A method for providing positional relationship data, which represent the position of a marker device relative to a bone which it is affixed to, to a medical navigation system, including receiving bone registration data which represent locations of points on the surface of the bone, calculating, from the bone registration data, the current relative position between the bone and a medical instrument which bears the marker device, determining an offset between the current relative position and a desired relative position； outputting indication information which is based on the calculated offset, receiving bone re-registration data which represent locations of points on the surface of the bone, and calculating the positional relationship data from the bone re-registration data.
