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.

Manipulation

Selection

o    Microfilled composites or hybrids for anterior class III, IV, V
o    Hybrids or midifills for posterior class I, II, III, V

Conditioning of enamel and / or dentin

Do not apply fluorides before etching.-->Acid-etch --> Rinse for 20 seconds with water --> Air-dry etched area for 20 seconds but do not desiccate or dehydrate --> Apply bonding agent and polymerize

Mixing (if required)--> mix two pastes for 20 to 30 seconds

o    Self-cured composite-working time is 60 to 120 seconds after mixing
o    Light-cured composite-working time is unlimited (used for most anterior and some posterior composite restorations)
o    Dual-cured composite-working time is > 10 minutes
o    Two-stage cured composite-working time is >5 minutes

Placement

use plastic instrument or syringe --> Light curing --> Cure incrementally in <2 mm thick layers. Use matrix strip where possible to produce smooth surface and contour composite .Postcure to improve hardness
 

Manipulation

1. Selection-based on strength for models, casts, or dies

2. Mixing
(1)Proportion the water and powder 
(2) Sift powder into water in rubber mixing bowl
(3) Use stiff blade spatula to mix mass on side of bowl
(4) Complete mixing in 60 seconds

3. Placement

(1) Use vibration to remove air bubbles acquired through mixing
(2) Use vibration during placement to help mixture wet and flow into the impression

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

Mercury bioactivity

• Metallic mercury is the least toxic from and is absorbed primarily through the lungs rather than the GI tract or skin
• Mercury in the body may come from air, water, food. dental (a low amount). Or medical sources
•  Half life for mercury elimination from body is 55 days .-
• mercury toxicity is <50 µm / m³ on average per 40-hour work week.
• Mercury hypersensitivity is estimated as less than 1 per 100,000,000 persons
• Indium-containing amalgams can have lower Hg vapor pressures than conventional dental amalgam

Pit-and-Fissure Dental Sealants

Applications/Use

Occlusal surfaces of newly erupted posterior teeth
Labial surfaces of anterior teeth with fissures
Occlusal surfaces of teeth in older patients with reduced saliva flow (because low saliva increases the susceptibility to caries)

Types

Polymerization method

Self-curing (amine accelerated)
Light curing (light accelerated)

Filler content

Unfilled-most systems are unfilled because filler tends to interfere with wear away from self-cleaning occlusal areas(sealants are designed to wear away, except where there is no self-cleaning action a common misconception is that sealants should be wear resistant)

Components

Monomer-BIS-GMA with TEGDM diluent to facilitate flow into pits and fissures prior to cure
Initiator-benzoyl peroxide (in self-cured) and diketone (in light cured)
Accelerator-amine (In light cured)
Opaque filler-I % titanium dioxide. or other colorant to make the material detectable on tooth surfaces
Reinforcing filler-generally not added because wear resistance is not required within pits and fissures

Reaction-free radical reaction 

Manipulation

Preparation

Clean pits and fissures of organic debris. Do not apply fluoride before etching because it will tend to make enamel more acid resistant. Etch occlusal surfaces, pits, and fissures for 30 seconds (gel) or 60 seconds (liquid) with 37% phosphoric acid . Wash occlusal surfaces for 20 seconds. Dry etched area for 20 seconds with clean air spray. Apply sealant and polymerize

Mixing or dispensing

Self-cured-mix equal amounts of liquids in Dappen dish for 5 seconds with brush applicator. Light cured-dispense from syringe tips 
Placement

-pits, fissures, and occlusal surfaces  --> Allow 60 seconds for self-cured materials to set. 

Finishing

Remove unpolymerized and excess material .Examine hardness of sealant. Make occlusal adjustments where necessary in sealant; some sealant materials are self-adjusting

Properties

Physical

Wetting-low-viscosity sealants wet acid etched tooth structure the best

Mechanical

Wear resistance should not be too great because sealant should be able to wear off of  self-cleaning areas of tooth
Be careful to protect sealants during polishing procedures with air abrading units to prevent sealant loss

Clinical efficacy

Effectiveness is 100% if retained in pits and fissures .Requires routine clinical evaluation for resealing of areas of sealant loss attributable to poor retention .
Sealants resist effects of topical fluorides