CS2-21 Lab Stages and Fabrication Techniques PDF

1 Assoc. Prof. Dr. Özay ÖNÖRAL Laboratory Stages of Metal-Ceramic Restorations Wax Modeling The first stage of fabrication of metal-ceramic restorations is the wax modeling of the infrastructure (coping, framework) to be produced. The dental technician performs the wax-modeling of the infrastructure...

1 Assoc. Prof. Dr. Özay ÖNÖRAL Laboratory Stages of Metal-Ceramic Restorations Wax Modeling The first stage of fabrication of metal-ceramic restorations is the wax modeling of the infrastructure (coping, framework) to be produced. The dental technician performs the wax-modeling of the infrastructure on the patient's cast model. Before the wax replica is prepared, the material called “die spacer”, which creates a space between the restoration and the tooth, is applied on the die according to the desired cement space and the shrinkage amount of the preferred alloy (Nesse et al., 2015). The experience and skill of the dental technician is important at this point (Örtorp et al., 2010). In contemporary dentistry, wax blocks or PMMA resin blocks from various companies are used in order to standardize fabrication, eliminate die spacer application stage, wax modeling stage and thus possible errors that may occur during these stages. The infrastructure designed in the computeraided design (CAD) unit is milled (subtractive technique) in the CAM unit using either wax or PMMA blocks. After the wax replica (model) is obtained from the CAM unit, the process continues with conventional lost-wax and casting technique (Farjood et al., 2016). Wax infrastructures are not only produced by the subtractive technique as above. There are also additive techniques that can be used for this purpose including (I) stereo lithography apparatus - SLA, (2) fused deposition modeling - FDM. SLA technique in which 3 dimensional models are manufactured by photosensitive polymer is the most widely used among the additive techniques. The technique defined by Hull in 1984, systematically consists of 3 basic components: (i) pool with a photosensitive liquid, (ii) platform where the model is manufactured, (iii) ultraviolet (UV) laser. When photopolymer resin is exposed to UV light, the first layer at a certain thickness hardens. Following the hardening of the first layer, the platform where the model is produced is lowered at a certain distance in the pool and a new layer of photopolymer resin is applied on the Near East University, Faculty of Dentistry 2 Assoc. Prof. Dr. Özay ÖNÖRAL previous layer. The basic processes continue until the construction of the three-dimensional solid material is completed. The self-adhesive feature of the material helps layers stick together. After the completion of the process, newly produced hard material is removed from the pool and kept in the UV cabinet for a while until the hardening process is completed. Fused deposition modeling is the second widely used fast prototyping technique after stereo lithography enabling the layer by layer extrusion of the thermoplastic material with the help of a nozzle controlled by heat. In this technique, the filament of the thermoplastic polymer material feeds the extrusion nozzle controlled by heat. The wire-like plastic material in strips leads to extrusion nozzle and here the material is melted by heating. The extrusion nozzle forms the first layer of the product by spraying the distilled molten material on the platform. At this stage, the heat of the platform is kept at lower degrees and thus, the sprayed thermoplastic material is immediately (in 0.1 second) hardened. The manufacturing platform is brought one step down in each layer and thus the product is constructed in layers. These procedures are repeated until the entire part is constructed. During construction, the parts have to be supported by support structures which are either manually or automatically designed. After the completion of the manufacturing, these support structures are removed from the part. Water-soluble materials can be used as support units. FDM equipment has low maintenance costs. However, it should be noted that it has disadvantages such as formation of a connection line among the layers, Near East University, Faculty of Dentistry 3 Assoc. Prof. Dr. Özay ÖNÖRAL requirement of a support unit, a long manufacturing duration and separation of layers depending on fluctuations in heat. Fabrication of Metal Copings 1. Traditional Casting Consolidated with Lost-Wax Technique For years, casting method has been used in the framework production of metal-ceramic restorations (Kim et al. 2016; Kim et al., 2013; Quante et al., 2008; Tamac et al., 2014). The fact that there have been developments in the production of prosthetic restorations from past to present and that the casting includes many stages that are prone to complication formation has decreased its popularity (Abduo et al., 2014; Kaleli and Saraç, 2016; Kim et al., 2017b; Kocaağaoğlu et al., 2017; Kocaağaoğlu et al. 2016; Nesse et al., 2015; Park et al., 2016). However, it is still a technique used in the production of restorations (Kim et al., 2017a; Kim et al., 2014; Kocaağaoğlu et al., 2016; Prabhu