Nasal cavity

The sella can be reached by several routes through the nasal cavity. The nasal cavity, wider below than above, is bounded above by the anterior cranial fossa, laterally by the orbit and the maxillary sinus, and below by the hard palate ( Figures 10-2 and 10-3 ). This cavity is divided sagittally by the nasal septum, which is formed anteriorly and superiorly by the perpendicular plate of the ethmoid, and inferiorly and posteriorly by the vomer with an anterior bony deficiency occupied by septal cartilage. The nasal cavity opens anteriorly onto the face through the anterior nasal aperture and posteriorly into the nasopharynx by way of the posterior nasal apertures. Each posterior nasal aperture, measuring about 25 mm vertically and 13 mm transversely, is bordered above by the anterior aspect of the sphenoid body, below by the posterior margin of the hard palate formed by the horizontal plate of the palatine bones, medially by the nasal septum formed by the vomer, and laterally by the medial pterygoid plate.

Stepwise dissection of the nasal pathway along which transsphenoidal surgery is directed. A, Sagittal section to the left of the midline and nasal septum. The nasal septum is formed anteriorly by the septal cartilage, above by the perpendicular plate of the ethmoid, and below and posteriorly by the vomer. The posterior inferior part of the septum is supplied by the branches of the sphenopalatine artery, a terminal branch of the maxillary artery. The upper part of the septum, below the cribriform plate, is supplied by the branches of the ethmoidal arteries, which arise from the ophthalmic artery. A septum divides the sphenoid sinus near the midline. The optic chiasm, optic and oculomotor nerves, third ventricle, and pituitary stalk are located above the pituitary gland. The gyrus rectus of the frontal lobe is located above the cribriform plate and olfactory tract. B, Midsagittal section of the sphenoid sinus and pituitary gland. Prominences overlie the optic canal, internal carotid artery, superior orbital fissure, and maxillary nerve in the wall of the sphenoid sinus. The opticocarotid recess extends laterally between the optic nerve, internal carotid artery, and the prominence medial to the superior orbital fissure, and extends into the optic strut, which separates the optic canal from the superior orbital fissure. The serpiginous prominence overlying the internal carotid artery is located anterior to and below the pituitary gland. C, The lateral wall of the nasal cavity is constituted below by the nasal surface of the maxilla and above by the nasal surface of the ethmoid sinuses. The inferior concha (turbinate) is an independent bone, which articulates with the nasal surface of the maxilla and the perpendicular plate of the palatal bone. The middle and superior concha are appendages of the ethmoid bone. The lacrimal duct opens below the anterior part of the inferior concha. The inferior, middle, and superior nasal meatus are located below their respective concha. The superior meatus is located between the middle and superior concha. The sphenoethmoidal recess, a narrow cleft located above the superior concha, separates the superior concha from the anterior surface of the sphenoid sinus and is the site of the ostium communicating the sphenoid sinus and nasal cavity. The eustachian tube opens into the nasopharynx below the sphenoid sinus. D, The concha has been removed. The maxillary and frontal sinuses drain into the middle meatus. The lacrimal duct opens below the inferior turbinate into the inferior meatus. The ethmoid bullas are rounded prominences overlying the middle ethmoidal air cells. The anterior ethmoidal air cells drain into the superior meatus. The posterior ethmoidal air cells and the sphenoid sinus drain into the sphenoethmoidal recess. E, The medial wall of the maxillary sinus has been removed to expose the sinus roof, which forms the orbital floor. The infraorbital sulcus and canal, which transmit the infraorbital nerve and are situated in the floor of the orbit, form a prominence in the roof of the maxillary sinus. The anterior and posterior ethmoidal arteries arise from the ophthalmic artery and pass through the anterior and posterior ethmoidal canals to reach the floor of the anterior fossa beside the cribriform plate where they again penetrated the bone to reach the walls of the nasal cavity. The Vidian canal is the site of passage of the Vidian nerve to the pterygopalatine ganglion. The Vidian nerve is formed by parasympathetic fibers from the greater petrosal nerve and sympathetic fibers from the deep petrosal branch of the carotid plexus. The frontal sinus is situated at the upper anterior part of the medial wall. F, Removing the lateral wall of ethmoid air cells, which forms the medial wall of the orbit, exposes the periorbita. The medial wall of the nasolacrimal duct has been removed to expose the interior of the duct.

