Pure Endoscopic Approaches to the Mandibular Nerve: An Anatomic Study Comparing Extradural Subtemporal to Endonasal Transpterygoid Approaches

Introduction: Trigeminal schwannomas may appear anywhere along the length of the nerve, and therefore its surgical management requires a spectrum of skull base approaches. In this study, we evaluate two purely endoscopic approaches to the peripheral segments of the mandibular nerve (V3): endoscopic extradural subtemporal approach (EESTA), and endoscopic endonasal transpterygoid approach (EETPA). The aim of this study was to compare the exposure that can be achieved by each of these approaches, and to define which of them provides a better access to different anatomical locations in relation to V3.

Methods: Fourteen EESTA and EETPA were performed on 7 fresh cadaveric heads. In all specimens, a 4-mm 0° endoscope was used to develop each surgical approach, and to identify anatomical landmarks and limitations associated with accessing V3. All Dissections were performed under endoscopy alone. EESTA was performed through mini craniotomy that was placed above the middle third of the zygomatic arch. EETPA was performed with a medial antrostomy, and without adding Caldwell-Luc incision. Anatomic exposure corresponded to the area within 4 anatomic barriers directly visible in the surgical field. The distance to the target, which was defined uniformly as V3 segment in the foramen ovale, was measured in all dissections.

Results: The trajectories of the EESTA and EETPA corresponded with the intracranial (proximal) and extracranial (distal) segments of peripheral V3, respectively. The EESTA revealed the course of the proximal segment from its exit from Meckel’s cave to its bifurcation into anteromedial and posterolateral divisions (in average, the bifurcation was located 0.85cm distal to the foramen ovale). This approach provides favorable access to the lateral aspect of proximal V3: Specifically, the anatomic exposure achieved by this approach is extended from the maxillary division of the trigeminal nerve (V2) and foramen rotundum anteriorly to the greater superficial petrosal nerve (GSPN) posteriorly, and from the retracted temporal lobe superiorly to the superior head of the lateral pterygoid muscle inferiorly. The mean distance from the cranium to the target was 3.1 cm. On the other hand, the EETPA revealed the course of the distal segment of V3 from the foramen ovale to the infratemporal fossa. This approach provides favorable access to the anteromedial aspect of distal V3: Specifically, the anatomic exposure achieved by this approach is extended from the temporal muscle in the infratemporal fossa laterally to the medial pterygoid plate and paraclival internal carotid artery medially, and from the infraorbital branch of V2 superiorly to the level of the nasal cavity floor inferiorly. The mean distance from the edge of the ipsilateral nostril to target was 8.4 cm.

Conclusion: The combination of EESTA and EETPA effectively followed the course of the peripheral V3, from Meckel’s cave to the infratemporal fossa. The anatomic exposure of EESTA and EETPA is better suited for a lesion that is located lateral to proximal V3 and anteromedial to distal V3, respectively. These endoscopic minimal invasive approaches might be incorporated in the management of trigeminal schwannoma.