Microsurgical Considerations of the Internal Capsule: Correlations with the Thalamic Peduncles

The dense fiber mass of the internal capsule (IC) has been parceled based on its relationship with the borders of the lentiform nucleus. We aimed to examine the microsurgical anatomy of the IC as a single compact massive fiber and reveal its relationship with thalamic peduncles.

Ten postmortem human brains were dissected to evaluate the relationship between the IC and the thalamic peduncles. Fiber microdissections were performed from lateral to medial and from inferomedial to superolateral.

The anterior and posterior limbs of the IC were effectively disconnected from the deeper thalamic peduncle fibers, and the distinct S-shaped curved trajection of the anterior and superior thalamic peduncles were traced until their cortical terminations. Temporopulvinar and temporopontine fibers constituted the sublenticular-IC, forming a tightly interwoven configuration with the inferior thalamic peduncle. Layers along the roof of the temporal horn were segregated into seven layers from superficial to deep: ventral part of the external capsule, anterior commissure, Meyer's loop, auditory radiations, sublenticular-IC, stria terminalis/caudate nucleus tail, and tapetal fibers. The retrolenticular IC and the posterior thalamic peduncle together constituted the superficial and deep layers of the optic radiations and formed the deeper components of the sagittal stratum, respectively.

The anterior and superior thalamic peduncles can be separated from more superficially lying IC components. Meyer's loop and the sublenticular-IC form an inseparable subcortical connection to the thalamic peduncles. Awareness of distinct connectional features of the IC and their intimate relations with the thalamic peduncles may prove helpful during intraaxial surgery.