et al., 2016). In lost-wax technique, wax replica (produced by the technique in which the dental technician is active, by subtractive techniques or by additive techniques) is melted by heat application after a prepared wax model is surrounded by a heat-resistant investment material, and subsequently, burn-out (wax elimination) is performed in a furnace. Molten metal is transferred from the casting channel, which is expressed as sprue (channel where the metal in the molten state will flow). In order to perform the casting process, it is necessary to prepare the wax replica first (Powers and Wataha, 2013, p. 150). In order to prevent possible distortions that Near East University, Faculty of Dentistry 4 Assoc. Prof. Dr. Özay ÖNÖRAL may occur in the wax replica, the replica should be immediately invested (Powers and Wataha, 2013, p. 152). The wax model is then combined with plastic wire, metal or wax-made sprue and positioned in the casting ring at the appropriate position. Wax sprues are preferred as they will often melt at the same speed as the wax replica. Sprues can be created in flat or similar shapes to U or Y letters (Baydaş, 2005, p. 115). The primary purpose here is to first allow the molten wax to be removed from the mold; and then to ensure that the molten metal moves into the casting cavity and fills the cavity without being obstructed by any obstacle. Although the casting ring may vary depending on factors such as the length and volume of the wax model; they usually have 1-4 mm width and 10-20 mm length (Baydaş, 2005, p. 115). The sprues are melted during the melting process (burn-out) of the wax model and thus reflect a shape in the investment and form a casting channel. This channel allows the molten metal to reach the casting cavity during casting. In order to facilitate this cycle, instead of cylindrical ones, they should be prepared in the form of a cone. Thanks to the channel formed by these sprues from thick to thin, the physical advantage is provided. Thanks to this situation, since the metal traction direction will be from thin to thin, thin surfaces pull the molten metal towards itself. The casting process ends when the molten metal fills the casting cavity (Baydaş, 2005, p. 116). Near East University, Faculty of Dentistry 5 Assoc. Prof. Dr. Özay ÖNÖRAL Casting devices are classified according to 3 different variables. In the first component, the metal alloy rod is melted at the point where it opens out or in a separate crucible located in the casting device. In the second component, the metal alloy is melted using gas-air torch, oxyacetylene torch, electric induction heating or one of the electric resistance melting methods. In the third component, the molten metal alloy is sent into the casting cavity with the help of gravity, air pressure, steam pressure or centrifugal / centrifugal force. In today's most popular system, metal alloy is melted in a separate crucible with electric induction heating and the molten metal is sent to the casting cavity with centrifugal force (Ulusoy & Aydın, 2010, p. 571-574). 2. Milling (Subtractive Manner) The foundations for the automation of the production of prosthetic restorations were laid with CAD / CAM systems that entered dentistry in the 1980s. The increase in patients' expectations regarding the lifespan, biocompatibility, and aesthetic properties of the restorations led to the rapid development of CAD / CAM systems (Beuer et al., 2008; Park et al., 2015; Strub et al., 2006). The subtractive method is also called milling technique (Liu et al., 2006; Van Noort, 2012). The production of the restoration desired to be created in the stated method is based on the principle of obtaining by engraving from the block. However, a large amount of material is wasted because an average of 90% of the prefabricated block is used to create a typical restoration, and about 10% of the block is wasted each time. In addition, 10% residual material cannot be recycled (Liu et al., 2006; Şeker & Ersoy, 2010; Van Noort, 2012). More than 20 milling systems have been developed from past to present and these systems are basically divided into 2 categories as hard machining and soft machining. Near East University, Faculty of Dentistry 6 Assoc. Prof. Dr. Özay ÖNÖRAL Fully sintered nonporous ingots are used in the hard machining method, which is defined as obtaining the restoration by milling from the sintered mono-block. In this method, milling is done quite hard. However, they do not need to be subjected to sintering again. Soft processing, on the other hand, finds expression as the process of obtaining restoration by milling from the block that will later be sintered. Pre-sinterized ingots are porous and allow a rapid milling process. In order for the porous structure to be eliminated, they must be subjected to a sintering process (Ebert et al. 2009; Griggs, 2007). The processing steps of soft metal blocks developed in recent years are almost similar to the processing steps of zirconia in pre-sinterized form (Kim et al., 2016). Since both systems are subtractive methods, the biggest disadvantage of both systems is significant material expenditure because the parts