2. ACKNOWLEDGMEN
T
We would like to express our special thanks of gratitude to
our teacher Dr. Gaurav Gupta, Dr. Sanjeev Joshi and Dr.
Parveen who gave us the golden opportunity to do this
wonderful project on the topic “Physical properties of
Dental M aterial”, which also helped us in doing a lot of
research and we came to know about so many new things
we am really thankful to them.
Without their guidance it would not have been possible.
3. CONTENT
S 1. Introduction
2. Physical properties of dental materials
a.Rheology
b. Viscosity
c. Creep and flow
d. Colour and optical effects
-Nature of light and role of human vision
- Nature of object under view
e. Three dimension of colour
-Hue
-Value
- Chroma
f. Measurement of colour
g. Colour matching
h. The effect of observer
i. Flourescence
j. Radiopacity
3. Thermal properties
a. Thermal conductivity
b. Thermal diffusivity
c. Co-efficient of thermal expansion
4. 4. Electrochemical properties
a. Tarnish and corrosion
b. Electromechanical mechanism of corrosion
c. Disimilar metals
d. Stress corrosion
e. Concentration cell corrosion
f. Protection against corrosion
5. Recent Advances
6. Conclusion
7. Acknowledgement
8. Reference
5. INTRODUCTIO
N
The principle goal of dentistry is to maintain to improve the quality of life
of dental patient. This requires the replacement of alteration of existing
tooth structure; the main challenges for centuries have been the selection
of good prosthetic materials that can withstand the adverse condition of
oral environment.
Physical properties are the measures of a material. These properties have
great significance in dental research because they provide the info needed
to assess the characteristic and improvement in material under
development.
Physical properties of a tooth, set the standard for materials attached to
a tooth. Theory suggest that if a restorative materials can be made to
hold property similar to those of natural tooth structure, it should
perform as well as original tooth.
6. RHEOLOG
Y
Definition :- It is the study of the deformation and flow characteristics
of matter, whether liquid or solid.
In dentistry study of rheology is necessary because many dental
materials are liquids at some stage of their use e.g-
Molten alloy
Freshly mixed impression materials and cements.
Gypsum products use in the fabrication of models and dies are
transformed extra orally from fluid slurries into solid .
Amorphous materials such as waxes and resin appear solid but actually
super cooled liquids that can flow plastically (irreversibly) under sustain
loading or deform elastically (reversibly) under small stresses.
7. Definition :- It is defined as the resistance of a
liquid to flow.
Most liquids, when placed in motion resist
imposed forces that cause them to move this
resistance to fluid flow (viscosity) is controlled by
internal frictional forces within the liquid .Thus
viscosity is a measure of the consistency of a
fluid and its resistance to flow.
VISCOSITY
:
8. When a spatula is moved through a pasty fluid such as dental
cement to blend two components on mixing pad.
The mixing surface is fixed and the upper surface (spatula
blade) moves to right at a given velocity (v) a force (F) is
required to overcome the frictional resistance with in the fluid
(viscosity) and cause the fluid to flow .
If the two surface have an area (A) in contact within the liquid,
a shear stress () can be defined as t= F/A.
The shear strain or rate of change of deformation is
E=v/2, where ‘d’ is the shear distance of upper surface relative
to fixed lower surface .
9. 1) Newtonian:- Viscosity is constant
2) Pseudoplastic:- Vicosity decreases with increasing strain until it reaches
constant value.
3) Dilatant : - it is opposite to pseudoplastic
4) Plastic :- initially rigid and than attains constant viscosity.
5) Bingham flow :- above certain stress, which behaves as newtonian fluid
The viscosity of many dental materials decreases with increasing strain rate
until it reaches a nearly constant value, thus faster they are stirred ,
forces through syringe or squeezed less viscous and more fluid they become
( pseudoplastic)
The viscosity of most fluids decreases rapidly with increasing temperature, it
becomes less viscous and more flowable upon repeated applications of
pressure known as thixotropic e.g. Plaster of Paris , resin cements and
impression material.