A–D, Comparison of osseous and mucosal structures in the nasal septum and conchae. E and F, Osseous relationships along the transsphenoidal and endonasal approaches to the sella. A, The structures anterior to the left orbital apex and the portion of the maxilla above the alveolar process have been removed to expose the nasal septum, which is formed posteriorly by the vomer, above by the perpendicular plate of the ethmoid, and anteriorly by the septal cartilage. B, The nasal septum and anterior wall of the sphenoid sinus have been removed. This exposes the superior, middle, and inferior conchae and a midline septum within the sphenoid sinus. The ethmoid air cells are exposed in the medial wall of the right orbit. The part of the sphenoid sinus medial to and below the orbital apex has been opened. C, The left half of the facial skeleton, including the left half of the maxilla and orbit, has been removed to expose the left side of the nasal septum, which is formed above by the perpendicular plate of the ethmoid bone and below by the vomer. The palate is formed anteriorly by the maxilla and posteriorly by the horizontal plate of the palatine bone. D, The nasal septum has been removed. The inferior concha is a separate bone, which protrudes into the nasal cavity from the maxilla. The middle and superior concha are appendages of the ethmoid bone. The maxillary ostium is located between the perpendicular plate of the palatine bone behind the ethmoid superiorly and the medial maxillary wall below. The maxillary and frontal sinus and the anterior ethmoid air cells drain into the middle meatus and the posterior ethmoid air cells drain into the superior meatus. E, The anterior nasal aperture is formed above by the nasal bones, and laterally and below by the maxilla. The anterior part of the osseous nasal septum is formed above by the perpendicular ethmoid plate and below by the vomer. The inferior concha, a separate bone, and the middle concha, an appendage of the ethmoid bone, are visible through the aperture. F, Posterior view of the posterior nasal aperture. The floor of the posterior aperture is formed by the horizontal plate of the palatine bone. The lateral margin is formed by the medial plate of the pterygoid process and is joined anteriorly by the perpendicular plate of the palatine bone, which forms the part of the lateral nasal wall between the maxilla and the medial pterygoid plate. Posteriorly, the middle concha is much more prominent than the inferior concha and often must be displaced laterally in the transsphenoidal approach to the sphenoid sinus and sella. The vomer extends from the upper surface of the hard palate to the body of the sphenoid bone and separates the paired nasal cavities at the posterior aperture.

The lateral nasal wall usually has three medially directed projections: the superior, middle, and inferior nasal conchae, below each of which is a corresponding superior, middle, or inferior nasal meatus (see Figures 10-2 and 10-3 ). The paired sphenoethmoidal recesses, located above and behind the superior nasal conchae and in front of the upper anterior aspect of the sphenoid body, are the site of the paired sphenoid ostia, which communicate between the nasal cavity and the sphenoid sinus. The upper half of the lateral nasal wall, corresponding to the medial orbital wall, is composed, from anterior to posterior, of the frontal process of the maxilla, the lacrimal bone, and the orbital plate of the ethmoid bone. The extremely thin lacrimal and ethmoid bones, occupied by the ethmoidal air cells, separate the nasal cavity from the orbit. The nasolacrimal groove and canal, the site of the lacrimal sac and nasolacrimal duct, respectively, pass downward in front of the anterior end of the middle nasal concha and open into the inferior nasal meatus. The frontoethmoidal suture, located at the junction of the roof and medial orbital wall, is situated at the level of the roof of the nasal cavity and the cribriform plate. The anterior and posterior ethmoidal foramina, which transmit the anterior and posterior ethmoidal arteries and nerves, are located in or just above the frontoethmoidal suture. These arteries and nerves exit the ethmoidal foramina and enter the anterior cranial fossa at the lateral edge of the cribriform plate. The anterior ethmoidal artery, a terminal branch of the ophthalmic artery, supplies the mucosa of the anterior and middle ethmoid sinuses and the dura covering the cribriform plate and the planum sphenoidale. It gives rise to the anterior falcine artery intracranially. The posterior ethmoidal artery, usually smaller than the anterior ethmoidal artery and absent in up to 30% of the ophthalmic arteries, feeds the mucosa of the posterior ethmoid sinus and the dura of the planum sphenoidale. The average distance between the anterior lacrimal crest of the maxilla’s frontal process and the anterior ethmoidal foramen is 22 to 24 mm; between the anterior and posterior ethmoidal foramina, 12 to 15 mm; and between the posterior ethmoidal foramen and the optic canal, 3 to 7 mm. In midline transfacial procedures, these arteries may be divided between the periorbita and the medial orbital wall. Care should be taken to prevent damaging the optic nerve, which is sometimes located immediately behind the posterior ethmoidal foramen.

