Blank for a dental prosthesis and method for its manufacture, use of the blank and method for manufacturing a dental prosthesis from the blank

Description

[0001] The invention relates to a blank for dental purposes, in particular for the production of dental restorations such as dental prostheses, with which the optical properties of natural tooth material and the natural oral mucosa can be very well imitated and which, due to its properties, is particularly suitable for the simple production of aesthetically demanding dental prostheses, such as full upper jaw dentures and / or full lower jaw dentures, with very good mechanical properties.

[0002] In the dental field, multi-unit and sometimes long-span implant-supported restorations are increasingly being fabricated for patients in the lower and upper jaw. In addition to the aesthetics of the teeth, the color matching of the transition to the oral mucosa (gingiva) and the optimal color imitation of the actual oral mucosa also play a key role in the overall aesthetic result. However, a significant amount of manual effort is required to restore the gingival area as lifelike as possible.

[0003] In the past, full dentures were typically manufactured by taking a silicone impression of the patient's edentulous jaw; creating a plaster model on this basis; placing an artificial row of teeth on the plaster model with the aid of a wax plate; performing a try-in on the patient and, if necessary, a subsequent adjustment; embedding the artificial teeth, the wax model, and part of the plaster model in plaster; melting out the wax; filling the resulting cavity with a PMMA-based prosthetic material; removing the plaster embedding; and finally cleaning and polishing the surface. This procedure generally leads to very good results, but is very time-consuming and labor-intensive.

[0004] Multi-colored blocks that simulate the color and translucency gradient from dentin to enamel and their use in dental technology are well known in the art. Blanks based on zirconium oxide ceramic and their use for fabricating dental prostheses in a CAD / CAM process using a computer-controlled milling system are also well known in the art. However, even when using such blanks, a high level of manual effort is required to restore the gingival area true to nature. This is essentially because, according to a first alternative, the gingival area must be applied subsequently, e.g., by firing or pressing the gingival portion onto a ZrO2 restoration by layering or pressing it using gingiva-colored press blanks and then realistically reproducing it using all-ceramic techniques.It is also possible to layer the gingival area with composite material after the restoration is completed. However, this carries the risk of color instability and plaque and odor formation.

[0005] According to a second alternative, a zirconium oxide blank with a pre-formed gingival area is used, which, however, still requires post-treatment to achieve optimal color imitation of the gingiva. When using tooth-colored, pre-shaded zirconium oxide discs with a pre-formed gingival portion, the tooth-colored shaded area is covered with glass-ceramic layering, veneering, staining, and characterization materials in several steps. When using unshaded or slightly shaded zirconium oxide discs with a pre-formed gingival portion, the problem is currently solved by infiltrating the gingival area with a highly concentrated solution based on water-soluble Er 3+< salts, such as ErCl 3 , ErCl 3 ·6H 2 O, or Er(NO 3 ) 3 ·5H 2 O.

[0006] The highly concentrated Er 3+< -based infiltration solutions have a strongly acidic pH value, often in the range of 1-3. For safety reasons, improper use poses a very high risk of burns or the inhalation of harmful gases / vapours, such as HCl or HNO 3 vapors, when opening the containers without using a suitable extractor or adequate room ventilation. Furthermore, such solutions are usually not stable over the long term and therefore change their properties over time. Concentration changes can also occur over time due to uncontrolled evaporation of the solvent. Uncontrolled complex formation can also occur, as these solutions usually contain certain organic compounds. In the worst case, this can lead to the precipitation of certain compounds.

[0007] Infiltration with coloring solution can also result in the coloring solution not being distributed evenly throughout the porous zirconia. This can depend on both the user and the surface quality of the restoration, such as the degree of dust-freeness and moisture content. Inhomogeneities can have a significant impact on the final result.

[0008] Infiltration in the gingival region also causes a change in the degree of stabilization and the phase composition of the zirconium oxide after dense sintering. During dense sintering, Er 3+< ions can be incorporated into the crystal framework in addition to the Y 3+< ions already incorporated into the zirconium oxide. This then leads to overstabilization and a corresponding change in the phase composition in the infiltrated areas. This results in a deterioration of mechanical properties, particularly fracture toughness and biaxial strength. Especially in the gingival region, implant-supported restorations absorb a very high masticatory load, so weakening of this area must be avoided.

[0009] In addition, doping with Er 2 O 3 in higher concentrations changes the sintering behavior of the zirconium oxide. Er 2 O 3 acts as a sintering activator and changes the theoretical density of the stabilized zirconium oxide. The overall shrinkage of the infiltrated area increases, so that the magnification factor taken into account when milling the blank is no longer correct and the accuracy of fit of the densely sintered restoration is negatively affected. If Er 2 O 3 is only used locally to color the gingival area, shrinkage occurs earlier locally during sintering, i.e. while the gingival area is already shrinking, the restoration in the tooth area lags behind in the shrinkage. This leads to stresses during the sintering process that remain in the overall construction and weaken it permanently. Sudden fractures often occur at the workplace or when inserting the restoration.

[0010] To simplify prosthesis production, WO 2010 / 057584 A1 proposes a milling block with a pink component and a tooth-colored component, wherein both the pink component and the tooth-colored component consist essentially of PMMA or a (meth)acrylate-based plastic. Furthermore, the use of such a milling block for the production of a full upper jaw denture or a full lower jaw denture using a CAD / CAM process by means of a numerically controlled milling system is described.

[0011] WO 2013 / 068124 also describes a milling block with two sections for the production of dental prostheses. The first section has a prosthesis base that can be machined according to the jaw shape and adapted to the patient's individual mucosa by milling. The second section has non-individualized, prefabricated artificial teeth that are arranged in a predetermined pattern and require no further finishing. Zirconium oxide ceramic is mentioned as a material for the first and second sections. However, adaptation to the individual patient situation in the opposing jaw is not possible with such a milling block.

[0012] US 2021 / 0128283 A1 describes a two-tone blank for the fabrication of a prosthesis using CAD / CAM. The blank has a lower pink layer to imitate the gingiva and an upper tooth-colored layer to imitate teeth. Zirconia ceramic is mentioned as a material for the upper and lower layers. The interface between the two layers has a multitude of convex and concave sections, so that the upper surface of the gingival layer has a wavy shape. It is described that this method makes it particularly easy to imitate the natural transition to the gingiva and reduces the number of shade correction steps.

