Introduction to Dental Materials PDF

INTRODUCTION TO DENTAL MATERIALS Lecturer: Dr. Sagon-Kendall  INTRODUCTION The overriding goal in dentistry is to improve the quality of life for the patient. Historically, several items has been used as tooth crown and root replacement such as animal teeth and bone, human teeth, sea shell...

INTRODUCTION TO DENTAL MATERIALS Lecturer: Dr. Sagon-Kendall  INTRODUCTION The overriding goal in dentistry is to improve the quality of life for the patient. Historically, several items has been used as tooth crown and root replacement such as animal teeth and bone, human teeth, sea shells and ceramics. Restorative materials for the replacement of loss tooth structure has evolved over the centuries. Modern dentistry began in 1728 when Fauchard published a treatise describing many types of dental restorations including a method of construction of artificial dentures using ivory. The four groups of materials used in dentistry today are metals, ceramics, composites and polymers. Dental Materials may be classified as : 1. Preventive Materials 2. Restorative Materials 3. Auxiliary Materials  PREVENTIVE DENTAL MATERIALS Preventive materials include pit and fissure sealants; sealing agents used to prevent leakage, materials with antibacterial effects and materials used primarily because they release fluoride. In some cases, it can be used as a restorative material RESTORATIVE DENTAL MATERIALS These materials are used to replace, or repair lost structure of the tooth. E.g. resin composites, metals, denture polymers. Can be classified as direct or indirect restorative materials depending on whether the restoration was fabricated intraorally or extraorally  AUXILIARY DENTAL MATERIALS These are substances used in the fabrication of dental prosthesis and appliance, but it is not part of these devices. E.g. impression materials, gypsum products, dental waxes  CRITERIA OF DENTAL MATERIALS Dental materials must not be poisonous or harmful to the body. They must not be harmful or irritating to the tissues of the oral cavity. The materials must help protect the tooth and tissues of the oral cavity. They must resemble the natural dentition as closely as possible to be aesthetically pleasing. The materials must be easily formed and placed in the mouth to restore natural contours. Dental materials must conform and function despite limited access, wet conditions and poor visibility. PROPERTIES OF DENTAL MATERIALS Dr. Sagon-Kendall  PHYSICAL PROPERTIES Physical properties are based on the laws of mechanics, acoustics, optics, thermodynamics, electricity, magnetism, radiation, atomic structure, and nuclear phenomena. Hue, value, and chroma relate to color and perception and are physical properties based on the laws of optics, which is the science that deals with the phenomena of light, vision, and sight. Thermal conductivity, diffusivity, and expansion are physical properties based on the laws of thermodynamics 9  As examples, the physical properties of color and thermal expansion are of particular importance to the performance of dental ceramics. Flow and viscosity (the resistance of a fluid to flow) are essential properties of impression materials. Creep (slow deformation under a static load) is relevant to the clinical performance of amalgam. Tarnish and corrosion are electrochemical properties that strongly affect the performance of metals and their alloys 10  RHEOLOGY Rheology is the study of the deformation and flow characteristics of matter, whether liquid or solid. Viscosity is the resistance of a fluid to flow. Dental professionals must manipulate a wide variety of dental materials in a fluid state to achieve successful clinical outcomes. Moreover, the success or failure of a given material may be as dependent on its manipulation and handling properties in the liquid state as it is on its performance properties as a solid 11  Most dental materials are initially in a fluid state so that they can be placed and shaped as required; then they undergo transformation to a solid state, in which they are durable and perform their function. Cements and impression materials undergo a fluid-to solid transformation in the mouth. Gypsum products used in the fabrication of models and dies are transformed extraorally from fluid slurries into solids. Amorphous materials such as waxes and resins appear solid but actually are supercooled liquids that can flow plastically (irreversibly) under sustained 12  VISCOSITY Viscosity is a measure of the consistency of a fluid and its resistance to flow. A highly viscous fluid flows slowly. Dental materials have different viscosities depending on their intended clinical application. A liquid occupies the space between two flat surfaces, as, for example when a spatula is moved through a pasty fluid such as a dental cement to blend two components on a mixing pad. The mixing surface is fixed, and the upper surface (e.g., a spatula blade) moves to the right at a given velocity (V). A force (F) is required to overcome the frictional resistance within the fluid (i.e., the viscosity) and cause 13  CREEP AND FLOW If a metal is held at a temperature near its melting point and is subjected to a constant applied stress, the resulting strain will increase over time. Creep is defined as the time dependent plastic strain of a material under a static load or constant stress. Dental amalgams contain from 42% to 52% of mercury by weight and begin melting