Applied Surgical Anatomy of the Posterior Approach to the Subaxial Cervical Spine‌

Overview

The muscles covering the posterior aspect of the cervical spine run longitudinally and are supplied segmentally. Although it is not critical to know the various individual posterior muscles of the cervical spine, being aware of these muscles and their layers is helpful. Because the approach itself is in the midline, it disturbs no vital structures and is relatively safe.

Landmarks and Incision

Landmarks

The spinous processes of the cervical spine, from C2 to C6, are bifid. C2 is the largest proximal cervical spinous process; the spinous processes of C3, C4, and C5 are relatively small. C7 is thicker, is not bifid, and has a tubercle at its end. Because it is the largest distal cervical spinous process, it is easy to palpate (see Fig. 6-63A).

All the spinous processes (except C7) are directed caudad and posteriorly, serving as points of attachment for the cervical muscles.

Incision

The skin on the back of the neck is thicker and less mobile than is the skin on the throat; it is attached directly to the underlying fascia. The incision runs perpendicular to the tension line of the skin, causing thicker scarring. Nevertheless, the wound usually heals well, and, because the nape of the neck is covered with hair, cosmetic concerns seldom are a problem.

Superficial Surgical Dissection

The ligamentum nuchae is a fibroelastic septum that takes origin from the occiput and inserts into the C7 spinous processes, sending septa down to each of the cervical spinous processes and dividing the more lateral paracervical muscles. The septum, which is almost vestigial in humans, is well developed in quadrupeds, because it helps the muscles support the head. It is the homologue of the supraspinous ligament in the rest of the spine. Dissection through it is safe, as long as it remains in the midline (see Fig. 6-63B).

The paracervical muscles in the cervical spine run in three layers. The most superficial layer consists of the trapezius muscle, which takes origin from the superior nuchal line and from all the spinous processes of the cervical spine. The trapezius covers the entire cervical area; in common with its counterpart in the lumbar spine, the latissimus dorsi, it is essentially an upper limb muscle (Fig. 6-59).

The intermediate layer is filled by the splenius capitis, a relatively large, flat muscle that takes origin from the midline (spine of C7, the lower half of the ligamentum nuchae, and the upper three thoracic spinous processes) and inserts into the occipital bone (Fig. 6-60).

The deep layer is subdivided into three portions: Superficial, middle, and deep. The superficial portion consists of the semispinalis capitis, a relatively large muscle that lies immediately beneath the splenius. The semispinalis capitis takes its origin from the transverse processes of the

cervical vertebrae and inserts into the occipital bone. The middle portion of the deep layer is filled by the semispinalis cervicis, which originates from the transverse processes of the upper five or six thoracic vertebrae and inserts into the midline spinous processes. The deepest portion of the deep layer consists of the multifidus muscles and the short and long rotator muscles (Fig. 6-61).

Figure 6-59 The superficial musculature of the cervical spine consists of the trapezius and the sternocleidomastoid muscles. Between these and deeper levels lies the intermediate layer, the splenius capitis.

Figure 6-60 The superficial layer has been resected to reveal the splenius capitis, which also is resected partially. Deep to this are the semispinalis capitis, the longissimus capitis, the splenius cervicis, and, most laterally, the levator scapulae.

The laminae of the cervical vertebrae are angled from medial to lateral at 45 degrees. Lateral to the laminae are the joint capsules, which completely surround the cervical facet joints. The facet joints are in a frontal plane (Figs. 6-63B and 6-64).

Unless the patient has a large spina bifida, the spinal canal is safe

during this phase of the dissection. A wide, flat instrument (such as a Cobb dissector) held transverse to the lamina helps to protect the canal (see Fig. 6-54).

Deep Surgical Dissection and Its Dangers

As it does elsewhere in the spine, the ligamentum flavum connects the lamina on one vertebra to the adjacent vertebra, filling the space between the two. The ligaments are paired, one on each side, and may be separated in the midline by a tiny space. They take origin from the leading edge of the lower lamina and insert proximally into small ridges on the anterior surface of the higher vertebra, about one-third up the anterior surface.

Figure 6-61 The semispinalis capitis has been resected to reveal the deepest layer, the semispinalis cervicis, and the multifidi muscles.

Each ligamentum flavum extends from the midline laterally to the joint capsule. The spinal cord is directly beneath the ligamentum flavum. Therefore, the ligament must be removed carefully, so that the coverings of the cord (the outer dura, the middle arachnoid, the inner pia) do not tear. The posterior longitudinal ligament lies on the posterior surface of the cervical vertebral bodies, within the vertebral canal, and extends down through the entire spinal canal. The ligament attaches to each vertebra and disc; it is broadest in the cervical region. Over the ligament, on the floor of

the canal, lie large vertebral veins, comprising a nonvalvular venous plexus. These may bleed and require cauterization.

The key vascular structure in the deep dissection is the vertebral artery. The artery runs upward in the neck through a series of foramina in the transverse processes of the cervical vertebrae. Vigorous decortication may breach the posterior walls of these foramina and damage the artery, which carries a blood supply that is vital to the hindbrain. The risks are far greater when the transverse processes are involved in pathology. If the artery is damaged during dissection, it should be packed to produce at least temporary control of bleeding. In most situations, this will also produce definitive control; occasionally, repair of the artery will be required (see Figs. 6-62 and 6-64).

Figure 6-62 The muscles of the suboccipital triangle of the neck consist of the rectus capitis posterior minor and major, and the obliquus capitis superior and inferior. Note the course of the vertebral artery on the superior border of the arch of C1. It is lateral to the midline. The course of the vertebral artery in the transverse foramen distal to C1 is anterior to the facet joints.

Rectus Capitis Posterior Major. Origin. Tendinous, from spinous process of axis. Insertion. Into lateral part of the area below the inferior nuchal line of occipital bone. Action. Extends head and rotates it to same side. Nerve supply. Nerve branch of posterior primary ramus of C1.

Rectus Capitis Posterior Minor. Origin. Tendinous, from tubercle of posterior arch of atlas. Insertion. Into medial part of nuchal line of occipital bone and surface beneath it, and foramen magnum (only muscle to take origin from posterior arch of C1). Action. Extends head. Nerve supply. A branch of posterior primary ramus of C1.

Obliquus Capitis Inferior. Origin. From apex of spinous process of axis. Insertion. Into inferoposterior part of transverse process of atlas. Action. Rotates atlas; turns head toward same side. Nerve supply. Branches of posterior primary ramusof C1.

Obliquus Capitis Superior. Origin. From tendinous fibers from upper surface of transverse process of atlas. Insertion. Into occipital bone between superior inferior nuchal lines; lateral to the semispinalis capitis. Action. Extends head and bends it laterally. Nerve supply. A branch of posterior primary division of first cervical nerve.

Figure 6-63 Osteology of the cervical spine in posterior (A) and lateral (B) views.

Figure 6-64 Cross section of the cervical spine. Note that the vertebral artery is anterior to the nerve.