Lecture 2 & 3 - Dental Materials: Physical, Optical, Rheological, Mechanical, and Biocompatibility Properties
Thermophysical properties
ADA framework: dental materials have three main types of properties—physical, chemical, and biological.
Today’s focus: physical properties which include mechanical properties as a subset.
Brief mention of chemical properties (bonding agents, resin composites) and biological properties (biocompatibility).
Physical Properties is the laws if mechanics, acoustics, optics, thermaldynamics, electricity, magnetism, atomic and nuclear phenomena.
Thermophysical properties.
Optical properties
Rheological properties
Mechanical properties.
Thermophysical properties include five key parameters:
Melting temperature: the transition from solid to liquid; extremely important for dental alloys used in RPD frameworks, metal-ceramic crowns, pure metallic crowns, and orthodontic wires.
Thermal conductivity: how much heat passes through a material per unit area per unit temperature gradient.
Thermal diffusivity: how fast a material reaches thermal equilibrium when heated; related to thermal conductivity, density, and specific heat.
Specific heat: the amount of heat required to raise the temperature of a unit mass by one degree.
Thermal expansion Coefficient: how much a material expands or contracts with temperature change.
Practical context in the mouth:
Thermophysical properties determine how heat from hot or cold foods/beverages is transferred to the dental pulp via the tooth structure.
The pulp is soft tissue enclosed by dentin/enamel; heat transfer can cause pain if the pulp is stimulated.
Different materials transfer heat differently; this affects patient comfort and the design of restorations.
Melting temperature in dentistry: ( Welding, RPD framework, Fixed prosthodontics)
RPD frameworks often use a cobalt-chromium alloy.
Gold restorations or crowns (historical) and silver/aluminum alloys have higher melting points relevant to manufacturing and processing.
Not a primary exam topic here, but useful for understanding alloy processing and applications.
Thermal conductivity concepts:
Definition: amount of heat transferred through a material by conductive flow. ( calories per second passing through the material)
In the mouth: materials with high thermal conductivity transfer heat to the tooth faster, potentially increasing sensitivity;
materials with low thermal conductivity insulate better.
Comparative trends:
Metals and alloys generally have higher thermal conductivity than ceramics.
Polymers have lower thermal conductivity than ceramics and metals.
Example comparisons:
Enamel vs dentin vs amalgam (a metallic alloy) vs gold vs resin composites vs glass ionomer cement.
Amalgam (metallic) transfers heat more quickly than composites; polymers (resins) are closer to enamel but still lower than metals.
Graphical reference: enamel thermal conductivity on top, dentin below, with amalgam higher than gold, and polymeric materials like resin composites closer to enamel (lower than metals) and glass ionomer cement in between.
Thermal diffusivity:
Definition: the speed with which a temperature change will spread through an object when one surface is heated.
Implications: high density and high specific heat tend to yield low thermal diffusivity (slower heating/cooling), while low density and low specific heat yield high thermal diffusivity (faster temperature changes).
Specific heat:
Definition: amount of heat required to raise the temperature of a given mass by a given temperature change.
Relationship to dental materials: affects how much heat is absorbed by a restoration during a thermal challenge (hot coffee, ice cream).
Coefficient of thermal expansion (CTE):
Definition: how much a material expands or contracts with a change in temperature.
Changes in size and volume of a material related to orginal dimensions due to temperature variations.
Clinical relevance: CTE compatibility with enamel/d dentin is important to minimize stresses at interfaces; liners/bases are chosen to have CTE values closer to dentin/enamel to reduce thermal shock.
Practical implications summary:
Materials with high thermal conductivity (metals) transfer heat quickly and may cause more sensitivity unless insulated.
Insulating liners/bases with CTE similar to dentin/enamel help reduce thermal stresses and pulp irritation.
Glass ionomer cement can act as an insulating liner with relatively favorable thermal properties.
