Treatment of dental caries:
AMALGAM.
Properties, indications, inserting,
carving, polishing.
3.Year - DM
AMALGAM
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Amalgam is the most widly used permanent filling in
dentistry. It is prepared by mixing the alloy with
mercury.
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The reaction between mercury and alloy is termed an
amalgamation reaction.
It results in the formation of a hard restorative material
of silvery – grey appearance
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Amalgam Use and Benefits
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Dental amalgam, in widespread use for over 150 years, is one of
the oldest materials used in oral health care.
Its use extends beyond that of most drugs, and is predated in
dentistry only by the use of gold.
Dental amalgam is the end result of mixing approximately equal
parts of elemental liquid mercury (43 to 54%) and an alloy
powder (57 to 46%)
composed of silver, tin, copper, and sometimes smaller amounts
of zinc, palladium, or indium.
Amalgam
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MERCURY (Hg) – is a liquid at room temperature and it is
able to form a workable mass when mixed with the alloy.
The reaction between mercury and alloy is termed an
amalgamation reaction.
Dental amalgam has been used for many years with a large
measure of success.
It is the most widely used of all available filling materials.
Composition
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The composition of the alloy powder particles varies from one
product to another.
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Composition of CONVENTIONAL AMALGAM ALLOY:
Metal:
Weight:
Silver (Ag) .......................... 65% min.
Tin (Sn)
..............................29% max.
Copper (Cu) ..............................6% max.
Zinc (Zn) ...............................2% max.
Mercury (Hg) ............................ 3% max.
Composition
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1. Silver - Increases strength, expansion and reactivity.
Decreases creep. Corrosion products are AgCl and
AgS.
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2. Tin - Increases reactivity and corrosion. Decreases
strength and hardness. Corrosion products are SnO,
SnCl, and SnS.
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3. Copper - Increases strength, expansion and
hardness. Decreases creep. Corrosion products are
CuO and CuS.
Composition
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4. Zinc - Increases plasticity, strength and the Hg:alloy
ratio. Decreases creep. Causes secondary expansion.
Corrosion products are ZnCl and ZnO.
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5. Mercury - Wets the alloy particles. Decreases
strength if in excess amounts. Implicated in toxic and
allergic reactions.
Effects on properties of an amalgam restoration
imparted by ingredients.
PROPERTY
INGREDIENT
Silver
Strength
Increases
Durability
Increases
Tin
Hardness
Copper
Increases
Expansion
Increases
Decreases
Increases
Flow
Decreases
Increases
Decreases
Color
Imparts
Increases
Decreases
Setting time
Workability
Cleanliness
Zinc
Decreases
Increases
Increases
Increases
Amalgam
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The role of zinc (Zn) :
is as a SCAVENGER during the production of the alloy. The
alloy is formed by melting all the constituent metals together.
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There are tendency for oxidation to occur.
OXIDATION of tin (Sn), copper (Cu) or silver would seriously
affect the properties of the alloy and amalgam.
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Zinc reacts rapidly and preferentially with the available oxygen,
forming a slag of zinc oxide (ZnO) which is easily removed.
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Many alloys contain no zinc. They are described as ZINCFREE ALLOYS.
Amalgam: properties
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1. DIMENSIONAL CHANGES
2. STRENGTH
3. PLASTIC DEFORMATION (CREEP)
4. CORROSION
5. THERMAL PROPERTIES
6. BIOLOGICAL PROPERTIES
1. DIMENSIONAL CHANGES
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A large contraction would result in a marginal gap
down which fluids could penetrate.
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A large expansion would result in the protrusion of the filling
from the cavity.
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Zinc reacts readily with water producing hydrogen:
Zn + H2O –––––––––––– ZnO + H2
The liberation of hydrogen (H2) causes a considerable
expansion.
2. STRENGTH
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The strength of dental amalgam is developed slowly. It may take
up to 24 hours to reach a reasonably high value and continues to
increase slightly for some time after that.
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15 – 20 minutes after placing the filling , the AM is relatively
weak. I tis necessary, to instruct patients not to apply undue
stress to their freshly placed AM fillings.
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There is good correlation between strength and mercury
content.
Optimum properties are produced for amalgams containing 4448% mercury.
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3. PLASTIC DEFORMATION (CREEP)
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Amalgam undergoes a certain amount of plastic deformation or
creep when subjected to dynamic intra-oral stresses.
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The gamma 2 phase of AM is responsible for the relatively high
values of creep.
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The copper- enriched amalgams, which contain little or no
gamma 2 in the set material, have significantly lower creep values
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Gamma 2 phase is responsible for high creep.
4. CORROSION
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Corrosion is a matter which may significantly affect the structure and
machanical properties. The heterogeneous, multiphase structure of AM makes
it prone to corrosion.
