Exam 1 Flashcards
Dental Materials
materials that are specifically designed for use in dentistry
One classification system for dental materials groups them in the following way
Preventive materials (ex. Sealants- cavities, fluoride- prevent caries)
Restorative materials
Auxiliary materials
Biomaterials
Preventive Dental Materials
Materials that slow or prevent the onset and progression of disease
Pit and fissure sealants
Materials that release therapeutic agents such as fluoride
Cements
Liners and bases
Glass ionomers (GI) restorative materials (release fluoride- change chemistry of mouth- in US used as preventative, not standard care due to history of it; other countries used as restorative)
Materials that release remineralizing agents such as casein phosphopeptide (CPP) and amorphous calcium phosphate (ACP)
Restorative Dental Materials
Consist of synthetic components that can be used to repair or replace damaged or missing tooth structure
Includes metals, polymers, ceramics and composites
Can be further subdivided into:
Direct restorative materials (amalgam; directly on tooth)
Indirect restorative materials (crown- made in a lab somewhere; use something else to be able to bond crown on prep)
Temporary (provisional) restorative materials (before permanent crown)
Direct Restorative Materials
Cements, metals or resin-based composite materials that are placed, formed, contoured and finished (cured) INTRAORALLY to repair disease, restore damaged teeth or improve esthetics
Two of the most commonly used direct restorative materials include dental amalgam and dental composite
Indirect Restorative Materials
A ceramic, metal, metal-ceramic or resin-based composite material that is fabricated EXTRAORALLY on casts or using other means (already cured), and repairs damaged teeth, replaces missing teeth or improves esthetics
Includes crowns, ceramic veneers, inlays, onlays, and composite restorations fabricated using indirect methods
Cemented (luted)
Temporary (provisional) Materials
Materials used to restore teeth for a short period of time, ranging from a few days to more than a year, with the understanding that they are to be replaced with more durable restorative materials.
Indications:
While a definitive restoration is being fabricated
When active disease is present (e.g. perio, caries)
To guide auxiliary procedures (e.g. perio, ortho)
Symptomatic teeth with unclear pulpal health
Esthetic and trial restorations, when substantial corrections to the occlusion or tooth position are part of the care plan
Definitive restorations
Temporary dental materials; restorative services that are provided to serve as final, long-term treatment
Prevent to go farther in the pulp (restorative) that may need a root canal for example
Auxiliary Dental Materials
Materials that are used to fabricate restorations, but do not become a part of the restoration
Examples: Dental stone Dental waxes Impression materials Tray and mouthguard acrylic Gypsum and phosphate-bonded investments Finishing and polishing abrasives Etc.
Biomaterials
Any material that interacts with biological systems
Generally, biomaterials are used to restore, maintain or improve tissue and organ function
Using biomaterials to repair, shape or direct the growth of host tissues is called tissue engineering
Deeper interaction ; injection in root canals- pulp grows back (working on it?) ; most of them are used in clinical trials- quite new
Tissue engineering
Employs several strategies for repairing tissues:
Injection of cells
Guided Tissue Regeneration
Cell induction
scaffolds (help to grow bone for example)
Injection of cells
Tissue engineering; Stem cells (progenitor cells) are capable of forming new tissue with one or more phenotypes
The stem cells are injected into the general vicinity of the site in which they are intended to propagate
The injected cells then migrate to the area of injury and replicate, thereby replacing lost tissue
ex. inject cells in pulp chamber
Guided Tissue Regeneration
Tissue engineering; A surgical procedure for regenerating tissue by enhancing the opportunity for one cell type to proliferate
In periodontal applications, a membrane is used to exclude unwanted cell types
Cell induction
Tissue engineering; Growth factors and developmental proteins are administered locally to induce progenitor cells to differentiate into desired tissues
Scaffolds
Tissue engineering; Promote new tissue formation by providing a surface and void volume that encourages the migration and proliferation of desired cell types
Many scaffolds are preformed and bioresorbable
Typically, scaffolds are seeded with progenitor cells that are allowed to attach and proliferate in vitro
After suitable time in vitro, the scaffold is grafted into a host site
The implanted scaffold must then look to the host vasculature for nutrient and metabolite exchange in order to survive
Gradually, the scaffold degrades (resorbed) until it is completely replaced by new tissues
Ex. 3 types of biomaterials studied as scaffolds and carrier systems: Natural (biological derived), ceramic or glass materials, polymeric materials
Ideal Dental Materials
