Сraig. Dental Materials

Fig, 4-36 lsochromatic fringes in a two-dimensional photoelastic model of a molar with a full crown under a concentrated force of 266 N; the numbers represent the fringe order of the isochromatic fringes.

(From Hood JAA, Farah JW, Craig RG: J Prosthet Dent

34:415, 1975.)

quarter-wave plate, and this polarized beam is split into two components traveling along the directions of principal stress in the model. Depending on the state of stress in the model, the two beams travel at different rates. After the light emerges from the model, it passes through a second quarter-wave plate, which is crossed with respect to the first, and an analyzer that is usually perpendicular to the polarizer. The interference pattern may be recorded photographically as shown in Fig. 4-36, which is an isochromatic fringe pattern. These isochromatic fringes, or dark lines, represent locations where the difference in the principal stresses is a constant. The magnitude of the stress can be determined by identifying the order of the isochromatic fringes. Some of the fringe orders (numbers) are indicated on Fig. 4-36. The fringe order multiplied by a constant and divided by the thickness of the model gives the value of the differences in the principal stresses. Areas in the model where the fringes are close together are under higher

stress gradients than areas where there are fewer fringes, and areas containing fringes of higher order are under higher stress than those having fringes of lower order.

The advantages of using photoelasticity are that it can quantify stresses throughout a threedimensional structure and determine stress gradients. However, a birefringent material is needed and the technique is more difficult with complex geometries.

FINITE ELEMENT ANALYSIS

The finite element method is a numerical method and offers considerable advantages over photoelasticity. The method is valuable for analyzing complex geometries, and it can determine stresses and strains throughout a threedimensional component. In this method, a finite number of discrete structural elements are interconnected at a finite number of points or nodes. These finite elements are formed when the original structure is divided into a number of appropriately shaped sections, with the sections retaining the actual properties of the real materials. The information needed to calculate the stresses and displacements in a finite element model is (1) the total number of nodal points and elements, (2) a numbering system for identifying each nodal point and element, (3 ) the elastic moduli and Poisson's ratio for the materials associated with each element, (4) the coordinates of each nodal point, ( 5 ) the type of boundary constraints, and (6) the evaluation of the forces applied to the external nodes. Note that finite element methods are purely numerical, are based on many limiting assumptions, and are potentially costly. Much more research is needed in this area before the numerical values can be accepted without question. In general, the finite element method is best suited for predicting trends and performing parametric analyses. There is an increased awareness that detailed knowledge of anisotropic material properties and constitutive relations is important in building a valid finite element model. There is also an increased emphasis on experimental validation of numerical results.

112 Chapter 4 MECHANICAL PROPERTIES

SUMMARY

The physical properties of oral restorations must adequately withstand the stresses of mastication. Several means may be used to ensure proper strength of a restoration. With a constant force, the stress is inversely proportional to the contact area; therefore stresses may be reduced by increasing the area over which the force is distributed. In areas of high stress, materials having high elastic moduli and strength properties should be used if possible. If a weaker material has desirable properties, such as esthetic qualities, one may minimize the stress by increasing the bulk of the material when possible.

As an example, consider cement bases used under amalgam restorations. Occlusal forces on the amalgam restoration create tensile stresses in the amalgam adjacent to the cement base. The tensile strength of amalgam is low, and if the cement base has a low modulus, it allows deflection of the amalgam adjacent to the cement base resulting in tensile stresses that are sufficient to initiate fracture of the amalgam. The use of zinc phosphate cement as a base rather than zinc oxide-eugenol cement reduces the probability of fracture of the amalgam because the zinc phosphate cement has a higher modulus. If zinc oxide-eugenol cement is necessary to protect the pulp, a minimum amount should be used, followed by the use of a zinc phosphate cement base to provide resistance to deflection.

