Dental Ceramics
Question 1. Write a short note on dental porcelain.
Or
Write briefl on dental ceramics.
Or
Write a note on dental ceramics.
Answer:
Dental ceramic is the generalized term for a product which is made from non-metallic inorganic material which is processed by firing at high temperature, while porcelain is the restrictive term used for mixture of kaolin, quart,z and feldspar which is when fied at high temperatures produces glassy, translucent fiish.
Dental porcelain is used for metal ceramic restorations and dental ceramics is used for all ceramic restorations Dental ceramic is defined as compounds of one or more metals with non-metallic element usually oxygen. They are formed of chemical and biochemical stable substances that are strong, hard, britte, and inert non-conductors of thermal and electrical energy.
Read And Learn More: Dental Materials Question And Answers
Classification of Dental Ceramic/Porcelain:
- According to Firing Temperature:
- High-fusing: 1300°C for denture teeth
- Medium-fusing: 1001° to 1300°C for denture teeth
- Low-fusing: 850 to 1100°C for denture teeth.
- Ultra-low-fusing: Less than 850°C used with titanium.
- According to Type:
- Feldspathic porcelains
- Leucite-reinforced glass ceramics
- Tetrasilicic flormica based glass ceramics
- Lithia disilicate based ceramics
- Alumina-reinforced ceramics
- Spinell-reinforced ceramics
- Zirconia-reinforced ceramics.
- According to its Function Within Restoration:
- Core ceramics: Supports and reinforces restoration
- Opaque ceramics: Mask or hide the metal.
- Veneering ceramics:
- Body or dentin: Simulates the dentine portion of natural teeth
- Incisal: Simulates enamel portion of natural teeth
- Gingival: Simulates the darker gingival portion of teeth
- Translucent: Simulates translucent incisal enamel seen sometimes in natural teeth
- Stains: Colored ceramic improves esthetics
- Glaze: Imparts a smooth glossy surface to the restoration.
- According to Microstructure:
- Glass-ceramics
- Crystalline ceramics
- Crystal-containing glasses.
- According to Fabrication Process:
- Condensable ceramics
- Slip-cast glass-infiltrated ceramics
- Heat pressed ceramics
- Machinable ceramics
- Various combinations of the above.
Clinical Applications of Dental Ceramic/Porcelain:
- Inlays and onlays.
- Aesthetic lamination (veneers) over natural teeth.
- Single (all ceramic) crown.
- Short span (all ceramic) bridge.
- As veneer for cast metal crown and bridge (metal ceramic).
- Artificial denture teeth (for complete denture and partial denture use).
- Ceramic orthodontic brackets.
- As intraradicular post in post-endodontic restorations.
- As biomaterial in the form of dental implant
Composition of Dental Ceramic/Porcelain:
Advantages of Dental Ceramics /Porcelain:
- Dental ceramics remain stable for the longer time period.
- They have excellent biocompatibility.
- They are resistant to corrosion.
- They do not interact with liquid, acid, alkalis, and gases in an oral environment.
- After glazing, it provides a smooth surface and increases the fracture resistance.
- Their compressive strength is excellent.
- They surpass all dental materials in mimicking natural tooth structure in both color and translucency.
Disadvantages of Dental Ceramics/Porcelain:
- They have low tensile strength, which makes them brittle.
- As ceramics are hard, they tend to abrade opposite enamel at the time of occlusal contact.
- During the cooling of fired ceramic sometimes microcracks occur on surface of ceramic which enhances the surface roughness. This decreases the strength.
Question 2. Write a short note on the condensation of porcelain.
Or
Write a note on methods of porcelain condensation.
Answer:
The process of packing the powder particles together and removing the excess water is known as “condensation”.
Objectives of porcelain condensation:
- To adapt the material to the requisite shape.
- To remove as much water as possible. The more water is , removed, the less is the contraction.
- To produce desired strong porcelain.
Condensation Techniques or Methods of Porcelain Condensation:
Condensation can be achieved by several methods:
- Vibration:
- It consists of applying the wet porcelain to the platinum matrix and then vibrating the die in which the matrix rests.
- As the particles condense, the water rises to the surface.
- The excess water is then blottd with a clean tissue paper or an absorbent medium.
- Speculation:
- The wet porcelain is applied with a spatula and then the surface is smoothened with the instrument.
- This will distribute the particle and cause them to become more closely packed.
- The water rises to the surface and it is removed with a lined cloth or blotting paper.
- Brush technique or capillary action:
- It consists of adding paste to the matrix and dry powder is sprinkled onto the wet surfaces.
- The dry powder removes the excess water by capillary action from the mixture already applied.
- The particles move close together as the water is withdrawn.
- Whipping:
- After the paste has been applied to the matrix, it may be whipped with a brush.
- The water is thus brought to the surface and it is removed.
Factors Affecting Condensation:
- The success of condensation depends on the skills of the operator and the range of size of the particles.
- If the particles are of the same size, 45% of a given volume will consist of voids.
- If a number of particles are blended with larger ones, void or space is considerably reduced.
- If three or more particle sizes are used that will achieve an even greater degree of compaction.
- Powder consisting of mixtures of particle sizes compact more easily than those with particles of one size.
- A well-compacted crown reduces firing shrinkage and shows regular contraction over the entire surface.
- Thus maintaining the original form on a slightly reduced space.
Question 3. Write a short note on the porcelain metal bonding mechanism.
Or
Write a short note on the bonding of porcelain to metals.
Or
Write a short note on metal-ceramic bond.
Or
Write a short note on porcelain-metal bonding.
Answer:
The primary requirement for the success of a metal-ceramic prosthesis is the development of a durable bond between the porcelain and the alloy.
Porcelain metal binding falls in three groups:
- Chemical bonding across the porcelain metal interphase.
- Mechanical interlocking between porcelain and metal.
- Compressive (Thermal) bonding.
- Chemical Bonding:
- Currently regarded as the primary bonding mechanism.
- The adherent oxide layer is essential for good bonding.
- In base metal alloys, chromic oxide is responsible for a good bond.
- Noble metal alloys do not have an oxide layer so they primarily depend on mechanical interlocking for bonding.
- Here addition of a small amount of tin to noble metal alloys leads to the formation of oxides on their surface. This is done by electrodeposition.
- In electrodeposition, a layer of pure gold is electrodeposited onto the cast metal surface.
- This is quickly followed by a quick flashing deposition of tin over the gold. This tin helps in chemical bonding through the formation of tin oxide.
