surgical anatomy of the skull base

Surgical Anatomy of the Skull Base

Cranial Base is divided into 3 Fossae

Middle Cranial Base•Floor of the middle cranial fossa•Posterior edge of the lesser sphenoid wing •To postero-sup edge of the petrous temporal bone

Inferior aspect Skull Base

•Posterior , central and anterior part by two lines•Anterior margin of foramen magnum•Posterior margin of hard palate

Middle cranial base (Extracranial)

•Roof of infratemporal fossa, middle ear, mastoid, condylar fossa & lateral wall of sphenoid sinus•Petrous part of temporal bone

Central & Lateral Compartments

• Central Compt : Area betn 2 parasagittal lines drawn from the Med Pterygoid plate to the occipital condyle.

Includes: Pitutary fossa, Sphenoid rostrum & sinus, Nasopharynx, Pterygopalatine fossa, Clivus.

• Lateral Compt : Includes IFTF, Parapharyngeal space, Petrous temporal bone

High density of neurovascular structures:

Extratemporal: IJV, ICA, Cranial nerves, ECA

Intratemporal: Labyrinth & VIII

Foramina

Foramina

Bones

Temporal Bone

Petrous part of Temporal Bone

• Triangular pyramid, wedged between Sphenoid & Occipital

• Lodges the middle & inner ear securely

• Base, Apex, 3 surfaces & 3 borders• Anterior surface : Trigeminal Impression,

roof of carotid canal & IAM, Arcuate eminence, tegmen tympani, groove for GSPN , LPN

• Posterior surface : IAM, Aqueduct of Vestibule, Subarcuate fossa

• Inferior Surface : Quad area near apex, Carotid canal, Jugular fossa, Mastoid, tympanic & cochlear canaliculi, Notch for IXth N Ganglion

Sphenoid Bone

Surgical Landmarks• Lateral Pterygoid Plate : Palpable guide for surgical orientation Easy exposure by medial dissection along Gr Wing

Easily identified on radiographic imaging

Root is 1mm post to F Rotundum & ant to F Ovale-Positions of V2, V3 F Spinosum just post to F Ovale

• Sphenoid Spine : Medial to condylar fossaPalpable, radiographically identifiableImm lateral to Carotid canal- identify highest part of cervical ICA

Infratemporal Fossa

Boundaries

• Roof: Greater wing of sphenoid, small part from squamous temporal

• No anatomical floor, continues with parapharyngeal space

• Anteriorly: Posterior wall of Maxilla, pterygomaxillary and inferior orbital fissure

• Posteriorly: Carotid sheath & Styloid apparatus

• Medially: Medial Pterygoid muscle• Laterally: Mandible

Contents

Medial and Lateral Pterygoid muscles

Maxillary artery and branchesPterygoid venous plexusMaxillary VeinsMandibular Nerve and branches

Muscles

Lateral pterygoidOrg: 2 Heads – Upper from the

infratemporal surface of skull, Lower from the outer surface of lateral pterygoid plate

Ins: into the pterygoid pit at the medial end of mandibular condyle

Ns: V3Ac: Bilateral Contraction opens

the mouth and protrudes the chin U/L cont swings jaw to opp

side

Medial Pterygoid Org: Medial surface of lateral

Pterygoid plate and the Pterygoid fossa

Small slip from tuberosity of maxilla and tubercle of palatine bone

Ins: into the angle of the mandible

Ns: V3Ac: Contraction closes the jaw

U/L cont swings jaw to opp side

Massetor Org: Anterior 2/3 of lower

border of zygomatic archIns: lower part of lateral

surface of ramus of mandible & coronoid process

Ns: V3Ac: Elevates the mandible to

clench the teeth

TemporalisOrg: The temporal fossa and

fasciaIns: margins of coronoid

process and anterior border of ramus

Ns: V3Ac: Elevates and retracts

the mandible

Digastric Org : Post belly from Digastric notch on medial surface of the mastoid process

Intermediate tendon passes thru fibrous sling on the hyoid near lesser cornu

Ins : Anterior belly inserts on the digastric fossa on lower border of mandible

N s : Post belly : VIIth N ( N to Digastric )

Ant belly : Vth N ( Mylohyoid N )

Ac : Depress & retract the chin

Maxillary Artery

• Terminal branch of external carotid• Enters the IFTF betw sphenomandibular lig and neck of mandible

• Leaves IFTF through pterygomaxillary fissure to enter pterygopalatine fossa

• Divided into 3 parts in relation to lateral pterygoid muscle

1st part Inferior AlveolarMiddle MeningealAccesory MeningealDeep AuricularAnt Tympanic

• 2nd PartArt to Lat PterygoidArt to Med PterygoidTemporalisLingualLong Buccal

• 3rd Part (in pterygopalatine fossa)

Postero superior alveolarInfra orbitalGreater (desc) PalatinePharyngeal BranchArt of the pterygoid canalSphenopalatine

