7.2 Amalgam, Composite Resin & Glass Ionomer
Key Takeaways
- Dental amalgam is mercury mixed with silver-tin-copper alloy; high-copper alloys minimize the weak, corrosive γ₂ (Sn–Hg) phase and reduce creep-related marginal failure.
- Composite resin is a filled methacrylate polymer; filler loading, degree of conversion, polymerization shrinkage, and cavity C-factor control stress, seal, and clinical success.
- Glass ionomer sets by acid–base reaction, bonds chemically to tooth mineral, and releases (and can recharge) fluoride—ideal for root caries, high-risk, and sandwich applications.
- Amalgam needs macromechanical retention and tolerates moisture better than bonding steps; composite needs isolation and adhesive technique; GIC trades strength for adhesion and fluoride.
- Material selection balances aesthetics, occlusal load, isolation quality, caries risk, and remaining tooth structure—not a single universal restorative for every cavity.
7.2 Amalgam, Composite Resin & Glass Ionomer
Quick Answer: Dental amalgam is a metallic restorative from silver-tin (and copper) alloy mixed with mercury; modern high-copper alloys minimize corrosive γ₂ (Sn–Hg). Composite resin is a filled polymer relying on adhesion, with properties dominated by filler, polymerization shrinkage, and C-factor. Glass ionomer cement (GIC) chemically bonds to tooth and releases fluoride, excelling in low-stress, high-caries-risk, and moisture-challenged sites. Choose materials by load, isolation, aesthetics, and caries risk—not brand slogans.
This section is the core direct-restorative materials triad for AFK. Biomaterials science (modulus, creep, corrosion, polymerization) was introduced earlier; here you apply those properties to clinical selection and failure modes.
Dental Amalgam
Composition and mixing
Amalgam = liquid mercury + alloy powder (triturated → plastic mass → sets in the cavity).
| Alloy component | Typical role |
|---|---|
| Silver (Ag) | Strength, corrosion resistance, sets expansion tendency |
| Tin (Sn) | Improves amalgamation; reduces expansion; involved in γ₂ in low-copper alloys |
| Copper (Cu) | Strength; in high-copper alloys suppresses γ₂ |
| Zinc (Zn) (if present) | Scavenger in manufacturing; moisture contamination of zinc-containing amalgam can cause delayed expansion |
| Mercury (Hg) | Reacts with alloy particles to form matrix phases |
Trituration mixes alloy and mercury. Under/over-trituration alters plasticity, working time, and strength. Condensation adapts material to walls and reduces excess mercury-rich matrix; carving establishes anatomy before full set; polishing (when indicated) is delayed until set is adequate.
Metallurgical phases (high yield)
Classic low-copper amalgam reaction (simplified teaching):
- Alloy particle (mainly γ = Ag₃Sn) + Hg → γ₁ (Ag₂Hg₃) matrix + γ₂ (Sn₇–₈Hg) + unreacted γ particles
| Phase | Identity | Clinical meaning |
|---|---|---|
| γ (gamma) | Ag–Sn alloy particle remnant | Unreacted core; strengthens if well distributed |
| γ₁ (gamma-1) | Ag–Hg | Principal matrix; relatively corrosion-resistant |
| γ₂ (gamma-2) | Sn–Hg | Weak, corrosion-prone; linked to breakdown and ditching in low-copper amalgams |
| η' (eta-prime) | Cu–Sn (e.g., Cu₆Sn₅) in high-copper systems | Replaces γ₂ pathway; improves corrosion resistance and strength |
High-copper amalgams (single-composition or admixed) are modern standard teaching: copper reacts preferentially with tin so γ₂ is minimized or eliminated, improving clinical performance.
Properties and clinical behavior
| Property | Amalgam teaching point |
|---|---|
| Strength | Adequate compressive strength for many posterior loads after full set; early strength lower—avoid heavy occlusion immediately |
| Creep | Time-dependent plastic deformation under load; high creep associated with marginal breakdown; high-copper alloys show lower creep |
| Dimensional change | Slight contraction or expansion depending on composition/technique; zinc + moisture → delayed expansion |
| Corrosion | Seals microgaps over time (corrosion products) but excessive corrosion weakens low-copper alloys |
| Thermal conductivity | High → deep amalgams may need base/liner for pulp thermal protection historically emphasized |
| Esthetics | Metallic gray—not for aesthetic zones |
| Bond to tooth | None chemically—needs mechanical retention form |
| Longevity | Excellent track record in stress-bearing posterior restorations when isolation and form are correct |
Advantages and limitations
| Advantages | Limitations |
|---|---|
| Durable in heavy occlusion | Poor aesthetics |
| Technique relatively forgiving of mild moisture vs composite bonding | Requires more tooth removal for retention (undercuts) |
| Self-sealing corrosion products | Mercury content—public concern; environmental handling rules |
| Good wear resistance | Can fracture tooth if large/weak cusps not protected |
| Radiopaque | Galvanic effects with dissimilar metals possible |
Mercury safety (exam level): Set amalgam is stable for clinical use; occupational risk is mainly from vapor during placement/removal—use water spray, high-volume suction, rubber dam, and proper scrap storage. True allergy is uncommon; lichenoid contact reactions are a separate Type IV discussion.
