A patient can be taken to the operating theatre where a measurement arm can be used to locate fiducial markers relative to a fixed substrate. As a result, the location of the region of interest is known. An instrument guide, adapted to constrain movement of an instrument thereon, can then be positioned at a suitable location, and its location checked by way of the measurement arm. A computing means connected to the measurement arm so as to receive information as to the location of the tip can be programmed to determine a desired necessary movement of the guide in order to locate the instrument at a desired location relative to the region of interest. The guide can then be moved in a controlled and accurate manner from its initial position to an accurately determined correct position.
