Biomechanical effects of different intervertebral height reconstructions on the intermediate segment in skip-level ACDF for discontinuous cervical spondylosis: a finite element analysis

Background Skip-level anterior cervical discectomy and fusion (ACDF) has been reported to provide favorable outcomes for discontinuous cervical spondylosis; however, intermediate segment degeneration (ASD) remains a concern. Reconstruction of intervertebral height at fused levels may affect postoperative biomechanics, yet the optimal reconstruction strategy remains unclear. Methods A three-dimensional finite element model of the cervical spine (C2–C7) was developed. Skip-level ACDF was simulated at C4/5 and C6/7. Six models were analyzed: an intact model (M0) and five postoperative models with reconstructed intervertebral heights of 100%, 125%, 150%, 175%, and 200% of a reference height (M1–M5). A 50 N axial preload and a 1.0 N·m pure moment were applied to simulate flexion, extension, left/right lateral bending, and left/right axial rotation. Outcome measures included cervical range of motion (ROM), peak Von Mises stress of vertebrae and implants, and stress in the intermediate disc (C5/6). Results ROM at the fused segments decreased markedly in all postoperative models. During lateral bending and axial rotation, ROM and disc stress at non-fused levels—particularly the intermediate segment—generally increased with greater reconstructed height, whereas disc stress tended to decrease or remain relatively stable during extension and right axial rotation. Across most motion directions, the 100% reconstruction model showed relatively smaller increases in intermediate-segment ROM and disc stress, while the 125% reconstruction model exhibited lower cage and facet joint stresses. Lateral bending produced notably higher cage and screw stresses compared with other motion directions. Conclusions Under finite element conditions, when the cage height was set to 100% of the reference intervertebral height, the intermediate segment exhibited relatively smaller biomechanical changes in most motion scenarios, suggesting that this reconstruction height may have a limited impact on the intermediate segment. Biomechanical responses of the postoperative intermediate segment and the fixation system varied among different motion directions; in particular, load variation patterns during extension and lateral bending suggest that postoperative motion may influence the mechanical environment of the intermediate segment and implant stability. The clinical relevance of these findings requires further investigation.