- Mechanical Bonding:
- In some systems, mechanical interlocking provides the principal bond.
- The fused ceramic flows over the metal covering adapts to minute irregularities present on the metal surface and forms micromechanical bonds.
- Sandblasting is often used to prepare the metal surface.
- Irregularities on the coping surface can be produced by sandblasting.
- The presence of the surface roughness on the metal oxide layer gives retention especially if undercuts are present.
- The ability of the fused porcelain to intimately adapt to the metal surface is called wetting which is important for bonding.
- Compressive (Thermal) bonding:
- A critical requirement for the adhesion is thermal expansion compatibility between the ceramic and metal.
- Ceramo-metallic systems are designed with a very small degree of mismatch in order to leave the porcelain in a state of compression.
- α Porcelain — 13 to 14 × 10-6/°C
- α Metal —13.5 to 14.5 × 10-6/°C
- The difference of 0.5 × 10-6/°C causes the metal to contract slightly more than the porcelain on cooling from firing temperature.
- This mismatch leaves porcelain in residual compression and makes it less sensitive to apply tensile forces which increases the bond strength.
Question 4. Write a short note on the classification of dental ceramics.
Or
Write brief on the composition of dental ceramics.
Or
Write a short note on the composition of dental ceramics.
Answer:
Dental ceramics are far stronger, wear-resistant, and virtually indestructible in the oral environment.
Classification of Dental Ceramic:
- According to Firing Temperature:
- High-fusing: 1300° for denture teeth
- Medium-fusing: 1001° to 1300°C for denture teeth
- Low-fusing: 850° to 1100°C for denture teeth.
- Ultra-low-fusing: Less than 850°C used with titanium.
- According to Type:
- Feldspathic porcelains
- Leucite-reinforced glass ceramics
- Tetrasilicic flormica based glass ceramics
- Lithia disilicate-based ceramics
- Alumina-reinforced ceramics
- Spinel-reinforced ceramics
- Zirconia-reinforced ceramics.
- According to its Function within the Restoration:
- Core ceramics: Supports and reinforces restoration
- Opaque ceramics: Mask or hide the metal.
- Veneering ceramics:
- Body or dentine: Simulates the dentine portion of natural teeth
- Incisal: Simulates the enamel portion of natural teeth
- Gingival: Simulates darker gingival portion of teeth
- Translucent: Simulates translucent incisal enamel seen sometimes in natural teeth
- Stains: Colored ceramic improves esthetics
- Glaze: Imparts smooth glossy surface to the restoration.
- According to Microstructure:
- Glass-ceramics
- Crystalline ceramics
- Crystal-containing glasses.
- According to the Fabrication Process:
- Condensable ceramics
- Slip-cast glass-infiltrated ceramics
- Heat-pressed ceramics
- Castable ceramics
- Machinable ceramics
- Various combinations of the above.
Question 5. Write a short note on aluminous porcelain.
Answer:
- Porcelain jacket crowns (PJC) were very brittle and fractured easily. The marginal adaptation was also quite poor.
- The “Mc Lean and Hughes” developed the PJC with an alumina-reinforced core in 1965.
- This crown was developed to improve the strength of earlier PJC.
- Aluminous porcelain is a ceramic compound of the glass matrix phase and it contains at least by volume 35% Al2O3.
- It is a type of core porcelain. Alumina strengthens the porcelain.
- Aluminous porcelain is available as body, dentin, or gingival porcelain.
- Aluminous porcelain is veneering ceramic for ceramic or metal-ceramic prostheses.
Composition of Aluminous Core:
Silica − 35%
Alumina − 53.8%
Soda − 2.8%
Potash − 4.2%
Zinc oxide − Rest
Boric oxide − 3.2
Calcium oxide − 1.12%
Zirconium oxide Rest
Types of Aluminous Core:
- Conventional or traditional PJC
- PJC with aluminous core.
Core Formation of Aluminous Core:
- On the prepared tooth the platinum foil is adapted on to which the core porcelain is condensed.
- The platinum foil along with core porcelain placed in the furnace and fired.
- After cooling the rest of the body is built up using dentine, enamel, and other porcelain.
- After completion of the restoration, the foil is gently teased out and discarded.
Advantages of Aluminous Core:
- A thick layer of ceramic can be applied which improves the esthetics.
- Require less removal of tooth structure as to PFM.
Disadvantage of Aluminous Core:
- Do not have sufficient strength for the posterior crown.
- Functions of Alumina
- As powdered alumina is added to porcelain, it provides sufficient strengthening.
- Alumina is slightly soluble in low-fusing porcelain allowing for continuity of atomic bonding through the ceramic.
- So interface between alumina particles and porcelain is stress-free and does not encourage crack propagation around alumina particles.
Question 6. Write a short note on all-ceramic restoration.
Answer:
All ceramic restorations are also known as metal-free ceramics. Metal-free ceramic restorations are made without a metallic core or sub-structure. This makes them esthetically superior to metal-ceramic restoration. Metal-free ceramic restorations had lower strength, thus, metal-ceramics continued to be the restoration of choice.
Continued research has led to improved systems with greater strength and fracture resistance. Manufacturers today claim new generation all-ceramic materials are capable of producing not only single crowns but anterior and even posterior ceramic crowns and bridges as well. Long-span fixed partial dentures have also been attempted.
Classification of Metal Free Ceramic:
All ceramic restorations are grouped according to their type and method of fabrication.
- Condensed Sintered
- Traditional feldspathic porcelain jacket crown
- Porcelain jacket crown with aluminous core (hi–ceram)
- Ceramic jacket crown with leucite reinforced core (optic HSP)
- Cast glass ceramics (dicor).
- Injection molded (leucite reinforced) glass ceramic (IPS Empress).
- Slip-cast glass infiltrated ceramics
- Glass infiltrated aluminous core restorations (in-ceram)
- Glass infiltrated spinal core restorations (in-ceram Spinell)
- Glass infiltrated zirconia core (in-ceramic zirconia).
- Milled ceramic restoration or cores:
-
- CAD/CAM restorations
- Copy-milled restorations.
Porcelain Jacket Crown:
These are made up completely of feldspathic porcelain. Their construction is done on a platinum foil matrix which is subsequently removed.
They are of three types, i.e.
- Porcelain jacket crown (traditional).
- Porcelain jacket crown with aluminous core.
- Porcelain jacket crown with Leucite reinforced core (OptekHSP).
- Porcelain Jacket Crown Traditional:
- The all-porcelain jacket crown has been around for a century (early 1900s).
