Biological, physical and chemical properties of dental materials

1. BIOLOGICAL, PHYSICAL AND
CHEMICAL
PROPERTIES OF DENTAL MATERIALS
By – Dr. Bhavika Nagpal
PG 1st year Prosthodontics

2. BIOLOGICAL PROPERTIES

3. CONTENTS
 Introduction
 Ideal Biological Properties of Dental Materials
 Classification of Dental Materials based on their
biocompatibility
 Classification of Adverse reactions of Dental
Materials
 Adverse Effects of Dental Materials
 Occupational Hazards for dental personnel
 Measurement of Biocompatibility
 Standards that regulate the measurement of
Biocompatibility

4.  Materials used in oral cavity are classed as
‘BIOMATERIALS”.
 A biomaterial can be defined as any substance other than a
drug that can be used for any period of time as a part of
system that treats, augments, or replaces any tissue, organ
or function of the body.
INTRODUCTION

5.  The importance of learning the biological properties of dental
materials is to assess the biocompatibility of the material in
use.
 Biocompatibility is defined as the ability of biomaterial to
perform its desired function w.r.t. its medical or dental therapy,
without eliciting any undesirable local or systemic effects in the
recipient and beneficiary of that therapy, but generating the
most appropriate beneficial cellular or tissue response, thus
optimizing the clinical relevance of that therapy. (Williams,
2008)

6. BIOCOMPATIBILITY INVOLVES TWO COMPONENTS
(i.e. concluding the definition……)
 "BIOSAFETY”
This concerns and deals with the exclusion of deleterious effects of a
biomaterial on the organism itself (toxicity at the cellular level) .
 “BIOFUNCTIONALITY”
This concerns and addresses the need of a material not only to be free
from damaging effects on the host at the cellular level, but also to be able
to elicit a beneficial host-response for optimal functioning of the medical
device.

7.  Placement of material in a body produce a :
Dynamic interface
Between
Material | the Host
 Biocompatibility relates to the overall performance of the
(bio) material : “Two-Way” concept
 The effect the body has on the material.
 The effect the material has on the body.

8.  Biocompatibility is like a color.
 It is not just the property of biomaterial, but also that how
the material interacts with environment.
 So biological response will change, if changes will occur in
host, application of material and material itself.

9. IDEAL BIOLOGICAL PROPERTIES OF DENTAL
MATERIALS
 Should not be harmful / irritant to pulp and soft tissue.
 Should not diffuse toxic substances.
 Should not produce allergic reactions.
 Should not be carcinogenic.
 Should not be mutagenic.

10. CLASSIFICATION OF DENTAL MATERIALS BASED
ON THEIR BIOCOMPATIBILITY

11. S.no CATEGORY EXAMPLES Dental materials
1. Materials for short term
superficial application OR
accidentally contacted /
inhaled
Impression materials,
Suture materials,
Rubber dam, Latex,
alginate dust, mercury
vapors
Zinc oxide Eugenol,
Impression compound
2. Materials contacting the
hard tissues of oral cavity
Dental Implants Ceramic, Titanium
3. Materials contact the soft
tissues (gingiva, oral
mucosa) of oral cavity
Crown restorations,
Denture base materials,
Tissue conditioners
Ceramic, casting alloys,
Composite resins, Acrylic
resin, Soft liners
4. Materials contacting the pulp Restorative materials,
Cements, Pre &
Postoperative materials
- Amalgam, composite,
GIC, liners
- Bonding agent,
etchent
- Fluorides, bleaching
agents
5. Materials contacting the
peri-apical area
Root canal filling and
irrigating solutions
Sodium hypochlorite,
Chlorhexidine, Ca(OH)2,
MTA, GP, Formocresol

12. REACTIONS OF PULP
MICROLEAKAGE
 Microleakage may be defined as the clinically undetectable
passage of bacteria, fluids, molecules or ions between a cavity
wall and the restorative material.
 Clinically, microleakage can lead to staining around the
margins of restorations, postoperative sensitivity, secondary
caries, restoration failure, pulpal pathology or pulpal death,
partial or total loss of restoration.
SOURCE - THE RELEVANCE OF MICRO-LEAKAGE STUDIES
ANDREA FABIANELLIA , SARAH POLLINGTONC , CAREL L. DAVIDSONB , MARIA
CHRYSANTI CAGIDIACOA , CECILIA GORACCIA

13. MICROLEAKAGE VS. NANOLEAKAGE
 Microleakage is usually associated with invasion from the
external environment through the margins of the restoration,
but microleakage can also occur internally.
 In the latest studies, a new form of leakage, nanoleakage,
has been described.
 i.e. the leakage specific to dentin bonding, occurs between
mineralized dentin and bonding material, where the
material does not sufficiently penetrate into demineralized
collagen matrix.
 Nanoleakage is independent of microleakage.

14.  Some studies suggested the cause of pulpal irritation due
to microleakage, is the dental materials themselves.
(DIRECT cause)
 Several others described the products leaching at the
interface of dental restorations and tooth structure as a
result of microleakage, be the cause of pulpal irritation, than
dental materials themselves. (INDIRECT cause)
 Ormocer showed significantly lower microleakage when
compared to giomer.
 Glass ionomer showing maximum leakage followed by
composite resin.
Ormocer < Giomer < Composite < GIC

15. HOW TO REDUCE MICROLEAKAGE ?
 Incremental placement and curing.
 Use of flowable composites.
 Using GIC as a liner below composite restorations
(Sandwich Technique), allow GIC to bind chemically to
dentin and micromechanically to composite, thus providing
better seal.
 Adhesive luting of inlays create better bond than direct
luting restorations.
 Surface application of restorations with varnish produce
good sealing properties.

