NEET MDS Lessons
Dental Materials
Properties of Amalgam.
The most important physical properties of amalgam are
- Coefficient of thermal expansion = 25-1 >ppm/ C (thus amalgams allow percolation during temperature changes)
- Thermal conductivity-high (therefore, amalgams need insulating liner or base in deep restorations)
- Flow and creep. Flow and creep are characteristics that deal with an amalgam undergoing deformation when stressed. The lower the creep value of an amalgam, the better the marginal integrity of the restoration. Alloys with high copper content usually have lower creep values than the conventional silver-tin alloys.
Dimensional change. An amalgam can expand or contract depending upon its usage. Dimensional change can be minimized by proper usage of alloy and mercury. Dimensional change on setting, less than ± 20 (excessive expansion can produce post operative pain)
- Compression strength. Sufficient strength to resist fracture is an important requirement for any restorative material. At a 50 percent mercury content, the compression strength is approximately 52,000 psi. In comparison, the compressive strength of dentin and enamel is 30,000 psi and 100,000 psi, respectively. The strength of an amalgam is determined primarily by the composition of the alloy, the amount of residual mercury remaining after condensation, and the degree of porosity in the amalgam restoration.
- Electrochemical corrosion produces penetrating corrosion of low-copper amalgams but only produces superficial corrosion of high copper amalgams, so they last longer
- Because of low tensile strength, enamel support is needed at margins
- Spherical high-copper alloys develop high tensile strength faster and can be polished sooner
- Excessive creep is associated with silver mercury phase of low-copper amalgams and contributes to early marginal fracture
- Marginal fracture correlated with creep and electrochemical corrosion in low-copper amalgams
- Bulk fracture (isthmus fracture) occurs across thinnest portions of amalgam restorations because of high stresses during traumatic occlusion and/or the accumulated effects of fatigue
- Dental amalgam is very resistant to abrasion
Solution Liners (Varnishes)
Applications
o Enamel and dentin lining for amalgam restorations
o Enamel and dentin lining for cast restorations that are used with non adhesive cements
o Coating over materials that are moisture sensitive during setting
Components of copal resin varnish
o 90% solvent mixture (e.g., chloroform, acetone, and alcohol)
o 10% dissolved copal resin
Reaction
Varnish sets physically by drying Solvent loss occurs in 5 to 15 seconds (a film forms the same way as drying fingernail polish)
Manipulation
Apply thin coat over dentin. enamel. And margins of the cavity preparation Dry lightly with air for 5 seconds Apply a second thin coat Final thickness is 1 to 5 µ.m
Properties
o Physical
Electrically insulating barrier that prevents shocks. Too thin to be thermally insulating. Decreases degree of percolation attributable to thermal expansion
o Chemical
Forms temporary barrier that prevents microleakage into dentinal tubules until secondary dentin formation occurs. Decreases initial tendency for electrochemical corrosion
o Mechanical
Very weak and brittle film that has limited lifetime
Film adheres to smear layer
Classification
Rigid impression materials
(1) Plaster
(2) Compound
(3) Zinc oxide-eugenol
Flexible hydrocolloid impression materials
(I) Agar-agar (reversible hydrocolloid)
(2) Alginate (irreversible hydrocolloid)
Flexible, elastomeric, or rubber impression materials
(1) Polysulfide rubber (mercaptan rubber)
(2) Silicone rubber (condensation silicone)
(3) Polyether rubber
(4) Polyvinyl siloxane (addition silicone)
Manipulation
Mixture of powder and liquid is painted onto working cast to create shape for acrylic appliance à After curing of mixture, the shape and fit are adjusted by grinding with burrs and stones with a slow-speed handpiece .Acrylic dust is irritating to epithelial tissues of nasopharynx and skin and may produce allergic dermatitis or other reactions. Grinding may heat polymer to temperatures that depolymerize and release monomer vapor. which may be an irritant
Mercury bioactivity
- Metallic mercury is the least toxic from and is absorbed primarily through the lungs rather than the GI tract or skin
- Mercury in the body may come from air, water, food. dental (a low amount). Or medical sources
- Half life for mercury elimination from body is 55 days .-
- mercury toxicity is <50 µm / m3 on average per 40-hour work week.
- Mercury hypersensitivity is estimated as less than 1 per 100,000,000 persons
- Indium-containing amalgams can have lower Hg vapor pressures than conventional dental amalgam
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
Tooth Polishing and Cleansing Agents
1. Cleansing-removal of exogenous stains, pellicle, materia alba, and other oral debris without causing undue abrasion to tooth structure
2. Polishing-smoothening surfaces of amalgam, composite, glass ionomers, porcelain, and other restorative materials
Factors influencing cleaning and polishing
- Hardness of abrasive particles versus substrate
- Particle size of abrasive particles
- Pressure applied during procedure
- Temperature of abrasive materials
Structure
Composition
-contain abrasives, such as kaolinite, silicon dioxide, calcined magnesium silicate, diatomaceous silicon dioxide, pumice. Sodium-potassium
-aluminum silicate, or zirconium silicate; some pastes also may contain sodium fluoride or stannous fluoride, but they have never been shown to produce positive effects
Reactions-abrasion for cleansing and polishing
Properties - Mechanical
- Products with pumice and quartz produce more efficient cleansing but also generate greater abrasion of enamel and dentin
-Coarse pumice is the most abrasive
-The abrasion rate of dentin is 5 to 6 times faster than the abrasion rate of enamel, regardless of the product
-Polymeric restorative materials, such as denture bases, denture teeth, composites, PMMA veneers, and composite veneers, can be easily scratched during polishing
-Do not polish cast porcelain restorations (e.g., Dicor) that are externally characterized or the color will be lost