Physical properties of dental materials

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Physical properties of dental materials presented byH.Mani Bernard 1st year P.G Physical properties are based on the laws of mechanics, acoustics ,optics, thermodynamics, electricity , atomic structure or nuclear phenomena.

(anusavice)Definition3Aphysical propertyis anypropertythat ismeasurablewhose value describes a state of aphysical system. The changes in the physical properties of a system can be used to describe its transformations or evolutions between its momentary states. Physical properties are often referred to asobservables.ABRASION AND ABRASION RESISTANCEVISCOSITYSTRESS RELAXATIONCREEP AND FLOWOPTICAL PROPERTIESTHERMAL PROPERTIESELECTRICAL PROPERTIESTARNISH AND CORROSIONSOLUBILITY AND SORPTIONSHELF LIFE.CONTENTSABRASION:Loss of material from a surface caused by a mechanical action.Abration is a complex mechanism in the oral environment that involves an interaction among numerous factors

MAJOR FACTOR :Hardness of the material

OTHER FACTORS :Biting forcesFrequency of chewing Abrasiveness of the diet Surface impuritiesRoughnessSurface irregularities ABRASION AND ABRASION RESISTANCEAbrasion is further divided into processes of two body and three body wear.

Two body abrasion-abrasive particles bonded firmly to abrasive instrument.

Diamond bur is an classic example of two body wearThree body abrasion-abrasive particles free to move between two surfaces.

Non bonded abrasives dental prophylaxis pastes are three body abrasives.

They are placed in the rotary cup,which is rotated against a tooth TWO BODY ABRASIVES DENTAL BURS ABRASIVE DISCS

THREE BODY ABRASIVEThe Moh's (Mohs) scale of hardness is the most common method used to rank gemstones and minerals according to hardness.Mohs hardness refers to a material's ability to resist abrasion or scratching.this scale grades minerals on a scale from 1 (very soft) to 10 (very hard) MEASUREMENT OF HARDNESS FOR ABRASIVES

Mohs Scale of HardnessIt should be irregular in shape so that it presents a sharp edge.

It should be harder than the work it abrades.

DESIRABLE CHARACTERISTICS OF AN ABRASIVE :Size of the abrasive particle- larger the size-greater the abration.

Pressure of the work against the abrasive.

Speed at which the abrasive particles travels across the work.Greater the speed ,greater the rate of abration.FACTORS AFFECTING RATE OF ABRASION:Ceramics are more abrasive to tooth enamel in the presence of high biting forces and rough ceramic surface.

MANAGEMENT: Broader contact areas polished surfacesClinical implication RHEOLOGYRheology is the study of flow characteristics of materials.

For liquids, flow is measured by the viscosity, where as for solids one considers creep and viscoelasticity. defined as resistance of a liquid to flow.

it is controlled by internal forces with in the liquid.

viscosity is a measure of consistency of the liquid and its ability to flow.VISCOSITY

Types of Fluid Behavior1. Newtonian (ideal) LiquidsThey are liquids with a constant viscosity regardless of shear rate. Examples include water and newly mixed zinc phosphate cement.2. Plastic LiquidsThey are rigid until a certain yield stress is applied to them.The application of this yield stress to cause flow in a plastic material is called the Bingham characteristic.

3. Pseudoplastic (shear-thinning) LiquidsShow a decrease in viscosity with an increase in shear rate. Examples include the polycarboxylate cements and non-water mixed glass ionomer cements.4. Dilatant (shear-thickening) LiquidsShow an increase in viscosity with an increase in shear rate. Traditional (conventional) resin composites are a good example.5. Thixotropic LiquidsThey are liquids whose viscosity depends upon their previous shear history. In other words, if you shear them, their viscosity will decrease; if you then allow them to remain undisturbed, their viscosity will increase to pre stressedlevels. Examples include topical fluoride gels. The opposite of thixotropic is rheopexic.

NewtonianThixotrophicDilatantPseudoplasticVISCOELASTISITY:

Describes materials that exhibit characteristics of both a viscous liquid and an elastic solid. Viscoelastic materials, like alginate, show little permanent deformation when loaded quickly but exhibit a great deal of permanent deformation if loaded slowly. Amount of permanent deformation of elastomeric impression material minimised by

Removing the impression after material adequately gelled Negligible pressure to tray during polymerizationRemoval of impression along path of loading of trayAvoid undercuts during tooth preparation 22Clinical Applications of RheologyThe aqueous and nonaqueous elastomeric impression materials, being viscoelastic, should be removed from the mouth with a snap to minimize permanent deformation.

Ideally, we want cements and light-body (syringe) impression materials to be pseudoplastic so that with increasing shear strain rate, they exhibit a decrease in viscosity.