Restorations and appliances should be designed so the resulting forces of mastication are distributed as uniformly as possible. Also, sharp line angles, nonuniform areas, and notched, scratched, or pitted surfaces should be avoided to minimize stress concentrations. For example, in the construction of a complete maxillary denture, the midline notch between the central incisors should remain at a minimum. This area is under repetitive stress during mastication as a result of the transverse bending of the denture. If a sharp notch is present in this area, the denture will be less resistant to fatigue or impact forces.

The maximum tensile stress in a metal partialdenture circumferential clasp is near the midpoint of the inside surface. Because brittle mate-

rials are generally weak in tension, this is a likely area of failure when a force is applied to the tip of the clasp. Other factors, such as the uniformity of the taper of the clasp, porosity in the clasp, or notches and scratches on the surface of the metal, may alter the stress pattern. If a thin or porous area exists between the junction and the tip of the clasp, failure may occur at this site rather than at the midpoint. Notched or scratched areas are especially subject to fracture from fatigue or impact. Because a partial denture clasp is flexed a great number of times at values well below the yield point, failure by fatigue is particularly significant.

The dentist is often concerned not so much with the fracture of an appliance as with the deflection that occurs when a force is applied. This is the case with a fixed bridge, which may be cast as a single unit or may consist of soldered units. As discussed earlier in this chapter, the deflection of a beam, or in this case a bridge, supported on each end with a concentrated load in the center depends directly on the cube of the beam length and indirectly on the cube of the beam thickness. Doubling the length of the beam, therefore, increases the deflection by eight times. This also indicates that decreasing the thickness of the beam by one half increases the deflection by eight times. If too much bulk is required to develop the stiffness desired, changing to a material with a higher elastic modulus would be beneficial. This is one advantage that nickel-chromium or cobalt-chromium alloys have over gold, because the elastic modulus is greater than 200 GPa, whereas the modulus of gold alloys is less than 100 GPa.

These isolated examples of applied knowledge of biting forces and stresses in dental structures indicate why an understanding of this subject is necessary to the practicing dentist.

In summary, three interrelated factors are important in the long-term function of dental restorative materials: (1) material choice, (2) component geometry (e.g., to minimize stress concentrations), and (3) component design (e.g., to

should he asked: (1)Why did it fail?(2) How did it fail? (3) Whose fault is it? and (4)Can such failures be prevented in the future? Lastly, remember that dental material behavior is dependent on interrelated physical, chemical, optical, mechanical, thermal, electrical, and biological properties, and improvement of one specific property often leads to a reduction in another property.

SPECIFICATIONS FOR RESTORN MATERIALS

The properties described in this and other chapters serve as the basis for a series of specifications that have been developed for restorative materials, instruments, and equipment. One group is the American National Standards Institute/ American Dental Association Standards Committee on Dental Products. Standards developed and approved by this committee are reviewed by the Council on Scientific Affairs of the ADA, which has responsibility for adopting specifications. Presently, 59 specifications have been adopted and another 27 are being developed. A larger group called Federal Specifications and Standards is designed to regulate requirements of federal government service agencies for the purchase and use of materials. Specifications of this type have been available for the past quarter of a century, and additional specifications continue to be added in each group. A series of similar specifications is available for products in Australia, Japan, and several other countries. In 1963, a program for international specifications was established that combined the efforts of the Fkdkration Dentaire Internationale and the International Organization for Standardization. The practice of using physical test controls through methods of applied specifications is well established and will likely continue. Both the dental student and the practitioner must not only recognize that specifications for certain materials are available, but learn to some extent the qualities that are controlled by each specification. Through the specifications the quality of each product is maintained and improved.

AMERICAN DENTAL ASSOCIATION SPECIFICATIONS

The first of the American Dental Association Specifications was for amalgam alloy, formulated and reported in 1930. Since that time other specifications have been or are being formulated, as indicated in Table 4-21.

Copies of the specifications and worksheets to assist in the recording of the required data are available from the Council on ScientificAffairs of the American Dental Association in Chicago. The website of the Council lists the trade names and manufacturers of accepted dental products. This publication can also be obtained from the American Dental Association.

