CERVICAL VERTEBRAE

The cervical vertebrae are the smallest of the movable vertebrae. They are identified by the presence of foramina transversia hence they can be easily differentiated from the other vertebrae. There are 7 cervical vertebrae, out of which 3^rd to 6^th (C3, C4, C5 & C6) are typical; while 1^st (C1), 2^nd (C2) and 7^th (C7) are atypical vertebrae. The C1 is located close to the skull and then proceeding away from the skull to spine rest of the vertebrae situated.

The cervical spine supports the weight of the cranium and head & neck motion. 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 like nodding & rotation. However, the increased mobility and flexibility carry a higher spinal cord injury risk in this region. The cervical spine also protects the spinal cord and the foramina transversia allows passage of the blood vessels and sympathetic nerves through it. The cervical portion of the spine is an important one anatomically and clinically. It is within this region that the nerves to the arms arise via the brachial plexus, and where the cervical plexus forms providing innervation to the diaphragm among other structures.

TYPICAL VERTEBRAE

The typical cervical vertebra has a small, cylindrical and somewhat broad vertebral body. It bears less weight than the other vertebrae. The ventral body has the point for intervertebral articulation. The superior surface is concave transversely with upward projecting lips on each side known as the uncinate processes. The ventral body has a convex anterior surface. The anterior and posterior surfaces are flattened and slightly concave, & the discal margin gives attachment to anterior and posterior longitudinal ligament. The inferior surface is saddle shaped, convex from side-to-side & concave from before backwards.

The two pedicles project posterolaterally and longer laminae posteromedially, encloses in a large roughly triangular vertebral foramen. The vertebral foramen is larger than the vertebral body and it accommodates the spinal cord. The pedicles attach to the body midway between the discal surfaces of the body. The two laminae are thin, slightly curved, with thin superior & inferior vertebral notches are of similar depth.

The spinous process is a bony projection which is short, bifid and has 2 tubercles which are unequal in the size. The process extends from the vertebral arch that is from the junction between lamina and pedicle bulges laterally between superior and inferior articular processes to from lateral mass on each side. The nuchal ligament attaches here at the spinous process.

The transverse process is having foramina transversia on each side. The anterior and posterior roots end in the tubercles which is joined by the costotransverse bar or costal lamella. The transverse process gives passage to vertebral arteries, veins and sympathetic nerves. The attachment of anterior root to the pediculolaminar junction and posterior root to the capitellar process. The anterior tubercle of the sixth cervical vertebra is large and is called the carotid tubercle or Chassaignac tubercle because the common carotid artery can be compressed against it.

The superior and inferior articular facets form the articular pillars, it projects laterally at the junction of pedicle and lamina. The facets are flat and ovoid; the superior facet faces backwards, upwards and slightly medially & the inferior facet faces forwards, downwards and slightly laterally. Both the surfaces have a smooth surface known as articular facet. Both the facets articulate with the vertebra above creating the zygapophyseal joints. The intervertebral foramen is a passage way created by the vertebral arch, articular processes and zygapophyseal joints of adjacent vertebra; it allows spinal nerves to exit the vertebral canal from each vertebral level.

Uncinate Processes are bony projections on the lateral sides of the vertebral bodies contribute to the formation of uncovertebral joints, which provide additional stability and limit excessive lateral movement.

ATYPICAL VERTEBRA

The atypical vertebrae are C1, C2 & C7.

1.      FIRST CERVICAL VERTEBRA (ATLAS or C1)

The atlas is the first cervical vertebra which supports the atlanto-occipital joint. It is unique in that it fails to incorporate a centrum, whose expected position is occupied by the dens, a cranial protuberance from the axis. It is in ring shaped, has no body or spine, has short anterior arch, long posterior arch, transverse processes and right & left lateral masses. Due to its ring shape, it helps in accommodating the spinal cord which exits from the foramen magnum. The lateral masses are connected by the anterior and posterior arch.

The anterior arch is convex anteriorly and has a roughened anterior tubercle to which anterior longitudinal ligament is attached. The posterior surface of the anterior arch has a concave, circular facet which articulates with the dens.

