Solution Liners (Varnishes)

Applications 

o    Enamel and dentin lining for amalgam restorations
o    Enamel and dentin lining for cast restorations that are used with non adhesive cements
o    Coating over materials that are moisture sensitive during setting

Components of copal resin varnish

o    90% solvent mixture (e.g., chloroform, acetone, and alcohol)
o    10% dissolved copal resin

 Reaction
 
Varnish sets physically by drying Solvent loss occurs in 5 to 15 seconds (a film forms the same way as drying fingernail polish)

Manipulation

Apply thin coat over dentin. enamel. And margins of the cavity preparation  Dry lightly with air for 5 seconds Apply a second thin coat Final thickness is 1 to 5 µ.m

Properties

o    Physical 

Electrically insulating barrier that prevents shocks. Too thin to be thermally insulating. Decreases degree of percolation attributable to thermal expansion

o    Chemical

Forms temporary barrier that prevents microleakage into dentinal tubules until secondary dentin formation occurs. Decreases initial tendency for electrochemical corrosion

o     Mechanical

Very weak and brittle film that has limited lifetime 
Film adheres to smear layer
 

Chromium Alloys for Partial Dentures

Applications - Casting partial denture metal frameworks

Classification

a. Cobalt-chromium
b. Nickel-chromium
c. Cobalt-chromium-nickel

Composition

a. Chromium-produces a passivating oxide film for corrosion resistance
b. Cobalt-increase~ the rigidity of the alloy
c. Nickel-increases the ductility of the alloy
d. Other elements-increase strength and castability

Manipulation

a. Requires higher temperature investment materials
b. More difficult to cast because less dense than gold alloys usually requires special casting equipment
c. Much more difficult to finish and polish because of higher strength and hardness

Properties

a. Physical-less dense_than gold alloys
b. Chemical-passivating corrosion behavior
c. Mechanical-stronger. stiffer. and harder than gold alloys
d. Biologic

-Nickel may cause sensitivity in some individuals (I % of men and 11 % of women)
-Beryllium in some alloys forms oxide that  is toxic to lab technicians

Spruing Technique:

Direct Spruing:

The flow of the molten metal is straight(direct) from the casting crucible to pattern area in the ring. Even with the ball reservoir, the Spruing method is still direct. A basic weakness of direct Spruing is the potential for suck-back porosity at the junction of restoration and the Sprue.

Indirect Spruing:

Molten alloy does not flow directly from the casting crucible into the pattern area, instead the alloy takes a circuitous (indirect) route. The connector (or runner) bar is often used to which the wax pattern Sprue formers area attached. Indirect Spruing offers advantages such as greater reliability & predictability in casting plus enhanced control of solidification shrinkage .The Connector bar is often referred to as a “reservoir .

Armamentarium :
1 . Sprue
2 . Sticky wax
3 . Rubber crucible former
4 . Casting ring 
5 . Pattern cleaner 
6 . Scalpel blade & Forceps 
7 . Bunsen burner

Solution Liners (Varnishes)

Applications 

o    Enamel and dentin lining for amalgam restorations
o    Enamel and dentin lining for cast restorations that are used with non adhesive cements
o    Coating over materials that are moisture sensitive during setting

Components of copal resin varnish

o    90% solvent mixture (e.g., chloroform, acetone, and alcohol)
o    10% dissolved copal resin

Reaction
 
Varnish sets physically by drying → Solvent loss occurs in 5 to 15 seconds (a film forms the same way as drying fingernail polish)

Manipulation

Apply thin coat over dentin. enamel. And margins of the cavity preparation → Dry lightly with air for 5 seconds Apply a second thin coat → Final thickness is 1 to 5 µ.m

Properties

o    Physical 

Electrically insulating barrier that prevents shocks. Too thin to be thermally insulating. Decreases degree of percolation attributable to thermal expansion

o    Chemical

Forms temporary barrier that prevents microleakage into dentinal tubules until secondary dentin formation occurs. Decreases initial tendency for electrochemical corrosion

o     Mechanical

Very weak and brittle film that has limited lifetime 
Film adheres to smear layer

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
 

PHYSICAL PROPERTIES OF MATERIALS

Definite and precise terms are used to describe the physical properties of dental materials.

a. Hardness. Hardness is the measure of the resistance of a metal to indentation or scratching. It is an indication of the strength and wearability of an alloy or metal.

b. Ductility. Ductility is the measure of the capacity of a metal to be stretched or drawn by a pulling or tensile force without fracturing. This property permits a metal to be drawn into a thin wire.

c. Malleability. Malleability is the measure of the capacity of a metal to be extended in all directions by a compressive force, such as rolling or hammering. This property permits a metal to be shaped into a thin sheet or plate.

d. Flexibility and Elasticity. These terms differ in their technical definition but they are very closely related. Flexibility is the characteristic of a metal, which allows it to deform temporarily. The elasticity of a metal is used when it returns to its original shape when the load or force is removed.

e. Fatigue. Fatigue is the property of a metal to tire and to fracture after repeated stressing at loads below its proportional limit.

f. Structure (Crystalline or Grain Structure). Metals are crystalline and many of their physical properties depend largely upon the size and arrangement of their minute crystals called grains.

(1) Grain size. The size of the grains in a solidified metal depends upon the number of nuclei of crystallization present and the rate of crystal growth. In the practical sense, the faster a molten is cooled to solidification, the greater will be the number of nuclei and the smaller will be the grain size. Generally speaking, small grains arranged in an orderly fashion give the most desirable properties.

(2) Grain shape. The shape of the grains is also formed at the time of crystallization. If the metal is poured or forced into a mold before cooling, the grains will be in a flattened state. Metal formed by this method is known as cast metal. If the metal is shaped by rolling, bending, or twisting, the grains are elongated and the metal becomes a wrought wire.

g. Crushing Strength. Crushing strength is the amount of resistance of a material to fracture under compression.

h. Thermal Conductivity. Thermal conductivity is defined as the ability of a material to transmit heat or cold. A low thermal conductivity is desired in restorative materials used on the tooth whereas a high thermal conductivity is desirable where the material covers soft tissue.