Bifrontal Craniotomy 16-4: Identification of landmarks. If orbital osteotomy is planned, it is important to dissect the periorbita away from the orbital bone using gentle dissection because tears in the periorbita, besides increasing the risk of orbital complications, cause the orbital fat to extrude outward, making the osteotomy much more difficult. For unilateral orbital osteotomies, the dissection should begin underneath the superior orbital rim slightly medial to the supraorbital foramen and notch and extend laterally underneath the lateral orbital rim down to the level of the frontozygomatic suture.
The dissection should continue as far posteriorly into the orbit as is possible. If the supraorbital nerve is restrained by a bony foramen, this can be freed using an oblique cut with an osteotome to convert the foramen to a notch. The scalp dissection should continue down until the nasofrontal suture is visualized because the detaching cut of a bifrontal orbital osteotomy would run slightly superior to this suture.
Bifrontal Craniotomy 16-5: Unilateral or bifrontal craniotomy. Ideally, two burr holes—one placed at the McCarty keyhole and one placed directly posterior to this under the temporalis—should be enough to turn a frontal bone flap on one side. The bone flaps should extend laterally a few centimeters below the temporalis muscle cuff and as far anteriorly as allowed by the footplate.
Bifrontal Craniotomy 16-6: Orbital osteotomy (if needed). The cuts needed to remove the orbit in these approaches are more straightforward than the cuts needed for the orbitozygomatic approach. After dissecting the dura away from the orbital roof with a No. 1 Penfield, the orbit is entered first laterally with a reciprocating saw, and the lateral orbital rim is cut just above the frontozygomatic suture extending posteriorly into the burr hole at the keyhole. The roof is cut from lateral to medial in a plane just anterior to the crista galli. For the unilateral osteotomy, the cut is directed anteriorl1y slightly medial to the supraorbital foramen and proceeds anteriorly through the anterior face of the superior orbital rim, which detaches the orbital piece. In the bifrontal approach, the lateral orbital rim is cut bilaterally, and the orbital rim is cut from keyhole to keyhole running just anterior to the crista galli. Finally, the supraorbital bar is disconnected with a horizontal cut across the nasofrontal process just above the nasofrontal suture.
Bifrontal Craniotomy 16-7: Dural incision. After the bone work is done, the dura is opened with each side opened horizontally and as close to the frontal floor as possible (steps 1 and 2). The cut on each side should stop short of the superior sagittal sinus anteriorly. For a unilateral approach, this is all the dural opening required. Bifrontal approaches require that the sagittal sinus and falx be divided anteriorly. After a horizontal incision on both sides of the sinus, the frontal lobes are gently retracted away from the falx (steps 3 and 4) so that two 2-0 silk sutures can be passed through the falx below the sinus (steps 5 and 6). These sutures should ideally be placed as far anteriorly as possible. After tying these sutures over the top of the sinus, the sinus and falx are divided (step 7), and the basal dura is put under mild tension so that it lays as flat as possible and does not obstruct vision.
Tips from the masters
The eyes are protected against the preparation solution by either suture tarsorrhaphy or Tegaderm dressings and ophthalmic ointment.
Care should be taken to avoid severing the supraorbital nerve anteriorly as it exits the orbit and to avoid going too laterally over the frontozygomatic process until the supratemporal fat pad has been dissected over the frontozygomatic process to protect the facial nerve.
It is important with unilateral orbital osteotomies that the periorbita not be stripped from the medial portions of the orbital roof because it is unnecessary and risks detaching the trochlear pulley of the superior 1oblique muscle, causing diplopia.
Bilateral approaches differ in that the periorbita should be stripped from the orbital roof medially and laterally. In our experience, patients do not notice diplopia from bilateral superior oblique trochlear detachment.
Removal of the orbital roof or lateral orbital wall is unnecessary (unless the orbital contents need to be exposed), in contrast to in the orbitozygomatic approach. The principal purpose of this osteotomy is to facilitate a low and flat trajectory toward the back of the anterior cranial fossa and less to remove visual obstructions around the sphenoid wing, as with lateral approaches. By avoiding excessive orbital bone removal, the risk of pulsatile enophthalmos is decreased.
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