الفهرس 
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Abstract
The vertebral artery (VA) is the first and the largest branch of first part of the subclavian artery. The course of the vertebral artery is divided into four parts. The third part (suboccipital) (V3) extends from the foramen transversarium (FT) of atlas to the foramen magnum. It is located in the suboccipital triangle on the superior surface of the posterior arch of atlas.
Surgical approaches to the posterolateral craniovertebral junction entail working in close proximity to the V3 segment of the VA. A detailed knowledge of VA anatomy, the normal variants of the VA, and its relationship to nearby osseous structures is critical when exposing the lateral craniovertebral junction, when performing procedures requiring exposure of the V3 segment, and when placing C-2 spinal instrumentation.
The aim of the present study was to study the course and relations of the third part of the vertebral artery to avoid its injury during surgical procedures.
The present study was conducted on twenty halves of formalin preserved cadaveric head and neck specimens (10 rights and 10 left), Thirty atlas vertebra and Twenty adult patients evaluated by craniocervical CT angiography for other causes rather than vertebral artery abnormalities.
Cadaveric part of study included measuring the length, diameter of the third part of the vertebral artery and the distance between the artery and superior cervical sympathetic ganglion.
Osseus part of the study included measurement related to the vertebral artery groove over the posterior arch of atlas, width, depth of foramen transversarium, pars interaricularis, detection of presence of bony spicules.
Radiological part of the study included measuring the diameter, width and depth of the vertebral artery curve over the arch of the atlas were measured.
The present study showed that no significant differences between right and left vertebral arteries but left vertebral artery dominace was shown in only one specimen.
The second part of the vertebral artery passes vertically from axis and then through foramen transversarium of C1, The third part of the vertebral artery was found to pass horizontally over the posterior arch of atlas accompanied with dorsal ramus of C1 in nineteen cadvaeric specimens.
The third part of the vertebral artery passes under the posterior arch of atlas, then pierces the dura and passes through foramen magnum in only one specimen.
Vertebral artery groove was converted into a complete foramen in two cadaveric specimens.
The present study showed that surgical exposure of the posterior arch of the atlas should remains within 5 mm from the midline to avoid injury of the vertebral artery.