 You look at the antiaclynoid it is attached to the lesser wing of the spinoid the lesser wing of the spinoid if you follow it it ends in the tip of the antiaclynoid ok. If you look turn the skull a little more it has 3 roots can you see it the antiaclynoid has 3 roots the one root is continuous with the lesser wing of the spinoid the second root arches over the optic foramen forming the roof of the optic canal the third root goes below attaches to the body of the spinoid and is called a optic strut. So what does the optic strut do it forms the floor of the optic canal and it forms part of the roof of the superior orbital fissure. So it separates the optic canal from the superior orbital fissure is that clear from this. So when you are removing the antiaclynoid you have to deal with 3 roots one root which goes over the optic canal one root which arches over the superior orbital fissure and the third root which goes beneath the optic canal. So these are 3 issues which have to be dealt with. Now when you look at the antiacrynole fossa floor apart from the ridges there is no ferraman for dura to go inside no cranial nose passes through the roof of the orbit. Hence you can separate the frontal dura extra durally at least laterally in the midline you have the cribriform plate where the olfactory bulb sends its filaments into the roof of the nose. So if you separate the frontal lobe extra durally there it will result in anosmia loss of sensation of smell. But apart from that you can separate the frontal lobe dura till the spinoid ridge but you look at the temporal fossa pole if you look at the temporal fossa pole you see the superior orbital fissure. So why is separation of the temporal lobe possible from the ACF when there is the superior orbital fissure which transmits third fourth and sixth cranial nose from the cavernous sinus into the orbit. So how is that possible you can pull the temporal lobe dura pulling the temporal lobe extra durally backwards. So how is that possible what is the anatomical peculiarity there which allows the temporal lobe dura to be pulled back even though there are 3 cranial nose which pass from the temporal lobe side into the superior orbital fissure that is the special arrangement of the dura it is a 2 layer dura. You have the temporal dura propria which covers the surface of the temporal lobe when it comes to the superior orbital fissure it splits into 2. So one layer of dura forms the peri-obita goes into the superior orbital fissure the second layer of the dura which is the endosteal dura covers the temporal lobe over the cavernous sinus. So when you make a superficial incision at the superior orbital fissure when you start pulling back the temporal lobe dura away from the spinoid ridge you reach the superior orbital fissure very quickly. So you saw the orbital meningeal artery being coagulated to release the temporal dura propria that needs to be done after that the cranial nose are in a separate layer they are not in the temporal dura propria only the meningo orbital artery orbital meningeal artery is in the temporal dura propria. Once you cut that you release the temporal dura propria from the cavernous sinus and from the peri-obita. So it is very easy to hold the temporal dura propria with the tooth forceps and start pulling it back to reveal what you see below the superior orbital fissure it is the ferraman rotundum where the second division of the trigeminal nerve enters and the ferraman ovale behind it where the mandibular nerve enters. So you can separate the temporal dura propria from the superior orbital fissure to expose the second division of the trigeminal nerve the third division of the trigeminal nerve ok. So this is the basis for doing an extradural anterior clinoidectomy you have to pull the temporal dura propria away from the spinoid ridge to expose the superior orbital fissure and pull the temporal dura propria behind that to give a wide exposure of the anterior clinoid process. If you do not do this what happens is you will drill only from the frontal side not from the temporal side and becomes a very deep well and drilling becomes progressively more difficult as the temporal dura propria keeps falling over the place. So coming to the key techniques there are several key techniques which assist in performing skull base surgeries these are anterior clinoidectomy, anterior petrocytomy, posterior petrocytomy and occipital condylectomy I will restrict myself to anterior clinoidectomy. So what is the logic of doing this small additional procedures in skull base surgery if you look at this diagram where a basilar aneurysm basilar apex aneurysm is being shown you see there are several obstacles which can potentially come in the way of this aneurysm. When you are doing a frontal temporal craniotomy and approaching this aneurysm through the silvan fissure you can have the anterior clinoid which can come in the way posterior clinoid sometimes the rigid internal carotid artery can come in the way it may not be so easy to retract it even a higher bifurcation even the optic tract can come in the way because the bifurcation can come up to this level and you see the third now this also can obstruct the view to the aneurysm in some way. So it is how we deal with this inner obstacles which sometimes determines the course of a outcome of a particular surgery. So anti-clinoidectomy is one such procedure we will look at it so this is the spinoid ridge the superior root over the optic canal this is the course of the optic nerve and this is the supra orbital