Fig. 1. The midline segment of the ASB is made up of the posterior plate of the frontal sinus (pink area), cribriform plate (CP, red area), crista galli (CG), ethmoidal roof (ER, green area), planum sphenoidale (PS, dark blue area), and tuberculum sellae (TuS, light blue area). The lateral segment of the ASB is formed by the orbital plate of the frontal bone (OPFB) and lesser wing of the sphenoid bone (LWSB), which serves as the roof of both the orbit (yellow area) and optic canal (orange area). The anterior and posterior ethmoidal arteries run below or within the ethmoidal roof (red dashed lines) and enter the anterior cranial fossa through the anterior (AES) and posterior (PES) ethmoidal sulci, branching in the anterior meningeal arteries (red continuous lines). The middle meningeal artery provides the blood supply for the dura of the anterior cranial fossa through some branches of the frontal division (red continuous line on the far left). It also gives rise to the meningo-ophthalmic/meningo-lacrimal artery (red continuous line on the left), which enters the orbit (red dashed line) and provides a recurrent branch for the dura of the lesser sphenoid wing. The black dashed line represents the frontosphenoidal suture. ACP, Anterior clinoid process; OC, optic canal; OS, optic strut.
Gross Anatomy
The ASB can be divided into one midline and two symmetric lateral segments. The midline and lateral segments separate the anterior cranial fossa from the nasoethmoidal complex and orbits, respectively.
The ASB has a slightly regular surface on the intracranial side, which differs from other regions of the cranial base. Indeed, it is flat and not pierced by any neurovascular structure except in the hollow area of the cribriform plate, which is crossed by the olfactory phyla (Fig. 1). The extracranial surface is regular and slightly concave in the lateral segments, while the median segment is irregular due to the presence of ethmoidal cells that have variably shaped the bone of the skull base. Moreover, the anterior and posterior neurovascular ethmoidal bundles cross the ASB from the orbit to the vertical lamella of the cribriform plate. The bony thickness is quite constant throughout the ASB, except for the median area, where the cribriform plate is remarkably thinner compared to surrounding bones.
Midline ASB
The midline segment of the ASB serves as a watershed between the sinonasal tract and intracranial space. On the sinonasal side, it lies above the olfactory fissure, anterior and posterior ethmoid air cells, and sphenoid sinus (Fig. 2, 3).
Fig. 2. Paramedian sagittal section of a specimen (right side, seen from lateral to medial). A1, precommunicating tract of the anterior cerebral artery; AE, anterior ethmoid; ER, ethmoidal roof; FS, frontal sinus; IT, inferior turbinate; MOG, medial orbital gyrus; MT, middle turbinate; ON, optic nerve; PE, posterior ethmoid; PG, pituitary gland; PS, planum sphenoidale; PSt, pituitary stalk; SpS, sphenoid sinus; TS, tuberculum sellae.
Fig. 3. Axial section at 1 cm below the ASB (seen from below). The anterior (AE), posterior ethmoid, and olfactory fissure (OlF) lie just below the midline ASB, namely the ethmoidal roof (ER) and cribriform plate, respectively. The lateral ASB is located just above the orbital content. Ey, eyeball; FOV, fronto-orbital vein; GR, gyrus rectus; MCA, middle cerebral artery; MOFA, medial orbitofrontal artery; MOG, medial orbital gyrus; MRM, medial rectus muscle; ON, optic nerve; OpA, ophthalmic artery; SOV, superior ophthalmic vein.
Fig. 4. The dural lining of the anterior cranial fossa is slightly regular except for the area of the olfactory groove (OlG), where it is depressed and thinner. At the level of the crista galli (CGa), the falx cerebri (FaC) inserts on the ASB dividing the anterior cranial fossa into two compartments that communicate through the area of the planum sphenoidale. III, oculomotor nerve; DoS, dorsum sellae; ICA, internal carotid artery; ON, optic nerve.
Fig. 5. Scheme showing the trans-sinus (A), supraorbital (B), and pterional (C) trajectories of Figures 6, 10, and 11, respectively.
The cribriform plate, a complex and delicate part of the ethmoid that constitutes the central segment of the ASB, is perforated by several olfactory phyla and some terminal branches of the ethmoidal arteries and nerves. The cribriform plate is composed of two symmetric subunits, separated in the midline by the crista galli. When viewed intracranially, each subunit creates a niche, called the olfactory groove, which is about 21 mm long and 5 mm wide and “houses” the olfactory bulb (Fig. 4–6) [1]. In a coronal section view, each subunit is formed by a horizontal and a vertical lamella: the former is traversed by the olfactory phyla, while the latter is crossed by the ethmoidal bundles entering the intracranial compartment. The junction between the horizontal and vertical lamella corresponds to the area where the middle and superior turbinates insert onto the skull base (Fig. 7). This area is remarkably delicate due to several factors: the bone is thin and pierced by the olfactory phyla and ethmoidal bundles, making the area likely to be injured if handled roughly; its shape is angulate and the surrounding anatomy can be extremely variable and complex, making this area a high-risk source of cerebrospinal fluid leak during endoscopic endonasal surgery [2]. Keros [3] proposed a classification of the depth of the olfactory fossa into three grades in terms of cranial to caudal length: type 1, 1–3 mm; type 2, 4–7 mm, and type 3, 8 mm or higher, with type 2 being the most common. In addition, the inclination of the vertical lamella can vary remarkably and may also differ from side to side in the same patient. From an endonasal perspective, in case of great depth of the olfactory groove (higher Keros type) and a steep inclination of the vertical lamella, when exposing this area patients are at a higher risk of injury to the skull base during endoscopic procedures (Fig. 7). Similar to the bone, the dura mater that lines the anterior cranial fossa becomes thinner the closer it gets to the olfactory groove, where a dural envelope follows the phyla towards the olfactory mucosa.
Fig. 6. Transfrontal sinus endoscopic view of the median ASB (the trajectory is shown in Fig. 5) before (a) and after (b) section of the outer arachnoid (OAr). Each olfactory bulb (OBu) is enclosed between the crista galli (CGa) medially, cribriform plate (CrP) inferiorly and inferior-laterally, and ethmoidal roof (ER) superior-laterally. Superiorly, each olfactory bulb is connected to the respective gyrus rectus (GR) and medial orbital gyrus (MOG) by a layer of outer arachnoid. The gyrus rectus and medial orbital gyrus of each side are divided by the olfactory sulcus. The medial