Indications (typical)
- Moderate-to-large posterior restorations where isolation for bonding is poor and aesthetics are not critical
- Core build-ups in non-aesthetic zones (composite/RMGI cores often compete)
- Patients where moisture control or cooperation limits adhesive success
- Situations needing a robust direct metallic restoration under heavy function
Composite Resin
Basic structure
Dental composite = resin matrix + inorganic filler + coupling agent (silane) + initiators/accelerators + pigments/stabilizers.
| Component | Role |
|---|---|
| Matrix (e.g., Bis-GMA, UDMA, TEGDMA) | Polymerizable organic phase; TEGDMA lowers viscosity but may increase shrinkage |
| Filler (silica, glass, zirconia-silica) | Strength, wear resistance, ↓ shrinkage relative to unfilled resin, radiopacity, handling |
| Silane coupling agent | Bonds filler to resin so stress transfers through the composite |
| Photoinitiator (e.g., camphorquinone systems) | Light-activated free-radical polymerization |
Classification by filler (common teaching)
| Type | Filler character | Clinical use notes |
|---|---|---|
| Macrofill (historical) | Large particles | Rough; rarely used today |
| Microfill | Submicron silica | Excellent polish/esthetics; weaker—anteriors, not heavy stress |
| Hybrid / microhybrid | Mixed particle sizes | Strength + decent polish; universal workhorses historically |
| Nanofill / nanohybrid | Nano-scale particles ± blends | High polish retention + strength; common modern universal composites |
| Flowable | Lower filler % | Lower viscosity; liners, small preparations, PRR; higher shrinkage stress risk |
| Packable / condensable | Higher viscosity | Posterior handling mimic of amalgam—less emphasized than early marketing suggested |
| Bulk-fill | Chemistry/translucency for thicker increments | Faster posterior placement if used per depth limits |
Filler loading roughly tracks mechanical properties: more filler (by volume) → higher strength/modulus, lower polymerization shrinkage strain of the paste, better wear—up to limits of handling.
Polymerization
Most direct composites are light-cured free-radical addition polymers:
- Light (typically blue, ~468 nm for CQ) activates initiator system
- Free radicals open methacrylate double bonds
- Polymer network forms; degree of conversion is never 100%
- Material hardens; residual monomer and incomplete cure at depth are clinical risks
| Factor improving cure | Factor impairing cure |
|---|---|
| Adequate light intensity and correct wavelength | Old/damaged light, wrong tip distance |
| Appropriate exposure time | Too short exposure |
| Incremental thickness within manufacturer limits | Oversized bulk beyond material capability |
| Light color-compatible shade/translucency | Very dark/opaque shades need more attention |
Chemical-cure / dual-cure systems are used for cores, cements, and deep areas where light access is limited.
Polymerization shrinkage and C-factor
Composite shrinks toward the center of the mass / toward bonded walls as monomers pack closer—volumetric shrinkage often taught in the ~2–5% range depending on formulation.
C-factor (configuration factor) = bonded surfaces / unbonded surfaces.
| C-factor | Example | Stress implication |
|---|---|---|
| High (e.g., ~5) | Class I box with five walls bonded, only occlusal free | High shrinkage stress → debond, cusp flexure, postoperative sensitivity, white line margins |
| Moderate | Class II | Intermediate |
| Low | Class IV or small chip with few bonded walls | Stress relieved more easily |
Clinical controls for shrinkage stress:
- Incremental placement (oblique layers) to lower effective C-factor per increment
- Soft-start or appropriate light protocols (material-dependent teaching)
- Bond to enamel margins when possible (stronger, more stable bond)
- Avoid overheating pulp with excessive continuous light on deep preps
- Use of liners/flowables judiciously (does not eliminate need for good technique)
- Bulk-fill materials only within their indicated depth and technique
Clinical strengths and weaknesses of composite
| Strengths | Weaknesses |
|---|---|
| Excellent aesthetics | Technique sensitive (isolation, bonding) |
| Conserves tooth (adhesion) | Polymerization shrinkage stress |
| Bonds and can reinforce remaining structure | Wear/degradation over time; secondary caries if seal fails |
| Versatile for III, IV, V, I, II | Requires enamel for best margins when possible |
| Repairable | Depth of cure limits; light access |
Indications: aesthetic restorations; conservative posterior restorations with good isolation; recontouring; diastema closure; core build-ups (appropriate materials); sealants/PRR (related resin systems).
