The portion of the spine encompassing neck region forms the cervical spine. The cervical spine is formed by the first seven vertebrae 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 the anterior aspect. It ends when C7 joins with the first thoracic vertebra.
Understanding the anatomy of cervical spine is vital to understand the various problems and abnormalities of the 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 the upper cervical spine include
- The 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 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 appears like a ring.
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 is the second vertebra and has a 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 the 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.
Ligaments of Upper Cervical Spine
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 ligament connects 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. It fills about 50% of the neural canal in the upper cervical spine.
- 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.
- 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 [below axis vertebra] cervical spine, includes the C3 to C7 vertebral segments. Lower cervical vertebrae have a relatively uniform anatomic configuration as compared to the anatomy of upper cervical spine.
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 endplates. The uncinate processes articulate with rounded inferolateral borders of the superior vertebral body and the articulation is called an uncovertebral joint.
This joint marks the lateral extent of the vertebral body.
C3-C6 vertebrae have a typical anatomical structure –
- Small body, and broader from side to side than from front to back.
- Flattened anterior and posterior surfaces
- The upper surface
- Concave transversely
- Presents a projecting lip on either side.
- The lower surface
- Concave from front to back
- Convex from side to side
- On either side laterally they have shallow concavities
- Concavities receive uncinate processes to form uncovertebral joints
- The vertebral foramen is large and of a triangular form.
- The transverse processes are each pierced by the foramen transversarium
Transverse foramina are present consistently in upper six vertebrae but variable in C7. Even when it is present, it is small.
The transverse foramen is formed by the fusion of the anterior and posterior part of the transverse process and part of the pedicle.
Transverse foramina give passage to the vertebral artery and vein in upper six cervical vertebrae. In C7 it contains only vein as well as a plexus of sympathetic nerves.
C7 has enlarged spinous process called vertebra prominence. It is the most prominent structure that can be palpated when we pass our finger downwards from the skull.
Neural foramina of cervical spine allow exit of cervical spinal nerves which are eight in number and are named as C1 to C8.
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.
Pedicles in lower cervical vertebrae project in a posterolateral to anteromedial orientation.
Articular facets are flat
- Superior articular facets face backward, upward, and slightly medially
- The Inferior facets are forward, downward, and slightly laterally.
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 are narrow, and thinner above than below.
The laminae arise from the posteromedial border of the lateral masses project posterior and toward the midline to form bifid spinous processes.
The spinous process is short and bifid.
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 cervical nerves control many bodily functions and sensory activities.
C1: Head and neck
C2: Head and neck
C4: Upper body (e.g. Deltoids, Biceps)
C5: Wrist extensors
C6: Wrist extensors
Injury to cervical spine or involvement of spinal nerves affects the area they supply.