Method for producing dental prosthesis

The invention relates to a method for producing dentures according to the preamble of claim 1.

The previously used method of producing dentures was based on reproducing the individually different clinical mouth conditions with impressions and models obtained afterwards. A process for the production of dentures must make it possible to achieve a high level of accuracy of fit of the dentures, since this accuracy of fit is a prerequisite for the secure anchoring and permanent seating of the dentures. All tissues involved in the chewing process are then able to compensate for the physiological stresses without damage.

All dental and all subsequent dental technology operations are carried out using methods that have been in use for decades, which will be briefly described using the example of a single crown:<ul id="ul0001" list-style="none" compact="compact"><li>1.) impression of the prepared tooth stump with a rubber-elastic or hydrocolloidal impression material;</li><li>2.) Temporary restoration;</li><li>3.) Pouring the impression with super hard plaster and making a working model and a counterbite model;</li><li>4.) placing the models in an articulator;</li><li>5.) Manufacture of an artificial tooth crown after wax modeling and metal casting;</li><li>6.) Fitting the casting in the mouth on the tooth stump and final fitting of the artificial crown.</li></ul>

This conventional production from metal or other materials requires a large number of dental and dental technical processes, which involve a large number of inaccuracies and possibilities of error. Inaccuracies in fit can result in repetitions of the work processes being required.

Material properties such as gypsum expansion, metal shrinkage and the like as well as improper processing and difficult mouth conditions are responsible for this.

Efforts have been underway for some time to replace the conventional impression method and numerous subsequent operations using other techniques. These techniques are based on mechanical or optical three-dimensional measuring and scanning devices. The information obtained in this way is stored and passed on to control devices of NC machine tools (numerically controlled machine tools) already used in the machine tool industry.

US Pat. No. 4,182,312 shows a mechanical scanning of three-dimensional surface information of teeth and surrounding tissues directly on the patient. The dentist guides a probe in the patient's mouth. However, mechanical scanning involves certain inaccuracies.

US Pat. No. 3,861,044 describes a method which photographs the cavity of the tooth. A wax filler is brought into the desired final shape by the dentist.

The method described in European patent 0 054 785 also requires constructive corrections before the necessary accuracy of fit is achieved. Before this, surfaces of body organs in their spatial and topographical form are captured optically without contact.

The method shown in European patent EP-0 040 165 tries to use holographic interferometry to acquire and forward data of a prepared tooth stump. Light generated by laser is processed optoelectronically and fed to a computer. This method is described in EP-0 040 165; such a method has not yet been carried out in practice.

From the documents US-A-4 663 720 and US-A-4 070 683 a method for the production of dentures is known, in which contour lines or contour lines are generated on the ground tooth and its surroundings, the lines are detected with an optoelectronic device are, the spatial structure of the tooth and the dental prosthesis is calculated from the recorded values ​​and the dental prosthesis is manufactured according to methods known per se.

A method for producing dentures according to the preamble of claim 1 is known from US-A-4 575 805. Contour line patterns are created. The distance between the patterns is 1/4 of the contour line distance.

The object of the invention is to enable a fully automatic production of the denture in a method according to the preamble of claim 1.

This object is achieved by the method defined in claim 1.

It is essential for the invention that contour lines or contour lines are applied to the tooth stump. The present invention differs fundamentally from the method described in European Patent EP-0 040 165 in that it is based on an optical method for generating contour lines or contour lines.

The shape of the ground tooth stump, that of the neighboring teeth and the antagonists is determined by contour lines or contour lines. These contour lines or contour lines are generated by already known methods such as projection methods or by moiré.

In the projection process, contour lines, i.e. lines that describe the external shape of the tooth stump, are projected directly (Figure 1). The contour lines can be generated interferometrically with a laser or by casting shadows. In the Moire method, the tooth stump is illuminated by a grating and observed through the same grating via a television camera at a distance from the light source. By superimposing the projected shadow pattern on the tooth stump with the grid, a moire is created that describes the contour of the tooth stump (Figure 2).

The contour lines can be recorded with a video camera, which can be done directly or via glass fibers.

The contour lines are evaluated in an image processing system. In contrast to line tracking programs, the evaluation is attributed to an intensity measurement, which can be easily carried out using a television camera. The intensity is contained in the video signal for each pixel.

The calculated contour can then be transferred directly to a numerically controlled milling machine, so that the dentures can be made to fit exactly. The dentures can be fixed and / or removable.

The method according to the invention has the advantage that a tooth stump, the neighboring teeth and the antagonists can be optically detected, evaluated immediately and the shape of the tooth replacement can be calculated. The procedure is actually executable, simple and quick. The dental prosthesis is manufactured directly without any time gap and without contact marks. The dentures can therefore be used during a treatment.

According to the invention, at least three different contour or contour line patterns are generated. This procedure enables the dentures to be manufactured fully automatically.

