Dental Amalgam

What Is It?

• Hard and durable
• Highly radiopaque on dental radiographs
• Silver-grey ("silver filling") in appearance
• Resistant to secondary caries, partly due to antibacterial properties

Composition

• Creep: maximum 1.0%
• Dimensional change: -0.15% to +0.20%
• Compressive strength: minimum 80 MPa at 1 hour, minimum 300 MPa at 24 hours

Radiographic Appearance

Brogdon's Forensic Radiology - Restorative materials on X-ray: C = composite resin, A = amalgam (highly radiopaque), P = porcelain-fused-to-metal crown, G = gutta percha root canal filling

Clinical Properties and Advantages

• Ease and speed of placement - faster to place than composite resin, especially in moisture-prone environments
• Longevity - well-placed amalgam restorations can last 10-15+ years
• Cost - generally less expensive than tooth-colored alternatives
• Antibacterial effect - release of metal ions inhibits biofilm at margins
• Tolerant of moisture - can be placed in difficult-to-isolate areas where composite bonding would fail
• Reparable - defective restorations can often be repaired rather than replaced

Mercury and Safety

Chemistry of Mercury in Amalgam

Mechanisms of Mercury Toxicity

1. Sulfhydryl binding - Hg²⁺ avidly reacts with -SH groups of proteins, altering tertiary structure and abolishing biological activity; the kidney is the primary target organ
2. Autoimmune/immunogenic effects - Structural protein changes render them immunogenic, triggering B-lymphocyte proliferation; collagen tissues are particularly sensitive
3. Lipophilic neuronal binding - Alkyl mercury species (e.g. methylmercury) bind proteins in lipid-rich tissue; myelin is particularly susceptible

Does Amalgam Cause Harm?

• Allergy to amalgam components (mercury, nickel, tin)
• Severe renal disease (impaired mercury excretion)
• Pregnant women and young children (some guidelines recommend avoiding new amalgam placements)
• Patients with neurological conditions where additional mercury exposure may be inadvisable

Oral Mucosal Complications

Amalgam Tattoo

• Well-defined, grayish-black macular discoloration
• Asymptomatic, benign
• Highly radiopaque fragments may be visible on X-ray

Lichenoid Contact Reactions

• Topographically related to the amalgam filling (adjacent mucosa)
• Patients may be patch-test positive to mercury or other metals
• Removal of the amalgam resolves the lesions in most cases

Regulatory and Environmental Status

• The EU provisionally agreed (February 2024) to prohibit dental amalgam use from January 1, 2025 due to environmental mercury concerns (Minamata Convention on Mercury)
• The US FDA issued rules in 2010 reclassifying amalgam and specifying special controls
• The BDA (British Dental Association, 2024) argues that a complete phase-out without adequate alternatives risks worsening oral health inequalities, as amalgam remains the only feasible option in some clinical scenarios (lack of isolation, disability, dementia)
• Environmental impact relates to amalgam waste in dental wastewater and cremation emissions

Alternatives

Recent Evidence (PubMed, 2024-2026)

• Al-Sulimmani et al. (2025) - Systematic review and meta-analysis comparing failure risk of composite resin vs. amalgam restorations [PMID: 40578032]
• Shaw et al. (2026) - Systematic review of health impacts of amalgam vs. alternative restorative materials [PMID: 41643924]
• Hatipoglu et al. (2024) - Meta-analysis on repair vs. replacement of defective direct restorations [PMID: 38796090]

• Goodman & Gilman's Pharmacological Basis of Therapeutics - Heavy Metals chapter
• Tietz Textbook of Laboratory Medicine, 7th Ed. - Mercury: Sources, Toxicokinetics
• Brogdon's Forensic Radiology - Dental Restorative Materials
• Andrews' Diseases of the Skin - Lichenoid Reactions
• Cummings Otolaryngology - Amalgam Tattoo
• IADR Safety of Dental Amalgam (2024)
• ADA Amalgam Topic
• BDA Position Paper, February 2024

Dental Amalgam - BDS Level Notes

Note: The textbook library does not include BDS-specific dental materials books (Phillips' Science of Dental Materials, Craig's Restorative Dental Materials, etc.), but the content below is compiled from standard BDS dental materials curriculum sources aligned with those texts.

Definition

According to Skinner: "Amalgam is a special type of alloy in which mercury is one of the metals."

