INTRODUCTION

PHYSICAL PROPERTIES OF DENTAL MATERIALS

ACKNOWLEDGMENTWe would like to express our special thanks of gratitude to our teacher Dr. Gaurav Gupta, Dr. Sanjeev Joshi and Dr. Parveen who gave us the golden opportunity to do this wonderful project on the topic Physical properties of Dental M aterial, which also helped us in doing a lot of research and we came to know about so many new things we am really thankful to them.Without their guidance it would not have been possible.

CONTENTS

1. Introduction

2. Physical properties of dental materials a.Rheology b. Viscosity c. Creep and flow d. Colour and optical effects -Nature of light and role of human vision - Nature of object under view e. Three dimension of colour -Hue -Value - Chroma f. Measurement of colour g. Colour matching h. The effect of observer i. Flourescence j. Radiopacity

3. Thermal properties a. Thermal conductivity b. Thermal diffusivity c. Co-efficient of thermal expansion

4. Electrochemical propertiesTarnish and corrosionElectromechanical mechanism of corrosionDisimilar metalsStress corrosionConcentration cell corrosionProtection against corrosion

5. Recent Advances

6. Conclusion7. Acknowledgement8. Reference

INTRODUCTIONThe principle goal of dentistry is to maintain to improve the quality of life of dental patient. This requires the replacement of alteration of existing tooth structure; the main challenges for centuries have been the selection of good prosthetic materials that can withstand the adverse condition of oral environment.Physical properties are the measures of a material. These properties have great significance in dental research because they provide the info needed to assess the characteristic and improvement in material under development.Physical properties of a tooth, set the standard for materials attached to a tooth. Theory suggest that if a restorative materials can be made to hold property similar to those of natural tooth structure, it should perform as well as original tooth.

RHEOLOGYDefinition :- It is the study of the deformation and flow characteristics of matter, whether liquid or solid. In dentistry study of rheology is necessary because many dental materials are liquids at some stage of their use e.g- Molten alloy Freshly mixed impression materials and cements.Gypsum products use in the fabrication of models and dies are transformed extra orally from fluid slurries into solid .Amorphous materials such as waxes and resin appear solid but actually are super cooled liquids that can flow plastically (irreversibly) under sustain loading or deform elastically (reversibly) under small stresses.

Definition :- It is defined as the resistance of a liquid to flow.Most liquids, when placed in motion resist imposed forces that cause them to move this resistance to fluid flow (viscosity) is controlled by internal frictional forces within the liquid .Thus viscosity is a measure of the consistency of a fluid and its resistance to flow.VISCOSITY :

When a spatula is moved through a pasty fluid such as dental cement to blend two components on mixing pad.The mixing surface is fixed and the upper surface (spatula blade) moves to right at a given velocity (v) a force (F) is required to overcome the frictional resistance with in the fluid (viscosity) and cause the fluid to flow .If the two surface have an area (A) in contact within the liquid, a shear stress () can be defined as t= F/A.The shear strain or rate of change of deformation isE=v/2, where d is the shear distance of upper surface relative to fixed lower surface .

Based on viscosity, the fluids are classified into 5 types:

NewtonianPseudoplasticDilatantPlasticBingham

Newtonian:- Viscosity is constantPseudoplastic:- Vicosity decreases with increasing strain until it reaches constant value.Dilatant : - it is opposite to pseudoplasticPlastic :- initially rigid and than attains constant viscosity.Bingham flow :- above certain stress, which behaves as newtonian fluidThe viscosity of many dental materials decreases with increasing strain rate until it reaches a nearly constant value, thus faster they are stirred , forces through syringe or squeezed less viscous and more fluid they become ( pseudoplastic) The viscosity of most fluids decreases rapidly with increasing temperature, it becomes less viscous and more flowable upon repeated applications of pressure known as thixotropic e.g. Plaster of Paris , resin cements and impression material.Viscosity is measured in units of megapascals (mpa) per second per centipose (cp), flow e.g.:-Pure water at 20 degree celsius :- 1.0 cpAgar:-28100 cpPolysuphide: - 109000 cpHeavy body sulphide :- 1360000 cp

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CREEP:-Definition:-it is defined as the time dependent plastic strain of a material under a static load or constant stressThe related phenomenon of sag occurs in the permanent deformation of long span metal bridge structures at porcelain firing temperature under the influence of the mass of prosthesis.Metal creep usually occurs as the temperature increases to within a few hundred degrees of melting range.Metals used in dentistry for cast restoration or substrates the porcelain veneers have melting ranges(for alloys) that are much higher that mouth temperatures and they are not susceptible to creep deformation intraorally.Dental amalgam contains from 42% to 50% of mercury by weight and begin melting at temperatures only slightly above room temperature. Because of its low melting point, dental amalgam can undergo creep at a restored tooth side under provide sustained stress. Creep provides continuing plastic deformation , process can over time be very destructive to dental amalgam filling. CREEP AND FLOW

