Gypsum Products

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
 

Dental Porcelain and PFM Porcelains

Applications/Use

a. Porcelain inlays and jacket crowns
b. PFM crowns and bridges
c. Denture teeth

Terms

PFM-porcelain fused to metal
Fusing-adherence of porcelain particles into a single porcelain mass

Classification

 Dental porcelain is manufactured as a powder. When it is heated to a very high temperature in a special oven, it fuses into a homogeneous mass. The heating process is called baking. Upon cooling, the mass is hard and dense. The material is made in a variety of shades to closely match most tooth colors. Baked porcelain has a translucency similar to that of dental enamel, so that porcelain crowns, pontics, and inlays of highly pleasing appearance can be made. Ingredients of porcelain include feldspar, kaolin, silica in the form of quartz, materials which act as fluxes to lower the fusion point, metallic oxide, and binders. Porcelains are classified into high-, medium-, and low-fusing groups, depending upon the temperature at which fusion takes place. 
 
High-Fusing Porcelains. High-fusing porcelains fuse at 2,400o Fahrenheit or over. They are used for the fabrication of full porcelain crowns (jacket crowns). 

Medium-Fusing Porcelains. Medium-fusing porcelains fuse between 2,000o and 2,400o Fahrenheit. They are used in the fabrication of inlays, crowns, facings, and pontics. A pontic is the portion of a fixed partial denture, which replaces a missing tooth. 

Low-Fusing Porcelains. Low-fusing porcelains fuse between 1,600o and 2,000o Fahrenheit. They are used primarily to correct or modify the contours of previously baked high- or medium-fusing porcelain restorations. Eg  for PFM restorations

Structure

Components

a. Large number of oxides but principally silicon oxide, aluminum oxide. and potassium oxide    
b. Oxides are supplied by mixing clay, feldspar, and quartz.

Manipulation

Porcelain powders mixed with water and compacted into position for firing
Shrinkage is 30% on firing because of fusing and so must be made oversized and built up by several firing steps

Properties

1. Physical

a. Excellent electrical and thermal insulation
b. Low coefficient of thermal expansion and contraction
c. Good color and translucency; excellent aesthetics

2. Chemical

a. Not resistant to acids (and can be dissolved by  contact with APF topical fluoride treatments)
b. Can be acid-etched with phosphoric acid or  hydrofluoric acid for providing microll1echanical retention for cements

3. Mechanical

a. Harder than tooth structure and ,will cause opponent wear
b. Can be polished with aluminum oxide pastes

Effects of Amalgam.

(1) The combined effects of the properties of its ingredients should provide the most satisfactory restorative material.

(2) Quantity of mercury. Too little mercury in the mix results in a grainy, weak, readily tarnished, and corroded amalgam. Too much mercury will cause excessive expansion and weakened amalgam.

 (3) Composition of the alloy. Composition of the alloy must include a minimum of 65 percent silver, a maximum of 29 percent tin, a maximum of 6 to 13 percent copper, and a maximum of two percent zinc by weight

 (4) Correct proportion important. Before use, the silver alloy is mixed with pure and uncontaminated mercury. There are some alloys that are completely zinc free. They can therefore be used more successfully in a moisture-contaminated environment.

 (5) Properties of the finished product.

Silver imparts strength, durability, and color, gives the alloy desirable setting expansion, decreases flow, and accelerates (decreases) the setting time.

Tin makes the amalgam easier to work, controls excessive setting expansion, and increases both flow and setting time.

Copper increases hardness, contributes to setting expansion, reduces flow, and decreases setting time.

Zinc increases workability, and unites with oxygen and other "impurities" to produce a clean amalgam.

COMPOSITE RESINS

Applications / Use

• Anterior restorations for aesthetics (class III, IV, V, cervical erosion abrasion lesions)
• Low-stress posterior restorations (small class I, II)
• Veneers
• Cores for cast restorations
• Cements for porcelain restorations
• Cements for acid-etched Maryland bridges
• Repair systems for composites or porcelains

Polymerization--reaction of small molecules (monomers) into very large molecules (polymers)

Cross-linking-tying together of polymer molecules by chemical reaction between the molecules to produce a continuous three-dimensional network