Anatomy of temporal bone and skull base

DR ROOHIA

ANATOMY OF

TEMPORAL BONE

SKULL BASE

SKULL BASE

Anatomy skull base 3

INTRODUCTION 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.


EMBRYOLOGY OF THE 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.


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.


THE CHONDRIFICATION CENTRES OF SKULL 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 PARACHORDAL CARTILAGES AND EARLY CHONDROGENESIS

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.


THE HYPOPHYSEAL CARTILAGES AND RUDIMENTARY CENTRAL SKULL BASE Mesenchymal condensations migrating to

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


THE HYPOPHYSEAL CARTILAGES AND RUDIMENTARY CENTRAL SKULL BASE

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


THE HYPOPHYSEAL CARTILAGES AND RUDIMENTARY CENTRAL SKULL BASE Together with the trabecular cartilages, the

hypophyseal chondrification centers fuse to form the precursors of the central skull base.


THE HYPOPHYSEAL CARTILAGES AND RUDIMENTARY 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.


THE NASAL PLACODES AND ANTERIOR CRANIAL 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.


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).


Palatovaginal canal

Vomerovaginal canal

III.FORAMEN ROTUNDUM


RELATION OF SKULL BASE TO THE DEEP FACIAL SPACES. 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.

ANATOMY OF TEMPORAL BONE AND IT’S SURGICAL

IMPORTANCE

EMBRYOLOGY

0-8 WEEKS 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.

8-16 WEEKS 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

DEVELOPMENT AFTER 16 WEEKS 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 canal.

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.

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

ANATOMY OF TEMPORALBONE

The temporal bone is a composite structure consisting of1. The Tympanic Bone2. The Mastoid Process3. The Squama(Squamous

portion of the temporal bone)4. The Petrosa(Petrous portion

of the temporal bone.

TEMPORAL BONE (LATERAL SURFACE)

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 canal3) styloid bone - in an anterior-inferior direction. 4) mastoid boneGlenoid fossaMacevens triangle

LATERAL SURFACE OF RIGHT TEMPORAL BONE

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

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.

SQUAMOUS PORTION OF TEMPORALBONE:

Latera wall of Middle cranial fossa Parietal bone superiorly Zygoma,TMJ anteriorly Medially –middle meningeal artery Laterally-temporal artery

MASTOID PORTION . 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.

STYLOID PROCESS 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 ligamentstylomandibular ligamentstyloglossus muscle (innervated

by the hypoglossal nerve)stylohyoid muscle (innervated

by the facial nerve)stylopharyngeus muscle

(innervated by the glossopharyngeal nerve)

SURGICAL IMPORTANCE 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.

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 ProcessThe stylomandibular ligamentEagle syndrome

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.

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).

TEMPORAL BONE (MEDIAL SURFACE) 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).

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

TEMPORAL BONE (POSTERIOR SURFACE) 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.

SURGICAL IMPORTANCE 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.

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.

The inferior surface of the temporal bone separates the upper neck from the skull base. Accordingly, many vitalneurovascular 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 nervosaThe 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.

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

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.

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.

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.

TEMPORAL BONE (SUPERIOR SURFACE)The superior surface (tegmenThe 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 boneThe 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.

Foramen lacerum

Carotid canalGesserian ganglion

Foramen ovaleForamen 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.

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.

SURGICAL IMPORTANCEARCUATE 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.

THANK YOU

Education

Anatomy of temporal bone and skull base