Erosion is caused by pulsatile flow as the vertebral artery passes through the foramina transversaria of the upper six cervical vertebrae between its origin from the subclavian artery and its entrance into the cranial vault through the foramen magnum.
Tortuous vertebral artery. (A) Frontal tomogram shows the enlarged foramen (arrows). (B) Arteriogram shows the tortuous vertebral artery (arrow) entering the enlarged foramen.