Сraig. Dental Materials

because colored substances penetrate the materials and continue chemical reactions in the composites.

Various in vitro tests have been proposed to study tarnish, particularly that of crown and bridge and partial denture alloys. Testing generally relies on controlled exposure of the alloy to a solution rich in sulfides, chlorides, and phosphates. Most recently the discoloration of alloys exposed to such solutions has been evaluated by spectrophotometric methods to determine a color-difference parameter discussed earlier in this chapter.

WATER SORPTION

Water sorption of a material represents the amount of water adsorbed on the surface and absorbed into the body of the material during fabrication or while the restoration is in service. Water sorption of denture acrylic, for example, is measured gravimetrically in pg/mm3 after 7 days in water. The tendency of plastic denture base materials to have a high degree of water sorption is the reason this quality was included in American National Standards Institute/American Dental Association (ANSVADA) Specification No. 12 for this type of material. Usually a serious warpage and dimensional change in the material are associated with a high percentage of water sorption. The tendency of hydrocolloid impression materials to imbibe water if allowed to remain immersed and then to change dimensions has been a serious problem associated with their use.

SOLUBILITY AND DISINTEGRATION

Solubility and disintegration of crown and bridge cements can be measured gravimetrically by suspension of two disks, 20 mm in diameter and 1.0 mm in thickness, for 24 hours or longer in water at 37" C. The units are pg/mm2. A conductimetric method for studying solubility and disintegration has the advantage of detecting the elution of volatile components and of using a smaller specimen. Care should be taken in predicting in vivo properties from tests in water

because abrasion and attack from other chemicals often occur intraorally.

The lack of correlation between in vivo testing of the degradation of cements and the aforementioned test of solubility and disintegration in water has led to the development of other in vitro tests. One test involves placing cement between two, round, plane-parallel glass plates (16 mm in diameter) and exposing this specimen to various acidic media. Changes in the cement are recorded photographically. Observations suggest that degradation may follow a sequence of absorption, disintegration, and solution. Variables such as cement composition, thickness, molarity, and pH of the medium are important. More recently, an acid erosion test has been used for testing cements (see Chapter 20).

SETTING TIME

Setting time characteristics are associated with the reaction rates and affect the practical applications of many materials in restorative dentistry. Materials such as cements, impression materials, dental plaster, stone, and casting investments depend on a critical reaction time and hardening rate for their successful application. From the practical standpoint of manipulation and successful application, the time required for a material to set or harden from a plastic or fluid state may be its most important quality. The setting time does not indicate the completion of the reaction, which may continue for much longer times. The time varies for different materials, depending on the particular application, but duplication of results from one lot to another or from one trade brand of material to another is highly desirable. The influence of manipulative procedures on the setting time of various types of materials is important to the dentist and the assistant.

SHELF LIFE

Shelf life is a term applied to the general deterioration and change in quality of materials during shipment and storage. The temperature, humidity, and time of storage, as well as the bulk of

material involved and the type of storage container, are significant factors that vary greatly from one material to another. A material that has exceptionally good properties when first produced may be quite impractical if it deteriorates badly after a few days or weeks. These qualities are discussed in chapters dealing with gypsum materials and impression materials. Some studies of these qualities of various materials have been made in recent years, and through accelerated

aging tests, improvements in quality can sometimes be made. Radiographic film, anesthetics, and a few other products carry dates of expiration beyond which the product should not be expected to be serviceable. This practice assures the user that the material is not deteriorated because of age. Most materials that meet the requirements of the American Dental Association Specifications carry a date of production as a part of the serial number or as a separate notation.

I SELECTED PROBLEMS

Problem 1

A hole was drilled in a gold crown to facilitate an endodontic procedure. Subsequently, the hole was filled with a dental amalgam. Af- ter several months the amalgam appeared discolored and corroded. What caused this problem, and how can it be avoided?

Solution

The dental amalgam is anodic to the gold alloy. Furthermore, the surface area of the gold restoration is much larger than that of the amalgam. Both of these factors will cause the amalgam to corrode by galvanic action. The hole should be filled with gold foil to minimize corrosion.

