32 Surgical Anatomy of the Petrous Bone

32.1 Introduction

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 pyramidshaped 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.

○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-flled 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 superfcial landmark for the mastoid antrum is the suprameatal triangle (MacEwen’s triangle), a depression delimitated by

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).

•The supramastoid crest is a bony ridge corresponding to the level of the middle fossa foor; it represents the inferior extension of the superior temporal line and continues anteriorly as the upper edge of the zygomatic arch.

32.2 Base of the Petrous Bone

•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.

32.3 Petrous Apex

•The apex of the petrous pyramid is directed anteromedially.

•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

dur he temporal fossa while preserving the superior petrosal sinus.

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

•It forms the posterior part of the foor 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 superfcial petrosal nerve (GSPN) courses poste- ro-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 fbers 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 foor 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-superi- orly to the genu of the petrous internal carotid artery.

32.5 Posterior Surface of the

Petrous Bone

•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 fssure; the pos- tero-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).

32.6 Inferior Surface of the

Petrous Bone

•It is exocranial and joins the clivus through the interposition of fbrocartilaginous 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

Fig. 32.2 Microsurgical view of the right cerebellopontine angle through the presigmoid area. The petrosal surface of

the cerebellum is retracted backward. Abbreviations: AICA = anterior inferior cerebellar artery culus; ITG = inferior temporal gyrus; PB = petrous bone; SS = sigmoid sinus; R = retractor; VII/VIII = facial and vestibulocochlear complex; IX = glossopharyngeal nerve; X = vagus nerve;

XI = accessory nerve.

a communication between the perilymphatic and subarachnoid space, and the small foramen for the tympanic branch of the glossopharyngeal nerve.

•On the lateral side of the inferior petrous surface, the stylomastoid foramen is situated between the styloid and mastoid processes and represents the external opening of the facial canal.

32.7 Superior Ridge

•It is grooved by the superior petrosal sinus, which runs into the sulcus where the tentorial margin attaches.

postero-laterally. The superior semicircular canal lies parallel a few millimeters behind.

○At the geniculate ganglion, the facial nerve gives rise to the

GSPN and then turns posteriorly and laterally along the medial wall of the tympanic cavity, where it courses between the lateral semicircular canal and the oval window. The deep location of the tympanic segment is overlaid on the surface by Henle’s spine (postero-superior margin of the external acoustic meatus) (Fig. 32.432.5).

○At the level of the stapes, the facial nerve turns downward and descends vertically through the mastoid process until it reaches the stylomastoid foramen (Figs. 32.6,32.7).

32.8 Anterior Border

•It joins medially the greater wing of the sphenoid bone delimitating the foramen lacerum and laterally the temporal squama at the petro-squamosal suture.

32.9 Postero-Inferior Border

•It is articulated with the occipital bone along the petroclival fssure, where the inferior petrosal sinus courses.

32.10 Facial Canal

•The intratemporal course of the facial nerve can be divided into four segments: intrameatal, labyrinthine, tympanic, and mastoid.

○The intrameatal part enters the internal acoustic meatus and passes through the anterosuperior compartment of the fundus. At this level, it is separated from the cochlear nerve by the transverse crest, while the Bill’s bar (or vertical crest) divides the facial canal from the superior vestibular area (Fig. 32.3).

○The labyrinthine segment runs from the meatal fundus to the genu, where the geniculate ganglion sits; it is related to the cochlea antero-medially and the vestibular labyrinth

32.11 Bony Labyrinth

•The auditory and vestibular labyrinths consist of hollow cavities within the petrous part of the temporal bone.

•These are surrounded by a dense bony shell, the otic capsule, that can be easily distinguished compared to the pneumatized petrous apex and mastoid process.

•These cavities contain the sensory organs of the inner ear for acoustic and vestibular functions.

•The osseous labyrinth can be divided into three sections: vestibule, cochlea, and three semicircular canals.

•The bony labyrinth is located medial to the tympanic cavity and above the jugular bulb (Figs. 32.4,32.5)

•The opening of the internal acoustic meatus on the posterior surface of the petrous bone allows entry to the cochlea ante- ro-medially and the vestibule postero-laterally.

32.12 Petrosal Segment of the

Internal Carotid Artery

•The ICA enters its canal in the petrous part of the temporal bone at the outer surface of the skull base, anterior and medial to the jugular foramen.

•It ascends vertically and, at the genu, it turns anteromedially in the horizontal portion directed toward the petrous apex.