The lower part of the lateral nasal wall is formed anteriorly to posteriorly by the maxilla, the perpendicular plate of the palatine bone, and the medial pterygoid plate. The eustachian tube opens into the nasopharynx along the posterior edge of the medial pterygoid plate. The root of the middle nasal concha attaches to the lateral nasal wall near the junction of the orbit and the maxillary sinus. Thus the medial wall of the maxillary sinus is bounded medially by the middle and inferior nasal meati and the inferior nasal concha (see Figures 10-2 and 10-3 ). The maxillary sinus communicates with the middle nasal meatus through an opening located in the medial wall just below the roof of the sinus.

The pterygopalatine fossa is situated just outside the lateral wall of the nasal cavity between the posterior wall of the maxillary sinus anteriorly and the pterygoid process posteriorly ( Figures 10-2 through 10-4 ). The pterygopalatine fossa contains the pterygopalatine ganglion, which receives the vidian nerve (nerve of the pterygoid canal), the segment of the maxillary nerve and its branches located just anterior to the foramen rotundum, and the internal maxillary artery and its terminal branches. This fossa communicates laterally with the infratemporal fossa through the pterygomaxillary fissure and medially with the nasal cavity via the sphenopalatine foramen, which transmits the corresponding nerve and vessels. The internal maxillary artery exits the infratemporal fossa to enter the pterygopalatine fossa by passing through the pterygomaxillary fissure. The greater and lesser palatine arteries and nerves arise from the maxillary artery and nerve and descend in the greater and lesser palatine canals, which are separated medially from the nasal cavity by the thin perpendicular plate of the palatine bone.

Transnasal route to the sphenoid sinus and sella. A, The cross section extends across the nasal cavity, superior and middle turbinates, maxillary sinuses, the orbits near the apex, and the ethmoid sinuses in front of the sphenoid sinus. The zygomatic and infraorbital nerves arise from the maxillary nerve in the pterygopalatine fossa, which is located behind the posterior maxillary wall. The nasal septum is formed above by the perpendicular ethmoid plate, below by the vomer, and anteriorly by the cartilaginous septum. B, The concha and posterior ethmoid air cells have been removed to expose the vomer and the anterior wall of the sphenoid sinus and the sphenoid ostia. The nasolacrimal duct descends along the lateral wall of the nasal cavity. C, Enlarged view. The perpendicular ethmoid plate joins the anterior sphenoid face and the vomer extends upward to join the inferior sphenoid wall, both in the midline. The posterior ethmoid air cells are located anterior to the lateral part of the sphenoid face and overlap the superolateral margins of the sphenoid ostia. D, The anterior face of the sphenoid has been removed to expose the multiseptated sphenoid sinus and the anterior wall of the sella. The bony prominences over the optic canals are situated in the superolateral margins of the sinus. E, The anterior wall of the sella and the lateral wall of the sphenoid sinus have been removed to expose the petrous and cavernous segments of the carotid artery and the pituitary gland. The posterior wall of the maxillary sinus has been removed to expose the maxillary nerve and artery and the pterygopalatine ganglion in the pterygopalatine fossa. The branches of the maxillary artery penetrate the lateral wall of the nasal cavity to course along the sphenoid face. The maxillary nerve sends communicating rami to the sphenopalatine ganglion. The vidian nerve enters the posterior aspect of the sphenopalatine ganglion. The pituitary gland is surrounded by the cavernous sinuses laterally and an anterior intercavernous sinus above. F, The optic nerves have been elevated to show the suprasellar area and the relationships between the orbital apex, optic canals, nasal cavity, pterygopalatine fossa, and the petrous and intracavernous segments of the internal carotid artery. The superior hypophyseal arteries pass to the lower margin of the optic chiasm and the pituitary stalk.