The reason many highly viscous cements (e.g., polycarboxylates, non-water mixed glass-ionomer cements) form thin film thickness layers is because they are pseudoplastic.

Solid - state diffusion process driven by a thermal energy ,atoms can move back slowly to their equilibrium positions. As a result material wraps or distorts due to trapped internal stresses.

clinical application: Non crystalline materials like waxes, resins and gels, when manipulated and cooled, can then under go relaxation at an elevated temperature.STRESS RELAXATIONCREEP AND FLOW Creep is defined as time dependent plastic strain of a material under a static load or constant stress. Slippage of molecules over each other is called FLOW.

Most metals do not creep as they have high melting temperatures, an exception is amalgam as it has components with melting points only slightly above room temperatures.The term flow has been generally used in dentistry to describe the rheology of amorphous materials such as waxes. The flow of wax is a measure of its potential to deform under a small static load.

IMPORTANT GOAL OF DENTISTRY-restore color and appearance of natural dentition. Ideal general purpose,direct filling,tooth colored material is one of the challenges of current dental materials researchCOLOURoptical propertiesLIGHT-electromagnetic radiation detected by human eye.

The eye is sensitive to wavelengths from approximately 400 nm(violet) to 700nm(dark red).

MOST SENSITIVE-55Onm[green-yellow]LIGHTWhen direct light falls upon an object, it is either absorbed, reflected, or transmitted. The spectral distribution of transmitted or reflected light resembles that of incident light, although certain wavelengths are reduced in magnitude.

Direct lightOPACITY AND TRANSLUCENCY As light strikes a surface it is either totally reflected (opaque), totally transmitted (transparent) or a combination of both (translucent).

Properties of colorTo accurately describe our perception of a beam of light reflected from a tooth or restoration surface, 3 variables must be measured. Color can be described in three dimensional color space measured as 1.HUE 2.VALUE 3.CHROMA

Dimensions of colorHUE-dominant color of an object.EXAMPLES-red,green,blue.Hue represents the dominant wavelengths present in the spectral distribution.

Hue In the Vita shade guide there are 4 hues'A" for reddish brown'B" for reddish yellow'C" for' grayish'D" for reddish-grey

The primary source of natural tooth colour is dentine and its hue either in the yellow or yellow-red range.

value

On a scale of black to white, white has" high value", black has "low value" and midway between the black and white is medium grey

Value is the only dimension of color that can exist by itself.

Chroma represents the degree of saturation of a particular hue.

In teeth, it is dictated by the dentine and influenced by the translucency and thickness of enamel.

chroma

MUNSELL COLOR CO-ORDINATE SYSTEM

Developed by- Albert Henry Munsell - 1915

L represents the value of an object.*a* represents red green axis*b* represents yellow blue axis CIE L*a*b color chart

Clinical implication Three factors-consideredBezold-brucke effectMetamerismflourescenceShade selection As the brightness become too intense color appears to change.

Patient is visualised against an intense colored background,a tooth shade may be selected with a hue that is shifted somewhat towards the complementary color of the back ground color.

Bezold-brucke effectMETAMERISM

The objects that appear to be color matched under one light source may appear different under another light source. This change in color perception is referred to as metamerism.

Spectral distribution of light reflected from or transmitted is dependent on the spectral content of incident light. The appearance of an object is quite dependent on the nature of light by which the object is viewed.

METAMERISMFLUORESCENCE

Natural teeth absorbs light at wavelengthsToo short to be visible to the human eye in the range of 300 to 400 nm referred to as near-ultraviolet radiation

Absorbed energy is converted in to light with longer wavelengthsThis phenomenon is called florescence

This property contributes to the vital appearance of teeth.

PRINCIPLES OF SHADE MATCHINGDetermine shade at the beginning of the appointment Clean the teeth and remove all stains and debris

A shade guide sample should be applied parallel with the tooth

Obtain value levels by squinting.After value select chroma and then hue.

Tooth should not be observed for not more than 5 seconds at a time.Dental operatory-common sources of light used are: 1.day light 2.incandascent lamps 3.florescent lampsColor matching should be done under 2 or more different light sources,one of which should be day light.

SOURCE OF LIGHT

Shade Guides Because an important factor in restorations is the shade determination, it is imperative that guides are available to assist in the correct shade matching. VITA Lumin Vacuum Shade Guide contains porcelains that are pre-fired set out in the colour tones that the company has available. They are arranged on porcelain pins that can be removed individually for maximum shade matching. The main tones groups are rose-brown, rose-white, greytone, and rose-grey. VITA VMK Individualised Shade Guide is a kit for the reproduction of intermediate and individual shades, and therefore demonstrates the offering of a multitude of possibilities of shade determination of the companies metal ceramics. This is the main communication method of shade determination between dentist and technician. The porcelains are layered onto a ceramic cap which corresponds in colour to the oxidised metal. These are also excellent for practice.

