Skull Base.pdf

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Skull Base


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Understanding the osteology of the skull base is a fundamental step in skull basesurgery . It allows for accurate topographic location and helps tailoring surgical routes tospecif ic skull base areas. This chapter rev iews the osseous anatomy of the skull base, itsma jor div isions and components.

The skull is div ided into cranium and facial skeleton. The cranium, by its turn, is

div ided into calvarium, which is the domelike superior portion of the cranium, formedby the frontal, parietal and squamous parts of the occipital and temporal bones, and thecranial base.

The cranial base has an endocranial surface, which faces the brain and is naturally div ided into anterior, middle and posterior fossae (Fig. 1) and an exocranial surface (Fig.2), which faces the nasal cav ity, sinuses, orbits, pharynx, infratemporal fossae andpterygopalatine, parapharyngeal and infrapetrosal spaces.

On the endocranial side of the skull base, the border between the anterior and middlefossa is marked the sphenoid ridge, joined medially by the chiasmatic sulcus, and the

border between the middle and posterior fossae is formed by the petrous ridges joinedby the dorsum sellae and posterior clinoid processes (Fig. 3).On the exocranial side, the anterior and middle fossae are div ided by a transverse line,

extending through the pterigomax illary f issures and pterygopalatine fossae at the upperlevel, and the posterior edge of the alveolar processes of max illa at a lower level.Medially, this corresponds to the attachment of vomer to the sphenoid bone. Themiddle and posterior cranial fossae are separated on each side, by a transverse linecrossing near the posterior border of vomer-sphenoid junction, foramen lacerum,carotid canal, jugular foramen, styloid process and mastoid tip (Fig. 4).

The anterior endocranial surface is formed by the combination of three bones:

Anterior Skull Base

Introduction

Anatomical Basis of Skull Base Surgery:Skull Osteology

CAROLINA MARTINS1,2

, MD, PhD, ALVARO CAMPERO2

, MD,ALEXANDRE YASUDA2, MD, PhD, SHIGEYUKI OSAWA2, MD, PhD,LUIZ FELIPE ALENCASTRO2, 3, MD, LUIZ CARLOS DE ALENCASTRO3, MD, PhD,ALBERT R HOTON JR .2, MD1Medical School of Pernambuco – IMIP, Recife/Brazil2Department of Neurosurgery, University of Florida, Gainesv ille3Hospital Mãe de Deus, Porto Alegre, Brasil

Key words: Skull Base Osteology, Microsurgical Anatomy

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frontal, ethmoid and sphenoid (Fig. 5). The orbital plates of the frontal bones form mostof the lateral portions of this fossa, contributing to the roof of orbital cav ities andgiv ing support to the dura and orbital gyri of the frontal lobe. The medial gap betweenthe orbital plates is f illed by the cerebral surface of the ethmoid bone, presenting the cristagalli, which gives attachment to the falx and the cribriform plates, which, by its turn, givessupport to the olfactory bulbs. The anterior fossa is closed posteriorly by the lesserw ings of the sphenoid bone laterally, and the sphenoid body medially . In this way, themedial portion of the anterior fossa is formed by three bones, while the lateral part, whichcovers the orbit and optic canals, is formed only by two, the orbital plate of frontal boneand the lesser sphenoid w ing on each side.

On the exocranial side, the lateral portion of the anterior skull base is on the top of theorbit and max illary sinus. Medially, it corresponds to the sphenoid sinus of sphenoid body and the ethmoid sinuses, on top of the nasal cav ity (Fig. 6). In fact, the most posteriorportion of the medial exocranial anterior surface is related w ith the sphenoid, while the

medial and anterior thirds are related w ith the ethmoid bone.The nasal septum, which is formed by vomer and perpendicular plate of ethmoid,

div ides the nasal cav ity along midline, while the lateral plates of the ethmoid bones div idethe nasal cav ity from each orbit (Fig. 7, 8). Some foramina and grooves connect the endoand exocranial surfaces in this area as the supraorbital grooves, on the superior orbitallimits, the ethmoid canals, located along the suture line formed by the frontal andethmoid bones, the superior orbital f issure between the lesser and greater sphenoidalw ings and optic canals, between the anterior and posterior roots of the anterior clinoidprocesses.

The endocranial surface of the middle fossa is formed by the sphenoid and temporalbones. The div ision between these bones usually is not well seen unless focusingattention to the sphenoid spine, the most posterior prominence of the sphenoid bone, just behind the foramen spinosum and follow ing the petro-sphenoidal and squamous-sphenoidal sutures (Fig. 9). The sphenoid contributes to the middle fossa mainly w ith thelateral parts of its body, the sphenoid crests and the greater w ings. The sphenoid crest is

the posterior edge of the lesser sphenoid w ings. The lesser w ings connect across midlinethrough the sphenoid planum. The chiasmatic sulcus is located posterior to the planum.On each side of the chiasmatic sulcus are the endocranial openings of the optic canals.Posteriorly, the chiasmatic sulcus is separated from the sella cav ity by the tubercullumsellae. The posterior limit of sellae is comprised by dorsum and posterior clinoidprocesses, which are the medial boundaries between the middle and posterior cranialfossae (Fig. 10).

The largest opening at the greater sphenoid w ing is the foramen ovale, which transmitsthe third trigeminal div ision and, most of the times, the accessory meningeal artery .Lateral to this opening is the foramen spinosum for the middle meningeal artery .Occasionally there may be an opening medial to foramen ovale: the emissary sphenoidforamen (foramen of Vesalius), which transmits a vein connecting the pterygoid venousplexus and the cavernous sinus and, on occasions, might transmit the accessory

Middle Skull Base

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meningeal artery . The lingula is a protrusion of the sphenoid bone located at the junction of body and greater w ing. As soon as the carotid artery leaves its canal on thepetrous portion of the temporal bone it is embraced by lingula, which holds the artery in place, and allow it to run along the carotid sulcus on each side of sellae. Anteriorly, thecarotid artery rests against the optic strut, in close relationship w ith the anterior clinoid.Lingula gives attachment to the petrolingual ligament that div ides the horizontal petrouscarotid from vertical cavernous carotid segment (Fig. 11).

The endocranial surfaces of the petrous and squamosal parts of the temporal bone alsoform the middle fossa (Fig. 12, 13). In this area, the greater petrosal nerve runs into thefacial hiatus just medial to tensor tympani muscle and lateral to the carotid canal. Nextto petrous apex there is the trigeminal impression which houses the trigeminal nerve.

Exocranially, the lateral middle fossa is related to the infratemporal, pterygopalatineand infrapetrosal spaces, while the central part is div ided into pharyngeal andparapharyngeal spaces by a line passing through the medial pterygoid plate (Fig. 14, 15).

The lateral pterygoid plate is the anteromedial boundary of the infratemporal fossa, whichis separated from the temporal fossa by the infratemporal crest. The pterigomax illary andinferior orbital f issures, the alveolar canals, foramen spinosum, ovale and emissary sphenoid foramen open into the infratemporal fossa. The exocranial surface of thetemporal bone is comprised between the mandibular fossa, petrous apex and mastoid tip.In the center of this area are the styloid process and the stylomastoid foramen for thefacial nerve. From this area two grooves radiate: digastric groove, for the posteriorbelly of the digastric muscle and the occipital groove for the occipital artery (Fig. 15). Thecarotid canal is anterior to the jugular foramen and medial to external auditory meatus.

The endocranial surface of the posterior fossa is formed medially by union of thesphenoid and clival portion of occipital bones. Laterally, it is composed by the appositionof mastoid and the posterior surface of petrous portion of temporal bone w ith thecondylar and basal portions of the occipital bone (Fig. 16). The occipital bone is the ma jorosseous component of the posterior fossa. It is formed by a squamosal, condylar and basalparts. The basal part fuses w ith the sphenoid to form clivus. Laterally, it articulates w ith

the temporal bone at the petroclival f issures. The squamosal part forms the posteriorboundary of the fossa and presents three angles. The superior angle f ills the gap betweenthe parietals along the lambdoid suture. The lateral, paired angles mark the most lateralextension of the lambdoid sutures and the ending point of the transverse sinus. It joinsthe occipitomastoid and parietomastoid sutures at asterion, an important surgicallandmark for posterior fossa craniotomies.

The condylar part of the occipital bone forms a bridge between the squamosal andbasal parts (Fig. 17 & 18). The posterior surface of the petrous portion of the temporalbone presents a medial area, containing the sulcus for the inferior petrosal sinus and alateral area comprising the jugular fossa and sulcus for the sigmoid sinus (Fig. 19). In thearticulated skull, the jugular fossa of temporal bone faces the jugular area of occipitalbone, while the anterior angle of occipital bone, located at the quadrilateral plate of the jugular process adapts to the temporal bone lateral to the jugular fossa (Fig. 20 & 21), in

Posterior Skull Base

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such a way that the jugular process of the occipital bone forms the posterior lip of the jugular foramen.

The occipital condyles are located on the exocranial surface of the condylar part of theoccipital bone (Fig. 22 & 23). In this area, basion is the midline point at the anterior archof foramen magnum, while the opisthion is the midline point at the posterior arch theforamen. If the basion is considered 12:00 o’clock and opisthion 06:00 o’clock, theoccipital condyles can be pro jected in a position between 1-3 and 9-11:00 o’clock .Posteriorly, on top of the condyles are the supracondylar fossae, which house theposterior opening of the posterior condylar canals. These canals transmit the posteriorcondylar veins, which connect the vertebral venous plexus to the jugular bulb. Theanterior condylar veins, also called hypoglossal veins, course through the hypoglossalcanals.