Creep may cause unacceptable deformation of a dental restoration.It may also lead to unacceptable fit of a FPD, when a cast alloy with porcelain at high temperature.CLINICAL SIGNIFICANCE:-

AMALGAM CREEP

Definition:- the term flow has been 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 even that associated with its own mass . Although creep or flow maybe measured under any types of stress, compression is usually employed in the testing of dental materials.A cylinder of prescribed dimension is subjected to a given compressive stress for a specified time and temperature the creep or flow is measured as the percentage decrease in length that occurs under these testing conditions.FLOW:-

An important goal of dentistry is to restore or improve extractive color and appearance of natural dentition. Esthetic consideration in restorative and prosthetic dentistry have received increasingly greater emphasis grown even greater in past few years following widespread use of bleaching and whitening technologies.Since esthetic dentistry impose severe demands on artistic abilities of dentist and technician knowledge of underlying specific principles of colour and other optical effects is essential.COLOR AND OPTICAL EFFECTS

COLOUR AND OPTICAL EFFECTSAn important goal of dentistry is to restore or improve esthetics- colour and apperance of natural dentition. Esthetic consideration in restorative and prosthetic dentistry have receined increasingly greater emphasis in recent decades and the challenges have grown greater in past few years following widespread use of bleaching and whitening technologies.Since esthetic dentistry imposes severe demands on artistic abilities of dentist and technician, knowledge of underlying the scientific peinciples of colour and other optical effects is essential.

Light is electromagnetic radiation that can be detected by human eye. The eye is sensitive to wavelengths from approx 400nm (violet) to 700nm (dark red).The combined intensities of wavelengths present in beam of light determine the property usually called color. For an object to be visible , it must be reflect or transmit light incident on it from external source which is usually polychromatic , i.e.. mixture of various wavelengths known as white light light from an object that is incident on the eye is formed in retina and is converted nerve impulses which are transmitted into brain . Cone shaped cells in retina are responsible for color vision. These cells have threshold intensity required for color vision and respond to wavelength for both normal color vision and deficient vision.

NATURE OF LIGHT AND THE ROLE OF HUMAN VISION:-

Esthetics is critically important in dealing with dental restorative materials electromagnetic radiation in visible region interacts with objects through reflection from its surfaces. Absorption, refraction or transmission. This phenomenon determine the opacity, translucency or transparency of an objects. Enamel consists of hydroxyapatite crystals when light strikes, some is reflected , some is absorbed , some refracted or some transmitted. Enamel has a refractive index of 1.65 and is translucent.NATURE OF OBJECT UNDER VIEW:-

Verbal description of color are not precise enough to describe the appearance of teeth. To overcome this problem, color perception is derived by three objectives variables:- hue, value, and chroma. These three parameters constitute the three dimensions of colour space.THREE DIMENTION OF COLOR:-

HUE:-The dominant color of an objects e.g. Red, green, or blue. This refers to dominant wavelength present in the special distribution.

It is also known as grey scale. It is the vertical or z axis . value increases towards higher end ( higher) and decreases toward low end ( darker) . Tooth or dental crown identifies lightness or darkness of color by hue. VALUE:-

It is the degree of saturation of a particular hue. For e.g. Red can vary from scarlet to light pink where scarlet has high saturation and pink has low saturation.Crown is not considered separately in dentistry, it is always associated with hue and value of dental tissue, restoration and prosthesis.CHROMA :-

One of the most commonly used method to define and measure colour quantitatively is the munsell systemIt is a co ordinate system which can be viewed as a cylinder. The lines are arranged sequentially around the perimeter of cylinder while chroma increases along a radius from the axis. The values co ordinate varies along length of cylinder from back at the bottom to neutral grey at the center to white at top.MEASUREMENT OF COLOR:

MUNSELL SYSTEM

In dental practice color matching is most often performed with use of shade guide to select color of ceramic ,veneers ,inlays or crowns. Shape guide tabs are used in much same way as paint chips to match color of house paint.These individual shade tabs are grouped according to hue followed by value and chroma. It is usually insufficient to simple select a shade from guide shade and transfer the information to lab. To ensure esthetic results additional information such as drawings, description and photographs should also be sent.Preference of patient must be taken in account patients usually prefer a lighter shade in restoration as compared to color of natural teeth.

