The present device provides an apparatus for securely positioning a medical instrument relative to a patient. The apparatus comprises a drive assembly for moving the medical instrument along a first axis that is substantially parallel to the length of the instrument; an adapter comprising an elongated member and a pair of cooperating hubs for connecting the drive assembly to a positioning system. The cooperating hubs serving to allow separation of sterile from non sterile components. The positioning system comprises two motors each for moving the instrument about a different axis than the drive assembly. The apparatus moving the instrument about a point which is external to the patient. The device further comprises a sterile bag which encloses the positioning system, the bag having at least one opening for receiving an axel from a sterile adapter assembly, thereby rendering the entire apparatus sterile.