A new System has been developed, comprising a frameless isocentric stereotactic mechanism and a three-dimensional (3-D) digitizer for intraoperative spa-tial monitoring. The 3-D digitizer''s multiarticulated arm has three joints related to Car-tesian coordinates, two quadrant arcs forming an isocenter system, a microdrive, and a probe holder. The frameless isocentric mechanism is useful for open stereotaxy. Routine CT- or MRI-guided stereotactic surgery is also possible, due to the high level of accuracy of the system. Before surgery, CT and/or MR images are acquired after placing on the scalp three or four external markers. For surgical procedures which require high accuracy, Laitinen''s noninvasive CT or MRI localizing markers are used. CT or MR images are entered into a computer using an image scanner, and are stored on a floppy disk. After the patient''s head is fixed to the operating table using a Mayfield clamp, the 3-D digitizer is used to read the spatial points and external markers on the scalp or the reference points of Laitinen''s localizing markers. During the procedure, the coordinates on the patient''s head are automatically entered into the computer and matched with those of the 3-D digitizer and CT/MR images on the CRT display. This system has been used in 22 cases of open craniotomy and 33 cases of burr hole surgery, both carried out using the stereotactic function and the 3-D spatial monitoring function in parallel. Errors in mechanical accuracy of the 3-D digitizer were less than 0.8 mm, and the maximum error during Operation was presumed not to exceed 2 mm.

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We have developed a frameless stereotactic multi-articulated arm suitable for image-directed microsurgery and surgical navigation. The system comprises a frameless isocentric mechanism and a three-dimensional (3 D) digitizer for intra-operative monitoring. Five joints of our multi-articulated arm are equipped with electromagnetic clutches, and the arms are provided with counterbalancing weights to ensure safe, smooth movement. Stereotactic microsurgical procedures were carried out with a laser beam guide mounted at the tip of the isocentric arm. We have used this system in 22 cases to resect tumours or epileptic foci. There were 12 cases of glioma, 5 cases of deep seated benign tumours, 3 cases of multiple metastatic tumours, and 2 cases of temporal lobe epilepsy. Postoperative enhanced CT images indicated total or subtotal resection in all cases. We also have used this system to determine the approach for clipping of a posterior cerebral artery aneurysm in 2 cases. The error of mechanical accuracy of the system was less than 1 mm. We believe that this system will prove useful in image-directed microsurgical procedures.