The portion of spine encompassing neck region forms cervical spine. Cervical spine is formed by first seven vertebra which are named as C1 to C7.
Anatomically, cervical spine starts where the top vertebra (C1) connects to the bottom of the skull.
Normal cervical spine has a lordosis.That means it is curved with convexity on anterior aspect. It ends when C7 joins with first thoracic vertebra.
Understanding cervical spine is vital to understand the various problems and abnormalities of neck. Anatomy of cervical spine is easier to understand by dividing it in upper and lower cervical regions
Anatomy of Upper Cervical Spine
The bones of upper cervical spine include
- Base of skull surrounding the foramen magnum
- Pair of occipital condyles
- Atlas vertebra (C1)
- Axis vertebra (C2)
The occipital condyles are under-surface facets of the occipital bone which articulates with the superior facets of the atlas vertebra.
The condyles are oval in shape, and their anterior extremities and they are directed forward and medialward.
The articular surfaces of the condyles are convex from before backward and from side to side, and look downward and lateralward.
Occipital condyles are covered by hyaline cartilage.
Atlas (C1) is the first cervical vertebra of the spine.
It is named for the Atlas of mythology, because it supports the globe of the head.
The atlas is along with the Axis (C2)– forms the joint connecting the skull and spine and are specialized to allow a greater range of motion. Together they are responsible for the nodding and rotation movements of the head.
Atlas has no body and consists of an anterior and a posterior arch and two lateral masses. It appear ring-like.
Two lateral masses on either lateral side provide the bulk of the atlas bone mass. The transverse foramina are located to the lateral aspect of the lateral masses
The axis has unique shape. It consists of a large bony protuberance and a pars articularis separating the superior from inferior articular processes.
The odontoid process or dens is a 2 to 3 cm long corticocancellous structure with narrowed waist and thickened cortical tip emanating in a rostral (towards the head) direction
from the vertebral body.
The odontoid rests in a recess behind the anterior arch of the atlas and the medial walls of the two lateral masses.
The superior articular surface of the axis is the cranial (towards head) surface of the vertebral body of the axis and lies separated by a narrow osseous recess lateral to the odontoid.
The two inferior articular processes of the axis are located on the inferolateral corner of the neural arch. The transverse foramen of the axis is located in the lateral aspect of the vertebral bodies on either side.
The joint surfaces of these three bony components of the upper cervical spine have little or no inherent stability. Thus the stability of this region is mainly determined almost entirely by ligamentous structures
The alar ligaments connect the medial surface of the occipital condyles to the lateral tip of the odontoid . A small portion also attaches obliquely along the medial upper wall of the atlas.
The apical ligament connects between a bony protuberance of the basion(The mid-point on the anterior margin of the foramen magnum on the occipital bone.) to the superior tip of the odontoid.
The transverse atlantal ligamentconnects the medial wall of the anterior third of either lateral mass of the atlas with one another.
Tectorial membrane is the rostral continuation of the posterior longitudinal ligament and is composed of a deep and a superficial layer.
Between the apical ligament and the deep layer of the tectorial membrane is the cruciate ligament bridging the basion and the axis body and a narrow transverse ligament, which extends between the upper ends of the lateral masses of the atlas.
Posteriorly, the upper cervical spine does not have well-developed ligamentous support structures.
The atlantoaxial and atlanto-occipital membranes, respectively form a thin protective barrier rather than an effective restraint mechanism.
Accessory atlantoaxial ligaments and the atlanto-occipital and atlantoaxial joint capsules are additional well-defined ligamentous support structures
The spinal cord at the craniocervical junction is located between the posterior halves of the lateral masses of the atlas and the pars interarticularis of the axis and fills not much more than 50% of the neural canal in the upper cervical spine.
The C-1 roots emerge from the spinal cord at a right angle and are located posterior to the occipital condyles superior to the lamina of the atlas.
The C-2 roots are larger than the C-1 roots and are located posterior and slightly caudal (towards foot) to the atlantoaxial joints.
The vertebral arteries emerge from the transverse foramina of the vertebral body of the axis in a cranial direction lateral to the pars interarticularis and the atlantoaxial joints.
At the level of the atlas the vertebral arteries will enter the transverse foramen, course medially in a shallow bony groove located on the superior surface of the lateral third of the atlas lamina, and then head into the cerebellar fossa.
Anatomy of Lower Cervical Spine or Subaxial Spine
The lower cervical spine or subaxial cervical spine [As it is below axis vertebra], includes the C3 to C7 vertebral segments. Lower cervical vertebrae have a relatively uniform anatomic configuration.
The vertebral body is larger in coronal (left to right) than its sagittal diameter (anterior to posterior). Bilateral prominences called uncinate processes are present along the lateral aspects of the superior end plates. The uncinate processes articulate with rounded inferolateral borders of the superior vertebral body and the articulation is called uncovertebral joint.
