dental amalgams Flashcards
what is the description of a dental amalgam?
A dental amalgam contains powdered alloy and mercury
Amalgamation is releasing the mercury from a sealed compartment into another that has the alloy, and mixing them in an amalgamator
The amalgam is condensed under pressure against the walls and floor of the prepared tooth
Alloy particles + Hg Dental amalgam + unreacted powder particles
Amalgams harden and strengthen with time
When amalgam dissolves the outer shell it forms another phase but you still have the inner part still in tact.
what is the alloy composition of amalgam? what does higher copper content up to 30% wt do? how are the particles produced?
Historically: 65 wt% Ag, 29 wt% Sn, and <1 wt% Zn added as a deoxidizer
what is the difference between fast set and slow set amalgam?
Fast set amalgam has less mercury, slow set has more mercury.
why do they acid etch the particles of the amalgam?
Acid etching of the particles is done to get rid of the oxide layer, also increase the surface area.
what are the factors governing the quality of a dental amalgam restoration?
Alloy selection
Alloy / Hg ratio
Trituration procedure
Condensation - should be about 13 - 15 pounds.
Marginal integrity - you don’t want marginal leakage
Anatomical characteristics
Finishing - a rough surface is a crack initiator
what are the metallurgical phases in dental amalgams?
γ (Ag3Sn) γ1 (Ag2Hg3) γ2 (Sn7-8Hg) ε (Cu3Sn) η (Cu6Sn5) Silver-Copper eutectic (Ag-Cu)
you don’t want amalgam to have gamma 2, it is the one most susceptible to corrosion and creep.
there is no melting range for the eutectic composition, the melting point is also lower than both of the constituents.
what are the amalgamation and resulting microstructures of low-copper alloy?
Low-Copper Alloys:
Alloy particles (β + γ) + Hg –> γ1 + γ2 + Unconsumed alloy particles (β + γ)
The more unconsumed Ag-Sn particles that are retained in the final structure, the stronger is the amalgam
The weakest component is the γ2 phase
the core of the particle will remain inside, the outside is dissolved.
The more the core, the stronger the amalgam.
what are the high-copper alloys like?
High-Copper Alloys:
Exhibit better mechanical properties, superior corrosion characteristics, better marginal integrity, and improved performance in clinical trials
Available in two types: admixed alloy powder and single composition alloy powder
what are admixed alloys?
Contains a mixture of spherical silver-copper eutectic alloy( 71.9 wt% Ag and 28.1 wt% Cu) particles and lathe-cut low-copper amalgam alloy particles.
Alloy particles (β + γ) + Ag-Cu eutectic + Hg –> γ1 + η + Unconsumed alloy of both types of particles
γ2 phase is eliminated
what are single-composition alloys?
Each particle has the same chemical composition (eg. 60 wt% Ag, 27 wt% Sn, and 13 wt% Cu)
Cu can reach concentrations of 30 wt%
Ag-Sn-Cu Alloy particles + Hg –> γ1 + η + Unconsumed alloy particles
if you add more copper it’s at the expense of tin.
what is the dimensional stability of amalgams?
Amalgams can expand or contract depending on manipulation
Ideally, dimensional change should be minimal. ANSI/ADA Spec. No. 1: Amalgams should neither contract nor expand more than 20 μm/cm, measured at 37oC, between 5 min and 24 hrs after the beginning of trituration
Severe contraction can cause microleakage, plaque accumulation, and secondary caries
Excessive expansion can produce pressure on the pulp and postoperative sensitivity
Expansion is better than contraction
how can you get different volume from two different metals being melted and combining, you would get a different volume from a different crystal structure.
what is the effect of moisture contamination?
Occurs if a zinc-containing low or high copper amalgam is contaminated by moisture during trituration or condensation
Starts after 3 to 5 days and may continue for months. That is why it is called delayed or secondary expansion.
The effect is caused by the reaction of zinc with water, where hydrogen is produced by an electrolytic action. The hydrogen collects within the amalgam, increasing the internal pressure to high levels causing the amalgam to creep.
what is creep?
zinc takes the oxygen away and leaves hydrogen behind because zinc is an oxygen scavenger.
what are 6 factors affecting amalgam strength
Loading: Amalgams perform better under compression.
Composition: Under compression, single composition amalgams exhibit superior properties to low-copper and admix amalgams
Trituration: Trituration time and speed of amalgamtor affects the strength of the amalgam. Both undertrituration and overtrituration decrease the strength of both traditional and high copper amalgams
Mercury Content: Amalgams with Hg content in excess of 54 % exhibit a marked reduction in compressive strength
Condensation: High condensation pressure and a good technique promotes amalgam strength
Porosity: Caused by low condensation pressures. The higher the porosity, the lower the strength
Hardening Rate: Patients should not subject the amalgam to biting stress for at least 8hrs to ensure amalgam strength. (so you tell the patient not to bite on it for a while)
what is the creep of dental amalgams?
Reduced in high-copper amalgams
γ2 phase is associated with creep
Excessive mercury contributes to increased creep
how is the clinical performance of amalgam restorations?
Microleakage: occurs as a result of amalgam contraction
Corrosion: γ2 is anodic to the other phases
Compositional effects on Amalgam Survival: Survival rate: high-copper-zinc amalgams > high- copper > low-copper-zinc > low copper