are no longer usable after milling, and it is not possible to recycle the material. While the restorations produced with the hard processing method offer advantages such as obtaining the correct shape and full size. One of the most important disadvantages of the method is that the processing of sintered blocks is an expensive and time-consuming process. Also in this method, the tools are exposed to severe abrasion (Kim et al., 2016; Kocaagaoglu et al., 2016). In addition, there is a risk of microscopic cracks forming on the metal surface during the processing of the material. This does not occur during soft machining because milling is done before sintering process. In addition, milling of pre-sintered mono-blocks ensures both shorter machining times and longer tooling cycles (Kim et al., 2016; Kocaagoglu et al., 2016). However, the accuracy and shape of the restorations produced by the soft processing method is a bit more troubled than the hard processing. The reason for this is the shrinkage that will occur during the sintering that will be done later (Bindl & Mörmann, 2007; Ebert et al., 2009; Wong & Hernandez, 2012). For example, after soft metal milling, metal infrastructures are subjected to high sintering in argon gas environment. This application results in 11% shrinkage (Kim et al., 2016; Kocaağaoğlu et al., 2016). With these techniques, many laboratory steps such as wax modeling (waxing), spruing, investing, wax elimination, and casting are eliminated (Lapcevic et al., 2016). Thus, technical complications that may occur at these stages are also eliminated. Although subtractive methods still have widespread use in the clinic; additive manufacturing techniques, which have important Near East University, Faculty of Dentistry 7 Assoc. Prof. Dr. Özay ÖNÖRAL advantages and superior features, continue to develop rapidly. This is the biggest indication that the additive method will soon replace milling techniques. 3. Layering (Additive Manner) Additive manufacturing systems are defined as systems that enable the production of an object with three dimensional designs (CAD) with computer support by adding a layer on the layer (Azari and Nikzad, 2009; Madhav and Daule, 2013; Ruiz et al., 2015; Torabi et al., 2015; Van Near East University, Faculty of Dentistry 8 Assoc. Prof. Dr. Özay ÖNÖRAL Noort, 2012). Additive manufacturing is expressed by the American Society for Testing and Materials (ASTM) as follows: “Contrary to subtractive production methods, it is the process of layering the materials (adding layers on the layer) in order to produce objects from the computer data of the three-dimensional model” Additive manufacturing system, which is widely used in dentistry today, can be referred as 3D printing, additive fabrication, rapid prototyping, layered manufacturing or solid freeform fabrication (Azari and Nikzad, 2009; Madhav & Daule, 2013; Sun & Zhang, 2012; Torabi et al., 2015; Van Noort, 2012). The additive method, which originated from the name “rapid prototyping / manufacturing” in the sense of rapid production, was first used in the 1980s for the production of prototypes, models, and casting elements. Although the operating cycles of additive systems are different from each other; they generally have 5 common stages. These stages are (Van Noort, 2012): • Creating a CAD model • Convert CAD model data to standard tessellation language (STL) file • Slicing the STL model mathematically into layers • Layer-upon-layer production • Additional operations after the completion of production Over time, the idea that additive manufacturing is a great opportunity for dentistry has been put forward and its use in the dental field has spread rapidly. Not all additive manufacturing techniques are used in dentistry. Among the techniques that are mostly used in engineering fields, those that have been routinely used in dentistry are: (i) Stereo-lithography, (ii) Selective Laser Sintering, (iii) Selective Laser Melting, (iv) Fused Deposition Modeling (Azari and Nikzad, 2009; Bartolo et al., 2012; Günsoy and Ulusoy, 2015; Madhav and Daule, 2013; Quante et al., 2008; Torabi et al., 2015 ; Van Noort, 2012). Near East University, Faculty of Dentistry 9 Assoc. Prof. Dr. Özay ÖNÖRAL 3.1. Selective Laser Sintering The selective laser sintering technique, in which powdered materials can be used, allows the fusion of powdered particles with the help of carbon dioxide laser. It was first described by Deckard in 1989. The powdered material distributed with the help of a roller melts selectively when exposed to a laser beam (Yıldırım & Bayındır, 2013). In other words, it is sintered. Then, the new layer is distributed on the platform, which is lowered one layer thickness and selectively melted (sintered) material is connected to the previous layer by laser beam. The steps are repeated until the final product is obtained (Bhatnagar et al., 2014; Goswami et al., 2014; Günsoy and Ulusoy, 2015; Heynick & Stotz, 2009; Kocaağaoğlu et al., 2016; Van Noort, 2012; Wong and Hernandez, 2012). The temperature applied by the laser throughout the process should be kept below the melting range of the material and the heat should be enough to