Skull base anatomy

1. By-
Dr. AdityaTiwari,
Resident, Dept. of E.N.T.

2.  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.
Introduction

3. The human skull consists of three components:
(1) Membranous neurocranium Forms at bones of skull
(2) Cartilaginous neurocranium / chondrocranium Forms majority of
skull base
(3) Viscerocranium or facial skeleton
The basicranium Develops from cartilage precursors, with a small
component from membranous bone.
Development of the cartilaginous skull base begins around the 40th day of
gestation, with conversion of mesenchyme into cartilage.
Occipital sclerotomal mesenchyme concentrates around the notochord &
extends cephalically forming the floor of brain.
Embryology of the skull base

4. 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.
The chondrification centres of skull

5. 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).

6.  Skull base boundaries:-
Anterior  Upper incisor teeth
Posterior  Sup. nuchal line of occipital bone,
Lateral Remaining upper teeth, the zygomatic
arch & its posterior root, the mastoid process
Composed of five bones:
1) Ethmoid, 2) Sphenoid, 3) Occipital ,
4) Paired temporal & 5) Paired frontal bones
Fossa :- 1) Anterior cranial fossa,
2) Middle cranial fossa, &
3) Posterior cranial fossa.

7. Anterior cranial fossa. Boundaries :-
Anterior: Posterior wall of the frontal sinus.
Posterior: lesser wing of the sphenoid & anteior clinoid processes.
Floor : roof of the nasal cavity & ethmoid sinuses medially.
Lateral wall : thick and strong orbital plates of the frontal bone

8. Middle cranial fossa Post. Cranial fossa
Anterior margin :- The
posterior surface of the
clivus.
Laterally:- superiorly the
posterior surface of the
petrous part of temporal
bone & inferiorly the
condylar part of the
occipital bone.
Posteriorly:- the mastoid
part of temporal bone &
the squamous part of
occipital bone.
Foramen magnum
The sphenoid bone
contributes to the most
of the CSB.
Anterior border :-
Tuberculum sellae, ant.
clinoid process, post.
margin of lesser wing of
sphenoid & anterior
superior rim of greater wing
of sphenoid.
Posterior border :- superior
border of petrous part of
temporal bone & dorsum
sellae of sphenoid.

9.  Behind the faciomaxillary bones, its made up of the occipital bone, temporal
bones and part of the sphenoid bones.
Occipital bone
 Has foramen magnum squamous part, petrous part & basi occiput.
 Ant margin of F. magnum gives attachment to membrana tectoria,
vertical limb of the cruciform ligament, and the apical & pair of alar
ligaments of the odontoid peg
 Occipital condyle attaches to atlas.
 Ant condylar canal12th CN
 Post.condylar canal vein from
the sigmoid sinus to the suboccipital
venous plexus.
Osteology of the skull base

10. It shows sup & inf nuchal line and ext occipital proturbance & crest.
Each half of the occipital region is subdivided into four areas. two alongside the
foramen magnum receive the recti.
Rectus capitis post minor Supplied by C1 Extend the head . Rectus capitis
post major supplied by C1 extend & rotate the head.
Bwt sup & inf nuchal lines, medial area
receives semispinalis capitis, supplied
segmentally by post primary rami of the
spinal nerves Chief extensor of the head.
Lateral area, superior oblique muscle
Supplied by C1 lateral flexor of the head

11. Temporal bone
 Its made up of squamosal, mastoid, petrous & tympanic parts which ossify
separately and later fuse creating squamotympanic & petrosquamos fissure
 Has styloid process Behind its base, stylomastoid foramen & post the mastoid
bone has the digastric notch, medial to which there is a groove for the occipital
artery. Stylomastoid foramen transmits facial nerve.


12. Foundation of MCF & 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 ; & two pterygoid processes, directed inferiorly.
The superior surface articulates ant with the cribriform plate of the ethmoid bone
& contains a smooth central surface, the planum sphenoidal.
Post, the chiasmatic sulcus forms
a slight depression & leads laterally
to the optic canals.
The tuberculum sellae, a bony
elvation, just posterior to this sulcus.
The sphenoid bone

13.  Followed, posteriorly by sella turcica & dorsum sellae.
 The dorsum sellae terminates laterally into the posterior clinoid processes
 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 & laterally
from both sides of the sphenoid
body-forms floor of MCF, posterolateral
orbit & lateral calvaria.

