NEET MDS Lessons
Dental Materials
Reaction
PMMA powder makes mixture viscous for manipulation before curing. Chemical accelerators cause decomposition of benzoyl peroxide into free radicals that initiate polymerization of monomer
New PMMA is formed into a matrix that surrounds PMMA powder. Linear shrinkage of 5% to 7% during setting. but dimensions of appliances are not critical
Classification of Dental amalgam
1. By powder particle shape .
- Irregular (comminuted, filing, or lathecut)
- Spherical (spherodized)
- Blends (e.g., irregular-irregular, irregularspherical, or spherical-spherical)
2. By total amount of copper
- Low-copper alloys (e.g., conventional, traditional); <5% copper
- High-copper alloys (e,g. corrosion resistant); 12% to 28% copper
3.By presence of zinc
Examples
- Low-copper, irregular-particle alloy-silver (70%)-tin (26%)-copper (4%)
- High-copper, blended-particles alloy-irregular particles, silver (70%) –tin (26%) -Copper (4%); spherical particles, silver (72%)-copper (28%)
- High-copper, spherical-particles alloy-silver (60%) - tin (27%)-copper (13%)
Casting ring
CASTING RING LINERS
Most common way to provide investment expansion is by using a liner in the casting ring .Traditionally asbestose was used .
Non asbestose ring liner used are :
1) Aluminosilicate ceramic liner .
2) Cellulose paper liner .
The aim of using a resilient liner is to
-. allow different types of investmentbexpansion (act as a cushion)
_. facilitate venting during casting procedure.
_. facilitate the removal of the investment block after casting.&. prevent the distortion by permitting the outward expansion of the mold.
The casting ring holds the investment in place during setting and restricts the expansion of the mold. Normally a resilient liner is placed inside the ring leaving about 2-3 mm from both ends to allow for supporting contact of the investment with the casting ring.
Purpose of Casting Ring Liner
Ringer liner is he most commonly used technique to provide investment expansion. To ensure uniform expansion , liner is cut to fit the inside diameter of the casting ring with no overlap.
Non-asbestos Ring Liners: Ceramic (aluminum silicate) Cellulose (paper) Ceramic-cellulose combination Safety of the ceramic ring liners remains uncertain, because aluminum silicate also appears capable of producing hazardous-size respirable particles
CAD/CAM Restorations
Applications-inlays, onlays, veneers, crowns, bridges, implants, and implant prostheses
Stages of fabrication
CSD-computerized surface digitization
CAD-computer-aided (assisted) design
CAM-computer-aided (assisted) machining
CAE-computer-aided esthetics (currently theoretic)
CAF-computer-aided finishing or polishing (which are currently theoretic steps)
Classification
Chairside or in-office systems
(1) Cerec (Siemens system)-inlays, onlays, veneers
(2) Sopha (Duret system)-inlays, onlays (and Crowns)
Laboratory systems
(1) DentiCAD (Rekow system)-inlay, onlays, veneers, crowns
(2) Cicero (Elephant system)-porcelain fused-to-metal crowns
Materials
a. Feldspathic oorcelains (Vita)
b. Machinable ceramics (Dicor MGC)
c. Metal alloys limited use)
Cementing
- Etching enamel and/or dentin for micromechanical retention
- Bonding agent for retention to etched surface
- Composite as a luting cement for reacting chemically with bonding agent and with silanated surface of restoration
- Silane for bonding to etched ceramic (or metal) restorations and to provide chemical reaction
- Hydrofluoric acid etching to create spaces for micromechanical retention on surface or restoration
Properties
1. Physical properties
a. Thermal expansion coefficient well matched to tooth structure
b. Good resistance to plaque adsorption or retention
2. Chemical properties-not resistant to acids and should be protected from APF
3. Mechanical properties
a. Excellent wear resistance (but may abrade opponent teeth)
b. Some wear of luting cements but self-limiting
c. Excellent toothbrush abrasion
4. Biologic properties-excellent properties
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
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
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.