Fig. 32.3 Enlarged view of the nervous bundle of the acoustic-facial complex within the internal acoustic meatus after removal of the posterior wall and dural opening. The vestibular nerve has been sectioned to expose facial and cochlear nerves. Abbreviations: IAM = internal acoustic meatus; MF = meatal fundus; VII = facial nerve; VIII coch = cochlear nerve, VIII vest = vestibular nerve.

•The ascending segment of the petrous ICA is separated by a thin osseous layer from the anterior wall of the tympanic cavity. At the genu, the cochlea is situated posterior and superior to the ICA.

•The GSPN courses parallel to the lateral aspect of the horizontal segment of the petrous ICA, posteriorly to the third division of the trigeminal nerve.

•Deeper within the temporal bone, the tensor tympani muscle and the osseous portion of Eustachian tube lie along the anterior surface of the horizontal segment.

•The intracranial end of the petrosal carotid artery can be recognized as it passes above the foramen lacerum, a bone defect obliterated by thick fbrocartilaginous tissue, to continue toward the cavernous sinus at the lateral surface of the sphenoid body.

•The main collateral branches arising from the petrous ICA are the vidian and the caroticotympanic arteries. The frst one passes antero-inferiorly through the foramen lacerum and crosses the pterygoid canal; the latter one reaches the tympanic cavity.

32.13 Surgical Considerations

The surgical corridors running through the superior and posterior surfaces of the petrous pyramid provide a panoramic view of middle cranial fossa, cerebellopontine angle (CPA), and petroclival region.

Fig. 32.7 Labyrinthectomy. The deep position of tympanic segment of the facial nerve and lateral semicircular canal is approximated by Henle’s spine. The upper end of the posterior semicircular canal merges with the posterior end of the superior semicircular canal in

the common crus, which opens in the bony vestibule and is located medially to the tympanic cavity.

Abbreviations: AICA = anterior inferior cerebellar artery; EAC = external acoustic canal; HS = Henle’s spine; ITG = inferior temporal gyrus; LSC = lateral semicircular canal; PSC = posterior semicircular canal;

SS = sigmoid sinus; SSC = superior semicircular canal; TD = temporal dura;

VII mast = mastoid segment of the facial nerve; VII tymp = tympanic segment of the facial nerve; VII/VIII = facial and vestibulocochlear complex.

•The temporal lobe is carefully retracted and dura mater is elevated, exposing the middle meningeal artery as it enters the cranial cavity through the foramen spinosum of the sphenoid bone.

•Antero-medially to the foramen spinosum, the mandibular nerve (V3) enters the foramen ovale.

•A further posterior extradural exposure of the superior aspect of the petrous bone allows identity of the GSPN, which can be followed backward to its exit through the hiatus fallopii, and the arcuate eminence harboring the superior semicircular canal.

•The middle portion of the angle delimitated by the GSPN and the arcuate eminence helps to identify the deep position of the internal acoustic canal.

•The geniculate ganglion and the petrous segment of the internal carotid artery may not be covered by the bone of the foor of the middle cranial fossa.

•Bone removal at the level of the tegmen tympani opens the roof of the tympanic cavity and exposes the head of the incus in the epitympanic recess (Figs. 32.4–32.6).

•The posterior fossa can be reached by splitting the tentorium behind the point where the trochlear nerve pierces the free tentorial edge.

•An anterior petrosectomy provides a wider exposure of the lateral side of the clival region by additional bone removing between the porus acusticus and trigeminal roots entrance in Meckel’s cave.

•The bony surfaces of temporal fossa, mastoid process and posterior fossa are exposed after elevation of the skin fap; the temporal muscle in retracted antero-inferiorly, while sternocleidomastoid muscle insertion on the mastoid process is detached and retracted postero-inferiorly.

•The asterion is a fundamental craniometric point at the lateral view of the skull. It is defned by the junction of the lambdoid, occipitomastoid, and parietomastoid sutures and corresponds to the meeting point of the transverse and sigmoid sinuses.

•The foor of the middle cranial fossa is exposed performing a temporal craniotomy.

•The suboccipital extension of the temporal craniotomy and a complete mastoidectomy allow exposure of the supraand infratentorial compartments.

•The anatomical limits of the mastoid drilling are the mastoid tip inferiorly, the asterion posteriorly, and Henle’s spine anteriorly.