Biocompatible (elicit a healthy biological response from the host)
Bond permanently to tooth structure or bone
Match the natural appearance of tooth structure and other visible tissues
Exhibit properties similar to enamel, dentin, and other oral tissues
Capable of initiating tissue repair, or regenerating missing or damaged tissues
People are living longer
Research and Industry Trends
People are living longer
Regenerative endodontic pulpal therapies
Periodontal therapies are needed that allow consumers to retain their teeth longer
Because people are retaining their teeth longer, consumer demands are shifting restorative care from replacement of teeth to long-term restoration and maintenance of teeth
Patients who are missing one or more teeth are requesting life-like replacement options
Research and Industry Trends
Dental implants (coating and surfaces, root form geometries, periodontal tissue response [perimucositis, perimplantitis, gingival recession]
Periodontal tissue and bone grafting materials
Patients are demanding more esthetic techniques and outcomes
Research and Industry Trends
Tooth whitening, ceramic materials, composite materials, invisalign
Patients are demanding shorter treatment times
Research and Industry Trends
CAD/CAM lab and chairside indirect restorations
Each tooth contains 3 specialized calcified tissues which are?
Enamel (mostly mineral), Dentin, Cementum (thin, only in the roots)
Enamel is the most highly calcified tissue in the body and contains the least organic content of any of these tissues. By weight, mature enamel is 96% inorganic material (mineralized), 1% organic material, and 3% water
Dentin, cementum, and bone are vital, hydrated, biological composite structures formed mainly from collagen type I matrix reinforced with calcium phosphate mineral (hydroxyapatite)
Composite
a collective term for materials that are
- Made from two or more constituent components with different properties
- When combined, the individual components remain separate and distinct in the finished product
- the composite has properties that are unique relative to the constituent materials
Enamel
Forms the outer shell of the anatomic crown of the tooth.
Dentin is joined to enamel at the dentinoenamel junction (DEJ).
Enamel is formed by cells called ameloblasts.
Enamel apposition begins at the DEJ and proceeds outward towards the surface of the tooth.
Enamel crystals
Enamel is made of very long hexagonal crystals about 40nm wide, but which span the entire enamel thickness. These crystals are then packed into enamel rods or prisms, that are about 5µm across
About 100 crystals of the mineral are needed to span the diameter of a prism- these prisms are easily revealed by acid etching, and are found in a closely-packed array
The individual crystals within a prism are further coated with a thin layer of lipid and/or protein that plays a role in mineralization; This protein coat appears to increase enamel toughness
The interface between prisms are the inter-rod enamel (contain the organic component of enamel, permit the flow of water and ions in enamel demineralization and remineralization, have physical properties essential for enamel bonding). These areas are known as prism sheaths
Enamel bonding
In enamel bonding, enamel is etched with phosphoric acid. Acid etching preferentially dissolves enamel crystals in each prism. In type I enamel etching, the prism core is preferentially etched
Creates mechanical retention; increases SA; removes junk on top of surface; due to hand instruments; breaks down part of enamel, dentin; cleans it up; creates pores- allows materials to have a better interaction
Must etch both surfaces (enamel and dentin)- used adhesives (bonding agent- have acid monomers that does the same)- need to clean it up to create better cross linking
In Type II, the prism peripherally is preferentially etched. Type III enamel etching results in a uniform or mixed pattern of etching. No difference in micro-mechanical bond strength of the different etching patterns has been established
As etching is established, the surface of the etched enamel develops a frosty appearance. This roughened surface provides a substrate for infiltration of bonding agents; After the bonding agents penetrate the roughened enamel surfaces, they are polymerized, forming a micromechanical bond to the enamel surface
Enamel structure
Enamel is not a tissue that is uniform in structure. Near the DEJ, enamel is commonly aprismatic, making it more difficult to etch
Similarly, fluoride concentration in surface enamel is significantly higher than subsurface enamel, making it more difficult to etch as well
The enamel mineral phase: hydroxyapatite
The mineral in all calcified tissues in the body is a highly defective version of the mineral hydroxyapatite (HA). HA has a hexagonal crystal structure, and the formula is Ca10(PO4)6(OH)2. Biological versions of apatites differ from ideal HA found elsewhere in nature in that biological apatite has many more defects and chemical substitutions. Defects and substitutions generally serve to make it less stable and more soluble in acids.
