Raj Rao and Satyajit V. Marawar
ANATOMY
Posterior Cervical Musculature
The muscles covering the posterior aspect of the cervical spine are arranged in three layers (FIG 1).
Superficial layer: The trapezius muscle originates from the superior nuchal line of the occiput, the ligamentum nuchae, and the spinous processes of the upper thoracic spine. It inserts into the spine of the scapula and the acromion.
Intermediate layer: The splenius capitis arises from the lower half of the ligamentum nuchae and upper six thoracic vertebrae, inserting onto the mastoid process and the lateral half of the superficial nuchal line under the sternocleidomastoid.
The deep layer consists of the semispinalis capitis, the semispinalis cervicis, the multifidus, and the rotators, arranged from superficial to deep layers respectively.
The semispinalis capitis arises from the transverse processes of the upper six thoracic vertebrae and the articular processes of the midcervical vertebrae and inserts onto the occiput between the superior and inferior nuchal lines.
The semispinalis cervicis arises from the transverse processes of the upper six thoracic vertebrae and inserts onto the spinous processes of C2 to C5.
The multifidus muscle lies deep to the semispinalis cervicis. It originates from the articular processes of the lower cervical vertebrae and inserts onto the spinous processes of the upper cervical vertebrae.
The rotators lie deep to the multifidus. They originate from the transverse process of one vertebra and ascend obliquely to insert on the spinous process of the vertebra one or two levels cranial to their origin.
Suboccipital Musculature
The rectus capitis posterior minor originates from the posterior tubercle of the atlas and inserts onto the medial half of the inferior nuchal line.
The rectus capitis posterior major originates from the spinous process of the axis and inserts onto the lateral half of the inferior nuchal line.
The obliquus capitis superior originates from the transverse process of the atlas and inserts onto the occiput laterally between the superior and inferior nuchal lines.
The obliquus capitis inferior muscle originates from the spinous process of the axis and inserts onto the transverse process of the atlas.
The suboccipital triangle lies between the rectus capitis posterior major and the superior and the inferior obliques.
The greater occipital nerve is the medial branch of the posterior division of the second cervical nerve at the medial angle of the suboccipital triangle. It runs cephalad between the semispinalis capitis and the obliquus inferior, toward the occiput, where it pierces the semispinalis capitis and the trapezius. It is responsible for cutaneous innervation of the back of the scalp (FIG 2).
Osteoligamentous Anatomy
The external occipital protuberance or inion is an easily palpable bony landmark in the midportion of the occiput. The superior nuchal line extends as a bony ridge on either side of this prominence. A small ridge or crest, called the median nuchal line, descends in the medial plane from the external occipital protuberance to the foramen magnum. The inferior nuchal line runs parallel to the superior nuchal line, midway between the inion and foramen magnum (FIG 3).
The atlas does not have a spinous process but has a posterior tubercle marking the center of the posterior arch.
The spinous process of the axis is tall, bifid, and broadest in the cervical spine.
A broad sheet of thick fibrous tissue called the posterior atlanto-occipital membrane extends from the posterior border of the foramen magnum to the superior border of the posterior arch of the atlas.
The posterior atlantoaxial membrane is a broad, thin membrane extending from the inferior border of the posterior arch of the atlas to the superior border of the lamina of the axis.
The tectorial membrane is the cranial extension of the posterior longitudinal ligament, running posterior to the transverse ligament to attach onto the anterior border of the foramen magnum.
The anterior atlantoaxial ligament is the continuation of the anterior longitudinal ligament, extending from the inferior border of the anterior arch of the atlas to the front of the body of the axis (FIG 4).
The pars interarticularis or isthmus of C2 is the waist of the posterior arch of C2, connecting the superior and inferior articular processes. The medial margin of the pars interarticularis along the superior border of the C2 lamina is a guide to the medial margin of the C2 pedicle.
The C1–2 facet joint is oriented largely in the axial plane, while the C2–3 and remaining subaxial cervical facet joints are coronally oriented 45 degrees to the plane of the spine.
The spinous processes from C3 to C6 are small and bifid. The C7 spinous process tends to be straight and long and terminates in a single tubercle. It is usually the longest of the cervical spinous processes.
The lateral mass of the cervical spine refers to the lateral column of each vertebral body that includes the superior and inferior articular processes and the transverse foramen on either side.
It offers a secure fixation anchor for screw insertion from C3 to C6, particularly when the spinous process and lamina are fractured or removed.
