Stereotactic neurosurgery

Stereotactic surgery or stereotaxy is a minimally-invasive form of surgical intervention which makes use of a three-dimensional coordinates system to locate small targets inside the body and to perform on them some action such as ablation (removal), biopsy, lesion, injection, stimulation, implantation, radiosurgery (SRS) etc. "Stereotactic" in Greek (another accepted spelling is "stereotaxic") means movement in space.

In theory, any organ system inside the body can be subjected to stereotactic surgery. Difficulties in setting up a reliable frame of reference (such as bony landmarks which bear a constant spatial relation to soft tissues), however, mean that its applications have been limited to brain surgery. Besides the brain, biopsy and surgery of the breast are done routinely to locate, sample (biopsy) and remove tissue. Plain X-ray images (mammography) and CT can be used to guide the procedure.

History
The stereotactic method was first developed by two British scientists in 1908, working at University College London Hospital, Sir Victor Horsley, a physician and neurosurgeon, and Robert H. Clarke, an engineer. The Horsley-Clarke apparatus they developed was used for animal experimentation and implemented a Cartesian (three-orthogonal axis) system. Improved designs of their original device came into use in the 1930s for animal experimentation and are still in wide use today in all animal neuroscience laboratories.

Using the Horsley-Clarke apparatus for human brains was difficult because of the high variability of spatial relations between the skull and the brain. However, by using contrasted brain radiography (particularly pneumoencephalograms and ventriculography) reference points inside the brain could be used, instead of bone landmarks. The first stereotactical devices for humans used the pineal gland as a landmark and the foramen of Monro. Later, other structures, such as the anterior and posterior commissures, became the most commonly used internal cerebral landmarks.

Using this approach between 1947 and 1949, two American neurosurgeons, Ernest A. Spiegel and Henry T. Wycis, and a Swedish neurosurgeon, Lars Leksell, developed the first stereotactic device used for brain surgery in humans. Spiegel and Wycis used the Cartesian coordinate system (also called the translational system) for their device. Leksell's device was of the polar coordinate type (also called spherical), and was far easier to use and calibrate in the operating room. The stereotactic localization system was also used by Leksell in his next invention, a device for radiosurgery of the brain. This system is also used by the Gamma Knife device, and by other neurosurgeons, using linear accelerators, Proton Beam Therapy and Neutron Capture Therapy. Lars Leksell went on to commercialize his inventions by founding Elekta.

In 1976, a system was developed to use computer assisted imaging to help make the location of the target more precise. It was developed jointly by the University of Toronto and Toronto General Hospital. The system used a pre-scanned image of the brain loaded into a Tektronix graphics terminal in the operating room. Low level voltages were applied to the brain and the co-ordinates and patient response entered by keyboard, the image on the screen showed the brain overlaid by graphics images (such as a hand) indicating the type of response.

The method continued to evolve, and at present uses a complex mixture of image-guided surgery using intraroom computed tomography, magnetic resonance imaging and stereotactic localization.

How it works
Stereotactic surgery works on the basis of three main components:


 * A stereotactic atlas of the targeted anatomical structures
 * A stereotactic device or apparatus
 * A stereotactic localization and placement procedure

The stereotactic atlas is a series of cross sections of anatomical structure (for example, a human brain), depicted in reference to a two-coordinate frame. Thus, each brain structure can be easily assigned a range of three coordinate numbers, which will be used for positioning the stereotactic device. In most atlases, the three dimensions are: latero-lateral (x), dorso-ventral (y) and rostro-caudal (z).

The stereotactic apparatus uses a set of three coordinates (x, y and z) in an orthogonal frame of reference (cartesian coordinates), or, alternatively, a polar coordinates system, also with three coordinates: angle, depth and antero-posterior location. The mechanical device has head-holding clamps and bars which puts the head in a fixed position in reference to the coordinate system (the so-called zero or origin). In small laboratory animals, these are usually bone landmarks which are known to bear a constant spatial relation to soft tissue. For example, brain atlases often use the external auditory meatus, the inferior orbital ridges, the median point of the maxilla between the incisive teeth. or the bregma (confluence of sutures of frontal and parietal bones), as such landmarks. In humans, the reference points, as described above, are intracerebral structures which are clearly discernible in a radiograph of tomogram.

Guide bars in the x, y and z directions (or alternatively, in the polar coordinate holder), fitted with high precision vernier scales allow the neurosurgeon to position the point of a probe (an electrode, a cannula, etc.) inside the brain, at the calculated coordinates for the desired structure, through a small trephined hole in the skull.

Currently, a number of manufacturers produce stereotactic devices fitted for neurosurgery in humans, as well as for animal experimentation.