An examination of each specification reveals a general pattern of standardization common to each material.

These features include an item on the scope and classification of the material, which defines the application and general nature of each material.

Each specification includes information on other applicable specifications.

The requirements of each material consider such factors as uniformity, color, or general working characteristics of the material, as well as the general limitations of test values.

The methods of sampling, inspection, and testing procedures include details of specimen preparation and physical tests to be performed.

Each specification includes information on preparation for delivery, with instructions concerning packaging, instructions for use, and marking with lot numbers and the date of manufacture.

Each specification includes notes that provide additional information on intended uses, and references to the literature or other special items.

The important features of each of these specifications are described appropriately in later chapters.

Title

Alloy for dental amalgam

Gypsum-bonded casting investment for dental gold alloys Dental inlay casting wax

Dental casting alloys Dental mercury

Dental agar impression materials

Denture base resins

Denture cold-curing repair resin

Dental base metal casting alloy

Acrylic resin teeth

Dental impression paste-zinc oxide-eugenol type Denture base temporary relining resin

Dental alginate impression material Elastomeric dental impression materials Dental duplicating material

Dental excavating burs Dental baseplate wax Dental gypsum products

Dental radiographic equipment and accessory devices Direct filling resins

Endodontic files and reamers

Zinc oxide-eugenol and noneugenol cements Orthodontic wire

Dental terminology Dental aspirating syringes

High-speed, air-driven handpieces

Dental diamond rotary cutting instruments

Dental abrasive powders

Metal-ceramic systems

Pit and fissure sealants

Dental implants

Biological evaluation of dental materials

Phosphate-bonded investments

Electrically powered dental amalgamators

Dental electrosurgical equipment

Dental porcelain teeth Dental patient chair Dental units

Dental activator, disclosing, and transillumination devices

AMERICAN DENTAL ASSOCIATION ACCEPTANCE PROGRAM

The American Dental Association, through the Council on ScientificAffairs, maintains an acceptance program for dental materials, instruments, and equipment. If a specification exists, the manufacturer may provide evidence that the product complies with the appropriate specification. If the product complies, its name is placed on the Accepted List and the manufacturer is allowed to place the Seal of Acceptance of the American Dental Association on the product. If a specification is in preparation or one does not exist, the manufacturer may provide evidence that the product functions as claimed, and after review by the Council it may be placed on the Accepted

List. Examples of such products are dental adhesives, denture adherents, dental floss, resilient reliners, and toothbrushes.

INDEX OF FEDERAL SPECIFICATIONS AND STANDARDS

The Index of Federal Specifications and Standards includes specifications for a number of restorative dental materials not described elsewhere. These specifications are used primarily by the federal services to maintain some quality control of dental products and are valuable for suppliers of these materials. In a few instances, reference is made to specific federal specifications and standards in later chapters.

I SELECTED PROBLEMS

Problem 1

With an average biting force of 565 N on the first or second molar, how is it possible for a patient to fracture a gold alloy bridge in service when the alloy has a tensile strength of 690 MPa?

Solution

The stress produced by the biting force is a function of the cross section of the bridge and the size of the contact area over which the force is applied. When the contact area from the opposing tooth is very small and

located near a portion of the bridge having a small cross section, bending produces tensile stresses that can exceed the tensile strength of the gold alloy. For example, in the above problem, relating the biting force of 565 N to the tensile strength of 690 MPa indicates that a minimum area of 0.82 mm2is necessary in this bridge:

Area = Forcehtress = 565 N/690 MPa

= 8.2 x lo-' m2= 0.82 mm2

Problem 2

Why is the yield strength of a restorative material such an important property?

Solution

The yield strength defines the stress at the point at which the material changes from elastic to plastic behavior. In the elastic range, stresses and strains return to zero after biting forces are removed, whereas in the plastic range some permanent deformation results on removal of the force. Significant permanent deformation may result in a functional failure of a restoration even though fracture does not occur.

