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2 CRAIG’S RESTORATIVE DENTAL MATERIALS Developments in materials science, robotics, and Some materials are cast to achieve excellent adapta- biomechanics have dramatically changed the way we tion to existing tooth structure, whereas others are l...

2 CRAIG’S RESTORATIVE DENTAL MATERIALS Developments in materials science, robotics, and Some materials are cast to achieve excellent adapta- biomechanics have dramatically changed the way we tion to existing tooth structure, whereas others are look at the replacement of components of the human machined to produce very reproducible dimensions anatomy. In the historical record, we find many and structured geometries. When describing these approaches to replacing missing tooth structure and materials, physical and chemical characteristics are whole teeth. The replacement of tooth structure lost to often used as criteria for comparison. To understand disease and injury continues to be a large part of gen- how a material works, we study its chemical struc- eral dental practice. Restorative dental materials are ture, its physical and mechanical characteristics, and the foundation for the replacement of tooth structure. how it should be manipulated to produce the best Form and function are important considerations performance. in the replacement of lost tooth structure. Although Most restorative materials are characterized by tooth form and appearance are aspects most easily physical, chemical, and mechanical parameters that recognized, function of the teeth and supporting are derived from test data. Improvements in these tissues contributes greatly to the quality of life. The characteristics might be attractive in laboratory stud- links between oral and general health are widely ies, but the real test is the material’s performance accepted. Proper function of the elements of the in the mouth and the ability of the material to be oral cavity, including the teeth and soft tissues, is manipulated properly by the dental team. In many needed for eating, speaking, swallowing, and proper cases, manipulative errors can negate the techno- breathing. logical advances for the material. It is therefore very Restorative dental materials make the reconstruc- important for the dental team to understand funda- tion of the dental hard tissues possible. In many areas, mental materials science and biomechanics to select the development of dental materials has progressed and manipulate dental materials appropriately. more rapidly than for other anatomical prostheses. Because of their long-term success, patients often expect dental prostheses to outperform the natural BASIC SCIENCES APPLIED TO materials they replace. The application of materials RESTORATIVE MATERIALS science is unique in dentistry because of the com- plexity of the oral cavity, which includes bacteria, The practice of clinical dentistry depends not only high forces, ever changing pH, and a warm, fluid on a complete understanding of the various clinical environment. The oral cavity is considered to be the techniques but also on an appreciation of the funda- harshest environment for a material in the body. In mental biological, chemical, and physical principles addition, when dental materials are placed directly that support the clinical applications. It is important into tooth cavities as restorative materials, there are to understand the ‘how’ and ‘why’ associated with very specific requirements for manipulation of the the function of natural and synthetic dental materials. material. Knowledge of materials science and biome- A systems approach to assessing the chemical, chanics is very important when choosing materials physical, and engineering aspects of dental materi- for specific dental applications and when designing als and oral function along with the physiological, the best solution for restoration of tooth structure pathological, and other biological studies of the and replacement of teeth. tissues that support the restorative structures pro- vides the best patient outcomes. This integrative approach, when combined with the best available SCOPE OF MATERIALS COVERED IN scientific evidence, clinician experience, patient RESTORATIVE DENTISTRY preferences, and patient modifiers results in the best patient-centered care. Restorative dental materials include representa- tives from the broad classes of materials: metals, polymers, ceramics, and composites. Dental materi- APPLICATION OF VARIOUS als include such items as resin composites, cements, SCIENCES glass ionomers, ceramics, noble and base metals, amalgam alloys, gypsum materials, casting invest- In the chapters that follow, fundamental charac- ments, dental waxes, impression materials, denture teristics of materials are presented along with numer- base resins, and other materials used in restorative ous practical examples of how the basic principles procedures. The demands for material characteristics relate to clinical applications. Test procedures and and performance range from high flexibility required techniques of manipulation are discussed briefly but by impression materials to high stiffness required not emphasized. Many of the details of manipulation in crowns and fixed dental prostheses. Materials have been moved to the book’s website at http:// for dental implants require integration with bone. evolve.elsevier.com/sakaguchi/restorative  1. ROLE AND SIGNIFICANCE OF RESTORATIVE DENTAL MATERIALS 3 A more complete understanding of fundamental implants are becoming a more popular option