NEET MDS Lessons
Dental Materials
Solution Liners (Varnishes)
Applications
o Enamel and dentin lining for amalgam restorations
o Enamel and dentin lining for cast restorations that are used with non adhesive cements
o Coating over materials that are moisture sensitive during setting
Components of copal resin varnish
o 90% solvent mixture (e.g., chloroform, acetone, and alcohol)
o 10% dissolved copal resin
Reaction
Varnish sets physically by drying Solvent loss occurs in 5 to 15 seconds (a film forms the same way as drying fingernail polish)
Manipulation
Apply thin coat over dentin. enamel. And margins of the cavity preparation Dry lightly with air for 5 seconds Apply a second thin coat Final thickness is 1 to 5 µ.m
Properties
o Physical
Electrically insulating barrier that prevents shocks. Too thin to be thermally insulating. Decreases degree of percolation attributable to thermal expansion
o Chemical
Forms temporary barrier that prevents microleakage into dentinal tubules until secondary dentin formation occurs. Decreases initial tendency for electrochemical corrosion
o Mechanical
Very weak and brittle film that has limited lifetime
Film adheres to smear layer
ACRYLIC RESINS
Use. Acrylic (unfilled) resins are used as temporary crown material. Temporary crowns are placed to protect the crown preparation and provide patient comfort during the time the permanent crown is being constructed
|
PROPERTY |
INGREDIENT |
|||
|
|
Silver |
Tin |
Copper |
Zinc |
|
Strength |
Increases |
|
|
|
|
Durability |
Increases |
|
|
|
|
Hardness |
|
|
Increases |
|
|
Expansion |
Increases |
Decreases |
Increases |
|
|
Flow |
Decreases |
Increases |
Decreases |
|
|
Color |
Imparts |
|
|
|
|
Setting time |
Decreases |
Increases |
Decreases |
|
|
Workability |
|
Increases |
|
Increases |
|
|
||||
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
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
Denture Liners
Use - patients with soft tissue irritation
Types
Long-term liners (soft liners)-used over a period of months for patients with severe undercuts or continually sore residual ridges
Short-term liners (tissue conditioners)-used to facilitate tissue healing over several days
Structure
Soft liners-plasticized acrylic copolymers or silicone rubber
Tissue conditioners-PEMA plasticized with ethanol and aromatic esters
Properties
Liners flow under low pressure, allowing adaptation to soft tissues, but are elastic during chewing forces.
Low initial hardness, but liner becomes harder as plasticizers are leached out during intraoral use
Some silicone rubber liners support growth of yeasts
Manipulation
Mixture of powder and liquid is painted onto working cast to create shape for acrylic appliance à After curing of mixture, the shape and fit are adjusted by grinding with burrs and stones with a slow-speed handpiece .Acrylic dust is irritating to epithelial tissues of nasopharynx and skin and may produce allergic dermatitis or other reactions. Grinding may heat polymer to temperatures that depolymerize and release monomer vapor. which may be an irritant