The variable composition of HA in enamel and dentin reflects its formative history and other chemical exposures during maturity. Enamel and dentin HA is calcium deficient, carbonate rich, and highly substituted (Mg and Na commonly substitute for Ca; Carbonate substitutes for the phosphate and OH groups) The most beneficial substitution is the fluoride ion (F-), which substitutes for the OH group; Applying fluoride changes HA- makes crystal stronger and less soluble in acids
Fluoroapatite Mineral
The most beneficial substitution is the fluoride ion (F-) which substitutes for the OH group; Formula: Ca10(PO4)6(F)2 which is less soluble than HA and more provides more protection against caries
Dentin
Complex hydrated composite structure composed of an organic matrix and a mineralized component, that forms the majority of the tooth; It is modified by aging, disease and other physiological processes resulting in a wide range of altered forms; The type of dentin you encounter performing restorative procedures may profoundly impact your ability to bond restorative materials to dentin; volumetrically dentin is composed of 50% carbonate rich, calcium deficient apatite, 30% organic material, primarily type I collagen, 20% fluid
The tubule lumen is lined by a highly mineralized cuff of peritubular dentin ~0.5-1µm thick. The peritubular dentin, also known as intratubular dentin by some, contains mostly apatite crystals and little or no organic (collagen) matrix; Tubules are separated by intertubular dentin, which has a composite structure, consisting of a matrix of type I collagen reinforced by apatite- this arrangement means that the volume of intertubular dentin in a given area of dentin will vary by location, because tubule density tends to increase as you approach the pulp from the DEJ
Odontogenesis
The development of teeth and their supporting structures from embryonic tissues; The cells responsible for dentin formation are called odontoblasts- they differentiate from the outer cells of the dental papilla in the Bell Stage of tooth development; the pulp develops from the central cells of the dental papilla; the basement membrane represents the location of the future DEJ; because dentin and pulp have similar embryonic origins, these 2 tissues are commonly referred to as the dentin-pulp complex
Odontoblasts continue to make dentin throughout the life of the cell; they can be found on the outer wall of the pulp, immediately medial to the advancing wall of dentin; Because odontoblasts rest just inside the dentin and line the walls of the pulp chamber, the dentin-pulp complex is considered to be a vital tissue
Dentin tubules
Tubules are a distinct feature of dentin; they represent the tracks taken by odontoblasts as they retreat from the DEJ and appear as tunnels, traversing the dentin from the DEJ to the pulp. The tubules converge as they approach the pulp chamber and therefore tubule density and orientation varies from location to location
pain is the only info they send to brain; close the the pulp wider; closer to DEJ, smaller (more difficult to report pain- more time to do restoration than root canal)
The content of the tubules are odontoblastic processes and fluid; the extent that the odontoblastic process occupies the tubules has not been conclusively established, but it is believed that it extends to the DEJ
Types of dentin
The morphology of dentin varies with location and is altered by aging and disease; Primary dentin is formed by odontoblasts before the tooth erupts while secondary dentin is formed by odontoblasts after tooth eruption; tertiary dentin is reparative dentin that forms quickly and in a very localized manner in response to trauma (caries, cavity prep, attrition, etc. ) [ it is only found under dentinal tubules that have been exposed to trauma, at the outer pupal wall
Transparent dentin (sclerotic) is a type of tertiary dentin that develops following trauma, and is characterized by the tubules being partially occluded with mineral (for pulp protection) ; many types of transparent dentin have been described; the nature of the mineralization is both heterogenous and complex, and has been described in detail elsewhere bonding is a problem because the tubules are closed- etching must be different]
Smear layer
When dentin is cut or abraded with a rotary instrument, a smear layer forms (junk on top); the smear layer is a collective term for any debris left on a tooth surface following tooth preparation; This debris consists of chips of enamel and dentin, and organic matter that are resting or burnished onto the surface or into the dentinal tubules; the smear layer serves to occlude the tubules and reduce dentinal permeability, thereby providing a protective effect; however it hinders dental bonding, so it should be either removed or modified before the bonding procedure
Acid etching
