Method of manufacturing a dental restoration blank using a single-stage sedimentation method

The present application relates to a method for producing a blank with a color gradient for dental restorations by means of a sedimentation process, a blank obtained by means of the method according to the invention and the use of a suspension for producing a dental restoration with a color gradient.

The term dental restoration refers to measures to restore defective teeth or to replace lost teeth. This can be done, for example, in the form of fixed dentures such as crowns, partial crowns, bridges or implants or in the form of removable dentures such as full or partial dentures. What these measures have in common is that the restorations used should come as close as possible to the appearance of natural teeth in order to create an aesthetically pleasing image.

In order to achieve an aesthetically pleasing restoration that blends into a patient's natural tooth environment, the restoration can be colored accordingly, for example by painting. Alternatively, to produce dental restorations, blanks can be used that already have a corresponding color gradient that is based on that of the natural tooth. In order to achieve this color gradient, the blank is usually made up of several layers, which differ in terms of their colour.

<patcit id="pcit0001" dnum="WO2020025795A"><text>WO 2020/025795</text></patcit> describes a blank for a dental restoration, which has a first and a second layer, which are constructed independently of one another from a glass, a glass-ceramic or a ceramic, the first layer and the second layer differing in terms of their color and forming an interface, positioned at an angle within the blank.

<patcit id="pcit0002" dnum="DE19714178"><text>DE 197 14 178</text></patcit> relates to a process for the production of a multi-colored shaped body for further processing into dental restorations, in which at least two differently colored starting materials are filled into a press die, which essentially defines the shape of the shaped body, and pressed to form a shaped body.

<patcit id="pcit0003" dnum="EP1900341A"><text>EP 1 900 341</text></patcit> discloses a multicolored shaped body with superimposed layers for creating dental restorations, which has (a) at least two consecutive and differently colored main layers and (b) between at least two consecutive and differently colored main layers at least two differently colored intermediate layers, wherein the main layers have a thickness of more than 0.5 mm and the intermediate layers have a thickness of 0.1 to 0.5 mm and wherein between the intermediate layers there is a change in color in a direction opposite to the direction of the change in color between opposite to the main layers.

According to<patcit id="pcit0004" dnum="EP3215820A"><text>EP 3 215 820</text></patcit> A computer-implemented method of making a dental restoration comprises the steps of: acquiring an image of a tooth at a color depth based on a multiplicity of color values; positioning the image by recognizing, in the tooth image, a contiguous first tooth color range with color values ​​within a predetermined first range of different color values ​​and assigning the first tooth color range to a common first false color value; and recognizing, in a tooth image, a coherent second tooth color range with color values ​​within a predetermined second range of different color values ​​and assignment to a common second false color value; determining a tooth color structure based on the first and the second false color value of the first and the second color area within the tooth image; providing information about a multicolor block having a predetermined color shade formed by at least a first block color zone and a second block color zone, the information comprising data about a block color structure in relation to dimensions and/or positions of the first and second block color zones; matching the tooth color structure to the block color structure; and based on matching; determining a position within the block where the tooth color structure and the block color structure match within predetermined limits; and machining the dental restoration from the block in the determined position.

<patcit id="pcit0005" dnum="EP3527166A"><text>EP 3 527 166</text></patcit> describes a milling blank made of plastic or a plastic-based one with a continuous color gradient for producing an indirect dental restoration and a method for producing such a blank by mixing two differently colored pastes while continuously varying the mixing ratio of the two pastes during the filling process.

The methods and techniques known in the state of the art for producing aesthetically pleasing dental restorations have in common that the color design is very complex and, in some cases, trained personnel with many years of experience are required in order to achieve satisfactory results. In those cases in which the restorations are made from appropriately colored blanks, there is the problem that the layered structure required to achieve the color gradient often means that the different layers can also be recognized in the finished restoration and thus a flowing Transition between the individual paint layers cannot be achieved.

There is therefore still a need for methods that allow the production of aesthetically pleasing dental restorations. It is therefore the object of the present invention to provide a method with which dental restorations are accessible, the visual appearance of which corresponds to that of a natural tooth and can be adapted according to the needs of the patient.

