seminar

2. PHYSICAL PROPERTIES
OF DENTAL MATERIALS
P R E S E N T E D BY -
D R . S H R U T I M I S H R A
M D S 1 S T Y E A R

3. GUIDED BY :
• Dr. Gopal Tawani (Reader and Guide)
• Dr. Manoj Chandak (Professor and HOD)
• Dr. Pradnya Nikhade (Professor)
• Dr. Ajay Saxena (Professor)
• Dr. Anuja Ikhar (Reader)
• Dr. Neelam Chandwani (Reader)
• Dr. Aditya Patel (Reader)
• Dr. Neha Shukla (Sr. Lecturer)
• Dr. Kaustubh Khatod (Sr. Lecturer)

4. SPECIFIC LEARNING OBJECTIVES
S/N Learning
Objectives
Domain Level Criteria
1. Introduction Cognitive Must know All
2. Mechanical
properties
Cognitive Must know All
3. Physicomechanica
properties
Cognitive Must know All
4. Rheological
Properties
Cognitive Must know All
5. Optical properties Cognitive Must know All
6. Thermal Cognitive Must know All
7. Electrical
properties
Cognitive Must know All

5. OUTLINE
• Mechanical properties
1. Force
2. Stress
3. Stress strain curves
4. Viscoelasticity
5. Dynamic mechanical properties
6. Surface mechanical properties
7. The colloidal state
8. Diffusion through membranes
and osmotic pressure
09. Surface considerations
10. Optical properties
11. Metamerism
12. Fluorescence
• Thermal properties
• Electrical properties
• Other properties
• Summary

6. INTRODUCTION
• Restorative dental materials are subjected to a very hostile environment, in which pH,
salivary flow and mechanical loading fluctuate constantly and often rapidly.
• The understanding of properties of polymers, ceramics, and metals is crucial to their
selection and design of dental restorations.
• No single property defines the quality of a material.
• The success of a restoration depends not only on its physical properties but also on
the biophysical or physiological qualities of the supporting tissues.

7. MECHANICAL PROPERTIES
• Biomechanics :- the science that studies how biological materials interact and deform
is called biomechanics
(craig)
• This section introduces concepts of elastic, plastic and viscoelastic deformation and
mechanical quantities including stress, strain , strength, toughness, hardness, friction
and wear in terms of performance of materials in the oral environment.

8. FORCE
• One body interacting with the other generates force.
• Forces may be applied through actual contact of the bodies or at a distance (e.g.
gravity).
• The result of an applied force on a body is translation or deformation of the body
depending on whether the body is rigid or deformable and whether the body is
constrained.
• If the body is constrained (i.e., does not move or translate), the force causes the body
to deform or change its shape.

9. • If the body is free of constraints, an applied force results in movement or translation.
• A force is defined by three characteristics : point of application, magnitude, and
direction of application .
• The direction of a force is characteristic of the type of the force.
• The SI unit is Newton.
• One pound-force (lb-f) = 4.4 Newtons (N).

10. OCCLUSAL FORCES
• Maximum occlusal forces range from 200N to 3500N.
• Occlusal forces are highest in the posterior region closest to the mandibular hinge axis
and decrease from the molar region to the incisors.
• Forces on the first molar varies between 400 to 800 N.
• Average force on the bicuspids, cuspids and incisors is about 300, 200 and 150 N,
respectively.
• A somewhat non linear but definite increase in force from 235 to 494 N occurs in
growing children, with an average yearly increase of about 22N.

11. FORCES ON RESTORATIONS
• Occlusal forces with dental prosthetic devices are generally lower than with the natural
dentition.
• Patients with RPD- the average force on bicuspids and molars is about 100N and that
on incisors is 40 N.
• Age and gender variations in the patient populations contribute to the large variation
in force values.
• Women – 90N less than that applied by men.

12. • Facial form and muscle definition are good predictors of occlusal force capacity.
e.g. patients with high mandibular angles generally exhibit lower occlusal forces than
patients with low angles and square mandibular form.
• When designing restorations and selecting materials, it is important to consider the
location, opposing dentition, and the force generating capacity of the patient.
• These factors can also be estimated by the success or failure of other restorations in
the patient’s mouth.
e.g. a material or design sufficient to withstand the forces of occlusion in the anterior
segment may not be sufficient for the posterior segment.

13. STRESS
• When a force acts on a constrained body, the force is resisted by the body. This
internal reaction is equal in magnitude and opposite in direction to the applied
external force, and is called stress, typically denoted as S or 𝜎.
• Both the applied force and the internal resistance are distributed over an area of the
body, so the stress in an object is defined as the force per area, or stress = force/ area.
• The unit of stress is the unit of force i.e. N divided by a unit of area , and is commonly
expressed as Pascal (1 Pa = 1 N/m2 = 1 MN/mm2).

