Accessing the Anterior Mesencephalic Zone: Orbitozygomatic versus Subtemporal Approach

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.