16. DENTIN BONDING
Though placing a bonding agent, increase the bond strength
and adhesive properties of dental materials ,
But from biocompatibility standpoint, removal of smear layer
may pose threat to pulp.
1. Barrier is removed – biomaterials can directly diffuse to
pulp.
2. Effects of microleakage become more significant.
3. Acids used to remove smear layer can produce pulp
irritation.
Various acids used are – Phosphoric, HFA, HCA, Citric,
Lactic acids.

17.  Factors producing effect on pulp tissue include –
 The strength of acid
 Duration of exposure
 RDT (minimum 0.5 mm)
 Penetration depth of acid into dentin is 100 µm.
 A 15 sec etching time is sufficient when using 32%-40% of
phosphoric acid.
 Studies have shown that these molecules (acids) may
serve as direct stimulants to up-regulate the production of
dentin remineralization.

18. RESIN BASED MATERIALS (COMPOSITES)
 Chemically cured and light cured resins often cause low to
moderate cytotoxicity, as a pulpal inflammatory response,
over 24-72 hours of exposure, with approx. 0.5 mm of RDT
present.
 The reaction diminishes as post operative period increase
to 5-8 weeks ; as it is accompanied by increase in
reparative dentin.

19. AMALGAM AND CASTING ALLOYS
 Implantation tests reveal the severe cytotoxic response of high-copper
amalgams, when compared to low-copper amalgams. As the unreacted
mercury and copper releasing from high-copper alloy, lead to adverse
tissue responses.
 Pain after amalgam restorations related to electrical and high thermal
conductivity of material is evident, post operatively in deep and unlined
cavity preparations after 3 days as a result of inflammatory response.
 Thus amalgam restorations are always lined for 2 reasons:
1. Insulation
2. Prevent marginal leakage
(Though at long terms, self sealing of margins occur as a result of
corrosion products)
Gallium based amalgam restorations are mercury free and are not cytotoxic,
unlike conventional high-copper amalgams.

20. GIC
 Biocompatibility is attributed by the weak nature and high
molecular weight of polyacrylic acid.
 Thus making it unable to diffuse into dentin.
 So, pulpal response is mild.
Bleaching Agents
 Cytotoxicity depends on concentration of peroxide.
 Bleaching agents penetrate intact enamel and pulp with few
minutes.
 If agent is not sequestered adequately in bleaching tray, it
can burn gingiva.

21. PATHWAYS THAT LEACHED IONS OR SUBSTANCES
FOLLOW DUE TO IN VIVO DEGRADATION OF DENTAL
RESTORATIONS

22. LINERS
Calcium Hydroxide
 The high alkaline pH (= 12) used as cavity liner produce caustic,
cytotoxic effect on underlying pulp tissue, by producing a necrosis to a
depth of 1mm or more.
 Ca(OH)2 form zones of healing :
1. Zone of Obliteration – Adjacent to the calcium hydroxide layer, containing
deranged pulpal tissue with dentinal fragments, debris, hemorrhage and
clot.
2. Zone of Coagulation Necrosis – As the hydroxyl ions leach more apically
they produce necrosis (0.3-0.7 mm).
3. Line of Demarcation – b/w Necrotic pulp tissue and subjacent vital tissue.
4. Zone of Dentin Bridge Formation – as underlying mesenchymal cells
stimulate odontoblast, reparative dentin begin to form.
Later necrotic zone disintegrate undergoes dystrophic calcification as a
stimulus from dentin bridge formation.

23.  Resin containing Ca(OH)2 lay down reparative dentin adjacent to liner,
without any zone of necrosis.
 Thus materials like Ca-Hydroxide liners and MTA stimulate odontoblast
and osteoblast like cells forming the mineralized dentin bridge formation.
Zinc Phosphate Cement
 Moderate to severe pulpal damage observed at first 3 days because of
initial low pH (4.2 at 3 minutes).
 pH of set cement reaches neutrality in 48 hrs.
 By 5-8 weeks only mild chronic inflammation is present, as reparative
dentin has already formed.
 Thermal conductivity is similar to enamel, so acts as a good insulator.

24. REACTIONS OF HARD TISSUES
Biocompatibility of Dental Implants :
 The success of endosseous dental implant is based on
biocompatibility of implant surface and the ingrowth of new
bone.
 The most common implant material include :
1. CP Ti.
2. Ti- Al- V alloy
3. Tantalum
4. Some types of ceramics- Zirconia

25. Intervening space present
Eg – Titanium, Non
Bioactive –Ceramic
implants
Intervening space absent
Eg – Bioactive ceramics -
Ceramic coated implants,
metallic implants

26. Ceramic Implants
Low toxic effects on tissues; Present in oxidized state; Corrosion
resistant; non immunogenic; non carcinogenic.
Available in two forms –
1. Bioactive (coated with Ca or hydroxyapatite)
2. Non Bioactive
Titanium Implants
Ti is pure when initially cast; in <1 sec. it forms surface oxide layer;
making it corrosion resistant; allows bone to osseointegrate.
They do release Ti in body; but no evidence of problem noticed yet.
Available as –
1. CP – Ti
2. Ti - 6AL – 4V

27. REACTIONS TO RESORBABLE MATERIALS
Resorbable Sutures
 Examples – Polylactic acid (PLA), Polyglycolic acid (PGA), cross-
linked collagen, starch and cellulose.
 These materials undergo – Hydrolytic decomposition into Carbon
dioxide and water.
 Generally well tolerated by tissues in vivo.
 The resorbability depends on amount of material implanted –
because these materials degrade into - acidic by products -
subsequent drop pH locally – may produce inflammatory
response.

28. CLASSIFICATION OF ADVERSE REACTION
FROM DENTAL MATERIALS
1. Toxic
2. Inflammatory
3. Allergic
4. Mutagenic

29.  Inflammation : The activation of host immune system to
ward off some challenge or threat.
 Allergic Reaction : occurs when body recognizes a
substance, molecule or ion as foreign. These rx. are
insensitive to amount of allergen.
It becomes difficult to distinguish the allergic reactions
from –
Non-allergic inflammatory processes &
Low grade toxicity.

