Сraig. Dental Materials

also an important factor in penetration. Fig. 2-9 describes the combined effects of gap width and contact angles on the capillary penetration of water between two plates. Two contact angles are involved, because one plate might be easily wetted, for example, glass, and the other might be a poorly wetted polymer.

y (73.2%Ag/26.8% Sn)

Eutectic (71.9%Ag/28.1% Cu)

Mynol

Dispersalloy

Ago

4420 SnO SnO,

From Baran G, O'Brien WJ:J A m DentAssoc 94398, 1977. Copyright by the American Dental Association. Reprinted by permission.

Most restorative materials used at present in dentistry do not adhere strongly to tooth structure. As a result, a crevice usually exists between the restoration and the tooth tissue into which mouth fluids penetrate because of capillary action. The importance of gap or width has long been recognized as a factor influencing the degree of marginal leakage. However, wetting is

PENETRATION COEFFICIENT

Another aspect of capillary phenomena involves the rate of penetration of a liquid into a crevice. An example is the penetration of a liquid prepolymer sealant into a fissure and the fine microscopic spaces created by etching of an enamel surface. The properties of the liquid affecting the rate of penetration may be related to the penetration coefficient (PC) where y is the surface tension, 71 is the viscosity, and 8 is the contact angle of the sealant on the enamel:

ycos 0

PC=-

2rl

The penetration coefficients for sealants have been shown to vary from 0.6 to 12 cm/sec. Narrow occlusal fissures can be filled almost completely if the penetration coefficient value is at least 1.30 cm/sec when the sealant is applied at a proximal edge of the fissure on the occlusal surface and allowed to flow to the other edge. If the sealant is painted over the occlusal surface, air trapped in the fissure prevents penetration beyond a certain depth. The same analysis applies to the penetration of liquid sealants into the

etched surface of enamel to form tags, as shown in Fig. 2-10.

ISOLATED CAPILLARIES

Still another aspect of capillary phenomena is the adhesion of liquid bridges between solids. Liquid bridges are considered a contributing factor in denture retention (Fig. 2-11, A) when a thin film of saliva was present between the denture material and the mucosa. The source of capillary adhesion is the arrangement called an isolated capillay.As illustrated in Fig. 2-11, C, the differential pressure between a capillary and a connected reservoir is balanced by the hydrostatic pressure of capillary elevation. In capillaries isolated from a reservoir, as shown in Fig. 2-11, A and B, a negative differential pressure exerts an adhesive force. This force is partly responsible for denture retention, but it operates only if the film of saliva is isolated at the periphery of the denture. If the saliva film beneath the denture is connected to a reservoir of saliva beyond the

Fig. 2-11 Two classes of capillaty systems.A and B, Isolated capillaries (isocaps).C, Connected capillaly.

(From O'Brien,WJ: J Dent Res 52:545, 1973.)

borders of the denture, a negative differential pressure does not develop. Viscosity of the saliva film, however, offers some resistance to separation of the denture from the mucosa and thus contributes to retention.

Isolated capillaries form around teeth when small quantities of saliva are trapped in inter-

Fig. 2-12 Formation of isolated capillaries around teeth.

(From O'BrienWJ: 1 Dent Res 52:547, 1973.)

proximal spaces and occlusal fissures, as shown in Fig. 2-12. It is interesting that the growth rate of bacteria has been found to increase under these conditions of negative pressure.

FORCES INVOLVED IN OF

RETENTION

The accuracy of fit of a denture has been cited as an important factor in the retention of denture bases without any explanation as to why a better fit results in better retention. A technical discussion of all factors involved in the retention of dentures is not within the scope of this text, but a qualitative discussion is helpful in understanding the role of fit in denture retention. Factors include: (1) capillary forces involving the liquid film between the oral tissues and the denture base; (2) surface forces controlling the wetting of the plastic denture base by the saliva; (3) the seating force applied to the denture, which, for the most part, determines the thickness of the saliva film between the denture and the oral tissues; (4)the surface tension of the saliva; ( 5 ) the viscosity of the saliva; and (6) the atmospheric pressure.