14. • The stress acting on the dental restorations in which the areas over which the forces
are applied often are extremely small.
e.g. cusp areas of contact may have cross sectional areas of only 0.16 to 0.016 cm2.
Numeric example:-
A 20 gauge orthodontic wire has a diameter of 0.8 mm and a cross sectional area of 0.5
mm2.
If a 220 N force is applied to a wire of this diameter, the stress developed is equivalent to
220N/0.5 mm2 or 440 N/mm2 (MPa).

15. • Stress is always normalized to a 1 m2 area, but a dental restoration such as a small
occlusal pit restoration may have no more than 4 mm2 of surface area, if the
restoration is 2mm on a side.
• If an occlusal force of 440 N is concentrated on this area, the stress developed would
be 100 MPa.
• Therefore , stresses equivalent to several hundred MPa occur in many types of
restorations.
• Stresses can be produced in the range of thousands of MPa when the contact area of a
cusp or dental explorer is used to apply the force.

16. TYPES OF STRESS
• A force can be applied from any angle or direction, and often several forces combine
to develop complex stresses in a structure.
• It is rare for forces and stresses to be isolated to a single axis.
• Individually applied forces can be defined as axial, shear, bending or torsional.

17. TYPES
Tensile
Results from two sets of
forces directed away
from each other in the
same straight line or
when one end is
constrained and the
other end is subjected
to a force directed away
from the constraint.
Compression
Results from two sets of
forces directed towards
each other in the same
straight line or when
one surface is
constrained and the
other is subjected to a
force directed toward
the constraint.
Shear
Occurs from two sets of
forces directed parallel
to each other , but not
along the same straight
line.

18. • TORSION: Results from twisting of a body , and bending or flexure results from an
applied bending moment.
• When tension is applied , the molecules making up the body resist being pulled apart.
• When compression is applied, they resist being forced more closely together.
• As a result of a shear stress application, one portion of the body must resist sliding
past another.
• These resistances of a material to deformation represent the basic qualities of elasticity
of solid bodies.

19. • Picture of stress example*

20. STRAIN
• Each type of stress is capable of producing a corresponding
deformation in a body.
• Strain , 𝜀, is described as the change in length per original length of
the body when it is subjected to a load.
• Strain (𝜀) = Deformation / original length
• e.g. if a load is applied to a wire with an original length of 2mm
resulting in a new length of 2.02 mm, it has deformed 0.02mm and
strain is 0.02/2 = 0.01 , or 1%.

21. • Strain is an important consideration in dental restorative materials, such
as orthodontic wires or implant screws, in which a larger amount of
strain can happen before failure.
• It is also important in impression materials, where the material needs to
recover without permanent distortion when removing it from the hard
tissue undercuts.

22. STRESS STRAIN CURVES
PL- Proportional limit
EL- Elastic limit
UTS- Ultimate tensile
strength
FS- Fracture
strength/Fracture point
YP- Yield point

23. • The strength of a material can be described by one or more of the following
properties:
• Proportional limit, highest stress at which, stress is linearly proportional to strain.
• Elastic limit, the maximum stress a material can withstand before it becomes plastically
deformed.

24. • The region of the stress strain
curve before the proportional
limit is called the elastic
region.
• When an object experiences
stress greater than the
proportional limit, permanent
or irreversible strain occurs.
The region of the stress strain
curve beyond the proportional
limit is called the plastic
region.

25. YIELD STRENGTH
• The yield strength or yield stress or yield point of a material is a property at which the
material begins to function in a plastic manner.
• At this point, a small defined amount of permanent strain has occurred in the material.
• Any dental restoration that is permanently deformed through the forces of mastication
is usually a functional failure to some degree.
• For eg. A FPD that is permanently deformed by excessive occlusal forces would exhibit
altered occlusal contacts. Here the restoration is permanently deformed because a
stress equal to or greater than the yield strength was generated.

26. ULTIMATE STRENGTH
• The ultimate tensile strength or stress is defined as the maximum stress that a material
can withstand before failure in tension.
• The ultimate compressive strength is defined as the maximum stress a material can
withstand in compression.
• The ultimate strength of an alloy as used in dentistry specifies the maximum load and
minimum cross sectional area when designing a restoration.
• The yield strength is often of greater importance than ultimate strength in design and
material selection because it is an estimate of when a material will start to deform
permanently.