allow the particles in the material to be sintered (Heynick & Stotz, 2009; Madhav & Daule, 2013; Wong & Hernandez, 2012). The material is not completely melted; therefore, only sintering saves considerable time. The technique, which has a wide range of materials, does not need a support unit during production (Bhatnagar et al. 2014; Heynick & Stotz, 2009). Thermoplastic materials such as casting waxes, metals, ceramics, and thermoplastic composites can be used in selective laser sintering. One of the most important features is that the material can now be recycled and reused. This significantly reduces the cost (Heynick & Stotz, 2009; Madhav & Daule, 2013; Wong & Hernandez, 2012). Since objects are porous, extra material infiltration may be required in this technique (Heynick & Stotz, 2009). Near East University, Faculty of Dentistry 10 Assoc. Prof. Dr. Özay ÖNÖRAL A number of precautions have been taken to minimize complications in the system. In this technique, all production steps are carried out in a vacuumed chamber. The aim is to eliminate difficulties such as moisture and oxidation (Vaezi et al., 2013). It should also be noted that, depending on the fluctuations in the size, speed or power of the laser beam, the properties of the polymer may deteriorate. Controlled use of the laser is important (Stansbury and Idacavage, 2016). With this technique, many laboratory steps such as wax modeling (waxing), spruing, investing, wax elimination, and casting are eliminated (Lapcevic et al., 2016). Thus, technical complications that may occur at these stages are also eliminated. 3.2. Selective Laser Melting In selective laser melting technique, which works in the same manner with selective laser sintering technique, the powder material is not sintered as in SLS but completely melted (Kim et al., 2016) and a melt pool is created. The system uses commercially available powder materials. The particle size can vary between 20-50 micrometers (Abduo et al., 2014; Gebhardt et al., 2010; Yıldırım & Bayındır, 2013). The first table type SLM machine was produced in 2009. SLM is a technique that produces a mass with the help of laser. Therefore, as in all laser-induced processes, problems such as shrinkage, fracture, distortion, and surface hardening are expected in SLM technique. To overcome all this, fine-grained powder materials are used and the process takes place in protective gas. Also, adjusting the diameter of the laser beam used and keeping the layer thickness thin minimizes both the stair type effect (Figure 2.14) and the abovementioned risks (Gebhardt et al., 2010). With the SLM technique, complex geometries can be produced in three dimensions (Zeng et al., 2015). In addition, it has been reported that metal-ceramic bond strength is higher in the SLM-fabricated restorations compared to the restorations produced by conventional casting technique (Xiang et al., 2012). In terms of corrosion properties, restorations produced with SLM technique were found to be more successful (Xin et al., 2013). With this technique, many laboratory steps such as wax modeling (waxing), spruing, investing, wax elimination, and casting are eliminated (Lapcevic et al., 2016). Thus, technical complications that may occur at these stages are also eliminated. Achievement of Durable Bond between Metal Coping and Porcelain The metal infrastructure must be able to connect strongly to ceramics. The properties of the alloys are important in terms of forming the strong connection and preventing mechanical Near East University, Faculty of Dentistry 11 Assoc. Prof. Dr. Özay ÖNÖRAL complications that may occur in restoration (Wataha and Messer, 2004). A strong metal-porcelain connection is the key factor for the long-term success of metal-ceramic restorations. The connection has four components: van der Waals forces, mechanical retention, chemical connection, and compression connection (Naylor, 1992, p. 83-85). Secondary forces (van der Waals forces) formed between charged atoms are important in metal-porcelain connection of metal-ceramic restorations. It can also be expressed as the minimal attraction force formed between the electrons and nuclei of atoms in a molecule and the nuclei and electrons of the atoms of neighboring molecules. Increased wettability of the metal surface also increases the effectiveness of van der Waals forces (Naylor, 1992, p. 83; Shillinburg et al., 1997, p. 456). The key-lock mechanism (micromechanical interlocking) is formed between the alloy and the porcelain in the mechanical connection. Ceramic is infiltrated on the metal surface, which is microscopically roughened with aluminum oxide particles (air-borne particle abrasion), during porcelain firing. Mechanical connection is important in terms of increasing the wettability of the metal by porcelain and helping chemical connection by increasing the surface area (Naylor, 1992, p. 84; Shillinburg et al., 1997, p. 456). However, in cases where excessive roughening is performed, stress accumulation occurs on the interface and sharp edges and corners caused by incorrect blasting reduce the wettability and cause gaps in the interface. The chemical connection between metal