**2. Anatomical aspects**

Anterior clinoid process (ACP) is tetrahedron in shape with the apex projected medio-posteriorly. Medially, it forms a superolateral wall of the optic canal. The optic strut is the posterior root of the ACP. Anteriorly it continues with the medial aspect of the sphenoid ridge.

As a rule, the process comprises the bony cortex. However, its pneumatization and bony connections could be variable and attention should be paid before the planned removal.

The removal of the process reveals the 2-6 cm long clinoid space [10]. The dural layer between this space and ACP is the deep extension from the roof of the cavernous sinus and covers the inferior surface of the clinoid process. Medially, this layer extends to surround the ICA as the proximal dural ring and turns upward along the clinoid segment of the ICA to fuse with the distal dural ring. The dural connection between the 3rd nerve and the lateral aspect of the distal dural ring is called the carotico-oculomotor membrane. From the inferolateral aspect of the ACP, the neural bundle consisting of 3rd, 4th, 6th and three branches of the ophthalmic nerve are running. Thus, manipulation in the inferior direction exposes these structures to danger and should be avoided [11, 12]. Meningiomas usually invade the outer (temporal) dural leaf and rarely spread to dura propria (DP), so the separation of dural leaves during surgery provides an increased removal rate as well as better visualization of anatomical structures [13, 14].

C2 and C3 segments of ICA (Bouthillier nomenclature) are traversing the horizontal and vertical portion of the carotid canal in the petrous bone. The cavernous C4 segment is forming a carotid siphon, surrounded by venous plexus. This portion ends with the dural entrance through the proximal dural ring. The number of veins surrounding the clinoid meningioma is not constant. The superficial middle cerebral vein (SMCV) drains the lateral part of the cerebral hemisphere into the cavernous sinus (CS) directly by penetrating its lateral wall and indirectly through the sphenoparietal sinus or through the latero-cavernous sinus. Sphenoparietal sinus runs medially just below the lesser sphenoid wing to empty into the anterior part of the CS [15, 16].
