Key Terms
RHEOLOGY
Dilatant Resistance to flow increases as the rate of deformation (shear strain rate) increases. The more rapidly dilatant fluids are stirred or forced through a syringe, the more viscous (thicker) and more resistant to flow they become.
Pseudoplastic Viscous character that is opposite from dilatant behavior in which the rate of flow decreases with increasing strain rate until it reaches a nearly constant value. Thus the more rapidly pseudoplastic fluids are stirred or forced through a syringe, the less viscous (thinner) and more easily they flow.
Rheology Study of the deformation and flow characteristics of matter (see viscosity ).
Thixotropic Property of gels and other fluids to become less viscous and flow when subjected to steady shear forces through being shaken, stirred, squeezed, patted, or vibrated. When the shear force is decreased to zero, the viscosity increases to the original value over a short time delay.
Viscosity Resistance of a fluid to flow (see rheology ).
STRUCTURAL RELAXATION
Creep Time-dependent plastic strain of a solid under a static load or constant stress.
Sag Irreversible (plastic) deformation of metal frameworks of fixed dental prostheses in the firing temperature range of ceramic veneers.
COLOR AND OPTICAL EFFECTS
Absorption The extent to which light is absorbed by the material in an object.
Chroma Degree of saturation of a particular hue (dominant color).
Color Sensation induced from light of varying wavelengths that reaches the eye.
Esthetics Principles and techniques associated with the development of the color and appearance required to produce a natural, pleasing effect in the dentition.
Hue Dominant color of an object (e.g . , red, green, or blue).
Metamerism Phenomenon in which the color of an object under one type of light source (e.g., room light) appears to change when illuminated by a different light source (e.g., sunlight).
Opacity The extent to which light does not pass through a material. No image and no light can be seen through an opaque object.
Reflection The amount of light that reflects from the surface of an object.
Refract/Refraction The degree to which light is bent when it passes from one medium to another. This makes a spoon appear bent in a glass of drinking water when light passes from air through glass into water, reflects from the spoon, and then passes back through water and glass into air. The index of refraction is a measure of this effect.
Translucency Property of an object in which light is scattered as it passes through, revealing a diffuse image.
Transmit/Transmission The amount of light passing through an object.
Transparency The extent to which light passes through a material and to which an undistorted image can be seen through it.
Value Relative lightness or darkness of a color. Also known as luminous reflectance and the gray scale .
THERMAL PROPERTIES
Coefficient of thermal expansion (linear coefficient of expansion) Change in length per unit of the original length of a material when its temperature is raised by 1 K.
Kelvin (K) The kelvin (K) temperature scale extends the degree Celsius scale such that zero degrees K is defined as absolute zero (= −273.15 °C). Temperatures on this scale are called “kelvins, ” not “ degrees kelvin,” kelvin is not capitalized, and the symbol (capital K) stands alone with no degree symbol; 1 K = 1 °C, K = °C + 273.15.
Thermal conductivity (coefficient of thermal conductivity) Property that describes the thermal energy transport in watts per second through a material 1 cm thick with a cross-sectional area of 1 cm 2 when the temperature differential between the surfaces of the material perpendicular to the heat flow is 1 K.
Thermal diffusivity Measure of the speed with which a temperature change will proceed through an object when one surface is heated.
ELECTROCHEMICAL PROPERTIES
Concentration cell Electrochemical corrosion cell in which the potential difference is associated with the difference in concentration of a dissolved species, such as oxygen, in solution along different areas of a metal surface. Pitting corrosion and crevice corrosion are types of concentration cell corrosion.
Corrosion Chemical or electrochemical process in which a solid, usually a metal, is attacked by an environmental agent, resulting in partial or complete dissolution.
Electromotive series Arrangement of metals by their equilibrium values of electrode oxidation potential. Used to judge the tendency of metals and alloys to undergo electrochemical (galvanic) corrosion.
Galvanic corrosion (electrogalvanism) Accelerated attack occurring on a less noble metal when electrochemically dissimilar metals are in electrical contact within a liquid corrosive environment.
Galvanic shock Pain sensation caused by the electrical current generated when two dissimilar metals are brought into contact in the oral environment.
Stress corrosion Degradation caused by the combined effects of mechanical stress and a corrosive environment, usually exhibited as cracking.
Tarnish Process by which a metal surface is dulled or discolored when a reaction with a sulfide, oxide, chloride, or other chemical causes surface discoloration through the formation of a thin oxidized film.
MAGNETIC MATERIALS
Magnet Metallic material in which the component atoms are so ordered that it attracts iron-containing objects or aligns itself in an external magnetic field.
