Anatomical and quantitative analysis of safe entry zones to the lower brainstem through the far-lateral approach

Safe entry zones (SEZs) in the medulla oblongata offer consistent anatomical corridors for accessing intrinsic lesions while minimizing damage to nuclei and tracts. Data quantifying the geometric parameters of surgical exposure to the lower brainstem are limited. This study aims to anatomically and quantitatively analyze the exposure area, working angles, and linear extent of access to the medullary SEZs via the far-lateral approach. Ten formalin-fixed human cadaveric heads were dissected using simulated operative positioning. The far-lateral approaches were performed under neuronavigation, with coordinates of seven standardized landmarks on the brainstem surface recorded. Customized software calculated exposure areas, horizontal and vertical working angles, and the vertical linear extent of exposure. The far-lateral route optimized exposure of the ventrolateral medulla and olivary zone. The average exposed area for the far-lateral approach was 856.8 ± 139.7 mm². The horizontal and vertical working angles toward the olivary zone were 40.8 ± 10.2° and 54.8 ± 6.8°, respectively, with a vertical linear extent of exposure of 60.0 ± 6.6 mm. These data quantitatively characterize the operative field and instrument maneuverability provided by each approach. This study offers a quantitative anatomical framework for the far-lateral approach to the ventrolateral medulla, outlining objective parameters of exposure area, working angles, and vertical linear extent to the olivary safe entry zone. By converting microsurgical medullary anatomy into measurable operative geometry, these data support the far-lateral route as an effective and reproducible corridor for accessing intrinsic medullary lesions and could improve preoperative planning, trajectory selection, and surgical safety.