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13/08/2017
Dental Material lecture
In the previous lecture:
Three Important Elements we obliged to know about DMs :
1) Biology
2) Chemistry
3) Physics
! A lot of factors affect the DMs and may produce an undesirable results. The most
important part of the oral cavity that would be bile for the DMs is the saliva.
Saliva Contents that affect the DMs :
1) glycoproteins and ions content that give arise to the corrosive nature of the
saliva (high pH)
2) it's microorganisms may lead to microleakage.
3) it's water content (most important because the properties of a metal when it's wet
is totally different when it's dry
! Water effect on the iron (steel)
In wet condition the steal is being oxidized forming corrosion. For this reason iron is
not a reliable material to use in restoration.
! Most of DMs used are noble alloys (= solid mixture of two or more metals)
The DMs are Divided into Three Categories (can be one of them or combination) :
1) Metals
2) Polymers
3) Ceramics
The Aims of this Lecture are:
1) Define some terms which are essential in understanding the properties of the
metals later.
2) Make a comparison between the bulk properties and surface properties. e.g. the
surface corrosion of the iron. In contrast, the glass fracture as a whole unit.
3) Stress-strain curve of DMs
The Occlusal Forces
The tooth is exposed to high occlusal forces. The hardest tissue in the body is the
enamel but under such a huge forces, it may become affected by attrasion (result
mainly from occlusal forces), erosion (result from occlusal forces combined with
acidic dietary or vomiting ), and abrasion. As a consequence, half of the tooth may
lost or it would be shorten almost to the gingival margin (= 10mm of the tooth
structure is lost).
Understanding the interaction result from the occlusal forces and DMs is a must to
avoid the failure in restoration process. Taking that into consideration, then we can
decide which material is better to be used.
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e.g. the plastic filling would be appropriate and aesthetic if the dental cavity is small
or medium in size, but if the filling covers the whole occlusal surface, it will fail to
function because it does not go along with the occlusal forces, so ceramics or crowns
are the solutions.
! Average Occlusal forces in the anterior region = 150 N and on the posterior teeth =
500 N
e.g. porcelain jacket crown –used in the early days- were put in the anterior region
and in the posterior region, as a consequence of the occlusal forces; these crowns
perform well anteriorly and become fractured posteriorly.
The solution for this dilemma is to increase the strength of this material via adding
alumina particles (increase the stress of the material to more than 500 N ).
Occlusal forces vary between individuals, depends on the physical and physiological
condition, for example : slim person with weak masticatory muscles has lesser
occlusal forces than athlete persons with strong muscles.
The occlusal forces may reach 3500 N and all of this forces are exerted on a small area
= 0.1 mm^2 so the pressure = force I area = 3.5 * 10^10 Pascal
Occlusal forces in edentulous patient is 15% of what non edentulous patient has.
If we want to know the effect of the occlusal forces on DMs, we have to understand
the various properties that materials show.
Physical or Bulk Properties
We assume that the material we deal with is a "beam" ( to bear the occlusal forces
applied ). Very important terms to know:
1) Stress
Force per unit area. It's an indication of the internal resistance to the applied external
forces. e.g. any weak hit to the woody table won't affect or fracture it, because it's
designed to resist any force that would deform or change it.
It's unit
Force I area = N I m^2 = Pascal (pressure unit)
Stress Forms:
a- axial (the forces applied are with the long axis of the beam)
1) compressive (the applied forces are in the same plane and at the same direction)
2) tensile (the applied forces are in the same plane opposite to each other, focus on
moving particles (grains) from one site to another when compared to the shearing
stress )
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Dental Material lecture
! The compressive stress is much important at the atomic level because in
compression we are shortening the distance between the electronic clouds between
adjacent atoms.
b- nonaxial (the forces applied are not in the same level)
1) shearing (line on the beam on which we affect the forces may be defected due to
e.g. presence of spaces and this would decrease the density and fracture in the
beam after all)
2) torsion
3) bending (approximate the 2 endings of the beam)
2) Strain
Deformation on the original level in the beam. This occur before the applied forces
reach a limit at which the beam is fractured and it can be:
1) elastic (transient deformation)
2) plastic (permanent deformation)
It's unitless; measured by dividing the change in the beam length of the on it's
original length.
Stress-strain curve
Can be :
1) engineering stress-strain curve
2) true stress-strain curve
Engineering Stress-strain Curve
Different forces are applied on the beam of the desired material, then sensors
attached to it to measure the minute changes in it's length. Thus, the strains plotted
on the X axis and the stress on the Y axis. Different materials have different curves.
Notes about the curve:
The curve shows the relation between the stress and strain.
Strain is proportional to the stress before we reach the A point, whenever there is a
stress there is a strain, if we remove the stress the metal will regain it's original
structure or shape.
At A
Elastic deformation of the beam, it can call back it's original length and shape
whenever we remove the stress.
At B ( Functional Failure)
The beginning of the plastic deformation (minimum value). The beam gains it's
plastic deformation in the form of length increment.
At C
Maximum plastic deformation occurs, the beam is not fractured yet.
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At D
Beam Fracture occurs.
The forces applied during B-C: the metal reach the fatigue limit and has an
unexpected behavior.
Fatigue
property of a metal to tire and to fracture after repeated stressing at loads below its
proportional limit.
Areas under the curve are : Resilience and toughness
Elastic limit and Proportional limit
e.g. tensile stress applied to a beam, whereat if we applied 10 N that will increase the
length 1 mm, 20 N the increment in length is 2 mm, 30 N the increment in length is 3
mm … 100 N the increment equal to 10 mm ,110 N the increment equals to 12 mm
Conclusions :
The increment in length until we reach 100 N is fixed 1 per 10 N ( proportional
limit).
At 110 N the increment does not equal the previous fixed values but still if we
remove the stress it will regain it's origin state ( elastic limit ).
! The elastic limit is higher than the proportional limit, but in dental material we
consider them as the same unless otherwise they are specified.
Elastic modulus or the Slope in the curve
Stress I strain
Increase the slope = increase in the stress (stiff material)
Decrease in the slope = decrease in the stress (brittle material)
It's unit is Pascal = stress (Pascal) I strain (unitless)
! Enamel is a stiff material designed that way to goes along with the occlusal forces,
so the dentin must be resilient to work as shock absorber to support the enamel.
enamel has elastic modulus 5-10 times of the dentin (= enamel is stiffer than the
dentin).
!! The amalgamation depends on using a material with elastic modulus between the
enamel and dentin.
Poisson ratio
When a tensile stress is applied on a beam on one dimension, the other dimensions
decrease (= stresses will form inside the beam at the other levels that were not
directly affected by the forces).
Done By : Rawan Hamdan