DM- Ch 10 Materials for Fixed Indirect Restorations and Prostheses PT 2

ADA Classification of Dental Casting Alloys

The differences between types is predominantly strength & elongation of the casting alloy

• Types I, II, III, IV

Type I

is weakest (soft); used for single surfaces

Type II

medium hardness, used for inlays/onlays

Type III

hard, used for crowns/bridges

Type IV hardest/strongest

used for bridges and partial denture frameworks

Gold, platinum, palladium

Metals classified as noble elements based on lack of chemical reactivity

Precious metals classified based on their cost

Include the noble metals above plus silver

Non-precious metal

Alloys with no noble metals (stainless steel for ex)

Gold content is described several ways

Percentage of parts per 100

• Carat is parts per 24

• Fineness is parts per 1,000

• 75% = 18 carat = 750 fine

Pure gold

is very soft, metals such as copper are added for strength

High-gold or High-noble alloys

Contain 60% or more gold & other noble elements

• Usually yellow metals

Low-gold or low-noble alloys

Contain at least 25% noble metals with gold typically being replaced by increased silver content

• Yellow - white metals

Silver-palladium alloys

70% silver, 25% palladium

• White-silver color

• Palladium increases tarnish resistance

Base Metals

less than 25% noble elements

Alloys for Ceramometal Restorations

Must be able to withstand high temps used to "fire" porcelain

Coping

is the metal understructure, layers of porcelain bonded to the metal coping, varying colors to mimic dentin and enamel translucency

Firing porcelain

causes powders to become solid, called "sintering"

• Powder isn't melted, just changes shape

• Firing reduces porosity, makes material denser and stronger

Types of Ceramometal Alloys

• Noble: Gold, Palladium

• Nonprecious: Nickel-chromium, Chromium-cobalt

Noble: Gold, Palladium

Higher melting temps., can withstand firing of porcelain

Nonprecious: Nickel-chromium, Chromium-cobalt

Stronger/harder than noble alloys, less expensive

Titanium

Most biocompatible metal

• Used for implants; will osseointegrate with bone

Partial Denture Frameworks Alloys

Nickle-chromium and Cobalt-chromium alloys used

• Inexpensive and have high-melting temperatures

Silicate and Phosphate-bonded Investments

Can withstand much higher burnout temperatures but are more difficult to use than gypsum-bonded investments (gypsum and silicate)

Porcelain

has been the favored ceramic material for more than a century

Porcelain jacket crowns

were the first all-ceramic restoration used

• Entire crown made of porcelain

• Very brittle, inadequate in most areas of mouth

Techniques to Strengthen Porcelain Materials

• Ceramometal: PFM

• Aluminous Porcelain

Ceramometal: PFM

Strong but metal is opaque, not natural looking

Aluminous Porcelain

Alumina oxide added to increase strength (like filler), still not very strong

Improvements in All-Ceramic Restorations

• Castable Glass

• Pressed Ceramics

• CAD/CAM

Castable Glass

Some improvement but still too weak

Pressed Ceramics

Stronger but still break, improved esthetics

CAD/CAM (Computer aided design/Computer aided

manufacture)

Still evolving, 3D printer

• Optical impression, digital not physical

• Machine makes restoration in-office from a solid piece of high-strength ceramic material, controlled by computer

Zirconium Oxide

used in CAD/CAM, high-strength ceramic

material

• Affordable alternative to ceramometal restorations

Advantages and Disadvantages of All-

Metal/Ceramometal/Ceramic Restorations

-Fracture

-Esthetics

-Wear

-Margins

Fracture

Porcelain/ceramic fracture; Metal fractures are rare

Esthetics

Choice depends on location

Wear

Porcelain/ceramic are harder than enamel, cause wear to opposing teeth

Margins

Casting decreases open margins; all ceramic restoration increases open margins

• Cements help to seal margins