[0013] EP 3 397 193 B1 discloses a blank for dental prostheses, which has a first layer based on zirconium oxide ceramic and a second layer based on zirconium oxide ceramic, wherein the first layer and the second layer differ in color and form an interface, wherein the interface is wave-shaped with alternating wave troughs and wave crests, and the crest lines of the wave crests, viewed in plan view of the interface, extend radially, wherein the blank is at least partially circular-arc-shaped. 1

[0014] US2020015944A1 discloses a blank for dental prostheses, which has a first layer based on PMMA and a second layer based on PMMA, wherein the first layer and the second layer differ in color and form an interface, wherein the interface is wavy in the dental arch course with alternating wave troughs and wave crests and the crest lines of the wave crests, viewed in plan view of the interface, extend radially from oral to vestibular in the area of ​​the anterior teeth, wherein the blank is at least partially circular in shape and the crest lines run rising from distal to mesial with an average gradient of 15 degrees or more.

[0015] EP 3 064 170 A1 and EP 3 597 143 A1 each disclose a prosthesis blank which is constructed from a flesh-coloured plastic material and a tooth-coloured plastic material, wherein the interface between the materials, viewed in the dental arch, is wave-shaped.

[0016] It is therefore a major challenge to provide blanks that meet the diverse requirements for use in dental technology, especially for the fabrication of dental prostheses. Such blanks should not only be easy to manufacture, but they should also be easily moldable to the desired geometry while still producing high-strength restorations. Finally, the blanks should produce an appearance that closely resembles that of natural tooth material and the natural oral mucosa, thus eliminating the need for complex subsequent creation of the desired optical properties of the dental prostheses.

[0017] The invention is intended to avoid the aforementioned problems. The invention is therefore based, in particular, on the object of providing a blank that is easy to manufacture, which can be easily machined into the shape of the desired dental restoration, and which, after shaping, can be converted into a precise and high-strength dental prosthesis. The blank can closely mimic the visual appearance of natural tooth material and natural oral mucosa.

[0018] This object is achieved by the blank according to claims 1 to 18. The invention also relates to the method for producing the blank according to claims 19 to 21, the use of the blank according to claim 22, and the method for producing dental prostheses according to claims 23 to 25.

[0019] The invention relates to a blank for dental prostheses, iea dental prosthesis blank. The blank according to the invention is characterized in that it has a first layer based on zirconium oxide ceramic and a second layer based on zirconium oxide ceramic, wherein the first layer and the second layer differ in color and form an interface, wherein the interface is wave-shaped in the dental arch course with alternating wave troughs and wave crests, and the apex lines of the wave crests, viewed in plan view of the interface, extend radially in the mesial-distal direction, wherein the blank is at least partially circular in shape, and the angle between a fictitious straight line connecting the lowest point of the wave trough for the second molar to be created with the lowest point of the wave trough for the central incisor to be created and the projection of this fictitious straight line onto a base surface of the circular blank is 2.0° to 4.5°.

[0020] The term "based on" means that the first and second layers of the blank predominantly contain zirconium oxide, i.e., ZrO2, based on the mass of the sum of all components of the layer. In addition to ZrO2 and small amounts of the impurity HfO2, the first and / or second layers may, for example, contain components for adjusting the color, such as Fe, Tb, Ce, Pr, Mn, Cr, Ni, Co, Nd, Dy, Eu, Er, V, and / or Ti, and / or components for adjusting the sintering kinetics, such as Mg, Al, Y, Ce, La, Yb, Gd, Ga, and / or In. Furthermore, the first and / or second layers may contain mixtures of zirconium oxide with other ceramics or spinels in the form of a composite and / or mixtures of zirconium oxide with pigments.

[0021] Differences in color refer to differences in shade in the narrower sense and / or differences in translucency, opalescence, or fluorescence. The term "translucency" describes the light transmission. The color can be characterized in particular by its Lab value or by a shade guide commonly used in the dental industry. Furthermore, it is not necessary that the differences in the color of the first and second layers in the blank be visible to the human eye. Rather, a difference in shade and / or translucency may only become visible after a sintering step or heat treatment. Likewise, the term "color gradient" encompasses not only a gradient in shade but also gradients in translucency, opalescence, or fluorescence.

[0022] The interface between the first and second layers is wavy in the dental arch of the dental prosthesis to be created. This means that in a side view of an arc-shaped, in particular parabolic or semicircular, cut surface through the blank, the interface between the first and second layers is wavy. If the blank is in the shape of a disc, for example, the cut surface extends from the upper base surface to the lower base surface of the disc and runs essentially parallel to the outer surface of the disc. The wave shape has alternating wave troughs and wave crests. The wave troughs can also be referred to as grooves or depressions, and the wave crests as ribs or elevations. The peaks of a wave crest and the - downward-facing - peaks of a wave trough each form a vertex line.In other words: The crests of a wave crest or trough each form a line, in particular a straight line. The crest lines of the wave crests, and preferably also the crest lines of the wave troughs, extend from the inside to the outside, i.e., from mesial to distal, when viewed from above onto the interface or base of the blank. The term "wave-like" is not used restrictively to describe purely sinusoidal waveforms, but generally includes any pattern of alternating raised and depressed areas.

[0023] Furthermore, the geometry of the interface between the first and second layers of the blank is designed such that the interface is radially shaped in the mesial-distal direction. This means that, in a plan view of the interface of the blank, the crest lines of the wave crests extend from a central area of ​​the blank in a radial direction, i.e., outward, in a ray pattern, preferably in the form of straight lines. The three-dimensional wave and ray geometry is preferably based on data from a large number of real patient cases.

[0024] The blank according to the invention is thus characterized in particular by the fact that the two differently colored layers and the integrated wave and ray geometry of the layers create a desired color gradient, so that the color of teeth and gingiva, as well as the transition from tooth material to gingiva, can be particularly well imitated in the finished dental prosthesis. The wave shape of the first layer can depict the gingival margin particularly well. Due to the radially extending wave structure, the gingival margin can always be created almost automatically, regardless of the size of the required dental arch. This represents a particular advantage over the checkerboard-like distribution of convex and concave areas known from US 2021 / 0128283 A1.

[0025] Furthermore, the blank according to the invention is characterized by the fact that, through the use of zirconium oxide ceramic-based layers, high-strength dental restorations can be produced that fully meet the mechanical property requirements of, for example, wide-span dental restorations such as dentures. In addition, the use of ZrO2 as a material leads to further advantages. For example, with zirconium oxide ceramics, the blank can be made flatter overall than with blanks based on plastic materials, thus reducing the overall height of the blank and simplifying machining of the blank in conventional CAM milling units, e.g., due to fewer undercuts in the design.

