Anatomy of Skullbase

1. ANATOMY OF SKULL BASE
BY :- Dr.JINU IYPE
2ND YEAR PG
DEPARTMENT OF ENT

2. BIBILIOGRAPHY:
• Scott Brown’s Otorhinolaryngology head
& neck surgery – 7th edition Vol -3
• Cummings Otolaryngology head and neck
surgery - 6th edition
• Glasscock Shambaugh surgery of the ear –
6th edition
• Grey’s Anatomy

3.  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 understanding several pathologic
conditions as well as for planning surgical procedures.
INTRODUCTION

4. Skull baseboundaries:-
Anterior►Upper incisor teeth
Posterior ►Sup. nuchal line of
occipital bone
Lateral►Remaining upper teeth
the zygomatic arch&
its posterior root,
the mastoid process

5. Three major areas
of the skull base
are described:
1. anterior
2. middle
3. posterior

6. Composed of fivebones:
1)Ethmoid
2)Sphenoid
3)Occipital
4) Paired temporal
5) Paired frontal bones

7. OSTEOLOGYOF THE SKULLBASE
The Frontal Bone

8. Orbital
part-
contain
2 openings-
Anterior &
Posterior
ethmoidal
foramen

9. The Ethmoid Bone
consists of :-
• horizontal plate(cribriform plate)
• and a vertical plate in
the midline, i.e. the
perpendicular plate.

10. • On closer examination, the cribriform plate
Horizontal medial lamella Vertical lateral lamella
This lateral lamella articulates
with the frontal bone.

11. The skull base in this region—the ethmoid
fovea —is formed
• medially by the lateral lamella of the
cribriform plate, which is very thin bone
• and laterally by the frontal bone, which in
contrast is a thicker bone.

12. • The frontal bone forming the ethmoid
fovea is 0.5 mm in thickness while the
lateral lamella of the cribriform plate is
0.2 mm.
• The region where the anterior ethmoidal
artery pierces the dura medially is the
thinnest area in the skull base and is only
0.05 mm in thickness.

14. KEROS CLASSIFICATION
The length of the lateral lamella and the depth
of the olfactory fossa are classified into 3
types:
• Type I — 1-3 mm
• Type II — 4-7 mm
• Type III — 8-17 mm

15. The Sphenoid Bone
• It separates the anterior and middle cranial
fossa.
• looks like a bat with outstretched wings.

16. • It consist of → a central body; two sets of wings– the
greater and lesser, which course laterally ; & two
pterygoid processes, directed inferiorly.

17. Posteriorly, the
chiasmatic sulcus
forms a slight
depression & leads
laterally to the optic
canal.
The tuberculum
sellae, abony
elevation, just
posterior to this sulcus

19. Followed by,
posteriorly by sella
turcica & dorsum
sellae.
The dorsum sellae
terminates
laterally into the
posterior clinoid
processes

21. The anterior surface ofthe body of sphenoid forms the roof
& posterior wall of nasopharynx
Thebody houses the sphenoidsinus .
Lesser wings→Forms
medial portionof
orbital apex.
Greaterwings → Course
upward & laterally from
both sides of the sphenoid
body-forms floor of
Middle Cranial Fossa.

22. The medial pterygoid plate
projects back , where it
articulates with the vertical
plate of the palatine bone.
Inferiorly, it ends in the
pterygoid hamulus and
superiorly in the pterygoid
tubercle which projects
back into the foramen
lacerum.
The lateral pterygoid plate
extends back and laterally
into the infratemporal fossa.
The plate gives attachment
to the pterygoid muscles.

23. Temporalbone
Its made up of squamous, mastoid, petrous & tympanic
parts which ossify separately and later fuse creating
squamotympanic & petrosquamous fissure


24. The squamous temporal bone contains the
hollow of the glenoid fossa joined laterally to
the zygomatic process.

25. The petromastoid bone projects forwards and
medially at 45°, wedged between the basiocciput
and the greater wing of the sphenoid; at the
apex of the wedge the three
bones do not quite meet, leaving a gap termed the
'foramen lacerum’

26. • Tympanic part has
styloid process →
behind its base,
stylomastoid
foramen &
posterior the
mastoid bone has
the digastric notch,
medial to which
there is agroove
for the occipital
artery.
• Stylomastoid
foramen transmits
facial nerve.

27. ANTERIOR CRANIAL FOSSA
• The anterior cranial base can be defined as that
portion of the skull base adjacent to the
anterior cranial fossa.

