Operative Lecture 8: Polymerization Shrinkage

What are the three steps after placing a composite?

1. Shape

2. Light cure

3. Lock

What is using light energy to turn soft paste into hard plastic called?

Light curing

What occurs to the shape of the composite during curing?

Shrinkage

Rank the three photoinitiators from shortest to longest wavelength

1. Lucerin

2. PPD

3. Camphorquinone

Which light curing unit has the shortest range of wavelengths?

Conventional LED

What is generated when photoinitiators are activated by a light source?

Reactive species (Free radicals)

What do reactive species initiate in light curing?

Polymer chain formation

How do polymer chains grow in light curing?

Sequential addition of monomer units

What are the four types of light curing units?

1. QTH

2. LED

3. Plasma arc

4. Laser units

What are the four disadvantages of using QTH curing lights?

1. Heavy

2. High energy consumption

3. Heat generation

4. Loud

What is the benefit of using QTH curing lights?

Wide spectrum

What is the tip shape for QTH curing lights?

Big and long

Why aer halogen curing lights not used anymore?

Does not cure photoinitiators with short wavelengths efficiently

What two traits make LED curing lights efficient?

1. Minimum energy consumption

2. Minimum heat generation

What are LED curing lights usually powered by?

Batteries

What is the tip shape for LED curing lights?

Short and thin

What are the four names for LED curing units that emit photons at two or more bands?

1. Polywave

2. Wide-band

3. Broad-band

4. Multi-chip

What is the term that measures how powerful the flashlight is?

Radiant power (flux)

What is the term for how much light energy is leaving curing tip surface?

Radiant exitance (emittance)

What is the term for the amount of light energy that hits composite?

Irradiance

What is the term for how much energy curing light emits at each wavelength?

Spectral radiant power

What are the three possible fates of photons when emitted from a light cure unit?

1. Lost

2. Reflected

3. Attenuated

What can some photons be lost in light curing?

Divergence angle of light beam

What are the two main processes that attenuate photons in light curing?

1. Absorption by photoinitiators

2. Scattering by the pigments and fillers

What angle do you want for light curing?

Perpendicular to tooth

What are the five factors that the depth of cure (polymerization) depends on?

1. Time of cure

2. Intensity

3. Distance between light source and resin

4. Increment thickness

5. Material related factors

What is the minimum intensity needed for light curing?

300 mW/cm²

What is the maximum increment thickness for light curing?

2 mm

What is the percentage of carbon double bonds converted into single bonds called? (Monomer to polymer)

Degree of conversion

What does the improvement of mechanical properties from increasing degree of conversion result in?

More color stability

What type of curing causes the highest rate of conversion?

Heat

After curing the composite for the optimal time, will over curing increase the degree of conversion?

No

What are the four negative effects of undercuring?

1. Low mechanical properties

2. Less color stability

3. Leaching of unreacted monomer

4. Possible recurrent caries

What are the three negative effects of overcuring?

1. Heat generation

2. Pulpal and soft tissue irritation

3. Longer chair time

Which type of photoinitiator has a simple bond-cleavage mechanism?

Norrish Type I

What type of photoinitiator is Camphorquinone?

Norrish Type II

What are the four steps for free-radical polymerization with photoinitiators?

1. Activation with light absorption

2. Free-radicals react with monomer

3. Polymer network formation

4. Termination of free radicals

What color is Camphorquinone?

Yellow-ish

Why can residual amines when using Camphorquinone cause color instability of RBCs?

Residual amines turn yellow over time

What forces initially hold close monomer molecules together?

Van-der Waals forces

What bonds are formed when monomers come closer together when being cured?

Covalent

What are the four phases of the polymerization process?

1. Autoacceleration

2. RP max

3. Autodeceleration

4. Limiting conversion

What is the transition between pre-gelation and post-gelation phases called?

Gel point

In the visco-plastic (pre-gelation) phase, what two things are taking place?

1. Chain growth

2. Plastic deformation

In the stress-building (post-gelation) phase, what two things are taking place?

1. Chain growth

2. More cross-linking

Is there a linear or exponential increase in stress as conversion increases?

Exponential

Which composites does shrinking occur in?

All composites

When does shrinking stress occur?

When composite is bonded to tooth

What is the ratio between the number of bonded walls to the unbonded ones?

C-factor

Does a higher C-factor indicate higher or lower stress?

Higher

What are the four advantages of the incremental technique?

1. Appropriate contact tightness

2. Optional restoration stains

3. Simulates natural enamel

4. Adequate curing energy for each increment

What are the three disadvantages of the incremental technique?

1. Time

2. Air bubbles

3. No evidence of reducing stress

What are the six consequences of shrinkage stress?

1. Marginal staining

2. Microleakage

3. Secondary caries

4. Enamel crack

5. Internal gap

6. Sensitivity

What should inspection clean up with adequate light be checked with?

Radiometer

What should be used to prevent cross contamination during curing?

Plastic sleeves

What should always be worn when using curing lights?

Amber eye protection

Why are multi-wave LEDs preferred?

Activates all currently used photo-initiators

How should compromised access and darker shades of composite be compensated by?

Increasing curing time

What are the two ways of reducing shrinkage stress related to material modification?

1. Increase filler loading

2. Bulk-fill composite

What are the three ways of reducing shrinkage stress related to stess-absorbing layers?

1. Adhesives

2. Low viscosity composites

3. RMGI

Why does increasing filler loading decrease shrinkage stress?

Reduces amount of resin used (less shrinkage)

What is the composite that can be placed without using an incremental technique?

Bulk-fill composite

What are the four benefits of using Bulk-fill composite?

1. Allowing deeper curing in single layer

2. Reducing shrinkage stress

3. Avoiding voids

4. Saving time

What is the photoinitiator for bulk-fill composite?

Ivocerin

What are the two differences of Ivocerin to camphorquinone?

1. Higher potency

2. Lower peak wavelength

What are the two main distinctive features of bulk-fill composites?

1. Increased translucency

2. Increased depth of cure

What is needed to finish off filling with flowable composites?

Capping with regular or bulk-fill composites

What are the three limitations of bulk-fill composites?

1. Limited long-term data

2. Esthetic limitations

3. Varying material properties

What kind of cure has applied energy remaining constant throughout the curing time?

Normal

What kind of cure has applied energy starting low then increasing to full intensity?

Soft-start

What kind of cure has applied energy gradually increasing from low to high intensity over time?

Ramp

What kind of cure has applied energy delivered in intermittent pulses with delays between applications?

Pulse delay

What are the two types of stress-absorbing liners?

1. Flowable composite

2. Resin modified glass ionomer

Which wall/floor can stress-absorbing liners be used on by itself?

Gingival floor

Which wall/floors must apply stress-absorbing liners be used on both?

Pulpal and axial walls

What technique is used with resin modified glass ionomer as a stress-absorbing liner?

Open sandwich

What is the benefit of RMGI?

Less moisture sensitive

Why is RMGI not sensitive to light curing?

Can undergo chemical curing parallel to light curing