Strength And Creep Of Dental Amalgam

Strength And Creep Of Dental Amalgam

 

The strength of an amalgam is extremely important since the restoration has to withstand the considerable load generated during mastication.

• Insufficient strength leads to the marginal breakdown of restoration or even gross fracture.
• Amalgam has good compressive strength but cannot withstand high tensile or bending stresses. Therefore, the cavity design should be such that the restoration will receive compressive forces and minimize tension or shear force.

Factors Affecting the Strength:

Effect of rate of hardening:  Amalgams do not gain strength as rapidly as might be desired. After 20 minutes, compressive strength may be only 6% of the 1-week strength. ISO specifications stipulate a minimum of 100 MPa at 1 hour and 350 MPa after 24 hours. Since the initial strength of amalgam is low, patients should be cautioned not to bite too hard for at least 8 hours after placement, the time at which at least 70% of its strength is gained.

The 1-hour compressive strength of high-copper single-composition amalgams is exceptionally high (262 MPa), so the chances of accidental fracture is less. Even after 6 months, some amalgams may still be increasing in strength, suggesting that the reactions between the matrix phases and the alloy particles may continue indefinitely.

Effect of trituration: The effect of trituration depends on the type of amalgam alloy, trituration time, and speed of the amalgamator. Both under- and trituration decrease strength in conventional as well as high-copper amalgams. Greater trituration energy leads to improved strength patterns.

Effect of mercury content: Sufficient mercury should be mixed with the alloy to wet each particle of the alloy. Excess mercury in the mix can produce a marked reduction in strength because of the higher γ2 content which is the weakest phase.

Effect of condensation:  Higher condensation pressure results in higher compressive strength (only for lathe-cut alloys). A good condensation technique will minimize porosity and remove excess mercury from lathe-cut amalgams. However, spherical amalgams condensed with lighter pressures produce adequate strength.

Effect of porosity:  Voids and porosities present in hardened amalgam are weak areas in restoration and cause a reduction in strength.

Effect of cavity design: The cavity should be designed to reduce tensile stresses. Amalgam has strength in bulk, therefore, the cavity should have adequate depth and width.

Effect of particle size: The smaller is the diameter of the original particle, the greater is the strength. Effect of temperature: Amalgam loses 15% of its strength when the temperature is elevated from room temperature to mouth temperature and loses 50% of strength when the temperature is elevated to 60°C.