dental amalgam

declining in use: mercury content - bad for environment, bioaccumulates

no evidence to suggest that release of mercury from materials is at a level that can cause harm to patients

phase down - can’t place in children or pregnant women


what is an amalgam?

a mixture of two or more metals one of which is mercury

difference between amalgam and dental amalgam is dental amalgam has silver and mercury

what is dental amalgam?

mercury + silver/tin alloy (amalgam alloy)

amalgamation reaction (combining mercury with metallic powder) results in hard shiny silver grey restorative

mercury is liquid at room temp so no need to heat up


in capsule form (encapsulated):

  • to protect clinician to exposure from mercury vapour

  • vapour has neurological affects

  • activate capsule - break seal between mercury and inter metallic alloy

  • place in amalgamator

  • process of amalgamating molten mercury with solid silver/tin alloy - trituration

  • mixing is important - react away all elemental mercury, get mixed with all solid alloy in capsule otherwise won’t set properly

  • solid shiny mass if mixed properly

  • follow manufacturers instructions by second - can overmix or undermix

  • temperature has big effect on setting reaction of material


composition of amalgam

general composition - will vary between manufacturer

  • intermetallic compound - Ag-Sn-Cu - alloy of the metallic components within the powder part - usually a mixture of two types of particles - spherical and lathe cut (irregular) particles

  • ^^ provide adv and disadv in terms of shape and size for handling and setting

  • modern amalgams - main constituent of inter metallic compound are silver, tin and copper

  • copper v important - improves properties of material - need at least 12% to optimise properties of material

  • normally mixed with 40-50% of mercury - dependant on shape and size of particles manufacturer uses

  • inter metallic phase referred to as gamma ᵞ

role of components:

Mercury - binds metals together

Silver - strength

Tin - controls physical change as material sets

^^too little and significant expansion, causes fracture of tooth structure (pain)

too much significant shrinkage/contraction (marginal gap)

Copper -

  • increases strength

  • reduces tarnish and corrosion,

  • decreases creep and marginal deterioration

  • improves mechanical properties

Zinc - optional

  • used by certain manufacturers

  • reduces oxidation reactions, keeps alloy clean during production

  • helps to delay corrosion

can be problematic

  • if zinc containing amalgam is moisture contaminated or too much zinc, can lead to:

    • delayed expansion

    • surface blistering

    • internal corrosion


High Cu Single Phase Amalgams

trying to wet every particle with as much mercury as possible

  • end up with some particles left

  • and other types of metal alloys that form crystallised from solution to harm material

  • copper stops weak phases forming e.g. low copper traditional amalgams Sn8Hg formed - weak phase

  • weak phases can form if material isn’t mixed properly!!!

  • copper prefers joining with tin than tin does with mercury - stops formation of gamma 2 to form eta phase

  • so modern amalgams - gamma 1 and eta phase formed

reaction that happens through mercury being soluble in the solid phase


General Setting Reaction

  • this all happens by vigorous mixing

  • mercury is diffusing into outer later of Ag-Sn-Cu particles

  • particle size decreases

  • discrete phases formed

Key Characteristics

  • ease of manipulation

    • mix, pick up with amalgam carrier, insert into cavity, condense with force

    • quicker materials to place

  • acceptable marginal adaptation

    • less physical change, over long period of time, condensing materials into cavity, more likely to create acceptable marginal adaptation

  • less technique sensitive

    • bulk placement, less issue with bonding/bonding system

  • self-sealing

    • over time as material corrodes and produces corrosion products, can fill gaps formed on placement

  • biocompatibility

    • mercury is a toxic element

    • if handled appropriately and placed correctly, then limited release of mercury with fully set restoration