Viscosity is measured in units of megapascals (mpa) per second per centipose
(cp), flow e.g.:-
Pure water at 20 degree celsius :- 1.0 cp
Agar:-28100 cp
Polysuphide: - 109000 cp
Heavy body sulphide :- 1360000 cp
10.
11.
12. CREEP:-
Definition:-it is defined as the time dependent plastic strain of a material under a
static load or constant stress
The related phenomenon of “sag” occurs in the permanent deformation of long
span metal bridge structures at porcelain firing temperature under the influence of
the mass of prosthesis.
Metal creep usually occurs as the temperature increases to within a few hundred
degrees of melting range.
Metals used in dentistry for cast restoration or substrates the porcelain veneers
have melting ranges(for alloys) that are much higher that mouth temperatures and
they are not susceptible to creep deformation intraorally.
Dental amalgam contains from 42% to 50% of mercury by weight and begin melting
at temperatures only slightly above room temperature. Because of its low melting
point, dental amalgam can undergo creep at a restored tooth side under provide
sustained stress. Creep provides continuing plastic deformation , process can over
time be very destructive to dental amalgam filling.
CREEP AND
FLOW
13. Creep may cause unacceptable deformation of a dental
restoration.
It may also lead to unacceptable fit of a FPD, when a
cast alloy with porcelain at high temperature.
CLINICAL
SIGNIFICANCE:-
17. Definition:- the term flow has been used in dentistry to describe
the rheology of amorphous materials such as waxes
The flow of wax is a measure of its potential to deform under a
small static load even that associated with its own mass .
Although creep or flow maybe measured under any types of
stress, compression is usually employed in the testing of dental
materials.
A cylinder of prescribed dimension is subjected to a given
compressive stress for a specified time and temperature the
creep or flow is measured as the percentage decrease in length
that occurs under these testing conditions.
FLOW:-
18. An important goal of dentistry is to restore or improve extractive –
color and appearance of natural dentition. Esthetic consideration
in restorative and prosthetic dentistry have received increasingly
greater emphasis grown even greater in past few years following
widespread use of bleaching and whitening technologies.
Since esthetic dentistry impose severe demands on artistic
abilities of dentist and technician knowledge of underlying
specific principles of colour and other optical effects is essential.
COLOR AND OPTICAL
EFFECTS
19.
20.
21.
22. COLOUR AND OPTICAL
EFFECTS
An important goal of dentistry is to restore or improve esthetics-
colour and apperance of natural dentition. Esthetic
consideration in restorative and prosthetic dentistry have
receined increasingly greater emphasis in recent decades and
the challenges have grown greater in past few years following
widespread use of bleaching and whitening technologies.
Since esthetic dentistry imposes severe demands on artistic
abilities of dentist and technician, knowledge of underlying the
scientific peinciples of colour and other optical effects is
essential.
23. Light is electromagnetic radiation that can be detected by human eye. The eye
is sensitive to wavelengths from approx 400nm (violet) to 700nm (dark red).
The combined intensities of wavelengths present in beam of light determine the
property usually called color. For an object to be visible , it must be reflect or
transmit light incident on it from external source which is usually polychromatic ,
i.e.. mixture of various wavelengths known as “white” light
light from an object that is incident on the eye is formed in retina and is
converted nerve impulses which are transmitted into brain . Cone shaped cells
in retina are responsible for color vision. These cells have threshold
intensity required for color vision and respond to wavelength for both normal
color vision and deficient vision.
NATURE OF LIGHT AND THE ROLE OF HUMAN
VISION:-
24.
25.
26. Esthetics is critically important in dealing with dental
restorative materials electromagnetic radiation in visible
region interacts with objects through reflection from its
surfaces. Absorption, refraction or transmission. This
phenomenon determine the opacity, translucency or
transparency of an objects.
Enamel consists of hydroxyapatite crystals when light
strikes, some is reflected , some is absorbed , some
refracted or some transmitted. Enamel has a refractive index
of 1.65 and is translucent.
