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Dental materialsDental materials

Summer courseSummer course

2007-20082007-2008

Dr. Hanan AlZraikatDr. Hanan AlZraikat

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Oral environment and patient Oral environment and patient considerationconsideration

• The oral environment represents a challenge to the success of dental materials.

• Understanding these challenges and limitations, is essential for a successful treatment.

• Materials must be biocompatible, aesthetic and durable.

• As a member of the allied dental team, you need to appreciate these factors & have a thorough knowledge of selection criteria and materials manipulation.

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Intraoral factorsIntraoral factors

• Force and stress:

a. Compressive: crushing biting forces

b. Tensile: biting force stretches a material

c. Shear: e.g. an incisor used for cutting

ca b

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– Resisting these forces creates stress within the material, which may lead to deformation, strain.

– Dental materials can withstand one type of stress while fail under another. But failure usually occurs after repetitive force application i.e. fatigue failure.

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• Moisture and acid levels:– Intraoral pH depends on diet and acid producing

bacteria. – Moisture affects materials during placement or over

time. Low solubility contributes to longevity.– Some materials take up water, color, odor, tastes of

foods and beverages (e.g. resins, acrylic).– Metals (except noble metals) are affected by acid and

moisture, i.e. corrosion: to wear out due to chemical rxn.

– Dental amalgam is susceptible to tarnish (discoloration caused by oxidation of metal).

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• Galvanism: an electric current transmitted between two dissimilar metals.

• Temperature:– Dimensional changes (expansion/contraction)– Coefficient of thermal expansion (CTE): measurement

of dimensional changes.– Percolation: opening and closing of a gap between

tooth and restoration due to expansion and contraction of restoration. This may lead to recurrent caries, staining, pulp irritation.

– Thermal conductivity and insulators (pulp sensitivity).– Exothermic rxn of restorative material.

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Retention: the ability of the material to maintain its position and resist displacement– Mechanical: undercuts e.g. amalgam– Chemical: e.g. glass ionomer cements– Bonding: a term used to describe how

composite is bonded to tooth surface (micromechanical/chemical). Affected by:

• Wetting

• Viscosity

• Film thickness

• Surface characteristics: cleanliness, moisture

contamination, texture, energy.

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• Microleakage: the seepage of harmful materials through the gap between tooth and restoration. Can cause:

– Staining– Recurrent caries– Sensitivity

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• Biocompatibility: dental material must not have an adverse effect on living tissue– Materials used on hard tissue vs. soft tissue

– Short term vs. long term exposure

– Small doses vs. high doses (fluoride treatment)

– Adverse effects maybe due to materials itself or the breakdown of its components.

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• Esthetics– Color components:

• Hue: dominant color of wavelength detected

(tooth color is seen in yellow and brown range)• Chroma: color intensity or strength• Value: how bright or dark a color is.

– Transparent vs. opaque– Shade guide

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• Conditions for assessing restorations:– Dry field– Good lighting– Sharp explorer– Radiographs– Magnification– Good knowledge of material

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End of part oneEnd of part one

Reference: Reference:

Dental materialsDental materials

Clinical applications for dental Clinical applications for dental assistants and dental hygienists (ch. 2)assistants and dental hygienists (ch. 2)

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Material propertiesMaterial properties

• Physical properties: properties based on the laws of mechanics, optics, thermodynamics, electricity etc. (Phillips’ science of dental materials)

A. Rheological properties (ref. Introduction to dental materials ch.1.8)

Definition: The study of flow or deformation of materials. Solids: elasticity and viscoelsticity Liquids: viscosity = shear stress/shear strain

• Consider extrusion of a fluid from syringe.

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• Viscosity: resistance of a liquid to flow. The ways in which materials flow or deform under stress are important to their use in dentistry.

• Thixotropic material: is a material that becomes less viscous when subject to repeated pressure (e.g. plaster, prophylaxis paste).

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B. Mechanical properties (ref. Dental Materials, properties and manipulation)

– Properties defined by the laws of mechanics; the physical science that deals with energy and forces and their effects on bodies.

– Maximum biting force decreases from molars to incisors. Average biting force– 1st and 2nd molars = 580 N – Bicuspids (premolars) = 310 N– Cuspids (canines) = 220 N– Incisors = 180 N

Artificial replacement of dentition decreases biting force (e.g. fixed bridge, partial and complete dentures)

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• To compare the performance of materials irrespective of their shape or size, an objective standard is needed. This standard is stress and strain. Description of mechanical properties depends on these two. – Stress = force/unit area (compressive, tensile, shear)– Strain: the deformation per unit of length as a result of

force = deformation/length (e.g. rubber vs. gold alloy)

Force (N) Area (mm²) Stress (MPa)

111 645 0.1724

111 64.5 1.724

111 6.45 17.24

111 0.645 172.4

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• Stress-strain curves are a convenient way to compare materials mechanical properties whether in compression, tension or shear, especially when strain is independent of the length of time the load is applied

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• Strain-time curves are sometimes used when strain depends on the time the load is maintained (e.g. alginate, rubber impression material)

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– Values resulting from stress-strain curves:• Elastic modulus = stress/ strain (MPa), a measure

of stiffness which is resistance to deformation measured by Young’s modulus

• Proportional limit: measure of stress allowed before permanent deformation occurs.