because principles of materials and mechanics is important they do not involve the preparation of adjacent teeth for the clinician to design and provide a prognosis for as for a fixed, multi-unit restoration. Research into restorations. For example, the prognosis of long-span implant coatings, surface textures, graded proper- fixed dental prostheses, or bridges, is dependent on ties, alternative materials, and new geometries will the stiffness and elasticity of the materials. When continue to grow. For those with less adequate access, considering esthetics, the hardness of the material removable prostheses will continue to be used. is an important property because it influences the An emphasis on esthetics continues to be popu- ability to polish the material. Some materials release lar among consumers, and this will continue to drive fluoride when exposed to water, which might be ben- the development of tooth whitening systems and eficial in high-caries-risk patients. When selecting a esthetic restorations. There appears to be an emerg- ceramic for in-office fabrication of an all-ceramic ing trend for a more natural looking appearance with crown, the machining characteristic of ceramics is some individuality as opposed to the uniform, spar- important. Implants have a range of bone and soft kling white dentition that was previously requested tissue adaptation that are dependent on surface tex- by many patients. This will encourage manufactur- ture, coatings, and implant geometry. These are just ers to develop materials that mimic natural dentition a few examples of the many interactions between the even more closely by providing the same depth of clinical performance of dental materials and funda- color and optical characteristics of natural teeth. mental scientific principles. With the aging of the population, restorations The toxicity of and tissue reactions to dental mate- for exposed root surfaces and worn dentitions will rials are receiving more attention as a wider variety become more common. These materials will need to of materials are being used and as federal agencies function in an environment with reduced salivary demonstrate more concern in this area. A further flow and atypical salivary pH and chemistry. Adhe- indication of the importance of the interaction of sion to these surfaces will be more challenging. This materials and tissues is the development of recom- segment of the population will be managing multi- mended standard practices and tests for the biologi- ple chronic diseases with many medications and will cal interaction of materials through the auspices of have difficulty maintaining an adequate regimen of the American Dental Association (ADA). oral home care. Restorative materials will be chal- After many centuries of dental practice, we con- lenged in this difficult environment. tinue to be confronted with the problem of replacing The interaction between the fields of biomaterials tooth tissue lost by either accident or disease. In an and molecular biology is growing rapidly. Advances effort to constantly improve our restorative capa- in tissue regeneration will accelerate. The develop- bilities, the dental profession will continue to draw ments in nanotechnology will soon have a major from materials science, product design, engineering, impact on materials science. The properties we cur- biology, chemistry, and the arts to further develop an rently understand at the macro and micro levels will integrated practice of dentistry. be very different at the nano level. Biofabrication and bioprinting methods are creating new structures and materials. This is a very exciting time for materials FUTURE DEVELOPMENTS IN research and clinicians will have much to look for- BIOMATERIALS ward to in the near future as this body of research develops new materials for clinical applications. In the United States over 60% of adults aged 35 to 44 have lost at least one permanent tooth to an acci- dent, gum disease, a failed root canal, or tooth decay. Bibliography In the 64- to 65-year-old category, 25% of adults have American Association of Oral and Maxillofacial Surgeons: lost all of their natural teeth. For children aged 6 to 8, Dental implants. http://www.aaoms.org/dental_implants 26% have untreated dental caries, and 50% have been.php. Accessed August 28, 2011. treated for dental decay. The demand for restorative Centers for Disease Control and Prevention: National care is tremendous. Advances in endodontology and Health and Nutrition Examination Study. http://www.cdc periodontology enable people to retain teeth longer,.gov/nchs/nhanes/nhanes2005-2006/nhanes05_06 shifting restorative care from replacement of teeth.htm. Accessed August 28, 2011. Choi CK, Breckenridge MT, Chen CS: Engineered materials to long-term restoration and maintenance. Develop- and the cellular microenvironment: a strengthening ment of successful implant therapies has encouraged interface between cell biology and bioengineering, patients to replace individual teeth with fixed, single Trends Cell Biol 20(12):705, 2010. tooth restorations rather than with fixed or remov- Horowitz RA, Coelho PG: Endosseus implant: the journey able dental prostheses. For those patients with good and the future, Compend Contin Educ Dent 31(7):545, access to dental care, single tooth replacements with 2010. 4 CRAIG’S RESTORATIVE DENTAL MATERIALS Jones JR, Boccaccini AR: Editorial: a forecast of the future National Institute of Dental and Craniofacial Research: for biomaterials, J Mater Sci: Mater Med 17:963, 2006. A plan to eliminate craniofacial, oral, and dental health dis- Kohn DH: Current and future research trends in dental parities, 2002. http://www.nidcr.nih.gov/NR/rdonlyres/ biomaterials, Biomat Forum 19(1):23, 1997. 