embryology,osteology.sugical importance

1. DR ROOHIA

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 under-standing several pathologic
conditions as well as for planning surgical
procedures.
3Anatomy skull base

4. 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.
4Anatomy skull base

5.  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.
5Anatomy skull base

6.  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.
Anatomy skull base 6

7.  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.
Anatomy skull base 7

8.  Mesenchymal condensations migrating to the
rostral end of notochord at the region of rathke’s
pouch form the polar or hypophyseal cartilages.
8Anatomy skull base

9.  Rostral extensions of these cartilages surround
the craniopharyngeal canal and join to create the
presphenoid.
9Anatomy skull base

10.  Together with the trabecular cartilages, the
hypophyseal chondrification centers fuse to form
the precursors of the central skull base.
10Anatomy skull base

11.  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.
11Anatomy skull base

12.  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.
Anatomy skull base 12

13. 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).
13Anatomy skull base

14. 14Anatomy skull base

20.  Palatovaginal
canal
 Vomerovaginal
canal

29.  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.
Anatomy skull base 29

30.  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.
30Anatomy skull base

33.  The adult temporal bone is made up of five
major components, namely the squamous part
(squama), the petrous part (petrosa), the
tympanic bone, the mastoid process, and the
styloid process.
 Both the squama and the tympanic bone are
products of membranous bone development.
 The petrous portion is represented by the
cartilaginous otic capsule until 20 weeks of
gestation during which ossification proceeds.
 The styloid process also is preformed in
cartilage.
 It is not until the eight-week stage that one can
first discern development of the squama of the
temporal bone as commencing from an
ossification center which extends into the
zygomatic process.

34.  The tympanic part of the temporal
bone begins its development at about 9
to 10 weeks of gestation.
 In the ninth week, the squama and
zygomatic process begin membrane
bone formation.
 By the end of the ninth week, the
superior wall of the middle ear
emerges as a projection of the otic
capsule; known as the superior periotic
process. It grows forward over the
ossicles forming the lateral aspect of
the tegmen tympani.
 The medial part of the tegmen tympani
consists of a fibrous tissue plate.
A coronal section of the Skull of a Foetus, 4 months old

35.  16 weeks-the postauditory process of
the squama extends posterior to the
tympanic ring forming the
anterosuperior portion of the mastoid
process.
 29th week- the tympanic process of the
squama joins the antral segment of the
periosteal otic capsule to form the lateral
wall of the antrum.
 At term an ossification center forms at
the dorsal aspect of Reichert’s cartilage
which fuses with the otic capsule to
create the styloid eminence in the floor
of the tympanic cavity and also part of
the distal segment of the bony fallopian

36. The external petrosquamous fissure demarcates the
border between that part of the mastoid derived from the
squama and the portion which arises from the petrosa.
This fissure is visible in the newborn, but generally
disappears by the second year of life.
At birth the mastoid antrum is large with a thin shell of
bone.
The mastoid process develops as a prominence on the
outer aspect of the petrous pyramid during the first year
of life. As the mastoid grows, the antrum shrinks in
relative size and assumes a more medial position, as does
the facial nerve. The mastoid, although well developed by
three years of age, does not achieve adult configuration
for several more years.
Postnatally, the styloid process forms as an ossification
center in the upper portion of Reichert’s cartilage;
concurrently, at its ventral aspect another ossification
center appears which will become the lesser horn of the
hyoid and the superior part of the body of the hyoid.
The fusion of the separate components of the temporal
bone then becomes the major process in its further
development.

37.  In children the pinna needs to be pulled backwards, downwards and
laterally to make the external auditory canal in line for examination as the
developing temporal bone is horizontally placed which becomes vertically
placed in adult.
 The mastoid process in children is not fully developed, thus cannot be
palpated easily. Hence the postauricular incision in children should be
given more horizontally to prevent injury to the facial nerve.
SURGICAL IMPORTANCE
POST AURICULAR INCISION IN ADULT VS INFANT

39. The temporal bone is a
composite structure consisting
of
1. The Tympanic Bone
2. The Mastoid Process
3. The Squama(Squamous
portion of the temporal bone)
4. The Petrosa(Petrous portion
of the temporal bone.