Surprisingly, it was found that this object is achieved by a method in which the blank is obtained by means of a sedimentation method.

As is known to those skilled in the art, the sedimentation behavior of solids contained in a suspension can be represented using the Stokes equation:<maths id="math0001"><math display="block"><mrow><msub><mi>v</mi><mi>p</mi></msub><mo>=</mo><mfrac><mn>2</mn><mn>9</mn></mfrac><mo>∗</mo><mfrac><mrow><msup><mi>right</mi><mn>2</mn></msup><mi>G</mi><mfenced><msub><mi>ρ</mi><mi>p</mi></msub><mo>−</mo><msub><mi>ρ</mi><mi>f</mi></msub></mfenced></mrow><mi>n</mi></mfrac><mo>,</mo></mrow></math><img file="EP4092007A1_D0001.tif" /></maths> With<ul id="ul0001" list-style="none"><li><i>v_p</i>: sedimentation speed</li><li>r: radius of sinking object</li><li>g: acceleration due to gravity</li><li>ρ_p: density of the particle</li><li>ρ_f: density of the fluid</li><li>η: dynamic viscosity of the fluid</li></ul>

In the context of the present invention, it was surprisingly found that this general principle can also be used as a basis for the production of dental restorations by means of sedimentation-based shaping processes in order to obtain blanks with a continuous color gradient.

Therefore, a first subject matter of the present application is a method for producing a blank for dental restorations with a color gradient, comprising the steps:<ol id="ol0001"><li>a) providing a suspension comprising<ol id="ol0002"><li>i) liquid dispersant,</li><li>ii) a base material and</li><li>iii) one or more coloring substances;</li></ol></li><li>b) sedimentation of the suspension to form a sediment with a color gradient;</li><li>c) solidification of the sediment; forming a gradient blank.</li></ol>

Surprisingly, it has been shown that the method according to the invention produces blanks that reproduce the natural color progression of a tooth without color transitions being discernible, such as those that occur with blanks that are produced by means of layer construction. Without being bound to a specific theory, it is assumed that the stepless development of the color gradient comes about in particular as a result of the different sedimentation behavior of the particles contained in the suspension.

Another advantage of the method according to the invention has turned out to be its broad applicability to different materials. All materials customary in the field of dental restorations can be used as the base material. An embodiment is therefore preferred in which the base material is selected from the group consisting of silicate glass ceramics and silicate glasses, aluminum oxides, spinels and zirconium oxides.

In a particularly preferred embodiment, the base material is selected from the group consisting of silicate glass ceramics and silicate glasses, in particular feldspars, foids, lithium silicates and leucite glass ceramics. In a further preferred embodiment, the base material is colored.

According to the method according to the invention, the color gradient in the blank is achieved by adding coloring substances, which can be put together as required. In a preferred embodiment, the coloring substances are color pigments, preferably oxides of elements selected from Zr, Hf, Zn, Ti, Fe, Al, Si, Cr, Sb, Mn, Cd, Sn, Ca, Na, K, Mg, Ni, Co, Sc, Y, Ce, Er, Yb, Pr, Eu, Dy, Tb and mixtures thereof.

The color pigments can be present in the suspension in different forms. Thus, an embodiment is preferred in which the color pigments are present in colloidal form, powder form or as a frit.

The coloring of the blank can be achieved using different techniques. In a preferred embodiment, the base material and the coloring substances are present as a mixture in the suspension.

In the context of the present invention, it was surprisingly found that by modifying the surface of the coloring substances, their sedimentation behavior can be influenced and the color progression of the blank can thus be controlled. For example, the coloring substances can be encapsulated. Therefore, as an alternative, preference is given to an embodiment in which surface-modified pigments are used as coloring substances. The surface modification can be achieved, for example, by subjecting the coloring substances to a thermal treatment in the presence of a suitable encapsulation material, during which the encapsulation material is melted and thus encloses the pigments that have remained intact.

In a further preferred embodiment, the base material and the coloring substances are present in an integrated form, for example as a frit. Such a material can be obtained, for example, in that the base material is melted together with the coloring substance or substances and the frit thus obtained is brought to the desired particle size by grinding.

