Figure 1: Harvey Cushing illustrated an osteoplastic craniotomy and cortical mapping in a patient who suffered from a gunshot wound (illustration courtesy of the Cushing Brain Tumor Registry at Yale University).

Figure 2: The skull bone contains the outer and inner cortical bone layers (tables) separated by cancellous bone. This anatomic relationship is important during creation of the burr hole because the operator can safely drill expeditiously through the outer table and cancellous bone, but should gently shell out the inner cortical layer to avoid direct transmission of the drill vibrations to the dura and cortex.

Figure 3: Placement of a hole along the root of the zygoma is shown here. After the outer table and cancellous bone (yellow arrow) have been drilled under ample irrigation, the inner cortical layer (blue arrow, right upper image) is carefully thinned out and a small area of the dura exposed (left middle image). Next, a Kerrison rongeur or curette removes additional pieces of the thin shell of inner cortical bone (right middle image). This tactic expands the inner rim of the burr hole beyond the edges of the outer rim (undercutting the inner table) to allow the tip of the dural dissector to achieve acute working angles and stay tangential to the inner table, avoiding an inadvertent dural tear (left lower image). These maneuvers allow the dural dissector to travel well beyond the burr hole to effectively separate the entire section of the dura undergoing a craniotomy (right lower image). These methodologies expand the working zone of the dissector and most often facilitate elevation of large bone flaps using only one or two burr holes.

Figure 4: The burr hole for a standard pterional craniotomy is placed at the junction of the frontotemporal line and the frontal process of the zygomatic bone. Because of the difference in the thickness of the skull at the frontal and temporal bones (the frontal bone is thicker), the tip of the acorn-shaped bit can reach the dura inside a portion of the burr hole before the entire base of the burr hole is removed. This phenomenon can lead to cortical injury, especially in patients with a “tight” brain. Careful use of the drill will minimize the risk of this complication. This is one of the reasons why I prefer to use a single burr hole just inferior to the superior temporal line for pterional craniotomies. Another reason is the need to minimize bone resection in cosmetically visible regions (such as the area in front of the hair line, such as the keyhole, that is not covered by the temporalis muscle).

Figure 5: I use only one burr hole for creation of a bifrontal craniotomy to minimize cosmetic deformity as long as the dura is easily separable. A B1 drill bit without a footplate may be used to create the most inferior osteotomy parallel to the skull base. This maneuver will allow controlled transection of the anterior and posterior frontal sinus walls. The use of the footplate in this region can be problematic because the skull is very thick near the air sinuses, and forceful maneuvering of the drill can lead to dural and cortical injuries.

Figure 6: The fear of a dural venous injury should not deter the operator from generously uncovering these structures to allow expansion of the supracerebellar operative corridor through mobilization of the dural sinuses superiorly. Placement of burr holes over the transverse sinuses is relatively safe, and correct use of the footplate tip pursuing the contours of the inner table leads to a single-piece bone flap exposing the dura and the roof of the venous sinuses.

Figure 7: The uneven skull thickness (for example, within the key hole or around the venous sinuses) can be a significant cause of cortical, dural and venous sinus injury because the cutting burrs along the equator of the acorn bit can reach the dura covered by the thinner temporal section of the skull or the wall of the sinus while the tip of the drill is still working on the adjacent thicker frontal or occipital bone within the burr hole, respectively.

This event is especially problematic when the surgeon is making burr holes over the dural sinuses for parasagittal and retromastoid craniotomies. Because the walls of the dural sinuses embed themselves within the inner table, the cutting burrs on the side of the drill bit can injure the sinus wall while the drill tip works on the thicker bone covering the sidewalls of the sinus.

Figure 8: Increased intracranial pressure is a real risk for subdural hematoma, subarachnoid hemorrhage, and cortical injury during drilling and consequent transmission of vibrations to the part of the brain pushed against the skull. The drill should be used carefully and the footplate maneuvered smoothly to minimize direct transmission of the unnecessary excessive vibrations to the brain. The use of lumbar CSF drainage can dramatically decrease the risk.

Figure 9: For the footplate to remain tangential to the contours of the skull, the drill handle should be perpendicular to the skull surface (top image). If the handle is not perpendicular (lower image), the drill will not advance smoothly and the operator’s struggle to push the drill forward will lead to dural tear and risk transmission of vibrations to the brain.

Figure 10: The footplate should stay tangential to the inner surface of the skull to avoid inadvertent dural tears, especially around the dural venous sinuses (inset image). This nuance is often ignored by junior residents. The dural sinuses can embed themselves within the inner table of the skull.

Figure 11: For parasagittal craniotomies, the last bony cut is performed over the dural sinus so that the bone flap can be quickly elevated if sinus injury occurs.