Instantaneous Center of Rotation and Instability of the Cervical Spine: A Clinical Study

Study Design. This study was designed to investigate flexion and extension of the cervical spine in patients with "instability" of the cervical spine. Objectives. To establish a new method of calculating the instantaneous center of rotation (ICR) of the skull relative to the thorax and to compare the differences in the ICR between normal volunteers and patients. Summary of Background Data. "Instability" of the cervical spine commonly is investigated using functional radiographs, which is an unreliable method. Although the ICR was suggested as an objective parameter in measuring spinal "instability," the ICR of the skull relative to the thorax has not been studied before. Method. Three groups of subjects (27 normal volunteers, 28 patients with chronic cervical spondylosis, and 17 patients undergoing fusion for cervical disc degeneration) were measured with a CA‐6000 system (OSI, CA). The ICR then were calculated and compared at various angles of flexion and extension. Results. The horizontal component of the ICR in the preoperative group was found to have shifted anteriorly from 5° to 25° of flexion (P < 0.05) and at 5° of extension (P < 0.05), compared with that of the healthy subjects. There also was a significant difference in the vertical component of the ICR at 15° of flexion (P < 0.05). However, there were no significant differences in the horizontal and the vertical components of the ICR between the normal and nonoperative groups. Conclusion. The ICR in patients with "instability" of the cervical spine was found to have shifted anteriorly; the method described in this study might be useful for identifying patients who suffer from this condition.