MECHANICAL PROPERTIES OF DENTAL MATERIALS

UNDER THE ABLE GUIDANCE OF:

DR RAVI DWIVEDIDR. N.K GUPTADR.SWATI GUPTADR. AMRIT TANDANDR. GARIMA AGGARWALDR. MANOJ UPADHYAYDR. SULABH

• SEMINAR PRESENTED BY: VIRENDRA VIKRAM

SINGH JR1 , PROSTHODONTICS

• Properties are descriptions of a materials interactions with the energy in its environment.

• The four common material property categories are Physical,Mechanical,Chemical & BiologicProperties.

• Mechanical properties are defined by the laws of

mechanics i.e the physical science that deals with Energy & Forces and their effects on

bodies.Thus all mechanical properties are measures of the resistanceof a material to deformation or fracture under an applied force.

RELATIONSHIP BETWEEN MECHANICAL PROPERTY AND STRENGTH

• MECHANICAL PROPERTY OF A MATERIAL ENSURES THAT PROSTHESIS SERVES ITS INTENDED FUNCTIONS EFFECTIVELY,SAFELY AND FOR A

REASONABLE TIME PERIOD.

STRESS• When a force acts on a body to produce

deformation a resistance develops to this external force which is called stress.

• Mathematically: Stress(S)=force/area.Stress is

expressed as LOAD/AREA i.e Pounds/ square of inch=PSI or newton/square

of mm=MPa.

DIGRAMATIC REPRESENTATION OF BONY TRABECULAE ALIGNMENT IN STRESS TREJECTORIES TO BETTER PREPARE THE FEMUR TO RESIST THE VARIETY OF FUNCTIONAL FORCES

TYPES OF STRESS• Based on forces acting on the specimen-• Simple stress -Tensile stress ,Compressive

stress ,Shear stress.• Complex stress-flexural stress. • Based on temperature changes on the

specimen• Thermal stress.

COMPRESSIVE STRESS-• If a body is placed under a load that tends to

compress or shorten,the internal resistance to such a load is called a compressive stress and it is associated with a compressive strain.

• Mathematically,calculated by dividing the applied force by the cross sectional area Perpendicular to the direction of applied force.

TENSILE STRESS• A tensile stress is caused by a load that tends

to stretch or elongate a body.A tensile stress is always accompanied by a tensile strain.Tensile stress is generated when structures are flexed.

• Mathematically, denoted by ( )=tensile 𝝈force /area

• Example-in clinics a sticky candy can be used to remove crown by means of a tensile force.

SHEAR STRESS-A shear stress tends to resist the sliding on a portion of a body over another. It can be produced by a twisting or torsional action on a material.

• Mathematically, shear stress is calculated by dividing the force by the area parallel to the force of direction.

FLEXURAL (BENDING STRESS)• Is Mathematically defined as force per unit

area of a material subjected to flexural loading.E.g-When a patient bites in to an object, the anterior teeth receive forces that are at an angle to their long axis,creates flexural stress.

• Tooth flexure( strain with in tooth structure)- Tooth flexure are either a lateral bending(as shown in figA or an axial bending of a tooth during occlusal loading(as shown in fig b). This flexure produces the maximum strain in cervical region, the strain appear to be resolved in tension or compression with in local regions, sometimes causing loss of bonded class V restorations.

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SOME NOTEWORTHY POINTS • Mechanical properties of a material describe

its response to loading.• Most clinical situations involve complicated 3-

D loading situations,it is common to describe the load in terms of a single direction (Compression,shear ,tensile).

• Combinations of these can produce TORSION(twisting) or FLEXION(transverse bending).

• During loading bonds are not compressed as easily as when they are stretched.

• Materials resist compression readily and are stronger in Compression than in Tension.

• As loading continues ,structure is ultimately deformed.

• Mechanical events are both temperature & time dependent, as the temperature increases the mechanical property values decrease.The stress-strain curve appears to move to the right & downward.