The posterior arch forms the 3/5^th of the circumference of the atlantal ring and is larger than the anterior arch. The superior surface bears a wide groove for the vessels and nerves. The posterior surface is marked by the median posterior tubercle.

The lateral masses are oval and bears a kidney shaped superior articular facet to articulate the occipital condyle. The inferior articular facet is circular, flat or slightly concave. On the medial surface of the lateral masses have a roughened area which bears vascular foramina and a tubercle for attachment of transverse ligament.

The transverse processes are longer than all other cervical vertebrae except the 7^th vertebra. These processes act as strong levers for muscles and to keep head balanced. The apex of the transverse process is broad, flat and palpable between mastoid process and ramus of mandible. A small anterior tubercle is sometimes visible on the anterior aspect of the lateral mass. The costal lamella is sometimes deficient, which leaves the foramen transversarium open anteriorly.

2.      SECOND CERVICAL VERTEBRA (Axis or C2)

The axis or the second cervical vertebra which acts as an axle for rotation of the atlas and head around strong dens or odontoid process which projects cranially from the superior surface of the body. The dens is usually believed to represent the centrum or body of the atlas which has fused with the centrum of the axis.

The dens is conical in shape, slightly tilted on the body of the axis. The posterior surface has a broad groove for transverse ligament and is covered in cartilage. The apex is pointed and from there the apical ligament arises. The anterior surface bears an oval articular facet for the anterior arch of the atlas and surface is pitted by many vascular foramina which are numerous near the apex.

The body is a less compact bone which is composite and consists of partly fused centra of the atlas and axis, rudimentary disc between them which usually remains detectable deep within the body of the axis throughout the life. Large ovoid articular facets are present on either side of the dens at the junction of the body and neural arch which are flat or slightly convex for articulation with the masses of the atlas. The facets lie in a plane anterior to the plane of the intercentral (Luschka) articulations, with which they are, in part, homologous.

The pedicles are stout, and the superior surface carries part of the superior articular facet, which also projects laterally and downwards on to the transverse process. The anterolateral surface is deeply grooved by the vertebral artery, running beneath the thin lateral part of the inferior surface of the superior articular facet, which can become quite thin. The inferior surface of each pedicle bears a deep, smooth inferior intervertebral notch, in which the large root sheath of the third cervical nerve lies. The interarticular part of the pedicle is short and lies between the relatively small inferior posterior articular process (which is located at the pediculolaminar junction and bears a small anteriorly facing facet) and the superior articular surface. The transverse process is pointed, projects inferiorly and laterally, and arises from the pediculolaminar junction and the lateral aspect of the interarticular area of the pedicle. The rounded tip is homologous with the posterior tubercle of a typical cervical vertebrae. The foramen transversarium is directed laterally as the vertebral artery turns abruptly laterally under the superior articular facet. Small anterior tubercles may be present near the junction of the costal lamella with the body. The laminae are thick and give attachment to the ligamenta flava. The spinous process is large, with a bifid tip and a broad base, which is concave inferiorly. The ligamentum nuchae is attached to the apical notch.

3.      SEVENTH CERVICAL VERTEBRA (Vertebra Prominens or C7)

The seventh cervical vertebra (C7), also known as the vertebra prominens, is a distinctive and atypical vertebra characterized by its prominent spinous process, which is the longest among the cervical vertebrae and can be easily palpated through the skin at the lower end of the nuchal furrow. This spinous process is thick, nearly horizontal, and non-bifid, ending in a rounded tubercle that provides attachment for the ligamentum nuchae. The transverse processes of C7 are relatively large, with the posterior roots being more prominent than the anterior roots. The transverse foramina are smaller compared to those of other cervical vertebrae and typically transmit vertebral veins but not the vertebral artery. These foramina may be asymmetrical, double, or absent. Anteriorly, the costal lamella is thin, occasionally deficient, and may present as a cervical rib, which can sometimes compress nearby blood vessels or nerves, causing thoracic outlet syndrome. The vertebra's transverse process is grooved superiorly for the seventh cervical nerve and may feature a small anterior tubercle, though the posterior tubercle is more pronounced. While C7 is the most prominent cervical vertebra in about 70% of individuals, it can sometimes be surpassed in prominence by the spinous processes of C6 or T1. These unique anatomical features make C7 distinct among the cervical vertebrae.

