Background: Despite the latest development of microsurgery, electrophysiological monitoring and
neuroimaging, the surgical management of intrinsic brainstem lesions remains a challenge.
Several safe entry points have been described to access the different surfaces of
the brainstem. The knowledge of this entry zones anatomy is critical to perform a
safe and less morbid approach. To access the anterior midbrain surface, a well-known
entry point is the anterior mesencephalic (AM) zone.
Objective: To quantify the surgical exposure through the orbitozygomatic (OZ) and subtemporal
(ST) approaches, currently used to access the anterior midbrain surface. In addition,
we analyze the angular exposure along the horizontal and vertical axis angles for
the AM zone.
Methods: Ten cadaveric heads were dissected using the OZ and ST approaches for the anterior
midbrain surface exposure. Neuronavigation system was used to determine tridimensional
coordinates. Area of surgical exposure, angular exposure, and anatomic limits of each
craniotomy were evaluated and determined by software analysis and compared for intersection
areas and AM safe zone exposure.
Results: The median surgical exposure was 164.7 ± 43.6 mm2 for OZ and 369.8 ± 70.1 mm2 for ST (p = 0.001). The vertical angular exposure was 37.7 ± 9.92 o for the OZ and 18.4 ± 2.8
o for the ST opening (P < 0.001). The horizontal angular exposure to the AM zone was
37.9 ± 7.3 o for the OZ and 47.0 ± 3.2 o for the ST opening (P 0.002).
Conclusion: Although the OZ craniotomy offers a reduced surgical exposure of the brainstem, it
provides a better trajectory to the AM zone than the ST approach.
