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DMD2 > Science of Dental Materials > Flashcards

Science of Dental Materials Flashcards

1
Q

Why is it important for dental practitioners to have an understanding of dental materials?

A

The dentist spends much of his professional career handling materials and the success or failure of many forms of treatment depends on the correct selection of materials with adequate properties combined with careful manipulation

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2
Q

What kind of materials are commonly used by dental practitioners as treatment biomaterials?

A

Metals and Alloys (amalgam)

Polymers (mouthguards and dentures)

Ceramics (crowns and bridges)

Composites (Tooth coloured restorations)

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3
Q

How is the correct material selected for when treating a patient?

A

Problem is analysed

Requirements are considered along with the availability of the materials.

Requirements and properties are compared and selected before undergoing review.

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4
Q

What materials have an amorphous structure and which have crystalline structures?

A

Amorphous structures include dental waxes and ceramic crowns.

Crystalline structures include

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5
Q

What is isomerism?

A

Arrangement of atoms or molecules of many elements or compounds can be changed by altering the surrounding physical conditions. Allotropy and polymorphism are types of isomerism.

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6
Q

What properties are used to characterize dental materials?

A

Mechanical properties (properties not concerned with the application of force)

Rheological properties (flow properties)

Thermal properties

Chemical properties

Surface phenomena and adhesion

Biological properties

Miscellaneous physical properties

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7
Q

What are the temperature related properties that can be used to characterize mateirals?

A

Heat of fusion (Heat required to convert 1g of a material from the solid to the liquid state at the melting temperature)

Thermal conductivity (Coefficient of thermal conductivity is the quantity of heat in calories or joules per second passing through a body of 1cm thickness and 1cm2 cross-sectional area when temperature difference is 1 degrees Celsius)

Thermal diffusivity (Measure of transient heat-flow and is defined as thermal conductivity divided by specific heat x density)

Thermal expansion (Change in length per unit length of a material for a 1 degree change in temperature)

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8
Q

What is the heat of fusion and how is it measured?

A

Heat of fusion (L) is the heat
in calories or joules required to convert 1 gm of a material from the solid to the liquid state at the melting temperature.

Heat of fusion =(L) = q/m
q= total heat
absorped
m=mass of substance melted

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9
Q

What is thermal conductivity and how is it measured? What is its significance?

A

Coefficient of thermal conductivity (K) is the quantity of heat in calories or joules per second passing through a body of 1 cm thickness and 1 cm2 cross –sectional area, when the
temperature difference is 1°C.

Cal/sec/cm²/(c°/cm) .

Significance: Denture base and metallic fillings

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10
Q

What is specific heat? Why is it important?

A

The amount of heat needed to raise the temperature of 1 gram of material by 1°C

Melting and casting alloys

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11
Q

What is thermal diffusivity and how is it measured?

A

The measure of transient heat flow and is defined as the thermal conductivity / Specific heat x Density

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12
Q

What is thermal expansion and how is it measured?

A

Thermal expansion
It is the change in length per unit length of a material for a 1°C change in temperature and it is calculated as follows:

(Lfinal - Loriginal) / Loriginal x (°C final - °C original)

Volumetric expansion = Linear x 3 (for isotropic
solids)

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13
Q

How can thermal expansion information be applied?

A

Wax patterns - Increasing coefficient of expansion affects accuracy or rest.

Decreasing in volume on cooling of an alloy needs to be compensated.

Investment, polymorphism of silica

Marginal percolation/leakage

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14
Q

What are the optical properties that are important to consider in dentistry?

A

Colour

Translucency

Surface texture

Metamerism (change in colour of an object due to change in light source)

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15
Q

What are the electrical properties that are relevant to dental materials?

A

Galvanism (generated electrical current a patient can feel resulting from dissimilar metals present in a solution that contains ions (oral cavity)) This can also lead to corrosion and pitting (due to metal dissolving)

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16
Q

What is solubility and sorption and how do they affect dental materials?