at temperatures only slightly above room temperature. Because of its low melting range, dental amalgam can undergo creep at a restored tooth site under periodic sustained stress, such as would be imposed by patients who clench their Presentation title 14 teeth. Because creep produces continuing plastic  The term flow, rather than creep, has generally been used in dentistry to describe the rheology of amorphous materials. Creep may cause unacceptable deformation of dental restorations (such as low-copper dental amalgam) made from a material that is used clinically at a temperature near its melting point for an extended period. Presentation title 15  COLOUR AND OPTICAL EFFECTS Another important goal of dentistry is to restore or improve esthetics—the color and appearance of natural dentition. The development of a general-purpose, technique insensitive, direct-filling, tooth-colored, color-stable restorative material remains one of the more serious challenges of current dental materials research. 16 Light is electromagnetic radiation that can be detected by the human eye. The eye is sensitive to wavelengths from approximately 400 nm (violet) to 700 nm (dark red). For an object to be visible, it must reflect or transmit light incident on it from an external source. The incident light is usually polychromatic; that is, a mixture of the various wavelengths, commonly known as “white” light. 70 0 60 0 50 0 Presentation title 400 17  For good esthetics, the interaction of light with restorative materials must mimic the interaction of light with natural teeth. The nature of the restorative material, or that of any object under view, determines how that object will appear. Electromagnetic radiation in the visible region interacts with an object through reflection from its surface, absorption, refraction, or transmission (i.e, by passing through unchanged.) These phenomena determine the opacity, translucency, or transparency of an object. Light from rough surfaces scatters in many directions because it is reflected at many angles by the uneven surface. This leads to an appearance that ranges from mirrorlike for a perfectly smooth surface (termed specular reflectance) to the flat, dull appearance (termed diffuse reflectance) of a surface such as chalk. 18  THREE DIMENSIONS OF COLOUR Verbal descriptions of color are not precise enough to describe the appearance of teeth. Color perception is described by three objective variables: hue, value, and chroma. 19  Hue: The dominant color of an object. This refers to the dominant wavelengths present in the spectral distribution. The continuum of these hues creates the 3-D color solid shown in the image. Value: Value is also known as the gray scale. It is the vertical, or Z-axis, of the image shown. Value increases toward the high end (lighter) and decreases toward the low end (darker). Value is also expressed by the “lightness” factor, with varying levels of gray between the extremes of white and black. Teeth and other objects can be separated into lighter shades The three dimensions of color space. Value (higher value) and darker shades (lower value). For increases from black at the bottom center example, the yellow of a lemon is lighter than the red to white at the top center. Chroma increases from the center radially outward, of a cherry. For a light-diffusing and light-reflecting and changes in hue occur in a object such as a tooth or dental crown, value circumferential direction. A, 3-D Munsell identifies the lightness or darkness of a color, which Color Space. B, Partial color space revealing hue, value, and chroma regions. can be measured independently of the hue (Courtesy of Minolta Corporation, Chroma: Chroma is the degree of saturation of a Instrument Systems Division, Ramsey, NJ.) particular hue. For example, red can vary from “scarlet” to light pink, where scarlet has a high saturation and pink has a low saturation. In other words, the higher the chroma, the more intense the 20 color.  THE EFFECT OF LIGHT SOURCE Objects that appear to be color-matched under one type of light may appear different under another type. This phenomenon is called metamerism. Thus, if possible, color matching should be done under two or more different light sources, one of which should be daylight, and the laboratory shade matching procedures should be performed under the same lighting conditions. 21 COLOUR MATCHING In dental practice, color matching is most often performed with the use of a shade guide to select the color of ceramic veneers, inlays, or crowns. The individual tabs according to hue (A, B, C, and D, where A = red-brown, B = red-yellow, C = gray, D = red-gray), followed by value (1 to 4, or lightest to darkest). This arrangement follows the “classical” order originated by Vita for porcelain. Recently, however, the trend is to arrange shade guides in decreasing order of value (lightest to darkest: B1, A1, B2, D2, A2, C1, C2, D4, A3, D3, B3, A3.5, B4, C3, A4, C4). Matching tooth shades simplified by the arrangement of tabs by value; this arrangement has been found to be easier and more reliable to use Presentation title 22  THERMAL PROPERTIES When restorative materials are placed in deep cavities, the heat transmitted to vital dental pulp must be limited so as to prevent thermal shock and trauma. In other circumstances, as with denture bases in contact with mucosal surfaces, the transmission of a certain amount of thermal energy is desirable to