metals used in prosthodontics

8/2/2019 Metals Used in Prosthodontics

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USED IN PROSTHODONTICS


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CONTENTS

INTRODUCTION

PERIODIC TABLE DEFINITIONS GENERAL CHARACTERSTICS OF METALS STRUCTURE AND PROPERTIES OF METALS

STRUCTURE AND PROPERTIES OF ALLOYS CLASSIFICATION OF ALLOYS CASTING ALLOYS ALLOYS USED FOR METAL CERAMICS METALS FOR PARTIAL DENTURES WROUGHT ALLOYS METALS FOR IMPLANTS BIOCOMPATIBILITY OF METALS CONCLUSIONS

REFERENCES


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INTRODUCTION


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PERIODIC TABLE


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GPT 8 (2005) defines METAL as any strongrelatively ductile substance that provides

electropositive ions to a corrosive environment

and that can be polished to a high lustre.

Characterized by metallic atomic bonding.

The metals handbook (1992) defines ametal as an opaque lustrous chemical

substance that is a good conductor of heatand electricity and, when polished, is a goodreflector of light

DEFINITIONS


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The science and art of the extraction of metalsfrom their ores together with the refinement of

these metals and their adaption to various uses.

METALLURGY


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Metallography is primarily the microscopicexamination of the internal structure of

metals. This metallographic examinationgives some indication of the physical

behavior which the metal can be expected to

exhibit.


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General characteristics of metals

A metal is any element that ionizes positively in

solution.

Metals have certain typical andcharacteristic

properties that tend to identify and distinguishthem from the nonmetallic elements, such as

lustre, opacity, density, thermal and electrical

conductivity.


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Extreme ductility andmalleability are often

desirable in metals used in dentistry andthese are found to predominate in pure

metals rather than in alloys.


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STRUCTURE AND PROPERTIES OF

METALS

Metals usually have crystalline structures in the solid

state.

A SPACE LATTICE can be defined as anyarrangement of atoms in space such that every atom

is situated similarly to every other atom. It is alsocalled a crystal.


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When a molten metal oralloy is cooled, the

solidification process is oneof crystallization and isinitiated at specific sitescalled nuclei. The nuclei areformed from impuritieswithin the molten mass ofmetal.

Each crystal is known as agrainand the area between

two grains in contact is thegrain boundary


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The arrangement adopted by any one

crystal depends on specific factors such as

atomic radius and charge distributions onthe atoms. Although there is a tendency

towards a perfect crystal structure,

occasional defects occur.


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Such defects are normallyreferred to as dislocationsand their occurrence has

an effect on the ductility ofthe metal or alloy.


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When the material isplaced under asufficiently high stressthe dislocation is able tomove through the latticeuntil it reaches a grainboundary. The planealong which thedislocation moves iscalled a slip planeandthe stress required to

initiate movement is theelastic limit.


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Grain boundaries form a natural barrier to the

movement ofdislocations. The concentration

of grain boundaries increases as the grain size

decreases.Metals with finer grain structure

are generally harder and have higher values of

elastic limit than those with coarser grain

structure. Hence it can be seen that materialproperties can be controlled to some extent by

controlling the grain size.


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A fine grain structure canbe achieved by rapidcooling of the molten metalor alloy following casting.

This process, oftenreferred to as quenching,ensures that many nucleiof crystallization areformed, resulting in a large

number of relatively smallgrains.


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For an applied tensile force the maximum degreeof extension is a measure the ductility of themetal or alloy.

For an applied compressive force the maximumdegree of compression is a measure ofmalleability.

These changes occur when the stress is greaterthan the elastic limit and at relatively lowtemperatures.

COLD WORKING


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Such cold working notonly produces a changein microstructure, withdislocations becomingconcentrated at grain

boundaries, but also achange in grain shape.

The grains are nolonger equiaxed but take

up a more fibrous.


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It is process ofheating a metal to reverse the

effects associated with cold working such as strainhardening, low ductility and distorted grains.

In general it has 3 stages.

1) Recovery

2) Recrystallization

3) Grain growth.

Annealing


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Recovery : is considered the stage at which the

coldwork properties begin to disappear before

any significant visible changes are observed

under the microscope.