The exocranial surface of the squamosal part of the occipital bone is marked by f ivelines arranged around the external occipital protuberance (Fig. 23). Four of these are

transverse lines. The supreme nuchal lines are the highest ones, and give attachment tothe occipital aponeurosis. The superior nuchal line is just inferior to the prev ious ones.From medial to lateral, it gives attachment to trapezius, splenius capiti andsternocleidomastoid muscles. The vertical, unpaired line that radiates vertically from theexternal occipital protuberance is the external occipital crest. It affords attachment to theligamentum nuchae and from its midpoint arise the inferior nuchal lines, which giveattachment to the superior oblique and rectus capitis posterior muscles.

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28 General Principle

Fig . 1-41. Cranial base: endocranial surface. 2. Cranial base: exocranial surface. The upper surface of the

anterior cranial base is formed by the frontal bone, which roofs the orbit, the ethmoid bone, which isinterposed between the frontal bones and is the site of the cribriform plate; and the lesser w ing and theanterior part of the body of the sphenoid, which forms the posterior part of the floor of the anterior

fossa. The upper surface of the middle cranial base floor is formed by the greater sphenoid w ing andposterior two thirds of the sphenoid body anteriorly and the upper surface of the temporal bone,posteriorly . The posterior part of the cranial base is formed by the temporal and occipital bones. Theexocranial surface is formed by the max illa, the zygoma, palatine, sphenoid, temporal and occipital bones

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Anatomical Basis of Skull Base Surgery: Skull Osteology 29

and vomer. The max illa, orbits and the nasal cav ity are located below the anterior fossa. The anteriorpart of the hard palate is formed by the max illa and the posterior part is formed by the palatinebone. The anterior part of the zygomatic arch is formed by the zygoma and the posterior part, by thesquamosal part of the temporal bone. The mandibular fossa is located below the posterior part of themiddle fossa. The vomer attaches to the lower part of the body of the sphenoid and forms the

posterior part of the nasal septum. The infratemporal fossa is located below the greater sphenoid w ingand is limited anteriorly by the infratemporal crest. 3. On the endocranial side of the skull base, the limitbetween the anterior and middle fossa is marked the sphenoid ridge, joined medially by the chiasmaticsulcus (dotted light blue line), and the limit between the middle and posterior fossae is formed by thepetrous ridges joined by the dorsum sellae and posterior clinoid processes (dotted dark blue line). 4. Onthe exocranial side, the anterior and middle fossae are div ided by a transverse line, extending throughthe pterigomax ilary f issures and pterygopalatine fossae at the upper level, and the posterior edge of thealveolar processes of max illa at a lower level. Medially, this corresponds to the attachment of vomer tothe sphenoid bone (dotted light blue line). The middle and posterior cranial fossae are separated on eachside, by a transverse line crossing near the posterior border of vomer-sphenoid junction, foramenlacerum, carotid canal, jugular foramen, styloid process and mastoid tip (dotted dark blue line).

Fig . 5-85. The frontal ethmoid and sphenoid bones combine to form the anterior fossa, which is div ided into

medial and lateral portions. The medial part, covering the upper nasal cav ity and sphenoid sinus, is

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formed by the crista galli and the cribriform plate of the ethmoid bone anteriorly and the planum of thesphenoid body posteriorly . The lateral part, which covers the orbit and the optic canal, is formed by thefrontal bone and the lesser w ing of the sphenoid bone, which blends medially into the anterior clinoidprocesses and point towards the middle fossa. 6. On the exocranial side, the anterior cranial base isdiv ided into a medial part related to the ethmoidal and sphenoidal sinuses and nasal cav ity below, and

a lateral part that corresponds to the orbit and max illa. The ethmoid bone forms the anterior and middlethirds of the exocranial surface and the sphenoid body forms the posterior third of the medial part. Theethmoid presents the perpendicular plate that joins the vomer in forming the nasal septum and twolateral plates located in the medial wall of the orbits. The lateral plates separate the lateral wall of the nasalcav ity and the orbit. The main foramina of the region are the anterior and posterior ethmoidalforamina located in the superomedial orbital wall, along the frontoethmoidal suture, which transmitthe ethmoidal nerves and arteries, the supraorbital and supratrochlear notches or foramina, transmittingthe arteries and nerves of the same name, and the optic canal which transmits the optic nerve andophthalmic artery . The superior orbital f issure is located between the lesser and greater sphenoidal w ingson the lateral side of the optic canal. It transmits the oculomotor, trochlear, ophthalmic, and abducensnerves, a recurrent meningeal artery, and the superior and inferior ophthalmic veins. 7. The osseous nasal

septum is formed by the attachment of the perpendicular ethmoid plate and vomer at the sphenoidalcrest. 8. Anterior norma. The orbital rim is formed by the frontal bone, zygoma and max illa. The nasalbone is interposed above the anterior nasal aperture, between the max illae. The nasal cav ity is locatedbetween the ethmoid bone above and the max illae, palatine bones, and sphenoid pterygoid processbelow . It is roofed by the frontal and ethmoid bones and the floor is formed by the max illae and palatalbones. The nasal septum forms the medial walls of the nasal cav ities. The nasal conchae are located onthe lateral walls of the nasal cav ity . The inferior concha (insert) is a separate bone, and the middle andsuperior concha are appendages of the ethmoid bone.

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Fig . 9-139. The endocranial surface of the middle cranial base is formed by the sphenoid and temporal bones

and can be div ided into three regions: a medial part, the sellar region (blue shaded area), formed by thesphenoid body ; a lateral part, the temporal fossa (pink shaded area), formed by the sphenoidal w ings

and the cerebral surface of the squamosal and petrous parts of the temporal bone, and an intermediatepart, the parasellar area (yellow shaded area), formed by the transitional part of the sphenoid bonebetween the greater w ing and body, and receiv ing posteriorly a small contribution of the petrous apex of the temporal bone. The greater w ing forms the largest part of the endocranial surface of the middlefossa, w ith the squamosal and the petrosal parts of the temporal bone completing this surface. 10.Enlarged v iew of the medial part of the middle fossa. The medial part of middle fossa is formed by thebody of the sphenoid bone. 11. Lateral v iew of the parasselar region. The course of the petrous,cavernous and supraclinoid carotid have been represented. The cavernous sinus sits on the lateral aspectof the body of the sphenoid bone. The carotid sulcus is the shallow groove on the lateral aspect of thebody of the sphenoid bone along which the cavernous carotid courses. The cavernous carotid sits against

and is separated from the carotid sulcus by the dura of the medial sinus wall. The carotid sulcus beginsinferior and lateral to the dorsum sellae at the intracranial end of the carotid canal, turns forward togroove the body of the sphenoid immediately below the lateral edge of the floor of the sella, and turnsupward to end medial to the anterior clinoid process. 12. The upper surface of the petrous bone isgrooved along the course of the greater and lesser petrosal nerves. The lesser petrosal nerve from thetympanic plexus passes through the tympanic canaliculus, which is located anterior to the facialhiatus and courses in an anteromedial direction parallel to the greater petrosal nerve, which courses alongthe facial hiatus. The carotid canal extends upward and medially and prov ides passage to the internalcarotid artery and carotid sympathetic nerves in their course to the cavernous sinus. The posteriortrigeminal root reaches the middle fossa and, at the trigeminal impression, on the upper surface of thepetrous bone, are located Meckel’s cave and the semilunar ganglion. The arcuate eminence approx imates

the position of the semicircular canals. The internal auditory canal can be identif ied below the floor of the middle fossa by drilling along a line approx imately 60 degrees medial to the arcuate eminence, nearthe middle portion of the angle between the greater petrosal nerve and arcuate eminence. The petrousapex, medial to the internal acoustic meatus, is free of important structures. A thin lamina of bone, the



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tegmen tympani, extends laterally from the arcuate eminence and roofs the mastoid antrum andtympanic cav ities and the canal for the tensor tympani. Opening the tegmen from above exposes theheads of the malleus, incus, the tympanic segment of the facial nerve, and the superior and lateralsemicircular canals. 13. The anterior surface of the temporal bone has been drilled to expose theinternal structure of the temporal bone. The carotid artery is represented in red, the facial nerve in

yellow, the cochlear nerve in black and the vestibular nerves in green. The arcuate eminenceapprox imates the position of the superior semicircular canal; however, the relationship between thesetwo structures is greater at their anterior end, from which their main ax is diverge. From the brainstemto its peripheral branches, the facial nerve can be div ided into six portions: cisternal, meatal (a),labyrinthine, tympanic, mastoid (c) and extracranial. The labyrinthine segment, which is located in thepetrous part, extends from the meatal fundus to the geniculate ganglion and is situated between thecochlea anteromedially and the semicircular canals posterolaterally . The labyrinthine segment ends atthe site at which the greater superf icial petrosal nerve arises from the facial nerve at the level of thegeniculate ganglion. From there, the nerve turns laterally and posteriorly along the medial surface of thetympanic cav ity, thus giv ing the name tympanic segment to that part of the nerve. The tympanic segmentruns between the lateral semicircular canal above and the oval w indow below . As the nerve passes below

the midpoint of the lateral semicircular canal, it turns vertically downward and courses through thepetrous part ad jacent to the mastoid part of the temporal bone; thus the third segment, which ends atthe stylomastoid foramen, is called the mastoid or vertical segment. Into the temporal bone the facialnerve gives off the greater petrosal (e) and chorda tympani nerves (f). The corda tympani nerve,which arises from the mastoid part, runs upwards, passes along the roof of the tympanic cav ity and ex itsthe cav ity through the anterior canaliculus. The greater petrosal nerve runs initially along the facial hiatusruns beneath the dura of the middle fossa, reaches the sphenopetrosal groove formed by the junctionof the petrous and sphenoid bones, immediately superior and anterolateral to the horizontal segmentof the petrous carotid and joins the sympathetic carotid nerves to help forming the v idian nerve intothe pterygoid canal. The cochlea lies below the floor of the middle fossa in the angle between the

labyrinthine segment of the facial nerve and the greater petrosal nerve, just medial to the geniculateganglion, anterior to the fundus of the internal acoustic meatus, and posterosuperior to the lateral genuof the petrous carotid artery .

