Physical Properties

Physical properties involve reversible interactions of a material with its environment. A few common physical properties are reviewed here with respect to important dental situations. Metals, ceramics, polymers, and composites have different types and numbers of bonds. During temperature changes, these materials respond differently. During temperature increases, more frequent atomic motions stretch the bonds and produce net expansion. During temperature decreases, solids undergo contraction. The relative rate of change is called the coefficient of thermal expansion (or contraction). If it is referenced to a single dimension, it is called the linear coefficient of thermal expansion (LCTE), symbolized by the Greek letter alpha (α). The LCTE is expressed in units of “inch/inch/°F,” “cm/cm/°C,” or “ppm/°C.” Because the rate of change is small, the actual value is typically a multiple of 10⁻⁶ cm/cm/°C and is reduced to ppm/°C. Ceramics typically have an LCTE of 1 to 15 ppm/°C. Metals typically have values of 10 to 30 ppm/°C. Polymers typically have values of 30 to 600 ppm/°C. The LCTE of tooth structure is approximately 9 to 11 ppm/°C. It is important that the LCTE of a restorative material be as near that of tooth structure as possible. Online Table 18-1 presents examples of values for biomaterials.

One of the consequences of thermal expansion and contraction differences between a restorative material and adjacent tooth structure is percolation. This process is typified by an intracoronal amalgam restoration. During cooling, amalgam contracts faster than the tooth structure and recedes from the preparation wall, allowing the ingress of oral fluids. During subsequent expansion, the fluid is expressed. Cyclic ingress and egress of fluids at the restoration margin is called percolation (schematically presented in Online Fig. 18-6).

Other important physical properties involve heat flow through materials. Enamel and dentin are composed primarily of finely packed ceramic crystals (i.e., hydroxyapatite, Ca₁₀[PO₄]₆[OH]₂) that make those structures act as thermal insulators. If the tooth structure is replaced by a metallic restoration, which tends to be a thermal conductor, it may be important to provide thermal insulation to protect the dental pulp from rapid increases or decreases in temperature in the mouth. Generally, dental cements that may be used as bases under metallic restorations act as insulators. An advantage of a composite is low thermal conductivity. Composites do not need liners and bases to provide thermal insulation. Heat flow through a material is measured in terms of either the relative rate of heat conduction (thermal conductivity) or the amount of heat conduction per unit time (thermal diffusivity). Thermal diffusivity is the more important property because it determines the amount of heat flow per unit time toward the pulp through a restoration. The dental pulp can withstand small temperature changes (37-42°C) for relatively short periods (30–60 seconds) without any permanent damage.5,34,35 Under most circumstances, the microcirculation of the pulp transports the heat entering the pulp away to other parts of the body, where it is dissipated easily. Extreme temperature changes or extended times of exposure to high temperatures cause pulpal changes, however.

Electrical conductivity is a measure of the relative rate of electron transport through a material. This concept is important for metallic restorations that easily conduct electricity. If a galvanic cell (electrochemical cell) is present, electrical current may flow, and that process would stimulate nerves in the pulp. This may occur accidentally, such as when a tinfoil wrapper of chewing gum contacts a cast gold restoration and produces a minor electrical shock.

Mass properties of materials involve density or specific gravity. Density is a material’s weight (or mass) per unit volume. Most metallic materials have relatively high densities (6–19 gram per cubic centimeter [g/cm³]). Ceramic densities are typically 2 to 6 g/cm³. Polymer densities generally range from 0.8 to 1.2 g/cm³. Density is an important consideration for certain dental processing methods such as casting. Dense metal alloys are much easier to cast by centrifugal casting methods. Density is important in estimating the properties of mixtures of different materials (composites) because the final properties of the mixture are proportional to the volume of mixed materials (and not the weight). Occasionally, the relative density (or specific gravity) may be reported. Relative density is the density of the material of interest compared with the density of water under a standard set of conditions. At 25°C at 1 atmosphere of pressure, the density of water is 1 g/cm³. A specific gravity of 1.2 translates into a density of 1.2 g/cm³ under the same conditions.