A faint longitudinal groove marks the separation between the laminae and lateral masses.
The exiting nerve root and posterior portion of the transverse process lie anterior to the lateral mass.
The anteroposterior depth of the lateral mass reduces gradually from C3 (about 8.9 mm) to C7 (about 6.4 mm).3
FIG 1 • Superficial, intermediate, and deep layers of the posterior cervical musculature are shown on the left. The suboccipital muscles lie deep to these muscles and are shown on the right.
FIG 2 • Anatomy of the suboccipital triangle. The suboccipital triangle lies between the rectus capitis posterior major, the obliquus superior, and the obliquus inferior. The greater occipital nerve is seen crossing the suboccipital triangle along its medial angle. The posterior arch of the atlas with the vertebral artery is seen in the floor of the suboccipital triangle.
FIG 3 • A. Bony anatomy of the occiput with muscular insertions. Superior, inferior, and median nuchal lines are the prominent bony ridges on the posterior occipital surface. The major posterior cervical muscles and muscles of the suboccipital triangle insert on these bony ridges and on the posterior occipital surface between these ridges. B. Sagittal cross-section showing the ligamentous architecture of the proximal cervical spine. Anterior and posterior atlanto-occipital as well as atlantoaxial ligaments and the ligaments stabilizing the odontoid process are depicted: the apical ligament of the dens and the transverse ligament of the atlas.
FIG 4 • Axial section showing nerve root anatomy. The spinal rootlets join to form the ventral and the dorsal roots of the spinal nerve. The dorsal root ganglion is seen as the enlargement of the dorsal root lying between the facet joint and the vertebral artery. The roots merge outside the intervertebral foramen to form the spinal nerve.
The lateral mass of C7 is elongated superoinferiorly but is thinner in the anterior posterior plane than the other cervical vertebrae.
The pedicles of the cervical vertebrae are smaller than those in the lumbar spine. The dimensions are generally appropriate for pedicle screw insertion at C2 and C7.
Computed tomography should be obtained in all patients before screw fixation to verify pedicle width and morphology, particularly between C3 and C6.
Nerve Root Anatomy
The dorsal and ventral nerve roots formed from the respective rootlets enter a common sleeve of the arachnoid and dura mater.
The nerve root runs 45 degrees anterolaterally and 10 degrees inferiorly to enter the intervertebral foramen by passing over the top of the corresponding pedicle.
The dorsal nerve root lies anterior to the superior articular process, positioned at the tip of the superior articular facet medially and then coursing inferiorly to lie on top of the pedicle laterally.
The ventral root lies anteroinferiorly adjacent to the uncovertebral joint.
The cervical nerve roots occupy the lower third of the intervertebral foramen, while the upper two thirds of the foramen is filled with fat.
In the lateral part of the intervertebral foramen, the dorsal nerve root is enlarged to form the dorsal root ganglion, which lies between the vertebral artery and a groove on the anterolateral aspect of the superior articular process (Fig 4).
The dorsal and the ventral nerve roots join distal to the dorsal root ganglion outside the intervertebral foramen to form the spinal nerve.
Vertebral Artery
The vertebral artery is a branch of the first part of the subclavian artery, lying anterior to the transverse process of the seventh cervical vertebra at its origin.
The vertebral artery courses medially and posteriorly through the subaxial cervical spine within the transverse foramina of the sixth through the first cervical vertebrae.
It is at risk of injury where it lies unprotected between the transverse foramina and during anterior procedures lateral to the disc space, particularly at the upper cervical levels (FIG 5).
Anatomic variations in the course of the vertebral artery are not infrequent. Following its origin off the subclavian artery, the vertebral artery typically enters the C6 transverse foramen. Bruneau et al reported entry into the C3, C4, C5, or C7 transverse foramina in 0.2%, 1.0%, 5.0%, and 0.8% of patients, respectively.1
A 2% incidence of tortuosity of the vertebral artery has been reported, leading to a potentially dangerous medial course of the vessel within the vertebral body.1,2
More cephalad, after emerging from the transverse foramen of C2, the artery lies lateral to the C1–2 facet joint before it enters the transverse foramen of the atlas.
The artery exits the transverse foramen of the atlas and continues posteromedially in a groove on the superior surface of the posterior arch of the atlas.
