Fig. 6.

a Position of the center of rotation (COR) during flexion–extension neck motion of normals at the level of the CDR and at the supra-adjacent level (green) in comparison with the position of the COR in patients with a CDR at their level of the prothesis (red) and the supra-adjacent level (blue). A positive COR-X value means the COR is anterior to the mid-point of the vertebral body endplate in the sagittal plane. A positive COR-Y value means the COR is below the endplate. b Visualization of the COR superimposed on a clinical example. Position of the center of rotation (COR) during flexion–extension neck motion of cervical normals (red ellipses) versus patients implanted with the CDR (yellow ellipses). Normal COR data are from Hipp and Wharton, 2008 [15]. The red ellipses represent the 95 % confidence interval for an asymptomatic, radiographically normal population. The cross-hairs represent the mean location of the COR. Data are reported on a level-specific basis (n = 125 at C4–5; n = 121 at C5–6; n = 76 at C6–7). The yellow ellipses represent the 95 % confidence interval for the CDR patients. All CDR-level data (n = 17 implantations at C4–5 through C6–7) are pooled and overlaid on the C5–6 level in the figure. All supra-adjacent data (n = 18, C3–4 through C5–6) are pooled and overlaid on C4–5. All caudal-adjacent data (n = 10, C5–6 and C6–7) are pooled and overlaid on C6–7. (Data are pooled due to low N. Ideally, if N were larger, data would be reported on a level-specific basis in the same manner as the normative reference data in red.). There is near-perfect overlap of the COR data at the superior and inferior adjacent (untreated) levels. Roughly 50 % of subjects have a COR-X that is shifted anteriorly outside of normal limits (as defined by the 95 % confidence interval for an asymptomatic population)