Also known as conditioning; removes the smear layer and alters the external lumen of tubules, allowing better infiltration by bonding agents; as dentinal surfaces are conditioned with acid, peritubular dentin is preferentially removed since it has no organic component. The tubule lumen widens with etching creating a funnel shape that can be less retentive due to its flared nature. Longer acid etching of dentin flares the tubule lumen even further
Etching transparent dentin
Transparent dentin has a different outcome in conditioning, because the tubules are occluded with mineral- this mineral in the tubules can be very heterogeneous in its response to acid conditioning, depending upon the stimulus which led to its formation; after etching transparent dentin, peritubular dentin is preferentially removed, but the tubules may continue to retain plugs of precipitated mineral which make bonding to transparent dentin even more difficult
Dentin conditioning
Dentin is a composite material, consisting of a mineralized component (HA), and an organic component (type I collagen); acid etching removes the smear layer; acid etching also removes the mineralized component of intertubular dentin, leaving a network of collagen fibrils; engaging this network of collagen fibrils and dentinal tubules with bonding agents forms the micro-mechanical basis for dentin bonding
An important element of dentin bonding technique is to keep the demineralized dentin moist; if demineralized dentin is dried, the dentin matrix shrinks and becomes matted, making the collagen fibrils and dentinal tubules difficult to penetrate with bonding agents
Physical and Mechanical properties of enamel and dentin
Look at slide 73 on PP 1
The DEJ Magne and Belser
“The assembly of two tissues with distinctly different elastic moduli requires a complex fusion for long term functional success. Stress transfer in simple bilaminate structures with divergent properties usually induces increased focal stresses at the interface.”
“If enamel and dentin at the functional surfaces of a tooth comprised such a simple bilaminate, enamel-initiated cracks would easily cross the DEJ and propagate into dentin.”
“In reality, the situation is quite different…multiple enamel cracks…seldom affect the structural integrity of the enamel-dentin complex.”
“The explanation lies in…a complex fusion at the DEJ, which can be regarded as a fibril-reinforced bond.”
“[At] the DEJ…parallel, coarse collagen bundles (probably von Korff fibers of mantle dentin) form massive consolidations that can divert and blunt enamel cracks through considerable plastic deformation.”
“The structure of the DEJ shows two levels of scalloping, which [serves to] increase the effective interfacial area and strengthen the bond between enamel and dentin. The scalloping is most prominent where the junction is subject to the most functional stresses.”
DEJ
The morphology of the DEJ begins at the earliest developmental stage of the tooth crown; Any other sequence would not allow the creation of such a complex dentinoenamel fusion;
During odontogenesis, before the cells of the inner enamel epithelium (IEE) differentiate into preameloblasts, and the outer cells of the dental papilla differentiate into odontoblasts, they are separated by a basement membrane (represents the future location of the DEJ)
Once the cells of the IEE differentitate into preameloblasts, they stimulate the outer cells of the dental papilla to differentiate into odontoblasts. The odontoblats then begin secreting dentin matrix on their side of the basement membrane, retreating into the central cells of the dental papilla as matrix and dentin is formed
At this point, the basement membrane disintegrates, preameloblasts differentiate into ameloblasts, and the ameloblasts begin secreting enamel matrix. The junction of enamel and dentin is established as these 2 hard tissues begin to form
Before enamel forms, some of the developing odontoblastic processes extend into the ameloblast layer; When enamel formation begins, these odontoblastic processes become trapped as enamel spindles. Von Korff fibers in mantle dentin also cross the DEJ into enamel to further integrate these tissues
DEJ photos
cracks in enamel do not extend into dentin but instead stop at the scalloped DEJ
Have large scallops in molars, but smaller scallops in anterior teeth
Summary of Oral Hard Tissues
Enamel and dentin are oral hard tissues with profoundly different morphological and mechanical properties
Bonding dental materials to enamel and dentin is technique-sensitive and requires treating each as a unique tissue
Enamel and dentin bonding is micromechanical
Natural teeth, through the optimal combination of enamel and dentin, demonstrate the perfect and unmatched compromise between stiffness, strength and resilience
Restorative procedures and alterations in the structural integrity of teeth can easily violate this balance
What hostile environments are restorative dental materials subjected to?
