Purpose of the study: The theoretical usefulness of a disc prosthesis in comparison with arthrodesis would be to restore physiological segmental motion without perturbing the kinematics of the adjacent levels. The purpose of this study was to determine the rotation centers of the lumbar segments before and after implantation of a disc prosthesis with a mobile insert (Mobidisc™).
Material and methods: Lateral flexion and extension views in the sitting position with a stabilized pelvis were obtained before and after implantation of the lumbar disc prosthesis in 32 patients. Spineview™ was applied to the digitalized images for semi-automatic recognition of the vertebral body contours and calculation of the rotation centers. The detection threshold for this automatic system was 5° motion.
Results: Rotation centers were difficult to determine preoperatively because of the absence of mobility. A pathological position was found for three patients. Postoperatively, at three and twelve months, the position was «physiological» in 13 patients, in the posterior half of the disc or inferior body near the vertebral end plate. IN 14 patients, the center could not be determined because motion measured 5° or less. For three patients, the center was too anterior on a prosthesis implanted to anteriorly. There were no changes in the rotation centers for the adjacent levels.
Discussion: Demonstration of an abnormal rotation center could be an additional indication of presumed instability. In certain cases, a disc prosthesis appears to restore the physiological rotation center. But the position and the thickness of the implant can influence their localization.
Conclusion: Restoration of a physiological rotation center for the instrumented intervertebral segment and the absence of change in the rotation centers for the adjacent centers are arguments in favor of disc prosthesis for reducing the incidence of osteoarthritic degradation of adjacent discs in comparison with fusion, under the condition that the implantation and the size are correctly adapted.
