INSTANTANEOUS CENTER OF ROTATION AND INSTABILITY OF THE CERVICAL-SPINE - A CLINICAL-STUDY

Study Design. This study was designed to investigate flexion and exten sion of the cervical spine in patients with ''instability'' of the cer vical spine. Objectives. To establish a new method of calculating the instantaneous center of rotation (ICR) of the skull relative to the th orax and to compare the differences in the ICR between normal voluntee rs and patients. Summary of Background Data. ''instability'' of the ce rvical spine commonly is investigated using functional radiographs, wh ich is an unreliable method. Although the ICR was suggested as an obje ctive parameter in measuring spinal ''instability,'' the ICR of the sk ull relative to the thorax has not been studied before. Method. Three groups of subjects (27 normal volunteers, 28 patients with chronic cer vical 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 ext ension. Results. The horizontal component of the ICR in the preoperati ve group was found to have shifted anteriorly from 5 degrees to 25 deg rees of flexion (P < 0.05) and at 5 degrees of extension (P < 0.05), c ompared with that of the healthy subjects. There also was a significan t difference in the vertical component of the ICR at 15 degrees of fle xion (P < 0.05). However, there were no significant differences in the horizontal and the vertical components of the ICR bet tween the norma l and nonoperative groups. Conclusion. The ICR in patients with ''inst ability'' of the cervical spine was found to have shifted anteriorly; the method described in this study might be useful for identifying pat ients who suffer from this condition.