Hybrid Constructs for Performing Three-level Hybrid Surgery: A Finite Element Study

Abstract

To systematically investigate the effect of 3-level hybrid constructs on the cervical spine biomechanics based on a validated model of the C3-C7 segments. Three hybrid constructs with 2 U-shaped dynamic cervical implants and 1 cage were simulated. The 3 constructs were 1) Cage-U-U (cage implanted at the C3-C4 level and U-shaped dynamic cervical implants implanted at the C4-C5 and C5-C6 levels), 2) U-Cage-U, and 3) U-U-Cage. Biomechanical parameters including moments, cervical motions, and stresses in the facet and implants were analyzed in flexion and extension. The flexion and extension motions at artificial cervical disc replacement levels increased for all hybrid constructs when compared with those of intact model. However, the maximum increase was 52% with U-U-Cage model. At the unoperated adjacent level, the maximum motion increase in extension was 23% with the U-U-Cage model. Also, the U-U-Cage and U-Cage-U model generated more than 40% increase in terms of flexion motion at the adjacent level. The facet stress at the adjacent level increased by 28%, 20%, and 39% with the Cage-U-U, U-Cage-U, and U-U-Cage models, respectively. The moments required to reach the same motion as the intact model were significantly increased. The study showed that the U-U-Cage model lead to more compensation in terms of motion and facet stress. Furthermore, the present results imply that when conducting the hybrid surgery, the segmental motions should be taken into account. Performing anterior cervical discectomy and fusion at the level whose motion is relatively small may decrease the compensation required at the adjacent level.