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A craniotomy is the surgical removal of a section of bone (bone flap) from the skull for the purpose of operating on the underlying tissues, usually the brain. The bone flap is replaced at the end of the procedure. If the bone flap is not replaced, the procedure is called a craniectomy. A craniotomy is used for many different procedures within the head, for trauma, tumor, infection, aneurysm, etc.

Anatomy

• At birth the bones that make up the cranium or skull are separated, allowing the head to pass through the birth canal. (Figure 1)
  
• As the individual matures, the bones fuse together so that by late teens the bones form a solid union
  
• The various bones of the skull are the frontal, parietal, temporal, occipital, and sphenoid, (Figure 2)
  
• The scalp covers the skull
  
• Within the skull lie:
  
  1. The brain, which is divided into four major parts- the right and left cerebral hemispheres, the cerebellum and the brainstem (Figure 3).
     
• The cerebral hemispheres form the largest portion of the brain and can be regarded as the 'thinking' part of the brain and are involved in movement, sensations, speech and creation of ideas
  
• Each cerebral hemisphere is divided into four lobes - frontal, parietal, temporal and occipital
  
• The surface of the hemispheres is folded upon itself and presents as various grooves (sulci) and bulges (gyri). The two cerebral hemispheres are connected across the midline by a large band of brain fibers called the corpus callosum that transmit nerve impulses between the hemispheres
  
• The cerebellum lies at the back of the brain under the occipital bone and is involved in fine tuning movement
  
• The brainstem lies in front of the cerebellum and is attached above to the cerebral hemispheres, behind to the cerebellum and below to the spinal cord
  
2. The meninges (the membranes that line the inside of the skull (dura) and cover the brain (pia-arachnoid). A large fold of dura called the falx lies above the corpus callosum and separates the cerebral hemispheres. (Figure 4) Another large fold of dura, the tentorium, separates the cerebral hemispheres from the cerebellum. The brainstem passes through a hole in the front of the tentorium. The space that lies beneath the tentorium, which contains the cerebellum and brainstem, is called the posterior fossa
   
3. The blood vessels that feed the brain
   
4. The cerebrospinal fluid (the fluid that bathes the brain) originates within the ventricles (spaces) within the brain
• Additional anatomy information

Figure 1 - Infant skull as seen from the side. The bones of the skull are labeled. © T. Graves	Figure 2a - The adult skull as seen from the front. © T. Graves
Figure 2b - The adult skull as seen from the side. © T. Graves	Figure 2c - The adult skull as seen from the back. © T. Graves
Figure 3 - Brain seen from the side. The four lobes of the brain are indicated as well as the cerebellum and the midline brainstem. © T. Graves	Figure 4 - The falx separates the cerebral hemispheres while the tentorium separates the cerebral hemispheres from the cerebellum. Note that on the left side the tentorium can be partially seen through. Note also the opening in the tentorium for the brainstem. © T. Graves

Pathology

There are a variety of lesions (abnormalities) that affect the skull and its contents that require a craniotomy or craniectomy for exposure or removal.

• Skull. The most common lesions of the skull are benign tumors.
  
• Brain. The most common lesions of the brain that require craniotomies are
  
  1. Tumors of the supporting cells of the brain called gliomas
     
  2. Cancers from other organs that have gone (metastasized) to the brain (Figure 5)
     
  3. Abscess (localized infection)
     
• Meninges. These tissues give rise to mostly benign tumors called meningiomas that may grow quite large and compress the brain causing damage (Figure 6)

Figure 5 - Two metastatic tumors to the brain removed using frameless stereotaxic image guided surgery through small craniotomies (4 by 4 cm). Cerebrospinal fluid fills the cavities left by removal of the tumors. The small craniotomies directly over the tumors allowed the patient to be discharged the day after surgery.	Figure 6 - Large benign tumor (meningioma) before and after removal. The cavity formed by removal of the tumor is filled with cerebrospinal fluid.