Optical properties:
Light basics:
Light is electromagnetic radiation that can be detected by human eye within a range of wavelengths from approximately 400\, \text{nm} to 700 nm
Wavelength and intensity depend on the light source.( sunlight or Fluorescent light)
Visual/clinical relevance of light:
The color of a restoration is perceived through interaction of light with dental substrates (enamel, dentin) and the material itself.
Visible light interaction during procedures (color matching, curing) is central to clinical outcomes.
Light-tissue interactions:
Reflection: light bounces off a surface; may be diffuse or specular (mirror reflection).
Absorption: light energy is absorbed by the material, contributing to color and potential heating.
Refraction: light changes direction as it passes through a material.
Transmission: light passes through a material; sometimes partially, depending on thickness and composition.
Surface finish effects:
Polished surfaces promote specular reflection (single direction), appearing shiny.
Rough surfaces produce diffuse reflection (scattered directions), appearing dull.
Color dimensions (hue, chroma, value):
Hue: dominant color name (blue, yellow, green, red, etc.).
Chroma (saturation): intensity of a hue ( different shades of colors).
Value (lightness): position on grayscale from dark to light.( transtion of grayscale either dark or light.
Color matching in dentistry:
Tooth color is not monochromatic; multiple shades are needed to mimic natural variation.
The most common color scale used clinically is the data scale with four major hues labeled A, B, C, D (often with additional categories for bleached teeth. (VITA shade guide most common)
A, B, C, D correspond to general hue families:
A: transition red-brown
B: red-yellow
C: gray
D: red-gray
Within each hue, chroma levels are denoted (e.g., A1, A2, A3, A3.5, A4; B1, B2, B3, B4; etc.).
Value or lightness: detected way to know lighted color.
Metamerism:
A phenomenon where two colors appear the same under one light source but differ under another light source.
Example demonstration: two balls appear same color under one light but different under another; light source example is the only change.
Clinical implication: color matching should use multiple light sources to ensure stable color perception; two light sources improve accuracy.
Color measurement tools and color spaces:
Some devices provide color readings in a perceptual color space such as CIELAB
Intraoral color matching devices (color meters) exist but clinicians often rely on trained eyes; intraoral scanners and AI-assisted color matching are evolving.
Fluorescence:
Natural teeth exhibit fluorescence; modern composites often incorporate fluorescence to mimic tooth appearance under various lighting (e.g., blue/violet light at night).
Fluorescence varies by brand and product; many contemporary composites include fluorescence to approximate natural tooth behavior.
Some older composites lacked fluorescence; today many include fluorescence within the expected range.
Fluorescence intensity and perception depend on restoration thickness; thicker layers may reduce visible fluorescence.
Practical note: if a restoration requires fluorescence, verify that the material’s fluorescence aligns with natural teeth for the expected lighting conditions.
Fluorescence and cosmetic considerations:
Fluorescence helps the restoration blend under UV/blue-light exposure (e.g., party/night lighting).
BPA-related safety considerations are separate from fluorescence but influence material selection.
Rheological properties
Rheology definition:
Study of flow dynamics and how materials deform under stress; flow is not limited to liquids; pastes, gels, and viscous solids also show rheological behavior.
Viscosity: A measure of the consistency of a fluid and its resistance to flow, resistance to fluid flow controlled by internal frictional forces. (ex: Filtek Bulk fill, Filtek Supreme Flow, Filtek Supreme)
Viscosity is dental material can behave in different ways responding to various stresses.
The clinical performance may depend on their properties in the liquid or solid state.
Four primary flow behaviors:
Newtonian: constant viscosity independent of shear rate; example liquids like water, milk, many beverages.
Pseudoplastic (shear-thinning): viscosity decreases with increasing shear rate; Example:elastomeric impression materials; viscosity reduces when mixed and handled.
Dilatant (shear-thickening): viscosity increases with increasing shear rate; Example: some denture-base resins at high shear.
