January 16, 2015
This video describes general principles for retromastoid craniotomy. This opening, as described here, is mostly for medium to a larger size posterior fossa and cerebellopontine angle tumors. However, the principles are the same regarding performing a smaller retromastoid craniotomy for microvascular decompression surgery. The patient is placed in a lateral position as you can see here. A curvilinear incision is used. And very importantly, the shoulder has been mobilized forward and caudally in order to increase the working zone of the surgeon in this area. This principle is very important especially for more heavy set patients where their bulky shoulders can significantly compromise the working angles of the surgeon within their posterior fossa. You can see the landmarks that has been previously described in our other videos, the transverse sinus mastoid groove in the curvilinear incision where whose summit is at the level of the burr hole or the transverse sigmoid junction. This curvilinear incision is now being completed. Bovie electrocautery is used to cut through the deeper part of the subcutaneous space and muscle attachments. These suboccipital muscles are disconnected from the superior nuchal line, which as you can see is here, and also approximates the location of the transverse sinus. The emissary vein often is intradiploic at this level as you can see here or here, and can be a source of significant bleeding. But as the muscle is immobilized away, the opening of the vein through the bone can be sealed using a piece of bone wax. You can see the tip of the mastoid was exposed. Now the scalp is mobilized more laterally and anteriorly to be able to give us enough space for the burr hole. Here is again the tip of the mastoid and also the mastoid groove located here. So the burr hole is placed again at a line joining the inion to the posterior root of zygoma, and another line from mastoid groove. So it would be just about in this region. The initial part of the burr hole exposes the dura. And then after which, the bone over the transverse sigmoid junction is egg shelled and removed using Kerrison rongeurs. Here you can see the initial opening was over the dura, and the more posterior aspect of the transverse sigmoid junction. After which, the bone was egg shelled over the sinus, and carefully removed. It is best to not use the drill directly over the sinus if it can be avoided. The location of the transverse sigmoid junction can be quite variable, and neuro navigation may help with adequate and precise localization of these important dural sinuses. Any slight bleeding from the bone can be stopped using thrombin soaked Gelfoam. We'll go ahead and stay as much as possible over the region of the dura. You can see we tried to bite the bone parallel to the sinus rather than perpendicular to it as much as possible, as this is the safest maneuver to prevent cutting the upper wall of the sinus. The tip of the mouth of the Kerrison rongeur is pointed to superiorly, and can be used as a dissector to dissect the wall of the sinus away from the inner skull bone. Here is a good exposure of the sigmoid sinus, or at least the start of the sigmoid sinus. This is the transverse sigmoid junction. We use a periosteal elevator to dissect the bone away from the dura. And then we use a handheld drill with a footplate to stay just along the inferior aspect of the edge of the transverse sinus. We curve inferiorly and then anteriorly, and then stop right along the edge of the sigmoid sinus. The footplate is not used over the area of the dura in close proximity of the sigmoid sinus, or on the sigmoid sinus itself. The bone can be quite thick, and the mastoid air cells will never be exposed. The drill is then backed out, and the second cut over the sigmoid sinus can be performed either using a bayonet without a foot plate over the sinus wall, or we can stay just posterior to the sigmoid wall and remove the bone over the sinus after it has been shelled out. Here you can see the bayonet without a footplate is used to carefully thin the blood bone over the sinus. And then the Kerrison rongeur is used to expose the sigmoid sinus. The bone flap is subsequently elevated. Additional thick bone over the sinus is first drilled out, egg shelled and removed in pieces. Any injury to the dural sinus should be repaired. Packing with hemostatic material would most likely lead to thrombosis of the sigmoid sinus, and is not advised. You can see again the bone over the sigmoid sinus is carefully shelled out, thinned out. And again, this mastoid bone is beveled out so we can mobilize this sigmoid sinus laterally with our retention sutures after the dura has been opened, and then removing these small pieces of bone over the sigmoid sinus can be quite effective. Depending on the pathology at hand, additional bone over the transverse sinus can be removed as well. Here is the end of our bony opening. You can see all the mastoid air cells are waxed very thoroughly on the way in and on the way out after the dura is closed. You can see the bone more posteriorly is also beveled out to advance the working angles of the surgeon toward the CP angle. And here is the final product for the bony opening. You can see the sigmoid sinus is generously unroofed. You can see the transverse sinus. Now we can go ahead and open the dura along the dural sinuses, and retention sutures on these dural sinuses nicely mobilizes the sinus out of our way, so we can have a more direct trajectory toward the CP angle. The lumbar puncture at the beginning of the procedure gives us significant relaxation in the tension in the posterior fossa, and facilitates both bony work as well as bleeding from the dural sinuses. The initial dural cut is done. Then we use scissors. Leave a sleeve of dura over the dural sinuses, and just follow the contours of the venous sinuses. Again, the lumbar puncture is critical to prevent cerebellar herniation at this juncture. We use three retention sutures. One along the transverse sinus, one at the level of the transverse sigmoid junction, and also one at the level of the petrous bone or the sigmoid sinus. If the sinus is injured during dural opening, it's best to avoid using bipolar to coagulation and use a suture to close a small tear within the wall of the sinus. Here is the crucial stitch that would mobilize the sinus out of our way, and would give us just a few more millimeters to be able to see around the cerebellum. Without any retraction at this time, we can use dynamic force of the suction apparatus to go around the cerebellum and expose the CP angle. Thrombin soaked Gelfoam is used before intradural work is started to halt all the small bleeding, and provide a clear view around the cerebellum. You can see the dural tack of sutures here have really nicely moved the sinus out of our way. We use a piece of rubber dam or a piece of glove cut to the size of a long carotenoid, and slide the carotenoid over the rubber dam to reach the CP angle.
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