Amalgam Restoration

Amalgam Restoration

• Amalgam is an alloy of mercury combined with one or more metallic elements
• Dental amalgam is an alloy of mercury with silver, tin, and varying amounts of copper, zinc, and other minor constituents
• Mercury's unique characteristic is that it is a metal that is liquid at room temperature.

Advantages of Amalgam Restorations

• Super adaptation to cavity walls improves with aging through self-sealing
• Self-sealing occurs as corrosion products deposit and occlude the tooth-restoration interface, decreasing microleakage, recurrent caries, and pulp affection
• High compressive strength allows the restoration to withstand occlusal forces without fracture
• Adequate form stability allows restoration to maintain surface polish, occlusal anatomy, and interproximal contact
• Insolubility
• High wear resistance
• Low creep of modern high copper alloy
• Low coefficient of thermal expansion, approximately twice that of the tooth ensures marginal integrity when subjected to thermal changes
• Ease of manipulation: Amalgam is easier to manipulate and less technique-sensitive
• Low cost and short time are required for constructing restorations
• Favorable long-term clinical results: Amalgam has a long history of successful use

Disadvantages of Amalgam Restorations

• Low tensile and shear strength, approximately only 25% of its compressive strength, makes it a brittle restoration
• This brittleness makes it liable to fracture under high tensile & shear stresses and can lead to isthmus fracture and marginal ditching
• Poor esthetics due to metallic color can lead to excessive discoloration due to tarnish & corrosion
• Cannot be used in esthetic areas due to poor color
• Lack of adhesion to tooth structure: Requires mechanical retention through undercuts and grooves in cavity preparation, which cannot reinforce weakened tooth structure
• Creep is time-dependent plastic deformation of set material in the mouth after setting, resulting in instability, marginal deterioration, flattening of contact, gingival overhang, and saucer-like deformation of occlusal anatomy
• Creep is decreased in high-copper alloy
• Flow is deformation of amalgam restoration under load before it has set
• High thermal conductivity can cause pulp irritation if not protected
• Adequate remaining dentin bridge or base material is needed in deep cavities to protect from thermal conductivity side effects
• Galvanism can occur if another metallic restoration is placed in close proximity, such as cast gold, resulting in patient discomfort or a metallic taste and accelerating corrosive breakdown of electro-negative metals
• Potential health hazards due to the presence of mercury in the amalgam should be considered

Indications for Amalgam Restorations

• Primary application is the restoration of posterior teeth
• Amalgam, along with posterior resin composite and indirect restorations are used in Class I & II cavities (Si/Sta 1.1 to 1.4 and 2.1 to 2.4)
• Class V restorations in posterior teeth are an option only when esthetics is not a primary concern
• Restoration of Class III cavities in the distal surface of upper & lower canines is less common
• Amalgam is a core material for build-ups under full coverage restoration

Contraindications for Amalgam Restorations

• Extensive lesions, especially those involving undermined cusps where the cast gold is a material of choice are contraindicated
• Esthetic areas are a contraindication
• Presence of gold restoration can cause galvanic shock
• Rampant caries, where glass ionomer can act as a control restoration, is a contraindication

Materials Selection Factors for Restoration of Posterior Teeth

• Extent of lesion: Amalgam is suitable for restoring small and medium sized Class I & II cavities (Si/Sta 1.2, 1.3, 2.2, 2.3)
• Extensive lesions (Si/Sta 1.4, 2.4) require indirect restorations as they provide support and easier contouring
• Esthetics: Resin composite is preferred for its esthetic properties
• Caries incidence: Amalgam is favored when there is a moderate to high caries incidence because its superior adaptation, low cost, and ease of repair
• Economic: Amalgam is a more cost-effective material

Classification of Amalgam Alloys

• Based on shape of particles: Lathe-cut, Spherical and Spheroidal (admixed) alloys
• Lathe-cut alloy: irregular shape and requires more mercury (50%) for mixing, resulting in less plasticity and requiring heavy condensation pressure. It’s manufactured by milling a cast ingot of amalgam alloy
• Spherical alloy: spherical smooth surface that requires less mercury (42%), leading to better properties and less condensation pressure. It's manufactured by atomization
• Spheroidal (admixed) alloy: mixing of lathe-cut and spherical particles
• Based on copper content: low copper and high copper
• Low copper alloy: contains copper in range 2-6%
• High copper alloy: contains copper in range 12-30%
• Admixed alloy is a mixture of lathe cut and spherical particles.
• Unicompositional alloy is when each particle of alloy has the same chemical composition
• Based on zinc content: zinc containing and zinc free
• Zinc containing alloys: Contains between 0.01% and 1% zinc
• Zinc free alloys: Contain less than 0.01% zinc
• Based on the presence of alloyed metals: Binary, Ternary, and Quaternary Alloys
• Binary allows contain two metals i.e silver and tin
• Ternary allows contain three metals i.e silver, tin, and copper
• Quaternary allows contain four metals i.e silver, tin, copper and zinc
• Based on presence of noble metals: Noble metal alloys and non-noble metal alloys
• Noble metal alloys: Contain a small amount of gold or platinum
• Non-noble metal alloys: Do not contain noble metals
• Based on size of powder particles: Micro-cut, Fine-cut, and Coarse-cut
• Fine-cut powder size has improved adaptation, easier carving and high strength