30.  Estrogenecity : Olea et al, 1996 claimed that dental sealants
released estrogenic substances in sufficient quantities to
warrant the concern.
 Estrogenicity is defined as the ability of a chemical to act as a
hormone estrogen. As these chemicals are not indigenous,
thus called xenoestrogen.
 Bisphenol A (BPA) – a starting synthetic point of bis-GMA
composites – is considered Xenoestrogen.
 That is why dental composites has also been questioned,
particularly for use in children.

31. LICHENOID REACTION
 Oral lichenoid lesions or reactions (OLLs/OLRs) are clinical and
histological contemporaries of the classical oral lichen planus . (OLP)
 In contrast to the idiopathic nature of OLP, OLLs are often associated
with a known identifiable inciting factor.
 Hypersensitivity presents as an oral lichenoid reaction affecting oral
mucosa in direct contact with an restoration and represents a delayed,
type IV, cell mediated immune response.
 Various Dental Materials associated with OLL are
Metal- Amalgam, Co, Cr, Ni, Gold
Non- Metal- Epoxy resins, Composites

32. THE ADVERSE EFFECTS OF DENTAL MATERIALS

33. ILLUSTRATION OF CRITICAL TISSUES AND ORGANS THAT
MAY BE SUSCEPTIBLE TO THE ADVERSE EFFECTS OF
DENTAL MATERIALS

35. 1. CLASS V RESTORATION 2. NI CONTAINING ORTHODONTIC WIRE
3.CROWN RESTORATION 4. AMALGAM BLUES
1 2
3 4

36. 1. LICHENOID REACTION
2. , 3. ALLERGIC RESPONSE TO
DENTURE RESIN
1
2
3

37.  Zinc Oxide Eugenol Impression materials :
 Studies pertaining to cytotoxicity of eugenol which can cause
tissue effects from low-grade reactions to the severe allergic
reactions.
 Allergic reactions may be contact urticaria, gingivitis,
stomatitis venenata, allergic eczema of skin, edema of lips,
tongue and may proceed to epiglottis.
 The first reaction can be gagging, which is not due to the
material overextension.
REFERENCE :A rare case report of a patient with allergic reaction
following use of a dental impression material
Department of Prosthodontics, Indira Gandhi Institute of Dental Sciences, Sri Balaji
Vidyapeeth, Puducherry, India

39. OCCUPATIONAL HAZARDS FOR DENTAL PERSONNEL
Pneumoconiosis : Severe fibrotic lung disease that is caused by
chronic exposure to inorganic dust.
 Lab Cause : Fabrication and Finishing of Base metal
prostheses.
 It is occupational hazard for Dental Technicians, but not for
dental personnel .
 Prevention – Installation of dust exhaust collectors is highly
recommended in labs.
 Other lab caused hazards for dental Technicians include :
o Silicosis
(particles from abrasive silica and silica carbides)
o Berylliosis

40. Contact type reactions : causing hand or facial dermatitis
because of exposure to acrylates, latex, formaldehydes,
fragrances and rubber additives.
“Latex”
 6%-7% surgical personnel are allergic to latex gloves and
rubber dams.
 Ammonia is added to white milky sap extracted from
growing trees to preserve it.
Ammonia hydrolyzes – degrades the sap proteins –
produce allergens.
 Sulfur compounds added to vulcanize liquid latex to
rubber latex, can itself act as an allergen.

41. Hydrofluoric acid Exposure: Etching of glass-phase ceramics
and glass ceramics is performed using HF both in lab and in
mouth, to promote the bonding of ceramics to resin based
composites.
Exposure of skin, mucosa and eyes to HF :
 HF is highly corrosive and destroys tissue.
 May result in blindness or permanent eye damage.
 Can decalcify the bone.
 Exposure to skin produce – pain & burning sensations.
 Because of delayed effect – stay symptomless for several hours after
exposure.

42. Inhalation of HF vapor :
 Can damage lungs.
 Long term chronic exposure result in – Fluorosis.
Management :
 Eyes irrigated with large volumes of water for atleast 15 minutes,
while eyelids are held apart. Ca gluconate is NOT applied.
 On hands or forearms, Ca gluconate is readily applied. Reapplied
after every 15 minutes. If not available, irrigated with large volumes of
water for atleast 15 minutes.
 To minimize inhalation, HF vapor, gels and liquid should be used with
adequate ventilation. Concentrations >5% should be handled under
chemical fume hood.

43. MERCURY WASTE TREATMENT
 Special properties of Hg make it difficult to dispose off
with conventional techniques.
 Dumping on a landfill – NOT possible – Because of its
high density – Hg waste quickly seep into ground water.
 Incineration – NOT possible – because Hg would
evaporate – would make it difficult to remove from huge
off-gas stream of normal waste - incineration plant.

44.  However, special characteristics of Hg do offer one
possibility to remove it from waste.
The solution is EVAPORATION.
 Due to its extremely low boiling point (compared to other
metals), it is possible to remove Hg by thermal desorption.
 Two methods that have been considered to be most
suitable for Hg treatment includes :
 Indirectly heated, discontinuous vacuum mixer
 Rotary kilns, directly fired

45. MEASURING
BIOCOMPATIBILITY
Variety of tests are classified as –
1. In vitro
2. Animal
3. Usage

46. IN VITRO TESTS
 In vitro tests are done outside a living organism, require placement of a
material in contact with a cell, enzyme or some isolated biological
system.
 It can be performed – Direct (contact the cell system) & Indirect (use
barriers)
 Two types of cells used – Primary (limited growth time) & Continuously
grown cells (modified; thus do not retain all in vivo characteristics)
In vitro Tests
Cytotoxicity
tests
Tests for cell Metabolism or
cell function
Mutagenesis
assays

47.  Cytotoxicity tests – assess cell death caused by material by
measuring cell number or growth before or after the
exposure. Membrane permeability tests are used.
 Tests for cell metabolism or cell function – use the
biosynthetic or enzymatic activity of cells to assess
cytotoxic response. Tests that measure DNA synthesis or
protein synthesis.
 Mutagenesis Assays – assess the effect of biomaterial on
cell’s genetic material. The Ames test is used.

