Сraig. Dental Materials

Brantley WA, Augat WS, Myers CL et al: Bending deformation studies of orthodontic wires, J Dent Res 57:609, 1978.

Dolan DW, Craig RG: Bending and torsion of endodontic files with rhombus cross sections, J Endodont 8:260, 1982.

Ruyter IE, Svendsen SA: Flexural properties of denture base polymers, J Prosthet Dent 43: 95, 1980.

Viscosity

Combe EC, Moser JB: The rheological characteristics of elastomeric impression materials, JDent Res 57:221, 1978.

Herfort TW, Gerberich WW, Macosko CW et al: Viscosity of elastomeric impression materials, J Prosthet Dent 38:396, 1977.

Koran A, Powers JM, Craig RG: Apparent viscosity of materials used for making edentulous impressions, J Am Dent Assoc

95:75, 1977.

Vermilyea SG, Huget EF, de Simon LB: Apparent viscosities of setting elastomers, J Dent Res 59:1149, 1980.

Vermilyea SG, Powers JM, Craig RG: Rotational viscometry of a zinc phosphate and a zinc polyacrylate cement, J Dent Res 56:762, 1977.

Vermilyea SG, Powers JM, Koran A: The rheological properties of fluid denture-base resins, J Dent Res 57:227, 1978.

Viscoelasticity

Bertolotti RL, Moffa JP: Creep rate of porcelainbonding alloys as a function of temperature, J Dent Res 59:2062, 1980.

Cook WD: Permanent set and stress relaxation in elastomeric impression materials, J Biomed Mater Res 15:449, 1981.

Duran RL, Powers JM, Craig RG: Viscoelastic and dynamic properties of soft liners and tissue conditioners, J Dent Res 58:1801, 1979.

Ellis B, Al-Nabash S: The composition and rheology of denture adhesives, J Dent 8109, 1980.

Ferracane JL, Moser JB, Greener EH: Rheology of composite restoratives, J Dent Res 60:1678, 1981.

Goldberg AJ: Viscoelastic properties of silicone, polysulfide, and polyether impression materials, J Dent Res 53:1033, 1974.

McCabe JF, Bowman AJ: The rheological properties of dental impression materials,

Br Dent J 151:179, 1981.

Morris HF, Asgar K, Tillitson EW: Stressrelaxation testing. Part I: A new approach to the testing of removable partial denture alloys, wrought wires, and clasp behavior, J Prosthet Dent 46:133, 1981.

Nikolai RJ, Crouthers RC: On the relaxation of orthodontic traction elements under interrupted loads, J Dent Res 59:1071, 1980.

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

Ruyter IE, Espevik S: Compressive creep of denture base polymers, Acta Odont Scand 38169, 1980.

Tolley LG, Craig RG: Viscoelastic properties of elastomeric impression materials: polysulphide, silicone and polyether rubbers, J Oral Rehabil5:121, 1978.

Wills DJ, Manderson RD: Biomechanical aspects of the support of partial dentures, J Dent 5310, 1977.

Xu HHK, Liao H, Eichmiller FC: Indentation creep behavior of a direct-filling silver alternative to amalgam, J Dent Res 77:1991, 1998.

Dynamic Properties

Impact resistance of plastics and electrical insulating material, D 256-92. In ASTM Standards 1993, Vol. 8.01, Philadelphia, American Society for Testing and Materials, 1993.

Koran A, Craig RG: Dynamic mechanical properties of maxillofacial materials, J Dent Res 54:1216, 1975.

Properties of Composite Materials

Bayne SC, Thompson JY, Swift EJ Jr et al:

A characterization of first-generation flowable composites, J Am Dent Assoc 129:567, 1998.

Braem MJA, Davidson CL, Lambrechts P et al: In vitro flexural fatigue limits of dental composites, J Biomed Mater Res 28:1397, 1994.

122 Chapter 4 MECHANICAL PROPERTIES

Braem M, Van Doren VE, Lambrechts P et al: Determination of Young's modulus of dental composites: a phenomenological model

J Mater Sci 22:2037, 1987.