Problem 3

Why is the elongation value for a casting alloy not always an indication of the burnishability of the margins of the casting?

Solution

Although the elongation of an alloy gives an indication of its ductility, or ability to be drawn into a wire without fracturing, to burnish a margin of a casting, sufficient force must be applied to exceed the yield strength. Therefore alloys with high yield strengths are difficult to burnish even though they have high values for elongation.

Problem 4

Why does a mesial-occlusal-distal (MOD) amalgam fail in tension when compressive

biting forces are applied from the opposing teeth?

Solution

The compressive load produces bending of the MOD amalgam, which results in compressive stress on the occlusal surface and tensile stresses at the base of the restoration. Amalgam is a brittle solid with much lower tensile than compressive strength, and therefore fails first at the base of the restoration, with the crack progressing to the occlusal surface of the amalgam.

Problem 5

Because the modulus of nickel-chromium alloys is about twice that of gold alloys, why is it not correct to reduce the thickness by one half and have the same deflection in bending?

Solution

Although the deflection in the bending equation is directly proportional to the modulus, it is inversely proportional to the cube of the thickness. Therefore only minimal reductions in thickness are possible for the nickel-chromium alloy to maintain the same deflection.

Problem 6

How is it possible to use a single elastomeric impression material and yet have the correct viscosity for use in the syringe and the tray?

Solution

Correct compounding of the polymer and filler produces a material that has the quality described as shear thinning. Such a material decreases in viscosity at high shear rates, such as during spatulation or syringing, and has a higher viscosity at low shear rates, as when it is placed and used as a tray material.

Problem 7

After an orthodontic latex band is extended and placed, the force applied decreases with

118 C h a ~ t e 4r MECHANICAL PROPERTIES

time more than expected for the distance the tooth moves. Why?

Solution

Latex rubber bands behave elastically, viscoelastically, and viscously. It is principally the viscous deformation that is not recoverable that accounts for the greater than expected decrease in force as the band shortens from the movement of the tooth. This effect is even more pronounced when plastic rather than latex bands are used.

Problem 8

If dental manufacturers showed you the compliance versus time curve for their rubber impression material and pointed out that it had a high elastic compliance, moderate viscoelastic compliance, and a very low viscous compliance, how would you characterize the product?

Solution

The material would be highly flexible, should recover from deformation moderately rapidly, and the recovery from deformation should be nearly complete.

Problem 9

If you wished to measure the surface hardness of a material that had small isolated areas of widely varying hardnesses, which hardness test would be most appropriate and why?

Solution

Diamond pyramid hardness. Only the Knoop and diamond pyramid are appropriate for surface hardness and a wide range of hardness. The selection of the diamond pyramid over the Knoop test is based on the information that there were isolated areas of different hardness and the diamond pyramid indentation can be placed in smaller areas. Chose nano-indentation for the study of micro-phases.

Problem 10

Why is the selection of a cement base with a high modulus so important for an amalgam restoration, whereas the selection is not as critical for a composite restoration?

Solution

The cement base under an amalgam should have a high modulus (stiffness) to provide support and prevent bending, thus minimizing tensile stresses. Also, low-stress gradients occur across the amalgam-cement base interface when the modulus values are similar.The composite has greater tensile strength and a lower modulus than does amalgam, allowing the use of a cement base with a somewhat lower modulus.

Forces on Dental Structures

Black GV: An investigation of the physical characters of the human teeth in relation

to their diseases, and to practical dental operations, together with the physical characters of filling materials, Dent Cosmos 37: 469, 1895.

Burstone CJ, Baldwin JJ, Lawless DT: The application of continuous forces in orthodontics, Angle Orthod 31:1, 1961.

Dechow PC, Carlson DS: A method of bite force measurement in primates, J Biomech 16:797, 1983.

Koolstra JH, van Euden TMGJ: Application and validation of a three-dimensional mathematical model of the human masticatory system in vivo, J Biomech 25:175, 1992.