54B65018-D3FE-4459-86DD-AAA0AD51C82B/0/ Nakamura M, Iwanaga S, Henmi C, et al: Biomatrices and hdplan.pdf. biomaterials for future developments of bioprinting and Oregon Department of Human Services, Public Health biofabrication, Biofabrication 2(1):014110, 2010 Mar 10. Division: The burden of oral disease in Oregon, Nov, Epub. 2006. National Center for Chronic Disease Prevention and Health U.S. Department of Health and Human Services: Oral health Promotion (CDC): Oral health, preventing ­cavities, gum in America: a report of the Surgeon General—executive disease, tooth loss, and oral cancers, at a glance, 2010. summary, Rockville, MD, 2000, U.S. Department of Health National Institute of Dental Research: National Institutes of and Human Services, National Institute of Dental and Health (NIH): International state-of-the-art conference on Craniofacial Research, National Institutes of Health. restorative dental materials, Bethesda, MD, Sept 8-10, 1986, NIH.  C H A P T E R 2 The Oral Environment O U T L I N E Enamel Oral Biofilms and Restorative Dental Materials The Mineral Dentin Physical and Mechanical Properties Difficulties in Testing The Dentin-Enamel Junction 5 6 CRAIG’S RESTORATIVE DENTAL MATERIALS The tooth contains three specialized calcified the calcified tissues, and these procedures rely on tissues: enamel, dentin, and cementum (Figure 2-1). detailed knowledge of the structure and properties Enamel is unique in that it is the most highly calci- of the adhesive tissue substrates. fied tissue in the body and contains the least organic content of any of these tissues. Enamel provides the hard outer covering of the crown that allows ENAMEL efficient mastication. Dentin and cementum, like bone, are vital, hydrated, biological composite Figure 2-1 shows a schematic diagram of a poste- ­structures formed mainly from a collagen type I rior tooth sectioned to reveal the enamel and dentin matrix reinforced with the calcium phosphate min- components. Enamel forms the hard outer shell of eral called apatite. Dentin forms the bulk of the tooth the crown and as the most highly calcified tissue is and is joined to the enamel at the dentin-enamel well suited to resisting wear due to mastication. junction (DEJ). The dentin of the tooth root is ­covered Enamel is formed by ameloblasts starting at the by cementum that provides connection of the tooth dentin-enamel junction (DEJ) and proceeding out- to the alveolar bone via the periodontal ligament. ward to the tooth surface. The ameloblasts exchange Although the structure of these tissues is often signals with odontoblasts located on the other side described in dental texts, the properties are often of the DEJ at the start of the enamel and dentin for- discussed only superficially. However, these proper- mation, and the odontoblasts move inward from the ties are important in regard to the interrelationships DEJ as the ameloblasts forming enamel move out- of the factors that contribute to the performance ward to form the enamel of the crown. Most of the ­necessary for the optimum function of these tissues. enamel organic matrix composed of amelogenins In restorative dentistry we are interested in pro- and enamelins is resorbed during tooth maturation viding preventive treatments that will maintain to leave a calcified tissue that is largely composed of tissue integrity and replace damaged tissues with mineral and a sparse organic matrix. The structural materials that ideally will mimic the natural appear- arrangement of enamel forms keyhole-shaped struc- ance and performance of those tissues when neces- tures known as enamel prisms or rods that are about sary. Thus knowledge of the structure and properties 5 μm across as seen in Figure 2-2. of these tissues is desirable both as a yardstick to The overall composition is about 96% mineral by measure the properties and performance of restor- weight, with 1% lipid and protein and the remainder ative materials and as a guide to the development being water. The organic portion and water probably of materials that will mimic their structure and func- play important roles in tooth function and pathology, tion. In addition, many applications, such as dental and it is often more useful to describe the composition bonding, require us to attach synthetic materials to on a volume basis. On that basis we see the organic components make up about 3% and water 12% of the structure. The mineral is formed and grows into very long crystals of hexagonal shape about 40 nm Enamel Dentin across; these have not been synthetically duplicated. There is some evidence that the crystals may span the Outer whole enamel thickness, but this is difficult to prove because most preparation procedures lead to frac- ture of the individual crystallites. It appears that they are at least thousands of nanometers long. If this is Inner true, then enamel crystals provide an extraordinary “aspect” ratio (length to width ratio) for a nanoscale Pulp material, and they are very different from the much Inner cervical smaller dentin crystals. The crystals are packed into enamel prisms or rods that are about 5 μm across as shown in Figure 2-2. These prisms are revealed easily by acid etching and extend in a closely packed array from the DEJ to the enamel surface and lie roughly FIGURE 2.1 Schematic diagram of a tooth cut longitudi- perpendicular to the DEJ, except in cuspal areas nally to expose the enamel, dentin, and the pulp chamber. where the rods twist and cross, known as decussation, On the right side are illustrations of dentin tubules as viewed from the top, which shows the variation in the which may increase fracture resistance. About 100 tubule number with location. At the left is an illustration of crystals of the mineral are needed to span the diam- the change in direction of the primary dentin tubules as eter of a prism, and the long axes of the crystals tend secondary dentin is formed. (From Marshall SJ, et al: Acta. to align themselves along the prism axes, as seen in Mater. 