NATURE OF OBJECT UNDER VIEW:-
27. Verbal description of color are not precise
enough to describe the appearance of teeth. To
overcome this problem, color perception is
derived by three objectives variables:- hue, value,
and chroma.
These three parameters constitute the three
dimensions of colour space.
THREE DIMENTION OF COLOR:-
28. HUE:-
The dominant color of an objects e.g. Red,
green, or blue. This refers to dominant
wavelength present in the special distribution.
29.
30. It is also known as grey scale. It is the vertical or
z – axis . value increases towards higher end (
higher) and decreases toward low end ( darker) .
Tooth or dental crown identifies lightness or
darkness of color by hue.
VALUE:-
31. It is the degree of saturation of a particular hue. For e.g.
Red can vary from “scarlet” to “ light pink” where scarlet
has high saturation and pink has low saturation.
Crown is not considered separately in dentistry, it is
always associated with hue and value of dental tissue,
restoration and prosthesis.
CHROMA :-
32.
33. One of the most commonly used method to define
and measure colour quantitatively is the “munsell
system”
It is a co ordinate system which can be viewed as a
cylinder. The lines are arranged sequentially around
the perimeter of cylinder while chroma increases
along a radius from the axis. The values co ordinate
varies along length of cylinder from back at the
bottom to neutral grey at the center to white at top.
MEASUREMENT OF COLOR:
35. In dental practice color matching is most often performed with use of
shade guide to select color of ceramic ,veneers ,inlays or crowns. Shape
guide tabs are used in much same way as paint chips to match color of
house paint.
These individual shade tabs are grouped according to hue followed by
value and chroma. It is usually insufficient to simple select a shade from
guide shade and transfer the information to lab. To ensure esthetic
results additional information such as drawings, description and
photographs should also be sent.
Preference of patient must be taken in account patients usually prefer a
lighter shade in restoration as compared to color of natural teeth.
COLOR
MATCHING:-
38. Signals of color are sent to humans from three sets of receptors in
retina called cones which are sensitive to red, blue ,and green.
Factors that interfere with true perception of color include low or
high level, fatigue of color receptors, sex , age, memory and
internal background.
At low level, rod in retina are dominant and color perception is
lost.
As it brightens it becomes more intense and color appear to
change.
e.g. observer looks at red object for long time than she sees green
on white background.
EFFECT OF THE
OBSERVER:-
39.
40. The spectral distribution of light reflected from or transmitted
through an object is dependent on spectral content of incident
light. The appearance of an object is dependent on nature of light
in which the object is viewed.
Day light, incandescent and fluorescent lamps are common
sources of light in dental operatory.
Objects that appear to be color matched under one type of light
may appear different under another type. This phenomenon is
known as “metamerism”.
If possible, color matching should be done under 2 or more
different light sources, one of which should be daylight and
laboratory shade matching procedure should be performed under
same light conditions.
THE EFFECT OF LIGHT
SOURCE :
41.
42. Natural tooth structure absorbs light at wavelengths too short to
be visible to human eye. Between 300 and 4000nm referred as
near ultraviolet radiation e.g. natural sunlight photoflash lamps.
The energy that tooth absorbs is converted into light with longer
wavelengths in which tooth becomes a light source known as
“fluorescence “
Flourescence makes a definite contribution to brightness and
initial appearance of a human tooth.
e.g. ceramic crowns or composite restoration that lack a
fluorescing agent appear as missing with when viewed under a
black light.
FLOURESCENC
E:
43.
44.
45.
46. X-rays a form of electromagnetic radiation lie at the high energy end of the
spectrum
Sufficient radiographic contrast is required in an x- ray image in order to assess
restoration for marginal defects or break down, help differentiate composite
restoration from dental caries and detect micro leakage.
A significant number of foreign bodies of dental origin with many injuries and
deaths are reported each year.
When radiopacity is too low, resin will not be visible on an x- ray when it is too high
it may black out and obscure details of adjacent anatomy.
