DE19642247C1 - Prosthesis production system for prepared tooth - Google Patents

Abstract

The profile of the prepared tooth is derived from tooth heights detected and digitally stored.To determine the shape of the prosthesis, a three-dimensional picture of an intact model tooth of the same type as the prepared one is distorted to align it with the latter. The result is matched to the prepared tooth, to restore the natural shape of the tooth. The surface data to produce the prosthesis from a blank is fed to a computer-controlled machine-tool. The three-dimensional picture of the model-tooth surface and of the inner datum points of its chewing surface can be distorted into line with that of the prepared tooth.

Description

The invention relates to a method for producing a tooth replacement part for a given prepared tooth, especially posterior.

To repair a damaged tooth, the caries or the rest of a previous filling removed, the damaged area smoothed and be prepared for a filling or crown. The shape of the ent standing cavity or cavity depends on the filling material, that should be used. Amalgams come as filling materials or plastic in question, which is introduced into the tooth in plastic form and only hardened after shaping, or materials such as Ke Ceramic and gold, some also plastics, in solid form in the tooth be used, the shape is already fixed before insertion. Whether amalgams have been used as filling material for over 100 years and from a dental perspective still in terms of workability and Firmness is one of the best restorative materials, lately become discredited due to the mercury pollution in the body. Except he became more and more interested in tooth-colored materials. This includes both special plastics, so-called composites, as well Ceramics. The strength of previously used composites is limited, so that these are mostly only used for smaller fillings. Against it is with Ceramic possible to replace larger parts of the tooth. Besides, is Ceramic in terms of its hardness with the outer layer of a healthy one  Tooth, the enamel, comparable.

With the fillings used in solid form, the dentist speaks of one warehouse restorations. These include inlays, onlays and overlays or partial crowns. Inlays cover only parts of the purchase area, onlays cover the whole. Both who which is only anchored in the tooth core, while in overlays the anchoring also takes place at the tooth edge. The following is simplified from Restau rations spoken.

The shape of a restoration is determined on the one hand by the shape of the Cavity in which it is to be inserted, on the other hand by the desired one outer shape of the occlusal surface. Once the restoration has been created, you can use a special cement can be attached to the existing tooth stump. In wide It is ground by hand in order to match it as well as possible To adapt conditions. In the case of hard ceramics, these are rework tedious and uncomfortable for the patient, since they are sometimes in the mouth must be executed. In addition, grinding is also automatic healthy tooth substance with removed.

There are several criteria to consider when designing the shape of the restoration:
The restoration must be fitted into the prepared cavity. Cavities and gaps can be leveled out with the cement if the restoration is too small. However, this washes out over time and harbors the risk of caries formation in the marginal fissures. If the restoration is too large, it cannot be used at all.

The restored tooth should be in its contact area with the neighboring teeth have the right distance and the right shape. Both are of the tooth type dependent.

The teeth of the opposing jaw (antagonists) must be in the sense be seen that when biting the teeth one if possible uniform pressure load is achieved. The dentist speaks of this Occlusion. The consequences of incorrectly positioned restorations can be chafing severe, damage to the tooth nerve associated with hypersensitive and toothache as well as TMJ problems.

Furthermore, the surface of the restoration should be a continuous surface have so that it continuously merges into the surface of the remaining tooth. Math From a mathematical point of view, this does not only mean continuity of the surface, but: also continuity of their first derivative.

The created purchase surface should also be in its optical on print the natural structures of the occlusal surface, i.e. elevations, ditches and contain deep, typical for a posterior tooth, so far this does not necessarily result from the criteria explained above.

The classic way of making a restoration is very lengthy, since he has multiple treatment sessions for the patient and multiple edits ter includes and is therefore very expensive. Prepared the dentist the tooth and creates a negative impression of the entire dentition. This The impression is passed on to a dental technician, who uses it to make a positive plaster model of the bit. On this he models with a pro visual plastic material, which is mostly wax  delt, the filling. To take into account the criteria explained above gen, the models of the upper and lower jaw using a so-called ten articulator assigned to each other. With the help of this articulator Chewing movements together with the side movements occurring simu be lated. The points of contact with the antagonists can be so mentioned occlusion film can be determined. From the provisional re Restoration is then carried out using several possible methods with several Partial steps molded and fired the filling from the ceramic material. It must again be noted that shrinkage when burning fung occurs, which is characterized by an intentionally too large shape before burning must be compared. A disadvantage of this classic manufacturing method is also that when burning the irregular shape different Ma material properties arise.

