Surgical Landmarks for the Internal Carotid Artery in Endoscopic Endonasal Approaches. M and N, 45-degree view. The inferior aspect of the petrous apex overlying the ICA has been drilled. This segment of the vessel traverses 3 planes simultaneously: (1) from inferior to superior; (2) from posterior to anterior; and (3) from lateral to medial. The petrous ICA may look shorter than it actually is when projected on a 2-dimensional monitor. The vidian canal provides a good depth gauge for the position of the genu of the ICA as it transitions from the petrous to the paraclival at the level of the foramen lacerum. The posterior opening of the vidian canal is filled with fibrocartilaginous tissue, which fuses with the similar tissue overlying the foramen lacerum medially. O–T, landmarks for the paraclival ICA. O and P, 0-degree and left-sided 45-degree views inside a sphenoid sinus after the completion of bilateral anterior and posterior ethmoidectomies and sphenoidotomies. In the midline, caudally to rostrally, the sphenoid floor, clival recess, sellar face, tuberculum sellae, and planum sphenoidale are shown. The paraclival protuberance forms the lateral borders of the clival recess and can be easily identified in well-pneumatized sinuses. The protuberance overlies the paraclival segment of the ICA. Lateral and anterior to the paraclival protuberance is the foramen rotundum. Due to its canalicular shape, especially when viewed endoscopically through the sphenoid sinus, we refer to it here as the rotundum canal. When identified, the canal can be followed posteriorly toward the paraclival ICA and hence is a landmark for it. The maxillary recess, the ventral representation of the maxillary strut as seen though the sphenoid sinus, forms the anterior part of the canal’s roof as well as the floor of the superior orbital fissure. Q, 45-degree view at the left side. The bone overlying the paraclival ICA has been removed. The maxillary and mandibular nerves as well as the nerves in the cavernous sinus have been exposed. The maxillary nerve can be followed posteriorly toward the gasserian ganglion, which lies in Meckel’s cave lateral to the paraclival ICA. R, 0-degree view directed at the right side. The medial wall of the maxillary sinus has been opened to expose the infraorbital nerve, which can be followed posteriorly toward the maxillary nerve, which itself serves as a landmark for identifying the paraclival ICA. Identifying the infraorbital nerve and following it as described here may be crucial in cases in which the sphenoid sinus is poorly pneumatized; the rotundum canal and maxillary nerve are difficult to identify. (Images courtesy of AL Rhoton, Jr.)