Mandibular Nerve

• Enters IFTF thru Foramen Ovale• Courses behind Lat Pterygoid• Gives off (sensory) Nervous Spinoses

• Divides into Ant and Post Divisions

• Anterior DivisionEntirely Motor except for

long buccal nerveMotor Branches to Temporalis,

Massetor, Med & Lat Pterygoid

• Posterior DivisionEntirely sensory except for

mylohyoid NAuriculo temporal nerve Inferior Alveolar N – Mylohyoid NLingual Nerve – Joined by Chorda

Tympani

• Otic GanglionClose to V3 just below F Ovale &

relay secretomotor fibres to parotid gland

Pterygopalatine fossa

• Shape of an inverted pyramid• Apex is the pterygopalatine canal• BoundariesAnt: Post wall of maxillaPost: Greater wing of sphenoidSup: Inferior surface of sphenoid bodyMedial: Perpendicular plate of palatineLateral: IFTF via pterygomaxillary fissure

• Contents can be divided into neural compt & vasc compt

• Space connects with 5 regions of the skullAnt: Via inf orbital fissure to the orbit

Transmits IO vessels, Ns, Asc branches of pterygopalatine

ganglionPost: Via F Rotundum to middle cranial fossa, transmits Maxillary N

Via pterygoid canal to F lacerumMed: Via sphenopalatine foramen to nasal

cavityLat: Via pterygomaxillary fissure to IFTF

Transmits Max vessels & sup alveolar Ns

Inf: Via grater palatine canal to roof of the mouth

Transmits palatine vessels & Nerves

Pterygoid Canal (Vidian canal)

Inferior & medial to F RotundumTransmits pterygoid N and ArteryPterygoid N formed by GSPN & DPNGSPN carries pregang parasym fibres from sup salivatory nucleus

DPN carries postgang sym fibres from cervical symp chain

Pterygomaxillary Fissure•The connection between the IFTF and the pterygopalatine fossa

Parapharyngeal Space

• Pyramidal space with base at skull base

• Apex at Hyoid• Relations: Med: Buccopharyngeal Fascia &ConstrictorsPost: Prevertebral FasciaLat: Medial Pterygoid Muscle, Mandible, Deep surface of parotid gland

Styloid Apparatus divides into 2 comptsPre styloid & Retrostyloid

• Pre styloid Compartment:Contains Tensor palati

Levator palati Asc Palatine Art Asc Pharyngeal Art

• Retrostyloid Compartment:Contains Carotid sheath & its contents

• Styloid Apparatus2 Ligaments – Stylohyoid & Stylomandibular3 Muscles- Stylopharyngeus, Styloglossus & Stylohyoid

Carotid Sheath

• It is a dense feltwork of areolar tissue surrounding ICA and the Xth N

• Virtually absent over the IJV• Attached to skull base around carotid foramen

• Continues till the aortic arch• Firmly attached anteriorly to deep surface of SCM

• Posteriorly free to slide over pre vertebral fascia

Contents:Carotid ArtIJVCranial Ns IX, X, XI, XIICervical Symp TrunkLymph Nodes

Ant

Med

Internal Carotid Artery•Common carotid divides at upper border of thyroid cartilage•ICA passes vertically upwards to enter the carotid foramen•Divided into 4 parts

CervicalPetrousCavernousCerebral

Branches of the ICA• Cervical – No Branches• Petrous – Caroticotympanic & Pterygoid

• Cavernous – Branches to trigeminal gang

Sup & Inf Hypophyseal• Cerebral – Opthalmic

ACA, MCA, P Comm & Ant Choroidal

Carotid Siphon

Internal Jugular Vein

• Jugular foramen is divided by 2 transverse septae of fibrous dura into 3 compts

Anterior – IXth N, Inf Petrosal sinusMiddle – Xth and XIth NsPosterior – Jugular Bulb, receives the Inf Petrosal Sinus

• Lies lateral to ICA in the neck• Deep cervical lymph nodes lie lateral to the vein

Jugular foramen from the inside

Nerves in the Carotid Sheath

• IXth N: emerges from lateral to inferior petrosal sinus in the anterior part of jugular foramen

Passes lateral to ICA and curves forward towards the tongue

• Xth N: emerges from its superior ganglion in the middle compartment of the jug foramenRuns straight downward betw ICA & IJVJust below the skull base, it dilates to form the inferior ganglion – receives connection from the XIth N ( N Ambiguus)

• XIth N: Lateral to vagus in Middle compartmentImmediately curves posteriorly between styloid and posterior belly of Digastric, gives branch to SCM, enters posterior Δle, to reach the trapezius

• XIIth N: Emerges from ant condylar foramenSpirals lat thru carotid sheath between IJV & ICA, swings further lat deep to styloid apparatus to tongue

• Cervical Symp Trunk: Lies behind carotid sheath just medial to the vagus & ends sup, at sup cervical ganglion.