Dr Deeksha Arya Assistant Professor Department of Prosthodontics

 Introduction  Physical properties of Dental Materials  References

Physical properties of Dental Materials are based on:  The laws of mechanics-Creep & flow, abrasion and abrasion resistance, viscosity  Thermodynamics e.g. Thermal conductivity, Thermal diffusivity and Coefficient of thermal expansion

 Electricity &Magnetism e.g tarnish and corrosion  Optics –Metamerism,Hue, chroma value  Radiation –Laser

Creepis defined as the time dependent plastic strain of a material under a static load or constant stress. The magnitude of deformation depends on  the materials  the temperature  the load  the time of application of the load.

 Creep may cause unacceptable deformation of dental restorations.eg.-low-copper dental amalgam.  According to ADA SP. No 1 creep should be below 3%.

Flow: Ability of material to displace itself under load. Flow is dependent on:  Temperature of the wax.  Force applied.  Length of time the force is applied.  Flow increases as the melting point of the wax is approached.

Abrasion is defined as the wearing away of a substance or structure through a mechanical process such as grinding, rubbing or scraping.

 Hardness has been used as an index of the ability of the material to resist abrasion or wear.

Viscosity -:  Resistance of a liquid to flow.  Dental material have different viscosities depending on the preparation for their intended clinical application.  measured in units of MPa per second, or centipoise (cP).  most liquids decreases rapidly with increasing temperature.

 Pseudoplastic behavior  Plastic  Thixotropic – Dental prophylaxis paste,Plaster of paris,Resin cements, impression material

Properties which are not related to force application-  Mass-related properties:  Density  Thermal properties:  Thermal conductivity  Thermal diffusivity  Coefficient of thermal expansion and contraction  Heat of fusion & latent heat of solidification  Melting and freezing temperature  Specific heat

 Less specific properties:  Water sorption.  Optical properties  Color

It is the mass per unit volume of the material. Units are gm/cm 3. Clinical importance: 1- Retention of the upper denture. 2- Weight of complete or partial denture. 3- During casting.

Thermal Conductivity  Defined as the ability of a material to transmit heat or cold.  A low thermal conductivity is desired in restorative materials used on the tooth  High thermal conductivity is desirable where the material covers soft tissue.

Thermal diffusivity=Thermal conductivity Density x specific heat Clinical Importance:  The value of thermal diffusivity of a materials controls the time rate of temperature change as heat passes through a material.  Cements which have low thermal diffusivity are used for pulpal protection.

MaterialDensity (g cm -3 ) Specific heat (Cal g -1 K -1 ) Thermal Conductivity (W m -1 K -1 ) Thermal diffusivity (cm 2 s -1 ) Water Dentin Glass ionomer Zinc Phosphate Composite Enamel Amalgam Pure gold

 Metallic fillings in close proximity to the dental pulp, causes thermal irritation of the pulp through conductors of heat and cold from food and drinks when not properly insulated.

 For effective thermal protection the base should have minimal thickness of 0.75 mm.

The change in length per unit length of the material for a 1°C change in temperature is called the linear coefficient of thermal expansion(α) α= L final - L original L original x (°C final - °C original )

Materialα (ppm K -1 )α material / α toothEnamel Aluminous porcelain Dentin Commercially pure titanium Type II glass ionomer Tooth enamel Gold-palladium alloy Gold(pure) Palladium-silver alloy Amalgam Composite Denture resin Pit and fissure sealant Inlay wax

Close matching of the coefficient of thermal expansion (α) is important between: 1-The tooth and the restorative materials to prevent marginal leakage.

2-Opening and closing of gap results in breakage of marginal seal between the filling and the cavity wall, this breakage of seal leads to:  Marginal leakage  Discoloration  Recurrent caries  Hypersensitivity.

Heat of fusion is the amount of heat in calories or joules required to convert l gm of a material from the solid to the liquid state at the melting temperature. Clinical Importance: In actual use of pure metal or casting alloy must have low specific heat and low heat of fusion, so it does not required prolonged heating to come to a molten state which may cause oxidation of the metal, under conventional procedure.

Latent heat of solidification is the amount of heat in calories or joules liberated when 1 gm of a material is converted from liquid to solid state.

Clinical Importance:  For the fabrication of indirect metallic restorations (casting), the melting temperature of metals and alloys is important in determining the melting machine used for casting  During casting metal must be heated 100°C above its melting temperature.

 The materials that are to be manipulated directly in the mouth eg-waxes, impression compound etc. should have a softening or melting point slightly above the body temperature and must harden to a desired degree of rigidity at body temperature.  An ideal solder for user on a particular alloy will have a melting temperature 30 C-100 C less than that of the alloy.

Specific heat is the quantity of heat needed to raise the temperature of one gram of the substance 1°C. Metals have low specific heat, while non metals have high specific heat.

Clinical Importance :  It is very important because it shows how much and how long a metal is to be heated to bring it to the melting point.  Because of the low specific heat of dental gold alloys, prolonged heating is unnecessary, during casting.

