Effects of a cervical disc prosthesis on segmental and cervical spine alignment.

OBJECT: Cervical arthroplasty offers the promise of maintaining motion of the functional spinal unit (FSU) after anterior cervical discectomy. The impact of cervical arthroplasty on sagittal alignment of the FSU needs to be addressed, together with its effect on overall sagittal balance of the cervical spine. METHODS: The authors prospectively reviewed radiographic and clinical outcomes in 14 patients who received the Bryan Cervical Disc prosthesis (Medtronic Sofamor Danek, Memphis, TN), for whom early (< 6 months) and late (6-24 months) follow-up data were available. Static and dynamic radiographs were measured by hand and computer to determine the angles formed by the endplates of the natural disc preoperatively, those formed by the shells of the implanted prosthesis, the angle of the FSU, and the C2-7 Cobb angle. The range of motion (ROM) was also determined radiographically, whereas clinical outcomes were assessed using the Neck Disability Index (NDI), and Short Form-36 (SF-36) questionnaires. The ROM was preserved following surgery, with a mean preoperative sagittal rotation angle of 8.96 degrees , which was not significantly different from the late postoperative value of 8.25 degrees . When compared with the preoperative disc space angle, the shell endplate angle in the neutral position became kyphotic in the early and late postoperative periods (mean change -3.8 degrees in the late follow-up period; p = 0.0035). The FSU angles also became significantly more kyphotic postoperatively, with a mean change of -6 degrees (p = 0.0006). The Cobb angles varied widely preoperatively and did not change significantly after surgery. There was no statistical correlation between the NDI and SF-36 outcomes and cervical kyphosis. CONCLUSIONS: Cervical arthroplasty preserves motion of the FSU. Both the endplate angle of the treated disc space and the angle of the FSU became kyphotic after insertion of the Bryan prosthesis. The overall sagittal balance of the cervical spine, however, was preserved.