fissure here the optic nerve and the carotid artery come there. Now you see all the three roots the lateral root over the supra orbital fissure the optic strut the anterior root the optic canal supra orbital fissure this is the optic strut which separates the optic canal from the supra orbital fissure. So when you drill out the anterior clinoid and remove it completely you expose a special arrangement of the dura the dura from the superior surface of the anterior clinoid arches over the optic nerve forming which ligament the falsiform ligament. So when you have aneurysms or tumours pushing the optic nerve above it gets squeezed between the falsiform ligament and the lesion pushing it above resulting in visual loss. So that is one mechanism of visual loss in tumours and aneurysms. The second mechanism is the optic nerve the chiasm or the optic tract can get squeezed between the anterior sabral artery and the lesion because the anterior sabral artery is rigid it cannot be moved and when the tumour is elevating it again you can get visual loss. So you have the falsiform ligament and the inferior surface of the dura what is the sulcus that is the sulcus formed by the carotid artery in the cavernous anus it is called as the anterior genus or the anterior bend where it turns and then comes out intradurally. So it takes a turn here and it produces a deep groove the dura from the superior surface attaches itself to the carotid artery and dura from the inferior surface of the clinoid process again attaches itself to the carotid artery. So two layers of dura one from the upper surface one from the lower surface attach itself to the carotid artery producing what they produce the dural ring see this from the upper surface which produces the distal dural ring and this is from the lower surface of the clinoid which forms the proximal dural ring. So, the cavernous sinus usually extends only till the proximal dural ring. What you have between these two dural rings is called as the carotid collar. So, you have the membrane extending from here to the oculomotor now from the lower ring to the oculomotor now which is called as carotid oculomotor membrane. So, between these two you have some venous plexus which extends under a thin layer of dura over the carotid artery and is called as the carotid collar. So, that is what produces the bleeding. Actually it may not be cavernous sinus which you are entering, but you may be opening a few channels in the carotid collar. What is the advantage of doing this dissection for example, carotid ophthalmic aneurysm? Why do you have to do antiocline odectomy for carotid ophthalmic aneurysm? Now the question is, do you see the neck without removing the clinoid process in a superior hyperfacial aneurysm or a carotid ophthalmic aneurysm? So, you do not see the entire neck in these aneurysms only after you do a clinoidectomy you display the neck. The second advantage is you can apply a temporary clip across the ICI here. If you are quite well versed here with this technique you can apply it here instead of having a proximal neck control when doing this surgery. So, cutting the distal dural ring makes the carotid artery more mobile. Cutting the falsiform ligament again makes the optic nerve more mobile. So, what does it do? It allows these structures to be retracted. So, without doing these maneuvers if you try retracting the optic nerve again you may produce visual loss and the carotid artery may not tolerate the retraction especially if it is rigid with atherosclerosis that kind of a retraction required to visualize structures deeper to the carotid artery. The other point you have to remember is the oculomotor now enters the superior orbital fissure in close approximation to the anti-aclyanoid process. So, when you are drilling towards the superior orbital fissure the optic strut you are drilling towards the superior orbital fissure you have to be very careful that you do not injure three structures. What are the three structures? One is the optic nerve of course because the optic strut is here forming the floor of the optic canal. Second the clinoidal segment of the carotid artery is underneath the clinoid process. Third the oculomotor nerve is just adjacent to the root of the clinoid process. Hence it can be damaged by the heat produced by the drilling. That is why the heat production is more maximum when you use a diamond drill. When you use a cutting drill the heat production is not that much especially when you are using a diamond drill to thin out the cancellous portion. This is the cancellous portion is the inner portion outside is the cortical layer of the clinoid process. So, when you are drilling the cancellous portion you have to irrigate continuously to avoid heat transfer to the oculomotor nerve. This is the frontal lobe dura which you can easily retract up till the spinoid ridge without reducing any cranial nerve damage. Then you retract the temporal dura from the temporal pole. So, if you look at the arrangement of the bone the temporal squama where it meets the lateral wall of the orbit. This is the lateral wall of the orbit and there is a groove where the temporal squama meets the lateral wall of the orbit. You look at the inner aspect that marks the extra dural temporal pole this groove. So, you could appreciate that the groove between the lateral orbital wall and the temporal squama marks the temporal extra dural pole intracranially agreed. So, in a normal exposure you do not dissect the temporalis muscle to expose this groove and the lateral orbital