Choi KK, Condon JR, Ferracane JL: The effects of adhesive thickness on polymerization contraction stress of composite, J Dent Res 79812, 2000.

Condon JR, Ferracane JL: Reduction of composite contraction stress through non-bonded microfiller particles, Dent Mater 14:256, 1998.

Ferracane JL: Current trends in dental composites, Crit Rev Oral Biol Med 6:302, 1995.

Ferracane JL, Berge HX, Condon JR: In vitro aging of dental composites in water-effect of degree of conversion, filler volume, and filler/matrix coupling, J Biomed Mater Res 42:465, 1998.

Ferracane JL, Condon JR: In vitro evaluation of the marginal degradation of dental composites under simulated occlusal loading, Dent Mater 15:262, 1999.

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

Goldberg AJ, Burstone CJ, Hadjinikolaou I et al: Screening of matrices and fibers for

reinforced thermoplastics intended for dental applications, J Biomed Mater Res 28:167, 1994.

McCabe JF, Wang Y, Braem M: Surface contact fatigue and flexural fatigue of dental restorative materials, J Biomed Mater Res 50:375, 2000.

Peutzfeldt A: Resin composites in dentistry: the monomer systems, Eur J Oral Sci 105:97, 1997.

Sakaguchi RL, Ferracane JL: Stress transfer from polymerization shrinkage of a chemical-cured composite bonded to a pre-cast composite substrate, Dent Mater 14:106, 1998.

Urabe I, Nakajima M. Sano H et al: Physical properties of the dentin-enamel junction region, Am J Dent 15129, 2000.

Van der Varst PGT, Brekelmans W M , De Vree JHP et al: Mechanical performance of a dental composite: probabilistic failure prediction, J Dent Res 72:1249, 1993.

Willems G, Lambrechts P, Braem M et al: Composite resins in the 21st century, Quint Internat 24:641, 1993.

Tear Strength and Tear Energy

Herfort TW, Gerberich WW, Macosko CW et al: Tear strength of elastomeric impression materials, J Prosthet Dent 39:59, 1978.

MacPherson GW, Craig RG, Peyton FA: Mechanical properties of hydrocolloid and rubber impression materials, J Dent Res 46:714, 1967.

Strength of conventional vulcanized rubber and thermoplastic elastomers, D 624-91. In ASTM Standards 1994, Vol. 9.01, Philadelphia, American Society for Testing and Materials, 1994.

Webber RL, Ryge G: The determination of tear energy of extensible materials of dental interest, J Biomed Mater Res 2:231, 1968.

Hardness

DeBellis A: Fundamentals of Rockwell hardness testing. In Hardness Testing Reprints, WD-673, Wilson Instrument Division, Bridgeport, Conn, 1967.

Doerner MF, Nix WD: A method for interpreting the data from depth-sensing indentation measurements, J Mater Res 1:601, 1986.

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

Lysaght VE: How to make and interpret hardness tests on plastics. In Hardness Testing Reprints, WD-673, Wilson Instrument Division, Bridgeport, Conn, 1967.

Lysaght VE: Indentation hardness testing,

New York, 1949, Reinhold.

Lysaght VE, DeBellis A: Microhardness testing. In Hardness Testing Reprints, WD-673, Wilson Instrument Division, Bridgeport, Conn, 1967.

Tirtha R, Fan PL, Dennison JB, Powers JM: In vitro depth of cure of photo-activated composites. J Dent Res 61:1184, 1982.

Van Meerbeek B, Willems G, Celis JP et al: Assessment by nano-indentation of the hardness and elasticity of resin-dentin bonding area, J Dent Res 72:1434, 1993.

Willems G, Celis JP, Lambrechts P et al: Hardness and young's modulus determined by nanoindentation technique of filler particles of dental restorative materials compared with human enamel, J Biomed Mater Res 27:747, 1993.

Xu HHK et al: Indentation damage and mechanical properties of human enamel and dentin, J Dent Res 77:472, 1998.

