The number of fusion levels as a potential factor influencing long-term complications of anterior controllable antedisplacement fusion: a biomechanical analysis.

BACKGROUND

Anterior controllable antedisplacement fusion (ACAF) is widely used for cervical ossification of the posterior longitudinal ligament, but long-term complications, such as adjacent segment degeneration (ASD), pseudarthrosis, cage subsidence, and implant failure, remain nonnegligible. This study aimed to explore the influence of the number of fusion levels (NFL) on these complications through finite element (FE) analysis, providing a biomechanical basis for optimizing surgical strategies for ACAF.

METHODS

Three FE ACAF models (two-level, three-level, and four-level) were established on the basis of a validated C2-T1 cervical spine model. A hybrid loading protocol with a 75 N follower load and physiological moments was applied to simulate physiological motions. Key parameters, including the range of motion (ROM) of the surgical and adjacent segments, disc stress, facet joint force (FJF), endplate stress, and the plate, screw, and screw-bone interface stresses, were compared among the three models.

RESULTS

An increase in the NFL led to significant increases in the ROM, disc stress, and FJF of adjacent segments, with the upper adjacent segment showing more prominent changes than the lower segment. The ROM of the surgical segment gradually increased with increasing NFL, and the fusion space micromotion correspondingly increased. Endplate stress and implant-related stresses (plate, screw, and screw-bone interface stresses) all tended to increase steadily with increasing NFL, reflecting a continuous increase in the mechanical load at the surgical site and in the adjacent segments.

CONCLUSIONS

The NFL is a potential risk factor for long-term complications of ACAF. An increase in the NFL raises the mechanical load in the surgical and adjacent segments, thereby potentially increasing the risks of ASD, pseudarthrosis, cage subsidence, and implant failure.