• As the rate of loading decreases ,the mechanical properties decrease called STRAIN RATE SENSITIVITY.

CLINICAL IMPLICATION: To momentarily make a material’s behaviour stiffer strain it quickly. For recording undercut areas in an elastic intraoral impression remove it rapidly so that it will be more elastic & accurately record the dimensions of the structures

STRAIN Is defined as change in length per unit initial length.

• Strain is deformation( per unit of length(L) i.e =𝜺 ∆L/L

• Is expressed in inch/inch or cm./cm. THE STRESS -STRAIN CURVE• With a constant increase in loading, the

structure is ultimately deformed• At first the deformation (strain) is reversible –

ELASTIC STRAIN.

• With increased loading, there is some irreversible strain which results in permanent deformation-PLASTIC STRAIN.

• ELASTIC STRAIN • THE deformation that is recovered upon

removal of an externally applied force or pressure.

PLASTIC STRAIN The deformation that is not recoverable when

an externally applied force is Removed

• The point of onset of plastic strain is called the Elastic limit( proportional limit, yield point).

• It is indicated on stress-strain curve as the point at which the straight line starts to become curved.

• Continuing the plastic strain leads to Fracture.• The highest stress before fracture is the

Ultimate Strength

ELASTIC LIMIT of a material is defined as the greatest stress to which a material can be subjected to, such that it returns to its original dimensions when force is released.

• Material that undergo extensive plastic deformation before fracture are called Ductile( in tension) and Malleable(in compression)

• Materials that undergo very little plastic deformation are called Brittle.

• ELASTIC LIMIT of a material is defined as the greatest stress to which a material can be subjected to, such that it returns to its original dimensions when force is released.

• Material that undergo extensive plastic deformation before fracture are called Ductile( in tension) and Malleable(in compression)

• Materials that undergo very little plastic deformation are called Brittle.

MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION

Elastic modulus(Youngs modulus ,Modulus of elasticity)

• It describes the relative stiffness or rigidity of a material which is measured by the slope of the elastic region of the stress strain graph.

• It represents the amount of strain produced in response to each amount of stress.

• Elastic modulus has a constant value that does not change and it describes the relative stiffness of a material.

• Elastic modulus of Enamel is higher than that of Dentin.

• Depending on the area of the tooth its value may be 3 to 7 times.

• ENAMEL is STIFFER AND more BRITTLE than dentin.• Dentin is more flexible & tougher and is capable of

sustaining significant plastic deformation under compressive loading before it fractures.

• Mathematically,E=STRESS/STRAIN= (F/A)/ ∆l/l CLINICAL SIGNIFICANCE• When a load is applied to a tooth it is

transmitted through the material giving rise to stresses and strain. If these exceed the maximum value the material can withstand , a fracture results.

CLINICAL APPLICATION• THE MOST USEFUL properties of a restorative

material are Modulus of elasticity(E) & Elastic limit.

• A restorative material should be very stiff so that under load, its elatic deformation should is minimal.

• An exception to this in class V Composites are used-they should be less stiff to accommodate for tooth flexure.

• When selecting a restorative material the clinician must bear in mind the stress level during function.This should not exceed the elastic limit,lest deformation occurs which may cause failure at some point of time.

RESILIENCE• Popularly the term resilience is associated with

“springiness”.• Def: Resilience can be defined as the amount of

energy absorbed within a unit volume of a structure when it is stressed to its proportional limit.

• The resilience of two or more material can be assessed by comparing the areas under the elastic region of their STRESS STRAIN PLOTS.

• For example- A proximal inlay cause excessive movement of the adjacent tooth if large proximal strain develops during compressive loading on occlusal table.

• Restorative material should exhibit a moderately high elastic modulus& relatively low resilience.

• TOUGHNESS- IS THE TOTAL AREA UNDER THE STRESS –STRAIN CURVE.

STRENGTH PROPERTIES-• Is the stress necessary to cause either

fracture(ultimate strength) or a specified amount of plastic deformation(yield strength).