Sphenoid bone

The sphenoid bone is located in the center of the cranial base ( Figures 10-3 and 10-5 ). The intimate contact of the body of the sphenoid bone with the nasal cavity below and the pituitary gland above has led to the transsphenoidal route being the operative approach of choice for most sellar tumors. The neural relationships of the sphenoid bone are among the most complex of any bone: the olfactory tracts, gyrus rectus, and posterior part of the frontal lobe rest against the smooth upper surface of the lesser wing; the temporal lobe rests against the inner surface of the greater wing; the pons and mesencephalon lie posterior to the clival portion; the optic chiasm lies posterior to the chiasmatic sulcus; and the second through sixth cranial nerves are intimately related to the sphenoid bone and all exit the skull through the optic canal, superior orbital fissure, foramen rotundum, or foramen ovale, all foramina located in the sphenoid bone ( Figure 10-6 ).

Types of sphenoid bone. Anterior views. A, Conchal type sphenoid bone. B, Bone with presellar type of sphenoid sinus. C, Bone with sellar type sphenoid sinus and well-defined sphenoid ostia. D, Bone with sellar type of sphenoid sinus with poorly defined sphenoid ostia and obliquely oriented sphenoidal septa

(From Rhoton AL Jr, Hardy DG, Chambers SM: Microsurgical anatomy and dissection of the sphenoid bone, cavernous sinus and sellar region. Surg Neurol 1979; 12:63-104).

Superior view of sellar region. A, The sella is located between the cavernous sinuses. The diaphragm, which usually separates the sella from the suprasellar cisterns, is absent in this case. The oculomotor nerves enter the roof of the cavernous sinus where there is a narrow cistern around the nerve. The oculomotor triangle, the triangular patch of dura through which the oculomotor nerve enters the dura in the cavernous sinus roof, is positioned between the anterior and posterior clinoid processes and the petrous apex. The roof of the cavernous sinus extends forward under the anterior clinoid process. B, The dura covering the anterior clinoid process and optic canal has been removed. The outer layer of dura in the lateral wall of the cavernous sinus has been removed to expose the thin inner layer of the lateral sinus wall and the lateral surface of Meckel cave. The falciform ligament, the dural fold extending above the optic nerve proximal to the nerve’s entrance into the optic canal, extends from the anterior clinoid to the tuberculum. C, The inner layer of the lateral wall of the cavernous sinus has been removed to expose the nerves coursing in the wall of the cavernous sinus and middle fossa. The dura covering the dorsum sellae, basilar sinus, and posterior clinoid process has been removed. The oculomotor nerve passes forward lateral to the posterior clinoid and below the anterior clinoid. An abnormal bony projection extends laterally from the right posterior clinoid below the oculomotor nerve toward the petrous apex. The basilar sinus crosses the back of the dorsum and upper clivus and communicates widely with the posterior edge of the paired cavernous sinuses. The abducens nerve passes through the lower margin of the basilar sinus. An anterior intercavernous passes along the anterior margin of the sella. D, The anterior clinoid process has been removed to expose the clinoid segment of the internal carotid artery defined by the upper and lower dural rings. The upper ring is formed by the dura extending medially from the upper surface of the anterior clinoid. The lower dural ring is formed by the dura, which extends medially from the lower margin of the anterior clinoid and separates the lower clinoid margin from the oculomotor nerve. E, Posterior superior view of the sella. The dorsum and posterior clinoid have been removed to expose the posterior lobe of the pituitary, which was hidden below the dorsum. The abducens nerve passes through the Dorello canal, which is roofed by the petrosphenoid ligament. The trigeminal nerve has been reflected forward to expose the petrolingual ligament, which extends above the internal carotid artery just proximal to the artery’s entry into the cavernous sinus. F, Enlarged view. The carotid artery protrudes medially to deform the lateral surface of the anterior lobe of the pituitary gland. A tongue of anterior lobe extends laterally above the intercavernous carotid. The inferior hypophyseal branch of the meningohypophyseal artery passes medially to reach the posterior lobe.

The sphenoid bone has many important arterial and venous relationships: the carotid arteries groove each side of the sphenoid bone and often form a serpiginous prominence in the lateral wall of the sphenoid sinus; the basilar artery rests against its posterior surface; the circle of Willis is located above its central portion; and the middle cerebral artery courses parallel to the sphenoid ridge of the lesser wing. The cavernous sinuses rest against the sphenoid bone and intercavernous venous connections line the walls of the pituitary fossa and dorsum sellae.

