bone.
Posterior to the ethmoidal roof, the midline ASB is formed by the planum sphenoidale, which corresponds to the area limited by the cranial insertion of the anterior walls of the sphenoid sinuses anteriorly, the tuberculum sellae posteriorly, and the optic canals laterally forming the cranial portion of the body of the sphenoid bone. The planum sphenoidale is a flat bony lamina that separates the sphenoid sinuses from the intracranial space. At the junction between the planum sphenoidale and the anterior sellar wall, the bone thickens forming the tuberculum sellae, which lies anteroinferiorly to the optic chiasm. Being the anterior insertion of the diaphragma sellae (i.e., the dural roof of the sellar region), the tuberculum sellae can be used as a landmark between the sella, below, and the suprasellar region, above. Within the suprasellar region, properly called chiasmatic cistern, the optic nerves merge together into the optic chiasm and exit as optic tracts running medially to the intracranial tract of the internal carotid artery. The pituitary stalk lies posterior to the optic chiasm and joins the pituitary gland piercing the diaphragma sellae. In this area, the superior hypophyseal arteries arise from the internal carotid arteries and supply the optic apparatus, dura, and pituitary stalk (Fig. 10).
Fig. 10. Transcranial endoscopic view of the ASB via a supraorbital subfrontal approach (the trajectory is shown in Fig. 5). The point of view is neurosurgical (upside down). a The endoscope is placed above and anterior to the anterior clinoid process (ACP) and below the posterior (POG) and medial (MOG) orbital gyri. The optic nerve (ON), medially, and fronto-orbital vein (FOV), middle cerebral artery (MCA), and lateral orbitofrontal artery (LOFA), laterally, are visible. b, c The endoscope is placed pointing at the planum sphenoidale (PS). The internal carotid artery (ICA), ophthalmic artery (OpA), and the optic nerves can be identified. More anteriorly, the olfactory tract (OlT) is identified while entering the outer arachnoid membrane parallel to the medial orbitofrontal artery (MOFA). d Turning the scope upwards towards the suprasellar region, the optic chiasm (OC), pituitary stalk (PS), and diaphragma sellae (DS) can be identified. All these structures are vascularized by the superior hypophyseal arteries (SHA).
At the lateral borders of the planum sphenoidale are the optic canals, which “house” the optic nerves and ophthalmic arteries. Of note, the ophthalmic artery usually runs inferior in the optic canal, and the safest area to open the optic sheath endoscopically is via the superomedial quadrant. The trajectory of the optic canals is from posteromedial to anterolateral and their orbital aperture lies about 7 mm posterior to the posterior ethmoidal foramen [11]. On its lateral side, the optic canal is separated from the superior orbital fissure by a bony structure, the optic strut, which serves as one of the three roots of the anterior clinoid process. The other roots derive from the posterolateral portion of the planum sphenoidale and from the posteromedial portion of the lesser wing of the sphenoid (Fig. 11). From an endonasal perspective, the optic strut corresponds to the lateral optic-carotid recess, while the medial optic-carotid recess serves as a landmark for the lateral edge of the tuberculum sellae. Within the optic canal and orbital cavity, the optic nerve is completely surrounded by dura, subarachnoid space, and arachnoid up to a few millimeters before joining the eyeball (globe).
Fig. 11. Transcranial microscopic view of the anterior clinoid process through a pterional approach (the trajectory is shown in Fig. 5). The point of view is neurosurgical (upside down). a The meningo-orbital fold (MOF) is identified between the lesser (LWSB) and greater (GWSB) wings of the sphenoid bone. b The fold is cut, paying attention not to damage the neurovascular structures that are located in the medial portion of the superior orbital fissure. c, d The anterior clinoid process (ACP) is exposed and drilled to expose the internal carotid artery (ICA). PS, planum sphenoidale.
The inferior surface of the frontal lobes and anterior portion of the inter-hemispheric fissure lie above the ASB. In particular, the gyrus rectus and medial orbital gyrus rest on the midline ASB and are separated by the olfactory sulcus, where the olfactory tracts run (Fig. 12). The medial orbitofrontal artery is a branch of the post-communicating tract of the anterior cerebral artery and provides blood supply to the gyrus rectus and medial orbital gyrus. The frontopolar artery arises a few millimeters after the medial orbitofrontal artery and runs on the medial surface of the frontal lobe to reach the frontal pole. These arteries and especially their related veins are frequently connected to the falx cerebri and dura of the anterior cranial fossa via some small bridge vessels that cross the subarachnoid space. During transnasal endoscopic approaches, special attention should be given to avoid injury to these vessels, which may be in contact with the cranial portion of the lesion that is being targeted for removal.
Fig. 12. Transnasal endoscopic view of the ASB via a transcribriform approach. a, b The olfactory fissures (OlF) and ethmoidal roofs (ER) have been exposed by removing the ethmoid complex, nasal septum (NS), middle (MT), and superior (ST) turbinates. c The bone of the median ASB has been removed exposing the dura of the crista galli (CGD), ethmoidal roof (ERD), and planum sphenoidale (PSD) together with the olfactory phyla (OPh). d The dura has been incised and displaced medially to identify the olfactory groove (OGr), olfactory bulb (OBu), and the outer arachnoid (OAr) that is attached to the gyrus rectus (GR) and medial orbital gyrus (MOG). e The falx cerebri (FaC) has been progressively sectioned towards the corpus callosum (white asterisk). f Scheme of the trajectory of the transnasal corridor towards the ASB. AEA, anterior ethmoidal artery; AFA, anterior falcine artery; FPA, frontopolar artery; FS, frontal sinus; LP, lamina papyracea; MOFA, medial orbitofrontal artery; PEA, posterior ethmoidal artery; SpR, sphenoid rostrum; SpS, sphenoid sinus.
Lateral ASB
On both sides, the lateral segment of the ASB is the orbital roof, which is formed by the orbital plate of the frontal bone anteriorly and the lesser wing of the sphenoid posteriorly (Fig. 1, 13). The periorbit and dura mater line the inferior and superior surfaces of the orbital roof, respectively, and merge at the superior orbital fissure forming the meningo-orbital fold.
Fig. 13. Lateral sagittal section of a specimen (right side, seen from lateral to medial). The lateral portion of the ASB is formed by the orbital plate of the frontal bone (OPFB) and lesser wing of the sphenoid bone (LWSB) and separates the intracranial structures from the frontal sinus (FS) and orbital content. V3, mandibular nerve; ET, eustachian tube; Ey, eyeball; GWSB, greater wing of the sphenoid bone; ICA, internal carotid artery; LOG, lateral orbital gyrus; LPM, lateral pterygoid muscle; MCA, middle cerebral artery; MS, maxillary sinus; ON, optic nerve; SRM, superior rectus muscle; TL, temporal lobe.
On the orbital side, the orbital roof is adjacent to several neurovascular and muscular structures. From medial to lateral, the trochlear (IV cranial nerve), frontal, and lacrimal nerves run within the extraconal fat just beneath the periorbit. The trochlear nerve reaches the posterior portion of the superior oblique muscle, which lies from the Zinn annulus (the common origin of the four rectus muscles, superior oblique muscle, and levator palpebrae superioris muscle surrounding the optic nerve at its entrance at the apex of the orbit) to the trochlea (a cartilaginous ring
