Anatomy, Head and Neck: Cervical Vertebrae (2024)

Introduction

The spine, or vertebral column, is a segmental set of 33 bones and associated soft tissuesin the subcranial portion of the axial skeleton. It is subdivided into5 regions based on curvature and morphology: cervical, thoracic, lumbar, sacral, and coccygeal (see Image. Vertebral Column).

The cervicalspine has 7 articulating vertebrae, whereas the thoracic has 12, and the lumbar has 5. Despite their similar morphology, these regionshave variable flexibility, movement, and joint articulation, giving rise to the spine's S-shaped curvature.

Meanwhile, the sacrum and the coccyx are2 sets of fused caudal vertebrae that convey no motion. The sacrum has 5 fused vertebrae, while the coccyx has 4.[1]

The cervical spine comprises 7 vertebrae (C1to C7) and is divided into2 major segments. The 2 most cephalad vertebrae, the atlas (C1) and the axis (C2), form thecraniocervical junction (CCJ) together with the occiput. The 5 cervical vertebrae caudad, C3to C7,comprise the subaxial spine and are referred to by number (see Image. Cervical Vertebrae). The cervical spine supportsthe weight of thecranium and head and neck motion.

Structure and Function

Typical vertebrae havehallmark anatomic structures that are conserved across the cervical, thoracic, and lumbar regions. Generally, each vertebrahas a ventral vertebral bodywith mostly trabecular (cancellous) bone and a denser, mostly cortical dorsal vertebral arch.

The vertebral body is the main site of intervertebral articulation and load-bearing. Each vertebral body is linked tothe next by an intervertebral disk.

The dorsal arch typically consists of a pair of pedicles arising from the dorsal vertebral body, which unite dorsally bytheir flat laminae. The spinous process arisesas a dorsal projection from the midline fusion of the laminae. The pedicles, laminae, and dorsum of the vertebral body form the vertebral foramen, a complete osseous ring enclosing the spinal cord. The vertebral canal, which houses thespinal cord and its associated vascular structures, forms from connecting the vertebral foramina.

The transverse processes extend posterolaterally from the junctions of the laminae and pedicles. The 2 superior and 2 inferior articular processes also project from thejunctions of the laminae and pedicles.In the cervical spine, the costal process becomes the anterior part of the transverse process, enclosing the vertebral artery foramen.[2]

Unique Features of the Cervical Spine

The cervical vertebral bodies are relatively smaller than their thoracic and lumbar counterparts due to the relatively lighter load at this spinal level. Consequently, the cervical spine is also more mobile, making it appropriate for supporting head and neck motion. However, the increased mobility and flexibility carry a higher spinal cord injury risk in this region.

Despitehaving most of the typical vertebral hallmarks, structural variability exists among the 7 cervical vertebrae. For instance,C2 to C6 have bifid spinous processes, but C7 has a larger, singular spinous process, and C1 has none. The C7 spinous process is called "vertebra prominens," which is palpable at the neck's base and similar to thoracic spinous processes. The C1 vertebra has a posterior tubercle insteadof a spinous process.

The cervical vertebrae also haveseveral anatomic variations. For example, all transverse processes have transverse foramina that, when connected, protect the vertebral arteries traveling at both sides of the neck. However, the C7 transverse foramina may be small or absent in some individuals.[3]

The Upper Cervical Spine – The Atlas (C1) and the Axis (C2)

The atypical cervical vertebrae C1 and C2 make up the upper cervical spine.

The C1 vertebra, aka the atlas,supportsthe skull at theatlanto-occipital joint (see Image.First Cervical Vertebra). Due to its unique role,C1 has features not sharedwith the rest of the spine.First, the vertebral body is absent,allowing the ring-shaped C1to accommodate the spinalcord as it exits the foramen magnum. Second, C1's concave, medially oriented articular facets make it suitable for receiving the occipital condyles to form the atlanto-occipital joint. The atlanto-occipital jointtransmits the head's weight to the spine, contributes greatly to the neck's flexion and extension, and limits the occiput's lateral displacement. Strong soft-tissue ligaments further stabilize the joint.[2][4]

The C2 vertebra, aka the axis,is the primary weight-bearing bone of the upper cervical region(seeImage.Second Cervical Vertebra). C2'shallmark feature is the odontoid process,aka dens, which is thebony projection extending cranially from its vertebral body. The odontoid process evolved from the body of C1and serves as the principal attachment point for the soft tissues, stabilizing the atlantoaxial junction.

