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A method for measuring occipitocervical angle.To develop a new method of measurement for assessing the occipitocervical angle using intraoperative fluoroscopic imaging, and to examine its reliability.To avoid postoperative complications following occipitocervical fusion, it is vital to obtain a suitable fusion angle between the occipital bone and the upper cervical spine.The subjects were 30 cases with occipito-atlanto-axial lesions and 30 healthy volunteers. Lateral plain radiographs of the cervical spine in neutral position were used to draw the McGregor line, the line between the external occipital protuberance and the most caudal point on the midline occipital curve (Oc line), the tangential line of the inferior endplate of the C2 vertebra (C2 line), and the posterior longitudinal line of the C2 vertebra (Ax line). The angles formed by these 4 lines and the horizontal line were measured. The O-C2 angle and the Oc-Ax angle, the new indicator, were measured by 3 doctors and reliability was evaluated.In the disease group, mean intraobserver variances of the McGregor, Oc, C2, Ax, O-C2, and Oc-Ax angles were 0.7, 1.3, 1.5, 1.2, 1.6, and 1.9 degrees. Mean intraobserver intraclass correlation coefficients were 0.997, 0.994, 0.994, 0.997, 0.989, and 0.988, showing high intraobserver reliability for all angles. Mean interobserver intraclass correlation coefficients were 0.998, 0.996, 0.994, 0.997, 0.988, and 0.990, showing high interobserver reliability for all angles. The same reliability was obtained in the healthy group.The Oc-Ax angle is as reliable an indicator as the conventional O-C2 angle, and could be used as a new intraoperative indicator for occipitocervical fusion. It may be particularly useful in cases where it is difficult to identify the McGregor line and/or the inferior endplate of the C2 vertebra.Level 3—diagnostic study.