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272 Cards in this Set
- Front
- Back
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What are the applications of polymers in Dentistry?
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Denture bases and artificial teeth, Restorative materials, Sealants, Varnishes, impression materials, Bonding Agents, Resin Based Cements, Provisional Materials and Athletic Mouth Guards.
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What is a polymer?
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A very large molecule formed by joining many small molecules
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What does Mer mean?
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Repeating Unit
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What are the general properties for Polymers?
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Low Density, Transparent, Translucent, capable of many colors, Low thermal diffusibility and electrical conductivity, Low strength and stiffness and hardness, and a High Coefficient of Thermal Expansion
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What are the basic structures of polymers?
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Linear and branched
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What is cross-linking?
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Where linear or branched molecules connect to form 3D structures. This may increase strength and hardness and decrease solubility and water sorption.
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What are some natural polymers?
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Natural Rubber, silk, cellulose (cotton), shellac, proteins, and carbohydrates.
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What is silk?
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A natural polymer that is an amorphous secrection from silk worm that is drawn and forms a crystalline structure. Hydrogen bonding creates stacks of parallel planes.
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What are the characteristics of silk sutures?
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Suture is made from braided filaments for higher strength and is not considered absorbable, but degrades slowly as protein material is vulnerable to enzymes and phagocytosis. Available in different diameters 5-0 through 11-0.
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What is gut suture?
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Made from animal intestines specifically the collagen from the submucosal connective tissue. Held together with hydrogen bonds. Absorbable
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What is chromic gut suture?
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It is collagen that is treated with acidic chromate salts that produces crosslingking between molecules to resist enzymatic degredation and increase the longevity of the suture. Inflammation is increased due to the chromium…will absorb, but takes longer.
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What is gutta percha?
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A natural polymer made form latex sap from tropical trees (palaquium) and combined with Zinc Oxide and other constituents to form endodontic points to fill root canals.
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What is vulcanized rubber?
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Natural rubber processed (cross linked) by sulfur..eg. rubber dam. It was also used previously in Vulcanite denture material.
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What is nitrated cellulose?
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It is a processed natural polymer. Combo with camphor for Celluloid (ping pong balls, film strips) Combo with NaOH and CS2 though a slit to get Cellophane and through a spinneret to get Viscose/rayon
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What is cellulose acetate?
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It is a processed natural polymer used for strip-off crown forms.
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What does thermoplastic mean?
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Many early polymers were thermoplastic….They can melt to a liquid when heated and freeze to a brittle glassy state and then e reheated to soften again.
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Why are polymers called “plastic”?
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Due to the characteristic of being able to be deformed into other shapes.
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What type of suture is Vicryl by Ethicon?
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It is a synthetic polymer that is polygalactin absorbable and braided. Vicryl is the Kleenex of synthetic sutures.
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What is polyethylene?
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It is a thermoplastic synthetic polymer used to make milk containers (HDPE), trash bags (LDPE), and artificial joints (UHDPE)
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What is polyethylene ribbond used for?
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Splinting teeth
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What is plexiglass?
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It is a thermoplastic synthetic polymer. PMMA (poly methyl methacrylate). It is used to make aquariums, acrylic pain, hard contact lenses, and bone cement.
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What are thermoset polymers?
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They become permanently hard when heated about the critical temp and they don’t soften upon reheating because the polymeric chains are crosslinked. Eg: Epoxy, Polyester (Mylar), and Vulcanized rubber.
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What are adhesives?
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Compounds that bond substances together via drying, temperature, reactant, and pressure sensitity. Eg. Epoxy resin, Cyanoacrylate/Super Glue
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What are fibers of synthetic polymers?
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Strands of resins can be made into cloth, rope, floss, suture and toothbrush bristles. Eg. Polyethylene and Polyamide nylon
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What is an elastomer?
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A synthetic polymer that will return to its original shape when a load is removed. It can be used interchangeably with the term rubber. Eg. Impression materials, and ethelene-vinyl acetate.
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What is one use of elastomers?
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Ethylene-vinyl acetate copolymer is used for Mouthguards.
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What is a homo polymer?
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Polymer with one type of repeating unit (mer)
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What are hetero and Co-polymers?
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Polymers with two or more types of mers.
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What is an example of a homopolymer?
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Poly (methyl methacrylate) a denture base material
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What is a random copolymer?
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A polymer with no swequential order between mer units.
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What is an alternating copolymer?
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A polymer with repeating mer units.
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How can you alter the flexibility of a denture?
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Add a small amount of ethl methacrylate to the methyl methacrylates.
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What is block copolymer?
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Identical monomer units occur in relatively long sequences along the main polymer. Block and graft polymers often show improved impact strength and can modify the surface properties of resins.