The atlantoaxial junction has3 articulation points (see Image. Atlantoaxial Junction).The first is the midline atlanto-odontoid (atlanto-dental) joint, which permits the anterior arch of the atlas to pivot on the odontoid process. The other2 are theatlantoaxialfacet joints, which are lateral and involve C1's inferior facets and C2's superior facets. Theatlantoaxialfacet joints are fairly shallow,thus allowing significant motion. Compared to the atlanto-occipital joint, the atlantoaxial junction contributes more to the head and neck's rotatory motion than flexion or extension.[2][3][4][5]

The Subaxial Cervical Spine – C3 to C7

Compared to the upper cervical spine, all5 subaxial cervical vertebrae share nearly identical morphological and functional features and have mostof the typical vertebral structures. The C3 to C7 vertebraepossess uncinate processes, which are bony lateral vertebral body projections articulating at the Luschka joints to prevent vertebral listhesis.[3]

However, C7 has features not found in the othersubaxial cervical vertebrae.As mentioned, the C7 transverse foramina are the smallest and sometimes absent. The vertebral arteries typically bypass the C7 transverse foramina andrun cephalad starting from the C6 transverse foramina. C7 is also considered a transitional vertebra with aspinous process, inferior facets, and a larger vertebral body like the thoracic vertebrae.[6]

Embryology

The vertebral columnoriginates from the somites, part of the paraxial mesoderm. More than40 somite pairs develop craniocaudallyalongside the notochord during the body axis elongation process. Local factors secreted by the notochord, neural tube, ectoderm, and visceral mesoderm induce the somites to undergo endothelial-to-mesenchymal transition (EMT) rapidly. The sclerotome, dermatome, and myotomeappear after the transition. The mesenchymal sclerotome gives rise to the vertebral column, whereas the dermomyotome forms the muscles and overlying dermis.[7][8]

During the 4th week of embryogenesis, sclerotome cells develop around the notochord and the neural tube, signifying the initialformation of the true vertebral column and skull base. Column segmentation beginsowing to the notochord's inductive influence. Cellsthat will give rise to the intervertebral discs' annulus fibrosus and nucleus pulposus coalesce between collections of sclerotome cells. The sclerotome aggregatesform thevertebrae duringthe 6th week ofembryogenesis.

Each vertebra initially has 3 ossification centers. However,2 moreossification centerswilldevelop during the first year of life andcontribute to vertebral growth plate formation.[5][7][8]The ossification centers of the odontoid process are significant since they are oftenmistaken for pediatric fractures. The junction between the dens andC2's vertebral bodyfusesaround6 years of age. Meanwhile, the secondary odontoid ossification centers appear around age 3but do not fuse to the cranial aspect of the densuntil age 12.[5]

Blood Supply and Lymphatics

The vertebral arteries course cranially to the base of the skull distal to their bifurcation from the subclavian arteries (see Image. Internal Carotid and Vertebral Arteries).They progress past the C7 vertebrae anteriorly before entering the C6 transverse foramina.After exiting the C2 transverse foramina, the vertebral arteries bend laterally to enter the C1 transverse foramina,revert medially over the C1 arch, then enter the skull base.The main tributaries of the vertebral arteries, the cervical radicular arteries, supply blood to the cervical vertebral bodies.[9]

The carotid arteries also course through thecervical spine region. The right common carotid arterydivergesfromthe brachiocephalic artery, while the left common carotid arises directly from the aortic arch. The common carotids then bifurcate into the internal and external carotids at the C3 vertebral level. Theinternal carotids do not perfuse any cervical spine structures despite their cervical location.[9]

Nerves

The cervical spine functions as the spinal cord's bony protection as it exits the cranium. Despite there being only 7 cervical vertebrae, there are 8 pairs of cervical nerves, termed C1 to C8.The first 7 cervical nervesexit the spine cranially totheir associated vertebrae, while the 8th cervical nerve exits caudally to C7 (see Image. Cervical Nerves).

Direct innervation of the spinal column is highly complex. The sympathetic trunk, sympathetic rami communicantes, and perineural vascular plexus of the vertebral arteries lie in the cervical spine's ventral compartment. Meanwhile, the posterior aspect of the cervical spine receives innervationfrom the medial branch of the posterior primary rami. Coursing through the intervertebral foramen with the vertebral arteriesare the vertebral nerves,whichsupply additional sympathetic innervation.[10]

Muscles

The cervical vertebrae serve as the origins and insertion points for the musclessupporting head and neck movement (see Image. Neck Muscles).

The erector spinae are deep back muscles traversing posteriorly along the entire spine. These musclesinsert on thespinous and transverse processes of the cervical and upper thoracic vertebrae. The erector spinae provide postural support and assist in flexing and extending the vertebral column.

The posterior neck musclesand the suboccipital triangle are associated with the cervical vertebrae and enable the neck's extension, rotation, and lateral bending. The anterior neck's deep muscles also originate at various cervical landmarks before attaching to the cranium or1stand 2nd ribs. Theanterior neck musclescontribute to neck flexion, rotation, lateral bending, and stabilization of the skull.[4]

Physiologic Variants

The vertebrae initiallyarisefrom3 primary ossification centers during embryogenesis: one in the centrum of the vertebral body and one in either vertebral arch. However, 5 ossification centersdevelop during the1st year of life: one at either transverse process, one at the spinous process margin, and oneeach at the superior and inferior portions of the vertebral body. Bones ossifyingfrom these5 areas contribute to growth plate formation. Consequently, the growth plate's absence or asymmetry can contribute to cervical vertebral congenital defects.[8]

Surgical Considerations

The cervical spine isadjacent to vital head and neck structures.Most surgical procedures involving the cervical spine are performed via an anterior or posterior midline approach.