• Strength of a material can be described by – 1) PROPORTIONAL LIMIT-The stress above

which stress is no longer proportional to strain.

2)ELASTIC LIMIT- The maximum stress a

material can withstand before it becomes plastically deformed.

3) YIELD STRENGTH OR PROOF STRESS-The stress required to produce a given amount of plastic strain.

4)ULTIMATE TENSILE STRENGTH, SHEAR STRENGTH,COMPRESSIVE STRENGTH & FLEXURAL STRENGTH entity is a measure of stress required to fracture a material.

STRENGTH is not a measure of individual atom to atom attraction or repulsion but rather is a measure of the interatomic forces collectively over the entire wire,cylionder,implants,crown, pin or whatever structure is stressed.

PROPORTIONAL LIMIT-• On plotting a stress strain diagram, with

material obeying hooks law the elastic stress will be proportional to elastic strain.

• In such material stress-strain diagram starts from origin O as a straight line ,along this line the material behaves elastically and it springs back to its initial shape & size when the force is removed.

• When stress value corresponding to point P is exceeded, the line becomes nonlinear and stress is no longer proportional to strain.

• The stress value at P, the point above which the curve digresses from a straight line is known as the proportional limit .

ELASTIC LIMIT- • EL of a material can be defined as the

greatest stress to which a material can be subjected such that it returns to its original dimensions when the force is released.

• If a small tensile stress is induced in a wire ,the wire will return to its original length on load removal, on increasing the load incrementally & then releasing after each increase in stress , a stress value will be reached at which the wire does not return to its original length after it is unloaded, this point the wire has been stressed beyond its elastic limit.

YIELD STRENGTH( PROOF STRESS)-• YIELD STRENGTH is a property that represents

the stress value at which a small amount of plastic strain has occurred.

• A value of 0.1 or 0.2% of the plastic strain is often selected and is referred to as the % offset.

• If yield strength values for two materials tested under the same conditions are to be compared, identical offset values should be used.

To determine the Yield Strength FOR A MATERIAL AT 0.2 % OFFSET ,a line is drawn parallel to the straight line region starting at a value of 0.002(0.2%) of the plastic strain along the strain axis and is extended until it intersects the stress strain curve.

• Stress corresponding to this point is the yield strength.

• STRESS-STRAIN PLOT for dental ceramics(a brittle material) is a straight line with no plastic region,hence is not practical at (0.1 or 0.2% strain offset since there can be no intercept of straight line offset parallel to the elastic deformation line.

PERMANENT PLASTIC DEFORMATION-• If material A is deformed by a stress at a point

above the proportional limit before fracture , the stress reduces to 0 numerically on removal of applied force, on the contrary strain does not decrease to 0 since the plastic deformation has occurred.

• Hence the object does not returns to its original dimension when the force is removed

COLD WORKING( Strain hardening or Work hardening)

• When a metal is stressed beyond its proportional limit the hardness & strength of metal increase at the area of deformation but the ductility of metal decreases .

• As DISLOCATIONS MOVE & PILE up along grain boundaries further plastic deformation in these areas become more difficult.

• As a result repeated plastic deformation (in orthodontic wire bending,clasp arm adjustment on a R.P.D can lead to BRITTLENESS AND SUBSEQUENT FRACTURE ON FURTHER adjustment (key to minimise the risk of reduced plasticity(EMBRITTLEMENT) is to deform the metal in small increments so as not to plastically deform the metal excessively.

DIAMETRAL TENSILE STRENGTH-• Tensile strength is determined by subjecting a

rod, wire or dumbbell shaped specimen to tensile loading(uniaxial tension test.).

• DIAMETRAL COMPRESSION TEST MEASURES THE DIAMETRAL TENSILE STRENGTH.

• D.C.T IS MEANT FOR MATERIAL that exhibit predominantly elastic deformation and little or no plastic deformation.