Dentin has same fragment radio density as that of aluminum while enamel has
double of it.
Denture resin fragment of 2mm should have a radiopacity equal or greater than that
of 0.2mm of copper. This requires the resin to have sufficient additives to produce
a radiopacity about three times that of aluminum.
RADIOPACIT
Y
47.
48. When restorative materials are placed in deep cavities, heat transmitted to vital dental
pulp must be limited so as to prevent thermal shock and trauma
While with denture bases its contact with mucosal surface is desirable to convey
sensation of heat and cold associated with food and beverages. Such attributes are
governed by properties of thermal conductivity and thermal diffusivity.
THERMAL
PROPRTIES:
THERMAL
CONDUCTIVITY:
Definition:- it is defined as the quantity of heat in calories per second passing through a
material 1 cm thick with a cross section of 1 cm square having temperature difference of
1k (1 degree Celsius) and is measure under steady state conditions in which
temperature gradient does not change.
The international system(si) unit is watts per meter per Kelvin (w*m-1*K-1).
Materials that have high thermal conductivity are called conductors whereas low thermal
conductivity are called insulator .
49.
50. It is the measure of speed with which a temperature
change will spread through an object when one surface
is heated. It is calculated from thermal conductivity
divided by product of density and heat capacity.
THERMAL
DIFFUSIVITY:
51.
52. Gold has about one length the heat capacity of dentin about 500 times the
thermal conductivity and about 600 times thermal diffusivity
Thus, a pure gold filing would provide tooth pulp with very little protection
against thermal shock as compared to natural dentin.
When the product of heat capacity and density is high, thermal diffusivity may
be low even though thermal conductivity is relatively high. Therefore both
parameter are important in predicting transfer of thermal energy.
Enamel and dentin are effective are effective thermal insulators.
Thermal conductivity or thermal diffusibility of cements- GIC , zinc, phosphate
composites are higher for metallic restorations.
53. Definition:- It is defined as the change in length per unit of original length of a material
when its temperature is raised 1 degree Celsius
Alpha=delta L/ L into delta T
L= original length
Delta L= change in length
Delta T= change in temperature
The unit is mm/mk or ppm/k.
It dictates procedure for using wax pattern, casting metal crown. Placing amalgam and
composite restoration and preparing metal ceramic crown and bridges.
The high thermal expansion co- efficient of inlay wax is important as it is susceptible to
temperatures changes
e.g. an accurate wax pattern that fits a prepared tooth contracts significantly when it is
removed from tooth or die in warmer areas and then stored in cooler area. This
dimensional change is transformed to a cast metal restoration which is made from lost
wax process.
COEFFICIENT OF THERMAL EXPANSION
54.
55.
56. Dentistry uses a wide variety of metals for restorations
and prosthetic device. The successful clinical
performance and long term durability requires adequate
corrosion resistance in oral environment .
Thus, an understanding of corrosion and
electrochemical properties of corrosion and
electrochemical properties is essential in order to control
electrochemical effects and minimizing corrosion.
ELECTROCHEMICAL
PROPERTIES:
57. TARNISH AND
CORROSION
TARNISH is surface discoloration on a metal or a
Slight loss or alteration of surface finish or luster.
In oral environment tarnish occurs due to:
1. Formation of hard substance- calculus and soft substance-plaque
2. Formation of thin films such as oxides, sulfides or chlorides
CORROSION is the deterioration of a metal caused by reaction with
environment. It can cause severe and catastrophic disintegration of
metals. Tarnish is forerunner to corrosion.
58.
59. Corrosion of a metal is either:
1. Chemical
2. Electrochemical
Chemical corrosion- direct combination of metallic and non-
metallic elements to yield chemical compound through
oxidation reactions. It is also known as dry corrosion.
Examples:
1. Discoloration of silver by sulphur
2. Corrosion product of dental gold alloys that contain silver.
3. Oxidation of silver-copper alloy particles that are mixed
with mercury to prepare certain dental amalgam products.
60.
