News Column

Patent Issued for Method, Apparatus and Computer Program for Producing a Dental Prosthesis

July 23, 2014



By a News Reporter-Staff News Editor at Journal of Engineering -- Schweiger; Josef (Bergen, DE) has been issued patent number 8775131, according to news reporting originating out of Alexandria, Virginia, by VerticalNews editors.

The patent's inventor is Schweiger, Josef (Bergen, DE).

This patent was filed on March 1, 2011 and was published online on July 8, 2014.

From the background information supplied by the inventors, news correspondents obtained the following quote: "A two-piece tooth replacement having a scaffold made of, for example, zirconium oxide and a veneer made of, for example, siliceous ceramics is described in DE 10 2005 042 091 A1, wherein the tooth replacement is produced with the CAD/Cam method from two individual components which are connected to each other in a joining step. In DE 10 2005 042 091 A1, no indication can be found how the resulting boundary surface is geometrically predetermined.

"DE 103 12 848 A1 discloses to form a tooth replacement part by using a tooth model and/or a tooth database, wherein the internal structure of the tooth model corresponds to the internal structure of an ideal tooth or is determined by means of an image prepared in advance of the individual tooth to be replaced/added, or, respectively, the tooth database reproduces tooth data of real existing teeth in digitalized form. In DE 103 12 848 remains unclear which aspects of the internal structure of a tooth are addressed, in particular, the dentino-enamel junction of a tooth is not addressed in DE 103 12 848 A1.

"DE 199 23 978 A1 discloses a method for computer-assisted, patient-specific illustration and planning of dental and/or dental prosthetic work, wherein a digitalized database comprising a plurality of views of tooth models is used. The inner structure of a tooth or a tooth replacement is not addressed here.

"In DE 102 52 298 B3, a method for producing tooth replacement parts or tooth restorations using electronic tooth representations is described. DE 102 52 298 B3 proposes a generic tooth model for modeling the outer geometry of a tooth or tooth replacement, wherein known data of the (remaining) tooth situation are used for adapting the generic tooth model to the individual case.

"In the known prior art, the inner structure of a natural tooth, in particular the inner structure of that tooth for which a tooth replacement to be prepared is provided, is virtually disregarded. The inner structure of the tooth replacement to be prepared, in particular the extension of a boundary surface between a core and an outer layer of the tooth replacement, without consideration of the natural tooth structure and without defined rules, depends on the skills and the experience, for example, of the dental technician who monitors or performs the preparation of the tooth replacement."

Supplementing the background information on this patent, VerticalNews reporters also obtained the inventor's summary information for this patent: "One of the objects on which the present invention is based on is now to provide an aforementioned method, a corresponding device and a corresponding computer program by means of which the tooth replacement prepared therewith can fulfill the technical, in particular mechanical requirements for the tooth replacement largely independently of skills and experience of the user and, in terms of aesthetics, comes as close as possible to the desired appearance of a natural tooth.

"According to the invention, one aspect of the aforementioned object is approached with the proposal of a method for producing a tooth replacement comprising the generation of the tooth replacement with, in particular, a one-piece and with, in particular, a one-piece outer layer, wherein the core and the outer layer define a boundary surface between each other, wherein the extension of the boundary surface represents a natural extension of the dentino-enamel junction of a tooth for which the tooth replacement is provided, wherein the extension of the boundary surface (i) is determined based on a recording of the dentino-enamel junction of the tooth for which the tooth replacement is provided, and/or (ii) is determined based on directly or indirectly recorded geometry data of at least a portion of the outer geometry of the tooth for which the tooth replacement is provided.

"According to the invention, a further aspect of the aforementioned object is approached with the proposal of a device for carrying out such a method or a method according to any one of the dependent claims.

"Furthermore, a further aspect of the aforementioned object is approached with the proposal of a computer program having program means which cause a device according to the invention to carry out a method according to the invention when the computer program is carried out on the device.

"One of the findings on which the present invention is based on is that when reproducing the natural extension of the boundary surface between dentin material and enamel material in the natural tooth as comprehensive as possible, the material of the core of the tooth replacement quasi replaces the dentin of the tooth and the material of the outer layer of the tooth replacement replaces the enamel of the tooth in the region of the tooth replacement, whereby an appearance corresponding to the natural tooth can be reproduced in a simple manner without having to compromise features such as strength, elasticity or hardness of the restored tooth.

