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45 Cards in this Set
- Front
- Back
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What are Noble metals |
Gold and platinum group metals (platinum, palladium, rhodium, ruthenium, iridium, and osmium), which are highly resistant to oxidation and dissolution in inorganic acids. |
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What is the Lost wax technique? |
Process in which a wax pattern, prepared in the shape of missing tooth struc-ture, is embedded in a casting investment and burned out to produce a mold cavity into which molten metal is cast. |
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What is the Karat system for gold? |
The carat system specifies the gold content of an alloy based on parts of gold per 24 parts of the alloy |
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What is fineness of gold |
Fineness is the unit that describes the gold content in noble metal alloys by the number of parts of gold per 1000 parts of alloy. |
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How much is 18 carat gold in fineness |
18 carat gold is 75% gold which is 750 fine |
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Why is silver considered a precious metal but not a noble metal |
Silver and all the noble metals are considered to be precious metals because of their expense. But silver reacts in the mouth that is why it is not considered as a noble metal. |
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What are ceramics? |
A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing a nonmetallic mineral, such as clay, at a high temperature. Common examples are earthenware, porcelain, and brick |
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What are the main properties of dental ceramics? |
Inert nature, esthetics, biocompatibility, refractory nature, high hardness, excellent wear resistance, low to moderate fracture toughness, susceptibility to tensile fracture and low electrical conductivity. |
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Three desirable properties of dental ceramics |
Hardness less than tooth enamel, an easily polishable surface and resistance to fracture |
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Main disadvantage of ceramics |
Brittleness to tensile stress. |
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2 properties that are used to measure ceramic brittleness |
Fracture toughness (Kic) and critical strain energy release rate (G) |
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Dental ceramics composition |
Nonmetallic inorganic structure, mainly oxygen compounds with metallic and semi metallic aluminum, boron, calcium, cerium, lithium magnesium phosphorus potassium silicon sodium titanium and zirconium. |
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Increased crystalline matrix can |
Make the ceramic more opaque. More glassy matrix will make it more translucent |
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What is feldspar? |
They are a group of rock-forming tectosilicate minerals that make up about 41% of the Earth's continental crust |
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Structure of feldspar |
The structure of a feldspar crystal is based on aluminosilicate tetrahedra. Each tetrahedron consists of an aluminium or silicon ion surrounded by four oxygen ions. Each oxygen ion, in turn, is shared by a neighbouring tetrahedron to form a three-dimensional network. The structure is open enough for cations (typically sodium, potassium, or calcium) to fit into the structure and provide charge balance. |
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What is kaolinite? |
is a clay mineral, with the chemical composition Al2Si2O5(OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica (SiO4) linked through oxygen atoms to one octahedral sheet of alumina (AlO6) octahedra. |
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What is the use of kaolinite in dental ceramics? |
It is the main component of porcelain |
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What is vitrification? |
Vitrification is the transformation of a substance into a glass, a non-crystalline amorphous solid. In ceramics, it reduces permeability to water. |
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How is vitrification achieved? |
Vitrification is usually achieved by heating materials until they liquidize, then cooling the liquid, often rapidly, so that it passes through the glass transition to form a glassy solid. |
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What is ceramic glaze? |
Ceramic glaze is an impervious layer or coating of a vitreous substance which has been fused to a ceramic body through firing. Glaze can serve to color, decorate or waterproof an item. |
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What is the difference between feldspar and quartz ? |
Quartz is harder than feldspar. Quartz breaks in irregular and curvy pieces while feldspar breaks along a cleavage Quartz is usually clear and has more crystals.
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What is dental feldspathic porcelain? |
Dental feldspathic porcelain is predominantly a glass material with an amorphous (non-crystalline) structure. It was derived from the natural mineral feldspar. |
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Structure of feldspathic porcelain |
Basic structure is silicondioxide tetrahedra. Adding metal oxides (calcium, Sodium, Potassium) reduces fusion temperature and gkves a more fluid behavior. |
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What is the addition to porcelain for pfm? |
Leucite crystals were added which increased the thermal expansion of porcelain and brought it nearer to to that of the metal substrate. This increases strength. |
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What is the metal used as a base in porcelain jacket crown? |
Platinum |
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What are the main components of feldspathic porcelain? |
Kaolin 3-5%Quartz (silica) 12-25%Feldspar 70-85%Metallic colourants 1%Glass up to 15% |
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Why was all ceramic crowns invented instead of pfm? |
The pfm metal base needed an extra metal blockout opaque layer. This reducwd esthetics and needed more tooth prep. |
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What is the weakness of the pjc? |
Internal micro cracking during the cooling phase of fabrication |
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What was done to reduce internal micro cracking of pjc? |
Pfm with metal base was invented. The bond between the metal and the porcelain prevented stress cracks from forming. |
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What is alumininous porcelain? |
Addition of alumina crystals to porcelain (40 to 50%). the dispersed Alumina particles are stronger with higher modulus of elasticity. Eg. High ceram used as coping under regular porcelain. |
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Disadvantages of aluminous porcelain |
Had a chalky white appearance which needed feldspathic porcelain as a veneer to hide the colour. Had high fracture rate for posterior teeth. Sintering shrinkage of core at high firing temperatures. |
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What are the proposed theories for metal ceramic bonding? |
1. Van der waal's forces 2. Mechanical retention 3.compression bonding 4. chemical bonding |
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How is mechanical bonding increased between metal and ceramic |
By air abrading the metal. |
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How is compression bonding increased between metal and ceramic? |
By placing the porcelain under compression while cooling. This is achieved by having a metal with higher thermal expansion than porcelain. The metal contracts faster on cooling putying compressive stress on the porcelain. But the difference shouldn't be significant as it can lead to shearing off. |
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How is chemical bonding achieved between metal and ceramic? |
By metal degassing. Creates an oxide layer on the metal. Thinner, the better |
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What were the first ceramic crowns used in dentistry? |
Pjc or porcelain jacket crown |
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Main disadvantage of porcelain jacket crown using platinum? |
Internal microcracking while cooling during fabrication |
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Why were metal ceramic crowns invented? |
The metal base bonds to the ceramic preventing internal micro cracking. Also the lost wax technique improved marginal fit. |
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What are the Metals used in pfm |
Base metals like Cobalt Nickel chromium, titanium alloys, gold alloys. |
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Advantage of base metals over gold alloys |
Base metals are cheaper but can cause metal allergy in some patients |
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Why was vacuum used while firing ceramics in the furnace? |
It reduces porosity and shrinkage |
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What is sintering? |
A process of compacting and forming a solid mass of material by heat and or pressure without melting it to the point of liquifaction. |
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What are refractory die material? |
Used for making dies on which ceramic restorations are constructed |
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What are the different techniques of strengthening ceramics? |
1. Optimal design of the prosthesis 2. Development of residual compressive stress 3. Ion exchange 4. reducing number of firing cycles 5. Thermal tempering 6. dispersion strengthening 7. Transformation toughening |
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What are the techniques of optimal design of a prosthesis? |
1. Use tougher ceramic material 2. Grinding of material should be reduced 3. Avoid sharp line angles during tooth preparation 4. Use bigger connectors between fpd |
