CS2-18. Dental Ceramics - Assoc. Prof. Dr. Özay Önöral.pdf

1 Assoc. Prof. Dr. Özay ÖNÖRAL COURSE ID Code of Committee: CS-2 Name of Committee: Fixed Prosthetic Restorations Lecturer: Assoc. Prof. Dr. Özay Önöral Topic of the Course: Dental Ceramics Duration of the Course: 150 minutes After completion of this course, the student will be able to: De...

1 Assoc. Prof. Dr. Özay ÖNÖRAL COURSE ID Code of Committee: CS-2 Name of Committee: Fixed Prosthetic Restorations Lecturer: Assoc. Prof. Dr. Özay Önöral Topic of the Course: Dental Ceramics Duration of the Course: 150 minutes After completion of this course, the student will be able to: Define the chemical structure of ceramics Express the condensation and baking processes Classify dental ceramics according to different parameters Optically and mechanically compare dental ceramics (existing commercial samples) classified according to their microstructures. Select case-specific dental ceramics and justify their choice. Contemporary Fixed Prosthodontics, 6th Edition. Book by Junhei Fujimoto, Martin F. Land, and Stephen F. Rosenstiel. Published by Elsevier in 2022. Anusavice KJ, Shen C, Rawls HR (2013). Phillips’ Science of Dental Materials. (12th ed.). St. Louis: Elsevier Mosby. Learning Objectives of the Course: Suggested References to Review: Near East University, Faculty of Dentistry 2 Assoc. Prof. Dr. Özay ÖNÖRAL DENTAL CERAMICS 1 Introduction to Ceramic Chemistry Ceramic, which is an earthy material, is generally silicate and takes its name from the word “keramikos”, which means pottery and burnt substance in Greek. Ceramic is a combination of one or more metals with a nonmetal element, usually oxygen. Silicon (Si), zirconium (Zr) and aluminum (Al) are the most common metals found in ceramics. The large oxygen atoms act like a matrix, wrapping smaller metal atoms and semi-metal silicon atoms with bonds of ionic or covalent character and form the SiO4 tetra-hedra structure (Figure I). These strong bonds provide properties such as stability, hardness, resistance to heat and chemicals. Moreover, ceramics exhibit superior aesthetic, biocompatibility, color stability, high wear resistance. However, this structure also causes undesirable properties such as fragility (brittleness) due to low flexural strength and wear on opposing dentition due to the high surface hardness. Silicone Silicone Oxide (SiO4) tetra-hedra structure The composition of dental ceramics mainly consists of feldspar, quartz and kaolin. Feldspar gives the ceramic a natural translucency and forms the main structure. It is a mixture of potassium aluminum silicate (K2O.Al2O2.6SiO2) and albite. With its combining feature, it melts during firing, wraps kaolin and quartz and ensures the integrity of the mass. Quartz, which is in the structure of silica (SiO2), acts as a filler in the matrix. It provides stability to the mass by preventing shrinkage that may occur as a result of firing process. Since the melting temperature is very high, it helps the restoration keep its shape at high temperatures. Kaolin, also called Chinese clay, is an aluminum hydrate silicate (Al2O3.SiO2.2H2O). It is quite resistant to heat, but it is very opaque and is used very little. Due to its adhesive feature, it acts as a binding agent for quartz and feldspar. It also facilitates hand-processing by giving elasticity to the ceramic dough (slurry). In the composition of the ceramic, besides these abovementioned three main substances, there may also be fluids or glass modifiers (fluxes), intermediate oxides, various color pigments, opacifiers or various agents that enhance the fluorescence property. Oxides, called fluxes, are sodium oxide, potassium oxide and calcium oxide. They reduce the softening temperature of the glass by reducing the amount of crosslinking between the glass-forming elements and oxygen. Iron oxide, nickel oxide, copper oxide, magnesium oxide, titanium oxide, and cobalt oxide are oxides used to give the ceramic the desired color. Zirconium, cerium, tin, and uranium oxides are also used to make the ceramic have the appropriate opacity. Near East University, Faculty of Dentistry 3 Assoc. Prof. Dr. Özay ÖNÖRAL 2 Condensation and Firing Protocols of Ceramics Ceramic slurry is obtained by mixing powder and sufficient amount of liquid. Too much liquid causes movement between dust particles, causing deterioration in the powder-liquid mass. The process of removing the liquid binder in the ceramic powder by bringing the particles together is called condensation. With the