Fig. 3From: A novel method for measurement of the occipital-cervical distance via the occiput-C4 distanceThe vertebral body of C4, an apex most visible on radiographs and least affected by the cervical curve, was designated as a landmark. The OC4D can be regarded as the radius of a circle positioned at the center of the C4 vertebral body and tangent to the McGregor’s lineBack to article page