Abstract

Skull base surgery as a field has witnessed an evolution through the three anatomical eras of neurosurgery, osseous, vascular, and now neural, under the guiding principles of microsurgical anatomy and the incremental development of optics.

The anterior endocranial surface is formed by the ethmoid, sphenoid, and frontal bones, and is divided into medial and lateral segments. The medial segment is perpendicular to the cribriform plate, with the ethmoid bone located anteriorly and the planum, limbus, and tuberculum sellae of the sphenoid body located posteriorly. The lateral segment consists of the orbit and optic canal, and is formed by the frontal bone and the lesser wing of the sphenoid bone, which blends medially into the anterior clinoid process.

Microsurgical anatomy of the anterior skull base, especially through the endoscopic endonasal view, is complex, and its organization can be daunting. The development of three-vector circumferential-radial corridor system was founded in an effort to consolidate the vast “laundry list” of conventional surgical approaches, and organize the relevant soft tissue, osseous, vascular, and neural structures into a more conceptual, simplified framework.

Keywords: Keywords: anterior cranial base, endoscopic endonasal approach, skull base, orbit

17.1 History

Skull base surgery as a field has witnessed an evolution through the three anatomical eras of neurosurgery, namely, osseous, vascular, and now neural, under the guiding principles of microsurgical anatomy and the incremental development of optics. The introduction of the microscope, in particular, paved the way for the creation of numerous skull base approaches, with the ability to visualize and access previously “un-accessible corridors.” In an effort to unveil the detailed anatomy within, Professor Rhoton was instrumental in allowing us to visualize and understand microneurosurgery with a “see-through X-ray-type” concept; explicitly, his work allowed us to visualize deep-seated structures in relation to cranial and superficial cerebral landmarks.

The subsequent development of the endoscope in the 1990s allowed access to areas of the skull base that were previously difficult to access with traditional transcranial approaches. To further build on this concept, a ventral modular codified access system was created—the endoscopic endonasal approach (EEA) which allowed access to virtually all three fossae of the cranial base. The ventral anterior cranial base is divided into two planes: (1) the sagittal (median) plane, which is located in between the internal carotid arteries (ICAs) and comprises the transfrontal, transethmoidal, and transplanum-transtuberculum corridors, and (2) the coronal (paramedian) plane, which is lateral to the ICA and comprises the transorbital corridor. We have since created a more organized, conceptual 360-degree anatomical corridor system, and in particular, have designated a ventral anteromedial (VAM) and ventral anterolateral (VAL) corridor for approaches to the median and paramedian planes, respectively.

17.2 Guiding Principles in Ventral Skull Base Surgery

The development of the EEA led to a revolution in intraoperative visual access to the skull base, and is guided by several key principles and concepts:

●Microsurgical anatomy: A thorough understanding of the relevant landmarks of the ventral cranial base, from the outer to inner layers, is required.

●Access: A key tenet in skull base surgery is that bone can be drilled, vessels can be manipulated, but nerves should not be crossed, whenever feasible.

●Resection: The principles of microsurgery are identical to that when using the microscope. With the development of image guidance, safer and more confident surgical resection can be now guided with neuronavigation and tractography.

●Reconstruction: Use of the vascularized pedicled nasoseptal flap in EEA surgery has greatly reduced surgical morbidity.

●Team surgery: EEA surgery is best performed with a multidisciplinary team of neurosurgeons, otolaryngologists, and neuroradiologists.

17.3 Microsurgical Anatomy of the Anterior Cranial Base

We have previously described the development of a conceptual and organized 360-degree framework for accessing the skull base with the creation of a three-vector circumferential-radial corridor system. This coordinate system was derived from key anatomical concepts and was organized into anteromedial, anterolateral, lateral, and posterolateral corridors for both dorsal (transcranial) and ventral (EEA) approaches. The three vectors in this system are defined as follows: Vector 1: Selection of either a dorsal or ventral approach, which consists of anteromedial, anterolateral, lateral, and posterolateral corridors; Vector 2: Formation of an outer circumferential-radial corridor (OCRC) formed by bone, muscles, and soft tissues and inner circumferential-radial corridor (ICRC) constituted by vascular, cranial nerves, and white matter tracts; and Vector 3: Represents the core or epicenter that corresponds to the relevant anatomical landmarks and decides the selection of the appropriate surgical circumferential-radial corridor.

●The ethmoid bone (EB): The EB is comprised of: (1) the crista galli, (2) the cribriform plate, (3) the perpendicular ethmoidal plate, which is located in the medial portion, and (4) two lateral masses or labyrinth (anterior, middle, and posterior ethmoidal cells) and the lamina papyracea. The crista galli is the site of attachment of the falx cerebri. The cribriform plate is the roof of the nasal cavity that transmits olfactory nerve fibers through its perforations on the way to superior turbinate, nasal septum, and MT, and it supports the olfactory bulb. The perpendicular plate is a thin, flattened lamina that descends from cribriform plate; it joins inferiorly with the quadrangular cartilage, anteriorly with the spine of the frontal bone and the crest of the nasal bones, and posteriorly by its upper half with the sphenoidal crest and by its lower with the vomer; all of which form the nasal septum. The agger nasi is the most anterior ethmoidal air cell of the agger nasi region; they are identified in 90% of patients. Agger nasi region is a small ridge located in the lacrimal bone anteriorly and superiorly to the axilla, which is the attachment to the lateral nasal wall of the MT. The Onodi cell is a posterior ethmoid cell that extends superiorly and often laterally to the sphenoid sinus; it is here where we can view the bony impression of the optic nerve in approximately 7 to 25% of cases. The uncinate process is a crescent bony lamina situated in the lateral nasal wall from the maxillary ostium to the frontal recess. Resection of uncinate process allows for visualization of the ostium of the maxillary sinus and is also entrance to the orbit. The ethmoid bulla is the largest anterior ethmoidal air cell and when it is removed, it exposes the lamina papyracea. It lies posterior to the uncinate process, anterior to the basal lamella, and superior to the infundibulum. The hiatus semilunaris is located between the posterior free edge of the uncinate and the ethmoid bulla and communicates with the ethmoid infundibulum. The frontoethmoidal suture represents two important landmarks, namely, the anterior and posterior ethmoidal foramina, in which the respective anterior and posterior ethmoidal arteries and nerves traverse. Lamina papyracea forms the lateral surface of the labyrinth of the ethmoid bone and is delimited by the fronthoethmoidal and the ethmoidomaxillary suture.