From US 4,766,704 a method is known in which the Re restoration worked directly out of a fired ceramic blank can be tet. Using a modern CAD system (Computer Aided Design) the well-known method allows modeling restoration on the screen and the subsequent milling of the re restoration in an integrated milling unit. The prepared tooth will be there recorded with a special camera in the patient's mouth. Next a gray-scale image of a conventional camera is created called depth image, which for every visible point of the recorded Be range indicates the distance to the camera plane. For the dentist on the picture The screen of the control unit either shows the gray scale image or the Depth picture of the prepared tooth. With a mouse or a trackball First the cavity edges and the tooth edge are marked in the picture. From this, the computer calculates the shape of the underside of the restoration  for edge adjustment. Furthermore, by entering auxiliary lines the Defined occlusal area of the future restoration. The structures like tooth Bumps, trenches and depressions must be specified by the dentist will. This work requires a high level of abstraction of the tooth doctor who, when modeling, on the 2-dimensional representation of the image is required to define a 3-dimensional object and to manufacture. The entered lines are made using a computer mathematically interpolated to create volume data of the restoration, which in turn are passed on to the milling machine (NC machine) the. The occlusal surface of a restoration created in this way is usually very rough and still needs to be reworked. The time required for treatment according to the known method depends on the size of the residual ration at an average of 60 minutes, of which about 15 minutes for Mon dell the restoration on the screen and about 30 minutes for the on setting and reworking of the restoration are required.

No. 4,742,464 describes a method for producing a tooth known in part, in which by means of an interferometric method Volume data of the cavity of a prepared tooth can be created. Around Modeling a crown becomes a solid model of a model tooth the depth image fitted. For this purpose, six delimitation levels are automatically calculated table that define the location of the model tooth to be used among other things, the delimitation planes to the neighboring teeth. By this displacement and rotation of the model as well as scaling and Shear can project this into the interior of the delimitation planes the.

From US 5,273,429 is a further developed process for the manufacture  ment of a dental prosthesis known to improve the Einpas The model first senses a variety of angles and distances the neighboring teeth are extracted. The Para meters for displacement, rotation, scaling and shear of the volume model calculated to adapt it to the existing tooth stump.

The two latter methods have in common that the adjustment of the Model tooth to the conditions of the prepared tooth only via linea re transformations happen like shift, rotation, scaling and Shear of the solid model of the model tooth or parts thereof. A real fit, for a seamless transition from the given Tooth stump to the adapted model tooth and thus to the one to be created Dental prosthesis, but can generally with such transformations mean not to be reached. In particular the production of inlays to whom only a part of the occlusal surface is restored is with these procedures not possible in the sense that the remaining part of the purchase area of the prepared tooth when determining the shape of the restoration is taken into account.

Proceeding from this, the object of the invention is a method to specify the manufacture of a dental prosthetic item in the case of considering the criteria mentioned above the manual effort none Recontour on the restoration inserted in the prepared tooth and is reduced while shaping. This applies in particular the creation of a naturally shaped occlusal surface (with smooth transitions between the dental prosthetic item and the remaining tooth) including the before existing chewing surface of the prepared tooth, as well as the consideration the neighboring and opposite teeth of the prepared tooth.  

To solve this problem, a method for Her provision of a tooth replacement part for a given prepared tooth with the Features of claim 1 proposed. So one of those prepared digitally stored depth image with a Depth image of an intact model tooth of the same type through image defor mation brought to cover, whereupon the height information for the prep area of the prepared tooth from the deformed depth image of the Model tooth is taken over. This is followed by an altitude guide the transferred height information to the height values of the tooth blunt on the edge of the prepared area of the prepared tooth, so that a smooth transition from the restored area to the groomed area Tooth is created and a naturally shaped tooth shape is created. The Surface data in the restored area are used to manufacture the be required denture part to a computer-controlled machine tool tet, by means of which the tooth replacement part is made from a material blank becomes. The surface data required for this on the underside of the Restoration to adapt to the shape of the cavity and, in addition, the Data of the areas of the restoration below the tooth equator can be determined in a manner known per se.

