Properties-improve with filler content

Physical

Radiopacity depends on ions in silicate glass or the addition of barium sulfate (many systems radiolucent)
Coefficient of thermal expansion is 35 to 45 ppm/C and decreases with increasing filler content
Thermal and electrical insulators

Chemical

Water absorption is 0.5 % to 2.5% and increases with polymer level)
Acidulated topical fluorides (e.g., APF) tend to dissolve glass particles, and thus composites should be protected with petroleum jelly (Vaseline) during those procedures
Color changes occur in resin matrix with time because of oxidation, which produces colored by-products

Mechanical

Compressive strength is 45,000 to 60,000 lb/ in2, which is adequate
Wear resistance-improves with higher filler content, higher percentage of conversion in curing, and use of microfiller, but it is not adequate for some posterior applications
Surfaces rough from wear retain plaque and stain more readily

Biologic

Components may be cytotoxic, but cured composite is biocompatible as restorative filling material

Mechanical properties

1.  Resolution of forces

Uniaxial (one-dimensional) forces-compression, tension, and shear

Complex forces-torsion, flexion. And diametral

2. Normalization of forces and deformatations

Stress

 Applied force (or material’s resistance to force) per unit area

Stress-force/area (MN/m²)

Strain

Change in length per unit of length because of force

Strain-(L- L_o)/(L_o); dimensionless units

3. Stress-strain diagrams

Plot of stress (vertical) versus strain (horizontal)

• Allows convenient comparison of materials
• Different curves for compression, tension, and shear
• Curves depend on rate of testing and temperature

4. Analysis of curves

• Elastic behavior
  • Initial response to stress is elastic strain
  • Elastic modulus-slope of first part of curve and represents stiffness of material or the resistance to deformation under force
  • Elastic limit (proportional limit)- stress above which the material no longer behaves totally elastically
  • Yield strength-stress that is an estimate of the elastic limit at 0.002 permanent strain
  • Hardness-value on a relative scale that estimates the elastic limit in terms of a material’s resistance to indentation (Knoop hardness scale, Diamond pyramid, Brinnell, Rockwell hardness scale, Shore A hardness scale, Mohs hardness scale

• Resilience-area under the stress strain curve up to the elastic limit (and it estimates the total elastic energy that can be absorbed before the onset of plastic deformation)

• Elastic and plastic behavior

• Beyond the stress level of the elastic  limit, there is a combination of elastic  and plastic strain
• Ultimate strength-highest stress  reached before fracture; the ultimate compressive strength is greater than the ultimate shear strength and the ultimate tensile strength
• Elongation (percent elongation)- percent change in length up to the point of fracture = strain x 100%

• Brittle materials-<5% elongation at fracture

• Ductile materials->5% elongation  at fracture
• Toughness-area under the stress strain  curve up to the point of fracture (it estimates the total energy absorbed up to fracture)

• Time-dependent behavior

the faster a stress is applied, the more likely a material is to store the energy elastically and not plastically

• Creep-strain relaxation
• Stress relaxation

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
 

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 setMix drop of each paste together for 5 secondsApply 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

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
 

SELECTION OF SPRUE 

1 . DIAMETER :
It should be approximately the same size of the thickest portion of the wax pattern .
Too small sprue diameter suck back porosity results .

2 . SPRUE FORMER ATTACHMENT :
Sprue should be attached to the thickest portion of the wax pattern .
It should be Flared for high density alloys & Restricted for low density alloys .

3 . SPRUE FORMER POSITION

Based on the
1. Individual judgement .
2. Shape & form of the wax pattern .

Patterns may be sprued directly or indirectly .
Indirect method is commonly used