Cervical Spine Anatomy - Medscape Reference

The cervical spine is made up of the first seven vertebrae, referred to as C1-C7 (see the images below). It functions to provide mobility and stability to the head while connecting it to the relatively immobile thoracic spine. The cervical spine may be divided into two parts: upper and lower.

Cervical spine anatomy. View Media Gallery Lateral radiograph of cervical spine showing all 7 vertebrae. View Media Gallery Cervical vertebra. View Media Gallery

Upper Cervical Spine

The upper cervical spine consists of the atlas (C1) and the axis (C2). [2, 3, 4, 5] These first two vertebrae are quite different from the rest of the cervical spine (see the image below). The atlas articulates superiorly with the occiput (the atlanto-occipital joint) and inferiorly with the axis (the atlantoaxial joint). The atlantoaxial joint is responsible for 50% of all cervical rotation; the atlanto-occipital joint is responsible for 50% of flexion and extension. The unique features of C2 anatomy and its articulations complicate the assessment of its pathology.

Cervical spine. Note uniquely shaped atlas and axis (C1 and C2). View Media Gallery

Atlas (C1)

The atlas is ring-shaped and does not have a body, unlike the rest of the vertebrae. The fused remnants of the atlas body have become part of C2, where they are called the odontoid process, or dens. The odontoid process is held in tight proximity to the posterior aspect of the anterior arch of the atlas by the transverse ligament, which stabilizes the atlantoaxial joint. The apical, alar, and transverse ligaments, by allowing spinal column rotation, provide further stabilization and prevent posterior displacement of the dens in relation to the atlas.

The atlas is made up of a thick anterior arch, a thin posterior arch, two prominent lateral masses, and two transverse processes. The transverse foramen, through which the vertebral artery passes, is enclosed by the transverse process.

On each lateral mass is a superior and inferior facet (zygapophyseal) joint. The superior articular facets are kidney-shaped, concave, and face upward and inward. These superior facets articulate with the occipital condyles, which face downward and outward. The relatively flat inferior articular facets face downward and inward to articulate with the superior facets of the axis.

According to Steele's rule of thirds, at the level of the atlas, the odontoid process, the subarachnoid space, and the spinal cord each occupy one third of the area of the spinal canal.

Studies have highlighted various anatomical variations in C1, such as accessory transverse foramina and ponticulus posticus, which can have clinical implications during surgical interventions or in diagnosing conditions affecting this region. [6]

Axis (C2)

The axis has a large vertebral body, which contains the odontoid process (dens). The odontoid process articulates with the anterior arch of the atlas via its anterior articular facet and is held in place by the transverse ligament. The axis is composed of a vertebral body, heavy pedicles, laminae, and transverse processes, which serve as attachment points for muscles that facilitate head and neck movement. The axis articulates with the atlas via its superior articular facets, which are convex and face upward and outward, allowing articulation with the atlas and supporting rotational movement. The axis also provides stability and serves as a pivot point for the atlas and skull to rotate around. [1]

Understanding variations in C2 anatomy is essential for managing conditions such as cervical spine instability or trauma. For instance, biomechanical studies have shown that injuries or degenerative changes can significantly affect motion dynamics at this level, influencing both clinical outcomes and treatment strategies. [7]

Embryology

C2 has a complex embryologic development. It is derived from four ossification centers: one for the body, one for the odontoid process, and two for the neural arches. The odontoid process fuses by the seventh gestational month.

At birth, a vestigial cartilaginous disc space called the neurocentral synchondrosis separates the odontoid process from the body of C2. The synchondrosis is seen in virtually all children aged 3 years and is absent in those aged 6 years. The apical portion of the dens ossifies by age 3-5 years and fuses with the rest of the structure around age 12 years. This synchondrosis should not be confused with a fracture.

Parts of the occiput, atlas, and axis are derived from the proatlas. The hypocentrum of the fourth sclerotome forms the anterior tubercle of the clivus. The centrum of the proatlas sclerotome becomes the apical cap of the dens and the apical ligaments.

The neural arch components of the proatlas are divided into rostral and ventral components. The rostral component forms the anterior portion of the foramen magnum and the occipital condyles; the caudal component forms the superior part of the posterior arch of the atlas and the lateral atlantal masses. The alar and cruciate ligaments are formed from the lateral portions of the proatlas.

