Dental prosthesis and method for manufacturing a dental prosthesis

Abstract: A method for manufacturing a dental prosthesis, such as a dental crown, wherein at least the visible part of the prosthesis (the outside) is subjected to a material removing operation by means of a numerically controlled micro machine tool and wherein the machining paths are visible on the final prosthesis. Said machining paths follow a three-dimensionally curved line and do no lie in a flat plane, therefore. The dental prosthesis which is manufactured in this manner has a natural-looking appearance. According to another aspect of the invention the shape of the dental prosthesis is such that near the edge of the prosthesis the cement layer is thicker than at some distance from said edge. Therefore the edge of the dental prosthesis is released from heavy forces.

Description: The invention relates to a dental prosthesis, such as a dental crown, an inlay, a veneer (facet) or a bridge, wherein at least the visible part of the prosthesis (the outside) is subjected to a material removing operation by means of a numerically controlled micro machine tool, and to a method for manufacturing such dental prosthesis. A method and an apparatus as defined in the preamble of claim 1 and 4, respectively is known from US Patent No. 5,027,281, wherein a dental prosthesis is made of a massive block of material. Material is thereby removed from said block of material by means of a numerically controlled micro milling machine. After the shape of the prosthesis has been determined and stored in the memory of a computer, the computer calculates the machining paths which the miller is to follow. Said machining paths are determined by means of flat, mutually parallel sections of the intended prosthesis. However, the machining path which the milling tool follows during the material removing operation may be a three dimensional curved machining path. In order to remove the machining paths which the final material removing operation leaves behind, the surface is polished, for example by means of a rotating brush. US Patent No. 4,937,928 describes a method for manufacturing a dental prosthesis, wherein the prosthesis is formed on a model in the shape of the part of the teeth on which the prosthesis is to be provided. The prosthesis is thereby produced by successively applying a number of layers of material on the model, whereby after the application of each layer the workpiece is worked by means of a numerically controlled machine tool. The machining paths which the tool follows during said operation are computed by means of a CAD/CAM system. A drawback of the known method is that either the machining paths must be spaced so closely that the individual paths on the final product are no longer visible, which is very laborious, or the workpiece must be subjected to an intensive polishing or grinding operation, in order to obtain an aesthetically sound result. Such a polishing operation is usually carried out by hand. An object of the invention is to provide a method for manufacturing a dental prosthesis, wherein an aesthetically sound dental prosthesis is obtained in an effective manner by carrying out a material removing operation by means of a micro machine tool. In order to accomplish the said objective of the invention the prosthesis is worked in such a manner, that the machining path follows a three-dimensionally curved line, and a portion of that line follows the cusp line (which runs over the highest parts of the prosthesis) and/or the fossa line (the deepest part of the upper side of the prosthesis). In practice it has become apparent that when a dental prosthesis is subjected to a material removing operation, a machining path following a natural line of the prosthesis creates a natural-looking result, which is aesthetically even better-looking than a dental prosthesis with a polished surface. So the three-dimensionally curved machining paths follow the natural lines of the surface of the teeth. Thereby a groove (fissure) is produced in the tooth surface by having the tool follow the path of the groove during the material removing operation. According to another aspect of the invention the outer side of a prosthesis, which is formed in the shape of a tooth, can be worked by following substantially circular machining paths around the tooth, whereby the mutual distance of said machining paths varies locally. At local protrusions in the tooth surface, where the radius of the substantially circular machining path is relatively smaller, the mutual distance of the machining paths can be slightly increased, which gives the prosthesis an natural-looking appearance. The outer surface of a dental prosthesis can be determined by taking a so-called library model as a starting point, i.e. making a selection from a number of typical models of teeth whose shape is laid down, for example in the memory of a computer. After a selection has been made with regard to the model which will serve as a starting point, the model in question will have to be adapted to the specific circumstances and the space which is available for the prosthesis. This may be done by hand by displaying the prosthesis in the shape of the library model on a computer screen, whereby the computer calculates and points out, on the basis of data of the teeth in question and the relevant jaw movements, at what locations the prosthesis needs to be adapted. Said adaptation may be effected by moving a pointer across the screen by means of a mouse and clicking the mouse at positions where problems occur, after which the computer calculates an adapted shape and displays it on the screen. These operations may be repeated until a shape of the prosthesis is obtained which satisfies all requirements. The machining paths associated with this shape, in order to manufacture the prosthesis by means of a material removing operation, may then be computed by the computer and be transferred to a numerically controlled machine tool. It is also possible to take the shape of the corresponding part of the teeth in the other jaw half as a starting point for the shape of the prosthesis. This shape may be registered by means of a three-dimensional scanner, be mirrored and subsequently recorded in the memory of the computer. Said scanning generally takes place by repeatedly determining in a flat plane the line of intersection with the surface of the teeth. The amount of data required for recording a surface scanned in such a manner can be considerably reduced by converting said data into data recording the intersection points of a network of lines. The number of these lines, and thus the amount of data, can be further reduced by having said lines follow the shape of the surface of the teeth, i.e. placing lines in particular in grooves that are present. This may take place by displaying the surface obtained by means of the scanner on a computer screen, whereby subsequently a pointer is clicked at characteristic locations in a groove by means of a mouse, after which the computer carries out a conversion computation, whereby intersection points of lines are put at the indicated places. In practice it has become apparent that in this manner it is possible to record the shape of a dental crown with a limited number of intersection points, for example from 500 - 2000. According to the invention a dental prosthesis may be formed in that during the material removing operation the lines are followed by means of which the computer has recorded the shape of the prosthesis in the above-described manner. The invention furthermore relates to a dental prosthesis, of which at least the visible outside of the prosthesis has undergone a material removing operation over its entire surface by means of a numerically controlled micro machine tool, whereby the machining path of the tool has followed a three-dimensionally curved line, whereby machining paths are visible on the surface of the prosthesis and whereby a portion of the machining path of the material removing tool follows the cusp line and/or the fossa line of the prosthesis. In order to more fully explain the invention embodiments of the invention will be described in more detail hereafter with reference to the drawing. Although a dental crown has been taken as an example, the invention similarly relates to other dental prostheses, such as an inlay, a facet or a bridge. Figure 1 shows a dental crown, on which machining paths are indicated; and Figure 2 shows a dental crown, which is undergoing a finishing operation. The Figures are merely diagrammatic illustrations, showing proportions deviating from the actual proportions for the sake of clarity. Figure 1 shows a dental crown, which is bounded by the preparation line 1 at the bottom side. The dental crown has undergone a material removing operation by means of for example a pointed burr 2 and a disc cutter 3. These tools are shown merely diagrammatically and not in proportion. The dental crown is shown to have machining paths 4, i.e. the paths along which the material removing operation has taken place. The machining paths 4 are shown diagrammatically and not in proportion for that matter. In practice the mutual distance of the machining paths will for example be 0.02 - 0.2 mm. A number of machining paths 4 follow a characteristic path of the crown, such as the equatorial line 5 (which constitutes the circumference of the crown in vertical projection), the so-called "cusp line" 6 (which runs over the highest parts of the crown) and the so-called "fossa" 7 (the deepest part of the upper side of the dental crown). The above-mentioned characteristic lines, as well as other characteristic lines of the dental crown, can be established by displaying the dental crown, whose shape is recorded in the memory of a computer, on a screen, whereby a pointer is moved across the screen by means of a mouse, whilst the mouse is clicked when the pointer is located in characteristic positions. The characteristic positions and lines may also be established by means of calculations by the computer, since said lines and positions are determined by the shape of the dental crown stored in the computer. As is apparent from Figure 1, the machining paths extend in an irregular-looking pattern, which is shown out of proportion for that matter. As a result of this irregular pattern the dental prosthesis has a natural appearance when fitted in the patient's mouth. Figure 2 shows a dental crown in elevational view, said dental crown at its bottom side being bounded by the preparation line 1 and exhibiting a number of primary and secondary fissures 8 at its upper side. Said fissures 8 constitute characteristic lines at the upper side of the dental crown and may form the machining path for e.g. the pointed burr 2 during the material removing operation of the crown. This milling operation may form part of the operation explained with reference to Figure 1. However, said milling may also be a finishing operation, which is carried out after the final form of the prosthesis has been substantially achieved. The invention is not restricted to the embodiment described. Many variations, for example as regards size and materials used, are possible when using the invention.