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The gamma 2 phase of a conventional AM is the most electrochemically
reactive and readily forms the anode in an electrolytic cell.
The rate of corrosion is accelerated if the AM filling contacts a gold
restoration.
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Smooth surfaces are less prone to concentration cell corrosion.
5. THERMAL PROPERTIES
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AM has a relatively high value of thermal diffusivity
Dentine is replaced by a good thermal conductor.
The coefficient of thermal expansion value for AM is about
three times greater than that for dentine.
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This results in considerably more expansion and contraction
in the restoration than in the surrounding tooth when a patient
takes hot or cold food or drink.
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Thermal expansion may cause microleakage around the
fillings since is no adhesion between AM and tooth substance.
Microleakage plays an important part in initiating such lessions.
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6. BIOLOGICAL PROPERTIES
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Certain mercury compounds are known to have a harmfull
effect on the central nervous system.
Some studies have shown a higher concentration of mercury in
the blood and urine of patients with AM fillings than those
without.
Another potential problem concerns allergic reactions, usually
manifested as a contact dermatitis.
Mercury or freshly mixed AM should never be touched by hand.
Mercury is readily absorbed by the skin.
Usage and preparation of amalgam
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The dental specialist has the direct responsibility for
the correct preparation and use of amalgam.
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Incorrect use may produce a faulty restoration that can
cause or contribute to the loss of a tooth.
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Therefore, the dental specialist must use extreme care
in preparing a good mix of amalgam that will
provide the best qualities obtainable from the alloy.
MANIPULATIVE VARIABLES
The manipulating of AM involves the following
sequence of events:
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1. Proportioning and dispensing
2. Trituration
3 Condensation
4.Carving
5. Polishing
1. PROPORTIONING AND
DISPENSING
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Alloy/mercury ratios vary between 5:8 and 10:8.
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Those mixes containing greater quantities of mercury
are „wetter“ and are generally used with hand mixing.
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Those mixes containing smaller quantities of mercury
are „drier“ and are generally used with mechanical
mixing.
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Spherical particle alloys,for example , require less mercury to
produce a workable mix.
1. PROPORTIONING AND
DISPENSING
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For optimum properties, the final set amalgam
should contain less than 50% mercury.
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The optimal final mercury content ranges from
an average of 45% for lathe-cut materials to an
average of 40% for spherical materials.
2. TRITURATION
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The mixing or trituration of AM may be carried out by
hand or in an electrically powdered machine
which vibrates a capsule containing the mercury and
alloy.
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Trituration by hand is not extensively practised in
developed countries nowadays. Mechanical mixing is far
more widely used - amalgamator.
Trituration times 5 - 20 seconds are normal.
2. TRITURATION
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The advantages of mechanical trituration are as follows:
1. A uniform and reproducible mix is produced.
2. A shorter trituration time can be used.
3. A greater alloy/mercury ratio can be used.
Amalgamator
3. CONDENSATION
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Material is condensed into the prepared cavity using a flat-ended,
steel hand instrument called an amalgam condenser.
The technique chosen for condensation must ensure the
following.
1. Adequate adaptation of the material to all parts of the cavity
base and walls.
2. Good bonding between the incremental layers of amalgam
3. Optimal mechanical properties in the set amalgam by
minimizing porosity and achieving a final mercury content of
44-48%.
3. CONDENSATION
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There should be a minimal time delay between
trituration and condensation.
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If condensation is commended too late, the
amalgam will have achieved a certain degree of
set and adaptation, and final mechanical
properties are all affected.
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There is a good correlation between the quality
of an AM restoration and the energy expended
by the operator who condenses it.
It needs to use a high condensating force.
Lower forces are required to condense spherical
particle amalgams than lathe-cut materials.
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Condensing instruments
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Amalgam carriers and condensers are used for this purpose.
4. CARVING
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Soon after condensing the AM, the surface layer , which is rich in
mercury, is carved away with a sharp instrument.
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If carving is delayed too long the material may become too hard
to carve and there is a danger of chipping at the margins.
4. CARVING
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Amalgam carvers
5. POLISHING
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Polishing is carried out in order to achieve a lustrous surface
having a more acceptable appearance and better corrosion
resistance.
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The fillings should not be polished untill the material has
achieved a certain level of mechanical strength, otherwisw there
is a danger of fracture , particularly at the margins.
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Many products require a delay of
24 hours between placing and polishing.
5. POLISHING
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AM polishing kits
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AM polishers
Black
Dark purple
Green
Amalgam indications
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In primary and permanent dentition
In stress bearing areas of the mouth
Small to moderate - sized cavities in the posterior teeth
As a foundation to crowns
When oral hygiene is bad
When moisture control is a problem
When cost is a concern
Amalgam contraindications
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When esthetics is important
When pation has a history of allergic reactions to the alloy
When cost is not a concern
Amalgam filling
Amalgam filling