A

Both are important criteria for dental materials selection. Lab studies are used to evaluate and rank materials.

Sorption includes absorption and adsorption. Absorption is the uptake of liquid by solid and adsorption is the concentration of molecules at the surface of solid or liquid (eg adsorption of saliva on tooth surface)

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17
Q

What is wettability?

A

Measure of the affinity of a liquid for a solid indicated by spreading of a drop (eg wetting of denture base by saliva. Wetting of enamel surface by pits and fissures)

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18
Q

How is wettability observed?

A

Based on the shape of the liquid on the solid surface as 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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19
Q

What are the most important biological properties of dental materials?

A

Primary requirements of any dental materials include:

It must be non-toxic, non-irritant, no carcinogenic or allergenic potential, and if used as a filling it should be harmless to the pulp

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20
Q

What happens to biting force from posterior to anterior teeth? What effect does artifical dentition have on the biting forces?

A

Biting force decreases.

1st and second molars = 580N

Premolars = 310N

Canines = 220N

Incisors = 180N

Artificial replacement of dentition decreases biting force.

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21
Q

What has stronger tensile strength dentin or enamel?

A

Dentin

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22
Q

What has higher compressive strength dentine or enamel?

A

Enamel

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23
Q

How is deformation observed in dental materials?

A

Deformation results in a change in length in any dimension of the material. The change in length is a measure of deformation.

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24
Q

What are the types of deformation?

A

Elastic deformation (reversible)

Plastic deformation (irreversible or permanent)

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25
Q

What is stress?

A

Internal reaction to the external applied force per unit area. It is equal to intensity and opposite in direction to the external applied force per unit area.

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26
Q

What is strain?

A

Defined as a change in length per unit length.

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27
Q

What are the types of stress that can affect materials?

A

Tensile stress (Results when the body is subjected to 2 sets of forces directed away from each other in the same straight line)

Compressive stress (results when the body is subjected to two sets of forces directed 
towards each other in the same straight line [normal or axial)

Shear stress (results when the body is subjected to two parallel sets of forces directed towards each other not in the same straight line)

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28
Q

What does the stress-strain curve tell us?

A

2 portions of the stress-strain curve:

Elastic portion (obeys hook’s law, the strain is directly proportional to the applied stress)

Plastic portion (does not obey hook’s law, strain is not linearly proportional to the applied stress, when stress is removed the original size and shape is not recovered)

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29
Q

How is the engineering stress-strain curve different to the normal stress-strain curve?

A

The engineering stress - strain curve is that curve where the applied forces are
always divided by the original cross sectional area to calculate the corresponding stress.

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30
Q

What data can we obtain from a stress-strain curve?

A

Stress related terms:

Proportional limit

Elastic limit

Yield stress

Ultimate stress

Fracture stress

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31
Q

What does the proportional limit tell us?

A

Hook’s law applies directly proportionally between stress and strain until a certain point. This is the proportional limit.

32
Q

What does the proportional limit tell us?

A

Hook’s law applies directly proportionally between stress and strain until a certain point. This is the proportional limit.

The greatest stress that a material will sustain without a deviation from the law of proportionality of stress to strain.

33
Q

What is the elastic limit?

A

It is defined as the maximum stress a material can withstand without permanent deformation. It therefore describes the elastic behavior of the material

34
Q

What is the difference between proportional limit and the elastic limit?

A

For all practical purposes the elastic limit and the proportional limit represents the same stress within the structures and the terms are often used interchangeably

35
Q

How could we determine the resistance of a material to permanent deformation?

A

Yield strength

36
Q

What is yield strength?

A

The stress at which the material exhibits a specified
limiting deviation from proportionality of stress to
strain.(0.2% is usually the limit)

37
Q

What is the ultimate strength of a material?

A

The maximum stress that a material can withstand before failure (fracture or rupture)

38
Q

What is the modulus of elasticity?