Research highlights that during prenatal and postnatal development, the formation of C2 involves several cartilaginous articulations, including: [8]

• Dentocentral synchondrosis - Separates the odontoid from the body
• Neurocentral synchondrosis - Separates the odontoid and body from the neural arches

These synchondroses remain significant until ossification is complete, which can extend into early childhood. [8]

Numerous genes associated with congenital vertebral malformations have been identified, emphasizing genetic pathways crucial for vertebral column formation. These findings suggest that disruptions in specific signaling pathways, such as Wnt and Notch, may influence normal embryonic development. [9]

Vasculature

The vasculature of the cervical spine is complex and is characterized by an extensive arterial network that provides blood supply to the region. This network includes contributions from various arteries, particularly around the dens (odontoid process) of the axis (C2). [1]

The primary arterial supply to the cervical spine arises from: [1]

Vertebral arteries: [1]

These paired arteries originate from the subclavian arteries and ascend through the transverse foramina of the cervical vertebrae C6 to C1 before entering the foramen magnum to supply blood to the brainstem and cerebellum. Around the C3 level, the paired anterior and posterior ascending arteries branch off from the vertebral arteries. These vessels contribute to the vascular supply of the atlantoaxial complex (C1-C2), particularly the dens (odontoid process).

Ascending cervical arteries: [1]

Branching from the thyrocervical trunk, these arteries further contribute to the blood supply, particularly around C3 and C4.

Anterior spinal artery: [1]

The anterior spinal artery (ASA) is a significant vessel that arises from the vertebral arteries and runs along the anterior surface of the spinal cord. It engages in providing blood supply to the anterior two thirds of the spinal cord and has a network of segmental radiculomedullary arteries that branch off from it, enhancing its vascularity in the cervical region. [10]

Occipital and ascending pharyngeal arteries:

These arteries also contribute to the vascular supply around the craniovertebral junction. The occipital artery supplies blood to the posterior aspects of the skull base, while the branches from the ascending pharyngeal artery join with other vessels to form anastomotic connections around the dens. [1]

The arterial anastomoses around the dens are particularly important as they provide collateral circulation, which can be critical in cases of vascular compromise or injury. The anastomotic network allows for redundancy in blood supply, ensuring that even if one pathway is obstructed, others can maintain perfusion. [1]

Ligaments

The craniocervical junction and the atlantoaxial joints are secured by the external and internal ligaments. The external ligaments consist of the atlanto-occipital, anterior atlanto-occipital, and anterior longitudinal ligaments (ALLs).

• Atlanto-occipital ligament - This ligament connects the atlas (C1) to the occipital bone, providing support and stability to the craniocervical junction. [1, 11]
• Anterior atlanto-occipital ligament - It extends from the anterior arch of the atlas to the anterior margin of the foramen magnum. It prevents excessive movement at the atlanto-occipital joint. [1, 11]
• ALL - Running along the anterior surface of the vertebral bodies, this ligament helps prevent hyperextension of the cervical spine, providing anterior stabilization to the cervical spine, including the atlanto-occipital region. [1, 11]

The internal ligaments have six components, as follows:

• The transverse ligament holds the odontoid process in place against the posterior atlas, which prevents anterior subluxation of C1 on C2.
• The accessory ligaments arise posterior to and in conjunction with the transverse ligament and insert into the lateral aspect of the atlantoaxial joint; the apical ligament lies anterior to the lip of the foramen magnum and inserts into the apex of the odontoid process.
• The apical ligament, located anteriorly at the lip of the foramen magnum, attaches to the apex of the odontoid process. Its functional significance in terms of craniocervical motion has been debated in studies. [1, 7, 11]
• The paired alar ligaments secure the apex of the odontoid to the anterior foramen magnum. They play a vital role in limiting the rotational movements and lateral bending of the neck. [1, 7, 11]
• The tectorial membrane is a continuation of the posterior longitudinal ligament (PLL) to the anterior margin of the foramen magnum. It provides additional support and stability to the upper cervical spine. [1, 7, 11]
• The 3 cm × 5 mm accessory atlantoaxial ligament not only connects the atlas to the axis but also continues cephalad to the occipital bone. Functionally, it becomes maximally taut with 5-8° of head rotation, lax with cervical extension, and maximally taut with 5-10° of cervical flexion. It seems to participate in craniocervical stability; future improvements in magnetic resonance imaging may lead to better appreciation of its structure and integrity of this ligament. [12]

Lower Cervical Spine

The five cervical vertebrae that make up the lower cervical spine, C3-C7, are similar to each other but very different from C1 and C2.

