MATERIAL and METHODS: According to the protocol of Klingler and Ludwig, six BSs were kept in 10% formalin solution for at least 2 months. After removing the arachnoid mater, pia mater, and vascular structures, the samples were frozen at ?16°C for at least 2 weeks. White matter (WM) pathways of the BS were gradually examined using fiber dissections under a surgical microscope.

RESULTS: Safe entry zones of the BS were defined and investigated, focusing on the ventral pontine region and pontomesencephalic junction. Because of the lack of fibers on the anterior surface of the pons, the MPA formed a safe surgical area. The MPA, strategically positioned between the descending corticospinal tracts and extending securely to the anterior limit of the medial lemniscus, serves as a protective pathway, creating a secure environment for accessing safe entry zones within the BS during surgery.

CONCLUSION: The position of the MPA has the potential to provide a combined surgical path with superiorly located BS entry zones, resulting in a larger surgical area. Entry to the BS via the MPA increases the accessible surface area in the ventral pons and can be combined with the other perioculomotor safe regions outlined. Our findings might lead to safer endoscopic endonasal transclival interventions for intrinsic pontine lesions. Keywords : Brainstem, Cerebellum, Endoscope, Entry zone, Microsurgical anatomy, Pons, Transclival approaches, White matter