• TENSILE STRESS for such a specimen is computed by,

• TENSILE STRESS= 2 P/∏DT where,• P= applied load, d=diameter,t =thickness

• FLEXURE STRENGTH(TRANSVERSE STRENGTH OR MODULUS OF RUPTURE)For a bar subjected to 3-point flexure (upper central loading).

• Mathematically for computing flexture strength WHERE,

• 𝝈=flexure strength, l= the distance between the supports, b= width of the specimen,d=depth or thickness of the specimen,p= max. Load at the point of fracture

• UNIT of the stress in s.i unit is( MPa).• For ceramic(a brittle material) flexure tests are

preferred to the diametral compressive stress.

FATIGUE STRENGTH• STRESS values well below the ultimate tensile

strength can produce premature fracture of a dental prosthesis because microscopic flaws grows slowly over many cycles of stress this is called FATIGUE FAILURE.

• A standard Engineering design limit for dental restorative materials is approx. 10 million cycles or approx. 10 years of intraoral service( working surfaces of teeth are mechanically cycled approx 1 million times per year.

MAX.SERVICE STRESS OR an ENDURANCE LIMIT- The max. Stress that can be maintained without failure over an infinite number of cycles, for brittle material with rough surfaces the endurance limit is lower than a highly polished surface.

STATIC FATIGUE- a phenomena attributed to the interaction of a constant tensile stresswith structural flaws over time.For e.g Prosthetic appliances,ceramic orthodontic brackets, activated wires with in the brackets.

DYNAMIC FATIGUE FAILURE-The delayed fracture of molar CERAMIC CROWNS that are subjected to periodic cyclic forces are caused by dynamic fatigue failure.

• In a nutshell,the failure begins as a flaw that propagates until catastrophic fracture occurs.

• DENTAL BIOMECHANICS-• Biomechanics is the study of the physical

behaviour of the biologic structures and the interactions between biologic & restorative systems.it is the application of mechanics to biologic systems.

• The biomechanical behaviour of a restored tooth is of immense significance to the clinician. A standard biomechanical unit involves1) tooth structure,restorative material,interfacial zone.

• In normal tooth loads are transmitted to dentin through Enamel.

• Dentin undergoes a small amount of deformation.(tooth flexure.).this amount of strain is proportional to the amount of load applied on the tooth.

• A restored tooth transmits stress differently, ENAMEL IS NOT CONTINIOUS & its resistance is lowered, therefore restoration be designed in a manner in which STRESS is distributed to the denrtin rather than ENAMEL.

OTHER MECHANICAL PROPERTIES-

HARDNESS-• In mineralogy the relative hardness of a

substance is based on its ability to resist scratching.In most other disciplines the concept of HARDNESS is ‘RESISTANCE TO INDENTATION’.

• Tests most frequently used in determining the hardness of dental materials are BRINELL, ROCKWELL,VICKERS ,KNOOP,BARCOL,SHORE.

• BRINELL TEST- A hardened steel ball is pressed under a specified load in to the polished surface of a material, load is divided by the area of the projected surface of the indentation & the quotient is called BHN. THE smaller the indentation larger is the BHN NO. This test is used in DENTISTRY for determining hardness of metals & metallic materials.

ROCKWELL HARDNESS TESTS- A STEEL BALL OR CONICAL DIAMOND POINT ,The depth of penetration is measured directly by a dial gauge, a number of indenting points with different sizes are available.RHN IS designated according to particular indenter & load applied.

• Above two tests are not suitable for brittle materials.

VICKERS HARDNESS TESTS-A square based pyramid is used , VHN is calculated by dividing the load by the projected area of the indentation.VHT IS USED FOR determining hardness of DENTAL CASTING GOLD ALLOYS.VHT Is suitable for determining brittle materials.

KNOOP HARDNESS TEST- Employs a diamond tipped tool. Impressions is rhombic in outline & length of largest diognal is measured. The area is divided in to to the load to yield KHN.

• The knoop & Vickers are MICROHARDNESS TESTS( Indentations are in the order of less than 19µ & EMPLOY LOADS LESS THAN 9.8 N. BRINELL & ROCKWELL are MACROHARDNESS TESTS.