Claims: A method for manufacturing a dental prosthesis, such as a dental crown, wherein at least the visible outside of the prosthesis is subjected to a material removing operation over its entire outer surface by means of a numerically controlled micro machine tool (2,3), whereby the machining path(4) of the tool (2,3) follows a three-dimensionally curved line and machining paths (4) are visible on the prosthesis after said material removing operation, characterised in that a portion of the machining path (4) of the material removing tool (2,3) follows the cusp line (6) and/or the fossa line (7) of the prosthesis. A method according to claim 1, characterized in that said machining paths (4) comprise substantially circular paths around a tooth, with a varying local radius, wherein the mutual distance of the machining paths is larger at locations with a smaller radius than at locations where the paths (4) have a larger radius. A method according to any one of the preceding claims, characterized in that the shape of the outside of the prosthesis is numerically stored as a network of lines whose points of intersection are fixed, wherein lines follow the natural grooves (8) of the surface and wherein the mutual distance of the lines substantially lies between 0.01 and 0.25 millimeter, and wherein paths (4) of the material removing operation follow said lines. A dental prosthesis, of which at least the visible outside of the prosthesis has undergone a material removing operation over its entire surface by means of a numerically controlled micro machine tool (2,3), whereby the machining path (4) of the tool (2,3) has followed a three-dimensionally curved line and machining paths (4) resulting from said material removing operation are visible on the surface of the prosthesis, characterised in that a portion of the machining path (4) of the material removing tool (2,3) follows the cusp line (6) and/or the fossa line (7) of the prosthesis.

Inventors: ZEL JOSEPH MARIA VAN DER, VAN DER ZEL, JOSEPH MARIA
Applicants: ELEPHANT DENTAL BV, ELEPHANT DENTAL B.V.
Application year: 1993
Main classification: A61C13
IPC codes: A61C13, A61C5