and porcelain is established through oxides on the metal surface. The thickness and composition of the metal oxide layer is very important in the long-term success of the connection. Metal oxide formation takes place spontaneously in many base metal alloys; sometimes the oxide layer formed can be very thick (Rosenstiel et al., 2016, p. 655; Wataha, 2002;). In these cases, the thick oxide layer should be thinned before veneer porcelain application and the layer thickness should be kept at optimum values. However, due to the low reactivity of gold, not enough oxide layers can be formed in high noble and some noble alloys to establish metal-porcelain connection. Metals such as iron, gallium, indium, and tin are added as trace elements to create an adequate oxide layer for these alloys. Since these elements burn easily during casting; these alloys do not overheat. While the thickness and composition of the metal oxide layer is important for the metal-porcelain connection, the color of the layer is also very important in terms of aesthetics. High gold alloys have a relatively light oxide layer. In this case, the layer can be easily masked with opaque porcelain. However, since many alloys based on silver, nickel, and cobalt form a darker, grayish oxide layer; thicker layer opaque porcelain application is required to mask the layer in these alloys. The thin or thick oxide layer increases the risk of failure in the metal-ceramic connection because occlusal stresses are concentrated in the Near East University, Faculty of Dentistry 12 Assoc. Prof. Dr. Özay ÖNÖRAL oxide layer. The thickest oxide layer formation occurs in nickel and cobalt based alloys because these alloys contain elements that can easily form oxides at the initial oxidation stage. As a result, sufficient oxide formation that does not endanger the expansion, durability, color, and opacity properties of porcelain is a desired feature (Powers and Wataha, 2013, p.142; Wataha, 2002; Wataha and Messer, 2004). The last component that plays an active role in metal-porcelain connection is compression. Ceramics are sintered and fused with alloys at temperatures ranging from 850°C to 1350°C, depending on the preferred ceramic. The metal substructure should be able to maintain its structure without being distorted during ceramic baking. Otherwise, the harmony of the restoration will be compromised. To prevent this situation, it is desired that the melting temperature of the alloys is higher than the sintering temperature of the ceramic. Metal elements (platinum, palladium) with high melting degrees are added to the structure in order to increase the melting degree of the alloys. Not all ceramics are compatible with alloys. Therefore, manufacturers indicate which ceramic can be used with which alloy (Powers and Wataha, 2013, p. 144). Connection in the compression style is provided depending on the thermal expansion of the materials. Alloys and ceramics expand when heated; they shrink when cooled. The dimensional increase (amount of expansion) of the material per unit of heat is expressed as the coefficient of thermal expansion and its unit is (ºC) -1 (Powers and Wataha, 2013, p. 144). It should be remembered that shrinkage will increase as the thermal expansion coefficient increases. The thermal expansion of the metal is greater than porcelain. After the porcelain is baked with metal, both begin to shrink during cooling (Wataha, 2002; Wataha and Messer, 2004). Metal shrinks faster than porcelain. A tension occurs in metal that shrinks faster than porcelain, and a compression occurs in porcelain. When metal and porcelain are connected, three possible relationships can occur in thermal expansions. These are; (1) the thermal expansion of the porcelain is greater than that of the metal, (2) the thermal expansion of the porcelain is equal to that of the metal, or (3) the thermal expansion of the metal is greater than that of the porcelain (Wataha, 2002). The porcelain is quite resistant to stresses in the style of tension. In the first relationship, since such stresses will occur, porcelain deformed with the formation of fast cracks and crack propagation (Wataha, 2002). In the second relationship, no stress occurs in porcelain. However, it is impossible to achieve such a relationship practically. In the third relationship, since the metal will shrink more than porcelain; compression stresses occur in porcelain. Porcelain is highly resistant to forces in compression style. Although this is the desired relationship; it does not mean Near East University, Faculty of Dentistry 13 Assoc. Prof. Dr. Özay ÖNÖRAL that cracks and fractures will never occur in porcelain. The difference between the thermal expansion coefficients of metal and porcelain should be approximately 0.5x10-6 degrees (Wataha, 2002; Wataha and Messer, 2004). In cases where the coefficient difference between the materials is high, cracks and fractures are observed again. Thermal expansion coefficients of alloys available in the dental market range from 13.5x10-6 degrees to 17.0x10-6 degrees. The choice of porcelain material with the appropriate thermal expansion coefficient according to the thermal expansion