Materials must withstand varying degrees of force through the muscular action of pushing and pulling an object during mastication, and for some patients from bruxism or clenching

Compressive: Pressure applied toward each other or in the same direction
Ex.) Posterior teeth ideally suited for this type of force
Tensile: Pressure applied away or in opposite directions
Ex.) An impression being pulled from mouth
Shearing: Pressure applied when two surface slide against each other or in a twisting rotating motion
Ex.) An incisor used for cutting
Torsion: Twisting force
Ex.) Turning a door knob
Bending: A combination of tension and compression
Ex.) Books piled one at a time on a table; causing stress

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Types of Stress: Review

Compression → Pushing stress
Tension → Pulling stress
Shear (slip) → Sliding stress
Torsion → Twisting stress
Bending → Compression + tension stress

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Mechanical Properties: Types of Forces

Stress: The internal force which resists the applied force
Reaction of an object to resist external force
Strain: Distortion or deformation occurring when an object cannot resist a stress
Enough stress placed on an object to cause change or deformation
The greater the stress, the greater the strain
Stress is the amount of force exerted from within an object; and object; and strain is the amount of change that the force has produced

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Mechanical Properties: Strain

The change in length divided by the original length
Measured as:
Fraction
Percentage
Examples of expansion of a material in terms of fraction and percentage → change in length:

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Mechanical Properties: Stress

Force that develops within a loaded object, and is:
Proportional to the applied load
Related to size of the object
Stress = load/area
Measured in:
Pounds per square inch (psi)
Pascals (metric)

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Relationship of Stress and Strain

Proportional
Always occur together
Modulus of elasticity (Young’s Modulus)
Stress/strain
Characteristic of a material and its atomic bonds
High modulus of elasticity = stiff material

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Modulus of Elasticity (Young’s Modulus)

It is the ratio of the stress to the strain (slope of stress/strain curve)
When the slope is:
More horizontal = springier
More vertical = the wire is stiffer

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Stress-Strain Plot

Elastic deformation
Plastic/permanent deformation
Elastic limit, proportional limit, or yield point
The point at which either a material will stay same or bend
Ultimate strength
The greatest point on stress plot that material is strongest and then will eventually breakdown

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Other Mechanical Properties of Interest- Optional Poisson’s Ratio

Ratio of strain in the direction of stress to the strain in the direction perpendicular to the stress
Materials change shape in three dimensions

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Resilience and Toughness

Resilience
Material has the ability to absorb energy without becoming deformed
Ability to absorb energy and not be deformed (mouthguard)
Toughness
Energy absorbed up to the failure point on stress/strain diagram (helmet)

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Fracture Toughness

Measure of the energy required to fracture a material when a crack is present
Energy it takes to fracture a material when a crack is present
Low fracture toughness:
Glass
Dental porcelain
High fracture toughness:
Metals

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Fatigue

Material/object failure due to repetitively being stressed for a long time

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Time-Dependent Properties- Optional

Creep
Small change in shape when an object is under continuous compression
Stress relaxation
Slow decrease in force, (stress) over time

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Flow and Creep

Time dependent plastic deformation
When a metal is placed under stress, it will undergo plastic deformation
The high copper alloys, as compared with conventional silver tin alloys, usually tend to have lower creep values

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Stress Concentration

Stress increases around defects
Increases likelihood of fracture
Ex.) Cutting glass & hairline fractures on teeth

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Under Physical Properties: Moisture & Acid Levels

Solubility: Susceptibility to being dissolved
Highly soluble- sugar in H₂O, enamel is not
Dental cements are tested for their strongest and most durable qualities
Most materials are adversely affected by moisture, either during placement or over time
The breakdown of most restorative materials is directly related to the effects of moisture, acid, and stress
Water sorption: The ability to absorb moisture. There is a saturation point for the tooth
Measured much like solubility
Weight gained is the water sorption
Acrylic has the highest sorption of all materials- it swells
Hydrocolloid impression materials will imbibe water if immersed in it leading to dimensional changes
It represents the amount of water adsorbed on the surface and absorbed into the body of the material
Straining: Of resins and acrylics from repeated exposure of tea, coffee, and other dyed beverages- due to water sorption

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Ideal Properties of Dental Cements