• BRITTLENESS- Is the relative inability of the material to sustain plastic deformation before fracture of a material occurs.

• Below are 3 stress strain curves-

• A brittle material fractures at or near its proportional limit.( as seen in material C. A brittle material is not necessarily week(the tensile strength of In –ceram alumina is 450 MPa but 0% elongation.

DUCTILITY & MALLEABILITY-• When a structure is stressed beyond its

proportional limit it becomes permanently deformed.if a material sustains tensile stress and considerable permanent deformation without rupture it is called DUCTILE.

• Materials that undergo extensive plastic deformation before fracture are called DUCTILE( IN TENSION ) OR MALLEABLE IN COMPRESSION.

• GOLD Is the most ductile and malleable pure metal & silver holds 2nd rank.

STRESS ANALYSIS-• Is an engineering discipline that determines

the stress in materials & structures subjected to static or dynamic forces or load.

• Aims & objective- To determine whether the structure can safely withstand the specified forces ,objective is achieved when the determined stress from the applied forces is less than the U.T.S,ultimate compressive strength or fatigue strength the material is known to withstand.

STRESS ANALYSIS OF DENTAL RESTORATION-• Mechanical properties of a material used in

dental restoration must be able to withstand stress & strains caused by the repetitive forcesof mastication. It is mandatory to use design that don’t result in stresses & strain that exceed the strength properties of dental material under clinical conditions.

HISTORICAL PERSPECTIVE-• HOPPENSTAND & Mc CONNEL used a model

simulation to study the mechanical failure of class 1 type Amalgam restorations.MAHLER ET AL used a similar technique to investigate design aspects of class two restorations.

TECHNIQUES USED FOR STRESS ANALYSIS- 1)THEORITICAL- Use mathematical formulation & solution of the resultant equations.e.g FINITE ELEMENT ANALYSIS. 2)EXPERIMENTAL – Involves measurements of various types made directly on the structures of interest.e.g STRAIN GAUGE, PHOTOELASTICITY.

Finite element analysis model of temporomandibular joint (TMJ) stress. (a) Three-dimensional finite element mesh of the TMJ, including the mandible, disc (i), cartilage (ii), condyle (iii), and coracoid process (iv), with the mandible angle fixed (v) and alveolar ridge loaded by two-dimensional occlusal force vectors (vi). (b) State of stress in an orthotropic element. Each of the three material axes was assigned a value for the elastic modulus ( ), and each of the three planes defined by those axes received values for the shear modulus ( ) and Poisson’s ratio ( ). Thus, one principle stress ( ) and two shear stress ( ) values could be calculated from each plane of the element. 1, 2, and 3 indicate the same corresponding directions as in Figure 1. (c) Twenty-nine observation points map the TMJ for stress value collection.

FINITE ELEMENT ANALYSIS-(RICHARD COURANT,1943)

• Is a computer simulation technique used in engineering analysis. It uses a numerical technique called Finite Element Method.FEA is commonly used for the determination of stresses & displacements in mechanical objects & systems.It has the advantage of being applicable to solids of irregular geometry & heterogenous material properties, hence ideally suits to examine the structural behaviour of teeth.

HOW DOES FINITE ELEMENT ANALYSIS WORK?

• FEA uses a complex system of points called nodes , which make a grid called a mesh. The mesh is programmed to contain the material & structural properties which define how the structure will react to certain loading conditions.

• Nodes are assigned at a certain density throuhout the material depending on the anticipated stress levels of a particular area.regions that will receive large amount of stress usually have a higher node density yhan those which experience little or no stress.

• stress area. The mesh acts like a spider web, whereby in each node there extends a mesh element , this web of vectors is what Points of interest consist of: fracture point of previously tested material, corners ,complex detail & high stress area.

