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The biting force acting in the premolar andmolar region in the oral cavity ranges from222 to 890 N.

In a sound tooth structure enamel easilysustains such large forces however when aperson needs to have a restoration in the

posterior teeth the primary requirement ishigh compressive strength.

Amalgam is one such material whichgained a lot of popularity because the

strengths were adequate for normalsituations within the oral cavity.

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However being viscoelastic in nature whenan amalgam restoration is subjected to

large dynamic masticatory forces it resultsin creep or “flow” of the restoration on the

occlusal surface forming thin sections.

Hence two important properties that are tobe kept in mind while selecting an amalgamalloy for a posterior restoration are--

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Strength :

Amalgam is weaker in tension than incompression and is a brittle materialand may sometimes require

reinforcement with pins embedded inthe dentin(pin retained amalgamrestoration).

The min. required compressivestrength is 310 Mpa.

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Typical values:

Type ofamalgamalloy

Compressive(1hr)

Strength(7 days)

Tensile strength

Low copperamalgam

145 300 60

High copperadmix

137 430 48

High copperunicompositional

262 500 62

All values in MPa

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Tensile strength of amalgam is very lowirrespective of its composition.

◦

 A compressive stress on an adjacentrestored cusp introduces complex stressesthat result in tensile stresses in the isthmusregion.

◦ AMALGAM IS SRONGEST IN COMPRE-SSION &MUCH WEAKER IN TENSION & SHEAR , THEPREPARD CAVITY DESIGN SHOULD MAXIMIZE

THE COMPRESSION FORCES IN SERVICE &

MINIMIZE TENSION \ SHEAR FORCES.

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Factors affecting Strength :

Triturition

Hg/alloy ratio

Condensation

Particle shape

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Microstructure of amalgam

Particle size

Rate of hardening

Corrosion

Porosity

temperature

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Measurement of strength: Usually a cylindrical sample of

height 8mm and diameter 4 mmis prepared keeping thevariable parameters constantand preserved at 37 degrees

for 7 days. These are tested by universal

testing machines at definitestrain rates. Being viscoelasticits properties like strength andmodulus of elasticity depend onstressing or straining rates. 

8mm

4mm

Instron universal testing machine

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Repair strength of dental

amalgams.Shen C, Speigel J, Mjör IA.(Department of Dental Biomaterials, College ofDentistry, Health Science Center, University ofFlorida, Gainesville, FL 32610-0446, USA.

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Abstract

This study tested the hypothesis that newly triturated amalgamcondensed vertically on old amalgam was essential forestablishing a bond between the new and old amalgams.

The study showed that amalgams repaired with a differentamalgam yielded higher strength values than those repaired withthe original amalgam, and the baseline specimens exhibited

significantly higher strength values than all the repairedspecimens.

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Influence of microstructure oncreep:

Low Cuamalgam

High Cuamalgam

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Manipulative variables:

Hg/alloyratio

Triturition

Delayedcondensation

Impropercondensation

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Clinical significance:

Creep causes the amalgam to flowsuch that the unsupported amalgamprotrudes out from margins of the

cavity. Those unsupported edges are weak

and may further weaken by corrosion.

This causes the formation of a ditcharound the margins of amalgamrestoration.

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Creep values at different magnitudes of applied dynamic stresses

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Creep values for different amalgam types at a fixed dynamic stress

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 Amalgam creep tester: 

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Amalgam creep tester has been designed to test creepof dental amalgam materials as per ADA specification no.1.

Instrument has been built in a solid aluminum blockcontaining two amalgam specimens test stations wherethe temperature of 37 degrees Celsius is precisely

controlled with a digital microprocessor controller and adesignated creep test weight is applied onto thespecimens.

The creep of the specimen is detected by the 2-LVDT

transducers which are connected to the two pen stripchart recorder for the creep test evaluation and analysis.

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Thank

you