32 Surgical Anatomy of the Petrous Bone A comprehensive knowledge of the complex anatomy of the petrous bone and its relationships with surrounding regions (petroclival region, middle and posterior cranial fossa) is the mainstay for lateral skull base surgical approaches directed through the temporal bone. The petrous bone is a pyramid-shaped structure that lies between the sphenoidal and occipital bones, participating to the formation of the skull base. • Petrous pyramid includes the following parts: ◦ Base. ◦ Apex. ◦ Antero-superior surface. ◦ Posterior surface. ◦ Inferior surface. ◦ Superior ridge. ◦ Anterior border. ◦ Postero-inferior border. • Critical anatomical structures of the petrous bone: ◦ Facial canal and nerve. ◦ Osseous labyrinth of the inner ear (cochlea, vestibule and semicircular canals). ◦ Petrous internal carotid artery. • The base of the petrous pyramid continues in the mastoid process, a grossly triangular bony prominence projecting downward and located behind the external auditory canal in the outer surface of the temporal bone • It gives attachment to the insertions of ◦ Sternocleidomastoid muscle. ◦ Splenius capitis and longissimus capitis muscles. ◦ Posterior belly of the digastric muscle at the mastoid notch on its medial side. • The occipital artery courses in a groove medially to the mastoid notch. • The mastoid process is variably pneumatized. ◦ The largest air-filled cell is the mastoid antrum and it communicates through the aditus ad antrum with the posterior part of the tympanic cavity of the middle ear at the epitympanic recess. ◦ An important superficial landmark for the mastoid antrum is the suprameatal triangle (MacEwen’s triangle), a depression delimitated by • The supramastoid crest is a bony ridge corresponding to the level of the middle fossa floor; it represents the inferior extension of the superior temporal line and continues anteriorly as the upper edge of the zygomatic arch. • The apex of the petrous pyramid is directed antero-medially. • It is wedged between the body and posterior border of the greater wing of the sphenoid bone and the basilar part of the occipital bone. • It faces the intracranial end of the carotid canal. Fig. 32.1 Lateral view of the middle and posterior cranial fossa on the right side. A temporal craniotomy has been performed and drilling of the mastoid process allows to expose the sinodural angle and Trautmann’s triangle. Access to the cerebellopontine angle is gained through dural opening in the presigmoid area; a supratentorial dural incision is made on the floor of the temporal fossa while preserving the superior petrosal sinus. • It forms the posterior part of the floor of the middle cranial fossa. • In the central part, the bulging of the arcuate eminence overlies the superior semicircular canal. • Farther laterally, a thin osseous layer, the tegmen tympani, separates the middle cranial fossa from the underlying tympanic cavity. • The trigeminal impression represents a shallow depression near the apex where the semilunar ganglion sits in Meckel’s cave. • The greater superficial petrosal nerve (GSPN) courses postero-laterally to the trigeminal impression until it reaches the facial hiatus, the opening through which the nerve leaves the geniculate ganglion. • A smaller canal anterolateral to the facial hiatus occasionally provides passage to the lesser petrosal nerve (LPN). • The GSPN joins the sympathetic fibers coming from the carotid plexus (deep petrosal nerve) to form the vidian nerve that passes forward in the vidian canal through the root of the pterygoid process. • The greater and lesser petrosal nerves run parallel beneath the dura of the middle fossa in the spheno-petrosal groove, immediately superior and lateral to the horizontal segment of the petrous internal carotid artery (ICA). • The anterior surface of the petrous ICA is separated by a thin osseous lamina from the tensor tympani muscle and the Eustachian tube. • The position of the cochlea below the middle fossa floor can be approximated by the angle between the GSPN and the labyrinthine segment of the facial nerve. • On the petrous surface, the cochlea corresponds to the lateral apex of Kawase’s triangle, which is delimitated by the GSPN, the lateral border of the third division of the trigeminal nerve (V3) and a line connecting the facial hiatus to Meckel’s cave. • The cochlea lies medially to the geniculate ganglion, anteriorly to the internal acoustic meatus (IAC) and postero-superiorly to the genu of the petrous internal carotid artery. • It delimitates the anterior wall of the posterior cranial fossa. • The internal acoustic meatus is located in the central part and transmits the facial, cochlear and vestibular nerves (Fig. 32.2). • Superolaterally, the subarcuate fossa divides the internal acoustic meatus by the vestibular aqueduct, which connects the vestibule to the endolymphatic sac located beneath the dura. • The anteromedial part of the jugular foramen is delimitated by the lower edge of the posterior surface of the petrous bone, and it receives the venous drainage of the inferior petrosal sinus descending through the petroclival fissure; the postero-lateral border corresponds to the notch on the jugular process of the occipital bone and receives the sigmoid sinus directed at the jugular bulb after coursing down the sulcus grooved on the intracranial surface of the mastoid process. • One or more emissary veins to the sigmoid sinus perforate the posterior border of the mastoid process. • Glossopharyngeal, vagus and accessory nerves traverse the jugular foramen through the intrajugular compartment (Fig. 32.2). • It is exocranial and joins the clivus through the interposition of fibrocartilaginous tissue. • Near the apex, an irregular surface gives attachment to the levator veli palatini and to the cartilaginous portion of the Eustachian tube. • Behind the external end of the carotid canal, the jugular fossa lodges the superior bulb of the internal jugular vein. • On the lateral edge of the jugular foramen, the mastoid canaliculus transmits the auricular branch of the vagus nerve. • The ridge between the carotid canal opening and the jugular foramen presents the cochlear canaliculus, which provides a communication between the perilymphatic and subarachnoid space, and the small foramen for the tympanic branch of the glossopharyngeal nerve.
32.1 Introduction
32.2 Base of the Petrous Bone
The postero-superior border of the external acoustic meatus (Henle’s spine).
The supramastoid crest.
A tangential line to the external acoustic meatus joining the two (Fig. 32.1).
32.3 Petrous Apex
Abbreviations: C = cerebellum; EAM = external acoustic meatus; HS = Henle’s spine; ITG = inferior temporal gyrus; MEV = mastoid emissary vein; MP = mastoid process; SPS = superior petrosal sinus; SS = sigmoid sinus; ST = suprameatal triangle; TS = transverse sinus; TT = Trautmann’s triangle.
32.4 Antero-Superior Surface of the Petrous Bone
32.5 Posterior Surface of the Petrous Bone
32.6 Inferior Surface of the Petrous Bone
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