27. FRACTURE STRENGTH
• Fracture strength: The stress at which a brittle material fractures is called the
fracture strength or fracture stress. Denoted as 𝜎𝐹.
• Shear strength: it is the maximum stress that a material can withstand before
failure in a shear mode of loading.
• Flexural strength: it is the maximum stress that a material can withstand before
failure under flexure or bending.

28. • Tensile strength : When a body is subjected to two axial forces along the
same plane but in opposite directions, it results in tension. The
resistance of material to this load is called tensile strength.
• Compressive strength: Compressive strength is most useful for
comparing materials that are brittle & generally weak in tension

29. SHEAR STRENGTH
MATERIAL SHEAR STRENGTH (Mpa)
ENAMEL 90
DENTIN 138
AMALGAM 188
PORCELAIN 111
ZINC PHOSPHATE CEMENT 13

30. COMPRESSIVE STRENGTH
MATERIAL COMPRESSIVE STRENGTH (MPa)
ENAMEL 384
DENTIN 297
AMALGAM 189
CALCIUM HYDROXIDE LINER 8
FELDSPATHIC PORCELAIN 149
HIGH STRENGTH STONE 81
RESIN COMPOSITE 225
ZINC PHOSPHATE CEMENT 110

31. ELONGATION
• The deformation that results from the application of a tensile force is elongation.
• The permanent deformation may be measured with extensometer while the material is
being tested and calculated from the stress-strain curve.
• Total elongation is commonly expressed as a percentage. The percent elongation is
calculated as follows :
• Elongation = ( increase in length / original length ) * 100%
• The total percent elongation includes both the elastic elongation as well as the plastic
elongation.
• The plastic elongation usually the greatest of the two, except in materials that are quite
brittle or those with very low stiffness.

32. • For example:-
A material that exhibits a 20% total elongation at the time of fracture has increased in
length by one fifth of its original length. Such a material , as in many dental gold alloys,
has a high value for plastic or permanent elongation, and in general, is a ductile type of
alloy, whereas a material with only 1% elongation would possess little permanent
elongation and be considered brittle.
An alloy that has a high value for total elongation can be bent permanently without
danger of fracture .
Clasps can be adjusted, orthodontic wires can be adapted, and crowns or inlays can be
burnished if alloys with high values for elongation are used.

33. ELASTIC MODULUS
• The measure of the elasticity of the material is described as elastic modulus, also
referred to as Young’s modulus and is denoted by the variable E.
• The word modulus means ratio.
• The elastic modulus represents the stiffness of a material within the elastic range.
• Elastic modulus = Stress/ Strain.
E = 𝜎/𝜀
• This equation is also called as Hooke’s law.
• Unit of elastic modulus is MPa.

34. • The elastic qualities of a material represent a fundamental property of the material.
• The interatomic or intermolecular forces of the material are responsible for the
property of elasticity.
• The property is generally independent of any heat treatment that a metal alloy has
received , but is quite dependent on the composition of the material.

35. TABLE

36. POISSON’S RATIO
• Within the elastic range, the ratio of the lateral to the axial strain is called POISSON’s
RATIO. Denoted by (v).
• In tensile loading , the Poisson's ratio indicates that the reduction in cross section is
proportional to the elongation during the elastic deformation. The reduction in the
cross section continues until the material is fractured.
• Poisson’s ratio is a unitless value because it is the ratio of two strains.

37. • Most rigid materials, such as enamel, dentin , amalgam , and dental composite, exhibit
a Poisson’s ratio of about 0.3.
• Brittle substances such as hard gold alloys and dental amalgam show little permanent
reduction in cross section during a tensile test.
• More ductile materials such as soft alloys, which are high in gold content, show a
higher degree of reduction in cross sectional area and higher Poisson’s ratios.

38. DUCTILITY AND MALLEABILITY
• Ductility is defined as the ability to be drawn and shaped into wire by means of
tension.
• Malleability of a substance represents its ability to be hammered or rolled into thin
sheets without fracturing. Malleability comes from the Latin word malleus, or hammer.
• A high degree of elongation indicates good malleability and ductility.
• Ductility is a property that has been related to the workability of a material in the
mouth
(e.g. burnishability of the margins of a casting).
• Gold and silver , used extensively in dentistry , are the most mallaeable and ductile of
the metals, but other metals do not follow the same order for both mallaeability and

39. RESILIENCE
• It is the resistance of a material to
permanent deformation.
• It indicates the amount of energy
necessary to deform the material to the
proportional limit.
• Resilience is therefore measured by the
area under the elastic portion of the
stress- strain curve.
A, the area indicating the resistance

40. • Resilience has a particular importance in evaluation of orthodontic
wires.
• An example is the amount of work expected from a particular spring to
move a tooth.

