Skull base : Development and anatomy

Dr. Sarbesh Tiwari

The skull base represents a central and complex bone structure of the

skull that forms the floor of the cranial cavity on which the brain lies.

It separates brain from facial structures and suprahyoid neck.

Anatomical knowledge of this particular region is important for

under-standing several pathologic conditions as well as for planning

surgical procedures.

2Anatomy skull base

The human skull consists of three components:

(1) the membranous neurocranium, which constitutes the flat bones of

the skull,

(2) the cartilaginous neurocranium or chondrocranium which forms the

majority of the skull base, and

(3) the viscerocranium or facial skeleton.

3Anatomy skull base

The basicranium develops primarily from cartilage precursors, with a

small component from membranous bone.

The development of the cartilaginous skull base begins around the

40th day of gestation, with the conversion of mesenchyme into

cartilage.

Occipital sclerotomal mesenchyme concentrates around the

notochord and extends cephalically forming the floor of brain.

4Anatomy skull base

The parachordal cartilage – Around the notochord.

Sclerotomal cartilage – Occipital bone.

2 hypophyseal cartilage – Fuse to form basisphenoid cartilage.

2 presphenoid cartilage – body of sphenoid. ‘

Orbitosphenoid and Alisphenoid – wings of sphenoid.

5Anatomy skull base

6Anatomy skull base

The chondrocranium begins to form when the collections of mesenchyme accumulating around and in front of the notochord condense into cartilage.

These chondrification centers, termed the parachordal cartilages, form early in the seventh week adjacent to the rostral end of the notochord and contribute to the creation of the basal plate.

The parachordal cartilage fuse with the sclerotomes arising from the occipital somites surrounding the neural tube.

7Anatomy skull base

Mesenchymal condensations migrating to the rostral end of notochord at the region of rathke’s pouch form the polar or hypophyseal cartilages.

Rostral extensions of these cartilages surround the craniopharyngealcanal and join to create the presphenoid.

Together with the trabecular cartilages, the hypophyseal chondrification centers fuse to form the precursors of the central skull base.

Laterally, the cartilages of the orbitosphenoid (lesser wing) and alisphenoid (greater wing) combine with the centrally positioned basisphenoid and presphenoid cartilages later to form the sphenoid bone.

8Anatomy skull base

The capsular tissue surrounding the nasal placodes chondrifies along with the trabeculi cranii, ossifies into the ethmoid and inferior nasal concha bones.

The midline segments of these bones create the nasal septum, which remains cartilaginous postpartum and acts as functional matrix for later midface growth.

9Anatomy skull base

Tissue surrounding the otic placodes also chondrifies then fuses with the parachordal cartilages to eventually create the petrous and mastoid segments of the temporal bone.

10Anatomy skull base


Dorsal view of the chondrocranium, or

base of the skull, in the adult showing

bones formed by endochondral

ossification.

Bones that form rostral to the rostral

half of the sella turcica arise from

neural crest and constitute the

prechordal (in front of the notochord)

chondrocranium (blue).

Those forming posterior to this

landmark arise from paraxial

mesoderm (chordal chondrocranium)

(red).



The skull base is composed of

five bones:

(1) ethmoid,

(2) sphenoid,

(3) occipital ,

(4) paired temporal , and

(5) paired frontal bones.

Fossa :-

1. Anterior cranial fossa

2. Middle cranial fossa.

3. Posterior cranial fossa. 14Anatomy skull base

It forms the bottom of the anterior skull , separating the anterior cranial

fossa from the paranasal sinuses and the orbits.

The boundaries :-

1. Anterior border :- posterior wall of the frontal sinus.

2. Posterior border :- lesser wing of the sphenoid bone and anterior clinoid

processes.

3. Floor :- roof of the nasal cavity and ethmoid sinuses medially.

4. Lateral wall :- thick and strong orbital plates of the frontal bone.



Frontal crest :- It’s a midline bony ridge that projects upwards and provide attachment to the falx cerebri.

Crista galli :- (latin for cock’s comb) Provides the site for anterior most attachment of the falx cerebri.

Cribriform plate :- It is a sheet of bone which contains numerous small foramina – these transmit olfactory nerve fibres (CN I) into the nasal cavity.

Anterior ethmoidal foramen transmits the anterior ethmoidal artery, nerve and vein. Posterior ethmoidal foramen transmits the posterior ethmoidal artery, nerve and vein.