Each has a vertebral body that is concave on its superior surface and convex on its inferior surface (see the image below). This design facilitates articulation with adjacent vertebrae. [1]

On the superior surfaces of the bodies are raised processes or hooks called uncinate processes, each of which articulates with a depressed area on the inferior lateral aspect of the superior vertebral body, called the echancrure or anvil.

Normal anatomy of lower cervical spine. View Media Gallery

These uncovertebral joints are most noticeable near the pedicles and are usually referred to as the joints of Luschka. [13] They are believed to be the result of degenerative changes in the annulus, which lead to fissuring in the annulus and the creation of the joint. [14] These joints can develop osteophytic spurs, which can narrow the intervertebral foramina. The uncovertebral joints are crucial for spinal stability but are also common sites for degenerative changes. Osteophyte formation at these joints can lead to the narrowing of the intervertebral foramina, potentially causing nerve root compression, which is prevalent in conditions such as cervical spondylosis. [15]

Studies emphasize that the degenerative changes in these joints often begin in midlife and progress with age, leading to symptoms such as pain and neurological deficits due to nerve compression. [15]

The growth of osteophytes can impinge on nearby structures, including spinal nerves and the vertebral artery, further complicating clinical presentations. [15]

The spinous processes of C3-C6 are usually bifid, providing attachment points for ligaments and muscles, [1] whereas the spinous process of C7 is usually non-bifid and somewhat bulbous at its end, making it a prominent anatomical landmark. [1]

Anterior and posterior columns

The subaxial cervical spine can conveniently be divided into anterior and posterior columns. The anterior column consists of the typical cervical vertebral body, sandwiched between supporting discs. The anterior surface is reinforced by the ALL and the posterior body by the PLL, both of which run from the axis to the sacrum.

The articulations include disc-vertebral body articulations, uncovertebral joints, and zygapophyseal (facet) joints. The disc is thicker anteriorly, contributing to normal cervical lordosis, and the uncovertebral joints in the posterior aspect of the body define the lateral extent of most surgical exposures. The facet joints are oriented at a 45º angle to the axial plane, allowing a sliding motion; the joint capsule is the weakest posteriorly. The supporting ligamentum flavum, posterior, and interspinous ligaments also strengthen the posterior column. [16]

Studies emphasize the importance of maintaining proper alignment within both columns to prevent degenerative changes and enhance functional outcomes. Disruption in this balance can lead to conditions such as cervical spondylosis or herniated discs, which may necessitate surgical intervention. [1, 17, 18, 19]

Nerve supply

In the neuroanatomy of the cervical spine (see the image below), the cord is enlarged, with lateral extension of the gray matter consisting of the anterior horn cells. The lateral dimension spans 13-14 mm, and the anterior-posterior extent measures 7 mm. An additional 1 mm is necessary for cerebrospinal fluid anteriorly and posteriorly, as well as 1 mm for the dura. A total of 11 mm is needed for the cervical spinal cord. Exiting at each vertebral level is the spinal nerve, which is the result of the union of the anterior and posterior nerve roots.

Cross-sectional anatomy of cervical spinal cord. View Media Gallery

The foramina are the largest at C2-C3 and progressively decrease in size down to C6-C7. The spinal nerve and spinal ganglion occupy 25-33% of the foraminal space. The neural foramen is bordered anteromedially by the uncovertebral joints, posterolaterally by the facet joints, superiorly by the pedicle of the vertebra above, and inferiorly by the pedicle of the lower vertebra. Medially, the foramina are formed by the edge of the end plates and the intervertebral discs.

Interconnections are present between the sympathetic nervous system and the spinal nerve proper. The spinal nerves exit above their correspondingly numbered vertebral body from C2-C7. Because the numbering of the cervical spinal nerves commences above the atlas, eight cervical spinal nerves exist, with the first exiting between the occiput and the atlas (C1) and the eighth exiting between C7 and T1.

Cervical nerves primarily conduct motor (ventral root) and sensory (dorsal root) information throughout the neck and upper limbs: [1]

• Ansa cervicalis - This loop of nerves (C1-C3) innervates infrahyoid muscles essential for speech and swallowing.
• Brachial plexus - Formed from C5-T1 roots, it supplies motor and sensory innervation to the upper extremity.
• Sympathetic supply (derived from thoracic segments of spinal cord) - The sympathetic trunk runs parallel to the vertebral column, connected through specific branches to the nerve roots.

Vasculature

The vascular anatomy consists of a larger ASA located in the central sulcus of the cord and paired posterior spinal arteries (PSAs) located on the dorsum of the cord. It is generally accepted that the blood to the anterior two thirds of the cord is supplied by the ASA and the posterior one third is supplied by the posterior arteries.