Problem 2

A ceramic veneer to be bonded on an anterior tooth matches the color of the shade guide but not the adjacent tooth. What most likely caused this problem, and how can it be avoided?

Solution a

If different light sources are used to match metameric shades, then the color could appear correct when observed under one light but not under the other. Be sure to match teeth and shade guides under appropriate lighting conditions.

Solution b

Ceramic is a translucent material, the color of which can be affected by the color of the cement retaining the restoration, particularly if the veneer lacks an opaque layer. Select a resin cement of an appropriate shade to bond the veneer.

Problem 3

A glaze applied to a ceramic restoration cracks on cooling. What caused the glaze to crack, and how can this problem be avoided?

Solution

Ceramics have a low thermal diffusivity and are subject to cracking as a result of thermal shock. Be sure to cool a ceramic restoration as recommended by the manufacturer to minimize larger thermal gradients.

Problem 4

A denture cleaned in hot water distorted and no longer fits the patient's mouth. Why?

Solution

If the temperature of the denture during cleaning exceeds the glass transition temperature of the resin, then distortion can occur readily. Be sure to use cool water to clean a denture.

Color and Optical Properties

Asmussen E: Opacity of glass-ionomer cements, Acta Odontol Sca~zd41:155, 1983.

Baran GR, O'Brien WJ, Tien T-Y: Colored emission of rare earth ions in a potassium feldspar glass, J Dent Res 56:1323, 1977.

Colorimetry, of$cial recommendations of the International Commission on Illumination (CIE), Publication CIE No 15 (E-1.3.1), 1971.

Crisp S, Abel G, Wilson AD: The quantita-

tive measurement of the opacity of aesthetic dental filling materials, J Dent Res

58:1585, 1979.

Dennison JB, Powers JM, Koran A: Color of dental restorative resins, J Dent Res 57:557, 1978.

Hall JB, Hefferren JJ, Olsen NH: Study of fluorescent characteristics of extracted human teeth by use of a clinical fluorometer, J Dent Res 49:1431, 1970.

Johnston WM, O'Brien WJ, Tien T-Y: The determination of optical absorption and scattering in translucent porcelain, Color Res Appl 11:125, 1986.

Johnston WM, O'Brien WJ, Tien T-Y: Concentration additivity of Kubelka-Munk optical coefficients of porcelain mixtures, Color Res Appl11:131, 1986.

Jorgenson MW, Goodkind RJ: Spectrophotometric study of five porcelain shades relative to the dimensions of color, porce-

lain thickness, and repeated firings, JProsthet Dent 42:96, 1979.

Judd DB: Optical specification of lightscattering materials, J Res Nat Bur Standards 19287, 1937.

Judd DB, Wyszecki G: Color i n business, science, and industry, ed 3, New York, 1975,John Wiley & Sons.

Koran A, Powers JM, Raptis CN, Yu R: Reflection spectrophotometry of facial skin, J Dent Res 60:979, 1981.

Kubelka P: New contributions to the optics of intensely light-scattering materials, Part

I, Opt Soc Am J 38:448, 1948.

Kubelka P, Munk F: Ein Beitrag zur Optik der Farbanstriche, Z Tech Phys 12:593, 1931.

Miyagawa Y, Powers JM: Prediction of color of an esthetic restorative material, J Dent Res 62:581, 1983.

Miyagawa Y, Powers JM, O'Brien WJ: Optical properties of direct restorative materials,

J Dent Res 60:890, 1981.

Nickerson D: The specification of color tolerances, Textile Res 6:509, 1936.

Noie F, O'Keefe KL, Powers JM: Color stability of resin cements after accelerated aging,

Int J Prosthodont 8:51, 1995.

O'Brien WJ, Johnston WM, Fanian F: Doublelayer color effects in porcelain systems,

J Dent Res 64940, 1985.

O'Brien WJ, Johnston WM, Fanian F et al: The surface roughness and gloss of composites, J Dent Res 63:685, 1984.

O'Keefe KL, Powers JM, Noie F: Effect of dissolution on color of extrinsic porcelain colorants, Int J Prosthodont 6:558, 1993. Panzeri H, Fernandes LT, Minelli CJ: Spectral fluorescence of direct anterior restorative

materials, Aust Dent J 22:458, 1977. Powers JM, Barakat MM, Ogura H: Color and

optical properties of posterior composites under accelerated aging, Dent Mater J 4:62, 1985.