• Blood vessels. The vessels at the base of the brain may give rise to aneurysms (weak areas that form like a blister on the vessel) that may rupture and cause bleeding around the brain (subarachnoid hemorrhage). (Figure 7 and click on Figure 8 for a video) An abnormal tangle of arteries and veins called an arteriovenous malformation (Figure 9) may bleed causing a clot within the brain (intracerebral hematoma). High blood pressure may rupture of a blood vessel in the brain and cause an intracerebral hematoma
  
• Fluid that bathes the brain (cerebrospinal fluid). Obstruction to the flow of cerebrospinal fluid produces hydrocephalus that on occasion requires craniotomy.

	Click Below to view Animation
Figure 7 - Angiogram of the vessels at the back of the brain showing an aneurysm at the junction of the vertebral artery and posterior inferior cerebellar artery (PICA). Inset shows the position of the temporary (dotted line) and permanent (solid line) clips shown in the video of Figure 8.	Figure 8 - Frame from a video of an operation for clipping of the posterior inferior cerebellar artery (PICA) aneurysm seen in figure 7. Note the use of a temporary clip across both the aneurysm neck and PICA. The temporary clip is used to protect the aneurysm from rupture while the aneurysm is manipulated for placement of the permanent clip that clips the aneurysm and leaves the PICA open.
Figure 9 - An arteriovenous malformation. The arterial blood flows directly into veins.

Procedure

• The craniotomy is labeled by which part of the skull is opened. A frontal craniotomy indicates the opening is in the frontal bone while a parietal craniotomy involves opening the parietal bone. If part of two adjacent bones is opened, then both bones are mentioned, for example, fronto-temporal craniotomy (Figure 10)
  
  1. In the temporal areas, which are covered by muscle, the neurosurgeon may carry out a craniectomy in which the bone is not replaced
     
  2. Surgery on the back part of the brain beneath the tentorium is usually carried out by removal of the lower part of the occipital bone (Figure 11). This is called a suboccipital craniectomy. The craniectomy may be in the midline or to one side or the other. When the bone removal is more to the side and just behind the mastoid bone it may be called a retromastoid craniectomy. Occasionally an abnormality is situated in the low brainstem or cerebellum and may extend to the upper spinal cord. In these instances a cervical laminectomy may also accompany the suboccipital craniectomy

Figure 10 - Outline of a fronto-temporal craniotomy. The small circles indicate bur holes. The darker blue area indicates where bone is removed and not replaced. The dark blue line indicates where the bone is cut. The light blue area is replaced after the surgery. © T. Graves	Figure 11 - Outline of a midline suboccipital craniectomy. Note the bur holes that are used to start the bone removal. The blue area indicates the bone removed. © T. Graves

• The incision in the scalp is designed to expose the skull over the lesion to be removed
  
• Removal of the bone flap is done in the following manner:
  
1. A series of small holes (bur holes) are made in the skull. The holes are positioned around the periphery of the proposed bone flap. Making the holes may be accomplished in one of three ways at the discretion of the surgeon
   
   • The oldest method, which is still used by many surgeons, involves a set of three drill bits and a hand drill. The first bit has a point and is used to just penetrate the bone. The second and third bits, which have more of a curvature, widen the hole without cutting the underlying dura, which lines the inner surface of the skull
     
   • Another method is by using a special air powered drill. The drill bit is made so that as soon as the center of the drill bit penetrates the bone, the drill stops
     
   • The last method uses an air driven burr to gradually remove bone until the dura is seen. This method allows the smallest holes, and the holes can also be tailored in shape. (Figure 12)
     
2. The skull is cut between each two adjacent burr holes in a progressive manner until the bone flap is separated from the surrounding skull. This is accomplished in one of two ways
   
   • The oldest method involves the passage of a thin metal strip (saw guide) between two adjacent holes. The strip is placed between the skull and the dura. A small hook on saw guide allows a wire saw (Gigli saw) to be drawn under the skull in the same path as the guide (Figure 13). The saw driven by hand then cuts the bone from inside out (Figure 14)
     
   • The air driven craniotome has for the most part replaced the manual method. The craniotome resembles an air drill with a protective footplate (Figure 15). Cuts are then made with the craniotome from hole to hole until the bone flap is free

Figure 12 - Photograph of an air drill making a bur hole.	Figure 13 - Wire Gigli saw for cutting bone.
Figure 14 - Operative photograph showing the Gigli saw being used.	Figure 15 - Air craniotome being used in surgery.

