NEET MDS Lessons
Dental Materials
Zinc Phoshate Cement
Uses. Zinc phosphate cement is used both as an intermediate base and as a cementing medium.
(1) Intermediate base. A thick mix is used under permanent metallic restoration. This layer of cement protects the pulp from sudden temperature changes that may be transmitted by the metallic restoration.
(2) Cementing medium. Zinc phosphate cement is used to permanently cement crowns, inlays, and fixed partial dentures upon the remaining tooth structure. A creamy mix of cement is used to seat the restoration or appliance completely into place. The cementing medium does not cement two objects together. Instead, the cement holds the objects together by mechanical interlocking, filling the space between the irregularities of the tooth preparation and the cemented restoration
c. Chemical Composition.
(1) Powder. primary ingredients - zinc oxide and magnesium oxide.
(2) Liquid. Phosphoric acid and water in the ratio of two parts acid to one part water. The solution may also contain aluminum phosphate and zinc phosphate Liquids exposed in open bottles will absorb moisture from the air in high humidity. The liquids will lose moisture if humidity is low. Water gain hastens setting; water loss lengthens setting time.
PROPERTIES OF ZINC PHOSPHATE CEMENT
a. Advantages. Some advantages of zinc phosphate cement as a cementing medium are:
o Inconspicuous appearance.
o Speed and ease of usage.
o Sufficient flow to form a thin layer for the cementing of closely adapted crowns, fixed partial dentures, and inlays.
o Low thermal conductivity beneath a metallic restoration.
b. Disadvantages. Some disadvantages of zinc phosphate cement as a cementing medium are:
o Low crushing strength that varies between 12,000 and 19,000 psi.
o Slight solubility in mouth fluids.
o Opaque material not suitable for visible surfaces.
c. Strength. The ratio of powder to liquid increases the strength of phosphate cements to a certain point. For this reason, the dental specialist must use as thick a mix as practical for the work being performed.
SETTING REACTIONS OF ZINC PHOSPHATE CEMENT
a. Chemical Reaction. The chemical reaction that takes place between the powder and liquid of setting phosphate cement produces heat. The amount of heat produced depends upon the rate of reaction, the size of the mix, and the amount of heat extracted by the mixing slab.
b. Powder to Liquid Ratio. The less powder used in ratio to the liquid, the longer the cement will take to harden. Good technique minimizes the rise in temperature and acidity of the setting cement that can injure the pulp. Generally, for increased strength, decreased shrinkage, and resistance to solubility, it is advisable to blend as much powder as possible to reach the desired consistencies.
c. Setting Time. The setting time of zinc phosphate cement is normally between 5 and 9 minutes.
Lower the temperature of the glass mixing slab to between 65° and 75° F (18° to 24° C), if the glass mixing slab is not already cooled below the temperature at which moisture will condense on it. → Blend the powder slowly. → Mix the powder over a large area of the cool slab. → Use a longer mixing time, within optimum limits.
Precautions. The following precautions should be observed.
o Prevent loss or gain of moisture in liquid cement by keeping bottles tightly stoppered.
o Dispense drops only when ready to mix.
o Use a cool, dry glass slab (65° to 75° F).
o Use the same brand of powder and liquid.
o Add increments of powder slowly.
o Use the maximum amount of powder to obtain the desired consistency.
(To incorporate the most powder, the material should be mixed with a moderate circular motion over a large area of the slab, turning the spatula often.)
Glass Ionomer Cements
Applications
a. Class V restorations-resin-modified glass ionomers for geriatric dentistry
b. Class II restorations-resin-modified glass ionomers, metal-modified glass ionomers in pediatric dentistry
c. Class III restorations-resin-modified glass ionomers
d. permanent cementing of inlays, crowns, bridges, and/or orthodontic band/brackets. In addition, it can be used as a cavity liner and as a base.