41. TOUGHNESS
• Toughness is the resistance of a material to
fracture, is an indication of the amount of
energy necessary to cause fracture.
• It is the area under the elastic and plastic
portions of a stress strain curve as shown in the
figure.
B, the area representing the
toughness of a material

42. FRACTURE TOUGHNESS
• The ability to be plastically deformed without fracture , or the amount of energy
required for fracture is called fracture toughness.
• Fracture toughness has been measured for a number of important restorative
materials, including amalgam, acrylic denture base materials, composites,
ceramics, orthodontic brackets, cements, and human enamel and dentin.

43. • The presence of fillers in polymers increase the fracture toughness .
• Similarly addition of upto 50% by weight of zirconia to ceramic
increases fracture toughness.

44. PROPERTIES AND STRESS STRAIN
CURVES
• The shape of the a stress strain
curve and the magnitudes of the
stress and strain allow classification
of materials with respect to their
general properties.
• It is clear that the properties of
stiffness, strength and ductility are
independent, and materials may
exhibit various combinations of
these three properties.

45. TENSILE PROPERTIES OF BRITTLE
MATERIALS
• Many restorative materials including dental amalgam, cements, ceramics, plaster and
stone are much weaker in compression.
• Although special grips have been used to provide axial tensile loading with a minimum
of localized stress, obtaining uniform results is still difficult, and such testing is
relatively slow and time consuming.

46. VISCOELASTICITY
• The mechanical properties of many dental materials such as alginate,
elastomeric impression materials, wires, amalgam, polymers, bone,
dentin , oral mucosa, and periodontal ligaments, depend on how fast
they are loaded.
• Materials that have mechanical properties independent of loading rate
are termed elastic.
• In these materials, strain occurs when the load is applied.

47. • Other materials exhibit a lag in response when a load is applied.
• This time lag is referred to as a viscous response.
• Materials that have mechanical properties dependent on loading
rate and exhibit both elastic and viscous behavior are termed as
viscoelastic.

48. FLUID BEHAVIOR AND VISCOSITY
• Many dental materials such as cements and impression materials , are in the fluid state
when formed. Therefore, fluid phenomena are important.
• Viscosity is the resistance of the fluid to flow and is equal to the shear stress divided by
the shear strain rate.
• When a cement or impression material sets, the viscosity increases , making it less
viscous and more solid like.
• The units of viscosity are poise , p (1p = 0.1 Pa s = 0.1 N s /m2)

49. CLASSIFICATION OF FLUIDS
DEPENDING ON THEIR VISCOSITY :-
Newtonian
Pseudoplastic
Dilatant

51. • The viscosity of a Newtonian fluid is constant and independent of shear rate. E.g. some
dental cements and impression materials are Newtonian.
• The viscosity of a pseudoplastic fluid decreases with increasing shear rate.
E.g.Monophase elastomeric impression materials are pseudoplastic.
e.g. ketchup is also pseudoplastic , which makes it difficult to remove from a bottle.
Shaking the bottle or rapping the side of the bottle increases its shear rate, decreases its
viscosity , and improves its pourability.
• The viscosity of a dilatant fluid increases with increasing shear rate. E.g. fluid denture
base resins.

52. VISCOELASTIC MATERIALS
• For viscoelastic materials, the strain rate can alter the stress-strain properties.
Example 1- the tear strength of alginate impression material, for example, is increased
about four times when the rate of loading is increased from 2.5 to 25 cm/ min. Alginate
impressions should therefore be removed from the mouth quickly to improve its tear
resistance.
Example 2- Another example of strain rate dependence is the elastic modulus of dental
amalgam is 21 GPa at slow rates of loading and 62 GPa at high rates of loading.

53. • There are two properties of
importance to viscoelastic
materials:-
1) Stress relaxation:-reduction
in stress in a material
subjected to constant strain.
2) Creep :- increase in strain in a
material under constant
stress.

54. • The importance of creep can be
seen in this figure, which shows
creep curves for low and high
copper amalgam.
• For a given load at a given time, the
low copper amalgam has a greater
strain.
• The implications and clinical
importance of this are that the
greater creep in the low copper
amalgam makes it more susceptible
to strain accumulation and fracture,
and also marginal breakdown,
which can lead to secondary decay.
• The higher creep behavior of low
copper amalgam contributes to its
decline in popularity.