[0026] The blank according to the invention not only closely replicates the visual appearance of natural tooth material and natural oral mucosa, but also allows the blank according to the invention to be given the shape of the desired dental prosthesis in a particularly simple manner. This is surprisingly achieved by a combination of the special design of the interface between the first and second layers and the use of zirconium oxide as the material of the first and second layers. The blank according to the invention enables efficient monolithic production in the areas of digital fixed and partially removable prosthetics, so that a partial or complete denture can be manufactured in one milling process and a few manual steps. Furthermore, after shaping, e.g.CAM milling eliminates the need for subsequent infiltration with a coloring solution in the gingival area, which has led to frequent restoration failures in vivo in the past. This guarantees stress-free monolithic fabrication and thus a long-term stable restoration.

[0027] In a preferred embodiment of the blank according to the invention, the geometry of the interface between the first and second layers of the blank is designed such that the interface, viewed in a top view of the interface, is fan-shaped in the mesial-distal direction in the area of ​​the anterior teeth of the prosthesis to be created. This means that in a top view of the interface of the blank, the apex lines of the wave crests extend from a center point of a partial section of the dental arch to be created in a radial direction, i.e. outwards, in a fan shape. This means that in the area of ​​the anterior teeth to be created, the wave crests, and preferably also the wave troughs, viewed in the oral-vestibular direction, preferably extend in a fan shape, i.e. radially starting from a ray center point.

[0028] Furthermore, it is preferred that the interface between the first and second layers in the area of ​​the molars to be created—viewed in a top view of the interface—feature radial crest lines of the troughs in the oral-buccal direction, and in particular, essentially parallel crest lines of the troughs, and preferably also crests. The term "essentially parallel" means that the crest lines of the troughs, and preferably also crests, extend parallel or deviate from parallelism by a maximum of 10 degrees, in particular a maximum of 5 degrees. The design of parallel crest lines of the troughs in the area of ​​the molars to be created offers the particular advantage of improved aesthetics in the posterior region.

[0029] Overall, the two above-mentioned embodiments of a fan-shaped design of the crest lines of the wave crests in the anterior tooth region and a parallel design of the crest lines of the wave troughs in the posterior tooth region allow both the function and the aesthetics of the final dental prosthesis to be improved. In particular, it is possible to provide suitable tooth sets for both small and large dental arches. With large dental arches, the dental arch is milled somewhat further radially outwards, i.e., in the vestibular direction, and with small dental arches, it is milled further inwards, i.e., more orally. Due to the inventive parallelization of the wave crests and troughs of the interface for the molar region, a comparatively large chewing surface is available even with small dental arches.

[0030] Thus, in a preferred embodiment, the blank according to the invention can have fan-shaped troughs and crests with a true ray center in the area of ​​the central incisors to be created. However, this crest line for the canine to be created no longer accurately matches this crest line. Rather, this crest line is less fan-shaped and more parallel to the adjacent crest line of the lateral incisor. This also applies to the first premolar crest line, which in turn has a course that is even more closely aligned to parallelism. The subsequent crest lines, i.e., the crest crest lines for the second premolars, the first molars, and the second molars, are completely parallel to one another.

[0031] Furthermore, the ridge lines preferably slope upwards from distal to mesial. The interface is therefore preferably arranged at an angle within the blank. This allows the color and translucency gradient of natural teeth, especially anterior teeth, to be particularly well imitated.

[0032] In a further preferred embodiment, the second layer of the blank according to the invention has a continuous, i.e., linear, or discontinuous, i.e., non-linear, color gradient. This also allows the color and translucency gradient of natural teeth, especially anterior teeth, to be particularly well imitated.

[0033] The zirconium oxide ceramic of the second layer particularly preferably contains yttrium, and the color gradient, in particular a translucency gradient, is preferably formed by a gradient in the amount of yttrium, i.e., by a gradually changing amount of yttrium. Thus, the blank preferably has a continuous, i.e., linear, or discontinuous, i.e., non-linear, gradient in the amount of yttrium. In particular, the yttrium content increases from the interface between the first and second layers to the outer surface of the second layer opposite the interface. This increase in the yttrium content can occur continuously or in stages.

[0034] In one embodiment, the second layer of the blank according to the invention has at least two discrete layers that differ in their yttrium content.

[0035] The second layer preferably has an inner layer and an outer layer, wherein the inner layer borders the interface between the first and second layers, and the outer layer borders the outer surface of the second layer opposite the interface. The inner layer serves in particular as a so-called dentin layer, i.e., to imitate the dentin area of ​​natural teeth. The outer layer serves in particular as an incisal layer, i.e., to imitate an enamel layer of natural teeth.

[0036] In this embodiment, it is particularly preferred that (a) the inner layer has an yttrium content of 2.0 to 6.0 mol%, in particular 3.0 to 5.0 mol%, and (c) the outer layer has an yttrium content of 3.5 to 8.0 mol%, in particular 4.0 to 7.5 mol%, where the yttrium content is a proportion of the amount of Y 2 O 3 related to the sum of the amounts of Y 2 O 3 , ZrO 2 and HfO 2 is defined.

[0037] Optionally, there is a so-called intermediate layer between the inner layer and the outer layer. If an intermediate layer is present, it is particularly preferred that (a) the inner layer has an yttrium content of 2.0 to 6.0 mol%, in particular 3.0 to 5.0 mol%, (b) the intermediate layer has an yttrium content of 3.0 to 7.0 mol%, in particular 3.5 to 4.5 mol%, and (c) the outer layer has an yttrium content of 3.5 to 8.0 mol%, in particular 4.0 to 7.5 mol%, where the yttrium content is a proportion of the amount of Y 2 O 3 related to the sum of the amounts of Y 2 O 3 , ZrO 2 and HfO 2 is defined.

[0038] Furthermore, it is preferred that the inner layer, the outer layer and optionally the intermediate layer contain, in addition to Y 2 O 3 , oxides for adjusting the color, in particular oxides of Fe, Cr, Mn, Tb, Pr, Ce, Ni, Co, Nd, Dy, Eu, Er, V and / or Ti, and oxides for adjusting the sintering kinetics, in particular Mg, La, Al, Ce, Yb, Gd, Ga and / or In.

[0039] Preferably, the inner layer, the outer layer, and optionally the intermediate layer contain, in addition to Y 2 O 3 , at least one oxide of Fe, Cr, Mn, Tb, and Pr to adjust the color, and particularly preferably oxides of all of these elements. Preferably, the inner layer, the outer layer, and optionally the intermediate layer contain, in addition to Y 2 O 3 , at least one oxide of Mg, La, Y, and Al to adjust the sintering kinetics, and particularly preferably oxides of all of these elements.