coefficient of the selected dental alloy has a critical role in success (Powers and Wataha, 2013, p. 144-145; Wataha, 2002). Porcelain Layering and Baking on Metal Infrastructure The metal infrastructure is subjected to a heat treatment before porcelain veneering. This application is generally done in vacuum at about 1000 degrees for 8-10 minutes. This is called degasing, by which all gases and contaminants come to the surface under heat and move away from metal. In this process, oxides in the metal alloy also form the oxide layer that will provide chemical bonding on the metal surface and hence is also called oxidation firing. The space available for porcelain in a metal-porcelain crown is often limited. For this reason, the opaque powder to be applied must have superior masking properties. By only this way, a good color reflection from the surface and metal masking can be achieved. Intense color powders of the color chosen from the scale should be prepared in the clinic before proceeding to the porcelain application. In addition, the porcelain application set should be ready. These sets include sable brushes and spatulas of different thicknesses. Porcelain powder is mixed with glass or agate spatulas on a glass or ceramic plate or container. Porcelain powders are used with Near East University, Faculty of Dentistry 14 Assoc. Prof. Dr. Özay ÖNÖRAL distilled water or with the modeling liquids in the set recommended by the manufacturers. Usually a special liquid is used for opaque powder. When mixed opaque powder with liquid, it should be of a creamy consistency. To understand the consistency of the opaque, the spatula is raised a little and if the mixture gives a dense elastic appearance, the consistency is good. Opaque application is generally recommended in two stages. In the first, the opaque mixture is applied to the metal surface with a brush, as if it is washed or painted (Wash Technique), and the excess moisture is absorbed with a desiccant paper. When opaque is baked, the connection between metal and ceramic is strong, air bubbles are eliminated, and a layer is provided to mask the metal surface well. Opaque firing is done at the temperatures and program recommended by the manufacturer. After firing, opaque-applied coping should not be left exposed for a long time. Because the surface is very matte and rough, and it can easily become contaminated. If it had to be exposed, it should be cleaned with steam. Then a second layer is applied on opaque-applied coping, excess moisture is removed with a desiccant paper and the second opaque firing is done. Opaque firing is usually done around 960-980 degrees. After 550 degrees, the vacuum is activated. After the second opaque firing, the metal should never be seen under the opaque. Also, the baked opaque surface should not have a rough irregular appearance. Cervical and dentin porcelain are placed on the glass table and mixed with distilled water or modeling liquid. The mixture should have a brush-like consistency and not too runny. Before applying the veneer porcelain, its opaque surface is moistened with a brush dipped in distilled water and the cervical parts are thinly covered with cervical porcelain by using brush. Then the applied porcelain is condensed by spatulization and the excess moisture is absorbed on a dry paper, this process is continued until the desired thickness is obtained in the cervical region. Then, the other part of the coping is completed in the same way. When staining the dentine porcelain, the excess liquid is removed with a desiccant paper that is contacted to the palatal part of the porcelain structure. However, the point that should be emphasized in the meantime is the necessity of keeping the porcelain always moist; porcelain is never left completely dry. While the porcelain is stacked and shaped with the help of a brush, the dimensions of the neighboring teeth are taken as basis. Then, detailed surface anatomy is studied. As a result, to fully compensate for the firing shrinkage, the fully processed crown or bridge needs to be processed approximately 20% larger than the normal form. Enamel porcelain can be applied by spatula removing the required parts after the original form has been formed by stacking enamel porcelain to all layers, or enamel porcelain can be stacked towards the incisal edge and proximal surfaces only. Then the porcelain surfaces are smoothed with a soft dry brush and the first dentine firing is performed by placing it on the crown Near East University, Faculty of Dentistry 15 Assoc. Prof. Dr. Özay ÖNÖRAL or bridge carrier plate. The firing program is applied as recommended by the manufacturer and when the firing cycle is over, the porcelain crown or bridge is removed from the furnace and left to cool in the open-air. This is called the first biscuit baking (about 920 degrees). Once cooled, it can be corrected or shaped by grinding. During this correction phase, occlusal relations and approximal contacts should be checked. Later, missing sections or shrinkage gaps for the second dentine firing are reshaped by stacking the porcelain. After giving the final corrections and color properties, the same firing