RINGLESS INVESTMENT TECHNIQUE
Used for phosphate bonded investments .
This method uses paper or plastic casting ring .
It is designed to allow urestricted expansion .
Useful for high melting alloys .

Composition of Acrylic Resins.

·        Powder. The powder is composed of a polymethyl methacrylate (PMMA), peroxide initiator, and pigments

·        Liquid. The liquid is a monomethyl methacrylate (MMA), hydroquinone inhibitor, cross-linking agents, and chemical accelerators (N, N-dimethyl-p-toluidine)

Properties

I. Physical

a. Excellent thermal and electrical insulator
b. Very dense
c. Excellent dimensional stability
d. Good reproduction of fine detail of hard and soft tissues

2. Chemical

a. Heating will reverse the reaction (decompose the material into calcium sulfate hemihydrate, the original dry component)
b. Models, casts, and dies should be wet during grinding or cutting operations to prevent heating

3. Mechanical

a. Better powder packing and lower water contents at mixing lead to higher compressive strengths (plaster < stone < diestone)
b. Poor resistance to abrasion

4. Biologic

a. Materials are safe for contact with external - epithelial tissues
b. Masks should be worn during grinding or polishing operations that are likely to produce gypsum dust

PHYSICAL PROPERTIES OF MATERIALS

Definite and precise terms are used to describe the physical properties of dental materials.

a. Hardness. Hardness is the measure of the resistance of a metal to indentation or scratching. It is an indication of the strength and wearability of an alloy or metal.

b. Ductility. Ductility is the measure of the capacity of a metal to be stretched or drawn by a pulling or tensile force without fracturing. This property permits a metal to be drawn into a thin wire.

c. Malleability. Malleability is the measure of the capacity of a metal to be extended in all directions by a compressive force, such as rolling or hammering. This property permits a metal to be shaped into a thin sheet or plate.

d. Flexibility and Elasticity. These terms differ in their technical definition but they are very closely related. Flexibility is the characteristic of a metal, which allows it to deform temporarily. The elasticity of a metal is used when it returns to its original shape when the load or force is removed.

e. Fatigue. Fatigue is the property of a metal to tire and to fracture after repeated stressing at loads below its proportional limit.

f. Structure (Crystalline or Grain Structure). Metals are crystalline and many of their physical properties depend largely upon the size and arrangement of their minute crystals called grains.

(1) Grain size. The size of the grains in a solidified metal depends upon the number of nuclei of crystallization present and the rate of crystal growth. In the practical sense, the faster a molten is cooled to solidification, the greater will be the number of nuclei and the smaller will be the grain size. Generally speaking, small grains arranged in an orderly fashion give the most desirable properties.

(2) Grain shape. The shape of the grains is also formed at the time of crystallization. If the metal is poured or forced into a mold before cooling, the grains will be in a flattened state. Metal formed by this method is known as cast metal. If the metal is shaped by rolling, bending, or twisting, the grains are elongated and the metal becomes a wrought wire.

g. Crushing Strength. Crushing strength is the amount of resistance of a material to fracture under compression.

h. Thermal Conductivity. Thermal conductivity is defined as the ability of a material to transmit heat or cold. A low thermal conductivity is desired in restorative materials used on the tooth whereas a high thermal conductivity is desirable where the material covers soft tissue.

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
 

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