Interpretation of Sagittal Plane Intervertebral Motion Abnormalities Using the Center of Rotation in the Cervical Spine.

STUDY DESIGN: Retrospective analyses of the center of rotation (COR) in asymptomatic volunteers and symptomatic patients. OBJECTIVE: Document (i) level-specific COR position at radiographically normal levels in asymptomatic volunteers, (ii) preoperative COR abnormalities in symptomatic patients, and (iii) correlations between COR and other intervertebral motion metrics. SUMMARY OF BACKGROUND DATA: The COR has been discussed in many prior publications; however, our understanding of the normal position of the COR, the types and prevalence of COR abnormalities in patients, and the association between COR and other more common measures of intervertebral motion remains limited. METHODS: Using automated, FDA-cleared software, we quantified COR and standard motion/alignment metrics from cervical flexion-extension radiographs of 341 asymptomatic volunteers and 1263 symptomatic preoperative patients. Level-specific 95% reference ellipses were derived from volunteers. Abnormality was defined as COR outside the level-matched ellipse. Associations between COR (cranial-caudal and anteroposterior) and rotation, translation, sagittal plane offset, disc height change, and standardized instability indices were evaluated. RESULTS: In volunteers, COR was level-dependent and well summarized by 95% reference ellipses. The cranial-caudal position of COR was chiefly determined by rotation and translation, whereas the anteroposterior position related to sagittal plane offset and disc height change. COR also correlated with translational (TI-Index) and vertical (AVI/PVI) instability metrics. In symptomatic patients, most levels fell within reference ellipses; when deviations occurred, they were infrequent and multidirectional. CONCLUSIONS: COR can be automatically derived from flexion-extension radiographs. Deviations from level-specific reference ellipses provide an intuitive visualization of abnormal kinematics, complementing established motion metrics and linking cranial-caudal and anteroposterior shifts to specific patterns of translation, alignment, and instability.