Thus, the method of image deformation known per se is used to from the existing data of a model tap that no longer exists To imitate an area of a prepared tooth as closely as possible. This is done with high accuracy, so that the effort for rework is reduced with the inventive method.

In a further embodiment of the invention, the depth image of the model number nes in the outer contour of the tooth and in the inner feature points of the occlusal surface  by image deformation with the depth image of the prepared tooth for Dec brought. By aligning the shape of the model tooth to the pre parried tooth in the outer contour becomes the required smooth transition from restoration to the remaining tooth. The choice more characteristic Points within the occlusal areas advantageously provide for the sheep a natural tooth shape that covers the remaining surfaces structures of the prepared tooth fully taken into account, and therefore the opening wall of recontouring and the risk of damage occurring healthy tooth structure further reduced.

In a further embodiment of the method, a number of Au Contour lines of the intact tooth areas of the prepared tooth and ent speaking outer contour lines of the model tooth lying in the Depth images of the prepared tooth and the model tooth determined and as control points required for performing the image deformation assigned in pairs. The control points required for the image deformation on the contour lines of the prepared tooth and the model tooth are advantageously known in the depth images with the help of the method of active contours (Kass et al: Snakes: Active Con tour models, proc. First Int. Couf. on Computer Vision (ICCV), London 1987, pp. 259-268). This has the advantage that on the one hand the exact shape in areas of the outer contours is automatically determined, on the other hand minor interruptions in the outer contour line of the prepared tooth be interpolated by the method. In the event of major interruptions mark one or two additionally by the operator in the depth image te bases to complete the outer shape of the restored tooth to determine. This advantageously minimizes manual opening wall of the operator with an optimal shape of the rest  ration.

In an advantageous embodiment of the method, the height adjustment by interpolating the unknown height differences between the above taken altitude information and the original altitude values from the known height differences at the edge of the prepared area of the prepared tooth, being carried out by adding the interpolated Difference values and the depth values of that taken from the model tooth Area a steady transition between the area to be restored prepared tooth and the adopted area of the model tooth will be produced. This way, starting from Data of a model tooth of the same type Data for the surface of the restoring tooth and thus won for the tooth replacement part without that this data should be known explicitly or by an operator or would have to be modeled on a dental technician.

In a further embodiment of the method, the height adjustment the shape of antagonists of the prepared tooth is also taken into account. This makes the required criterion of the occlusal fit of the restaurant tion fulfilled.

The shape of the antagonists is advantageously taken into account of the prepared tooth by providing additional support when adjusting the height points are used, which are derived from depth images of the antagonists of the pre parried tooth and from depth images of a bite plate, which by Aufbei is plastically deformed between the affected teeth, or out data obtained from a sheet-like pressure sensor will.

The method is described below using an exemplary embodiment Reference to the drawing explained in detail.

Fig. 1 shows a schematic representation of a method for manufacturing a dental prosthesis.

Fig. 2 shows a depth image of a prepared posterior tooth with already marked and masked cavity.

Fig. 3 shows a three-dimensional representation of the prepared Seitenzah nes of FIG. 2nd

FIG. 4 shows a contour segmentation of the prepared posterior tooth of FIG. 2.

Fig. 5 shows the prepared posterior tooth of FIGS . 2 and 4 with characteristic points drawn in.

FIGS. 6 and 7 show the depth image of another tooth prepared sides and serve to illustrate the contour segmentation with the support of an operator.

Fig. 8, the isolated and labeled feature points is low image of a model tooth used.

FIG. 9 shows the mapping of detected feature points on the model tooth of FIG. 8.

FIG. 10 shows the depth image of the model tooth of FIG. 8 after an image deformation.

Fig. 11, the depth image of the restored occlusal surface facing a Höhenanpassungsinterpolation.

FIG. 12 shows a three-dimensional representation of the depth image of FIG. 11.