[0040] The second layer may preferably contain at least one and in particular all of the following components in the specified amounts: Component ZrO 2 + HfO 2 Y 2 O 3 Fe 2 O 3 Cr 2 O 3 Mn 2 O 3 Pr 2 O 3 Wt.% 82.5 - 96.5, in particular 84.5 - 95.0 3.5 - 11.0, in particular 5.0 - 9.0 0 - 0.15, in particular 0.005 - 0.12 0 - 0.025, in particular 0.0002 - 0.01 0 - 0.002, in particular 0.00005 - 0.0012 0 - 0.02, in particular 0.0001 - 0.015 Tb2O3 0 - 0.02, especially 0.00005 - 0.015 He 2 O 3 0 - 1.0, especially 0.01 - 0.65 MgO 0 - 0.05, especially 0 - 0.025 Al2O3 0 - 0.25, especially 0 - 0.15 La 2 O 3 0 - 1.0, especially 0 - 0.55

[0041] Furthermore, the inner layer may preferably contain at least one and in particular all of the following components in the specified amounts: component % by weight ZrO 2 + HfO 2 85.0 - 96.5, especially 86.5 - 95.5 Y 2 O 3 3.5 - 8.5, especially 4.5 - 7.0 Fe 2 O 3 0 - 0.15, especially 0.005 - 0.12 Cr2O3 0 - 0.025, especially 0.0002 - 0.01 Mn2O3 0 - 0.002, especially 0.00005 - 0.0012 Pr2O3 0 - 0.02, especially 0.0001 - 0.015 Tb2O3 0 - 0.02, especially 0.00005 - 0.015 He 2 O 3 0 - 1.0, especially 0.01 - 0.65 MgO 0 - 0.025, especially 0 - 0.005 Al2O3 0 - 0.25, especially 0 - 0.15 La 2 O 3 0 - 1.0, especially 0 - 0.55

[0042] Furthermore, the outer layer may preferably contain at least one and in particular all of the following components in the specified amounts: component % by weight ZrO 2 + HfO 2 81.5 - 95.0, especially 82.5 - 93.5 Y 2 O 3 5.0 - 12.0, especially 6.5 - 11.0 Fe 2 O 3 0 - 0.1, especially 0.0035 - 0.085 Cr2O3 0 - 0.025, especially 0.0002 - 0.01 Mn2O3 0 - 0.002, especially 0.00005 - 0.0012 Pr2O3 0 - 0.02, especially 0.0001 - 0.015 Tb2O3 0 - 0.02, especially 0.00005 - 0.015 He 2 O 3 0 - 0.6, especially 0.01 - 0.5 MgO 0 - 0.025, especially 0 - 0.005 Al2O3 0 - 0.25, especially 0 - 0.15 La 2 O 3 0 - 0.55, especially 0 - 0.15

[0043] Furthermore, the intermediate layer may preferably contain at least one and in particular all of the following components in the specified amounts: component % by weight ZrO 2 + HfO 2 83.0 - 96.0, especially 84.0 - 94.5 Y 2 O 3 4.0 - 11.0, especially 5.0 - 9.0 Fe 2 O 3 0 - 0.12, especially 0.005 - 0.1 Cr2O3 0 - 0.025, especially 0.0002 - 0.01 Mn2O3 0 - 0.002, especially 0.00005 - 0.0012 Pr2O3 0 - 0.02, especially 0.0001 - 0.015 Tb2O3 0 - 0.02, especially 0.00005 - 0.015 He 2 O 3 0 - 1.0, especially 0.01 - 0.65 MgO 0 - 0.02, especially 0 - 0.005 Al2O3 0 - 0.25, especially 0 - 0.15 La 2 O 3 0 - 1.0, especially 0 - 0.55

[0044] The zirconium oxide ceramic of the first layer contains zirconium oxide, which is preferably stabilized with erbium and / or yttrium. The zirconium oxide ceramic of the first layer preferably has an erbium content of 0.0 to 4.5 mol%, in particular 0.5 to 4.25 mol%, particularly preferably 1.5 to 3.5 mol%, wherein the erbium content is defined as the proportion of the molar amount of Er 2 O 3 relative to the sum of the molar amounts of Er 2 O 3 , ZrO 2 and HfO 2 . Furthermore, the zirconium oxide ceramic of the first layer preferably has an yttrium content of 0.0 to 4.5 mol%, in particular

[0045] 0.5 to 4.25 mol-%, particularly preferably 0.75 to 2.0 mol-%, wherein the yttrium content is defined as the proportion of the molar amount of Y 2 O 3 based on the sum of the molar amounts of Y 2 O 3 , ZrO 2 and HfO 2 . Furthermore, it is preferred that the sum of the erbium and yttrium contents is 1.5 to 6.0 mol-%, in particular 2.0 to 4.5 mol-%, particularly preferably 2.5 to 4.0 mol-%. Since the first layer of the blank serves to form the gingival region of the dental restoration to be created, the formation of a color gradient or material gradient of yttrium in the first layer is not advantageous. Thus, the yttrium content within the first layer is preferably essentially constant. Alternatively, the first layer can have a gradient intended to represent fixed and mobile gingiva.

[0046] The zirconium oxide ceramic of the first layer preferably contains at least one and in particular all of the following components in the specified amounts: component % by weight ZrO 2 + HfO 2 86.0 - 97.0, preferably 90.0 - 94.0 He 2 O 3 0,5 - 10,0, preferably 4.0 - 8.0 Y 2 O 3 0,5 - 8,0, preferably 1.0 - 4.0 first coloring oxide 0,0 - 0,5, preferably 0.0 - 0.25, particularly preferred 0.0 - 0.1 second coloring oxide 0,0 - 0,1, preferably 0.0 - 0.05, particularly preferred 0.0 - 0.025 wherein the first coloring oxide is selected from the group consisting of Fe 2 O 3 , Tb 2 O 3 , Pr 2 O 3 and V 2 O 5 and in particular can cause an additional yellowing of the zirconium oxide ceramic, and wherein the second coloring oxide is selected from the group consisting of Mn 2 O 3 , Cr 2 O 3 and CoO and in particular can cause an additional grey colouration of the zirconium oxide ceramic.

[0047] In one embodiment, it is preferred that the first layer of the blank contains zirconium oxide ceramic or a mixture of zirconium oxide ceramic with one or more materials selected from the group consisting of alumina toughened zirconia (ATZ), zirconium toughened alumina (ZTA), spinels, pigments, or mixtures thereof.

[0048] Alumina reinforced zirconia (ATZ) contains 5 to 40 wt.%, in particular about 20 wt.%, Al 2 O 3 and 60 to 95 wt.%, in particular about 80 wt.%, ZrO 2 . The Al 2 O 3 portion may be doped with MgO, such as 50 wt. ppm to 3 wt.%. The ZrO 2 portion may be doped with Y 2 O 3 , such as 1 to 3 mol.%.

