Weiner grades 0–1) were more frequently associated with PJFs, whereas higher grades (≥2) were predominantly associated with DJFs (p = 0.024). Multivariate analysis showed that preoperative thoracolumbar kyphosis (hazard ratio [HR] = 1.164), preoperative T1 pelvic angle (HR = 1.269), and postoperative pelvic incidence–lumbar lordosis mismatch (HR = 0.877) as significant risk factors for PJF. Postoperative proximal junctional angle (HR = 0.899) and lumbar lordosis (HR = 0.920) were independently associated with DJF. Conclusions: Sagittal spinopelvic alignment parameters appear to have a greater influence on junctional failure patterns than residual L5–S1 disc degeneration in long fusions terminating at L5. Adequate sagittal correction should be prioritized to reduce the risk of both proximal and distal junctional failures.