"Furthermore, it was found that the determination of the dentino-enamel junction or the extension of the same can take place in a suitable manner, thus without completely destroying the tooth or the remainder of the tooth, directly on the tooth for which the tooth replacement is provided so as to allow a restoration as natural as possible. Such a determination of the extension of the boundary surface between natural dentin and natural enamel can be carried out, on the one hand, at a sufficiently early time and on the tooth, thus (long) before a damage or, on the other, after the damage and on the remainder of the tooth which is still available.

"However, it was also identified that the present invention is not limited to the case in which the dentino-enamel junction itself can be recorded. It was found that for determining the extension of the boundary surface to be prepared it is also possible, i.e. additionally or alternatively, that the outer geometry or at least a portion of the still available outer geometry of the tooth can be used. Through the knowledge and in consideration of the processes by means of which a natural tooth is formed it was found that from the outer geometry of a tooth, the geometry of the boundary between enamel and dentin can be concluded.

"It should be noted that with the invention, no reproduction of the boundary surface between natural dentin and natural enamel of the tooth to be replaced is intended which lies for example in the order of magnitude of individual cells or even finer, although this seems to be possible with appropriate effort. In a practical implementation it was found that the accuracy in the range from 20 .mu.m to 100 .mu.m, which can be achieved with the present technologies for preparing, for example, ceramic or plastic bodies, provides already very satisfying results, wherein a lower resolution or contour accuracy, for example in the range of 100 .mu.m or even 200 .mu.m, was also found to be sufficient.

"It is known to use databases comprising tooth data sets in the field of dental restorations. However, the previously known databases relate only to a partial or also complete outer geometry of the teeth or tooth models. One aspect of the present invention considers using a database which is suitable for producing an inner tooth structure and was created, among other things, on the basis of an examination or measurement of natural teeth. The access to inner tooth structures is in particular of great advantage for a realistic reproduction of anterior teeth but also posterior teeth, as has been found by the inventors. The reason for this particular advantage is that by a multipart layer structure of the tooth replacement which is based on the structure of a natural tooth, an aesthetically appealing tooth replacement crown or dental bridge can be achieved particularly well.

"Up to now, if layering in a conventional manner was provided for the structure of a replacement tooth, such layers were carried out manually by the dental technician. Thereby, the dental technician forms an inner structure as core onto which then an outer layer with transparent incisal compounds is applied. However, the resulting structure of the replacement tooth is exclusively based on the dental technician's skills and feeling gained through experience and is in no way associated with the natural inner structure of a tooth or even the tooth for which the tooth replacement is provided. The dental technician is focused on preparing a finished tooth replacement with the appearance of a natural tooth without having the will or the necessary information to reproduce the inner structure of the tooth.

"Through an extensive use of computer-assisted manufacturing systems, the present invention allows to produce dental crowns from glass ceramics or lithium silicate ceramics using the CAD/CAM method without being limited here to the reproduction of only the desired outer shape. A desired aesthetic for the anterior tooth region can be achieved because a tooth provided with the tooth replacement produced according to the invention comes close to the natural tooth not only with respect to the outer geometry but also in terms of its inner structure.

"While in conventional methods which provide a tooth replacement structure with core and outer layer, the inner tooth structures (which in the dental technician's jargon were sometimes called 'dentin cores' without actually reproducing or representing natural dentin structures) were prepared in manual production by the dental technician, for example by growing and digitally copying or by reducing the anatomical outer shape by means of abrasive bodies and thus were based substantially on the experience of the dental technician, the present invention allows a completely computer-assisted production of a tooth replacement including the inner tooth structure, for example based on data from a database comprising information on natural teeth.

"In one configuration of the method according to the invention, recording the dentino-enamel junction of the tooth takes place by means of computer tomography, in particular microcomputer tomography, sonography, radiography, close-range photogrammetry and/or photography.

"The mentioned methods allow advantageously a non-invasive and non-destructive determination of the boundary surface between dentin and enamel, which boundary surface is located inside the tooth and thus is not directly accessible so that such a determination can take place independently of the necessity for a tooth replacement thus, while the tooth is not damaged yet. For example, the image of the dentino-enamel junction can be taken as soon as the tooth is fully developed.

"In a further configuration of the method according to the invention, the indirectly recorded geometry data are determined from geometry data of the situation of the remainder of the teeth.

