Fig. 4.

Posterior macroscopic view (left panel) and corresponding frontal pQCT sectional image through the posterior arch of C1 at the vertebral artery groove (right panel) of a dissected specimen. D3 width of the posterior arch at the vertebral artery groove; D4 height of the posterior arch at the vertebral artery groove, D7 height of mid-portion of C1 lateral mass, D8 distance from the posterior tubercle to optimal screw insertion point, red dotted line (OIP, marked with a black asterisk); A5 medial inclination angle of the cranial facet of C1; A6 medial inclination angle of the caudal facet of C1