Water Sorption: It represents the amount of water adsorbed on the surface and absorbed into the body of the material. Clinical Importance: 1- Acrylic resin denture base materials have the tendency for water sorption. 2- Hydrocolloid impression materials will imbibe water if immersed in it leading to dimensional changes.

Tarnish: Surface discoloration on a metal or even a slight loss or alteration of the surface finish or lusture. Tarnish usually occurs due to- Formation of hard and soft deposits on the surface of the restorations.e.g.-calculus, mucin and plaque.

Corrosion is not a surface discoloration but actual deterioration of a metal by reaction with the environment.

Corrosion may be classified as:  Chemical corrosion is a direct effect on the metal surface by oxidation, halogenations or sulfurization reaction.  Acidity and alkalinity may give rise to corrosion by direct attack, particularly if there is any defect in the constitution of the alloy.

Electrolytic corrosion-  Chemical corrosion leads to electrolytic corrosion.  Electrolytic corrosion may takes place in the mouth with saliva acting as an electrolyte. when the following condition are present in it:  Different metals and alloys  Heterogeneous composition of alloys  Cold work metal  Difference in oxygen tension

 Galvanic corrosion  Stress corrosion  Concentration cell corrosion or Crevice corrosion

Galvanic Corrosion:  Galvanic corrosion occurs when dissimilar metals lie in direct physical contact with each other.  Eg- if a gold restoration comes in contact with an amalgam restoration, the amalgam forms the anode and starts corroding.  When two restoration touch causes sharp pain.  The best precaution is to avoid dissimilar metals in contact.

The perception of the Color of an object is the result of a physiological response to a physical stimulus (light). Light is an electromagnetic radiation that can be detected by the human eye. It can be seen that the visible electromagnetic radiation is in the range from nanometers.

Transparency is a property of a material, that allows the passage of light in such a manner that little distortion takes place so that objects can be clearly seen through them e.g. glass, pure acrylic resin.

Translucency : Property of the material, which allows the passage of some light and scatters or reflects the rest. In such manner, the object cannot be clearly seen through them Translucency decreases with increasing the scattering centers. e.g. tooth enamel, porcelain, composite and pigmented acrylic resin natural teeth.

Opacity is a property of the material that prevents the passage of light. Opaque material absorbs all of the light. Objects cannot be seen through them. Eg. metal-ceramic restoration Black color materials absorb all light colors. White color materials reflect all light colors. Blue color materials absorb all light colors but reflect its color.

 Ideal restorative material should match the color of the tooth it restores.  In maxillofacial prosthetics the color of the gums,external skin and eyes have to be duplicated.  Clinically in the operatory or dental lab, color selection is usually done by the use to shade guides.

COLOR Color parameters: A- Hue: It is the dominant wave length. It represents the color of the material, i.e yellow, green, red and blue. B- Chroma: It represents the strength of the color or degree of saturation of the color (color intensity).

C- Value: It represents the lightness or darkness of color (the amount of grayness). A tooth of low value appears gray and non–vital = DEAD, Therefore, it is the most important parameter. Because it is intimately related to the aspect of vitality in human teeth.

Shade guide is used for color matching. So, it is important to match colors under appropriate conditions. 1- Source: Different sources have different color content. i.e. Incandescent light has a color content different from that of fluorescent light.

2- Surrounding: Colors of wall, lips or clothes of the patient modify the type of light reaching the object. 3- Object: A- Translucency: It controls lightness or darkness of color. High translucency gives a lighter color appearance (higher value) i.e more vital tooth appearance

B- Surface texture (surface finish): This determines the relative amount of light reflected from the surface, smooth surface appears brighter than rough surface. C- Presence of scattering centers as inclusions or voids: This increase opacity and lower the value (more dark)

D- Fluorescence:  It makes the teeth bright and vital, as it increases the brightness.  Light used in “discotheques” or even under some fluorescent light under such conditions artificial teeth or restorations without any fluorescence completely black out.

E- Thickness: The thickness of a restoration can affect its appearance. Increase in thickness, increase opacity, and lower the value. F- Metamerism: It is the change of color matching of two objects under different light sources.

4- Observer: A- Color response: Eye responds differently among individuals. B- Color Vision: Some individuals may have color blindness and inability to distinguish certain colors. C- Color Fatigue: Constant stimulus of one color decreases the response to that color.

Light Amplification by Stimulate Emission of Radiation. The principle of laser production is simply that an element or compound (medium) can be excited by high energy to produce a special type of light called laser

Characteristics of laser beam: a- Monochromatic :.  all photons have the same wave length. b- Coherent :  all waves are bin phase (have the same speed ). c- Collimated :  all waves are parallel ( minimum divergence )

 Surgery for removal of soft tissues.  Removal of initial carious lesions.  Curing of composite resin.

 Philips’ Science Of Dental Material by Kenneth J. Anusavice DMD PhD (Jun 17, 2003)  Notes on Dental Materials (Dental Series) by E. C. Combe (Nov 1992)