1. Characteristics of Materials
Chapter 2
Dental Materials
DAE/DHE 203

2. Characteristics of Materials:
Four Classes of Materials
Structure of Materials
Physical Characteristics
Mechanical Characteristics
Biologic Characteristics

3. Classes of Materials: METALS: High thermal & electrical conductivity
High ductility (bend without breakage)
High opacity (do not transmit light)
High luster (reflect light; appear shiny)
Crystalline arrangement of atoms (solid)
Strong metallic bond (high melting point - except Mercury)
Metals: Oldest of the materials used in dentistry
When metals are in a solid state, their atoms are arranged in perfect cubes that create a crystalline structure.
Metals have a high melting point – the atoms are so strongly bonded that it takes high temperatures to break them and make them turn to a liquid state.
Mercury is a metal that is liquid at room temperature and therefore is used in amalgam fillings so that the filling can be constructed directly in the patient’s mouth.

4. Classes of Materials: METALS: Strong, rigid, and stable materials
Can be “cast” or formed into various shapes
What restorations can you name that are constructed with metal?
Metallic restorations that we have already discussed:
Crowns – gold, SSC & space mtnrs., PFM
Bridges – the metal substructure beneath the porcelain
Partial – metal substructure and clasps
Fillings – amalgams, inlays/onlays
What about orthodontic appliances and wires?

5. Classes of Materials: CERAMICS: Compound of metal with non-metal
High melting points
Low thermal & electrical conductivity
Crystalline or Non-crystalline
Inert – biologically compatible
Used as fillers to reinforce composites
Esthetic - porcelains
Ceramic materials commonly used in dental ceramics are OXIDES – oxygen + metals =
Silicon, aluminum, calcium, magnesium oxides.
Some example of ceramics are: Glass, Concrete, Fine Crystal, and Gypsum
Porcelains are a special type of ceramic used in dentistry.
Ceramics tend to be very brittle – cannot be deformed or bent without cracking
Ceramics can be good insulators
Ceramics are manufactured by fusing oxide powders together in ovens at high temperatures – able to “paint” various shades using ceramic oxides.
Glass ceramics are used as fillers for composite filling materials.

6. Classes of Materials: POLYMERS:
Man-made, long-chain, organic molecules (carbon atoms linked together)
Low thermal & electrical conductivity
Low strength and stability
Dental acrylics – dentures, sealants, temps
Impression materials
Adhesives – dental cements
Resin base for dental composites
Long-chain organic molecules – like thousands of carbon atoms linked together in a string of beads.
The strength of a polymer is only in one direction – example: breaking a nylon string, but generally they have low overall strength and durability and therefore have not been used alone as a restorative material.

7. Classes of Materials: COMPOSITES:
Mixtures of two or more of the other classes (metals, ceramics, polymers)
Example: Dental Composite filling material
Resin matrix (polymer) + glass filler (ceramic)

8. Structure of Materials:
BONDS: forces holding atoms together
Primary Bonds – solids
Covalent: sharing electrons
Ionic: interaction of + and – charges
Metallic: share electrons of outer shell
Secondary Bonds – liquids
Less stable; weaker attractions
Bond liquid to liquid, or liquid to solid
Primary Bonds – hold solids together – these are known to be STRONG & STABLE
Covalent: example is the carbon chain linked together – electrons are shared to create their bond; this is strong and stable
Ionic: The bond that holds glass ionomers and tooth OR polycarboxylate cement and tooth are the interaction of the positive charges of the tooth with the negative charges of the cement.
Metallic: also a strong & stable bond – electrons are free to move about to atoms in the solid – creates ability for a metal to be able to conduct electricity and heat.
Secondary – less stable

9. Structure of Materials:
ATOMIC ARRANGEMENTS: (when a material is in a solid state)
Amorphous – irregular pattern of atoms; “frozen liquids”
i.e. glass, polymers
Crystalline – a regular pattern of columns and rows, stacked upon each other; “cubic” or other crystal patterns
i.e. metals
The structure of materials is important since it determines the physical and mechanical properties of the material.