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Fig . 14-15The exocranial surface of the middle cranial base is also div ided into medial (blue shaded area),

intermediate (yellow shaded area) and lateral (pink shaded area) parts. The medial part encompassesthe sphenoid body and the upper portion of the basal (clival) part of the occipital bone and correspondsto the sphenoid sinus and the nasopharynx . The lateral part is formed by the greater sphenoid w ing, the

petrous, tympanic, and squamous and styloid parts of the temporal bone and the zygomatic, palatine,and max illary bones. The intermediate part corresponds to the area comprised between the pterygoidplates, encompassing the area inferior to the cavernous sinuses, which extend from the pterygopalatinefossa in front to the pterygoid fossa posteriorly . 15. Temporal bone. Exocranial surface.The temporalbone is div ided into squamosal, petrous, mastoid, tympanic, and styloid parts. The tympanic andsquamosal parts, which form the roof of the mandibular fossa, are located anteriorly to the styloid part,the mastoid part is postero-lateral, radially grooved by the occipital groove and mastoid notch, and thepetrous part is located medial to the styloid part.

Fig . 16-1916. The posterior cranial base is formed by three bones: sphenoid, temporal and occipital bones. The

posterior fossa can be div ided into medial and lateral portions. Medially, the sphenoid and the basal

(clival) portion of occipital bone fuse at the sphenoclival syncondrosis. Laterally, the posterior fossa iscomposed by the apposition of mastoid and posterior surface of petrous portion of temporal bone w ithcondylar and basal portions of the occipital bone. The occippital bone is the main component of theposterior cranial fossa. It has three parts, clival, condylar and squamosal; three borders, petrous,mastoid and parietal and three angles: paired, anterior and lateral angles, and an unpaired, superiorangle. The anterior angle marks the combination of the different parts of temporal and occipitalbones: medial to the anterior angle, the petrous border meets the petrosal part of the temporal bone atthe petroclival f issure, and the jugular fossa of the temporal bone combines w ith the jugular notch of the occipital bone to form the jugular foramen. Lateral to the anterior angle, the mastoid border meetsthe mastoid part of the temporal bone to form the occipitomastoid suture. The parietal border,

between the lateral and superior angle, combines w ith the parietal bone forming the lambdoid suture.17. Superior v iew of the endocranial side of the condylar and basal (clival) parts of the occipitalbone. The condylar part can be considered a bridge connecting the clival and squamosal parts of theoccipital bone. The clival part is concave from side do side and present the sulcus for the inferior petrosal



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sinus. The inferior petrosal sinus links the cavernous sinus to the medial part of the jugular foramen.On the exocranial side, there is a similar sulcus for the inferior petrosal vein. The condylar partcomprises a quadrilateral plate of bone, the jugular process, whose anterior border presents the jugular notch and the anterior angle of the occipital bone. Medial to the jugular notch, on theendocranial surface is the jugular tubercle, a protrusion related to the course of the lower cranial nerves.

18. Exocranial surface of the condylar part of the occipital bone. The jugular notch is locatedsuperolateral to the hypoglossal canal and occipital condyle. On its posterior wall there is the openingof the posterior condylar canal, which transmits an emissary bvein connecting the vertebral plexus tothe sigmoid sinus. The hypoglossal canal can be div ided by a f ibrous or a bony septum. 19. Theposterior surface of the temporal bone forms the anterolateral limit of the posterior fossa. It extends fromthe petrous apex medially to the sigmoid sulcus laterally and from the sulcus for the superior petrosalsinus and petrous ridge superiorly, to the sulcus for the inferior petrosal sinus and jugular fossainferiorly . The nerves passing through the internal acoustic meatus have been represented w ithcolored material: the facial nerve (yellow) is located anterosuperior, the cochlear nerve (black) isanteroinferior, and the vestibular nerves are located posterolaterally . The petrous carotid (red) isrepresented in the carotid canal.

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1. Rhoton Al Jr. The posterior cranial fossa. Microsurgical anatomy and surgical approaches.Neurosurgery 47(1):S1-S298, 2000.2. Rhoton AL Jr. The supratentorial cranial space. Microsurgical anatomy and surgical

approaches. Neurosurgery, 51(1):S375-S410, 2002.

REFERENCES


Fig . 20-2320. Petroclival area. The combination of the basal (clival) part of the occipital bone and petrosal part

of the temporal forms the petroclival f issure. The combination of the jugular notch of the occipital boneand the jugular fossa of the temporal forms the jugular foramen. The intra jugular processes of thetemporal and occipital bones pro ject into the jugular foramen and div ide this area into petrosal andsigmoid parts. The intra jugular process of the temporal bone is usually more prominent. 21.Arrangement of parietal, temporal and occipital bones on the right side of the skull. Lateral to the jugularforamen, the mastoid border of the occipital bone meets the mastoid part of the temporal bone,forming the occipitomastoid suture. The parietal bone meets the mastoid part of the temporal at theparietomastoid f issure and the parietal border of the occipital bone combines w ith the parietal bone atthe parieto-occipital or lambdoid suture. The meeting point of the occipitomastoid, parietomastoid andlambdoid sutures forms the asterion. The asterion is related to the lateral angle of the occipital bone andmarks the transition between the transverse and sigmoid sinuses at the most lateral part of theposterior cranial fossa. 22. The occipital bone surrounds the foramen magnum. The occipital bone isdiv ided into a squamosal part located above and behind the foramen magnum, a basal (clival) partsituated in front of the foramen magnum, and paired condylar parts, located lateral to the foramenmagnum. Basion is the most anterior part of the anterior edge of foramen magnum. Opisthion is themost posterior point along the posterior edge of foramen magnum. 23. Exocranial surface of squamosalpart of the occipital bone.The exocranial surface is marked by four paired transverse ridges and onevertical crest that radiate from the external occipital protuberance. The supreme nuchal lines are thehighest ones, and give attachment to the occipital aponeurosis. The superior nuchal lines are just inferiorto the prev ious ones, and usually more marked. From medial to lateral, they give attachment totrapezius, splenius capiti and sternocleidomastoid muscles. The superior nuchal lines have a transversepart, close to the external occipital protuberance. Laterally, close to the mastoid, they describe an archedcourse, related to the mastoid emissary foramina. The transverse part of the superior nuchal line marksexternally the position of the transverse sinuses and the transition between the supratentorial andinfratentorial compartments. The external occipital crest radiates vertically from the external occipital

protuberance close to midline. It affords attachment to the ligamentum nuchae. From the midpoint of the external occipital crest, an arched paired ridge, the inferior nuchal line radiates, just above theposterior edge of foramen magnum. The inferior nuchal line on each side gives attachment to thesuperior oblique and rectus capitis posterior ma jor and minor. The occipital bones sit over thecerv ical atlas adapting the convex occipital condyles adapt to the concav ity of the lateral masses of atlasand piling on top of the cerv ical column.

Ant.: Anterior, Ac.: Acoustic, Arc.: Arcuate, Can.: Canaliculus, Car.: Carotid, Cav.: Cavity, Chiasm.:Chiasmatic, Clin.: Clinoid, Cond.: Condyle, Condylar, Em.: Emmissary,Emin.: Eminence, Eth.: Ethmoid,Ethmoidal, Eust.: Eustachian, Ext.: External, Fiss.: Fissure, For.: Foramen, Front.: Frontal, Gr.: Greater,Horiz.: Horizontal, Hypogl.: Hypoglossal, Impres: Impression, Inf.: Inferior, Infratemp.: Infratemporal, Int.:

Internal, Intrajug.: Intrajugular, Jug.: Jugular, Lat.: Lateral, Less.: Lesser, Mand.: Mandibular, Mast.:Mastoid, Mid.: Middle, Occip.: Occipital, Occipito, Orb.: Orbital, Palat.: Palatine, Par.: Parietal, Pet.:Petrous, Perp.: Perpendicular, Pharyng.: Pharyngeal, Post.: Posterior, Proc.: Process, Pteryg.: Pterygoid,Pterygomax.: Pterygomaxillary, Subarc.: Subarcuate, Sem.: Semicircular, Sphen.: Sphenoid, Sphenoidale,Squam.: Squamous, Squamotymp.: Squamotympanic, Sup.: Superior, Supraorb.: Supraorbital, Surf.:Surface, Sut.: Suture, Supracond.: Supracondylar, Supraorb.: Supraorbital, Sig.: Sygmoid, Stylomast.:Stylomastoid, Temp.: Temporal, Trig.: Trigeminal, Tymp.: Tympanii, Tympanic, Tuberc.: Tubercle,Transv.: Transverse, Vest.: Vestibular, Zyg.: Zygomatic.