condensation process, the contact force between the ceramic and the substrate is provided as much as possible, the connection force is increased, the formation of air bubbles is prevented, the porosity of the ceramic mass is reduced, and the durability of the structure is increased. The condensed ceramic mass is placed in front of the preheated oven, allowing the remaining water to be removed. If the condensed structure is placed directly in a hot oven, as a result of sudden heating, voids and large fracture areas occur on the restoration. After the preheating process, the ceramic is placed into the furnace and the baking process is started. During firing, ceramic goes through different stages according to the temperature of the furnace: • Low-bisque firing • Medium-bisque firing • High-bisque firing Near East University, Faculty of Dentistry 4 Assoc. Prof. Dr. Özay ÖNÖRAL One of the important features that occurs during the firing of the ceramic is its shrinkage, which depends on the particle size of the ceramic powder, condensation, humidity, and the firing temperature. Generally, this shrinkage is reported to be 30-38% by volume and 11-15% linearly. Ceramics are processed larger in the shrinkage rate predicted according to their type and feature. Colorless glass powders applied to the baked ceramic surface to provide a polished surface are called “glaze” and this process is called “glazing”. The thermal expansion coefficient of the glaze ceramic should be slightly lower than the ceramic mass to which it is applied. If the glaze has a higher coefficient of expansion than dentin ceramic, it cools under radial tension. The resulting stresses tend to form fine cracks on the surface. Near East University, Faculty of Dentistry 5 Assoc. Prof. Dr. Özay ÖNÖRAL 3 Classification of Dental Ceramics Dental ceramics can be classified in different ways according to baking temperatures, usage areas, translucency levels, microstructures, and manufacturing techniques. Classification according to microstructures of ceramics is the most frequently used. 4 4.1 Classification of Dental Ceramics with Respect to Microstructures Glassy Ceramics Dental ceramics that mimic the optical properties of enamel and dentin are glassy ceramics. Glasses are formed by the formation of three-dimensional network of atoms and they do not have a regular structure. Glasses in dental ceramics are made from a group of minerals called feldspar and are built on silica (silicon oxide) and alumina (aluminum oxide). Therefore, feldspathic porcelains are subset of alumino-silicate glasses. The glasses in Feldspar structure are resistant to crystallization (de-vitrification) during firing, have a long firing temperature range, and are biocompatible. Glassy ceramics are used in veneering of ceramic infrastructures, inlays, onlays, and veneers. Glass ceramics can be produced by mixing the powder and liquid and preparing the ceramic slurry, or they can be produced with the CAD / CAM (Computer Aided Design / Computer Aided Manufacturing - Computer Near East University, Faculty of Dentistry 6 Assoc. Prof. Dr. Özay ÖNÖRAL Aided Design / Computer Aided Production) systems that have been developed in the last 20 years and have become very popular today. In CAD / CAM systems, digital data of the tissues from which the restoration will be produced are obtained primarily by intraoral scanners or by scanning the conventional impression / model with the laboratory scanner. While the restoration design is done in computer environment using this data in the CAD unit; the design is obtained by subtracting from a prefabricated ceramic block in the milling device guided by a computer. Different opacity and translucency options are available. In addition, there are also polychromatic blocks, which show opalescence and fluorescence properties and simulate the optical properties of different dental tissues. 