Acute Disc Herniation at the Proximal Junctional Segment (UIV/UIV + 1) Following Long Segment Spinal Fusion With Cement-Augmented Instrumentation: A Retrospective Study

<sec><title>Background</title> Prophylactic cement augmentation at the uppermost instrumented vertebra (UIV) and the adjacent supravertebra (UIV + 1) is commonly used during long-segment fusion for adult spinal deformity to reduce the risk of proximal junctional failure. While this strategy may protect against osteoporotic fracture, its impact on adjacent soft tissues—particularly the intervertebral disc—remains poorly characterized. Acute disc herniation (ADH) at the proximal junction represents a rare but potentially catastrophic complication. </sec><sec><title>Methods</title> In this retrospective cohort study, 173 patients underwent long-segment posterior spinal fusion (≥5 levels) with prophylactic polymethylmethacrylate (PMMA) cement augmentation at UIV/UIV + 1 between January 2016 and January 2024. Nine patients developed ADH at the proximal junction. After 1:3 propensity score matching on age, <italic>T</italic> score, UIV level, and preoperative pelvic incidence–lumbar lordosis mismatch, 9 ADH cases were compared with 27 matched controls without ADH. Radiographic parameters at UIV/UIV + 1 (UIV Cobb angle, adjacent segment Cobb angle, disc slope, and disc height) were measured preoperatively and at ADH diagnosis (or equivalent follow-up in controls). </sec><sec><title>Results</title> Baseline characteristics were well balanced between matched groups, with no significant differences observed for matched or unmatched variables (all <italic>P</italic> > 0.05) except for smoking, which was more prevalent in the ADH group (<italic>P</italic> = 0.03). ADH patients presented with acute neurological deterioration, including lower-extremity weakness, gait instability, myelopathy, and urinary dysfunction, often requiring urgent revision surgery. Radiographically, the ADH group demonstrated significantly greater junctional deterioration: UIV Cobb angle increased by approximately +20.3° ± 8.5° compared with +1.2° ± 7.8° in controls (<italic>P</italic> < 0.001); adjacent segment Cobb increased by +18.5° ± 6.5° vs +1.7° ± 4.8° (<italic>P</italic> < 0.001); and disc height decreased by −3.5 ± 2.0 mm vs −0.9 ± 1.1 mm (<italic>P</italic> = 0.006). Disc slope change was similar between groups (<italic>P</italic> = 0.42). </sec><sec><title>Conclusions</title> Despite preserved bone quality, ADH was associated with profound junctional collapse and rapid neurological decline, suggesting a biomechanical failure mechanism driven by PMMA-related overstiffening. Strategies that preserve a gradual stiffness transition at the proximal junction may be necessary to mitigate this risk. </sec><sec><title>Clinical Relevance</title> Although rare, acute proximal junctional disc herniation can result in rapid neurological deterioration following long-segment fusion with prophylactic UIV/UIV+1 cement augmentation. Spine surgeons should maintain a high index of suspicion for this complication in patients presenting with new neurological symptoms and consider strategies that minimize abrupt stiffness transitions at the proximal junction. </sec><sec><title>Level of Evidence</title> 3. </sec>