46, 2529-2539, 1998.) Figure 2-2.  2. THE ORAL ENVIRONMENT 7 Interrod enamel Head Tail A B 40.0 30.0 20.0 10.0 0 0 10.0 20.0 30.0 40.0 C FIGURE 2.2 Enamel microstructure showing a schematic diagram of keyhole-shaped enamel prisms or rods about 5 μm in diameter (B). Atomic force microscopy (AFM) images showing prism cross sections in A and along axes of the prisms in C. Crystallite orientation deviates in the inter-rod and tail area, and the organic content increases in the inter-rod area. (Modified from Habelitz S, et al: Arch. Oral Biol. 46, 173-183, 2001.) The crystals near the periphery of each prism dissolves persisting layers of prismless enamel in deviate somewhat from the long axis toward the deciduous teeth, and differentially dissolves enamel interface between prisms. The deviation in the tail crystals in each prism. The pattern of etched enamel of the prism is even greater. The individual crystals is categorized as type 1 (preferential prism core etch- within a prism are also coated with a thin layer of ing, Figure 2-2, A); type 2 (preferential prism periph- lipid and/or protein that plays important roles in ery etching, Figure 2-3, C), and type 3 (mixed or mineralization, although much still remains to be uniform). Sometimes these patterns appear side by learned about the details. Recent work suggests that side on the same tooth surface (Figure 2-3, E). No this protein coat may lead to increased toughness of differences in micromechanical bond strength of the the enamel. The interfaces between prisms, or inter- different etching patterns have been established. In a rod enamel, contain the main organic components standard cavity preparation for a composite, the ori- of the structure and act as passageways for water entation of the enamel surfaces being etched could and ionic movement. These areas are also known as be perpendicular to enamel prisms (perimeter of the prism sheaths. These regions are of vital importance cavity outline), oblique cross section of the prisms in etching processes associated with bonding and (beveled occlusal or proximal margins), and axial other demineralization processes, such as caries. walls of the prisms (cavity preparation walls). Dur- Etching of enamel with acids such as phosphoric ing the early stages of etching, when only a small acid, commonly used in enamel bonding, eliminates amount of enamel crystal dissolution occurs, it may smear layers associated with cavity preparation, be difficult or impossible to detect the extent of the 8 CRAIG’S RESTORATIVE DENTAL MATERIALS A B C D E 25 m FIGURE 2.3 Etching enamel. A, Gel etchant dispensed on the enamel portion of the preparation. B, Frosty ­appearance after etching, rinsing and drying. C, Magnified view of etch pattern with preferential prism periphery etch (type 1). D, Bonding agent revealed after dissolving enamel. E, Mixed etch patterns showing type 1 (light prisms with dark periphery) and type 2 (dark cores with light periphery) etching on same surface after Marshall et al, 1975 JDR. Marshall GW, Olson LM, Lee CV: SEM Investigation of the variability of enamel surfaces after simulated clinical acid etching for pit and fissure sealants, J Dent Res 54:1222–1231, 1975. Part C from Marshall, Olson and Lee, JDR 1975 (same as above) and Part E from Marshall, Marshall and Bayne, 1988: Marshall GW, Marshall SJ, Bayne SC: Restorative dental materials: scanning electron microscopy and x-ray microanalysis, Scanning Microsc 2:2007–2028, 1988. process. However, as the etching pattern begins to enamel structure so that they form micromechanical develop, the surface etched with phosphoric acid bonds to the enamel when polymerized. With self- develops a frosty appearance (Figure 2-3, B), which etching bonding agents, this frosty appearance can- has been used as the traditional clinical indicator for not be detected. sufficient etching. This roughened surface provides There are two other important structural varia- the substrate for infiltration of bonding agents that tions of enamel. Near the DEJ the enamel prism can be polymerized after penetration of the etched structure is not as well developed in the very first  2. THE ORAL ENVIRONMENT 9 enamel and dentin has a much more variable compo- Hardness Hard (GPa) 6 sition that depends on its formative history and other chemical exposures during maturity. Thus the min- Buccal B Bucca cca al al Lingual 5.5 eral in enamel and dentin is a calcium-deficient, car- 5 bonate-rich, and highly substituted form related to HA. Metal ions such as magnesium (Mg) and sodium 4.5 (Na) may substitute for calcium, whereas carbonate 4 substitutes for the phosphate and hydroxyl groups. These substitutions distort the structure and make it 3.5 more soluble. Perhaps the most beneficial substitu- tion is the fluorine (F) ion, which substitutes for the 3 hydroxyl group (OH) in the formula and makes the 2.5 structure stronger and less