Plesh 0,Bishop B, McCall Jr WD: Kinematics of jaw movements during chewing at different frequencies, J Biomech

26:243, 1993.

Southard TE, Southard KA, Stiles RN: Factors influencing the anterior component of occlusal force, J Biomech 23:1199, 1990.

Stress Analysis and Design of Dental Structures

Chen J, Xu L: A finite element analysis of the human temporomandibular joint, J Biomech Eng 116:401, 1994.

Craig RG: Dental mechanics. In Kardestuncer H: Finite element handbook, New York, 1987, McGraw-Hill.

Craig RG, Farah JW: Stress analysis and design of single restorations and fixed bridges, Oral Sci Rev 10:45, 1977.

Craig RG, Farah JW: Stresses from loading distal-extension removable partial dentures,

J Prosthet Dent 39274, 1978.

Farah JW, Craig RG: Distribution of stresses in porcelain-fused-to-metal and porcelain jacket crowns, J Dent Res 54:255, 1975.

Farah JW, Craig RG, Sikarskie DL: Photoelastic and finite element stress analysis of a restored axisymmetric first molar, J Biomech 6:511, 1973.

Farah JW, Hood JAA, Craig RG: Effects of cement bases on the stresses in amalgam restorations, J Dent Res 54:10, 1975.

Farah JW, Powers JM, Dennison JB et al: Effects of cement bases on the stresses and deflections in composite restorations, J Dent Res 55:115, 1976.

Hart RT, Hennebel W, Thonpreda N et al: Modeling the biomechanics of the mandible: a three-dimensional finite element study,

J Biomech 25:261, 1992.

Hylander WL: Mandibular function in galago crassicaudatus and macaca fascicularis: an in vivo approach to stress analysis of the mandible, JMolph 159253, 1979.

Kohn DH: Overview of factors important in implant design, J Oral Implantol

18:204, 1992.

KO CC, Kohn DH, Hollister SJ: Micromechanics of implant/tissue interfaces, J Oral Implantol 18:220, 1992.

Koran A, Craig RG: Three-dimensional photoelastic stress analysis of maxillary and mandibular complete dentures, J Oral Rehabil 1:361, 1974.

Korioth TWP, Hannam AG: Deformation of the human mandible during simulated tooth clenching, J Dent Res 7356, 1994.

Properties from Stress-StrainCurves

Flinn RA, Trojan PK: Engineering materials and their applications, ed 4, New York, 1995, Wiley.

Park, JB, Lakes, RS: Biomaterials: An introduction, New York, 1992, Plenum Press.

Titelman AS, McEvily AJ, Jr: Fracture of stmctural materials, New York, 1967,John Wiley & Sons.

von Recum AF, editor: Handbook of biomaterials evaluation: scient@c, technical, and clinical testing of implant materials, Philadelphia, 1999, Taylor and Francis.

Fracture Toughness

Cruickshanks-Boyd DM, Lock WR: Fracture toughness of dental amalgams, Biomaterials 4:234, 1983.

de Groot R, Van Elst HC, Peters MCRB: Fracture mechanics parameters for failure prediction of composite resins, J Dent Res 67:919, 1988.

Dhuru VB, Lloyd CH: The fracture toughness of repaired composite, J Oral Rehabil 13413, 1986.

El Mowafy OM, Watts DC: Fracture toughness of human dentin, J Dent Res 65:677, 1986.

Ferracane JL, Antonio RC, Matsumoto H: Variables affecting the fracture toughness of dental composites, J Dent Res 66:1140, 1987.

Ferracane JL, Berge HX: Fracture toughness of experimental dental composites aged in ethanol, J Dent Res 74:1418, 1995.

Ferracane JL, Marker VA: Solvent degradation and reduced fracture toughness in aged composites, JDent Res 71:13, 1992.

Ferracane JL: Current trends in dental composites, Crit Rev Oral Biol Med 6302, 1995.