28. ANTERIOR CRANIAL BASE
Boundaries :- Anterior: Posterior wall of thefrontal sinus
Posterior: Lesser wing of the sphenoid & anteior clinoid processes.
Floor : roof of the nasal cavity & ethmoidsinuses medially.
Lateral wall : Orbital plates of the frontal bone

29. • An important visible landmark is the foramen
cecum, which is the site of a communication
between veins of the nasal cavity and the origin
of the superior sagittal sinus.
• The next landmark, the crista galli, protrudes
upward from the midline to provide attachment
for the falx cerebri.
• On either side of the crista are the openings of
the cribriform plate, through which olfactory
nerves are transmitted

30. Just posterior to the last of these olfactory
foramina is a smooth-surfaced area known as the
planum sphenoidale, which forms the roof of the
sphenoid sinus when the sinus is well
pneumatized.
1 foramen cecum 2crista
galli; 3 cribriform plate;
4 planum sphenoidale;
5 optic canal
6chiasmatic sulcus
7anterior clinoid process
8pituitary fossa
9, dorsum sellae;
10, foramen ovale;
11, foramen lacerum.

31. 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.
Thecontents & foramina'sofACF

32. Cribriform plate :- Sheet of bone contaning
many small Olfactory foramina →Transmit
olfactory nerve fibres into the nasal cavity.

33. The orbits contain several landmarks
that can help surgical orientation
during cranial base operations.
 The superior orbital fissure
transmits the oculomotor, trochlear,
ophthalmic, and abducens nerves and
the ophthalmic vein, and it
communicates with the middle
cranial fossa
 The inferior orbital fissure contains
the maxillary nerve (V2) and
communicates primarily with the
pterygopalatine fossa; the lateral end
of this fissure is an important
landmark for the placement of lateral
orbital osteotomies.

34. The optic canal transmits the optic nerve and the
ophthalmic artery.
Ant & post ethmoidal foramen:-TransmitsAnt &
post. ethmoidal artery, nerve,vein
These ethmoid foramina mark the position of the
frontoethmoid suture line, a valuable guide to the
level of the ethmoid roof and the anterior fossa
floor.
The posterior ethmoid foramen is of additional
significance because of its consistent relationship
with the optic canal, found 4 to 7 mm posteriorly.

36. Optic foramen

37. MIDDLE CRANIAL BASE
• The middle cranial base
forms the floor of the
middle cranial fossa.
• Boundaries (from the
intracranial perspective)
Anteriorly - the
posterior edge of the lesser
sphenoid wing
Posteriorly - ends at the
posterosuperior edge of the
petrous part of the temporal
bone.

38. Subdivisions of the lateral skull base
1. Pharyngeal;
2. Tubal;
3. Neurovascular;
4. Auditory;
5. Articular;
6. Infratemporal fossa.

39. PHARYNGEAL AREA
• Situated
centrally in the
skull base, this
area forms the
roof of the
nasopharynx.

40. 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 posteriorly into a pharyngeal ligament
that continues inferiorly as the pharyngealraphe.
The pharyngobasilar fascia is separated from
the prevertebral muscles by the prevertebral
fascia.

42. TUBAL AREA
The tubal area lies just lateral to the pharyngeal
area and occupied by the Eustachian tube.
Anteriorly, the scaphoid fossa
at the base of the medial
pterygoid plate, between the
petrous bone and the greater
wing of the sphenoid

43. The pharyngobasilar fasciais attached to undersurface of the tube, &
two 'paratubal' muscles ariseone on eachside of it.
The levator palati arisesmedially (within the pharynx) & the tensor
palati arises laterally (outside thepharynx).
Both musclesare partly attached to the tube, and open it during swallowing

44. NEUROVASCULAR AREA
• Posterior to the tubal area lies the neurovascular
area:-
• Carotid sheath;
• Styloid apparatus;
• Facial nerve

45. Carotid sheath
• It is attached to the skull base around the
carotid foramen and continues downwards as
far as the aortic arch.
• Content :
Internal carotid artery
Vagus nerve.
Internal jugular vein

46. • In the neck, the carotid sheath + pretracheal
fascia, is firmly attached anteriorly to the
deep surface of the sternomastoid.
Posteriorly, it is not attached to the
prevertebral fascia, but is free to slide over it.
This means that pus tracking laterally from a
parapharyngeal abscess passes behind the
sheath and behind the sternomastoid, to point
in the posterior triangle.

47. • The cervical sympathetic trunk lies behind the
carotid sheath in front of the prevertebral
fascia, just medial to the vagus nerve.
• It ends superiorly at the superior cervical
ganglion.
The DEEP PETROSAL NERVE arises
from this ganglion and passes superiorly to
join the greater superficial petrosal nerve
to form the VIDIAN NERVE.

