Anatomy

The CCJ is a complex interface of joints and ligaments through which the head achieves substantial mobility upon the neck while the lower brainstem and spinal cord are safeguarded. To accomplish these two functions, the CCJ harbors several structures that are unique to this region of the spine. These include two uniquely shaped bones, the atlas (C1) and axis (C2), and a complex ligamentous network.

The atlas is a ring-shaped bone composed of two lateral masses and an anterior and posterior arch. It lacks a body, and in its absence articulates with the peglike superior process of C2, the odontoid, or dens. The superior and inferior articulating processes, which articulate with the occipital condyles and superior facets of C2, respectively, are located within the lateral masses. The transverse processes extend laterally from the lateral masses and are fenestrated by the transverse foramen, through which the vertebral artery courses. This artery ascends through the transverse foramen, then it turns medially and slightly posteriorly around the posterior arch of C1 in its sulcus, and then turns anteriorly to enter the dura. Both the anterior and posterior arches of C1 form a platform through which ligaments attach to the bones above and below. Just medial to the lateral masses are two small tubercles that serve as attachment sites for the transverse atlantal ligament; this ligament drapes across the posterior aspect of the odontoid process of C2, allowing for rotation of C1 around the odontoid.

The odontoid process of the axis (dens) sits just behind the anterior arch of C1 and is held in position by the transverse atlantal ligament. It serves as an attachment for several important ligaments that course from C2 to the skull base. The pedicles are broad and are covered by the superior articulating processes; the transverse processes are diminutive and are pierced by the transverse foramen and the vertebral arteries that course through these canals. The laminae are strong and, in contrast to the atlas, join in the midline as the prominent spinous process, a useful landmark when navigating the upper cervical spine through a posterior approach.

The ligamentous complex surrounding the upper cervical spine is important in providing stability while at the same time allowing for rotation and flexion–extension ( Fig. 5-1 ). The anterior longitudinal ligament extends from the skull base to S1 and is adherent to the anterior vertebral bodies. The superior segment, which courses from C1 to the anterior basion, is called the anterior atlantooccipital membrane. The posterior longitudinal ligament extends from C1 to S1 along the posterior vertebral bodies of the spine. A continuation of this ligament superiorly, known as the tectorial membrane, extends from the posterior aspect of the anterior arch of C1 to the posterior basion.

A, The brainstem and vertebral arteries have been sectioned to display the ligaments traversing posteriorly on the C2 vertebral body and dens. The tectorial membrane has been removed to reveal the alar ligaments and the vertical and horizontal cruciate ligaments. B, After removal of the vertical cruciate ligament, the apical ligament is seen traveling superiorly from the odontoid to the margin of the foramen magnum.

The transverse ligament of C1 extends from the tubercles of C1 and divides its vertebral foramen into an anterior compartment, composed of the dens process of C2, and a posterior compartment, through which the spinal cord courses. Two cruciate ligaments arise from the midline of the transverse ligament immediately posterior to the dens: the superior cruciate ligament extends superiorly to the posterior basion; the inferior cruciate ligament extends inferiorly to the body of the axis. The apical ligament is a single ligament that extends from the tip of the dens to the basion. Finally, the paired alar ligaments course from the lateral dens superiorly to the lateral margin of the foramen magnum. Together these ligaments are responsible for maintaining stability of the CCJ while allowing the flexibility of this complex in rotatory and flexion-extension movements. Specifically, the joint between the occipital condyles and the superior articulating processes of C1 offers approximately 50% of the total flexion-extension movement of the head, and the articulation of the dens on C1 offers approximately 50% of the total rotation movement of the head. As such, the complex ligamentous relationships, unique bony structures, and inherent mechanical properties of the CCJ result in an elegant yet complicated anatomic setting to effectively navigate surgically. Furthermore, because of the important mechanical relationships of the CCJ, stability must be accounted for at all times.

Posterior approaches to the cervical spine traverse a series of symmetric muscles that may be categorized into groups based on their depth ( Fig. 5-2 ). The most superficial layer is the trapezius muscle, followed by a middle layer composed of the lesser rhomboid muscle. Finally, the deepest layer is composed of the intrinsic muscles of the neck and includes the splenius capitus, splenius cervicis, semispinalis capitis, semispinalis cervicis, rotator muscles, and the multifidus muscles. The short nuchal muscles are located more laterally and consist of the rectus capitis major and minor and the superior and inferior oblique capitis muscles ( Fig 5-3 ). These muscles form the suboccipital triangle and are useful in locating the vertebral artery, which courses through its center.

Posterior view of the muscles overlying the CCJ. A, The trapezius (most superficial) and sternocleidomastoid muscles have been removed on the left and preserved on the right. Underlying the trapezius muscle are the semispinalis capitis and splenius capitis. B, After removal of these muscles, the suboccipital triangle is seen containing the vertebral artery ( left ). The triangle is composed of the superior oblique muscle, which runs from the occipital bone to the transverse process of C1; the inferior oblique muscle, which travels from the transverse process of C1 to the spinous process of C2; and finally the rectus capitis posterior major, which arises from the C2 spinous process and courses superiorly to the occipital bone near the superior nuchal line. CN, cranial nerves; Longiss. cap. m., longissimus capitis muscle. C, The muscles of the suboccipital triangle have been removed bilaterally to reveal the bony arches of C1 and C2 and the vertebral arteries and C2 nerve roots. The vertebral artery travels superiorly and laterally from the transverse process of C2 to the transverse process of C1 and then turns medially to course on the posterior arch of C1 before entering the dura.