3. After the bone flap is removed (Figure 16), the underlying dura is cut to expose the lesion. The dura is then cut within the margins of the skull opening. If the lesion is a meningioma that is attached to the dura, the dura is cut around the tumor leaving a margin of normal dura. When there is a loss of dura, various substitutes can be used such as bovine pericardium (covering of the heart), banked human dura, Gortex plastic or an absorbable collagen matrix
   
4. What occurs next depends on the specific lesion that is found. When the surgery is for a malignant brain tumor, the surgeon may wish to line the cavity left by removal of the tumor with an absorbable wafer impregnated with an anticancer drug. This has been shown to extend life by two to four months. (Figure 17)

Figure 16 - The cut bone is elevated.	Figure 17 - MRI obtained after partial removal of a malignant brain tumor (glioblastoma multiforme) in which the tumor was treated with absorbable wafers (arrows) impregnated with an anticancer drug. Courtesy A. Sloan, M.D.

5. Following removal of the lesion, all bleeding is secured, the dura is sutured closed and the bone flap restored to the skull with wire sutures or titanium miniplates and screws. Burr holes in cosmetically exposed areas are covered with small titanium plates. If the bone cannot be replaced (infected or invaded by tumor) a prosthesis can be used. These are usually made of titanium mesh or plastic. (Figure 18)
   
6. The scalp is then sutured closed

Figure 18 - Titanium mesh cranioplasty used for replacement of an infected bone flap.

• There are several instruments that have improved the ease and accuracy of a craniotomy:
  
  1. Operating Microscope. The human hand can make very small and accurate movements as long as the eye can see it. The magnification provided by the operating microscope has added another dimension to operating. The magnification varies between 4 and 16x. This allows magnification of small brain structures particularly the blood vessels and nerves at the base of the brain. The microscope has markedly improved the surgery of aneurysms of the brain arteries and tumors at the brain base
     
2. Ultrasonic Aspirator. The ultrasonic aspirator is used to remove tumors from the brain with a minimum of brain movement. The small tip of the instrument vibrates back and forth at thousands of times per second, thus liquefying the tumor tissue and allowing it to be easily sucked away with a minimum of injury to the surrounding brain
   
3. Intraoperative Doppler Ultrasound. The intraoperative ultrasound is used for localizing a lesion below the surface of the brain. It is similar to the ultrasound used by an obstetrician to image a fetus in the womb. Sound waves are sent out from the instrument (transducer) that strike the target lesion and bounce back to the recording portion of the transducer. A picture is thus produced which can guide the surgeon to the lesion
   
4. Stereotaxic Image Guided Craniotomy. (Figure 19, a-c) In the last few years, a significant improvement in brain surgery is made possible by the marriage of modern imaging studies (CT and MRI) and computer graphics. This frameless stereotaxic (three dimensional) image guided surgery is a major advance in the removal of lesions inside the skull, particularly small lesions and lesions beneath the surface of the brain. It has only slightly affected large lesions, diffuse brain lesions and surgery for ruptured cerebral aneurysm
   
   • Prior to surgery, small markers (feducials) that show up on CT or in the MRI are applied to the head of the patient. The patient is then placed in the CT or MRI unit and a series of images are obtained. The electronic data that are the source of the images are transferred to a computer in the operating room. This computer reconstructs the CT or MRI images and produces a three dimensional picture of the head containing the lesion as well as a reconstruction of the head and lesion in three planes
     
