The Sellar Region
The pituitary gland and sella are located below the center of the brain in the center of the skull base ( Figure 10-1 ). Access to the sella is limited from above by the optic nerves and chiasm and the circle of Willis, from laterally by the cavernous sinuses and internal carotid arteries, and from behind by the brain stem and basilar artery. The vital structures protecting its superior, lateral, and posterior borders have led to the preferred surgical routes to tumors of the gland being from below through the nasal cavity and sphenoid sinus or from anteriorly between the frontal lobe and the floor of the anterior cranial fossa. This chapter focuses on the microsurgical anatomy important in performing the various subcranial and transcranial approaches to the sellar region. It is divided into two sections; one dealing with the relationships in the cranial base around and below the sella, and another dealing with the relationships in the suprasellar and third ventricular regions. Special emphasis is placed on the transnasal route to the sella because this route is the one most commonly selected for dealing with pituitary tumors.
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.
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.
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 ).
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.
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.
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.