10. Physical Characteristics:
Electrical & Thermal Conduction:
No need for materials to be conductive
Metals ARE good conductors
Galvanism – a “shock” created by 2 unlike metals in contact + saliva
Heat Capacity – metals have a low capacity
Protect teeth from stimulation; insulate!
Galvanism or “galvanic shock” – metals in contact, example: fork on amalgam, aluminum foil on gold, etc. Saliva acts as the electrolyte that carry the electrical current.
Heat Capacity is the measurement of heat needed to raise the temperature of a material – example: amalgam has a LOW capacity since NOT much heat is required (degrees of change in temp) to transmit heat through the metal to the tooth.
The tooth needs “insulation” to protect the pulp if the prep is deep and metal extends deep into the tooth – a base will protect the tooth from electrical and thermal conduction.

11. Physical Characteristics:
Thermal Expansion: temperature change causing a material to expand or contract
can create change in dimension
Coefficient of Thermal Expansion:
the amount of dimensional change as a material expands/contracts
Ideally, choose a restoration that expands & contracts same as tooth.
Choosing a material with a similar Coefficient of Thermal Expansion as tooth only looks at ONE characteristic and property by which we would choose a material. It may be an important consideration – as temperatures fluctuate in your mouth, so do they affect the restorations – the restorations and the teeth are expanding & contracting with the temp changes.

12. Coefficient of Thermal Expansion
Expansion: Amount of expansion & contraction taking place in material at a given temperature change.
Conductivity: the rate heat flow thru a material
What might happen in a tooth with an amalgam filling that is exposed to an extreme temperature change? How will the metal respond? How will the tooth respond?
It is possible that if the material and tooth change at different rates a gap may be created at the interface of the restoration and tooth.
What materials are most similar to tooth in their ability to conduct heat?
What materials are most similar to tooth in the expansion & contraction to temps?
Ferracane; 2001; Materials in Dentistry; Table 2-1, p. 21
Thermal Expansion:
Thermal Conductivity:

13. Physical Characteristics:
Solubility:
Ability to dissolve
Cause allergic rxn?
Least soluble = porcelains & ceramics
Most soluble = polymers & acrylics
Sorption:
Uptake of fluids or substances
Highest sorption = polymers
Swelling of material
Distort or disintegrate
If a material is soluble and dissolves elements into the oral cavity, might it be harmful to the patient? Create a gap in the tooth/restoration interface (dissolving cement)? Allergic rxn? Sensitivity? Harmful? Beneficial (glass ionomer – fluoride)?
If a material can take up fluids into it, it will distort thereby leaving it dimensionally changed.
Ideally, dental materials are resistant to both.

14. Physical Characteristics:
ADHESION:
The force of attraction between
molecules or atoms of two different surfaces brought into contact.
COHESION:
molecules or atoms within a material.
Adhesive – the cement in most dental restorations is the “adhesive”; bonding agents, luting cements, etc.
Adherend - the tooth surface –
the crown cement is adhered to the tooth by the polycarboxylate cement to the tooth
Adhesive: that which is being attached (“glue”)
Adherend: the surface to which it will be attached

15. Physical Characteristics:
Factors influencing ADHESION, (adhesion):
Wettability – the ability of the surface to become wet; ( wettability)
Surface Energy – the available energy at the surface; ( surface energy)
Surface Tension – amount of attraction the molecules have for one another; ( surface tension)
Viscosity – resistance to flow; ( viscosity)
Wettability will be looked at more closely in just a minute.
The higher the surface energy, the better the potential adhesion will be. To improve the available energy / available chemical bonds on a surface, the surface should be free of plaque & pellicle.
Adhesion is improved when the surface tension of a material is lower. When molecules have an affinity for eachother the material will tend to clump together rather than spread-out over the surface adherend.
The lower the viscosity the better the flow of a material/adhesive – the surface will be wetted better.

16. Wettability: Contact Angle
adherend
adherend
Poor Wetting:
High contact angle;
Over 90°
Good Wetting:
Low contact angle;
Less than 90°

17. Adhesion:
Why is adhesion an important factor in the success of restorative dentistry?
We want the most successful adhesion between the restoration and the tooth that can possibly be achieved, to eliminate the loss of a restoration (high retention); minimize the removal of tooth structure (less reliance on mechanical retention); maximize the “seal” of the margin thereby preventing leakage.

