Alloys for Inlays, Crowns and Bridges

Inlay – indirect restoration; occlusal surface excluding cusps

Onlay – indirect restoration; occlusal surface plus cusp(s)

Crown – usually covers the clinical crown of the natural tooth Full crowns/ ¾ crowns

Bridge – replaces missing tooth/teeth Abutment vs. Pontic Cantilever, Maryland

Cantilever Bridge

Maryland Bridge

Materials for Indirect Restorations:

Dental Ceramics – Porcelains Composites Alloys

materials Requirement

Composition

Properties

Clinical usage/application

Requirements Biocompatibility – non-allergic, not toxic

Good corrosion + tarnishing resistance

Corrosion tarnishing

Suitable mechanical properties

High yield stress

Sufficient ductility

hardness

Ease of casting – high density, good fluidity

Cost – inexpensive material/labour

Gold alloys with at least 75% noble metals Proportion of gold in alloy

Carat rating – number of 24th parts of gold

Finess rating – parts/thousand of gold

Gold alloys

Alloy Composition: Noble Metals

Gold (Au) * Platinum (Pt) * Palladium (Pd) * Iridium, Ruthenium, Niobium, Osmium

Resistant to corrosion and tarnish Gold was the first metal successfully used

copper & silver added to enhance it

Precious Metal = containing metals of high economic value such as gold, platinum, palladium, silver, (rhodium), (iridium), (rhuthenium), and (osmium).

Noble Metal = a precious metal that is resistant to tarnish. This excludes “silver” by definition

Classification and compositionType

Description

Minimum total

amount of noble metals

Au (%) Ag (%) Cu (%) Pt (%) Pd (%) Zn (%)

I (A)

Soft 83 80-90 3-12 2-5 Little/none

Little/none

Little/none

II (B)

Medium 78 75-78 12-15 7-10 0-1 1-4 0-1

III (C)

hard 78 62-78 8-26 8-11 0-3 2-4 1

IV (D)

Extra hard

75 60-70 4-20 11-16 0-4 0-5 1-2

Effects of Alloys Components:

(1) Gold (Au) (2) Copper (Cu) (3) Silver (Ag) (4) Palladium (Pd) (5) Platinum (Pt) (6) Zinc (Zn)

Corrosion resistanceHardnessCounteract orange color of copperIncrease MP and hardnessIncrease MPPrevent oxidation during melting (O2 getter)

Classification and compositionType

Description

Minimum total

amount of noble metals

Au (%) Ag (%) Cu (%) Pt (%) Pd (%) Zn (%)

I (A)

Soft 83 80-90 3-12 2-5 Little/none

Little/none

Little/none

II (B)

Medium 78 75-78 12-15 7-10 0-1 1-4 0-1

III (C)

hard 78 62-78 8-26 8-11 0-3 2-4 1

IV (D)

Extra hard

75 60-70 4-20 11-16 0-4 0-5 1-2

Dental uses Type I – situations where there is no

great stress Cl III Cl V

Type II - inlays

Type III

Crowns & bridges Situations where there is great stress

Type IV

Cast partial dentures clasps

Mechanical properties

1) brittle, strong2) flexible, strong, tough3) ductile, medium strength4) ductile, weak, small toughness

Hardness (Brinell)

Hardness (Knoop)

type Proportional limit (MN/m2)

Ultimate tensile strength (MN/m2)

Elongation (%) Hardness (BHN)

I (as cast) 85 200 25 40-75

II (as cast) 160 345 24 70-100

III (softened) 195 365 20 90-140

III (hardened) 290 445 10 120-170

IV (soft) 360 480 15 130-150

IV (hard) 585 790 10 210-230

1 – ultimate tensile strength2 - yield strength3 – proportional limit4 – fracture point

Heat treatment Properties of alloy depend on

Composition Mechanical history (work hardened) Thermal history (heated T, rate of cooling)

Practical precautions Melting with air/gas torch

To heat alloy sufficiently to completely molten

Avoid overheating Use reducing zone Apply flux

Re-use of surplus alloy

Don’t mix surplus alloys of different types

Don’t melt alloy > 2-3 times (Zn lost)

Mix with new alloy

Other alloys Medium and low gold alloys

Au (50%), Pd, Ag, Cu, Zn

Silver-palladium alloys Ag, Pd, Au, Cu, Id, Zn

Nickle-chromium alloys Ni (75%), Cr (20%)

Miscellaneous materials Cu-Zn (brass) + Id, Ni Ag-Id+Pd Al-Bronze