Dental Amalgam Properties

Dental Amalgam Properties

Explain the physical properties of dental amalgam.
Answer:

ADA specification no. 1 for amalgam alloy lists three physical properties as a measure of the quality of amalgam, i.e. strength, dimensional change, creep, and flow.

Strength: The compressive strength of dental amalgam is strong but it is weak in tension and shear.

Compressive Strength:

• High-copper single compositional alloys have the highest compressive strength of 510 MPa. This value is obtained after 7 days.
• As 1-hour compressive strength is also important due to clinical significance, it is much less than half the final strength.
• Patients should be told not to use hard masticatory forces for 6 to 8 hours after restoration gets done, at this time 70 to 80% of total strength is achieved.
• Compressive strength is the least for the low-copper alloys both in 1 hour duration or 7 days duration.

The following are the compressive strengths of various amalgams:

Tensile Strength:

• Amalgam cannot withstand high tensile or bending stresses and can fracture easily in improperly designed restorations.
• So the cavity should be designed so the restoration will receive minimal tension or shear forces.
• The tensile strength of both the low and high-copper amalgam ranges from 48 MPa to 64 MPa.

Dental Amalgam Separator

Following is the tensile strength of various amalgams:

Dimensional Change:

• As mercury gets combined with the amalgam it undergoes two dimensional changes, i.e. contraction and expansion.
• Excessive contraction can lead to microleakage, sensitivity, and secondary caries, while excessive expansion can produce pressure on the pulp and post-operative sensitivity and protrusion of restoration.
• As per ADA specifications no. 1, amalgam should neither contract nor expand more than 20 µm/cm when measured at 37°C between 5 minutes and 24 hours after the beginning of trituration.
• Expansion is more for low-copper than high-copper alloys.
• Modern amalgam shows net contraction, while old amalgam always shows expansion.

Creep and Flow:

• Creep is defined as time-dependent plastic deformation.
• A creep of dental amalgam is a slow progressive permanent deformation of set amalgam which occurs under static or dynamic loading over a period of time.
• High-copper alloys as compared to conventional silver tin alloys have low creep values.
• It is mainly associated with low-copper amalgam.
• Low creep values lead to marginal deterioration while very high creep values result in the ditching of amalgam restoration.

Various creep values of amalgam are:

Flow refers to the deformation which occurs during the setting of amalgam. The greater is the flow, the greater the chances of restoration failure