- These early crowns are also referred to as traditional or conventional PJCs.
- They were made from conventional high-fusing feldspathic porcelains.
- These were very brittle and fractured easily. The marginal adaptation was also quite poor.
- Because of these problems they gradually lost popularity and are no longer used presently.
- Porcelain Jacket Crown with Aluminous Core:
- Problems associated with traditional porcelain jacket crowns led to the development of porcelain jacket crowns with an alumina-reinforced core.
- The increased content of alumina crystals (40 to 50%) in the core strengthened the porcelain by interrupting of crack propagation.
- In spite of the increased strength they were still brittle and therefore not indicated for posterior teeth and their use was restricted to anterior teeth.
- Leucite Reinforced Porcelain (Optec HSP):
- Optec HSP is a feldspathic porcelain with a higher leucite crystal content (leucite reinforced).
- Its manipulation, condensation, and firing is quite similar to the alumina-reinforced porcelain jacket crowns (using a platinum foil matrix).
Uses of Metal-Free Ceramic:
Inlays, onlays, veneers, and low-stress crowns.
Advantages of Metal Free Ceramic:
- They are more esthetic because the core is less opaque (more translucent) when compared to aluminous porcelain.
- Higher strength.
- No need for special laboratory equipment.
Disadvantages of Metal-Free Ceramic:
- Fit is not as good as metal-ceramic crowns.
- Potential marginal inaccuracy.
- Not strong enough for posterior use.
Castable Glass Ceramic:
- It is the first commercially available castable glass ceramic for dental use made by Corning Glass Works named it as Dicor.
- It consists of 55 vol% of tetra silicic Formica crystals inside a glass matrix.
- The ceramic prosthesis is fabricated the same as the lost wax casting technique used for metals.
- Casted glass core should be gently sandblasted and coated by a protective material and is subjected to heat treatment.
- This causes the growth of microscopic plate-like crystals of mica inside the glass matrix. This is known as cramming.
- Now cerammed glass coping is coated with veneering porcelain to fabricate prostheses.
- The advantages of cramming are increased toughness and strength, increased abrasion as well as thermal resistance, and chemical durability.
- Disadvantages of castable glass ceramic are, it has inadequate strength for posterior use and internal characterization is not possible
- It is available as CAD/CAM material in the form of blanks and ingots.
Heat Pressed Ceramics:
- This ceramic material has a unique way of fabrication, i.e. by injection molding.
- It is a pre-crammed glass having a high concentration of reinforcing crystals.
- The material supplied in the form of an ingot is softened under high temperatures and forced into a mold created by a lost wax process.
- This pressed ceramic is then contoured and subsequently stained and glazed for the final finish of the restoration.
- As the ceramic is cast from a single ingot, restoration is monochromatic, though staining can be done to alter the color.
- Heat-pressed ceramics are used for inlays, onlays, single crowns, and veneers.
- Advantages are better fit; and better esthetics because of the absence of metal or an opaque core.
- Disadvantages are the need of costly equipment and the potential of fracture in posterior areas.
Microstructure of Heat-Pressed Ceramics:
- IPS Empress: It consists of 35 to 40% volume of leucite crystals
- IPS Empress 2: It consists of 65 to 70% by volume of interlocked elongated lithia disilicate crystals. Crystal size varies from 0.5 to 4 µm in length.
Glass Infitered of Heat Pressed Ceramics:
They are specialized core ceramics that are reinforced by a unique glass infiltration process. They are also known as split-cast ceramics.
Types of Glass Infitered:
They are of three types based on the core material used:
- Glass infitrated alumina core (In-ceram alumina)
- Glass infitrated spinell core (In-ceram spinell)
- Glass infiltrated zirconia core ( In-ceramic zirconia)
1. Glass Infitrated Alumina Core (In-Ceram Alumina)
This has a unique glass infiltration process and first of its kind for anterior fixed partial denture fabrication.
Composition:
- Alumina powder:
- Al2O3 – 99.7%
- MGO – 0.03%
- Infiltration glass powder:
- La2O3 – 49.6
- SiO2 – 19.1
- TiO2 – 6.16
- CaO – 3.14
- Others–2.0
Properties Of Alumina Core :
- Flexural strength: 256 to 500 MPa
- Fracture toughness: 4.4 to 4.8 MPa.m1/2
Indications Of Alumina Core:
- Single anterior and posterior crowns
- Short span anterior 3 unit fixed partial denture.
Advantages Of Alumina Core :
- Good fi and marginal adaptation
- Good strength as compared to earlier all ceramic crowns
- Minimal fie shrinkage
- Use to cover dark teeth or post and core
- Wearing of opposite teeth is less.
Disadvantages Of Alumina Core :
- Less aesthetic because of the opacity of the alumina core.
- Quite tedious to fabricate.
- Insufficient long-term results.
2. Glass Infitrated Spinell Core (In-Ceram Spinell):
- It is an offhoot of in-ceram alumina.
- These ceramics use magnesium aluminate crystals instead of alumina crystals for strength.
- It is more translucent and is more aesthetic as compared to the in-ceram alumina core.
- As strength is lower, its use is limited to low-stress situations.
- Indication: Because of its high translucency it is the material of choice for crown and restoration in esthetic and stress-free zones.
- Contraindication: In underlying tooth which is severely discolored; in posterior teeth and as fixed partial dentures.
Glass Infitrated Zirconia (In-Ceram Zirconia):
- It is the mixture of zirconium oxide/aluminum oxide in the framework material.
- It is the strongest of all three glass-infiltrated core materials.
- The final core of ceramic zirconia consists of 30% wt% zirconia and 70 wt% aluminas.
- It can be used in posterior fixed partial dentures, and cores for anterior crowns to mask discoloration.
CAD/CAM Ceramics:
Constructing a dental ceramic restoration is technique sensitive, labor-intensive, and time-consuming. CAD/CAM ceramics were introduced to overcome some of these problems. They are also known as milled or machined ceramics.
Machinable ceramic systems can be divided into two categories, which are as follows:
- CAD/CAM systems
- Copy milled systems
1. CAD/CAM Systems:
These are systems that can design and produce restorations out of blocks or blanks of ceramics with the aid of a computer. CAD/CAM is an acronym for computer-aided design/computer-aided manufacturing.
Materials for CAD/CAM:
The fabrication process is system and material-specific. The prepared tooth or teeth is scanned directly from the mouth or from a model made from a regular impression. Next, the restoration or sub-structure is designed on the computer. The blank is attached to the milling station and the bar code.