FIG. 13 shows a three-dimensional representation of the tooth replacement part to be created for the prepared posterior tooth of FIG. 2.

In the schematic illustration of FIG. 1 of the sequence of the method is shown an inlay for the occlusal surface side of a tooth for the production. The operation of this method is explained with reference to the illustrations in FIGS . 2 to 13.

Fig. 2 shows a posterior tooth 10 to be restored with an already prepared cavity 12 and parts of neighboring teeth 14 , 16 in the form of a depth image. After the posterior tooth 10 to be restored has been prepared by forming a cavity 12 , the depth image of the prepared tooth 10 shown is recorded. Then, in the depth image, the visible teeth 10 , 14 , 16 are roughly localized using a mask process and the cavity is marked ( FIG. 2). As a preprocessing step for localization, the use of an edge operator on the deep image is necessary, such as the Sobel operator known per se (H. Niemann: Classification of Patterns, Sprihger, Berlin, 1983). In the area of an annular mask, which is placed on the depth image at various positions in a predetermined search area, the edge strength values of the edge operator are added up and the sum is assigned to the point of the image which forms the center of the mask. Points in the search area, which are assigned very high values in this way, indicate tooth centers. The advantage of this procedure is that teeth can also be localized, the edge of which is only partially visible, be it that they lie on the edge of the image or that the tooth edge lies in the area of the cavity. FIG. 3 shows a three-dimensional representation of the prepared posterior tooth 10 with cavity 12 from FIG. 2.

Then a segmentation of the External tooth contour made. This is advantageously done by means of known method of active contours. For this the Kon initialized at the previously detected positions and their shape becomes changed using an iterative process until the edge evaluators tion of the contour points is maximum, while at the same time the local curvature the contour line is minimized. There will be breaks in the contour line interpolated automatically or by an operator. Especially smaller parts of the tooth rim that are affected by the defect automatically interpolated.

As an additional limitation of the contour line shape, overlaps of the contour lines of neighboring teeth are automatically detected in the image and counteracted by further influencing the active contours by pulling the points in the overlap area to their center point. As a result, the contact point to the neighboring tooth, the so-called approximal contact, is advantageously determined automatically by the method. The result of such a contour segmentation is shown in FIG. 4 for the depth image of FIG. 2. 80 contour points were selected and linearly interpolated to create the contour line 18 for the prepared posterior tooth 10 . The teeth 14 , 16 adjacent to the prepared posterior tooth 10 were also subjected to a corresponding contour segmentation in the area lying in the depth image.

In the case of cavities in which a larger part of the tooth edge is affected, the active contour can also be influenced by an operator by setting a few support points in the image, which are then interpolated by the contour line. Such a contour segmentation with the support of an operator is illustrated in FIGS . 6 and 7. In the depth image of FIG. 6 and 7, a prepared tooth side 50 is shown which has a relatively large cavity 52nd The contour line provided with the reference numeral 54 in FIG. 6 was determined with the method described above, but this needs to be corrected in the area of the cavity located at the top right in the figure, since it does not reflect the entire area of the previously existing tooth circumference. For this purpose, an operator specifies a certain number of points, in the present case two points, which are shown in FIG. 6 as white crosses. After a few iterations of the method of active contours, the contour curve shown in FIG. 7 and with the reference numeral 54 'is then automatically created.

Next, selected points in the occlusal surface of the model tooth and the remaining occlusal surface of the prepared tooth are determined. This is illustrated in Fig. 5. Fig. 5 shows the prepared tooth side of Fig. 2 and 4 in isolated from the depth image representation of these figures, where characte in the region of the occlusal surface 20 of the prepared tooth side 10 teristic points 22 have been marked 24th The most salient features of a occlusal surface 20 of a posterior tooth 10 are the tooth bumps 26 . The positions of the tips 22 of the tooth bumps 26 can be determined in an automatic process by searching for local maxima in the depth image. In addition, points 24 in trenches 28 between the bumps 26 are determined. The search for these can be based on the previously detected cusp tips 22 , for example by searching for the lowest points 24 on connecting lines between the cusp tips 22 .