The calculation of the spatial structure of the tooth and the denture is based on the following formula:<maths id="math0001" num="(1)"><math display="block"><mrow><mtext> I = ax (1 + mx cos Θ)</mtext></mrow></math><img file="EP0299490B2_D0001.tif" /></maths>    In this formula:    I - intensity    a - background brightness    m - contrast    Θ - angle    The intensity I can be measured. It is fixed for each pixel when recording with a video camera. If the video image consists of 512 x 512 pixels, this intensity can therefore be clearly determined or measured for each of the pixels from the video signal. So three unknowns remain in the equation: the background brightness a, the contrast m and the angle Θ. The size we are looking for is the angle θ. If the angle Θ is fixed for each individual pixel, the height coordinate (z coordinate) can also be calculated from this for each individual pixel. This height coordinate z is a function of θ. By determining the height coordinate z for each individual pixel x, y, the spatial shape of the tooth and the dental prosthesis is clearly defined. The goal is therefore to calculate the height coordinate for each individual point x, y. For this it is necessary and sufficient to calculate the angle Θ for each individual pixel x, y. This does not succeed with equation (1) because this equation contains three unknowns. So you need three equations. In order to obtain these three equations, one first displaces the contour or contour line pattern by a certain path in order to obtain a pattern of contour or contour lines which differs from the first pattern. The following equation then applies to this second pattern:<maths id="math0002" num="(2)"><math display="block"><mrow><mtext> I (2) = ax (1 + mx cos (Θ + Θ (2))</mtext></mrow></math><img file="EP0299490B2_D0002.tif" /></maths>    The displacement angle Θ (2) is known because it corresponds to the measure of the line displacement. So it remains with the three unknowns a, m and Θ. Then the contour or contour line pattern is shifted again. The heights of these displacements are preferably a quarter to a third of the grid spacing; however, other (well-defined) shifts are also possible. The equation for the third pattern is:<maths id="math0003" num="(3)"><math display="block"><mrow><mtext> I (3) = ax (1 + mx cos (Θ + Θ (3))</mtext></mrow></math><img file="EP0299490B2_D0003.tif" /></maths>    The angle Θ (3) is also known because it is predetermined. The system of equations resulting from equations (1), (2) and (3) now enables the calculation of the unknown Θ and thus the calculation of the height coordinate z. When this procedure has been carried out for each of the pixels, the entire spatial structure is calculated.

The "phase shift" or "line shift" method just described makes it possible to produce the denture fully automatically. If only a single contour or contour line pattern were to be recorded and evaluated, it would also have to be entered manually in which direction ascending contour lines and in which direction descending contour lines run. Furthermore, the areas in which shadows or fissures are located would have to be entered. According to the method according to the invention, three different contour or contour line patterns are used, so that this additional information no longer has to be entered specifically. The z coordinates for each pixel x, y can then be calculated from the three equations (1), (2) and (3) using the method described above. A separate information about the course of the elevation of the contour lines is not necessary. The shadows and fissures can be recognized by the fact that they are always in the same place in each of the three different pattern images.

The contour is evaluated by entering at least three images into the computer, the line pattern being shifted by a predetermined amount. The contour can then be clearly calculated from the distance of the shift and the change in intensity in each pixel.

An embodiment of the invention is explained below with reference to the accompanying drawings. In the drawings shows<dl id="dl0001" compact="compact"><dt>Image 1</dt><dd>the generation of contour lines by projection and</dd><dt>picture 2</dt><dd>the generation of contour lines by moire.</dd></dl>

As can be seen in Figure 1, a projector 1 generates light beams that hit a grating 2. This grating 2 has opaque areas and horizontal, translucent areas 3 at a predetermined, preferably equal distance from each other. The light rays therefore leave the grating 2 in horizontal planes 4 running parallel to one another. They meet the tooth stump 5 and generate contour lines 6 there. These contour lines 6 on the tooth stump 5 are recorded by the television camera 7 and passed on to the monitor 8 and to the computer 9. The contour lines 6 can be made visible on the monitor 8, as indicated by the reference number 10. The computer 9 calculates the spatial structure of the tooth stump 5 from the intensity values ​​for each of the pixels of the monitor. The monitor can, for example, consist of 512 x 512 pixels.

If only a single image with contour lines is taken, the computer still has to be given the information in which direction the higher and in which direction the lower areas of the tooth lie. Furthermore, the computer must still be entered the information which areas contain shadows and / or fissures.

To make the system fully automatic, a phase shift or line shift is required. First, an image is taken with certain contour lines. Then the projector 1 is displaced perpendicular to the projection axis 11, that is to say perpendicular to the planes 4, specifically by a well-defined height which preferably corresponds to approximately a quarter to a third of the grating distance, that is to say the distance between the light-transmitting regions 3 of the grating 2. This creates a second, shifted line pattern on the tooth stump 5 and removes it from the television camera 7. This process is then repeated a third time. In this way, three different contour lines arrive in the computer 9. This computer 9 can use this to calculate the spatial shape of the tooth stump completely automatically. The shadows and / or fissures are recognized by the fact that they lie in the same place in all three exposures and do not shift - like the contour lines. These shadows and / or fissures can therefore be automatically omitted later by the computer 9 during the calculation.

Figure 2 shows the generation of contour lines by moire. Light rays are emitted from the light source 21 and thrown onto a grating 22. This grating consists of opaque areas and linear translucent areas 23. The linear translucent areas are arranged parallel to each other and run at a well-defined, preferably equal distance. The grating plane runs perpendicular to the axis 24 of the tooth stump 5. The grid generates 5 contour lines on the tooth stump, of which only a single contour line 25 is shown in FIG. The television camera 7 is at a distance from the light source 21. The tooth stump 5 is thus illuminated by the grating 22 and observed through the same grating 22 via the television camera 7. By superimposing the projected shadow pattern (contour lines 25) on the tooth stump 5 with the grid 22, a moiré is created which describes the contour of the tooth stump 5. This moire can be made visible on the monitor 8, as indicated by the lines 26. The television camera 7 is connected to the monitor 8 and to the computer 9.

The procedure shown in Figure 2 differs from that in Figure 1 only in that in Figure 1, contour lines are generated by projection, while in Figure 2, contour lines are created by moiré. Otherwise, the explanations for the procedure according to Figure 1 apply to the procedure according to Figure 2.