Strength and creep of dental amalgam

Prateek Mathur
Student
2nd year B.D.S
Manipal college of dental sciences.

1. The biting force acting in the premolar and
molar region in the oral cavity ranges from 222
to 890 N.
In a sound tooth structure enamel easily
sustains such large forces however when a
person needs to have a restoration in the
posterior teeth the primary requirement is high
compressive strength.
Amalgam is one such material which gained a
lot of popularity because the strengths were
adequate for normal situations within the oral
cavity.

2. However being viscoelastic in nature when an
amalgam restoration is subjected to large
dynamic masticatory forces it results in creep
or “flow” of the restoration on the occlusal
surface forming thin sections.
Hence two important properties that are to be
kept in mind while selecting an amalgam alloy
for a posterior restoration are--

3. Prateek Mathur

4. Strength :Strength :
Amalgam is weaker in tension than in
compression and is a brittle material and
may sometimes require reinforcement
with pins embedded in the dentin(pin
retained amalgam restoration).
The min. required compressive strength
is 310 Mpa.

5. Typical values:Typical values:
Type of
amalgam
alloy
Compressiv
e
(1hr)
Strength
(7 days)
Tensile
strength
Low copper
amalgam
145 300 60
High copper
admix
137 430 48
High copper
unicomposition
al
262 500 62
All values in MPa

6. Tensile strength of amalgam is very low
irrespective of its composition.
◦ A compressive stress on an adjacent restored
cusp introduces complex stresses that result in
tensile stresses in the isthmus region.
◦AMALGAM IS SRONGEST IN COMPRE-SSION &
MUCH WEAKER IN TENSION & SHEAR , THE
PREPARD CAVITY DESIGN SHOULD MAXIMIZE
THE COMPRESSION FORCES IN SERVICE &
MINIMIZE TENSION \ SHEAR FORCES.

7. Factors affecting Strength :Factors affecting Strength :

9. Measurement of Measurement of
strength:strength:
Usually a cylindrical sample of
height 8mm and diameter 4 mm is
prepared keeping the variable
parameters constant and
preserved at 37 degrees for 7
days.
These are tested by universal
testing machines at definite strain
rates. Being viscoelastic its
properties like strength and
modulus of elasticity depend on
stressing or straining rates.
8mm
4mm
Instron universal testing machine

10. Repair strength of dental
amalgams.
Shen C, Speigel J, Mjör IA.
(Department of Dental Biomaterials, College of
Dentistry, Health Science Center, University of
Florida, Gainesville, FL 32610-0446, USA.

11. Abstract
This study tested the hypothesis that newly triturated amalgam
condensed vertically on old amalgam was essential for establishing a
bond between the new and old amalgams.
The study showed that amalgams repaired with a different amalgam
yielded higher strength values than those repaired with the original
amalgam, and the baseline specimens exhibited significantly higher
strength values than all the repaired specimens.

12. Creep:Creep:
DEFINED AS A TIME DEPENDENT PLASTIC
DEFORMATION UNDER CONSTANT STRESS
OCCURING IN CRYSTALLINE MATERIALS.


 ACCORDING TO ADA SPECIFICATION NO 1
CREEP SHOULD BE BELOW 3%.

CREEP OF LOW Cu AMALGAM IS 0.8-8% &
HIGH Cu IS <0.1%.

13. Influence of microstructure on Influence of microstructure on
creep: creep:

14. Manipulative variables:Manipulative variables:

15. Clinical significance:Clinical significance:
Creep causes the amalgam to flow such
that the unsupported amalgam protrudes
out from margins of the cavity.
Those unsupported edges are weak and
may further weaken by corrosion.
This causes the formation of a ditch
around the margins of amalgam
restoration.

16. Creep values at different magnitudes of applied dynamic stresses

17. Creep values for different amalgam types at a fixed dynamic stress

18. Amalgam creep tester:Amalgam creep tester:

19. Amalgam creep tester has been designed to test creep
of dental amalgam materials as per ADA specification no.
1.
Instrument has been built in a solid aluminum block
containing two amalgam specimens test stations where
the temperature of 37 degrees Celsius is precisely
controlled with a digital microprocessor controller and a
designated creep test weight is applied onto the
specimens.
The creep of the specimen is detected by the 2-LVDT
transducers which are connected to the two pen strip chart
recorder for the creep test evaluation and analysis.

20. Thank you…Thank you…