Composition of Amalgam

• Amalgam comprises amalgam alloy and mercury
• Amalgam alloy is composed of silver-tin alloy with varying levels of copper, zinc, indium, and palladium
• Silver increases strength, reduces setting time, resists tarnish and corrosion, and decreases flow
• Tin helps form a silver/tin compound (gamma-phase) that readily undergoes amalgamation with mercury, increases setting time, reduces strength, hardness and setting expansion
• Copper reduces tarnish and corrosion, decreases creep, and increases the strength of the amalgam
• Zinc acts as a scavenger for oxygen to impede oxidation of Ag, Sn, or Cu during alloy ingot manufacturing
• If zinc-containing alloys are contaminated with moisture, delayed expansion of amalgam occurs which can lead to pain, marginal fracture, or ditching of the amalgam restoration
• Pallidium improves corrosion resistance and enhances mechanical properties
• Indium decreases mercury evaporation and the amount of mercury needed to wet the alloy particles

Setting Reaction of Amalgam

• Low copper amalgam (Lathe cut): Ag3Sn + Hg results in Ag2Hg3 + Sn8Hg + unreacted Ag3Sn
• The Sn8Hg (gamma 2) phase is the weakest, softest, most brittle, and corrodible phase in amalgam
• Admixed High copper amalgam (Lathe-cut & Spherical) occurs in two phases
• The initial reaction is similar to that of low copper alloy: Ag3Sn + Ag-Cu + Hg results in Ag2Hg3 + Sn8Hg + unreacted Ag3Sn
• The second phase involves the silver-copper eutectic alloy (Ag-Cu): Sn8Hg + Ag-Cu results in Ag2Hg3 + Ag3Sn + Cu6Sn5
• A disadvantage for the admixed, high copper amalgam is that is that it takes time to replace (Υ2) with (Υ1), (Υ) and (η) eta phase, about one week
• Unicompositional High copper amalgam alloys are produced by melting together all components of the high copper system (silver, tin, and copper) creating a single composition, spherical or lathe-cut alloy
• The difference in admix and unicompositional alloys is absent of eutectic alloy Ag-Cu as the reaction is directly with silver, copper, and tin phases. In these, only silver reacts with mercury and the tin remains bound to copper
• Ag3Sn + Cu3Sn + Hg result in Cu6Sn5 + Ag2Hg3

Cavity Preparation for Amalgam Restorations

• Conservative outlines are needed
• Must provide adequate bulk for strength of material given through depth of cavity and at isthmus area
• Saucering of pulpal floor at the isthmus area is needed
• Incline axial wall toward isthmus
• Round axio-pulpal line angles.
• Preparation should have a cavosurface angle (CSA) of 90 degrees (butt joint) for adequate strength
• If CSA is obtuse, fracture is likely
• If CSA is acute, a fracture tooth risk is likely (brittle unsupported enamel rods marginal ditch)
• Cavity walls need to be parallel or perpendicular to occlusal loads
• Undermined enamel should be eliminated to avoid fracture under load
• Smooth walls and floor and rounded line angles help avoid stress concentration
• Sufficient retention features, such as undercuts & grooves, should be applied
• Each portion of compound cavities should provide adequate self-retention & resistance
• In compound cavities, the gingival seat should be definite, with proper width and depth
• Reduce undermined cusps— reduced to a minimum 2mm for cusp building with amalgam

IManipulation of Amalgam:

• Consists of selection of alloy, proportioning, trituration, condensation, carving, finishing and polishing

Selection of Alloy:

• Several variables should be considered during selection of amalgam alloy, including shape, size, its copper, zinc content and form

• Spherical alloys:

  • Advantage: provides the amalgam of soft consistency -----> it contains less mercury concentration which needs light condensation force; Provide smooth surface;
  • Indication: pulp capped teeth where pressure is contraindicated; around pin in pin retained restoration
  • Contraindication: not recommended for extensive restoration--------> cannot establish proper contour and contact areas and development of marginal overhangs.
  • Blended or admixed alloy shape is indicated in extensive restoration especially that needs cusp building and extensive restoration of contact and contouring.

• ii. Copper content amalgam:

  • High copper amalgam:
    • Advantage: Low creep value, high strength properties and high corrosion resistance;
    • Indication: in extensive restoration and is used when margins involve centric holding areas

• Zinc containing amalgam:

  • zinc free used in areas of moisture control is difficult;
  • zinc free amalgam showed less plasticity and workability;
  • if used zinc containing amalgam in moisture contamination area lead to delay expansion of amalgam, surface blistering and pain; ZnO+ H (reaction take place in24-72hrs after amalgam insertion)

• Iv. Form supply of alloy mercury: May be supplied in powder, tablets or in capsules

• Capsules eliminate chance of mercury spills and exposure to mercury vapor and provide more reliable and standardized pre weighted amalgam alloy.

Proportioning of alloy and Hg

• Capsules are more accurate proportioning as it pre-proportioned and not allow for individual variations in amount alloy/mercury ratio.
• Dispensing of alloy is always more accurate by weight rather than by volume due to volume is affected by variations in shape and size of particles.
• The amount of alloy and Hg to be used depend on:

a-amount required to overfill cavity b- speed and dexterity of operator

• amount which can be condensed within its limited working time Less Hg : Non - coherent, friable mix of amalgam. The mix have low plasticity and workability. The restoration will be Weak and corrodible restoration. Excess Hg : Formation Y2 phase. Reduction in Strength and hardness Increased Expansion increased flow and creep Techniques of proportioning:

1- Wet mix techni