Anatomy of Skull & base in a Nutshell

Anatomy of Skull & base

Definitions:

• Skull: bones of the head
  • Neurocranium (brain-box)

1. 1. • Large, flat, curved bones that interlock via sutures
      • Responsible for protecting the brain

1. • Viscerocranium (Facial Skeleton)

1. 1. • Bones of the face
      • Includes: nasal bones, orbit bones, mandible, maxilla etc.

• Cranium: the skull without the mandible (lower jaw)
• Calvaria: Dome-like cap of the skull
• Cranial vault:
  • The interior of the neurocranium
  • Remove the calvaria
  • Supports the brain (brain rests on it)
• Vertex:
  • The highest point on the skull
  • Usually in the midline along the sagittal suture

Frontal bone:

• Makes up the forehead
• Has significant contribution to the roof of the orbit with its anterior / inferior portion curving back into the orbit.
• Structures:
  • Glabella: A slight elevation that happens between the supraciliary ridges / arches (deep to eyebrows)
  • Supraorbital margin: At border between orbit and forehead. This is where the forehead becomes the orbital.
  • Nasion: Intersection of the frontal bone with the nasal bones. On the “bridge” of the nose.
  • Supraorbital foramen:

1. 1. • Right above the supraorbital margin
      • usually not a complete foramen, maybe just a notch.

• • • It transmits the supraorbital nerve, artery, and vein.

Maxilla:

• Is the upper jaw
• Articulates (touches) with the frontal bone, nasal bones, and zygomatic bones.
• Is a fusion of two bones in which you cannot see the suture (too fine to see)
• Structures:
  1. Superior Alveolar processes: Bony sockets for the upper teeth
  2. Infraorbital Foramen: Transmits the infraorbital nerve, artery, and vein.

Zygomatic bones:

• Are the cheek bones
• Contribute to the orbit
• Articulates with frontal bone and maxilla
• Temporal Process of the Zygomatic bone: Process that stretches toward the temporal bone.

Mandible:

• Structures:
  • Inferior Alveolar processes: bony sockets for the lower teeth
  • Mental foramen: transmits the mental nerve, artery, and vein.

Parietal bones (2):

• Large curved bone
• Articulates with frontal bone, sphenoid, temporal bone, and occipital bone
• Temporal Lines:
  • Superior temporal line / Inferior temporal line
  • Serve as attachment point for Temporalis muscle and its fascia

Temporal bones (2):

• Has 4 parts:
  • Squamous portion: Flat portion
  • Petrous portion:

1. 1. • Dense part of the bone (“rock-like”)
      • Projects into the cranial vault

• • Mastoid portion: the mastoid process
  • Tempanic part: includes External auditory meatus- opening to the ear canal
• Zygomatic process of the temporal bone: contributes to the zygomatic arch
• Zygomatic arch: is made up of the zygomatic process of the temporal bone + the temporal process of the zygomatic bone.

Sphenoid bone:

• Can see the Greater Wing of the sphenoid bone superficially
• The most complicated bone of the skull
• Most is interior to the cranial vault

Occipital bone:

• External occipital protuberance
• Lines:
  • Superior / inferior occipital (nucchal) lines
  • Points of attachment for muscles of the neck
• Foramen magnum: transmits the spinal cord

Pterion: An small line (suture) of the skull in which 4 bones come together

• Frontal, parietal, sphenoid, temporal
• The skull is particular thin here
• Middle meningial artery:
  • Supplies the meninges
  • Skull fractures to the pterion area can damage the branches of this artery

Sutures of the skull:

• Immovable joints for the bones of the skull
• Flat and curved bones are interlaced together rather than have flat surfaces for union.
• Coronal Suture:
  • Between the frontal bone and the parietal bones
  • Also called Frontoparietal suture
• Sagittal suture:
  • Goes right down the middle
  • Between the two parietal bones (called interparietal suture)
• Lambdoidal suture:
  • Between two parietal bones and the occipital bone
  • Also called occipitoparietal suture
• Intersection points between sutures
  • Bragma: Anterior intersection point
  • Lambda: Posterior intersection point
  • These are key structures for neurosurgeons.  The anatomy deep to these intersections is very similar amongst the population even though brains / skulls vary in general.
• Sutures are not closed in a newborn
  • Allows for brain growth
  • Flexibility for passage through vaginal canal
  • Fontanelles:

• Membranous structure (areas) where bones have not fused (soft spots)
• Fontanelles:
  • Anterior fontanelle: Bragma
  • Posterior fontanelle: Lambda
• Can tell pressure of fluid by physical exam of these soft spots.

1. • Premature suture closing is badà impending brain constriction

Cranial vault:

• Has three fossae
  • Anterior cranial fossa

1. 1. • The most anterior
      • Is the shallowest of the three

• • • Formed primarily from the frontal bone, ethmoid bone, and the lesser wing of sphenoid bone.

• • • Supports the frontal lobes of the brain
    • Important structures:
      1. Crista Galli:
         1. Crest of bone in midline of ethmoid bone
         2. Triangular projection
         3. Anterior attachment point for the dural fold called Falx Cerebri
      2. Cribiform plate:
         1. Also part of ethmoid bone
         2. Plate of sieve-like bone, many holes
         3. The olfactory nerves (CN I) projects up through this

1. • Middle Cranial Fossa

• • • Sphenoid bone:
      1. Very distinct shaped bone that looks like a butterfly
      2. makes up the bulk of the middle cranial fossa
    • Temporal bone makes up the remainder of this fossa.

• • • Supports the temporal lobes of the Cerebral hemispheres

• • • Pituitary fossa:
      1. a very distinct fossa in the center
      2. where the pituitary gland sits
      3. the gland itself sits in the Sella Turcica, which is a bony depression between two bony processes.
    • 5 or 6 Cranial nerves have foramen in this fossa

• • Posterior Cranilal Fossa

• • • The largest and the deepest of the three fossae
    • Composed primarily from the occipital bone

• • • Supports the cerebellum, pons, medulla

• • • Jugular Foramen: Laterally located.
    • This is the starting point for the internal jugular vein.
    • cranial nerves IX,X,XI exit here
  • Foramen Magnum:
    • Very large opening
    • Transmits:
      1. Spinal cord
      2. Vertebral arteries
      3. Spinal portion of spinal accessory nerve (thus cranial nerve XI goes through both jugular foramen and foramen magnum)