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Most cranial base tumors are benign and do not invade ad jacent structures. Theircomplete removal may lead to cure of the patient. However, because of the complex structure of the cranial base and the close prox imity of the lesion to cranial nervesand vessels, surgery in this area is associated w ith considerable risk of morbidity and evenmortality . The management of such lesions still remains a challenge, despite theenormous progress over the past few decades, which made it more eff icient and safe.Some four decades ago many areas in the skull base were regarded as “no mans land”. Theneurosurgeon was unable to prov ide adequate treatment for a considerable number of

basal lesions. At that time a close cooperation w ith specialists in the area of neurosurgery,ENT, max illofacial surgery, and ophthalmology was initiated. Receiv ing the input fromeach one of them, skull base surgery was established, soon prov ing to be one of the mostrapidly developing surgical f ields. The principle of interdisciplinary cooperation remainsa main feature of this f ield of surgery . The establishment of endoscopy, either as a soleprocedure or as endoscopic assistance to the microsurgical technique, is a recentexample of how the ongoing cooperation leads to new concepts of treatment.

The goal of skull base surgery is the achievement of complete tumor removal.However, another generally accepted principle is the preservation of neurological

functions and quality of life after surgery . In case of malignant tumors, it might benecessary to sacrif ice some functions in order to achieve oncological resection. Inbenign tumors, however, sound judgment is required to decide when to stop and avoidsevere def icits. An integral part modern skull base surgery is the restoration orreconstruction of lost functions.

Nowadays, any skull base lesion can be successfully approached and removed w ith very low mortality risk, acceptable morbidity rates and increasing rate of preservation of neurological functions. Multiple approaches to each part of the skull base have beendeveloped and put into practice, ranging from small modif ications of more traditionalapproaches to complex and sophisticated new techniques. One tendency was to promotew ide bone resections, characteristic of the skull base approaches, and thus to avoidexcessive brain retraction and the related complications. Another tendency was tomake smaller craniotomies and to be less invasive in regard to the skull opening, at the

Principles and practice of skull basesurgery

MADJID SAMII1

, MD, PhD; VENELIN M. GERGANOV2

, MD, PhD1President - International Neuroscience Institute, Rudolf Pichlmayr Str. 4, 30625Hannover, Germany

2Associate Neurosurgeon, Department of Neurosurgery, International NeuroscienceInstitute-Hannover

Key words: scull base surgery, endoscopic assistance, intraoperative neuronav igation,frontolateral approach, retrosigmoid suprameatal approach, Samii’sapproach

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expense of restricting the operative v iew, increasing the risks of in jury to ad jacentstructures and limiting the completeness of tumor removal. The senior authors (MS)experience in this area, encompassing more than 40 years, has shown that achievementof optimal outcome requires utilization of approaches that are not by themselves relatedto signif icant approach-related morbidity and prov ide suff icient access (1). Simplerapproaches, indiv idually targeted to each lesion and patient, lead to optimal outcomes.Thus, the ref inements of the traditional retrosigmoid suboccipital approach have madeit the most effective and safe approach, the “golden standard” for lesions in thecerebellopontine angle (CPA). On the other hand, in some basal tumors, e.g. chordomasand chondrosarcomas, the approach has to be selected indiv idually and always tailoredto the characteristics of the particular tumor, its location and patient's expectations.

Achievements in neuroradiology, neuroanesthesia, electrophysiology, microsurgery,as well as the technological progress, all contributed to the establishement of skull basesurgery . The advances of neuroimaging contributed signif icantly to decreasing the risks

of surgery . Besides information on the tumor type, an adequate neuroradiologicalstudy should allow an accurate v isualization of the tumor location and extension, as wellas the relation to surrounding structures. A detailed study of the structure of ad jacentareas of the skull base is an essential component of the approach planning. Theknowledge of the anatomical variations is a prerequisite for the safe performing of theprocedure. Preoperative angiography prov ides essential information regarding thevascular supply to the tumor, relation to ma jor vessels or sinuses, and hemisphericdominance. In case of highly vascular tumors, such as hemangiopericytomas, glomus jugulare tumors, and some menigiomas, preoperative embolisation of tumor vessels

might be of great help. The application of intraoperative neuronav igation has become a routine in

neurosurgery . In skull base surgery it allows for accurate approach planning and forintraoperative localization of important bone, brain and vascular structures, thusincreasing the safety of surgery (Fig.1). The tumor location and extension can bev isualised in real time and the completeness of resection can be controlled. It is especially helpful in those cases when the normal anatomical landmarks are diff icult to identify orhad been lost: when work ing v ia a narrow corridor such as the endonasal (Fig.2); intumors causing w ide bone destruction; in case of prev ious radiotherapy ; or in

reoperations. The technique is very reliable and accurate throughout the procedure dueto the absence of the “brain shift” at the skull base level. Radiosurgery cannot lead to a cure; however, it can prov ide long-term tumor control

and is an additional option in case surgery cannot lead to complete tumor removal or istoo risky . The cooperation between neurosurgeons and radiosurgeons allows for theoptimal selection of an indiv idual patients management. The increasing experiencehas shown that operative treatment after prev ious radiosurgery is much more diff icultand dangerous. Further, it frequently hinders radical tumor removal; therefore, surgery should have a priority . In case a tumor portion cannot be completely removed w ithoutma jor def icits, this residual can be treated w ith radiosurgery . In other cases, such as smallmeningiomas or tumor remnants limited to or invading the cavernous sinus, radiosurgery may be the f irst treatment choice (2,3).

Principles and practice of skull base surgery 145Principles and practice o skull base surgery 261


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Tumors of the anterior cranial base are either primarily intracranial or extracranial w ithsecondary involvement of the skull base. Surgery is the primary treatment mode forbenign tumors, while in malignant tumors it is a part of the multimodal management.Extracranial approaches are indicated if the tumor does not penetrate the dura. Extensivetransbasal or combined intra- and extracranial approaches even w ith orbital exenterationmay be indicated in aggressive tumors. The introduction of endoscope assistance and theelaboration of the extended endonasal surgical technique is a recent advancement. Thepossibility of the endoscope to look around the corner, their great illumination power andw ide v iew ing angle, allow access to tumors of the whole cranial base, from the crista gallito C2 level (4,5,6). Its ma jor advantage are the avoidance of craniotomy and of brain

Anterior and middle cranial base

146 Neuro-oncology

Fig . 1a Fig . 1b

Fig . 1c

Fig . 1d

Fig . 1 Petrous apex chondrosarcoma - T1- weighted MRI (Fig.1a), bone-w indow CT (Fig.1b). The tumor was approached v ia the subtemporal extradural route. Thebone overly ing the tumor in the petrous apex is resected and the petrous internalcarotid artery (ACI) and tumor are v isible (Tu) - Fig. 1c. The neuronav igation imageshows good corresponds to the intraoperative v iew (red - petrous internal carotidartery ; yellow - tumor) - Fig.1d. The bone structures are also well demonstrated dueto the fusion between MRI and bone-w indow CT data.

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retraction and the direct approach to the feeding vessels of the tumor. The technique,however, has several drawbacks that make manipulations and dissection close to criticalstructures more dangerous. Its actual indications, especially in the surgery of theanterior skull base, remain to be def ined.

The intracranial approaches to this part of the skull base are either extradural orintradural. For tumors w ith both intra- and extracranial extension, such asesthesioneuroblastomas, a combined approach may be necessary . The intracranialtumor part is removed v ia the frontolateral route which allows for safe dissection fromthe brain under direct observation and for reconstruction of the dural defect w ith a galea-periosteal flap or fascia temporalis. The extracranial extension is removed v ia anendoscopic or endoscope-assisted endonasal approach at a second stage.

The traditional intradural approach to the anterior skull base and the suprasellar arearequired a bifrontal or unilateral subfrontal craniotomies because the w ide exposure was

considered essential. In some cases, however, it can cause severe neurological impairmentdue to frontal lobe retraction, especially if bilateral. The occlusion of the superiorsagittal sinus and of the bridging veins may have unpredictable sequelae. To avoid thiscomplication, low basal approaches w ith removal of the supraorbital rim have beenintroduced. Increasing experience has shown that the simple frontolateral andfrontotemporal (pterional) approaches prov ide suff icient access to tumors of theanterior cranial fossa, and the supra- and parasellar regions. Either of these approachesavoids the approach-related morbidity of the more extensive and complex approaches,which are time consuming, require inev itable opening of the frontal sinus, and endangerthe venous structures. The craniotomy is relatively small but allows to v isualize theimportant structures and prov ides enough room for safe tumor dissection. Withadequate positioning of the patient's head and early cerebrospinal fluid release, theneed for brain retraction is obv iated.

Principles and practice of skull base surgery 147

Fig . 2 Extended endonasal endoscopicapproach w ith a nav igational guidanceto a extensive skull base chordoma.

