Changes in the Centers of Rotation in Different Prosthetic Segments Following Continuous Two-Level Cervical Disc Replacement: A Retrospective Study.

OBJECTIVE:To observe the short to medium-term therapeutic effects and radiological outcomes of adjacent two-level cervical spondylosis treated with artificial cervical disc replacement (ACDR) using Mobi-C and Bryan discs. To observe changes and discrepancies in the flexion-extension centers of rotation (FE-COR) of different ACDR segments. METHODS:Twenty-four patients undergoing 2-level (C4/5 and C5/6) ACDR were analyzed retrospectively (11 patients in the Mobi-C group and 13 patinents in the Bryan group). Clinical outcomes were assessed using modified Japanese Orthopedic Association (mJOA) score, Neck Disability Index (NDI), and visual analog scale (VAS). Preoperative and 2 years' follow-up radiographs were collected to compare total cervical spine curvature (C2-C7) range of motion (ROM), upper and lower ACDR segmental ROM, and the operated adjacent segmental ROM. The FE-COR of the ACDR segment was measured using the mid-plumb line method. The degree of disc degeneration in the adjacent segment were observed. RESULTS:At the 2 years' follow-up, in both group the mJOA score increased significantly, and the NDI, neck and upper extremity VAS scores decreased significantly compared with pre-operative (P<0.05). There were no significant statistical differences in post-operative scores between the two groups (P>0.05). The overall cervical ROM, the upper and lower segmental ROM at the 2 years' follow-up showed no significant statistical differences compared with the pre-operative period (P>0.05). There was no statistically significant difference in the adjacent segmental ROMs compared with the pre-operative period (P>0.05). Furthermore, the statistical analysis revealed no significant differences in the measurements of the ROM at each time points between the Mobi-C Group and the Bryan Group (P>0.05). There was no significant difference in the pre-operative FE-COR-X and FE-COR-Y of upper or lower ACDR segments between the two groups (P>0.05). At the 2 years' follow-up, there were significant differences both in FE-COR-X and FE-COR-Y between the two groups (P<0.05). For the Mobi-C group, in both the upper and lower segment, the FE-COR-X significantly increased compared with pre-operative (P<0.05), while the FE-COR-Y decreased compared with pre-operative (P<0.05). For the Bryan group, no significant changes were observed in the upper or lower segment in both FE-COR-X and FE-COR-Y compared with pre-operative (P>0.05). Fourty-eight adjacent segments (24 superior and 24 inferior segments) were included in the studies of ASD. Four segments showed imaging ASD (4/48, 8.33%) in 4 patients, of which 2 were mild, and 2 were moderate according to the grading criteria. Among them 2 were from the Mobi-C group and 2 were from the Brayn group. No severe imaging degeneration was observed. CONCLUSION:In continuous two-level cervical ACDR surgery, both Mobi-C and Bryan artificial cervical discs achieved satisfactory clinical outcomes in the short to medium term postoperatively. The FE-COR exhibited different trends of change. In the Mobi-C group, the FE-COR for both upper and lower segments shifted anteriorly and inferiorly, whereas in the Bryan group, whether upper or lower, the FE-COR remained closer to the pre-operative state. The changes in FE-COR did not significantly affect the short to medium term clinical outcomes postoperatively.