How do you determine the tooth shade with VITAPAN 3D-MASTER?The logical configuration of the shades makes working with VITAPAN 3D-MASTER very easy.

The shade selection is a logical progression of three simplified choices - the desired shade is found very quickly:In the first step of the shade taking procedure the value (lightness) is determined. Select the value level from the five value groups (levels 1 - 5) that is closest to the value of the tooth to be compared. Pull out the medium shade sample (M) from the selected value group. In the second step the chroma (levels 1 2,3) is determined. Select the color sample of the selected M group that is closest to the tooth to be compared. In the third step the hue (L, M, R) is determined. Check whether the natural tooth displays a "more yellowish" (L) or "more reddish" {R) shade than the color sample of the M-group that has been selected in the second step. Now the best matching shade sample is determined and the information is recorded in the color communication form.

VITAPAN 3D-MASTER divides the tooth color space into 5 levels of value.

In the medium (M) hue there are three levels of color samples for the chroma. Deviations towards the more yellowish hues (L) or more reddish hues (R) exist in 2 chromas.

A deviation of the medium hue (M) towards yellow or red can be compared with the corresponding hue groups (L,

Value (lightness) levelIn the first step the value or lightness is determined.

ChromaIn the second step the chroma or color saturation is determined.

HueIn the third step the hue is verified. RECENT ADVANCES IN SHADE SELECTIONBasic designAll color-measuring devices consist of a detector, signal conditioner, and software that process the signal in a manner that makes the data usable in the dental operatory or laboratory. Because of the complex relationship between these elements, accurate colorimetric analysis is difficult at best.The first system to combine digital color imaging with colorimetric analysis was introduced by Cynovad (Saint-Laurent, Canada).The recent devices works on the principle of - Colorimeter- Spectrophotometer- Digital color analyzerCan transfer esthetic plan from dentist to labComprehensive optic shade communicationIt displays the patient tooth in terms of Lightness,chroma&Hue

Currently Available Shade matching devices SPECTROSHADE

Patended windows based system

Allows taking images in any lighting.

Shade eye NCC SHADE SCAN

SHADE RITE DENTAL VISION

THERMAL PROPERTIESThermal conductivity is the rate of heat flow through a material occurs through the interaction of crystal lattice vibrations,motion of electrons and interaction with atom. The thermal conductivity (k) or coefficient of thermal conductivity is the quantity of heat in calories/second that passes through a specimen 1 cm thick having a cross sectional area of 1cmsq when the temperature differential between the surfaces perpendicular to the heat flow of the specimen is 1c.

THERMAL CONDUCTIVITYmaterialsThermal conductivity[W.m .k]1.enamel0.872.dentin0.593.silver3854.copper3705.water0.55materials with high thermal conductivity conductors.

Materials with low thermal conductivity-insulators.

SI unit-watts per meter per degree kelvin.

Such filling material as cements are relatively poor conductors and insulate the pulp area.

THERMAL DIFFUSIVITY

It is a measure of the rate at which a body with a non uniform temperature reaches a state of thermal equilibrium.The mathematical formula that relates thermal conductivity and thermal diffusivity is h=(k/Cp X P) K-thermal conductivity, Cp-temperature dependent specific heat capacity,P-temperature dependent density.SI Unit:mms

materialsThermal diffusivity mm sq/sec1.enamel0.472.dentin0.183.amalgam9.64.composite0.685.porcelain

6.Acrylic resin0.64

0.13Thermal diffusivity of different materialsThe specific heat of the substance is the quantity of heat required to raise the temperature of 1gm of the substance by 1 degree centigrade.

The specific heat of both enamel and dentin has been found to be higher than that of metals used for fillings .

During melting and casting procedures,specific heat of metal or alloy is important,the total amount of heat must be applied to the mass to raise temperature to the melting point.SPECIFIC HEATLINEAR COEFFICIENT OF THERMAL EXPANSION

The linear coefficient of thermal expansion is defined as the change in length per unit of the original length of a material when its temperature is raised by 1c.

MaterialsCoefficient x 10-6 / oCInlay waxes 350 450Silicone impression materials 210Polysulfide impression materials140Acrylic resin 76.0Mercury 60.6Composite resins Anterior 17 50Posterior 14 40Zinc oxide eugenol cement 35Amalgam 22.1 28.0Gold 14.4Porcelain

12.0

The high expansion coefficient of inlay wax is also important because it is highly susceptible to temperature changes.

