NEET MDS Lessons
Dental Materials
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PROPERTY |
INGREDIENT |
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Silver |
Tin |
Copper |
Zinc |
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Strength |
Increases |
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Durability |
Increases |
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Hardness |
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Increases |
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Expansion |
Increases |
Decreases |
Increases |
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Flow |
Decreases |
Increases |
Decreases |
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Color |
Imparts |
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Setting time |
Decreases |
Increases |
Decreases |
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Workability |
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Increases |
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Increases |
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Denture Teeth
Use-complete or partial dentures
Type
a. Porcelain teeth
b. Acrylic resin teeth
c. Abrasion-resistant teeth (microfilled composite)
Structure and properties
1. Porcelain teeth (high-fusing porcelain)
Only bonded into denture base mechanically. Harder than natural teeth or other restorations and abrades those surfaces. Good aesthetics.Used when patients have good ridge support and sufficient room between the arches
2. Acrylic resin teeth (PMMA [polymethyl methacrylate])
Bonded pseudochemically into the denture base. Soft and easily worn by abrasive foods . Good initial aesthetics
Used with patients with poor ridges and in cases where they oppose natural teeth
3. Abrasion-resistant teeth (microfilled resins)
Bonded pseudochemically into the denture base.Better abrasion resistance then acrylic resin teeth
Bonding Agents
Applications-composites, resin-modified gIass ionomers, ceramic bonded to enamel restorations, veneers, orthodontic brackets, and desensitizing dentin by covering exposed tubules (Maryland bridges, composite and ceramic repair systems, amalgams and amalgam repair, and pinned amalgams)
Definitions;-
Smear layer - Layer of compacted debris on enamel and/or dentin from the cavity preparation process that is weakly held to the surface (6 to 7 MPa) , and that limits bonding agent strength if not removed
Etching (or, conditioning)- smear layer removal and production of microspaces for micromechanical bonding by dissolving –minor amounts of surface hydroxyapatite crystals
Priming..- micromechanical (and chemical) bonding to the microspaces created by conditioning step.
Conditioning/priming agent-agent that accomplishes both actions
Bonding- formation of resin layer that connect the primed surface to the overlying restoration (e.g., composite) .. –
Enamel bonding System-for bonding to enamel (although dentin bonding may be a Second step)
Dentin bonding system for bonding to dentin (although enamel bonding may have been a first step)
• First-generation dentin bonding system for bonding to smear layer
• New-generation dentin bonding system- for removing smear layer and etching intertubular dentin to allow primer and/or bonding agent to diffuse into spaces between collagen and form hybrid zone
Enamel and dentin bonding system-for bonding to enamel and dentin surfaces with the same procedures
Amalgam bonding system for bonding to enamel, dentin, and amalgam, dentin and amalgam during an amalgam placement procedure or for amalgam repair
Universal bonding system-for bonding to enamel, dentin, amalgam, porcelain , or any other substrate intraorally that may be necessary for a restorative procedure using the same set of procedures and materials
Types
Enamel bonding systems
Dentin bonding systems
Amalgam bonding systems
Universal bonding systems
Structure
o Components of bonding systems
o Conditioning agent-mineral or organic acid
Enamel only 37% phosphoric acid
Dentin only or enamel and .dentin---37% phosphoric acid, citric acid, maleic acid, or nitric acid
o Priming agent
Hydrophobic-solvent-soluble, light cured monomer system
Hydrophilic-water-soluble, light-cured monomer system
Bonding agent
BIS-GMA-type monomer system
UDMA-type monomer system
Reaction
Bonding occurs primarily by intimate micromechanical retention with the relief created by the conditioning step
Chemical bonding is possible but is not recognized as contributing significantly to the overall bond strength
Manipulation-follow manufacturer's directions
Properties
Physical-thermal expansion and contraction may create fatigue stresses that debond the interface and permit micro leakage
Chemical-water absorption into the bonding agent may chemically alter the bonding
Mechanical-mechanical stresses may produce fatigue that debonds the interface and permits microleakage
Enamel bonding-adhesion occurs by macrotags (between enamel prisms) and microtags (into enamel prisms) to produce micromechanical retention
Dentin bonding-adhesion occurs by penetration of smear layer and formation of microtags into intertubular dentin to produce a hybrid zone (interpenetration zone or diffusion zone) that microscopically intertwines collagen bundles and bonding agent polymer
Biologic
Conditioning agents may be locally irritating if they come into contact with soft tissue
Priming agents (uncured), particularly those based on HEMA, may be skin sensitizers after several contacts with dental personnel
Protect skin on hands and face from inadvertent contact with unset materials and/ or their vapors
HEMA and other priming monomers may penetrate through rubber gloves in relatively short times (60 to 90 seconds)
RINGLESS INVESTMENT TECHNIQUE
Used for phosphate bonded investments .
This method uses paper or plastic casting ring .
It is designed to allow urestricted expansion .
Useful for high melting alloys .
Reaction
PMMA powder makes mixture viscous for manipulation before curing. Chemical accelerators cause decomposition of benzoyl peroxide into free radicals that initiate polymerization of monomer
New PMMA is formed into a matrix that surrounds PMMA powder. Linear shrinkage of 5% to 7% during setting. but dimensions of appliances are not critical
Properties-improve with filler content
Physical
Radiopacity depends on ions in silicate glass or the addition of barium sulfate (many systems radiolucent)
Coefficient of thermal expansion is 35 to 45 ppm/C and decreases with increasing filler content
Thermal and electrical insulators
Chemical
Water absorption is 0.5 % to 2.5% and increases with polymer level)
Acidulated topical fluorides (e.g., APF) tend to dissolve glass particles, and thus composites should be protected with petroleum jelly (Vaseline) during those procedures
Color changes occur in resin matrix with time because of oxidation, which produces colored by-products
Mechanical
Compressive strength is 45,000 to 60,000 lb/ in2, which is adequate
Wear resistance-improves with higher filler content, higher percentage of conversion in curing, and use of microfiller, but it is not adequate for some posterior applications
Surfaces rough from wear retain plaque and stain more readily
Biologic
Components may be cytotoxic, but cured composite is biocompatible as restorative filling material
Dental Solders
Applications-bridges and orthodontic appliances
Terms
Soldering -joining operation using filler metal that melts below 500° C
Brazing -joining operation using filler metal that melts above 500°C
Welding-melting and alloying of pieces to be joined
Fluxing
-Oxidative cleaning of area to be soldered
- Oxygen scavenging to prevent oxidation of alloy being soldered
16- 650 -- 650 fineness solder to be used with 16-karat alloys; fineness refers to the gold content
Classification
a. Gold solders-bridges
b. Silver solders-gold-substitute bridges and orthodontic alloys
Structure of gold solders
Composition-lower gold content than of alloys being soldered
Manipulation-solder must melt below melting temperature of alloy
Properties
1. Physical-similar to alloys being joined
2. Chemical-more prone to chemical and electrochemical corrosion
3. Mechanical-similar to alloy being joined
4. Biologic-similar to alloys being joined