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What are graft or branch polymers?
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Sequences of one type of mer unit with branches of another type. Eg. Polybutadiene with grafted polystyrene can absorb energy when hit making a less brittle polystyrene called High Impact Plystryene (HIPS)
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What happens to the physical properties of Crosslinked, branched and linear polymers as the molecular weight is increased?
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The go from gummy soft/ gas to hard and rigid where the Crosslinked are the most affected, then branched followed by linear. (graph)
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What does a high degree of crystallinity lead to?
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Tensile strength, Higher Tg, but lower ductility (more brittle).
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What does higher MW and more polar bond site equate to?
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Higher Tg and higher strength
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What will polar bond sites lead to?
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More chain entanglements causing higher Tg and strength.
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Do Branched or linear polymers have lower Tg and lower strength?
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Branched have lower Tg and Strength unless the branches contain cross-links with main chains, then the strength is increased.
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What are the longer chain diacrylate prepolymers?
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Bis GMA and urethane dimethacrylate
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What substances are shorter chain diluents monomers?
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EGDMA ad TEGMA
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What happens at Tg?
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Molecular motion begins to break secondary bonds to start softening. The strength and modulus of elasticity decrease and thermal expansion increases.
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Are thermoplastic polymers fusible (melt) when heated?
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Yes and they can do so repeatedly.
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Are thermosetting polymers fusible?
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No, they are usually crosslinked and infusible (decomposes). They generally have better wear resistance.
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Name 3 types of deformation and their recovery response?
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Elastic Deformation (reversible and recover when stress is removed), Plastic deformation (permanent and irreversible), and Viscoelastic deformation (a combo of elastic and plastic deformation where only elastic strain is recovered over time)
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What are rheometric properties?
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Flow behavior eg. Viscoelasticity (a combo of plastic deformation viscous flow and elastic recovery.
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What are rheometric properties influenced by?
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Chain length, Cross-links, Temperature, and Rate of Force application
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What influences the solvation properties of polymers?
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increased Mw and cross-linking = insolubility, crystallinity, and Chain branching.
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How do polymers react to solvents?
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They tend to absorb them, swell, and soften rather than dissolve.
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What happens when polymers absorb a solvent?
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Elastomers swell more than plastics, solvent/water spread polymer chains apart and cause slip between chains which reduces strength, hardness, and Tg. Eg. Temporary soft denture lining.
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What are the two basic types of polymerization reactions?
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Addition polymerization and condensation (step-growth) polymerization.
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What happens in condensation polymerization?
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Monomers are all reactive and often small molecule by-products are formed, polymer obtained by the stepqise intermolecular condensation of reactive groups. Eg. Condesation silicon rubbers, polysulfide rubbers, proteins, DNA, Carbs etc.
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What is the condensation by product in the making of silicone rubber and polysulfide Rubber?
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Alcohol and Water respectively.
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What are characteristics of condensation polymerization?
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Generates small molecule by-products, slower than free radical polymerization, and Mw is lower than addition polymerization.
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What are the stages in free radical polymerization?
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1. Induction including activation and initiation 2. Propagation and 3. Chain transfer 4. Termination
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How does polymerization activation and initiation occur?
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An initiator decomposes into free radical under external energy. This radical reacts with monomer to extract an electron and form a new free radical site at the other end of the monomer.
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What are the general characteristics for polymerization reactions?
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They are fast and exothermic so there is usually a small amount of inhibitor or stabilizer added to prevent spontaneous polymerization
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What is DC?
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The % conversion of doublebonds to single bonds. Higher DC = increased strength, wear resistance and decreased solubility and leaching.
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What causes polymerization shrinkage?
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Volume decrease after polymerization as distance between monomers is reduced as covalent bonds are formed.
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What can be done to prevent/deal with the oxygen inhibition layer?
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Oxygen inhibition layer is where the surface layer cannont be thouroughly cured because O2 reacts with free radicals and retards polymerization. You can use a mylar strip, glycerin (K-Y jelly), Cure in N2 or Ar or Cure in H20 (self-cure)
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What is ring opening polymerization?
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Eg. Polyethers, epoxy resins. A reaction of a difuntional epoxide with a difuntional amine.
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What do low volume shrinkage/expansion and high photopolymerization rates of restorative dental resin lead to?
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No marginal gaps and only short curing time.
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What are the common polymers and monomers used in restoratitve dentistry?
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Methacrylates (dentures) and dimethacrylates (direct tooth colored restorative material. They are easily processed and polymerized with good esthetic and physical properties, are chemically stable in the oral cavity and have low costs.
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What is a metal?
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An element that liberates positive ions in an electrolyte. All metals are crystalline and exhibit long-range ordering of atoms
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How many crystal configurations are there and what are the three most common?