55. DYNAMIC MODULUS
• The dynamic modulus is defined as the ratio of stress to strain for small cylindrical
deformations at a given frequency and at a particular point on the stress-strain curve.
• It is measured in a dynamic oscillation instrument, the dynamic modulus is computed
by:
E= mqp2
Where m is mass of the loading element.
q is height divided by twice the area of the cylindrical specimen.
p is the angular frequency of the vibrations.

56. • In conjunction with the dynamic modulus, values of internal friction and
dynamic resilience can be determined.
• For example, cylindrical stretching or compression of an elastomer results
in irreversibly lost energy that is exhibited as heat.
• The internal friction of an elastomer is comparable with the viscosity of a
liquid.
• The value of internal friction is necessary to calculate the dynamic
resilience, which is the ratio of energy loss to energy expanded.

57. SURFACE MECHANICAL PROPERTIES
Hardness :- may be broadly defined as the resistance to permanent
surface indentation or penetration.

58. • Hardness is therefore a measure of the resistance to plastic deformation and is
measured as force per unit area of indentation.
• Hardness influences ease of cutting, finishing and polishing an object and its resistance
to in service scratching.
• Finishing and polishing of a structure is important for esthetic purposes as scratches
can compromise fatigue strength and lead to premature failure.
• The common methods for testing the hardness are the Brinell, Knoop , Vickers,
Rockwell , Barcol and Shore.

59. FRICTION
• Friction is the resistance
between contacting
bodies when one moves
relative to the other.

60. • An example of the importance of friction in dentistry lies in the concept
of sliding mechanics used in orthodontics. A known and controlled
frictional force is required when an orthodontic wire is slid through the
bracket. Combinations of different materials results in different frictional
forces.
• Friction is also an important consideration when dissimilar restorative
materials contact and slide against each other in the oral cavity such as
in protrusive or working movements of the mandible.

61. WEAR
• Wear is a loss of material resulting from removal and relocation of
materials through the contact of two or more materials.
• When two solid materials are in contact, they touch only at the tips of
their most protruding asperities.
• In general, there are four types of wear:-
1. Adhesive wear
2. Corrosive wear
3. Surface fatigue
4. Abrasive wear

62. • Adhesive wear involves a harder material cutting or plowing into a
softer material.
• There can be two types of abrasive wear :-two and three body abrasive
wear.
• This type of wear can be minimized if surfaces are smooth and hard
if third party particles are kept off the surfaces.
• Corrosive wear is secondary to physical removal of a protective layer
and is therefore related to the chemical activity of the wear surfaces.
• In general, metals  adhesive, corrosive and three body wear.
• Polymers  abrasive and fatigue wear.

63. THANK YOU!

64. THE COLLOIDAL STATE
Substances are called colloids when they consist of two or more phases, with units of
at least one of the phases having dimension slightly greater than simple molecular size.
(craig)
A homogeneous non-crystalline substance consisting of large molecules or
ultramicroscopic particles of one substance dispersed through a second substance.
Colloids include gels, sols, and emulsions; the particles do not settle, and cannot be
separated out by ordinary filtering or centrifuging like those in a suspension.

65. • Thus colloidal systems can be fine dispersions, emulsions or foams.
Examples :- 1) colloidal silica as a filler in resin composites.
2) colloidal silica in water to be mixed with high strength dental stone
3) fillers used in elastomeric impression materials to control viscosity.
4) agglomerates of detergent molecules in water that serve as wetting
agents.

66. TYPICAL COLLOID SYSTEMS
Sol Gel
Resembles a solution, but it is made up of
colloidal particles dispersed in a liquid.
When a sol is chilled or caused to react by
the addition of suitable chemicals, it may
be transformed in to a gel.
Semisolid , or jelly like, quality.
• Systems that have water as one component are described hydrosols
and hydrogels.
• A more general term is hydrocolloid, which is often used in dentistry
to describe the alginate gels used as a flexible impression materials.
• Two examples of material that involve gel structures are the agar
and alginate hydrocolloid impression materials.

67. DIFFUSION THROUGH MEMBRANES
AND OSMOTIC PRESSURE
• Osmotic pressure is the pressure developed by diffusion of a liquid
or solvent through a membrane.
• The solvent passes from the dilute to the more concentrated
solution through the membranes separating the two solutions.

68. • Osmotic pressure is a concept that has been used to explain the
hypersensitivity of dentin. The change in pressure in carious, exposed
dentin from contact with saliva or concentrated solutions causes
diffusion through out the structure that increases or decreases the
pressure on the sensory system.
• Diffusion of salts and acids through some cavity liners is a potential
problem.