PRE-PROCESSING STEPS IN FINITE ELEMENT ANALYSIS-

• is the 1ST step in FEA , whereby construction of a finite element model of the structure to be analysed is made. This can be 1-D ,2-D or 3-D form modeled by line , shape or surface representation(3-D models are mostly used) , objective of the model is to realistically replicate the important parameters & features of the real models in to small elements.

• The simplest mechanism to achieve modelling similarity in structural analysis is to utilize preexisting digital blueprints, design files,CAD models & by importing that in to FEA environment after the creation of finite element geometric model a meshing procedure is used to define & break up the model in to small elements.

• A finite element model is a mesh network made up of the geometric arrangement of elements and nodes.NODES REPRESENT POINTS at which features as displacement are calculated. Elements are bounded by set of nodes &define localised mass & stiffness properties of the model.

NEXT STAGE IS ANALYSIS( COMPUTATION OF THE SOLUTION)-

• The FEM coducts a series of computational procedures involving applied forces & the properties of the element which produce a model solution.such a structral analysis allows determination of effects as DEFORMATIONS, STRAIN, STRESSES caused by applied structural load

NEXT STAGE IS- POST PROCESSING (VISUALISATION)-

• THESE results can be studied using visualisation tools within the FEA environment to view and to fully identify implications of the analysis.numerical & graphical tools allow the precise location of the data such as stresses & deflections to be identified.VISUALISATION- results can be studied by the visualisation tools with in FEA based computers.to view & identify the implications of the analysis, Numerical & Graphical tools are used.these tools allow the precise location of the data such as STRESSES & DEFLECTIONS.

• RESULTS OF FEA- FEA predicts failure of material which results due to unknown stresses by showing their problem areas, this method of product design & testing is far superior to the manufacturing costs that would accrue if each sample was actually built & tested.

DIFFICULTIES IN USING FEA IN DENTISTRY-• The difficulty involved in obtaining the M.P of

the tooth’s constituent materials viz-ENAMEL, DENTIN, CEMENTUM & PULP.

• EXPERIMENTAL TECHNIQUE FOR STRESS ANALYSIS-By STRAIN GAUGE & PHOTOELASTICITY.

• STRAIN GAUGE- (Edward E. simmons & Arthur c.ruge, 1938)

• A STRAIN GAUGE is a device used to measure deformation ( strain ) of an object. The most common strain gauge consists of an insulating flexible backing which supports a metallic foil pattern,the gauge is attached to the object by a suitable adhesive. A strain gauge is a long length of conductor arranged in a zigzag pattern on a membrane.

• When it is stressed its resistance increases , strain gauges are mounted in the same direction as the strain and often in

fours to form a full Wheatstone bridge.

• STRAIN GAUGE use the principle that when a certain electrical resistance is subjected to an object, it produces strain.TENSION produces an increase in resistance; compression causes a decrease in resistance,therefore, if such a strain gauge were bonded to the surface of a structure under a load , monitoring the resistance changes would yield knowledge of the strain characteristic at the point.

PHOTOELASTIC STRESS ANALYSIS-• PSA is based on the property of some

transparent materials to exhibit colourful patterns when viewed with polarised light.These pattern occur as the result of alteration of the polarised light by internal stresses into two waves that travel at different velocities

• The pattern that develops is consequently related to the distribution of the internal

stresses & is called PHOTOELASTIC EFFECT.PREREQUISITE- model of the structure of interest be fabricated in right dimension & proportions model be made from transparent

material exhibiting a photoelastic response. Stress developing in model due to applied load can be visualised by examining model with polarising filter.

AUTOMATED PHOTOELASTIC STRESS ANALYSIS-

• AUTOMATED PHOTOELASTICITY uses a computer to calculate principal strain differences & directions without the need to count the fringes or rely on the subjective interpretation of fringe colours.Photoelastic coating allow for full field strain measurements to be made on structure under load.analysis involves bonding a special plastic coating on to the structure, shining polarised light on to the plastic & analysing the resultant images.