Fujishima A. Ferracane JL: Comparison of four modes of fracture toughness testing for dental composites, Dent Mater 1238, 1996.

120 Chapter 4 MECHANICAL PROPERTIES

Hassan R, Vaidyanathan TK, Schulman A: Fracture toughness determination of dental amalgams through microindentation, J Biomed Mater Res

20:135, 1986.

Hill RG, Bates JF, Lewis TT, Rees N: Fracture toughness of acrylic denture base, Biomater 4:112, 1983.

Kon M, Ishikawa K, Kuwayam N: Effects of zirconia addition on fracture toughness and bending strength of dental porcelains, Dent Mater J 9:181, 1990.

Lloyd CH: The fracture toughness of dental composites. 11. The environmental and temperature dependence of the stress intensification factor (K,,), J Oral Rehabil 9133, 1982.

Lloyd CH: The fracture toughness of dental composites. 111. The effect of environment upon the stress intensification factor (K,,) after extended storage, J Oral Rehabil 11: 393, 1984.

Lloyd CH, Adamson M: The fracture toughness (K,,) of amalgam, J Oral Rehabil 12:59, 1985.

Lloyd CH, Adamson M: The development of fracture toughness and fracture strength in posterior restorative materials, Dent Mater 3225, 1987.

Lloyd CH, Iannetta RV: The fracture toughness of dental composites. I. The development of strength and fracture toughness. J Oral Rehabil9:55, 1982.

Lloyd CH, Mitchell L: The fracture toughness of tooth coloured restorative materials, J Oral Rehabil 11:257, 1984.

Marcos Montes-G G, Draughn RA: Slow crack propagation in composite restorative materials, J Biomed Mater Res 21:629, 1987.

Morena R, Lockwood PE, Fairhurst CW: Fracture toughness of commercial dental porcelains, Dent Mater 2:58, 1986.

Mueller HJ: Fracture toughness and fractography of dental cements, lining, build-up, and filling materials, Scanning Microsc 4:297, 1990.

Neihart TR, Li SH, Flinton RJ: Measuring fracture toughness of high-impact poly (methyl methacrylate) with the short rod method,

J Prosthet Dent 60:249, 1988.

Pilliar RM, Smith DC, Maric B: Fracture toughness of dental composites determined using the short-rod fracture toughness test, J Dent Res 65:1308, 1986.

Pilliar RM, Vowles R, Williams DF: The effect of environmental aging on the fracture toughness of dental composites, J Dent Res 66: 722, 1987.

Roberts JC, Powers JM, Craig RG: Fracture toughness of composite and unfilled restorative resins, J Dent Res 56:748, 1977.

Roberts JC, Powers JM, Craig RG: Fracture toughness and critical strain energy release rate of dental amalgam, J Mater Sci 13965, 1978.

Rosenstiel SF, Porter SS: Apparent fracture toughness of metal ceramic restorations with different manipulative variables, J Prosthet Dent 61:185, 1989.

Rosenstiel SF, Porter SS: Apparent fracture toughness of all-ceramic crown systems, J Prosthet Dent 62:529, 1989.

Sih GC, Berman AT: Fracture toughness concept applied to methyl methacrylate,

J Biomed Mater Res 14:311, 1980.

Taira M, Nomura Y, Wakasa K et al: Studies on fracture toughness of dental ceramics, J Oral Rehabil 17:551, 1990.

Uctasli S, Harrington E, Wilson HJ: The fracture resistance of dental materials, J Oral Rehabil 22:877, 1995.

Shear Strength

Black J: "Push-out" tests, J Biomed Mater Res 23:1243, 1989.

Johnston WM, O'Brien WJ: The shear strength of dental porcelain, J Dent Res 59:1409, 1980.

Bending and Torsion

Asgharnia MK, Brantley WA: Comparison of bending and torsion tests for orthodontic wires, Am J Orthodont 89:228, 1986.