61. Electrochemical corrosion also known as galvanic corrosion requires
presence of water or electrolytes and a pathway for transport of electrons. It is
also referred as wet corrosion.
ELECTROCHEMICAL MECHANISM OF CORROSION
When a metal is in contact with fluid electrolyte, chemical potential causes
enough ions to dissolve to form a saturated solution and produce an equal
number of free electrons.
The Anode is the surface where positive ions are formed production of free
electrons.
M M+ + E-
The cathode consumes free electrons produced anode.
Electrolytes supplies ions needed at cathode and caries away corrosion
products at anode. External circuit serves as a pathway. The difference in the
electrical potential is known as electromotive force (EMF)
62. DISIMILAR
METALS
It is a type of electrochemical reaction occurs when combinations
of dissimilar metals are in direct physical contact.
The alloy combination that may produce galvanic corrosion or
electro galvanism through the flow of galvanic currents maybe in
either continuous or intermittent contact
Example: dental amalgam restoration placed on occlusal surface
of tooth directly opposing a gold inlay
This results in sharp pain known as galvanic shock which is well
known by dentist.
63.
64. STRESS
CORROSIONThe imposition of stress increases to internal energy of an alloy either through elastic
displacement of atoms or creation of micro strain fields associated with dislocation. The
tendency to undergo corrosion will be increased called stress corrosion.
It is most likely to occur during fatigue or cyclic loading in oral environment. Thus any
cold working of an alloy by bending, burnishing or mallaeting causes localized
permanent deformation. This is why excessive burnishing of margins of metallic
restoration is contraindicated
65.
66. CONCENTRATION CELL
CORROSION
An important type of electrochemical corrosion is called concentration cell corrosion,
which occurs whenever there is variation of electrolytes or in composition of given
electrolytes
Example:
1. Accumulation of food debris in interproximal areas between the teeth particularly
if oral hygiene is poor.
2. Difference in oxygen concentration between parts of same restoration with
greatest attack at areas containing least oxygen. Oxygen deprived becomes and as it
is covered by food debris while surface around rim of a scratch or pit becomes
cathode.
3. Irregularities likes pits, scratches and reacts are an excellent example.
Thus to protect such pitting corrosion all metallic restoration must be polished
67.
68. PROTECTION AGAINST
CORROSION
1. A highly effective protection utilizes certain metals that develop thin, adherent
highly protective film by reaction with environment such metal is said to be
passive
A thin surface oxide forms on chromium is a good examples
Example: stainless Steel which contains sufficient amount of chromium added
to iron and its alloy to passviate alloy.
2. Titanium and its alloy are widely used because of its favorable combination of
physical chemical and biological properties as well as their resistance to
corrosion,
3. Noble metal resist corrosion because their electromotive force is positive with
regard to any of the common reduction reaction formed in oral environment, it is
important that is more active constituents of alloy be uniformly dispersed in
random solid solutions.
69. RECENT ADVANCEMENT
RHEOMETER- The new ARES-G2 is the most
distinctive rotational rheometer for advance
research. It represents the culmination of five
years of ground up intensity effort, the ARES-G2
remains the only commercial available
rheometer to offer unique separate motor and
transducer technology for the rheological
measurements.
70. ADVANCES IN COLOUR
MATCHING
SHADE GUIDES- the first shade guide
introduced in market by vita zahnfabrik
for measurement of color for ceramic
system. In late 1990s one of the first
clinical result was development of vita
3D master shade guide.
71. SHADE TAKING DEVICES-In order to reduce color
mismatch , two main categories of electronic
instruments were developed for dental color analysis.
1-colorimeters-it is simple and designed to measure
color on basis of three axes by using a filter that
stimulates the human eye.
2- spectrophotometer- it includes the ability to
convert spectrophotometric measures to various
color measures to various color measures.
72. CONCLUSION:
It is very important to know the properties of materials we use in
dentistry, especially in restorative materials. This will enable us to
select a material that will have properties close to that of natural
tooth surface.
Also we will be able to better understand and select materials
from wide range that coming into market.
