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--
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.
Typical values: All values in MPa Type of amalgam alloy Compressive (1hr) Strength (7 days) Tensile strength Low copper amalgam 145 300 60 High copper admix 137 430 48 High copper unicompositional 262 500 62
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 .
Factors affecting Strength :
Measurement of 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
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.
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.
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%.
Influence of microstructure on creep:
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.
Creep values at different magnitudes of applied dynamic stresses
Creep values for different amalgam types at a fixed dynamic stress
Amalgam creep tester:
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.