Direct Dental Restorative Materials: Dental Amalgam W6 Flashcards
What is the difference between an alloy, amalgam and dental amalgam
Alloy: Combination of >1 metals
Amalgam: Alloy of HG + 1 or more other metals ie. Alloy with Hg in it
Dental Amalgam: Mix of liquid Hg with solid alloy particles (mainly Ag, Sn and Cu) = Amalgam Alloy
What are the advantages of Amalgam
- Good toughness and wear resistance
- Good compressive strength (for stress bearing areas)
- Good longevity
- Pervents Marginal leakage
- Easy to use
- Economical
What are the disadvantages of Amalgam
- Not aesthetically pleasing
- Brittle
- Loss of sound tooth structure (if small restoration req.)
- Discolouration of tooth structure
- Metallic taste
- Corrosion and galvanism
- Limited support for weakened tooth structure
- Concerns about waste disposal and Hg toxicity
What are the indications and contraindications of Amalgam
Indications
- Class 1, 2, 3 and 5 in unaestetic areas or moderate to large restorations
- As core build up - for crowns and bridges
- Restorations covering cusps - using pins
- Restorations with a heavy occlusal contact
- Restorations that cannot be isolated easily
- in teeth acting as abutments (anchors of bridge) for removal appliances
Contraindications
- Anterior teeth or anywhere aesthetic
- Small-moderate class 1,2,6 that can be well isolated
How can you classify amalgams?
By…
# of alloy metals
unmixed or admixed alloys
particle shape
amount of Zn
particle size
copper content*
noble metals
generation of development
What are the main components of dental analgam alloy*
Main components…
Silver (Ag)
Tin (Sn)
Copper (Cu) - 13-30% high Cu alloy, <5% low Cu alloy
Lesser amounts of…
Indium, Palladium, Platinum, Zinc (>0.01% = labelled as zinc containing), Mercury
Note: amalgam as a whole main component = Hg - 50%
Describe the various shapes of dental amalgams (Cu classified)
Low Cu Alloys
Lathe-cut or spherical particles
High Cu Alloys
Unicompositional - spherical particles
Admixed - irregular spherical ie. different sizes but same composition or mixture of particles (Ag+Sn vs Ag+Cu) = MOST PROMINENT NOW
Differences in shape lead to different compositions
Besides Hg, __ is the next major component of dental amalgam. What are the benefits of including it?
Ag (Silver)
- ⅔ of amalgam alloy
- Increases strength, expansion on setting and resistance to corrosion
___ is the second largest alloy component of dental amalgam, why do we include it and what are the drawbacks?
Tin (Sn)
- ¼ of Amalgam alloy
- Causes setting contraction, decreases stength and resistance to corrosion
- Readily combines with Hg to form Y2 - contributes to failure
Cu, Zn and In/Pd are present as alloy components of dental amalgam. What are the benfits of including them?
Copper (Cu) - preferred
- Increases hardness, strength and setting expansion
Zinc (Zn) - not preferred
- During manifacture it decreases the oxidation of other alloys and decreases marginal failure
- Causes delay in setting expansion
Indium (In)/Palladium (Pd)
- Increases plasticity and resistance to deformation
What happens in the amalgamation reaction for low-copper amalgam?*
Ag3Sn + Hg -> Ag3Sn (Y) + Ag2Hg3 (Y1) + Sn8Hg (Y2)
Tituration mixes Ag3Sn with Hg = Ag2Hg3 (Y1) + removal of surface oxides that could have formed on alloy particles
Y2 = Sn8Hg (phase we dont want!)
Early in working time = few Y1 and Y2 = soft, condensable, carvable
Late in working time = more Y1 and Y2 = harder and stronger
Which phases are favourable/unfavourable in an amalgamation reaction - low copper amalgam
Y (Ag3Sn): 27-35% - strongest, most corrosion resistant
Y1 (Ag2Hg3): 54-56% - somewhat strong and corrosion resistant, but brittle. The matrix phase that holds this multi phase material together
Y2 (Sn8Hg): 11-13% - weakest and most corrosion prone
What is the main difference between a low copper amalgam and a high copper amalgam ie. how does this effect the phases that are present?
Increased Cu % = no Y2 phase
instead Sn + Cu form multiple products (ie. Sn is completely occupied by the Cu)
= increased strength, corrosion resistance, decerased marginal breakdown
What is Creep?
A slow change in shape caused by compression - Sliding of Y1 grains.
-> presence of Y2 predicts the extent of marginal fracture ie. High copper (no Y2) = low creep value
What are the changes in Amalgam during formation? Ie. Dimensional changes
Stage 1: Initial contraction (within first 20 mins)
-> Alloy particles dissolve in Hg = contraction
Stage 2: Expansion
-> formation and growth of crystal matrix around unconsumed alloy particles
Stage 3: Limited delayed contraction (after 6-8h)
-> If contamination with H20 occurs with Zn containing alloy = dimesional changes = sensitivity
How do dimentional changes differ in Low-high copper alloys
High-copper admix = smallest dimentional change
Low-copper lathe-cut alloy = largest dimentional change
Delays in expansion cause…
Post-operative sensititiy
Protrusion of restoration from cavity
Microleakage
Perforations
Increased flow and creep
- can occur 10-12 days following placement
What are the factors that affect dimensional changes?
Particle size and shape (homogeneous=smoother=coat with Hg easier=faster amalgamation=limited expansion)
Hg content (more expansion when high Hg)
Manipulation
Corrosion definition in relation to amalgams…
Progressive destruction of a metal by chemical or electrochemical reactions in the environment
= increased porosity, reduced marginal integrity, decreased strength, release of metallic products into oral cavity
What factors affect manipulation (& performance) of amalgam
Manufacturing
- Alloy composition; Hg content, particle size, shape, distribution. Alloy form (powder, tablet, capsule)
Dentist
- Trituration (technique, speed and time, too much tituration), Condensation, burnishing and carving, finishing and polishing
Define condensation (in relation to amalgam manipulation)
Incremental placement and compression of amalgam into cavity prep = removes excess Hg, adapts to prep walls, reduce voids = homogeneous mass
- Within 3 minutes following tritration
What are the objectives of burnishing and carving amalgam
Removes overhangs, restore physiological contours, maintain interproximal contacts
Burnishing: reduces size and number of voids, excess Hg at surface discarded
When does finishing and polishing occur for amalgam and what are the objectives
24 hours after restoration is placed (to allow fully set)
- reduce concentration of stress, minimise corrosion, prevent adherance of plaque = extend lifespan of restoration
What are the 3 different forms of Hg and relative toxicity issues
Elemental - liquid (No issues) and vapour (absorbed=Issue)
Inorganic - low toxicity, harmless if swallowed
Organic - toxic at low concentrations (not used for amalgam)
Hg released from dental procedures does not elict toxic reactions as it is not enough (small quantity)
