Fig. 1

Surgical corridors outlined based on bony landmarks. A The top of zygomatic process of frontal bone (white thick long arrow) was used to outline plane parallel to the exposed triangle. The intersection (black thin long arrow) of the plane and the axis was the vertex of triangular pyramid for corridor 1. B The exposed triangle was outlined with the ipsilateral (black thin short arrow) and contralateral (white thin short arrow) anterior edge of jugular tuberculum, and anterior edge of foramen magnum (white thick short arrow) as vertexes. The axis was drawn to connect the center (black thick short arrow) of exposed triangle and the posterior and superior edge of occipital condyle (white arrow head). C The intersection (black thick long arrow) between axis and calvarium was the vertex of triangular pyramid for corridor 2. D The line on the triangular pyramid through contralateral anterior edge of jugular tuberculum form intersection (white thin long arrow) on the calvarium. Posterior view of corridor 1 (E) and corridor 2 (F)