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 lying at the superior-medial-anterior corner of the orbital cavity and serving as the anchor point for the superior oblique muscle). The frontal nerve, coming from the ophthalmic branch of the trigeminal nerve, forms the supratrochlear nerve (medial) and supraorbital nerve (lateral), and both provide the sensitive nerve supply of the forehead. The lacrimal nerve (branch of the ophthalmic nerve) goes towards the lacrimal gland, whose orbital portion is located in the superolateral angle of the orbital rim. The superior ophthalmic vein usually reaches the superior orbital fissure in an area between the frontal and lacrimal nerves. The proximal tract of the intraorbital ophthalmic artery, its main branches, and other orbital nerves are not adjacent to the skull base as they run caudally to the levator palpebrae superioris and superior rectus muscle. After entering the orbit from the inferior portion of the optic canal, the ophthalmic artery usually turns lateral and above the optic nerve (below the superior rectus muscle), and gives rise to the anterior, middle (when present), and posterior ethmoidal arteries. The anatomy of the ophthalmic artery is considerably variable: in particular, it can arise from other portions of the internal carotid artery, thus changing its relationships with neighboring structures (especially with the optic apparatus) [12, 13]. The nasociliary nerve (branch of the ophthalmic nerve) runs within the annulus of Zinn and then close to the orbital beak, parallel to the ophthalmic artery and inferomedial to the superior oblique muscle.
On the intracranial side, the orbital roofs are in contact with the inferior surface of the frontal lobes. The anterior, posterior, and lateral orbital gyri overlie the orbital roof and are supplied by the lateral orbitofrontal artery, which comes from the middle cerebral artery. These circumvolutions are separated by the orbital sulci, which has an “H” shape. The olfactory vein (posterior-lateral), fronto-orbital vein (posteromedial), and prefrontal veins provide