The Lateral Mesencephalic Sulcus as a Layered Posterolateral Microsurgical Corridor: A Cadaveric Fiber Dissection and MRI Morphometric Study

Background/Objectives: This study aimed to describe the sequential microsurgical relationships surrounding the lateral mesencephalic sulcus (LMS) through fiber dissection and characterize regional anatomical variation using MRI morphometry in an independent adult cohort. Methods: Eight formalin-fixed adult specimens underwent stepwise fiber dissection. T2-weighted MRI examinations from 100 adults were retrospectively analyzed. Measurements included the inferior colliculus–straight sinus angle (ICSSA), corpus callosum–pontomesencephalic junction distance (CCPJD), LMS–midline angle (LMS-MA), LMS–midline distance (LMS-MD), and LMS–cerebellar surface distance (LMS-CSD). The splenium–mesencephalon relationship (SMR) was categorized descriptively. Comparisons and age correlations were evaluated using Holm-adjusted p values. Results: Dissection documented sequential relationships between the LMS, lemniscal pathways, superior cerebellar peduncle, and deep tegmental structures. The MRI cohort included 50 women and 50 men aged 23–88 years. Mean ICSSA was 29.6 ± 5.3° and CCPJD was 24.17 ± 3.85 mm. Right and left values were 21.9 ± 3.9° and 22.2 ± 4.0° for LMS-MA, 9.77 ± 0.81 and 9.91 ± 0.84 mm for LMS-MD, and 6.12 ± 1.87 and 6.28 ± 1.68 mm for LMS-CSD. No right–left differences remained significant after correction. Males had greater left LMS-MD and right LMS-CSD (adjusted p = 0.002 and 0.004, respectively). Age correlated negatively with CCPJD (ρ = −0.389; adjusted p < 0.001) and positively with right and left LMS-CSD (ρ = 0.268 and 0.379; adjusted p = 0.042 and <0.001, respectively). SMR Types A, B, and C occurred in 70%, 29%, and 1%, respectively. Conclusions: Fiber dissection and MRI morphometry provide complementary descriptive anatomical reference data. The MRI parameters characterize regional variation; their correspondence with deep microsurgical boundaries and clinical applicability remain to be established.