4.2 Particle Reinforced (Filled) Ceramics Filler particles are added to the basic glass composition to improve mechanical properties and control optical effects such as color and opacity. Glass ceramics reinforced with leucite The first filler used in dental ceramics is crystalline mineral particles of potassium alumina silicate (SiO2.Al2O3.K2O) structure, called leucite. This filler has been added in metal-supported ceramic restorations to ensure that ceramic can be fired successfully on the metal substructure. Ceramics that are thermally compatible with dental alloys are obtained during baking by adding leucite with a higher thermal expansion / shrinkage coefficient of 17-25% compared to feldspatic glasses. The use of leucite for full ceramics was realized with the IPS Empress (Ivoclar Vivadent, Schaan, Liechtenstein) system, which was developed at the University of Zurich in 1983 and introduced to the market in 1990. The most important advantage of this system is that the leucite crystals distributed homogeneously inside the material form a barrier that counteracts the stress that causes the formation of micro-cracks. The wax modeling of the restoration that is desired to be produced in the IPS Empress system is prepared, invested with the phosphate-bonded investment material, and the negative is obtained with the lost wax technique. Ceramic tablets containing 35-55% leucite by mass are placed into the furnace and sent into the mold at 12000C heat and under pressure. The combination of heat and pressure allows the shrinkage of the ceramic to be reduced and to have a higher bending strength. However, 120-140 MPa flexural resistance limited the indications of this ceramic to inlay, onlay and single-unit crowns. In 2006, IPS Empress CAD blocks were developed for use in CAD / CAM systems. Blocks with higher flexural resistance (160-180 MPa) have high (HT) and low (LT) translucency options. The 'Multi' type blocks developed in recent years are produced to provide a more translucent appearance towards the incisal area in the cervical region in order to mimic the optical property differences between the dental tissues. Near East University, Faculty of Dentistry 7 Assoc. Prof. Dr. Özay ÖNÖRAL Glass ceramics reinforced with lithium disilicate In all-ceramic restorations, one of the fillers used to strengthen the infrastructure is lithium disilicate (Li2Si2O5). The microstructure of lithium disilicate consists of very small crystals that are dispersed and interlocked in a versatile way. These crystals increase the durability by preventing the spread of cracks in the material. IPS Empress 2 (Ivoclar Vivadent, Schaan, Liechtenstein) system was developed with the addition of 70% by volume lithium disilicate to the felspatic glass structure. In this system, as in IPS Empress, the material is sent to the mold by injection method under heat and pressure. The resulting substructure (infrastructure, coping terms can also be used) is veneered with a ceramic whose thermal expansion coefficient is compatible. The difference in the microstructures of IPS Empress and IPS Empress 2 resulted in IPS Empress 2 having a higher bending strength (360 MPa) but lower translucency. IPS Empress 2 can be used in single member crowns and anterior 3 member bridges. In 2005, IPS e.max Press (Ivoclar Vivadent, Schaan, Liechtenstein), which was strengthened with lithium disilicate, was introduced to the market. In this restorative material, improved physical properties and translucency in comparison with IPS Empress 2 were achieved. IPS e.max Press contains 70% lithium disilicate crystals embedded in glassy matrix and these crystals are 3-6 μm long. Compared to IPS Empress 2, IPS e.max Press has smaller crystals in its structure and its bending resistance is reported as 370-460 MPa. IPS e.max Press is indicated for inlay, onlay, veneers, anterior and posterior crowns, 3-unit anterior and premolar-region bridges, and implant superstructures. The homogeneous distribution of coloring ions in the structure of the material inside the glass structure prevents color pigment errors in the microstructure. Apart from this, different translucency options such as high, medium and low translucency, high and medium opacity increase the aesthetic success of the final restoration. Apart from production under heat and pressure, blocks of lithium disilicate ceramics, called IPS e.max CAD, have been developed for use in CAD / CAM systems with different color, opacity and translucency options. There are also perforated CAD / CAM blocks for the production of a custom abutment. IPS e.max CAD blocks are partially crystallized (pre-crystallized) called 'blue state' and contain 40% metasilicate in addition to lithium disilicate crystals. Pre-crystallized blocks have a bending resistance of about 130 MPa, easy to mill and take a short time in CAD / CAM systems; it also ensures less wear of the milling tools. After the milling process is over, the restoration is subjected to heat treatment (840 ° -850 ° C, 10 min) in the ceramic furnace, and the metasilicate crystals turn into lithium disilica (70%), allowing the bending resistance to increase to 450-500 