The time taken for milling depends on the size and complexity of the restoration as well as the material used. For example, pre-sintered zirconia is easier to mill than sintered zirconia. It also reduces the wear of the milling tools. After milling, the structure is separated from the blank using water-cooled cutting and grinding discs or burs.
Subsequent processing procedures are then initiated depending on the material and system used.
- Feldspathic blanks:
- Feldspathic restorations can be milled to full contour. The restoration is glazed after milling.
- Optional processing includes veneering and staining. It is indicated for inlays, laminates, and anterior crowns.
- Leucite reinforced:
- These blanks can be milled to full contour. The restoration is glazed after milling.
- Optional processing includes veneering and staining.
- It is indicated for inlays, onlays, laminates, and anterior crowns.
- Lithium disilicate:
- The ceramic is machined in an intermediate crystalline state in which the material shows its characteristic bluish shade.
- In this stage, the material is easier to shape and can be tried in the mouth.
- This is followed by a simple, quick crystallization process (30 minutes) in the conventional ceramic oven in which it reaches its final strength and the desired esthetic properties such as tooth color, translucence, and brightness.
- Optional processing includes veneering and staining. Uses—inlays, onlays, and anterior and posterior crowns.
- Glass infiltrated ceramics:
- These are usually machined as cores or FDP substructures.
- Subsequent processing includes glass infiltration, veneering, and glazing.
- It is indicated that in-ceram Spinell is recommended for anterior single crown copings.
- In-ceram alumina is indicated for anterior and posterior crowns and 3-unit anterior FDP substructures.
- In-ceram zirconia can be used for anterior and posterior crowns and 3-unit FDP substructures.
- Presented zirconia:
- Fully dense zirconia is extremely difficult to machine, taking up to 2 hours just to fabricate a single unit.
- Therefore, most restorations with zirconia frameworks are fabricated by machining a porous or partially fied block of zirconia known as pre-sintered zirconia.
- These are usually used as cores for crowns or FPDs.
- In pre-sintered condition, they are usually softer and easier to mill.
- Sintered zirconia:
- Since these materials are already fully sintered, post-sintering is not required of this material.
- It is milled in 1:1 ratio as no shrinkage is expected.
- Because of its extreme hardness milling takes more time and causes more wear on the milling tool.
- Subsequent processing includes build-up with compatible veneering ceramics.
- It is indicated for core construction of crowns and long-span anterior and posterior fixed partial dentures.
2. Copy Milled Systems:
They use a copy-milling technique to produce ceramic cores or substructures for fixed partial dentures. In copy milling a wax pattern of restoration is scanned and a replica is milled out of the ceramic blank. Examples are:
- Clay
- Cercon
- Ceramill system.
Question 7. Write their composition and failure in porcelain restoration.
Or
Classify and describe composition along with role of each constituent.
Answer:
Composition Of Porcelain Restorations:
Failures in Porcelain Restorations:
- It is susceptible to brittle fracture particularly when flow and tensile stress exist in the same region of ceramic restoration.
- If an adequate shoulder margin is not present mainly in anterior teeth, achieving the aesthetic margin without a dark shadow of metal is difficult.
- Metal base can affect the aesthetic of porcelain which makes its appearance unnatural because of decreased light transmission.
- Porcelain shows high degree of shrinkage on firing.
- In porcelain, there are problems in matching the exact color and texture of natural tooth.
Question 8. Write a short note on castable ceramic.
Answer:
Composition Of Castable Ceramic:
The first commercially available castable ceramic material for dental usage is decor which was named on the names of its manufacturers, i.e. Dentsply International and Corning Glass Works, it is supplied as silicon glass plate ingot containing MgF2
Dicor contains 55 vol% of tetra silicic Formica crystals in the glass matrix.
Major Ingredients Of Castable Ceramic:
- Sio2 − 45 to 70% forms the glass matrix
- KO2 − 20%
- MgO − 13 to 30% decreases the viscosity
- Mgf2 − 4 to 9% act as nucleating agent and flux
Minor Ingredients Of Castable Ceramic:
- Al2 O3 − 2% enhances durability and hardness
- ZrO2 − 7% fluorescent agent for esthetics
- BaO− 1 to 4% provides radiopacity.
Dicor Of Castable Ceramic:
Castable ceramics are also known as glass ceramics.
- Its properties are closer to that of glass and its construction is quite diffrent.
- This the only porcelain crown made by a centrifugal casting technique.
- The ‘ceramming’ process is also quite unique to this porcelain.
Types of Castable Ceramics:
There are four types of castable ceramics, i.e.
- Dicor
- Apatite glass ceramic—cera pearl
- Lithia based
- Calcium phosphate.
Composition Of Castable Ceramic:
Dicor contains 55 vol% of tetra silicic Formica crystals in a glass matrix
Fabrication of a Dicor Crown:
- The wax crown is first constructed in wax and then invested in investment material like a regular cast metal crown.
- After burning out the wax, nuggets of Dicor glass is melted and cast into the mould in a centrifugal machine.
- The glass casting is carefully recovered from the investment by sandblasting and the spores are gently cut away.
- The glass restoration is then covered with a protective ‘embedment material’ to prepare it for the next stage called cramming.
- Ceramming is a heat treatment process by which the glass is strengthened. It also reduces the transparency of the glass making it more opaque and less glass-like.
- The cramped glass is now built up with enamel and dentin, condensed, and fired to complete the restoration.
Advantages Of Castable Ceramic:
Ease of fabrication
- Esthetics is good
- Marginal fi is good
- Improved strength and fracture toughness
- Processing shrinkage is very low
- Opposing teeth undergo low abrasion.
- Features: The Dicor glass ceramic crown is very aesthetic.
- Uses: Inlays, onlays, veneers, and low-stress crowns.
Disadvantages Of Castable Ceramic:
- Strength is inadequate for posterior use.
- Internal characterization is impossible, so it should be stained externally to improve esthetics.
Lithia based
- They are developed by Uryu.
- Crystals of LiO. Al2O3.4SiO2 is formed after heat treatment.
- It is available as Olympus castable ceramic.
Calcium Phosphate
- It is reported by Kihara.
- It is a combination of calcium phosphate and phosphorus pentoxide plus trace elements.
- It is cast at 1050°C in a gypsum investment mold.
- Clear cast crown is converted to crystalline ceramic by heat treating at 645°C for 12 hours.
Question 9. Write in detail methods of strengthening ceramic.