48. • The jugular foramen is
divided by two transverse
septa(Jugular spine) of
fibrous dura (which may
ossify) into three
compartments.
• The anterior compartment
is occupied by the
Glossopharyngeal nerve
and the inferior petrosal
sinus.

49. The middle compartment is shared by the
vagus (X) and accessory (XI) nerves
The posterior
compartment is
filled by the
emerging
internal jugular
vein.

50. IX & XITH CNlie more laterally thanXTH CNin the foramen
The right jugular foramen is larger than the left in 75%of
thepopulation.
When theroof of thejugular bulb isseen abovethelevel of
floor of IAC, it is called a high riding jugular bulb, which is
more common on the righ tside.
This is adangerous variant & exposing during
translabyrinthine surgery.

51. 3muscles : Stylopharyngeus, Stylohyoid,
Styloglossus.
Stylopharyngeus:- Passlateral toICA.
Origin- Deep aspect of baseof
styloid process.
Insertion-Thyroid cartilage & side
wall of pharynx.
Nerve supply-Ninthnerve.
Function:- Elevates
larynx &pharynx.
Styloidapparatus

52. Stylohyoid:-
 Passlateral to ECA.
Origin- Back of the base of styloid process
Insertion- Base of greater cornu of hyoid
Nerve supply-
Facial nerve.
Function:- Elevates
& retracts the hyoid.

53. Styloglossus:- Lateral to ICA
Origin- front of the styloid process &upper part of
stylohyoid ligament
Insertion-
Side of thetongue
Nerve supply-
Hypoglossalnerve
Function:-
Retract thetongue .

54. STYLOMASTOID FORAMEN
It transmits the facial nerve
and the stylomastoid artery.
 As soon as it emerges from
the foramen, VII gives off the
posterior auricular nerve
(supplying the occipital belly
of occipitofrontalis)
and a muscular branch
(supplying the posterior belly
of digastric and stylohyoid).
 It then swings forward
into the parotid gland,
dividing zygomaticofacial
& cerviciofacial division

55. AUDITORY
AREA
This small area
anterolateral to
the neurovascular
area
forming the floor
and anterior wall
of the external
auditory canal
and middle ear.

56. Petrotympanic fissure of Glaser –
Transmits the chorda tympani and anterior
tympanic branch of the maxillary artery,
and the corresponding veins which drain
into the pterygoid plexus.

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

61. 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 asin dia
Superior orbitalfissure

62. Extends from pterygopalatine fossaalong orbital floor.
Separatesgreater 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 orbitalfissure

63. .
Dural invagination at posterior aspect of cavernous sinus.
Contains gasserian ganglion (trigeminal).
Dural layers shows thinperipheral enhancement
In MRI, 3sensorydivisions of
trigeminal nerve can be
visualized leaving the
gasserianganglion
MECKEL’S CAVE

64. Optic canal
Formed by the lesser wing of sphenoid.
The contents
are-
Optic nerve
Ophthalmic
Artery.
Sympathetic fibers
from carotid plexus

65. ForamenRotundum
Is actually a canal in the base of the
greater sphenoid wing, is situated just
inf & lateral tosuperior orbital fissure.
It extends obliquely forward & slightly
inferiorly, connecting the MCFto
pterygopalatine fossa.
Transmits the maxillary nerve (V2),
artery of the foramen Rotundum &
emissary veins.
Best visualized by coronalCT

66. Foramen Ovale
Endocranially, its situatedpost-lat to F.rotundum;
exocranially, it is found at the base of lateral
pterygoid plate.
Contents :-
1)Mandibular Nerve
2)Accessory meningeal
nerve
3) Lesser petrosal nerve
4)Emissary vein
5)Occasionally ant. trunk of middle meningealartery

67. Foramenspinosum
Its an aperture in the greater wing of the sphenoid
posterolateral to foramen ovale.
Contents :-
1) Middlemeningeal artery
& vein.
2)Emissary vein.
3)Nervous spinosus
(Meningeal branch of
mandibularnerve)

68. ForamenLacerum
Its located at the base of medial pterygoid
plate, ant to the petrous apex.
Structures passing wholelength:
1)Meningeal branch of Ascending pharyngeal
artery 2) Emissaryvein
Other structures partiallytraversing:
3)Internal carotid artery
4)Greater petrosal nerve.