Should by strong and hard
Able to protect pulp
Should be insoluble in saliva and liquids taken in mouth
Should be dimensionally stable
Should be adhesive
Should be non porous
Should be biocompatible and non irritant
Coefficient of thermal expansion should be equal to the tooth structure
Should not be affected by thermal changes and moisture
Should be easy to manipulate

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Strong Dental Cements with Low Solubility

Glass Ionomer
Powder is an acid-soluble calcium. The slow release of fluoride from this powder aids in inhibiting recurrent decay
Causes less trauma or shock to the pulp than many other types of cements
Low solubility in the mouth
Adheres to a slightly moist tooth surface
Has a very thin film thickness, which is excellent for seating ease

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Cements with High Solubility Qualities

Zinc phosphate
Over 100 years of clinical experience
Routine application
Post-op sensitivities
Low hardness
High solubility
pH
No bond with tooth

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Physical Properties: Moisture & Acid Level

Enlargement of material → Imbibition theory: ability of material to absorb moisture
Ex.) Impression materials
On the other hand… when dentures are left out of water they dry out and shrink! It’s best to leave them in for an adequate period of time will allow for a comfortable fit
Syneresis Theory
All metals, except noble metal (gold, platinum, or palladium) suffer from the effects of moisture & acidity

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How Materials Breakdown

Tarnish: Discoloration caused by oxidation of a metal just on the surface
Corrosion: To wear away, especially by a chemical reaction. The material corrodes inside & out and the surface gets pitted and rough (dental amalgams)

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Tarnish

The oxidation that attacks the surface of the amalgam and extends slightly below the surface
Results from contact with oxygen, chlorides, and sulfides in the mouth
Causes a dark, dull appearance that is not destructive to the amalgam

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Corrosion

Excessive corrosion can lead to:
Increased porosity
Reduced marginal integrity
Loss of strength
Release of metallic products into the oral environment

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Causes for Distortion and Dimensional Change of Alginate

If an alginate impression is stored in water or in a very wet paper towel, the alginate will absorb additional water and expand. This condition is called imbibition
If an alginate impression remains in the open air, moisture will evaporate from the material, causing it to shrink and distort. This condition is called syneresis

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Physical Properties → Temperature

Dimensional change: Expansion and contraction of matter when heated
Expansion and contraction: Measured using the coefficient of thermal expansion (CTE)
Measurement of change in volume or length (composites and amalgams- have different rates of CTE)
Percolation: Space between the restoration and the tooth caused by continual shrinkage and expansion of the restoration when heated or cooled
Highest for composites & amalgam
Staining
Recurrent decay
Pulpal

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Coefficient of Thermal

Measurement of change in volume in relation to a change in temperature
Cooling results in shrinkage/contraction
Compare dental material to tooth
Restoration will shrink with cold and expand with heat
Opens gaps between restoration and tooth = microleakage (may cause recurrent decay)
Opening and closing gap = percolation
Dental amalgam → percolation decreases over time due to corrosion products from the amalgam filling the space
Percolation: Opening and closing of the gap between tooth structure and a restoration due to differences in coefficients of thermal expansion (when heated/cooled)
Results in:
Microleakage
Tooth sensitivity
Recurrent decay
Thermal conductivity: Rate at which heat flows through a material
Insulator: Matter that prevents the passage of electricity, heat or sound through an object
Exothermic reaction: The reaction of certain components when they are mixed
Resulting in the production of heat (Certain gypsum products create more heat than other like yellow dental lab stone)

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Microleakage

The seepage of harmful materials into the restoration/tooth surface interface
Thus, can lead to staining, recurrent decay, and postoperative sensitivity

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Electrical Conductivity

Good electrical conductors
Metals
Gold (one of the best thermal conductors)
Poor electrical conductors (but great insulators)
Polymers
Ceramics
Cements
Dentin (a natural insulator)

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Galvanism

An environment containing moisture, acidity, and dissimilar metals makes the generation of an electric current possible. Electric charge can further breakdown material
Galvanism: An electric current transmitted between two dissimilar metals
Galvanic shock: Current may result in stimulation to pulp
A pain sensation cause by electric current generated by a contact between two dissimilar metals forming a battery in the oral environment

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Galvanic Corrosion/Electrogalvanism

This type of electrochemical corrosion occurs when two or more dissimilar metals are in direct physical contact with each other
Ex.) Two adjacent or opposing restorations made of different alloys. Saliva acts as an electrolyte