Powers JM, Capp JA, Koran A: Color of gingival tissues of blacks and whites, J Dent Res 56:112, 1977.

Powers JM, Dennison JB, Koran A: Color stability of restorative resins under accelerated aging, J Dent Res 57:964, 1978.

Powers JM, Dennison JB, Lepeak PJ: Parameters that affect the color of direct restorative resins, J Dent Res 57:876, 1978.

Powers JM, Koran A: Color of denture resins, J Dent Res 56:754, 1977.

Powers JM, Yeh CL, Miyagawa Y: Optical properties of composite of selected shades in white light, J Oral Rehabil10:319, 1983.

Ruyter IE, Nilner K, Moller B: Color stability of dental composite resin material for crown and bridge veneers, Dent Mater 3:246, 1987.

Seghi RR, Johnston WM, O'Brien WJ: Spectrophotometric analysis of color differences between porcelain systems, J Prosthet Dent 56:35, 1986.

Specifying color by the Munsell system, D1535-68 (1974). In ASTM Standards, 1975, Part 20, Philadelphia, 1975, American Society for Testing and Materials.

Sproull RC: Color matching in dentistry. Part 111. Color control, J Prosthet Dent 31:146, 1974.

Van Oort RP: Skin color and.facia1 prosthetic-a colorinzetric study, doctoral dissertation, The Netherlands, 1982, Groningen State University.

Wyszecki G, Stiles WS: Color science, New York, 1967, John Wiley & Sons.

Yeh CL, Miyagawa Y, Powers JM: Optical properties of composites of selected shades,

J Dent Res 61:797, 1982.

Yeh CL, Powers JM, Miyagawa Y: Color of selected shades of composites by reflection spectrophotometry,J Dent Res 61:1176, 1982.

Thermal Properties

Antonucci JM, Toth EE: Extent of polymerization of dental resins by differential scanning calorimetry, J Dent Res 62:121, 1983.

Brady AP, Lee H, Orlowski JA: Thermal conductivity studies of composite dental restorative materials, J Biovzed Mater Res 8:471, 1974.

Brauer GM, Termini DJ, Burns CL: Characterization of components of dental materials and components of tooth structure by differential thermal analysis, J Dent Res 49:100, 1970.

Brown WS, Christiansen DO, Lloyd BA: Numerical and experimental evaluation of energy inputs, temperature gradients, and ther-

mal stress during restorative procedures, J Am Dent Assoc 96:451, 1978.

Brown WS, Dewey WA, Jacobs HR: Thermal properties of teeth, J Dent Res 49752, 1970.

Civjan S, Barone JJ, Reinke PE et al: Thermal properties of nonmetallic restorative materials, J Dent Res 51:1030, 1972.

Craig RG, Eick JD, Peyton FA: Properties of natural waxes used in dentistry, J Dent Res 44:1308, 1965.

Craig RG, Peyton FA: Thermal conductivity of tooth structure, dental cements, and amalgam, J Dent Res 40:411, 1961.

Craig RG, Powers JM, Peyton FA: Differential thermal analysis of commercial and dental waxes, J Dent Res 46:1090, 1967.

Craig RG, Powers JM, Peyton FA: Thermogravimetric analysis of waxes, J Dent Res

50:450, 1971.

Dansgaard W, Jarby S: Measurement of nonstationary temperature in small bodies, Odont Tskr 66:474, 1958.

de Vree JH, Spierings TA, Plasschaert AJ:

A simulation model for transient thermal analysis of restored teeth, J Dent Res 62:756, 1983.

Fairhurst CW, Anusavice KJ, Hashinger DT et al: Thermal expansion of dental alloys and porcelains, J Bionzed Mater Res 14:435, 1980.

Henschel CJ: Pain control through heat control, Dent Dig 47:294, 444, 1941.

Lisanti VF, Zander HA: Thermal conductivity of dentin, J Dent Res 29:493, 1950.

Lloyd CH: The determination of the specific heats of dental materials by differential thermal analysis, Biomaterials 2:179, 1981.

