Figure 2.

(A,B) The Leksell frame is assembled opposite the traditional manner, with the short fixation posts flanking the curved nasal piece, and long fixation posts flanking the straight piece. Female fixation screws are used and serve as skull fiducials for robot registration after O-arm imaging. (C) After frame placement, the patient is initially positioned supine while still on the stretcher, with the head supported on a radiolucent plastic board, and a pre-operative O-arm image is obtained for registration to the pre-operative CT. (D) The patient is then flipped prone on gel rolls on the OR table, and the Leksell frame is affixed to the robot using the goalpost-shaped Leksell holder. The reverse orientation of the frame allows the three straight edges of the frame to fit within the beveled clamps of the Leksell holder, with the curved nasal piece over the occipital region. (E) Registration points are chosen on the merged fluoroscopic CT image including the frame. Four points are chosen, one for each frame pin, such that the registration marker sits in middle of the divot on the pin with its equator flush with the flat surface of the pin. (F) Registration is then performed using the ball-tip probe robot attachment.