Specifications

Council on Scientific Affairs: Clinical products

in dentistry; a desktop reference, Chicago,

1996, American Dental Association.

United States General Services Administration:

Index of Federal Specifications and Stan-

dards, Washington, DC, 1994, Superinten-

dent of Documents, U.S. Government

Printing Office.

Wear

Abe Y, Sato Y. Akagawa Y. Ohkawa S. An in vitro study of high-strength resin posterior denture tooth wear. Int J Prosthodont 10:28, 1997.

Barbakow F, Lutz F, Imfeld T: A review of methods to determine the relative abrasion of dentifrices and prophylaxis pastes,

Quint Internat 1823, 1987.

Condon JR. Ferracane JL: Factors effecting dental composite wear in vitro. J Biomed Mater Res 38:303, 1997.

Condon JR, Ferracane JL: In vitro wear of composite with varied cure, filler level, and filler treatment. J Dent Res 76:1405, 1997.

Draughn RA, Harrison A: Relationship between abrasive wear and microstructure of composite resins, J Prosthet Dent 40:220, 1978.

Ferracane JL, Mitchem JC, Condon JR, Todd R: Wear and marginal breakdown of composites with various degrees of cure, J Dent Res 76:1508, 1997.

Hu X, Harrington E, Marquis PM et al: The influence of cyclic loading on the wear of a dental composite, Biomat 20:907,

1999.

Hu X, Marquis PM, Shortall AC: Two-body in vitro wear study of some current dental composites and amalgams, J Prosthet Dent 82:214, 1999.

Knibbs PJ: Methods of clinical evaluation of dental restorative materials, J Oral Rehabil 24109, 1997.

Koczorowski R, Wloch S: Evaluation of wear of selected prosthetic materials in contact with enamel and dentin. J Prosthet Dent 81:453, 1999.

Powers JM, Craig RG: Wear of dental tissues and restorative materials. In proceedings of national symposium on wear and corrosion, June 4-6, 1979,

Washington, DC, 1979, American Chemical Society.

Powers JM, Fan PL, Craig RG: Wear of dental restorative resins. In Gebelein CG, Koblitz FF, editors: Biomedical and dental applications of polymers: Polymer science and technology, vol 14, New York, 1981, Plenum Press.

Roberts JC, Powers JM, Craig RG: Wear of dental amalgam, J Biomed Mater Res 11:513, 1977.

Teoh SH, Ong LF, Yap AU et al: Bruxing-type dental wear simulator for ranking of dental restorative materials, J Biomed Mater Res 43:175, 1998.

Wu W, McKinney JE: Influence of chemicals on wear of dental composites, J Dent Res 61: 1180, 1982.

Yap AU, Ong LF, Teoh SH et al: Comparative wear ranking of dental restoratives with the BIOMAT wear simulator, J Oral Rehabil 26:228, 1999.

B iocompatibility is formally defined as the ability of a material to elicit an appropriate biological response in a given application in the body. Inherent in this definition is the idea that a single material may not be biologically acceptable in all applications. For example, a material that is acceptable as a full cast crown may not be acceptable as a dental implant. Also implicit in this definition is an expectation for the biological performance of the material. In a bone implant, the expectation is that the material will allow the bone to integrate with the implant. Thus an appropriate biological response for the implant is osseointegration. In a full cast crown, the expectation is that the material will not cause inflammation of pulpal or periodontal tissues, but osseointegration is not an expectation. Whether a material is biocompatible is therefore dependent on what physical function we ask of the material and what biological response we require from it. Using this definition, it makes little sense to say that any given material is or is not biocompatible, because we need to define how the material will be used before we can assess this. In this sense, biocornpatibility is much like color. Color is a property of a material interacting with its environment (light), and the color of a material depends on the light source and the observer of the light. Similarly,biocornpatibility is a property of a material interacting with its environment. The biological response may change if the host, the application of the material, or the material itself

are changed (Fig. 5-11,

Dentistry shares concerns about biocompatibility with other fields of medicine, such as orthopedics, cardiology, and vascular biology, among others. Today, in the development of any biomaterial, one must consider not only the strength, esthetics, or functional aspects of the material, but its biocornpatibility as well. Furthermore, demands for appropriate biological responses are increasing as we ask materials to perform more sophisticated functions in the body for longer periods. Thus considerations of biocompatibility are important to manufacturers, practitioners, scientists, and patients. The field of biocompatibility is interdisciplinary, and draws knowledge from materials science, bioengineer-

