In-vivo study of three-dimensional, cervical segmental motion characteristics following anterior cervical fusion with different numbers of fusion levels

Background: The pathogenesis of adjacent segment degeneration (ASD) and the biomechanical influence following anterior cervical fusion (ACF) surgery with different number of fusion levels remains controversial. The objective of this study was to investigate the effect of different numbers of fusion levels on adjacent segments and overall cervical kinematics. Methods: From October 2018 to August 2020, patients who underwent ACF at our department were screened for enrollment in the study and divided into three groups based on the number of fusion levels: one-, two- and three-level. All subjects underwent pre- and post-operative cervical spine cone beam computed tomography(CBCT)scans at 7 functional positions. The range of motion (ROM) of the superior and inferior adjacent segments as well as the overall cervical, were measured using a 3D-3D registration technique. Changes in the pre- and post-operative ROMs were calculated for each patient. Clinical outcomes were assessed using the cervical spine stiffness scale, VAS and the JOA Scores. Results: At the superior and inferior adjacent segment, the ROM of three-level were all greater than one-level during left-right bending and flexion-extension (p＜0.05). The global ROM of three-level were significantly smaller than one-level during flexion-extension (p＜0.05). The postoperative ROM of C4-C5 segment was significantly greater than that of C2-C3 (p＜0.05) at flexion-extension. The post-operative VAS、JOA scores showed no significant differences between the three groups(p > 0.05). But the post-operative cervical spine stiffness scores of the three-level was higher than that of the one-level (p＜0.05). Conclusion: Three-level ACF had increased compensatory motion at the superior or inferior adjacent segments than one-level group at last follow-up and were probably to develop adjacent segment degeneration, which is more common in C4-C5 segment. Compared with the one-level fusion, the global ROM of the three-level decreased slightly and had little effect on the postoperative function of cervical spine.