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Dental Materials - NEETMDS- courses
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Dental Materials

Mouth Protectors

Use - to protect against effects of blows to chin, top of the head, the face, or grinding of the teeth

Types

o    Stock protectors-least desirable because of poor fit
o    Mouth-formed protectors-improved fit compared with stock type
o    Custom-made protectors-preferred because of durability. low  speech impairment, and comfort


I. Components

a. Stock protectors-thermoplastic copolymer of PYA-PE (polyvinyl acetate-polyethylene copolymer)
b. Mouth-formed protectors-thermoplastic copolymer
c. Custom-made protectors- thermoplastic copolymer, rubber. or polyurethane
2. Reaction-physical reaction of hardening during cooling
3. Fabrication

Alginate impression made of maxillary arch. High-strength stone cast poured immediately. Thermoplastic material is heated in hot water and vacuum-molded to cast .

Mouth protector trimmed to within ½ inch of labial fold, clearance provided at the buccal and labial frena, and edges smoothed by flaming. Gagging, taste, irritation. and impairment of speech are minimized with properly fabricated appliances

4. Instructions for use

a. Rinse before and after use with cold water
b. Clean protector occasionally with soap and cool water
c. Store the protector  in a rigid container
d. Protect from heat and pressure during storage
e. Evaluate protector routinely for evidence of deterioration

Properties

1. Physical-thermal insulators
2. Chemical-absorbs after during use
3. Mechanical-tensile strength, modulus, and hardness decrease after  water absorption, but elongation, tear strength, and resilience increase
4. Biologic-nontoxic as long as no bacterial, fungal, or viral growth occurs on surfaces between uses
 

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)

Cement Bases

Applications

•    Thermal insulation below a restoration
•    Mechanical protection where there is inadequate dentin to support amalgam condensation pressures

Types

•    Zinc phosphate cement bases
•    Polycarboxylate cement bases
•    Glass ionomer cement bases (self-curing and light-curing)

Components

o    Reactive powder (chemically basic)
o    Reactive liquid (chemically acidic)

Reaction

o    Acid-base reaction that forms salts or cross linked matrix
o    Reaction may be exothermic

Manipulation-consistency for basing includes more powders, which improves all of the cement properties

Properties

Physical-excellent thermal and electrical insulation

Chemical-much more resistant to dissolution than cement liners

    Polycarboxylate and glass ionomer cements are mechanically and chemically adhesive to tooth structure

    Solubility of all cement bases is lower than cement liners if they are mixed at higher powder- to-liquid ratios

Mechanical- much higher compressive strengths (12,000 to 30,000 psi)
  
 Light-cured hybrid glass ionomer cements are the strongest
    Zinc oxide-eugenol cements are the weakest

Biologic (see section on luting cements for details)

    Zinc oxide-eugenol cements are obtundent to the pulp
    Polycarboxylate and glass ionomer cements are kind to the pulp
 

Physical reaction-cooling causes reversible hardening

Chemical reaction-irreversible reaction during setting

Composition of Acrylic Resins.

·        Powder. The powder is composed of a polymethyl methacrylate (PMMA), peroxide initiator, and pigments

·        Liquid. The liquid is a monomethyl methacrylate (MMA), hydroquinone inhibitor, cross-linking agents, and chemical accelerators (N, N-dimethyl-p-toluidine)

Casting ring

CASTING RING LINERS

Most common way to provide investment expansion is by using a liner in the casting ring .Traditionally asbestose was used .
Non asbestose ring liner used are :
1) Aluminosilicate ceramic liner .
2) Cellulose paper liner .

The aim of using a resilient liner is to

-. allow different types of investmentbexpansion (act as a cushion)
_. facilitate venting during casting procedure.
_. facilitate the removal of the investment block after casting.&. prevent the distortion by permitting the outward expansion of the mold.
The casting ring holds the investment in place during setting and restricts the expansion of the mold. Normally a resilient liner is placed inside the ring leaving about 2-3 mm from both ends to allow for supporting contact of the investment with the casting ring.

Purpose of Casting Ring Liner

Ringer liner is he most commonly used technique to provide investment expansion. To ensure uniform expansion , liner is cut to fit the inside diameter of the casting ring with no overlap. 

Non-asbestos Ring Liners: Ceramic (aluminum silicate) Cellulose (paper) Ceramic-cellulose combination Safety of the ceramic ring liners remains uncertain, because aluminum silicate also appears capable of producing hazardous-size respirable particles
 

ACRYLIC RESINS

Use. Acrylic (unfilled) resins are used as temporary crown material. Temporary crowns are placed to protect the crown preparation and provide patient comfort during the time the permanent crown is being constructed

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