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Recrystallization :

when a severely cold worked metal is

annealed, recrystallization occurs after the

recovery stage. The old grains disappear

completely and are placed by a new set of

strain free grains.


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Grain growth:The crystallized structure has a certain

average grain size, depending on the

number of nuclei .The more severe the coldworking, the greater the number of such

nuclei. Thus, the grain size for completely

recrystallized material can range from

rather fineto fairly coarse.


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CLASSIFICATION

OF

METALS AND ALLOY SYSTEMS


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Metals can be broadly classified according tocomposition as

NOBLE METALS

The term noble identifies elements in terms of their

chemical stability i.e., they resist oxidation and areimpervious to acids.

Gold, Platinum, Palladium, Rhodium, Ruthenium,

Iridium, Osmium, and Silver are the eight noblemetals.

In the oral cavity Silver is more reactive andtherefore is not considered as a noble metal.


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BASE METALS

These are Ignoble elements. These remain

invaluable components ofdental casting alloysbecause of their influence on physicalproperties, control of the amount and type of

oxidation, or for their strengthening effects.e.g., Chromium, Cobalt, Nickel, Iron, Copperetc.


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The bureau of standards established gold casting alloystype i through type iv according to function, withincreasing hardness from type i to iv (1927)


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In 1984, ADA proposed a simple classification

for Dental casting alloys


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Alloy types bydescription


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Removable partial denture alloysAlthough type IV noble metal alloys may be

used, majority of the removable partial frame

works are made from base metal alloys.

E.g., Cobalt-chromium,

Nickel-chromium.


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DENTAL CASTING ALLOYS

The history of dental casting alloys has beeninfluenced by three major factors.

a)The technological changes of dental prosthesis.

b)Metallurgic advancements

c)Price changes of noble metals since1968.

In 1932, the dental materials group at nationalbureau of standards surveyed the alloys being usedand roughly classified them type I-IV.


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Uses

1) Fabrication of inlay, onlays

2) Fabrication of crowns, conventional all metalbridges, metal-ceramic bridges, resin bonded

bridges.

3) Endodontic posts.

4) Removable partial denture frameworks.


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Desirable properties

1) Biocompatibility.

2) Ease of melting.

3) Ease of casting, brazing and polishing.

4) Less solidification shrinkage.

5) Minimal reactivity with the mould material.

6) Good wear resistance.

7) High strength and sag resistance.

8) Excellent tarnish and corrosion resistance.


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Nickel-chromium and Cobalt-chromium

Alloys

Dental applications:

1) Partial denture framework: Co-Cr, Ni-Cr

2) Porcelain - metal restorations: Co-Cr, Ni-Cr3) Crowns and bridges: Ni-Cr


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During the years since the Co-Cr casting alloysbecame available for cast removable partial

denture constructions, they have continued to

increase in popularity.


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Function of various alloying elements:

Chromium is responsible for the tarnish resistanceand stainless properties of these alloys.

When chromium content of alloy is over 30% , thealloy is difficult to cast. With this percentage ofchromium, the alloy also forms a brittle phase, known

as sigma phase. Therefore cast base metal dentalalloys should not contain more than 28-29% ofchromium.


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Cobalt increases the elastic modulus, strength and

hardness of alloy more than does nickel.

One of the effective ways of increasing their hardness is

by altering carbon content.0.2% increase changes the properties such that

alloy would no longer be used in dentistry.

[Too brittle]

0.2% decrease will reduce yield and ultimate

tensile and yield strengths.


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Aluminum in nickel containing alloys increases theultimate tensile and yield strengths.


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Base metal alloys

Compared with ADA certified type IV gold alloys.

Cobalt based alloys, Nickel based alloys, and Pure

titanium have the following advantages.1) Low cost

2) Low density

3) Greater stiffness4) Higher hardness

5) High resistance to tarnish and corrosion.


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CompositionCo-Cr 53-67% of cobalt

25-32% of chromium

02-06 wt % molybdenum.Ni-Cr 61-81 wt % Nickel

11-27% chromium

02-05 wt of molybdenum.

Chromium provides passivation and

corrosion resistance.


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Properties:1) Higher hardness and stiffness.

2) More sag resistant at elevated temperatures.

3) It deforms only less than 25 m when porcelain isfired over it.


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metals used in prosthodontics

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