Or
Write methods of strengthening dental ceramics
Answer:
Methods used for strengthening of dental ceramic are based on the following mechanism:
Development of residual compressive stress:
- Ion exchange or chemical tempering
- Thermal tempering
- Thermal compatibility.
Interruption of crack propagation.
- Dispersion of crystalline phase
- Transformation toughening.
Development of Residual Compressive Stresses within the Surface of Metal:
In this method, residual compressive stresses are introduced within the surface of glass and ceramic objects. Strengthening is gained by virtue of the fact that developing of tensile stresses before any net tensile stress develops; must first negate these residual stresses.
Chemical Tempering or Ion Exchange:
- It is one of the most effective methods of introducing residual compressive stresses into the ceramic.
- In this process, the larger K+ ions exchange the smaller Na+ ions.
- When dental porcelain possessing sufficient soda (Na2O) is immersed in a KNO3 salt bath at 400°C for 4 hours, K+ ions will replace or exchange some of the Na+ ions located close to the surface layers.
- The K+ ions are 35% larger than the Na+ ions, creating larger residual compressive stresses in the surface of the glass subjected to this treatment.
- This surface compression results in increased strength of porcelain.
Thermal Tempering:
- On rapid cooling, the surface of the object from the molten state can introduce residual compressive stresses.
- The rapid cooling produces skin of glass surrounding a soft (molten) core.
- During solidification, the molten core tends to shrink, but the outer skin remains rigid.
- This shrinkage in the molten core creates residual tensile stress in the core and residual
compressive stresses within the outer surface.
Thermal Compatibility: This method applies to porcelain fused metals:
- The metal and porcelain should be created with a slight mismatch in their thermal contraction coefficient.
- Typically porcelain coefficient of thermal expansion coefficient between 13.0 – 14.0 × 10 -6 /°C and metals have between 13.5 – 14.5 × 10 -6/°C.
- The difference of 0.5 × 10-6/°C in the thermal expansion between metals and porcelain causes the metal to contract slightly more than does the ceramic during cooling after firing the porcelain.
- This condition puts the ceramic under slight residual compression which makes it less sensitive to tensile stresses. ‘These are known as thermally compatible system.
Interruption of Crack Propagation through the Material:
A dispersed phase which is capable of obstructing the crack propagation through the material is reinforced into the glasses or ceramics to strengthen them. Two different types of dispersions are used to interrupt crack propagation.
Dispersion of Crystalline Phase:
- Al2O3 is added to glasses as a dispersed phase to strengthen them.
- Al2O3 is a tough crystalline material which can prevent crack propagation through them and strengthen the glass.
- The technique has found application in dentistry in the development of aluminous porcelain for porcelain jacket crowns.
Transformation Toughening:
- A crystalline material such as Partially Stabilized Zirconia is incorporated into glasses or ceramics.
- Partially stabilized zirconia is capable of undergoing change in crystal structure when placed under stress and can improve the strength.
- As the refractive index of partially stabilized zirconia is much higher than that of surrounding glass matrix results in scattering of light.
- As it passes through the bulk of porcelain, this scattering produces an opacifying effect that may not be esthetic in most restorations.
Question 10. Write a short note on metal-ceramic restorations.
Answer:
It is also known as porcelain fused to metal or metal bonded restoration or ceramometal.
Composition of Porcelain Powder for Metal Ceramics:
Types of Metal-ceramic Systems
As previously mentioned the metal-ceramic systems can be divided into:
- Cast metal-ceramic restorations:
- Cast noble metal alloys
- Cast base metal alloys
- Cast titanium:
- Swaged metal-ceramic restorations:
- Gold alloy foil coping
- Bonded platinum foil coping.
Technical Consideration of Metal Ceramic Restoration:
- Because of the high melting temperature of the alloy, phosphate or silica-bonded investment material must be used.
- A gas-oxygen flame or induction is generally employed for melting the alloy.
- Surface treatment of metal casting before porcelain application is important for good bonding.
- This treatment is used to toughen the casting and form surface oxides.
- The surface may be roughened by sandblasting with a fie alumina abrasive.
- In most cases, the metal casting is heat treated either in air or under a partial vacuum to produce surface oxide to improve bonding.
- The opaque porcelain is condensed with a thickness of approximately 0.1-0.2 mm.
- It is then fired, the translucent enamel is applied and tooth form is built. The unit is again fied.
- A final glaze is then obtained.
Uses of Metal Ceramic Restoration:
- Single anterior and posterior crowns
- Short and long span anterior and posterior FPDs.
Advantages of Metal Ceramic Restoration:
- If proper fabrication of metal-ceramic crowns is done they are stronger and more durable than all ceramic crowns.
- Long-span bridges can be done with an excellent fit.
- In metal-ceramic restoration, tooth preparation should be less as compared to all-ceramic restorations.
- Metal ceramic restorations withstand the forces of occlusion without wear.
- Metal ceramic restorations do not undergo staining along the interface between metal and ceramic veneer.
- Metal ceramic restoration is hard and resists wear extremely well.
- Metal ceramic restoration is compatible with soft tissues.
Disadvantages of Metal Ceramic Restoration:
- If an adequate shoulder margin is not present mainly in anterior teeth, achieving the aesthetic margin without a dark shadow of metal is difficult.
- Metal base can affect the aesthetic of porcelain which makes its appearance unnatural because of decreased light transmission.
- Patients are allergic to metal and alloys which warrant the use of metal-free ceramic.
- Due to aesthetic demands, patients are opting for All ceramic restoration.
- There is a high degree of shrinkage on firing.
- Along with time metal gets discolored.
- The appearance of translucency is difficult to produce.
Question 11. Give methods of condensation, stages in firing, and properties of dental porcelain.
Answer:
Methods Of Condensation:
Condensation can be achieved by several methods:
- Vibration: It consists of applying the wet porcelain to the platinum matrix and then vibrating the die in which the matrix rests. As the particles condense, the water rises to the surface. The excess water is then blotted with a clean tissue paper or an absorbent medium.
- Spatulation: The wet porcelain is applied with a spatula and then the surface is smoothened with the instrument. This will distribute the particles and cause them to become more closely packed. The water rises to the surface and it is removed with a lined cloth or blotting paper.
- Brush technique or capillary action: It consists of adding paste to the matrix and dry powder is sprinkled onto the wet surfaces. The dry powder removes the excess water by capillary action from the mixture already applied. The particles move close together as the water is withdrawn.
