Сraig. Dental Materials

Problem 8

You are a dentist who has been practicing for 20 years, and you like to do a lot of your own laboratory work. You have noticed that when you handle methacrylate denture base and monomer, you develop a rash on your hands. What is the problem, and what can you do about it?

Solution

You are probably hypersensitive to the monomer and should wear rubber gloves around monomer and freshly polymerized methacrylate. Try to avoid contact because monomer can penetrate latex rubber.

Problem 9

You have a patient who has an endosteal blade that had been implanted by her previous prosthodontist before she moved to town. The implant has been in place 1 year and appears to be somewhat mobile. What are your options for analyzing the problem?

Solution

Radiographs of the implant and tissue in the region should be done to look for areas of radiolucencies, both at the cervix and at the mesial and distal tips of the blade. A periodontal probe may be used gently to determine if there is tissue attachment around the implant. However, if the implant is mobile at all, failure is probably imminent.

Problem 10

A dental patient comes to you with concerns about the estrogenicity of dental composites you have placed. What will you tell her?

Solution

First, it is true that some starting components (bis-phenol A and its dimethacrylate) show estrogenic types of reactions in in vitro tests. However, the concentrations of these compounds in today's commercially available resins are vey low. Second, there is no verifiable clinical evidence that these compounds or others are released in sufficient concentrations

to cause estrogenic reactions. For example, the concentration of bis-phenol A required to cause an estrogenic response is 1000 times greater than that of the naturally occurring hormone estradiol. Finally, there are no documented cases of these materials having any estrogenic-type of reactions in dental patients, despite the placement of millions of these restorations.

AAMI Standards and Recommended Practices:

Biological Evaluation of Medical Devices, vol 4, Arlington, VA, Association for the Advancement of Medical Instrumentation, 1994.

American Dental Association: Addendum to American National StandarddAmerican Dental Association Document No. 41 for recommended standard practices for biological evaluation for dental materials, Chicago, 1982, American Dental Association.

American Dental Association: American National Standards Institute/American Dental Association Document No. 41 for recommended standard practices for biological evaluation of dental materials, J Am Dent Assoc 99697, 1979.

Barile FA: In vitro cytotoxici[y: mechanisms and methods. CRC Press, Boca Raton, 1994.

Bergenholtz G: In vivo pulp response to bonding of dental restorations, Trans Acad Dent Mater November: 123, 1998.

Bouillaguet S, Ciucchi B, Holz J: Potential risks for pulpal irritation with contemporary adhesive restorations: an overview, Acta Med Dent Helv 1:235, 1996.

Brannstrom M: Dentin andpulp in restorative dentisty, London, 1982, Wolfe Medical.

Cox CF, Keall CL, Keall HJ, Ostro EO: Biocompatibility of surface-sealed dental materials against exposed pulps, J Prosthet Dent 57:1, 1987.

deSouza Costa CA, Beling J, Hanks CT: Current status of pulp capping with dentin adhesive systems: a review, Dent Mater 16:188, 2000.

162 Chapter 5 BlOCOMPATlBlLlM OF DENTAL MATERIALS

Ecobichon DJ: B e basis of toxicity testing, Boca Raton, 1992, CRC Press.

Geurtsen W: Substances released from dental resin composites and glass ionomer cements, Eur J Oral Sci 106:687, 1998.

Geursten W: Biocompatibility of resin-modified filling materials, Crit Rev Oral Biol Med 11333, 2000.

Geurtsen W, Leyhausen G: Biological aspects of root canal filling materialshistocompatibility, cytotoxicity, and mutagenicity, Clin Oral Invest

1:5, 1997.

Hanks CT, Wataha JC, Sun ZL: In vitro models of biocompatibility: a review, Dent Mater 12:186, 1996.

Hensten-Pettersen A: Skin and mucosal reactions associated with dental ~naterials,

E u r J Oral Sci 106:707, 1998.

Hodgson E, Levi PE, eds: A textbook of modern toxicology, New York, 1987, Elsevier Science.