Anterior Surgical Approach

The anterior approachexposes theC2 to T1 vertebral levelsandis indicated for anterior cervical diskectomy and fusion, corpectomy and fusion, cervical disk replacement, tumor resection, fracture, and infection. Surgeon preference dictates the side of access. A left-sided approach is often preferred, where the recurrent laryngeal nervehas aless variablecourse and posesa lower iatrogenic injury risk. Recurrent laryngeal nerve injury results invocal cord paralysis and hoarseness.However, the clinicalvalue of this approach remains controversial.

Superficial dissection transects the platysma muscle at the desired level. The sternocleidomastoid muscle is then identified and retracted laterallyto protect the carotid sheath and expose strap musculature at the medial aspect. Superficial dissectionmust beperformed carefully to avoid injuring the structuresinsidethe carotid sheath.

Deep dissection retracts the trachea and esophagus medially, allowing access to the longus colli muscles. The longus colli muscles are then divided longitudinally and retracted laterally, exposing the anterior longitudinal ligament that overlies the vertebral bodies and disks. Careful retractor placement and dissectionhelp avoid injury to the trachea, esophagus, and stellate ganglion, which are typically lateralto the longus colli muscles at the C6 level.

The stellate ganglion is a sympathetic structure. Injuries can lead toHorner syndrome, characterized by ptosis, anhydrosis, miosis, enophthalmos, and loss of ciliospinal reflex ipsilateral to the lesion.

A postoperative lateral cervical spine x-ray isobtained to assesssoft tissue swelling. The soft tissue shadow anterior to the cervical spine should be less than 10 mm at the C1 level, less than 5 mm at C3, and less than 15 to 20 mm at C6.[2]

Posterior Surgical Approach

The posterior approachdivides the paraspinal muscles dorsally at the midline. Adequate hemostasis and proper retractor placement ensureminimal blood loss.Access to the cervical spine's posterior elementsis necessary during procedures like laminectomy, laminoplasty, foraminotomy, and posterior cervical instrumentation.

Uppercervical spine fracturesare also most frequently accessed posteriorly.The unique anatomy of this spinal segment warrants careful consideration during surgical intervention. For example, pedicle screws may be safely used on C2 and C7 but not C3 to C6 because of theirsmall size and proximity tothevertebral arteries. Lateral mass screw placement with radiographic navigation is an alternative, having a lower risk of injuring the vertebral arteries.

Posterior arch dissection during C1 lateral mass screw placement should not be exposed more than 1.5 cm awayfrom the midline to minimize the risk of vertebral artery injury. To avoid the vertebral artery, the screw should be angled 10 to 15 degrees medially.[2]

Trans-articular screw placement at the C1-C2 level alsorisks vertebral artery injury as the screwmust pass C2'spars interarticularis. C2 pedicle screw placement carries less vertebral artery injury risk but confers some risk to the internal carotid artery, depending on screw length.

In cases involving upper cervical spine fusion, surgeons mustinform the patient about the risk of potential mobility reduction when rotating, flexing, or extending the cervical spine.[2]

Clinical Significance

Neck pain presents a significant socioeconomic burden.Studies estimate the prevalence of neck pain in the adult population to be between 10% and 30%, making it one of the leading causes of disability globally.Neck pain is often classified based on duration (acute, subacute, or chronic)and etiology (eg, mechanical, neuropathic, rheumatologic).Frequent neck pain is definedas havingat least 3 neck pain episodes in a year.[3][11]

Acute neck pain often arises from trauma or sports injury. Fractures and dislocations are common and, in severe cases, may be fatal.Cross-table radiography and computed tomography (CT) scans are invaluable tools for assessing cervical fractures and other bone abnormalities (see Image. CT Scan of C6-C7 Fracture). In contrast, magneticresonance imaging (MRI) is preferred forspinal cordor soft tissue evaluation (see Image. Cervical Spine MRI). Given the high neurological stakes, missed or incorrect diagnoses can have serious long-term costs. Regardless of location, the specific characteristics of cervical spine injury dictate the management.[11][12]

Subacute and chronic neck pain have diverse causes. Cervical facetarthritisis a common cause of mechanical neck pain. Neuropathic neck painfrequentlyhas peripheral origins such asnerve root compression, intervertebral disk herniation, and spinal stenosis. A comprehensive history and physical examination canhelp differentiate between mechanical and neuropathic etiologies. Diagnostic imaging and further workup arenecessary in some cases.[11]

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Disclosure: Joshua Kaiser declares no relevant financial relationships with ineligible companies.

Disclosure: Vamsi Reddy declares no relevant financial relationships with ineligible companies.

Disclosure: Marjorie Launico declares no relevant financial relationships with ineligible companies.

Disclosure: Julian Lugo-Pico declares no relevant financial relationships with ineligible companies.