Study Design: Kinematic study of a single site in an investigational device exemption trial. Objective: Evaluate the center of rotation (COR) after Bryan cervical arthroplasty and compare adjacent segment motion after cervical disk arthroplasty and fusion using validated radiographic analysis. Summary of Background Data: The goal of cervical arthroplasty is to reestablish spinal kinematics after anterior decompression. Excellent maintenance of range of motion has been reported for a variety of the prostheses; however, the manner the prostheses perform this task is different. A parameter that may be as important as range of motion is restoring the quality of motion. One of the important components is the COR that is easily studied biomechanically but has not been reported from in vivo studies. Furthermore, the effects on the quality of motion at adjacent levels have not been studied. The purpose of this study is to determine the quality of motion after Bryan cervical disk arthroplasty at the target level and the adjacent segments. Methods: The first 48 patients diagnosed with single level cervical disk degenerative disease and associated myelopathy or radiculopathy from a single institution enrolled in the Bryan disk investigational device exemption trial were selected for inclusion. Twenty-two investigational patients and 26 anterior cervical discectomy and fusion controls were evaluated radio-graphically preoperatively and 3, 6, 12, and 24 months postoperatively. These results were analyzed using Quantitative Motion Analysis software manufactured by Medical Metrics Inc. Kinematic parameters included translation, sagittal rotation, anterior/posterior disk height, and the calculation of the COR both in the sagittal and coronal planes. Results: At the arthroplasty level, the COR shifted more posterior (0.3 mm, 1% end plate width) and cephalad (4.9 mm, 20% end plate width) compared with the preoperative position, however, this change was not statistically significant (P = 0.06). The variability of the COR, however, was less after arthroplasty compared with preoperative values. There was no significant difference in the short term between the adjacent levels after fusion compared with the prosthesis. At later time points (12 and 24 mo), however, the COR was significantly posterior at the level above arthroplasty compared with fusion (P < 0.01). COR X was not significantly (P > 0.3) different below fusions compared with arthroplasty. Sagittal rotation significantly increased at the level above for both the fusion and prosthesis groups. A trend was noted for increased translation (2.5% end plate width) at the level above a fusion compared with the Bryan disk at 24 months postoperatively, but this did not reach statistical significance. Conclusions: Sagittal rotation increases above the level of the arthroplasty and fusion. In the long term, the arthroplasty group had a more posterior COR at the level above, compared with the level above a fusion. Translation at the level above a fusion was slightly increased but not statistically compared with the level above the arthroplasty with similar amounts of sagittal rotation (flexion/extension). Although not reaching statistical significance, the COR seemed to shift more posterior and cephalad at the arthroplasty level with less variability compared with the preoperative position.
