Dental casting alloys

Casting is the most commonly used method for the fabrication of metal

structures (inlays, crowns, partial denture frames, etc) outside the mouth.

A pattern of the structure is first made in wax. This is then surrounded by

an investment material. After the investment hardens, the wax is removed

(burnt out) leaving a space or mould. Molten alloy is forced into this

mould. The resulting structure is an accurate duplication of the original

wax pattern.

Terminology

Several terms are used frequently when describing the elements or

compositions or casting alloys.

Precious metals

Precious metals (or elements) refer to those that are of high economic

value (i.e., are most expensive), such as gold, platinum, palladium, silver,

(rhodium), (iridium), (ruthenium), and (osmium).

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Noble Metal

These have been used for inlays, crowns, and bridges because of their

resistance to corrosion in the mouth.

Noble metal is resistant to tarnish and corrosion even under extreme

conditions such as gold, palladium, platinum, rhodium, ruthenium,

iridium, and osmium.

All of these except silver are also noble metals. Nobility has the same

meaning as corrosion and electrochemical corrosion resistance. Since

silver is prone to tarnish, it is not included under this definition.

Silver is not a noble metal, because of its tendency to corrode in the

mouth.

Base-Metals

These are not noble metals; these metals include titanium, nickel, copper,

chromium, cobalt, iron, silver, zinc, and many others .

They are requiring in alloys for the strength and wear properties

necessary for dental restorations.

They are important components of dental casting alloys because of their

influence on physical properties, control of the amount and type of

oxidation, and their strengthening effect.

Such metals are reactive with their environment, and are referred to as

“base metals”. Some of the base metals can be used to protect an alloy

from corrosion by a property known as passivation.

Alloy

An alloy is defined as a metal containing two or more elements, at least

one of which is metal, and all of which are mutually soluble in the molten

state.

Metals are materials composed of metallic elements that possess the

characteristics of high thermal and electrical conductivity, ductility,

opacity, andluster. They also have relatively high strengths and generally

melt at high temperatures.

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General requirements of casting alloys

All cast metals in dentistry have some basic common requirements

These will be reviewed in categories of physical, chemical, mechanical,

and biological properties.

1- Physical Properties include:

b. Moderately high density (castability)

The density of an alloy is the amount of mass in grams that take up a

volume of one cubic centimeter (g/cm3).

Metal Density g/cm3

Metal Density g/cm3

Platinum 12.45 Cobalt 8.90

Gold 18.88 Nickel 8.90 Palladium 12.02 Iron 7.87

Silver 10.50 Chromium 7.17 Copper 8.96 Titanium 4.5

The densities for dental casting alloys range from 4.5 g/cm3 for titanium-

based alloys to more than 18 g/cm3 for some of the high-noble alloys.

The density of an alloy is important:

1. in the casting of the alloy

2. and its final cost.

Because alloys are generally sold by mass, high-density alloys cost more

because more mass is present in any given volume of restoration.

High-density alloys are generally easier to cast because gravity

(heaviness) can accelerate the molten metal more easily into the casting

mold.

Gold alloys need to have a relatively low melting temperature so that

simple heating equipment can be used for casting.

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Dental restorations (inlays, onlays, crowns, bridges, partials) usually

include fine surface details. It is important to force the molten alloy into

the mold firmly against the mold walls with sufficient force (or pressure)

that the alloy adapts well to all the intricacies and fills up the mold space.

An alloy of high density is easier to cast in low-cost centrifugal casting

machines. Gold has high density (approximately 18 g/cm3) and works

well. Most of the other alloying elements that we use are medium-to-

high density as well but not as helpful

2- Chemical Properties include:

a. Chemical corrosion (tarnish resistance)

b. Electrochemical corrosion resistance

c. Solubility (solderability)

3. Mechanical Properties include:

a. High elastic modulus E (stiffness)

b. Moderately high yield strength YS and hardness H (resistance to

plastic deformation)

c. Hard by heat treatment (retention of polish)

4. Biological Properties:

The biocompatibility of dental alloys is related primarily to their

corrosion.

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a. Biocompatibility: no toxic soluble phases

b. Non-reactive in the oral environment

Finally, it is critical that the elements in the alloy not be released by

corrosion or wear procedures at levels that could be considered toxic to a

patient. All metal elements and ions could be toxic at sufficient levels but

are generally not a problem for dentistry. However, some low gold

systems are more prone to corrosion and may be suspect.

If an alloy corrodes more, it releases more of its elements into the mouth

and increases the risk for unwanted reactions in the oral tissues .

