The utmost superior aspect of the clivus is represented by the dorsum sellae and the posterior clinoids on both sides.

The anterior aspect of this area encloses posteriorly the sella: two layers of dura cover its bony surface. the periosteal and meningeal layers, the same two layers covering the sellar floor, between which run the superior, inferior, and posterior intercavernous sinuses (PIS) [54]. This latter venous channel is located behind the pituitary gland and could be seen upon elevation of the gland; the dorsum sellae could be identified posterior to the PIS. Upon bone removal, the clival dura harboring the basilar venous plexus is exposed (see Fig. 25.4): dural opening at this level gives access to the interpeduncular cistern encompassed laterally by Liliequist membrane and the posterior communicating arteries, the respective perforating arteries, and the third cranial nerves (see Figs. 25.5 and 25.6a, b). The mesencephalon, the basilar bifurcation, the posterior cerebral arteries, and the superior cerebellar arteries are noble neurovascular structure encountered in a deep level, being the epicenter of this area. The horizontal lamina of Liliequist membrane represents the inferior limit of this area [52, 53].

The bony aspect of this clival area is enclosed between the sellar floor superiorly and the sphenoid floor inferiorly; the protuberances of the ascending segments of ICAs, the so-called paraclival tract, represent the lateral limits of the area [41]. The identification of this region from a ventral endonasal route depends on the degree of pneumatization of the sphenoid sinus: it is well represented in sellar and presellar types, while it could be troublesome to recognize its boundaries in case of conchal sinuses (see Fig. 25.4).

Once the bone has been removed, the dura is exposed with the basilar venous plexus within: a segment of the sixth cranial nerve’s root runs in between the two layers of clival dura just before piercing Dorello’s canal [31]. The dural opening reveals the prepontine cistern with the sixth cranial nerves laterally; the pons with the basilar artery and its branches and the anteroinferior cerebellar arteries lie deeply (see Fig 25.6b). The pontomedullary junction and the vertebrobasilar junction (VBJ) are considered as the inferior edge of this region [7, 35].

The inferior third has its superior border at the level of the floor of the sphenoid sinus and reaches down the foramen magnum; on both sides, the inferior third of the clivus is not limited directly by the ICA as in the middle third, so that dissection can be safely extended further laterally. Upon the lateral aspects of the clival bone, the petroclival synchondrosis can be identified and followed all the way to the jugular foramen (see Fig. 25.6c). The occipital condyles are found in the anterior portion of the foramen magnum: the lateral exposure can be increased by removing the condyles up to the hypoglossal canal, wherein the twelfth cranial nerve runs, this latter being fixed as the limit of maximum lateral extent [7, 35]. Once the dura has been opened, the premedullary cistern and the medulla oblongata are identified as the epicenter of this area. Whether a supracondylar approach is extended through the jugular tubercle, the ninth, tenth, and eleventh cranial nerves are exposed, in the lateral aspect of the cistern [49].

Once the main landmarks, i.e., clival indent, carotid protuberances, and sellar floor, have been identified, the bone removal can start at the level of the sellar floor: dura is exposed from the superior intercavernous sinus (SIS), down to inferior intercavernous sinus (IIS) and, posteriorly, at the sella-clival junction. Once circular sinuses have been managed, the dura is opened up to the supradiaphragmatic space to allow freeing the pituitary stalk. In order to gain a more comfortable corridor, Kassam et al. [36] introduced the so-called pituitary transposition/trans-dorsum sellae technique: the gland is mobilized superiorly after ligaments connecting the pituitary capsule to the medial cavernous sinus have been dissected. The posterior sellar dura is coagulated and transected so that the dorsum sellae and posterior clinoids are exposed; these are then drilled and carefully removed, minding attention to avoid injuries to the ICA and third and sixth cranial nerves. The retroclival dura harboring the basilar plexus is visualized: basilar plexus can determine intense venous bleeding, which can be controlled with hemostatic agents such as oxidized cellulose or thrombin/gelatin matrix.

Once the dura has been opened tumor, debulking is first performed and then the dissection from surrounding neurovascular structures completes the removal maneuvers. Chordomas in this location can be tightly attached to the branches of the superior hypophyseal artery, which should be preserved in order to avoid vascular damages to the pituitary stalk and the optic chiasm; as well, the basilar apex and its perforators are pushed posteriorly and attached to tumor capsule, while, laterally, the lesion can impinge the third cranial nerve and the posterior communicating artery from which it should be dissected carefully. Owing that, these maneuvers should be performed sharply under direct closeup view. Finally, it is useful to remark that when performing removal of the inferior aspects of tumor involving this area, the preservation of the inferior horizontal lamina of Liliequist membrane helps to decrease subarachnoid blood dissemination to other cisterns.