Material

Fig. 5-1 Like color, biocompatibility is not a property of just a material, but rather a property of how a material interacts with its environment. A material's color depends on the character of the light source, how the light interacts with the material, and how the observer interprets the reflected light. In this sense, the material's color depends on its environment. The biocompatibility of a material is similar in the sense that

it depends on its environment.

ing, biochemistry, molecular biology, and others. This chapter surveys the tests used for evaluating the biocornpatibility of dental materials, the specifications that govern such testing, and the strengths and weaknesses of the testing methods. In addition, the biocornpatibility of various materials used in dentistry are discussed within a framework of principles. Because an understanding biocornpatibility requires an understanding of the biological system into which materials are placed, this chapter will first summarize the anatomical and pathological aspects of the oral

tissues relevant to dental materials.

THE TOOTH

Enamel Mature human enamel is highly mineralized (96% by weight), with only 1%of its weight being organic molecules and 3% being water. The organic matrix of enamel consists of at least two types of glycoproteins: amelogenins and enamelins. After synthesis by ameloblasts, the calcified organic matrix of enamel does not appear to be maintained in any way by cellular synthetic mechanisms, contrary to other calcified

tissues such as dentin, bone, and cementum. Enamel rods have a specific orientation with each other, and this orientation provides maximal strength. Because of its high mineral (hydroxyapatite) content, enamel is much more brittle than dentin and is solubilized to a greater extent by acid solutions. This property is used to advantage with bonding agents, where acids are used to etch the enamel to provide micromechanical retention of resin composite materials. The differential etching that occurs is a consequence of the different orientation of enamel rods on the enamel surface. The permeability of enamel to most oral molecules is quite low, and in this sense enamel "seals" the tooth to outside agents. However, recent evidence indicates that enamel is not impermeable. Peroxides in bleaching agents have been shown to permeate intact enamel in just a few seconds.

Dentin and Pulp Because of their close anatomical relationship, most researchers consider the dentin and pulp to be a single organ. The dentinal matrix (both calcified and uncalcified) forms the greatest bulk of the tooth. Calcified dentin is about 20% organic, 70% inorganic, and 10%aqueous by weight. Collagen constitutes approximately 85% of the organic portion of dentin, and hydroxyapatite is the main inorganic compound. The dentinal matrix also contains many proteins, including collagenous proteins (mainly type I collagen with smaller amounts of type V and type I trimer collagens), noncollagenous dentin-specific proteins (phosphophoryns, dentin sialoprotein, and dentin matrix protein-l), and several nonspecific proteins associated with mineralized tissues (e.g., osteocalcin and osteopontin).

The dentinal matrix surrounds dentinal tubules that are filled with the odontoblastic processes. These processes stem from odontoblasts that reside in the pulp of the tooth. The tubules traverse the region between the dentoenamel junction (DEJ) and the pulp. The numbers of tubules per cross-sectional area range from about 20,000/mm2near the DEJ to 50,000/mm2near the pulp. The tubule diameter varies from about 0.5 pm at the DEJ to about 2.5 pm near the pulp

Fig. 5-2 Diagram of dentinal tubules. Dentinal tubules occur throughout the dentin, but their size and number vary as a function of proximity to the pulp of

the tooth. Near the dentin-enamel junction (A), the tubules are small in diameter and relatively few in number per centimeter squared. As the depth approaches the pulp (B, D), the tubules become larger in diameter and are more dense in number. At the pulp (C), the tubules are very dense and have the largest diameters. Furthermore, the tubules do not follow a straight path from the pulp to the enamel, but curve as a function of the shape of the tooth. Thus

a cavity preparation may section the tubules in either a cross-section (D) or a longitudinal section (B). The tubular structure of dentin is critical to biocompatibility because components of materials may use the tubules as conduits to pulpal tissues.