COLOR MATCHING:-

SHADE GUIDE

Signals of color are sent to humans from three sets of receptors in retina called cones which are sensitive to red, blue ,and green. Factors that interfere with true perception of color include low or high level, fatigue of color receptors, sex , age, memory and internal background.At low level, rod in retina are dominant and color perception is lost.As it brightens it becomes more intense and color appear to change.e.g. observer looks at red object for long time than she sees green on white background.

EFFECT OF THE OBSERVER:-

The spectral distribution of light reflected from or transmitted through an object is dependent on spectral content of incident light. The appearance of an object is dependent on nature of light in which the object is viewed.Day light, incandescent and fluorescent lamps are common sources of light in dental operatory.Objects that appear to be color matched under one type of light may appear different under another type. This phenomenon is known as metamerism.If possible, color matching should be done under 2 or more different light sources, one of which should be daylight and laboratory shade matching procedure should be performed under same light conditions.THE EFFECT OF LIGHT SOURCE :

Natural tooth structure absorbs light at wavelengths too short to be visible to human eye. Between 300 and 4000nm referred as near ultraviolet radiation e.g. natural sunlight photoflash lamps.The energy that tooth absorbs is converted into light with longer wavelengths in which tooth becomes a light source known as fluorescence Flourescence makes a definite contribution to brightness and initial appearance of a human tooth.e.g. ceramic crowns or composite restoration that lack a fluorescing agent appear as missing with when viewed under a black light. FLOURESCENCE:

X-rays a form of electromagnetic radiation lie at the high energy end of the spectrumSufficient radiographic contrast is required in an x- ray image in order to assess restoration for marginal defects or break down, help differentiate composite restoration from dental caries and detect micro leakage.A significant number of foreign bodies of dental origin with many injuries and deaths are reported each year.When radiopacity is too low, resin will not be visible on an x- ray when it is too high it may black out and obscure details of adjacent anatomy.Dentin has same fragment radio density as that of aluminum while enamel has double of it.Denture resin fragment of 2mm should have a radiopacity equal or greater than that of 0.2mm of copper. This requires the resin to have sufficient additives to produce a radiopacity about three times that of aluminum.RADIOPACITY

When restorative materials are placed in deep cavities, heat transmitted to vital dental pulp must be limited so as to prevent thermal shock and traumaWhile with denture bases its contact with mucosal surface is desirable to convey sensation of heat and cold associated with food and beverages. Such attributes are governed by properties of thermal conductivity and thermal diffusivity.THERMAL PROPRTIES:THERMAL CONDUCTIVITY:Definition:- it is defined as the quantity of heat in calories per second passing through a material 1 cm thick with a cross section of 1 cm square having temperature difference of 1k (1 degree Celsius) and is measure under steady state conditions in which temperature gradient does not change. The international system(si) unit is watts per meter per Kelvin (w*m-1*K-1).Materials that have high thermal conductivity are called conductors whereas low thermal conductivity are called insulator .

It is the measure of speed with which a temperature change will spread through an object when one surface is heated. It is calculated from thermal conductivity divided by product of density and heat capacity.THERMAL DIFFUSIVITY:

Gold has about one length the heat capacity of dentin about 500 times the thermal conductivity and about 600 times thermal diffusivityThus, a pure gold filing would provide tooth pulp with very little protection against thermal shock as compared to natural dentin.When the product of heat capacity and density is high, thermal diffusivity may be low even though thermal conductivity is relatively high. Therefore both parameter are important in predicting transfer of thermal energy.Enamel and dentin are effective are effective thermal insulators.Thermal conductivity or thermal diffusibility of cements- GIC , zinc, phosphate composites are higher for metallic restorations.

Definition:- It is defined as the change in length per unit of original length of a material when its temperature is raised 1 degree Celsius Alpha=delta L/ L into delta TL= original lengthDelta L= change in lengthDelta T= change in temperatureThe unit is mm/mk or ppm/k.It dictates procedure for using wax pattern, casting metal crown. Placing amalgam and composite restoration and preparing metal ceramic crown and bridges.The high thermal expansion co- efficient of inlay wax is important as it is susceptible to temperatures changes

e.g. an accurate wax pattern that fits a prepared tooth contracts significantly when it is removed from tooth or die in warmer areas and then stored in cooler area. This dimensional change is transformed to a cast metal restoration which is made from lost wax process.

COEFFICIENT OF THERMAL EXPANSION

Dentistry uses a wide variety of metals for restorations and prosthetic device. The successful clinical performance and long term durability requires adequate corrosion resistance in oral environment .Thus, an understanding of corrosion and electrochemical properties of corrosion and electrochemical properties is essential in order to control electrochemical effects and minimizing corrosion.ELECTROCHEMICAL PROPERTIES:

TARNISH AND CORROSIONTARNISH is surface discoloration on a metal or aSlight loss or alteration of surface finish or luster.In oral environment tarnish occurs due to:

1. Formation of hard substance- calculus and soft substance-plaque2. Formation of thin films such as oxides, sulfides or chlorides

CORROSION is the deterioration of a metal caused by reaction with environment. It can cause severe and catastrophic disintegration of metals. Tarnish is forerunner to corrosion.