pH Saliva Chewing- occlusion Mechanical loads- depending where food is at (temp too)
Research is necessary to develop products that are clinically appropriate materials
Must understand the properties of the different materials
Polymers- composites (powder/liquid mixed –>activated; light cure)
Ceramics
Metals
In order to conclude that a material is appropriate for clinical use we need to evaluate all kind of properties.
In vitro research is just a simulation of what we might get clinically … So… to be able to have a clinical relevance we need to set up clinical trials.
In vitro research allow us to standardize measures to compare materials and guiding the interpretation of clinical trials.
In the lab we try to simulate as much as we can the clinical conditions.
-Size
-Shape
-Conditions (temperature, proportions, humidity, etc)
When we restore a (caries, fractured) tooth it is important to select a materials that is adequate, but we need to take into account the quality of the substrates (enamel, dentin, cementum, proximity to the pulp) that we will restore
Different material properties
Mechanical Thermal Electrical Electromechanical Optical
Mechanical Properties
When we restore a tooth, we expose that restoration to challenges:
chemical- acids (lemonade, OJ)
Thermal- different temp
Mechanical- wear
Force
One body interacting with another generates a force.
Applied
- Contact of the bodies
- Distance (Eg. Gravity)
Results of Force
Deformation (rigid or deformable and constrained)
- Constrained (no movement or translate) = deformation or change its shape - Free of constrains = movement or translate
Occlusal forces
The force applied on opposing teeth when the jaws are closed; posterior teeth are stronger (surface area, anatomy, muscles)
Max occlusal forces ranges from 200- 3500N
Occlusal forces are increased in posterior regions and decreased in anterior regions.
- Forces on 1st and 2nd molar vary from 400- 800 N.
- Average forces on the bicuspids, cuspids, and incisors is about 300, 200, 150 N respectively
Children growing up show an increase in force from 235- 494 N with an average increase yearly of about 22N
Forces on restorations
Patients with dental prosthetic devices decrease occlusal forces. about 65-235 N (partial dentures), about 100 N molars and bicuspids (complete dentures) and about 40 N incisors (complete dentures)
dentures cause face changes- lose bone therefore lose strength
Age and gender variations on occlusal forces
Women occlusal forces is about 90 N less than men; facial form and muscles can be predictors of occlusal forces (low angles and square mandibular form=high occlusal forces)
Males stronger than female; female have a more delicate facial structure; some people have strong masseters (usually square shaped)
Changes of max occlusal forces
Max occlusal forces and response of underlying tissues changes with:
- Age
- Anatomic location
- Occlusal Scheme (contacts that upper teeth have with lower teeth; as the form and the arrangement of the occlusal contacts in natural and artificial dentition)
- Placement of dental prosthesis (removal vs permanent)
Stress
When a force acts on a constrained body, the force is resisted by the body and the internal reaction is equal in magnitude and opposite in direction to the applied external force and is called stress (S or σ)
The applied force and internal resistance (stress) are distributed over an area of the body
stress= Force (N)/area
- Pascal (1Pa -1N/m2=1MN/mm2)
- mega Pascals (MPa) or millions of Pascals = 1MPa = 106 Pa
Types of stress
A force can be applied from any angle or direction
Complex stress= several forces
It is rare for forces and stress to be isolated to a single axis
Individually applied forces can be defined:
Axial- applying load along the axis (occlusion)
- Shear- removal of bracket; like cut - Bending- - Torsional- endo files
All stresses can be resolved into combinations of 2 basic types: axial and shear
Tension stress
Results from 2 sets of forces directed away from each other in the same straight line; bonding agents (hydrophobic/ hydrophilic)?
Compression stress
results from 2 sets of forces directed toward each other in the same straight line
Axial- Compression stress
Compress or shorten
Axial- tensile stress
stretch or elongate
shear stress
2 sets of forces directed parallel to each other, but not along the same straight line
A stress that tends to resist the sliding or twisting of one portion of a body over another
Ex. orthodontics bracket removal ; like cutting?