48. ANIMAL TEST
 Usually involving animals like mice, rats, hamsters, guinea pigs.
 Distinct from usage test (that also use animals) in that material is NOT
placed in animal with regard to its final use.
 Variety of animal tests include :
1. Mucous membrane irritation test – whether a material causes
inflammation to MM or abraded skin.
2. Skin sensitization test – Materials are injected intra-dermally to check
for hypersensitivity reactions.
3. Implantation test – used to evaluate the materials that will contact
subcutaneous tissue or bone.

49. USAGE TEST
 May be done in animals or humans.
 They differ from animal tests in that they require material be placed in a
situation identical to its intended clinical use.
 When humans are used, usage tests are k/a Clinical trials.
 Gold Standard tests – as they ultimately determine whether the material is
biocompatible for use or not.
 Most expensive and time consuming tests.
 The main targets of usage tests in dentistry include –
1. Dental pulp
2. Dental implants
3. Mucosa and gingival usage tests

50.  The basic concept in the implication of this
philosophy was that no single test will be adequate
completely to characterize the biocompatibility of a
material.
USING IN-VITRO, ANIMAL & USAGE
TESTS TOGETHER

51.  Early strategies for use of biocompatibility tests to assess the safety of
materials – Pyramid Testing Protocol, in which all materials were tested
at the bottom of pyramid and the materials were ‘weeded out’ as the
testing continued to the top of pyramid.
 This scheme funneled the safer materials for clinical trials.

52. FUTURE STRATEGIES TO TEST BIOCOMPATIBILITY
A. PRIMARY & SECONDARY TEST PLAY CONTINUOUS BUT DECREASED
ROLE AS PROGRESS CONTINUES
B. ASSESSING THE BIOCOMPATIBILITY IS AN ONGOING PROCESS

53. STANDARDS THAT REGULATE THE MEASUREMENT OF
BIOCOMPATIBILITY
1. ANSI / ADA (American National Standards Institute /
American Dental Association) specification described 3
categories of tests in 2005 –
Initial
Secondary
Usage
(Usage tests in humans are allowed only after having
approval from Food & Drug administration)
2. ISO (International Organization for Standardization)

54. PHYSICAL PROPERTIES OF
DENTAL MATERIALS

55. CONTENTS
 Introduction
 What are physical properties
 Abrasion & abrasion resistance
 Viscosity
 Creep and flow
 Color & color perception
 Thermal properties
 Tarnish & corrosion
 Conclusion
 References

56.  Physical properties determine how materials
respond to changes in their environments.
 Properties are the basis for the selection of
materials to be used in particular dental
procedures and restoration.
INTRODUCTION

57. WHAT ARE PHYSICAL PROPERTIES
 Properties based on laws of
 Mechanics,acoustics,optics,thermodynamics,
electricity,magnetism,radiation,atomic structure or
nuclear phenomena
 Hue,value,chroma & translucency- based on laws of
optics
 Thermal conductivity & coefficient of thermal expansion –
based on laws of thermodynamics
 Viscosity- related to material science & mechanics

58. ABRASION & ABRASION RESISTANCE
 Hardness : index of ability of a material to resist wear or
abrasion
 In oral cavity, abrasion is a complex mechanism, with
interaction of numerous factors.
 So, hardness can be used to compare similar materials (one
brand of metals with other) but invalid for dissimilar
materials(resins & metals)
 According to GPT 9
◦ Abrasion 1: the wearing away of a substance or structure (such as
the skin or the teeth) through some unusual or abnormal
mechanical process 2: an abnormal wearing away of the tooth
substance by causes other than mastication.
◦ Thus Hardness is considered abrasion resistance of a material.

59.  A reliable in vitro test for abrasion resistance should
simulate particular abrasion in vivo
 Due to complex clinical environment, in vitro and in vivo
tests will differ
 For example, apart from hardness, abrasion of enamel of a
tooth opposing ceramic crown is affected by :
1. frequency of chewing
2. abrasiveness of diet
3. composition of liquids
4. temperature changes
5. surface roughness
6. physical properties of materials
7. surface irregularities
8. Bite Force

60. VISCOSITY
 Rheology :Is the study of deformation and flow characteristics of matter.
 Viscosity is the resistance of a liquid to flow
 Success of given dental material depends on its properties in liquid state
as much as it is in solid state
 Dentist has to manipulate many materials in liquid state to achieve
successful clinical outcomes
e.g. cements, impression materials, gypsum
 Some amorphous materials such as waxes and resins appear solid but
they are super cooled liquids that can flow -
plastically(irreversibly) with sustained loading or
elastically(reversibly) with small stresses.