It enters the foramen magnum by piercing the atlanto-occipital membrane about 10 mm from the midline.1
In approaches to the posterior cervical spine, the vertebral artery is at risk of injury during exposure of the posterior arch of the atlas and in the transverse foramina of C1 and C2 during screw insertion for occipitocervical or atlantoaxial fusion procedures.
To protect the vertebral artery during these procedures, dissection should be limited to within 12 mm of the midline on the posterior aspect of C1 and within 8 mm of the midline on the superior surface of the posterior arch of the atlas.1 Further lateral dissection can be performed on the inferior surface of the C1 arch versus the superior surface because the vertebral artery runs on the superior surface of the C1 arch.
The width of the lateral mass of the atlas averages 11.6 ± 1.4 mm. The height of the portion of the lateral mass of the atlas inferior to its posterior arch averages 4.1 ± 0.7 mm.2 The lateral mass of C1 thus can generally safely accommodate a 3.5-mm screw below its attachment to the posterior arch.
FIG 5 • Origin and course of the vertebral artery. The vertebral artery branches out from the first part of the subclavian artery. It passes through the transverse foramina of the upper six cervical vertebrae and has a significantly tortuous course in the proximal cervical spine.
SURGICAL MANAGEMENT
Indication.
Posterior spinal cord decompression via laminoplasty or laminectomy
Nerve root decompression via foraminotomy
Occipitocervical or atlantoaxial decompression, fusion, and instrumentation
Posterior cervical fusion
Cervical pedicle or lateral mass instrumentation
Positioning
The patient's cervical spine should be ranged in flexion and extension in the preoperative area to determine a safe range that does not produce symptoms. Movements of the neck during intubation should be minimized, particularly in myelopathic patients.
Awake intubation and positioning should be considered in myelopathic patients with markedly reduced canal dimensions.
In patients undergoing occipitocervical and atlantoaxial procedures, the chin should be tucked to facilitate exposure of the occipitocervical region. For subaxial procedures, slight flexion of the neck reduces overlap of the laminae and facet joints, making deep dissection easier and facilitating decompression of the central and lateral canal. The neck should be brought back into neutral position for fusion or instrumentation procedures.
Hyperextended or hyperflexed positions under anesthesia, particularly when held for prolonged periods of time, may contribute to spinal cord injury.
We recommend use of the Mayfield three-point clamp to hold the cranium during posterior occipitocervical and posterior cervical surgery. The clamp is secured to the operating table with an adaptor.
We infrequently use intraoperative tong traction because we believe the amount of traction transmitted to the operative site is variable.
The shoulders are pulled down and taped to the distal end of the bed to facilitate intraoperative radiographic visualization (FIG 6). Excessive traction on the shoulders should be avoided to minimize the risk of brachial plexus injury.
The reverse Trendelenburg position reduces epidural venous congestion and intraoperative bleeding. We avoid the sitting position to minimize the risk of intraoperative air embolism.
Bony prominences and peripheral nerves in the upper and lower extremities should be well padded to protect against intraoperative decubiti or neurapraxia.
Allowing the abdomen to hang free facilitates venous return to the heart, maintains cardiac output, and decreases the required peak inspiratory pressure.
Radiographs are obtained after positioning to verify cervical alignment. Placement of a radiopaque marker before obtaining these radiographs facilitates planning of the incision.
FIG 6 • Positioning of the patient for posterior cervical surgery. In the prone position, the patient's head is stabilized with a Mayfield three-point clamp while traction is applied through the shoulders by taping them down. The patient is in the reverse Trendelenburg position with the abdomen allowed to hang free.
TECHNIQUES
POSTERIOR APPROACH TO SUBAXIAL SPINE
A midline skin incision is used for most surgical procedures to the subaxial spine. Palpation of the prominent spinous processes of C2 and C7 beneath the skin or the use of intraoperative radiographs can help restrict the incision to the area that requires exposure.
The incision is deepened through the relatively avascular median raphe, which is a condensation of the deep fascia. This appears as a “white line” in the midline.
Electrocautery is then used to incise the ligamentum nuchae.
Troublesome bleeding from the paraspinal muscles can be minimized by staying within the avascular median raphe.
Intermittent palpation of the spinous processes helps the surgeon stay oriented to the midline. The posterior cervical paraspinal musculature generally originates laterally and caudally, passing obliquely cephalad.
Reduction of intraoperative bleeding is facilitated by dissecting caudal to cephalad in a subperiosteal fashion.