• Ultimate strength: maximum amount of strength a material can withstand without breaking.

*Note: some materials can be classified as clinical failure when significant permanent deformation occurs even if the material does not fracture

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– Other mechanical properties• Elasticity: the ability to stretch and not break

(impression material and undercuts)– Elastic (recovery immediate) vs. viscoelastic

(recovery slow or with some degree of permanent deformation

• Toughness: ability of the material to resist fracture• Resilience: the ability of the material to resist

permanent deformation• Creep: time-dependent plastic strain of a material

under a static load or constant stress.

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• Hardness: resistance to wear or abrasion (enamel and porcelain are among the hardest). Hardness is measured using several tests such as Knoop, or Vickers hardness tests

• Fatigue properties (refer to slides only):

Materials are subjected to intermittent stress over long period of time, stress is small, but over time, failure may occur by a fatigue process. This involves the formation of microcracks, resulting from stress concentration at a surface fault, so crack propagates until fracture occurs. Final fracture occurs at a low stress level.

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• Fatigue is studied in 2 ways:1. Fatigue life: application of stress cycles at a certain

amount and frequency and observe number of cycles needed to cause failure.

2. Fatigue limit: select a number of cycles (e.g. 10 000) and determine the value of the cyclic stress which is required to cause fracture within this number of cycles.

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• Ultimate strength values of selected dental materials

Material Tensile strength Compressive strength

Dentine 98 297

Enamel 10 400

Amalgam 48-69 310-483

Gold alloys 414-828 -----

Composite 34-62 200-345

Porcelain 40 150

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C. Thermal properties: (ref. Dental Materials, properties and manipulation)

Materials have different rates of conducting heat. (Metals have higher values compared to plastics and ceramics). Therefore patients may experience postoperative sensitivity in association with amalgam restorations for instance.

• Thermal conductivity: it’s a measure of heat transferred through a material or rate of heat flow. Enamel and dentine are poor thermal conductors compared to amalgam and gold alloys. Therefore insulators are required in some cases to protect the pulp.

• Coefficient of thermal expansion (explained previously)

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D. Electrical properties: (ref. Dental Materials, properties and manipulation)

• Galvanism: generated electrical current a patient can feel resulting from dissimilar metals present in a solution that contains ions (e.g. oral cavity)

• Corrosion: can result from• adjacent dissimilar metals. Galvanic action can cause the

metal to dissolve resulting in pitting and roughness. • Chemical corrosion

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E. Solubility and sorption: (ref. Dental Materials, properties and manipulation)

• important criteria for dental materials selection. Laboratory studies are used to evaluate and rank materials.

• Sorption includes:• Absorption: uptake of liquid by solid e.g. uptake

of water by acrylic plastics• Adsorption: concentration of molecules at the

surface of solid or liquid e.g. adsorption of saliva on tooth surface

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F. Wettability: (ref. Dental Materials, properties and manipulation)

• measure of the affinity of a liquid for a solid indicated by spreading of a drop e.g. wetting of denture base by saliva. Wetting of enamel surface by pits and fissures.

wettability is observed by shape of a drop of liquid on solid surface identified by contact angle:

• Low contact angle = high wettability (hydrophilic if liquid is water)

• High contact angle = low wettability (hydrophobic if liquid is water)

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liquidθ

solid solid

Good wetting of a solid by a liquid with low contact angle (left), poor wetting forming a high contact angle (right).

liquidθ

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G. Optical properties: (ref. Introduction to dental materials)

• Every object we see is as a result of reflectance of light from that object reaching an extremely sensitive photodetector, namely the eye. This is characterized by:

• Color (Hue, value, chroma)

• Translucency:• Translucent materials allows some light to pass, absorbs

some, and scatters the rest• Opaque material does not transmit light, but absorbs and

scatters it. • Surface texture: the polishability of a material is an important

criteria for selection

*Metamerism: change of color of an object due to a change in light source

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Shade guide and selectionShade guide and selection

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H. Biological properties: (ref. applied dental materials ch.2)

• Primary requirements of any dental material:– Non-toxic– Non-irritant– Should not have carcinogenic or allergic potential– If used as filling material should be harmless to pulp

• Biological evaluation of dental materials:– Level 1– Level 2– Level 3

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