Hume WR: A new technique for screening chemical toxicity to the pulp from dental restorative materials and procedures, J Dent Res 64:1322, 1985.

International Standards Organization: Biological evaluation of medical devices,

IS0 10993, ed 1, Geneva, Switzerland, 1992, ISO.

Jontell M, Okiji T, Dahlgren U, Bergenholtz G: Immune defense mechanisms of the dental pulp, Crit Rev oral Biol Med 9:179, 1998.

Kawahara H, Yamagami A, Nakamura M: Biological testing of dental materials by means of tissue culture, Int Dent J 18:443, 1968.

Mackert JR: Dental amalgam and mercury, J Am Dent Assoc 122:54, 1991.

Mackert JR, Bergland A: Mercury exposure from dental amalgam filling: absorbed dose and the potential for adverse health effects, Crit Rev Oral Biol Med 8:410, 1997.

Meryon SD: The influence of dentine on the in vitro cytotoxicity testing of dental restorative materials, J Biomed Mater Res 18:771, 1984.

Mjor IA, Hensten-Pettersen A, Skogedal 0: Biologic evaluation of filling materials: a comparison of results using cell culture techniques, implantation tests and pulp studies, Int Dent J 27:124, 1977.

National Institutes of Health: Consensus development statement on dental implants, June 13-15, 1988,J Dent Educ 52:824, 1988.

Pashley DH: The effects of acid etching on the pulpodentin complex, Oper Dent 17:229, 1992.

Pashley DH: Dynamic of the pulp-dentin complex, Crit Rev Oral Biol Med 7:104, 1996.

Schmalz G: Modern concepts in biocompatibility testing of restorative materials, Trans Acad Dent Mater 9:170, 1996.

Schmalz G: Concepts in biocompatibility testing of dental restorative materials, Clin Oral Invest 1:154, 1997.

Schmalz G : The biocompatibility of nonamalgam dental filling materials, Eur J Oral Sci 106:696, 1998.

Schuster GS, Lefebvre CA, Wataha JC, White SN: Biocompatibility of posterior restorative materials, Calif Dent J 24:17, 1996.

Tennant RW, Margolin BH, Shelby MD, Zeigler E, Haseman JK, Spalding J, Caspary W, Resnick M, Stasiewicz S, Anderson B, Minor R: Prediction of chemical carcinogenicity in rodents from in vitro genetic toxicity assays, Science 236933, 1987.

Wataha JC: Biocompatibility of dental casting alloys: a review, J Prosthet Dent 83:223, 2000.

Wataha JC: Materials for endosseous dental implants, J Oral Rehabil23:79, 1996.

Wataha JC, Hanks CT: Biological effects of palladium and risk of using palladium in dental casting alloys, J Om1 Rehabil23309, 1996.

Wataha JC, Hanks CT, Craig RG: Precision of and new methods for testing in vitro alloy cytotoxicity, Dent Mater 8:65-71, 1992.

Williams DF: Toxicology of ceramics. In Williams DF, editor: Fundamental aspects of biocompatibility, vol 2, Boca Raton, Fla, 1981, CRC Press.

Metals and alloys play a prominent and important role in dentistry. These materials are used in almost all aspects of dental prac-

tice, including the dental laboratory, direct and indirect dental restorations, and instruments used to prepare and manipulate teeth, Metals and alloys have optical, physical, chemical, thermal, and electrical properties that are exploited to advantage in dentistry and are unique among the basic types of materials (metals, polymers, and ceramics). Although popular press dental journals have occasionally promoted "metal-free" dentistry as desirable, the metals remain the only clinically proven materials for many long-term dental applications. This chapter will summarize the aspects of metals and alloys that are relevant to dentistry. The concepts of crystallization,

phase diagrams, alloy microstructure, and alloy strengthening will be described and related to the clinical practice of dentistry. Finally, this chapter will present a foundation for understanding the major classes of alloys used in dentistry: amalgam (see Chapter 11); noble alloys and solders (see Chapter 15); base metal alloys (see Chapter 161, and ceramic-metal restorations (see Chapter 19).