A

Modulus of elasticity (E)
It is the constant of proportionality between
stress and strain. It represents the slope of the
elastic portion of the stress -strain curve

This tells us the stiffness of the material.

39
Q

How is the elastic modulus calculated?

A

It is determined from a stress strain curve by calculating the ratio of stress to its corresponding strain within the elastic portion.

40
Q

What is the unit of the elastic modulus?

A

Same units as stress since the strain has no units.

41
Q

What provides the elasticity of the materials?

A

The inter - atomic or intermolecular forces of the
material are responsible for the property of elasticity.
The stronger the basic attraction forces, the greater
the values of the elastic modulus and the more rigid
the material.

42
Q

What is the significance of the rigidity of the materials used in dentistry and why is it of prime importance?

A

The rigidity of major connectors in partial dentures controls stability of the whole design

More rigid alloys have thinner sections that can be used without bending

Base under restoration should be rigid in order to increase the fracture resistance of the filling.

43
Q

What is the flexibility of a material?

A

Flexible material, when subjected to small stress,

large amount of elastic deformation will occur.

44
Q

How is the flexibility of a material calculated?

A

Em = PL/E

Em= maximum flexibility

PL= proportionality limit

E= modulus of elasticity

45
Q

How is the maximum flexibility defined?

A

Maximum flexibility is defined as the strain which
occur when a material is stressed to its proportional
limit

46
Q

What is the clinical significance of flexibility?

A

Flexibility is an important property in elastic
impression materials, since it represents the ease by which the impression can be removed from the mouth.

Orthodontic wires

Endodontic files as well as reamers

47
Q

What is ductility and malleability?

A

The ability of a metal to undergo plastic deformation once the proportional limit has been exceeded.

It is known as malleability. When the material is in compression

and

It is known as ductility when the material is in tension.

In other words: the
malleability of a metal is its ability to be hammered into thin sheets without fracture, while ductility is its ability to be drawn into wires under force of tension without fracturing

48
Q

How is ductility measured?

A

Measured by the % elongation reduction in CSA or the cold bend test

49
Q

What is brittleness?

A

If a material demonstrates no or very little plastic
deformation on application of a load it is described
as being brittle

*Brittle materials are weak in tension for example;
dental amalgam has a compressive strength which
is nearly six times higher than its tensile strength.

50
Q

How are the tensile properties of brittle material tested?

A

Compressive testing (Diametrical compression test for tension, the indirect tensile test or the brazilian test)

51
Q

What are the resilience and toughness of a material?

A

Resilience = Amount of energy absorbed by material when stressed to proportional limit.

Toughness: Energy absorbed or required to stress the material to the point of fracture.

52
Q

What are the types of bending that can take place?

A

2 points bending

3 points bending

4 points bending

53
Q

What is the impact strength?

A

Total energy absorbed by material before fracture when struck by sudden load.

54
Q

What are the types of impact strength testers?

A

2 types available: Charpy and izod

55
Q

What is fatigue strength?

A

When a material is subject to stresses below the yield stress in a repeated manner for many times (cyclic loading) the strength of the material may be reduced leading to failure.

56
Q

What is fatigue?

A

Fatigue is a progressive
fracture under repeated loading or it is the stress at which a material fails under
repeated loading.

57
Q

How is fatigue strength tested?

A

Fatigue tests are performed by subjecting the specimen to alternating stress application below the yield stress until fracture occurs

58
Q

What is failure under repeated loading or cycling dependent on?

A

Magnitude of the load

Number of cycles

59
Q

What is the clinical significance of fatigue?

A

Lots of alternating forces in the oral cavity due to mastication, grinding, as well as presence of prosthetic materials.

60
Q

What is hardness?

A

A surface property. Numerous factors influence hardness making the term difficult to define.

In mineralogy it is the ability to resist scratching

In metallurgy it is the resistance to indentation or penetration

61
Q

How is hardness measured?

A

Many methods are used to measure hardness. All of the tests depend on the penetration of some small symmetrically shaped indentor into the surface of the material being tested.

