Biomechanical Effects of Zero-P Height on Anterior Cervical Discectomy and Fusion: A Finite Element Study

The principle of selecting a Zero-P implant of an appropriate height remains a topic of debate, particularly when similarly sized implants seem to appropriately fit the intervertebral space. Thus, this study compared the biomechanical performance of smaller and larger Zero-P implants within an appropriate height range with that of oversized Zero-P implants for anterior cervical discectomy and fusion (ACDF).

A three-dimensional finite element (FE) model of the C2-C7 cervical spine was constructed and validated. The implants were categorized as smaller (6 mm), larger (7 mm), and oversized (8 mm) according to the average intervertebral height and implant specifications. Thus, the following four FE models were constructed: the intact cervical spine model (M1), the 6 mm model (M2), the 7 mm model (M3), and the 8 mm (M4) Zero-P implant C5/6 segment ACDF surgical model. Then, a pure moment of 1.0 N·m combined with a follower load of 75 N was applied at C2 to simulate flexion, extension, lateral bending, and axial rotation.

The results indicated that the maximum stress on the vertebral body, intervertebral disc, and facet joints under self-weight increased with increasing Zero-P height. Under six different loading conditions, the maximum stress on the vertebral body in the surgical segment of the M4 model was generally greater than that in the M2 and M3 models. Following an increase in the height of the implant from 6 mm to 8 mm, the maximum stress increased, and the intervertebral disc stress of both segments reached its peak in the M4 model. In the M4 model, the implant experienced the highest stress, whereas the M2 model exhibited the lowest stress on the implant under both self-weight and loading conditions. Furthermore, the stress on the posterior facet joints of the surgical segment increased with increasing Zero-P height. The range of maximum stress on the posterior facet joints for the M3 model was situated between that of the M2 and M4 models.

In summary, after determining the appropriate height range for the implant in accordance with the mean height of the intervertebral space, opting for a larger size appears to be more advantageous. This approach helps maintain the height of the intervertebral space and provides greater stress, promoting a tighter fit between the upper and lower endplates and the Zero-P. This tighter fit is crucial for maintaining spinal stability, enhancing the early bony fusion rate, and potentially leading to better postoperative outcomes.