14. Its the skull base part situated between the foramen magnum and the dorsum
sellae.
Formed from sphenoid and occipital bones. Normal fat signals in adult (late
teens) in MRI.
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.
CLIVUS

15. Situated centrally in the skull base forming roof of the nasopharynx,
Boundaries Formed by the line of attachment of the pharyngeal wall.
The pharyngobasilar fascia is attached to the skull base and medial pterygoid
plates thickened post into a pharyngeal ligament that continues inferiorly as
the pharyngeal raphe.
Separated from the prevertebral
muscles post by prevertebral fascia.
 Tubal area lies just lateral to
pharyngeal area, comprises the
region occupied by eustachian tube
Pharyngeal &Tubal area

16. The pharyngobasilar fascia is attached to undersurface of the tube, & two
'paratubal' muscles arise one on each side of it.
The levator palati arises medially (within the pharynx) & the tensor palati arises
laterally (outside the pharynx).
Both muscles are partly attached to the tube, and open it during swallowing

17. 3 muscles : Stylopharyngeus, Stylohyoid, Styloglossus.
Stylopharyngeus:- Pass lateral to ICA.
Origin Deep aspect of base of styloid process.
InsertionThyroid cartilage & side wall of pharynx.
Nerve supply Ninth nerve.
Function:- Elevates larynx & pharynx.
Stylohyoid:- Pass lateral to ECA.
Origin Back of the base of styloid process
Insertion2 slips over base of greater cornu of hyoid
Nerve supply Seventh nerve.
Function:- Elevates & retracts the hyoid.
Styloid apparatus

18. Styloglossus:- Pass lat. To ICA & then swings forwards med. to lingual nerve
Origin front of the styloid process &upper part of stylohyoid ligament
Insertion Side of the tongue
Nerve supply Hypoglossal nerve,
Function:- Retract the tongue .

19. Frontal crest :- Midline bony ridge that projects upwards & provide attachment to
the falx cerebri.
Foramen caecum:-Transmits emissary vein from nose to superior sagittal sinus
Crista galli :- Provides site for ant. most attachment of the falx cerebri.
Cribriform plate :- Sheet of bone contaning many small Olfactory foramina 
Transmit olfactory nerve fibres into the nasal cavity.
Ant & post ethmoidal foramen:-Transmits Ant & post.ethmoidal artery,nerve,vein
The contents & foramina's of ACF

20. Cavernous Sinus
Situated on each side of the body of sphenoid bone & extend
from sup. orbital fissure ant to petrous apex post.
Receives :- Sup.& inf. ophthalmic vein , Sphenoparietal sinus.
Drains into:- Petrosal sinus, Pterygoid plexus, Basilar plexus.
Contents:- 1) CN III, IV, V1, V2 & VI 2) ICA
The contents & foramina's of MCF
Only anatomic
location in the
body in which an
artery travels
completely
through a
venous structure

21. Triangular shaped fissure bounded med. body of sphenoid, sup. lesser wing,
& inf. greater wing and is completed lat frontal bone as greater & lesser
wings converge.
Optic strut separates optic canal from superior orbital fissure.
Optic canal & superior orbital fissure together form the orbital apex.
Contents are as in dia-
Superior orbital fissure
Abducent nerve is most likely to damage 1st in
sup. orbital fissure syndrome

22. Extends from pterygopalatine fossa along orbital floor.
„Separates greater wings of the sphenoid from the maxilla.
Content –1) Maxillary branch of trigeminal nerve 2)Infra orbital vessels.
3) Emissary veins connecting inf ophthalmic vein to pterygoid venous plexus.
4) Zygomatic nerve.
Inferior orbital fissure

23. A fat filled space between the pterygoid plates and the posterior wall of maxillary
sinus. Shaped like an inverted pyramid.
Borders :- Med - Perpendicular plate palatine bone, Lat - Narrowing to
pterygomaxillary fissure, Ant - Post wall of maxillary sinus, Post - Med & Lat
pterygoid plates; inferior aspect of lesser wing of sphenoid bone.
It contains:-1) Pterygopalatine ganglion 2) Terminal third of the maxillary artery,
3) CN V2 4) Greater & deep petrosal nerve.
Pterygopalatine fossa

24. The PPF is an important pathway for the spread of neoplastic and infectious
processes:
Med - with nasal cavity via sphenopalatine F.
Lat - with masticator space (or infratemporal
fossa) via the pterygomaxillary fissure.
Ant - with orbit via the inferior orbital fissure.
Post & sup - with Meckel cave & cavernous
sinus (of MCF) via the F. rotundum.
Post & inf- with MCF via the vidian
canal, which transmits the Vidian nerve.
Inferiorly - with palate via the greater
and lesser palatine canals
Pterygopalatine fossa Communications

