Abstract / Summary
Zero-profile anterior cervical discectomy and fusion (ACDF) constructs vary in screw number and trajectory, but their effects on immediate stability and load transfer remain unclear. This study compared two-, three-, and four-screw constructs and examined middle-screw orientation. A validated C2-C7 finite-element model was used to simulate single-level C4-C5 ACDF with five zero-profile configurations: one two-screw construct, three-screw constructs with cranial or caudal middle-screw orientation, and four-screw constructs with the same two orientations. C4-C5 range of motion (ROM) and peak von Mises stress in the C3-C4 and C5-C6 adjacent discs and the fixation system were evaluated under flexion, extension, lateral bending, and axial rotation. All constructs reduced C4-C5 ROM to 0.080–0.852 degrees, representing reductions of 83.2%-98.4% relative to the intact model. The two-screw construct produced the highest fixation-system stress in every loading mode: 84.974 MPa in flexion, 84.818 MPa in extension, 109.190 MPa in lateral bending, and 94.848 MPa in axial rotation. Compared with caudal orientation, cranial middle-screw orientation reduced fixation-system stress by 6.9%-18.0% in the three-screw design and 9.3%-23.0% in the four-screw design. Adjacent-disc stress changes varied by motion, level, and configuration, without a uniform screw-number or orientation trend. Although all constructs provided strong immediate motion restriction, the two-screw construct showed the highest fixation-system stress across all four loading modes. Cranially directed middle screws reduced fixation-system stress in both three- and four-screw designs under the present modeling conditions, without a uniform advantage in C4-C5 ROM or adjacent-disc stress.