40. The four parts visible here are:
1) squamous bone : origin for the
temporalis muscle. zygomatic
process
2) tympanic bone - bony portion of
the external auditory canal
3) styloid bone - in an anterior-
inferior direction.
4) mastoid bone
Glenoid fossa
Macevens triangle
LATERAL SURFACE OF RIGHT TEMPORAL BONE

41. TYMPANIC BONE.
 It interfaces with
 1) the squama at the tympanosquamous suture,
2) the mastoid at the tympanomastoid suture
3) the petrosa at the petrotympanic fissure

42.  Posterior wall of the glenoid fossa for the
temporomandibular joint (TMJ).
 The chorda tympani nerve, anterior process of
the malleus, and anterior tympanic artery
traverse the petrotympanic fissure.
 Laterally, the tympanic bone borders the
cartilaginous EAC,
 the annular sulcus.
 the notch of Rivinus,.
 in referred otalgia, owing both to the
proximity of the EAC and the shared innervation
by the mandibular division of the trigeminal (fifth
cranial) nerve.

43.  SQUAMOUS PORTION OF
TEMPORALBONE:
 Latera wall of Middle cranial fossa
 Parietal bone superiorly
 Zygoma,TMJ anteriorly
 Medially –middle meningeal artery
 Laterally-temporal artery

44.  . It is composed of a squamous
portion (laterally) and a petrous
portion (medially) separated by
Körner’s (petrosquamous)
septum
 The fossa mastoidea
(Macewen’s triangle)
 The fossa mastoidea, a cribrose
(cribriform) area, is identified
by its numerous, perforating
small blood vessels.
 The mastoid foramen
 Inferiorly, the
sternocleidomastoid muscle
attaches to the mastoid tip.

45.  Normal length-2.5cm
 Its proximal part (tympanohyal) is ensheathed
by the vaginal process of the tympanic portion.
 Its distal part (stylohyal) gives attachment to
the following:
 stylohyoid ligament
 stylomandibular ligament
 styloglossus muscle (innervated
by the hypoglossal nerve)
 stylohyoid muscle (innervated
by the facial nerve)
 stylopharyngeus muscle
(innervated by the
glossopharyngeal nerve)

46.  The tympanosquamous and tympanomastoid sutures are
landmarks for the “vascular strip” incisions used in
tympanomastoid surgery. The elevation of EAC skin
and periosteum at these two sutures often requires sharp
dissection to divide the contained periosteum,
particularly at the tympanosquamous suture.
 The tip of the mastoid process is easily palpated and is a
landmark for the positioning of postauricular incisions.
 On occasion, posterior bulging of the anterior canal wall
may obscure full visualization of the tympanic
membrane. Anterior canalplasty can improve surgical
visualization but if overzealous may result in prolapse
of the TMJ into the EAC with, for example, opening the
mouth.

47.  The tympanomastoid fissure is anterior to the tip
of the mastoid and can be traced medially to the
stylomastoid foramen, which is the exit point of
the facial nerve
 Vestibular schwannoma, Middle cranial fossa
approach- A small window of squamous part of
temporal bone is removed to allow exposure of the
tumor from the upper surface of the internal
auditory canal, preserving the inner ear structures.
 Styloid Process
The stylomandibular ligament
Eagle syndrome

48. LINEA TEMPORALIS
• The linea temporalis is an avascular
plane, a feature that makes it an ideal
location for the superior limb of the
“T” musculoperiosteal incision used in
the postauricular approach to the
tympanomastoid compartment.
• The squamous portion of the
temporal bone (the squama)
extends above the temporal
line, whereas inferiorly and
anteriorly is the tympanic ring
and posteriorly the mastoid.
• The temporal line also
approximates the position of
the floor of the middle cranial
fossa.

49. TYMPANOMASTOID SUTURE
• The posterior meatal skin is firmly adherent to the
tympanmastoid suture, as such sharp and careful
dissection should be carried out in this region to
prevent tear of the tympanomeatal flap.
• The facial nerve lies 8mm medial to the
tympanomastoid line.
• The tympanomastoid suture is traversed by
Arnold’s nerve (auricular branch of vagus nerve).