Optical properties of bulk materials include interactions with electromagnetic radiation (e.g., visible light) that involve reflection, refraction, absorption (and fluorescence), or transmission (Online Fig. 18-7). The radiation typically involves different intensities for different wavelengths (or energies) over the range of interest (spectrum). Any of these interactive events can be measured using a relative scale or an absolute scale. When the electromagnetic radiation is visible light, the amount of reflection can be measured in relative terms as gloss or in absolute terms as percent reflection. Visible light absorption can be measured in absolute terms as percent absorption (or transmission) for every wavelength (in the visible spectrum). Color is a perception by an observer of the distribution of wavelengths. The same color sensation may be produced by different absorption spectra (metamerism). An individual’s eye is capable of sensing dominant wavelength, luminous reflectance (intensity), and excitation purity. Variations among individuals’ abilities to sense these characteristics give rise to varying perceptions of color.

Color measurement techniques do not measure these quantities directly. Color traditionally has been measured using the Munsell color system in terms of hue, value, and chroma. These terms correspond approximately to wavelength, intensity, and purity. The relationships of these quantities are represented schematically in Online Figure 18-8. Shade guides for matching restorative biomaterials to the tooth structure are based on this system of describing color. The quality of color also is measured by the Commission Internationale de l’Eclairage system as tristimulus values and reported as color differences (ΔL*, Δa*, and Δb*) compared with standard conditions.

Color is more than a property of a material. It is coupled with the electromagnetic spectrum involved (and the relative intensity of every wavelength in the spectrum) and the perceptive abilities of the observer. A practical example of the importance of the spectrum and the observer would be the appearance of anterior dental porcelain crowns in a nightclub in which the lighting involves low-level fluorescent lamps. The crowns fluoresce differently in that light and appear different from the adjacent natural teeth compared with a natural appearance in full-spectrum visible daylight.

Radiation of another wavelength may be preferentially absorbed (e.g., x-rays). Composites that contain lithium, barium, strontium, or other good x-ray absorbers may appear radiopaque (radiodense) in dental radiographs. Materials that are good absorbers (for whatever form of radiation) are described as opaque.

The appearance of a dental restoration is a combination of events of surface reflection, absorption, and internal scattering. The scattering simply may deflect the path of the radiation during transmission (refraction), or it may reflect the radiation internally from varying depths back out of a solid to the observer (translucency). Enamel naturally displays a high degree of translucency; translucency is a desirable characteristic for restorative materials attempting to mimic enamel.

A wet tooth that is isolated from the wetting by saliva soon has a transient whiter appearance. Most of this shade change is the effect of loosely bound water lost from subsurface enamel (by dehydration) between hydroxyapatite crystals. This increases the internal scattering of light, with much of it reflected back to the observer (see internal reflection in Online Fig. 18-7). This probably explains why it takes 15 to 20 minutes for the isolated tooth to develop the whiter appearance and 30 minutes or more for it to regain its original appearance after isolation is terminated. Larmas et al showed that 0.8% to 1% by weight of pulverized moist enamel is exchangeable water and that it can be removed at 4% relative humidity and 20°C.³⁶ Loosely bound water also provides channels for diffusion through enamel of ions and molecules. The direction of radiation may be perturbed as it crosses an interface from a medium of one type of optical character to another. Refractive index is the angle of changed path for a standard wavelength of light energy under standard conditions.

Another group of physical properties comprises surface properties. Surfaces are important because all restorative biomaterials meet and interact with the tooth structure at a surface. Also, all dental surfaces interact with intraoral constituents such as saliva and bacteria. Changing a material’s surface properties can mitigate the extent of that interaction. The type of interaction between two materials at an interface is defined as the energy of interaction, and this is conveniently measured for a liquid interacting with a solid under a standard set of conditions as the contact angle (θ). The contact angle is the angle a drop of liquid makes with the surface on which it rests (Online Fig. 18-9, A). This angle is the result of an equilibrium between the surface tensions of the liquid–gas interface (γ_LG), solid–gas interface (γ_SG), and solid–liquid interface (γ_SL). These relationships can be expressed as an equation (see Online Fig. 18-9, A). If the energy difference of the two materials in contact is large, they have a large contact angle. If the energy difference is very small, the contact angle is low, and the liquid appears to wet the solid by spreading. Wetting is a qualitative description of the contact angle. Good wetting, or spreading, represents a low contact angle. Partial (poor) wetting describes a contact angle approaching 90 degrees. Nonwetting is a contact angle approaching 180 degrees (see Online Fig. 18-4, B).

