Computer implemented method for modifying a digital three-dimensional model of a dentition comprising: displaying an image of the dentition on a display; receiving user input information modifying the shape of at least a portion of the dentition by markings which are input on the image on the display; transferring the shape modifying information to the three-dimensional model and modifying the three dimensional model accordingly; characterized by displaying the image of the dentition by applying a virtual camera to the three-dimensional model to render the image for the display; acquiring a line drawn by the user on the image on the display; forming a projected line as a projection of the acquired line, using the virtual camera, to a three-dimensional projection surface approximating the shape of the dental arches in the three-dimensional model of the dentition, the three-dimensional projection surface extending the upper and lower dental arches towards each other 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 in the three-dimensional model of the dentition, 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. displaying an image of the dentition on a display; receiving user input information modifying the shape of at least a portion of the dentition by markings which are input on the image on the display; transferring the shape modifying information to the three-dimensional model and modifying the three dimensional model accordingly; characterized by displaying the image of the dentition by applying a virtual camera to the three-dimensional model to render the image for the display; acquiring a line drawn by the user on the image on the display; forming a projected line as a projection of the acquired line, using the virtual camera, to a three-dimensional projection surface approximating the shape of the dental arches in the three-dimensional model of the dentition, the three-dimensional projection surface extending the upper and lower dental arches towards each other 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 in the three-dimensional model of the dentition, 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.