The CSB makes up the floor of the

middle cranial fossa.

The sphenoid bone contributes to the

most of the CSB.

Anterior border :-

- tuberculum sellae, anterior clinoid

process, posterior margin of lesser

wing of sphenoid & anterior superior

rim of greater wing of sphenoid.

Posterior border :-

- superior border of petrous part of

temporal bone and the dorsum sellae

of sphenoid.


For diagnostic imaging purposes, it

is useful to subdivide the CSB

further into

midline sagittal,

off-midline parasagittal, and

lateral compartments

by drawing vertical lines passing

medially to the petroclival fissure

and just lateral to the foramen

ovale, respectively


Includes the body of the

sphenoid and the portion of the

clivus anterior to the spheno-

occipital synchondrosis

(basisphenoid),

contains the sphenoid sinus,

and

bordered superiorly by the sella

turcica and inferiorly by the

roof and posterior wall of the

nasopharynx20Anatomy skull base

It includes the petroclival synchondrosis, foramen lacerum, and

medial aspect of the greater sphenoid wing.

It is bordered superiorly and medially by the parasellar region

containing the cavernous sinus, superiorly and laterally by the basal

temporal lobes, and inferiorly by the parapharyngeal and masticator

spaces of the suprahyoid neck.

Many crucial neurovascular structures lay in this compartment ,

including cavernous sinus, superior orbital fissure, foramen

rotundum, vidian canal , and foramen lacerum.


formed by sphenoid triangle, squamous part of temporal bone, and

temporomandibular joint.


Anterior margin :- The posterior surface of the clivus.

Laterally – superiorly the posterior surface of the petrous part of temporal bone and inferiorly the condylar part of the occipital bone.

Posteriorly , the mastoid part of temporal bone and the squamous part of occipital bone.

Foramen magnum


The sphenoid bone is the foundation of the central skull base.

The shape of the sphenoid bone resembles that of a bird with wings

outstretched.

It consist of

a central body;

two sets of wings– the greater and lesser, which course laterally ;and

two pterygoid processes, which are directed inferiorly.


The superior surface articulates anteriorly with

the cribriform plate of the ethmoid bone and

contains a smooth central surface, the planum

sphenoidal.

Posteriorly, the chiasmatic sulcus forms a slight

depression and leads laterally to the optic canals.

The tuberculum sellae, a bony elvation, is found

just posterior to this sulcus.

Followed, posteriorly by sella turcica & dorsum

sellae.

The dorsum sellae terminates laterally into

the posterior clinoid processes.25Anatomy skull base

The anterior surface of the body of sphenoid

forms the roof & posterior wall of

nasopharynx.

„ The body houses the sphenoid sinus .

„ Lesser wings- forms medial portion of

orbital apex

„ Greater wings – course upward and

laterally from both sides of the sphenoid

body- forms floor of middle cranial fossa,

posterolateral orbit & lateral calvaria.


The pterygoid processes descend inferiorly from the sphenoid body.

The lateral pterygoid plate forms a portion of the medial wall of

the infratemporal fossa and provides attachment for the lateral

pterygoid muscle.

The medial pterygoid plate terminates inferiorly as a hook-like

process, the pterygoid hammlus around which the tendon of the

tensor veli palatini is slung.

Attachment to medial pterygoid muscles and pharyngobasilar

fascia.


The clivus is that part of the skull base situated between the

foramen magnum and the dorsum sellae.

Formed from sphenoid and occipital bones.

The petroccipital fissure forms the anterior lateral margin of the

clivus, while the synchondrosis between the basioccipital and

exoccipital bones forms the posterior lateral margins.

Normal fat signals in adult (late teens) in MRI.



The cavernous sinuses are situated on each side of the body of the

sphenoid bone and extend from the superior orbital fissure anteriorly to

the petrous apex posteriorly.

Receives :-

Superior ophthalmic vein

Inferior ophthalmic vein

Sphenoparietal sinus.

-Drains into

Petrosal sinus.

Pterygoid plexus.

Basilar plexus. Contents

1. III, IV, V1, V2 & VI

2. ICA. 30Anatomy skull base

Dural invagination at posterior aspect of

cavernous sinus

„ Contains gasserian ganglion (trigeminal).

Dural layers demonstrate thin peripheral

enhancement.

