Mouth Protectors

Use - to protect against effects of blows to chin, top of the head, the face, or grinding of the teeth

Types

o    Stock protectors-least desirable because of poor fit
o    Mouth-formed protectors-improved fit compared with stock type
o    Custom-made protectors-preferred because of durability. low  speech impairment, and comfort

I. Components

a. Stock protectors-thermoplastic copolymer of PYA-PE (polyvinyl acetate-polyethylene copolymer)
b. Mouth-formed protectors-thermoplastic copolymer
c. Custom-made protectors- thermoplastic copolymer, rubber. or polyurethane
2. Reaction-physical reaction of hardening during cooling
3. Fabrication

Alginate impression made of maxillary arch. High-strength stone cast poured immediately. Thermoplastic material is heated in hot water and vacuum-molded to cast .

Mouth protector trimmed to within ½ inch of labial fold, clearance provided at the buccal and labial frena, and edges smoothed by flaming. Gagging, taste, irritation. and impairment of speech are minimized with properly fabricated appliances

4. Instructions for use

a. Rinse before and after use with cold water
b. Clean protector occasionally with soap and cool water
c. Store the protector  in a rigid container
d. Protect from heat and pressure during storage
e. Evaluate protector routinely for evidence of deterioration

Properties

1. Physical-thermal insulators
2. Chemical-absorbs after during use
3. Mechanical-tensile strength, modulus, and hardness decrease after  water absorption, but elongation, tear strength, and resilience increase
4. Biologic-nontoxic as long as no bacterial, fungal, or viral growth occurs on surfaces between uses
 

PFM Alloys

Applications-substructures for porcelain-fused-to-metal crowns and bridges
 
Classification
o    High-gold alloys
o    Palladium-silver alloys
o    Nickel-chromium alloys

Structure

Composition
o    High-gold alloys are 98% gold. platinum. And palladium
o    Palladium-silver alloys are 50% to 60% palladium and 30 to 40% silver
o    Nickel-chromium alloys are 70% to 80% nickel and 15% chromium with other metals

Manipulation
o    Must have melting temperatures above that of porcelains to be bonded to their surface
o    More difficult to cast (see section on chromium alloys)

Properties - Physical

Except for high-gold alloys, others are less dense alloys
Alloys are designed to have low thermal expansion coefficients that must be matched to the overlying porcelain

Chemical-high-gold alloys are immune, but others passivate

Mechanical-high modulus and hardness
 

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.

METALLURGICAL TERMS

a. Cold Working. This is the process of changing the shape of a metal by rolling, pounding, bending, or twisting at normal room temperature.

b. Strain Hardening. This occurs when a metal becomes stiffer and harder because of continued or repeated application of a load or force. At this point, no further slippage of the atoms of the metal can occur without fracture.

c. Heat Softening Treatment (Annealing). This treatment is necessary in order to continue manipulating a metal after strain hardening to prevent it from fracturing. The process of annealing consists of heating the metal to the proper temperature (as indicated by the manufacturer's instructions) and cooling it rapidly by immersing in cold water. Annealing relieves stresses and strains caused by cold working and restores slipped atoms within the metal to their regular arrangement.

d. Heat Hardening Treatment (Tempering). This treatment is necessary to restore to metals properties that are decreased by annealing and cold working. Metals to be heat hardened should first be heat softened (annealed) so that all strain hardening is relieved and the hardening process can be properly controlled. Heat hardening is accomplished in dental gold alloy by heating to 840^o Fahrenheit, allowing it to cool slowly over a 15-minute period to 480^o Fahrenheit, and then immersing it in water.

Stages of manipulation

Definitions of intervals

• Mixing interval-length of time of the mixing stage.
• Working interval-length of time of the working stage
•  Setting interval-length of time of the setting stage

Definitions of times

• Mixing time-the elapsed time from the onset to the completion of mixing
• Working time-the elapsed time from the onset of mixing until the onset of the initial setting time
• Initial setting time-time at which sufficient reaction has occurred  to cause the materials to be resistant to  further manipulation
• Final setting time-time at which the material practically is set as defined by its resistance to indentation

[All water-based materials lose their gloss at the time of setting]

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
 

Components 

a. Fillers added to most to control shrinkage
b. Matrix