soluble. Complete substi- tution of F for (OH) in hydroxyapatite yields fluo- roapatite mineral, Ca10(PO4)6(F)2, that is much less FIGURE 2.4 Nanoindentation mapping of the mechani- soluble than HA or the defective apatite of calcified cal properties of human molar tooth enamel. (From Cuy JL, et al: Arch. Oral Biol. 47(4), 281-291, 2002.) tissues. It is worth noting that HA has attracted con- siderable attention as an implantable calcified tissue replacement. It has the advantage of being a purified enamel formed, so that the enamel very close to the and stronger form of the natural mineral and releases DEJ may appear aprismatic or without the prism no harmful agents during biological degradation. Its like structure. Similarly, on the outer surface of the major shortcoming is that it is extremely brittle and enamel, at completion of the enamel surface, the sensitive to porosity or defects and therefore frac- ameloblasts degenerate and leave a featureless layer, tures easily in load-bearing applications. called prismless enamel, on the outer surface of the The approximate carbonate contents of the enamel crown. This layer is more often observed in decidu- and dentin apatites are significantly different, about ous teeth and is often worn off in permanent teeth. 3% and 5% carbonate, respectively. All other factors However, if present, this causes some difficulty in being equal, this would make the dentin apatite more getting an effective etching pattern and may require soluble in acids than enamel apatite. Things are not roughening of the surface or additional etching treat- equal, however, and the dentin apatite crystals are ments. The outer surface of the enamel is of great much smaller than the enamel crystals. This means clinical significance because it is the surface sub- that the dentin crystals present a higher surface area jected to daily wear and undergoes repeated cycles of to attacking acids and contain many more defects per demineralization and remineralization. As a result of unit volume and thus exhibit considerably higher these cycles, the composition of the enamel crystals solubility. Finally, as discussed further below, the may change, for example, as a result of exposure to dentin mineral occupies only about 50% of the den- fluoride. Thus the properties of the enamel might be tin structure, so there is not as much apatite in the expected to vary from the external to the internal sur- dentin as there is in enamel. All of these factors mul- face. Such variations, including a thin surface veneer tiply the susceptibility of dentin to acid attack and of fluoride-rich apatite crystals, create differences in provide insight into the rapid spread of caries when the enamel properties within the enamel. Enamel is it penetrates the DEJ. usually harder at the occlusal and cuspal areas and less hard nearer the DEJ. Figure 2-4 shows an exam- ple of the difference in hardness. DENTIN Dentin is a complex hydrated biological compos- THE MINERAL ite structure that forms the bulk of the tooth. Fur- thermore, dentin is modified by physiological, aging, The mineral of all calcified tissues is a highly and disease processes that result in different forms defective relative of the mineral hydroxyapatite, of dentin. These altered forms of dentin may be the or HA. The biological apatites of calcified tissues precise forms that are most important in restorative are different than the ideal HA structure in that the dentistry. Some of the recognized variations include defects and chemical substitutions generally make it primary, secondary, reparative or tertiary, sclerotic, weaker and more soluble in acids. Hydroxyapatite transparent, carious, demineralized, remineralized, has the simple formula Ca10(PO4)6(OH)2, with an and hypermineralized. These terms reflect altera- ideal molar ratio of calcium to phosphorus (Ca/P) of tions in the fundamental components of the struc- 1.67 and a hexagonal crystal structure. The apatite of ture as defined by changes in their arrangement, 10 CRAIG’S RESTORATIVE DENTAL MATERIALS interrelationships, or chemistry. A number of these controlling crystallite size and orientation; however, may have important implications for our ability to these functions are not discussed further in this text. develop long-lasting adhesion or bonds to dentin. The major components are distributed into distinc- Primary dentin is formed during tooth develop- tive morphological features to form a vital and com- ment. Its volume and conformation, reflecting tooth plex hydrated composite in which the morphology form, vary with the size and shape of the tooth. Den- varies with location and undergoes alterations with tin is composed of about 50 volume percent (vol%) age or disease. carbonate-rich, calcium-deficient apatite; 30 vol% The tubules, one distinct and important feature organic matter, which is largely type I collagen; and of dentin, represent the tracks taken by the odonto- about 20 vol% fluid, which is similar to plasma. Other blastic cells from the DEJ or cementum at the root to noncollagenous proteins are thought to be involved the pulp chamber and appear as tunnels piercing the in dentin mineralization and other functions such as dentin structure (Figure 2-5). The tubules converge on the pulp chamber, and therefore tubule density and orientation vary from location to location (see Figure 2-1). Tubule number density is lowest at the DEJ and highest at the predentin surface at the junc- tion to the pulp chamber, where the odontoblastic cell bodies lie in nearly a close-packed