CHEMICAL AND ATOMIC

OF METALS

A metal is any element that ionizes positively in solution; metals constitute nearly two thirds of the periodic table (Fig. 6-1). During ionization,

metals release electrons. This ability to exist as free, positively charged, stable ions is a key factor in the behavior of metals and is responsible for many metallic properties that are important in dentistry. Another important group of elements in Fig. 6-1 are the metalloids, including carbon, silicon, and boron. Although metalloids do not always form free positive ions, their conductive and electronic properties make them important components of many alloys.

ATOMIC STRUCTURE

At the atomic level, pure metals exist as crystalline arrays (Fig. 6-2) that extend for many repetitions in three dimensions. In these arrays, the nuclei and core electrons occupy the atomic centers, whereas the ionizable electrons float freely among the atomic positions. The mobility of the valence electrons are responsible for many properties of metals, such as electrical conduc-

tivity. It is important to note that the positively charged atomic centers are held together by the electrons and their charge is simultaneously neutralized by the electrons; thus pure metals have no net charge.

The relationships between the atomic centers in a metallic crystalline array are not always uniform in all directions. The distances in the x, y (horizontal), and z (vertical) axes may be the same or different, and the angles between these axes may or may not be 90 degrees. In all, there are six crystal systems that occur (Fig. 6-3), and these can be further divided into 14 crystalline arrays. Metallic nuclei may occur at the center of faces or vertices of the crystal. Within each array, the smallest repeating unit that captures all of the relationships among atomic centers is called a unitcell (see Fig. 6-2). The unit cells for the most common arrays in dental metals are shown in Fig. 6-4. In the body-centered cubic (BCC) array, all angles are 90 degrees and all atoms are equidistant from one another in the horizontal and vertical directions. Metallic atoms are located at the corners of the unit cell, and one atom is at the center of the unit cell (hence the name

Fig. 6-2 A typical metallic crystal lattice, in this case a body centered cubic lattice. Every lattice has a unit cell (shown in bold) that extends (repeats)in three dimensions for large distances. Electrons are only relatively loosely bound to atomic nuclei and core electrons. The nuclei occupy specific sites (shown as dots in the unit cell) in the lattice, whereas the electrons are relatively free to move about the lattice. In reality, the metal atoms are large enough to touch each other.

of which can be subdivided. Each family is defined by the distances between vertices and the angles at the vertices. The body-centered cubic, face-

centered cubic, and hexagonal lattices (asterisks) are the most common in dental alloys and pure metals.

body-centered cubic). This is the crystal structure of iron and is common for many iron alloys. The face-centered cubic (FCC) array has 90degree angles and atomic centers that are equidistant horizontally and vertically (as does the BCC), but atoms are located in the centers of the faces with no atom in the center of the unit cell (hence the nameface-centered cubic). Most pure metals and alloys of gold, palladium, cobalt, and nickel have the FCC array. The more complex hexagonal close-pack array occurs with titanium. In this array, the atoms are equidistant from each other in the horizontal plane, but not in the vertical direction.

In a metallic crystal, the atomic centers are positively charged because the valence electrons have been released to float about the crystal. At first glance, it seems unlikely that these positively charge atoms could exist so close together. Rather, a repulsion between these centers would seem more likely. The metallic bond is a fundamentally important type of primary bond that holds the atomic centers together in a metal lattice. The freely floating electrons bind the atomic centers together and, although it is not directed between specific centers, this metallic bond has a formidable force between atomic centers. The metallic bond is fundamentally dif-

of metals act by impeding the movement of dislocations. These methods will be discussed later in the chapter.

The fracture of metals occurs when the atomic centers cannot slide past one another freely. For example, this can happen when impurities block the flow of dislocations (Fig. 6-6). The inability of the dislocation to be moved through the solid results in the lattice rupturing locally. Once this small crack is started, it takes little force to propagate the crack through the lattice. An example

illustrates this idea. Consider a plate of steel 15 cm wide and 6 mm thick. Suppose it has a 5-cm crack running into one side. The force required to make the crack run the remaining 10 cm would be about 180 kg. Without the aid of the crack, 230,000 kg would be required if the steel were the best grade commercially available. If the steel were a single, flawless crystal, 4.5 million kg would be necessary! The fracture of metals depends heavily on dislocations and the local rupture ofthe crystal lattice.