The development of the color gradient in the blank obtainable by the process according to the invention is determined in particular by the different sedimentation behavior of the components of the suspension, which in turn can be influenced by the choice of a suitable dispersant. In this respect, an embodiment is preferred in which the dispersant is selected from the group consisting of water, organic solvents, in particular alcohols, liquid polymerizable monomers and mixtures thereof.

The sedimentation behavior of solids contained in a suspension is influenced, among other things, by the solids content of the suspension. In the process according to the invention, it has proven to be particularly advantageous if the solids content of the suspension used is not more than 70% by volume, based on the total volume of the suspension, since this allows a seamless color gradient to be obtained in the blank. An embodiment is therefore preferred in which the suspension has a solids content of 15 to 65% by volume, preferably 35 to 60% by volume, based in each case on the total volume of the suspension.

The color gradient in the blank obtained according to the invention can also be determined by the density of the particles contained in the suspension. It has proven to be advantageous if the base material has a density that deviates from the density of the coloring substances, since in this way the natural color gradient of a tooth can be imitated. Therefore, an embodiment is preferred in which the base material has a density different from that of the coloring substances. Preferably, the density of the base material is lower than the density of the coloring substances. In a further preferred embodiment, the base material has a density of 2 to 6 g/cm³, preferably 2 to 3 g/cm³ on. The one or more coloring substances used in the suspension preferably have a density of 3 to 13 g/cm³, particularly preferably from 4 to 8 g/cm³ on. In this case, the density can be determined, for example, by means of helium pycnometry.

Another factor that can be used to influence the sedimentation behavior is the particle size of the components contained in the suspension. In the context of the present invention, it has surprisingly been shown that the selection of base material and coloring substances with suitable particle sizes can prevent visible transitions from forming between the differently colored areas of the blank. An embodiment is therefore preferred in which the base material and/or the coloring substances have an average particle size D50 of 0.03 to 100 μm, preferably 1 to 20 μm, determined by means of static and/or dynamic light scattering. The D50 value of the particle size refers to the number of particles that are larger and smaller than the specified value and is therefore also referred to as the average or mean particle size.

The particle size distribution of any powdered material is usually given using the values ​​D10, D50 and D90, where D10 denotes the 10% of the particles that have a particle size smaller than the given value, D50 denotes the average particle size and D90 the 90% of the particles that have a particle size that is below the specified value. The percentages relate to the total volume of the particles.

In a preferred embodiment, the method according to the invention is characterized in that the base material has a particle size distribution in which the D10 value is one fifth, preferably half, of the D50 value. Furthermore, the D90 value of the particle size distribution is four times, preferably twice, the D50 value. The particle size distribution of the base material is preferably expressed as a D10 value of 0.2^∗D50, preferably 0.5^∗D50 and a D90 value of 4^∗D50, preferably 2nd^∗D50.

The coloring substances used in the process according to the invention are preferably distinguished by the following particle size distribution: D10 of 0.1^∗D50, preferably 0.4^∗D50, and D90 from 5^∗D50, preferably 2nd^∗D50. In this way, a natural color gradient of the blank could be achieved without excessive color intensity occurring in the lower area of ​​the blank.

The desired color gradient in the blank could be further optimized by adding other components to the suspension. An embodiment of the method according to the invention is therefore preferred in which the suspension also comprises additives, preferably wetting agents and/or dispersants. The wetting agent is preferably selected from the group of nonionic surfactants. The dispersant is preferably selected from the group consisting of water glass, polyelectrolytes, surfactants, carboxylic acid esters, amines, amino alcohols, polycarbonates, polycarboxylic acid derivatives, silanes, silicate polycarbonates and mixtures thereof.

The blank obtained by the method according to the invention is intended as a dental restoration, and it has proven to be advantageous to achieve the shape of the blank by filling the suspension into appropriate casting molds. It is therefore important to ensure that the desired color gradient is achieved and that the casting mold is filled without bubbles, particularly with regard to the viscosity of the suspension. Therefore, an embodiment is preferred in which the suspension has a viscosity of 1 mPas to 10⁵ mPas, preferably 1 mPas to 10³ mPas, determined at 25° C. using a rheometer (eg Physica MCR 101 rheometer from Anton Paar).