(Courtesy Avery JK: Ann Arbor, 1987, University of Michigan School of Dentistry)

(Fig. 5-21. Some odontoblastic processes extend through the dentin tubules to the DEJ. The percentage of processes that reach the DEJ is a matter of some conjecture.

A serum-like fluid fills the dentinal tubules. This fluid has continuity with the extracellular fluid of the pulp tissue. The pulpal circulation maintains an intercellular hydraulic pressure of about 24 mm Hg (32.5 cm H20), which causes fluid flow in the tubules to be directed from the

128 Chapter 5 BIOCOMPATIBiLlTY OF DENTAL MATERIALS

pulp outward toward the DEJ when enamel is removed. External hydrostatic and osmotic pressures can also cause fluid movement toward or away from the pulp. The positive or negative displacement of this fluid through exposed dentinal tubules is capable of affecting either odontoblasts or pulpal nerve endings. These effects are the basis of the hydrodynamic theory of hyperalgesia (pulpal hypersensitivity).

During cavity preparation by the dentist, a "smear layer" is formed by the action of the bur or hand instruments on the calcified dentin matrix (Fig, 5-3). This mat of organic and inorganic particles occludes the dentinal tubules to some extent. The smear layer is quite effective in reducing hydrostatic pressures, but less effective in reducing diffusion, especially if the layer is interrupted or defective. The smear layer can be removed by acid etching, which also demineralizes the openings of the tubules (Fig. 5-4). The dentinal tubules establish continuity with the pulpal fluid to facilitate the diffusion of molecules, both natural or from materials, into and out of the pulp. The smear layer, dentinal tubules, and dentinal matrix are all important in the application of dentinal bonding agents and the ability of components of the bonding agents to reach and affect pulpal tissues.

Moderately deep cavity preparation will damage odontoblasts by severing the odontoblastic processes. Deep cavity preparation may destroy most of the dentin and kill the primary odontoblasts. A number of investigators think that the extracellular matrix (ECM) of dentin and pulp is largely responsible for the differentiation of secondary odontoblasts that form reparative dentin. The source of secondary odontoblasts is not known, but much of the proliferative activity of granulation tissue following pulpal insult is found in perivascular areas proximal to the core of the pulp. In monkeys, the minimal amount of time between pulp injury and replacement odontoblast differentiation is about 5 days. Nerves and blood vessels, which arborize from the core of the pulp as they approach the odontoblastic layer, may influence the extent of the inflammatory response and the amount of new dentinal matrix formed during dentin repair.

Fig. 5-3 Scanning electron micrograph of cut dentin. When a dentin surface is cut with a bur, a layer of debris, called the smear layer (S), remains on the surface. The smear layer consists of organic and inorganic debris that covers the dentinal surface and the tubules (TI. Often, the debris fills the distal part of the tubules in a smear plug (PI.

(From Brannstrom M: Dentin and puip in restorative dentistry, Stockholm, 1981, Dental Therapeutics AB.)

In the absence of a smear layer, components of materials or bacterial products diffuse toward the pulp against the pressure gradient (diffusional permeability, see later discussion). Bacteria can sometimes be seen within tubules below a carious lesion or at the base of a prepared cavity, with or without a restoration (Fig. 5-5). When toxic bacterial or chemical products traverse the dentin, odontoblasts and the pulpal connective tissue usually respond first by focal necrosis (0 to 1 2 hours), which may be followed by an acute,

Fig. 5-4 Scanning electron micrograph of acid-etched dentin. When a smear layer (as seen in Fig. 5-3) is etched with an acid, the smear debris is removed. The rate of etching depends on the acid and the character of the dentin. The layer shown above was etched for 5 seconds with 37% phosphoric acid. The smear layer is completely gone and the tubules (T) are open. Further etching will open the tubules even further.

