Occipitocervical Spine Approaches for Decompression

Dorsal Surgical Anatomy of the Occipitocervical Region

Dorsal approaches to the OC area are most commonly used for OC fusions. During the approach, dissection through several muscular layers is required. The trapezius muscle constitutes the first superficial layer. The trapezius arises from the external occipital protuberance, the ligamentum nuchae, and the spines of the seventh cervical and all thoracic vertebrae. The upper fibers insert into the lateral third of the clavicle and form the curve of the shoulder. The middle fibers insert into the medial edge of the acromion and the superior margin of the spine of the scapula, and the lower fibers insert onto the spine of the scapula ( Fig. 101.1 ). Acting together, the upper and lower fibers rotate the scapula so that the glenoid cavity is turned upward in the movement of raising the arm above the head. The nerve supply of the trapezius muscle is the spinal accessory nerve (cranial nerve XI).

Moving deeper, the next muscle layer consists of a few muscle groups. Laterally, the levator scapulae originates from the transverse processes of the upper four cervical vertebrae. The muscle inserts onto the medial border of the scapula. The levator scapulae is commonly innervated by a combination of the ventral rami of the third, fourth, and fifth cervical nerves, as well as the dorsal scapular nerve. More medially, the splenius muscle originates from the lower aspect of the ligamentum nuchae and the spines of the seventh cervical and upper six thoracic vertebrae. Its fibers course rostrally, and it is divided into cervical and cranial components. The splenius cervicis is the lateral component, which inserts into the transverse processes of the upper three cervical vertebrae, deep to the levator scapulae muscle; whereas the splenius capitis muscle inserts on the lower aspect of the mastoid process and the lateral aspect of the occipital bone. Its nerve supply is the dorsal rami of the cervical nerves.

Beneath the levator scapulae and the splenius lie the deep muscles of the posterior neck. These include the semispinalis capitus, recuts capitis posterior major and minor, obliquus capitis superior and inferior, and the cervical components of the erector spinae muscle. The recuts capitis posterior major, along with the obliquus capitis superior and inferior, forms the suboccipital triangle. This triangle is a landmark for identifying the V3 segment of the vertebral artery. The erector spinae muscle is composed of three main columns (from lateral to medial): the iliocostalis, longissimus, and spinalis muscles. The only muscle from this group that significantly impacts the OC spine is the longissimus capitis muscle. This is a long muscle that lies under the splenius muscle immediately dorsal to the transverse processes. It arises from the transverse processes of the upper four thoracic vertebrae and passes upward to be inserted into the back of the mastoid process. A recent cadaveric study found that using the subatlantic triangle (levator scapulae and splecius cervicis inferolaterally, longissimus capitis medially, and inferior oblique capitis superiorly) provides a consistent and straightforward way to identify and expose the atlantoaxial vertebral artery. The ligamentum nuchae is a strong fibrous substance, which is the median between the muscles of the two sides. It is considered a continuation of the superior spinous and interspinous ligaments from the spine of the seventh cervical vertebra through the external occipital protuberans.

The main vessels in the dorsal OC area are the occipital artery and the vertebral artery. The occipital artery arises from the external carotid artery in the front of the neck, runs dorsally and rostrally deep to the mastoid process, and then courses dorsally immediately deep to the muscles attached to the superior nuchal line. It then pierces the trapezius muscle 2.5 cm from the midline to ramify on the back of the head (see Fig. 101.1 ). As for the vertebral artery, only the third part of this artery is significant during the approach. It emerges from the foramen and the transverse process of the atlas and hooks dorsomedially around the dorsal surface of the lateral mass of the atlas (see Fig. 101.1 ). It is partly separated from the arch of the atlas by the first cervical nerve ( Fig. 101.2 ; see also Fig. 101.1 ). It then passes ventromedially in front of the thickened lateral edge of the dorsal atlantooccipital membrane, which forms an arch over the artery. Occasionally, this arch may be ossified and is referred to as the ponticulus posticus . This condition must be recognized preoperatively, because failure to do so can lead to catastrophic results if the lateral mass C1 screws are placed through the vertebral arteries. The artery then pierces the dura mater and enters the vertebral canal. The suboccipital plexus of veins is a network of veins that drains into the deep cervical vein and into the vertebral venous plexus around the vertebral artery. The greater occipital nerve is the medial branch of the dorsal ramus of the second cervical nerve, which is the thickest cutaneous nerve in the body. It appears at the middle of the lower border of the inferior oblique muscle and curves superior medially across the suboccipital triangle. It runs rostrally on that muscle and then pierces the trapezius muscle about 2 cm lateral to the occipital protuberans (see Fig. 101.1 ).

Ventral Anatomy of the Occipitocervical Junction

Three muscles originate from the ventral aspect of the atlas: longus colli, rectus capitis anterior, and rectus capitis lateralis ( Fig. 101.3 ):

• 1.

  The longus colli muscle is the longest and most medial of the muscles. It extends from the anterior tubercle of the atlas to the lower part of the body of the upper thoracic vertebrae. Between these points it is attached to all the vertebral bodies and into the third to sixth cervical transverse processes.

• 2.

  The rectus capitis anterior is a short, wide muscle that originates from the ventral surface of the lateral mass of the atlas and is inserted into the base of the skull ventral to the occipital condyle.

• 3.

  The rectus capitis lateralis is a short muscle that runs vertically between the rostral surface of the transverse process of the atlas and jugular process of the occipital bone. It lies dorsal to the jugular foramen and is separated from the rectus capitis anterior by the ventral ramus of the first cervical nerve, which supplies both muscles. The function of these muscles is to stabilize the skull on the vertebral column (see Fig. 101.3 ).

Ventral to the prevertebral muscles is the retropharyngeal space. The anterior tubercle of the atlas may be palpated through the dorsal pharynx during a transoral approach. There are many other muscles in the neck that play a role in head and neck movement and can be encountered in larger anterior exposures (sternocleidomastoid, omohyoid, among many others); however, these are less critical for occipitocervical exposures and will be covered in subaxial spine approaches.