A craniotomy is a surgical operation in which part of the skull, called a bone flap, is removed in order to access the brain. Craniotomies are often a critical operation performed on patients suffering from brain lesions or traumatic brain injury (TBI), and can also allow doctors to surgically implant deep brain stimulators for the treatment of Parkinson's disease, epilepsy and cerebellar tremor. The procedure is also widely used in neuroscience for extracellular recording, brain imaging, and for neurological manipulations such as electrical stimulation and chemical titration.

Human craniotomy is usually performed under general anesthesia but can be also done with the patient awake using a local anaesthetic; the procedure generally does not involve significant discomfort for the patient. In general, a craniotomy will be preceded by an MRI scan which provides a picture of the brain that the surgeon uses to plan the precise location for bone removal and the appropriate angle of access to the relevant brain areas. The amount of skull that needs to be removed depends to a large extent on the type of surgery being performed. Most small holes can heal with no difficulty. When larger parts of the skull must be removed, surgeons will usually try to retain the bone flap and replace it immediately after surgery. It is held in place temporarily with metal plates and rather quickly reintegrates with the intact part of the skull, at which point the metal plates are removed.

Craniotomy is distinguished from craniectomy, in which the skull flap is not replaced, and from trepanation, which is performed voluntarily without medical necessity.

From http://en.wikipedia.org/wiki/Craniotomy