Plastic: requires a yield stress to begin flowing; once yielded, material behaves as a fluid with a defined viscosity.
Thixotropy:
Viscosity decreases with time under constant shear; material becomes easier to flow with time under continued shear (common in some toothpaste or prophylaxis pastes and flowable resin systems).
Relevance to dentistry:
Impression materials: often designed to be plastic (yielding at tray insertion) and then flow (pushed into tooth structures) under pressure, providing accurate negative impressions.
Flowable composites and denture-base materials exhibit varying viscosities; thixotropic properties aid handling and material placement.
Thixotropy improves handling by becoming more fluid under stress (placement) and then recovering viscosity when at rest (stability in position).
Creep
Describes the rheology of amorphous solids.
Example: amalgam creep can lead to marginal overhangs and leakage if margins are not periodically monitored; long-term failure includes secondary caries due to microleakage, fracture, or wear.
Prevention: regular follow-up appointments to monitor margins and restoration integrity of amalgam resin.
Practical takeaways:
Many impression materials are designed to be plastic during loading and then set, preventing premature flow or displacement.
Resin composites and other viscous materials benefit from controlled rheology to optimize handling and adaptation.
Mechanical properties
stress, strain, deformation, and the response of materials to loading.
Stress : Measure of force per unit of area acting on an object.
Stress: \sigma = \frac{F}{A} where F is force and A is the cross-sectional area.
Small area is higher stress
Large area is lower stress.
4 Types of stress:
Tensile stress: elongates an object (pulling apart).
Compressive stress: shortens or compress an object (pushing together).
Shear stress: forces sliding one surface past another (orthodontic wire movements).
Flexural stress: combination of tensile and compressive stresses and shear stress during bending of material body.
Mechanical behavior concepts:
Elastic deformation: reversible deformation; the material returns to its original shape after the load is removed.
Plastic deformation: permanent deformation after the yield point; the material does not return to its original shape.
Elastic limit / limit of proportionality: the point at which the stress-strain relationship ceases to be linear; up to this point deformation is elastic.
If the load is removed before the limit of proportionality, the material recovers its original dimensions (elastic behavior).
If the limit is exceeded, plastic deformation occurs and some permanent change remains.
Stress-strain relationship and material properties:
Modulus of Elasticity also called Young's modulus: the slope of the linear portion of the stress-strain curve it is Elastic strain and the curve in graph where it is Plastic strain, at the end of graph is Fracture point where the material fractures under stress.
The higher the inclination —> the higher the angle —> The higher the elastic modulus—→ the Higher Rigidity
Resilience: the amount of elastic energy that is sustained by a material on loading and released after unloading without going over its elastic limit.
Ductility: ability to undergo significant plastic deformation under tensile stress (e.g., orthodontic wires).
Maleability: ability to deform under compressive stress (e.g., forming metal plates in crowns).
Toughness:amount of plastic and elastic deformation energy required to fracture a material, it increases with increase strength and ductility. ( combine of plastic and elastic together into stress strain.
Brittleness: little to no plastic deformation before fracture (typical of many ceramics).
Practical implications for dentistry:
Fixed prosthodontics: choice of material depends on the forces, geometry, and required structural support; flexural and tensile stresses influence margin integrity.
The balance between elastic and plastic deformation (toughness) determines how well a material can withstand chewing forces without fracturing.
Materials differ in their interface behavior at margins; high brittleness increases fracture risk, especially under complex loading.
Practical takeaways:
Ceramic materials tend to be brittle with high elastic modulus but limited plastic deformation, contributing to fracture risk.
Metals and metal alloys generally show higher toughness and ductility, making them suitable for certain load-bearing restorations and frameworks.
Elastomeric impression materials exhibit resilience and controlled flow during impression taking.
Biological properties and biocompatibility
Biocompatibility: the ability of a material to perform its desired function in the body without causing a negative or undesirable local or systemic response.