50.  It features the porus of the
internal auditory canal
(IAC).
 internal carotid foramen
 The sigmoid portion of the
lateral venous
 superior petrosal
 The petrous portion of the
temporal bone houses part
of the middle ear (e.g.,
ossicles) and inner ear (i.e.,
cochlear and vestibular end
organs).

51.  Cochear aqueduct- connect scala tymani in
basal turn to CSF space around the brain
 Vestibular aquduct- bony passage runs from
vestibule to subarachnoid space,
 IAM
 Mastoid process
 Sigmoid sinus sulcus
 Styloid process
 Jugular fossa

52.  The vertically oriented posterior face of the petrosa
dominates the posterior view of the temporal bone as it
delimits the anterolateral aspect of the posterior cranial
fossa and lies between the superior and inferior
petrosal sinuses.
 The porus of the IAC, operculum, endolymphatic
fossette cradling the endolymphatic sac, and
subarcuate fossa are the key anatomic features on this
surface.

53.  The posterior surface of the temporal bone
forms the anterior border of the posterior
cranial fossa.
 The sigmoid sulcus is an indentation at the
lateral aspect of the posterior surface .
 Anterior to the sigmoid sulcus is the foveate
fossa for the intradural portion of the
endolymphatic sac.
 the operculum, covers the intraosseous
portion of the endolymphatic sac. The
vestibular aqueduct runs anteriorly, superiorly,
and medially from the operculum to end at the
medial wall of the vestibule.
 The superior petrosal sulcus, located at the
interface of the posterior and middle cranial
fossa plates of the temporal bone, carries the
superior petrosal sinus from the sigmoid sinus
to the cavernous sinus anteriorly.

54. A The internal auditory canal penetrates the
posterior surface of the petrous ridge,
branch of the inferior vestibular nerve, the posterior
ampullary nerve or singular nerve , which innervates the
ampulla of the posterior semicircular canal, exits the
internal auditory canal through the singular canal.
In rare cases of chronic persistent positional vertigo
which do not respond to physiotherapy singular nerve
neurectomy is a new surgical procedure for treatment.

55. The inferior surface of the temporal bone
separates the upper neck from the skull base.
Accordingly, many vital
neurovascular structures traverse this surface.
 Anteriorly and medially, the carotid foramen
 the jugulocarotid crest, separates the carotid
canal from the jugular foramen.
 Jugular foramen ,pars venosa,pars nervosa
 The hypoglossal nerve exits the occipital bone
by the hypoglossal canal, medial to the pars
nervosa of the jugular foramen.
 Lateral to the jugular foramen is the styloid
process.
 stylomastoid foramen.
TEMPORAL BONE (INFERIOR SURFACE)
 The triangular opening of the cochlear aqueduct is
located medial to the jugular foramen.
 The inferior tympanic canaliculus runs in the
jugulocarotid crest and carries the inferior
tympanic artery (a branch of the ascending
pharyngeal artery) and the tympanic branch of the
glossopharyngeal nerve (Jacobson’s nerve) into the
tympanic cavity.

56. Posterior retraction of the
internal jugular vein and
resection of the jugular bulb
allow visualization of the lower
cranial nerves exiting the skull
(IX,X,XI).
Glomus jugulare tumors are
rare, slow-growing,
hypervascular tumors that arise
within the jugular foramen of the
temporal bone.
PHELP'S SIGN - loss of crest of
bone as seen in CT-scan between
carotid canal and jugular canal in
glomus jugulare.
SURGICAL IMPORTANCE

57. From the transmastoid perspective, the cochlear aqueduct is
encountered when drilling medial to the jugular bulb; opening the
aqueduct results in the flow of cerebrospinal fluid into the mastoid,
a useful maneuver in translabyrinthine cerebellopontine angle
tumor surgery as it decompresses cerebrospinal fluid pressure.
 the cochlear aqueduct can be used as a guide to the lower limits of
IAC dissection in, for example, the translabyrinthine approach as it
allows full exposure of the IAC without risking the lower cranial
nerves.