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There are 14 configurations. The three most common are body-centered cubic (bcc), Face-centered cubic (fcc), Hexagonal close packed (hcp)
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How are metallic bonds characterized?
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Loose diffuse electron cloud held together tightly by an aggregate of positive ions, cloud of electrons contributes to bonding, heat and electrical conduction
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Why use an alloy over a pure metal?
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Many pure metals are weak and corrode easily. In an alloy metals are missed together or mixed with non-metals to optimize properties
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What is the latent heat of fusion?
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The heat absorbed as metal is transformed from solid to liquid. The temperature stays constant until the transformation is complete then the temp will rise again
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How does solidification happen?
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Nuclei of crystallization initiate the solidification process, atoms are deposited forming crystals, solid area grows until all the liquid is frozen, each nuclei gives rise to and individual crystal or grain
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What is the grain boundary?
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A narrow region between grains, atoms in this transition zone are disorganized with no specific orientation
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As far as grain size goes what makes a metal stronger?
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The smaller the grain size the stronger the metal
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What is homogenous nucleation?
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Temperature/ time dependent nucleation
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How is grain size affected in homogeneous nucleation?
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Grain size determined by the number of nuclei of crystallization. Solidification is time-dependent so: slow cooling results in coarse grain structure, and rapid cooling yields a fine grain structure
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What can be used to seed nucleation?
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Impurities and foreign body additives. Ex. Iridium
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What is slip?
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Movement of atoms due to applied force. The atoms would be required to move simultaneously. The total energy to move an entire homogeneous plane of atoms is too great and cannot occur
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Why are cast metals not as strong as theoretically possible?
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Because of imperfections
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What are examples of macro imperfections?
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Voids, inclusions, cracks, surface pits, and scratches
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What defects in crystal structure cause dislocations?
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Point defects – vacancy, line defect – row of atoms out of alignment, Area defect – irregularity in stacking of planes
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What happens in a line defect?
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An incomplete row of atoms interspersed among complete rows of atom in a crystal structure causes propagation of dislocation due to shear stress resulting in a change of shape
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What is inhomogeneous slip?
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Sliding of the dislocations due to external forces leads to deformation, one line of atoms moving at a time requires less energy than moving an entire plane.
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What is slip the basis for?
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Ductility, malleability and burnishability
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Where do dislocations accumulate?
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They pile up at the grain boundary and resist further movement. Impeding slip with strengthen the metal
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What is an Alloy?
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Metal containing two or more elements that are mutually soluble in the molten state.
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How are alloys classified?
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By use, major element, nobility, principle three elements, dominant phase system
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What are the 3 possible results of solidification of liquid metals?
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1. Solid solution 2. Partial solid solubility 3 additionally or alternatively an intermetallic compound may be formed
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How is a solid solution characterized?
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Crystal structure of parent metal is unchanged by solute, a single homogeneous phase remains, most noble metal dental alloys are solid solutions
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What is a substitution type of solid solution?
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Two different types of atoms occur randomly in the same crystal structure
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What are the requirements for substitution solid solution?
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Have less than 15% difference in atomic diameters, preferably have the same valence and crystal structure, show no appreciable difference in electronegativity
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How is an interstitial solid solution characterized?
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Very small atoms are accommodated in the spaces between large atoms. Ex. Carbon in iron = steel
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What is a solid solution cooling curve?
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Solid solutions do not have a definite melting point, solidification occurs over a range of temperatures. The cooling range will vary as the percentage of each metal is altered
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How is a phase diagram read?
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Everything above the L line (liquidus) is liquid, everything below the S line (solidus) is solid. Between the lines is a combination of solid and liquid
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What is partial solubility?
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Complete liquid solubility, but limited solid solubility
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What are eutectic alloys?
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Ones that have limited solubility in the solid state. They have a % composition point at which the liquid solidifies to a solid. Eutectic means well-melting. Ex. Solders
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What is a peritectic alloy?
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One in which instead of a liquid changing to a solid at one point, liquid and solid phase changes to solid phase at a point
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What are intermetallic compounds?
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Metals with chemical affinity but devoid of any solid solubility. Miscible in the liquid state, during solidification they unite to form chemical compounds
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What properties do intermetallic compounds have?
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Alloys are bound covalently and are hard and brittle.
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What is an alloy used commonly in dentistry that is an intermetallic compound?
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Amalgam. Ag3Sn is called gamma because it is the 3rd phase to solidify
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What are the uses of phase diagrams?
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Offers hints to combinations of metals for certain applications, knowing the composition we can predict the structure, knowing the structure we can predict the properties. Certain alloy compositions can pass form on crystal phase to another by heat treatment, or pass form one phase region to another at a given temp by changing alloy proportions
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What increases yield strength and how can it be done?