Torsion stress
Resulting from the twisting of a body ex. endo files
bending or flexural
results from an applied bending movement
Modulus of rupture, bend strength or fracture strength is a material property, defined as the stress in a material just before it yields in a flexure test.
Depending when apply the load you can have different types of stress
Elasticity of solid bodies
Compression- resist to be forced more closely
shear- one portion of the body must resist sliding past another
Strain
Deformation in a body due to stress
Strain = ε = change in the length (ΔL= L-Lo)
Strain (ε)= Deformation/Original Length
= (L-Lo)/Lo= ΔL/Lo
It is reported as a percentage
*Lo=Change in length
Stress- Strain Curve
Amount of deformation (strain) at distinct intervals of tensile or compressive loading (stress)
If a bar of a material is subjected to an applied force (F) the magnitude of the force and the resulting deformation (δ) can be measured….. But we can apply the same F to another bar with different dimensions and that other bar will suffer different force-deformation characteristics.
If the applied F is normalized by the cross-sectional area (A) of the bar (stress) and the deformation is normalized by the original length of the bar (strain), the resulting stress-strain curve is independent of the geometry of the bar
It is better to report the stress-strain curve of a material rather than the force-deformation characteristics.
Stress-strain relationship = measure load and deformation and then calculate the corresponding stress-strain
Universal testing machine
If we assumed that the cross-sectional area of a specimen remains constant during the testing then S-ε curve is engineering S-ε
Stress- Strain curve Example
- Look at graph
A=PL proportion limit- will come back to original (marble will have a higher vs glass) – stress and strain are linearly proportional [after exceed PL can not come back (deformation); straight line)
B=yield point (constants: 0.1, 0.2. 0.3, 0.5 [doesn’t change the results]; most common is 0.2)- way to calculate in a more accurate way- (constant of the behavior of the material) starts acting like a plastic material
Difficult to calculate proportional limit (arrrow in graph)
C= ultimate tensile strength- max stress before failure
D= fracture point (strength)
Elastic limit is not known on this graph
Elastic region- can return to original shape, size
Plastic- will never return to initial dimensions
Proportional Limits on Stress Strain Curve
Plastic deformation from PL to failure point (FP)
The value of the stress at A is known as proportional limit (PL)
- (Spl or σpl) below the PL = no permanent deformation, material is elastic in nature.
After PL= permanent deformation, irreversable strain occurs = plastic region = non-linear
Super elastic materials (exception)
Elastic Limit on Stress Strain Curve
Elastic deformation it is up to the proportional limit (PL)
(SEL or σEL)
Is the maximum stress that a material will withstand without permanent deformation
Elastic limit and proportional limit differ in fundamental concepts
-Elastic limit: describes the elastic behavior of the material region)
-Proportional limit: deals with the proportionality of strain to stress in the structure (slope)
***So they are different values
Yield strength (YS or σY) on stress strain curve
The stress at which the materials begin to function in a plastic manner
At YS a small defined amount of permanent deformation has occurred in the material.
Amount of permanent strain is indicated as 0.1%, 0.2% (often) or 0.5% = offset
Stiff material (small elongation) will have a greater offsets than material with larger elongation or deformation. Elastic limit and yield strength define the transition from elastic to plastic behavior
Ultimate strength on stress strain curve
Ultimate tensile strength or stress (UTS): Maximum stress that a material can withstand before failure in tension
Ultimate compressive strength (UCS): Maximum stress a material can withstand in compression before failure in compression
Fracture strength or fracture stress
Point at which a brittle material fractures
if the material does not fracture at the point at which the max stress occurs they might elongate resulting in necking
Elongation
Deformation as a result of the application of a tensile force.
Divided in 2:
1. Increase in length of specimen below the PL (0-A) = not permanent
2. Increase in length of specimen after PL (A-D) = permanent
Total elongation is reported as a %
%Elongation = (Increase in length/original length) *100%
An alloy with high value for total elongation can be bent permanently without danger of fracture
Elastic modulus or Modulus of elasticity or Young modulus
The measure of elasticity of a material is descirbed by the term elastic modulus = E; modulus = ratio.
