Method for extracting margin line at artificial crown designing

Abstract: Rate of variation of data representing the shape of an abutment tooth are determined, and a train of points is extracted from the rate of variation. Then a developed view of the surfacial shape of the abutment tooth is displayed, and the obtained train of points is also displayed in the developed view. The margin line for designing the artificial crown is determined, based on thus displayed train of points.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method for extracting the margin line in the designing of an artificial crown, which is used for complementing the defective part of a defective tooth. Related Background Art Conventionally, the artificial crown has been prepared by a manual process called waxing-up method, as shown in Figs. 6A to 6C. At first a plaster cast of the abutment tooth, as shown in Fig. 6A, is prepared. The dentist forms the abutment tooth by shaping the diseased part, and said plaster cast is prepared from an impression of said abutment tooth. Then a margin line 61 is drawn on said plaster cast of the abutment tooth. The position of said margin line is determined by the dental technician, who watches the plaster cast and attaches marks where he thinks the margin line exists. Subsequently, as shown in Fig. 6B, molten wax is heaped on the plaster cast of the abutment tooth in such a manner that the wax does not overflow the margin line to the side of the plaster cast of the abutment tooth and that the wax is smoothly shaped in the vicinity of the margin line. In this manner the shape of the artificial crown is prepared as shown in Fig. 6C. An impression is prepared by thus prepared model of the artificial crown, and the artificial crown is prepared by said impression. In such conventional method, the determination of the margin line depends greatly on the skill of the technician. An inadequate margin between the abutment tooth and the artificial crown leads to the detachment of the artificial crown or to the dental caries inside the artificial crown. Also such inadequate margin requires additional adjustment by the dentist at the attachment of the artificial crown to the diseased part, thus necessitating a longer time of treatment. The preparation of the artificial crown, which has been manually conducted up to now, will surely depend upon CAD/CAM in the near future. A CAD based modelling procedure is known from the Journal of Manufacturing Systems vol. 5, No. 10, 1991, Dearborn pages 383 - 395, XP243565, Motavalli et al, Background, Smoothing, Curve fitting. More particularly, it is known from this document to extract the shape of a rotationally symmetrical part by determining the amounts of variation of plural points on a curved plane of the part and to calculate the inclination between adjacent points. SUMMARY OF THE INVENTION Therefore, it is a main object of the present invention to provide a method for extracting the margin line in the designing of an artificial crown, that is to say of a non-rotationally symmetrical part. In this connection, also care should be taken to provide a method for extracting the margin line, capable of easy determination thereof. This object is attained by the features of claim 1 and is further developed by the features of the subclaims. The above-mentioned method allows to determine a train of points that can be identified as the margin area by calculation. Also the margin line can be easily determined by plotting said train of points on a development view of the surface. BRIEF DESCRIPTION OF THE DRAWING Figs. 1A and 1B are views showing display states of the train of points in an embodiment of the margin line extracting method of the present invention; Figs. 2A and 2B are views showing a calculational method for determining the train of points of the margin area and a method for preparing a development view in an embodiment of the margin line extracting method of the present invention; Fig. 3 is a development view with shaded display, in an embodiment of the margin line extracting method of the present invention; Fig. 4 is a development view with line display in an embodiment of the margin line extracting method of the present invention; Fig. 5 is a view, showing a three-dimensional shape and a development view, both shown in a display device, in an embodiment of the margin line extracting method of the present invention; and Figs. 6A to 6C are views showing the steps of preparation of an artificial crown by the waxing-up method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now the present invention will be described in detail by a preferred embodiment thereof. At first, the shape of a plaster cast of the abutment tooth, prepared in the same manner as in the conventional art, is measured to obtain shape data of said abutment tooth. Said measurement can be achieved by a known technology, such as in the CT scanning or by a three-dimensional measuring instrument. Thus the data indicating the three-dimensional shape of the abutment tooth are obtained from the measurement data. A train of points in the margin area is extracted from said data representing the three-dimensional shape of the abutment tooth, for example, by a method explained in the following: (1) At first, as shown in Fig. 2A, a central axis 22 of the three-dimensional shape 21 of the abutment tooth is determined: (2) There is considered a crossing line between a plane including the central axis 22 and a curved plane constituting the shape 21 of the abutment tooth. In Fig. 2A, a line 23 corresponds to said crossing line. Then calculated is the distance between the central axis 22 and each point on said crossing line 23, contained in the curved plane constituting the abutment tooth 21. Then the inclination Si between the adjacent points is determined, for each point i on the crossing line 23 in the following manner: Si = Ij - Ii (j = i + 1) wherein Si is the inclination, and Ii is the distance from the central axis. (3) Based on the above-explained calculation, the margin area is defined at a point where the inclination crosses zero or where the variation of the inclination exceeds a certain value. More specifically, the margin area is defined at a point j satisfying: Si · Sj < 0, or |a| ≥ constant, a = Sj - Si in which a represents the amount of variation of the inclination. The margin area can be extracted in the above-explained manner. The foregoing explanation is limited to the calculation on the crossing line 23, but according to the invention there must be selected plural planes containing the central axis 22. Consequently a train of points is obtained by effecting the above-explained calculation on the crossing line corresponding to each plane. Then the train of points representing the margin area, determined in the above-explained manner, is displayed on a display unit. In this state, a development view of the shape of the abutment tooth is displayed, and said point train of the margin area is superposedly displayed. Said development view is obtained by developing the lateral plane of the three-dimensional shape of the abutment tooth and displaying said lateral plane in planar form. More specifically the developed data of the abutment tooth are prepared by developing the cylindrical plane from a broken line L shown in Fig. 2B, and the development view is displayed, based on said developed data. The train of points representing the margin area is displayed on the corresponding position on said development view. Fig. 1A shows the display of said point train of the margin area on the developed view, wherein said points are connected by straight lines, and such connection can be made by hitting the points with a mouse. The development view can be displayed in a shaded display or a bent-line display as if seen from an oblique direction, selectable by a menu. Fig. 1A illustrates the shaded display. Subsequently there is executed the correction for the point train of the margin area extracted by calculation and the lines connecting the points. Said correction is conducted by hitting the displayed point and moving said point by giving an amount of variation by keys or a dial. Said correcting operation will be explained in the following, with reference to Fig. 1A, in which shaded areas 12a, 12b slope gradually upward to an area 13. The area 13 constitutes a ridge line of a mountain. The actual margin line is considered present in the vicinity of the peak of the heaped portion in the developed view, namely in the area 13 in Fig. 1A. The point train 11 in Fig. 1A indicates the margin area determined by the aforementioned calculation. Most of the points of the train 11 are in the area 13, and can be considered close to the actual margin line. However points indicated by arrows are in the area 12b, namely on the sloped portion. These points, indicated by arrow, are considered deviated from the actual margin line, having been improperly obtained by the aforementioned calculational method, and have to therefore be corrected in positions. As explained above, the correction can be achieved by hitting the point to be corrected with the mouse and moving said point to a desired position by setting the amount of movement for example by keys or a dial. The developed view of the abutment tooth and the three-dimensional shape thereof can be simultaneously displayed as shown in Fig. 5, wherein in addition to the display of the three-dimensional shape of the abutment tooth, the developed view is displayed in the upper part. The point train and the connecting lines, representing the margin area, can be displayed both in the three-dimensional shape and in the developed view. The above-mentioned correcting operation for the points may be conducted on the display of the three-dimensional shape, but is more easily conducted on the two-dimensional developed view. The amount of movement of the point for correction is reflected both on the developed data and on the three-dimensional data. Consequently, when the point is moved on the developed view, the corresponding point in the display of the three-dimensional shape may also be moved in corresponding manner. After the points are corrected in position, they are again connected with straight lines. Fig. 3 shows a state in which the corrected points are again connected by the lines, and Fig. 4 shows a bent line display corresponding to the state shown in Fig. 3. Naturally a display as shown in Fig. 4 may be employed for the correction of point positions, instead of the display as shown in Fig. 1A. The point train thus determined is converted into three-dimensional data, and formed as a curve as shown in Fig. 1B, and this curve corresponds to the margin line. Said margin line is registered on the shape data of the abutment tooth. The designing of the artificial crown can be made on the shape data of the abutment tooth, in which thus determined margin line is registered. The margin line of the artificial crown to be designed can be made to coincide with the margin line registered in said shape data of the abutment tooth. As explained in the foregoing, the present invention simplifies the extraction of the margin line in the designing of dental compliment. As a result it is rendered possible to reduce the designing time and to improve the productivity. Also the dental compliment provides an improved durability, since the accuracy of the margin line is improved in comparison with that in the prior art.