wall. So, when you want to do a pre temporal approach that is an approach called as pre temporal approach where you go to the interpedant killer fossa area in front of the temporal lobe. This pre temporal corridor elevates the uncuss of the temporal lobe out of the tentorial notch. Normally in a transillian approach what hides the approach to the interpedant killer fossa is the uncuss which herniates over the edge of the tentorium into that area. So, when you pull back the temporal pole extra duraly this whole area becomes clear. So, you have a wide exposure into the interpedant killer fossa. So, this is the meningo orbital artery and this is where the temporal dura propia enters the superior orbital fissure to form the peri orbital of the orbit. This is the lesser wing of the spinoid when you cut this and make a small superficial incision here at the level of the supere orbital fissure. So, as soon as you see that you make a very small incision there the dura is freed and you can pull the entire dura and the temporal lobe back. See this is the supere orbital fissure after making that incision the whole temporal dura propia has been pulled back. So, you can continue the same in the temporal base and that forms the approach for kavases approach. So, when you go from front it is the pre temporal approach when you go from below it is kavases approach where you do an anterior petrosectomy. The principle is the same you incise the superficial layer of the dura pull back the temporal dura propria elevate the temporal dura propria from the base coagulate and cut middle meningeal artery. So, the entire dura is freed you saw that drilling you can use a cutting drill or a diamond drill. Diamond drill is hemostatic, but slow and produces lot of heat cutting drill is faster and produces less heat. But the most important thing to remember in drilling is do not use force use it like a paint brush. So, when you retract the frontal lobe side and drill over the roof of the orbit you can see the indentation of the extradural optic nerve as you pull back the frontal dura. So, you drill over the roof of the orbit optic canal you expose the optic nerve and then this is the remnant of the anterior clinoid and the optic strut which separates it from the carotid artery. And when the clinoid is removed you see the extradural clinoid segment of the carotid artery along with the optic nerve extradurally. So, when you go in this is a case of a meningeoma clinoidal meningeoma which is arising from the under surface of the clinoid process. It is producing visual loss one by elevating the optic nerve against the this is the falsiform ligament and also extending into the optic canal. So, you can see this indentation mark on the optic nerve this is the tumor. So, you can cut the dura here cutting the falsiform ligament thereby relieving the pressure on the optic nerve and this is after removal of the tumor the optic nerve is free. So, though it is a small meningeoma it is still producing visual loss because of its location. Examples like this again when you do this pre temporal dissection in a tumor like this, this is a medial spinoid wing meningeoma. What is the advantage of pulling back the dura? The meningeoma has a dural vascular supply. So, when you pull back the dura from the supia orbital fissure the lesser wing of the spinoid the blood vessels entering the tumor of extra dural wing they get coagulated and cut. So, you de-vascularize the tumor before removing the tumor. Second when you do a pre temporal approach you remove the temporal squama quite anteriorly and you reduce the spinoid ridge. So, what happens to the tumor? The tumor working distance from the scalp to the tumor becomes less because you have now a direct perpendicular approach rather than a slanting approach if you do not do this work. So, what happens? It reduces the working distance and it convexitizes deeper seated lesions. So, they become more superficial because of the reduced working distance. So, this is the post-op scan. Now, I just want to show this as an example of microsurgical technique in aneurysms. In this particular aneurysm it is a low bacillabifacation aneurysm. One side P1 is hyperplastic this has a daughter blood here wide neck. So, there are challenges in endovascularly coiling this aneurysm. This is a blood aneurysm point number one and second is it has a wide neck. So, you may have to do a complex intervention like a strength assisted coiling or a flow diverter. If you want to use a flow diverter you have to put this patient on five days of either aspirin, clopidogrel or prosagrel one of the platelet anti inhibitors. So, what happens is you expose the patient to more risk of re-blading in the four to five days when they have to be given anti platelet agents. You see here this is the documsella the neck of the aneurysm is below the documsella. So, traditionally one would use a Drake's approach for clipping this aneurysm which is a subtemporal approach. But the disadvantage from a subtemporal approach is you do not see both the P1s. You see only one side P1 from the side which you approach. So, you have to be extra careful about the other P1 to not clip it along with the neck of the aneurysm. If you come from the front you have the advantage of seeing both the P1s and the P1 perforators and you can take care of them. And if you see this side posterior clientoid and this side posterior clientoid the posterior clientoid on this side is more open. So, it gives a direct view when you come from the pre temporal approach. This is the