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Early immobilization before the onset of slip, this can be accomplished by solid solution hardening, precipitation hardening and order hardening
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What are the 3 ways alloying improves strength?
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1) adhesive forces between the dissimilar atoms are higher than the cohesive forces between the same atoms 2) Solute atoms are larger or maller than solvent atom lattice. This increases the bond energy, the greater the misfit the greater the energy, the more solute atoms the greater the energy 3) The presence of solute atoms increases the energy to move dislocations
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How does precipitation hardening work?
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Solubility limit is exceeded, precipitation of a second phase takes place. Slip is impeded by the precipitate. The degree of hardening depends on the amount and size of the precipitate, distribution and the nature of the precipitation/matrix
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How is order hardening done?
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Most substitutional solid solutions are disordered Ordering of atoms in a methodical array may occur at temps below casting temperature
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What is the result of order hardening?
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A less ductile alloy that is stiffer, stronger and harder
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Which types of ADA gold alloys are heat hardenable and why?
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ADA type III and IV because they have sufficient copper content ( at least 9-10% copper is needed for order hardening in gold alloys)
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How is softening heat treatment done?
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1) Quenching in water immediately after casting to trap the copper atoms in a random position 2) heat hardened casting to 700-750 C ofr 10-30 min then quench to achieve trap atoms in random pattern
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How is heat hardening done with gold alloys?
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1) after casting bench cool – slowly cooling to room temp copper atoms will migrate to an ordered position 2) reheat softened cast to 350 to 400 C for 10-20 min then quench or cool slowly. Random copper atoms will order themselves at temps below 400 C
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What leads to strengthening know as work hardening or strain hardening?
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Immobilization during slip
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How is work hardening done?
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Metal or alloy is strengthened by cold working – bending or plastically deforming
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What are wrought metals?
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Metals that have been worked or deformed
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What is the result of cold working a metal?
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Metal becomes stronger but less ductile
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What is the microstructure of wrought metals?
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Fibrous – fragmented and elongated grain structure
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What is the microstructure of cast gold?
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Equiaxed – sphere shaped grains or crystals
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What is the mechanism of hardening by cold working a metal?
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1. Deformation leads to grain fragmentation and increase in grain boundaries. Grain boundaries act as a barrier to slip. 2. Deformation generates dislocations which entangle with each other interfering with their motion. 3. Deformation adds more energy to the bond energy that keeps atoms together
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What is annealing?
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Cold-worked metals have distorted grain structure that will try to return to their original position at elevated temperatures
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What are the 3 stages of annealing?
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Recovery, Re-crystallization, and grain growth
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How does stress relief occur?
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Low temperature short time anneal that frees dislocations from entanglement – removing strain energy. Decreases strength and increases ductility slightly
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When does recrystallization occur?
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If temperature of anneal is increased to ½ to 2/3 solidus temp, new grains (free of strain) form within the deformed grains and grow. Results in drastically reduced strength and increases ductility
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How is grain growth accomplished?
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Continued annealing past recrystallization leads to more grain growth. Results in slight decrease in strength and increase in ductility – little benefit and undesirable
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What is the practical application to annealing?
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There is a limit to how much orthodontic wires can be worked before they fracture, if further deformation is necessary metal must be annealed.
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What are 3 point defects?
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Vacancy, substitution and Interstitial
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What is corrosion?
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Destruction of metallic material by an oxidation reaction with its environment.
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What is tarnish?
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It is a mild form of corrosion involving discoloration due to deposition of corrosion products on the surface. It may be a forerunner to corrosion
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What is oxidation?
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Metals dissolve in an electrolyte, giving off positive ions with electrons left behind.
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What is reduction?
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Where metal ions in solution tend to plate back to the electron rich metal plate. Ie Ag+ + e- = Ag.
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What is the corrosive status in a closed system where oxidation = reduction?
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This is equilibrium, a no corrosion situation.
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What occurs when the oxidation/reduction equilibrium is upset and the forward oxidation reaction is favored?
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Corrosion begins.
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What are the two ways to favor the forward reaction causing corrosion to occur?
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Remove electrons as they are generated via an electrical circuit or remove the metal ions as the are formed via chemical reactions or by changing the electrolytes constantly in an open system.
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How does corrosion affect dentistry?
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Restorations must be corrosive resistant in the moderately corrosive oral environment and Metal ions released by metals may elicit toxic responses in the tissues of the body.
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Why do many of the metals in dentistry corrode?
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They are refined and not in their lowest energy state and will react with oxygen, sulfur, or chlorine to form lower energy compounds.
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What is a galvanic electrochemical/ corrosion cell?
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When two metals are immersed in an electrolyte they compete to dissolve ions in the electrolyte.
