Anterior cervical decompression and fusion (ACDF) is a surgical procedure typically utilized in cervical spine disc herniation to alleviate spinal cord compression. There have been several implants used in ACDF surgeries [6,11]. Common implants include standalone cages that insert in the intervertebral space (zero-profile) and cages with an anterior cervical plate and screws that connects the adjacent vertebral bodies. The anterior cervical plate and screw placement has been the preferred method due to increase in fusion rate, reduced graft subsidence, and overall improved lordotic alignment [1]. Multiple studies have shown that standalone cages like the zero-profile result in higher incidence of failure and cage subsidence [16, 17, 18, 10, 19, 20]. This study aims to evaluate the mechanical stress induced by a zero-profile construct throughout the spine as a means to understand device risk and predict mechanical failure.

There are specific levels within the cervical spine that fail more often if adjacent levels are fused. When the level above the construct fails, it tends to be a process of subsidence and collapses of the above vertebral body onto the construct. This can be due to poor bone quality, or failure of bone growth to induce bony fusion between the two levels adjacent to the construct. Conversely, levels below the construct do not subside, but have an increased chance for disc herniation, requiring re-operation and extension of the fusion. The interaction of these levels and implanted cages remains poorly understood from a mechanical standpoint, and even less so in instances of multilevel ACDF [2].

Our computational model geometry included a zero-profile construct placed at a single level, followed by the application of physiological loading to the vertebral column. Parametric studies that probe various load magnitudes predict that resultant stress fields are concentrated at the level above the construct. High stresses immediately above the construct suggest an increased likelihood for subsidence in comparison to the levels below, which is in concert with clinical findings. Interestingly, the levels below the contrast experienced minimal stress shielding in comparison to referent normal simulations. Additional studies examined multidirectional forces that mimic flexion and extension of the cervical spine, with qualitatively similar findings of elevated stress above the construct. This model enables mechanical assessment of cervical spine instrumentation and provides a framework for understanding multilevel ACDF and predicting the performance of new cages/approaches to stabilize the spine.

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