(From Brannstrom M: Dentin and pulp in restorative dentistry,

Stockholm, 1981, Dental Therapeut~csAB.)

but more widespread, pulpitis (12 hours to several days). This response may resolve naturally if the injurious agent is removed or the tubules are blocked. If the pulpitis does not resolve, it may spread to more completely involve the pulp in liquefaction necrosis (especially if the pulpitis results from bacterial products) or in chronic inflammation. Finally, both acute complete pulpitis and acute exacerbation of chronic pulpitis may lead to sequelae such as dental periapical lesions

and osteomyelitis, which may be reviewed in oral pathology textbooks.

Dentin Permeability Much has been learned about dentin permeability in the last three decades. In practical terms, two types of dentin permeability occur. The first is fluid convection; that is, movement of fluid through the dentinal tubules. Fluid convection toward the pulp will occur under positive hydraulic pressure when a crown or inlay is being seated. If the dentinal tubules are open, this produces a sharp, localized pain in the pulp from stimulation of A-fibers. Fluid convection away from the pulp will occur with negative osmotic pressures when concentrated solutions, such as sucrose or saturated calcium chloride, are exposed to open dentinal tubules. Clinically, this situation occurs with cervical abrasion or carious lesions. Convection of fluids across dentin varies with the fourth power of the radius of the dentinal tubule (r4), and thus is very sensitive to the diameter of the tubules. In general, coronal dentin exhibits greater convective permeability than root dentin. Axial wall dentin is more permeable than dentin in the floor of cavities, and dentin near pulp horns (where tubule diameter is greatest) is more permeable than dentin at a distance from the pulp horns. The presence of a smear layer or of cavity liners, sealers, crystals such as calcium oxalate, and even debris and bacteria in the dentinal tubules can dramatically reduce fluid convection.

The second type of dentin permeability is diffusion. Patent dentinal tubules, no matter how small the diameter, establish a diffusion gradient through which ions and molecules can move, even against positive hydraulic pressure. Diffusion is proportional to the length of the dentinal tubules and thus, roughly, to the thickness of the dentin between cavity preparation and the pulp. Smear layers created in cavity preparation are better than cavity liners and sealers at limiting diffusional permeability. However, if the smear layer is incomplete, interrupted, or removed, or if there is a disruption in a cavity liner, sealer, or base, then diffusion of molecules toward the pulp will occur.

Diffusion of natural and synthetic molecules through dentin has been studied. In general, diffusion through a given thickness of dentin is proportional to the molecular size of the molecule. Consequently, molecules the size (molecular weight [mwl) of urea (mw 60), ,phenol (mw 941, and glucose (mw 180) diffuse more easily than molecules the size of dextran (mw 20,000)and albumin (bovine serum albumin, mw 68,000). Through diffusion, small or globular molecules such as albumin, gamma globulin, and the bis-glycidyldimethacrylate (Bis-GMA, an oligomer of dental composites) are diluted 2000 to 10,000 times on the pulpal side of the dentin by 0.3 to 0.4 mm of dentin. Large, fibrous molecules such as fibrinogen are diluted 25,000 to 125,000times by the same dentin thickness. Most molecules are probably adsorbed to some extent by dentin. Some molecules and atoms or ions, such as tetracycline, zinc, H,O,, and fluorescein, are adsorbed to a greater extent than the biological molecules and resin monomers mentioned

above. Finally, the capillary beds and vascular dynamics in most healthy pulps are probably capable of removing relatively large amounts of cytotoxic chemicals and bacterial products once they diffuse through the dentin. However, if the pulp is already damaged (inflamed because of caries or trauma), edema and sluggish circulation probably compromise the removal of these materials. Much still needs to be learned about the dynamics and significance of diffusion and adsorption of both host and foreign molecules through dentin.

BONE

Bone is an extracellular matrix (ECM) with accompanying cells and tissue. The ECM of bone is a mineralized tissue composed of about 23% organic substances and about 77% hydroxyapatite. Like dentin, most (86%) of this organic matrix is type I collagen, which gives elastic and viscoelastic qualities to bone. The hydroxyapatite