MUSCLE & LIGAMENT ATTACHMENTS

The cervical vertebrae, forming the neck region of the spine, exhibit a complex network of muscle and ligament attachments essential for head movement and spinal stability.

Muscle Attachments

• Anterior Tubercle: The superior oblique parts of the longus colli muscle attach to the anterior tubercle on each side.
• Lateral Mass: The anterior surface of the lateral mass provides attachment for the rectus capitis anterior muscle.
• Posterior Tubercle: Just lateral to the posterior tubercle, the rectus capitis posterior minor muscle attaches.
• Transverse Process: The rectus capitis lateralis muscle attaches to the superior surface of the transverse process, while the obliquus capitis superior is located more posteriorly. The obliquus capitis inferior attaches laterally on the apex, below which are slips of the levator scapulae, splenius cervicis, and scalenus medius muscles.
• Body: The anterior surface of the body carries deep depressions on each side for the attachment of the vertical part of the longus colli muscle. The levator scapulae, scalenus medius, and splenius cervicis muscles attach to the tips of the transverse processes, and the intertransverse muscles attach to their upper and lower surfaces.
• Spinous Process: The lateral surfaces of the spinous process give origin to the obliquus capitis inferior muscle, and the rectus capitis posterior major muscle attaches a little more posteriorly. The inferior concavity of the process receives the semispinalis and spinalis cervicis, multifidus more deeply, and the interspinales near the apex.
• Tubercle of the Spinous Process: The trapezius, spinalis capitis, semispinalis thoracis, multifidus, and interspinales muscles all attach to the tubercle of the spinous process.
• Anterior Border of the Transverse Process: The scalenus minimus (pleuralis) muscle, when present, attaches to the anterior border of the transverse process. The first pair of levatores costarum muscles also attaches to the transverse processes.

Ligamentous Attachments

• Anterior Longitudinal Ligament (ALL): Attaches to the anterior and lateral aspects of the vertebral bodies and intervertebral discs, extending from the sacrum to C2.
• Posterior Longitudinal Ligament (PLL): Runs along the posterior aspect of the vertebral bodies or the anterior aspect of the vertebral canal, extending from the sacrum to C2.
• Ligamentum Flavum: Connects the laminae of adjacent vertebrae, limiting hyperflexion and enclosing the posterior aspect of the vertebral canal.
• Intertransverse Ligament: Connects the transverse processes of adjacent vertebrae.
• Interspinous Ligament: Weak ligament connecting the spinous processes of adjacent vertebrae.
• Nuchal Ligament: Continuation of the supraspinous ligament in the cervical spine, resisting hyperflexion.

Unique Ligaments of the Cervical Spine

• Alar Ligament: Connects the dens of C2 to the lateral margins of the foramen magnum.
• Apical Ligament: Connects the tip of the dens of C2 to the anterior aspect of the foramen magnum.
• Transverse Ligament: Holds the dens against the posterior aspect of the anterior arch of C1, forming part of the cruciate ligament.

These intricate muscle and ligament attachments work in concert to provide stability, support, and facilitate a wide range of head and neck movements.

NERVOUS SUPPLY

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 nerves exit the spine cranially to their associated vertebrae, while the 8th cervical nerve exits caudally to C7. 

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 innervation from the medial branch of the posterior primary rami. Coursing through the intervertebral foramen with the vertebral arteries are the vertebral nerves, which supply additional sympathetic innervation.

BLOOD SUPPLY

The cervical vertebrae are richly vascularized, with blood supply derived from both arterial and venous systems. These blood vessels play essential roles in supporting the metabolic needs of the cervical vertebrae, associated soft tissues, and the spinal cord. Below is a detailed description of the major blood vessels associated with the cervical vertebrae.