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What determines an elements tendency for oxidation?
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Differences in atomic structure.
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How do you form a spontaneous electrochemical cell?
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Two different metals in contact with each other and an electrolyte.
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Which electrode is known as the cathode?
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The one associated with reduction
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Which electrode is known as the anode?
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The one associated with oxidation.
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What happens if there is resistance to electron flow in a galvanic cell?
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There will be a backlog of electrons. A surplus of electrons at the anode give a negative charge and a deficit at the cathode leads to a positive charge.
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What does electrode potential measure?
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The tendency to lose electrons and ionize(oxidize). This is measured in volts and a more negative number = easier for that metal to ionize.
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How would you rank the following according to their corrosive prowess: Type III Gold alloy, Type IV Gold alloy, Co-Cr, Stainless Steel, NiCr, Amalgam, and Zinc?
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This is the order from least corrosion prone to the most.
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How does the corrosion prowess relate to how cathodic or anodic a metal is?
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The more corrosive the more anodic a metal is.
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What are 5 types of microgalvanism that can occur?
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Grain-grain boundaries, Crystallographic orientation, compositional difference, non-uniform stress distribution, and concentration cells.
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How are micorgalvanisms and grain-grain boundaries related?
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Atoms at a grain boundary are more active (anodic) than in the interior due to higher energy at grain boundaries due to lack or perfect crystals.
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How are microgalvanism and crystallographic orientation related?
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Crystals oriented in different directions have different electrode potentials.
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What does compositional differences in microgalvanism refer to?
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Minor segregations in an alloy.
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How can non uniform stress distribution work as a micro galvanic cell?
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High energy strained conditions are more anodic.
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How can a concentration cell be a type of microgalvanism?
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Differences in concentration of oxygen in one area of an alloy compared to another may create galvanic action. Eg. A restoration that is partially supragingival and partially subgingival.
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How can crevice corrosion increase microgalvanic conditions?
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Besides the difference in Oxygen concentration, crevices can have increased plaque and decreased pH.
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According to the clinical situation #1, what is causing the pain when the patient bites down and contacts a new amalgam with an existing gold inlay?
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Oral electrogalvanism or galvanic shock, you could replace amalgam with a non-metal or gold restoration, do surface treatments, or see if time takes care of it.
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What is going on in situation #2 with an MO amalgam that contacts a full gold crown?
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She is probably having galvanic shock, you can separate the two to see if it subsides. If it is shock, you can replace amalgam with non-metal or gold.
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What are some other situations that can cause painful eletrogalvanism?
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Allowing temporary metal crowns to contact opposing gold restorations or amalgam restorations, Utensils or tin foil chewing gum wrapper that contact metal restorations, silver soder and stainless steel ortho wires,
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What are some other examples of galvanism?
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Autoclaving handpieces where the heat and steam accelerate corrosion and carbon steel burs corrode quickly vs. stainless steel with its passivating oxide layer.
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What is passivation?
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It allows for an oxide layer once in contact with air and this layer limits the reactivity of the metal. The layer is coherent, continuous and impermeable to water and oxygen at normal temperatures.
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What does it mean to anodize aluminum?
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You get a thicker layer of aluminum oxide by making the metal the anode of an electrolytic cell with a carefully designed solution eg. Our plastic/resin instruments.
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Why is chromium added to metals?
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To create a passive surface layer of Cr2O3 eg. Stainless steel 18-8 for ortho wires, CoCr alloy for removable partials.
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How will acids and alkalis react with Cr2O3 coated materials?
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They will attack the surface and corrode them…this is why we don’t clean RPD frames with sodium hypochlorite.
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Why is titanium good for dental implants?
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It’s strength, biocompatibility and corrosion resistance where TiO2 is formed on the surface to passivate the metal.
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How can corrosion be minimized?
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Use noble metals or ones the form a passive oxide film, Use homogenous alloys, take care not to overheat metals, keep surfaces clean and polished, prevent contact between dissimilar metals and limit bending or stretching of metals.
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What are some uses of deliberate corrosion?
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Electroetching, Electropolishing, Electroplating, and degassing metal from PFMs.
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What is electroetching used for?
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Used to roughen metal surfaces to promote adhesion especially in resin-bonded bridges (Maryland bridges)
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What is electropolishing?
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It is deliberate corrosion, a controlled galvanic process that is used to smooth very hard alloys like NiCr or CoCr
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What is electroplating?
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A controlled galvanic process used to add a layer of metal to the surface like when plating impressions with metal for metal dies and plate PFM retainers.
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How is electroplating accomplished?
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Place it in a solution of metal ions, connect negative pole (cathode) current passes from material to the solution of metal ions and metal ions are reduced to atomic metal by electrons.