- Whipping: After the paste has been applied to the matrix, it may be whipped with a brush. The water is thus brought to the surface and it is removed.
Stages of Firing:
- The process of firing is carried out for fusing the porcelain and is known as sintering.
- Stages of firing: During firing the porcelain moves through different stages.
- The temperature at which each stage occurs depends on the type of porcelain used.
The following are the stages:
- Drying/Green Stage: Initially build-up of the ceramic should be placed at the open entrance of the furnace and the temperature should be raised slowly to 100°C.This leads to the remaining water binder evaporating and prevents the formation of steam in the mass which may lead to compact powder particles cracking. There is a slight contraction reported in this stage.
- Low Bisque Stage: During this stage, the temperature increases gradually and the surface of the particles begins to soften and loose particles just begin to join. The material becomes rigid and is very porous. Particles lack cohesion and they do not have any translucency and glaze. Shrinkage is very minimal.
- Medium Bisque Stage: In this stage, the time of exposure to high temperature still is going on and the fusion of particles increases more and more which brings them closer. This leads to the maximum shrinkage at this stage and now porcelain is non-porous.
- High Bisque Stage: In this, the surface of porcelain is fully sealed and presents a smooth surface. Any of the corrections can be made during this stage before glazing.
- Cooling Stage: Cooling should be carried out at a very slow rate which is very necessary in order to avoid crazing or cracking of ceramic.
Properties of Porcelain:
- Strength: Porcelain is a material having good strength. However, it is brittle and tends to fracture. The strength of dental porcelain is usually measured in terms of its flexure, strength, or modulus of rupture.
- Flexure strength: It is a combination of compressive, tensile as well as shear strength.
- Glazed porcelain
- Compressive strength: Porcelains have a good compressive strength of 331 MPa.
- Diametral tensile strength is 34MPa which is low because of surface defects.
- Shear strength is 110 MPa which is low due to its brittle nature or lack of ductility
- Transverse strength is 62 to 90 MPa.
- Surface hardness: Porcelain is much harder than natural teeth (460 KHN).
- Modulous of elasticity: Porcelain has a high modulus of elasticity of around 69 GPa.
- Wear resistance: Porcelain is more resistant to wear than natural teeth. So it cannot be placed opposite to natural teeth
- Thermal properties: Porcelain has low thermal conductivity. Coeffient of thermal expansion is 12×10–6/°C
- Thermal insulation: It is a good thermal insulator. Its thermal conductivity is 0.0030 and its thermal diffusivity i 0.64 mm2/Sec.
- Dimensional stability: Porcelain becomes dimensionally stable after firing but before firing it is not stable.
- Specifi gravity: Specifi gravity of porcelain is 2.2 to 2.3 gm/cm3. The specific gravity of fired porcelain is usually less because of the presence of air voids.
- Chemical stability: It is insoluble and impermeable to oral fluids
- Biocompatibility: It is quite compatible with the oral tissues.
- Aesthetic properties: The aesthetic qualities of porcelain are excellent. It is able to match adjacent tooth structures in color, intensity, and translucency.
Question 12. Describe the manipulation of dental ceramics.
Answer:
Manipulation Of Dental ceramics:
- As soon as there is the construction of cast metal coping is done metal preparation is over and degassing is completed, opaquer, i.e. a dense yellow-white powder is dispensed on ceramic palette and mixed with a special liquid to a paste-like consistency.
- It is applied by brush on a metal frame and is condensed. Excess liquid is removed with blotting paper.
- Opaquer is built up to a thickness of 0.2 mm. Casting with opaquer is placed in a porcelain furnace and is fired at the appropriate temperature.
- Dentin powder is mixed with distilled water or supplied liquid.
- For mixing glass spatula is used. The bulk of the tooth is buildup with dentin.
- A portion of dentin in the incisal area is cut back and enamel porcelain is added building the restoration.
- After build-up and condensation, it is again returned to the furnace for sintering.
Question 13. Write a short note on recent advances in ceramics.
Answer:
Following are the recent advances in ceramics:
- Magnesia core
- Alumina-reinforced ceramic (Hi-Ceram)
- Castable ceramics.
- Ceramic nanopowders.
Alumina Reinforced Ceramic (Hi-Ceram):
This is based on the dispersion strengthening of ceramic. Alumina crystals are dispersed uniformly in a glass matrix to increase the strength, toughness, and elasticity of the material. In the case of aluminous ceramic, the concentration of alumina crystals and glass powder are mixed and prefritted at 1200°C. Then this crystal glass is grounded and incorporated in a glass matrix, for example, Hi-cream.
Ceramic Nanopowders:
- Nanotechnology in dental ceramics leads to the preparation of nanopowders from zirconia, alumina, and ceria.
- Ceramic blocks prepared from these nanopowders have smooth surfaces which is reduced to porosities and internal defects.
- They have high flexural strength.
- As the size of nanoparticles is below the wavelength of light, they allow light to pass via material resulting in bettr optical properties.
Question 14. Describe the steps in the casting procedure.
Answer:
Following are the steps in the casting procedure:
1. Tooth/teeth preparation: Teeth are prepared by the dentist to receive a cast restoration.
2. Impression: An accurate impression of tooth/teeth is placed with elastomers.
3. Dye Preparation: A dye is prepared from dye stone or the impression is electroformed. A dye spacer is coated or painted over the dye which provides space for the luting cement.
4. Wax Pattern: A pattern of the final restoration is made with type II inlay wax or other casting waxes with all precautions to avoid distortion. Before making the pattrn, a dye lubricant is applied to help separate the wax pattern from the dye.
5. Sprue Former: A sprue former is made of wax, plastic or metal. Thickness is in proportion to the wax pattern. A reservoir is attached to the sprue or the attachment of the sprue to the wax pattern is flared. The length of the sprue is adjusted.
- The functions of sprue former or sprue are as follows:
- To form a mount for the wax pattern
- To create a channel for the elimination of wax during burnout
- Forms a channel for entry of molten alloy during casting
- Provides a reservoir of molten metal that compensates for alloy shrinkage during solidification.
6. Casting Ring Lining: A ring liner is placed inside of the casting ring. It should be short at one end. Earlier asbestos liners were used. Its use has been discontinued due to health hazards from breathing its dust.
- Types of non-asbestos ring liners used are as follows:
- Fibrous ceramic aluminous silicate
- Cellulose
- Ceramic-cellulose combination.
The functions of the ring liner are as follows:
-
- Allows for mold expansion (provides cushion effect).