MPa. Monolithic (full contour) restorations can be produced with IPS e.max CAD blocks, and highly aesthetic restorations can be achieved by veneering the lithium disilicate substrate with glassy ceramics. Near East University, Faculty of Dentistry 8 Assoc. Prof. Dr. Özay ÖNÖRAL Glass ceramics reinforced with lithium silicate and zirconia First lithium silicate ceramic reinforced with zirconia ceramics is the Vita Suprinity developed by Vita Zahnfabrik in 2013. The purpose of developing this ceramic is to combine the advantageous properties of zirconium oxide, such as high resistance, and the aesthetic properties of glassy ceramics. Ceramic contains 10% zirconium oxide; fracture stiffness and bending resistance are higher than lithium disilicate ceramics. Vita Suprinity CAD / CAM systems have honey color and pre-crystallized blocks. For crystallization after milling, it should be subjected to heat in the ceramic furnace. With this ceramic, full contour (monolithic) restorations can be produced without the need for veneer ceramic application. The 3-point bending strength of the material is 420 MPa. It has two translucency options as translucent and highly translucent. It is indicated for anterior and posterior crowns, implant-supported crowns, and laminate veneer restorations. Vita Ambria (Vita Zahnfabrik), developed in 2019, is a lithium silicate ceramic reinforced with zirconia, and its ingots are produced under heat and pressure. The bending resistance is specified by the manufacturer as 550 MPa. Anterior and posterior crowns, 3-unit bridge up to the premolar region are the indications. Celtra Duo, developed by Dentsply Sirona, is also lithium silicate glass ceramics reinforced with zirconia. This ceramic is also produced using CAD / CAM systems. After milling, the surface of the restoration can be polished by mechanical polishing or by baking after glaze ceramic application. When firing is applied, the 3-point bending strength of the ceramic is 370 MPa, while the bending resistance is 210 MPa only when mechanical polishing is performed. It is indicated for crown, inlay, onlay, and laminate veneer restorations. Glass ceramics reinforced with fluormica Dicor (Dentsply International Inc, York, PA), which was obtained by strengthening the glass ceramic structure with 55% by volume tetracilicic fluormica crystals (K2Mg5SiO2OF4) and called castable glass ceramic, was developed in 1983. The lost wax technique is used in the production of Dicor full ceramic restorations. The wax modeling is put into phosphate-bonded investment and after eliminating the wax, glass tablets containing fluormica are casted at 13800C and a glass structure is obtained. This glass structure is heated at 1070 0C for 6 hours to turn it into a crystal structure. This process is called ceramming. Ceramming process increases the resistance, hardness, and chemical stability of the structure, while decreasing its translucency. All ceramic restorations produced with Dicor system are not preferred today because of their insufficient physical and optical properties. Glass ceramics reinforced with alumina (aluminum oxide) Alumina was first used in 1965 to strengthen its ceramic structure. Aluminous ceramic contains 40-50% alumina by mass and is obtained by sintering method. The infrastructure is baked on a platinum foil and veneered with ceramic whose thermal expansion properties are compatible. The bending resistance is 139 MPa and the shear resistance is 145 MPa. Near East University, Faculty of Dentistry 9 Assoc. Prof. Dr. Özay ÖNÖRAL In-Ceram Alumina (Vita, Bad Säckingen, Germany) was developed in 1989, which is a glass infiltrated ceramic and contains 70% by volume alumina. In the In-Ceram Alumina full ceramic system, a duplicate of the working model is obtained first. Alumina suspension (slip material) is applied with a brush on refractory day and this process is called “slip-casting”. The porous structure obtained by slip-casting method is baked at 11200C and a partially sintered infrastructure is obtained. In order to eliminate the porosity and to resist the infrastructure, the glass containing lanthane is infiltrated into the partially sintered structure at 11000C for 4 hours. The resistant infrastructure is veneered with feldspatic ceramics. In-Ceram Alumina has a bending resistance of 450 MPa and can be used in anterior 3-unit bridges and single-unit crowns. The development of In-Ceram Alumina blocks also allowed production with a CAD / CAM system, CEREC (Sirona Dental Systems GmbH, Bensheim, Germany). Glass