Claims: A method for extracting the margin line in the designing of an artificial crown by extracting a train of points in the area of the margin line from said data representing the three-dimensional shape of an abutment tooth, comprising the steps of: (a) determining a central axis (22) of the three-dimensional shape (21) of the abutment tooth; (b) considering a crossing line (23) between a plane including the central axis (22) and a curved plane constituting the shape (21) of the abutment tooth; (c) calculating the distance between the central axis (22) and each point on said crossing line (23), contained in the curved plane constituting the abutment tooth(21); (d) determining the inclination Si between the adjacent points for each point i on the crossing line (23) in the following manner: Si = Ij - Ii (j = i + 1) wherein Si is the inclination, and Ii is the distance from the central axis; (e) definining a point j of the margin line satisfying: Si · Sj < 0, or |a| ≥ constant, a = Sj - Si in which a represents the amount of variation of the inclination; (f) repeating steps (b) to (e) so as to define the complete margin line by using a plurality of different crossing lines each of which yielding one point of a point train; (g) displaying a developed view of the surfacial shape of said abutment tooth and displaying said train of points in said developed view; (h) connecting said points with a straight line; and (i) correcting the margin line by hitting the deviated points and moving the points in said train in position by giving an amount of variation by keys or a dial and connecting thus moved points again with straight lines in the display. A method according to claim 1, wherein said developed view of the surfacial shape of the abutment tooth is displayed in shaded representation. A method according to any one of claims 1 or 2, wherein said abutment tooth and said train of points are three-dimensionally displayed on said display. A method according to claim 3, wherein a movement of a point in said train displayed in said developed view of the surfacial shape causes a movement of the point of the train displayed in said three-dimensional shape. A method according to any one of claims 2 to 4, further comprising a step for converting said train of points into three-dimensional data. A method according to claim 5, further comprising a step for registering the train of points, converted into said three-dimensional data, in the shape data of said abutment tooth.

Inventors: BABA MASAMI, KAWAI MASAHARU, MIYOSHI KATSUYA, BABA, MASAMI, KAWAI, MASAHARU, MIYOSHI, KATSUYA
Applicants: NIPPON KOGAKU KK, NIKON CORPORATION
Application year: 1993
Main classification: A61C13
IPC codes: G06F17, G05B19, A61C9, A61C5, A61C13