the intracranial compartment from the orbital content. Several peculiar anatomical areas make up the midline segment (posterior frontal plate, cribriform plate, ethmoidal roof, planum sphenoidale, and tuberculum sellae), while the lateral segments are more regular, formed by flat laminae (orbital plates of the frontal bones and lesser wings of the sphenoid). Here we detail each segment of the anterior skull base, emphasizing major landmarks, providing classifications and measurements of key areas, and cautioning the endoscopist about areas to avoid or minimize the occurrence of cerebrospinal fluid leaks, as well as providing recommendations and tips. Several endoscopic and sectional macroscopic anatomical images provide the reader with an informative, illustrative, and broad perspective of anterior skull base anatomy.
© 2020 S. Karger AG, Basel
The anatomy of the human skull has not changed in millennia, but following the introduction of lighting, optics, and endoscopes, innovative changes and approaches to the management of diseases and conditions of the nose, paranasal sinuses, and adjacent anatomical areas of the mid-face and anterior skull base (ASB) have occurred. This minimally invasive surgery approach, contrary to open approaches, requires that surgeons acquire detailed knowledge of local anatomy as viewed from in to outwards, allowing for a safe, targeted, and bloodless approach while pursuing the view seen from an endoscope. This surgical approach involves not only the nasoethmoidal box, but also the adjacent anatomic areas, which are not infrequently the target of so-called extended transnasal endoscopic approaches.
For this reason, the authors considered it important that the reader be updated with a review of the ASB together with photographic documentation that allows for detailed understanding of the anatomic structures encountered in a transnasal endoscopic journey toward the ASB and beyond the nose. In addition, details on the endoscopic view of the ASB via extra-nasal endoscopic approaches, as the transorbital or the supraorbital, are included.
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
