The presence of cervical spine instability with respect to preoperative and postoperative changes in angular, horizontal, and rotational displacement of the vertebral body were studied. With the anterior approach, the instability in the remaining unfused segments, and their relation to the kyphotic or lordotic fused segment were studied. With the posterior approach, postoperative ROM (range of motion) could be better maintained, and horizontal displacement was improved in more cases by laminoplasty compared with laminectomy. With the anterior approach, the compensatory function for the loss of motion of the segments resulting from fusion was most remarkable at the levels of C2-3 and C6-7. In the alignment of the anterior fused segments, it appears important that the physiologic lordotic position be maintained.