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What is the process used to galvanize steel?
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Electroplating with Zinc that protects the steel.
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Is electroplating recommended with gold restoration?
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No. if the plating is damaged the metal substruction will corrode.
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How is degassing for PFM’s achieved?
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Metal frameworks are fired in a porcelain oven to create metal oxides on the surface that will chemically bond once the porcelain is applied and fired
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How prevelant is painful oral electrogalvanism?
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One study showed .4%.
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What type of amalgam is most likely to cause painful oral electrogalvanism?
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Amalgams that contain Zinc are more likely to cause electrogalvanic shock.
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What are dental ceramics based on?
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SiO2
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What is traditionally contained in dental porcelain?
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Glass and more ordered elements
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What are a few applications of dental ceramics?
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Porcelain denture teeth, full porcelain crowns, PFM crowns and bridges, inlays, onlays and laminates
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What is the structure of silica?
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Tetrahedral unit with silicon in the middle and oxygen at the corners
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What is soda glass?
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Sodium disrupts the silica crystalline structure, certain metal oxides in dental ceramics lower the melting range and decrease the viscosity by disrupting the crystalline structure creating a glassy matrix
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What are constituents of feldspathic ceramic?
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Feldspar, Leucite, Tridymite, and crystobalite
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What are the classifications of ceramic material?
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Glass based – feldspathic, high leucite, lithium disilicate, mica. Alumina-based – Alumina and alumina MgO spinell. Zirconia-based – Zirconia
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How are feldspathic crowns made?
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Built-up and sintered
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Why are build-up ceramics vacuum fired?
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To reduce porosity
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What are the physical properties of ceramic?
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Low Electrical conductivity, low thermal diffusivity, low coefficient of thermal expansion, high melting temp, and intermediate density.
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What are the chemical properties of ceramic?
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Low chemical reactivity, and low absorption and solubility
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What are the mechanical properties of ceramic?
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High modulus of elasticity, much stronger in compression than tension (~10x), Brittle (low plastic deformation), and High hardness
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What does it mean for a ceramic to be brittle?
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Unable to flex without breaking, .1% deformation before fracture, flexural strength is a more meaningful measure than compressive strength
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Why do ceramics usually fail?
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Propagation of internal cracks caused by repeated stress
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What is the compressive and tensile strength of ceramic?
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Compressive 862 Mpa, tensile 34 MPa
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How can brittle materials be strengthened?
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Interruption of crack propagation, introduce residual stresses on the surface of the material, addition of high-fusing tough particles, rigid metal substructure, intentional expansion mismatch of layers, and resin-bonded cementation for most all-ceramic crowns
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How does introducing tough particles into ceramic improve strength?
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Particles will inhibit the growth of microcracks
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What does the rigid metal substructure do?
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Inhibits crack propagation
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How does mismatch of layers improve strength?
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A slight mismatch of the coefficient of thermal expansion of porcelain and the metal substructure creates residual compression in the porcelain. Residual compressive forces must be overcome before tensile forces can lead to crack propagation
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How does cement bonding improve strength?
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Bonding to tooth structure with resin can inhibit micro cracks that initiate from the prepared tooth surface. A good bond also transmits forces to the tooth to help absorb energy
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What is Ion strengthening?
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Apply potassium-containing liquid to surface of sodium-containing ceramics and fire, this creates residual compression
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What is transformation toughening?
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Zirconia transforms its crystal structure to reduce crack propagation
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What is thermal tempering?
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Rapid cooling of exterior surface, creating residual compression
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How does porcelain transmit stress?
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Isotropically (radially) which creates higher shear at any interface
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How do enamel and dentin transmit stress?
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Anisotopicaly (more in one direction) in the long axis of the rods and tubules
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At what thickness will the chances of fracturing the porcelain be greatly increased?
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2.5- 3mm beyond its support
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How do surface flaws effect strength?
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Near perfect surfaces give increased strength, because stresses do not concentrate in one site. Glazes and polishing improve strength
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Why is APF contraindicated for ceramics?
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Acidulated phosphate fluoride (APF) and other acids are contraindicated because low pH environments induce decomposition of matrix
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How does the hardness of porcelain compare?
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Porcelain =450, Enamel =350, Dentin =70, Acrylic =20. Porcelain is abrasion resistant but may abrade opposing dentition and it is difficult to polish
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What is Devitrification?
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The vitreous (amorphous) portions of porcelain may crystallize due to repeated firing at high temps resulting in translucent porcelain turning opaque
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What are the design and application considerations for dental ceramics?
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Minimize tensile stress and minimize stress concentration
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How can tensile stress be minimized?
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Metal copings reduce the amount of internal stress. Certain all-ceramic crowns are contra-indicated on posterior teeth and anterior teeth with extreme vertical overlap
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How is stress concentration minimized?