Corrosion of a metal is either:ChemicalElectrochemical Chemical corrosion- direct combination of metallic and non-metallic elements to yield chemical compound through oxidation reactions. It is also known as dry corrosion.Examples:Discoloration of silver by sulphurCorrosion product of dental gold alloys that contain silver.Oxidation of silver-copper alloy particles that are mixed with mercury to prepare certain dental amalgam products.

Electrochemical corrosion also known as galvanic corrosion requires presence of water or electrolytes and a pathway for transport of electrons. It is also referred as wet corrosion. ELECTROCHEMICAL MECHANISM OF CORROSION

When a metal is in contact with fluid electrolyte, chemical potential causes enough ions to dissolve to form a saturated solution and produce an equal number of free electrons.

The Anode is the surface where positive ions are formed production of free electrons.M M+ + E-The cathode consumes free electrons produced anode.

Electrolytes supplies ions needed at cathode and caries away corrosion products at anode. External circuit serves as a pathway. The difference in the electrical potential is known as electromotive force (EMF)

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DISIMILAR METALSIt is a type of electrochemical reaction occurs when combinations of dissimilar metals are in direct physical contact.

The alloy combination that may produce galvanic corrosion or electro galvanism through the flow of galvanic currents maybe in either continuous or intermittent contact

Example: dental amalgam restoration placed on occlusal surface of tooth directly opposing a gold inlayThis results in sharp pain known as galvanic shock which is well known by dentist.

STRESS CORROSIONThe imposition of stress increases to internal energy of an alloy either through elastic displacement of atoms or creation of micro strain fields associated with dislocation. The tendency to undergo corrosion will be increased called stress corrosion.

It is most likely to occur during fatigue or cyclic loading in oral environment. Thus any cold working of an alloy by bending, burnishing or mallaeting causes localized permanent deformation. This is why excessive burnishing of margins of metallic restoration is contraindicated

CONCENTRATION CELL CORROSIONAn important type of electrochemical corrosion is called concentration cell corrosion, which occurs whenever there is variation of electrolytes or in composition of given electrolytes

Example:Accumulation of food debris in interproximal areas between the teeth particularly if oral hygiene is poor.Difference in oxygen concentration between parts of same restoration with greatest attack at areas containing least oxygen. Oxygen deprived becomes and as it is covered by food debris while surface around rim of a scratch or pit becomes cathode.3. Irregularities likes pits, scratches and reacts are an excellent example.

Thus to protect such pitting corrosion all metallic restoration must be polished

PROTECTION AGAINST CORROSION1. A highly effective protection utilizes certain metals that develop thin, adherent highly protective film by reaction with environment such metal is said to be passiveA thin surface oxide forms on chromium is a good examplesExample: stainless Steel which contains sufficient amount of chromium added to iron and its alloy to passviate alloy.

2. Titanium and its alloy are widely used because of its favorable combination of physical chemical and biological properties as well as their resistance to corrosion,

3. Noble metal resist corrosion because their electromotive force is positive with regard to any of the common reduction reaction formed in oral environment, it is important that is more active constituents of alloy be uniformly dispersed in random solid solutions.

RECENT ADVANCEMENTRHEOMETER- The new ARES-G2 is the most distinctive rotational rheometer for advance research. It represents the culmination of five years of ground up intensity effort, the ARES-G2 remains the only commercial available rheometer to offer unique separate motor and transducer technology for the rheological measurements.

ADVANCES IN COLOUR MATCHINGSHADE GUIDES- the first shade guide introduced in market by vita zahnfabrik for measurement of color for ceramic system. In late 1990s one of the first clinical result was development of vita 3D master shade guide.

SHADE TAKING DEVICES-In order to reduce color mismatch , two main categories of electronic instruments were developed for dental color analysis.1-colorimeters-it is simple and designed to measure color on basis of three axes by using a filter that stimulates the human eye.2- spectrophotometer- it includes the ability to convert spectrophotometric measures to various color measures to various color measures.

CONCLUSION:It is very important to know the properties of materials we use in dentistry, especially in restorative materials. This will enable us to select a material that will have properties close to that of natural tooth surface.

Also we will be able to better understand and select materials from wide range that coming into market.

REFERENCEPHILLIPSScience of dental material12th edition

2. Google for images.

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