NEET MDS Lessons
Dental Materials
Wax elimination (burnout):
Wax elimination or burnout consists of heating the investment in a thermostatically controlled furnace until all traces of the wax are vaporized in order to obtain an empty mold ready to receive the molten alloy during procedure.
• The ring is placed in the furnace with the sprue hole facing down to allow for the escape of the molten wax out freely by the effect of gravity .
• The temperature reached by the investment determines thethermal expansion. The burnout temperature is slowly increased in order to eliminate the wax and water without cracking the investment.
•For gypsum bonded investment, the mold is heated to650 -6870 c )to cast precious and semiprecious
precious alloys.
• Whereas for phosphate-bonded investment, the mold is heated up to 8340 c to cast nonprecious alloys at high fusing temperature.
The ring should be maintained long enough at the maximum temperature (“heat soak”) to minimize a sudden drop in temperature upon removal from the oven. Such a drop could result in an incomplete casting because of excessively rapid solidification of thealloy as it enters the mold.
• When transferring the casting ring to casting, a quick visual check of the sprue in shaded light is helpful to see whether it is properly heated. It should be a cherry-red color .
WAX BURNOUT AND HEATING THE RING
After the investment has set hard, the crucible former and the metal sprue former is removed carefully, and any loose particles at the opening of the sprue hole are removed with small brush.
The purpose of the wax burnout is to make room for the liquid metal. The ring is placed in the oven at 250C with the sprue end down, thus allowing the melted wax to flow, out for 30min or even up to 60min may be a good procedure to ensure complete elimination of the wax and the carbon.
Heating the ring: The object is to create a mold of such dimension, condition and temperature so that it is best suited to receive the metal.
Hygroscopic Low-Heat Technique.
After the wax elimination the temperature of the same furnace can be set to a higher temperature for heating or else, the ring can be transferred to another furnace, which has already set to the higher temperature. In any case accurate temperature control is essential and therefore these furnaces have pyrometer and thermocouple arrangement. The ring is placed in the furnace with the sprue hole down and heated to 500C and kept at this temperature for 1 hour. In this low heat technique the thermal expansion obtained is less but together with the previously obtained hygroscopic expansion the total expansion amounts to 2.2 percent, which is slightly higher than what is required for gold alloys.
So this technique obtains its compensation expansion from three sources:
(1) The 37º C water bath expands the wax pattern
(2) The warm water entering the investment mold from the top adds some hygroscopic expansion
(3) The thermal expansion at 500' C provides the needed thermal expansion.
High-Heat Thermal Expansion Technique.
After the wax elimination, the ring should be placed in the furnace which is at room temperature and then the temperature is gradually raised, until it comes to 700C in 1 hour. Then the ring is heat soaked at this temperature for ½ hour. This slow rise in temperature is necessary to prevent
This approach depends almost entirely on high-heat burnout to obtain the required expansion, while at the same time eliminating the wax pattern. Additional expansion results from the slight heating of gypsum investments on setting, thus expanding the wax pattern, and the water entering the investment from the wet liner, which adds a small amount of hygroscopic expansion to the normal setting expansion.
Acrylic Denture Bases
Use - used to support artificial teeth
Classification
a. PMMA/MMA dough systems
b. PMMA/MMA pour resin systems
1. Components
a. Powder-PMMA polymer, peroxide initiator, and pigments
b. Liquid-MMA monomer, hydroquinone inhibitor, and cross-linking agents
2. Reaction
a. Heat (or chemicals) is used as an accelerator to decompose peroxide into free radicals
b. Free radicals initiate polymerization of MMA into PMMA
c. New PMMA is formed as a matrix around residual PMMA powder particles
d. Linear shrinkage is 5% to 7% of monomer on polymerization
3. Manipulation
a. P/L mixed to form dough or fluid resin to fill mold
b. Mold heated to start and control reaction
Principles of cutting, polishing, and surface cleaning
- Surface mechanics for materials
Cutting-requires highest possible hardness materials to produce cutting
Finishing-requires highest possible hardness materials to produce finishing, except at margins of restorations where tooth structure may be inadvertently affected
Polishing- requires materials with Mohs ./ hardness that is 1 to 2 units above that of substrate
Debriding-requires materials with Mohs hardness that is less than or equal to that of substrate to prevent scratching
- Factors affecting cutting, polishing. and surface cleaning
- Applied pressure
- Particle size of abrasive
- Hardness of abrasive
- Hardness of substrate
- Precautions
- During cutting heat will build up and change the mechanical behavior of the substrate from brittle to ductile and encourage smearing
- Instruments may transfer debris onto the cut surface from their own surfaces during cutting, polishing, or cleaning operations (this is important for cleaning implant surfaces)
Casting of glass or ceramic
A castable ceramic is prepared in a similar manner as metal cast preparation .
Glass is heated to 1360 degrees & then cast.
Phosphate bonded investments are used for this purpose .
ZINC OXIDE AND EUGENOL
This material is used for many dental purposes ranging from temporary restorative material to pulp capping. The material is composed of a powder that is basically zinc oxide and a liquid that is called eugenol.
Chemical Composition.
The powder must contain between 70 and 100 percent zinc oxide. The manufacturer may add hydrogenated resins to increase strength and zinc acetate to hasten the set.
Eugenol is usually derived from oil of cloves. The oil of cloves contains more eugenol (82 percent) Eugenol is an obtundent (pain-relieving agent). It is a clear liquid that gradually changes to amber when exposed to light.
Physical Properties.
This material relieves pain, makes tissue less sensitive to pain, is slightly antiseptic, and is low in thermal conductivity. It provides a good marginal seal when placed in tooth cavities. The crushing strength (compression strength) of pure zinc oxide and eugenol is about 2,000 psi, which is low in comparison to other cements. The addition of hydrogenated resin increases the crushing strength to 5,000 psi.
CLINICAL USES OF ZINC OXIDE AND EUGENOL
Treatment Restoration. It helps prevent pulpal irritation in carious teeth, lost restorations, advanced caries, or pulpitis. This dental material also exerts a palliative effect on the pulp.
Temporary Cementing Medium. Zinc oxide and eugenol is used as a temporary cementing medium for crowns, inlays, and fixed partial dentures.
Intermediate Base. Zinc oxide and eugenol is used as an intermediate base. This material provides insulation between metallic restorations and vital tooth structure. Because of the low crushing strength, its use is sometimes contraindicated.
Surgical Packing or Dressing. The surgical dressing applied and adapted over the gingival area after a gingivectomy. This dressing protects the area and makes the tissue less sensitive.
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
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
|