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Rounded line angles and preferred for preparations and copings, adequate thickness for porcelain is necessary, frameworks should allow uniform thickness of the porcelain
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What are the advantages of dental ceramics?
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Good esthetics, High compressive strength, Good chemical durability, Biocompatible, and stain resistant
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What are disadvantages of dental ceramics?
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Brittle, Low tensile strength, and high hardness
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What are porcelain laminates?
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Thin veneers of porcelain that are bonded to acid etched enamel with resin cement
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Why choose porcelain laminates over RBC?
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Laminates are more esthetic than direct RBC, Laminates are more resistant to stain and wear
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What are the disadvantages of porcelain laminates?
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Fragile until bonded, Expensive, Lower anterior may abrade upper teeth
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Can porcelain be repaired?
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Yes with resin-based composite bonded to porcelain through mechanical nad chemical means
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How is a mechanical bond to porcelain achieved?
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Roughen surface with diamond bur, micro etch, and etch with 1.23% acidulated phosphate-fluoride gel or 9.5% hydrofluoric acid
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How does hydrofluoric acid help?
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HF acid creates roughness through differential reaction rates on matrix and core phases. High stress area and cracks will be attacked more quickly, stress will be relieved and cracks blunted
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What are the characteristics of HF?
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Corrosive to metals and silicates, diffusion through soft tissue is fast and deep (if HF comes in contact with soft tissue neutralize with calcium gluconate gel and rinse with water)
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How is the chemical bond achieved for porcelain repair?
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With silane coupling agent
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Why are porcelain repairs considered interim procedures?
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All porcelain repair systems result in a significant loss of bond strength after 3 months immersion in water
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What is abrasion?
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The wearing away of a surface by rubbing, cutting, or scraping where the object that does the abrading is the abrasive.
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What does cutting refer to?
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Removing material by shearing off with a bur or a hand instrument.
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What are slow and high speed burs made of?
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Slow-carbon steel, and high-speed are made of tungsten carbide w/a Moh’s hardness of 9.
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What is Moh’s hardness based on?
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Evaluates mineralogical material hardness by using one material to scratch another.
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What is Moh’s hardness value for diamond?
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10 (the hardest mineral)
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How many blades are on a cutting bur?
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6-10
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How many blades are on a finishing and polishing bur?
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12-40
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How would you define finishing?
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Contouring a restoration or appliance by removal of material.
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How do you define polishing?
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The final removal of material from a restoration or appliance to give a smooth, highly reflective surface, by reducing the size of scratches through a series of smaller abrasives.
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What are the benefits to polishing?
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Improved esthetics, and reduce adhesion for bacteria or stains, make surface feel smooth to patient, improve marginal adaptation, and reduce corrosion in metal restorations.
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What 6 factors affect abrasion?
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Hardness, Size, Shape, Pressure, Speed, and Lubrication.
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What are the knoop hardness values for cementum, Dentin, and enamel?
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40, 70, and 340 vs. 7000 for diamonds
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What is hardness?
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A measure of a materials ability to resist indentation
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How does hardness affect abrasion?
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The greater the difference between the abrasive and the substrate, the more rapid and efficient the abrasion.
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How does size affect abrasion?
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The larger the particle, the greater the abrasion
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How does shape affect abrasion?
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An irregular shaped object with sharp edges will abrade more efficiently than a spherical shaped object.
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How does pressure affect abrasion?
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Greater forces cause more rapid removal of material and also increases temperature.
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How does speed affect abrasion?
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More speed = more abrasion, but also increased temperature.
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How can lubricants improve the efficiency of abrasion?
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By cooling and removing debris.
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What are some common mechanism/tools that abrasives are attached to?
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Diamond burs, stones, rubber wheels, cups, and points, disks and strips, brushes, felt cones, and cloth wheels.
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What are a dozen types of abrasives?
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Diamond, Silicon Carbide, Aluminum Oxide, Emery, Garnet, Sand/Quartz (cuttle), Ziroconium silicate, tin oxide, Tripoli, pumice, Rouge, and Chalk.
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Where do Dentin and Enamel lie on Moh’s hardness scale?
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Dentin – 3-4, and Enamel 5-6…..diamonds-10, aluminum oxide –9, Quartz/sand—7,
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What are most polishing pastes made of?
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Aluminum oxide. Including “diamond” pastes.
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What are some characteristics of silicon Carbide?
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Synthetic, green and blue-black. Come in disks and points. (joe dandy disks)
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What is aluminum oxide?
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A synthetic abrasive used in white stones to adjust metal alloys, RBC, and ceramic materials. Also found in disks, strips, rubber points, and polishing pastes.
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What are the characteristics of Zirconium Silicate?
