Dental amalgam properties

Microleakage, Corrosion, Dimensional changes, thermal and biological Properties of dental Amalgam have been discussed

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Dental Amalgam
Properties
Professor
Dr Tahir Ali Khan

2. Objective
At the end the student will be able to
know
The properties of amalgam
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Material-Related Variables
Leakage
Dimensional change
Strength
Corrosion
Creep
Thermal Properties
Biological Properties

5. MICRO or Nano LEAKAGE
OCCURS DUE TO PENETRATION OF FLUIDS OR
DEBRIS AROUND THE MARGINS THAT CAN
LEAD TO RECUURENT/SECONDARY CARIES
AMALGAM HAS GOT A SELF SEALING
PROPERTY –CORROSION PRODUCT WILL FILL
THE TOOTH RESTORATION INTERFACE &
PREVENT LEAKAGE
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MODERN AMALGAM ALWAYS SHOWS CONTRACTION
OLDER AMALGAM SHOWS EXPANSION
DIMENSIONAL CHANGES
CONTRACTION EXPANSION

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Dimensional Changes
Contraction or expansion depends on
Its manipulation
Ideally it should be small
Severe contraction can lead to
Micro leakage, plaque accumulation and recurrent/secondary caries

8. Dimensional Changes
Excessive expansion
Pressure on the pulp and post-operative
sensitivity, and protrusion of a restoration
ADA/ANSI Specification No.1 requires ±20µm/cm measured
at 37Cº b/w 5 min and 24 hr after beginning of Trituration
with a device that is accurate to at least 0.5µm
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Mixing of Alloy and Mercury Results in Contraction
The dissolution of particles begins and the Gamma 1 grows
Contraction continues as long as the growth of Gamma 1 phase
continues
As gamma 1 crystal grow, they impinge against one another
If condition appropriate, this impingement of Gamma 1 can
produce an outward pressure, tending to oppose the contraction

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Plastic matrix in case of sufficient mercury
Gamma 1 crystals impingement causes expansion
Once the matrix rigid matrix, growth of gamma 1 crystals cannot
force the matrix to expand
If sufficient mercury is present, net expansion will occur

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Net contraction
Type of alloy used
spherical alloys have more
contraction
Less mercury
Zinc (Delayed Expansion)
Zn+H
2O →Zno+H
2
Hydrogen causes considerable delayed
expansion (40 micrometer, during trituration
and condensation process)
Condensation technique
Greater condensation = higher contraction
Trituration time
Over triturationcauses higher contraction
Phillip’s Science of Dental Materials 2003

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Strength
Develops slowly
•1 hr: 40 to 60% of maximum (50MPa)
•24 hrs: 90% of maximum (300MPa)
Spherical alloys and Cu enriched alloy develop strength
more rapidly than conventional lathe cut alloy
Fine grain lathe cute alloys develop more rapidly than
coarse grain prodcts
•Require less mercury (Optimum Properties = 44 –48% Hg)
Phillip’s Science of Dental Materials 2003

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Greater the condensation pressure the higher the strength (lathe
cut)
Condensation with lighter pressure produces adequate strength
(spherical alloy)
Higher compressive vs. tensile strength

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AMALGAM IS SRONGEST IN COMPRESSION &
MUCH WEAKER IN TENSION & SHEAR
THE PREPARD CAVITY DESIGN SHOULD MAXIMIZE THE
COMPRESSION FORCES IN SERVICE & MINIMIZE TENSION \
SHEAR FORCES

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Weak in thin sections
•Unsupported edges fracture
•Voids and porosity are possible factors influencing the
compressive strength
•In case of delayed condensation and under tritutation
The porosity will more and strength will be reduced

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PropertyEnamel DentineAmalgam
Modulus of
elasticity GPa
50 12 30
Compressive
strength at 7
day
250 280 350
Tensile strength
at 7 day
35 40 –260 60
Vickers
Hardness
350 60 100

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Hardness of amalgam is lower than enamel
Surface hardness is 3 times lower than enamel
Appears to have adequate resistance to intra-oral abrasion
and rarely fails by this mechanism

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Corrosion
Should not be confused with tarnish. Reduces strength
Seals margins
low copper
6 months
SnO
2, SnCl
gamma-2 phase
high copper
6 -24 months
SnO
2, SnCl, CuCl
eta-phase (Cu
6Sn
5)

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Why Dental amalgam behaves as a self sealing
restoration because
SnOhelps to seal the space against
microleakage (Along the margins)
Electrochemical corrosion of high-copper
amalgams produce both Cu & Sn oxides &
oxychloride

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The most electrochemically reactive is 2
phase, readily forms the anode in an
electrolytic cell
The 2phase breaks down
Hg which may be able to combine with
unreacted alloy (phase).
Small quantity is inevitably ingested which
is of concern

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Creep or plastic deformation
DEFINED AS A TIME DEPENDENT PLASTIC DEFORMATION
UNDER CONSTANT STRESS.
ACCORDING TO ADA SPECIFICATION NO 1 CREEP SHOULD
BE BELOW 3%.
CREEP OF LOW Cu AMALGAM IS 0.8 -8% & HIGH Cu IS 0.4-
1%.

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Amalgam undergoes a certain amount of creep when
subjected to dynamic intra-oral stresses
Creep causes the amalgam to flow, such that unsupported
amalgam protrudes from the margin of the cavity
These unsupported edges are weak and may be further
weakened by corrosion

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Fracture cause the formation of ditch around the margin of
amalgam restorations
Gamma 2 phase in case of low copper alloys is responsible for
the relatively high values of creep
High copper alloys got low creep values

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Material type Creep %
Convention amalgam alloy 2.5
Admixed 0.2
Copper enriched 0.6
containing 0.5 %
palladium

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Creep
High-copper amalgams have creep resistance
prevention of gamma-2 phase
requires >12% Cu total
single composition spherical
eta (Cu
6Sn
5) embedded in gamma-1 grains
interlock
admixture
eta (Cu
6Sn
5) around Ag-Cu particles
improves bonding to gamma 1

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Thermal Properties
Relatively high value of thermal
diffusivity.
A cavity liner or base is
recommended to be used under
amalgam restoration.
The coefficient of thermal
expansion is 3 times greater
than that for dentine

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Biological Properties
2 aspects: on Patient and on Dentist & Auxiliary staff.
Neurotoxicity
Renal dysfunction
Birth defects
Reduce immunocompitence
Compromised general health.

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Thanks

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