[0049] Zirconia reinforced alumina (ZTA) contains 60 to 95 wt.%, in particular about 80 to 90 wt.%, Al 2 O 3 and 5 to 40 wt.%, in particular about 10 to 20 wt.%, ZrO 2 . The Al 2 O 3 portion may be doped with MgO, such as 50 wt. ppm to 3 wt.%. The ZrO 2 portion may be doped with Y 2 O 3 , such as 1 to 3 mol.%.

[0050] In mixtures of zirconium oxide ceramics with ATZ and ZTA, discrete Al 2 O 3 and ZrO 2 crystallites can be detected in the final microstructure by SEM. In contrast, in current zirconium oxide ceramics for dental applications, Al 2 O 3 crystallites are generally not detectable in the microstructure by SEM. This is probably due to the lower amount of Al 2 O 3, usually around 0.05 to 0.1 wt.%, and its homogeneous distribution. ATZ and ZTA composites can have a positive effect on the mechanical properties of the blank, in particular by increasing both the biaxial strength and fracture toughness. An increase in fracture toughness is achieved in particular when the ZrO 2 content of ATZ or ZTA is stabilized by 1.5 to 2.5 mol.% Y 2 O 3.

[0051] Particularly suitable spinels are spinels selected from the group consisting of MgAl 2 O 4 , SrAl 2 O 4 , La spinel, Y spinel and mixtures thereof, such as LaMg spinels. Spinels can positively influence the mechanical properties of the blank, in particular by increasing both the biaxial strength and the fracture toughness. An increase in fracture toughness is particularly achieved when the ZrO 2 content of the zirconium oxide ceramic is stabilized by 1.5 to 2.5 mol.% Y 2 O 3 . In the composite of zirconium oxide ceramic and spinel, the spinel usually grows in a rod-like manner, especially at high sintering temperatures, which has a positive effect on the fracture toughness of the composite.

[0052] While the second layer of the blank according to the invention is preferably tooth-colored and retains a tooth-colored shade even after heat treatment of the blank, the first layer can be whitish or pinkish and retain such a shade even after heat treatment. In the case of a first layer with a whitish shade, it is customary to subsequently coat the gingival area after shaping and heat treatment in order to imitate the aesthetics of the gingival area in the finished dental restoration as optimally as possible. Such a step can be omitted in the case of a first layer with a pre-colored first layer, so that in this embodiment, a simpler production of, for example, dental prostheses is possible.

[0053] Particularly advantageous are blanks according to the invention in which the zirconium oxide ceramic of the first layer and the zirconium oxide ceramic of the second layer are pre-sintered. This improves processability and precision during subsequent machining to produce dental restorations. In particular, the lower strength of zirconium oxide in the pre-sintered state enables simple, time-saving shaping of the blanks that protects the milling tool. The zirconium oxide ceramic of the first layer and the zirconium oxide ceramic of the second layer preferably have a density of 1.8 to 4.4 g / cm 3 , in particular 2.5 to 4.0 g / cm 3 .

[0054] For the production of dental restorations, such pre-sintered blanks require the blanks to be subjected to a sintering step, iea heat treatment to achieve the desired mechanical properties, especially high strength and hardness. Such heat treatment results in sintering shrinkage of the zirconium oxide ceramic. Therefore, when using pre-sintered blanks, it is necessary that the wave and beam geometry of the blanks according to the invention be compared to blanks based on materials without sintering shrinkage. e.g. Plastic materials. Preferably, in a blank according to the invention in the pre-sintered state, the dimensions of the geometry are enlarged by a factor of 1.200 to 1.250, in particular 1.22 to 1.25. In a blank according to the invention in the green state, the dimensions of the geometry are enlarged by a factor of 1.250 to 1.350, in particular approximately 1.275.

[0055] The blank according to the invention preferably has a biaxial fracture strength of 10 to 150 MPa, in particular 20 to 120 MPa, particularly preferably 25 to 80 MPa. The biaxial fracture strength was determined according to ISO 8672 (2008) (piston-on-three-ball test).

[0056] Furthermore, the blank according to the invention preferably has a Vickers hardness Hv 2.5 of 50 to 1000 MPa, in particular 300 to 850 MPa, particularly preferably 300 to 700 MPa. The Vickers hardness was measured according to ISO 14705:2016 under a load of 2.5 kg.

[0057] Furthermore, it is preferred that the first layer and the second layer of the blank according to the invention are joined together by a one-piece production. This enables efficient monolithic production, ie A patient-specific complete denture can be manufactured from a blank in one milling process.

[0058] The shape of the blank according to the invention is at least partially circular. In particular, the blank has the shape of a disk, particularly preferably the shape of a circular disk.

[0059] Furthermore, it is preferred that the circular-arc-shaped blank has a projection on its outer circumference, in particular an outwardly projecting clamping edge. This clamping edge serves as a holder in grinding and milling devices, such as conventional CAD / CAM devices. The projection can be made of the same material as the blank. In this case, the blank can initially be provided with a larger circumference, and the projection can be removed by turning processes, e.g. on a milling machine. Alternatively, the projection can be made of a different material, for example, a plastic ring that is subsequently applied to the blank.

[0060] It is further preferred that the blank according to the invention has at least two markings, grooves, and / or flattened portions on the outer circumference, which are arranged asymmetrically and not rotationally symmetrically to each other, i.e., not diametrically opposite each other, and serve as rotation protection or anti-twist protection. The position of the shaft geometry relative to the disk must be known and fixed in all spatial directions so that the restoration can be positioned relative to the shaft in the subsequent machining step.

[0061] The blank according to the invention based on zirconium oxide ceramic is used in particular for fixed prosthetics. This means that the blank according to the invention is particularly suitable for the production of fixed dental prostheses that are screwed or bonded to implants or residual tooth parts, such as e.g.a tooth stump. As a result, in blanks according to the invention for the production of an upper jaw prosthesis, a fold and palatal plate, which are necessary for the adhesion of the prosthesis to the patient's palate, can be omitted. This, in turn, means that the height of the first layer of the blank, which serves to imitate the gingival area, can be reduced. Furthermore, by omitting a palatal plate, it is not necessary to make the entire first layer pink or otherwise color it. Rather, in a top view of the circular base of the blank, an inner region can be omitted or left uncolored.

