The precise spatial reposition of the brain to millimeter accuracy is necessary for implementing certain therapeutic techniques, for example surgery in stereotaxic conditions, or repeated irradiation for tumour-related illnesses or vascular malformations (angiomas).
Positioning of a reference frame, which may or may not be metal, fixed to the neurocranium allows investigations to be performed in a topographically superposable manner: arteriography, ventriculography, body scanning, MRI, and performing treatments, which constitutes the principle of stereotaxy. But this assumes that the frame, and therefore the fastening of the cranial anchoring, is repositioned each time.
Two technical methods are possible for this purpose:
either a position-finding process which is redone each time, using three-dimensional coordinate computation. This computation is currently relatively easy but the refitting does not always have millimeter precision, taking into account that the needles of the frame are difficult to reposition in the holes of the bone. PA1 Or precise repositioning, to millimeter accuracy, of the frame in the osseous structure of the neurocranium using percutaneous bone screws.
This has obvious resulting drawbacks from the point of view of the patient: his neurocranium must actually be subjected to piercings, incisions of the scalp and local anaesthetics repeated on each refitting, giving rise to infection risks and painful discomfort. For the surgeon, repeating these fitting operations requires considerable time trying to find the position accurately.
The latter technique, undoubtedly the most accurate, demands iterative refitting of the frame on the neurocranium, on percutaneous screws (that is to say, screws whose head projects out of the skin of the patient) which are anchored in the osseous structure, for several days on end or possibly several days per week for several weeks, during sequential irradiations.