The capillary force, F: responsible for retention of a denture can be expressed by the following equation:

yA(cos0, + cos 0,)

F =

dg

where y is the surface tension of saliva, A is the surface area of the tissue surface of the denture, 0, and 0, are the contact angles of saliva against the plastic and the oral mucosa, d is the film thickness of the saliva between the denture and the tissue, and g is the gravitational constant.

Certain factors, such as wetting, are more important in the retention of a denture under static conditions, whereas other factors, such as capillarity and atmospheric pressure, are more effective when a force tends to dislodge the denture. The wetting of the plastic surface depends on the energy relationship between the solid and the liquid. If complete wetting occurs, the liquid will spread on the solid. If partial wetting occurs, the liquid will form droplets on the surface. The plastics used in dentistry are only partially wetted by saliva, but the wetting may improve after contact with oral fluids because of the adsorption of certain components of the saliva by the plastic surface.

The capillary force, which helps restrain any dislodging force on the denture, is increased by complete wetting of the denture surface, high surface tension of the saliva, and large tissue contact area of the denture. Patients with stringy (low surface tension) saliva experience difficulty with retention of dentures. The viscosity of saliva is low, and little difference in retention is observed regardless of whether the load is applied slowly or rapidly. The use of denture pastes, however, provides a film of increased thickness and viscosity; if they are used, greater retention is observed when the load is applied rapidly. It has been shown that the use of adhesives has no detrimental effect on the health of the supporting tissues.

The viscosity of saliva from dentate and edentulous patients differs, with edentulous patients demonstrating a lower viscosity. The lower vis-

cosity may be caused by stimulation of salivary flow; the denture, acting as a foreign object, causes a larger flow of saliva with a lower mucin content. Another possibility is that the mandibular denture may obstruct the ducts of the submandibular glands, which produce a greater proportion of mucous glycoprotein. Saliva also appears to act as a non-newtonian fluid, exhibiting lower viscosities at higher shear rates or shear thinning.

The capillary force is diminished when the distance between the denture and the oral tissues is increased. It is reduced to a very small value if the periphery of the denture is immersed in saliva or other fluids. This explains some of the difficulty patients may encounter when drinking fluids and the variation in retention of the maxillary compared with the mandibular denture. At best, the role of atmospheric pressure is that of a temporary restraining force, because the pressure in the saliva film is only slightly less than atmospheric pressure. During the application of a dislodging force, however, a reduced pressure may occur under the denture, which may temporarily retard its movement.

The two factors just mentioned may explain in part the function of the peripheral seal. If all other factors are held constant for a particular patient, the fit of the denture controls the distance between it and the oral tissues, which in turn controls the force necessary to dislodge a denture. As a patient continues to wear a denture for a period of time, changes in the contour of the oral tissues and bone structure may eventually result in a poorer fit and decreased retention.

ADHESION

Adhesion is the bonding of dissimilar materials by the attraction of atoms or molecules. Because there is always some attraction between atoms, adhesive strength is a matter of magnitude.

Stresses that weaken adhesive bonds are caused by differences in thermal expansion coefficients and dimensional changes during setting of the adhesive. Adhesion can also be reduced by hydrolytic degradation of the bond.

Two mechanisms of adhesion may be distinguished: chemical and mechanical. Chemical adhesion involves bonding at the atomic or molecular level. Mechanical adhesion is based on retention by the interlocking or the penetration of one phase into the surface of the other. In many cases chemical and mechanical adhesion occur together.

Adhesion with composites has been achieved by etching tooth enamel with acids such as phosphoric or acrylic acid. Adhesive bond strengths approaching the tensile strength of enamel have been found even after storage in water. Examination under high magnification shows the etched enamel to be greatly roughened. The adhesion of resins to etched enamel is a result of capillary penetration into surface irregularities. These polymer projections into enamel have been called tags. Enamel etching has been applied in the use of pit-and-fissure sealants to obtain adhesion and with composite filling materials to obtain adhesion to enamel margins.