In order to be able to use the image deformation method, the corresponding target position of these points in the deformed image is required for a number of selected points of the depth image of the model tooth 30 used, these target positions being determined by the determined characteristic or feature points 22 , 24 of the prepared posterior tooth 10 are given (see FIG. 8). For this purpose, the identified feature points 22 , 24 of the occlusal surface 20 of the tooth 10 to be restored and the target positions 32 , 34 of the occlusal surface 31 of the model tooth 30 must be assigned to one another in pairs. Since, as described above, possible interruptions in the contour line 18 of the prepared tooth 10 were closed by means of interpolation ( FIG. 4), the contour points of two complete contour lines can be assigned to one another.

The model tooth 30 is selected so that it is a tooth corresponding to the type of the prepared tooth 10 , since the contour line of two teeth of the same type are similar in shape. This similarity can be described via the local curvature at each contour point, its distance from the contour center of gravity and its distance from the closest feature points within the occlusal surface, these criteria being independent of the position and position of the teeth in the depth images. The criteria mentioned can be calculated for each contour point. A method for mapping the contour points onto one another uses the data series obtained by shifting the data series of the model tooth along the contour line in relation to that of the prepared tooth and distorting it non-linearly in such a way that a distance measure over the data series is minimized. When the assignment of the contour points has been made, the feature points within the purchase surface can also be assigned to one another on the basis of these. Points of the model tooth 30 , the corresponding point of which lies in the prepared area (points 36 in FIG. 8), are either ignored or the corresponding positions in the prepared area can be estimated by a further partial method or specified by an operator.

In Fig. 9, the result of the mapping of the detected feature points is shown. Both the contour points of the contour line 18 of the prepared posterior tooth 10 and the inner feature points 22 , 24 within the occlusal surface 20 were assigned to the corresponding points of the model tooth 30 in an automatic process. The feature points 32 , 34 of the model tooth 30 are shown in the illustration in FIG. 9 as black dots, with the outline of the model tooth 30 being shown in a reduced manner for clarification and the assignments being indicated by white lines.

After this "matching" of the feature points of the prepared tooth and the model tooth was made, the image deformation of the Depth image of the model tooth.

For the feature points determined as explained above in the depth image of the model tooth, there are target positions through the assigned feature points in the prepared tooth to which the feature points of the model tooth are to be shifted by the image deformation, which is illustrated by the white lines in the illustration in FIG. 9. Points between the detected feature points are "dragged" by a suitable interpolation method. This step can be clearly illustrated by imagining the depth image of the model tooth printed on a rubber skin, which is drawn to the respective target positions at the determined feature points and fixed there. The resulting picture approximates that of the tooth to be restored.

Methods of digital image deformation that work as described are known per se (G. Wolberg: Digital Image Warping, WEE CS Press, 1990). Fig. 10 shows the result 30 'of an image deformation of the depth image of the model tooth 30 of Fig. 8th

The preliminary height profile of the inlay to be created is now given by the partially congruent depth image of the model tooth 30 'in the region of the cavity 12 , so that this height information can be obtained.

The next step is to adjust the height of the height profile taken from the deformed model tooth 30 'to the tooth stump of the prepared posterior tooth 10 .

The height values of the area taken over generally form a jump edge at the transitions to the tooth stump 11 of the prepared posterior tooth 10 . The image deformation does cover both occlusal surfaces, but without taking into account direct height differences between them. In a further step, this height difference is minimized and the surface used is adapted to the height level of the tooth stump 11 . For this purpose, the differences in height of the two occlusal surfaces, especially at the edge of the cavity, are used as bases for an interpolation method that interpolates the area of the cavity 12 .

The result of this interpolation thus forms an estimate of the height difference between the occlusal surface 31 of the model tooth 30 and the original one inside the prepared area of the tooth 10 to be restored. Adding the interpolated values to the depth values of the deformed model tooth 30 ′ results in a continuous transition to the tooth stump 11 . Due to the previous alignment of the occlusal surfaces, there is also a natural continuation of the surface structure of the prepared tooth 10 .

