Dental Restorative Materials: Properties and Requirements

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Questions and Answers

Describe how restorative materials are subjected to a harsh oral environment.

Restorative materials are subjected to mechanical forces (200-3500N), pH variations (acidic or alkaline drinks & foodstuff), temperature variations (hot and cold food and drinks), and moist conditions (saliva, oral fluids).

What are the structural properties of enamel and dentine?

Enamel is hard, brittle, and wear-resistant, cracking but generally not failing. Dentine is soft, compliant, and can deform under axial loading, leading to bucco-lingual and mesio-distal expansion.

What are the compositions of enamel and dentine?

This card is missing a definition.

What features make up an ideal restorative material?

<p>An ideal restorative material should be biocompatible, exhibit properties similar to enamel and dentine, be able to perform in the oral environment, and assist in tissue regeneration or repair of missing/damaged tissues.</p> Signup and view all the answers

What are the desired properties of restorative materials?

<p>Desired properties include: restoration remains integral and in place, restore occlusion and withstand masticatory forces, aesthetics are maintained over time, prevent formation of caries and recurrence, restore aesthetics, provides patient comfort and restore function.</p> Signup and view all the answers

Why do dental materials sometimes fail?

<p>Dental materials sometimes fail because the oral environment exposes them to chemical, thermal, and mechanical challenges. Occlusal forces range from 200 to 3500 N and are highest in the posterior region.</p> Signup and view all the answers

What are the consequences of replacing a restoration?

<p>Replacing a restoration may lead to an increase in cavity size, may weaken the tooth, and constitutes a large part of operative work in dental practice.</p> Signup and view all the answers

What are the different types of forces that a material feels in the mouth?

<p>This card is missing a definition.</p> Signup and view all the answers

What zones can be formed as a result of compression on a material?

<p>Tensile zones and shear zones.</p> Signup and view all the answers

How can materials physically fail?

<p>Materials can physically fail when the critical stress is exceeded. The magnitude of the critical stress depends on the loading conditions. E.g. in general a material loaded in shear has a lower critical stress than one loaded in tension.</p> Signup and view all the answers

What is stress?

<p>When an external force is applied on a test specimen an internal force, equal in magnitude but opposite in direction, is set up in the body.</p> Signup and view all the answers

What is the formula for stress?

<p>Stress = force/area (units are Pascals)</p> Signup and view all the answers

What does Hooke's Law state about stress and strain?

<p>They are proportional so it would be a straight line on a graph, with the gradient equating to the elastic modulus of the material. Note: in the second diagram, point 2 represents up to where the relationship is directly proportional and point 3 represents the elastic limit.</p> Signup and view all the answers

What happens beyond the elastic limit?

<p>The material will experience a permanent deformation.</p> Signup and view all the answers

What is the definition of elastic limit?

<p>The maximum stress a material can withstand without permanent deformation.</p> Signup and view all the answers

What is the definition of proof stress?

<p>The point at which a particular degree of permanent deformation occurs in the sample Typically, the stress needed to produce 0.1 to 0.2% of plastic deformation is considered proof stress.</p> Signup and view all the answers

What is Young's modulus?

<p>A mechanical property that measures the stiffness of a solid material.</p> Signup and view all the answers

What is the stiffness of a material determined by?

<p>Determined by interatomic and intermolecular forces of material Stronger force- stiffer, more rigid material Same in compression and tension Independent of heat treatment.</p> Signup and view all the answers

What does the stiffness of a material also depend on?

<p>Stiffness will depend on dimensions If the Young's modulus of a material is known by using standard specimens then it is possible to determine stiffness of any structure from that material.</p> Signup and view all the answers

Why is it important to know the yield point in dentistry?

<p>Because we often need to use materials in their elastic mode e.g. orthodontic wires.</p> Signup and view all the answers

Compare the elastic moduli of different materials in dentistry.

<p>This card is missing a definition.</p> Signup and view all the answers

What is the definition of tensile strength?

<p>The ability of a material to resist a force that tends to pull it apart The tensile strength of a material is the maximum amount of tensile strength it can withstand before failure (failure being either breaking or plastic deformation.</p> Signup and view all the answers

What is important to remember about tensile strength testing?

<p>Difficult specimen preparation Alignment is crucial However, it is easy to analyse.</p> Signup and view all the answers

What is the definition of compressive strength?

<p>The maximum compressive load it can withstand without undergoing failure.</p> Signup and view all the answers

How do different types of objects fare under high compressive loads?

<p>Brittle materials tend to show fragmentation of the specimen when max compressive strength is exceeded Ductile materials show either rebarreling or thinning of the cross-section.</p> Signup and view all the answers

Why is true stress different from engineering stress?

