Index and Adjacent Level Kinematics After Cervical Disc Replacement and Anterior Fusion

STUDY DESIGN: Prospective, randomized trial. OBJECTIVE: To assess the in vivo kinematics of the cervical spine after cervical fusion and arthroplasty. SUMMARY OF BACKGROUND DATA: Compared with spinal fusion, disc replacement may impede the development of adjacent segment disease. To achieve this goal, any arthroplasty device should achieve desired spinal alignment while maintaining physiologic spinal motion at both the operated and surrounding motion segments. METHODS: As part of a multicenter, prospective, randomized Food and Drug Administration IDE clinical evaluation of the porous coated motion artificial cervical disc, patients underwent either a single-level total disc replacement (TDR; 272 patients) or anterior cervical discectomy and fusion (ACDF; 182 patients) for treatment of cervical radiculopathy or myelopathy. Neutral, flexion, and extension radiographs of the cervical spine obtained before surgery, and at 3, 6, and 12 months after surgery were assessed. Quantitative assessments and comparisons of motion patterns were produced using validated computer-assisted methods. Kinematic parameters, including segmental rotation, translation, center of rotation (COR), disc height, and disc angle were calculated. RESULTS: Cervical TDR preserved angular motion at the operated level, although the range was reduced from 8.0° before surgery to 6.2° at 12 months after surgery (P < 0.001). Significantly after TDR, adjacent-level angular motion was unchanged. In patients treated with fusion, angular motion at the superior adjacent level increased from 9.6° before surgery to 11.0° (P = 0.003) at 12 months, with a trend toward increased postoperative translation (P = 0.07). For the TDR group, the horizontal COR averaged 0.8 mm posterior to the disc center before surgery and 0.2 mm anterior to the center at 12 months after TDR (P < 0.001), and the vertical COR averaged 2.5 mm below the endplate before surgery and 4.0 mm below at 12 months (P = 0.001). COR at the adjacent levels was unaltered by fusion or TDR. Lordotic alignment and disc height at the index level increased after intervention in both groups. CONCLUSION: TDR with the porous coated motion implant is able to restore and maintain lordotic alignment and disc height and maintain angular motion while allowing for similar translation to that seen before surgery. In contrast, after ACDF, the superior adjacent level developed increased angular motion compared with preoperative range of motion. This study provides in vivo data regarding the functioning of TDR and ACDF and their impact on adjacent-level kinematics.