Reappraising the Anatomy of the Ansa Lenticularis in the Human Brain: A Cadaveric, Focused Fiber Micro-Dissection Study Perspective

Background: The objective of this study was to map the topography, morphology, connectivity, and correlative anatomy of the ansa lenticularis (AL) in the human brain since there is a paucity of direct structural evidence from cadaveric studies. Methods: Twenty normal adult formalin-fixed cerebral hemispheres were treated with Klingler’s method and subsequently explored through the fiber microdissection technique. Basal and medial dissections focusing on the anterior perforated substance, subthalamic, and mesencephalic areas were carried out in a stepwise manner. Hemispheric asymmetries were recorded, and the spatial relationship of the AL with the surrounding fiber tracts and nuclei was investigated. Results: The AL and its segments were consistently identified sweeping around the cerebral peduncle in a medial direction posterior to the anterior commissure and dorsal to the optic tract and ansa peduncularis after arising from the ventral-posterior margin of the globus pallidus. Then it made an almost right angle to reach the thalamus (dorsal segment), subthalamic nucleus (dorsal segment), red nucleus (middle segment), and substantia nigra (ventral segment), respectively. Additionally, the dorsal segment of the AL intermingled with the fasciculus lenticularis (FL) at the level of the zona incerta (ZI) to form the thalamic fasciculus (H1 field of Forel), which travelled slightly lateral to the cerebellothalamic fibers, ascended through the prerubral field, and terminated in the area of the anterior and ventral thalamus. Conclusions: We provide structural evidence of the topography, morphology, and connectional anatomy of the ansa lenticularis. From our review of the literature this is the first cadaveric study using white matter microdissection to delineate the comprehensive composition of the ansa lenticularis. Fiber microdissection studies are integral for the extrapolation of accurate anatomical conclusions which subsequently inform clinical practice. Combined with tractography and histology, these studies enhance our understanding of delicate pathways that can act as surgical targets in fields such as stereotactic neurosurgery.