61.  Most liquids when placed in motion, resists
imposed forces that cause them to move.
 The resistance of fluid flow is controlled by internal
frictional forces with in the liquid.
 Thus viscosity is a measure of consistency of a
fluid and its inability to flow
 Highly viscous liquids flows slowly compared to
less viscous liquids
 Example of viscosity difference-zinc
polycarboxylate cement and resin cements are
more viscous as compared to zinc phosphate
cement

62. Liquid occupying space between two plates – lower plate fixed and upper being
moved to right with a velocity V and a force F is required to overcome viscosity
Stress: force per unit area that develops with in a structure
when external force is applied
Shear Stress = F/A
A – area of plates in contact with liquid
Strain: deformation caused by stress. It is calculated as a
change in length divided by initial reference length
Strain rate = V/d
d– distance moved by upper plate relative to lower plate
Viscosity = shear stress/shear strain rate

63. SHEAR STRESS VERSUS SHEAR STRAIN CURVE
 According to GPT 9
◦ stress-strain curve: the graphic representation of the tensile or
compressive stress and associated strain of a material
 Used to explain viscous nature of some materials
 Based on Rheological behavior, fluids can be
 classified into
◦ Newtonian
◦ Pseudo plastic
◦ Dilatant
◦ Plastic
 Nature of this curve is important in determining best way to
manipulate a material

65.  Newtonian behaviour
‣ An ideal fluid
‣ Demonstrates shear stress is proportional to
strain rate
‣ Fluid has constant viscosity
‣ Plot is straight line
‣ Slope is constant

66.  Pseudoplastic –
 When the flow index value is less than unity, an increase
in strain rate produces a less than proportionate increase
in shear stress. Thus the viscosity decreases with
increasing strain rate until it reaches nearly constant
value.
 Faster they are stirred, forced through syringe or
squeezed, the less viscous and more fluid they become.
Elastomeric impression
materials when loaded into a
tray, in mouth shows a higher
viscosity, whereas the same
material when extruded under
pressure through a syringe tip
shows more fluidity.

67.  Dilatant –
 Opposite to pseudoplastic
 Viscosity increases with increase in shear strain rate
 when the flow index value is greater than unity an
increase in strain rate produces a more proportionate
increase in shear stress, thus viscosity increases
 Faster they are stirred, more viscous they become.

68.  Plastic –
 These materials behave like a rigid body until some
minimum value of shear stress is reached(OFFSET) &
then attain constant viscosity
 e.g. ketchup-a sharp blow to the bottle is usually required
to produce an initial flow

69.  Thixotropic
‣ Viscosity of most liquids decreases rapidly
with increasing temperature
‣ Liquid that becomes less viscous & more
fluid under repeated applications of
pressure is referred as thixotropic
 e.g. dental prophylaxis pastes, plaster of
Paris, resin cements & some impression
materials

70.  Dental prophylactic pastes - they will not flow
out of a rubber cup until it is rotated against the
teeth to be cleaned

71.  Impression materials - does not flow out of an
impression tray until placed over dental tissues
which is beneficial for mandibular impression
 Plaster of Paris - if stirred rapidly and viscosity is
measured, the value is lower than the value for a
sample that was left undisturbed due to thixotropic
property

72. CREEP & FLOW
 Creep is defined as time-dependent plastic strain of a
material under a static load or constant stress
 According to GPT 9
◦ Creep: to change shape permanently due to
prolonged stress or exposure to high temperature
◦ Creep (1818): the slow change in dimensions of an
object due to prolonged exposure to high
temperature or stress

73. Clinical significance of creep:
Greater creep in low-copper amalgam makes it more
susceptible to strain accumulation and fracture, and also
marginal breakdown, which can lead to secondary decay
Inaccurate fit of an FPD results when a cast alloy with
poor creep resistance is veneered with porcelain at
relatively high temperatures
Flow is measurement of potential to deform under a small
static load

74.  In metal, creep usually occurs at very high temperature,
near to metal melting point
 Metal used for cast restorations or substrates for porcelain
veneers are not susceptible to creep deformation due to
melting points.
 The most important exception is dental amalgam, which
has components with melting points only slightly above the
room temperature.

75. Significance of creep on amalgam performance
Dental amalgam usually contain 42-52% of Hg and
begin melting at a temperature only slightly above room
temperature
Restored tooth with amalgam
Clenching & biting
Periodic sustained stress
Destruction of dental prosthesis

76. COLOR AND COLOR PERCEPTION
 Color
1: a phenomenon of light or visual perception that enables one to
differentiate otherwise identical objects
2: the quality of an object or substance with respect to light reflected or
transmitted by it. Color is usually determined visually by measurement of
hue, saturation, and luminous reflectance of reflected light.
3: a visual response to light consisting of the three dimensions of hue,
value, and saturation (GPT 9)
 An important goal of dentistry is to restore the color and appearance
of natural dentition
 Among the important factors that influence esthetic appearance of
restorations are color, translucency, gloss and fluorescence

77. Each of these factors is influenced by:
•Illuminant(light source)
•Object
•Interpretation of the observer
It is interaction of light with the object that
allows perception of color

78.  Light: electromagnetic radiation that can be detected by
human eye.
 Eye is sensitive to wavelengths of 400nm(violet)-
700nm(red).
 The reflected light intensity and the combined intensities
of wavelengths present in incident and reflected light
determines the appearance properties i.e. hue, value
,and chroma

79. PERCEPTION
 Light from an object that is incident on the eye is
focused on the retina and is converted into nerve
impulses that are transmitted to the brain
 Rods perceive brightness of color i.e. the intensity
of light rays reaching the eye. (Value)
 Cones perceive hue i.e. the color
 Image on retina is focused, then energy in visual
spectrum is converted to electric potential by rods
and cones through chemical reaction

80.  Eye is most sensitive to light in green-yellow region (550
nm) and least sensitive at red or blue regions of color
spectrum

81.  Color fatigue: decrease in eyes response to color
because of constant stimulation by single color.
 Color blindness: defect in certain portion of color
sensing receptors .
 Thus humans as observers greatly differs in their
ability to distinguish colors.

82.  Color measurement devices are used to capture,
communicate, and evaluate color.
 There are basically two types of color measurement
instruments: colorimeters and spectrophotometers.
COLOR MEASUREMENTS

83. Colorimeters
 A colorimeter is a device that mimics the way humans
perceive color.
 Using an internal light source, a colorimeter shines light down
onto the surface of the sample.
 As the light reflects back up to the device, it passes through
three filters: red, green and blue. These filters distill tri-
stimulus (RGB) values that match how our eyes see color.