In high resolution MR, 3 sensory divisions

of trigeminal nerve can be visualized

leaving the gasserian ganglion.


The optic canal is formed by the lesser

wing of sphenoid.

The contents are :-

Optic nerve .

Ophthalmic Artery.

Sympathetic fibers from carotid plexus.


The triangular superior orbital fissure is

bounded medially by the body of the sphenoid,

above by the lesser wing, and below by the

greater wing and is completed laterally by the

frontal bone as the greater and lesser wings

converge.

Optic strut separates the optic canal from the

superior orbital fissure.

The optic canal and the superior orbital

fissure together form the orbital apex.


Content –

•Nerves 3,4,1st division of 5 , 6.

•„Orbital branch of middle meningeal art; sympathetic

nerve; recurrent meningeal art, sup. ophthalmic vein.


Extends from PPF along orbital floor.

„Separates greater wings of the sphenoid from the maxilla.

Content –

Maxillary branch of trigeminal nerve. Infraorbital vessels. Emissary veins connecting inferior

ophthalmic vein to pterygoid venous plexus.

Zygomatic nerve.


Is actually a canal in the base of the greater sphenoid

wing, is situated just inferior and lateral to the

superior orbital fissure.

The canal extends obliquely forward and slightly

inferiorly, connecting the middle cranial fossa to the

pterygopalatine fossa.

The canal transmits the maxillary nerve ( V2) the

artery of the foramen Rotundum and emissary veins.

This foramen is best visualized by means of coronal

CT


• Endocranially, the foramen ovale is situated

posterolateral to foramen rotundum;

exocranially, it is found at the base of the

lateral pterygoid plate.

Contents :-

1. Mandibular Nerve (CN V3)

2. Accessory meningeal nerve

3. Lesser petrosal nerve

4. Emissary vein (Cavernous sinus to

pterygoid plexus)

5. Occasionally anterior trunk of middle

meningeal artery


It is an aperture in the greater wing of the

sphenoid posterolateral to foramen ovale.

Contents :-

1. Middle meningeal artery & vein.

2. Emissary vein.

3. Nervous spinosus (Meningeal branch

of mandibular nerve)


The vidian canal (pterygoid canal) is located in

the floor of the sphenoid sinus at the junction

of the pterygoid process and the sphenoid body

connecting the pterygopalatine fossa anteriorly

and the foramen lacerum posteriorly.

Contents :-

- Vidian Artery ( Br. Of Maxillary

Artery).

- Vidian Nerve (greater superficial

petrosal nerve and deep petrosal nerve )


It is located at the base of medial pterygoid

plate, anterior to the petrous apex.

The contents are :-

Structures passing whole length:

1. Meningeal branch of Ascending pharyngeal

artery

2. Emissary vein

Other structures partially traversing:

3. Internal carotid artery

4. Greater petrosal nerve.


The carotid canal is a passage within the petrous

temporal bone and transmits the internal carotid

artery and sympathetic plexus.

The carotid canal is initially directed superiorly,

then turns anteromedially to reach up to

the petrous apex.

It approximately runs 2cm and opens to foramen

lacerum.


A bony conduit that transmits VII & VIII

cranial nerves from pontomedullary

junction to inner ear.

Divided by a bony lamina (falciform crest)

into :-

A. Smaller superior part

• Superior vestibular N.

• Facial Nerve

B. Larger Inferior part

• Inferior vestibular N.

• Cochlear nerve. 42Anatomy skull base

The jugular foramen is divided by a fibrous

or bony septum, the jugular spine, into:

A. the pars nervosa: smaller and

anteromedial-

i. Inferior petrosal sinus.

ii. Glossopharyngeal nerve with its

tymapnic branch (Jacobson’s Nerve).

B. the pars vascularis: larger and

posterolateral-

i. Jugular bulb.

ii. Vagus Nerve with its auricular

branch (Arnold’s nerve).

iii. Spinal accessory nerve 43Anatomy skull base

The appearance of the jugular foramen is anatomically variable, and

sometimes both cranial nerves IX and X traverse through the pars

nervosa.

The right jugular foramen is larger than the left in 75% of the

population.

When the roof of the jugular bulb is seen above the level of floor of

internal auditory canal , it is called a highriding jugular bulb, which is

more common on the right side.