<p>Compressive strengths calculated from equations may not yield accurate results because: -the cross sectional area changes and the sample decreases in height as testing is conducted -frictional forces due to clamping have to be overcome -the barrelling effect occurs as the frictional force is not uniform across the specimen.</p> Signup and view all the answers

Contrast the strengths of ceramics and composites.

<p>Ceramics typically have a much higher compressive strength than tensile strength Composite materials tend to have higher tensile strengths than compressive strengths.</p> Signup and view all the answers

What is diametral compressive strength (indirect tensile strength) used for?

<p>It's a property for characterising dental composite restorations.</p> Signup and view all the answers

What is the definition of flexural strength?

<p>The ability to resist deformation under load.</p> Signup and view all the answers

What is the difference between a flexure test and a bend test?

<p>The difference is the type of material that is used and the info obtained from it A flexure test is designed to measure the bend strength of a brittle material A bend test is designed to measure the crack resistance of a ductile material.</p> Signup and view all the answers

What is hardness defined as?

<p>The resistance to permanent surface indentation Hardness measurement can be defined as macro, micro or nano- scale according to the forces applied and displacements obtained.</p> Signup and view all the answers

How can we measure hardness of dental materials?

<p>This card is missing a definition.</p> Signup and view all the answers

What is the Vickers hardness of some dental materials?

<p>Enamel- 350 Dentine- 60 Acrylic resin- 20 Dental amalgam- 100 Porcelain- 450</p> Signup and view all the answers

What are the structural properties of enamel?

<p>Enamel is hard, brittle, and wear resistant. It cracks but does not generally fail.</p> Signup and view all the answers

Flashcards

Oral Environment Impact

Restorative materials in the mouth endure mechanical forces (200-3500N), pH variations from acidic/alkaline substances, temperature changes from hot/cold items, and constant moisture from saliva and oral fluids.

Enamel vs. Dentine: Structure

Enamel is hard, brittle, and wear-resistant, resisting failure despite cracking. Dentine is softer and deforms under axial loading, enabling expansion.

Ideal Restorative Material

An ideal material is biocompatible, mimics enamel/dentine properties, performs well in the mouth, and aids tissue regeneration or repair.

Desired Material Properties

Restorations should stay intact, withstand chewing forces, maintain aesthetics, prevent decay, provide comfort, and restore function effectively.

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Material Failure Reasons

Dental materials fail due to chemical, thermal, and mechanical stressors in the oral environment, including occlusal forces ranging from 200-3500 N.

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Replacement Consequences

Replacing restorations can enlarge cavities, weaken teeth, and constitutes a large amount of operative work in dental practices.

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Physical Material Failure

Physical failure occurs when critical stress is exceeded, varying with loading conditions. Shear loading generally causes failure at lower stress than tension.

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Stress Definition

Stress is the internal force per unit area within a material that arises from externally applied forces; measured in Pascals.

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Strain Definition

Strain is the change in dimension (length) of a material relative to its original dimension, resulting from applied stress.

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Hooke's Law

Stress and strain are directly proportional within the elastic region, according to Hooke's Law. The gradient represents the elastic modulus.

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Beyond Elastic Limit

Beyond the elastic limit, a material undergoes permanent deformation, indicating non-recoverable structural changes.

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Elastic Limit

The elastic limit is the maximum stress a material can withstand without undergoing permanent deformation.

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Proof Stress

Proof stress is the stress required to produce a specified amount of plastic deformation, typically 0.1% to 0.2%.

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Young's Modulus

Young's modulus is a mechanical property that quantifies a solid material's stiffness, indicating resistance to elastic deformation under stress.

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Material Stiffness

Material stiffness is determined by interatomic and intermolecular forces: stronger forces result in stiffer, more rigid materials, irrespective of heat treatment.

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Yield Point Importance

Yield point knowledge is crucial in dentistry because materials often need to function elastically, like orthodontic wires.

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Tensile Strength

Tensile strength is a material's capacity to resist being pulled apart, representing the maximum tensile stress it can endure before failure, either by breaking or plastic deformation.

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Compressive Strength

Compressive strength represents the maximum compressive load a material can withstand before failure.

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Ceramics vs. Composites

Ceramics have higher compressive than tensile strength, whereas composites tend to have higher tensile versus compressive strengths.

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Diametral Compressive Strength

Diametral compressive strength (indirect tensile strength) is utilized to characterize dental composite restorations.

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Flexural Strength

Flexural strength is the ability of a material to resist deformation under a bending load.

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Flexure vs. Bend Test

A flexure test measures the bend strength of brittle materials, while a bend test assesses the crack resistance of ductile materials.