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Retention

Retention: The ability of a material to maintain its position without displacement under stress
Retention may be mechanical or through undercuts, which lock the material in place
The are chemical bonds and mechanical bonds (most of today’s restorative materials use a combination of mechanical and chemical or bonding adhesion systems for optimum retention)
Chemical adhesion or bonding
Bonding: Ability to hold together, commonly used to describe the retention of materials to each other
Advantages of Bonding:
1.) Requires less removal of tooth surface structure because no undercuts are necessary
2.) It produces a stronger retentive force between tooth & restoration
3.) Seals the restoration margin to prevent seepage of bacteria & fluids through the process of percolation
Today’s restorative materials use a combo of mechanical, chemical, or bonding adhesion for maximum retention
Factors that affect bonding include:
1.) Viscosity
2.) Wetting
3.) Film thickness
4.) Surface characteristics of the tooth, the restoration, and the adhesive
Viscosity: The ability of a liquid material to flow
Wetting: The degree to which a liquid adhesive is able to spread over the surface of a tooth and restoration (poor wetting → Teflon surface)
Retention: The better the adhesive is able to spread and retain to surface (good cooking oil, bonding adhesives → agents used to bond composite resin)
Film thickness: The minimal thickness obtainable by a layer of material
Cement should have low film thickness

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Viscosity

Ability of a material to flow
Temperature dependent
High viscosity = high contact angle = poor wetting
Low viscosity = low contact angle = good wetting
Examples of clinical significance of pseudoplastic properties, hydrophilicity, and high mechanical resistance (tear strength) of impression materials

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Surface Characteristics Determine How Well Bonding Occurs

1.) Cleanliness of surface
2.) Moisture contamination
3.) Surface texture
4.) Surface energy of the restoration & tooth
Surface energy: The attraction of atoms to a surface relates to wettability
Low surface energy bead up (wax or many plastics)
High surface energy flows (metals, ceramics, and enamel)

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Biocompatibility

Biocompatible: Material that does not impede or adversely affect living tissue
Other words, a material may be acceptable for use or fabrication on hard tissues (tooth structure), but may not be acceptable for use on soft tissues
Example: Etchant used for sealants or certain cements are of great benefit when used according to the manufacturer's directions, but can be irritating to soft tissues and can excessively etch enamel if used improperly
Significant skin allergies or hypersensitivity to some metals, particularly nickel and acrylics can occur, and patients should avoid these materials. A complete health history and questioning of the patient can help identify such individuals

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Esthetics

Materials used in dentistry must be ethically acceptable. Esthetic dentistry is a growing priority, and esthetics may be of great concern to the patient
The color of teeth varies: (The human eye senses light through cone cells…)
Hue: Dominant color of the wavelength. Tooth colors are predominantly seen in the yellow and brown range
Chroma: Refers to the intensity or strength of the color; teeth are rather pale in color. Pink vs red range
Value: Describes how light or dark the color is
Teeth have value ranges in the light scale (gray range)
Also, the color of teeth is determined by way they reflect light
Book goes into principals: transparency, opaqueness, translucency, and vitality

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Color

Psychological response to physical stimulus (light)
Perception varies between individuals
Standardization by using shade guides

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Interaction of Materials with X-Rays

Radiolucent
Some ceramic materials
Denture acrylic resin
Radiopaque
Metal restorations

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Detection of Restorative Materials

It is important that oral healthcare professionals are able to identify restorative materials within the oral environment to treat them appropriately. Heavy pressure during scaling, the use of sonic and ultrasonic scaling or air polishing, and inappropriate use of polishing agents may gouge or scratch the surface of a restoration
Tactile evaluation of the tooth surface may be the most reliable means of clinical assessment
Radiographs are a valuable tool for detection of restorations and the assessment of restorative components
For more difficult cases a combination of methods would be required

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Physical Properties of Dental Materials