69. VidianCanal
Also c/a pterygoidcanal.
Located in the floor of sphenoid sinus at the junction of the pterygoid
process& the sphenoid body connecting the pterygopalatine fossa
anteriorly & the foramen lacerumposteriorly.
Contents:- 1) VidianArtery ( Br.Of MaxillaryArtery). 2)Vidian
Nerve (greater superficial petrosal nerve & deeppetrosal nerve )

70. Infratemporal fossa
• It is the space between the skull base,
lateral pharyngeal wall & the ramus of
mandible.

71. BOUNDARIES :- 1) Lat. - Ramus
and condylar process of the mandible.
2)Med.-Medial pterygoid muscle.
3)Ant. –Posterolateral wall of
maxilla.
4)Post. –Carotid sheath&
styloid apparatus
5)Sup. - Greaterwing of the
sphenoidbone.
6)Inf.–No anatomical.Continued
down with neck

72. • The contents of the fossa are :-
1. Lateral and Medial pterygoid muscles
2. The maxillary artery and its branches
3. The pterygoid venous plexus and maxillary
veins
4. Branches of the mandibular nerve

73. 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
 Insertion into the angle of the mandible

74. • Action: To close the mouth and move the
mandible towards the opposite side in
chewing.
Blood Supply:- Branch of Maxillary artery
Nerve supply:- Medial pterygoid nerve
branch of mandibular nerve

75. Origin:- Upper head → infra-temporal surface & crest
of greater wing of sphenoid
Lower head → Lateral surface of the lateral
pterygoid plate
 Insertion into the neck of the mandible
Lateral Pterygoidmuscles

76. Action: Depression & Protrusion of
the mandible, Side to side movement
Blood Supply:- Pterygoid vessels from
Maxillary artery
Nerve supply:- Branches from the
masseteric or buccal nerve, branch of
the ant. trunk of the mandibular nerve

77. Its the larger of the 2 terminal branches of
ECA→Enters the infratemporal fossa→
Pass forward bwt ramus of mandible &
sphenomandibular ligament → Then runs
sup or deep to the lateral pterygoid muscle
→ Pterygomaxillary fissure →
Pterygopalatine fossa.
MAXILLARYARTERY

78. BRANCHESOFMAXILLARYARTERY

79. The pterygoid plexus & maxillary veins
Plexus → lies within and on the lateral surface of
the lateral pterygoid muscle, and receives
branches of the maxillary artery.
Drains into two short, large
maxillary veins → superficial
temporal vein and
retromandibular vein .

80. The pterygoid plexus has three important
communicating veins.
1. The inferior
ophthalmic vein
→ the facial vein.
2. A connecting vein
passes vertically
down →cavernous
sinus
3. The deep facial
vein → join the
anterior facial vein.

81. The mandibular nerve
It passes through the foramen ovale.
The main trunk :
1.Nervus Spinosus
(which reenters the
middle fossa
through the
foramen
spinosum)
2. Nerve to the
Medial pterygoid
→ the tensor palati
and tensor tympani.

82. The main trunk divides into
anterior posterior divisions
• The anterior division:-
1. Buccal nerve → buccinator to supply the
mucous membrane of the cheek.
2. Temporalis
3. Masseter
4. Lateral pterygoid.

83. The posterior division :-
1. The Mylohyoid nerve.
2. The Auriculotemporal nerve.
3. The Inferior alveolar nerve
4. The Lingual nerve is joined by the
chorda tympani 2 cm below the base of the
skull

84. The otic ganglion lies close
to the mandibular nerve
just below the foramen
ovale.
It relays secretomotor fibres
for the parotid gland,
which it receives by way of
the lesser
superficial petrosal nerve
and transmits to the
auriculotemporal nerve..

85. A space between the pterygoid plates and the
posterior wall of maxillary sinus. Shaped like an
invertedpyramid.
Pterygopalatine fossa

86. Boundaries :-
Medially - Perpendicular plate palatine bone
Laterally - Narrowing to pterygomaxillary fissure
Anteriorly - Post wall of maxillary sinus
Posteriorly - Med & Lat pterygoid plates
Superiorly- Undersurface of body of sphenoid.

87. .
The PPFis an important pathway for the spread of
neoplastic and infectious
processes:
Med - with nasal cavity via
sphenopalatine F.
Lat - infratemporal fossa
via the pterygomaxillary
fissure.
Ant - with orbit via the
inferior orbital fissure
Pterygopalatine fossaCommunications

88. 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

89. Contains:-
1)Pterygopalatine ganglion
2)Terminal third of the maxillary artery
3)MaxillaryNerve–CNV
4)Greater & deep petrosal nerve.

90. Posterior Cranialfossa
• Anterior margin :- The posterior surface ofthe
clivus.
• Laterally:- superiorlythe posterior surface of
the petrous part oftemporal bone
• Posteriorly :- mastoid
portion of temporal
bone & the squamous
partof occipital bone.