"In particular, if in the case of an affected, damaged or even missing tooth, no complete or sufficient determination of the original geometry data of the original tooth can take place, it is advantageously still possible to use the geometry of the remainder of the teeth, thus, in particular adjacent and/or corresponding teeth, to draw conclusions on the geometry of the tooth to replaced.

"In a further configuration of the method according to the invention, determining the extension of the boundary surface comprises virtually deforming the outer model geometry of a predetermined tooth data set for adapting to the outer geometry of the tooth for which the tooth replacement is provided, and virtually deforming a model boundary surface of the predetermined tooth data set corresponding to the deformation of the outer geometry for determining the boundary surface.

"It was found that the geometry of the boundary surface between dentin and enamel of a natural tooth and the outer geometry of the natural tooth correlate in a sufficient manner so as to be able, based on the outer geometry of the tooth to be replaced and via an adaptation of the tooth model with data on geometry to the detected outer geometry, to determine the geometry of the boundary surface to be generated by adequate adaptation of the tooth model data on the dentino-enamel junction.

"In a further configuration of the method according to invention, determining the extension of the boundary surface comprises determining a group of tooth data sets, wherein a predetermined average value of the outer model geometry of the group of tooth data sets corresponds to the outer geometry of the tooth for which the tooth replacement is provided, and determining the extension of the boundary surface by means of the predetermined average value of model boundary surfaces of the group of tooth data sets.

"Alternatively or additionally to the aforementioned determination of the boundary surface geometry of the tooth data set to be prepared by an adapting 'deformation' of model data, a combination of a plurality of tooth data sets can also be used. Said combination of a plurality of tooth data sets is determined in such a manner that the result of the application of an averaging algorithm for the data of the outer geometry of the tooth data sets corresponds to the indirectly or directly determined outer geometry of the tooth to be replaced (i.e. the outer geometry of the tooth to be replaced in the healthy, original state or the outer geometry of the combination of remaining tooth with prepared tooth replacement in the ideal case). If said averaging algorithm is applied accordingly also to the information provided via the tooth data sets on the boundary surface between enamel and dentin, this results in the desired extension of the boundary surface for the tooth replacement to be prepared.

"In one embodiment of this configuration of the method according to the invention, the average value is determined by averaging the absolute and/or relative positions of predetermined reference points.

"By selecting suitable reference points, for example in the form of characteristic points of the surfaces, the determination of the average value for a plurality of three-dimensional surfaces can be transferred into a determination of a plurality of position values, wherein characteristic properties of the respective tooth type are maintained.

"In a further configuration of the method according to the invention, determining the extension of the boundary surface comprises determining a tooth data set, the outer model geometry of which meets a predetermined similarity criterion with respect to the outer geometry of the tooth for which the tooth replacement is provided, and determining the extension of the boundary surface based on the model boundary surface of the tooth data set.

"In particular if the pool of available tooth data sets is large, it can be expected that within the plurality of these tooth data sets at least one data set exists which describes a tooth or a tooth model which, on the outside, corresponds to the tooth to be replaced so that the inner geometry of the same, thus the boundary surface between dentin and enamel stored in this data set, defines the boundary surface between core and outer layer for the tooth to be prepared. The higher the number of available tooth data sets, accordingly, the higher is the probability of a suitable tooth data set or the number of suitable tooth data sets, wherein the term 'suitable' describes a similarity between the natural tooth to be replaced (in its original shape) and the geometry of the tooth data set, which similarity exceeds a predetermined degree. Different similarity degrees are known to the person skilled in the art which can be selected and/or combined according to the respective requirements of implementing the present invention.

"In a further configuration of the method according to the invention, the model boundary surface of at least one tooth data set is determined based on a recording of the dentino-enamel junction of a natural tooth by means of computer tomography, in particular microcomputer tomography, sonography, radiography, close-range photogrammetry and/or photography and/or a recording of the outer geometry of the dentin of a natural tooth after removing the enamel.

"Even if the data collection of tooth geometry data in the database of tooth data sets can be based entirely on a theoretical determination based on suitable models, the data collection on the basis of actual teeth including a data collection which is not entirely non-destructive is associated with the advantage that independent of potential deficits of a theoretical modeling, the preparation of tooth replacements according to the present invention is based on geometries of actual teeth.

"In a more specific configuration of the method according to the invention, recording the dentino-enamel junction takes place by means of close-range photogrammetry and/or photography using light for which the enamel of the tooth is translucent, in particular using red light or light having a longer wavelength.