18. Physical Characteristics:
COLOR & ESTHETICS:
Created by light’s interaction with material
Hue – dominant color
Value – lightness of a color
Chroma – intensity of a color
Translucency - teeth permit light to transmit through them
High demand today for materials to match natural tooth
Color shade matching a tooth is an important role in dental prosthetics procedures. Teeth are generally not a single color shade from the incisal edge to the gingival third of the tooth. The color shades should be taken BEFORE a rubber dam is placed so that light is reflected and interacting with the natural shade of the gingiva as the background.

19. Physical Characteristics:
CORROSION:
The deterioration of a metal by a chemical or electrochemical reaction; irreversible
Tarnish: surface deterioration; discoloration
Methods of Corrosion:
“Battery” created: 2 metals + saliva =
release of metallic ions & destruction of metal
Crevice corrosion: saliva & plaque acids seep into filling interface
The environment of the mouth being – warm, salty, & acidic makes for a good environment for corrosion of metals.
Tarnish is different in that the only result of the deterioration process is that the surface discolors – much like unpolished fine silver; once polished the piece is intact and fine
Some metals can be made corrosion-resistant by adding CHROMIUM (like in stainless steel) – creating a film, called “Passive Barrier” that protects the surface of the metal.

20. Physical Characteristics:
What happens when the interface or gap between an amalgam and tooth corrodes?
How can this type of corrosion be prevented?
Corrosion at Amalgam interface:
The products of the corrosion fill in the gap or interface, thereby making it difficult later for the saliva and acids to penetrate it. Preventing further leakage & potentially reducing secondary decay; “self-sealing”.
The margin of the restoration is more prone to breakdown with corrosion.
Corrosion prevention:
Well placed, carved and polished amalgams.
2. Good oral hygiene practices to eliminate the presence of the acids, etc.

21. Mechanical Characteristics:
Types of FORCE:
Tensile: pulling
Compressive: crushing, squeezing
Shear: sliding
STRESS: a material’s response to force; generated within it to counter the force
STRAIN: change in dimension as a result of the applied force; deformation

22. Mechanical Characteristics:
Modulus of Elasticity – proportion of stress is equal to strain (a material can resume its shape when the stress is removed)
Elastic Limit – maximum stress level tolerated by a material without deformation (a material will be deformed if any more stress is put upon it)
AKA “proportional limit”; “yield point”
Ultimate Strength – the highest stress tolerated before failure (any more stress will result in breakage of the material)

23. Stress-Strain Curve: Stress Strain Ultimate Strength Failure
Elastic Limit Proportional Limit
Stress
Modulus of Elasticity
Strain

24. Mechanical Properties:
Hardness – ability to resist indentation
Toughness – ability to resist fracture
Fatigue – repeated stress over time creating small cracks/weakness; may lead to failure
Creep – gradual, permanent change in dimension under constant load
Wear – direct contact of 2 objects deteriorates surface
Attrition – wearing of occlusal/incisal surfaces
Abrasion – “toothbrush abrasion”
Hardness of materials is important to resist the forces of chewing or toothbrushing. (porcelains are very hard; some metals – like gold – not very resistant to indentation)
Toughness – filling materials – composites – being able to withstand chewing forces (metals may be tough, porcelain & ceramics may be less tough)
Fatigue – metals, plastics & ceramics can all fail by fatigue
Creep – most materials only creep at temperatures close to their melting points – not a concern for metals and ceramics in dentistry; Concern for composites and amalgam, however.
Wear can occur on dental materials just as it does on natural tooth – could even be caused by materials (porcelains – very hard – may wear opposing dentition)

25. Biologic Characteristics:
The most prevalent concern: Sealing the interface between restoration & tooth!
Prevent leakage of bacteria, saliva, by-products
Prevent seepage of fluids into & out of the tooth
Dentinal tubules carry fluids, sealed by enamel
Hydrodynamic Theory – tooth pain is a result of the fluid flow around odontoblastic processes, stimulating nerve fibers
Temperature change can expand/contract these fluids
Besides materials being NON-toxic and NON-allergenic, the most prevalent concern is their ability to seal open spaces in tooth. As previously discussed, many clinical trials are performed to ensure the safety and non-toxicity of dental materials.
Prevention of secondary decay around margins of restorations.
Prevent sensitivity – prevent seepage of fluids into and out of tooth
(odontoblasts – create dentin; processes extend through dentin, close to nerves; tubules end in enamel)