Lloyd CH: A differential thermal analysis (DTA) for the heats of reaction and temperature rises produced during the setting of tooth coloured restorative materials, J Oral Rehabil 11:111, 1984.

McCabe JF, Wilson HJ: The use of differential scanning calorimetry for the evaluation of dental materials. I. Cements, cavity lining materials and anterior restorative materials, J Oral Rehabil7:103, 1980.

McCabe JF, Wilson HJ: The use of differential scanning calorimetry for the evaluation of dental materials. 11. Denture base materials, J Oral Rehabil7:235, 1980.

McLean JW: Physical properties influencing the accuracy of silicone and thiokol impression materials, Br Dent J 110:85, 1961.

Murayama T: Dynamic mechanical analysis of polymeric materials, New York,

1978, Elsevier Science.

Pearson GJ, Wills DJ, Braden M et al: The relationship between the thermal properties of composite filling materials, J Dent

8:178, 1980.

Peyton FA: Temperature rise and cutting efficiency of rotating instruments, N Y J Dent 18:439, 1952.

Peyton FA: Effectiveness of water coolants with rotary cutting instruments, J Am Dent Assoc 56:664, 1958.

Peyton FA, Morrant GA: High speed and other instruments for cavity preparation, Int Dent J 9:309, 1959.

Peyton FA, Simeral WG: The specific heat of tooth structure, Alum Bull U Mich School Dent 56:33, 1954.

Powers JM, Craig RG: Penetration of commercial and dental waxes, J Dent Res

53402, 1974.

Powers JM, Hostetler RW, Dennison JB: Thermal expansion of composite resins and sealants, J Dent Res 58:584, 1979.

Rootare HM, Powers JM: Determination of phase transitions in gutta-percha by differential thermal analysis, J Dent Res 56:1453, 1977.

Soderholm KJ: Influence of silane treatment and filler fraction on thermal expansion of composite resins, J Dent Res 63:1321, 1984.

Souder WH, Paffenbarger GC: Physical properties of dental materials, National Bureau of Standards Circular No C433, Washington, DC, 1942, US. Government Printing Office.

Soyenkoff BC, Okun JH: Thermal conductivity measurements of dental tissues with the aid of thermistors, J Am Dent Assoc 57:23, 1958.

Tay WM, Braden M: Thermal diffusivity of glass-ionomer cements, J Dent Res 66:1040, 1987.

Walsh JP, Symmons HF: A comparison of the heat conduction and mechanical efficiency of diamond instruments, stones, and burs at 3,000 and 60,000 rpm, NZ Dent J

45:28, 1949.

Watts DC, Smith R: Thermal diffusivity in finite cylindrical specimens of dental cements,

J Dent Res 60:1972, 1981.

Watts DC, Smith R: Thermal diffusion in some polyelectrolyte dental cements: the effect of powder/liquid ratio, J Oral Rehabil

11:285, 1984.

Wilson TW, Turner DT: Characterization of polydimethacrylates and their composites by dynamic mechanical analysis, J Dent Res 66:1032, 1987.

Electrical and Electrochemical Properties

Arvidson K, Johansson EG: Galvanic series of some dental alloys, Scand J Dent Res 85:485, 1977.

Bergman M, Ginstrup 0,Nilner K: Potential and polarization measurements in vivo of oral galvanism, Scand J Dent Res 86:135, 1978.

Bjorn H: Electrical excitation of teeth, Svensk Tandlak T 396uppl):1946.

Braden M, Clarke RL: Dielectric properties of zinc oxide-eugenol type cements, J Dent Res 53:1263, 1974.

Braden M, Clarke RL: Dielectric properties of polycarboxylate cements, J Dent Res 547, 1975.

Cahoon JR, Holte RN: Corrosion fatigue of surgical stainless steel in synthetic physiological solution, J Biomed Mater Res 15:137, 1981.

Clark GCF, Williams DF: The effects of proteins on metallic corrosion, J Biomed Mater Res 16:125, 1982.

Fairhurst CW, Marek M, Butts MB et al: New information on high copper amalgam corrosion, J Dent Res 57:725, 1978.