This joint marks lateral extent of the vertebral body.
C3-C6 vertebrae have a typical structures –
- The body of these four vertebrae is small, and broader from side to side than from front to back.
- Anterior and posterior surfaces are flattened and of equal depth former being on a lower level than the latter.
- The upper surface is concave transversely and presents a projecting lip on either side.
- The lower surface is concave from front to back and convex from side to side. On either side laterally they have shallow concavities which receive the corresponding projecting lips of the underlying vertebra.
- The pedicles are directed laterally and backward.
- The laminae are narrow, and thinner above than below
- The vertebral foramen is large and of a triangular form.
- The spinous process is short and bifid.
- The articular facets are flat. Superior articular facets face backward, upward, and slightly medially and inferior face forward, downward, and slightly laterally.
- The transverse processes are each pierced by the foramen transversarium, which, in the upper six vertebrae, gives passage to the vertebral artery and vein, as well as a plexus of sympathetic nerves.
C7 has enlarged spinous process called vertebra prominence. It is most prominent structure that can be palpated when we pass our finger downwards from skull.
Neural foramina of cervical spine allow exit of cervical spinal nerves which are eight in number and are named as C1 to C8.
In between two adjacent vertebrae is interposed intervertebral disc. The longus colli muscles lie directly over and insert onto the anterolateral aspects of each cervical vertebra.
The sympathetic plexus lies on top of the lateral muscle belly and may be injured aggressive dissection or retraction which can lead to Horner’s syndrome.
The prevertebral (deep) and alar (superficial) fascial layers separate the spine from the overlying esophagus.
Transverse processes of cervical spine are unique. They contain foramen for vertebral arteryformed by fusion of anterior and posterior part of transverse process and part of pedicle.
Pedicles in lower cervical vertebrae project in a posterolateral to anteromedial orientation.
The facet joints are formed by the interaction of superior and inferior articular processes. They are also known as the zygapophyseal articulations. The articular surfaces are angled approximately 45 degrees in relation to the transverse axis. The pillar of bone between the superior and inferior articular processes is commonly referred to as the lateral mass and is It is a useful site for posterior screw or wire stabilization.
The laminae arise from the posteromedial border of the lateral masses project posterior and toward the mid line to form bifid spinous processes. Ligamentum flavum or yellow ligament is present between space between two laminae.
Ligaments like interspinous and supraspinous ligaments or ligamentum nuchae form a posterior ligamentous complex and its disruption may lead to mechanical instability.
The spinal canal can be compromised by retropulsed vertebral body fragment, translational displacement in dislocations disc herniations or epidural hematoma.
The cervical nerves control many bodily functions and sensory activities.
C1: Head and neck
C2: Head and neck
C4: Upper body muscles (e.g. Deltoids, Biceps)
C5: Wrist extensors
C6: Wrist extensors
Injury to cervical spine or involvement of spinal nerves affect the area they supply.
Anatomy of First Cervical Vertebra or Atlas
First cervical vertebra sits just below skull. It is called the atlas after the mythological hero that bore weight of shoulder on its shoulders. It can be identified by the following features.
- It is ring shaped. It has no body. It also has no spine.
- The atlas has a short anterior arch, a long posterior arch, right and left lateral masses, and transverse processes.
- The anterior arch is marked by a median anterior tubercle on its anterior aspect. Its posterior surface bears an oval facet which articulates with the dens.
- The posterior arch forms about two fifths of the ring and is much longer than the anterior arch. Its posterior surface is marked by a median posterior tubercle. The upper surface of the arch is marked (behind the lateral mass) by a groove.
- Each lateral mass shows the following important features.
- Its upper surface bears the superior articular facet. This facet is elongated (forwards and medially), concave, and is directed upwards and medially. It articulates with the corresponding condyle to form an atlanto-occipital joint.
- The lower surface is marked by the inferior articular facet. This facet is nearly circular, more or less flat, and is directed downwards, medially and backwards. It articulates with the corresponding facet on the axis vertebra to form an atlantoaxial joint.
- The medial surface of the lateral mass is marked by a small roughened tubercle.
- The transverse process projects laterally from the lateral mass. It is unusually long and can be felt on the surface of the neck between the angle of the mandible and the mastoid process. Its long length allows it to act as an effective lever for rotatory movements the head. The transverse process is pierced by the foramen transversarium.
Attachments & Relations of Atlas
- The anterior tubercle provides attachment (in the median plane) to the anterior longitudinal ligament, and provides insertion (on each side) to the upper oblique part of the longus colli muscle.
- The upper border of the anterior arch gives attachment to the anterior atlanto-occipital membrane.
- The lower border of the anterior arch gives attachment to the lateral fibres of the anterior longitudinal ligament.
- The posterior tubercle provides attachment to the ligamentum nuchae (in the median plane) and gives origin to the rectus capitis posterior minor (on each side).