cycle is repeated with the temperature about 10-20°C lower than that of first dentine firing. This is referred to as the second biscuit baking. The firing temperatures of each porcelain powder are carried out at different temperatures according to the composition prepared by the manufacturer. The porcelain surface is once again abraded before it is finished. All surfaces are abraded with stones and discs in order to apply equal amount of glaze in every area. Fissures on the chewing surface are never prepared too deep; otherwise the crown's resistance will be reduced. These parts can be painted with yellowish, brownish paints to determine the lines of the fissures, to individualize the restoration, and to give a more natural-looking appearance. In addition, the desired external colors can be obtained by painting in the desired regions. However, it should be remembered that the actual natural color and natural effects can be given better with the color of the internal dentin. At the last stage, glaze powder is mixed with its own special liquid and applied to all surfaces with a brush as thin layer as possible. The wet glazing presenting a bright appearance indicates the appearance of the crown to be after firing and allows to be compared with the color scale. If more color adjustments are required, the appropriate paint is prepared in dark consistency with glaze liquid, applied simultaneously with the glaze, and baked. Glaze firing is applied without vacuum, and the fabrication of metal-ceramic crowns is completed in this way. Meanwhile, the number of firings should be at the optimum limit. Oven over firing negatively affects porcelain quality. Near East University, Faculty of Dentistry 16 Assoc. Prof. Dr. Özay ÖNÖRAL Precautions during Laboratory Stages 1. During mastication, compressive, flexural, shear, and tensile stresses are exerted on the alloy framework. The ceramic veneering material tolerates torsional movements of the metal framework only to a certain extent. In connector areas between bridge pontics and abutment teeth, the framework material must demonstrate adequate thickness. The framework construction must be able to withstand the mentioned forces. 2. Heat treatment (oxidation and ceramic firing) may result in deformation of too thin and too delicate framework constructions. This may lead to considerable problems affecting the marginal integrity during firing and during the cooling phase due to residual thermal stress caused by the difference between the coefficient of thermal expansion of the alloy and that of the ceramic material. 3. Base metal alloys with a higher modulus of elasticity and a higher proof stress may be used for more delicate framework designs. Since base metal alloys oxidize more easily and intensively and show a darker oxide on their surface, they have to be appropriately masked with a thicker opaquer layer. 4. If the alloy features a high modulus of elasticity and proof stress, a high amount of force is required to deform the alloy framework. Near East University, Faculty of Dentistry 17 Assoc. Prof. Dr. Özay ÖNÖRAL 5. The larger the span width between the abutment teeth, the higher the risk of deformation for the bridge reconstruction. 6. The dimensions of the framework cross-section must be large enough, especially in the direction of loading. For the posterior area, an adequate height of the connectors is important, while a horizontal reinforcement in the lingual direction should be provided in the anterior area. Near East University, Faculty of Dentistry 18 Assoc. Prof. Dr. Özay ÖNÖRAL 7. In bridge restorations with a shorter span width, the connector cross-section must comply with the minimum requirements. If the same deformation load is applied to two beams, one of which is half the length of the other, the curvature of the shorter one will be four times more pronounced than that of the longer one. The maximum tensile strength not only depends on the absolute deflection value, but also on the corresponding curvature. An increased curvature of the beam and/or a thicker ceramic layer at the underside of the restoration increases the tensile stress. 8. The cross-section of the interdental connector area decisively influences both the strength of the restoration during the dental-lab procedures and the clinical long-term success after cementation. 9. The cross-section of the interdental connector area decisively influences both the strength of the restoration during the dental-lab procedures and the clinical long-term success after cementation. Near East University, Faculty of Dentistry 19 Assoc. Prof. Dr. Özay ÖNÖRAL In addition to the 0.2 % proof stress, a number of other factors also influence the connector crosssections: – Modulus of elasticity - the higher the modulus of elasticity, the lower is the elastic deformation – Span of the metal-ceramic restoration – Layer thickness of the veneering ceramic – the thicker the layer of the veneering ceramic, the higher is the tensile stress (especially in the basal area) 10. Framework connectors between the anterior and the posterior region, therefore, also have to be given an adequately stable design in a horizontal lingual direction, as well as the suitable height. 