Fig. 3
Vertical atlanto-axial subluxation, measurement methods. (a) Lateral normal radiograph in neutral position showing the location of McGregor’s line (black) between the postero-superior aspect of the hard palate and the most caudal point of the occipital curve. Migration of the tip of the dens >4.5 mm above McGregor’s line indicates vertical subluxation. The distance indicated by the white line between McGregor’s line and the midpoint of the inferior margin of the body of axis is used to evaluate vertical subluxation according to Redlund-Johnell and Pettersson’s method. A distance less than 34 mm in men and 29 mm in women indicates vertical subluxation. (b) Sagittal CT reconstruction of a normal cervical spine showing the location of McRae’s line corresponding to the occipital foramen and the division of the axis into three equal portions used by Clark’s method for diagnosing vertical subluxation. If the anterior arc of the atlas is in level with the middle or caudal third of the axis there is slight and pronounced vertical subluxation, respectively. (c) Ranawat’s method includes determination of the distance between the centre of the second cervical pedicle and the transverse axis of the atlas. A distance less than 15 mm in males and 13 mm in females indicates vertical subluxation [4]