Force application to the indentor produces a symmetrically shaped indentation which can be measured under a microscope for depth, area or width of the indentation produced. By applying certain equation relating the force to the area or depth of
indentation, a hardness
number is obtained.

The smaller the indentation the higher is the number,
the harder is the material and vice versa

62
Q

What types of indentors can be used for hardness testing? How does this affect how the material responds to the test?

A

The indentor may be made of steel, tungsten carbide or diamond, and shaped as a sphere, cone or pyramid.

Each tool will give rise to an indentation area of definite shape and dimensions.

Different loads can be applied to the tool
depending on the material (range from 1-3000 Kg)

63
Q

What is the brinell test?

A

A hardened steel ball is pressed into a polished surface of a material under a specified load.

Load is divided by the area of the surface of the indentation.

64
Q

What are the advantages and disadvantages of the brinell test?

A

Advantages: The Brinell test is used to determine the hardness of the dental metallic materials

Disadvantages: The Brinell test is not suitable for determining the hardness of brittle material; the steel ball tends to fracture brittle materials thus the indentation is not well defined.

Not suitable for those that exhibit elastic recovery; resilient materials tend to recover upon removal of the steel ball, and the indentation is inaccurate.

65
Q

What is the Rockwell test?

A

The Rockwell hardness test
is similar to Brinell test in
that a steel ball or cone is
used.

The depth is measured
directly by a dial gauge on
the instrument.

66
Q

What are the advantages and disadvantages to using the rockwell test?

A

Advantages:

It is a rapid method for hardness determination.

It is good for testing viscoelastic materials.

Disadvantages:

A preload is needed

Greater time is required

The indentation may
disappear immediately on
removal of load.

67
Q

What is the vicker test?

A

Diamond shape of a square based pyramid is used

Vickers hardness number is computed by dividing the load by the area of indentation.

68
Q

What are the advantages and disadvantages of the vicker test?

A

Advantages:

Test is suitable for brittle materials

It is especially useful in measuring the hardness of small areas and very hard materials.

Disadvantages:

Not suitable for materials with elastic recovery.

69
Q

What is the knoop test?

A 
The Knoop hardness test 
employs a diamond 
indenting tool ground to a 
pyramid shape thus giving 
a diamond or rhomboid 
indentation having a long 
and short diagonal of an 
approximate ratio of 7 : 1
70
Q

What are the advantages and disadvantages to the knoop test?

A

Advantages:

Can be employed to examine material that vary in hardness. Hardness of tooth enamel can be compared to that of gold, porcelain, resin, and other tooth restorative materials by this test

It is suitable for materials with elastic recovery because when indentation is made the stresses are distributed in such a way that only the minor axis gets smaller.

71
Q

What are rheological properties?

A

The science which study the flow and deformation of matter.

72
Q

In which instances are rheological properties most important?

A

In fluid pastes which subsequently solidify

Mixed pastes that are adapted to required shapes

Setting of such materials involves a change in viscosity with time, and then the development of an elastic modulus as solification proceeds.

73
Q

What is working time?

A

Period during which the material can be manipulated ideally with no thickening taking place.

74
Q

What is setting time?

A

Time when the material has reached a given degree of rigidity.

75
Q

What are the types of creep and how are the different?

A

Static creep (time dependent plastic strain on a material in response to a constant or static load that produces stresses below proportional limit)

Dynamic creep (is the creep of a material when the load (below proportional limit) is 
applied in a cyclic manner. )
76
Q

How does creep occur?

A

Creep usually occurs at temperatures near the
softening point of the material.
•
Since the softening temperatures of most metals and
ceramics are far above mouth or room temperatures
they do not creep in dental application.
•
However, many polymers such as waxes, rubber and
plastics are near their softening point at room or
mouth temperatures and can creep considerably.

The term creep has also become useful in describing
the flow of set amalgam samples under an applied
load.

77
Q

When are strain-time curves used?

A

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

DMD2 (89 decks)

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