11. The homogeneous bond between the alloy and the ceramic layers not only increases the strength of ceramic-veneered frameworks, but also substantially reduces the fracture susceptibility of ceramic materials. In fact, it is this sound bond between alloys and veneering ceramics that permits minimum alloy framework thicknesses of 0.3–0.5 mm. Near East University, Faculty of Dentistry 20 Assoc. Prof. Dr. Özay ÖNÖRAL 12. The most important factors determining the strength of the metal-ceramic restoration include the bonding strength at the interfaces, the strength of the framework, and the different coefficients of thermal expansion of the alloy and the ceramic. 13. Mechanical failures can be seen in different zones. Near East University, Faculty of Dentistry 21 Assoc. Prof. Dr. Özay ÖNÖRAL 14. As a rule, the CTE of the veneering ceramic must be lower than the CTE of the alloy. In this way, the veneering ceramic is put under compressive stress. 15. During oxidation and firing of the veneering ceramic, the alloy must demonstrate adequate heat resistance to prevent the alloy framework from sagging (sag resistance). This is particularly important for long-span bridges. Therefore, alloys are usually required to have a solidus temperature that is approximately 100 °C higher than the firing temperature of the veneering ceramic. 16. An adequately stable and aesthetic metal-ceramic restoration is also achieved by a preparation with ample available space. If the minimum space requirements cannot be met because the abutment tooth cannot be adequately reduced (prepared), a metalceramic restoration is contraindicated. Near East University, Faculty of Dentistry 22 Assoc. Prof. Dr. Özay ÖNÖRAL 17. The larger the marginal angle, the more ideal are the conditions for creating a ceramic shoulder or for neatly veneering the metal margin with ceramic. With angles smaller than 50°, the metal margin may only be covered with an over-contoured ceramic layer. A visible alloy margin is then almost unavoidable. 18. The crown design in the marginal area and the accuracy of fit on the natural tooth stump directly influence the periodontal health. For ceramic veneers, the materials at the crown margin must demonstrate a minimum thickness in order to achieve ample stability and appropriate shade reproduction. Near East University, Faculty of Dentistry 23 Assoc. Prof. Dr. Özay ÖNÖRAL 19. An inadequate crown margin design may lead to (secondary) caries and injury to the marginal periodontium. 20. With fused-on ceramic shoulders, it must be made sure that the framework, as well as the veneer is supported by the prepared tooth. Reduce the framework exactly up to the inner edge of the chamfer or shoulder preparation. In this way, functional support of the framework by the preparation is ensured. 21. With fused-on ceramic shoulders, it must be made sure that the framework, as well as the veneer is supported by the prepared tooth. Reduce the framework exactly up to the inner edge of the chamfer or shoulder preparation. In this way, functional support of the framework by the preparation is ensured. Near East University, Faculty of Dentistry 24 Assoc. Prof. Dr. Özay ÖNÖRAL 22. With fused-on ceramic shoulders, it must be made sure that the framework, as well as the veneer is supported by the prepared tooth. Reduce the framework exactly up to the inner edge of the chamfer or shoulder preparation. In this way, functional support of the framework by the preparation is ensured. 23. Varying layer thicknesses of the veneering material result in undesired shade differences and uncontrollable shrinkage of the ceramic material coupled with tensile stress. Near East University, Faculty of Dentistry 25 Assoc. Prof. Dr. Özay ÖNÖRAL 24. The framework reflects the reduced tooth shape. It must be contoured in such a way that it provides incisal and occlusal support. An even, proportional layer thickness should be achieved in the incisal area, as well as in the cusp and fissure areas. In this way, the forces that are applied during static and functional masticatory loading are mainly transmitted to the framework and not to the veneering ceramic alone. 25. In bridge frameworks, the stability in the connector areas between the bridge abutments and bridge pontic in particular must be ensured by the framework design and adequate framework thickness. 26. The proximal areas must enable adequate oral hygiene and individualized preventive treatment regimes. During framework design, an adequate opening of the interdental area must be taken into consideration so that oral hygiene with interdental brushes and dental floss can be performed. No black triangles should be created. Near East University, Faculty of Dentistry 26 Assoc. Prof. Dr. Özay ÖNÖRAL 27. In order to achieve light transmission, the framework construction is reduced in a targeted fashion. Important! – The framework reduction must not result in unacceptable weakening of the framework– The available space in the proximal and occlusal areas, as well as the alloy type used, must be taken into consideration in this context. Near East University, Faculty of Dentistry