The correlative positions of the entry-exit zones of the cranial nerve roots (V, VII-VIII, and IX-X-XI medullary root) were investigated in 12 adult cadaver brains to determine the most appropriate cut planes of vertebral angiotomography (VAT) for preoperative evaluations of microvascular decompression cases for trigeminal neuralgia or hemifacial spasm. The proximal zones of the left V, VII-VIII, and IX-X-XI medullary roots, 5 mm in length, were marked using Myodil-stained cotton sheets.

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... ments were made in several angles on plane X-ray films. Statistical analysis on the position of these nerve roots yielded measurements for practical use. A convenient procedure was developed for locating the optimal cut plane for VAT. Clinical cases examined by this procedure were presented and the diagnostic usefulness was discussed. Measurements for practical use, corrected for magnification, were as follows. For the V root proximal zone, the center of the entry zone in the lateral view was 12 mm from the clivus, and on a line crossing the external auditory canal (EAC) and perpendicular to the anthropologic base line. The zonal radius was 2.5 mm. In the A-P view, the most medial part was 16 mm from the midline. For the VII-VIII root proximal zone, the center of the zone in the lateral view was 14 mm from the clivus, and on a line 5 mm posterior from the posterior margin of the EAC and perpendicular to the anthropologic base line. The zonal radius was 2.5 mm. In the A-P view, the most medial part was 13 mm from the midline. For the IX-X-XI medullary root proximal zone, the entry zone in the lateral view was on a line from the VII-VIII root proximal zone to the midpoint of the foramen magnum. The lower border was 10 mm from the foramen magnum. In the A-P view, the medial part was 9-10 mm from the midline.