NEET MDS Lessons
Dental Materials
METALLURGICAL TERMS
a. Cold Working. This is the process of changing the shape of a metal by rolling, pounding, bending, or twisting at normal room temperature.
b. Strain Hardening. This occurs when a metal becomes stiffer and harder because of continued or repeated application of a load or force. At this point, no further slippage of the atoms of the metal can occur without fracture.
c. Heat Softening Treatment (Annealing). This treatment is necessary in order to continue manipulating a metal after strain hardening to prevent it from fracturing. The process of annealing consists of heating the metal to the proper temperature (as indicated by the manufacturer's instructions) and cooling it rapidly by immersing in cold water. Annealing relieves stresses and strains caused by cold working and restores slipped atoms within the metal to their regular arrangement.
d. Heat Hardening Treatment (Tempering). This treatment is necessary to restore to metals properties that are decreased by annealing and cold working. Metals to be heat hardened should first be heat softened (annealed) so that all strain hardening is relieved and the hardening process can be properly controlled. Heat hardening is accomplished in dental gold alloy by heating to 840o Fahrenheit, allowing it to cool slowly over a 15-minute period to 480o Fahrenheit, and then immersing it in water.
Stages of manipulation
Definitions of intervals
- Mixing interval-length of time of the mixing stage.
- Working interval-length of time of the working stage
- Setting interval-length of time of the setting stage
Definitions of times
- Mixing time-the elapsed time from the onset to the completion of mixing
- Working time-the elapsed time from the onset of mixing until the onset of the initial setting time
- Initial setting time-time at which sufficient reaction has occurred to cause the materials to be resistant to further manipulation
- Final setting time-time at which the material practically is set as defined by its resistance to indentation
[All water-based materials lose their gloss at the time of setting]
CRUCIBLE FORMER
It serves as a base for the casting ring during investing .Usually convex in shape.
May be metal , plastic or rubber .
Shape depends on casting machine used .
Modern machines use tall crucible to enable the pattern to be positioned near the end of the casting machine .
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.
Properties of Acrylic Resins.
- They have a low thermal conductivity. These resins are not easily washed out by the acids of the oral cavity (low solubility). Acrylic resins are also resilient, which allows them to be used in stress-bearing areas.
- Acrylic resins exhibit a moderate shrinkage of from 3 to 8 percent. This shrinkage and low marginal strength can lead to marginal leakage. Acrylic resins have a low resistance to wear. Acrylic resins cannot be used over a zinc oxide and eugenol-type base because eugenol interferes with the acrylic curing process.
- Mixing. Insufficient mixing will cause an uneven color or streaks in the mixture. Overmixing will cause the material to harden before it can be placed
- Poor distortion resistance at higher temperatures, therefore dentures should not be cleaned in hot water
- Good resistance to color change
- Absorbs water and must be kept hydrated (stored in water when not in mouth) to prevent dehydration cycling and changes in dimensions
- Not resistant to strong oxidizing agents
- Low strength; however, flexible, with good fatigue resistance
- Poor scratch resistance; clean tissue-bearing surfaces of denture with soft brush and do not use abrasive cleaners
Mercury hygiene
- Do not contact mercury with skin
- Clean up spills to minimize mercury vaporization
- Store mercury or precapsulated products in tight containers
- Only triturate amalgam components-in tightly- sealed capsules
- Use amalgam with covers
- Store spent amalgam under water or fixer in a tightly sealed jar
- Use high vacuum suction during amalgam alloy placement, setting, or removal when mercury may be vaporized
- Polishing amalgams generally causes localized melting of silver-mercury phase with release of mercury vapor, so water cooling and evacuation must be used
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
Reaction
a. Calcium sulfate hemihydrate(one-half water) crystals dissolve and react with water
b. Calcium sulfate dihydrate(two waters) form and precipitate new crystals
c. Unreacted (excess) water is left between crystals in solid