Viscoelastic Materials in Dentistry

Mechanical Properties

• Mechanical properties describe how materials react to forces or loads.
• These include characteristics such as strength, stiffness, and ductility.

Force

• Force is defined by speed (static or dynamic), magnitude, the point of application (normal or tangential), and direction.
• Applying force can result in displacement, acceleration, and deformation of a body.

Stress

• Stress is the internal reaction to an external force.
• It is equal in intensity but opposite in direction to the external force.
• Stress is calculated as Force/Area (σ = F/A).
• The unit for stress is Pascals (Pa) or Mega Pascals (MPa).
• Factors affecting stress include force (directly related) and area (inversely related).
• Types of stress include tensile, compressive, shear, and complex stresses.

Tensile Stress

• Tensile stress involves two opposing forces on the same line, pulling away from each other.
• This type of stress can cause elongation.

Compressive Stress

• Compressive stress involves two opposing forces on the same line, pushing toward each other.
• This type of stress can cause shortening.

Shear Stress

• Shear stress involves two opposing forces that are parallel but not on the same line.
• This type of stress can cause sliding or tearing.

Complex Stress

• Complex stress is a combination of two or more types of stress.
• Examples include the stresses found in oral cavities.

Strain

• Strain is the change in length per unit length, resulting from the application of stress.
• Strain = Deformation/Original length (ε = I final - I original / I original)

Types of Strain

• Elastic strain (Temporary strain) is recovered after load removal.
• Plastic strain (Permanent strain) is not recovered after load removal.

Poisson's Ratio (μ)

• Poisson's Ratio is used for axial loading (tension or compression).
• It relates lateral strain to axial strain.
• For most dental materials, μ = 0.3.

Stress-Strain Curve

• Plots stress against strain to study material properties.
• Stress is plotted on the vertical axis, and strain on the horizontal axis.

Proportional Limit

• The maximum stress a material can withstand without deviating from Hooke's law, where stress is directly proportional to strain.
• Doubling the stress will double the strain.

Elastic Limit

• The maximum stress a material can withstand without permanent deformation.
• Often the same value as the proportional limit but differs in fundamental concept.

Yield Strength

• The stress at which a material begins to sustain permanent deformation (plastic behavior)
• The amount of permanent deformation is often measured as 0.1%, 0.2%, or 0.5% offset.

Ultimate Strength

• The maximum stress a material can withstand before fracture.
• Yield strength is usually more clinically important because it represents a functional failure of a restoration.

Modulus of Elasticity (Young's Modulus, "E")

• A constant that measures a material's stiffness within the elastic range.
• Represented by the slope of the elastic portion of the stress-strain curve.
• Not affected by whether the test is tensile or compressive.
• Depends on interatomic/intermolecular forces, composition, heat treatment, and mechanical treatment of a material.

Clinical Importance of Modulus of Elasticity

• Denture bases should be rigid to prevent bending in thin sections.
• Long-span bridges necessitate rigid materials for proper stress distribution.
• Rigid restorative materials enhance the fracture resistance of fillings.

Flexibility

• The maximum strain a material experiences when stressed to its proportional limit.
• Important in elastic impression materials (easy removal), partial denture clasps (easy removal), and endodontic files (easy preparation).

Brittleness

• Brittle materials show little to no plastic deformation before fracturing.
• Brittle materials are typically weak in tension but strong in compression.
• Fracture occurs via crack propagation.
• Dental amalgam exhibits significantly higher compressive strength than tensile strength.

Malleability and Ductility

• Malleability: The ability of a material to be hammered into thin sheets without fracturing (resistance to compressive stresses).
• Ductility: The ability of a material to be drawn into wires without fracturing (resistance to tensile stresses).
• These properties demonstrate the workability of a material.

% Elongation

• Represents the deformation result of tensile forces.
• Measured as the indication of an alloy's workability.
• % elongation = (increase in length)/(original length) x 10
• Dental gold alloys typically have high elongation.
• Nickel-chromium alloys, comparatively, have low elongation.

Brittle vs. Ductile Fracture

• Brittle fracture occurs without significant plastic deformation in materials like resins and ceramics, primarily characterized by crack propagation.
• Ductile fracture features more extensive plastic deformation (necking) in materials like metals, often resulting in different fracture patterns.

Resilience

• The amount of energy a material absorbs to deform to its proportional limit.
• Its value corresponds to the area under the straight-line portion of the stress-strain curve.
• Important in dental applications such as orthodontic wires, removable dentures, and mouth guards.

Toughness

• The total energy absorbed by a material before fracture, representing the area under the stress-strain curve's elastic and plastic regions.

Fracture Toughness

• The energy required to fracture a material containing flaws (e.g., cracks).
• In brittle materials, this is more apparent compared to ductile materials.

Dental Importance of Fracture Toughness

• Factors that influence fracture toughness in dental materials include the presence of fillers in resin composites, crystalline phases in ceramics, and the presence of zirconia particles.

Other Mechanical Properties and Tests:

• Diametral Compression Test (Brazilian Test): Used to determine the tensile strength of brittle materials via an indirect method.
• Three-point Bending Test: Measures flexural strength (resistance to bending for beams) using a static load in the middle of a supported beam. Deformation is crucial as it signifies restoration failure.
• Cantilever Bending: Evaluating bending properties of materials that are clamped at one end for application of load at the other free end, helpful in endodontic files.

Fatigue Strength

• The stress level at which a material fractures progressively under repetitive cyclical loading conditions.
• Fatigue strength tests subject a specimen to repeated stresses below the proportional limit until failure happens. This limit can be called endurance limit or fatigue limit. Dental materials should have this limit that is above the masticatory forces to resist endless cycles of loading.

Impact Strength

• Measured using tests like Charpy or Izod to determine a material's resistance to sudden shock loading.
• A notched specimen is either struck at its midpoint (Charpy) or at one end (Izod).

Hardness

• Describes the resistance of a material to permanent deformation, scratching, and/or penetration.
• Hardness tests (Brinell, Rockwell, Shore A, Vickers, Knoop) utilize an indenter to quantify the resistance.

Wear

• Defined as material loss due to mechanical actions.
• Types include physiological (normal tooth wear during chewing and biting), mechanical (improper brushing), and pathological (bruxism). Teeth or restorations experiencing significant wear are undesirable except when involved with polishing applications.

Viscoelasticity

• Viscoelastic materials exhibit a combination of elastic and viscous behavior.
• The response to loading depends on the loading rate(slow is ductile, rapid is brittle).
• They exhibit immediate and delayed recovery upon stress removal.
• Various components of the elastic and viscous behavior and recovery patterns result in material-dependent responses to stress during use.

Dental Applications of Viscoelasticity

• Elastic impression materials should be removed quickly from the patient's mouth (sharp snap removal) to decrease deformation that is produced by viscous parts but increase the tear strength of the impression material.
• Elastic materials should ideally have time to recover between impressions before being poured into gypsum molds to allow the material to recover from anelastic parts.
• For materials involved in the application (creep): materials such as polymers and dental amalgam can creep if stresses are sustained over time due to their similar softening temperature characteristics as oral temperatures.

This quiz focuses on the behavior of viscoelastic materials in dental applications, including their response to loading rates and the implications for impression materials. Explore key concepts such as yield strength, modulus of elasticity, and clinical implications of material flexibility.