In the anterior view the sphenoid bone resembles a bat with wings outstretched (see Figure 10-5 ). It has a central portion called the body; two lesser wings, which spread outward from the superolateral part of the body; two greater wings, which spread upward from the lower part of the body; and two pterygoid processes with their medial and lateral pterygoid plates directed downward from the body. The body of the sphenoid bone is more or less cubical and contains the sphenoid sinus. The superior orbital fissure, through which the oculomotor, trochlear, abducens, and ophthalmic nerves pass, is formed on its inferior and lateral margins by the greater wing and on its superior margin by the lesser wing. The inferior surface of the lesser wing forms the posterior part of the roof of each orbit and the exposed surface of the greater wing forms a large part of the lateral wall of the orbit, the floor of the middle fossa, and the roof of the infratemporal fossa. The optic canals are situated above and are separated from the superomedial margin of the superior orbital fissure by the optic strut, a bridge of bone that extends from the lower margin of the base of the anterior clinoid process to the body of the sphenoid. The narrowest part of the optic canal is closer to the orbital than the intracranial end. The optic canals average 5 mm in length, and are of a conical configuration tapering to a narrow waist near the orbit end. The sphenoid ostia open from the nasal cavity into the sinus. The infratemporal crest divides the inferior from the lateral parts of the greater wing and separates the temporal and infratemporal fossa. The lateral pterygoid muscles arise between the infratemporal crest and the lateral pterygoid plate. The area lateral to the infratemporal line gives origin to the temporalis muscle. The pterygoid (vidian) canal courses from anterior to posterior through the junction of the pterygoid process and the sphenoid body.

In the superior view, the pituitary fossa occupies the central part of the body and is bounded anteriorly by the tuberculum sellae and posteriorly by the dorsum sellae (see Figures 10-1 and 10-6 ). The chiasmatic groove (sulcus), a shallow depression between the optic foramina, is bounded posteriorly by the tuberculum sellae and anteriorly by the planum sphenoidale. The frontal lobes and the olfactory tracts rest against the smooth upper surface of the lesser wing and the planum sphenoidale. The posterior margin of the lesser wing forms a free edge, the sphenoid ridge, which projects into the sylvian fissure to separate the frontal and temporal lobes. The anterior clinoid processes are located at the medial end of the lesser wings, the middle clinoid processes are lateral to the tuberculum sellae, and the posterior clinoid processes are situated at the superolateral margin of the dorsum sellae. The dorsum sellae is continuous with the clivus. The upper part of the clivus is formed by the sphenoid bone and the lower part by the occipital bone. The carotid sulcus extends along the lateral surface of the body of the sphenoid.

The depth of the sella turcica is the greatest distance between the floor and a perpendicular line connecting the tuberculum and dorsum. Sellar length, defined as the greatest anteroposterior diameter of the pituitary fossa, may occur at the level of the tuberculum sellae or below. Sellar width is defined as the width of the horizontal plateau of the sellar floor between the carotid sulci. The volume is calculated by applying the simplified mathematical formula for the volume of an ellipsoid, namely, volume (cm ³ ) = 0.5 (length × width × depth in mm)/1000. The upper limit of normal depth is 13 mm; length, 17 mm; width, 15 mm; and volume, 1100 mm.

The superior aspect of each greater wing is concave upward and is filled by the pole of each temporal lobe. The foramina rotundum, ovale, and spinous, from anterior to posterior, are located near the junction of the body and greater wing. When viewed from inferiorly, the vomer, a separate bone, frequently remains attached to the anterior half of the body of the sphenoid, and its most anterior portion separates the sphenoid ostia.

The pterion and the “keyhole” are two important anatomical landmarks in the region of the greater wing in the lateral view (see Figure 10-5 ). The pterion is located over the upper part of the greater wing and approximates the site of the lateral end of the sphenoid ridge. The keyhole is located behind the junction of the temporal line and the zygomatic process of the frontal bone several centimeters anterior to the pterion. A burr hole placed over the pterion will be located at the lateral end of the sphenoid ridge. A burr hole placed at the keyhole will expose the orbit in its lower part and dura over the frontal lobe in its upper part.

