the invention includes a rigid frame that is positioned around the skull to define a multi - dimensional coordinate system . multiple arms are coupled to the frame , each arm having an attachment assembly movable along a longitudinal axis directed toward the center of the rigid frame assembly . multiple receptors are either attached to the skull or drilled into the skull for receiving an associated attachment assembly . the receptors provide a mounting base upon which the frame is repeatedly attached and detached from the skull at the same reproducible reference location . each arm has a multi - station receptacle that receives various attachments used for mounting and attaching the receptors . special bushings are used to lock the attachment assembly in a preset longitudinal position in relation to the arm . the locked attachment assembly can then be removed from the arm allowing the frame to be removed from the skull while maintaining frame position information .