The present invention relates to a computer implemented method for modifying a digital three-dimensional model (3) of a dentition comprising:displaying an image of the dentition by applying a virtual camera to the three-dimensional model to render the image for a display (1)；acquiring a line drawn by a user on the image on the display (1)；projecting the acquired line, using the virtual camera (4), to a three-dimensional projection surface (7) approximating the shape of the dental arches and extending the upper and lower dental arches to cover any opening therebetween；obtaining visible teeth arch contour curves, namely incisal curve and gingiva border curve, of the upper and lower teeth arches, and projecting the teeth arch contour curves to the projection surface；selecting one of the teeth arch contour curves as teeth arch contour curve to be modified and selecting the other teeth arch contour curve of the same teeth arch or a curve derived therefrom as a baseline curve；deforming all teeth belonging to the selected teeth arch contour curve along the projected line in the three-dimensional model of the dentition in a length direction such that at least parts of the selected teeth arch contour curve, after deformation and projection to the projection surface, coincide with the projected line and any remaining parts of the selected teeth arch curve are located between the projected line and the baseline curve, whereas the baseline curve remains unaffected by the deformation.