convey the sensations of heat and cold associated with food and beverages. Such attributes are governed by the properties of thermal conductivity and thermal diffusivity. Another category of thermal behavior is the expansion when heated and contraction when cooled. Presentation title 23  THERMAL CONDUCTIVITY Thermal conductivity is the physical property that governs heat transfer through a material by conductive flow. The conduction of heat within a solid involves the transfer of thermal energy from one part of a material to another across a temperature gradient. It is defined as the quantity of heat in calories per second passing through a material l cm thick with a cross section of 1 cm2 having a temperature difference of l K (1 °C) and is measured under steady-state conditions in which the temperature 24  In general, thermal conductivities increase in the following order: polymers < ceramics < metals, although there are exceptions. Materials that have a high thermal conductivity are called conductors, whereas materials of low thermal conductivity are called insulators. The higher its thermal conductivity, the greater the ability of a substance to transmit thermal energy. Presentation title 25  THERMAL DIFFUSIVITY Thermal diffusivity is a measure of the speed with which a temperature change will spread through an object when one surface is heated. A material with a high density and high specific heat will likely have a low thermal diffusivity. Such a material changes its temperature very slowly. Low heat capacity and high thermal conductivity lead to high diffusivity, and temperature changes transmit rapidly through the material. 26 Material Density Specific Thermal Thermal heat conductivity diffusivity (cm s- Water 1.00 1.00 0.44 0.0014 Dentin 2.14 0.30 0.57 0.0018- 0.0026 Glass 2.13 0.27 0.51-0.72 0.0022 Ionomer Zinc 2.59 0.12 1.05 0.0030 phosphate Composite 1.6-2.4 0.20 1.09-1.37 0.0019- 0.0073 Enamel 2.97 0.18 0.93 0.0047 Amalgam 11.6 0.005 22.6 0.96 Gold 19.3 0.03 297 1.18 27  COEFFICIENT OF THERMAL EXPANSION When materials undergo a temperature increase, the vibrational motion of atoms and mean interatomic (bond) distances increase. This results is an increase in volume—an expansion. The increase is described by the coefficient of thermal expansion. This parameter is extremely important in dental applications as broad ranging as producing cast restorations that fit and maintaining the seal at a restoration margin. Its influence often dictates the procedures that have been developed for using wax patterns, casting metal crowns, placing amalgam and composite resin restorations, and preparing metal- ceramic crowns and bridges. A tooth restoration may expand or contract more than the tooth during a change in temperature; thus there may be marginal microleakage adjacent to the restoration, or the restoration may debond from the tooth. 28  ELECTROCHEMICAL PROPERTIES Tarnish and Corrosion Tarnish is a surface discoloration on a metal or a slight loss or alteration of the surface finish or luster. In the oral environment, tarnish often occurs from the formation of deposits on the surface of a restoration. Tarnish also arises from the formation of thin films, such as oxides, sulfides, or chlorides. The latter phenomenon may be only a simple surface deposit, and such a film may even be protective, as discussed subsequently. However, it is often an early indication and precurser of corrosion. 29  Corrosion is a process whereby deterioration of a metal is caused by reaction with its environment. In due course, corrosion can cause severe and catastrophic disintegration of metals. Even if highly localized, corrosion may cause mechanical failure of a structure even though the actual volume of material lost is quite small. Corrosive disintegration can take place through the action of moisture, atmosphere, acid or alkaline solutions, and certain chemicals. Corrosion of a metal is either a chemical or an electrochemical process, in each of which the first step is the loss of an electron. 3  Chemical corrosion is the direct combination of metallic and nonmetallic elements to yield a chemical compound through oxidation reactions. This mode of corrosion is also referred to as dry corrosion, since it occurs in the absence of water or another fluid electrolyte. Electrochemical corrosion, also known as galvanic corrosion, requires the presence of water or some other fluid electrolyte and a pathway for the transport of electrons (i.e., an electrical current). It is also referred to as wet corrosion, since it requires a fluid electrolyte. Electrochemical corrosion is seldom isolated and almost invariably is accompanied by chemical corrosion. However, the electrochemical mode of corrosion is the more important for dental materials 31 PROPERTIES OF DENTAL MATERIALS- MECHANICAL PROPERTIES Knowing the mechanical properties of dental materials will allow you to differentiate the potential causes of clinical failures that may be attributed to material deficiencies, design features, dentist errors, technician errors, or patient factors such as diet, biting force magnitude, and force orientation. Mechanical properties are defined by the laws of mechanics—that is, the physical science dealing with forces that act on bodies and the resultant motion, deformation, or stresses that those bodies experience. Presentation title 33 Mechanical properties are expressed most often in units of stress and/or strain. The