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A natural mineral Zircon used in polishing strips, disks, and prophy pastes.
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What is emery?
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The natural form of aluminum oxide, grayish or black and used in arbor bands to adjust acrylic.
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What are Garnet abrasives?
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Natural silicates of Mg, Mn, Fe, Co, and Al red in color and used on coated disks to adjust acrylic, RBC and metals.
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How are Quartz and Sand abrasives used?
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They are natural silicate oxides of various colors used in disks or strips to finish gold, acrylic and RBC.
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What is tin oxide used for?
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Mixed with water or glycerin to polish metal restorations.
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What is Tripoli?
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A natural siliceous rock ground to powder and added to soft compound to form a bar that is used to polish metals and acrylic.
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What is pumice?
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A natural gray volcanic rock rich in silica. Used with a rag wheel to polish acrylic, brushes to polish amalgam, and rubber to polish teeth.
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How is grit size determined?
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If they pass through a certain size mesh screen. The larger the number, the smaller the particle size.
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What is perlite?
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Volcanic glass sheet like particles 2-3 microns thick with a Moh’s hardness of 5.5-7.0. the particle reduce in size and edges dull under load. Good cleaning with low abrasion….Clinpro is brand name.
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What is Rouge?
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Natural iron oxide formed into a block and used with a rag will to polish metal.
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How is chalk used as an abrasive?
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It is natural calcium carbonate used with a rotary brush to polish metal and teeth.
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What is the key to efficient polishing?
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Recognition of unfinished or unpolished areas.
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What is a good rule of thumb with abrasives?
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Use the more abrasive materials longer than finer abrasives and don’t move to a finer abrasive until all the voids and pits are removed.
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How is roughness measured?
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Profilometer, using a stylus recording the average height from a center line. Different profiles can have the same Ra value.
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What is the roughness (Ra) in microns after a 12 bladed carbide finishing bur vs. a fine diamond?
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.6-.7 vs. 1.0 microns.
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What is the difference between the grits of the different polishing disks?
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Coarse-black-2.0, Medium-dark blue-.6, Fine-Medium blue-.3, and Super fine-light blue-.1
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What is the roughness range on the rubber with embedded abrasives polishing points?
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1.0-.3
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What type of brushes can be used to polish RBC?
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SiC abrasive impregnated brushes.
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To what roughness can polishing pastes polish to?
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.1
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At what speed should you use the Shofu disks?
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Slow speed 10-12K
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How do the RPM of our Kavo Handpieces read?
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Slow speed is a 1 to 1 ratio (@1= 1K, @40= 40K), Fast speed is a 1 to 5 ratio (@1=5K, @40= 200K)
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How does fine pumice polish compare to the astropol polish?
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The astropol looks much more polished in the split picture.
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Which polishes better, Astropol or toothpaste with a prophy cup?
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Astropol looks much more polished in picture.
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What can be used to do the intitial finishing of amalgam?
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Finishing burs, green or white stones.
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What are two ways to polish amalgams?
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1. Rubber polishing points Brown, Greenie, super greenie and then tin oxide 2. Pumice and brushes: Medium, Fine, Flour, and Tin Oxide and brush.
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To what level of roughness do white stones finish amalgams?
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.5-1.0 microns
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What should you do 1st after finishing amalgams?
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Clean surface to see if all areas have the same luster, if not continue with the same abrasive.
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What speed should you polish with greenies, brownies and super greenies?
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Slow 5-7K
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What level of roughness will brownies, greenies and supergreenies leave?
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.3-.1
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How do you use a crescent brush to polish amalgams?
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With wet SnO, light pressure, and fast speed until dry.
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What should you use for the final polish of an amalgam?
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Rubber points and tin oxide.
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How can you polish pits and groove on an amalgam?
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Cut crescent brushes to form a point.
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At what speed should you polish with pumice?
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5-10K (slow)
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How do you finish cast alloys?
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Finishing burs or green or white stones, then rubber abrasive wheels or points, then bar buffing compounds/Tripoli, then Rouge with a rag or chamois wheel.
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How do you adjust the occlusion on a metal cast crown?
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A course diamond bur.
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What do you use to finish a metal cast crown?
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Fine diamonds on high speed or green stone on slow then use a white stone and then polish with a burlew wheel and a brownie, or a greenie and Tripoli and a robinson wheel, or rouge and a soft robinson wheel, or with a rag or Chamois wheel.
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What are the two ways to finish and polish ceramic restorations?
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Extraoral before cementation esp on PFM’s and Intra oral (after cementation, all porcelain empress onlays and crowns.
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What is a good way to do the final polish of ceramic restorations?
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Polishing paste and stiff robinson wheel and polishing brushes on slow speed (5-7K)
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