It is important that film formers such as varnishes, liners, cements, and bonding agents (all of which are discussed later in this chapter) have good wetting on tooth preparation surfaces or other restorative materials on which they may be placed so that they adapt to the microscopic interstices of the surfaces. In other instances, poor wetting may be an advantage. Experimental posterior composites have been formulated to have high contact angles to retard water or bacterial interactions or both. In most cases, wetting can be anticipated on the basis of the hydrophilicity (water-loving) or hydrophobicity (water-hating) of materials. Hydrophilic surfaces are not moistened well by hydrophobic liquids.

Mechanical Properties

The mechanical properties of a material describe its response to loading. Although most clinical situations involve complicated three-dimensional loading situations, it is common simply to describe the external load in terms of a single dimension (direction) as compression, tension, or shear. Combinations of these can produce torsion (twisting) or flexion (transverse bending). These modes of loading are represented schematically in Online Figure 18-10, with respect to a simple cylinder and a mesio-occlusal amalgam restoration. For testing purposes, it is often impossible to grip and pull a specimen in tension without introducing other, more complicated stresses at the same time. To circumvent problems for tensile testing of cylinders, it is possible to compress the sides of a cylinder and introduce stresses equivalent to tension. This variation of tension is called diametral tension (or diametral compression).

When a load is applied, the structure undergoes deformation as its bonds are compressed, stretched, or sheared. The load-deformation characteristics are useful only if the absolute size and geometry of the structure involved are known. It is typical to normalize load and deformation (in one dimension) as stress and strain. Stress (abbreviated σ) is load per unit of cross-sectional area (within the material). It is expressed in units of load per area (lb/in² = psi, or N/mm² = MPa). Strain (abbreviated ε) is deformation (ΔL) per unit of length (L). It is expressed in units of length per length (inch/inch, or cm/cm), which is a dimensionless parameter. A schematic summary is presented in Online Figure 18-11. During loading, bonds generally are not compressed as easily as they are stretched. Materials resist compression more readily and are said to be stronger in compression than in tension. Materials have different properties under different directions of loading. It is important to determine what the clinical direction of loading is before assessing the mechanical property of interest.

As loading continues, the structure becomes deformed. At first, this deformation (or strain) is completely reversible (elastic strain). Increased loading finally produces some irreversible strain as well (plastic strain), however, which causes permanent deformation. The point of onset of plastic strain is called the elastic limit (proportional limit, yield point). This point is indicated on the stress–strain diagram (see Online Fig. 18-11) as the point at which the straight line starts to become curved. Continuing plastic strain ultimately leads to failure by fracture. The highest stress before fracture is the ultimate strength (see Online Fig. 18-11, C). The total plastic tensile strain at fracture is called elongation; this also may be expressed as the percent elongation. Materials that undergo extensive plastic deformation before fracture are called ductile (in tension) or malleable (in compression). Materials that undergo very little plastic deformation are called brittle.

The slope of the linear portion (constant slope) of the stress–strain curve (from no stress up to the elastic limit) is called modulus, modulus of elasticity, Young’s modulus, or stiffness of the material and is abbreviated as E. It represents the amount of strain produced in response to each amount of stress. Ceramics typically have much higher modulus values (high stiffness) than polymeric materials (low stiffness). Because the slope of the line is calculated as the stress divided by the strain (E = σ/ε), modulus values have the same units as stress (i.e., pounds per square inch [psi] or megapascals [MPa]).

Two of the most useful mechanical properties are the modulus of elasticity and the elastic limit. A restorative material generally should be extremely stiff so that under load, its elastic deformation is extremely small. An exception is a Class V composite, which should be less stiff to accommodate tooth flexure. If possible, a material should be selected for an application so that the stress level during function usually does not exceed the elastic limit. If the stress exceeds the elastic limit by a small amount, the associated plastic deformation tends to be very small. If the stress is well beyond the elastic limit, the resulting deformation is primarily plastic strain, which at some point results in failure.