69. ADSORPTION, ABSORPTION AND
SORPTION
Adsorption
• In this process, a liquid or gas adheres to the surface of the solid or
firmly by the attachment of molecules, thus reducing their surface free
energy.
• Two pieces of the same metal in the solid state pressed closely together,
mass is said to COHERE.
• When dissimilar substance, such as gas or liquid, is in intimate contact
with the surface of the solid, it is said to ADHERE.
• Example : Degree to which saliva will wet or adhere to the enamel
of tooth depends on the tendency for surface adsorption.

70. ABSORPTION
• In the process of absorption, the substance absorbed diffuses into the
solid material by diffusion process, and the process is characterized by
concentration of molecules at the surface.
• Example: the process of absorption of water by alginate impression
materials.

71. SORPTION
• When both absorption & adsorption are known to exist & it is not clear
which predominates, the whole process is known as SORPTION.
• EXAMPLE : Measurement of moisture content of dental resins, it is
described as sorption of moisture by the resin.

72. SURFACE TENSION AND WETTING
• Surface tension is the elastic tendency of a fluid surface which makes it
acquire the least surface area possible. (craig)
• The increase in energy per unit area of surface is referred to as the surface
energy or surface tension. (phillip’s)
• It is measured in terms of force(dynes) per centimeter of the surface of
liquid.
e.g. the surface tension of water is 76 at 0º C , 72 at 25℃ ,68 at 50℃ and 59 at
100℃.

73. • The increased wettability of solids with liquids of reduced
surface tension is important in numerous dental
applications.
• The tendency for wetting surfaces can be studied by
measuring the angle of contact between the liquid and
the solid surface.

75. CONTACT ANGLES OF WATER AND
SALIVA IN DENTAL MATERIALS
• The determination of contact angle is important in number of clinically
relevant situations.
• For e.g.. The contact angle of water and saliva on complete denture
relates to the retention of the denture.

77. OPTICAL PROPERTIES
"Color is the sensation resulting from stimulation of the retina
of the eye by light waves of certain lengths”
(Webster)

78. • According to one of the grassman’s laws, the eye can distinguish
differences in only 3 parameters of color.
1) DOMINANT WAVELENGTH
2) LUMINOUS REFLECTANCE
3) EXCITATION PURITY

79. • The dominant wavelength of color is monochromatic light, when mixed
in suitable proportions with achromatic color will match the color
perceived.
• The eye is sensitive to wavelengths from approximately 400 nm(violet)
to 700nm(dark red).
• MOST SENSITIVE-55O nm[green-yellow]
• Luminous reflectance of a color classifies an object as equivalent to a
member of a series of achromatic, grayscale objects ranging :-
1) From black to white- light diffusing objects
2) From black to perfectly clear & colorless- transmitting objects

80. • Hue is the tone. Basic color
depending on the wavelength it
reflects red, purple, blue, etc.
• Value(lightness) describes overall
intensity to how light or dark a color
is. It is the only dimension of color
that may exist by itself.
• Chroma(Saturation) may be defined
as the strength or dominance of the
hue. Towards the center of the
color wheel, no hue dominates and
they becomes less and less
saturated.

81. HUE
• It is the quality by which we distinguish one color family from another.
(Munsell )
• In simple words, hue is the type of colour.

82. VALUE
• The relative darkness or lightness of a color, or brightness of an object.
• Range = 0-10 (0=black, 10 = white)
• Amount of light energy an object reflects or transmits.
• Restorations too high in value are easily detected.
Low value  high value

83. CHROMA
Low chroma  high chroma
• It is that quality by which we distinguish a strong colour
from a weak one.
• In simpler words, it is the saturation or intensity of a
hue.

84. OPACITY, TRANSLUCENCY,
TRANSPARENCY AND OPALESCENCE

85. OPACITY
• Opacity is the property of a material that prevents the passage of the
light.
• When all of the colors of the spectrum from a white light source such as
sunlight are reflected from an object with the same intensity received ,
the object appears white.
• When all the spectrum colors are absorbed equally , the object appears
black.
• An opaque material may absorb some of the light and reflect the
remainder.

86. TRANSLUCENCY
• Translucency is a property of substances that permits the passage of
light but disperses the light, so objects cannot be seen through the
material.
• Some translucent materials used in dentistry are ceramics, resin
composites and acrylics.

87. TRANSPARENT
• Transparent materials allow the passage of light so little distortion takes
place and objects may be clearly seen through them.
• Transparent substances such as glass may be colored if they absorb
certain wavelengths and transmit others.
• For e.g. if a piece of glass absorbed all wavelengths except red, it would
appear red by transmitted light.

