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, comprising the steps of: a) receiving bone registration data which represent locations of points on the surface of the bone; b) calculating, from the bone registration data, the current relative position between the bone and a medical instrument which bears the marker device; c) determining an offset between the current relative position and a desired relative position; d) outputting indication information which is based on the calculated offset; e) receiving bone re-registration data which represent locations of points on the surface of the bone; and f) calculating the positional relationship data from the bone re-registration data.