58.  Medial to the mastoid tip is
the digastric groove for the
posterior belly of the digastric
muscle.
 1) This is an important
landmark for the
identification of facial nerve
during parotid surgery.
 2) This projects as the
digastric ridge in the mastoid
cavity which anteriorly traced
leads to the stylomastoid
foramen which delineates the
vertical portion of the facial
nerve.

59. TEMPORAL BONE (ANTERIOR SURFACE)
The petrous apex is the wedge of bone that
separates the greater wing of the sphenoid
from the occipital bone.
The most prominent feature of this surface is
the internal carotid foramen, through which
the carotid artery exits the temporal bone.
The impression for the trigeminal ganglion is
located on the lateral surface of the petrous
apex.
The semi canal for the tensor tympani is
lateral to the carotid canal; the bony portion
of the Eustachian tube runs inferior and
parallel to the tensor tympani muscle.
The thin medial wall of the eustachian tube
forms the lateral wall of the carotid canal and
is frequently dehiscent. Thus, the carotid
canal is vulnerable to injury in the course of
surgical manipulations in the anterior
tympanic cavity and in the medial wall of the
eustachian tube.

60. TEMPORAL BONE (SUPERIOR SURFACE)
The superior surface (tegmen
The tegmen can be divided into
1) an anterior tegmen tympani
(covering the tympanic cavity) and
2) a posterior tegmen
mastoideum (covering the mastoid air
cells).
The petrotympanic suture line forms
the medial boundary of the tegmen.
Petrous bone
The greater petrosal nerve (GPN)
separates from the geniculate
ganglion and emerges through the
facial hiatus to run in a groove that is
slightly medial to the petrotympanic
suture and that parallels the petrous
ridge.
 Lateral to and paralleling the greater petrosal
nerve is the lesser petrosal nerve, which runs
in the petrosquamous suture (superior
tympanic canaliculus).
 The tensor tympani muscle is inferior to the
lesser petrosal nerve.

61. Foramen lacerum
Carotid canal
Gesserian
ganglion
Foramen ovale
Foramen lacerum
* Meckel’s cave impression,
AE Arcuate eminence,
AFL Anterior foramen lacerum,
FM Foramen magnum,
FO Foramen ovale,
FR Foramen rotundum,
FS Foramen spinosum, GPN Groove for the greater
petrosal nerve,
PR Petrous ridge, SS Sigmoid sinus sulcus, ZP
Zygomatic process
A superior view of an articulated temporal bone.

62. THE PETROSA
 It is evident on superior,
medial, and posterior
views of the temporal
bone.
 The term “petrous”
(Greek for “rocklike”)
stems from the extreme
density of its bone,
which guards the sensory
organs of the inner ear.
 Arcuate eminence
 Meatal plane
 Foramen spinosum
 Facial hiatus for GSPN
The lesser petrosal nerve, accompanied by
the superior tympanic artery, occupies the
superior tympanic canaliculus, lying lateral to
and paralleling the path of the greater petrosal
nerve to the petrous apex. The petrous apex
points anteromedially and is marked by the
transition of the intrapetrous to the intracranial
internal carotid artery, orifice of the bony
eustachian tube, and, anterolaterally, ganglion
of the trigeminal nerve in Meckel’s cave.

63. ARCUATE EMINENCE
 key landmark in middle cranial fossa surgery.
 in case of brain abscess following chronic suppurative otitis
media with complications the pus elevates the dura and tracts
anteriorly thereby causing a swelling in the preauricular region
known as POTT’S PUFFY TUMOUR.
 landmark for identification of the internal auditory meatus. The
bone anteromedial to the arcuate eminence and greater
superficial petrosal nerve is termed the ‘meatal plane’ and lies
above the inernal auditory canal. It is often marked by a
shallow depression.
 Superior canal dehiscence syndrome (SCDS by a thinning or
complete absence of the arcuate eminence.
MECKEL’S CAVE
 For relief of pain in trigeminal neuralgia glycerol injection is
given in the gasserian ganglion in this region.