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Skull Base Anatomy Fatih Kökdere Overview • Forms the floor of the cranial cavity • Separates the brain from other facial structures Overview • Formed by ethmoid, sphenoid, occipital, paired frontal, and paired temporal bones. Overview • Subdivided into 3 regions: the anterior, middle, and posterior cranial fossae. Anterior Cranial Fossa • Most shallow and superior of the three cranial fossae • Lies superiorly over the nasal and orbital cavities • Accommodates the anteroinferior portions of the frontal lobes of the brain Anterior Cranial Fossa: Borders • Anteriorly and laterally bounded by the inner surface of the frontal bone • Posteriorly and medially bounded by the limbus of the sphenoid bone • Posteriorly and laterally it is bounded by the lesser wings of the sphenoid bone • The floor consists of the frontal bone, ethmoid bone and the anterior aspects of the body and lesser wings of the sphenoid bone Anterior Cranial Fossa : Contents • Frontal crest: acts as a site of attachment for the falx cerebri (a sheet of dura mater that divides the two cerebral hemispheres) • Crista galli: midline of ethmoid bone, acts as another point of attachment for the falx cerebri • Cribriform plate: on either site of the crista galli, supports the olfactory bulb and has numerous foramina that transmit vessels and nerves. • Anterior clinoid processes: rounded ends of the lesser wings, serve as a place of attachment for the tentorium cerebelli (a sheet of dura mater that divides the cerebrum from the cerebellum) Anterior Cranial Fossa : Foramina • Cribriform plate: numerous small foramina, transmiting olfactory nerve fibers. 2 larger foramen; • Anterior ethmoidal foramen transmits the anterior ethmoidal artery, nerve and vein • Posterior ethmoidal foramen transmits the posterior ethmoidal artery, nerve and vein Anterior Cranial Fossa : Clinical Relevance • Cribriform plate: thinnest part, most likely to fracture • Anosmia • CSF rhinorrhoea: leakage of CSF into the nasal cavity Middle Cranial Fossa • • • • • • • As its name suggests, centrally in the cranial floor Butterfly shaped Anteriorly and laterally: lesser wings of the sphenoid bone Anteriorly and medially: limbus of the sphenoid bone Posteriorly and laterally: superior border of the petrous part of the temporal bone Posteriorly and medially: dorsum sellae of the sphenoid bone Floor: the body and greater wing of the sphenoid, and the squamous and petrous parts of the temporal bone. Middle Cranial Fossa: Central Part • Formed by the body of the sphenoid bone • Contains the sella turcica(turkish saddle), acts to hold and support the pituitary gland • Tuberculum sellae (horn of the saddle): anterior wall of the sella turcica, and the posterior aspect of the chiasmatic sulcus • Hypophysial fossa or pituitary fossa (seat of the saddle): a depression in the body of the sphenoid, which holds the pituitary gland • Dorsum sellae (back of the saddle): posterior wall of the sella turcica, separates the middle cranial fossa from the posterior cranial fossa. Middle Cranial Fossa: Lateral Parts • Formed by the greater wings of the sphenoid bone, and the squamous and petrous parts of the temporal bones • Support the temporal lobes of the brain • The site of many foramina Foramina of the Sphenoid Bone • Optic canals: optic nerves (CN II) and ophthalmic arteries, connected by the chiasmatic sulcus • Superior orbital fissure: opens anteriorly into the orbit; contains the oculomotor nerve (CN III), trochlear nerve (CN IV), ophthalmic branch of the trigeminal nerve (CN V1), abducens nerve (CN VI), opthalmic veins and sympathetic fibers • Foramen rotundum: transmits the maxillary branch of the trigeminal nerve (CN V2). • Foramen ovale: the mandibular branch of the trigeminal nerve (CN V3) and accessory meningeal artery • Foramen spinosum: transmits the middle meningeal artery, middle meningeal vein and a meningeal branch of CN V3 Foramina of the Temporal Bone • Hiatus of the greater petrosal nerve: transmits the greater petrosal nerve (a branch of the facial nerve), and the petrosal branch of the middle meningeal artery • Hiatus of the lesser petrosal nerve – transmits the lesser petrosal nerve (a branch of the glossopharyngeal nerve). • Carotid canal – located posteriorly and medially to the foramen ovale. This is traversed by the internal carotid artery, the deep petrosal nerve also passes through this canal • Foramen lacerum: At the junction of the sphenoid, temporal and occipital bones, filled with cartilage Clinical Relevance: Pituitary Surgery • • • • The pituitary gland lies in the sella turcica of the sphenoid bone In cases of a pituitary tumour, it may need to be removed surgically. usually by a endoscopic transsphenoidal approach The sphenoid sinus is opened and the endoscope passes through to the pituitary gland • Complications of pituitary surgery include CSF rhinorrhoea, meningitis, diabetes insipidis, haemorrhage and visual disturbances Posterior Cranial Fossa • • • • • Most posterior and deep of the three cranial fossae Accommodates the brainstem and cerebellum Three bones: the occipital bone and the two temporal bones. Anteriorly and medially: dorsum sellae of the sphenoid bone Anteriorly and laterally: superior border of the petrous part of the temporal bone. • Posteriorly: internal surface of the squamous part of the occipital bone • Floor: mastoid part of the temporal bone and the occipital bone Posterior Cranial Fossa - Foramina • Internal acoustic meatus: transmits the facial nerve (CN VII), vestibulocochlear nerve (CN VIII) and labrynthine artery • Foramen magnum: transmits the medulla of the brain, meninges, vertebral arteries, spinal accessory nerve (ascending), dural veins and anterior and posterior spinal arteries • Jugular foramina: transmits the glossopharyngeal nerve, vagus nerve, spinal accessory nerve (descending), internal jugular vein, inferior petrosal sinus, sigmoid sinus and meningeal branches of the ascending pharyngeal and occipital arteries. • Hypoglossal canal: Hypoglossal nerve Clinical Relevance: Cerebellar Tonsillar Herniation • Downward displacement of the cerebellar tonsils through the foramen magnum • Produced by a raised intracranial pressure; causes include hydrocephalus, space occupying lesions, and a malformed posterior cranial fossa • Cerebellar tonsillar herniation results in the compression of the pons and medulla, which contain the cardiac and respiratory centers References • TeachMeAnatomy. (2017). The Anterior Cranial Fossa. [online] Available at: http://teachmeanatomy.info/head/areas/crani al-fossa/anterior/ [Accessed 30 Mar. 2017]. • Skull Base Anatomy. (2016, June 28). Retrieved March 30, 2017, from http://emedicine.medscape.com/article/8826 27-overview