Gjerdet NR, Brune D: Measurements of currents between dissimilar alloys in the oral cavity, Scand J Dent Res 85:500, 1977.

Holland RI: Galvanic currents between gold and amalgam, Scand J Dent Res 88:269, 1980.

Maijer R, Smith DC: Corrosion of orthodontic bracket bases, Am J Orthodont 81:43, 1982.

Marek M, Hochman R: i%e corrosion behavior of dental amalgam phases as a function of tin content. Microfilmed paper no 192, delivered at the Annual Meeting of the International Association for Dental Research, Dental Materials Group, Washington, DC, April 12-15, 1973.

Mohsen NM, Craig RG, Filisko FE: The effects of different additives on the dielectric relaxation and the dynamic mechanical properties of urethane dimethacrylate, J Oral Rehabil27:250, 2000.

Mumford JM: Direct-current electrodes for pulp testing, Dent Pract 6:236, 1956.

Mumford JM: Direct-current paths through human teeth, master's thesis, Ann Arbor, Mich, 1957, University of Michigan School of Dentistry.

Mumford JM: Electrolytic action in the mouth and its relationship to pain, J Dent Res 36:632, 1957.

Mumford JM: Resistivity of human enamel and dentin, Arch Oral Biol12:925, 1957.

Mumford JM: Path of direct current in electric pulp-testing, Br Dent J 106:23, 1959.

O'Brien WJ: Electrochemical corrosion of dental gold castings, Dent Abstracts 7:46, 1962.

Phillips LJ, Schnell RJ, Phillips RW: Measurement of the electric conductivity of dental cement. 111. Effect of increased contact area and thickness: values for resin, calcium hydroxide, zinc oxide-eugenol, J Dent Res 34597, 1955.

Phillips LJ, Schnell RJ, Phillips RW: Measurement of the electric conductivity of dental cement. IV. Extracted human teeth; in

vivo tests; summary, J Dent Res 34:839, 1955. Rootare HM, Powers JM: Comparison of

zeta-potential of synthetic fluorapatite obtained by stepwise and continuous methods

of streaming, J Electrochem Soc 126:1905, 1979. Schreiver W, Diamond LE: Electromotive forces

and electric currents caused by metallic dental fillings,JDent Res 31:205, 1952.

Shaw DJ: Electrophoresis, New York, 1969, Academic Press.

Tay WM, Braden M: Dielectric properties of glass ionomer cements, J Dent Res 47:463, 1968.

Wilson AD, Kent BE: Dental silicate cements. V. Electrical conductivity, J Dent Res 47:463, 1968.

Zitter H, Plenk H, Jr: The electrochemical behavior of metallic implant materials as an indicator of their biocompatibility, J Biomed Mater Res 212381, 1987.

Other Properties

German RM, Wright DC, Gallant RF: In vitro tarnish measurements on fixed prosthodontic alloys, J Prosthet Dent 47:399, 1982.

Koran A, Powers JM, Lepeak PJ et al: Stain resistance of maxillofacial materials, J Dent Res 58:1455, 1979.

Mesu FP: Degradation of luting cements measured in vitro, J Dent Res 61:655, 1982.

Raptis CM, Powers JM, Fan PL et al: Staining of composite resins by cigarette smoke, J Oral Rehabil9:367, 1982.

Solovan DF, Powers JM: Effect of denture cleansers on partial denture alloys and resilient liners, Mich Dent Assoc J60:135, 1978.

Walls AW, McCabe JF, Murray JJ: An erosion test for dental cements, J Dent Res 64:11OO, 1985.

Wilson AD, Merson SA, Prosser HJ: A sensitive conductimetric method for measuring the material initially water-leached from dental cements. I. Zinc polycarboxylate cements, J Dent 8:263, 1980.

Biomaterials Database:

www.lib.umich.edu/dentlib/dental~tables

(University of Michigan)

68 Chapter 4 MECHANICALPROPERTIES

Most restorative materials must withstand forces during either fabrication or mastication. Mechanical properties are therefore important in understanding and predicting a material's behavior under load. Because no single mechanical property can give a true measure of quality, it is essential to understand the principles involved in a variety of mechanical properties to obtain the maximum service in a material. Quantities of force, stress, strain, strength, toughness, hardness, friction, and wear can help identify the properties of a material. In general, the stability of a solid under applied load is determined by the nature and strength of atomic binding forces. In this chapter, the concepts of elastic, viscoelastic, and surface mechanical properties are introduced, and the importance of these concepts in

dentistry is emphasized.

