NEET MDS Lessons
Dental Materials
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.
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
Manipulation
Mixture of powder and liquid is painted onto working cast to create shape for acrylic appliance à After curing of mixture, the shape and fit are adjusted by grinding with burrs and stones with a slow-speed handpiece .Acrylic dust is irritating to epithelial tissues of nasopharynx and skin and may produce allergic dermatitis or other reactions. Grinding may heat polymer to temperatures that depolymerize and release monomer vapor. which may be an irritant
Temporary Filling Materials
Applications / Use
While waiting for lab fabrication of cast restoration
While observing reaction of pulp tissues
Objectives
Provide pulpal protection
Provide medication to reduce pulpal inflammation
Maintain the tooth position with an aesthetic restoration
Classification
Temporary filling cements
Temporary filling resins
Components
Temporary filling cements
1. Zinc oxide-eugenol cement with cotton fibers added
2. Polyme r powder-reinforced zinc oxide eugenol cement
Temporary filling resins
• MMA / PMMA filling materials
• Polyamide filling materials
• BIS-GMA filling materials
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
Cement liners
Applications (if remaining dentin thickness is <0.5 mm)
o Used for thermal insulation where cavity preparation is close to the pulp
o Used for delivering medicaments to the pulp
• Calcium hydroxide stimulates reparative dentin or
• Eugenol relieves pain by desensitizing nerves
• Used to deliver F ion to enamel and dentin
Components
o Paste of calcium hydroxide reactant powder, ethyl toluene sulfonamide dispersant, zinc oxide filler, and zinc stearate radiopacifier
o Paste of glycol salicylate reactant liquid, titanium dioxide filler powder, and calcium tungstenate radiopacifier
Reaction
Chemical reaction of calcium ions with salicylate to form methylsalicylate salts Moisture absorbed to allow calcium hydroxide to dissociate into ions to react with salicylate Mixture sets from outside surface to inside as water diffuses
Manipulation
Dentin should not be dehydrated or material will not setMix drop of each paste together for 5 secondsApply material to dentin and allow I to 2 minutes to set
Properties
o Physical-good thermal and electrical insulator
o Chemical-poor resistance to water solubility and may dissolve
o Mechanical-low compressive strength (100 to 500 psi)
o Biologic-releases calcium hydroxide constituents, which diffuse toward the pulp and stimulate
o reparative dentin formation
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