14.1 Properties & Testing of Dental Materials
Key Takeaways
- Mechanical properties of clinical relevance include compressive strength, tensile strength, elastic modulus, hardness, and fracture toughness; brittleness and ductility determine how a material behaves under load.
- Stress is force per unit area (Pa), strain is proportional deformation; the stress–strain curve defines the elastic (reversible) region, proportional limit, yield stress, and ultimate strength.
- The oral environment is hostile: temperature swings (5–55 °C thermal cycling), pH 4–8 cycles, masticatory loads, and moisture — all drive material degradation and testing standards (ISO).
- Dimensional stability, setting contraction (polymerisation shrinkage), and water sorption govern marginal seal and clinical longevity.
- Wettability and viscosity control adaptation and impression detail; surface energy and roughness drive plaque retention and wear of opposing teeth.
Why Material Properties Matter
A restoration fails when applied stress exceeds the material's strength, or when the oral environment degrades it faster than it wears. Choosing a material means matching its mechanical, physical, and biological properties to the clinical situation: a Class II amalgam needs compressive strength and long-term dimensional stability; a Class IV composite needs tensile strength, modulus matching dentine, and wear resistance; a luting cement needs film thickness and solubility control.
Mechanical Properties — The Core Definitions
| Property | Definition | Clinical meaning |
|---|---|---|
| Stress (σ) | Force per unit cross-sectional area (Pa, MPa) | Internal resistance to applied load |
| Strain (ε) | Change in length per original length (dimensionless) | Deformation produced by stress |
| Elastic modulus (E) | Stress / strain in the elastic region (slope of linear part) | Stiffness — high E = stiff (ceramics), low E = flexible (polymers) |
| Proportional limit | Stress beyond which stress–strain is no longer linear | Reversible behaviour ends here |
| Yield stress (yield point) | Stress at onset of plastic (permanent) deformation | Where the material bends and stays bent |
| Ultimate tensile/compressive strength | Maximum stress sustained | Catastrophic failure point |
| Ductility | Plastic deformation before fracture | Metals are ductile; ceramics brittle |
| Fracture toughness (K₁c) | Resistance to crack propagation | Ceramics low (brittle), metals high |
| Hardness | Resistance to indentation/scratching | Wear and polishability |
Stress–Strain Curves
A ductile material (e.g. gold alloy) shows a long linear elastic region, a clear yield, then considerable plastic deformation before fracture. A brittle material (e.g. dental ceramic) shows little or no plastic region — it fractures near the elastic limit. This is why ceramics crack rather than dent.
Elastic vs Plastic Deformation
- Elastic deformation is fully reversible (Hooke's law region); the modulus is its slope.
- Plastic deformation is permanent; metals are valued because they yield plastically, absorbing energy without catastrophic fracture (toughness).
Physical Properties
| Property | Importance |
|---|---|
| Thermal conductivity / diffusivity | Metallic restorations conduct heat to the pulp — liners/bases protect; composites and cements are insulating |
| Coefficient of thermal expansion (CTE) | Mismatch with tooth (enamel CTE ~17, composite ~25–50 ppm/°C) causes percolation at margins on thermal cycling |
| Setting contraction / polymerisation shrinkage | Composites shrink 1.5–5% by volume → marginal gap, microleakage, postoperative sensitivity |
| Water sorption / solubility | Hydrolytic degradation, discoloration; cements must resist dissolution |
| Dimensional stability | Impressions and casts must be dimensionally stable over time |
| Radiopacity | Restorations and cements must be radiopaque for caries detection at margins (EN ISO 4049 requires ≥Al equivalent) |
Surface and Handling Properties
- Wettability (contact angle) — a low contact angle (good wetting) lets an impression capture fine detail and lets a cement adapt; a high contact angle (poor wetting) traps voids. Polyether is hydrophilic (good wetting but absorbs water); addition silicone is hydrophobic.
- Viscosity — determines flow: low-viscosity materials (light-body impression materials, luting cements, flowable composites) adapt to detail; high-viscosity materials (putty, condensable composites) resist flow.
- Surface energy / roughness — low surface energy and low roughness resist plaque; polished surfaces wear the opposing dentition less.
Standards and Testing (ISO)
Laboratory data are only comparable under standardised test conditions. Key standards:
- ISO 4049 — polymer-based restorative materials (sets water sorption/solubility and radiopacity requirements).
- ISO 6871 — zinc phosphate and zinc polycarboxylate cements.
- ISO 9917 — water-based cements (GIC).
- ISO 24234 — amalgam.
- ISO 6872 — dental ceramics (flexural strength by biaxial bend test).
Thermal cycling between 5 °C and 55 °C, water storage at 37 °C, and defined dwell times simulate the oral environment. Flexural strength (often via the three-point bend test) is a clinically relevant strength measure for brittle ceramics because it combines tensile, compressive, and shear stresses.
Clinical Take-Home
No single material dominates every property. Amalgam wins on compressive strength, longevity, and technique tolerance; composite wins on aesthetics and adhesion but loses on polymerisation shrinkage and technique sensitivity; ceramic wins on aesthetics and wear resistance but is brittle and abrasive to opposing teeth. The clinician trades properties against the demands of each site — and against moisture control, occlusion, and patient factors.
Biocompatibility and Handling
A material in the mouth must be biologically acceptable as well as mechanically adequate. Biocompatibility covers cytotoxicity (leached monomers from inadequately cured composite, fluoride release from glass ionomer), allergic potential (nickel in base-metal alloys, methacrylate allergy in dental staff), and pulpal response (deep restorations need a liner or base to insulate the pulp from conductive and chemical insult). Moisture control dominates handling: hydrophilic materials such as glass ionomer and dentine adhesive are unforgiving of contamination, and the bond of an etch-and-rinse or self-etch system fails predictably if saliva or blood wets the prepared surface. Technique sensitivity - the dependence of clinical success on operator-controlled variables such as isolation, placement, light-curing protocol, and mix ratio - is a recurring reason one material is chosen over another in a given practice setting.
Which mechanical property best describes a dental ceramic's tendency to fracture with little prior deformation when loaded beyond its elastic limit?
A composite with a coefficient of thermal expansion much higher than tooth structure is most likely to cause which clinical problem over time?
In a stress–strain curve, the slope of the linear (Hooke's law) portion corresponds to which property?
Which statement about dental impression materials and wettability is correct?