Force is generated through one body pushing or pulling on another. Forces may be applied through actual contact of the bodies or at a distance (e.g.,gravity). The result of an applied force on a body is a change in position of rest or motion of the body. If the body to which the force is applied remains at rest, the force causes the body to deform or change its shape. A force is defined by three characteristics: point of application, magnitude, and direction of application. The direction of a force is characteristic of the type of force. The unit of force is the Newton, N.

OCCLUSAL FORCES

One of the most important applications of materials science in dentistry is in the study of forces applied to teeth and dental restorations. Numerous reports in the dental literature describe the measurement of biting forces on teeth. The maximum forces, measured by strain gauges, telemetric devices, or numerical simulations, range from 200 to 3500 N .

Biting forces on adult teeth decrease from the

molar region to the incisors, with forces on the first and second molars varying from 400 to 800 N . The average force on the bicuspids, cuspids, and incisors is about 300, 200, and 150 N , respectively. A somewhat irregular but definite increase in force from 235 to 494 N occurs in growing children, with an average yearly increase the order of 22 N.

FORCES ON RESTORATIONS

Equally important to the study of forces on natural dentition is the measurement of forces and stresses on restorations such as inlays, fixed bridges, removable partial dentures, and complete dentures. One of the first investigations of occlusal forces showed that the average biting force on patients who had a fixed bridge replacing a first molar was 250 N on the restored side and 300 N on the opposite side, where they had natural dentition. For comparison, the average biting forces on permanent teeth were 665, 450, and 220 N on molars, bicuspids, and incisors, respectively.

Force measurements on patients with removable partial dentures are in the range of 65 to 235 N . For patients with complete dentures, the average force on the molars and bicuspids was about 100 N, whereas the forces on the incisors averaged 40 N. The wide range in results is possibly caused by age and gender variations in the patient populations. In general, the biting force applied by women is 90 N less than that applied by men.

These studies and others indicate that the chewing force on the first molars of patients with fixed bridges is about 40% of the force exerted by patients with natural dentition. A further decrease in force is obtained in patients with complete or removable partial dentures. Patients who wear such appliances exert only about 15% of the force applied by persons with normal dentition.

Recent measurements made with strain gauges are more precise, but, in general, the conclusions are similar. The distribution of force between the first bicuspid, second bicuspid, and the first molar

of a complete denture is about 15%, 30%, and 55% of normal, respectively. The average force on the first bicuspid, second bicuspid, and first molar while the patient chewed peanuts, coconut, or raisins was 6.6, 12.0, and 22.6 N, respectively. These values are low because they are forces required to chew the food rather than average maximum forces. A patient wearing a complete denture therefore may facilitate the chewing of tough foods by increasing the force or the number of chewing thrusts or by shifting the food to the small bicuspids, where the stress is greater. Because the range of force application is small,shifting the food forward would be a better solution.

SUMMARY OF OCCLUSAL FORCES

The studies cited above were for small patient populations or patients of different ages. Based on the range of data reported, research on forces of mastication should be conducted on a large number of controlled patient groups for more accurate quantification. However, we may surmise that the forces of occlusion and the response of underlying tissue change with anatomic location, age, malocclusion, and placement of a restorative appliance. Therefore a material or design sufficient to withstand the forces of occlusion on the incisor of a child may not be sufficient for the first molar of an adult with a malocclusion or bridge.

STRESS

When a force acts on a body tending to produce deformation, a resistance is developed to this external force application. The internal reaction is equal in intensity and opposite in direction to the applied external force, and is called stress. Both the applied force and internal resistance (stress) are distributed over a given area of the body, so the stress in a structure is designated as the force per unit of area. In this respect, stress resembles pressure, because both stress and

pressure are represented by the following equation:

Force

Stress = -

Area

Because the internal resistance to force applications is impractical to measure, the more convenient procedure is to measure the external force

(F) applied to the cross-sectional area (A), which can be described as the stress, typically denoted as S or o.The unit of stress therefore is the unit of force (N) divided by a unit of area or length squared, and is commonly expressed as Pascal (1 Pa = 1 TX/m2 = 1 M ~ / m m ~It) .is common to report stress in units of megaPascals (MPa), where 1 MPa = lo6 Pa.