•The transverse-sigmoid junction is skeletonized in correspondence of the asterion and the sigmoid sinus is followed down to the roof of the jugular bulb (Fig. 32.1).

•The sinodural angle (Citelli’s angle) is used as a landmark for the position of the superior petrosal sinus draining in the sigmoid sinus (Fig. 32.1).

•The mastoid air-cells behind the posterior wall of the external acoustic canal are resected until the mastoid antrum is identifed.

•Medially to the antrum, the compact bone that encloses the otic capsule can be easily recognized and the mastoid

segment of the facial nerve can be followed toward the stylomastoid foramen, where it exits (Figs. 32.5,32.7).

•The digastric ridge represents an important landmark for the initial extracranial course of the facial nerve.

•In the presigmoid area, the dura mater of the posterior fossa is exposed as described by the Trautmann’s triangle; this triangular dural patch gives access to the pontocerebellar angle and is bounded anteriorly by the posterior semicircular canal of the vestibular labyrinth, posteriorly by Citelli’s sinodural

angle and superior petrosal sinus, and inferiorly by the jugular bulb (Figs. 32.1,32.2).

•Drilling of the petrous bone can continue performing a translabyrinthine or transcochlear approach, depending on lesion location or whether a wider surgical access is required.

•The labyrinthectomy consists in the removal of the posterior and lateral semicircular canals and vestibule to gain the exposure of the internal acoustic meatus and fundus (Figs. 32.3,32.8.32.10).

•The lateral semicircular canal is medial to the epitympanic recess of the middle ear cavity and marks the tympanic segment of the facial nerve.

•The extension of the drilling antero-medially to the fundus of the internal meatus through the cochlea defnes the transco-

Fig. 32.8 Enlarged view of the bony labyrinth and of the course of facial nerve. The tympanic segment of the facial nerve courses between the lateral semicircular canal and the stapes in the oval window. The facial nerve eventually turns downward as the mastoid segment toward the stylomastoid foramen. The superior semicircular canal bulges upward in the

the middle cranial fossa corresponding to the superior surface of the petrous bone. The lateral semicircular canal is located postero-medially to the epitympanic area, and the posterior semicircular canal is situated laterally to the posterior wall of the internal acoustic meatus. Abbreviations: CC = common crus; ITG = inferior temporal gyrus; LSC = lateral semicircular canal; PSC = posterior semicircular canal; SS = sigmoid sinus; SSC = superior semicircular canal; St = stapes; VII mast = mastoid segment of the facial nerve; VII tymp = tympanic segment of the facial nerve.

Fig. 32.9 Removal by drilling of the petrous bone through the vestibular labyrinth and exposure of the dura mater lining the posterior wall of the internal acoustic meatus.

Abbreviations: AICA = anterior inferior cerebellar artery; D = dura mater;

culus; IAM = internal acoustic meatus; ITG = inferior temporal gyrus; JB = jugular bulb seen through the bone; SPS = superior petrosal sinus; SS = sigmoid sinus; VII/VIII = facial and vestibulocochlear complex;

VII tymp = tympanic segment of the facial nerve.

chlear approach and allows widening the exposure of the petroclival region.

References

1.Alonso F, Dekker SE, Wright J, et al. The retrolabyrinthine presigmoid approach to the anterior cerebellopontine region: expanding the limits of Trautmann triangle. World

Neurosurg 2017;104:180–185

Fig. 32.10 Dural opening within the internal acoustic canal.

Abbreviations: AICA = anterior inferior cerebellar artery; F = culus; IAM = internal acoustic meatus; ITG = inferior temporal gyrus; JB = jugular bulb seen through the bone; MF = meatal fundus; SS = sigmoid sinus; VII/VIII = facial and vestibulocochlear complex; VII mast = mastoid segment of the facial nerve; VII tymp = tympanic segment of the facial nerve.

2.Rhoton AL. The Temporal Bone and Transtemporal Approaches. Cranial anatomy and surgical approaches.Neurosurgery. Rhoton’s anatomy. Part 3. 2003:643–697

3.Troude L Jr, Carissimi M Jr, Lavieille JP, Roche PH. How I do it: the combined petrosectomy. Acta Neurochir (Wien) 2016;158(4):711–715

4.Luo Z, Zhao P, Yang K, Liu Y, Zhang Y, Liu H. The microsurgical anatomy of the modifed presigmoid trans-partial bony labyrinth approach. J Craniofac Surg 2015;26(5):1619–1623

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Transpetrosal Approachesxxxx