Bone work and craniotomy during a retromastoid corridor is very much tailored to the pathology at hand. I'm going to review the basic principles for compression of a retromastoid craniotomy. You can see the curvilinear incision has been used. This is a left-sided approach. You can see the mastoid groove and the mastoid tip here. The initial burr hole is placed where two lines cross. The first line is from posterior to the zygoma to the inion. And the second line is along the mastoid groove. This first line is also the same line that defines the superior nuchal line. At the junction of these two lines, a burr hole has been placed. You can see fish hooks have been used to extend our boney exposure more laterally and anteriorly. The initial burr hole is placed at the edge of the dura is exposed. We prefer to expose mainly first the dura and then move our boney opening over the transverse-sigmoid junction. You can see the tip of the drill is used. The side cutting piece of the drill is avoided in order to avoid any injury to the dura or sinuses. You can see the edge of the transverse-sigmoid junction. The first part of the burr hole exposed mainly the dura in this region. And then we carefully thin the bone or eggshell the bone more laterally and anteriorly until the sinus was carefully exposed. A number three Penfield is used to strip the dura away, specially over the transverse sinus and sigmoid sinus. Sigmoid sinus can be very much embedded in the bone as you can see here, and the craniotomy should avoid being placed right over the sinus specially with the use of a foot plate. The first osteotomy or boney cut for the craniotomy is complete just along the inferior edge of the transverse sinus. The drill's perpendicular to the surface of the skull. We continue medially and then inferiorly and then anteriorly and stop right along the posterior-medial edge of the sigmoid sinus. Again, the foot plate is not used directly over the sigmoid sinus. After this osteotomy or boney cut is completed, a number one bit without a foot plate may be used to complete the boney cut over the sigmoid sinus or a foot plate may be used just posterior to the sigmoid sinus over the dura. The drill bit is packed up. Here is the second osteotomy just over the dura, just posterior to the sigmoid sinus. Mastoid or cells may be exposed during this maneuver. Bone over the trans-sigmoid sinus may be used using Kerrison rongeurs. You can see the emissary vein at the tip of my suction. This should not be mistaken as the sigmoid sinus, or the bleeding from the emissary vein should not be mistaken as bleeding from sigmoid sinus. However, this emissary vein has to be carefully handled, as its aggressive manipulation may lead to injury to the sigmoid sinus. Again, the bone over the sigmoid sinus is first shelled out, and they're removed using Kerrison rongeurs while the mouth of the Kerrison is pointed superiorly to avoid catching the wall of the dural sinus. I also bevel out or bevel away the edge of the bone here. So after the dura is open along the dural sinuses, the sigmoid sinus can be mobilized anteriorly and laterally, and this maneuver will improve the working zone of the surgeon toward the CP angle. The emissary vein may be coagulated.
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