The method according to the invention provides for the sedimentation of the suspension provided. For the purposes of the present invention, sedimentation is understood to mean the depositing or settling of the solids contained in the suspension. The particles settled depending on their sedimentation rate, the particles with the greatest sedimentation rate settling first, ie being arranged at the bottom in the blank obtained according to the invention.

In a preferred embodiment, the sedimentation can be effected by means of centrifugal force.

Step c) of the method according to the invention provides for the solidification of the sediment obtained in step b). This solidification can take place, for example, by means of drying or, in cases where polymerizable monomers are used as dispersants, by means of polymerization of the monomers. During the hardening in step c) of the method according to the invention, a strength is preferably achieved which allows the blank to be handled, for example in order to release it from the casting mold or to process it using CAD/CAM methods.

In a preferred embodiment of the method according to the invention, this further comprises a step d) in which the sediment solidified in step c) is sintered. In cases where polymerizable monomers are used as dispersants, the desired strength is already achieved through the polymerization of the monomers. This can be done, for example, under high-pressure conditions, so that the sintering step is no longer necessary. In cases where a sintering step is provided, sintering can be carried out to a strength that allows further processing of the blank using CAD/CAM processes or to the final density that is desired for its use as a dental restoration.

In a further preferred embodiment, the method according to the invention also includes a step of infiltrating the blank. During the infiltration, the blank is preferably infiltrated with a liquid, hardenable composition, which is hardened after the end of the infiltration. The infiltration step is intended primarily for cases where liquid polymerizable polymers are not used as dispersants. The liquid, curable composition preferably comprises organic monomers and/or prepolymers, preferably based on acrylates and/or methacrylates. In a further preferred embodiment, the liquid, curable composition is a glass or a glass ceramic. In one embodiment, the infiltrating step may be performed after the sediment has dried. In another embodiment, the infiltration step can be preceded by a sintering step, with the sintering aiming for a density of the blank that is, for example, 65 to 85% of the theoretical density. Suitable compounds and methods will be apparent to those skilled in the art<patcit id="pcit0006" dnum="WO02076907A"><text>WO 02/076907</text></patcit> and<patcit id="pcit0007" dnum="WO2010029515A"><text>WO 2010/029515</text></patcit> known.

In a further preferred embodiment, the liquid, curable composition is a glass or a glass ceramic.

In the context of the present invention, it was surprisingly found that blanks for dental restorations can be obtained by means of sedimentation of appropriate dispersions. A further object of the present invention is therefore a blank produced by means of the method according to the invention, the blank preferably having a color gradient. In a particularly preferred embodiment, the blank is a blank infiltrated with a liquid, curable composition. Also preferred is an embodiment in which the curable composition is cured. In a particularly preferred embodiment, the blank is a blank made from a ceramic-polymer composite with interpenetrating networks, which is also referred to as hybrid ceramic in the dental industry

Another object of the present invention is the use of a suspension for the production of a blank or a dental restoration, the suspension comprising a liquid dispersant, a base material and one or more coloring substances, the blank or the dental restoration being characterized in that it /It has a color gradient that mimics the natural appearance of a tooth. The suspension used is preferably one as described above.

The blank obtained can be processed into a dental restoration. The dental restoration, either obtained by further processing the blank or directly by using the suspension according to the invention, is preferably inlays, onlays, table tops, veneers, crowns, partial crowns, implant-supported dentures or bridges.

Examples:

The present invention is to be described in more detail with reference to the following examples, which are in no way to be understood as limiting the inventive concept. The data in % by volume relate in each case to the total volume of the suspension.

1. Color gradient control via particle size

Uncolored feldspar powder with a particle size D50 of 3 μm was mixed with red and yellow inorganic pigments each with a mean particle size D50 of 6 μm and a white pigment with a mean particle size D50 of 1 μm. The mixture was dispersed in water, a nonionic surfactant and water glass using a ball mill, the solids content of the resulting suspension being 53% by volume. The suspension was poured into a plastic mold, sedimented and dried. The dried sediment was sintered into a shaped body.