To predict how a material will react under oral conditions, it is necessary to have an understanding of its physical properties. Their physical structure, application, manipulation, composition, and reaction can classify materials
Physical structure: The physical structure of a material may take on three basic forms: solid, liquid, or gas
Solids are described by their:
Density
Hardness
Elasticity
Stiffness
Ductility
Resilience
Toughness
Malleability
Brittleness
Density: A measure of compactness of matter
Hardness: Resistance of a solid to penetration
Ultimate strength: The maximum amount of stress a material can withstand w/o breaking
Elasticity: The ability to stretch w/o breaking
Stiffness: The resistance to deformation of a material measured by elastic modulus
Proportional limit: When a material has reached its limit of elasticity
Below the proportional limit no permanent deformation occurs, and the structure returns to its original shape
Resilience: The resistance of a material to permanent deformation
Ductility: The amount of dimensional change a material can withstand without breaking when it is under tension
Malleability: The ability for a material to be compressed without breaking
Edge strength: The combination of malleability and ductility gives a metal the ability to resist fracture or abrasion even at fine margins

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Liquids

Viscosity: Of a liquid is its resistance to flow
Thin liquids have low viscosity and thick liquids have high viscosity
Thixotropic materials: Liquids that flow more easily under mechanical force (fluoride gels)

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Application

Preventative
Fluoride, sealant, mouth guard, cavity varnish
Therapeutic
Medicated bases such as cement (IRM)
Treat disease
Restorative
Amalgam, composite (direct) *fabricated in the mouth
Inlays, crowns, bridge, implants (indirect) *fabricated outside mouth
So, restorations may be further classified as direct restorative materials or as indirect restorative materials

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Composition

Most materials combine two components at chair side to form resulting materials
Many components are classified as catalyst and base; the catalyst is responsible for the speed with which the reaction occurs and is often the liquid component and the base is responsible for the strength and insulation properties to be provided- often powder form
Water & powder
Liquid & powder
Paste & powder
Paste & paste
Paste & indicator- light
Components may be measured = C/B or prepackaged in pre-dosed amounts for which standardization of measurements eliminates the errors produced in measuring

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Reaction

Reaction: When mixed together, may be physical or chemical. Most reactions of the two components result in a solid structure
Mixing time: The amount of time to bring the components together into a homogeneous mix
Mixing times must be strictly adhered to in order to allow the clinician the full working time
Working time: The time permitted to manipulate the material in the mouth
Initial set time: Begins when the material no longer can be manipulated in the mouth
Final set time: When the material has reached its ultimate state
It is important to remember that temperature of water, rate of mixing, and moisture contamination affect final mixing time

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Manipulation

Technique
Ratio of component
Temperature and humidity
Shelf life
Storage

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General Handling and Safety

How chemicals enter the body:
1.) Inhalation
2.) Absorption through the skin
3.) Ingestion (eating or drinking)
4.) Invasion directly through a break in the skin
Control of chemical spills
General precautions for storing chemicals
Disposal of chemicals
Hazardous waste disposal
Occupational Safety and Health Administration Hazard Communication Standard (OSHA)
Hazard Communication Program
Bio-aerosols in the dental setting
Patient safety

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Acute and Chronic Toxicity

Acute toxicity: Result from high levels of exposure over a short period of time
Chronic toxicity: Results from repeated exposures, generally to lower doses, over a much long time period
Personal chemical protection:
Hand protection
Eye protection
Protective clothing
Inhalation protection

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Chapter 20: Infection Control and Safety in the Dental Office

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Introduction

Risk
Safety
Ionizing radiation

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Infectious Diseases

Infection control
Personal protective equipment and barriers

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Personal Protective Equipment and Barriers

Gloves
Handwashing
Preventing or managing latex irritation
Facial masks and shields
Eye protection
Protective attire

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Preventing or Managing Latex Irritation

Latex allergy in dental staff
Latex allergy in patients

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Eye protection

Hazards
Equipment
Patient protection
Curing light hazards
Lasers

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Instrument Sterilization and Surface & Equipment Disinfection

Degree of contamination
Critical objects
Semi-critical objects
Noncritical objects

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Other Aseptic Techniques

Distribution of supplies and materials
Bulk supplies
Kits
Unit dose
Use of disposable items

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Managing Aerosol and Splatter

Rubber dam
Pretreatment mouthrinse
Pretreatment toothbrushing

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Safe Handling and Disposal of Sharp Items Contaminated with Blood and Saliva

Sharps containers
Recapping needles

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Office and Laboratory Housekeeping Practices

Eating and cosmetics
Food storage
Separate contaminated items
Infectious wastes
Exposure control plan

