The bones of upper cervical spine include
- Base of skull surrounding the foramen magnum
- Pair of occipital condyles
- Atlas vertebra (C1)
- Axis vertebra (C2)
Occipital Condyles
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 Vertebra
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
Axis Vetebra
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.
Ligaments
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
Neurovascular 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.
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