For example,an accurate wax pattern that fits a prepared tooth contracts significantly when it is removed from the tooth or from a die in a hot area and then stored in a cooler area. This dimensional change is transferred to a cast restoration that is made from the lost wax process.

CLINICAL IMPLICATIONThermal stresses produced from the thermal expansion or contraction difference are also important in the production of metal ceramic restorations.

Consider a porcelain veneer that is fired to a metal substrate .it may contract to a greater etent than the metal during cooling,and induce tangential tensile stresses or tensile hoop..

Tensile stresses in the porcelain may cause immediate or delayed crack formation .

Although these thermal stresses cannot be eliminated completely,they can be reduced appreciably by selection of materials whose expansion or contraction coefficients are matched fairly closely(within 4%)Knowledge of coefficient of thermal expansion is essential because a tooth restoration may expand or contract more than the tooth during a change in temperature,thus the restoration may leak,or it may debond from the tooth.

When coefficients of thermal expansion of a restorative material and tooth structure are different,

PERCOLATION

i.e ingress and egress of fluids between the tooth and the restoration.

Polymerised resins have strong bonds but weak chain links,atoms can move freely vibrates in two direction perpendicular to long axis of polymer chain.ELECTRICAL PROPERTIESThe ability of a material to conduct electric current is called as conductivity or ,conversely ,as the specific resistance or resistivity.

The carious tooth has been observed to have a lower electrical resistance compared to normal tooth.Zinc oxide eugenol have high resistivity,followed by zinc polycarboxylate and zinc phosphate.GIC most conductive of cements close to dentin.ELECTRICAL CONDUCTIVITY Corrosion occurs whenever chemically different sites act as an anode and cathode. This corrosion requires the presence of an electrolyte,typically saliva.Electrochemical corrosion seen in dental amalgam is: 1.galvanic corrosion 2.crevice corrosion 3.stress corrosion

Galvanic corrosion associated with macroscopically different electrode sites.eg:Au &amalgam restorations.

Other corrosions occur microscopically. Cathode-residual amalgam alloy particles,anode-Sn-Hg or Cu-Sn in low copper & high copper amalgams.

TARNISH AND CORROSION CORROSION

Presence of dissimilar fillings in opposing or adjacent teeth

Fillings in conjunction with saliva,as an electrolyte,make up an electric cell

When the fillings contact each other,the cell is short circuted and there is a flow of current that occurs through pulp.this phenomenon is called galvanism.GALVANISMALL MATERIALS ARE SOLUBLE IN ORAL FLUIDS TO SOME EXTENT.

PORCELAIN AND CERAMICS ARE LEAST SOLUBLE.

LOSS OF SMALL ORGANIC MOLECULES FROM DENTURE LINERS AND TISSUE CONDITIONERS LEADS TO MATERIAL HARDENING IN THE MOUTH, THEREBY TISSUE IRRITATION. SOLUBILITYUPTAKE OF FLUID OR SUBSTANCE BY A MATERIAL.

OCCURS MOSTLY IN POLYMERIC MATERIALS PMMA 1 TO 2% BY WEIGHT COMPONSATES FOR POLYMERISATION SHRINKAGE , BUT ALSO CAN OCCUR IN INTERFACE OF METAL AND PORCELAIN LEADS TO SUBTLE DISCOLORATION OF THE PORCELAIN.

UPTAKE OF FOREIGN MATERIALS MAY LEADS TO WASH OUT OF CEMENTS EVENTUALLY LOOSENING OF RESTORATION. ABSORPTIONGENERAL DETEROITATION AND CHANGE IN QUALITY OF MATERIAL DURING SHIPMENT OR STORAGE.

TEMPERATURE ,HUMIDITY AND TIME OF STORAGE ARE SIGNIFICANT.

THIS IS IMPORTANT IN GYPSUM PRODUCTS AND IN IMPRESSION MATERIALS.SHELF LIFEThe physical properties of dental materials are an apt indicator towards of their clinical performance in the oral cavity.

A thorough working knowledge of each of these materials enable dentist to choose best material suited to particular clinical situation.conclusionPHILIPS SCIENCE OF DENTAL MATERIALS: ELEVENTH EDITION ANUSAVICE.

CRAIGS RESTORATIVE DENTAL MATERIALS: TWELFTH EDITION

ART & SCIENCE OF OPERATIVE DENTISTRY:FIFTH EDITION STURDEVANT.

GLOSSARY OF PROSTHODONTIC TERMS.Dent Clin N Am 48 (2004) 341358 Advances in color matching Jane D. Brewer, DDS, MSa, Alvin Wee, BDS, MSb,c, Robert Seghi, DDS, MS

REFERENCESTHANK YOU