NEET MDS Lessons
Dental Materials
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
ACID ETCH TECHNIQUE
Cavities requiring added retention (to hold firmly) are treated with an acid etching technique. This technique improves the seal of the composite resin to the cavity wall. The enamel adjacent to the margins of the preparation is slightly decalcified with a 40 to 50 percent phosphoric acid solution. This etched enamel enhances the mechanical retention of the composite resin. In addition, the acid etch technique is used to splint unstable teeth to adjacent teeth. The acid is left on the cut tooth structure only 15 seconds, in accordance with the directions for one common commercial brand. The area is then flushed with water for a minimum of 30 seconds to remove the decalcified material. Etched tooth structure will have a chalky appearance.
PFM Alloys
Applications-substructures for porcelain-fused-to-metal crowns and bridges
Classification
o High-gold alloys
o Palladium-silver alloys
o Nickel-chromium alloys
Structure
Composition
o High-gold alloys are 98% gold. platinum. And palladium
o Palladium-silver alloys are 50% to 60% palladium and 30 to 40% silver
o Nickel-chromium alloys are 70% to 80% nickel and 15% chromium with other metals
Manipulation
o Must have melting temperatures above that of porcelains to be bonded to their surface
o More difficult to cast (see section on chromium alloys)
Properties - Physical
Except for high-gold alloys, others are less dense alloys
Alloys are designed to have low thermal expansion coefficients that must be matched to the overlying porcelain
Chemical-high-gold alloys are immune, but others passivate
Mechanical-high modulus and hardness
Impression Material
|
Materials |
Type |
Reaction |
Composition |
Manipulation |
Initial setting time
|
|
Plaster |
Rigid |
Chemical |
Calcuim sulfate hemihydrate, water |
Mix P/L in bowl |
3-5 min
|
|
Compound |
Rigid |
Physical |
Resins, wax, stearic acid, and fillers |
Soften by heating
|
Variable (sets on cooling) |
|
Zinc oxide-eugonel |
Rigid |
Chemical |
Zinc oxide powder, oils, eugenol, and resin |
Mix pastes on pad
|
3-5 min
|
|
Agar-agar |
Flexible |
Physical |
12-15% agar, borax, potassium sulfate, and 85% water |
Mix P/L in bowl
|
Variable (sets on cooling)
|
|
alginate |
Flexible |
Chemical |
Sodium alginate, calcium sulfate, retarders, and 85% water |
Mix P/L in bowl
|
4-5 min
|
|
Polysulfide |
Flexible |
Chemical |
Low MW mercaptan polymer, fillers, lead dioxide, copper hydroxide, or peroxides |
Mix pastes on pad
|
5-7 min
|
|
Silicone |
Flexible |
Chemical |
Hydroxyl functional dimethyl siloxane, fillers, tin octoate, and orthoethyl silicate |
Mix pastes on pad
|
4.5 min
|
|
Polyether |
Flexible |
Chemical |
Aromatic sulfonic acid ester and polyether with ethylene imine groups |
Mix pastes on pad
|
2-4 min
|
|
Polyvinyl siloxane |
Flexible |
Chemical |
Vinyl silicone, filler, chloroplatinic acid, low MW silicone, and filler |
Mix putty or use two-component mixing gun
|
4-5 min
|
Investment Materials
Investment is mold-making material
Applications
a. Mold-making materials for casting alloys
b. Mold-making materials for denture production
Classification
a. Gypsum-bonded investments (based on gypsum products for matrix)
b. Phosphate-bonded investments
c. Silicate-bonded investments
Components
a. Liquid-water or other reactant starts formation of matrix binder by reacting with reactant powder
b. Powder-reactant powder, filler, or modifiers
Manipulation
a. P/L mixed and placed in container around wax pattern
b. After setting, the investment is heated to eliminate the wax pattern in preparation for casting
Denture Liners
Use - patients with soft tissue irritation
Types
Long-term liners (soft liners)-used over a period of months for patients with severe undercuts or continually sore residual ridges
Short-term liners (tissue conditioners)-used to facilitate tissue healing over several days
Structure
Soft liners-plasticized acrylic copolymers or silicone rubber
Tissue conditioners-PEMA plasticized with ethanol and aromatic esters
Properties
Liners flow under low pressure, allowing adaptation to soft tissues, but are elastic during chewing forces.
Low initial hardness, but liner becomes harder as plasticizers are leached out during intraoral use
Some silicone rubber liners support growth of yeasts
The Sprue :
Its a channel through which molten alloy can reach the mold in an invested ring after the wax has been eliminated. Role of a Sprue: Create a channel to allow the molten wax to escape from the mold. Enable the molten alloy to flow into the mold which was previously occupied by the wax pattern.
FUNCTIONS OF SPRUE
1 . Forms a mount for the wax pattern .
2 . Creates a channel for elimination of wax .
3 .Forms a channel for entry of molten metal
4 . Provides a reservoir of molten metal to compensate for the alloy shrinkage .
SELECTION OF SPRUE
Sprue former gauge selection is often empirical, is yet based on the following five general principles:
1. Select the gauge sprue former with a diameter that is approximately the same size as the thickest area of the wax pattern. If the pattern is small, the sprue former must also be small because a large sprue former attached to a thin delicate pattern could cause distortion. However if the sprue former diameter is too small this area will solidify before the casting itself and localized shrinkage porosity may result.
2. If possible the sprue former should be attached to the portion of the pattern with the largest cross-sectional area. It is best for the molten alloy to flow from the thick section to the surrounding thin areas. This design minimizes the risk of turbulence.
3. The length of the sprue former should be long enough to properly position the pattern in the casting ring within 6mm of the trailing end and yet short enough so the molten alloy does not solidify before it fills the mold.
4. The type of sprue former selected influences the burnout technique used. It is advisable to use a two-stage burnout technique whenever plastic sprue formers or patterns are involved to ensure complete carbon elimination, because plastic sprues soften at temperatures above the melting point of the inlay waxes.
5. Patterns may be sprued directly or indirectly. For direct sprueing the sprue former provides the direct connection between the pattern area and the sprue base or crucible former area. With indirect spruing a connector or reservoir bar is positioned between the pattern and the crucible former. It is common to use indirect spruing for multiple stage units and fixed partial dentures.