array. Lower tubule densities are found in the root. The contents of the tubules include odontoblast processes, for all or part of their course, and fluid. The extent of the odontoblast process is still uncertain, but evidence is mounting that it extends to the DEJ. For most of its course, the tubule lumen is lined by a highly min- eralized cuff of peritubular dentin roughly 0.5 to 1 30kv 2.00kx 5.0 959 μm thick (Figure 2-6). Because the peritubular den- tin forms after the tubule lumen has been formed, FIGURE 2.5 Scanning electron microscopy (SEM) image some argue that it may be more properly termed of normal dentin showing its unique structure as seen intratubular dentin and contains mostly apatite crys- from two directions. At the top is a view of the tubules, tals with little organic matrix. A number of studies each of which is surrounded by peritubular dentin. Tubules have concluded that the peritubular dentin does not lie between the dentin-enamel junction (DEJ) and converge contain collagen, and therefore might be considered on the pulp chamber. The perpendicular surface at the bottom shows a fracture surface revealing some of the a separate calcified tissue. The tubules are separated tubules as they form tunnel-like pathways toward the pulp. by intertubular dentin composed of a matrix of type The tubule lumen normally contains fluid and processes of I collagen reinforced by apatite (see Figures 2-5 and the odontoblastic cells. (From Marshall GW: Quintessence Int. 2-6). This arrangement means that the amount of 24, 606-617, 1993.) intertubular dentin varies with location. The apatite Peritubular dentin P Intertubular dentin I A B 20kv 5.0kx 2.00 956 FIGURE 2.6 Fracture surface of the dentin viewed from the occlusal in A and longitudinally in B. Peritubular (P) (also called intratubular) dentin forms a cuff or lining around each tubule. The tubules are separated from one another by intertubular dentin (I). (Courtesy of G. W. Marshall.)  2. THE ORAL ENVIRONMENT 11 crystals are much smaller (approximately 5 × 30 × important to understand altered forms of dentin and 100 nm) than the apatite found in enamel and con- the effects of such clinical interventions. tain 4% to 5% carbonate. The small crystallite size, When dentin is cut or abraded by dental instru- defect structure, and higher carbonate content lead ments, a smear layer develops and covers the surface to the greater dissolution susceptibility described and obscures the underlying structure (Figure 2-7). above. The bur cutting marks are shown in Figure 2-7, A, Estimates of the size of tubules, the thickness of and at higher magnification in Figure 2-7, B. Figure the peritubular region, and the amount of intertubu- 2-7, C, shows the smear layer thickness from the side lar dentin have been made in a number of studies. and the development of smear plugs as the cut den- Calculations for occlusal dentin as a function of posi- tin debris is pushed into the dentin tubule lumen. tion from these data show the percent tubule area The advantages and disadvantages of the smear and diameter vary from about 22% and 2.5 μm near layer have been extensively discussed for several the pulp to 1% and 0.8 μm at the DEJ. Intertubular decades. It reduces permeability and therefore aids matrix area varies from 12% at the predentin to 96% in maintaining a drier field and reduces infiltration near the DEJ, whereas peritubular dentin ranges of noxious agents into the tubules and perhaps the from over 60% down to 3% at the DEJ. Tubule den- pulp. However, it is now generally accepted that it sities are compared in Table 2-1 based on work by is a hindrance to dentin bonding procedures and, various investigators. It is clear that the structural therefore, is normally removed or modified by some components will vary considerably over their course, form of acid conditioning. and necessarily result in location-dependent varia- Acid etching or conditioning allows for removal tions in morphology, distribution of the structural of the smear layer and alteration of the superficial elements, and important properties such as perme- dentin, opening channels for infiltration by bonding ability, moisture content, and available surface area agents. Figure 2-8 shows what happens in such an for bonding and may also affect bond strength, hard- etching treatment. The tubule lumens widen as the ness, and other properties. peritubular dentin is preferentially removed because Because the odontoblasts come to rest just inside it is mostly mineral with sparse protein. The widened the dentin and line the walls of the pulp chamber lumens form a funnel shape that is not very retentive. after tooth formation, the dentin-pulp complex can Figure 2-9 shows these effects in a slightly differ- be considered a vital tissue. This is different than ent way. Unetched dentin in Figure 2-9, A, has small mature enamel. Over time secondary dentin forms tubules and peritubular dentin, which is removed in and the pulp chamber gradually becomes smaller. the treated dentin at the exposed surface after etching The border between primary and secondary dentin (bottom). The two-dimensional network of collagen is usually marked by a change in orientation of the type I fibers is shown after treatment in Figure 2-9, A. dentin tubules. Furthermore, the odontoblasts react Figure 2-9, B, shows progressive demineralization of to form tertiary dentin in response to insults such as a dentin collagen fibril in which the external mineral caries or tooth preparation, and this form of dentin is and proteins are slowly