Acute Disc Herniation at the Proximal Junctional Segment (UIV/UIV + 1) Following Long Segment Spinal Fusion With...

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.

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, T 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).

Baseline characteristics were well balanced between matched groups, with no significant differences observed for matched or unmatched variables (all P > 0.05) except for smoking, which was more prevalent in the ADH group (P = 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 (P < 0.001); adjacent segment Cobb increased by +18.5° ± 6.5° vs +1.7° ± 4.8° (P < 0.001); and disc height decreased by -3.5 ± 2.0 mm vs -0.9 ± 1.1 mm (P = 0.006). Disc slope change was similar between groups (P = 0.42).

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.

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.