History (Key Points)

• 1826 - Auguste Taveau (France) first used silver-mercury paste
• 1833 - Crawcour brothers introduced it in USA ("Royal Mineral Succedaneum")
• 1895 - GV Black standardized cavity preparation and amalgam formula
• 1963 - Innes & Youdelis introduced high-copper admixed alloy
• 1970s - Single-composition high-copper alloys developed

Classification

1. By Copper Content

2. By Particle Shape

• Lathe-cut (irregular) - ground ingot, irregular particles; requires more mercury
• Spherical - atomized particles; needs less mercury, less condensation pressure
• Spheroidal (admixed) - mix of lathe-cut + spherical

3. By Zinc Content

• Zinc-containing (>0.01% Zn) - Zinc acts as scavenger for oxygen during manufacturing; more prone to delayed expansion if contaminated with moisture
• Zinc-free - no delayed expansion risk

4. By Particle Size

• Coarse cut, Fine cut, Microcut (finer particles = faster reaction, faster strength gain)

5. By Generation

Composition

Alloy Powder Composition (pre-mercury)

Phases in Amalgam (VERY IMPORTANT for BDS exams)

Greek Phase Notation

Setting Reaction

Low-Copper (Conventional) Amalgam

γ (Ag₃Sn)  +  Hg  →  γ (Ag₃Sn)  +  γ₁ (Ag₂Hg₃)  +  γ₂ (Sn₇₋₈Hg)

• Hg dissolves Ag and Sn from the alloy surface
• Intermetallic compounds (γ₁ and γ₂) precipitate out
• γ₂ remains in the set amalgam → weakness and corrosion

High-Copper Amalgam (Admixed)

γ + Cu-Ag eutectic + Hg → γ + γ₁ + γ₂ → γ₁ reacts with γ₂ → η (Cu₆Sn₅) + γ₁

• After initial γ₂ formation, the copper-silver eutectic reacts with γ₂
• Cu preferentially reacts with Sn → forms Cu₆Sn₅ (eta phase)
• γ₂ is consumed/eliminated → superior properties

High-Copper Single Composition (Unicomposition)

γ (Ag₃Sn) + ε (Cu₃Sn) + Hg → γ + γ₁ (Ag₂Hg₃) + η (Cu₆Sn₅)

• No γ₂ formed at all
• Highest strength, lowest creep

• γ₂ is the weakest and most corrosion-prone phase
• It is responsible for marginal ditching (gap formation at cavity margins over time)
• High-Cu alloys have better creep resistance, strength, and longevity

Properties of Set Amalgam

Mechanical Properties

• Compressive strength - Highest of all properties; ~310-520 MPa (high-Cu > low-Cu)
• Tensile strength - Low (~60 MPa); amalgam is brittle under tension
• Shear/torsion strength - Low; do not carve too aggressively
• Requires bulk of material at margins (never feather edges)

Dimensional Change

• Initially slight contraction (as Hg reacts with alloy surface)
• Then slight expansion (as γ₁ crystals grow)
• Net change should be minimal (-0.15% to +0.20% per ADA standard)
• Excessive contraction → marginal gap → microleakage → secondary caries
• Excessive expansion → post-operative pain, extrusion, tooth fracture

Creep

• Slow plastic deformation under sustained load
• Low-Cu alloys: creep up to 8% → marginal ditching
• High-Cu alloys: creep 0.09-1.8% → far less ditching
• Formula: Higher γ₂ = higher creep

Corrosion / Tarnish

• Tarnish = surface discoloration (sulfide layer)
• Corrosion = electrochemical degradation; most at γ₂ phase
• High-Cu alloys are more corrosion-resistant (no γ₂)
• Corrosion products can expand and seal marginal gaps (self-sealing) - a debated advantage of conventional alloys

Thermal Properties

• Thermal conductivity is high (metal) → needs liner/base under deep cavities
• Coefficient of thermal expansion higher than tooth structure → marginal stresses

Manipulation of Amalgam (Clinical Steps)

1. Proportioning

• Eames Technique (1:1) - Mercury:Alloy ratio of 1:1 by weight (less mercury = better properties)
• Older techniques used higher Hg ratios (8:5)
• Pre-capsulated systems are most common now (correct ratio pre-set)