"In connection with the present invention it was found that--although a natural tooth is usually opaque for the human eye in such a manner that inner structures are not visible--by selecting light which is able to pass sufficiently unhindered through the outer layer, thus the enamel of the tooth, recording inner structures in the form of the boundary surface between dentin and enamel is possible.

"In a further configuration of the method according to the invention, the material or the materials for core and outer layer are selected such and/or generating core and outer layer is carried out such that the translucence of the outer layer is higher than the one of the core. Preferably, with respect to their translucence, the materials correspond to the natural materials dentin and enamel in terms of absolute values or in their relative ratio. The average light transmittance (direct transmission coefficient (tc) [%]) in the wavelength range from 400 to 700 nm for a sample with a thickness of 1 mm (measured with a spectral photometer DU-7 (Beckmann) after polishing with a 3 .mu.m diamond for removing scratches) for the core lies preferably in the range from 20 to 65%, particularly preferred in the range from 40 to 60%. In contrast, for the outer layer as veneer layer, a translucence in the range of 65 to 95% is preferred, in particular in the range of 76.5 to 85%. In 'Composite Resin Restorations-Natural Asthetics and Dynamics of Light' by S. Duarte et al. (Pract. Proced. Aesthet. Dent. 2003, 15(9); A-H) is specified that enamel (as material of the natural enamel layer) has a light transmittance/translucence of 70.1%, and dentin of 52.6%.

"When setting the translucence of the outer layer in such a manner that it exceeds the translucence of the core, the aesthetic appearance of the tooth replacement can be adapted in a simple manner to the one of a natural tooth (in particular the one of the adjacent teeth).

"In a further configuration of the method according to the invention, the core comprises or consists of an oxide-ceramic material, a silicate-ceramic material and/or a plastic material and/or the outer layer comprises an oxide-ceramic material, a silicate-ceramic material and/or a plastic material or consists thereof. Here, according to the above, a translucent, silicate-ceramic, in particular translucent glass-ceramic material is particularly preferred for the outer layer.

"In the present context, silicate ceramics include in particular the following: Glass ceramics, leucite-reinforced glass ceramics, feldspar ceramics, leucite-reinforced feldspar ceramics, lithium silicate ceramics and fluoroapatite ceramics.

"The aforementioned materials have already been used in practice so that adequate experience in terms of handling and use is available for the person having relevant skills, which experience can advantageously be used during the implementation of the present invention.

"It should be noted that the hardness of the outer layer is of importance for the functionality of the restoration. It should not be too high because otherwise the opposing tooth would be excessively abraded. It should not be tool low either because otherwise the veneer would be ground off.

"In a conventional restoration, the core of the present invention finds its equivalent in the combination of scaffold and the so-called 'dentin layer'. It is preferred that the core of the present invention has a strength which corresponds to the strength of the conventional scaffold or is higher. Preferred in the present case is a minimum strength of 100 MPa for individual treatments, 300 MPa for three-unit bridges in the anterior teeth region and 500 MPa in the posterior teeth region (values according to standard).

"When selecting the materials and properties for the core and the outer layer and in case that ceramic materials are used which are bonded by firing in the furnace, preferably, the coefficient of thermal expansion (CTE) should be considered which can play an important role for the function of the treatment (i.e. the tooth replacement). If, during the cooling phase, the veneer ceramic (i.e. the outer layer) is subjected to compressive stress, this increases the stability of the veneer. Tensile stress in the veneer ceramic can result in cracks and spalling, thus in a failure. Therefore, the CTE of the veneer is preferably lower than the one of the inner core.

"When selecting the material and the corresponding production, the abrasion resistance of the outer layer or, respectively, the abrasiveness of the material can also be considered and, preferably, should be considered. The selection of the material for the outer layer is preferably made in such a way that the material's abrasion values are largely approximated to the abrasion values of the natural tooth enamel so that the teeth in the opposite jaw are not excessively abraded during mastication."

For the URL and additional information on this patent, see: Schweiger, Josef. Method, Apparatus and Computer Program for Producing a Dental Prosthesis. U.S. Patent Number 8775131, filed March 1, 2011, and published online on July 8, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8775131.PN.&OS=PN/8775131RS=PN/8775131

Keywords for this news article include: Patents, Dentistry, Prosthetics, Medical Devices.

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Source: Journal of Engineering


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