Oral Environment and Patient
Considerations
Chapter 2
Oral Environment and Patient
Considerations
 To understand why we manipulate materials the
way we do, we first must understand the
challenges of the oral environment.
 Materials to be used must be biocompatible,
durable, nonreactive in acid or alkaline conditions,
compatible with other materials, and esthetically
acceptable.
 Limitations regarding what can and cannot be
used safely will vary from patient to patient.
 Materials must be compatible in an environment
of moisture and differing stresses, temperatures,
and acid levels.
Biocompatibility
 Materials must be biocompatible.
 This means that they must NOT adversely
affect living tissue.
 Adverse responses may include postoperative
sensitivity, toxicity, and hypersensitivity.
 Short-term and long-term functionality and
biocompatibility must be considered when
new dental materials are developed.
 Adverse responses may be caused by the
material itself or by the breakdown of its
components in the oral environment
Biomechanics
 Biomechanics applies the principles of
engineering and biological systems to the
design of new dental materials.
 Application of a material will depend on the
properties of the material and how they
relate to the needs of the patient, along with
the oral environment.
 Excessive wear of a material may be due to
variations in forces applied by stronger to
weaker antagonists.
Force
 When materials are subjected to the forces of
mastication, they must withstand varying degrees
of force and load.
 Normal biting force varies among individuals and
from one area of the mouth to another.
 Biting force is largely a measurement of the
strength of the muscles of mastication during the
normal chewing of foods.
 Normal masticatory force on the occlusal surfaces
of molar teeth averages 90 to 200 pounds per
square inch (psi) and can increase by as much as
28,000 pounds psi on a cusp tip while decreasing
in the incisor area.
Types of Force
 Three basic types of force have been identified:
1.
2.
3.
Compressive force: the force applied to compress
an object. Posterior teeth are able to deal with this
type of force because of their broad occlusal
surfaces and multiple roots.
Tensile force: the force applied in the opposite
direction as an object is pulled or stretched.
Shearing force: the force applied when two
surfaces slide against one another. The function of
incisors is an ideal example of the cutting action
associated with shear.
3 Basic Force Types
Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Stress and Strain
 When force is exerted on a tooth, the tooth or
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material creates resistance to counteract the
force.
The internal force that resists the applied
force is called stress.
If the stress within an object cannot resist
the force, distortion or deformation occurs.
The distortion or deformation produced by
stress is referred to as strain.
Stress is the amount of force exerted from
within an object, and strain is the amount of
change that the force has produced
Stress and Strain (cont’d)
 When we chew during mastication, many types of
stress are involved and form complex stress
combinations.
 One such combination, tension and compression,
is known as flexural stress, which is seen as
bending.
 An example of flexural stress in restorative
dentistry involves dental bridges, when tension is
placed on the occlusal surface, bending the bridge
toward the tissue. The tissue side of the bridge
then stretches upward in response.
Stress and Strain (cont’d)
 Dental materials are structured for multiple
purposes, but not every material is suited to all
types of stress.
 If force is exerted over a large area, tooth
structures can more adequately handle the stress.
When the force is exerted over a small area, the
increase in pressure may result in a fracture
 During mastication, stresses occur repetitively
over time; this may cause failure of a material.
 Microscopic flaws occur until the material fails,
resulting in fracture. This is known as fatigue
failure.
Moisture and Acid Levels
 The oral cavity is continually in contact with
moisture that varies from acid to alkaline.
 The type of moisture in our mouths depends
on what we eat or drink, what medications
we take, and what quantity of acid-producing
bacteria is present.
 The normal pH of saliva is 6.2 to 7.0; this is
fairly neutral but fluctuates during the day.
Effects of Moisture
 During placement or over time, most
materials react adversely to moisture.
 The breakdown of most dental materials is
brought about by moisture, acid, and stress.
 Materials that are needed to last a long time
must withstand this hostile environment.
Solubility
 A soluble material dissolves in fluid.
 Desirable materials should have a low
solubility.
 Gold and porcelain are retained in the oral
environment for many years because they are
insoluble.
 Materials that are tooth-colored are more
soluble; therefore they do not last as long.
Water Sorption
 Some materials have the undesirable
characteristic of water sorption.
 These materials may stain or may enlarge
slightly because they absorb moisture.
 Some materials stain as the result of water
sorption caused by repeated exposure to
coffee, tea, cola, and foods that contain dyes.
Metals
 Metals, with the exception of noble metals
such as gold, suffer the effects of moisture
and acidity.
 Many metals corrode because of their
continued contact with an acidic
environment.
 Others discolor as the result of oxidation of
metal surfaces. This is known as tarnish.
 Dental amalgam is highly susceptible to both
corrosion and tarnish.
Galvanism
 An environment that contains moisture,
acidity, and dissimilar metals makes possible
the generation of electrical current. This is
known as galvanism.
 The current may result in stimulation of the
pulp, called galvanic shock.
 It also may create a metallic taste in the
mouth.