Sphenoid sinus

The sphenoid sinus separates the cavernous sinuses, the cavernous segments of the carotid arteries, and the optic, extraocular, and trigeminal nerves. In addition, it separates the pituitary gland from the nasal cavity. The sphenoid sinus is subject to considerable variation in size and shape and to variation in the degree of pneumatization (see Figures 10-5 , 10-7 , and 10-8 ). It is present as minute cavities at birth, but its main development takes place after puberty. In early life, it extends backward into the presellar area and subsequently expands into the area below and behind the sella turcica, reaching its full size during adolescence. As the sinus enlarges, it may partially encircle the optic canals. When the sinus is exceptionally large, it extends into the roots of the pterygoid processes or greater wing of the sphenoid bone and may even extend into the basilar part of the occipital bone. As age advances, the sinus frequently undergoes further enlargement associated with absorption of its bony walls. Occasionally there are gaps in its bone with the mucous membrane lying directly against the dura mater.

Inferior view of the sellar region and surrounding skull base. A, The right half of the floor of the sphenoid sinus has been removed to expose the sellar floor and the part of the sphenoid sinus below the planum and tuberculum. On the specimen’s left side, the eustachian tube, pterygoid process, and posterior part of the maxillary sinus have been preserved. On the right side, the medial portion of the eustachian tube and the pterygoid process have been removed. This exposes the right mandibular nerve exiting the foramen ovale and the maxillary nerve exiting the foramen rotundum and passing forward as the infraorbital nerve. The pterygopalatine ganglion is located in the pterygopalatine fossa behind the maxillary sinus in the lateral wall of the nasal cavity. The right pterygoid process has been removed to expose the vidian canal, in which the vidian nerve travels to reach the pterygopalatine ganglion. The bone below the petrous carotid has been removed up to the point where the artery turns upward to enter the posterior part of the cavernous sinus. B, Part of the vomer, perpendicular ethmoid plate, and floor of the sphenoid sinus have been removed to expose the cavernous sinus, intracavernous carotid, and the pituitary gland. The floor of the optic canals has been removed to expose the ophthalmic arteries coursing below the optic nerves. The cavernous sinus surrounds the intracavernous carotid. An anterior intercavernous sinus crosses the anterior margin of the gland. Some of the upper clivus has been removed to expose the basilar sinus, which sits on the back of the dorsum and is the largest connection between the cavernous sinuses. C, The venous spaces around the pituitary gland have been cleared to expose the petrous and intracavernous carotid segments. D, Enlarged view of the pituitary gland, intracavernous carotid, and the optic nerves and ophthalmic arteries. The inferior hypophyseal arteries pass to the posterior lobe. The superior hypophyseal arteries arise in the chiasmatic cistern and pass medially to reach the stalk and chiasm.

Nasal pathway to the sphenoid sinus. Stepwise dissections showing the structures that form the lateral limit of the transnasal route to the sphenoid sinus and sella. A, Sagittal section to the right of the midline. The nasal septum, along which the transsphenoidal approach is directed, is formed above by the perpendicular plate of the ethmoid, anteriorly by the nasal septal cartilage, and below by the vomer. The vomer articulates with the anterior-inferior part of the sphenoid body, and the perpendicular plate articulates with the anterior face. The sphenoid sinus is located in the body of the sphenoid bone. B, The sagittal section has been extended to the right of the midline. The nasal concha and meati and the eustachian tubes are in the lateral margin of the exposure. C, A portion of the middle and inferior turbinates has been removed. The ostia of the maxillary and frontal sinuses opens into the middle meatus located below the middle turbinate. The nasolacrimal duct opens below the lower turbinate into the inferior meatus. The Rosenmuller fossa is located behind the eustachian tube. D, The mucosa in the lateral margin of the nasal cavity and the posterior part of the inferior and middle turbinates have been removed to expose the pterygoid process and the posterior maxillary wall, which form the posterior and anterior boundaries of the pterygopalatine fossa, respectively. The eustachian tube opens into the nasopharynx at the posterior edge of the pterygoid process. The terminal branches of the maxillary artery pass through the pterygopalatine fossa, located between the posterior maxillary wall and the pterygoid process, to enter the posterior superior part of the nasal cavity at the anterior inferior margin of the sphenoid sinus. The medial wall of the pterygopalatine fossa is formed by the perpendicular plate of the palatine bone. E, The medial wall of the maxillary sinus has been opened to expose the infraorbital nerve, which arises in the pterygopalatine fossa and passes forward in the sinus roof. The maxillary nerve passes through the foramen rotundum to enter the pterygopalatine fossa where it gives rise to the infraorbital, zygomatic, and greater palatine nerves, plus communicating rami to the pterygopalatine ganglion. F, Enlarged view. The bone and dura covering the optic canal in the superolateral part of the sphenoid sinus has been opened to expose the optic nerve and ophthalmic artery in the optic canal. The junction of the petrous and cavernous carotid limits the exposure below the level of the sella. Terminal branches of the maxillary artery intermingle with the neural structures in the pterygopalatine fossa, and exit the fossa to supply the tissues on the sphenoid face.