[0062] Thus, in the case of blanks according to the invention, their height can be reduced so that it remains well below 30 mm, which is a critical limit for machining a blank in a commercially available dental milling machine. In a preferred embodiment, the height of the blank, i.e. the distance between the circular-arc-shaped upper side of the blank and the opposite circular-arc-shaped lower side of the blank, is not more than 30 mm, preferably 20 to 30 mm, particularly preferably 25 to 29 mm. In addition, the height of the first layer is preferably 5.0 to 9.0 mm, in particular 6.0 to 7.5 mm, particularly preferably about 7.0 mm, and / or the height of the second layer is preferably 15.0 to 25.0 mm, in particular 18.0 to 22.0 mm, particularly preferably about 20 mm.The height of the first layer is defined as the smallest distance between the outer surface of the first layer opposite the interface and the deepest wave trough of the first layer in a side view of the lateral surface of the preferably disc-shaped blank. The height of the second layer is defined as the smallest distance between the outer surface of the second layer opposite the interface and the highest wave crest of the second layer in a side view of the lateral surface of the preferably disc-shaped blank, as described below as distance 31 in . Figure 3 is illustrated.

[0063] The ratio of the height of the first layer to the height of the second layer is preferably 1:1 to 1:5, in particular 1:2.5 to 1:3.5.

[0064] The inventive combination of the special wave geometry of the blank and the zirconium oxide material to be used allows for further advantages in addition to reducing the overall height of the blank. For example, it has surprisingly been found that, compared to prosthetic blanks known from the prior art, it is possible to reduce (i) the angle between the interface between the first and second layers and the surface of the blank and / or (ii) the angle between the crest line of the wave crests of the interface and the surface of the blank.

[0065] In particular, to make it suitable for producing a prosthesis in the upper jaw, the blank is characterized in that the angle between a fictitious straight line connecting the lowest point of the trough for the second molar to be created with the lowest point of the trough for the central incisor to be created and the projection of this fictitious straight line onto the circular arc-shaped base surface of the blank is 2.0° to 4.5°, preferably 2.0° to 4.0°, particularly preferably 2.5° to 3.5°, and most preferably approximately 3.0°. Such a small angle allows for better utilization of the advantages of this color gradient, particularly in blanks with a color gradient in the second layer, and the color and translucency gradient of the anterior and molar teeth of the prosthesis to be created can be improved.

[0066] In a further preferred embodiment, the blank is characterized in that the angle between the circular arc-shaped base surface of the blank and the crest line of the wave crests of the wave-shaped interface is 7° to 13°, preferably 9° to 11°, and particularly preferably about 10°.

[0067] In particular, in combination with a reduction in the overall height of the blank, the reduction of the angle of the interface to the surface of the blank and / or the reduction of the angle of the crest lines of the wave crests to the surface of the blank lead to easier machinability of the blank in a milling machine, since fewer undercuts are necessary in these embodiments.

[0068] The invention also relates to a method for producing the blanks according to the invention.

[0069] The method for producing a blank according to the invention is characterized in that (i) a first and a second green body are provided, wherein the first green body is provided for forming the first layer of the blank and the second green body is provided for forming the second layer of the blank, and wherein a first side of the first green body has the wave geometry of the first layer formed from the wave trough and wave crest, and a first side of the second green body has the corresponding inverse wave geometry; (ii) the first and second green bodies are placed on top of one another such that the first side of the first green body and the first side of the second green body come into close contact with each other over their entire surface; (iii) the first and second green bodies are pressed together; and (iv) at least one heat treatment is carried out in order to convert the zirconium oxide into pre-sintered zirconium oxide.

[0070] Particularly preferably, in step (i), a first green body and a second green body are provided separately by separately filling the zirconium oxide ceramic starting materials for the first layer and the second layer into a mold and then subjecting the masses to uniaxial and / or isostatic compression. The pressure during this compression is preferably less than 100 MPa.

[0071] Subsequently, the desired shaping of the surface of the first and second green bodies can preferably be achieved by milling.

[0072] Furthermore, it is preferred that the first and the second green body are pressed together isostatically in step (iii), in particular at a pressure of more than 100 MPa, preferably at a pressure of 150 to 1000 MPa, preferably 150 to 500 MPa.

[0073] Furthermore, it is preferred that in step (iv) the heat treatment takes place at a temperature of 700 to 1200°C and preferably at a temperature of 800 to 1100°C. The duration of the heat treatment in step (iv) may preferably be 5 to 600 minutes, in particular 10 to 300 minutes, in a temperature range of 700 to 1200°C and preferably 800 to 1100°C.

[0074] Due to the described special properties of the blanks according to the invention, they are particularly suitable for producing dental restorations, in particular dental prostheses. The invention therefore also relates to the use of the blanks according to the invention for producing a dental restoration and in particular a dental prosthesis, wherein the dental prosthesis is preferably selected from the group consisting of maxillary complete dentures, mandibular complete dentures, maxillary partial dentures and mandibular partial dentures. In particular, the said dental prostheses can advantageously be an implant restoration. The interfaces of the dental prosthesis to the implants can be manufactured directly, so that the dental prosthesis can be connected directly to the implant, e.g. by screwing. Alternatively, the dental prosthesis can be connected to a base, e.g. a base made of titanium, e.g.by gluing, which is then connected to the implant by screwing.

[0075] In a further aspect, the invention also relates to a method for producing a dental restoration, in which (i-1) a blank according to the invention is given the shape of the dental restoration by machining, (i-2) at least one heat treatment is carried out in order to convert the zirconium oxide of the first and second layers into densely sintered zirconium oxide, and (i-3) optionally the surface of the resulting dental restoration is finished.

[0076] The blanks according to the invention can be used in a simple manner to machine the desired shaped dental restorations, in particular dental prostheses.

[0077] The machining in step (i-1) is typically carried out by material-removing processes, in particular by milling and / or grinding. It is preferred that the machining be carried out using computer-controlled milling and / or grinding devices. Particularly preferably, the machining is carried out using a CAD / CAM process.

[0078] In step (i-2), the blank is subjected to a heat treatment to induce the formation of densely sintered zirconium oxide ceramic. The heat treatment takes place in particular at a temperature of 1050 to 1700°C, and preferably 1100 to 1600°C. The heat treatment is carried out in particular for a duration of 0 to 240 minutes, preferably 5 to 180 minutes, particularly preferably 30 to 120 minutes, whereby the term "duration" refers to the holding time at the maximum temperature.

[0079] The dental restorations produced according to the invention are in particular dental prostheses and are particularly preferably selected from the group consisting of maxillary complete dentures, mandibular complete dentures, maxillary partial dentures and mandibular partial dentures.

[0080] The dental prostheses produced according to the invention are characterized not only by excellent optical properties but also by particularly high strength. Preferably, the dental prosthesis has a biaxial fracture strength in the gingival region, i.e., in the first layer, according to ISO 6872 (2208) (piston-on-three-ball test) of more than 800 MPa, in particular more than 900 MPa, and most preferably more than 1000 MPa.