Classification by composition
a. Glass ionomer-limited use
b. Metal-modified glass ionomer-limited use
c. Resin-modified glass ionomer-popular use
Components
a. Powder-aluminosilicate glass
b. Liquid-water solution of copolymers (or acrylic acid with maleic, tartaric, or itaconic acids) and water-soluble monomers (e.g., HEMA)
Reaction (may involve several reactions and stages of setting)
a. Glass ionomer reaction (acid-base reaction of polyacid and ions released from aluminosilicate glass particles)
- Calcium, aluminum, fluoride, and other ions released by outside of powder particle dissolving in acidic liquid
- Calcium ions initially cross-link acid functional copolymer molecules
- Calcium cross-links are replaced in 24 to 48 hours by aluminum ion cross-links, with increased hardening of system
- If there are no other reactants in the cement (e.g., resin modification), then protection from saliva is required during the first 24 hours
b. Polymerization reaction (polymerization of double bonds from water-soluble monomers and/or pendant groups on copolymer to form cross-linked matrix)
- Polymerization reaction can be initiated with chemical (self-curing) or light-curing steps
- Cross-linked polymer matrix ultimately interpenetrates glass ionomer matrix
Manipulation
a. Mixing-powder and liquid components may be manually mixed or may be precapsulated for mechanical mixing
b. Placement-mixture is normally syringed into place
c. Finishing-can be immediate if system is resin-modified (but otherwise must be delayed 24 to 72 hours until aluminum ion replacement reaction is complete)
d. Sealing-sealer is applied to smoothen the surface (and to protect against moisture affecting the glass ionomer reaction)
Properties
1. Physical
-Good thermal and electrical insulation
-Better radiopacity than most composites
-Linear coefficient of thermal expansion and contraction is closer to tooth structure than for composites (but is less well matched for resin-modified systems)
-Aesthetics of resin-modified systems are competitive with composites
2. Chemical
-Reactive acid side groups of copolymer molecules may produce chemical bonding to tooth structure
-Fluoride ions are released
(1) Rapid release at first due to excess fluoride ions in matrix
(2) Slow release after 7 to 30 days because of slow diffusion of fluoride ions out of aluminosilicate particles
-Solubility resistance of resin-modified systems is close to that of composites
3. Mechanical properties
-Compressive strength of resin-modified systems is much better than that of traditional glass ionomers but not quite as strong as composites
- Glass ionomers are more brittle than composites
4. Biologic properties
- Ingredients are biologically kind to the pulp
- Fluoride ion release discourages secondary canes
INVESTING
Mixing investment with distilled water is done according to the manufacturers ratio in a clean dry bowl without entrapment of the air into the mix.
Mixing methods:
a. Hand mixing and the use of the vibrator to remove air bubbles.
b. Vacuum mixing- This is the better method because it removes air bubbles as well as gases that are produced and thus produces a smoother mix.
Methods of investing:
a. Hand investing
b. Vacuum investing
Hand investing:
First the mixed investment is applied on all the surfaces of the pattern with a soft brush. Blow off any excess investment gently, thus leaving a thin film of investment over the pattern, then apply again.
Then the coated pattern can be invested by two methods;
1. Placing the pattern in the ring first and then filling the ring full with investment.
2. Filling the ring with the investment first and then force the pattern through into it.
Vacuum investing :
Vacuum investing unit: This consists of the chamber of small cubic capacity from which air can be evacuated quickly and in which casting ring can be placed.
Evacuation of air can be done by electrically or water driven vacuum pump.
Procedure:
The ring filled with investment is placed in the vacuum chamber. Air entry tube is closed. Then the vacuum is applied. The investment will rise with froth vigorously for about 10-15 sec and then settles back. This indicates that air has been extracted from the ring. The pressure is now restored to atmospheric by opening the air entry tap gradually at first and then more rapidly as the investment settles back around the pattern. Then the ring is removed from the chamber and the investment is allowed to set. Modern investing unit does both mixing and investing under vacuum and is considered better than hand mixing and pouring.
Then there are two alternatives to be followed depending upon what type of expansion is to be achieved in order to compensate for metal shrinkage. They are:
1. If hygroscopic expansion of the investment is to be achieved then immediately immerse the filled ring in water at the temperature of 37C.
Or “under controlled water adding technique”. A soft flexible rubber ring is used instead of usual lined metal ring. Pattern is invested as usual. Then specified amount of water is added on top of the investment in the rubber ring and the investment is allowed to set at room temperature. In this way only enough water is added to the investment to provide the desired expansion.
2. If thermal expansion of the investment is to be achieved, then investment is allowed to set by placing the ring on the bench for 1 hour or as recommended by the manufacturer.
Physical reaction-cooling causes reversible hardening
Chemical reaction-irreversible reaction during setting
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
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
Properties
I. Physical
a. Excellent thermal and electrical insulator
b. Very dense
c. Excellent dimensional stability
d. Good reproduction of fine detail of hard and soft tissues
2. Chemical
a. Heating will reverse the reaction (decompose the material into calcium sulfate hemihydrate, the original dry component)
b. Models, casts, and dies should be wet during grinding or cutting operations to prevent heating
3. Mechanical
a. Better powder packing and lower water contents at mixing lead to higher compressive strengths (plaster < stone < diestone)
b. Poor resistance to abrasion
4. Biologic
a. Materials are safe for contact with external - epithelial tissues
b. Masks should be worn during grinding or polishing operations that are likely to produce gypsum dust