NEET MDS Lessons
Dental Materials
Model. Cast. and Die Materials
Applications
- Gold casting, porcelain and porcelain-fused–to metal fabrication procedures
- Orthodontic and pedodontic appliance construction
- Study models for occlusal records
Terms
a. Models-replicas of hard and soft tissues for study of dental symmetry
b. Casts-working replicas of hard and soft tissues for use in the fabrication of appliances or restorations
c. Dies :- working replicas of one tooth (or a few teeth) used for the fabrication of a restoration
d. Duplicates-second casts prepared from original casts
Classification by materials
a Models :- (model plaster or orthodontic stone; gypsum product)
b. Stone casts (regular stone; gypsum product)
c. Stone dies (diestone; gypsum product)-may electroplated
d. Epoxy dies (epoxy polymer)-abrasion-resistant dies
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)
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
Suspension liners
Applications
o Dentin lining under amalgam restorations
o Stimulation of reparative dentin formation
Components
-Calcium hydroxide powder
-Water
-Modifiers
Manipulation
Used as W/P or pastes Paint thin film on dentin → Use forced air for 15 to 30 seconds to dry → Film is thicker (15 µm) than varnishes → Do not use on enamel or cavosurface margins
Properties
Physical
-Electrically insulating barrier
-Too thin to be thermally insulating
Chemical
-High basicity for calcium hydroxide (pH is II)
-Dissolves readily in water and should not be used at exposed cavosurface margins or gaps may form
Mechanical - weak film
Biologic - calcium hydroxide dissolves, diffuses, and stimulates odontoblasts to occlude dentin tubules below cavity preparation
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
COMPOSITE RESINS
Applications / Use
- Anterior restorations for aesthetics (class III, IV, V, cervical erosion abrasion lesions)
- Low-stress posterior restorations (small class I, II)
- Veneers
- Cores for cast restorations
- Cements for porcelain restorations
- Cements for acid-etched Maryland bridges
- Repair systems for composites or porcelains
Polymerization--reaction of small molecules (monomers) into very large molecules (polymers)
Cross-linking-tying together of polymer molecules by chemical reaction between the molecules to produce a continuous three-dimensional network
POLISHING MATERIALS
1 Tin Oxide. Tin oxide is used in polishing teeth and metal restorations. Tin oxide is a fine, white powder that is made into a paste by adding water or glycerin.
2. Pumice. Pumice is used as an abrasive and polishing agent for acrylic resins, amalgams, and gold. It consists mainly of complex silicates of aluminum, potassium, and sodium. Two grades--flour of pumice and coarse pumice--are listed in the Federal Supply Catalog.
3. Chalk (Whiting). Chalk is used for polishing acrylic resins and metals. It is composed primarily of calcium carbonate.
4.Tripoli. Tripoli is usually used for polishing gold and other metals. It is made from certain porous rocks.
5. Rouge (Jeweler's). Rouge is used for polishing gold and is composed of iron oxide. It is usually in cake or stick form.
6. Zirconium Silicate. Zirconium silicate is used for cleaning and polishing teeth. It may be mixed with water or with fluoride solution for caries prevention treatment. For full effectiveness, instructions must be followed exactly to obtain the proper proportions of powder to liquid.