Is zero-profile fixation safe in consecutive two-level anterior cervical discectomy and fusion with partial uncinate resection? A finite element analysis

BACKGROUND: The necessity and extent of uncinate process resection (UPR) during anterior cervical discectomy and fusion remains controversial. The Zero-Profile device offers a low-profile alternative to traditional plate-cage systems but its biomechanical stability following various degrees of UPR, particularly in two-level ACDF, has not been clearly established. METHODS: This computational finite element analysis used a validated three-dimensional C2–T1 cervical spine model to simulate Two-Level ACDF (C4–C6) with a Zero-Profile construct under three unilateral uncinate process resection (UPR) conditions (0%, 30%, 50%). No clinical or cadaveric experiments were performed. Under six physiological motions (flexion, extension, left/right lateral bending, left/right axial rotation), we quantified global and segmental range of motion (ROM), adjacent-segment ROM, von Mises stress in the implant, vertebral body and endplate stresses, intradiscal pressure (IDP), and maximum displacement. RESULTS: Unilateral UPR led to a dose-dependent deterioration in biomechanical stability. In the 50% UPR model, the ROM of the C4/5 segment increased by 29.4% in flexion and 16.7% in lateral bending, while adjacent levels (C3/4 and C6/7) also exhibited elevated ROMs, indicating reduced segmental rigidity and increased compensatory motion. Stress on the fixation components rose significantly, with peak screw stress increasing by 47.1% and cage stress by 90.0% under flexion. Endplate stresses also increased, particularly in bending and rotation, with the C5 superior and C6 inferior endplates showing stress increments of 41.5% and 20.3%, respectively. Moreover, IDP at adjacent segments increased with greater UPR extent, reflecting enhanced load transmission to non-fused levels. The maximum construct displacement rose by 50.0% in flexion and 48.8% in axial rotation in the 50% UPR model, underscoring the progressive compromise of structural integrity. In contrast, the 30% UPR model demonstrated only mild changes in motion, stress, and displacement, suggesting that biomechanical performance remained within acceptable limits. CONCLUSIONS: Extensive UPR (≥ 50%) substantially compromises biomechanical stability in two-level ACDF using the ZP fixation, increasing mechanical load on implants and adjacent segments. ZP fixation appears acceptable when UPR is limited to 30%, but caution is warranted when higher resection is required. Surgeons should consider supplemental fixation strategies or alternative systems when extensive UPR is anticipated.