 Insulation of the restoration may help to
decrease the effects of galvanic shock.
Temperature
 With few exceptions, all forms of matter
expand and contract with temperature
changes. These changes result in
dimensional change within the restoration.
 Ingestion of foods and beverages and
smoking may alter the temperature of the
oral cavity.
 Dental materials should have expansion and
contraction rates similar to those of teeth.
Temperature (cont’d)
 Expansion and contraction are measured by
using the coefficient of thermal expansion
(CTE).
 The CTE measures changes in volume or
length related to changes in temperature.
 Materials such as composites or amalgams
should be similar to tooth structure so that
marginal integrity can be maintained.
Temperature (cont’d)
 If the CTE is significant, then repeated
shrinkage and expansion of the material
within a tooth will open gaps in the interface
between the restoration and the tooth.
 This action, known as percolation, will allow
bacteria and oral fluids to breach the
interface, causing recurrent caries, staining,
and pulpal irritation.
Temperature (cont’d)
 Another consideration associated with
temperature is thermal conductivity.

This is the rate at which heat flows through a
material.
 Metals such as gold and amalgam are good
conductors of temperature; nonmetals are
poor conductors of temperature.
Other Temperature Considerations
 Aside from the temperature changes
associated with food and drink, one must
consider chemical reactions within the
material itself.
 Some materials when mixed will produce
heat.
 This exothermic reaction must be minimized
to protect tooth structures from excessive
heat, which may cause sensitivity or pulpal
damage.
Retention
 One important factor related to the selection of materials is
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how they will be retained in the preparation or on the tooth
surface.
Retention may be mechanical or chemical through adhesion,
or it may reflect a combination of the two.
Mechanical retention places undercuts in the preparation.
Chemical retention works through adhesion or bonding.
Chemical adhesion offers several advantages in producing
retention:
 Requires less removal of tooth structure b/c no undercuts
are necessary
 Produces stronger retentive force between tooth and
restoration
 It can seal the margin of restoration to prevent seepage of
bacteria and fluids through percolation
Retention (cont’d)
 Most of today’s dental materials use a
combination of mechanical and chemical
retention.
 This may be helped or hindered by the
viscosity of the material.
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Viscosity is a material’s ability to flow.
The thicker the material, the harder it is for the material
to flow.
 Film thickness, the minimum thickness attainable, is
important when dental cements are used.
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Wetting
 Wetting is the degree to which a liquid
adhesive is able to spread over the surface of
the tooth and the restorative material.
 The better the adhesive is able to coat the
surface, the better the retention will be.
Surface Preparation
 Another consideration that affects retention
is the condition of the tooth or restoration
surface.
 Moisture contamination, surface texture, and
energy will affect how well the two bond.
 Even slight contamination of moisture or
debris will compromise the integrity of the
process.
Surface Energy
 Surface irregularities may prevent complete
wetting of the surface.
 Microscopic irregularities trap air as the
adhesive flows over them.
 This diminishes the surface energy and the
numbers of atoms attracted to the bonding
surface.
Microleakage
 The space between the walls of the
preparation and the preparation is called the
interface.
 If the interface is not sealed, fluids and
microorganisms can penetrate between the
tooth surface and the restorative material.
 This seepage of harmful materials, or
microleakage, results in tooth sensitivity,
recurrent decay, and marginal staining.
Esthetics
 Materials used in dentistry must be
esthetically acceptable.
 The human eye senses light through the cone
cells in the retina in three different ranges of
wavelength: red, green, and blue.
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Mixture of the three is interpreted by the brain to
determine the color we see.
Color
 The three components of color result in hue,
chroma, and value.
 Hue is the dominant color of the wavelength
detected.
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Teeth are seen predominantly in the yellow and
brown ranges.
 Chroma is the intensity or strength of a
color.
 Value describes how light or dark a color is.
Color (cont’d)
 The color of teeth is also determined by the
amount of light that passes through them.
 If light passes easily through a tooth, it is
said to be transparent.
 If light is completely absorbed, then the
tooth is opaque.
 A combination of the two is typical; the term
used is translucent.
Detection of Restorative Materials
 It is important that oral health care
professionals are able to identify restorative
materials within the oral cavity.
 Metal restorations are easier to identify than
esthetic restorations.
 Heavy pressure during scaling and polishing
may damage the surface of some restorations
(e.g., porcelain, composite); therefore it is
vital that the restoration is identified before
patient care is provided.
Detection of Restorative Materials
(cont’d)
 When tooth color is well matched, it can be
difficult to identify restorations.
 This may be accomplished by using location,
tactile sensitivity, and radiographs, as well as
air, magnification, and adequate illumination
of the areas in question.
Summary
 The oral environment is hostile, and the
materials we use must be compatible, must
last for the intended length of time, and must
be of the highest esthetic and functional
quality.
 Materials are constantly changing,
improving, and evolving.
 Keeping abreast of these changes allows
dental professionals to educate their patients
and provide the highest quality of care.