(from Greek, stereos: solid, tassein: to arrange) The term stereotaxic (also spelt stereotactic) refers to the location of an object within a 3-dimensional space. In the neurosciences, stereotaxic surgery involves the placement within the brain of devices of various sorts, with 0.01 mm precision in each dimension. It is not species-specific: stereotaxic surgery can be performed experimentally on animals or therapeutically on humans. In the following text, a standard stereotaxic surgical operation on an experimental animal is described, this serving to illustrate the basic principles involved. The operation involves several stages.
All experimental stereotaxic surgery is carried out on anaesthetized animals. (1) The first step in the operation therefore is the induction of general ANAESTHESIA, using an anaesthetic agent (usually combined with an analgesic; see ANALGESIA) that will maintain anaesthesia throughout the surgical procedure. Once this has been achieved the animal’s head is shaved on the top surface, where a scalp incision will be made. As with human surgery, hair is cleared away from the site of the operation to maintain cleanliness. Prior to making an incision in the scalp it will be swabbed with an appropriate cleansing agent. (2) The animal is placed in a STEREOTAXIC FRAME. This is a device that has two functions: it holds the subject’s head in a fixed position, and it permits manipulation of one or more arms (also called [micro-]manipulators or [micro-] positioners) in three dimensions: ANTERIOR-POSTERIOR, MEDIAL-LATERAL and DORSAL-VENTRAL. Normally any arm is at right angles to the stereotaxic frame in each dimension, An animals undergoing stereotaxic surgery has its head held by two devices: EAR BARS and INCISOR BARS. The ear bars should always be appropriate for the species being used, and be ATRAUMATIC: that is, they should penetrate the AUDITORY CANAL only to the degree necessary for supporting the animal’s head. No damage to the auditory apparatus should be produced. The incisor bar (or TOOTH BAR) is a bar over which the front teeth are placed. These two devices do not produce any TRAUMA, but do hold the animal’s head in a fixed position. For human stereotaxic surgery, ear and incisor bars are not appropriate. Nevertheless, the same principles operate: the head is held in a fixed position using clamps. While on the stereotaxic frame, animals body temperature can be maintained, if necessary, using a HOMEOTHERMIC BLANKET. (3) Once the animal has been fitted in the stereotaxic frame, and its scalp cleaned, an incision along the midline is made; the scalp is reflected (folded back) and the membranes (the fascia) between the scalp and skull exposed. Cutting directly through these invariably produces bone bleeding, which is not helpful, but blunt dissection (literally, scraping the fascia away) avoids bone bleeding and has the benefit of producing a ‘raggy wound’: these heal more quickly than clean cuts. Once the incision is made and the fascia removed, the surface of the SKULL is exposed and cleaned.
If bone bleeding is produced, it can be controlled using BONE WAX (a rather old-fashioned approach), cauterization (a rather dramatic approach) or, for preference, by the application of a surgical gelatin sponge which absorbs blood and provides a gelatin-based matrix to facilitate blood clotting. (4) The manipulators on the stereotaxic frame can be moved to an appropriate position in 3-dimensional space within the animal’s brain: a STEREOTAXIC ATLAS provides coordinates for brain structures. These coordinates are expressed in terms of distance in three dimensions from a zero reference point. Typically these reference points are: (a) anterior-posterior: either BREGMA (a skull landmark) which is best suited for structures relatively anterior in the brain; or EAR BAR ZERO, more properly referred to as the INTERAURAL LINE (a hypothetical line that runs, literally right between the ears: it is measured from the position of the ear bars before the animal is placed in the frame). This is best suited for locating structures relatively anterior in the brain. (b) Medial-lateral: the midline SUTURE along the skull is typically used. (c) Dorsal-ventral: either skull surface or the surface on the midline at the anterior-posterior level at which surgery will be conducted; or the DURA MATER (see MENINGES) (a reading of the depth of this can be taken once the appropriate portion of skull has been removed). (Note: when rats or other small animals are being operated on their heads are typically held at one of two orientations: LEVEL SKULL, which is as it says (and is determined by ensuring that the dorsal-ventral measurement at two skull landmarks, bregma and LAMBDA, are identical: typically the incisor bar will be ~3.3 mm below the interaural line); or the DE GROOT ORIENTATION, in which the incisor bar is set 5 mm above the interaural line. Typically, level skull is chosen, being a more direct route to any given structure.) The manipulator will be carrying a device: an ELECTRODE for recording, a CANNULA for making infusions, a GUIDE CANNULA carrier for implanting cannulae or whatever. This device can be used to locate the chosen zero reference points—for example, bregma, the midline suture and skull surface. For each of these three points, a numerical value can be taken from vernier scales on the manipulators. These three numbers—one for each direction—represent the zero reference point. A stereotaxic atlas will provide coordinates for each structure. These can be added to or subtracted from the zero reference point coordinates. Moving the manipulator in the anterior-posterior and medial-lateral planes to these new coordinates will position the manipulator above the skull at an appropriate point for the structure of interest. (5) Having determined the point on the skull surface that lies directly above the structure of interest, a small hole is drilled, usually using a dental drill. Once this has been done, and any necessary coordinate taken from the surface of the dura mater, the dura can be cut (to permit smooth access of whatever is to be inserted in brain). The instrument attached to the manipulator—electrode, cannula or whatever—can be lowered slowly and carefully into position and the appropriate operation performed: passage of fluid for example (see MICROINJECTION), or electrical recording (see SINGLE-UNIT ELECTROPHYSIOLOGICAL RECORDING), MICRODIALYSIS, or the induction of a LESION. (6) Once this is done, the device is retracted and the wound cleaned and sealed. The hole made by the dental drill can be packed with sterile gelatin foam, or left clean for tissue to grow in and fill, and the scalp wound sealed, either with suture silk (stitches) or with specialized wound clips (Michel clips). The animal is then removed from the frame, incisor bar before ear bars to prevent accidental damage to the teeth, and allowed to recovered from the general anaesthetic (while of course being carefully monitored).
This has described in general terms, the typical approach to experimental stereotaxic surgery using animals. Clearly, specific operations might need to adapt this. For example, operations that run close to the cerebral VENTRICLES or aqueducts might require a cannula or electrode to approach the brain not at 90° but at a more acute angle, in order to avoid penetrating the ventricle. However, the basic principles remain the same in the vast majority of cases.