removed to reveal the typi- often less well organized than the primary or second- cal banded pattern of type I collagen. In Figure 2-9, ary dentin. C, this pattern is seen at high magnification of the Early enamel carious lesions may be reversed treated dentin in Figure 2-9, A. by remineralization treatments. However, effective If the demineralized dentin is dried, the remain- re­mineralization treatments are not yet available for ing dentin matrix shrinks and the collagen fibrils dentin and therefore the current standard of care become matted and difficult to penetrate by bonding dictates surgical intervention to remove highly dam- agents. This is shown in Figure 2-10, which compares aged tissue and then restoration as needed. Thus it is demineralized and dried dentin with demineralized and hydrated dentin. Most restorative procedures involve dentin that TABLE 2.1 Comparison of Mean Numerical Density has been altered in some way. Common alterations of Tubules in Occlusal Dentin* include formation of carious lesions that form vari- Outer Dentin Middle Dentin Inner Dentin ous zones and include transparent dentin that forms under the caries infected dentin layer. Transparent 15,000/mm2 35,000/mm2 65,000/mm2 dentin results when the dentin tubules become filled 20,000/mm2 35,000/mm2 43,000/mm2 with mineral, which changes the refractive index of the tubules and produces a translucent or transpar- 24,500/mm2 40,400/mm2 51,100/mm2 ent zone. 18,000/mm2 39,000/mm2 52,000/mm2 Figure 2-11 shows a section through a tooth with *From data reported in the literature (Marshall GW: Quintessence Int. a carious lesion, which has been stained to reveal its 24, 606-617, 1993.) zones. The gray zone under the stained and severely 12 CRAIG’S RESTORATIVE DENTAL MATERIALS A B C FIGURE 2.7 Smear layer formation. A, Bur marks on dentin preparation B, Higher magnification showing smear layer surface and cutting debris. C, Section showing smear layer (SL) and smear plugs (S.P.). (A and B from Marshall GW, et al: Scanning Microsc. 2, 2007-2028, 1988; C from Pashley DH, et al: Arch. Oral Biol. 33, 265-270, 1988.) demineralized dentin is the transparent layer (Figure transparent dentin in which the tubule lumens are 2-11, A). Figure 2-11, B, shows the transparent dentin completely filled. in which most of the tubule lumens are filled with The properties of the transparent dentin may dif- mineral. After etching, as shown in Figure 2-11, C the fer from one to another depending on the processes peritubular dentin is etched away, but the tubules that lead to deposit of the mineral in the tubules. retain plugs of the precipitated mineral, which is Several studies have shown that elastic properties more resistant to etching. This resistance to etching of the intertubular dentin are not altered by aging, makes bonding more difficult. although the structure may become more suscepti- Several other forms of transparent dentin are ble to fracture. Similarly, arrested caries will contain formed as a result of different processes. A second transparent dentin and this has often been called scle- form of transparent dentin results from bruxism. rotic dentin, a term that implies it may be harder than An additional form of transparent dentin results normal dentin. However, other studies have shown from aging as the root dentin gradually becomes that the elastic properties of the intertubular den- transparent. In addition noncarious cervical lesions tin may actually be unaltered or lower than normal (NCCLs), often called abfraction or notch lesions, form dentin. at the enamel-cementum or enamel-dentin junction, usually on facial or buccal surfaces. Their etiology is not clear at this point; their formation has been Physical and Mechanical Properties attributed to abrasion, tooth flexure, and erosion or The marked variations in the structural elements some combination of these processes. Nonetheless of dentin when located within the tooth imply that these lesions occur with increasing frequency with the properties of dentin will vary considerably with age, and the exposed dentin becomes transparent as location. That is, variable structure leads to variable the tubules are filled. Figure 2-12 shows examples of properties.  2. THE ORAL ENVIRONMENT 13 20 s 60 s 10 15 5 A B 10 15 15 5 5 10 C D FIGURE 2.8 Stages of dentin demineralization. A, Schematic showing progressive stages of dentin demineralization. B to D, Atomic force microscopy (AFM) images showing stages of etching. The etching leads to wider lumens as peritu- bular dentin is dissolved and funnel-shaped openings are formed. (AFM images from Marshall GW: Quintessence Int. 24, 606-617, 1993.) Because one major function of tooth structure is and special tests have been developed to obtain to resist deformation without fracture, it is useful to insights into these properties. From the previous dis- have knowledge of the forces that are experienced by cussion of structural variations, it is also clear that teeth during mastication. Measurements have given testing such small inhomogeneous specimens means values on cusp tips of about 77 kg distributed over that the properties will not be uniform. the cusp tip area of 0.039 cm2 , suggesting a stress of Another problem is the great variation in struc- about 200 MPa. ture in both tissues. Enamel prisms are aligned generally perpendicular to the DEJ, whereas dentin Difficulties in Testing tubules change their number density with depth as In Table 2-2, values are presented for some impor- they course toward the pulp chamber. Preparing a