- When the ring is transferred from the furnace to the casting machine it reduces heat loss as it is a thermal insulator.
- Permits easy removal of the investment after casting.
7. Investing:
- Apply a wetting agent (to reduce air bubbles) on the wax pattern.
- Seat the casting ring into the crucible former taking care that it is located near the center of the ring.
- Mix the investment and vibrate.
- Some investment is applied to the wax pattern with a brush to reduce trapping air bubbles.
- The ring is reseated on the crucible former placed on the vibrator and gradually filed with the remaining investment mix.
- Allow it to set for 1 hour.
8. Wax Elimination (burnout) and Thermal Expansion:
The purpose of burnout is:
- To eliminate the wax (pattern) from the mold
- To expand the mold (thermal expansion)
- The crucible is formed from the ring is separated. If a metallic sprue former is used, it should be removed before burnout.
- Burnout is started when the mold is wet.
- If burnout is delayed store the mold in a humidor.
- If burnout is delayed heating should be gradual.
- Rapid heating produces steam which causes the walls of the mold cavity to fake.
- In extreme cases, an explosion may occur.
- Rapid heating also causes cracks in investment due to uneven expansion. Follow the manufacturer’s instructions.
- The ring is placed in a burnout furnace and heated gradually to 400°C in 20 minutes.
- Maintain it for 30 minutes. In the next 30 minutes, raise the temperature to 700°C and maintain it for 30 minutes.
9. Casting: It is a process by which molten alloy is forced into the heated investment mold. Fusion in the case of noble metal alloy.
- Alloys are melted by the following ways:
- Blow torch
- Electrical resistance or induction.
10. Blow Torch:
- The fuel used is a combination of:
- Natural or artificial gas and air
- Oxygen and acetylene gas (high fusion alloys).
- The flame has four zones as follows:
- Mixing zone: In this air and gas are mixed. No heat is present and it is dark in color.
- Combustion zone: This surrounds the inner zone. It is green in color. It is a zone of partial combustion and has an oxidizing nature.
- Reducing zone: It is a blue zone just beyond the green zone. It is the hottest part of the flame. This zone is used for the fusion of casting alloy.
- Oxidizingzone: Outermost zone in which final combustion between the gas and surrounding air occurs. This zone is not used for fusion.
The air and gas mixture is adjusted to get a reducing flame which is used to melt the alloy because it does not contaminate the alloy and is the hottest part of the flame. The hot casting ring is shifted from the burnout furnace to the casting machine. The ring is placed in the casting cradle so that the sprue hole adjoins the crucible.
Slide the crucible against the ring to avoid spilling of molten metal. The alloy may be melted by a torch or by induction heating. In an induction casting machine, the molten metal is derived into the mold by centrifugal force.
One arm of the machine has a counterweight which balances the weight of the arm carrying the crucible and mould as it rotates. The red hot crucible and the casting ring are visible in the machine. The induction coil (copper colored) is half visible and is used to melt the metal.
Sprinkle flax powder over the molten metal to reduce the oxides and increase fluidity for casting. When the alloy is molten it has a mirror-like appearance like a ball of mercury. Release the arm and allow it to rotate till it comes to rest. This creates centrifugal force which forces the liquid metal into the mold cavity. The ring is allowed to cool for 10 minutes till the glow of the metal disappears.
11. Quenching (for Gold Alloys): The ring is then immersed in water. This leaves the casting metal in an annealed (softened) condition and also helps to fragment the investment. Base metal alloys should not be quenched.
12. Recovery of Casting: The investment is removed and the casting is recovered. A pneumatic (air-driven) chisel may be used to remove the investment. The final bits of investment are removed by sandblasting.
13. Sandblasting: Sandblasting is the process by which particles of an abrasive are projected at high velocity using compressed air in a continuous stream. The casting is held in a sandblasting machine to clean the remaining investment from its surface.
14. Pickling: The surface oxides from the casting are removed by pickling in 50% hydrochloric acid. HCl is heated, but not boiled with the casting in it. Pickling is not a routine procedure and is performed only when indicated.
15. Polishing: Minimum polishing is required if all the procedures from the wax pattern to casting are followed meticulously
Question 15. Write different types of castable ceramics in detail.
Answer:
Castable ceramics are also known as glass ceramics
Types of Castable Ceramics:
There are four types of castable ceramics, i.e.
- Dicor
- Apatite glass ceramic—cera pearl
- Lithia based
- Calcium phosphate
1. Dicor:
The first commercially available castable ceramic material for dental usage is Dicor which was named on the names of its manufacturers, i.e. Dentsply International and Corning Glass Works, it is supplied as silicon glass plate ingot containing MgF2
- Its properties are closer to that of glass and its construction is quite different.
- This is the only porcelain crown made by a centrifugal casting technique.
- The ‘cramming’ process is also quite unique to this porcelain.
Composition Of Dicor:
Dicor contains 55 vol% of tetra silicic Formica crystals in the glass matrix.
Fabrication of a Dicor Crown:
- The wax crown is first constructed in wax and then invested in investment material like a regular cast metal crown.
- After burning out the wax, nuggets of Dicor glass are melted and casts into the mold in a centrifugal machine.
- The glass casting is carefully recovered from the investment by sandblasting and the spores are gently cut away.
- The glass restoration is then covered with a protective ‘embedment material’ to prepare it for the next stage called cramming.
- Cramming is a heat treatment process by which the glass is strengthened. It also reduces the transparency of the glass making it more opaque and less glass-like.
- The cramped glass is now built up with enamel and dentin, condensed, and fixed to complete the restoration.
Advantages of a Dicor Crown:
- Ease of fabrication
- Esthetics is good
- Marginal is good
- Improved strength and fracture toughness
- Processing shrinkage is very low
- Opposing teeth undergo low abrasion.
- Inherent resistance to bacterial plaque and is more biocompatible.
- Thermal conductivity is low
- Radiographic density is similar to dentin.
Uses of a Dicor Crown:
Inlays, onlays, veneers, and low-stress crowns.
Disadvantages of a Dicor Crown:
- Strength is inadequate for posterior use.
- Internal characterization is impossible, so it should be stained externally to improve esthetics.
- Require special equipment
2. Apetite glass-ceramic – In-ceram:
It is developed by Sumiya Hobo and Iwata in 1985. Apatite glass-ceramic melts (1460°C) and flows like molten glass and when cast (1510°C) it has an amorphous microstructure.
- The amorphous CaPO4 formed after casting changes into a crystalline oxy apatite on heat treatment (cramming) at 870°C for 1 hour.