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Physical Hazards

Trips, slips, hits, and spills
Floors
Patients
Lifting injuries
Lathes
Model trimmers
Burns, scalds, and fires
Sterilization equipment
Personal protective equipment
Flammables
Oxygen
Electrical hazards
Outlets near sinks
Maintaining equipment
Pacemakers
Hearing protection
Harmful noise levels
Protection
Respiratory hazards
Aerosols, dusts, allergies, and asthma
Allergic sensitization
Protection
Dermal hazards
Allergic contact dermatitis
Latex gloves
Musculoskeletal problems
Ergonomics
General recommendations
Hand and wrist recommendations
Instrument recommendations
Preventing injuries

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Chemicals

Material safety data sheets (MSDS)
Labels
Disinfectants
Mercury
Mercury toxicity
Dental personne
Nitrous oxide
Reproductive problems
Exposure
Measures to reduce exposure to nitrous oxide
Disposal of chemicals

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Final Thoughts

Emergencies
Putting risk in perspective

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Chapter 21: Disinfection of Impressions, Dentures, and Other Appliances and Materials

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Disinfection of Impressions

Personal protective equipment
Rinse the impresion
Disinfection techniques
Disinfection of an impression by immersion
Disinfection of an impression by spraying
Pour the impression

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Disinfection Techniques

Disinfection of an impression by immersion
Disinfection of an impression by spraying

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Disinfecting Dentures and Other Appliances

Chairside adjustments
Clean the prosthesis
Disinfect the prosthesis
Rinse the prosthesis
Store the prosthesis

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Chapter 22: General Rules for Handling Dental Materials

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Follow the Manufacturer’s Directions

Read, understand, and follow all directions
Save copies of directions
Practice using material before using clinically
Store materials in a cool, dry place

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Mixing and Setting Times

Use a clock with a second hand for timing
Materials set faster in the warmth of the mouth
Some materials set faster due to the humidity of the mouth

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Dispensing Materials

Dispense as directed by instructions
Dispense equal lengths, not volumes, of pastes
Dispense consistent drops
Fluff powders

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Mixing

Mix aggressively
For cements, force the powder into the liquid
For some materials, setting rate depends on the mixing method

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Light-Activated Materials

You can undercure but never overcure
Maintain space between curing light and oral tissues
Cover materials if not to be used immediately

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Contamination

By oral fluids, is bad for materials
All adhesive restorative materials
Keep teeth clean, and sometimes dry when using adhesive materials

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Summary

Follow and save manufacturer’s directions
Manipulation factors:
Proper dispensing
Correct mixing
Adequate curing
Avoiding contamination

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PowerPoint Chapter 20 Review Questions:

1.) Immediately after an impression is taken, it must be rinsed under runningwatertoremovesalivaorblood. This step is essential before the impression can be disinfected

The 1st statement is true; the 2nd statement is false
The 1st statement is false; the 2nd statement is true
Both statements are true
Both statements are false

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2.) It is recommended that all of the following types of impression materials are immersed in a disinfectant for 15 minutes. If not true than which can be,

Alginate
Polysulfide
Compound
Wax Bites

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3.) Chairside adjustments may be performed with a unit dose of abrasive, a sterile handpiece and bur, and a sterile rag wheel for polishing. This eliminates the need for disinfecting the appliance before handling.

The 1st statement is true; the 2nd statement is false
The 1st statement is false; the 2nd statement is true
Both statements are true
Both statements are false

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4.) After chairside adjustments are made to a dental prosthesis, it is cleaned, disinfected, rinsed, and stored. The recommended storage method is:

Place the prosthesis back on the patient’s stone cast
An individualized, plastic closed container for each patient, similar to those used for overnight soaking
A zippered plastic bag
A zippered plastic bag containing sterile water

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Note: The clinic may have slightly modified sequences for denture care. In office may have other/additional protocols

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5.) The primary goal of infection control when handling and disinfecting impressions, dentures and appliances is to:

Protect the patient from the laboratory personnel
Protect the laboratory personnel from the patient
Prevent cross- contamination
Prevent contamination of equipment in the dental laboratory

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6.) Disinfection by immersion is preferred over disinfection by spraying. The most important reason is because:

Office personnel don’t have the necessary time available to spray thoroughly
The spraying procedure must be performed twice, with ten minutes in between
Constant contact of the spray with all surfaces of the impression cannot be assured
The aerosol product is usually toxic

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Note