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This simple and safe procedure could be regarded as a modif ication of thefrontotemporal (pterional) approach, including only its frontal part, or as a minimally invasive version of the unilateral subfrontal approach (7). The main advantages of thefrontolateral approach are its simplicity and flex ibility . The sk in is incised behind the hairline, starting anterior to the tragus and reaching the midline at the anterior edge of thehairline. The scalp is elevated to the supraorbital rim and the frontozygomatic suture. Thesize of the craniotomy, which is cut just above the supraorbital margin, is usually 25 to35 mm in w idth and 20 to 25 mm in height (Fig.3). Laterally, the craniotomy extends tothe key point where a single burr hole is made. In the medial direction, it should reachthe supraorbital notch, avoiding in jury to the supraorbital nerve and artery . Thecraniotomy could be modified according to the size of the frontal sinus - its openingshould be avoided. In case of large sinus that is w idely entered, its mucosa should be

exenterated, and its integrity should be reconstructed at the end w ith a pericranialflap. The dura is incised in a semicircular manner w ith an inferior base and sutured

The frontolateral approach

148 Neuro-oncology

Fig . 3a frontolateral craniotomy .

Fig . 3c complete tumor removal w ith preservation of theoptic nerve (II) vaculature (ICA- internal carotid artery).

Fig .3b a meningioma (Tu) arising from the anterior clinoidprocess (ACP) is seen.

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forward. Excessive retraction of the frontal lobe can be avoided by initial draining of suff icient amount of CSF from the basal cisterns. Via this route, meningiomas originatingfrom the olfactory groove, tuberculum sellae, planum sphenoidale, diaphragm sellae, andanterior clinoid, as well as other tumors, such as pituitary macroadenomas andcraniopharyngiomas, can be successfully removed, and this has been our favoriteapproach in the past decade.

Tumors expanding toward the middle cranial fossa, such as medial sphenoid w ingmeningiomas, are better accessed v ia the classical frontotemporal route (8).

The retrosigmoid suboccipital approach is the most flex ible and safe approach to thecerebellopontine angle and allows for adequate access and removal of any les iondeveloping in that area (1). The technique of the approach has been presented in detail

in the Vestibular Schwannoma Chapter. Tumors of the petroclival area - typically meningiomas or trigeminal schwannomas

- remain diff icult to treat w ith acceptable mortality and morbidity rates. On the otherhand, the natural evolution of these tumors is characterized by progressive growthand neurological deterioration, and an attempt at surgical removal is therefore justified(9). The history of petroclival meningioma surgery to a large extent reflects the evolutionof skull base surgery . Initially complete v isualization of both the tumor and all ad jacentstructures was regarded as a prerequisite for mak ing surgery more feasible and safer.Multiple extensive skull base approaches have been described for their management: the

combined supra-infra-trans-tentorial approaches; the combined retrosigmoid-subtemporal-trans-tentorial approach w ith and w ithout ligation of the sigmoid sinus; theintradural or epidural subtemporal approach w ith resection of petrous apex ; the petrosalapproaches differing in the degree of removal of the pyramid: the presigmoid approach,the retrolabyrinthine petrosectomy, the translabyrinthine petrosectomy, and thecomplete petrosectomy which includes removal of the whole labyrinth and requirestransposition of the facial nerve (10-16). The advantages of lateral skull base approachesare: shorter distance to the tumor and surrounding neurovascular structures, improvedv isualization, and minimazed brain retraction (17,18). These extended approaches,

however, have an unacceptably high approach-related mortality, w ith a high rate of facialnerve palsy, hearing loss and CSF leaks. Another main disadvantage is the risk of in jury to the highly variable venous structures, e.g. the vein of Labbe´, or any of the ma jordraining veins, might have unpredictable and devastating consequences.

The retrosigmoid-suprameatal or Samii`s approach was developed as an effectivealternative w ith few approach-related complications (1,19,20). It prov ides access tothe Meckels cave, petroclival area and the middle fossa including the posterior cavernoussinus. Another advantage of the approach is the possibility of identify ing the abducentnerve early at its brain stem ex it zone, and follow ing it during tumor removal up to theDorello's canal.

Cerebellopontine angle and Petroclival area



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The approach that was introduced in 1982, makes use of the simple and saferetrosigmoid craniotomy, combined w ith a resection of the petrous apex and eventually incision of the tentorium. The positioning of the patient and the details of theretrosigmoid craniotomy are similar to those described for the cerebellopontine angletumors. The removal of petroclival tumors is based on the concept of the CPA levels orcorridors: between the tentorium and the trigeminal nerve; between the trigeminaland the 7-8th nerves complex ; between the 7-8th nerves and the caudal nerves; or- morerarely - between the caudal nerves and foramen magnum. Their removal should startinitially at the most expanded level, w ith the w idest distance between the respectivestructures. The tumor is debulked partially using the CUSA, and only after that is the nextlevel approached.

The petrous apex is then resected by intradural drilling of the suprameatal tubercle -

the bone located above and anterior to the IAM. The drilling permits to extend theretrosigmoid approach as far as 13.0 mm anterior when compared to the retrosigmoidapproach alone. Drilling permits extension of the retrosigmoid approach as far as 13.0mm anterior, compared w ith the retrosigmoid approach alone (21). Opening Meckel'scave allows for mobilization of the trigeminal nerve, which further increases the work ingspace. The incision of the tentorium prov ides access to tumor extension in the middlecranial fossa.

A staged surgery may be indicated if the tumor has a large supratentorial extension,which compresses or engulfs the optic nerve, the carotid artery, or the oculomotor

nerve. Initially, the tumor part in CPA, clivus, and/or Meckels cave is removed v ia theSamii`s approach, the brain stem is decompressed and the risk of severe neurologicaldeterioration is thus prevented; the supratentorial portion is removed at a second stagev ia frontotemporal route. If the v ision is deteriorating, the supratentorial part of theprocedure is performed f irst.

Frequently the tumor has already signif icantly displaced the brain stem and anadequate route even to the contralateral side or supratentorially is prov ided. Utilizing the“transtumoral routes” approach, the risks of additional morbidity of complex skullbase approaches could be avoided. In petroclival meningiomas, the access to the rostral

tumor part is prepared by the tumor itself and can be further enlarged by resection of thetentorium.

The selection of an approach to this area depends on several factors, such as tumor size,location and type, extent of osseous involvement, and neurological status of the patient(22). Nonetheless, of paramount importance is the location of the tumor in relation tothe dura. Posteriorly located tumors are approached v ia a medial bilateral suboccipitalcraniectomy and C1 laminectomy if necessary . All other intradural tumors can besuccessfully removed v ia the lateral suboccipital route, which combines a retrosigmoidcraniectomy and a C1 hemilaminectomy/laminectomy (23) - Fig.4. To achieve adequateaccess to the tumor C1 lamina should be resected suff iciently and the foramen magnum

Foramen magnum

Retrosigmoid- suprameatal approach (Samii`s approach)

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should be opened w ide.Complete dissection or transposition of the vertebral artery is dangerous and time-

consuming procedures of less practical significance. The far lateral approaches includeextensive posterior and lateral bone resection w ith vary ing degrees of drilling of theoccipital condyle (the transcondylar approaches) and are claimed to prov ide the bestavenues to the lower clivus, anterior foramen magnum, and premedullary area (24,25).The morbidity rate w ith these approaches, however, is high and is related to vascular

complications, in jury to cranial nerves, CSF leak, or instability . The lateral suboccipitalapproach w ith the patient in the semisitting position (described above) is a safer andsimpler alternative. The tumor itself creates the corridor sufficient for its removal, andextensive bone removal w ith the associated morbidity can usually be avoided. Only inexceptional cases, such as small meningiomas located purely in the midline, have wefound it necessary to resect the posterior third of the occipital condyle. The selection of the optimal approach to extradural tumors of the anterior foramen magnum depends ontheir mediolateral and craniocaudal growth. In clival lesions that do not extend laterally from the condyle or into the CPA, we favor the endonasal approach w ith endoscopic

assistance. It prov ides suff icient exposure to the whole clivus and down to the C2 level.Tumors that grow more laterally or penetrate the dura are accessed v ia a lateralsuboccipital approach, and the tumor is removed both extra- and intradurally . Foraggressive tumors w ith significant bone involvement, such as some chordomas, thepyramid or the condyle, which is already partially eroded, is w idely drilled off .

1. Al-Mefty O, Fox JL, Smith RR : Petrosal approach for petroclival meningiomas.Neurosurgery 22:510-517, 1988

2. Bertalanffy H, Seeger W: The dorsolateral, suboccipital, transcondylar approach tothe lower clivus and anterior portion of the cranio-cerv ical junction. Neurosurgery 29:815-821, 1991.

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Fig . 4a Fig . 4b

Fig . 4a Foramen magnum meningioma, exposed v ia a retrosigmoid craniectomy and a C1 laminectomy ; Fig.4b: v iew after complete tumor removal. (C- cerebellum, Tu- tumor, *- vertebral artery).

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3. Boulton MR, Cusimano MD: Foramen magnum meningiomas: Concepts, classif ications,and nuances. Neurosurg Focus 14:E10, 2003.