2. Trituration (Mixing)

• Mechanical mixing in an amalgamator/triturator (capsule spun at high speed)
• Optimal mix: homogeneous, cohesive, slightly warm, not sticky - called "Zipper test" / "snap test"
• Over-trituration → too much heat, rapid set, reduced working time, excess expansion, reduced strength
• Under-trituration → dry, crumbly, granular mix → weaker, more porous, increased corrosion

3. Mulling

• Kneading the amalgam mass in chamois leather or plastic bag after trituration
• Produces smoother, more homogeneous mix

4. Condensation

• Packing amalgam into prepared cavity in small increments
• Use progressively larger condensers
• Pressure drives out excess mercury → lower final Hg:alloy ratio → stronger restoration
• Spherical alloys require less condensation pressure than lathe-cut
• No amalgam should be placed >3 minutes after start of mixing
• Condensing a partly-set amalgam → poor adaptation, weak margins

5. Pre-carve Burnishing

• Using large burnisher for ~15 seconds
• Light force, from center outward to margins
• Reduces surface voids, brings excess Hg to surface for removal during carving

6. Carving

• Within the working time (before set)
• Carve occlusal anatomy, define margins
• Remove excess amalgam
• Do not create thin fins at margins (no feather edges) - they fracture

7. Post-carve Burnishing (Polishing)

• Done 24 hours later (after full set)
• Smooths surface, reduces tarnish and plaque adhesion
• Use slow speed with light, intermittent pressure
• Avoid heat generation (can volatilize mercury)

Indications and Contraindications

Indications

• Class I and II (posterior) restorations that can be well-isolated
• Class VI restorations (cusp tips)
• Large multi-surface restorations (when composite bonding is questionable)
• Patients with high caries risk (moisture control difficult)
• Core build-ups under crowns
• Patients where composite placement technique cannot be adequately performed

Contraindications

• Allergy to mercury, silver, copper, tin, or zinc
• Anterior (visible) areas - aesthetics
• Inability to isolate (relative - amalgam is more forgiving than composite)
• Pregnant women - avoid new placements (precautionary)
• Patients with severe renal disease
• Small Class III/IV restorations (use composite)
• Children < 6 years - FDA advises avoiding in deciduous teeth

Advantages vs. Disadvantages

Cavity Design for Amalgam

• Retention form - parallel or slightly convergent walls (dovetail, undercuts)
• Resistance form - flat pulpal floor perpendicular to occlusal forces
• Minimum depth - 1.5-2.0 mm (minimum bulk for strength)
• Cavosurface angle - 90° butt joint margin (not beveled - thin amalgam at bevel fractures)
• No feather edges

Mercury Safety Precautions (Important for Clinical Practice)

• Use pre-capsulated alloys (avoid open mercury bottles)
• Work in well-ventilated areas
• Wear gloves, mask, eye protection
• Use high-volume evacuation during placement and removal
• Never heat amalgam (increases mercury vapor)
• Store scrap amalgam in sealed containers with water or fixative
• Handle old amalgam removal carefully (rubber dam, water spray, evacuation)
• Amalgam separators required in dental unit wastewater

Bonded Amalgam

• Amalgam can be bonded to tooth using dentin bonding agents
• Benefits: reduced microleakage, reinforces weakened tooth structure, reduces post-op sensitivity
• Not universally adopted but useful in large restorations and cusp replacements

Comparison: Low-Copper vs. High-Copper Amalgam

Exam Memory Tips

• γ₂ = BAD (weak, corrodes, causes ditching) → High-Cu alloys eliminate it
• Gamma phases: γ = alloy particle, γ₁ = matrix (good), γ₂ = weak link (bad)
• Eames technique = 1:1 Hg:Alloy ratio
• Creep = sustained deformation under load → worst in low-Cu
• Zinc in amalgam = scavenger; moisture contamination → delayed expansion (can be >24 hrs later)
• ADA standard: Creep max 1.0%, Dimensional change -0.15 to +0.20%, Compressive strength min 80 MPa (1 hr), 300 MPa (24 hr)
• Never condense amalgam after 3 minutes from start of mix

• Phillips' Science of Dental Materials (Anusavice, Shen & Rawls) - Chapter on Amalgam
• Craig's Restorative Dental Materials (Sakaguchi & Powers)
• Dental Materials: Properties and Manipulation (Craig & Ward)
• Pocket Dentistry - Dental Amalgam (free online)