88. OPALESCENCE
• Opalescent materials , such as dental enamel, are able to scatter shorter
wavelengths of light.
• Under transmitted light, they appear brown/yellow, whereas shades of
blue are perceptible under reflected light.
• To produce highly esthetic restorations that truly mimic the natural
appearance of a tooth , materials with opalescent properties should be
used.
• This has popularized the use of porcelain veneering materials, as well as
direct restorative composites.

90. COLOR PERCEPTION
• Light from an object incident on human eye
• focused on the cone shaped cells of the retina
• cone shaped cells convert light in to nerve impulses
• nerve impulses transmitted to the brain

91. • Rods
• Scotopic (gray scale) vision.
• Interpret brightness, not color.
• Highest concentration on
peripheral retina.
• Cones
• Photopic (color) vision.
• Interpret color.
• More active under high light.
• Highest concentration on central retina (macula), most color
perceptive area of eye

92. SHADE SELECTION
Three factors-considered
• Bezold-brucke effect
• Metamerism
• Flourescence

93. BEZOLD-BRUCKE EFFECT
• A B-B effect appears when stimuli similar in dominant
wavelength, but of different intensity, are perceived as
different hues.
• It is a luminescence dependent change in color appearance.
• As the brightness become too intense color appears to
change.
• Patient is visualized against an intense colored background, a
tooth shade may be selected with a hue that is shifted
somewhat towards the complementary color of the back
ground color.

94. METAMERISM
• The change in color perception of two objects under different lights is
called metamerism.
• Spectral distribution of light reflected from or transmitted is dependent
on the spectral content of incident light
The appearance of an object is quite
dependent on the nature of light by which the object is viewed.

96. • Dental operatory-common sources of light used are:
1. Day light
2. Incandescent lamps
3. Fluorescent lamps
• Color matching should be done under 2 or more different light sources,
one of which should be day light.

97. FLUORESCENCE
• It is the emission of luminous energy by a material when a beam of light
is shone on it.
• The wavelength of the emitted light is usually longer than that of the
exciting radiation.
• Typically, blue or ultraviolet light produces fluorescent light that is in the
visible range.

98. • Sound human teeth emit fluorescent light when excited by ultraviolet
radiation (365 nm), the fluorescence being polychromatic with the
greatest intensity in the blue region (450 nm) of the spectrum.
• Some anterior restorative materials and dental porcelains are formulated
with fluorescing agents to reproduce the natural appearance of tooth
structures.

99. • This property contributes to the vital appearance of teeth.
• It primarily occurs in dentin because of higher amount of organic
material present.
• Thus the researcher developing a tooth colored restorative material and
the dentist who fabricates them must be concerned with color matching
under light sources that contain a sufficient near-ultraviolet radiation.
• Sunlight, photoflash lamps, vapor lamps-important sources.

100. THERMAL CONDUCTIVITY
• Thermal conductivity is the rate of heat flow through a material.
• The thermal conductivity (k) or coefficient of thermal conductivity is the
quantity of heat in calories/second that passes through a specimen 1 cm
thick having a cross sectional area of 1cm2 when the temperature
differential between the surfaces perpendicular to the heat flow of the
specimen is 1˚c.

101. • Materials with high thermal conductivity –conductors.
• Materials with low thermal conductivity- insulators.
• SI unit- watts per meter per degree kelvin.
• Dental application: Metallic dental restoration→ conduct heat→
transferred to pulp → discomfort and pain.
• Prevented by placing an insulating base.

102. SPECIFIC HEAT
• The specific heat of the substance is the quantity of heat required to
raise the temperature of 1gm of the substance by 1 degree centigrade.
• The specific heat of both enamel and dentin has been found to be
higher than that of metals used for fillings .

103. THERMAL DIFFUSIVITY
• It is a measure of the rate at which a body with a non uniform
temperature reaches a state of thermal equilibrium.
• Unit- mm²/sec
• E.g.: a gold crown or a dental amalgam, the low specific heat combined
with the high thermal conductivity creates a thermal shock more readily
than normal tooth structure does.

104. CLINICAL SIGNIFICANCE
• Thermal diffusivity of enamel & dentin is found to be low compared to
other restorative materials.
• So the thickness of the remaining dentin and the base are important
parameters in providing proper thermal insulation.

105. THERMAL INSULATION
• Thermal insulation is directly proportional to the thickness of the
cement base and inversely proportional to square root of thermal
diffusivity.
• This may be a more important property in dentistry than thermal
conductivity because temperatures change rapidly in the oral cavity.
• For example patient drinking ice water with amalgam filling, high
thermal diffusivity of amalgam provides a high risk of thermal shock
provided if improper base is given.