- The groove on the upper surface of the posterior arch is occupied by the vertebral artery and by the first cervical nerve. Behind the groove the upper border of the posterior arch gives attachment to the posterior atlanto-occipital membrane.
- The lower border of the posterior arch gives attachment to the highest pair of ligamenta flava.
- The tubercle on the medial side of the lateral mass gives attachment to the transverse ligament of the atlas.
- The anterior surface of the lateral mass gives origin to the rectus capitis anterior.
- The transverse process gives origin to the rectus capitis lateralis (upper surface anteriorly), the superior oblique (upper surface posteriorly), the inferior oblique (lower surface of the tip), the levator scapulae (lateral margin and lower border), the splenius cervicis, and the scalenus medius.
Anatomy of Second Cervical Vertebra or Axis
Second cervical vertebra is also knows as axis. It is identified by the presence of the dens (see below).It has following features.
- The superior surface of the body is fused with the dens, and is encroached upon on each side by the superior articular facets. The dens articulates anteriorly with the anterior arch of the atlas, and posteriorly with the transverse ligament of the atlas.
- The inferior surface has a prominent anterior margin which projects downwards.
- The anterior surface presents a median ridge on each side of which there are hollowed out impressions.
Dens (also called odontoid process)is a strong, tooth-like process projecting upwards from the body of axis. The dens is usually believed to represent the centrum (body) of the atlas or first cervical vertebra which has fused with the centrum of the axis.
- The pedicles are concealed (superiorly) by the superior articular process. The inferior surface presents a deep and wide inferior vertebral notch, placed in front of the inferior articular process. The superior vertebral notch is very shallow and is placed on the upper border of the lamina, behind the superior articular process.
- The laminae are thick and strong.
- Articular facets. Each superior articular facet occupies the upper surfaces of the body and of the massive pedicle. Laterally it overhangs the foramen transversarium. It is a large, flat, circular facet which is directed upwards and laterally. it articulates with the inferior facet of the atlas vertebra to form the atlanto-axial joint. Each inferior articular facet lies posterior to the transverse process and is directed downwards and forwards to articulate with the 3rd cervical vertebra.
- The transverse processes are very small and represent the true posterior tubercles only. The foramen transversarium is directed upwards and laterally.
- The spine is large, thick and very strong. It is deeply grooved inferiorly. Its tip is bifid, terminating in two rough tubercles.
- The dens provides attachment at its apex to the apical ligament, and on each side (below the apex) to the alar ligaments.
- The anterior surface of the body receives the insertion of the longus colli. The anterior longitudinal ligament is also attached to the anterior surface.
- The posterior surface of the body provides attachment, from below upwards, to the posterior longitudinal ligament, the membrana tectoria and the vertical limb of the cruciate ligament.
- The laminae provide attachment to the ligamenta flava.
- The transverse process gives origin by its tip to the levator scapulae, the scalenus medius (anteriorly) and the splenius cervicis (posteriorly). The intertransverse muscles are attached to the upper and lower surfaces of the process.
- The spine gives attachment to the ligamentum nuchae; the semispinalis cervicis, the rectus capitis posterior major, the inferior oblique; the spinalis cervicis, the interspinalis and the multifidus.
Seventh Cervical Vertebra
Cervical spine is formed by seven vertebras named as C1 to C7. First vertebra is known as atlas and second as axis. Third to sixth cervical vertebra are typical in anatomical structure. But as the cervical spine approaches its end, its last vertebra or seventh cervical vertebra, C7 changes in shape.
Seventh also known as the vertebra prominens because of its long spinous process, the tip of which can be felt through the skin at the lower end of the nuchal furrow or neck.
This vertebra prominens is used as landmark in clinical examination for numbering the vertebra.
It spine is thick, long and nearly horizontal. It is not bifid, but ends in a tubercle.
The transverse processes are comparatively large in size, the posterior root is larger than the anterior. The anterior tubercle is absent. The foramen transversarium is relatively small, sometimes double, or may be entirely absent.
The tip of the spine of the vertebra provides attachments to the ligamentum nuchae, the trapezius, the rhomboideus minor, the serratus posterior superior, the splenius capitis, the semispinalis thoracis, the spinalis cervicis, the interspinales, and the multifidus.
In the transverse process, the foramen transversarium usually transmits only an accessory vertebral vein. The posterior tubercle provides attachment to the suprapleural membrane. The lower border provides attachment to the levator costarum.
The anterior root of the transverse process may sometimes be separate. It then forms a cervical rib of variable size.
Cervical rib is an accessory rib in some individuals. It may be symptomless but frequently it leads t to pain and produces discomforts of varying degrees. This occurs due to compression of nerves and vessels in the neck by the rib.
Text adapted from: Human Anatomy by BD Chaurasia
Image Credit: Wikipedia
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