The step of height adjustment is advantageously expanded in such a way that the occlusion is also taken into account in the restored occlusal surface. In addition, a depth image of the antagonists of the tooth to be restored is created. Furthermore, a bite plate made of plastically deformable material is deformed by biting the plate on the affected teeth, so that the impressions on the front and back of the bite plate can be utilized. As an alternative to this, the data of a flat pressure sensor known per se (US Pat. No. 4,856,993 and US Pat. No. 4,734,034) can also be used. With the help of a correlation process, the values of the depth images are mapped onto one another, so that guide values for the maximum height of the area to be restored can be determined. These are used as further bases in the interpolation method explained above. Fig. 11 shows the depth image and Fig. 12 shows the corresponding three-dimensional representation of an occlusal surface 20 'restored according to the invention as a result of the explained height adjustment interpolation.

The depth values generated in the area of the cavity 12 are now used to calculate the volume data for the inlay and to control a computer-controlled machine tool for producing the inlay. Fig. 13 shows a three dimensional representation of the occlusal surface of the thus produced dental prosthesis 40, wherein the depth data corresponding to the data shown in FIGS. 11 and 12, limited to those in the Dar position of Fig. 2 labeled region of the cavity 12.

The method makes it possible to have previously saved saved volume data of intact model teeth for the restoration of teeth to use occlusal surfaces, all for the cavities to be found in practice Different variants in size and shape are possible. The prepared tooth is first scanned to obtain a volume model of the tooth. The volume data of a model tooth of the same type is attached to this tooth bluntly adapted to the upper using the procedure Define the area of a dental prosthesis. The procedure is particularly suitable for the production of inlays that only part of the Replace occlusal surface, with the occlusal surface of the inlay smoothly into the occlusal surface of the remaining tooth is embedded. The process works largely automatic, but gives the treating dentist the opportunity possibilities of corrective action.

Claims (9)

1. Process for producing a dental prosthetic item for a given pre parried tooth, its height profile as from point-sampled heights values composite digitally stored depth image is present, whereby to determine the shape of the tooth replacement part, a depth image of an intact Model tooth of the same type as the prepared tooth by deformation with the depth image of the prepared tooth, the Hö hen information for the prepared area of the prepared posterior tooth taken from the deformed depth image of the model tooth and the Height information adopted by height adjustment to the Höhenwer te of the tooth stump at the edge of the prepared area of the prepared Tooth is adjusted so that a naturally shaped tooth shape is restored is whose surface data for the manufacture of the dental prosthesis from a Ver material blank by means of a computer-controlled machine tool be applied. 2. The method according to claim 1, in which the depth image of the model tooth in the outer contour of the tooth and in the inner feature points of the occlusal surface Image deformation with the depth image of the prepared tooth to cover brought. 3. The method of claim 1 or 2, in which a number of outside contour lines of the intact tooth areas of the prepared tooth and correspond corresponding outer contour lines of the model tooth in the points  Depth images of the prepared tooth and the model tooth determined and as control points required for performing the image deformation can be assigned in pairs. 4. The method according to claim 3, in which interruptions in the outer con Guidelines of the prepared tooth automatically or manually by a Be the person to be interpolated. 5. The method according to claim 3 or 4, in which to determine the outside contour lines uses the known process of active contours becomes. 6. The method according to any one of claims 3 to 5, in the characteristic Points of the purchase area are determined, after which the determined points in another step can be assigned to each other in pairs. 7. The method according to any one of claims 1 to 5, in which the Höhenanglei by interpolation of the unknown height differences between the transferred altitude information and the original altitude values the known differences in height at the edge of the prepared area of the prepared tooth is carried out, by adding the interpo difference values and the depth values of the ent from the model tooth area taken a steady transition between the area to be restored Area of the prepared tooth and the area taken over from the mo dellzahnes is produced. 8. The method of claim 7, in which the form of antagonists of the pre parried tooth is taken into account.   9. The method according to claim 8, in which taking into account the shape of the Antagonists of the prepared tooth take place in that at high adjustment additional bases are used, which are from depth images of the antagonists of the prepared tooth as well as from deep images of one Bite plate that becomes plastic by biting between the affected teeth is deformed, or won by a sheet-like pressure sensor calculated data.