E= stiffness of a material within the elastic range.
E= stress/strain; E=S/ε
Units GPa or MPA (1GPa=1000MPa)
To calculate E select 2 stress and 2 strains coordinates in the elastic or linear range .
The slope is therefore:
(σ1 -σ2 )/(ε1- ε2)
Relative rigidity or stiffness of the material within elastic range
Young’s modulus of elasticity
Elastic modulus = Stress/Strain
Poisson’s Ratio
During axial loading in tension or compression there is a simultaneous strain in the axial and transverse or lateral directions.(Relative change in lateral dimension and elongation in the longitudinal direction)
Tensile loading: as material elongates in load direction = reduction in cross-section known as “necking” during elastic deformation and deformation continues until the material is fractured.
Compressive loading: increase in cross sectional within elastic range, the ratio of the lateral to the axial strain is called “Poisson’s Ratio”.
Is unitless value because it is ratio of two strains
Ductility
Ductility: Its ability to be drawn and shaped into a wire by means of tension ex. Orthodontics
Eg. Gold and silver: are used extensively in dentistry because are the most malleable and ductile materials
Malleability
Malleability: Its ability to be hammered or rolled into thin sheets without fracturing
Eg. Gold and silver: are used extensively in dentistry because are the most malleable and ductile materials
Resilience
Resistance of a material to permanent deformation
Is an indicator of the amount of energy necessary to deform the material to the PL.
Area under the elastic portion of the S-εcurve
Units: mMN/m3 or mMPa/m
Ex. Mouth guards- need resilient material
Toughness
Resistance of a material to fracture
Indication of the amount of energy necessary to cause fracture.
Units: mMN/m3 or mMPa/m
includes all aspects of the SS curve
Fracture toughness (KIc)
The ability to be plastically deformed without fracture, or the amount of energy required for fracture.
It is proportional to the energy consumed in plastic deformation
Stiffness
Is the RIGIDITY of an object
The extent to which it resists deformation in response to an applied force
The more flexible an object is less stiff
Properties and S- E curves
The slope of S-ε curve and the magnitude of the stress and strain allow classification of materials with respect to their general properties
The properties of stiffness, strength and ductility are independent
Materials may exhibit various combinations of these 3 properties
*** look at graphs on slide 52
Tensile properties of brittle materials
Brittle materials are difficult to test because they fracture easily. Grips Slow loading Axial tensile loading Low stress on the material
Viscosity
The mechanical properties of many dental materials (alginate, elastomeric, waxes, amalgam, polymers) and substrates (bone, dentin, etc) depend on how fast they are loaded
Materials with mechanical properties dependent of loading rate and with viscous and elastic behavior are = Viscoelastic
Materials with mechanical properties independent of loading rate = Elastic (strain is produced when load is applied)
Other materials exhibit a “Lag” response to load = Viscoresponse.
Fluid behavior and Viscosity
Same materials are solid dental materials that show some fluid characteristic or many dental materials, such as cements and impression materials are in the fluid state when formed
Therefore, fluid (viscous) phenomena are important.
Viscosity (η) is the resistance of a fluid to flow and is equal to the shear stress/shear strain rate
Or,
η = τ/[dε/dt]
Units: poise, p (1p= 0.1 Pa; S= 0.1Ns/m2) or centipoise (100cp = 1p)
Viscoelastic materials
In this case the strain rate can alter the stress-strain properties
Eg. Tear strength of alginate increases ≈ 4 times when rate of loading is increased from 2.5 to 25 cm/min (remove quickly from mouth to increase tear resistance)
Dynamic Mechanical Properties
Involves cycling loading or loading at high rates (impact).
Dynamic Modulus (ED)
The ratio of stress to strain for small cyclical deformations at a given frequency and at a particular point on the stress-strain curve.
ED=mqp2
m= mass of the loading element
q= height/twice the area of the cylindrical specimen
p= angular frequency of the vibrations
Ex. endo files
Hardness
Surface Mechanical Properties
Resistance to permanent surface indentation or penetration
-A measure of the resistance to plastic deformation and is measured as a force per unit area of indentation
Friction
Surface Mechanical Properties
The resistance between contacting bodies when one moves relative to another.