There are three types of sphenoid sinus in the adult: conchal, presellar, and sellar types, depending on the extent to which the sphenoid bone is pneumatized (see Figure 10-5 ). In the conchal type, the area below the sella is a solid block of bone without an air cavity. In the presellar type of sphenoid sinus, the air cavity does not penetrate beyond a vertical plane parallel to the anterior sellar wall. The sellar type of sphenoid sinus is the most common, and here the air cavity extends into the body of sphenoid below the sella and as far posteriorly as the clivus. In our previous study in adult cadavers, this sinus was of a presellar type in 24% and of the sellar type in 76%. The conchatype is most common in children before the age of 12 years, at which time pneumatization begins within the sphenoid sinus. In the conchal type, which is infrequent in the adult, the thickness of bone separating the sella from the sphenoid sinus is at least 10 mm.

The depth of the sphenoid sinus is defined as the distance from the ostium of the sphenoid sinus to the closest part of the sella (see Figure 10-8 ). In the adult, the average anteroposterior diameter of the cavity has been found to be 17 mm (range 12 to 23 mm). This measurement defines the length of the path within the sinus through which instruments must pass to reach the sellar wall, and is important when selecting instruments for transsphenoidal surgery. The speculum most commonly used for transsphenoidal surgery is 9 cm in length and its tip should be placed anterior to the sphenoid sinus. In reaching the floor of the sella turcica, the depth of the sphenoid sinus (2 cm or more) is added to the 9 cm length of the speculum. Thus after traversing a distance of 11 to 12 cm, the dissecting instruments must then enter the sella turcica and be able to reach above the sella if a suprasellar tumor is present. The distance may be greater in the presence of acromegaly; therefore, it is important that transsphenoidal instruments have shafts at least 12 cm in length. Some transsphenoidal instruments have shafts 9.5 cm in length, barely long enough to reach through the speculum into the sphenoid sinus. The fact that important neural and vascular structures are exposed either in the lateral sinus wall, directly lateral to the sella, or above the diaphragma sellae, especially if the latter is defective, has led the authors to prefer blunt rather than sharp ring curettes for dissection in these areas.

Another measurement important in transsphenoidal surgery is the thickness of the anterior sellar wall and sellar floor. In the sellar type of sinus, the thickness of the anterior sellar wall ranged from 0.1 to 0.7 mm (mean 0.4 mm) as compared with 0.3 to 1.5 mm (mean 0.7 mm) for the presellar type. The thickness of bone covering the sinus was defined at the planum sphenoidale, tuberculum sellae, anterior sellar wall, sellar floor, and the clivus. The thickest bone was found at the clivus and tuberculum sellae and the thinnest along the anterior sellar wall.

The septa within the sphenoid sinus vary greatly in size, shape, thickness, location, completeness, and relation to the sellar floor (see Figure 10-9 ). The cavities within the sinus are seldom symmetrical from side to side and are often subdivided by irregular minor septa. The septa are often located off the midline as they cross the floor of the sella. In our previous study, a single major septum separated the sinus into two large cavities in only 68% of specimens, and even in these cases the septa were often located off the midline or were deflected to one side. The most common type of sphenoid sinus has multiple small cavities in the large paired sinuses. The smaller cavities are separated by septa oriented in all directions. CT or MRI of the sella provide the definition of the relationship of the septa to the floor of the sella needed for transsphenoidal surgery. Major septa may be found as far as 8 mm off the midline. The septa are not to be used as a guide to the midline but may be used as landmarks based on where the preoperative CT and MRI show them to be located in relation to the sella and the tumor.

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