84. Spectrophotometers
 Spectrophotometers allow for more sophisticated color
measurements and can capture more data related to color.
 A spectrophotometer works almost same way, except for
one main difference – the filters.
 Instead of using three filters to determine the RGB values
of the color like a colorimeter, modern day
spectrophotometers typically have 31 filters to measure the
full color spectrum.
 These filters measure light in each of 31 different
wavelengths to determine the color of the sample.

86.  Advantages : colorimeter is more precise than
human eye in measuring slight differences in
colored objects
 Disadvantages: it is extremely inaccurate when
used on rough or curved surfaces
 Eye can differentiate colors seen side by side on
smooth or irregular surfaces, whether curved or flat

88. COLOR SYSTEMS:
Several different color systems are available with the most
popular being
•MUNSELL SYSTEM
•CIE (Commission Internationale de I’Eclairage) l*a*b*
International Commission on Illumination
Munsell system uses a three dimensional system with
hue, value and chroma as coordinates

89. Three dimensions of color
Hue  Dominant color of an object (e.g.) red, blue,
green etc.
Value  Value identifies the lightness or darkness of
object. Lighter shades have higher value whereas
darker shades have lower value.
Chroma  It represents the degree of saturation of a
particular hue. Higher the Chroma more intense the
color. Colors in center are dull.
Example – Red can vary from scarlet to pink. Scarlet is more intense,
having higher saturation and has higher chroma; Pink is dull and has
low saturation, so comparatively dull color.

91. It uses a three-
dimensional system
With HUE ,
VALUE and
CHROMA.

92.  Hue changes occur in circumferential direction
 Value varies vertically
 increases towards the top(whiter)
 decreases towards bottom(darker or more black).
 Chroma varies radially
 Increases from center outwards (centifugal direction)

93. CIE l*a*b* SYSTEM
CIE system uses color chart to define how much red,
blue, green or yellow a certain object appears to contain.
Color chart is based on the CIE L*a*b* color space in
which
L* for the lightness from black (0) to white (100)
a* from green (−) to red (+)
b* from blue (−) to yellow (+)
CIELAB is a chromatic value (refers to the intensity of
the opponent process responses) color space.

96. THE OPPONENT COLOR THEORY
 the cone photoreceptors are linked together to form three
opposing color pairs:
red versus green,
blue versus yellow,
black versus white
(the last type is achromatic and detects light-dark variation,
or luminance).
When people stare at a bright color for too long, for example, red,
and look away at a white field they will perceive a green color.
Explanation - Red became fatigued and over stimulated when you
focused on the red for long. Consequently, the opposing cones
kicked into action.

98. FUN FACT
 Yellow is the most visible color of all the colors, it is the
first color that the human eye notices.
 Yellow appears bright to us because it stimulates both
the long-wavelength and medium-wavelength cone cells
very well
 More light is reflected by bright colors, resulting in
excessive stimulation of the eyes. Therefore, yellow is
an eye irritant.
 Some claim that babies cry more in yellow rooms,
husbands and wives fight more in yellow kitchens

99. DENTAL SHADE GUIDES:
Shade guides are used in determining the color of a
natural teeth so that artificial restorations possess the
similar color and esthetics.

100. SHADE MATCHING IN DENTAL OPERATORY:
Shade match under lights of similar spectral distribution
and intensity, both in dental operatory and the laboratory
is important. Lighting conditions should be similar to day
light.

101. Anything on the patient that influences the shade matching
including brightly colored clothing should be draped
If patient is observed against intense-colored background, dentist
may select tooth shade with a hue that is shifted toward
complimentary color of background color
Teeth to be matched should be clean
Patient should be viewed at eye level; viewed at a distance of
approx 10 inches
Shade matching should be made quickly(<5sec)
Dentist should rest his eyes between viewing by looking on a
neutral grey surface immediately before a matching

102. Remove the individual shade tab from the guide and
hold it close to the tooth for shade matching.
The surface texture of the restoration should match
that of the remaining dentition as closely as possible

103. SQUINT TEST
 An important part of this procedure is to squint the eyes.
 Squinting causes the black and white sensitive rods in the eye to
become more active than the color sensitive cones.
 The rods are helpful in determining the VALUE.
 This test helps in selection of artificial teeth according to
complexion of face.
 Dentist compares the prospective colors of artificial teeth held
along the face.
 The color that FADES FIRST from the view is the color that is
least conspicuous in comparison with the color of face.

104. 3D MASTER SHADE GUIDE

105. •Select group 0, 1, 2, 3, 4 or 5
•Start selection with darkest group first

106.  Spread the samples out like a fan
 Select one of the three shade samples
Determining the Chroma

107. Determining the hue
•Check whether the natural tooth
comparedto the shade sample selected
is more reddish or more yellowish as

108. Determining intermediate values
An even more precise method of shade determination is
possible by specifying the lightness and chroma of intermediate
shades. Use the two neighboring shades to help convince
yourself that the correct tooth color is neither the one nor the
other shade.
 All shade samples of an M-group feature the same hue and
lightness. Only the chromavaries.

110. BEZOLD-BRUCKE EFFECT
 At low levels, rods of human eye are more dominant than
cones, and color perception is lost.
 As brightness becomes more intense, color appears to
change.
 Also, if one looks at a red object for a long time, receptor
fatigue of cone cells causes a green hue to appear when one
looks at a white background.
As explained earlier, under The Opponent Process theory.

111. CLINICAL LIGHTING CHALLENGES
LIGHTING CONFLICTS
 Light coming from window mixes with fluorescent light coming from
hallway & color-corrected lighting in dental operatory
 If the clinician has to access to natural light source, it is best to
perform shade matching at 10am or 2pm on a clear, bright day
when the ideal color temperature of 5,500K is present
 Color-corrected lighting tubes that burn at about 5,500K (D50
illuminants) should be installed
Lighting conflicts Metamerism

112. Metamerism
Objects that appear to color matched under one type of
light may appear different under another light source. This
is called Metamerism.
Different sources of light in dental operatory are day light,
incandescent, fluorescent lamps etc.