Because the stress in a structure varies directly with the force and inversely with area, it is necessary to recognize that the area over which the force acts is an important consideration. This is particularly true in dental restorations in which the areas over which the forces are applied often are extremely small. For example, the clasps on removable partial dentures, orthodontic wire structures, or small occlusal restorations may have cross-sectional areas of only 0.16 to 0.016 cm2.

As a numerical example, a 20-gauge orthodontic wire has a diameter of 0.8 mm and a cross-sectional area of 0.5 mm2. If a 220 N force is applied to a wire of this diameter, the stress developed is equivalent to 220 N/0.5 mm2, or 440 ~ / m m '(MPa).

Stress is always stated as though the force were equn dent to that appl~edto a 1-m2section, but a dental restoration obviously does not have a square meter of exposed occlusal surface area. A small occlusal pit restoration may have no more than 4 mm2 of surface area, if it were assumed that the restoration were 2 mm on a side. If a biting force of 440 N should be concentrated on this area, the stress developed would be 100 MPa. Therefore stresses equivalent to several hundreds of MPa occur in many types of restorations.

Fig. 4-1 Schematic of the different types of stresses and their corresponding deformations.

TYPES OF STRESS

A force can be directed to a body from any angle or direction, and often several forces are combined to develop complex stresses in a structure. In general, individually applied forces may be axial (tensile or compressive), shear, bending, or torsional. These directional forces are illustrated in a simplified manner in Fig. 4-1. All stresses, however, can be resolved into combinations of two basic types-axial and shear.

Tension results in a body when it is subjected to two sets of forces directed away from each other in the same straight line. Cowzpression results when the body is subjected to two sets of forces directed toward each other in the same straight line, and shear is the result of two sets of forces directed parallel to each other. Torsion results from the twisting of a body, and bending results from an applied bending moment. When tension is applied, the molecules making up the body must resist being pulled apart. When compression is applied, they resist being forced more closely together. As a result of a shear stress application, one portion of the body must resist sliding past another. These resistances of a material to deformation represent the basic qualities of elasticity of solid bodies.

An example of the complexity and varying direction and magnitude of stresses in the oral cavity is shown in Fig. 4-2, in which a photoelastic model of a three-unit bridge has been loaded

in compression by the opposing occlusion. The arrows in Fig. 4-2, A, indicate locations of contact that are under compressive stress. Fig. 4-2, B, shows the type of stress at the periphery of the model and illustrates that the occlusal surface of the bridge is subjected alternately to areas of compression and tension, whereas the gingival portion of the pontic is under tensile stress. The soldered joints, however, are under both tensile and shear stress.

In the discussion of force, it was pointed out that a body undergoes deformation when a force is applied to it. It is important to recognize that each type of stress is capable of producing a corresponding deformation in a body (see Fig. 4-1). The deformation resulting from a tensile or pulling force is an elongation of a body in the direction of applied force, whereas a compressive or pushing force causes compression or shortening of the body in the direction of loading. Strain, E, is described as the change in length (AL = L - L,) per unit length (L,,) of the body when it is subjected to a stress. Strain has no unit of measurement, but is represented as a pure number obtained from the following equation:

Thus if a specimen with an original length of 2 mm is pulled to a new length of 2.02 mm, it has deformed 0.02 mm and the strain is 0.02/2 = 0.01, or 1%. Strain is therefore reported as an absolute value or as a percentage. The amount of strain will differ with each type of material subjected to stress and with the magnitude of the stress applied. Note that regardless of the composition or nature of the material, and regardless of the magnitude and type of stress applied to the material, deformation and strain result with each stress application. The importance of strain in dentistry is as follows: a restorative material, such as a clasp or an orthodontic wire, which can withstand a large amount of strain before failure,