It is often convenient to determine the elastic limit in a relative manner by comparing the onset of plastic deformation of different materials using scratch or indentation tests, called hardness tests. The Mohs hardness scale ranks scratch resistance of a material compared with a range of standard materials (Online Table 18-2). Rockwell, Brinell, and Knoop hardness tests employ indenters instead. The energy that a material can absorb before the onset of any plastic deformation is called its resilience (see Online Fig. 18-11, C) and is described as the area under the stress–strain curve up to the elastic limit. The total energy absorbed to the point of fracture is called toughness and is related to the entire area under the stress–strain curve (see Online Fig. 18-11, C).

Mechanical events are temperature dependent and time dependent. These conditions must be described carefully for any reported mechanical property. Generally, as the temperature increases, the mechanical property values decrease. The stress–strain curve appears to move to the right and downward. The opposite occurs during cooling. As the rate of loading decreases, the mechanical properties decrease. This is described as strain rate sensitivity and has important clinical implications: To make a material’s behavior momentarily stiffer or more elastic, the material should be strained quickly. For recording undercut areas in an elastic intraoral impression, the material should be removed rapidly so that it is more elastic and more accurately records the absolute dimensions of the structures. This is an excellent example of applied materials science.

Other time-dependent responses to stress or strain also occur. Deformation over time in response to a constant stress is called creep (or strain relaxation). Materials that are relatively weak or close to their melting temperature are more susceptible to creep. Dental wax deforms (creeps) under its own weight over short periods. Traditional amalgam restorations are involved in intraoral creep. Deformation over time in response to a constant strain is called stress relaxation.

During loading, for all practical purposes, the strain below the elastic limit is all elastic strain. The amount of plastic strain is infinitesimal—so small that it is ignored. During multiple cycles, these very small amounts of plastic strain begin to accrue. After millions of cycles, the total plastic strain accumulated at low stress levels may be sufficient to represent the strain required to produce fracture. This process of multiple cycling at low stresses is called fatigue (Online Fig. 18-12, A). A standard engineering design limit for dental restorative materials is approximately 10 million cycles (or approximately 10 years of intraoral service). A rule of thumb is that materials on working surfaces of teeth are mechanically cycled approximately one million times per year on average. The curve correlating cyclic stress levels (S) to the number of cycles to failure (N) is called fatigue curve (S-N curve). These curves have been determined only for a few biomaterials because conducting the tests requires such a long time.³⁷ The compressive fatigue curves for Tytin and Dispersalloy amalgams are shown as part of Online Figure 18-12, B.

Mechanical properties can be used to describe the behavior of liquids and solids. As the temperature of a solid is increased, its stress–strain curve shifts downward and to the right. At the melting point, the stress–strain curve is a horizontal line lying at zero stress along the strain axis. Rather than examining the stress–strain behavior of liquids, it is more meaningful to examine the shear stress (τ) versus shear strain rate (γ). Well-behaved liquids (newtonian behavior) form a straight line (Online Fig. 18-13). Departures may occur that produce lines curving down (pseudoplastic behavior) or curving up (dilatant behavior). In some cases, the starting point of the line is shifted up along the shear stress axis, representing a material that does not start to flow until a critical shear stress has been reached (Bingham body behavior). Pseudoplastic and Bingham body behaviors are typical for biomaterials. The lines on these diagrams are described by a relatively simple equation, ηⁿ = τ/γ, which is similar to the equation for elastic modulus, E = σ/ε. The term η, or viscosity, is the resistance to flow or stiffness of the liquid. As the temperature is increased above the melting point, the viscosity behavior tips down and toward the right. A 37% phosphoric acid solution gel used for etching displays pseudoplastic Bingham body behavior. It does not flow until a critical shear stress is exceeded, and as the shear stress is linearly increased, the shear strain rate increases even more rapidly, producing more flow.