ceramics reinforced with alumina and magnesium spinell In-Ceram Spinell (Vita, Bad Säckingen, Germany) was introduced to the market in 1994 to increase the translucency of In-Ceram Alumina. In-Ceram Spinell is also glass-infiltrated ceramic, the crystalline structure consists of a mixture of magnesium spinel (MgAl2O4) and alumina. This difference in microstructure causes the bending resistance of In-Ceram Spinell (350 MPa) to be lower than In-Ceram Alumina and its indications are limited to inlay, onlay, anterior single-unit crowns, and veneers. In-Ceram Spinell full ceramic restorations can be produced with slip-casting method or they can be produced with developed blocks using CEREC system. The obtained infrastructure is veneered with feldspatic ceramics. Glass ceramics reinforced with alumina and zirconia With the attempt to increase the resistance of In-Ceram Alumina, In-Ceram Zirconia (Vita, Bad Säckingen, Germany) consisting of 30% partially stabilized zirconium oxide and 70% alumina was developed. Although InCeram Zirconia has high bending resistance (600-700 MPa) compared to other glass infiltrated ceramics, the high opacity caused by its crystal structure prevents its use in the anterior region. As in the production of other glass infiltrated ceramics, slip-casting method can be applied in the production of In-Ceram Zirconia restorations or partially sintered prefabricated blocks can be shaped with CAD / CAM systems. 4.3 Polycrystalline Ceramics There is no glassy structure in the content of polycrystalline ceramics. Crystal atoms form a more regular and dense structure compared to glassy ceramics. Therefore, polycrystalline ceramics are harder and more resistant than glassy ceramics. However, polycrystalline ceramics, which are more opaque than glassy ceramics, are used in infrastructure in full ceramic restorations and are veneered with glassy ceramics to enhance translucency. Near East University, Faculty of Dentistry 10 Assoc. Prof. Dr. Özay ÖNÖRAL Aluminum Oxide Polycrystalline Ceramics Procera AllCeram (Nobel Biocare, Gothenburg, Sweden), developed by Andersson and Oden, contains 99.9% high purity dense sinterized aluminum oxide (Al2O3) crystal. In the Procera AllCeram system, the working model is scanned with a sapphire tip, and the 3D form of the preparation is obtained and the data are sent electronically to Procera's headquarters in Sweden or America. In order to compensate the shrinkage that occurs during sintering, using digital data, the day model is prepared approximately 20% larger than normal and high purity aluminum oxide powders are pressed on the enlarged die. Then, the infrastructure obtained in desired dimensions by fully sintering at 1550 0C is veneered with lowtemperature ceramic. Procera AllCeram allows full ceramic restorations to be used in anterior and posterior single-unit crowns, inlays, onlays, and veneers, due to its high bending resistance (450 MPa) and sufficient optical properties. Zirconium Oxide Polycrystalline Ceramics Zirconium oxide (ZrO2-zirconia), which has been used as a biomaterial since the 1970s, was introduced to dentistry in 1998. The zirconia, which has superior mechanical and favorable physical properties, has changed its composition in the last 20 years and its optical properties have been improved and its area of use has expanded. Zirconium oxide is a polymorphic material that can be found in 3 different phases as monoclinic, cubic and tetragonal depending on the temperature of the environment in which it is located. Pure zirconia is in the monoclinic phase at room temperature and remains stable in this phase up to 1170 0C. When it rises above this temperature, it begins to transform into the tetragonal phase and passes into the cubic phase at 2370 0C. When re-cooled to room temperature, it changes from tetragonal phase to monoclinic phase. This phase change causes a 3-5% volume increase in zirconia and increases tendency to internal stresses and fractures. 