Occipital bone

The occipital bone (also known as C0) is a trapezoid skull bone that contributes to the posteroinferior part of the cranial vault. It is pierced by the foramen magnum, permitting communication from the cranial cavity to the vertebral canal.

Terminology

Occiput is a noun referring to the back of the head, it is not a synonym for the occipital bone. The occipital bone is also known as "C0" because it joins the skull to the first cervical vertebra or C1, forming the atlanto-occipital joint.

Gross anatomy

The occipital bone is composed of four parts:

• squamous part: external/internal surfaces
• basilar part (basiocciput): lower/upper surfaces
• lateral (jugular) parts (two): under/upper surfaces

The squamous part is the curved, expanded plate located behind the foramen magnum.

The external surface of the squamous part features:

• external occipital protuberance, the tip is known as the inion
• highest nuchal line
• superior nuchal line: occipitalis, trapezius, sternocleidomastoid, splenius capitis
• inferior nuchal line: rectus capitis posterior major and minor
• median nuchal line: attachment of ligamentum nuchae
• attachment of posterior atlanto-occipital membrane

The internal surface of the squamous part features:

• cruciate eminence divide region into four fossa; the upper two contain the occipital lobes while the lower two contain the cerebellar hemispheres
• internal occipital tuberosity at intersection
• sagittal sulcus for superior sagittal sinus, falx cerebri attaches to margins
• internal occipital crest gives attachment to falx cerebelli
• transverse grooves hold transverse sinuses

The basilar part is the quadrilateral piece in front of the foramen magnum.

The lower surface of the basilar part features:

• pharyngeal tubercle: attachment to fibrous raphe of pharynx
• either side of middle line: longus capitis, and rectus capitis anterior
• anterior atlanto-occipital membrane

The upper surface of the basilar part features:

• the clivus has a broad, shallow groove, for medulla oblongata
• lateral grooves for the inferior petrosal sinus

The under surface of the lateral part features:

• occipital condyles: articulate with the atlas (C1), permitting flexion and extension (nodding) with minimal lateral flexion. No rotation occurs at the atlanto-occipital joint.
  • the condyles are egg shaped and sloped inferomedially from anterior to posterior which helps limits mobility ²
  • a third condyle may be a variant
• hypoglossal canal: at base of occipital condyles
• condylar fossa: behind condyle, receives posterior margin of superior facet of atlas, can be perforated by condyloid canal which contains an emissary vein from the transverse sinus
• jugular process, excavated in front by jugular notch, forming posterior part of jugular foramen

The upper surface of the lateral part features

• jugular tubercle: overlies hypoglossal canal, sometimes crossed by oblique groove for CN 9, 10, 11

Articulations

The occipital bone articulates with 6 bones:

• unpaired
  • sphenoid (spheno-occipital synchondrosis)
  • atlas (atlanto-occipital joint)
• paired bones
  • parietal bone (lambdoid suture)
  • temporal bone (petroclival suture and occipitomastoid suture)

Development

The apical portion of the squamous part (above highest nuchal line): ossifies in membrane. Otherwise, the four parts ossify in cartilage. 

Four parts are present at birth. Fusion occurs:

• 2^nd year: squamous and jugulars
• 6^th year: jugulars with basilar
• 25^th  year: basilar with basisphenoid