The invention relates to a surgery assistance system for guiding a surgical instrument. The surgical instrument (3) is fastened lo an arm system (10, 12, 14), the tip (S) of the surgical instrument (3) can be moved in a controlled manner by means of the arm system (10, 12, 14) in a Cartesian patient coordinate system (PKS), one of the three spatial axes (x, y, z) of the Cartesian patient coordinate system (PKS) extends through the surgical opening or the trocar point (T) receiving the surgical instrument (3). Advantageously, the angle of inclination (w) of the surgical instrument (3) is determined with respect to the spatial axis (z) of the Cartesian patent coordinate system (PKS) extending through the trocar point (T), the angle of inclination (w) so determined is compared with a predetermined set angle of inclination (ws) for the purpose of guiding the tip (S).