  • mechanical properties/wear resistance

    • metal alloys are stronger materials, tougher, and wear resistance

  • low cost

    • refers to time and placement

    • quicker, easier to place, so lower cost implication

Mercury Content

40-50% by weight

  • dependant on composition of material itself

  • if reacted out, not left with elemental mercury, different phases lock in elemental mercury

residual Hg may be left

  • trituration consumes all Hg so less is released

  • so need to mix properly

to optimise reaction, need to condense material (amalgam condenser), apply force to condense material into cavity - if don’t do properly then might not get rid of excess mercury

if delay time between mixing and condensation, don’t bring particles close together by condensing and reducing volume - can lead to problems

ideal range is 44-48 wt.%

>55 wt% gives decrease strength, increase corrosion, increase creep

  • poor condensation techniques

  • delay between trituration and condensation

highest residual Hg at surface and margins


Mechanical Properties : Strength

strength

  • 24h+ to reach 100%

  • by 1h, 40-60%

  • if on occlusal surface, try not to eat anything on that side for hour or so

set amalgam exhibits brittle failure

  • weak in thin sections

  • unsupported edges fracture easily

  • can snap if thin enough

  • important that amalgam is placed with supported remaining tooth structure

Physical Properties : Creep

progressive deformation of a material under constant stress

not a problem anymore

can be a problem if don’t mix or place material properly

time dependant response to force or stress

creep increases as melting point is reached

  • phases containing increased Hg results in inc creep over time

  • can be through masticatory force, through gravity

  • start to deform over time

consequences

  • amalgam flows over marginal ridge

  • thin sections fracture easily

  • marginal deterioration; ditching

creep of traditional amalgams » modern types due to less copper

if don’t condense material properly then chances of creep increases


Micro leakage

Key factors

  • 2-20 micrometer gap

  • poor condensation technique

  • corrosion by-products

  • differences in TEC (thermal expansion coefficient)

    • materials that have different thermal properties to tooth tissue

    • restorative material may expand and contract differently to tooth tissue that surrounds it

    • can create gaps at interface if happens too much

    • resin has this issue more

Operator Variables

Trituration

  • coat each particle with Hg

  • ensure thorough amalgamation

  • use recommended times

condensing

  • process of reducing volume of material, packing it into cavity

  • create marginal adaptation

  • eliminate voids within material (porosity)

  • remove excess Hg

  • Spherical alloys more difficult to condense/easier to polish

carving/polishing

  • function of particle morphology

  • becomes naturally polished (>6months)

  • polishing - inc hardness, dec porosity

Trends in use of amalgam

most popular direct restorative

  • ease of use

  • less technique sensitive than resin composites

  • cost effectiveness

  • adequate mechanical properties

generally, clinical performance > RBCs

usually outlast resin composites however if good at placing resin composites, then last a long time

use of amalgam is declining

  • different rates internationally

  • aesthetic demand

  • alleged health effects

  • environmental concern

Dental amalgam use

  • no single study has categorically confirmed or allayed fears in relation to dental fillings

Environmental Hazards

  • Natural emission

  • Human activity

  • low amount of mercury used in dentistry compared with total use of mercury in every industry

  • ethical obligation to minimise and eliminate mercury waste

  • major amalgam particles

    • surplus in trituration capsules

    • carved surplus

  • minor amalgam particles

    • produced during carving, polishing (sucked up)

minimata convention

minimata convention on mercury is global treaty which aims to reduce neg impacts of mercury on environment and human health

eu regulations on restrictions of how dental amalgam should be disposed of properly

specfies dental amalgams shouldn’t be placed in primary teeth in children under 15 yrs old or pregnant or breastfeeding women unless deemed strictly necessary by dental practitioner due to specific medical need

dental amalgam presents no direct risk to patients who have amalgam restorations

elemental mercury in amalgam is more stable form than methylmercury which is found in aquatic organisms - main source of exposure to general population

dental amalgam can indirectly cause harm when released into environment and converted into methylmercury by aquatic microorganisms




active recall questions

what is an amalgam

what is a dental amalgam

what does amalgamation result in

describe the encapsulated form of amalgam

describe the composition of amalgam

describe the role of each component

describe the high copper setting reaction

describe the general setting reaction

what are the key characteristics of amalgam

describe mercury content

what must be done to optimise trituration reaction

what is the ideal mercury weight

describe the mechanical property of strength

what does set amalgam exhibit

describe the physical property of creep

what are the key factors for micro leakage

what are operator variables for tituration

what are operator variables for condensing

what are operator variables for polishing

what are the trends in the use of amalgam

why is the use of amalgams declining

when are major amalgam particles produced

when are minor amalgam particles produced

what is the minimata convention