Dental Alloys

Question 1

Why use gold alloys as casting material

Answer 1

1.Corrosion resistant
2. Biocompatible
3. Easily melted & cast
4. Full compensation possible for shrinkage on cooling

Question 2

Type 1 gold

Answer 2

Soft

Used for dental restorations and appliances which are subject to very slight stress and where burnishing is required

Question 3

Type 2 gold

Answer 3

Medium

used for dental restorations and appliances which are subject
to moderate stress: 3/4 crowns, abutments, pontics, full crowns and saddles)

Question 4

Type 3

Answer 4

Hard
used for dental restorations and appliances which are subject to high stress: thin 3/4 crowns, thin cast backings, abutments, pontics, full crowns and saddles)

Question 5

Type 4

Answer 5

Extra Hard

Used for dental restorations and appliances which are thinin cross-section and subject to very high stress: saddles, bars, clasps, crowns

Question 6

There are used for

Answer 6

Types I-III as INLAY GOLDS (soft-medium-hard)
•Type IV as PARTIAL DENTURE GOLD, the latter based on the previous use of this material as the framework for a partial denture.

Question 7

ISO standard for gold alloys

Answer 7

must be at least 75% (wt) gold and metals of the platinum group

Type IV, with correspondingly higher values for the other types.
•This is to ensure that the materials do not tarnish, or corrode, in the mouth

Question 8

Constituents jobs

Answer 8

Silver: Reduces the melting range of the alloy

Palladium: May replace platinum

Copper & Platinum: Order hardening effects (for Cu content of > 8% (wt) approx)
Gold: is highly biocompatible, dense, etc.

Question 9

Porcelain

Answer 9

good aesthetics
•relative inertness in the mouth, make for a desirable restorative material.
However,
•It is brittle and will only tolerate relatively small strains (about 0.1%) before failure.
•A thin layer of dental porcelain is fused to a metal casting which then presents an aesthetic crown, bridge, etc.

Question 10

THE COEFFICIENT OF THERMAL EXPANSION

Answer 10

of the porcelain must be suitably matched with that of the alloy
•the melting range of the alloy must be raised sufficiently above the fusion temperature of the porcelain for a successful enamelling operation.

Answer 11

Suitably-formulated high-content gold alloys are available for the technique

Important that the alloy does not contain components which form coloured oxides, at the interface, or which give rise to colour effects within the
porcelain.

Question 12

COBALT-CHROMIUM CASTING ALLOYS

Answer 12

prone to oxidation on melting
•castings produced are hard and brittle.
•The high melting temperature requires the use of a phosphate-bonded investment
•shrinkage, on cooling (1.9% linear), is such that dimensional accuracy is difficult to achieve.

Answer 13

unsuitable for precision castings, however, techniques have improved to enable technicians to cast crowns in this type of alloy.
•over-oxidation, on heating makes them unsuitable for the fusion of porcelain to the surface.
However,
•the materials are corrosion resistant and well tolerated in the mouth.
•Typical application of Co-Cr-Mo alloys in dentistry is with respect to the framework of partial dentures.

Question 14

NICKEL-CHROMIUM CASTING ALLOYS

Answer 14

investigated in an attempt to overcome some of the limitations of cast Co-Cr,
ie the minimal ductility, high cooling shrinkage, and tendency towards over-oxidation.
Industrially, these alloys are known as NIMONICS and find application in jet-engine technology.

Question 15

COMPOSITION

Answer 15

suitable for the porcelain fused to metal technique.
• The Be (beryllium) -containing alloys present a toxic hazard in the laboratory, through the possible inhalation of dust from the grinding procedures.
•Suitable precautions must be taken.

Question 16

SILVER-PALLADIUM CASTING ALLOYS (WHITE GOLD)

Answer 16

full solid solubility. (Mutual solubility)
•While palladium (Pd) features a strong affinity for hydrogen gas, it is one of the platinum group metals and is relatively high melting
(MP = 1552oC).
•Silver (Ag) is lower melting
(MP = 962oC), making it more suitable as the basis of a dental casting alloy, but tarnishes easily in the presence of hydrogen sulphide.

Question 17

WROUGHT ALLOYS cold working

Answer 17

Increases
• yield strength,
•tensile strength
•surface hardness at the expense of ductility
•Young’s Modulus, is not much changed.
• Alloys might be deficient in featuring a relatively low yield strength, as cast (such as Co-Cr-Mo), might be more acceptable in a work-hardened condition.
• For example, it might be decided to solder clasps fabricated from Co-Cr-Ni wire onto a partial denture base, in this alloy, rather than to cast them integrally with the unit. The increased yield strength would ensure that the clasp arms are less likely to deform plastically, on straining (or fracture, as the case might be).

Question 18

GOLD WIRES

Answer 18

These may be similar to Type IV dental casting gold alloys in composition (either with high-gold content, or medium-gold content).

Question 19

COBALT-CHROMIUM-NICKEL WIRES

Answer 19

COBALT-CHROMIUM-NICKEL WIRES
These wires contain about Co, 40%; Cr, 20% & Ni, 16%, together with lesser components, including carbon. Young’s modulus is similar to that of the Ni-Cr casting alloys.

Question 20

STAINLESS STEEL WIRES

Answer 20

Austenitic stainless steel 18-8 is commonly used in orthodontic devices. Care must be taken is soldering such wires to avoid loss of corrosion resistance.

Question 21

NICKEL-TITANIUM WIRES

Answer 21

These wires are based on the intermetallic NiTi (Nitinol, Titanal, Titanol) and feature a shape-memory effect, hence SHAPE MEMORY ALLOYS (SMA).

Question 22

Gold alloys are good for the oral environment

Answer 22

Biocompatible
Good marginal fit
Low corrsosional fit

Question 23

Grain boundary in dental alloys

Answer 23

The interface between adjacent grains in a polycrystalline meta

Question 24

ideal properties of dental alloys

Answer 24

nontoxic and biocompatible and have mechanical properties suitable for uses subject to stress and wear