Detailed Anatomy of the Skull and Facial Bones

FIFTH LECTURE

Cranial bones

Mandible and maxillary bones

Paranasal sinuses

The Skull

Nas

Skull: Anterior View

  • Function

    • Protect the brain and its processes

    • Framework of the head

    • Provides attachment of the cranial and facial muscles

Subdivisions

  • Cranial Bones

  • Facial Bones

The Cranial Bones

  • The cranial bones form the roof and the base of the skull.

  • They house and protect the brain.

  • This structure is also known as the neurocranium.

  • Consists of paired and unpaired bones.

  • The cranial skeleton consists of 8 bones:

    • Frontal

    • Parietal

    • Temporal

    • Occipital

    • Sphenoid

    • Ethmoid

Cranial Bones

Bone

Number

Type of Development

Frontal

1

Intramembranous ossification

Parietal

2

Intramembranous ossification

Temporal

2

Intramembranous ossification

Occipital

1

Endochondral ossification

Sphenoid

1

Intramembranous/Endochondral ossification

Ethmoid

1

Endochondral ossification

  • Total: 8 bones

Facial Bones (14 bones)

  • 2 nasal bones

  • 2 maxillas

  • 2 zygomatic bones

  • Mandible

  • 2 lacrimal bones

  • 2 palatine bones

  • 2 inferior nasal conchae

  • Vomer

Cranial Bones

  • The cranial bones are separated by fibrous joints called sutures.

  • Sutures are immovable fibrous joints.

Fossae Cranii

Structure

  • Optic groove

  • Crista galli

  • Frontal sinus

Cranial Fossae

  • Anterior cranial fossa

  • Middle cranial fossa

  • Posterior cranial fossa

Nasal Conchae

  • Superior, middle, and inferior nasal conchae

Sphenoidal Sinus
Other Features

  • Jugular foramen

  • Hypoglossal canal

  • Internal acoustic meatus

  • Sella turcica

Frontal Bone

  • Forms:

    • Forehead, most of the roof of the orbit, and the floor of the anterior cranial fossa.

  • Articulates with:

    • Nasal and zygomatic bones inferiorly.

  • Supraorbital margin has a supraorbital foramen for the passage of the supraorbital vessels and nerves.

Parts of Frontal Bone

  1. Squamous part

  2. Nasal part

  3. Two orbital plates

  4. Two zygomatic processes

Detailed Structures of Frontal Bone

Squamous Part

  • Forms largest portion of frontal bone.

  • Consists of:

    • External Surface: Convex and corresponding to forehead.

    • Hosts a curved elevation known as suprciliary arch.

    • Internal Surface: Deeply concave with a midline ridge known as frontal crest.

    • Foramen Cecum: Two small foramina at the posterior end of frontal crest.

Nasal Part

  • Small portion projecting inferiorly between right and left supraorbital margins.

  • Contains:

    • Nasal Notch: Articulates with nasal bones and frontal processes of the maxilla.

Orbital Part

  • Contains orbital plates, triangular-shaped flat sheets of bone:

    • Extend posteriorly from the supraorbital margin.

    • Inferior surface forms most of the roof of the orbit.

Zygomatic Process

  • Projects inferolaterally from each side of frontal bone.

  • Extends from lateral aspect of supraorbital margin to zygomaticofrontal suture.

Frontal Sinus

  • Description: Paired air-filled cavities in frontal bone.

  • Functions:

    • Warm and humidify inhaled air from the nasal cavity.

    • Lighten skull mass for more efficient head and neck muscle functioning.

    • Enhance voice resonance.

Fractures of Frontal Sinus

  • Type 1: Comminuted fractures of frontal sinus without a vertical trajectory.

  • Type 2: Vertical fractures involving the orbit but not frontal sinus.

  • Type 3: Vertical fractures involving frontal bone and sinus but not the orbit.

  • Type 4: Both frontal sinus and ipsilateral orbit involved.

  • Type 5: Cross midline, involving frontal sinus and contralateral orbit or both orbits.

The Parietal Bones

  • Two parietal bones form the greater portion of the side and roof of the cranial cavity.

  • Internal surfaces contain many protrusions and depressions accommodating blood vessels supplying the dura mater.

  • No foramina present in parietal bones.

The Temporal Bones

  • The temporal bone is a paired bone located on the sides and base of the skull.

  • Functions:

    • Protection of the brain

    • Houses organs of hearing and balance

    • Provides attachment for muscles

Parts of Temporal Bone

  1. Squamous Part: Forms side of skull above the ear.

  2. Tympanic Part: Surrounds external acoustic meatus.

  3. Mastoid Part: Located behind the ear, contains air cells.

  4. Petrous Part: Dense, pyramid-shaped part containing inner ear structures.

Structures of Temporal Bone

  • Consists of internal and external acoustic meatus, jugular fossa, carotid canal.

  • External Acoustic Meatus: Canal leading to tympanic membrane (eardrum).

  • Internal Acoustic Meatus: Canal with facial and vestibulocochlear nerves within the petrous part.