Anterior spinal artery:

The ASA is formed by the anastomosis of the two anterior spinal branches of the vertebral arteries at the level of the foramen magnum. It descends along the anterior surface of the spinal cord within the anterior median fissure, supplying the anterior two thirds of the spinal cord. The ASA is reinforced by segmental medullary arteries that enter through intervertebral foramina, contributing to a complex vascular network. [1, 20]

Studies have highlighted the role of the ASA in various spinal vascular diseases such as arteriovenous malformations and fistulas. The ASA can function as both a feeder and a collateral channel in these conditions, affecting endovascular treatment approaches. [1, 10]

Posterior spinal arteries:

The paired PSAs, right and left, run along the dorsum of the spinal cord, supplying its posterior one third. They originate directly from the vertebral arteries and contribute to a pial plexus that encases the cord, facilitating communication between the anterior and posterior supplies. [1]

Collateral circulation:

The vascular network within the cervical spine is rich with collateral circulation. These segmental vessels play a critical role in maintaining blood flow in the event of compromised major spinal arteries. The anastomoses provide a buffer against ischemia, though insufficiencies in this network can result in localized cord ischemia, highlighting the fragility of spinal cord vasculature. [1, 20]

Research has uncovered significant anatomical variations in both the ASA and PSAs. Approximately 20% of vertebral arteries exhibit variations such as accessory vessels and lateral loops, which can complicate procedures such as cervical transforaminal epidural steroid injections. These variations increase the risk of intra-arterial injection due to their proximity to neural structures during such interventions. [21]

Additionally, studies have documented various anomalies in the vertebral artery's course, including lateral loops that may impinge on nerve roots, leading to compressive radiculopathy. Such findings show the importance of understanding vascular anatomy in clinical practice to avoid iatrogenic injuries during surgical procedures. [22]

Facet Joints

The facet joints in the cervical spine are diarthrodial synovial joints with fibrous capsules. They are lined with hyaline cartilage, which allows for smooth movement. [1] The joint capsules are more lax in the lower cervical spine than in other areas of the spine, allowing gliding movements of the facets. The joints are inclined at an angle of 45° from the horizontal plane and 85° from the sagittal plane. This alignment helps prevent excessive anterior translation and is important in weight-bearing. [23] These joints play a significant role in maintaining spinal stability and permitting a range of motion, including flexion, extension, lateral bending, and rotation. [1]

Research emphasizes that these joints contribute significantly to spinal load transmission, accounting for 3-25% of the load under normal conditions. The degeneration of facet joints can lead to instability and is often associated with conditions such as spondylolisthesis. Furthermore, the mechanical environment surrounding these joints is vital for maintaining cartilage health as changes in loading can affect cartilage homeostasis and lead to degeneration. [24]

Nerve supply

The fibrous capsules are innervated by mechanoreceptors (types I, II, and III), and free nerve endings have been found in the subsynovial loose areolar and dense capsular tissues. [25] In fact, there are more mechanoreceptors in the cervical spine than in the lumbar spine. [2] This neural input from the facets may be important for proprioception and pain sensation and may modulate protective muscular reflexes that are important for preventing joint instability and degeneration.

The facet joints in the cervical spine are innervated by both the anterior and posterior rami. The atlanto-occipital and atlantoaxial joints are innervated by the anterior rami of the first and second cervical spinal nerves. The C2-C3 facet joint is innervated by two branches of the posterior ramus of the third cervical spinal nerve innervate, a communicating branch and a medial branch known as the third occipital nerve.

The remaining cervical facets, from C3-C4 to C7-T1, are supplied by the posterior rami medial branches that arise one level cephalad and caudad to the joint. [26, 27] Therefore, each joint from C3-C4 to C7-T1 is innervated by the medial branches above and below. These medial branches send off articular branches to the facet joints as they wrap around the waists of the articular pillars.

Intervertebral Discs

Intervertebral discs are located between the vertebral bodies of C2-C7. Intervertebral discs are located between each vertebral body caudad to the axis. These discs are composed of four parts: the nucleus pulposus in the middle, the annulus fibrosis surrounding the nucleus, and two end plates that are attached to the adjacent vertebral bodies. They serve as force dissipators, transmitting compressive loads throughout a range of motion. The discs are thicker anteriorly and therefore contribute to normal cervical lordosis.