26. Dural invagination at posterior aspect of cavernous sinus. Contains gasserian
ganglion (trigeminal).
Dural layers shows thin peripheral enhancement.
In MRI, 3 sensory divisions of trigeminal nerve
can be visualized leaving the gasserian ganglion
Optic canal
 Formed by the lesser wing of sphenoid.
 The contents are :- Optic nerve .
Ophthalmic Artery.
Sympathetic fibers from carotid plexus
MECKEL’S CAVE

27. Foramen Rotendum Foramen ovale
Endocranially, its situated post-lat
to F. rotundum; exocranially, it is
found at the base of lateral
pterygoid plate.
Contents :- 1) Mandibular Nerve (CN
V3) 2) Accessory meningeal nerve
3) Lesser petrosal nerve 4)Emissary
vein 5)Occasionally ant. trunk of
middle meningeal artery
Is actually a canal in the base of the
greater sphenoid wing, is situated just
inf & lateral to supe orbital fissure.
It extends obliquely forward & slightly
inferiorly, connecting the MCF to
pterygopalatine fossa.
Transmits the maxillary nerve ( V2),
artery of the foramen Rotundum &
emissary veins.
Best visualized by means of coronal CT

28. Foramen spinosum Foramen Lacerum
Its located at the base of medial
pterygoid plate, ant to the petrous
apex.
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.
Its 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)

29. Vidian Canal
Also c/a pterygoid canal.
Located in the floor of sphenoid sinus at the junction of the pterygoid process &
the sphenoid body connecting the pterygopalatine fossa ant & the foramen
lacerum posteriorly.
Contents:- 1) Vidian Artery ( Br. Of Maxillary Artery). 2) Vidian Nerve (greater
superficial petrosal nerve & deep petrosal nerve )

30. Its a passage within petrous temporal bone & transmits the ICA & sympathetic
plexus enters the MCF from the neck.
 Its initially directed superiorly, then turns anteromedially to reach up to
the petrous apex.
CAROTID SHEATH
 Fibrous connective tissue that surrounds
vascular compartment of the neck.
 Layers:- Investing fascia, pretracheal fascia &
prevertebral fascia.
 Cervical part of the sympathetic trunk is embedded in prevertebral fascia
immediately post. to sheath.
Carotid canal

31. CONTENTS:-A) Above post belly of diagastric ICA, IJV,CN IX,X,XI,XII
B) Below post belly of diagastric ICA,IJV, CN X
 The carotid artery lies medial to the IJV, and the vagus nerve is situated post
between the two vessels.
The ansa cervicalis is embedded in
anterior wall of sheath. It is formed
by "descendens hypoglossi" (C1) &
"descendens cervicalis" (C2-C3).

32. For diagnostic imaging purposes, MCF further divided into
1) Midline sagittal, 2) Off-midline parasagittal, & 3) Lateral compartments
By drawing vertical lines passing medially to the petroclival fissure (medial 2 red
lines) and just lateral to the foramen ovale (lateral 2 red lines) respectively
Lateral compartment of MCF
 Formed by sphenoid triangle,
squamous part of temporal bone,
& temporomandibular joint

33. Midline sagittal compartment Parasaggital compartment
It includes the petroclival
synchondrosis, foramen lacerum &
medial aspect of the greater sphenoid
wing.
boundary sup & medparasellar region
containing cavernous sinus, sup & lat.
basal temporal lobes, and inf.
parapharyngeal & masticator spaces
Includes cavernous sinus, sup. orbital
fissure, F. rotundum, vidian canal , & F.
lacerum.
Includes body of sphenoid &
portion of clivus anterior to the
basisphenoid.
contains the sphenoid sinus
bordered superiorly by sella turcica
and inf by roof & posterior wall of
the nasopharynx

34. Foramen Magnum Internal acoustic canal
TransmitsVII & VIII cranial nerves
from pontomedullary junction to
inner ear
Divided by a bony lamina (falciform
crest) into :- 1) Smaller superior
part:Superior vestibular N. & Facial N
2) Larger Inferior part:- Inferior
vestibular N. & Cochlear nerve
The foramen magnum is entirely formed
within the occipital bone.
Contents :- 1. Medulla oblongata.
2.Vertebral arteries and veins.
3. Anterior & posterior spinal arteries.
4. spinal component of CN XI.
5. Tectorial membrane & alar ligaments.
The contents & foramina's of PCF

35. Located behind the carotid canal & formed in front by petrous portion of the
temporal, & behind by occipital.
Imaging based subclassification by bony ridge,
jugular spine, separate jugular foramen into 2 parts
A) Pars nervosa: smaller & ant-med compart.-
i) Inf petrosal sinus. Ii) Jacobson’s Nerve.
B) Pars vascularis: larger & post-lat compart.-
i) IJV. ii) Arnold’s nerve. iii) Spinal accessory nerve
Jugular foramen

36. IX & XITH CN lie more laterally than XTH CN in the foramen
It has anatomical variations & sometimes both cranial IX & XTH CN 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 IAC , it is
called a high riding jugular bulb, which is more common on the right side.
This is a dangerous variant & exposing during translabyrinthine surgery.

