Veneering ceramic for dental restorations made of yttrium-stabilized zirconium dioxide, and method for veneering dental restorations made of yttrium-stabilized zirconium dioxide

Veneering ceramics for dental restorations made of yttrium-stabilized zirconium dioxide and process for veneering dental restorations made of yttrium-stabilized zirconium dioxide.

The invention relates to veneering ceramics for dental restorations, the framework ceramic consisting of yttrium-stabilized zirconium dioxide.

Yttrium-stabilized zirconium dioxide is a high-performance material with extremely high strength, which is increasingly used in restorative dentistry for framework ceramics for crowns, inlays and bridges. The fine adjustment to the variety of natural teeth requires the use of veneering ceramics. The veneering ceramics were previously a weak point in the resilience of the restored teeth.

Veneering ceramics should enable good modeling, be color-matched to the neighboring teeth, have a high chemical resistance, have high bending strength after targeted temperature treatment, and be characterized by intensive adhesion with the framework ceramic.

Powders or pastes are generally used as starting materials in the production of the veneering ceramics. The properties of the veneering ceramic are determined by the chemical and crystallographic characteristics as well as the grain size of the starting materials.

Dental ceramics containing leucite are manufactured according to patent US 4,798,536 A via the glass melt. The leucite content is in the range from 35 to 60% by weight. The high expansion coefficient of the leucite-containing dental ceramic from 13 to 15 x 10^"6/ K is used for veneering metal crowns. The flexural strength of the veneering ceramic with leucite crystals is 80 MPa.

US Pat. No. 4,189,325 A proposes the use of lithium disilicate for restorative dentures. It will focus on the material system Li₂O-CaO-AI₂Os- SiO₂ concentrated. The nucleating agents Nb₂O₅ and Pt added. The patent US 4,515,634 A proposes Li in the basic system₂O-CaO-AI₂O₃-SiO₂ to improve nucleation and crystallization the nucleating agent P₂Add oe.

The published patent application DE 197 50 794 A1 describes the use of lithium disilicate glass ceramics for use in the hot pressing process. However, it has been shown that when this method is used, the edge strength on the restored tooth is not sufficient and that there is a high level of tool wear during reworking. DE 103 36 913 A1 proposes to manufacture the tooth to be restored in two stages. In the first stage, lithium metasilicate is crystallized, which is processed into dental products by machine processing. With a second temperature treatment, the lithium metasilicate is converted into the stronger lithium disilicate. The entire restored tooth is made of glass ceramic with lithium disilicate crystals. Patent specification DE 196 47 739 C2 describes a sinterable lithium disilicate glass ceramic and glass. The starting material is sintered into blanks. These blanks are at 700⁰C to 1200 ° C pressed to dental products. In the case of plastic deformation, the lithium disilicate glass ceramic described has only a slight reaction with the neighboring investment. On the basis of yttrium-stabilized zirconium dioxide, EP 1 235 532 A1 describes a process for the production of high-strength ceramic dentures. The framework ceramics manufactured according to this process have 4-point bending strength of greater than 1200 MPa.

The invention has for its object to provide a possibility for a translucent veneering ceramic with high flexural strength, the veneering ceramic should also have very good adhesion to the framework ceramic made of yttrium-stabilized zirconium dioxide.

According to the invention, this object is achieved in the case of veneering ceramics of dental restorations made from yttrium-stabilized zirconium dioxide in that it is produced by the following components: a) SiO₂ 58.0 - 74.0% by weight b) AI₂O₃ 4.0-19.0% by weight c) Li₂O 5.0 - 17.0% by weight d) Na₂O 4.0 - 12.0% by weight e) ZrO₂ 0.5 - 6.0% by weight

It can be advantageous if, in addition to the nucleating agent ZrO₂ another

Nucleating agents, for example TiO₂ is added within the limits of 0.2 to 8.0% by weight.

The veneering ceramic is used as a powdered starting glass with crystalline additives or without separate crystalline additives and is used in the area using a defined temperature program_doic_"h between 800 and 94O⁰C on dental

Products sintered from yttrium-stabilized zirconium dioxide and crystallized in a controlled manner.

Surprisingly, it has been shown that with specific glass ceramics and a defined temperature program, a very high adhesive strength on dental

Products made of yttrium-stabilized zirconium dioxide is achieved. The

Veneering ceramics are translucent and have a very good chemical

Resistance. The main crystal phase of the glass ceramic consists of lithium disilicate.

The veneering ceramic can be used as the starting product in addition to the powdered one

Starting glasses of glass ceramics also contain powdered crystals. The powdered veneering ceramic passes through the through a defined temperature treatment

Processes nucleation, sintering and fusion with the yttrium-stabilized

Zirconium dioxide and crystallization with the formation of microcrystals.

It is also preferred that powdered lithium disilicate is added to the starting glasses. The lithium disilicate can be produced via a solid-state reaction.