Theses unwanted reactions include: unpleasant tastes ،irritation ،allergy,

or other reactions.

All alloys release some amounts of elements into the mouth. Although a

wide range of release of elements from these alloys occurs, little evidence

recommends that the released elements cause significant problems for

most people.

The one exception is for people who are allergic to released metals.

Of these, nickel is by far the most common, 8 to 15% of the

population is allergic to nickel.

In general, alloys available on the market today are safe.

-Classification of alloys

The American Dental Association (ADA) classifies dental casting alloys

into three groups:

High-noble alloys: must have a noble metal content of at least 60% by

weight and a gold content of at least 40% .

Noble alloys: must have a noble metal content of at least 25%, but no

stipulation exists for gold content.

Predominately base-metal alloys: these alloys have a noble metal

content of less than 25%.

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1- High-noble alloy

High-noble alloys must contain at least 60% by weight of gold,

palladium, or platinum and at least 40% gold. These are:

The most expansive

Relatively high densities (above 13 g/cm3)That make them easier to cast

But also increase the cost of a restoration

Excellent corrosion resistance in the mouth.

These alloys are high gold-content alloys; they are used for full metal

restorations (inlays, onlays, crowns and bridges) and for metal-ceramic

restorations.

2- Noble metal alloys

Nobel alloys have at least 25% noble metal content but have no

stipulation for gold content.

Moderate densities (10 to 12 g.cm3).

yield strength and hardness greater thanhigh-noble alloys (higher

contents ofpalladium)

Corrosion resistance good.

The cost may be less than that of high-noble.

Used for crowns or bridges with or withoutporcelain coverings.

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3- Predominately base-metal alloy

The predominately base-metal alloys may have minor amounts of noble

elements, but more commonly they contain mainly nickel, cobalt, or

titanium.

These alloys are the most complex and may contain six to eight elements

in addition to the primary elements including molybdenum (Mo),

chromium (Cr), aluminum (Al), vanadium (V), iron (Fe), carbon (C),

beryllium (Be), manganese (Mn), gallium (Ga), and silicon (Si).

They have:

extremely high yield strengths and hardness

relatively low densities

the most difficult to cast and polish

Require special machines and techniques.

They may be used for:

1. crowns and bridges (with or without ceramic) -Ni/Cr alloys

2. removal Partial dentures - Co/Cr alloys

3. Dental implants- Titanium and titanium alloys

Base metal alloys are developed in the early 1970s; most of the base

metal alloys are based on nickel and chromium, why?

Because they are not noble metals, their corrosion resistance depends on

other chemical properties.

Cobalt-chromium and nickel-chromium alloys are very strong and hard.

Because of this they are generally difficult to work with (cutting,

grinding, polishing).

The main purpose of the chromium is to further harden the alloy by

solution hardening and also to impart corrosion resistance by the

passivating effect. Chromium exposed at the surface of the alloy rapidly

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becomes oxidized to form a thin, passive, surface layer of chromic oxide

which prevents further attack on the bulk of the alloy. These alloys have

very good corrosion resistance by virtue of the passivating effect. The

alloys are covered with a thin layer of chromic oxide which protects the

bulk of the alloy from attack.

Cobalt-chromium alloys generally are used for cast removal partial

denture.

Nickel-chromium alloys are normally used for crown and bridge with or

without porcelain.

Titanium and titanium alloys

The implants are normally constructed from titanium which has excellent

biocompatibility. Titanium is used as an implant metal, either in a pure

form, such as commercially pure Ti (cp Ti; 99.75%), or as an alloy with

aluminum (6%) and vanadium (4%).

Titanium is a strong, corrosion-resistant metal. It is lightweight, having a

density only half as great as Co/Cr. This characteristic is one reason it is a

preferred metal for the aerospace industry, where the combination of high

strength and lightweight are so important.

The stiffness of titanium is about 10 times greater than bone, however, so

it is still adequate for use as an implant.

The corrosion resistance of titanium is the result of its tremendous

reactivity with oxygen. A freshly exposed titanium surface reacts with

oxygen to form a tough, tenacious oxide film. This oxide film makes the

metal passive in the same way chromium oxide passivates cobalt or

nickel alloys. It is believed this oxide film is responsible for the excellent

biocompatibility of titanium implants.

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Most recently, titanium and titanium alloys have been developed as

dental casting alloys.

These metals can be used for all-metal and metal ceramic restorations, as

well as partial dentures.

Titanium is very reactive to oxygen and has a very high melting point

(2000oC). Therefore, it requires special casting equipment to produce

dental prostheses.

The problem with the use of titanium is rapid oxidation, and reactions

with investments.