Metals can be mixed together much as liquids are. A mixture of metals is called an alloy, and the study of metals and alloys is called metallurgy. Alloys may be a mixture of as few as two or (in the case of dental casting alloys) as many as nine

or more different metals. As with liquids, not all rnet~l.,\vd1 dlsiol\ c In one another freely, some metals \n 111 not d~ssolreat all mto other metals The concept of phases and phase diagrams n as developed-to help understand the nature of alloys and metal solubility. Alloys may have crystal structures like the pure metals previously discussed, or they may have other atomic structures, such as eutectics or intermetallic compounds.

These concepts will be discussed later in this chapter.

PHASE DIAGRAMS AND DENTAL ALLOYS

Aphase is a state of matter that is distinct in some way from the matter around it. In a mixture of ice and water there are two phases because although ice and water are the same chemically, they each have distinct arrangements of atoms. Ice has the crystalline arrangement of a solid whereas water has the random atomic arrangement of a liquid. A solid dental alloy may also have one phase if the composition of the alloy is essentially homogeneous throughout. If the alloy has areas where compositions are different, it is called a multiple-phasealloy.The distinction between singleand multiple-phase alloys is important to the strength, corrosion, biocompatibility, and other alloy properties.

Phase diagrams are "maps" of the phases that occur when metals are mixed together. If there are two metals in an alloy, a binary phase diagram is used. If three metals are in the alloy, a ternary phase diagram may be used. Phase diagrams describing alloys with more than three metals are not used because they are too complex; the vast majority of phase diagrams describe only binary alloys. For dental alloys that contain four or more metals, the binary phase diagram of the two most abundant metals in the alloy typically describe the alloy. In practice, phase diagrams are determined by meticulous examination of a series of binary alloys cooled slowly and monitored for composition.

A typical phase diagram for a theoretical binary alloy AB is shown in Figure 6-7. The x-axis describes the composition of the elements in either weight percent or atomic percent (see Chapter 15). The y-axis is the temperature of the alloy system. It is important to remember that a phase diagram shows the composition and types of phases at a given temperature and at equilibrium. Every phase diagram divides an alloy system into at least three areas: the liquid phase, the liquid + solid phases, and the solid phase. For example, everything above the temperatures shown by line ACB in Fig. 6-7 is liquid. This

*OOI1 Solid ~ o l u t i o n l$----

Composition

Fig. 6-7 A phase diagram for two theoretical components A and B. The liquidus for this system follows the line ACB. The solidus follows ADB. The melting points for each pure metal occur at A and 6.

line is therefore called the liquidus. Everything below line ADB is solid. This line is thus called the solidus. The area between these two lines contains some liquid and some solid. At 0% B, a single phase exists (100% A) that has a single melting point (800" C). At 100% B, a single phase exists that has a melting point (210" C). At any composition between these extremes, the melting range is defined as the temperature difference between the liquidus (ACB) and solidus (ADB).

The phase diagram can also be used to find the composition of liquid and solid between the liquidus and solidus (see Fig. 6-71. Consider an alloy that is 20% B (and 80% A) at a temperature of 800" C. This alloy is all liquid. When the temperature drops to 700" C and the system equilibrates, there will be some liquid and some solid. If a line parallel to the x-axis is projected at 700" C to the liquidus, the composition of the liquid can be determined by projecting down to the x-axis. In this case, the liquid will be 60% A (and 40% B). By projecting at 700" C to the solidus then to the x-axis, the composition of the solid is 10% B (and 90% A). Below about 540" C, the alloy is 80% A and (20% B), or all solid. Thus the phase diagram can map the types and compositions of phases at any temperature and over any alloy composition.

When metals are mixed together in the molten state, then cooled to the solid state, there are