3 mol% of yttrium oxide (Y2O3) is added to the structure to ensure that the zirconia remains in the tetragonal phase at room temperature by preventing unwanted phase change and to prevent its expansion. This structure is called yttrium tetragonal zirconia polycrystalline (Yttrium Tetragonal Zirconia Polycrystals-Y-TZP) or partially stabilized zirconia with yttrium. Near East University, Faculty of Dentistry 11 Assoc. Prof. Dr. Özay ÖNÖRAL Y-TZP, which stands out with its high mechanical resistance, chemical and dimensional stability, is the most used material in the infrastructure of all ceramic restorations today. Since Y-TZP is partially stabilized, although it is in the tetragonal phase at room temperature, there is energy in its internal structure to transform into monoclinic phase. If Y-TZP is subjected to stress or any force, and cracking occurs in its structure, tetragonal crystals begin to move to the monoclinic phase and a 3-5% increase in volume occurs. This volume increase prevents the progression of the crack by creating compressive stresses. This phenomenon, called "transformation toughening", provides superior mechanical properties to YTZP. Apart from the transformation hardening, another factor affecting the mechanical properties of Y-TZP is its particle size. The fact that the particles are larger than a certain size causes Y-TZP to change from tetragonal phase to monoclinic phase by itself. The phase change tendency decreases as the particle size decreases (<1 µm). However, having a particle size of less than 0.2 µm prevents conversion and causes the fracture resistance to drop. Since sintering conditions determine the particle size, it significantly affects the stability and mechanical properties of the ceramic. High sintering temperature and long sintering time causes the particle sizes to grow. The production of Y-TZP full ceramic restorations is possible by shaping partially-sintered or fully-sintered Y-TZP blocks with CAD / CAM systems. Partially-sintered Y-TZP blocks, which are common and recommended by many manufacturers, are sintered at high temperatures after being formed with CAD / CAM systems. The day model or wax modeling of the restoration is scanned with the scanner of the CAD / CAM system and is designed to be larger than it should be with computer software (CAD) to compensate the shrinkage that occurs during sintering. The partiallysintered block is milled (CAM) according to this design and sintered at high temperature. This process differs depending on the scanning type of the CAD / CAM system used and the shrinkage of the Y-TZP (25%) block during sintering. The infrastructure shaped by the CAD / CAM system is placed in the programmed furnace which is Near East University, Faculty of Dentistry 12 Assoc. Prof. Dr. Özay ÖNÖRAL special for sintering. Sintering shrinkage starts at 10000C. Depending on the different sintering conditions of each product, the sintering temperature reaches 1350-15,500C and its duration varies between 2-5 hours. To minimize residual stresses after sintering, the restoration is cooled in the furnace to a temperature below 200 0C. Cercon Smart Zirconia (Dentsply International), Lava Frame (3M ESPE), Procera AllZirkon (Nobel Biocare), In-Ceram YZ (Vident), IPS e.max ZirCAD (Ivoclar Vivadent) and Everest ZS (Kavo) are available semi-sintered blocks on the market. Fully sintered Y-TZP blocks are ensured to reach a density of 95% by sintering below 15000C before milling. Then, the blocks are subjected to hot-isostatic pressing (HIP) process at 1400-15000C and under high pressure and the density is achieved to reach 99%. Fully sintered blocks are milled in specially designed devices, but their high hardness makes them difficult to shape. Fully sintered blocks are difficult to process, affecting their mechanical properties badly. However, the absence of sintering shrinkage is also shown as the advantage of these blocks. DC Zircon (DCS Dental AG), Denzir (Cadesthetics AB), Digizon (Digident GmbH) and Everest ZH (Kavo) are fully sintered blocks on the market. CAD / CAM systems or copy milling systems are used in the production of full ceramic restorations from YTZP blocks. Since the low translucency of zirconia limits its aesthetic features, it was used only as an infrastructure ceramic until the last years and its aesthetic appearance was improved with the application of veneer ceramic. However, with the translucent zirconia developed by the modifications made in the particle sizes and sintering temperatures of the material, "monolithic" or "full contour" zirconia restorations that do not need to be applied to veneer ceramic can be produced today. Although the aesthetic properties of translucent zirconia are superior to conventional zirconia, their mechanical resistance is also lower. Near East University, Faculty of Dentistry 13 Assoc. Prof. Dr. Özay ÖNÖRAL Useful Charts Near East University, Faculty of Dentistry 14 Assoc. Prof. Dr. Özay ÖNÖRAL Near East University, Faculty of Dentistry 15 Assoc. Prof. Dr. Özay ÖNÖRAL Near East University, Faculty of Dentistry