Surgical Anatomy of the Vertebral Artery

The vertebral artery is divided into four segments. V₁ runs from the origin at the subclavian artery to the transverse process of C6. V₂ lies between the transverse foramen from C6 to C2. V₃ is from the transverse foramen of C2 to the dural entrance at the foramen magnum. V₄ is the intradural part ( Fig. 17.1 ).

Understanding the microanatomy of V₃ is especially essential for surgical approaches to lesions at the craniovertebral junction (CVJ). After passing through the transverse foramen of C2, V₃ runs upward and passes through the C1 transverse foramen (vertical portion), then turns medially to traverse above the lateral aspect of the posterior arch of C1. It enters the vertebral canal by passing below the lower, arched border of the posterior atlanto-occipital membrane (horizontal portion) and finally goes obliquely upward and pierces the dura mater at the foramen magnum (oblique portion). All along V₃, the vertebral artery is surrounded by the vertebral venous plexus and a periosteal sheath covering the plexus ( Fig. 17.2 ). At the dural penetration of the vertebral artery, the periosteal sheath is in continuity with the outer layer of the dura and also tightly adherent to the adventitia of the artery, making a sort of distal fibrous ring. The adventitia of the vertebral artery is fixed at only two segments; one is at this part, and the other is at the transverse process of C6. This anatomical characteristic prevents injury to the vertebral artery from various neck movements.

Muscular branches originate from the horizontal portion of V₃ to supply the deep paravertebral muscles. Some of them may need to be divided to expose the entire course of V₃ or to mobilize and transpose the segment. The posterior meningeal artery also originates at V₃ just before dural penetration. In rare cases, the posteroinferior cerebellar artery has an extradural origin. This may be evaluated preoperatively to avoid an ischemic complication7,8 ( Fig. 17.2 ).

Surgical Anatomy of the Suboccipital and Posterior Paravertebral Muscles

Although individual muscles need not be dissected separately at surgery, precise anatomical knowledge of all sub-occipital and posterior paravertebral muscles, including their origin and insertion, is indispensable for operative exposure and management of the vertebral artery.1–10 The suboccipital triangle is the most important surgical landmark for exposure of V₃. This triangle comprises three deep muscles: the superior oblique, rectus capitis posterior major, and inferior oblique. The superior oblique muscle originates at the inferior nuchal line and inserts on the transverse process of C1, the rectus capitis posterior major muscle originates from the inferior nuchal line and inserts on the spinous process of C2, and the inferior oblique muscle connects the transverse process of C1 to the spinous process of C2. The floor of the triangle is formed by the posterior atlanto-occipital membrane and the posterior arch of C1. In the triangle, the horizontal portion of V₃ and the C1 nerve root lie in a groove on the upper surface of the lateral part of the posterior arch of C1 ( Fig. 17.3a ). Understanding of the anatomy of the deep muscles inserting on the C1 transverse process is also essential when the vertebral artery is exposed from the anterolateral or lateral direction. The rectus capitis lateralis muscle running from the jugular process to the C1 transverse process serves as a natural barrier to the internal jugular vein located just in front of the C1 transverse process. The splenius cervicis and levator scapulae muscles are landmarks for the vertical portion of V₃ and the C2 ventral root7,8 ( Fig. 17.3b ).