stressing rate is also of importance since the strength of brittle materials increase with an increase in the rate at which stress is induced within their structures. They represent measures of (1)elastic or reversible deformation (e.g., proportional limit, resilience, and modulus of elasticity); (2) plastic or irreversible deformation (e.g., percent elongation and hardness); or (3) a combination of elastic and plastic-deformation (e.g., toughness and yield strength). Presentation title 34  STRESSES AND STRAINS Stress is the force per unit area acting on millions of atoms or molecules in a given plane of a material. For dental applications, there are several types of stresses that develop according to the nature of the applied forces and the object’s shape. These include tensile stress, shear stress, and compressive stress. The strength of a material is defined as the average level of stress at which it exhibits a certain degree of initial plastic deformation (yield strength) or at which fracture occurs (ultimate strength) in test specimens of the same shape and size. 35 Strength is dependent on several factors, including the (1)stressing rate, (2)shape of the test specimen, (3)size of the specimen, (4)surface finish (which controls the relative size and number of surface flaws) (5)number of stressing cycles, and (6)environment in which the material is tested. However, the clinical strength of brittle materials (such as ceramics, amalgams, composites, and cements) is reduced when large flaws are present or if stress 36 concentration areas exist because of Stresses greater than the proportional limit cause permanent deformation and, if high enough, may cause fracture. For brittle materials that exhibit only elastic deformation and do not plastically deform, stresses at or slightly above the maximal elastic stress (proportional limit) result in fracture. 37  MECHANICAL PROPERTIES Strain may be either: i. Elastic Strain: It is reversible, it disappears after the stress is removed. ii. Plastic Strain: It is permanent deformation of the material; it does not decrease when the force is removed. iii. Plastic and elastic: iv. Visoelastic: deform by exhibiting both viscous and elastic characteristics. These materials  ELASTIC PROPERTIES ELASTIC MODULUS MODULUS (YOUNG’S MODULUS OR OF ELASTICITY) Elastic modulus describes the relative stiffness or rigidity of a material. The elastic modulus of enamel is about three times greater than that of dentin and, depending on the study considered, it can be as much as seven times higher. Dentin is capable of sustaining significant plastic deformation under compressive loading before it fractures. Thus, enamel is a stiffer and more brittle material than dentin and unsupported enamel is more susceptible to fracture. Conversely dentin is more flexible and tougher. 39  STRENGTH PROPERTIES Strength is equal to the degree of stress necessary to cause either fracture (ultimate strength) or a specified amount of plastic deformation (yield strength). When we describe the strength of an object or a material, we are most often referring to the maximum stress that is required to cause fracture. Keep in mind, however, that strength values reported by manufacturers usually represent mean values, which means that 50% of the tested specimens have failed below this strength. 40 The strength of a material can be described by one or more of the following properties: (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; and (4)ultimate tensile strength, shear strength, compressive strength, and flexural strength, each of which is a measure of stress required to fracture a material. 41  TOUGHNESS Toughness is defined as the amount of elastic and plastic deformation energy required to fracture a material. Fracture toughness is a measure of the energy required to propagate critical flaws in the structure 42  BRITTLENESS Brittleness is the relative inability of a material to sustain plastic deformation before fracture of a material occurs. For example, amalgams, ceramics, and composites are brittle at oral temperatures (5 to 55 °C). They sustain little or no plastic strain before they fracture. In other words, a brittle material fractures at or near its proportional limit. The failure of these materials in clinical usage is most often associated with their low tensile strengths and the presence of flaws within the tensile stress region. 43  DUCTILITY AND MALLEABILITY Ductility represents the ability of a material to sustain a large permanent deformation under a tensile load up to the point of fracture. For example, a metal that can be drawn readily into a long thin wire is considered to be ductile. Malleability is the ability of a material to sustain considerable permanent deformation without rupture under compression, as in hammering or rolling into a sheet. Gold is the most ductile and malleable pure metal, and silver is second. Presentation title 44  HARDNESS Hardness is an index of the ability of the material to resist abrasion or wear. However, hardness alone may not be appropriate to evaluate the abrasion or wear resistance of the different classes of material such as metallic materials compared with a synthetic resin. Other factors that affect the wear or abrasion resistance are biting force, chewing frequency, abrasiveness of the diet, composition of intraoral liquids, temperature changes, surface roughness, physical properties of the material and surface