The friction force (Fs) is proportional to the normal force (FN) between the surfaces and the (static) coefficient of friction (μs)
Fs=μs FN
Wear
Surface Mechanical Properties
Is a loss of material resulting from removal and relocation of materials through the contact of two or more materials
Types of wear: Adhesive wear Corrosive wear Surface fatigue wear Abrasive Wear
Examples:
Metals: are susceptible to adhesive, corrosive and three body wear
Polymers: are susceptible to abrasive and fatigue wear
Adhesive wear
formation and disruption of micro junctions.
Corrosive wear
Secondary to physical removal of protective layer and it is related to the chemical activity of the wear surfaces (common in metals)
Surface fatigue wear
free particles with small areas of contact contribute to high localized stress and produce surface cracks.
Abrasive Wear
Involves a harder material cutting into a softer material ex. crowns
The colloidal state
Colloid is used now to describe a state of a matter rather than a kind of matter.
The main characteristic of colloidal materials is their high degree of microsegmentation.
These fine particles also have certain physical properties, such as electrical changes and surface energies that control the characteristics of the colloids.
Nature of colloids
Substances are called colloids when they consist of two or more phases, with the units of at least one of the phases having a dimension slightly greater than simple molecular size.
Size ≈1 to 500 nm in maximum dimensions
Colloidal state represent a highly dispersed system of the fine particles
They can be fine dispersions:
-Gels (hand sanitizer) – Films – Emulsion - Foams
Typical Colloid Systems
Divided in Sol and Gel
Sol resembles a solution, it is made up of colloidal particles dispersed in a liquid. When we add suitable chemicals the Sol can be transformed to GEL = semisolid, jelly like quality.
The liquid phase of a GEL or Sol is usually water or an organic liquid such as alcohol.
When it is water = hydrosols or hydrogels = hydrocolloids
Ex: In dentistry (alginate gels)
When the system has an organic liquid as a component = organosol or organogel.
Diffusion through membranes and Osmotic pressure
Osmotic pressure is the pressure developed by diffusion of a liquid or solvent through a membrane.
Osmotic pressure has been used to explain the hypersensitivity of dentin.
The changes in pressure in carious, exposed dentin from contact with saliva causes diffusion throughout the structure that increases or decreases the pressure on the sensory system.
Adsorption
A liquid or gas adheres to the surface of a solid or liquid firmly by the attachment of molecules, decreases their surface free energy
Ex. Alginate (viscoelastic material)- don’t want it too wet; external
Absorption
The substrate diffuses into the solid material by a diffusion process, and the process is characterized by concentration of molecules at the surface (Eg. alginate = swelling)
fuses into- in alginate
Sorption
When adsorption and absorption exist at the same time and it is unknown which one predominates.
surface tension and wetting
Measured in terms of force (dynes) per centimeter of the surface of liquid.
Surface tension is a contractive tendency of the surface of a liquid that allows it to resist and external force.
The wetting power of a liquid is represented by its tendency to spread on the surface of the solid. And it measure by the degree to which something can be wet.
Color
The perception of color is the result of a physiological response to a physical stimulus.
The sensation is a subjective experience, whereas the beam of light, which is the physical stimulus that produces the sensation is entirely objective
color of an object is determined on the spectrum and the intensity of the incident light on it and the changes of the light in contact with it as well. Consequently, the color is determined on the reflected light composition (wavelengths). A body, reflecting particular spectrum range to the white light and absorbing the rest, is colored with the color of the reflected light
Grassman’s Law
The eye can distinguish differences in only 3 parameters of color.
These parameters are dominant wavelength, luminous reflectance, and excitation purity.
The dominant wavelength (λ) of a color is the wavelength of a monochromatic light that, when mixed in suitable proportions with an achromatic color (gray), will match the color perceived.
Light
Electromagnetic radiation resulted in changes of the state of electron cover, including ultraviolet, infrared and x ray radiation
Dual character
steam of material particles- photons wide spreading as electromagnetic wave
Visible light
has wavelength in a range of ~380 nm to about 760 nm with a frequency range of ~405THz to 790 THz