temporal extradural for pole right. So, when you come along this pole you have a direct line of sight to the aneurysm. So, here I had to use an antiopetrocectomy approach also in order to get proximal control of the basilar. So, here I did the antioclineodectomy and an antiopetrocectomy to give a multi axial approach to the aneurysm, subtemporal, trans tentorial, pre temporal and trans clientoidal trans sylvian approach. So, you get multi axial view to the aneurysm and you can use any of the corridors to either get a temporary occlusion of the basilar artery or use the corridor for clipping the aneurysm or you can visualize the aneurysm in one corridor and approach with the clip from another corridor. When you pull back the temporal duropropria from the supia orbital fissure you expose the cavernous sinus. So, this approach is also a very good approach for the superior and lateral wall of the cavernous sinus. So, this is the pre temporal corridor intradurally. Nowadays if I have to do this approach I would do a extradural approach rather than an intradural approach because what do you see here from the temporal pole you see veins tethering the temporal lobe to the spinoidal ridge venous sinus. So, that restricts the amount of retraction of the temporal lobe. This is the trans sylvian approach you see between the optic nerve and the carotid artery the carotico optic space is narrow the lateral carotid space has a big dominant picom and then there is the oculomotor nerve. So, you have crowding of these three structures the optic nerve the ICA the dominant picom oculomotor nerve this is the tentorial edge this is the temporal lobe the frontal lobe anti sabral artery middle sabral artery. So, you see this is the oculomotor now being dissected but then the space between lateral to the carotid artery and oculomotor nerve again is very much restricted and when you retract the oculomotor nerve you get a view of the aneurysm the hyperplastic p1 but the tentorium is hiding the view to the neck completely and the proximal vesilar artery this the p1 being dissected and what else do you see you see these fine vessels know these fine vessels are the perforators from the p1 and they are present on both sides here it is closely applied to the neck of the aneurysm the most important task in this aneurysms is preservation of perforators the poor results from basilar bifurcation aneurysm surgeries occlusion of the perforators by clip. So, you see the perforators stuck to the neck the p1 the oculomotor now the tentorium and because the antipeterosectomy has been done here there is bone. So, you can cut the tentorium after the antipeterosectomy. So, you see the tentorial incision being done the perforators the p1 and this is through a pre temporal sub temporal corridor after cutting the temporal lobe has been pulled back. So, this is not through the transilvian corridor and then including the p1 in a ring clip the opposite side of the neck is taken care of this is the opposite p1 and the portion of the aneurysm neck in the ring is included in this clip. So, this is the post-op NGO showing so, the applications of an anterior clenodectomy can be in clenoidal meningioma medial spinaud wing meningioma at least even if you do not do the clenodectomy completely in the medial spinaud wing meningioma the very fact that you do this extradural approach helps in petroclival meningioma basilar apex aneurysm as shown and then trigeminal schwannoma. So, the advantages is it convexitizes a deep seated lesion divascularization of tumors like meningioma extradural interdural approach you give brain protection because you protect all the veins instead of cutting them you retract the brain and the dura along with the veins draining veins it gives brain protection it helps in dethethering of the optic nerve or unlocking the optic nerve and the internal carotid artery it gives a wide angle of approach and you can get proximal control of the basilar artery. So, this is starting after as I left it after the anterior clenodectomy, anterior petrocectomy, pre temporal approach and cutting the dura with sections the superior petrocell sinus you get bleeding which can be quickly controlled with surgery cell and stitching and then you see the perforator here an initial tentative clipping of the neck is being done to soften the aneurysm it is very important to free the oculomotor now otherwise the arachnoid strands with attached to it will produce oculomotor pulse even you retract it and now the perforator is being pulled off from the neck of the aneurysm that is the opposite side P1 you have to visualize the opposite side P1 you have perforators coming from there also. So, this is the opposite side P1 and this perforator is being dissected off from the aneurysm. So, it is quite stuck to the aneurysm if you clip the aneurysm without freeing this perforator you will produce either a midbrain or a thalamic infarction after freeing that since this clip is occluding a part of the P1 on the opposite side this clip is being applied above that another in clip to include the hyper plastic P1 this is the postage sabral artery this is the dominant P com and then the P2 segment of the postage sabral artery and the first clip is removed and then the portion of the neck left behind in the fenestration is clipped with another straight clip. So, you see now you have to see the opposite P1 here is the opposite P1 and this is the perforator coming from the opposite P1 this one it is free from the clip you saw the post op angiograms already. Thank you.