Chapter 2 Three-Dimensional Anatomy of the Skull Base: The Ventral Pathway

10.1055/b-0037-143508

Chapter 2 Three-Dimensional Anatomy of the Skull Base: The Ventral Pathway

Alberto Prats-Galino, Matteo de Notaris, Marija Mavar Haramija, Juan Antonio Juanes Méndez, Joaquim Enseñat

Introduction

In recent years, advances in computer technology and medical imaging techniques have acquired a significant impact on different branches of surgery, medical education, and research. In the fields of neurosurgery and otorhinolaryngology, progress in neuroimaging studies, such as high-resolution computed tomography (CT) scans, magnetic resonance imaging studies, and digital subtraction angiography data, has certainly refined the visualization of anatomic structures within the brain and the skull. At the same time, the evolution of minimally invasive techniques and the introduction of the endoscope have led to endoscopic endonasal (EE) approaches rapidly becoming the standard of care for pituitary and other skull base tumors, thus requiring the acquisition of new surgical skills. Even experienced neurosurgeons and rhinologists face a steep learning curve in EE approaches, as the operation is performed through a tight space, where both the lack of maneuverability and the conventional mucosal bleeding may hinder the visualization of the surgical field. An additional burden is represented by the lack of stereoscopic vision of most endoscopes. Even though the advent of three-dimensional (3D) scopes in endonasal surgery has improved depth perception, such new tools still face difficulties in hand–eye coordination. For such reasons, endoscopic transsphenoidal surgery can take a long time to learn and execute, especially for those surgeons not fully confident with the scope.

Anatomic cadaveric dissection has been, for many years, the only training tool available for guiding the improvement of technical skills in endoscopic transsphenoidal surgery. However, during the last decade, with the diffusion of 3D computer-generated models that provide accurate patient-specific 3D reconstructions from neuroimaging data, allowing interaction with such models, and the simulation of the different steps of the surgical intervention, the training in EE neurosurgery has gained further developments.

Combined with anatomic laboratory dissection, 3D computer-based reconstructions represent a unique opportunity for research and educational purposes when applied to the transsphenoidal perspective, guiding the acquisition of specific visual information for endoscopic approaches to the skull base.

2.1 Surgical Simulation Methodology

2.1.1 Data Acquisition (Predissection CT Scan)

A CT scan (SOMATOM Sensation 64; Siemens AG) of each specimen was performed prior to each dissection at the Department of Neuroradiology, Hospital Clinic, Barcelona. The heads were positioned in the scanner according to the Frankfurt plane. All studies were performed with a multislice helical acquisition protocol, with 0.625-mm slice thickness, using a gantry angle of 0 degrees. The images obtained were stored into the hospital PACS (Picture Archiving and Communication System) network in a DICOM (Digital Imaging and Communications in Medicine) format.

2.1.2 Anatomic Cadaveric Dissection

All dissections were performed in the Laboratory of Surgical NeuroAnatomy (LSNA), at the University of Barcelona, Spain. The position of the head on the dissection table was adjusted to match the usual position in the operating room, to better emulate a true surgical intervention. An EE transsphenoidal approach to the midline skull base was performed in three specimens using a rigid 0-degree endoscope, 18 cm in length and 4 mm in diameter (Karl Storz Endoscopy), as the sole visualizing instrument during the whole procedure.

2.1.3 Image Processing: Generating a Virtual 3D Model

The 3D model was developed from the CT scans of the anatomic specimens (postdissection CT), using a specific software for visualization and manipulation of biomedical data (Amira Visage Imaging Inc.). The 3D reconstructions of the bone volume of each surgical procedure were compared with those obtained in the dissection laboratory. The methodology followed at the LSNA to generate the 3D models is described in detail elsewhere.1–3

2.2 Simulation of the Different Steps of the EE Approach

In the present section, the main steps usually used to reach the midline skull base through a ventral approach are described. Specifically, the EE approach using different computer-based 3D models generated from the high-resolution CT scans is modeled in a step-by-step sequence, including the different nasal steps,4,5 the transsphenoidal and extended approaches,5–8 and the surgical corridors to the ventral brainstem.9,10

2.2.1 Nasal Steps

Once the endoscope is progressively introduced through the nostrils, the nasal vestibule, and the piriform aperture, a general view of the nasal cavity is obtained. The key nasal structures to be identified include the nasal septum (S), the head of the inferior (IT) and middle turbinates (MT), and the respective inferior and middle meati ( Fig. 2.1 ).

**Fig. 2.1** General frontal view of the nasal cavity in a 3D model of the skull. For comparative purposes, an endoscopic view is added. IT, inferior turbinate; MT, middle turbinate; S, nasal septum.

In a second step, a bilateral middle turbinectomy (MT, triangulated models) is performed to allow a wide exposure ( Fig. 2.2 ). Once both middle turbinates are removed, the bulla ethmoidalis (BE, Fig. 2.3 ) is easily identified.

**Fig. 2.2** Bilateral middle turbinectomy step. The right part of the figure shows at higher magnification a 3D model of the structures to be removed (triangular models). EC, ethmoidal cells; IT, inferior turbinate; MT, middle turbinate; S, nasal septum.

**Fig. 2.3** Septectomy step. The right part of the figure shows at higher magnification a 3D model of the structure to be removed (triangular model). BE, bulla ethmoidalis; EC, ethmoidal cells; IT, inferior turbinate; S, nasal septum.

In a third step, a septectomy is performed. This is accomplished by the resection of the posterior part of the nasal septum (S, triangulated model) ( Fig. 2.3 ).

The removal of the nasal septum may require the disarticulation of the sphenoidal crest from the perpendicular plate of ethmoid ( Fig. 2.4 ).

**Fig. 2.4** Disarticulation of the posterior nasal septum. The right part of the figure shows at higher magnification a 3D model of the structure to be removed (triangular model). An axial CT section has been added as reference. Note that the anterior part of the nasal septum is not removed. IT, inferior turbinate; MS, maxillary sinus; MT, middle turbinate; S, nasal septum.

In a fourth step, a bilateral ethmoidotomy is performed. It is important to avoid damaging the lamina papyracea ( Fig. 2.5 ).

**Fig. 2.5** Bilateral ethmoidotomy step. The right part of the figure shows at higher magnification a 3D model of the structure to be removed (triangular model). An axial CT section has been added as reference. The lamina papyracea at the internal wall of the orbit must be preserved (arrow). EC, ethmoidal cells; LP, lamina papyracea; MT, middle turbinate; S, nasal septum.

The final nasal step of the approach corresponds to an anterior sphenoidotomy. Once the anterior wall of the sphenoid is removed, several sphenoid septa can be observed within the sphenoid sinus. These septa could maintain a very close relationship with the internal carotid artery, so their removal must be performed very carefully ( Fig. 2.6 ).