Tesla Unit of flux density (T) of the magnetic field produced by a magnet.
Every dentist in the process of making or delivering a restoration must select various materials to complete the task. In making the choice, the dentist must have a thorough knowledge of the properties and behavioral characteristics of the materials chosen, namely, the physical and chemical properties. Physical properties are based on the laws of mechanics, acoustics, optics, thermodynamics, electricity, magnetism, radiation, atomic structure, and nuclear phenomena. Chemical properties are based on the ways in which substances interact, combine, and change at the molecular level, as governed by their outer orbital electrons.
From the categories of dental materials and challenges discussed in Chapter 1, there are specific behaviors of the materials during processing and use that dentists should consider, such as the flowability (or formability), esthetic appearance, thermal expansion, chemical durability, and magnetism of the materials. As examples, flow and viscosity (the resistance of a fluid to flow) are essential properties for mixing direct restorative materials ( Chapter 1, Direct Restorative Materials ) and gypsum products and impression materials ( Chapter 13 ). Creep (slow deformation under a static load) is relevant to the clinical performance of amalgam ( Chapter 8, Creep ) and the fabrication of metal-ceramic prostheses ( Chapter 10, Requirements of Metal Component ). The physical properties of color and thermal expansion are of particular importance to the performance of all restorations, especially for dental ceramics ( Chapter 10, Physical Properties ). Tarnish and corrosion are electrochemical properties that strongly affect the performance of metals and their alloys ( Chapters 8 and 9 ). Magnetic-able prostheses materials are often used for retaining implant-borne prostheses and for orthodontic tooth movement in place. Mechanical properties, a subset of physical properties based on the laws of mechanics, are discussed in Chapter 4 .
These phenomena will be discussed in greater detail in later chapters. However, having a firm grasp of the underlying principles that govern the properties is critical.
Rheology
Rheology is the study of the deformation and flow characteristics of matter under stress, whether liquid or solid. Dental professionals must manipulate a wide variety of dental materials in a fluid state to achieve successful clinical outcomes. Moreover, the manipulation and handling of a given material in the liquid state determine this material’s performance as a solid. 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 (see Chapter 13, Gypsum Products ).
Although a liquid at rest cannot support a shear stress (shearing force per unit shearing area), most liquids, when placed in motion, resist imposed forces that cause them to move. This resistance to fluid flow, termed viscosity, is controlled by internal frictional forces within the liquid. Thus viscosity is a measure of the consistency of a fluid and the fluid’s resistance to flow. Therefore highly viscous fluids flow slowly. Dental materials exhibit differing viscosities depending on their intended clinical application. In everyday life, we find wide differences in viscosities among such fluids as water, syrup, ketchup, moisturizing cream, and toothpaste. We will be discussing dental materials of various viscosities in later chapters.
The concept of viscosity is illustrated in Figure 3-1 . A liquid occupies the space between two flat surfaces, such as when a spatula is moved through a pasty fluid, for example, 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) with sufficient force (F) to overcome the frictional resistance within the fluid and cause the fluid to flow. As discussed in Chapter 1, What Is Force? , stress is the force per unit area that develops within a structure when an external force is applied. If the two surfaces have an area (A) in contact with the liquid, a shear stress (τ) can be defined as τ = F/A. The shear strain rate is <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='ε˙’>?˙ε˙
ε ˙
= V/d, where d is the shear distance of the upper surface relative to the fixed lower surface, and V is the velocity of the moving surface. As the shear stress τ increases, <SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='ε˙’>?˙ε˙
ε ˙
increases, and a curve can be obtained for shear stress versus strain rate.
ε ˙
( = V/d) of a viscous liquid between two plates caused by translation of the top plate at a velocity, V, relative to the stationary lower plate. This illustrates, for example, a spatula blade mixing dental cement on a mixing pad.