37. Also c/a ant condyloid canal. Located within occipital bone.
Its paired bone passage that runs lateral & slightly forward from PCF to
nasopharyngeal carotid spaceTransmits hypoglossal nerve.
Intracanalicular enhancement is always present (emissary veins), with linear
filling defects ( nerve rootlets).
Hypoglossal canal.

38. It is the space between the skull base, lateral pharyngeal wall & the ramus of
mandible.
Boundaries :- 1) Lat.- Ramus and condylar process of the mandible.
2) „Med.- Lateral pterygoid plate.
3) Ant. – Posterolateral wall of maxilla.
4) Post. – Carotid sheath.
5) Sup. - Greater wing of the sphenoid bone.
6) „Inf. – Medial pterygoid muscles.
Infratemporal fossa

39. Medial Pterygoid muscles
 Origin:- Superficial head Maxillary tuberosity & pyramidal process of palatine
bone
 Deep head Medial surface of lateral pterygoid plate of the sphenoid bone
 Action: Elevates & Protrusion of the mandible Side to side movement
 Nerve supply:- Medial pterygoid nerve branch of mandibular nerve
 Blood Supply:- Branch of mandibular nerve
Contents Of Infratemporal fossa

40. Origin:- Upper head infra-temporal surface & crest of greater wing of sphenoid
 Lower head Lateral surface of the lateral pterygoid plate
Action: Depression & Protrusion of the mandible, Side to side movement
Nerve supply:- branches from the masseteric or buccal nerve, branch of the ant.
trunk of the mandibular nerve
Blood Supply:- Pterygoid vessels from Maxillary artery
Lateral Pterygoid muscles

41. Its the larger of the 2 terminal branches of ECA Arises behind neck of the
mandible Imbedded in the substance of parotid gland Pass forward bwt
ramus of mandible & sphenomandibular ligament Then runs sup or deep to the
lateral pterygoid muscle Pterygomaxillary fissure Pterygopalatine fossa.
MAXILLARYARTERY

42. Described in 3 parts: before, on & beyond the lat. pterygoid muscle, with 5
branches coming from each part.
From 1ST & 3RD parts all branches enter bony foramina; from 2nd part none of
the branches go through bony foramina
BRANCHES OF MAXILLARY ARTERY

43. PLEXUS:- Lies within & on lat surface of lat pterygoid muscle, & receives
branches of maxillary artery.
Drains into two short, large maxillary vein to join superficial temporal vein &
form retromandibular vein.
Communicating veins: 1) Inferior orbital fissure  Inferior opthalmic vein
2) a connecting vein
passes vertically
down from the
cavernous sinus
3) deep facial vein to
join ant facial vein.
The pterygoid plexus & maxillary veins

44. The mandibular nerve
Course:- Lat. wall of cavernous sinus Passing through foramen rotundum
Enter pterygopalatine fossaThe pterygomaxillary fissure Infratemporal
fossa
Divided into 4 groups: 1) In cranium,2) In pterygopalatine fossa, 3) In infraorbital
canal, 4) On the face.
In the cranium:- Middle meningeal nerve in the meninges

45. From the pterygopalatine fossa:- 1) Infraorbital nerve via Infraorbital canal
2)Zygomatic nerve (zygomaticotemporal nerve, zygomaticofacial nerve) via
Inferior orbital fissure 3) Nasal Branches (nasopalatine) via Sphenopalatine
foramen 4) Superior alveolar nerves (Ant, mid. & post. superior alveolar nerves)
5) Palatine Nerves (Greater & lesser palatine nerve)including the Nasopalatine
nerve 6)Pharyngeal nerve
In the infraorbital canal Anterior superior alveolar nerve, Infraorbital nerve

46. Parapharyngeal , masticator, carotid & retropharyngeal spaces seen in close
contact with the skull base along their cephalad aspect .
Parapharyngeal space extends caudally to the submandibular space & cranially
abuts the base skull Contains fat, which acts as a medium for infection.
Relation of skull base to the deep facial spaces

