Fig. 7. Anatomical variability of the cribriform plate. a Tilted (right side) and high (left side) vertical portions of the cribriform plate. b Non-tilted and low vertical portions of the cribriform plate. Note the asymmetry between the vertical lamellas of different sides in both cases. The white dashed line shows the vertical lamella, and the white dotted line depicts the horizontal lamella. CG, crista galli; NS, nasal septum; T, common lamella of the middle and superior turbinate.
The anatomy of the arachnoid of this region has been described in two different ways. Seeger [4] reported that the olfactory bulbs lie in a subdural area because the outer arachnoid overlies them as a tent; consequently, they are not directly in contact with cerebrospinal fluid. Conversely, Key and Retzius [5] proposed that the arachnoid envelops the olfactory bulbs and phyla following the dura. Thus, the olfactory bulbs are in contact with cerebrospinal fluid. In all likelihood, the truth probably lies somewhere in between: the anatomy of the arachnoid of the olfactory region is variable and shaped by the physiology and hydrodynamics of cerebrospinal fluid, as observed in other skull base areas (i.e., sella turcica).
Fig. 8. Transorbital macroscopic view of the ASB. The orbital roof is formed by the orbital portion of the frontal bone (OPFB) anteriorly and lesser wing of the sphenoid bone (LWSB) posteriorly. The anterior (AEF) and posterior (PEF) ethmoidal foramina lie on the frontoethmoidal suture, between the lamina papyracea (LP) and frontal bone. The white asterisk identifies the optic canal. FR, foramen rotondum; GWSB, greater wing of the sphenoid bone; MT, middle turbinate; NS, nasal septum (perpendicular process of the ethmoid bone); SOF, superior orbital fissure; UP, uncinate process; VC, Vidian canal; Vo, vomer.
The crista galli is a triangular, median process of the ethmoid bone where the falx cerebri inserts anteriorly and inferiorly. It is usually composed of compact bone, but can be pneumatized by surrounding paranasal sinuses in <13% of cases [6]. The foramen caecum lies anterior to the crista galli and can harbor a small vein connected to the superior sagittal sinus. In this area, some nasofrontal dysembriogenic lesions such as dermal sinuses, dermoid cysts, meningoencephaloceles, or nasal gliomas may develop [7]. The anterior portion of the falx cerebri, which extends from the cranial vault to the corpus callosum, divides the anterior cranial fossa into two compartments, which “houses” the frontal lobes and related vessels.
Lateral to the cribriform plate, the midline ASB is formed by the ethmoidal roof, a thick portion of the frontal bone that joins with the ethmoidal box. The basal lamellae of the middle and superior turbinates inserts onto the ethmoidal roof and separates the anterior ethmoid from the posterior ethmoid, and the posterior ethmoid from the sphenoethmoidal recess, respectively. In the area where the lamina papyracea joins with the ethmoidal roof, the skull base tilts superiorly as a consequence of the convex shape of the orbit. The dihedral angle where the skull base turns from horizontal (ethmoidal roof) to convex (orbital roof), called the orbital beak, can be pneumatized by the superior-lateral extension of a suprabullar cell (supraorbital ethmoid cell) or the posterior extension of the frontal sinus (supraorbital recess of the frontal sinus). The ethmoidal arteries are collateral branches of the ophthalmic artery and enter the ethmoidal foramina located along the suture between the lamina papyracea and ethmoidal roof (Fig. 8, 9). In the majority of cases two arteries are found, one per ethmoidal compartment (anterior and posterior). In addition, a middle ethmoidal artery has been reported in 29–38% of cases [8–10]. Anterior ethmoidal arteries usually follow a caudal course with respect to the ethmoidal roof, while the middle and posterior ethmoidal arteries usually run into the skull base. The ethmoidal bony canals can show focal or wide dehiscent areas. The ethmoidal arteries divide into several small arteries for the nasal septum, middle and superior turbinate, and external nose, while the terminal branch perforates the vertical portion of the cribriform plate, forming a bony defect called ethmoidal sulcus, and contributes to the vascularization of the dura of the anterior cranial fossa (anterior meningeal arteries). Because of their relevance in ASB surgery, the anterior meningeal arteries running within the falx cerebri along the anterior surface of the crista galli have been specifically called anterior falcine arteries [1]. The skull base anterior to the ethmoidal roofs and cribriform plates is composed of the posterior plates of the frontal sinuses, which tilt from coronal to axial above the frontal recesses, constituting the transition area from cranial vault and cranial base.
Fig. 9. Transorbital endoscopic view of the ASB via a superior eyelid approach. The endoscope is placed in a posterior-medial (a), posterior (b), and posterior-lateral (c) direction through the superior orbital quadrant, between the orbital roof and periorbit (Per). d The yellow, green, and red circles show the areas targeted in a, b, and c, respectively. Several landmarks can be identified from anterior-medial to posterior-lateral: the anterior (AEF) and posterior (PEF) ethmoidal foramen along the frontoethmoidal suture (FES), the optic canal (OC), the superior orbital fissure (SOF), and Hyrtl’s foramen (HF), when present. GWSB, greater wing of the sphenoid bone; OPFB, orbital plate of the frontal bone; LWSB, lesser wing of the sphenoid 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.