The rheologic behaviors of four types of fluids are shown in Figure 3-2 . Curves depicting shear stress versus shear strain rate are used to characterize the viscous behavior of fluids, and the viscosity ( η ) is defined as the shear stress divided by the strain rate, η = t / <SPAN role=presentation tabIndex=0 id=MathJax-Element-4-Frame class=MathJax style="POSITION: relative" data-mathml='ε˙’>?˙ε˙
ε ˙
<SPAN role=presentation tabIndex=0 id=MathJax-Element-5-Frame class=MathJax style="POSITION: relative" data-mathml='ΔEab*=[(ΔL*)2+(Δa*)2+(Δb*)2]12′>??∗??=[(??∗)2+(??∗)2+(??∗)2]12ΔEab*=[(ΔL*)2+(Δa*)2+(Δb*)2]12
Δ E a b * = [ ( Δ L * ) 2 + ( Δ a * ) 2 + ( Δ b * ) 2 ] 1 2
. The straight line from the origin in Figure 3-2 shows that the force applied is proportional to the strain rate of the fluid. This is known as Newtonian viscosity. A Newtonian fluid has a constant viscosity independent of the strain rate. Viscosity is measured in units of pascal second (Pa⋅s) or centipoise (cP; 1Pa⋅s = 10,000 cP). Pure water at 20 °C has a viscosity of 1.0 cP, whereas the viscosity of molasses is approximately 300,000 cP (30 Pa⋅s). This value is similar to that of tempered agar hydrocolloid impression material (281,000 cP or 28.1 Pa⋅s at 45 °C). Of the elastomeric impression materials, light-body (“thin” consistency) polysulfide has a viscosity of 109,000 cP (10.9 Pa⋅s), compared with 1,360,000 cP (136 Pa⋅s) for heavy-body (“thick” consistency) polysulfide at 36 °C. The viscosity of most fluids decreases rapidly with increasing temperature, which is the reason the reported values for material viscosity often include the temperature at measurement.
The viscosity of many dental materials decreases with increasing strain rate until the viscosity reaches a nearly constant value. That is, the faster materials are stirred, forced through a syringe, or squeezed, the less viscous and more fluid they become. This is called pseudoplastic viscosity and is illustrated by the change in the slope of the plot in Figure 3-2 . Liquids that show the opposite behavior are dilatant and become more rigid as the rate of deformation (shear strain rate) increases. That is, the faster they are stirred, squeezed, and so forth, the more viscous and resistant to flow they become. Some classes of materials behave like a rigid body until some minimum value of shear stress is reached. This is represented by the offset along the shear-stress axis. These fluids, which exhibit rigid behavior initially and then attain constant viscosity, are referred to as “plastic.” Ketchup is a familiar example—a sharp blow to the bottle is usually required to produce an initial flow.
Viscosity may also depend on the previous deformation of the liquid. Thixotropic fluids become less viscous upon repeated applications of pressure and stay at that lower viscosity for a short period of time before the viscosity of the previous state is regained. Dental prophylaxis pastes, plaster of Paris, resin cements, and some impression materials are thixotropic. The thixotropic nature of impression materials is beneficial because the material dispensed from the syringe will maintain a lower viscosity for a while, allowing better wetting of the tissue. The material then stops moving when a state of higher viscosity is regained. For instance, prophylaxis paste does not flow out of a rubber cup until this is rotated against the teeth to be cleaned. For pseudoplastic materials, the material regains the viscosity as soon as the stressing stops.
Structural Relaxation
Structural relaxation is a rheological phenomenon of solids that occurs so slowly that it is not noticed until the process completes. The two categories of interest in dentistry are (1) stress relaxation and (2) creep and flow.
Stress Relaxation
After an object or substance has been permanently deformed, a process of shape changing by force, the atoms and molecules are displaced and are no longer in equilibrium positions. There are trapped internal stresses that make the structure unstable. The substance can relieve these stresses through a solid-state diffusion process driven by thermal energy, where the atoms can slowly return to their equilibrium positions. The result is a change in the shape or contour of the solid as the atoms or molecules change positions. The process is known as stress relaxation and may cause warping or distortion of the object. Such stress relaxation can occur with elastomeric impression materials and can lead to distortions in the impression and a subsequent lack of fit of the prostheses, as discussed in Chapter 13, Elasticity and Viscoelasticity .
The rate of relaxation increases with an increase in temperature. For example, if a wire is bent, the wire may tend to straighten out if it is heated to a high temperature. At room temperature, any such relaxation caused by the rearrangement of metal atoms may be negligible. On the other hand, there are many noncrystalline dental materials (e.g., waxes, resins, and gels) that, when manipulated and cooled, can then undergo relaxation (with consequent distortion) at a slightly elevated temperature.
Creep and Flow
If a solid metal is held at a temperature near this metal’s melting point and is subjected to a constant load, 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. Metals for cast restorations or metal-ceramic prostheses have melting points (for pure metals) or melting ranges (for alloys) that are much higher than mouth temperatures, and they are not susceptible to creep deformation intraorally. However, some alloys used for long-span metal-ceramic bridge structures can sag under the influence of the mass of the prosthesis at porcelain-firing temperatures. This phenomenon is discussed further in Chapter 9, Requirements of Alloys for Metal-Ceramic Applications .