[0081] Furthermore, the dental prosthesis in the dentin area, ie in the inner layer of the second layer, preferably has a biaxial fracture strength according to ISO 6872 (2208) (piston on three balls test) of more than 600 MPa, in particular more than 600 MPa and particularly preferably more than 800 MPa.

[0082] In addition, the dental prosthesis in the cutting area, ie in the outer layer of the second layer, preferably has a biaxial fracture strength according to ISO 6872 (2208) (piston on three balls test) of more than 300 MPa, in particular more than 400 MPa and particularly preferably more than 500 MPa.

[0083] In the optional step (i-3), the surface of the dental restoration can be finalized. In particular, in the case of a dental prosthesis with a whitish gingival area, i.e., when using a blank with a whitish first layer, it is possible to cover the gingival area with layering, veneering, staining, or characterization materials to closely mimic the color of natural oral mucosa. Alternatively or additionally, a surface polishing of the dental restoration can be performed in step (i-3).

[0084] Further features and advantages will become apparent from the following description of embodiments of the present invention with reference to the drawings. Fig. 1 is a schematic perspective view of a blank according to the invention; Fig. 2 is a schematic plan view of the first layer of the blank according to Figure 1illustrated blank; Fig. 3 is a schematic side view of a blank according to the invention; and Fig. 4 is a further schematic side view of a blank according to the invention.

[0085] In Fig. 1 1 shows a blank 1 according to the invention for a dental maxillary prosthesis. The blank 1 is disc-shaped in its basic structure. It has a first layer 3 based on zirconium oxide ceramic and a second layer 5 based on zirconium oxide ceramic. The first layer 3 and the second layer 5 differ in color and form an interface 7. In the illustration according to Fig. 1 the top side of the first layer 3 and the bottom side of the second layer 5 represent the interface 7 of the blank. An exemplary schematic course of a dental arch, ie the course of the teeth to be created for the prosthesis, is shown in Fig. 1represented as line 9. In the course of the dental arch 9, the interface 7 is wave-shaped, with alternating wave troughs 11 and wave crests 13. In the area 15 of the anterior teeth to be created, the wave crests 13 extend in a fan-shaped manner in the oral-vestibular direction. This means that the crest lines of the wave crests 13 extend from a center point 17 of the dental arch 9 in a radial direction, i.e., outward, i.e., toward the vestibular side, in a ray shape.

[0086] Fig. 2 shows a plan view of the top side of the first layer 3 of the Fig. 1 shown blank 1. In the top view, the fan-shaped course of the wave crests 13 starting from the beam center 17 can be seen in the anterior tooth area 15.

[0087] Out of Fig. 2It is also evident that the apex lines of the wave troughs 11 and the wave crests 13 in the area 19 of the molars to be created run essentially parallel in the oral-buccal direction. As described above, this parallel alignment of the wave crests and wave troughs in the molar area ensures that a comparatively large occlusal surface is available in the finished dental prosthesis, even with small dental arches.

[0088] The blank according to Fig. 2 On its outer circumference, i.e., the lateral surface 21, it also has three flattened portions 23 that are not arranged rotationally symmetrically to one another. The flattened portions 23 serve as markings to define the position of the wave and beam geometry within the disc relative to the disc itself, so that the disc can be correctly positioned during the fabrication of the dental prosthesis. Thus, the flattened portions 23 serve as anti-rotation or anti-twist protection during the subsequent milling process.

[0089] The inner region 25 of the first layer 3 represents the region of the prosthesis to be created which would come into contact with the patient's palate after insertion of the prosthesis, i.e. the so-called palatal plate. In the case of implant- or tooth stump-supported prostheses, the region 25 can be omitted or can remain unstained.

[0090] Fig. 3 shows a side view of the Fig. 1illustrated blank 1. The blank has a total height 27 which, in the specific embodiment of the blank illustrated in the figures, is 26.80 mm. The height 29 of the first layer is 6.80 mm and the height 31 of the second layer is 20.00 mm. In comparison to the blanks based on plastic materials known from EP 3 064 170 A1 and EP 3 597 143 A1, in which the height of the gingival layer and the tooth layer is usually each approximately 19.00 mm, the total height of the blank 1 could be significantly reduced and is below the critical limit of 30 mm, which enables simple processing in commercially available dental milling machines. In addition, the ratio of tooth layer to gingival layer could be significantly increased, whereby better color and translucency gradients can be achieved.

[0091] Furthermore, blank 1 is Fig. 3characterized in that the angle 33 between a fictitious straight line 35, which connects the lowest point of the trough 11a for the second molar to be created with the lowest point of the trough 11b for the central incisor to be created, and the projection of this fictitious straight line 35 onto a base surface of the blank, here the upper side 37 of the blank, is 3° and is thus smaller than the corresponding angle in blanks based on plastic materials known from EP 3 064 170 A1 and EP 3 597 143 A1, which is usually 5° there. By reducing the angle 33, the color and translucency gradient of the anterior and molar teeth of the prosthesis to be created can be further improved.

[0092] Fig. 4 shows another side view of the Fig. 1The angle 39 between the surface 37 and the crest line of the wave crests 13a is 10° in the illustrated embodiment and is thus significantly smaller than the corresponding angle in the plastic-based blanks known from EP 3 064 170 A1 and EP 3 597 143 A1, which is typically 15°. Reducing the angle 39, like reducing the angle 33, leads to easier machining of the blank in a milling machine, since fewer undercuts are necessary in these embodiments.

Claims

1. Blank (1) for dental prostheses, which has a first layer (3) based on zirconium oxide ceramic and a second layer (5) based on zirconium oxide ceramic, the first layer (3) and the second layer (5) differing in color and forming a boundary surface (7), wherein the boundary surface (7) is formed in the course of the dental arch (9) in an undulating shape with alternating wave troughs (11) and wave crests (13), and the vertex lines of the wave crests (13), viewed in plan view of the boundary surface, extend radially in the mesial-distal direction, wherein the blank is at least partially circular-arc-shaped and the angle (33) between a fictitious straight line (35) connecting the lowest point of the wave trough (11a) for the second molar to be produced with the lowest point of the wave trough (11b) for the central incisor to be produced, and the projection of this fictitious straight line onto a base surface (37) of the circular-arc-shaped blank is 2.0° to 4.5°.

2. Blank (1) according to claim 1, wherein the second layer (5) has a continuous or discontinuous color gradient.

3. Blank (1) according to claim 2, wherein the zirconium oxide ceramic of the second layer (5) comprises yttrium and the color gradient is formed by a gradient of the content of yttrium.

4. Blank (1) according to claim 3, wherein the content of yttrium in the second layer (5) increases from the boundary surface (7) to the outer surface of the second layer opposite the boundary surface.

