BACKGROUND: Although resection of the anterior clinoid process (ACP) is valuable 
in the surgical treatment of aneurysms of the ophthalmic (C6) segment of the 
internal carotid artery (ICA), quantitative assessment of this adjunct is 
incomplete. Our morphometric study assesses the effectiveness of the anterior 
clinoidectomy for exposure of the C6 segment of the ICA.
METHODS: Ten formalin-fixed adult cadaveric heads were dissected bilaterally and 
pterional craniotomies were performed bilaterally. Measurements before and after 
resection of the ACP included the length of C6 segment of the ICA on its lateral 
aspect; C6 segment length on its medial aspect; and medial length of the optic 
nerve from the optic chiasm to falciform ligament (before ACP resection) then to 
the annulus of Zinn (after ACP resection).
FINDINGS: Height and width of the intradural ACP were 8.67 +/- 2.63 and 6.57 +/- 
1.68 mm, respectively. After clinoidectomy, mean length of the lateral C6 
segment of the ICA increased 60% and mean exposure of the medial C6 segment of 
the ICA increased 113% (p < 0.001). Exposure of the optic nerve increased 150% 
(p < 0.001) after clinoidectomy and sectioning of the falciform ligament. No 
correlations were found between the lengths of the ACP and entire C6 segment, or 
the ACP size and amount of the C6 segment covered by the clinoid.
CONCLUSIONS: Exposure of the C6 segment of the ICA is markedly increased by 
increase of the mobility of the optic nerve with clinoidectomy and section of 
the falciform ligament.