Chemical Properties

Chemical properties of a material are properties that involve changes in primary or secondary bonding. Primary bonding changes occur during chemical reactions and electrochemical reactions. Secondary bonding changes occur during processes such as adsorption and absorption. For metallic materials in the oral environment, the principal changes in primary bonding occur as a result of chemical corrosion (tarnish) or electrochemical corrosion. Chemical corrosion involves direct reaction of species by contact in solution or at an interface. An example of this process is the sulfide tarnishing of silver in amalgams to produce a black surface film. Another example is the oxidation of casting alloys containing very high copper to produce a green patina.

For any material, many electrochemical corrosion processes also may occur. Electrochemical corrosion involves two coupled chemical reactions (half cells) at separate sites, connected by two paths. One path (a circuit) is capable of transporting electrons, whereas the other path (an electrolyte) is capable of transferring metallic ions.³⁸ The basic components required for any electrochemical cell are (1) an anode (site of corrosion), (2) a cathode, (3) a circuit, and (4) an electrolyte (Online Fig. 18-14).

Electrochemical corrosion occurs intraorally when these four components are present. The conditions define which of the metallic sites acts as an anode. Many types of electrochemical cells are possible. Examples are shown schematically in Online Figure 18-15. Many of these electrochemical cells are possible in a single restorative dentistry situation. When an amalgam is in contact with a gold alloy restoration, galvanic, local galvanic, crevice, and stress corrosion are possible. Galvanic corrosion is associated with the presence of macroscopically different electrode sites (amalgam and gold alloy). Local galvanic corrosion (structure-selective corrosion) is caused by the electrochemical differences of different phases in a single material (e.g., amalgam). Electrochemical cells may arise whenever a portion of the amalgam is covered by plaque or soft tissue. The covered area has a locally lowered oxygen or increased hydrogen ion concentration, making it behave more like an anode and corrode (concentration cell corrosion). Cracks and crevices produce similar conditions and encourage concentration cell corrosion. Both corrosion processes are commonly termed crevice corrosion. When the restoration is under stress, the distribution of mechanical energy is not uniform, and this produces different corrosion potentials. This process is called stress corrosion.

Ceramics and polymers do not undergo chemical or electrochemical corrosion in the same sense. Most of their changes are related to chemical dissolution, absorption, or adsorption. Chemical dissolution normally occurs as a result of the solubilization created by hydrogen bonding effects of water and locally high acidity. The tooth structure is dissolved by high concentrations of lactic acid under plaque. Dental ceramics may be dissolved by extremely acidic fluoride solutions (acidulated phosphate fluoride) used for protecting outer layers of enamel against caries.

Adsorption involves the addition of molecules to a surface by secondary bonding, and absorption involves the penetration of molecules into a solid by diffusion. Protein adsorption alters the behavior and reactivity of dental material surfaces. Water absorption into dental polymers affects their mechanical properties.

Biologic Properties

Biologic properties of biomaterials are concerned with toxicity and sensitivity reactions that occur locally, within the associated tissue, and systemically. Most biomaterials interface locally with a variety of tissues (enamel, dentin, pulp, periodontium, cheek, tongue). Local reactions may vary. It is possible to evaluate local toxic effects on cells by clinical pulp studies or by tissue culture tests. Unset materials may release cytotoxic components. In clinical situations, however, this problem is rarely evident. Two important clinical factors determining toxicity are the exposure time and the concentration of the potentially toxic substance. Generally, restorative materials harden quickly or are not readily soluble in tissue fluids (or both). Potentially toxic products do not have time to diffuse into tissues. Even more importantly, the concentration makes the poison. Some authorities believe that if the amount of material involved is small, the pulp or other tissues can transport and excrete it without significant biochemical damage. Others believe that no threshold exists. A threshold level for toxicity is one below which no effect can be detected.

Systemic changes resulting from biomaterial interactions have been difficult, if not impossible, to monitor. Most evidence of biocompatibility has come from long-term usage and indirect monitoring. This is an area of increasing concern for understanding potential risks of new or alternative restorative biomaterials.

Finally, toxicology is undergoing rapid evolution. In the 1970s, most toxicologic screening involved the use of the Ames test for determining mutagenicity. The inventor of that test has now withdrawn support for the conclusions derived from that screening procedure.39,40 Results from earlier screening tests of biomaterials may need to be reconsidered.