4. Bruneau M, George B. Foramen magnum meningiomas: detailed surgical approaches andtechnical aspects at Lariboisière Hospital and rev iew of the literature. Neurosurg Rev 31:19-32, 2008

5. Erkmen K, Pravdenkova S, Al-Mefty O: Surgical management of petroclivalmeningiomas: Factors determining the choice of approach. Neurosurg Focus 19:E7, 20056. Hakuba A, Nishimura S, Jang BJ: A combined retroauricular and preauricular

transpetrosal-transtentor ial approach to clivus meningiomas. Surg Neurol 30:108-116,1988

7. House WF, De la Cruz A, Hitselberger WE: Surgery of the skull base: transcochlearapproach to the petrous apex and clivus. Otolaryngology 86:770-779, 1978

8. Kassam AB, Gardner P, Snyderman C, Mintz A, Carrau R : Expanded endonasalapproach: Fully endoscopic, completely transnasal approach to the middle third of the clivus, petrous bone, middle cranial fossa, and infratemporal fossa. NeurosurgFocus 19:E6, 2005.

9. Kassam A, Snyderman CH, Mintz A, Gardner P, Carrau RL: Expanded endonasalapproach: The rostrocaudal ax is-Part I: Crista galli to the sella turcica. NeurosurgFocus 19:E3, 2005. Nakamura M, Roser F, Jacobs C, Vorkapic P, Samii M: Medialsphenoid w ing meningiomas: Clinical outcome and recurrence rate. Neurosurgery 58:626-639, 2006.

10. Kawase T, Shiobara R, Toya S: Anterior transpetrosal-transtentorial approach forsphenopetroclival meningiomas: surgical method and results in 10 patients. Neurosurgery 28:869-876, 1991

11. Kondziolka D, Nathoo N, Flick inger JC, et al: Long-term results after radiosurgery for benign intracranial tumors. Neurosurgery 53: 815-822, 2003

12. Lee JY, Kondziolka D, Flick inger JC, Lunsford LD. Radiosurgery for intracranialmeningiomas. Prog Neurol Surg 20:142-149, 200713. Malis LI: Suboccipital subtemporal approach to petroclival tumors, in Wilson CB (ed):

Neurosurgical Procedures: Personal Approaches to Classic Operations. Baltimore:Williams & Wilk ins, pp 41-51, 1992

14. Nakamura M, Roser F, Jacobs C, Vorkapic P, Samii M: Medial sphenoid w ingmeningiomas: Clinical outcome and recurrence rate. Neurosurgery 58:626-639, 2006.

15. Nakamura M, Struck M, Roser F, Vorkapic P, Samii M: Olfactory groove meningiomas:Clinical outcome and recurrence rates after tumor removal through the frontolateral andbifrontal approach. Neurosurgery 60:844-852, 2007.

16. Samii M, Ammirati M: The combined supra-infratentorial presigmoid sinus avenue to

the petro-clival region. Surgical technique and clinical applications. Acta Neurochir 95:6-12, 1988

17. Samii M, Klekamp J, Carvalho G: Surgical results for meningiomas of the craniocerv ical junction. Neurosurgery 39:1086-1095, 1996.

18. Samii M, Carvalho GA, Tatagiba M, et al: Surgical management of meningiomasoriginating in Meckel's cave. Neurosurgery 41:767-775, 1997

19. Samii M, Tatagiba M, Carvalho GA: Retrosigmoid intradural suprameatal approach toMeckel's cave and the middle fossa: surgical technique and outcome. J Neurosurg92:235-241, 2000

20. Samii M, Gerganov MV: Surgery of extraax ial tumors of the skull base. Neurosurgery 62

(Suppl 3):1153-1166, 200821. Seoane E, Rhoton AL Jr: Suprameatal extension of the retrosigmoid approach:microsurgical anatomy . Neurosurgery 44:553-560. 1999

22. Spetzler RF, Daspit CP, Pappas CT: The combined supra- and infratentorial approach

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for lesions of the petrous and clival regions: experience w ith 46 cases. J Neurosurg76:588-599, 1992

23. Stippler M, Gardner PA, Snyderman CH, Carrau RL, Prevedello DM, Kassam AB.Endoscopic endonasal approach for clival chordomas. Neurosurgery 64:268-277, 2009

24. Tedeschi H, Rhoton AL Jr: Lateral approaches to the petroclival region. Surg Neurol

41:180-216, 199425. Van Havenbergh T, Carvalho G, Tatagiba M, et al: Natural history of petroclivalmeningiomas. Neurosurgery 52:55-64, 2003



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Basic principles of contemporaryskull base surgery:

microsurgical and endoscopic assistedtechniques.Matteo de Notaris1,2 MD, PhD; Vita Stagno2,3 MD; Alberto Prats-Galino2 MD, PhD; Domenico Solari3 MD; Enric Ferrer1 MD, PhD;Joaquim Enseñat1 MD, PhD.1 Department o Neurosurgery, Hospital Clinic o Barcelona, Barcelona,Spain.

2 Laboratory o Surgical Neuroanatomy (LSNA), Faculty o Medicine,

Universitat de Barcelona, Barcelona, Spain.3 Department o Neurological Science, Division o Neurosurgery,Università degli Studi di Napoli “Federico II”, Naples, Italy.

Te skull base represents the basis o the cranium upon which the brain rests; itssurace is complex and irregular due to the presence o oramina, depressions andbone indentations that create several compartments. Neurovascular structures that arecrucial to the unction and integrity o the entire central nervous system are contained,and travel, therein.

It can be involved in a variety o lesions, neoplastic and/or inammatory, vascular,congenital and/or traumatic. Te complexity o this anatomical region makes skull basesurgery a challenging discipline or neurosurgeons. For such reason, the managemento such lesions may require special techniques or their successul treatment.

It has to be said that cranial base surgery has rapidly developed since the beginningo the past century when the surgery itsel was still inadequate and incomplete, thusattended by a high mortality and morbidity rate. Te first pioneers o this disciplinewere neurosurgeons, head and neck surgeons and otolaryngologists. From the firstdecade o the 20th century, Harvey Cushing and Charles Harrison Frazier were some o

the surgeons that with audacity attempted the resection o lesion originating rom thebase o the skull, considered “inaccessible” at that time (1, 2). We’re indebted to them, asthrough their efforts and mistakes they enabled the implementation and developmento contemporary cranial base surgery.

With the remarkable progress o computer hardware and sofware, modern visualization tools such 3D operative microscopes and high definition endoscopeshave becoming a powerul assistive technology or neurosurgery. echnical progresslead to the development o new devices and equipment such as advanced MRI and Cneuroimaging techniques, rameless neuronavigation systems, pre-operative planningsofware and intraoperative imaging tools have enhanced the saety o each interventionand improved the outcome.

Te new millennium was ushered in by a dramatic technological revolution and atthe same time we assisted to a progressive decline o the invasiveness o the surgical

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procedure, with the ambition to decrease the morbidity and mortality rate and offera reasonable and significant chance or cure patients. Concerning such concept, itmust be remarked that modern minimally invasive surgery does not only imply thatthe size o the craniotomy or craniectomy is reduced, it must be the perect balancebetween adequate maneuverability and simultaneously reduce the injury o normalbrain tissue.

In recent years, new concepts have been introduced in neurosurgery towardminimally invasive and more tailored approaches and away rom traditional cranialbase surgery dogmas. Te advent o keyhole surgery and endoscopic techniques hasdefinitely confirmed the establishment o minimally invasive neurosurgery “era”. As anatural consequence the concept o skull base surgery has evolved. Contemporary skullbase surgery tailors the site, the extent and the shape o the craniotomy to avoid largebone openings and to reduce the needed or brain retraction in a refined balance.

Modern techniques have been compared or years to balance the need to minimize

brain retraction and maximize surgical exposure. Te development o the contemporaryneurosurgical operative environment is driven principally by parallel evolution inscience and technology. Important tools including neuroendoscopic techniques as wellas neuronavigation and advanced intraoperative imaging, reect the effort to reducemorbidity and brain trauma, and to optimize the patient position, refine the size andtailor the shape o the craniotomy.

Trough new instruments as the endoscope, used as a sole visualizing tool or as acomplement to the classical microsurgical techniques, has allowed a superior close-up view o the vascular and neural structures and related pathology. Its application

to skull base surgery has demonstrated effective or the visualization and dissectiono blind corners between complex neurovascular structures. Indeed, during the lastdecades, endoscopic assisted techniques have been widely adopted with success orthe treatment o several different lesions o the skull base, previously considered to bealmost inaccessible. Modern skull base centers must be equipped with microscopicand endoscopic instrumentation and the planning o any skull base surgery shouldconsider the possibility to switch the microsurgical procedure to a pure or endoscopicassisted surgery.

In this chapter we describe the basic principles or the main microsurgical and

endoscopic assisted approaches to the skull base in order to provide an overview oits complex anatomy and to describe the technical steps o each specific approach.Indications and surgical strategies will be also discussed.

Preoperative studies and surgical planning

Te knowledge o surgical anatomy is imperative or complex neurosurgicalprocedures in regions with vital structures nearby, as in skull base surgery.

In the field o neurosurgery, progress in neuroimaging studies such as highresolution C scans, MRI imaging studies and digital subtraction angiography data,has certainly refined the visualization o anatomical structures within the brain andthe skull. On the other hand, the progress in computer technology and medical imageprocessing techniques has enabled stereoscopic display o anatomical structures rom

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computed imaging data. Indeed, three-dimensional (3D) imaging, which allows imagemanipulation and surgical simulation on screen, has become an indispensable part othe neurosurgical planning and training.

