Sequential sagittal alignment changes in the cervical spine after occipitocervical fusion

Figure 1 Representation of the radiographic measurements. O-C2a: The angle between the inferior endplate of C2 and the McGregor line; O-EAa: The angle formed by the McGregor line and the EA-line; C2Ta: The angle formed by the inferior endplate of C2 and the EA-line; C2-7a: The Cobb angle between the lower endplate of C2 and C7; T1 slope: The angle between the horizontal and the T1 superior endplate; C2-7 SVA: The horizontal distance between the C2 plumb line and the posterior corner of C7; PIA: The angle between McGregor line and the line that links the center of the C1 anterior arch and the apex of cervical sagittal curvature. O-C2a: O-C2 angle; O-EAa: Occipital and external acoustic meatus to axis angle; C2Ta: C2 tilting angle; C2-7a: C2-7 angle; SVA: Sagittal vertical axis; PIA: Pharyngeal inlet angle.