Frameless stereotactic neurosurgery: two steps towards the Holy Grail of surgical navigation.

The holy grail of surgical navigation is to provide precise continuous feedback during surgery about the target and its surrounding structures. The first step was the ability of hardware and software technology to allow patient-to-image registration using a multi-potentiometer position-sensing articulated arm system. We used such a system (OAS; Radionics, Burlington, Mass., USA) in 169 consecutive patients with common intracranial lesions. We achieved a mean application accuracy of 2.5 mm, which was sufficiently reliable for most neurosurgical procedures. However, to get the feedback information, the surgeon has to look away from the operative field to the workstation monitor. As psychological studies of manual workers including surgeons indicated that performance is better when the worker is looking in a downward gaze at his hands, the natural progression was to project feedback information between the eyes and the hands. Therefore, the second step was to link tracking technology to the surgical microscope with head-up display. We used such a system (SMN-Zeiss, Germany) in 65 consecutive patients with a mean application accuracy of 1.4 mm. This was again sufficiently reliable for neuronavigation. The head-up display provided continuous feedback to the surgeon about the target, risk zones and areas of interest without the need to interrupt the procedure to get such information. Furthermore, the use of the focal length of SMN with autofocus to perform the registration improved the application accuracy of this technology. The ability of the software to process all MRI sequences (T(1), T(2), MPR and CISS) allowed us to use a variety of image sequences to delineate the lesion more exquisitely.