5. Blank (1) according to any one of claims 2 to 4, wherein the second layer (5) comprises an inner layer and an outer layer, the inner layer being adjacent to the boundary surface (7) between the first and second layers, and the outer layer being adjacent the outer surface of the second layer (5) opposite the boundary surface.

6. Blank (1) according to claim 5, wherein (a) the inner layer has an yttrium content of from 2.0 to 6.0 mol%, preferably from 3.0 to 5.0 mol%, and (c) the outer layer has an yttrium content of from 3.5 to 8.0 mol%, preferably from 4.0 to 7.5 mol%, wherein the yttrium content is defined as the proportion of the amount of substance of Y2O3 relative to the sum of the amounts of substance of Y2O3, ZrO2, and HfO2.

7. Blank (1) according to any one of claims 1 to 6, wherein the first layer (3) is made of zirconium oxide ceramic or a mixture of zirconium oxide ceramic with one or more materials selected from the group consisting of alumina-reinforced zirconium oxide, zirconium oxide reinforced alumina, spinels, pigments, or mixtures thereof.

8. Blank (1) according to any one of claims 1 to 7, wherein the zirconium oxide ceramic of the first layer (3) comprises erbium and / or yttrium, the first layer preferably having an erbium content of 0.0 to 4.5 mol%, in particular 0.5 to 4.25 mol%, particularly preferably 1.5 to 3.5 mol%, wherein the erbium content is defined as the proportion of the amount of substance of Er2O3 relative to the sum of the amounts of substance of Er2O3, ZrO2, and HfO2, and / or an yttrium content of 0.0 to 4.5 mol%, in particular 0.5 to 4.25 mol%, particularly preferably 0.75 to 2.0 mol.%, wherein the yttrium content is defined as the proportion of the amount of substance of Y2O3 relative to the sum of the amounts of substance of Y2O3, ZrO2 and HfO2, the sum of the erbium and yttrium content being furthermore particularly preferred 1.5 to 6.0 mol%, in particular 2.0 to 4.5 mol%, particularly preferred 2.5 to 4.0 mol%.

9. Blank (1) according to any one of claims 1 to 8, wherein the first layer (3) is whitish or pinkish and the second layer (5) is tooth-colored.

10. Blank (1) according to any one of claims 1 to 9, wherein the zirconium oxide ceramic of the first layer (3) and the zirconium oxide ceramic of the second layer (5) are pre-sintered, wherein the zirconium oxide ceramic of the first layer (3) and the zirconium oxide ceramic of the second layer (5) independently of one each other, preferably have a density of 1.8 to 4.4 g / cm3, in particular 2.5 to 4.0 g / cm3.

11. Blank (1) according to any one of claims 1 to 10, wherein the first layer (3) and the second layer (5) are connected to each other by integral manufacture.

12. Blank (1) according to any one of claims 1 to 11, which has the shape of a disc, in particular the shape of a circular disc.

13. Blank (1) according to claim 12, wherein a protrusion, preferably an outwardly projecting clamping edge, is formed on the outer circumference of the blank (1).

14. Blank (1) according to one of claims 1 to 13, wherein the boundary surface (7), viewed in plan view of the boundary surface, in the region (15) of the anterior teeth to be produced has, in the oral-vestibular direction, vertex lines of wave crests in a fan shape.

15. Blanks (1) according to any of claims 1 to 14, wherein the boundary surface (7), viewed in plan view of the boundary surface, in the region (19) of the molars to be produced, has in the oral-buccal direction radiating crest lines of the wave troughs or vertex lines of the wave troughs which are substantially parallel to one another.

16. Blank (1) according to claims 1 to 15, wherein the angle between a fictitious straight line (35) connecting the lowest point of the wave trough (11a) for the second molar to be produced with the lowest point of the wave trough (11b) for the central incisor to be produced, and the projection of this fictitious straight line onto a base surface (37) of the circular arc-shaped blank is 2.0° to 4.0°, particularly preferably 2.5° to 3.5°, and most preferably about 3.0° .

17. Blank (1) according to one of claims 1 to 16, wherein the height (27) of the blank is not more than 30 mm, preferably 20 to 30 mm, more preferably 25 to 29 mm, wherein preferably (i) the height (29) of the first layer (3) is 5.0 to 9.0 mm, in particular 6.0 to 7.5 mm, more preferably about 7.0 mm, and / or (ii) the height (31) of the second layer (5) is 15.0 to 25.0 mm, in particular 18.0 to 22.0 mm, more preferably about 20 mm.

18. Blank (1) according to one of claims 1 to 17, wherein the angle (39) between a base surface (37) of the circular arc-shaped blank and the vertex line of the wave crests (13a) of the undulating boundary surface is 7° to 13°, preferably 9° to 11°, and more preferably about 10°.

19. Process for preparing a blank (1) according to any one of claims 1 to 18, comprising (i) providing a first and a second green body, wherein the first green body is provided for forming the first layer (3) of the blank (1) and the second green body is provided for forming the second layer (5) of the blank (1), and wherein a first side of the first green body has the wave geometry of the first layer (3) formed by wave trough (11) and wave crests (13), and a first side of the second green body has the corresponding inverse wave geometry; (ii) putting the first and second green bodies on top of each other so that the first side of the first green body and the first side of the second green body come into close contact with each other over their entire surface; (iii) pressing the first and second green bodies together; and (iv) carrying out at least one heat treatment to convert the zirconium oxide into pre-sintered zirconium oxide.

20. The process according to claim 19, wherein in step (iii) the first and second green bodies are isostatically compressed together at a pressure of more than 100 MPa, preferably at a pressure of 150 to 1000 MPa, preferably 150 to 500 MPa.

21. Process according to claim 19 or 20, wherein in step (iv) the heat treatment takes place at a temperature of from 700 to 1200°C and preferably at a temperature of from 800 to 1100°C.

22. Use of a blank (1) according to one of claims 1 to 18 for preparing a dental restoration and, in particular, a dental prosthesis, wherein the dental prosthesis is preferably selected from the group consisting of maxillary full dentures, mandibular full dentures, maxillary partial dentures, and mandibular partial dentures.

23. Process for preparing a dental restoration, comprising (i-1) providing the shape of the dental restoration to a blank (1) according to any one of claims 1 to 18 by machining, (i-2) carrying out at least one heat treatment to convert the zirconium oxide of the first and second layers into densely sintered zirconium oxide, and (i-3) optionally, finishing the surface of the dental restoration obtained.

24. Process according to claim 23, wherein the machining is carried out with computer-controlled milling and / or grinding devices.

25. Process according to claim 23 or 24, wherein the dental restoration is selected from the group consisting of maxillary full dentures, mandibular full dentures, maxillary partial dentures, and mandibular partial dentures.