For laminoplasty or multilevel laminectomy, the interspinous tissues are cauterized to minimize bleeding and then stripped off the spinous processes.
Deep retractors are inserted beneath the fascial layers directly on bone. Deep dissection is carried further laterally along the laminae.
Localization of level is facilitated by identifying the large C2 and C7 spinous processes and the bifid spinous processes from C2 to C6.
An intraoperative lateral radiograph should be obtained to confirm the operative levels.
If facet fusion is not planned, the dissection should stop at the medial third of the facet joint and the facet joint capsule should be preserved.
If facet fusion or instrumentation is required, the dissection is extended to the lateral border of the lateral mass.
POSTERIOR APPROACH TO OCCIPITOCERVICAL REGION
The external occipital protuberance and the prominent bifid C2 spinous process can be palpated in most patients beneath the skin. A midline skin incision is made extending from just above the occipital protuberance to the cervical level required.
The incision on the scalp is deepened down to bone, and the occiput is exposed in subperiosteal fashion from the inion down to the foramen magnum.
The dissection is carried laterally for a distance of 2.5 cm on either side of the median occipital crest. Excessive lateral dissection or retraction can injure the greater occipital nerve.
The incision is extended caudally through the ligamentum nuchae in the midline. Staying in the midline reduces blood loss.
Self-retaining retractors are applied at both ends of the incision.
The large bifid spinous process of C2 is easily identified. It is exposed subperiosteally by dissecting the attachments of the rectus capitis posterior major and obliquus capitis inferior from these structures.
The greater occipital nerve exits posteriorly along the inferior border of the obliquus capitis inferior muscle and can be preserved by keeping the dissection on the C2 posterior arch.
Preserving the soft tissue attachments to the distal and lateral aspects of C2 and the C2–facet joint helps maintain subaxial stability postoperatively.
The C1 ring lies deep in the space between the occiput and C2. The posterior arch of C1 has no muscular attachments.
Soft tissue from the posterior arch of C1 is dissected subperiosteally, taking care to stay within 12 mm of the midline on the posterior aspect of the posterior ring of C1 and within 8 mm of the midline on the superior aspect of the posterior ring of C1 to avoid vertebral artery injury (TECH FIG 1).
TECH FIG 1 • The vertebral artery emerges from the transverse foramen of the atlas and courses medially in the groove on the superior surface of the posterior arch of the atlas. At the medial end of the groove it turns anteriorly and pierces the atlanto-occipital membrane about 10 mm from the midline.
EXPOSURE OF C1–2 FACET JOINT
Exposure of the articulation between the lateral mass of C1 and the superior articular process of C2 is occasionally required for screw fixation of the lateral mass of C1 and fusion of this joint.
After dissection and retraction of the muscles off the posterior aspects of C1 and C2 of the upper cervical spine, the lamina of C2 is identified.
Soft tissue is carefully dissected off the lamina of C2 using a Freer elevator or dissector.
Tracing the lamina of C2 proximally exposes the pars interarticularis of C2 and the superior medial corner of the C2 pedicle.
Exposure of the C1 lateral mass can be obtained by following the inferior arch of C1 laterally until the lateral border of the spinal canal is identified by visualizing its corresponding location on C2.
From this point on C1, ventral dissection with a Penfield or Freer will allow palpation of the C1 lateral mass. The greater occipital nerve is encountered and swept distally. A large venous plexus is present and must be controlled with Gelfoam and bipolar cautery.
REFERENCES
1. Bruneau M, Cornelius JF, Marneffe V, et al. Anatomical variations of the V2 segment of the vertebral artery. Neurosurgery 2006;59: 20–24.
2. Curylo LJ, Mason HC, Bohlman HH, et al. Tortuous course of the vertebral artery and anterior cervical decompression: a cadaveric and clinical case study. Spine 2000;25:2860–2864.
3. Ebraheim NA, An HS, Xu R, et al. The quantitative anatomy of the cervical nerve root groove and the intervertebral foramen. Spine 1996;21:1619–1623.
4. Ebraheim NA, Xu R, Ahmad M, et al. The quantitative anatomy of the vertebral artery groove of the atlas and its relation to the posterior atlantoaxial approach. Spine 1998;23:320–323.
5. Hong X, Dong Y, Yunbing C, et al. Posterior screw placement on the lateral mass of atlas: an anatomic study. Spine 2004;29:500–503.