Our preoperative plan or skull base tumors provides 2 different procedures:1. A virtual preliminary exploration o patient’s anatomy using the 3D reconstruction

modules supported by the OsiriX sofware (Osirix®, Advanced open-Source PACSWorkstation DICOM viewer) in order to analyze the individual variability o theanatomy. Te Maximum Intensity Projection, the Volume rendering and the Suracerendering were the 3D reconstruction modules are used to explore each case (Fig.1A-B).

2. A dynamic 3D stereoscopic surgical planning using a medical visualization system(Dextroscope, Volume Interaction) that, through the processing o multimodalityimages, such as C, MRI and PE/SPEC, allows building up 3D interactiveprocedures with which to simulate surgical approaches. Such tool is composed o

a display platorm and an interaction console with two hand-held electromagneticsensors that allow manipulation o the 3D data. At the end o the process, thesegmented objects (representing skin, tumor mass, vascular system, ventricularcavities) are displayed by a combination o volume rendering and polygonal iso-suraces, ready to be manipulated (Fig. 1-C).

Both types o models have been previously utilized rom recently published workso our group concerning the anatomy o microscopic and endoscopic skull baseapproaches.

Fig. 1 (A): MRI 3D reconstruction modules supported by OsiriX sofware (Osirix®, Advanced open-Source PACS Workstation DICOM viewer);(B): C scan 3D reconstruction using the Volume rendering

Projection (C): 3D virtual reality simulation o the microsurgical clipping o an aneurysm using amedical visualization system (Dextroscope, Volume Interaction)

The frontotemporal approach

Te rontotemporal approach, popularized by Yasargil (3) or the treatment ointracranial aneurysms, is still one o the most versatile and widely used approachesin contemporary neurosurgery. It allows a wide exposure o the sellar, parasellar,suprasellar and retrosellar areas and it has also been employed or a large number oother applications, such as lesions o the sellar region, the cavernous sinus, and theanterior and middle cranial ossa and as a basic step or more complex approaches tothe skull base. Te widely recognized advantage o the rontotemporal craniotomy isthe enhanced exposure o deep neurovascular structures, which offer a shorter and

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wider view o the surgical target. Te immediate release o cerebrospinal uid romthe basal cisterns and the early visualization o the neurovascular structures result ina significant reduction in brain retraction and preservation o the normal vascularanatomy. Furthermore, it allows the early identification o the homolateral internalcarotid artery and the anterior circle o Willis, thus providing an early proximal vascularcontrol.

Technique

Te patient is placed in the supine position on the operating table with the legspropped up on pillows. Te head is rotated to the side contralateral to the approachby approximately 15-20° and fixed to the Mayfield-Keys pin headrest, the definitiveposition o the head will depend upon the type and location o the lesion to be treated.Te vertex o the head is then tilted down 10–15° such that the malar eminence is

almost the highest point o the operative field. Te skin incision is made 0.5–1 cmanterior to the tragus advancing in a curvilinear shape through the hairline–oreheadinterace until to reach approximately the midpupillary line (Fig.2A). When dissectingthe subcutaneous tissue the temporoparietal ascia or supercial temporal ascia comeinto view. At this point the integrity o the superficial temporal artery should berespected, as this vessel is essential to obtain a vascularized galeopericranial ap and orextracranial-intracranial bypass surgery. Te temporoparietal ascia can be divided intwo laminae with at lying in between. Just beneath, a dense, tough and uniorm tissueorms the temporal ascia, also called deep temporal ascia. Cranial to the superior

temporal line, it is in continuity with the pericranium and caudally, over temporalmuscle, it splits into two layers approximately 2–3 cm above the zygomatic arch calledsuperficial and deep laminae, the superficial temporal at pad is in between. Te rontalbranch o the acial nerve lies just above superficial lamina, which cover such at pad.In order to avoid inadvertent damage to this nerve the incision over the temporalisascia should be perormed close to the temporal line, parallel to the skin incision andin a semilunar shape, in such a way to include both superficial and deep laminae. Oncethe temporalis ascia is opened the temporal muscle dissection can be perormed inthree different ways: interascial, subascial, or submuscular. We preer the subascial

dissection as, in our experience, it represents the saest approach to preserve the rontalbranch o the acial nerve (Fig.2B).Te keyhole is perormed ollowing Dandy description and is placed just above

the rontosphenoidal suture, below the superior temporal line and posterior to therontozygomatic suture. Te craniotomy is directed toward the supraorbital notch toapproximately 2 cm above the orbital rim and continued downward in a curvilinearashion, over the parietal bone finishing over the sphenotemporal suture. Once thebone ap is detached, the bone removal can extend by drilling the lesser wing o thesphenoid, in order to optimize the basal trajectory to the skull base. Te dural incisionis made in the usual semicircular ashion. A slight retraction o the rontal lobe allowsexposure o the carotid, chiasmatic and lamina terminalis cisterns (Fig.2C). With theopening o the sylvian fissure, in a typical proximal to distal ashion, the entire circle oWillis and its related arteries, optic nerves and chiasm, the pituitary stalk, the anterior



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and posterior clinoid are exposed. Moreover, the opening o the Liliequist’s membranepermits to gain access to the interpeduncular cistern with the basilar artery and itsbranches come into view.

Endoscope assisted techniqueTe application o the endoscope assisted technique to classical transcranial

approaches was first described by Apuzzo et al. (4). A used side-viewing, angled-rodlenses was used to assess the adequacy o the placement o aneurysm clips and toassist in visualizing structures during pituitary surgery both or transsphenoidal andsubrontal approaches. Te endoscope lens can be guided through narrow corridorsand placed immediately adjacent to the region o interest, effectively bringing theoperator to the anatomy.

We believe that the use o an endoscope in conjunction with the operating microscope

can enhance the surgeon’s ability to visualize and control deep located neurovascularstructures.

In our experience, the endoscopic assisted technique has been ound extremelyuseul to complement the classic rontotemporal approach or the treatment o complexintracranial aneurysms. As a matter o act, the endoscope implements the visualizationo blind corners around the aneurysm and o perorating arteries, acilitating thesurgeon to accurately position the clip under microscopic view. Aferwards, theendoscope can be introduced again to check the final surgical result and i necessary,clip can be adjusted again under microscope. In selected cases the superior angle and

close-up view offered by the scope allows to realize a direct surgical clipping.Te endoscopic assisted microsurgical technique is a sae, minimally invasive and

effective alternatives to the traditional open cranial base approaches in selected supraand parasellar tumors as well as or complex intracranial aneurysms (Fig. 2D).

Fig. 2 (A): Te rontotemporal approach.Position o the patient on the operating tablewith the head fixed to the Mayfield-Keys pinheadrest; the skin incision is marked; (B):Anatomic cadaveric dissection showing the

first phase o the procedure, with the temporalisascia kept in place and exposure o thetemporal muscle; noted how the pericraniumhas been dissected rom the skin ap; (C):Intraoperative microscopic view showingthe optocarotid region (D):Intraoperativeendoscopic view demonstrating clipping oan anterior communicating aneurysm.

P:pericranium; F: temporalis ascia; M: temporalis muscle; FB: rontal bone; ON: optic nerve; ICA:internal carotid artery; O: optic tract; AcoA: Anterior communicating artery; L: lamina terminalis

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The retrosigmoid approach

Krause developed the retrosigmoid approach at the beginning o the 20th century,but only ew decades later, with the introduction o intracapsular tumor removal byCushing, the surgery o the cerebellopontine angle become saer and less invasive. Tis

approach was ideated mainly or the treatment o acoustic neurinoma and other space-occupying lesions as meningiomas, epidermoids and chordomas, but it can be usedor a wide spectrum o pathological entities as trigeminal nevralgia, hemiacial spasmand selected cases o vertebral artery and PICA aneurysms. Different variant o thisapproach have been proposed and are currently used worldwide. Te main advantageo the retrosigmoid approach is related to the possibility to widely and saely accesslesions within the cerebellopontine angle region, the petrous bone and the petroclivalarea.

More recently, the retrosigmoid-suprameatal approach popularized by Samii in 1982,

makes use o the simple and sae retrosigmoid craniotomy, combined with a resectiono the petrous apex and incision o the tentorium. Such approach has demonstrated theeffectiveness and saety to approach the cerebellopontine angle and allows or adequateaccess and removal o any lesion developing in that area.

Furthermore, the advent o modern intraoperative neurophysiological monitoringhas reduced the procedure-related complications rate, very requent in the beginningo the 20th century, leading to a marked improvement o post-operative results.

Finally, with the diffusion o endoscopic skull base surgery, the idea to use theendoscope or improve the visualization in the cerebellopontine angle area, either alone

or in combination with the microsurgical technique, has expanded, increasing thesaety o the procedure and urther reducing the post-operative complications rate.

Surgical technique

Te position o the patient in the operating table varies rom the supine to thesemisitting or the park-bench position, with each one offering its own benefits anddisadvantages. We use a lateral decubitus or park bench position or the majority opatients undergoing retrosigmoid surgery or posterior ossa tumors (Fig. 3A).

