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.
Casting Alloys
Applications-inlay, onlay, crowns, and bridges
Terms
a. Precious-based on valuable elements
b. Noble or immune-corrosion-resistant element or alloy
c. Base or active-corrosion-prone alloy
d. Passive -corrosion resistant because of surface oxide film
e. Karat (24 karat is 100% gold; 18 karat is 75% gold)
f. Fineness (1000 fineness is I00% gold; 500 fineness is 50% gold)
Classification
High-gold alloys are > 75% gold or other noble metals
Type 1- 83% noble metals (e.g., in simple inlays)
Type II-≥78% noble metals (e.g.,in inlays and onlays)
Type IlI-≥75% noble metals (e.g., in crowns and bridges)
Type IV-≥75% noble metals (e.g., in partial dentures)
Medium-gold alloys are 25% to 75% gold or other noble metals
Low-gold alloys are <25% gold or other noble metals
Gold-substitute alloys arc alloys not containing gold
(1) Palladium-silver alloys-passive .because of mixed oxide film
(2) Cobalt-chromium alloys-passive because of Cr203 oxide film
(3) Iron-chromium alloys-passive because of Cr203 oxide film
Titanium alloys are based on 90% to 100% titanium ; passive because of TiO2 oxide film
Components of gold alloys
- Gold contributes to corrosion resistance
- Copper contributes to hardness and strength
- Silver counteracts orange color of copper
- Palladium increases melting point and hardness
- Platinum increases melting point
- Zinc acts as oxygen scavenger during casting
Manipulation
- Heated to just beyond melting temperature for casting
o Cooling shrinkage causes substantial contraction
Properties
Physical
- Electrical and thermal conductors
- Relatively low coefficient of thermal expansion
Chemical
- Silver content affects susceptibility to tarnish
- Corrosion resistance is attributable to nobility or passivation
Mechanical
- High tensile and compressive strengths but relatively weak in thin sections, such as margins, and can be deformed relatively easily
- Good wear resistance except in contact with Porcelain
Denture Cleansers
Use - for removal of soft debris by light brushing and then rinsing of denture; hard deposits require professional repolishing
a. Alkaline perborates-do not remove bad stains; may harm liners .
b. Alkaline peroxides-harmful to denture liners
c. Alkaline hypochlorites-may cause bleaching, corrode base-metal alloys, and leave residual taste on appliance
d. Dilute acids-may corrode base-metal alloys
e. Abrasive powders and creams-can abrade denture surfaces
Denture cleaning Method
a. Full dentures without soft liners-immerse denture in solution of one part 5% sodium hypochlorite in three parts of water
b. Full or partial dentures without soft-liners immerse denture in solution of 1 teaspoon of hypochlorite with 2 teaspoons of glassy phosphate in a half of a glass of water
c. Lined dentures -- clean any soft liner with a cotton swab and cold water while cleaning the denture with a soft brush
Properties
1. Chemical-can swell plastic surfaces or corrode metal frameworks
2. Mechanical-can scratch the surfaces of denture bases or denture teeth
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
POLYCARBOXYLATE CEMENT
Use:. The primary use of polycarboxylate cement is as a cementing medium of cast alloy and porcelain restorations. In addition, it can be used as a cavity liner, as a base under metallic restorations, or as a temporary restorative material.
Clinical Uses
Polycarboxylate cement is used in the same way as zinc phosphate cement, both as an intermediate base and as a cementing medium.
c. Chemical Composition.
(1) Powder:. It generally contains zinc oxide, 1 to 5 percent magnesium oxide, and 10 to 40 percent aluminum oxide or other reinforcing fillers. A small percentage of fluoride may be included.
(2) Liquid. Polycarboxylate cement liquid is approximately a 40 percent aqueous solution of polyacrylic acid copolymer with other organic acids such as itaconic acid. Due to its high molecular weight, the solution is rather thick (viscous).
d. Properties.
The properties of polycarboxylate cement are identical to those of zinc phosphate cement with one exception. Polycarboxylate cement has lower compressive strength.
e. Setting Reactions:
The setting reaction of polycarboxylate cement produces little heat. This has made it a material of choice. Manipulation is simpler, and trauma due to thermal shock to the pulp is reduced. The rate of setting is affected by the powder-liquid ratio, the reactivity of the zinc oxide, the particle size, the presence of additives, and the molecular weight and concentration of the polyacrylic acid. The strength can be increased by additives such as alumina and fluoride. The zinc oxide reacts with the polyacrylic acid forming a cross-linked structure of zinc polyacrylate. The set cement consists of residual zinc oxide bonded together by a gel-like matrix.
Precautions.
The following precautions should be observed.
o The interior of restorations and tooth surfaces must be free of saliva.
o The mix should be used while it is still glossy, before the onset of cobwebbing.
o The powder and liquid should be stored in stoppered containers under cool conditions. Loss of moisture from the liquid will lead to thickening.
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
Structure of gypsum products
Components
a. Powder (calcium sulfate hemihydrate = CaSO4½H2O)
b. Water (for reaction with powder and dispersing powder)