  • Jugular Fossa: Contains jugular bulb.

  • Carotid Canal: Passage for carotid artery to brain.

Clinical Relevance of Temporal Bone

  • Fractures: Can result in hearing loss, facial nerve damage, cerebrospinal fluid leaks.

  • Otitis Media: Infection of middle ear causing pain, fever, hearing loss.

  • Mastoiditis: Infection of mastoid air cells leading to ear pain.

  • Cholesteatoma: Abnormal skin growth, can erode bone, cause infections, hearing loss, dizziness.

The Occipital Bone

  • The bone of the occiput or back.

  • It has two parts:

    • Squamous Part

    • Basilar Part

    • Contains external and internal surfaces.

Occipital Condyle

  • Located on either side of the foramen magnum, where the skull rests on the vertebral column.

  • Hypoglossal Canal: Passageway for CN XII, located anterolateral to occipital condyle.

  • Condylar Canal: Location for emissary vein and meningeal branch of ascending pharyngeal artery.

Squamous Part - Internal Aspect

  • Consists of:

    1. Cruciate Eminence: Divides into upper (triangular-shaped for cerebrum) and lower (quadrilateral for cerebellum) fossae.

    2. Internal Occipital Protuberance: Located at intersection of the four divisions of the cruciate eminence.

Basilar Part - External Surface

  • Thick, quadrilateral shaped bone that articulates with the sphenoid bone.

    • Pharyngeal Tubercle: Attachment for superior constrictor muscle.

The Sphenoid Bone

  • Complexity: Known as the 'wasp bone', contributes to the base of skull.

  • Functions include providing foramina and canals for neurovascular structures.

Structure

Body

  • Median portion with air sinuses, cubical shape.

Greater Wings

  • Strong processes curving upward and laterally.

Lesser Wings

  • Triangular plates projecting laterally.

Pterygoid Processes

  • Directed inferiorly, consist of lateral and medial plates.

Borders and Relations

  • Sphenofrontal Suture: Articulation with frontal bone.

  • Sphenoparietal Suture: Articulation with parietal bone.

  • Sphenosquamosal Suture: Articulation with temporal bone.

  • Spheno-occipital Suture: Articulation with occipital bone (disappears by age 25).

Ethmoid Bone

  • Anterior cranial bone located between eyes, contributes to the medial walls of the orbit and the nasal cavity.

  • Forms nasal septum with vomer.

  • Very porous, delicate bone with three major portions:

    1. Perpendicular Plate: Superior 2/3 of nasal septum.

    2. Cribriform Plate: Forms roof of nasal cavity, contains olfactory foramina.

    3. Labyrinth: Large mass, contains ethmoidal cells (sinus).

Nasal Conchae

  • Involved in ensuring that inhaled air is cleansed, humidified, and warmed.

  • Damage to ethmoid can lead to leakage of cerebral fluid, infection chances, and potential anosmia.

Paranasal Sinuses

  • Contains mucosa-lined air-filled sinuses that maintain resonance:

    • Frontal Sinus

    • Sphenoid Sinus

    • Ethmoid Sinus

    • Maxillary Sinus

Facial Skeleton

  • Composed of paired bones: maxilla, zygomatic bones, nasal bones, lacrimal bones, palatine bones, nasal conchae and unpaired bones: mandible, vomer.

Maxilla

  • Largest bones of the face supporting upper teeth, contributing to the face’s structure.

  • Consists of a body and four processes - frontal, zygomatic, alveolar, palatine, developed through ossification centers.

Maxillary Fractures

  • Unilateral Fracture: Often involves alveolar process.

  • Bilateral Fractures: Classified as Lefort I, II, III depending on their severity and regions involved.

Mandible

  • Largest, strongest facial bone from the first pharyngeal arch.

  • Forms the lower jaw holding lower teeth.

Components of Mandible

  • Body: Curved horizontal portion.

  • Rami: Two upward projections perpendicular to body.

  • Condylar Process: Articulates with temporal bone.

  • Coronoid Process: Muscle attachment point.

  • Mental Foramen: Passage for blood vessels and nerves.

Clinical Anatomy of Mandible

  • Most fractures occur at the canine socket. Can lead to neuralgic pain or involvement with inferior alveolar nerve causing issues.

Nerve and Vascular Relations

  • Mandibular Foramen: Transmits the inferior alveolar nerve.

  • Mental Foramen: Transmits the mental nerve.

  • Areas around these foramina are significant for their relation to major nerves and vascular structures.

Types of Skull Fractures

  1. Closed Fracture: Skin over the fracture is intact.

  2. Open Fracture: Skin broken, bone exposed.

  3. Depressed Fracture: Skull indents into the brain cavity.

  4. Basal Fracture: Occurs in the floor of the skull around eyes, ears, nose.

Cleft Lip and Cleft Palate

  • Cleft Lip: Birth defect causing unilateral or bilateral opening in the upper lip.

  • Cleft Palate: Birth defect marked by opening in the mouth roof due to tissue development issues.