During brain surgery, surgeons cut the skull using a two-step process: first drilling small holes through the bone, then connecting those holes with a specialized saw that has a built-in guard to protect the brain underneath. The entire technique is called a craniotomy, and it’s far more precise and controlled than most people imagine. Here’s how it works from start to finish.
Planning the Cut Before Surgery
The location, shape, and size of the skull opening are mapped out well before the surgeon picks up a drill. Preoperative CT and MRI scans are fed into a neuronavigation system that builds a three-dimensional model of the patient’s brain. This model shows exactly where a tumor, blood vessel, or other target sits relative to the skull’s surface. The surgeon uses this 3D map to outline the planned craniotomy, sometimes drawing directly on the reconstructed image of the patient’s head, with the opening traced in one color and the target highlighted in another.
During surgery, the neuronavigation system tracks instruments in real time and displays their position on a monitor overlaid on the patient’s brain anatomy. Think of it as GPS for the skull. This means the bone opening can be kept as small as possible while still giving the surgeon a clear path to the area that needs treatment.
Scalp Incision and Skin Folding
With the patient under general anesthesia and the head secured in a rigid frame, the scalp is cleaned with antiseptic. The surgeon then cuts through the skin with a scalpel, typically placing the incision behind the hairline to minimize visible scarring. In many cases, a hair-sparing technique is used that only shaves a quarter-inch strip along the planned incision line, though sometimes a larger area is shaved.
Once the skin is cut, it’s peeled back and folded to expose the bare bone underneath. Small clips along the edges of the scalp flap control bleeding, since the scalp has a rich blood supply.
Drilling Burr Holes
The first tool to touch the skull is a cranial perforator, a specialized drill bit attached to a powered handpiece. The perforator creates one or more small round holes called burr holes. What makes this drill safe is its automatic release mechanism: it only spins while the surgeon presses it against bone, and it stops the moment it breaks through to the other side. This prevents the bit from plunging into the soft tissue below.
The drill bit itself has a carefully angled cutting lip that shaves away bone and spiral grooves (called flutes) that channel bone dust out of the hole, similar to how a wood drill bit ejects shavings. Depending on the size of the planned opening, the surgeon may drill two, three, or more burr holes spaced around the perimeter.
Connecting the Holes With a Saw
This is the step that actually creates the opening. A craniotome, a thin saw blade with a protective footplate on its underside, is inserted into one of the burr holes. The footplate slides between the inner surface of the skull and the tough membrane covering the brain (the dura mater), acting as a physical barrier that keeps the blade from touching anything it shouldn’t. As the surgeon guides the craniotome along the skull, the blade cuts through bone from above while the footplate glides safely below.
The surgeon connects each burr hole to the next, essentially tracing a line between the dots until the cuts form a complete shape. The freed piece of bone, called the bone flap, is then gently lifted away and set aside in a sterile solution. What’s revealed underneath is the dura mater, a thick, leathery membrane that the surgeon carefully opens to access the brain itself.
How Keyhole Approaches Differ
Not every brain surgery requires a large opening. Keyhole craniotomies use a single small hole, sometimes as small as a coin, placed through a natural crease or fold to hide the scar. Several variations exist depending on where the surgeon needs to reach:
- Supraorbital: A small cut in or just above the eyebrow reaches the front of the brain.
- Retrosigmoid: A small opening behind the ear accesses the brainstem area at the back of the brain.
- Transorbital: A cut inside the eyelid or around the eye reaches frontal structures.
- Suboccipital: An opening at the base of the skull targets the back of the brain.
- Endoscopic endonasal: This approach goes through the nose to reach the skull base, often for pituitary tumors. No skull bone is removed at all.
In each case, the same drill-then-saw sequence applies, just on a smaller scale. The drill is designed to remove a small disc of bone precisely without damaging surrounding bone.
Burr Holes Alone
Sometimes the burr holes are the entire procedure. For conditions like chronic bleeding between the brain and skull (chronic subdural hematoma), surgeons may place one or two burr holes, wash out the collected blood, and leave a small drain. Most patients in studies of this technique received two burr holes, one near the front of the skull and one toward the top. A full craniotomy with a bone flap of 5 to 7 centimeters is reserved for cases where the surgeon needs wider access to break up clotted blood or open membranes that have formed.
Putting the Skull Back Together
After the brain surgery is complete, the bone flap is placed back into its original position like a puzzle piece. Surgeons secure it using small titanium plates and screws. Pinwheel-shaped titanium mini plates are commonly placed over the burr holes for a smooth cosmetic result, and additional plates bridge the edges of the flap to the surrounding skull. Four screws per plate is typically enough for stable fixation, though using all available screw holes provides extra protection against the plate bending over time.
In some situations, the bone flap is intentionally left out, a procedure called a craniectomy. This is done when the brain is swollen and needs room to expand. The flap is stored (often frozen) and replaced in a later surgery once swelling resolves.
How the Skull Heals
The bone flap gradually fuses back to the surrounding skull over the following weeks and months. Full recovery from a craniotomy takes about six to eight weeks on average, though it can stretch to two months. During this time, the scalp incision heals, swelling subsides, and the bone edges begin to knit together.
The bone flap doesn’t always heal perfectly. In about 11% of cases, the replaced bone gradually thins and is reabsorbed by the body, a process called bone flap resorption. When this happens, the patient may need a second surgery to replace the weakened bone with a synthetic implant made from materials like titanium mesh or a custom plastic composite. Infection is another risk, occurring in roughly 5 to 7% of cases and sometimes requiring implant removal and a course of antibiotics before a new plate can be placed.