DHY 204

Chapter 3: Physical and Mechanical Properties of Dental Materials

Oral Environment and Patient Considerations

• Dental materials should be:

• Biocompatible

• Durable

• Therapeutic

• Restorative

• Nonreactive in acid or alkaline conditions

• Compatible with other materials

• Esthetically acceptable

Properties of Material

• Physical

• Mechanical

• Chemical

• Biologic

• The focus will be primarily on: physical and mechanical properties

Physical Properties

• Used to observe or describe characteristics of matter

• How would you observe or describe this object:

• Size

• Shape

• Density

• Materials its made of

• Color

• Based on the laws of physics

• Describe mass, energy, force, light, heat, electricity

Mechanical Properties

• Describe a material’s ability to resist forces

• Dependent on:

• Amount of the material

• Size of the object

• Shape of the object

Chemical Properties

• Describes:

• Setting reaction of materials

• Decay or degradation of materials

Biologic Properties

• Describe the effects the material has on living tissue

Mechanical Properties

• Elasticity

• Strain

• Types of forces

• Stress

• Poisson’s ratio

• Resilience and toughness

• Fracture toughness

• Fatigue

• Time-dependent properties

Mechanical Properties: Forces and Stress

• 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

Types of Stress: Review

• Compression → Pushing stress

• Tension → Pulling stress

• Shear (slip) → Sliding stress

• Torsion → Twisting stress

• Bending → Compression + tension stress

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

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:

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)

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

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

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

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

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)

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

Fatigue

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

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

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

Stress Concentration

• Stress increases around defects

• Increases likelihood of fracture

• Ex.) Cutting glass & hairline fractures on teeth

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

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

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

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

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

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)

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

Corrosion

• Excessive corrosion can lead to:

• Increased porosity

• Reduced marginal integrity

• Loss of strength

• Release of metallic products into the oral environment

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

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

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)

Microleakage

• The seepage of harmful materials into the restoration/tooth surface interface

• Thus, can lead to staining, recurrent decay, and postoperative sensitivity

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)

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

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

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

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

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)

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

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

Color

• Psychological response to physical stimulus (light)

• Perception varies between individuals

• Standardization by using shade guides

Interaction of Materials with X-Rays

• Radiolucent

• Some ceramic materials

• Denture acrylic resin

• Radiopaque

• Metal restorations

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

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

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)

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

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

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

Manipulation

• Technique

• Ratio of component

• Temperature and humidity

• Shelf life

• Storage

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

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

Chapter 20: Infection Control and Safety in the Dental Office

Introduction

• Risk

• Safety

• Ionizing radiation

Infectious Diseases

• Infection control

• Personal protective equipment and barriers

Personal Protective Equipment and Barriers

• Gloves

• Handwashing

• Preventing or managing latex irritation

• Facial masks and shields

• Eye protection

• Protective attire

Preventing or Managing Latex Irritation

• Latex allergy in dental staff

• Latex allergy in patients

Eye protection

• Hazards

• Equipment

• Patient protection

• Curing light hazards

• Lasers

Instrument Sterilization and Surface & Equipment Disinfection

• Degree of contamination

• Critical objects

• Semi-critical objects

• Noncritical objects

Other Aseptic Techniques

• Distribution of supplies and materials

• Bulk supplies

• Kits

• Unit dose

• Use of disposable items

Managing Aerosol and Splatter

• Rubber dam

• Pretreatment mouthrinse

• Pretreatment toothbrushing

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

• Sharps containers

• Recapping needles

Office and Laboratory Housekeeping Practices

• Eating and cosmetics

• Food storage

• Separate contaminated items

• Infectious wastes

• Exposure control plan

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

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

Final Thoughts

• Emergencies

• Putting risk in perspective

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

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

Disinfection Techniques

• Disinfection of an impression by immersion

• Disinfection of an impression by spraying

Disinfecting Dentures and Other Appliances

• Chairside adjustments

• Clean the prosthesis

• Disinfect the prosthesis

• Rinse the prosthesis

• Store the prosthesis

Chapter 22: General Rules for Handling Dental Materials

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

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

Dispensing Materials

• Dispense as directed by instructions

• Dispense equal lengths, not volumes, of pastes

• Dispense consistent drops

• Fluff powders

Mixing

• Mix aggressively

• For cements, force the powder into the liquid

• For some materials, setting rate depends on the mixing method

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

Contamination

• By oral fluids, is bad for materials

• All adhesive restorative materials

• Keep teeth clean, and sometimes dry when using adhesive materials

Summary

• Follow and save manufacturer’s directions

• Manipulation factors:

• Proper dispensing

• Correct mixing

• Adequate curing

• Avoiding contamination

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

1. The 1st statement is true; the 2nd statement is false

2. The 1st statement is false; the 2nd statement is true

3. Both statements are true

4. Both statements are false

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,

1. Alginate

2. Polysulfide

3. Compound

4. Wax Bites

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.

1. The 1st statement is true; the 2nd statement is false

2. The 1st statement is false; the 2nd statement is true

3. Both statements are true

4. Both statements are false

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

1. Place the prosthesis back on the patient’s stone cast

2. An individualized, plastic closed container for each patient, similar to those used for overnight soaking

3. A zippered plastic bag

4. A zippered plastic bag containing sterile water

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

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

1. Protect the patient from the laboratory personnel

2. Protect the laboratory personnel from the patient

3. Prevent cross- contamination

4. Prevent contamination of equipment in the dental laboratory

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

1. Office personnel don’t have the necessary time available to spray thoroughly

2. The spraying procedure must be performed twice, with ten minutes in between

3. Constant contact of the spray with all surfaces of the impression cannot be assured

4. The aerosol product is usually toxic