91. Thecontents & foramina's of PCF
FORAMEN MAGNUM
• The foramen magnum is entirely formed within the
occipital bone.
Contents :-
1. Medullaoblongata.2.Vertebral arteries andveins.
2.Anterior & posterior spinal arteries. 3.spinal
component ofCNXI.

92. Also c/aant condyloid canal. Located within occipital bone.
Its paired bone passagethat runslateral & slightly forward from PCF
to nasopharyngeal carotidspace→Transmits hypoglossal nerve.
Intracanalicular enhancement isalwayspresent (emissaryveins), with linear
filling defects ( nerverootlets).
HYPOGLOSSALCANAL

93. INTERNALACOUSTICCANAL
• Transmit VII &VIII from pontomedullary
junctionto inner ear
Divided by a bony lamina (falciform crest) into
:- 1)Smaller superior part:Superior vestibular
Nerve &Facial Nerve
2) Larger Inferior part:- Inferior vestibular
Nerve & Cochlearnerve

95. Parapharyngeal, masticator, carotid &
retropharyngeal spacesseen in close contact with
the skullbase along their cephalad aspect.
Relationof skullbaseto the deepfacial spaces

96. s
.
e
Also c/a Lateral pharyngeal space,
Pharyngomaxillaryspace.
Shape like an invertedpyramid
Parapharyngealspace

97. Boundaries:-
Superior→Skull base,sphenoid & temporal bone
Inferior → Greater cornu of hyoid bone
Anterior → Pterygomandibularraphe.
Medial→Sup.constrictor,buccopharyngeal
fascia
Lateral→ Ramus ofmandible, deep lobe of
parotid gland, medial pterygoid muscle
Posterior lateral→ Carotid sheath
Posteriormedial→ Retropharyngeal space

99. Has two compartments Prestyloid compartment
Contains2 muscles→Tensor palati & Levator palati muscles
2 artery →Ascending palatine & ascending pharyngeal artery
& int. jugular vein.
Retrostyloid compartment :It is neurovascularspace, &
contains the carotid sheath.

100. 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.
Structureswithin the skullbase

101. Jugularbulb
It is the point at which sigmoid
sinus feeds the upper end of IJV.
Lies below posterior part of the
floor of the middleear.
Inferior petrosal sinus joints
jugular bulb atthe skull base
 GSPN+deep petrosal nerve=
vidianN.
Pterygoid canal→PterygopalatineG.
CNIX,X,IX, Jacobsons&Arnold
nerve lies in & around the
jugular foramen.

102. FRACTURES SUTURES
Less than 2 mm inwidth
Samewidth throughout the
length
Lighter onX-ray compared
with
fracture lines
At specific anatomicsites
Doesnot run in straight line
Curvaceous
Greater than 3mm inwidth
Widest at the centre & narrow at
the ends
Runsthrough both the outer & the
inner lamina of bone, hence appears
darker
Usually over temporoparietalarea
Usually runs in astraightline
Angularturns

103.  There are 4 muscles asfollow
Massetermuscle
 Origin:- From zygomaticarch
 Insertion:-Lateral aspect of
mandible from the angle forwards
along the lower border, & upwards
over the lower part ofthe ascending
ramus.
 Nerve supply:- massetericbranch
Fr of ant. division of the mandibular N.
Action:-Elevation &
protrusion of mandible
Musclessuperficialto thelateralskullbase

104. Largest muscleof mastication & fan shape.
Origin: From inf. temporal line , floor of temporal fossa & from overlying
temporal fascia of the side of the skull.
Insertion:Tothecoronoid process
Action: Elevation (anteriorfibers)
& Retraction (posteriorfibers)
Nerve supply:Ant div. of
mandibular N.
BloodSupply:- middle temporal
artery, branch ofsup. temporal artery
deep temporal arteries,branches of
the maxillaryartery
Temporalismuscle

105. Origin:- from 2heads: manubrium & clavicle.
Inserted:-Curved line extending from tip of the mastoid processto
superior nuchal line of theocciput.
Nerve supply:- Accessory nerve
Action:-Toprotract the head (moving it forwards while keeping it
vertical with a horizontal gaze).
Sternocleidomastoidmuscle

106. Two bellies united bytendon
Origin –Anterior belly from diagastric fossa of mandible. Posterior
belly from mastoid notch of temporal bone.
Insertion –into the digastric fossa on the lower edge of the mandible.
Nerve supply:- Post. belly
is supplied by Facial
nerve (nerve todigastric)
& the ant. belly by the
mylohyoid nerve( CN
V3).
Action:-Todepress & retract
the chin
Digastricmuscle