Te skin incision is usually realized medially to the mastoid process in a slight curvedC shape; the muscular layer is incised in a longitudinal ashion and retracted until to visualize the occipitomastoid area. Te exposure o the occipitomastoid, the squamosaland the lambdoid sutures is o utmost importance or exactly locates the sigmoid sinus.Te choice to perorm a craniotomy rather than a craniectomy strictly depends romthe confidence o the surgeon with regards o its own technique. In both cases thekeyhole is placed ineromedially to the asterion, a dissector is used to dissect the duracareully rom the overlying bone and to expose the edge o the sigmoid sinus. Usuallythe craniotomy is placed at the transition o the transverse sinus into the sigmoid sinus,but an extensive exposure o both sinuses should be avoided in order to prevent any

inadvertent laceration o such venous structures. Te dura mater is incised, accordingto Samii’s principle, in a curvilinear ashion medial to the sigmoid and inerior to thetransverse sinus. Once the cerebellomedullary cistern is opened and the cerebellar



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hemisphere is slight retracted, at this point, the neural and vascular structures o thecerebellopontine area come into view (Fig. 3B). Depending on lesion location and size,the intradural drilling o the suprameatal tubercle, the opening o the Meckel’s cave orthe incision o the tentorium can be perormed in order to increase the width o thesurgical field.

Fig. 3 (A): Te retrosigmoid approach.Position o the patient on the operatingtable with the head fixed to the Mayfield-Keys pin headrest; the skin incision ismarked; (B): Intraoperative microscopicintradural image. Te cranial nervescome into view afer the cerebellomedullary cistern has been opened andthe cerebellum gently retracted.

V: trigeminal nerve; VII-VIII: acial and vestibulocochlear nerves; IX: glossopharyngeal nerve.

Endoscope assisted technique

Te use o the endoscope at early or late stage o a tumor removal can minimize theapproach related morbidity, as it offers a wide view over the whole cerebellopontine areaand the lateral surace o the brainstem without significant brain manipulation. In caseo neurovascular decompression the endoscope allows disclosing blind corners behindcritical structures and helping the surgeon to ensure the appropriate placement o the

selected graf (eon o other materials) implant between the neurovascular conict.Usually, in order to minimize the risk o damaging major neurovascular structures,

the endoscopic exploration ollows natural corridors between the cranial nerves,avoiding prolonged close-up view to such structures. Furthermore, constant salineirrigation o the surgical field prevents thermal injury to the cranial nerves. Somesurgical corridors, as the one sited between the tentorium and trigeminal nerve andthe one between the V nerve and the VII-VIII complex are commonly encountered(Fig.4A-C). Finally, the lower cranial can be exposed moving the endoscope caudallyand the hypoglossal nerves can be reached by angling the endoscope ineromedially

in the direction o the vertebrobasilar junction, passing through the corridor between vagus superiorly and the accessory spinal nerves ineriorly.

Fig. 4 (A): Intraoperative endoscopic view during tumor removal o cerebello pontine epidermoidtumor (B): preoperative and (C) postoperative 2-weighted at suppressed MRI showing completeremoval o the lesion.

EP: epidermoid; :tentorium V:fh cranial nerve ; B:brainstem

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The far lateral approach

When compared with others skull base approaches the routes to the ventrolateralaspect o the brainstem and the cervicomedullary junction are considered pioneering,probably due to the complex anatomy and the high morbidity o lesions developing or

extending within this area.As a matter o act the craniovertebral junction approach has been described or the

first time ew decades ago and presented as a lateral variation o the classic unilateralsuboccipital approach. More recently, Bertalanffy et al reported their experience withextended bone drilling towards the occipital condyle as a trial to obtain increasedangled surgical exposure (5-7). With the advent o skull base techniques and withclear understanding o the anatomy o the oramen magnum region, lesion ventraland ventrolateral to the oremen magnum can be accessed in most cases via ar lateraltranscondylar approach. Te advantage o the approach is that the procedure enhance

exposure o the brainstem while markedly reducing the need or brain retraction and therisk o air embolisms using either a three-quarter prone or parch bench position(8)

Surgical technique

In the majority o cases, a modified park-bench position with a ull lateral decubitusand the head slightly rotaded cranially and exed. Te skin is incised in a slight curvilinearor in a hockey-stick ashion behind the ear, in the retroauricular area (Fig.5A). Teinsertion o the splenius capitis and sternocleidomastoid muscles is detached rom the

superior nuchal line and the mastoid. Te occipital artery is usually encountered at anearly stage o the procedure and should be adequately coagulated. Te exposure o thesupracondylar ossa and the posterior edge o the oramen magnum proceeds with thedissection along the deep suboccipital area until the exact location o C1 arch and itstransverse process are confirmed. Te transverse process o C1 is an essential landmarkor confirming the location o the vertebral artery that can be isolated (Fig.5B). Usuallymobilization o the artery is not strictly recommended except or selected cases asmalign tumor with extension in the midbrain and low cervicomedullary region. Tedefinitive extension o the bone exposure depends on the location and the craniocaudalgrowth pattern o the underlying lesion. Te craniotomy has variable size and can beextended rom the sigmoid sinus up to the first cervical segments. Partial or completedrilling o the occipital condyle can be helpul in augmenting the surgical exposure othe anterior portion o the oramen magnum, while hemilaminectomy o C1 and/or C2may become necessary to access cervicomedullary area (Fig.5C-D). Te dura openingis realized, depending on the extension o the craniotomy, in a C or Y ashion, takingcare to avoid excessive manipulation o the vertebral artery at its entrance point closeto the oramen magnum. Te ar lateral approach provides an adequate exposure and viewing angle to the ventral and ventrolateral brainstem with minimal retraction oimportant neurovascular structures.



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Fig. 5 (A): Far lateral approach. Position o thepatient on the operating table with the head fixedto the Mayfield-Keys pin headrest; the skin incisionis marked; (B): Intraoperative image showing theexposure o the vertebral artery and the dura mater

over the craniocervical junction;(C): Osirix® 3DVolume Rendering reconstruction showing theshape and the boundaries o the craniotomy; (D)Anatomic cadaveric image showing the extent othe craniotomy; noted the hemilaminectomy oC1.

VA: vertebral artery; dm: dura mater; Hc: hypoglossal canal; C1: atlas

Endoscope assisted technique

During the last years continuous advancements in endoscopic skull base surgeryhave ported great advantages to the surgery o craniovertebral junction. Te close-upand angled view offered by the scope has been valuable during the resection o bothextradural and intradural lesions, so that endoscopic assisted techniques are nowadaysconsidered undamental while approaching lesions within this area.

Using the natural surgical corridors between the lower cranial nerves and the first

cervical roots it is possible to gain a paramount view over the most anterior portiono the petroclival area and the cervicomedullary region, thus limiting the amount odrilling o the occipital condyle.

In selected cases an extended endoscopic endonasal approach can be complementaryto access lesion extended ventrally to the lower cranial nerves. In such cases, a combinedmicroscopic ar lateral approach/transclival endoscopic endonasal approach(9, 10) in asingle or in two stages can be used to achieve a complete exposure o the lower cerebellopontine angle region. (Fig.6A-B)

Fig. 6 Illustrative case showing a clivalchordoma

extending into the craniocervical junction (A):Preoperative and (B) Postoperative 1 post-contrast high resolution MRI, axial sequences,showing the combined microscopic ar lateralapproach/transclival endoscopic endonasalapproach used to gain access to the entire lesion.

Subfrontal approachSir.Victor Horsley perormed the first subrontal approach to operate a pituitary

adenoma in 1889(1). Since then many advancements have been made in the development

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o this approach(3, 11, 12).It offers a wide exposure over the anterior cranial base with a wide overview o the

cribriorm plate, sellar, suprasellar, and parasellar areas. Using such route it is possibleto gain an excellent midline orientation in order to expose the main neurovascularstructures: the cavernous sinus, optic and olactory nerves and internal carotidarteries.

Surgical technique

Patient is placed supine with head slightly elevated 20 degrees or rontal lobes toall away rom the anterior skull baseand kept in neutral position in Mayfield clamp.A bicoronal skin incision is realized starting 1 cm anterior to the tragus and extendingbehind the hairline, in a curvilinear ashion, to the opposite side. Te ap is then elevatedin a single skin-aponeurosis layer and retracts with temporary fishhooks (Fig.7A). As

the dissection is carried anteriorly, care is taken to preserve the supraorbital nerve. Awide pericranium ap can be easily obtained, at any time during the procedure, orsubsequent skull base repair. A birontal craniotomy upper the orbital rims is generallyperormed, with the burr hole placed at the Mc-Carty’s point bilaterally, just below thesuperior orbital ridge. One or two more burr holes are placed close to the midline, insuch a way to avoid superior sagittal sinus injury (Fig.7B).

Te rontal sinus, whenever opened, is repaired being covered with the pericraniumap, afer the mucosa is completely removed and the cavity is careully packed withmuscle. wo separate dura opening are realized on both sides, with the base o the

opening being at the orbital roo. On the midline, two ligatures are passed through thealx in such way to saely cut the sinus on its anterior margin along with the alx untilits inerior border. Finally, in order to get access to the anterior skull base region therontal lobes are gently retracted, thus permitting the visualization o the suprasellararea (Fig.7C). Te subrontal bilateral approach allows exposure o the whole surace othe anterior cranial base, offering the best view over the suprachiasmatic area. Te opticnerves, the chiasm, the internal carotid artery, its biurcation and the anterior Circle oWillis are under symmetric panoramic view.

Fig. 7 (A): Subrontal approach. Intraoperative image showing the skin-aponeurosis ap elevated andretracted with temporary fish hooks.(B): Intraoperative image showing the birontal craniotomy withthe burr hole placed at the Mc-Carty’s point bilaterally and one close to the midline to avoid superiorsagittal sinus injury

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