Adhesion of hydrophobic composites to hydrophilic enamel and dentin, even with micromechanical etching, is improved by the use of an intermediate layer of a compound like hydroxyethyl methacrylate. One end of this molecule is hydrophilic while the other end is hydrophobic and has a polymerizable carbon double bond. Thus good wetting of tooth structure is obtained from the hydrophilic end, and good compatibility and reactivity with the resin in the composite at the hydrophobic end.

Glass ionomers with polymer acid groups can react with adsorbed water on the surface of teeth to produce a moderate and quite stable bond. However, the mechanical strengths of these materials are not as high as those of bonded composites.

I SELECTED PROBLEMS

Problem 1

Why is mercury difficult to handle without contamination in the operatory?

Solution

The high surface tension of mercury and high contact angles on most surfaces cause mercury to cohere and roll off most surfaces. The vapor pressure of mercury at room temperature is high enough so that its concentration in air can be toxic. The solution is to handle free mercury over surfaces with lipped edges that can catch any spills or to use precapsulated amalgam systems.

Problem 2

Gold inlay castings made with the lost wax process were rough. What could have been the problem?

Solution

There could be several causes for the rough castings.A detergent or wetting agent may not have been used on the wax pattern prior to the investing procedure. Wax patterns are not readily wetted by the water-based gypsum investment unless a wetting agent is used; if one is not used, rough internal mold surfaces produce rough castings.

On the other hand, too much wetting agent placed on the wax will interfere with the setting of the investment and a rough surface will result. The wetting agent is painted on the wax pattern and the excess removed by painting with a dry brush. Very little wetting agent is needed (see Table 2-1).

Problem 3

The bond between a pit-and-fissure material that had just been removed from the refrigerator and etched enamel was found to be poor. Why?

Solution

The bonding of pit-and-fissure sealants to enamel depends on the capillary penetration of the sealant into the fine microscopic spaces produced by etching. The rate of capillary penetration is dependent upon the wetting and viscosity of the sealant. At lower temperatures, the viscosity of sealants is too high for rapid penetration. Therefore it is necessary to allow a refrigerated sealant to warm to room temperature before application.

Problem 4

An addition-silicone impression was poured in high-strength stone and it was difficult to reproduce the fine margins of cavity preparations. What might have been the problem?

Solution

In all probability, a hydrophobic additionsilicone impression material was used and wetting of the surface by the mix of highstrength stone was troublesome. Check the manufacturer's literature and it will probably not indicate that it is a hydrophilic type. Manufacturers will specify that it is hydrophilic, but not if it is hydrophobic.

Problem 5

A complete-denture patient is having difficulty with retention of a maxillary denture. What factors should you check to improve the retention?

Solution

Make sure there is adequate extension at the periphery so that on movement the seal is not broken.

The fit, especially at the periphery, is important because it will control the thickness of the saliva film between the denture and the tissue and the force needed to dislodge the den-

ture. The thinner the film of saliva the greater the retention.

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Restorative dental materials are developed by the producer and selected by the dentist on the basis of characteristic physical, chemical,

mechanical, and biological properties of the material.

No single property can be used as a measure of quality of materials. Often several combined properties, determined from standardized laboratory and clinical tests, are employed to give a measure of quality. The information gained from an orderly laboratory investigation can assist greatly in the clinical evaluation of the particular product or technique by shortening the time required for clinical testing.

There are times when it is not possible to develop a test that is identical with clinical conditions because of the nature of the material or the equipment involved. In such instances a systematic study is conducted with as practical an approach as possible, and the results are then interpreted on a comparative basis.

Standardization of test practices is essential, however, to control quality and allow for duplication of results by other investigators. When possible, the test specimens should approach the size and shape of the structure employed in practice, with mixing and manipulating procedures comparable with routine clinical conditions.