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Hardness Definition

Hardness is defined as a material's resistance to permanent surface indentation.

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Vickers Hardness Values

Enamel has a Vickers hardness of 350, dentine is 60, acrylic resin is 20, dental amalgam is 100, and porcelain is 450.

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Study Notes

  • Restorative materials face a challenging oral environment.

Mechanical Forces

  • Occlusal forces range from 200-3500N.
  • First and second molars experience 400-800N of force.
  • Incisal forces range from 150-300N, increasing in growing children.

Environmental Factors

  • pH varies due to acidic or alkaline foods and drinks.
  • Temperature fluctuates with consumption of hot and cold items.
  • Moist conditions are present due to saliva and oral fluids.

Enamel Properties

  • Enamel is hard, brittle, and wear-resistant.
  • It cracks but generally does not fail.

Dentine Properties

  • Dentine is soft and compliant.
  • It deforms under axial loading, leading to expansion.

Ideal Restorative Material

  • Must be biocompatible.
  • Should mimic properties of enamel and dentine.
  • Must perform well in the oral environment.
  • Assists in tissue regeneration or repair.

Desired Restorative Material Properties

  • Remains integral and in place.
  • Restores occlusion and withstands masticatory forces.
  • Maintains aesthetics over time.
  • Prevents caries formation and recurrence.
  • Provides patient comfort and restores function.

Reasons for Dental Material Failure

  • The oral environment presents chemical, thermal, and mechanical challenges.
  • Occlusal forces range from 200-3500 N, highest in the posterior region.

Consequences of Restoration Replacement

  • May increase cavity size.
  • May weaken the tooth.
  • Represents a significant portion of dental practice work.

Force Zones From Compression

  • Compression creates tensile and shear zones within a material.

Physical Material Failure

  • Occurs when critical stress is exceeded.
  • Critical stress magnitude depends on loading conditions.
  • Materials generally have a lower critical stress under shear than tension.

Stress Definition

  • Internal force within a body that resists an external force.

Stress Formula

  • Stress = Force/Area, measured in Pascals.

Strain Definition

  • Change in dimension of a material due to external force.

Strain Formula

  • Strain = Change in Length/Original Length.

Hooke's Law

  • Stress and strain are proportional up to the elastic limit.
  • A graph of stress vs. strain shows a straight line with the gradient representing the elastic modulus.

Beyond the Elastic Limit

  • Materials experience permanent deformation.

Elastic Limit

  • The maximum stress a material can withstand without permanent deformation.

Proof Stress

  • Stress at which a specific amount of permanent deformation occurs.
  • Typically, the stress to produce 0.1-0.2% plastic deformation is considered proof stress.

Young's Modulus

  • Measures the stiffness of a solid material.

Material Stiffness

  • Determined by interatomic and intermolecular forces.
  • Stronger forces result in stiffer, more rigid materials.
  • Stiffness is similar in compression and tension.
  • Independent of heat treatment.

Stiffness and Dimensions

  • Stiffness depends on dimensions.
  • Young's modulus allows determining stiffness of any structure made from that material.

Importance of Yield Point in Dentistry

  • Dental materials often need to function within their elastic range, e.g., orthodontic wires.

Tensile Strength

  • A material's ability to resist being pulled apart.
  • Maximum tensile stress a material can withstand before failure (breaking or plastic deformation).

Tensile Strength Testing

  • Specimen preparation is difficult, and alignment is crucial.
  • Analysis however, is easy.

Compressive Strength

  • Maximum compressive load a material can withstand without failure.

Material Behavior under Compression

  • Brittle materials fragment when max compressive strength is exceeded.
  • Ductile materials exhibit rebarreling or thinning.

True Stress vs. Engineering Stress

  • Equations may not yield accurate compressive strength results due to changing cross-sectional area and height.
  • Frictional forces also need to be overcome.
  • Barrelling occurs due to non-uniform frictional force.

Ceramics vs. Composites Strength

  • Ceramics have higher compressive strength than tensile strength.
  • Composites tend to have higher tensile strength than compressive strength.

Diametral Compressive Strength

  • Indirectly measures tensile strength.
  • Used for characterising dental composite restorations.

Flexural Strength

  • The ability to resist deformation under load.

Flexure Test vs. Bend Test

  • Flexure test measures bend strength of brittle materials.
  • Bend test measures crack resistance of ductile materials.

Hardness Definition

  • Resistance to permanent surface indentation.
  • Measured as macro, micro, or nano-scale based on applied forces and obtained displacements.

Vickers Hardness of Dental Materials

  • Enamel: 350
  • Dentine: 60
  • Acrylic resin: 20
  • Dental amalgam: 100
  • Porcelain: 450

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