Glass Ionomer Cement (GIC) and Resin-Modified GIC
Chemistry and bond
Conventional GIC: fluoroaluminosilicate glass powder + polyalkenoic acid liquid (e.g., polyacrylic acid). Setting is an acid–base reaction (not free-radical polymerization).
Chemical adhesion: carboxyl groups of polyacid chelate/bond to calcium in enamel and dentin—true ionic adhesion without light (though conditioning improves performance).
| Property | Conventional GIC | Resin-modified GIC (RMGI) |
|---|---|---|
| Set | Acid–base | Acid–base + resin polymerization (light/dual) |
| Moisture | More tolerant than composite bonding, still not “wet field ignore” | Improved early strength/handling |
| Strength / wear | Lower—avoid heavy stress-bearing bulk alone | Better than conventional; still < hybrid composite for high stress |
| Fluoride | Significant release/recharge potential | Fluoride release retained |
| Aesthetics | Opaque/acceptable | Improved translucency |
| Finish | Sensitive early | Can finish earlier after light cure (product-dependent) |
Fluoride release
GIC releases fluoride into adjacent tooth and plaque fluid and can be recharged by topical fluorides—useful for high caries risk, root caries, and sandwich techniques. Fluoride release alone does not replace plaque control or justify leaving open margins.
Clinical uses
| Use | Why GIC/RMGI fits |
|---|---|
| Root caries / Class V in elders | Adhesion to dentin/cementum, fluoride, moisture tolerance |
| High caries-risk temporary or transitional restorations | Fluoride; easy repair/replace |
| Liner/base under composite (open/closed sandwich) | Seals dentin, fluoride, reduces configuration stress somewhat |
| Atraumatic restorative treatment (ART) | Hand excavation + GIC in public health settings |
| Luting (some GICs) | Chemical bond for crowns/ortho bands (select products) |
| Pediatric restorations when isolation poor | Handling and fluoride advantages |
Not ideal as sole material for large stress-bearing posterior restorations in adults when composite or amalgam/indirect options are feasible—wear and fracture risk.
Comparative Selection Table
| Criterion | Amalgam | Composite | GIC/RMGI |
|---|---|---|---|
| Aesthetics | Poor | Excellent | Fair–good (RMGI better) |
| Primary retention | Mechanical | Micromechanical/chemical adhesion systems | Chemical ± light resin |
| Moisture tolerance | Moderate | Poor for bonding steps | Better than composite |
| Fluoride | None meaningful | None (unless giomer-type hybrids) | Yes |
| Shrinkage stress | N/A (sets metallurgically) | Significant issue | Low polymerization stress (conventional) |
| Heavy occlusion bulk | Strong | Good with proper design | Weakest |
| Tooth conservation | Least (retention form) | Best | Good |
Failure Modes Worth Memorizing
- Amalgam: marginal ditching (historical γ₂/creep), bulk fracture, tooth cusp fracture, recurrent caries at margins, galvanism
- Composite: debonding from shrinkage stress, secondary caries from leakage, wear, discoloration, postoperative sensitivity from open margins or aggressive etch on deep dentin without protection strategy
- GIC: wear, fracture in stress, early moisture contamination of conventional set, desiccation cracking if unprotected
AFK Integration Scenarios
- Large MOD in a second molar, poor isolation, heavy bruxism, low aesthetic demand → amalgam (or indirect) often more reliable than a heroic multi-surface composite.
- Class IV fracture central incisor → composite with enamel bevel and adhesive protocol.
- Root caries on buccal of mandibular premolar, high risk → RMGI/GIC excellent first-line direct choice.
- High C-factor deep Class I → incremental composite, excellent bond to enamel rim, consider liner; do not bulk improperly.
- Why high-copper amalgam is preferred → eliminates/reduces γ₂ → less corrosion, less creep, better margins.
Rapid review list
- Amalgam phases: γ, γ₁, γ₂; high-copper → η′ pathway, little γ₂
- Creep ↑ with poor alloys → marginal failure
- Composite = resin + filler + silane; light cure; shrinkage + C-factor drive stress
- GIC = acid–base glass + polyacid; chemical bond; fluoride release
- Match material to load, isolation, aesthetics, and caries risk
Section 7.3 details how adhesives create the hybrid layer that makes composite dentistry work, and how liners/bases and pulp caps protect the pulp when preparations deepen.
In low-copper dental amalgam, which phase is most associated with corrosion susceptibility and weaker clinical performance?
C-factor is defined as:
Which direct material is best characterized by an acid–base setting reaction, chemical adhesion to tooth structure, and sustained fluoride release?
A major clinical disadvantage of posterior composite relative to amalgam is: