2. Oral environment and patient
consideration
• The oral environment represents a challenge to the
success of dental materials.
• Understanding these challenges and limitations, is
essential for a successful treatment.
• Materials must be biocompatible, aesthetic and
durable.
• As a member of the allied dental team, you need to
appreciate these factors & have a thorough
knowledge of selection criteria and materials
manipulation.
2
3. Intraoral factors
•
Force and stress:
a. Compressive: crushing biting forces
b. Tensile: biting force stretches a material
c. Shear: e.g. an incisor used for cutting
a
b
c
3
4. – Resisting these forces creates stress within
the material, which may lead to deformation,
strain.
– Dental materials can withstand one type of
stress while fail under another. But failure
usually occurs after repetitive force
application i.e. fatigue failure.
4
5. • Moisture and acid levels:
– Intraoral pH depends on diet and acid producing
bacteria.
– Moisture affects materials during placement or over
time. Low solubility contributes to longevity.
– Some materials take up water, color, odor, tastes of
foods and beverages (e.g. resins, acrylic).
– Metals (except noble metals) are affected by acid and
moisture, i.e. corrosion: to wear out due to chemical
rxn.
– Dental amalgam is susceptible to tarnish
(discoloration caused by oxidation of metal).
5
6. • Galvanism: an electric current transmitted between two
dissimilar metals.
• Temperature:
– Dimensional changes (expansion/contraction)
– Coefficient of thermal expansion (CTE): measurement
of dimensional changes.
– Percolation: opening and closing of a gap between
tooth and restoration due to expansion and
contraction of restoration. This may lead to recurrent
caries, staining, pulp irritation.
– Thermal conductivity and insulators (pulp sensitivity).
– Exothermic rxn of restorative material.
6
7. Retention: the ability of the material to
maintain its position and resist
displacement
– Mechanical: undercuts e.g. amalgam
– Chemical: e.g. glass ionomer cements
– Bonding: a term used to describe how
composite is bonded to tooth surface
(micromechanical/chemical). Affected
by:
•
•
•
•
Wetting
Viscosity
Film thickness
Surface characteristics: cleanliness,
moisture
contamination, texture, energy.
7
8. • Microleakage: the
seepage of harmful
materials through the
gap between tooth
and restoration. Can
cause:
– Staining
– Recurrent caries
– Sensitivity
8
9. • Biocompatibility: dental material must not
have an adverse effect on living tissue
– Materials used on hard tissue vs. soft tissue
– Short term vs. long term exposure
– Small doses vs. high doses (fluoride treatment)
– Adverse effects maybe due to materials itself or the
breakdown of its components.
9
10. • Esthetics
– Color components:
• Hue: dominant color of wavelength detected
(tooth color is seen in yellow and brown range)
• Chroma: color intensity or strength
• Value: how bright or dark a color is.
– Transparent vs. opaque
– Shade guide
10
11. • Conditions for assessing restorations:
– Dry field
– Good lighting
– Sharp explorer
– Radiographs
– Magnification
– Good knowledge of material
11
12. End of part one
Reference:
Dental materials
Clinical applications for dental
assistants and dental hygienists (ch. 2)
12
13. Material properties
•
Physical properties: properties based on the laws of
mechanics, optics, thermodynamics, electricity etc.
(Phillips’ science of dental materials)
A. Rheological properties (ref. Introduction to dental
materials ch.1.8)
Definition: The study of flow or deformation of materials.
Solids: elasticity and viscoelsticity
Liquids: viscosity = shear stress/shear strain
•
Consider extrusion of a fluid from syringe.
13
14. • Viscosity: resistance of a liquid to flow. The ways
in which materials flow or deform under stress are
important to their use in dentistry.
• Thixotropic material: is a material that becomes
less viscous when subject to repeated pressure
(e.g. plaster, prophylaxis paste).
14
15. B. Mechanical properties (ref. Dental Materials,
properties and manipulation)
– Properties defined by the laws of mechanics; the
physical science that deals with energy and
forces and their effects on bodies.
– Maximum biting force decreases from molars to
incisors. Average biting force
– 1st and 2nd molars = 580 N
– Bicuspids (premolars) = 310 N
– Cuspids (canines) = 220 N
– Incisors = 180 N
Artificial replacement of dentition decreases biting force
(e.g. fixed bridge, partial and complete dentures)
15
16. • To compare the performance of materials irrespective of
their shape or size, an objective standard is needed.
This standard is stress and strain. Description of
mechanical properties depends on these two.
– Stress = force/unit area (compressive, tensile, shear)
– Strain: the deformation per unit of length as a result of
force = deformation/length
(e.g. rubber vs. gold alloy)
Force (N) Area (mm²) Stress (MPa)
111
645
0.1724
111
64.5
1.724
111
6.45
17.24
111
0.645
172.4
16
17. • Stress-strain curves are a convenient way to compare
materials mechanical properties whether in compression,
tension or shear, especially when strain is independent
of the length of time the load is applied
17
18. • Strain-time curves are sometimes used when strain
depends on the time the load is maintained (e.g.
alginate, rubber impression material)
18
19. – Values resulting from stress-strain curves:
• Elastic modulus = stress/ strain (MPa), a measure
of stiffness which is resistance to deformation
measured by Young’s modulus
• Proportional limit: measure of stress allowed
before permanent deformation occurs.
• Ultimate strength: maximum amount of strength a
material can withstand without breaking.
*Note: some materials can be classified as clinical failure
when significant permanent deformation occurs even if
the material does not fracture
19
20. – Other mechanical properties
• Elasticity: the ability to stretch and not break
(impression material and undercuts)
– Elastic (recovery immediate) vs. viscoelastic
(recovery slow or with some degree of permanent
deformation
• Toughness: ability of the material to resist fracture
• Resilience: the ability of the material to resist
permanent deformation
• Creep: time-dependent plastic strain of a material
under a static load or constant stress.
20
22. • Hardness: resistance to wear or abrasion (enamel
and porcelain are among the hardest). Hardness is
measured using several tests such as Knoop, or
Vickers hardness tests
• Fatigue properties (refer to slides only):
Materials are subjected to intermittent stress over
long period of time, stress is small, but over time,
failure may occur by a fatigue process. This
involves the formation of microcracks, resulting
from stress concentration at a surface fault, so
crack propagates until fracture occurs. Final
fracture occurs at a low stress level.
22
23. •
Fatigue is studied in 2 ways:
1. Fatigue life: application of stress cycles at a certain
amount and frequency and observe number of
cycles needed to cause failure.
2. Fatigue limit: select a number of cycles (e.g. 10
000) and determine the value of the cyclic stress
which is required to cause fracture within this
number of cycles.
23
25. C. Thermal properties: (ref. Dental Materials, properties and
manipulation)
Materials have different rates of conducting heat.
(Metals have higher values compared to plastics and
ceramics). Therefore patients may experience
postoperative sensitivity in association with amalgam
restorations for instance.
•
•
Thermal conductivity: it’s a measure of heat transferred
through a material or rate of heat flow. Enamel and dentine
are poor thermal conductors compared to amalgam and
gold alloys. Therefore insulators are required in some
cases to protect the pulp.
Coefficient of thermal expansion (explained previously)
25
26. D. Electrical properties:
(ref. Dental Materials, properties and
manipulation)
•
Galvanism: generated electrical current a
patient can feel resulting from dissimilar metals
present in a solution that contains ions (e.g.
oral cavity)
•
Corrosion: can result from
•
•
adjacent dissimilar metals. Galvanic action can cause the
metal to dissolve resulting in pitting and roughness.
Chemical corrosion
26
28. E. Solubility and sorption: (ref. Dental Materials,
properties and manipulation)
•
important criteria for dental materials selection.
Laboratory studies are used to evaluate and
rank materials.
• Sorption includes:
•
•
Absorption: uptake of liquid by solid e.g. uptake
of water by acrylic plastics
Adsorption: concentration of molecules at the
surface of solid or liquid e.g. adsorption of saliva
on tooth surface
28
29. F.
•
Wettability: (ref. Dental Materials, properties and manipulation)
measure of the affinity of a liquid for a solid indicated
by spreading of a drop e.g. wetting of denture base by
saliva. Wetting of enamel surface by pits and fissures.
wettability is observed by shape of a drop of liquid on
solid surface identified by contact angle:
• Low contact angle = high wettability (hydrophilic
if liquid is water)
• High contact angle = low wettability (hydrophobic
if liquid is water)
29
30. Good wetting of a solid by a liquid with low
contact angle (left), poor wetting forming a high
contact angle (right).
θ liquid
solid
θ
liquid
solid
30
31. G. Optical properties: (ref. Introduction to dental materials)
•
•
Every object we see is as a result of reflectance of light from
that object reaching an extremely sensitive photodetector,
namely the eye. This is characterized by:
Color (Hue, value, chroma)
Translucency:
•
Translucent materials allows some light to pass, absorbs
some, and scatters the rest
• Opaque material does not transmit light, but absorbs and
scatters it.
•
Surface texture: the polishability of a material is an important
criteria for selection
*Metamerism: change of color of an object due to a change in light
source
•
31
33. H. Biological properties: (ref. applied dental materials
ch.2)
•
Primary requirements of any dental material:
–
–
–
–
•
Non-toxic
Non-irritant
Should not have carcinogenic or allergic potential
If used as filling material should be harmless to pulp
Biological evaluation of dental materials:
–
–
–
Level 1
Level 2
Level 3
33