The color differences between the upper and lower areas of the blank obtained were clearly pronounced. After a region of constant whitening at the top of the block, yellowing increased gradually and constantly along the direction of sedimentation.<figref idref="f0001">figure 1</figref> shows the Lab color values ​​along the direction of sedimentation.

The color gradient of the blanks was determined using the Lab values ​​(L^∗a^∗b) checked. For this purpose, disks with a thickness of 3 mm were sawn from the samples obtained, and images of these disks positioned on a black background were taken using a light microscope. With the help of an appropriate image processing program, the color values ​​were then determined along a profile.

2. Color gradient control by solids content

Uncolored feldspar powder with a particle size D50 of 3 μm was mixed with a red and a yellow pigment, which also had a particle size D50 of 3 μm, and a white pigment with a particle size D50 of 1 μm. The mixture was dispersed in water, a nonionic surfactant and water glass using a ball mill. In this way, suspensions with a solids content of 52% by volume, 50% by volume and 46% by volume were produced.

<figref idref="f0001 f0002">Figure 2a to c</figref> shows the Lab values ​​of the shaped bodies obtained from the suspensions by sedimentation, drying and sintering along the direction of sedimentation. As can be seen from the samples, the color gradient of the shaped body increased with decreasing solids content of the suspension. In this way, the coloring of the blanks can be adjusted from monochrome to infinitely variable multichrome.

3. surface modified pigments

A red, a yellow and a white pigment were each mixed with a silicate-based powder and subjected to a thermal treatment in which the silicate-based powder and the pigments were fused into a frit, which was then ground to an average particle size was crushed by 5 µm.

The powders obtained in this way were mixed together with an undyed feldspar powder having an average particle size D50 of 3 μm, and the mixture was dispersed in water, a nonionic surfactant and water glass using a ball mill. Shaped bodies were obtained from the suspensions produced in this way by sedimentation, drying and sintering, and their color gradient was analyzed.

<figref idref="f0003">Figure 3a and b</figref> shows the Lab color values ​​of the blanks in the direction of sedimentation, with the solids content of the suspension being 50% by volume (<figref idref="f0003">Figure 3a</figref>) and 48% by volume (<figref idref="f0003">Figure 3b</figref>) fraud. As can be seen from the data provided, the encapsulation of the pigments enables a significantly finer gradation of the color gradient to be achieved. In addition to a continuous increase in the proportion of yellow, the red coloring could also be gradually increased in a targeted manner along the direction of sedimentation.

4. Pre-colored base material

A red, a yellow and a white pigment were mixed together with the uncolored base material and thermally treated to obtain a frit. The frit was comminuted to the desired particle size by grinding and dispersed in water, a nonionic surfactant and water glass using a ball mill. The suspensions with a solids content of 52% by volume, 48% by volume and 46% by volume were sedimented and the sediment was dried and sintered. The color gradient of the moldings obtained in this way was measured along the direction of sedimentation.

<figref idref="f0004">Figure 4a</figref> shows the Lab values ​​of the blank obtained from the suspension with a solids content of 52% by volume,<figref idref="f0004">Figure 4b</figref> that obtained from the suspension with a solids content of 48% by volume and<figref idref="f0005">Figure 4c</figref> that obtained from the suspension with a solids content of 46% by volume.

As can be seen from the data provided, blanks with a very fine color gradient can be obtained in this way.

5. comparative example

A molding was produced by methods customary in the prior art, by producing four mixtures from an uncolored base material and different concentrations of red pigment, yellow pigment and white pigment. Four pressed granules were produced from these mixtures by means of spray drying, which were pressed in a 4-chamber filling shoe by means of uniaxial multi-layer pressing. The molding obtained in this way was debound and sintered and the color gradient was measured. How from the in<figref idref="f0005">figure 5</figref> As can be seen from the Lab values ​​shown, the sudden changes in the color values ​​are clearly visible despite the use of thin layers, which disturb the natural appearance of the restorative material.