Stress distribution in adjacent segments following anterior cervical discectomy and fusion: a finite element biomechanical study based on anterior plate length and intervertebral disc height

Anterior cervical discectomy and fusion (ACDF) is the standard surgical treatment for cervical degenerative diseases. However, adjacent segment disease (ASD) remains one of its most important long-term complications. Anterior cervical plate length and intervertebral disc height are key surgical parameters affecting stress distribution in adjacent segments, yet their biomechanical effects under different physiological loading conditions have not been systematically investigated. A validated finite element model of single-level C4/5 ACDF was established using computed tomography (CT) data from a healthy 56-year-old male volunteer. Six anterior plate lengths and three intervertebral disc heights were evaluated. Physiological loading conditions, including flexion, extension, lateral bending, and axial rotation, were simulated under axial compressive loads ranging from 50 to 200 N combined with a 1 Nm moment. The distribution of Von Mises stress in adjacent segments was quantitatively analyzed. During flexion, anterior plate length was negatively correlated with adjacent segment stress, with a pronounced diminishing marginal effect in which the largest stress reduction occurred within the first plate-length interval. In contrast, during extension, lateral bending, and axial rotation, plate length was positively correlated with adjacent segment stress. The most prominent increasing marginal effect was observed during axial rotation, with the final plate-length interval (59 mm) showing stress increases of 23.48%–27.31% under 50 N. The effect of intervertebral disc height on adjacent segment stress was approximately linear during flexion and lateral bending, with mean percentage differences ranging from 3.30% to 20.28%, whereas nonlinear relationships were observed during extension and axial rotation, with mean percentage differences ranging from 35.65% to 67.29%. Across all loading conditions, linear regression analyses demonstrated a strong linear relationship between applied load and Von Mises stress (R² = 0.91–0.99). This finite element analysis demonstrates that selection of anterior plate length requires balancing its stress-shielding effect during flexion against its stress-enhancing effects during extension, lateral bending, and axial rotation. In addition, intervertebral disc height should not be over-distracted beyond the native anatomical height, as this was associated with a disproportionate increase in adjacent segment stress.