Biomechanical effects of supplemental posterior cervical stabilization in one-to three-level anterior cervical discectomy and fusion: a finite element analysis of construct length-dependent stress shielding and adjacent segment mechanics.

Objective: This finite element analysis study aimed to compare the biomechanical behavior of anterior cervical discectomy and fusion (ACDF) with circumferential cervical fusion (CCF) across one-, two-, and three-level constructs, to elucidate the length-dependent contribution of supplemental posterior fixation. Methods: A C3-C7 finite element model was developed from CT scans. Seven scenarios were simulated: intact spine; 1-, 2-, and 3-level ACDF; and corresponding CCF constructs (ACDF plus bilateral posterior facet cages). A 73.6 N axial load with ±1.0 Nm moments simulated flexion, extension, lateral bending, and axial rotation. Stress on vertebrae, instrumentation, and adjacent segments (C3/4, C5/6, C6/7 facet capsules and discs) was analyzed. Results: The biomechanical benefit of CCF was length-dependent. It reduced anterior plate stress relative to ACDF, with increasing reduction for longer constructs (8%-12% for 1-level, 10%-18% for 2-level, 15%-25% for 3-level). CCF provided greater vertebral stress shielding. All fusions increased adjacent facet capsule stress, slightly more with CCF. However, CCF resulted in lower increases in adjacent disc stress than ACDF (e.g., at C6/7 disc: +1.6% for 3-level CCF vs. +5.8% for ACDF). Conclusions: One-level ACDF provides sufficient stability. For two-level constructs, posterior fixation offers measurable protection, beneficial for high-risk patients. For three-level constructs, CCF effectively mitigates the significant anterior stress concentration and elevated adjacent disc stress associated with standalone ACDF, despite a minor increase in facet capsule stress. Posterior augmentation should be considered based on construct length.