The invention relates to a medical registration apparatus (1), comprising •two flanks (2a, 2b); •a pivot portion (3) around which at least one of the flanks (2a, 2b) is rotatable with respect to a rotation centre (3c, 3d) (FIG. 1, FIG. 3); •a contacting portion (4a, 4b) on each of the flanks (2a, 2b), each contacting portion (4a, 4b) being spaced apart from the rotation centre (3c, 3d); and •a sensor (5, 6) being arranged with an offset (r, FIG. 4 A) to a line (a) connecting the contacting portions (4a, 4b). The invention also relates to a data processing method for use with the medical registration apparatus.