essay 20 - dental amalgam - definition, types of amalgam, physico-mechanical and medico-biological properties of amalgam restorations

define dental amalgam

• a metal like restorative material composed of a mixture of silver, tin, copper alloy and mercury. The unset mixture is pressed into a specifically prepared undercut tooth form and contoured to restore the tooth’s form and function. When material hardens the tooth is functional again, restored with a silver coloured restoration

ways in which the different types of amalgam is categorised

Based of copper content

based on particle shape of the alloy

Based on composition alternatives (new types)

Based on zinc content

types of amalgam based on zinc content

Based on zinc content

1. Zinc containing

— 0.01% or more zinc

— zinc acts as an oxidation scavenger during manufacturing

— if contaminated with moisture it causes delayed expansion due to hydrogen gas formation!

2. non-zinc containing

— less than 0.01% or no zinc

— preferred in setting where moisture control is difficult as it avoid delayed expansion

types of amalgam based on composition alternatives (new types)

Based on composition alternatives (new types)

1. New amalgam alloys

— developed to address the mercury toxicity concerns

— not widely used, as it lacks clinical success and reliability

— eg. gallium based or indium based alloys

types of amalgam based on particle shape of the alloy

1. spherical form

— small, rounded alloy particles

— requires less condensation pressure

— has high early strength (high strength very quickly after placement)

— ideal for large restorations

2. Lathe cut

— irregular shape

— more condensation pressure required

— don’t pack efficiently as there is lots of space between them, therefore they require a higher amount of mercury content in order to fill the gaps

3. Admixed

— irregular particles, sometimes a combination of spherical and lathe cut

— needs more condensation pressure

— helps displace matrix bands, creating better proximal contact

types of amalgam based on copper content

1. Low copper amalgam

— less than 12% copper

— forms gamma 2 phase which is highly corrosive

— leads to rapid corrosion and breakdown of the restoration

— rarely used today

2. High copper amalgam

— 12% or more copper

— copper reacts with tin, preventing gamma 2 phase formation

— reduced corrosion, improved strength durability

— may help sealing the restoration margin via minimal corrosion layer formation

physico-mechanical properties of amalgam

strength and fracture behaviour

• compressive strength is high, similar to tooth structure

• Tensile strength is low, amalgam is prone to bulk fractures (usually high copper types)

• edge strength is low, susceptible to marginal fractures, especially in low copper types

Thermal expansion

• linear coefficient of thermal expansion of amalgam is 2.5x greater than that of tooth structure.

• May cause percolation and marginal gaps if not sealed by corrosion products

Thermal conductivity

• high

• requires use of liners and bases to protect the pulp

Creep and flow

• creep and flow; plastic deformation under low stress over time. This is minimal in good quality high copper amalgam types

Setting reaction and microstructure

• reaction forms

— gamma: unreacted alloy (Ag-Sn phase)

— gamma 1: strong amalgam (Ag-Hg phase)

— gamma 2: weak and corrosive amalgam (Sn-Hg phase)

Cu-Sn is formed in high copper amalgams instead of gamma 2, gives better corrosion resistance

• Brittleness

— amalgam is brittle and requires specific tooth preparation for mechanical retention

— minimum 1-2mm bulk

— 90 degree cavosurface margin

Medico-biological properties

• Biocompatibility

— biocompatible when set properly

— unreacted mercury can be a concern during placement/removal, not when set

• Mercury release

— set amalgam does not release much mercury

— when mercury is unset it is more hazardous, therefore we should use precapsulated amalgam and proper hygiene tools

• Pulpal protection

— amalgam must be insulated from the pulp using liners (calcium hydroxide) and bases (glass ionomer cement)

• Allergic reactions

— very rare and not life threatening

• Environmental and occupational hazards

— mercury vapour exposure during mixing, placement or removal is a risk

— protective measures include: ventilation, hygiene, proper disposal, capsulated systems