The addition of TiO₂ supports the process of nucleation as well as the

Crystallization of lithium disilicate. The veneering ceramic is then advantageously formed from a mixture which contains the following components:

a) SiO₂ 58.0-72.0% by weight b) AI₂O₃ 4.0-0% by weight c) Li₂O 5.0-0.17% by weight d) Na₂O 4.0-11.0% by weight e) ZrO₂ 0.5 - 5.5% by weight f) TiO₂ 0.2 - 8.0% by weight

Zirconium dioxide or a mixture of zirconium dioxide and titanium dioxide is used as the nucleating agent for the controlled crystallization of the veneering ceramic based on lithium silicate materials. The addition of titanium dioxide favors the conversion of lithium metasilicate into lithium disilicate. The veneering ceramic is also preferred if, in addition to lithium disilicate, lithium titanium oxide silicate Li₂TiOSiO₄ , Crystallize lithium aluminosilicate (ß - spodumene) and smaller amounts of lithium metasilicate.

The veneering ceramic can also be designed so that the crystalline content is below 40%. The veneering ceramic is applied thinly. In this case, the veneering ceramic is used to adapt the color and gives the dental framework ceramic a special aesthetic shine. The strength of the veneering ceramic can be increased further by targeted compressive stresses.

The oxides of the elements Ce, Fe, Mn, Sn, V, Cr, In and the rare earths Pr, Nd, Sm, Eu, Tb, Dy and Er can be used as coloring or fluorescent additives. To modify the technology, the additives La₂θ₃, B₂θ₃, P₂Oe, CaO, MgO, ZnO and fluoride are added, the maximum concentration being in the range up to 4.0% by weight.

For the production of the veneering ceramic, nucleation takes place in the temperature range from 500 to 680 ⁰C and the melting and crystallization in the temperature range from 800 to 940 ⁰C. The nucleation and crystallization process can be interrupted by cooling the veneering ceramic between nucleation and crystallization to room temperature, storing and then heating to the crystallization temperature. The adhesive strength between the yttrium-stabilized zirconium dioxide and the veneering ceramic is determined in the bending test. For this purpose, the powdered veneering ceramic is applied to the face of two round rods made of zirconium dioxide and subjected to the defined temperature treatment. The adhesive strength is determined in a three-point bending test. Veneering ceramics are preferred whose bond strength to zirconium dioxide is at least 150 MPa.

The invention will be explained in more detail below using exemplary embodiments. The drawings show:

1 X-ray diffractogram (XRD) after the solid-state reaction of lithium oxide and silicon dioxide (four hours at 940 ⁰C),

2 shows a typical temperature profile for the production of the veneering ceramic and FIG. 3 XRD of a veneering ceramic according to the invention.

In Table 1, twelve syntheses are shown as exemplary embodiments of the veneering ceramics according to the invention.

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The starting glasses were in platinum or platinum - rhodium crucibles at a

Temperature of 1530 ⁰C melted and poured into water to make a frit (Figure 2).

To support the controlled crystallization, the fritted starting glasses are kept at 580 ± 100 for about four hours ⁰C annealed and after

Cool powdered. The grain size used is in the range of

0.6 μm to 20 μm.

Powdered lithium disilicate can be added to the starting glasses. The lithium disilicate is produced by a solid-state reaction. FIG. 1 shows the X-ray diffractogram (XRD) of the lithium disilicate produced by the solid-state reaction.

The moistened starting materials are applied as veneering ceramics to the dental framework ceramics made of yttrium-stabilized zirconium dioxide and then

890 ± 50 ⁰C fused and crystallized under control. FIG. 2 shows the typical temperature profile during the manufacturing process of the veneering ceramic.

All twelve examples of the veneering ceramics shown in Table 1 are translucent.

Both the visual effect and the mechanical resistance of the veneering ceramic are influenced both by the structure of the veneering ceramic and by the interaction of the veneering ceramic and the framework ceramic.

The coefficient of expansion (α) of the veneering ceramic and the framework ceramic made of yttrium-stabilized zirconium dioxide (TZ3Y) must be coordinated.

Based on the examples in Table 1, the expansion coefficients (α) of the veneering ceramics are documented in Table 2 and compared with yttrium-stabilized zirconium dioxide. <img file="WO2008106958A2_D0002.tif" />

Table 2

The adhesive strength between the framework ceramic made of yttrium-stabilized zirconium dioxide and the veneering ceramic was determined in a three-point bending test. For this purpose, the powder of the veneering ceramic was applied between two cylindrical specimens made of zirconium dioxide and subjected to a temperature treatment corresponding to FIG. 2.

The adhesive strength for selected samples is shown in Table 3, where σ = adhesive strength in MPa after the three-point bending test and m = Weibull parameters.

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Table 3

In the veneering ceramics according to the invention with high adhesive strength, different nucleation and crystallization processes are possible depending on the composition and the temperature treatment. Based on Table 1 and a temperature of 890 ± 50⁰C crystallize the examples of veneering ceramics 1, 4, 8, 9 and 10 to the crystal phases lithium silicate and zirconium dioxide. The zirconium dioxide serves as a nucleating agent. The crystallization of the lithium silicate takes place in two stages. Lithium metasilicate Li forms first₂SiO₃ and the subsequent reaction with the surrounding silicate phase converts lithium metasilicate to lithium disilicate Li₂Si₂O₅ around. Based on Table 1 and a temperature of 890 ± 50 ° C, the examples of veneering ceramics 2, 5 and 7 crystallize into the crystal phases lithium disilicate, β-spodumene, lithium titanium oxide silicate Li₂ (TiO) (SiO₄) and lithium metasilicate. By adding titanium dioxide, the crystallization of lithium disilicate Li₂Si₂O₅ accelerates. The XRD image is shown in FIG.