Oblique view of the left CCJ. The trapezius, splenius capitis, and semispinalis capitis muscles have been reflected downward to expose the vertebral artery in the suboccipital triangle. The superior and inferior oblique muscles can be seen attached to the C1 transverse process laterally. Note the proximity of the carotid artery to the C1 transverse process.

Positioning

Stabilization of the head in a neutral position is of utmost importance during posterior approaches. A Mayfield pinion head holder is placed after intubation, and the patient is rotated to the prone position. The majority of posterior approaches are performed with the patient prone, usually with a Wilson frame under the chest. The head may be elevated relative to the body to enhance venous drainage and minimize blood loss. The sitting position is an additional option for posterior stabilization procedures; however, this position has become less popular given the inherent risk of air embolism. Lateral decubitus or three-quarter prone positions may also be used depending on individual patient anatomy and surgeon preference. It is also not uncommon for patients undergoing posterior craniocervical fixation to be maintained in preoperative traction to obtain reduction before their procedure.

Before securing the patient’s head in position for surgery, it may be necessary to make adjustments to the patient’s position to optimize preoperative alignment. In cases where the CCJ is unstable, fiberoptic intubation may be necessary to ensure that the head is not passively extended during placement of the endotracheal tube. Finally, the need for monitoring of somatosensory and motor-evoked potentials should be determined before surgery. In patients at risk for spinal cord compromise during surgery, monitoring of these potentials may be helpful in providing feedback to the surgeon during intraoperative maneuvers.

Surgical Technique

Prior to prepping the patient, the incision is planned, and the involved skin is shaved with trimmers. In most cases, the incision extends from just above the external occipital protuberance to the lower cervical vertebrae, depending on individual patient factors and treatment goals. The dermis and subcutaneous tissues are injected with a local anesthetic mixed with epinephrine to enact vasoconstriction and minimize skin bleeding. The patient is prepped widely, and sterile drapes are placed. The fluoroscopic device may be draped in the field at this time.

Next, the skin is incised from roughly two fingerbreadths above the external occipital protuberance caudally to the desired level, and monopolar cautery is used to divide the subcutaneous tissue and obtain hemostasis. Self-retaining retractors are placed to gently retract and facilitate cautery dissection. The nuchal ligament is then identified on the cranial aspect of the incision. A midline avascular plane bisects the nuchal ligament and the cervical musculature; by staying within this plane, bleeding is minimized during muscle division. The occiput is easily exposed using cautery, and the ligamentous attachments of the bone are swept laterally while retraction is performed with a Cobb or periosteal elevator. Venous bleeding from emissary veins is frequently encountered along the occiput and may easily be stopped by using bone wax. The foramen magnum is identified and demarcated using curettes.

The spinous processes of the cervical vertebrae are palpated, and dissection is continued until the tips of the processes are exposed. The divided muscle layers are retracted using larger self-retaining retractors. Next, the spinous process of C2 is identified, and dissection of the deeper muscle layers from the bone is performed. Subperiosteal dissection of the prominent ligamentous attachments may be performed initially using monopolar cautery or a scalpel, but it is best performed later using blunt dissection with a Cobb or periosteal elevator. During this maneuver, the tip of the instrument is placed on the spinous process, and then the flat end, facing downward, is pulled in a medial to lateral direction. This maneuver is continued caudally along the spinous process of the additional cervical vertebrae with the vertebral facet joints serving as the lateral boundary.

Next, the small muscles on C1 and C2 are then dissected from the bone, most commonly with a rasp, with the zygapophyseal joints serving as a lateral boundary. This dissection amounts to approximately 1.5 cm from the midline from C1 to the most caudal level. Further lateral dissection along C1 or C2 may result in inadvertent injury to the vertebral plexus surrounding the vertebral artery or to the second cervical ganglion and nerve. In general, lateral exposure is best performed with blunt dissection using a rasp or bipolar cautery to prevent inadvertent injury to the venous plexus or the vertebral artery. Care should be taken during blunt dissection to keep the instrument directly on bone to avoid arterial injury, because the vertebral artery courses medially in its sulcus on C1. Venous bleeding from the vertebral plexus is usually easily controlled with bipolar cautery or Gelfoam and cotton pledgets. The second cervical ganglion may be identified laterally near the sulcus of the vertebral artery, and it should be spared.

At this point, the spinous processes, laminae, and occiput have been exposed. If cervical fusion is to be performed, the spinous processes and interspinous ligaments may be removed using rongeurs. Curettes are used to expose the yellow ligament between adjacent vertebral arches and the posterior atlantooccipital membrane between the occiput and C1. At this point, exposure of the dura, occipital bony removal, laminectomy, or fixation maneuvers may be performed based on the goals of surgery. Broad fascial attachments usually connect the foramen magnum periosteum to the dura, and these should be respected during dissection. Furthermore, a midline dural sinus, or venous lake, is found in this position that should be respected and cauterized, if needed.

After the goals of the surgery have been met, closure is obtained by achieving hemostasis during slow removal of the retractors. A surgical drain may be left in place based on operator preference. Reapproximation of the cervical musculature is performed through numerous fascial sutures. The skin is then closed, and the patient is removed from pins.