This is a dangerous variant and compromises the exposure during

translabyrinthine surgery.


Located within occipital bone.

The hypoglossal (or anterior condyloid)

canal is a paired bone passage that runs

lateral to and slightly forward from the

posterior cranial fossa to the

nasopharyngeal carotid space.

It transmits the hypoglossal nerve.

Intracanalicular enhancement is always

present (emissary veins), with linear

filling defects ( nerve rootlets).


The foramen magnum is entirely formed

within the occipital bone.

Contents :-

1. Medulla oblongata.

2. Vertebral arteries and veins.

3. Anterior and posterior spinal arteries.

4. spinal component of spinal accessory

nerve.

5. Tectorial membrane and alar ligament.



48

Parapharyngeal , masticator, carotid, and retropharyngeal spaces are

seen in close contact with the skull base along their cephalad aspect .

Parapharyngeal space extends caudally to the submandibular space

and cranially abuts the base skull. It contains fat within, which acts as

a medium for infection.


Masticator space connects the mandible to the skull base. Odontogenic

infections and oropharyngeal squamous cell carcinoma can tract along

masticator space to the base skull.

Intracranial extension of the tumor can occur

via third division of trigeminal nerve, mandibular nerve (perineural

spread) through the foramen ovale.

Vascular lesions such as jugular vein thrombosis and neural tumors such

as Schwannoma, Neurofibromas, and Paraganglioma are seen in the

carotid space.



A fat filled space between the pterygoid

plates and the posterior wall of maxillary

sinus.

Shaped like an inverted pyramid.

Borders :-

The walls of the PPF are as follows:

Medial - perpendicular plate palatine bone.

Lateral - narrowing to pterygomaxillary fissure

Anterior - posterior wall of maxillary sinus

posterior - Medial and lateral pterygoid plates;

inferior aspect of lesser wing of sphenoid bone52Anatomy skull base

The PPF is an important pathway for the spread of neoplastic and infectious

processes:

Medially - communicates with the nasal cavity via the sphenopalatine foramen.

Laterally - communicates with the masticator space (or infratemporal fossa) via

the pterygomaxillary fissure.

Anteriorly - communicates with the orbit via the inferior orbital fissure.

Posteriorly and superiorly - communicates with the Meckel cave and cavernous sinus (of

the middle cranial fossa) via the foramen rotundum.

Posteriorly and inferiorly - communicates with the middle cranial fossa via the vidian

canal, which transmits the Vidian nerve.

Inferiorly - communicates with the palate via the greater and lesser palatine canals 53


The infratemporal fossa is the space between the skull base, lateral

pharyngeal wall and the ramus of mandible.

Boundaries :-

Lateral- Ramus and condylar process of the mandible.

„Medial- Lateral pterygoid plate.

Anterior – Posterolateral wall of maxilla.

Posterior – Carotid sheath.

„Superior - CRANIAL BASE (greater wing of the sphenoid bone).

„Inferior – Medial pterygoid muscles.


Pterygoid muscles, medial and Lateral

„ Maxillary artery and vein

„ Pterygoid plexus of veins

„ Mandibular division of trigeminal nerve

„ Otic ganglion










1. Imaging of skull base: Pictorial essay Abhijit A Raut , Prashant S Naphade, and Ashish

Chawla. Indian J Radiol Imaging. 2012 OctDec; 22(4): 305–316.

2. Imaging of the Anterior Skull Base :Hemant Parmar,MD, SachinGujar,MD et.al. Neuroimag Clin

N Am 19 (2009) 427–439.

3. CT and MR Imaging of the Central Skul Base. Part 1: Techniques, Embryologic Development, and

Anatomy. Fredj Lame, MD, Lyn Nadel, MD, Ira F. Braun, MD. RadloGraphics 190; 10:59 1-602

4. Imaging of the Central Skull Base Alexandra Borges, MD. Neuroimaging Clinics of North

America Volume 19, Issue 3, August 2009, Pages 441–468.

5. Skull base embryology: a multidisciplinary review. Di Ieva A1, Bruner E, Haider T et.al Childs

Nerv Syst. 2014 Jun;30(6):991-1000.

6. Som and curtin.

7. http://www.med.wayne.edu/diagRadiology/Anatomy_Modules/axialpages/Overview.html

THANK YOU 65Anatomy skull base


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Skull base : Development and anatomy