   • After the patient is anesthetized, the head is pinned in a head holder to rigidly hold it in place. The feducials are registered on the CT or MRI are matched to the corresponding feducials on the patient's head. The latter is accomplished with a pointer containing an array of light emitting diodes. A receiver positioned near the operating table registers the position of the diodes and thus the position of the head feducials. This information is transferred directly to the computer. The pointer or any other instrument containing the diode array can then be used to direct the surgeon to the lesion with no more than a 1-2 mm. error
     
   • Using this technique, the surgical trauma to the brain is reduced and the size of the craniotomy is minimized. This is translated into a faster and better recovery with discharge from hospital frequently occurring in 24 hours

Figure 19 MRI of left frontal metastatic brain tumor (arrow). Note: MRI images show the left side to the viewer's right Computer screen as seen by the neurosurgeon during image guided surgery. Note the images have been flipped from side to side so that the surgeon has a left sided image to his own left side. The arrows point to a yellow line that represents the direction of 'attack' chosen by the surgeon. The red 'cross hairs' is the position of the instrument being used by the surgeon. The right lower image shows the skin surface of the patient with multiple donut shaped feducials on the surface. The red asterisk lies on the tumor imaged in blue Post-operative MRI showing complete removal of the tumor

Complications

Complications following craniotomy are primarily related to involvement of the brain and its coverings. Some of the complications are:

• Complications of anesthesia
  
• Infection
  
• Hemorrhage and/or post-operative hematoma
  
• Leak of cerebrospinal fluid
  
• Brain swelling
  
• Raised intracranial pressure (pressure inside the head)
  
• Paralysis
  
• Hydrocephalus
  
• Loss of sensation
  
• Loss of vision
  
• Loss of speech
  
• Memory loss

Recovery

• Following surgery the patient is usually admitted to the intensive care unit
  
• Level of consciousness is carefully observed for any change
  
• Blood pressure is carefully monitored along with the pulse. A catheter inserted in an artery may be used to continuously monitor the blood pressure
  
• Intracranial pressure may be monitored through a small catheter placed within the head and connected to a pressure gauge
  
• Blood may be drawn to determine to determine the level of red blood cells, and to determine the concentration of sodium and potassium
  
• In some cases, a tube may be left in the windpipe to control respiration.
  
• Antibiotics are usually given to prevent infection
  
• Medication is frequently given to suppress the possibility of seizures
  
• If there are no serious problems, the patient may be discharged the following day, however, hospitalization may be considerably longer depending on the lesion and the difficulty of the procedure
  
• If there are problems such as weakness, loss of speech, hospitalization may be delayed
  
• Transfer to a rehabilitation unit may be necessary

Further care

• The patient returns to the surgeon's office 7-10 days following discharge. At this time sutures or staples may have to be removed. Continued care depends on the lesion. Prolonged follow up is usually required for infection and tumor
  
• Infections. A craniotomy for an infection is usually for a brain abscess. Frequently the patient must be kept on specific antibiotics for the infectious agent causing the abscess. On occasion antibiotics may be necessary for several months.
  
• Brain tumors. The after care for a brain tumor differs depending on whether it is benign or malignant
  
  1. Patient with benign tumors usually have to be followed for several years to be sure there is no recurrence. If there is recurrence, the alternatives are usually repeat surgery or radiation therapy
     
  2. Malignant tumors of the brain usually have a gloomy outlook. Additional therapies include
     
     • Radiation therapy is usually given following removal of both metastatic tumors and tumors that originate in the brain such as a glioblastoma multiforme. Survival following surgery doubles if radiation therapy is given
       
     • Chemotherapy has been used for glioblastoma but often helps only slightly and frequently has unwanted side-effects
       
     • Immunotherapy involves stimulating the patient's own immune system to fight the tumor. The patient's tumor (glioblastoma multiforme) taken at the time of surgery is used to make a vaccine (like the polio vaccine). The vaccine is given to the patient, which stimulates blood cells to create lymphocytes that will find and attack the tumor. Early trials have shown that immunotherapy improves survival in some patients with minimal side-effects