Dental amalgams, because of their mercury content, begin melting at temperatures a few hundred degrees above room temperature. Because of amalgam’s low melting range, this material can undergo creep at a restored tooth site under periodic sustained stress, such as would be imposed by patients who clench their teeth. Because creep produces continuing plastic deformation, the process can, over time, be very destructive to a dental amalgam filling. The relationship of this property to the behavior of amalgam restorations is discussed in Chapter 8, Creep .
The term flow, rather than creep, has generally been used in dentistry to describe the rheology of amorphous materials such as waxes ( Chapter 13, Flow of Dental Wax ). The flow of wax is a measure of the potential to deform under a small static load, which includes its own mass. Creep or flow characteristics are determined using a cylinder of prescribed dimensions subjected to a given compressive load for a specified time and temperature. The creep or flow is measured as the percentage decrease in length that occurs under these testing conditions. Creep may cause unacceptable deformation of dental restorations (e.g., low-copper dental amalgam) made from a material that is used clinically at a temperature near its melting point for an extended period.
Color and Optical Effects
Esthetic considerations in restorative and prosthetic dentistry have received increasingly greater emphasis in recent decades, and the challenges have grown even greater in the past few years following the widespread use of bleaching and whitening technologies. Thus 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.
Because esthetic dentistry imposes severe demands on the artistic abilities of the dentist and technician, knowledge of the underlying scientific principles of color and other optical effects is essential. This is especially true for the increasingly popular restorations that involve ceramic materials (see Chapter 10, Color Matching Ability and Aesthetic Qualities ).
Nature of Light and the Role of Human Vision
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), as shown in Figure 3-3 . For an object to be visible, this object must reflect or transmit incident light from an external source. The incident light is usually polychromatic, that is, a mixture of the various wavelengths, commonly known as “white” light. Incident light is selectively absorbed or scattered (or both) at certain wavelengths. The spectral distribution of the transmitted or reflected light resembles that of the incident light, although certain wavelengths are reduced in magnitude.
The phenomenon of vision, and certain related terminology, can be illustrated by considering the response of the human eye to light reflected from an object. Light from an object that is incident on the eye is focused in the retina and is converted into nerve impulses, which are transmitted to the brain. The retina has two types of cells that gather light: rods and cones. The rods are around the outer ring of the retina and are active in dim light. Cone-shaped cells in the retina are responsible for color vision.
Signals from the retina are processed by the brain to produce the psychophysiological perception of color. Because a neural response is involved in color vision, constant stimulation by a single color may result in color fatigue and a decrease in the eye’s response. Defects in certain portions of the color-sensing receptors result in the different types of color blindness; thus humans vary greatly in their ability to distinguish colors. In a scientific sense, one might liken the normal human eye to an exceptionally sensitive differential colorimeter, a scientific instrument that measures the intensities and wavelengths of light. Although colorimeters are more precise than the human eye in measuring slight differences in colored objects, they are inaccurate for rough or curved surfaces. The eye is highly sensitive in comparing two colors seen side by side, whether on rough, smooth, flat, or curved surfaces.
The Nature of the Object Under View
Esthetics is critically important in dealing with dental restorative materials. 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 the surface, absorption, refraction, or transmission (i.e., by passing through unchanged). These phenomena determine the opacity, translucency, or transparency of an object. Light reflected from rough surfaces scatters in many directions because light 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.
The opacity of a material is related to the amount of light the material can absorb and/or scatter. The opposite of opacity is translucency. For example, if 1-mm thicknesses of each of two materials absorb light passing through them, the material that absorbs 20% of the light is less opaque and more translucent than the material that absorbs 50%. Transparent materials are at the far end of the translucency scale, absorb no light, and transmit 100% of the light that passes through them.
Enamel is a composite material consisting of hydroxyapatite crystals in a protein matrix. When light strikes enamel, some is reflected, some refracted, some absorbed, and some transmitted. Enamel has a refractive index of 1.65 and is translucent.
How is color described objectively and quantitatively?
Three Dimensions of Color
Verbal descriptions of color are not precise enough to describe the appearance of teeth. For example, the definition of puce is “a brilliant purplish-red color,” according to Microsoft’s Encarta World English Dictionary, whereas Webster’s Third New International Dictionary defines puce as “a dark red that is yellower and less strong than cranberry, paler and slightly yellower than average garnet, bluer, less strong, and slightly lighter than pomegranate, and bluer and paler than average wine.” These definitions are too variable, complex, and imprecise to describe a desired color of a dental crown to a laboratory technician. Such a written description is subjective and does not clearly and unambiguously allow someone to perceive the color. To overcome this problem, color perception is described by three independent and objective variables shown in the Munsell color system: value, hue, and chroma ( Figure 3-4 ).