Arterial Supply

The arterial supply to the cervical vertebrae is primarily derived from branches of the subclavian arteries, with contributions from the external carotid arteries.

1. Vertebral Arteries

• The vertebral arteries arise as the first branches of the subclavian arteries and ascend through the transverse foramina of the cervical vertebrae, typically from C6 to C1.
• At the atlas (C1), the vertebral arteries curve posteriorly around the superior articular facet before entering the foramen magnum to supply the brain.
• These arteries provide critical branches to the spinal cord, meninges, and cervical vertebrae through:
• Anterior spinal arteries: Supply the anterior portion of the spinal cord.
• Posterior spinal arteries: Supply the posterior aspect of the spinal cord.
• Radicular arteries: Arise as small branches along the spinal cord and help vascularize the spinal nerve roots and nearby vertebrae.

2. Ascending Cervical Arteries

• Branches of the inferior thyroid artery (a branch of the thyrocervical trunk).
• These arteries ascend along the anterior surface of the cervical vertebrae and contribute to the blood supply of the deep cervical musculature, vertebrae, and spinal cord.

3. Deep Cervical Arteries

• Branches of the costocervical trunk (a branch of the subclavian artery).
• These arteries supply the posterior cervical musculature and contribute to the vascularization of the posterior aspects of the cervical vertebrae.

4. Occipital Arteries

• Branches of the external carotid artery, the occipital arteries supply the suboccipital region and upper cervical vertebrae.

5. Small Segmental Branches

• Other small branches from the external carotid artery and thyrocervical trunk provide supplemental blood supply to cervical vertebrae and adjacent soft tissues.

Venous Drainage

The venous system of the cervical vertebrae is interconnected and drains into larger systemic veins. It includes:

1. Vertebral Venous Plexus

• A complex network of interconnected veins located around the vertebral column.
• Composed of two main components:
• Internal vertebral venous plexus: Located within the vertebral canal and drains the spinal cord and meninges.
• External vertebral venous plexus: Located around the outer surface of the vertebrae, draining the vertebral bodies and adjacent muscles.
• The plexuses communicate freely with each other through intervertebral veins that pass through the intervertebral foramina.

2. Vertebral Veins

• These veins accompany the vertebral arteries and ascend through the transverse foramina of the cervical vertebrae.
• They drain blood from the vertebral bodies, cervical spinal cord, and deep neck muscles.
• Vertebral veins eventually empty into the brachiocephalic veins.

3. Deep Cervical Veins

• Run alongside the deep cervical arteries and drain the posterior neck and cervical vertebrae.
• These veins join the vertebral and brachiocephalic veins.

4. Jugular Venous System

Blood from the cervical vertebrae also drains into the internal jugular vein via smaller tributaries, such as those from the occipital veins.

OSSIFICATION

The cervical vertebrae, like other vertebral structures, develop from the paraxial mesoderm, specifically the sclerotome. During the fourth week of embryogenesis, sclerotome cells aggregate around the notochord and neural tube, initiating the formation of the vertebral column. Each vertebra initially forms from three primary ossification centers, one for each half of the neural arch and one for the centrum.

The ossification process in the cervical vertebrae is characterized by several key features:

• Atlas (C1): Ossifies from three primary centers: one for each lateral mass and one for the anterior arch. The lateral mass centers appear around the seventh week of intrauterine life and fuse posteriorly by the age of three. The anterior arch center appears around the first year of life and fuses with the lateral masses by the age of seven.
• Axis (C2): Ossifies from five primary centers: two for the vertebral arch, one for the centrum, and two for the dens. The dens ossifies separately and may not fuse completely with the body of the axis, leading to conditions like os odontoideum.
• Typical Cervical Vertebrae (C3-C7): Ossify from three primary and six secondary centers. The primary centers appear during the ninth to tenth week of fetal life for the neural arches and the third to fourth month for the centrum. The secondary centers, which include epiphyseal discs and tips of transverse processes, appear during puberty and fuse with the rest of the vertebra by the age of 25.