47. Masticator space connects the mandible to the skull base.
Odontogenic infections & oropharyngeal sq. cell ca. can tract along masticator
space to the base skull.
Intracranial extension of the tumor can occur via mandibular nerve (perineural
spread) through the foramen ovale.
Jugular vein thrombosis & neural tumors such as schwannoma, neurofibromas &
paraganglioma are seen in the carotid space.
Rt masseter space abscess PerineuralV3 spread via
F.ovale
Rt acoustic neuroma

48.  Also c/a Lateral pharyngeal space, Pharyngomaxillary space.
Boundaries:- Shape like an inverted pyramid
SupSkull base, sphenoid & temporal bones.
Inferior Greater cornu of hyoid bone
Anterior Pterygomandibular raphe.
Posterior Carotid sheath post-lat.
& retropharyngeal space post-med.
Med Sup. Constrictor,
Buccopharyngeal fascia
Lat Ramus of mandible, deep lobe
of parotid gland, medial pterygoid
muscle
Parapharyngeal space

49. Has two compartments Prestyloid compartment
Contains 2 musclesTensor palati & Levator palati muscles
2 artery Ascending palatine & ascending pharyngeal artery & int. jugular vein.
Retrostyloid compartment
It is neurovascular space,
& contains the carotid sheath.

50. Internal carotid artery: Carotid foramen curves upwards into F.lacerum in
MCF apex of petrous bone enters the cavernous sinus
It lies in front of cochlea & middle ear cavity, separated by thin plate of bone
(may be dehiscent) gives off small intrapetrous branches, including carotico-
tympanic artery feeding vessels for a glomus tumour.
Structures within the skull base

51. Jugular bulb
It is the point at which sigmoid sinus
feeds the upper end of IJV.
Lies below posterior part of the floor
of the middle ear.
Inferior petrosal sinus joints jugular
bulb at the skull base
 GSPN+ deep petrosal nerve= vidian N.
Pterygoid canalPterygopalatine G.
 CN IX,X,IX, Jacobsons & Arnold nerve
lies in & around the jugular foramen.

52. SUTURES OFTHE SKULL BASE

53. FRACTURES SUTURES
Less than 2 mm in width
Same width throughout the length
Lighter on X-ray compared with
fracture lines
At specific anatomic sites
Does not run in straight line
Curvaceous
Greater than 3 mm in width
Widest at the centre & narrow at the
ends
Runs through both the outer & the inner
lamina of bone, hence appears darker
Usually over temporoparietal area
Usually runs in a straight line
Angular turns

54.  There are 4 muscles as follow
Masseter muscle
 Origin:- From zygomatic arch
 Insertion:-Lateral aspect of mandible
from the angle forwards along the lower
border, & upwards over the lower
part of the ascending ramus.
 Nerve supply:- masseteric branch
from ant. division of the mandibular N.
 Action:-Elevation & protrusion
of mandible
Muscles superficial to the lateral skull base

55. Largest muscle of mastication & fan shape.
Origin: From inf. temporal line , floor of temporal fossa & from overlying
temporal fascia of the side of the skull.
Insertion: Superior border & medial tip of the coronoid process.
Action: Elevation (anterior fibers)
& Retraction (posterior fibers)
Nerve supply: Ant div. of mandibular N.
Blood Supply:- middle temporal artery,
branch of sup. temporal artery
deep temporal arteries, branches
of the maxillary artery
Temporalis muscle

56. Origin:- from 2 heads: manubrium & clavicle.
Inserted:- Curved line extending from tip of the mastoid process to superior
nuchal line of the occiput.
Nerve supply:- eleventh CN
Action:-To protract the head (moving it forwards while keeping it vertical with a
horizontal gaze).
Sternocleidomastoid muscle

57. Two bellies united by tendon
Origin – Anterior belly from diagastric fossa of mandible. Posterior belly from
mastoid notch of temporal bone.
Insertion –Both meet at the intermediate tendon and held by the fibrous pulley.
Nerve supply:- Post. belly is supplied
by seventh nerve (nerve to digastric)
& the ant. belly by the fifth nerve
(mylohyoid nerve).
Action:-To depress & retract the chin
Digastric muscle

58. Mastoid proces is absent It forms during 1st year as SCM muscles
complete their devp.& pull on petromastoid parts of temporal bones.
Styloid process is absent
Stylomastoid foramen is exposed on the lateral surface of the skull – (facial
nerve is vulnerable to injury)
Glabella and superciliary arches are not developed.
Paranasal sinuses are rudimentary or absent, (only maxillary sinus are
usually identifiable. Frontal sinus is absent)
Ext. acoustic meatus is short, straight and wholly cartilagenous
Ossification is incomplete – many bones are still in several pieces united by
fibrous tissue or cartilage
Characteristics Of Fetal Skull