The intervertebral discs are involved in cervical spine motion, stability, and weight-bearing. The annular fibers are composed of collagenous sheets (lamellae) that are oriented at a 65-70° angle from the vertical and alternate direction with each successive sheet. As a result, they are vulnerable to injury by rotation forces because only one half of the lamellae are oriented to withstand the force applied in this direction. [2]

The middle and outer one third of the annulus is innervated by nociceptors. Phospholipase A2 has been found in the disc and may be an inflammatory mediator. [28, 29, 30]

Ligaments

Although the cervical spine consists of seven cervical vertebrae interspaced by intervertebral disks, the complex ligamentous network keeps the individual bony elements behaving as if they were a single unit.

As noted, the cervical spine can be viewed as being made up of anterior and posterior columns. It can also be useful to think in terms of a third (middle) column, as follows:

• The anterior column consists of the ALL and the anterior two thirds of the vertebral bodies, the annulus fibrosus, and the intervertebral discs.
• The middle column is composed of the PLL and the posterior one third of the vertebral bodies, the annulus fibrosus, and the intervertebral discs.
• The posterior column is made up of the posterior arches, including the pedicles, transverse processes, articulating facets, laminae, and spinous processes.

The longitudinal ligaments are vital for maintaining the integrity of the spinal column. Whereas the anterior and PLLs maintain the structural integrity of the anterior and middle columns, the posterior column alignment is stabilized by a complex of ligaments, including the nuchal and capsular ligaments, and the ligamentum flavum.

If one of the three columns is disrupted as a result of trauma, stability is provided by the other two, and cord injury is usually prevented. With the disruption of two columns, spinal cord injury is more likely because the spine may then move as two separate units.

Several ligaments of the cervical spine that provide stability and proprioceptive feedback are worth mentioning and are briefly described here. [31, 32]

The transverse ligament, the major portion of the cruciate ligament, arises from tubercles on the atlas and stretches across its anterior ring while holding the odontoid process (dens) against the anterior arch. A synovial cavity is located between the dens and the transverse process. This ligament allows rotation of the atlas on the dens and is responsible for stabilizing the cervical spine during flexion, extension, and lateral bending. The transverse ligament is the most important ligament for preventing abnormal anterior translation. [33]

The alar ligaments run from the lateral aspects of the dens to the ipsilateral medial occipital condyles and to the ipsilateral atlas. They prevent excessive lateral and rotational motion while allowing flexion and extension. If the alar ligaments are damaged, as in whiplash, the joint complex becomes hypermobile, which can lead to kinking of the vertebral arteries and stimulation of nociceptors and mechanoreceptors. This may be associated with the typical complaints of patients with whiplash injuries (e.g., headache, neck pain, and dizziness).

ALL and the PLL are the major stabilizers of the intervertebral joints. Both ligaments are found throughout the entire length of the spine; however, the ALL adheres more closely to the discs than the PLL does, and it is not well developed in the cervical spine. The ALL becomes the anterior atlanto-occipital membrane at the level of the atlas, whereas the PLL merges with the tectorial membrane. Both continue onto the occiput. The PLL prevents excessive flexion and distraction. [34]

The supraspinous ligament, the interspinous ligaments, and the ligamentum flavum maintain stability between the vertebral arches. The supraspinous ligament runs along the tips of the spinous processes, the interspinous ligaments run between adjacent spinous processes, and the ligamentum flavum runs from the anterior surface of the cephalad vertebra to the posterior surface of the caudad vertebra.

The interspinous ligament and (especially) the ligamentum flavum control for excessive flexion and anterior translation. [34, 35, 36] The ligamentum flavum also connects to and reinforces the facet joint capsules on the ventral aspect. The ligamentum nuchae is the cephalad continuation of the supraspinous ligament and has a prominent role in stabilizing the cervical spine.

Children have significant anatomical variations in the craniocervical junction compared with adults. Besides the weaker ligaments and paraspinal muscles, the greater mobility of the articulations and the unfused dentocentral synchondrosis (between the odontoid and C2 body), may contribute to the instability of the craniocervical junction in young children as compared to older children and adults. [37] Surgical management of craniocervical junction instability in children poses unique challenges. While the indications for cervical fusion are similar to those in adults with regard to technique, in children, significant anatomical variations in the craniocervical junction complicate the approach and limit the use of internal fixation. Treatment is hindered by the diminutive bone and ligamentous structures, which are often complicated by syndromic craniovertebral abnormalities. Advances in imaging have improved outcomes. Menezes reviewed 850 children who underwent craniocervical fusion. The author presents a detailed review of the technique of fusion, as well as the indications and means of avoidance of complications, their prevention, and their management. [38]