113. THERE ARE TWO TYPES OF METAMERISM:
OBJECT METAMERISM AND OBSERVER METAMERISM
 Object metamerism occurs when the two items appear the
same in one lighting condition, but appear differently when
the light source is changed. In dental terms, it occurs when
crown is matched to the natural dentition under
incandescent light, but, when viewed under color-corrected
or fluorescent light, appears not to match the natural teeth.
 To obtain an acceptable shade determination, it is advisable
for the viewer (technician, clinician and assistant) to observe
the color matching under three different lighting conditions –
daylight, color – corrected light and dim light.

114.  Observer metamerism occurs when the light source
remains the same and the observer changes, caused by
human visual stimulus.
 Recommended that a third observer (assistant,
technician, friend, or family member) evaluate the
selected color prior to cementation of any final restoration

115. FLUORESCENCE
 Natural tooth absorbs wavelength too short to be visible to
human eye.
 The decorative lights, photoflash lamps, natural sunlight etc.
are sources containing near UV (300-400nm) radiation.
 The energy tooth absorbs is converted into light with long
wavelengths, in which case the tooth actually becomes light
source. This phenomenon is k/a Fluorescence.
 The emitted light, a blue white color, is primarily in the 400 to
450 nm range.
 As an example, ceramic crowns or composite restorations
that lack a fluorescing agent appear as missing teeth when
viewed under a black light

116. THERMAL PROPERTIES
 Wide temperature fluctuations occur in the oral cavity
due to ingestion of hot or cold food and drink.
 Dental pulp is very sensitive to temperature change and
the healthy tooth is surrounded by dentin and enamel,
which are relatively good thermal insulators.

117. THERMOPHYSICAL PROPERTIES
 Conduction through metal crystal lattice vibrations
motion of electrons interaction with atoms.
 Thermal conductivity is a thermophysical measure of how
well heat is transferred through a material by conductive
flow
 Measured under steady state conditions (constant T)
 Thermal conductivity is proportional both to
area(perpendicular to heat flow ) and temperature
gradient across the structure

118.  Coefficient of thermal conductivity: is the quantity of heat in
calories per second, that passes through a specimen 1cm
thick having a cross sectional area of 1 cm sq. when the
temperature difference between the surfaces perpendicular
to heat flow is 1 K.
 High conductivity - conductors
 Low conductivity – insulators
 units of measurement of thermal conductivity–
watt/meter/sec/kelvin

119. COEFFICIENT OF THERMAL EXPANSION
 Change in length per unit length of a material when its
temperature is raised 1K

120.  Other applications
 inlay wax removed from tooth or die in warmer area
and stored in cooler area
 Denture teeth arranged in wax in warm area and
stored in cooler area causing shift in positions of
teeth
 stresses produced from metal ceramic restorations
when porcelain veneer is fired to a metal substrate

121. CRACK FORMATION AT METAL CERAMIC
RESTORATION

122. CLINICAL SIGNIFICANCE
Metallic filling materials
Metallic denture bases

123.  Thus, good conductors have high values of conductivity.
 More heat is conducted through metals and alloys than
through polymer such as acrylic resin.
 High value of conductivity for dental amalgam indicates
that this material could not provide satisfactory insulation
of the pulp.
 Therefore, cavity base of a cement is used such as zinc
phosphate which has a lower thermal conductivity value.

124. TARNISH AND CORROSION
 Tarnish: the process by which a metal surface is dulled or
discolored when a reaction with a sulfide ,oxide, chloride,
or other chemical causes a thin film to form
(or)
 Tarnish is observable as a surface discoloration on a
metal , or as a slight loss or alteration of the surface finish
or luster.
 ‘Tarnish is often a forerunner of corrosion’

125. TARNISH
 Often occurs from formation of hard and soft
deposits on the surface of restoration.
 Soft deposits - plaque, films of bacteria,mucin,
stains from pigment producing bacteria, drugs
containing iron or mercury, adsorbed food debris
 hard deposits - calculus.
 Also from formation of thin films such as oxides,
sulfides, chlorides which is an early indication of
corrosion

126. CORROSION
 Corrosion: chemical or electrochemical process in which a
solid usually a metal, is attacked by an environmental agent,
resulting in partial or complete dissolution.
 (or)
 It is a process in which deterioration of a metal is caused by
reaction with its environment and is not merely a surface
deposit.
 Corrode 1: deterioration of a metal due to an electrochemical
reaction within its environment
 2: to eat away by degrees as if by gnawing
 3: to wear away gradually usually by chemical action
 Corrosion : the action, process, or effect of corroding; a
product of corroding; the loss of elemental constituents to the
adjacent environment (GPT 9)

127.  Disintegration of a metal by corrosion may occur in
mouth because of
 warmness and moistness
 fluctuations in temperature
 ingested foods with wide range of pH
 acids liberated from localized attachment of debris

128.  A tarnish film may in time accumulate elements and
compounds that chemically attack metal.
 For e.g. egg and certain foods containing sulfur. Sulfides
of such as hydrogen, or ammonium corrode Ag,Cu,Hg
and similar metals present in dental alloys and amalgam.
 Specific ions play role in corrosion of certain alloys .
 For e.g.; Oxygen and chloride in corrosion of amalgam

129. CLASSIFICATION OF CORROSION
Broadly classified into
1.chemical /dry corrosion
2.electrochemical/wet corrosion
 galvanic corrosion
 stress corrosion
 concentration cell corrosion

130. ELECTROCHEMICAL CORROSION
 Requires presence of water/fluid electrolyte and
also a pathway for transport of electrons.
 Electrochemical cell: composed of 3 components
 1. anode.(e.g. dental amalgam)
 2. cathode.(e.g. Gold alloy restoration)
 3. electrolyte.(e.g. saliva)