• ADVANTAGES-STRESSES can be determined in models of complicated 3-D shapes such as the oral structures therby facilitating the location & magnitude of stress concentration. Stresses from complex loading conditions as forces of mastication & forces produced by restorative appliances can also be determined.P.S.A has facilated the design of complicated structures & machinery& has wide application in dentistry.

2-D PHOTO ELASTIC STRESS ANALYSIS OF TRAUMATIZED INCISOR-

• In this study stress of traumatised incisor & effect of stress on tooth & alveolar bone was studied with 2-D photoelasticity,

2 homogenous ,2-D maxillary central incisor model were prepared,loads were applied to the labial side of incisal edge & middle third of crown at angles of 45° & 90°,it was observed that stress was increased on teeth & alveolar bone when load was applied 90° on labial side of incisal edge

FINITE ELEMENT STRESS ANALYSIS OF A NORMAL TOOTH-

• Enamel presents a similar behaviour to ceramics, being a fragile material & crystalline (Hydroxyapatite crystals).

• Anisotropic characteristic of enamel.( Enamel is thought to have highly anisotropic stiffness characteristics, because of its prismatic structure. It is probable that the enamel is stiffer in the prism direction compared with a direction perpendicular to it. The prisms are thought to run approximately perpendicular to the enamel-dentin junction.)

• SPEARS(1993) & LAS CASAS(2003) presented difference of Enamel’s mechanical behaviour when submitted to occlusal loads depending on the load directions in relation to the Enamel.

• Motta et al analysed the influence of Enamel anisotropy on stress distribution on sound tooth.In order to describe the results paths were created.

• Results showed that there is a difference in stress distribution between the Isotropic & Anisotropic enamel model.

FEA ON A MANDIBULAR PREMOLAR-• Normal mastication generates considerable

reactionry stresses in teeth & supporting tissue.

• Enamel is assumed to be Isotropic , has greater stiffness over that of dentin.

• The masticatory forces tend to “flow” around the Enamel cap although the dentin core remain lightly stressed.This is the cause for Isotropic or orthotropic Enamel under single or two point loading.

• Enamel near the DEJ is highly stressed because the reacted forces have to flow in to & through this thin wedge of tissue for them to be transmitted in to the root of the tooth.

• Therefore evident that restorations inserted in the cervical region of tooth can be subjected to direct contact stresses of mastication, this may be reason for pain in cervically placed restoration.

• Under masticatory type loading high stresses are generated in fissure.Tensile stresses tend to pull Enamel prism apart in this region & thereby assist the attack by caries in the fissures of premolar & molar teeth on the initiation of chemical demineralisation of Enamel has been initiated.

• The model is divided in to a number of triangles, the smaller triangles are located in areas of great interest.

• The ability of various types of cement bases to support the Amalgam was also studied.The stress induced in amalgam restoration was 4-5 times higher when the Amalgam was supported by 2mm ZnO Eugenol base as compared with an equal thickness of Zinc phosphate cement base.

• The fracture of the amalgam is influenced more by the modulus of elasticity of the base material than by the compressive strength of base.Hence the cement base must have the modulus of elasticity equal to that of restorative material.

TEETH WITH DIRECT COMPOSITE RESTORATION-• In vitro analysis was done in teeth with Class one

cavity preparation by using different axial wall inclinations(convergent or parallel) & by inserting composite resin.

• Teeth were sliced in the middle & the defects on the tooth restoration interface were measured based on it 2-D model was designed for each cavity. The objective of this study was to evaluate the influence of axial wall inclinations in relation to defect presence & stress distribution when submitted to occlusal loading.

• Stress analysis technique are invaluable for manufacturers of dental material as they help in evaluating critical stress levels, they help in evaluating the MP of dental materials under lab conditions & also give a 3-D view.

REFERENCES:• PHILLIPS SCIENCE OF DENTAL MATERIALS• STURDEVANTS ART & SCIENCE OF OPERATIVE

DENTISTRY• ORTHODONTICS PRINCIPLES & PRACTICE• INTERNET