Value is also known as the gray scale and is the vertical, or z -axis, on Figure 3-4 . 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 (higher value) and darker shades (lower value). For example, the yellow of a lemon is lighter than the red of a cherry. For a light-diffusing and light-reflecting object such as a tooth or dental crown, the value identifies the lightness or darkness of a color, which can be measured independently of the hue and chroma.
Hue is the dominant color of an object, for example, red, green, or blue. This refers to the dominant wavelengths present in the spectral distribution. The continuum of these hues creates the three-dimensional (3-D) color solid shown in Figure 3-4 .
Chroma is the degree of saturation of a particular hue. For example, red can vary from “scarlet” to light pink, where scarlet has a high saturation and pink has a low saturation. The yellow color of a lemon is a more saturated, “vivid,” color than that of a banana, which is a less saturated, “dull” yellow. Chroma varies radially, perpendicular to the value axis ( Figure 3-4 ). Colors in the center are dull (gray). In other words, the higher the chroma, the more intense the color. Chroma is not considered separately in dentistry and is always associated with the hue and value of dental tissues, restorations, and prostheses. The components of a color space can be more easily visualized by the individual parts in Figure 3-5 , here seen as discs stacked along the value axis on a scale of 0 to 10 from black to white. Around the periphery are 10 basic hues (dominant wavelength/color). Chroma radiates out from the value axis like the spokes of a wheel.
The color space can also be quantified by the CIE (Commission Internationale de l’Eclairage) L*a*b* color space, in which L* represents the value of an object from darker to lighter, a* is the measurement along the red (+a*)-green (–a*) axis, and b* is the measurement along the yellow (+b*)-blue (–b*) axis ( Figure 3-4 ).
Figure 3-6 represents a horizontal plane, perpendicular to the L* axis, through a CIE L*a*b* color space. The dot above the letter A is the color of a red apple, which is expressed by L* = 42.8, a* = 47.1, and b* = 13.9. In comparison, the dot next to the letter B is for a dental body (gingival) porcelain of shade A2 with a higher (lighter) L* of 73.0, a lower a* of 1.0, and a higher b* of 14.4.
In the CIE L*a*b* color space, the difference between two colors (ΔE* or ΔE* ab ) would be the distance between the two points representing the two colors and can be determined by the following formula:
ΔEab*=[(ΔL*)2+(Δa*)2+(Δb*)2]12
The color difference formula is designed to provide a quantitative representation of the color difference between a pair of colored specimens under a given set of experimental conditions. The value, however, is of little clinical significance without an understanding of the magnitude of color difference considered perceptible or acceptable by observers. The smallest perceptible color difference or perceptibility threshold (PT) refers to the smallest color difference that can be detected by 50% of observers under standardized conditions. The range of PT values reported in the literature is 0.4 to 4.0, with ΔE* = 1.0 being the value reported most often. Similarly, the difference in color that is esthetically acceptable by 50% of observers is known as the acceptability threshold (AT). The range of AT values reported in the literature is 2.0 to 6.8, with a range of 3.3 to 3.7 being the value reported most often. International Standards Organization (ISO) 28642:2016 uses a PT of ΔE* = 1.2 and an AT of ΔE* = 2.7, established in a prospective multicenter research project.
Color Matching
In dental practice, color matching is most often performed with the use of a shade guide, such as the one shown in Figure 3-7 , to select the color of ceramic veneers, inlays, or crowns. Shade-guide tabs are used in much the same way as paint chips to match the color of house paint. The individual shade tabs shown in the top row of Figure 3-7 are grouped 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), as shown in the middle row of Figure 3-7 . Matching of tooth shades is simplified by the arrangement of tabs by value; this arrangement has been found to be easier and more reliable to use. The bottom row of Figure 3-7 shows the arrangement according to the color difference as calculated by equation (1) , with respect to the lightest tab. Although a reasonable match can be achieved clinically between a tooth (or restoration) and one of the shade-guide tabs, relaying the information to a lab or technician can prove to be a challenge. To ensure esthetic results, additional information, such as drawings, descriptions, and photographs ( Figure 3-8 ), should also be sent. Obviously, if the technician can see the actual teeth, the probability of achieving an acceptable color match will be even greater.