- The chemically unstable oxy apatite when exposed to moisture (water) further converts to crystalline hydroxyapatite
- Similar to natural enamel in composition, density, refractive index, coefficient of thermal expansion, and hardness.
Composition Of Appetite glass-ceramic
- CaO − 45%
- Phosphorous − 15%. Aids in glass formation
- Magnesium oxide − 5%. Decreases the viscosity
- Silicon dioxide− 35% ~ Forms the glass matrix
- Other trace elements—nucleating agents.
Properties Of Appetite glass-ceramic:
- Knoop hardness number (KHN) − 350
- Tensile strength − 150 Mpa
- Young’s modulus of elasticity− 103 /15.0 × 106 Gpa
- Compressive strength − 590/0.08 × 106 Mpa.
3. Lithia based:
- They are developed by Uryu.
- Crystals of LiO. Al2O3.4SiO2 is formed after heat treatment.
- It is available as Olympus castable ceramic.
4. Calcium phosphate:
- It is reported by Kihara.
- It is a combination of calcium phosphate and phosphorus pentoxide plus trace elements.
- It is cast at 1050°C in a gypsum investment mold
- Clear cast crown is converted to crystalline ceramic by heat treating at 645°C for 12 hours.
Question 16. Write a short note on CAD/CAM.
Or
Write short on CAD/CAM technique.
Answer:
The meaning of CAD/CAM is computer-aided designing and computer-aided manufacturing.
Dental copings, crowns, and fixed partial denture frameworks are machined from metal blanks via CAD/CAM.
Technique / Essentials of CAD/CAM System: It consists of 5 essentials, i.e.
- Scanner or digitizer −virtual impression
- Computer − virtual design
- Milling station − produces the restoration of the framework
- Ceramic blanks− the raw material for the restoration
- Furnace − for post-sintering, cramming, etc.
Scanner of CAD/CAM System:
- Dimensions of the prepared tooth are picked up and digitized to create a three-dimensional image of a prepared tooth in a computer.
- Scanning and preparation of dye is done. Scanners are of two types, i.e. contact probes and scanners.
Computer or CAD Process:
- Either the restoration or the core is designed in the computer.
- Manufacturers consist of their own software for the CAD process.
- CAD process aids in designing the restoration, coping, or the FDP substructure. The computer automatically detects the finish line.
- Recording of bite registration is also added to the data.
- All the information with 3D optical impression of the prepared tooth establishes an approximate zone in which new restoration can form.
- Formed restoration is morphed to fi in this zone in an anatomically and functionally correct position.
Milling Station of CAD/CAM System:
Signals from the computer control the milling tool which shapes the ceramic block as per the computer-generated design. For starting this ceramic block is attached to the machine with the help of a frame or built-in handle. Milling is done by a diamond or carbide milling tool.
Ceramic Blanks of CAD/CAM System:
For milling a variety of ceramic blanks are available in various sizes, shapes,s, and shades. If the blank is larger multiple units are produced. Smaller blanks can produce a single coping or restoration. The ceramic blank is attached with the help of a frame to the machine or by one or more handles on the blank itself.
Furnace of CAD/CAM System:
Furnaces are available based on the type of blank used. The furnace for the sintering of zirconia is highly specialized as it uses very high temperatures. Sintering of zirconia involves a temperature greater than 1500°C.
Advantages of CAD/CAM System:
- Fit is improved.
- It is very strong.
- In some systems, scanning can be done in the mouth, so there is no need to take impressions.
- Single-visit prosthesis delivery is possible.
- Formed structures are homogenous with minimum porosity and defects.
- Complex castings, i.e. full arch fixed partial dentures, overdenture frames, and partial denture frames are manufactured with great ease and accuracy.
- Laboratory equipment can be decreased as the equipment involved with metal casting and processing is not required.
- Since it copies the original form of the tooth, it can produce the restoration that duplicates the original tooth.
Disadvantages of CAD/CAM System:
- It is very costly.
- Scanning of preparation is technique sensitive.
- It is still not as strong as PFM restorations.
- In the case of zirconium core ceramics, there is the problem of chipping of veneering ceramics.
Question 17. Write Mode supply, Manufacturing procedure
Answer:
Mode of Supply:
- Enamel porcelain powders in various shades (in bottles)
- Dentin porcelain powders in various shades (in bottles)
- Liquid for mixing enamel, dentin, gingival and transparent
- Opaquer powders in various shades/and liquids for mixing
- Gingival porcelain powder in various shades
- Transparent porcelain powder
- A variety of stain (color) powders
- Glaze powder
- Special liquid for mixing stains and glaze.
Manufacturing Procedure:
- Ceramic raw materials are mixed together in a refractory crucible and heated to high temperature (above 1000°C)
- At these temperatures, the water of crystallization is lost and the flux reacts with grains of silica, kaolin, and feldspar and partly combines them together.
- Feldspar undergoes decomposition to form a glass and a crystalline material known as leucite.
- In the next phase, the molten glass will dissolve the kaolin and quartz
- Continuous heating results in total dissolution and forms a homogeneous glass.
- The fused mass is then quenched in water and a frit is formed.
- The fruit is ground to a fine powder and can be used for the fabrication of porcelain restorations.
- During subsequent firing in the dental lab, the powder is fused to form restoration.
Question 18. Write a short note on the CAD/CAM process for ceramic and
metal.
Answer:
Following is the CAD/CAM process for ceramic and metal:
- Prepared tooth or teeth are scanned directly from the mouth or from a model made from a regular impression.
- Now the restoration or substructure is designed on the computer.
- Attach the metal or ceramic blank/ingot to the milling station and scan the bar code. In cases of metal, titanium metal blanks are commonly used.
- Now the designed image data is retrieved immediately to mill or grind metal or ceramic by computer control.
- The time taken for the milling depends on the size and complexity of restoration as well as the material used.
- As milling is completed, the structure should be separated from the blank by using water-cooled cutting and grinding discs or burs.
Question 19. Explain its applications of dental ceramics.
Answer:
Applications of Dental Ceramics:
- Dental ceramics are used as an indirect intracoronary restorative material for inlay, onlay, etc.
- They are used as an indirect extra-coronal restorative material for constructing crowns and bridges.
- They are used as intra–radicular posts for post-endodontic restoration.
- They are used as biomaterials in the form of dental implants.
- Ceramics orthodontic brackets are used in orthodontics.
- They are used for constructing artificial denture teeth for complete as well as par
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