Biocompatibility is the interaction of HOST, Properties of material and the function.
Ideal biocompatible properties:
Non-toxic, non-irritant, non-allergenic, non-mutagenic, non-carcinogenic.
Local vs systemic effects:
Local effects: at the site of contact in the oral cavity (mucosa, dentin, pulp, gingiva).
Systemic effects: potential effects beyond the local tissues, may arise from materials released or absorbed.
Factors influencing biocompatibility:
Host factors: patient-specific allergies, immune response, and overall health.
Material factors: composition, potential release of substances, and surface properties.
Application factors: how the material is used and placed in the mouth (e.g., curing depth, exposure to tissues).
Common materials and associated biocompatibility issues:
Amalgam: contains mercury; mercury vapor can be toxic; phase-down discussions; proper handling reduces risk to clinicians.
Methyl methacrylate (MMA) and resin monomers: unreacted monomers can irritate tissues or cause allergic reactions; proper polymerization is essential.
Imflammatory reaction to MMA such as processing mistake, too much uncured residue in contact with the oral mucosa.
Bisphenol A (BPA) in resin composites: potential carcinogenicity concerns; BPA-free formulations are increasingly available.
Nickel in wire and RPD:allergy reaction; Nickel-chromium alloys for RPD framework, Nickel-titanium alloys for orthowires.
Titanium implants: titanium shows high biocompatibility due to osseointegration, forming a stable bond with bone rather than eliciting adverse tissue responses.
Specific clinical considerations and scenarios:
Allergic reactions: patch testing and patient history inform material choices (e.g., nickel allergy guiding alloy selection).
Inflammatory or mucosal reactions: polishing and polymerization quality affect tissue response; some reactions can be due to residual monomers or heat from curing units.
Light-induced thermal injury: curing light sources can cause soft tissue burns if not properly managed.
Exposure risk for dental personnel: mercury exposure risk higher for clinicians than for patients; PPE and proper ventilation reduce risk.
Silicosis risk from alginate dust: dental personnel should use PPE due to inhalation risk; dust-free or low-dust formulations recommended.
Whenever possible, amalgam filling shouldn’t place in or remove from the teeth of pregnant women.
Amalgam should not placed in patients with impaired kidney function.
Existing amalgam should be place with another material where this is recommended by physician.
New amalgam should not place in contact with existing metal devices in mouth such as braces.
Dentist should inform the pt of the material using in resin fill in their teeth, including information of risk and benefits of the material and suitable alternatives.
Additional implant and bone considerations:
Osseointegration: titanium’s biocompatibility enables direct bone-implant integration without a periodontal ligament; used in dental implants and other orthopedic devices.
Bone grafting and alveolar preservation: graft materials help maintain bone volume for later implant placement; timing (immediate placement vs staged) depends on bone loss and restoration plan.
Miscellaneous and clinical implications
Practical clinical notes and scenarios mentioned in the transcript:
Color and patient perception: color-matching challenges require attention to hue, chroma, and value; two light sources improve color matching accuracy (to mitigate metamerism).
Layering strategy for tooth-colored restorations: dentin layer + translucent enamel layer + outer enamel/transparent layer to mimic natural tooth structure.
Fluorescence in tooth-colored restorations: aligning fluorescence properties with natural dentition improves esthetics under various lighting conditions.
Simplified clinical decisions: thermal properties, CTE matching, and the insulating effect of liners/base materials can prevent pulp sensitivity and improve longevity of restorations.
Cavity design considerations for amalgam vs composite: amalgam requires mechanical retention designs (undercuts) and proper isolation; composites bond to tooth structure and require different bonding strategies.
Environmental and safety considerations in practice: PPE, proper disposal, ventilation, and following guidelines for mercury-containing materials.
Dentistry’s broader context: biocompatibility intersects with patient health, regulatory guidelines, and evolving material technologies (e.g., BPA-free composites, advanced bonding agents).