106. LINEAR COEFFICIENT OF THERMAL
EXPANSION
• The linear coefficient of thermal expansion is defined as the change in
length per unit of the original length of a material when its temperature
is raised by 1˚c.
• SI UNITS - μm /cm ℃.
• Knowledge of coefficient of thermal expansion is essential because a
tooth restoration may expand or contract more than the tooth during a
change in temperature, thus the restoration may leak, or it may de-bond
from the tooth.

107. Materials Coefficient x 10-6- / oC
Inlay waxes 350 – 450
Silicone impression materials 210
Polysulfide impression materials 140
Acrylic resin 76.0
Mercury 60.6
Composite resins 14-50
Tooth (crown portion) 11.4
Glass ionomer (type 2) 10.2-11.4
Zinc oxide eugenol cement 35
Amalgam 22.1 – 28.0
Gold 14.4
Porcelain 12.0

108. • When coefficients of thermal expansion of a restorative material and
tooth structure are different.
i.e ingress and egress of fluids between the tooth and the restoration.

109. • The high expansion coefficient of inlay wax is also important because it
is highly susceptible to temperature changes.
• For example, an accurate wax pattern that fits a prepared tooth
contracts significantly when it is removed from the tooth or from a die
in a hot area and then stored in a cooler area.

110. • Inlay wax has high coefficient of thermal expansion, so it is susceptible
to the change in temperature.
Wax pattern prepared at the mouth temperature
↓
brought down to room temperature for laboratory work
↓
decrease in temperature
↓
significant change in dimensions.

111. ELECTRICAL CONDUCTIVITY AND
RESISTIVITY
• The ability of a material to conduct electric current is called as conductivity or
conversely ,as the specific resistance or resistivity.
• The carious tooth has been observed to have a lower electrical resistance compared
to normal tooth.

112. ELECTROCHEMICAL CORROSION
• Electrochemical corrosion occurs whenever chemically different sites
act as an anode and cathode.
• This corrosion requires the presence of an electrolyte, typically saliva.
The anode corrodes producing soluble and insoluble reaction products.
• Electrochemical corrosion seen in dental amalgam is:
1. Galvanic corrosion
2. Crevice corrosion
3. Stress corrosion.

113. GALVANIC CORROSION
• When combination of dissimilar metals are in direct physical contact.
• Saliva with its salt provides a weak electrolyte.

114. STRESS CORROSION
• When a dental appliance is subjected to stress in the presence of an
electrolyte like saliva it develops stress corrosion.
• Electrochemical cells consisting of more deformed metal regions
(anodic), less deformed metal regions (cathodic), saliva are created and
deformed metal regions will experience corrosion attack.
• Example : Regions within the amalgam that are under stress

115. CREVICE CORROSION
• It occurs whenever there are variations in the electrolytes or the
composition of the given electrolyte within the system.
• Example: Plaque or soft tissue debris covers the portion of enamel. The
covered area has locally lowered oxygen or hydrogen ion concentration,
making it behave more anodically & corrode.

116. GALVANISM
• Presence of dissimilar fillings in opposing or adjacent teeth.
• Fillings in conjunction with saliva, as an electrolyte, make up an electric
cell.
• When the fillings contact each other, the cell is completed and there is a
flow of current.
• This phenomenon is called galvanism.

117. • Sensitivity of the patient to the current has a greater influence on the pain
perception.
• Galvanic current developed from contact of 2 metallic restorations depend on:
1) Composition of fillings
2) Their surface area
• Alloy of stainless steel develops a higher current density than Au or Co-Cr alloys
when in contact with an amalgam restoration.
• As the size of cathode increases compared to anode[amalgam] the current density
may increase.

118. DENTAL APPLICATION
• Galvanic shock – gold restoration comes in contact with
amalgam restoration.
• Painting a varnish on the surface of amalgam.
• Best procedure is to avoid dissimilar metal in contact.

119. CONCLUSION
• Physical properties of oral restorations must adequately withstand the
stresses of mastication.
• Restorative materials are generally weaker in tension than in
compression. Restorations should be designed to minimize areas of
tension.
• Appliances and restorative materials should be designed in such a way
that masticatory forces are distributed uniformly.

120. TAKE HOME MESSAGE
• In summary, there are three interrelated factors that are important in the
long term function of dental restorative materials:
1. Material choice.
2. Component geometry (to minimize stress concentration).
3. Component design ( to distribute stresses uniformly).

121. REFERENCES
• Phillip’s SCIENCE OF DENTAL MATERIALS: ANUSAVICE 10th adition &
11th edition
• CRAIG’S restorative dental materials 13th edition
• Mahalaxmi dental materials.
• Vimal K sikri operative dentistry.

122. THANK YOU