132.  Anode is a surface or site where positive ions are formed
(surface undergoing oxidation and corroding) with the
production of free electrons
 At cathode, reduction reaction occurs that consumes free
electrons produced at anode

133.  Electrolyte supplies the ions needed at cathode ,and
carries away the corrosion products at the anode.
 External circuit – path to carry electrons from anode to
cathode.
 For corrosion to be an ongoing process, - production of
electrons at anode must be balanced by consumption in
reduction reaction at cathode.
 Cathodic reaction is primary driving force for electro-
chemical corrosion & an important consideration in
determining rate of corrosion

134.  If two metals are immersed in an electrolyte, connected by
an electrical conductor,
 metal with lower electrode potential - anode
 metal with higher electrode potential – cathode
 for e.g. electrochemical cell with Cu and Zn electrodes in
aqueous acidic solution Zn becomes anode and undergoes
surface dissolution.
 Magnitude of resulting corrosion influenced by salivary,
 concentration of its components
 pH
 surface tension
 buffering capacity

135. Usually, the dissolved ions from dental restorations removed
by food, fluids and brushing, there by corrosion continues

136. GALVANIC CORROSION OR ELECTROGALVANISM
 An important type of electrochemical corrosion occurring
when combinations of dissimilar metals are in direct
physical contact

137. GALVANIC SHOCK
Amalgam restoration on lower tooth opposing upper tooth with gold alloy
Electric circuit (because of saliva) with potential difference between
metals
When teeth brought into contact
Short circuit through alloys
Sharp pain

138.  When teeth are not in contact - still electric circuit is
present due to potential difference between metals.
 Saliva forms electrolyte and hard and soft tissues forms
external circuit.
 Electric current generated between gold and amalgam
restorations, when they are not in contact
 0.5 – 1 μA with potential difference -500mV.
 Coating with varnish tends to eliminate galvanic shock

139.  Single metallic restoration: current also associated with single
isolated metallic restoration.
 Electric cell is generated as a result of potential difference
created by two electrolytes - - saliva and tissue fluid.
(tissue fluid : used to denote , dentinal fluid, soft tissue fluid,
blood that provides a means of external circuit.)
 Because chloride concentration several times higher than
saliva, - interior surface of restoration exposed to dentinal fluid
will have a more active electric potential

141. HETEROGENEOUS SURFACE
COMPOSITION
 Associated with heterogeneous composition of dental alloys.
 Corrosion resistance of multiphase alloys is generally less than
that of single phase solid solution.
 e.g.: when alloy containing a two phase microstructural
constituents immersed in electrolyte, the lamellae of phase with
lower electrode potential are attacked and corrosion lower
potential are results
 Solder joints - corrode because of difference in composition of
alloy and solder.
 Impurities enhance corrosion

142. STRESS CORROSION
 Imposition of stresses increases internal energy of an alloy
through elastic displacement of atoms or creation of micro
strained fields associated with dislocation then the tendency to
undergo corrosion increases called stress corrosion.
 Likely to occur during fatigue or cyclic loading.
 Any cold working of alloy causes localized permanent
deformation. Electro-chemical cell forms with more deformed
metal regions (anode), less deformed metal regions(cathode),
and saliva.
 So excessive burnishing of margins of metallic restorations in
contraindicated

143. CONCENTRATION CELL CORROSION
 Occurs whenever there are variations in the electrolytes or in
the composition of the given electrolyte with in system.
e.g. : difference in electrolyte composition contacting restoration
on occlusal and proximal surfaces.
 Similar type of corrosion occurs due to difference in oxygen
concentration between parts of same restoration.
e.g. : pits in restorations

144.  Deepest portion of pit
 low o2 concentration because of debris - anode.
 Alloy surface around rim of the pit - cathode.
 Crevice corrosion: corrosion at the junction of tooth and
restoration because of presence of food debris causing
changes in o2 concentration and change in electrolyte

146. PROTECTION AGAINST CORROSION
 Metallic and non metallic coatings over gold alloy
restorations are ineffective because
 were too thin
 were incomplete
 did not adhere to metal
 were readily scratched
 were attacked by oral fluids
 When dissimilar metals in contact, a non-
conductive film can be painted

147.  Certain metals develop a thin ,adherent, highly
protective film by reaction with environment, such a
metal is said to be passive
 A thin surface oxide forms on chromium
 Stainless steel contain sufficient amounts of chromium
added to iron to passivate the alloy
 Titanium also forms passivating titanium oxide film
 Chromium passivated metals are susceptible to
stress corrosion and pitting corrosion

148. CORROSION OF DENTAL RESTORATIONS
 Corrosion resistance is very important consideration in
dental alloys because release of corrosion products
affect biocompatibility.
 A guideline that has been employed by manufacturers
for many dental alloys is at least 50% atoms should be
noble metals.
 Palladium – prevents sulfide tarnishing of silver alloys.
 Base metal alloys are immune to sulfide tarnishing but
susceptible to tarnish with chloride

149. CLINICAL SIGNIFICANCE OF GALVANIC
CURRENTS
 Many base materials below restorations lose the property of
insulation when they becomes wet through micro leakage or
dentinal fluid
 Practical method of eliminating galvanic currents - application
of varnish on surface of metallic restoration
 It has been suggested that Galvanic currents may account for
many types of dyscrasias such as lichenoid reactions, ulcers,
leukoplakia, cancer, and kidney disorders but research has
failed to find correlation

150. CONCLUSION
Several factors must be taken into account when considering
which properties are relevant to the successful performance of
a material.
It is very important to know the properties of dental materials
so that before performing any clinical procedure we choose a
material which is best suited.
No dental operatory is free from problems but a thorough
understanding of potential factors affecting them allows dental
professional to compensate for them