Although it is important to know the comparative values of properties of different restorative materials, it is also essential to know the quality of the supporting tissue. Whereas many restorations fail clinically because of fracture or deformation, it is not uncommon for a wellconstructed restoration to be useless because the supporting tissue fails. Consequently, in designing restorations and interpreting test results, remember that the success of a restoration depends not only on its physical qualities but also on the biophysical or physiological qualities of the supporting tissues.

The physical properties described in this chapter include color and optical properties, thermal properties, and electrical and electrochemical properties. The color and optical properties are color and its measurement, pigmentation, metamerism, fluorescence, opacity, index of

refraction, and optical constants. The thern~al properties are temperature, heat of fusion, thermal conductivity, specific heat, thermal diffilsivity, and coefficient of thermal expansion. The electrical and electrochemical properties are electrical conductivity, dielectric constant, electromotive force, galvanism, corrosion, and zetapotential. Other, less specific properties are tarnish and discoloration, water sorption, solubility and disintegration, setting time, and shelf life. These properties generally are not concerned with the application of force to a body as mechanical properties are.

OPTICAL PROPERTIES

COLOR

The perception of the color of an object is the result of a physiological response to a physical stimulus. The sensation is a subjective experience, whereas the beam of light, which is the physical stimulus that produces the sensation, is entirely objective. The perceived color response results from either a reflected or a transmitted beam of white light or a portion of that beam. According to one of Grassmann's laws, the eye can distinguish differences in only three parameters of color. These parameters are dominant wavelength, luminous reflectance, and excitation purity.

The dominant wavelength (h)of a color is the wavelength of a monochromatic light that, when mixed in suitable proportions with an achromatic color (gray), will match the color perceived. Light having short wavelengths (400 nm) is violet in color, and light having long wavelengths (700 nm) is red. Between these two wavelengths are those corresponding to blue, green, yellow, and orange light. This attribute of color perception is also known as hue.

Of all the visible colors and shades, there are only three primary colors: red, green, and blue (or violet). Any other color may be produced by the proper combination of these colors. For example, yellow may be obtained by a correct mixture of green and red lights.

The luminous reflectance of a color permits an

object to be classified as equivalent to a member of a series of achromatic objects ranging from black to white for light-diffusing objects and from black to perfectly clear and colorless for transmitting objects. A black standard is assigned a luminous reflectance of 0,whereas a white standard is assigned 100. This attribute of color perception is described as valuein one visual system of color measurement.

The excitation purity or saturation of a color describes the degree of its difference from the achromatic color perception most resembling it. Numbers representing excitation purity range from 0 to 1. This attribute of color perception is also known as chroma. Typical quantities for dominant wavelength, luminous reflectance, and excitation purity of materials and human tissues determined in reflected light are listed in Table 3-1.

MEASUREMENT OF COLOR

The color of dental restorative materials is most commonly measured in reflected light by instrumental or visual techniques.

Instrumental Technique Curves of

spectral reflectance versus wavelength can be

obtained over the visible range (405 to 700 nm) with a recording spectrophotometer and integrating sphere. Typical curves for a composite resin before and after 300 hours of accelerated aging in a weathering chamber are shown in Fig. 3-1. From the reflectance values and tabulated color-matching functions, the tristimulus values (X, Z] can be computed relative to a particular light source. These tristimulus values are related to the amounts of the three primary colors required to give, by additive mixture, a imtch with the color being considered. Typically, the tristiinulus values are computed relative to the Commission Internationale de 1'Eclairage (C.I.E.) Source A (gas-filled incandescent lamp) or Source C (average daylight from overcast sky). The ratios of each tristimulus value of a color to their sum are called the chromaticity coordinates (x,y, 2) . Dominant wavelength and excitation purity of a color can be determined by referring its chromaticity coordinates to a chromaticity diagram such as the one shown in Fig. 3-2. The luminous reflectance is equal to the value of the second (Y) of the three tristimulus values. Some typical quantities for color of dental materials are listed in Table 3-1.

A diagram of the C.I.E. L8a8b*color space is shown in Fig. 3-3. The L*a*b* color space is