tant properties of enamel and dentin. The wide uniform sample with the structures running all in spread of values reported in the literature is remark- one direction for testing is challenging. In addition, able. Some of the reasons for these discrepancies properties generally vary with direction and location should be appreciated and considered in practice or and the material is not isotropic; therefore, the best when reading the literature. a single value can tell you is some average value for First, human teeth are small, and therefore it is dif- the material. ficult to get large specimens and hold them in such a Storage and time elapsed since extraction are also way that you can measure properties. This makes the important considerations. Properties that exist in a use of standard mechanical testing such as tensile, natural situation or in situ or in vivo are of greatest compressive, or shear tests difficult. When testing interest. Clearly this condition is almost impossible bonded teeth, the problem is even more complicated, to achieve in most routine testing, so changes that 14 CRAIG’S RESTORATIVE DENTAL MATERIALS 484 s Unetched 360 s Treated 0s 100 nm A B 600 400 200 C FIGURE 2.9 Etching of dentin removes mineral from the intertubular dentin matrix leaving a collagen-rich layer and widening the dentin tubule orifices. A, After etching the tubule lumens are enlarged and the collagen network surround- ing the tubules can be seen after further treatment. B, Isolated dentin collagen fiber is slowly demineralized revealing the typical 67 nm repeat pattern of type I collagen. C, High magnification view of collagen fibers in A. (A and C from Marshall GW, et al: Surface Science. 491, 444-455, 2001; B modified from Balooch M, et al: J. Struct. Biol. 162, 404-410, 2008.) A B FIGURE 2.10 Demineralized dentin is sensitive to moisture and shrinks on drying. A, Demineralized dentin ­undergoes shrinkage when air dried forming a collapsed layer of collagen that is difficult to infiltrate with resin bonding agents. B, When kept moist, the collagen network is open and can be penetrated by bonding agents. (From Marshall GW, et al: J. Dent. 25, 441-458, 1997.)  2. THE ORAL ENVIRONMENT 15 Trans 10 20 30 A 40 B 10 20 30 40 C FIGURE 2.11 Transparent dentin associated with carious lesions. A, Carious lesion showing dentin carious zones revealed by staining, including the grayish transparent zone. B, Atomic force microscopy (AFM) of carious transparent dentin before etching. C, After etching the tubule lumens remain filled even as the peritubular dentin is etched away. (A from Zheng L, et al: Eur. J. Oral Sci. 111, 243-252, 2003; B and C from Marshall GW, et al: Dent. Mater. 17, 45-52, 2001b.) have occurred as a result of storage conditions prior is the issue of convenience. It is much more difficult to testing must be considered. It is also important to to test the tissues in a fully hydrated condition than consider biological hazards because extracted teeth in a dry condition. All of these factors and a number must be treated as potentially infective. How do you of others, such as temperature of testing, will influ- sterilize the teeth without altering their properties? ence the results and contribute to a spread in the val- Autoclaving undoubtedly alters the properties of ues reported for the properties. proteins, and is therefore not appropriate for dentin, Despite these limitations, some generalizations and might also affect enamel. about the properties of these tissues are useful (see Finally, the fluid content of these tissues must be Table 2-1). Root dentin is generally weaker and softer considered. Moisture is a vital component of both than coronal dentin. Enamel also appears to vary in tissues and in vivo conditions cannot be replicated its properties, with cuspal enamel being stronger and if the tissues have been desiccated (see Figure 2-10). harder than other areas, presumably as an adaption This becomes a critically important consideration in to masticatory forces. Dentin is less stiff than enamel bonding to these tissues, as is discussed further in (i.e., has a lower elastic modulus), and has a higher Chapter 13. In contrast to the importance of this issue fracture toughness. This may be counterintuitive but 16 CRAIG’S RESTORATIVE DENTAL MATERIALS A 15kv 2.0kx 5.00 523 B 15kv 2.00kx 5.0 519 FIGURE 2.12 Transparent dentin. As seen from the facial, A, and longitudinal, B, directions. The transparent dentin results from filling of the tubules with mineral deposits that alter the optical properties of the tooth. (Courtesy of Marshall GW.) TABLE 2.2 Properties of Enamel and Dentin softer and tougher, they need to be joined together to provide a biomechanically compatible system. Property Enamel Dentin Joining such dissimilar materials is a challenge, and Density 2.96 g/cm3 2.1 g/cm3 it is not completely clear how nature has accom- plished this. However, the DEJ not only joins these Compressive two tissues but also appears to resist cracks in the Modulus of elasticity 60-120 GPa 18-24 GPa enamel from penetrating into dentin and leading to Proportional limit 70-353 MPa 100-190 MPa tooth fracture as shown in Figure 2-13, A. Many such cracks exist in the enamel but do not seem to propa- Strength 94-450 MPa 230-370 MPa gate into the dentin. If the DEJ is intact, it is unusual Tensile to have tooth fracture except in the face of severe Modulus of elasticity 11-19 GPa trauma. In Figure 2-13, B, microhardness inden- tations have been placed to drive cracks toward Strength 8-35 MPa 30-65 MPa the DEJ (orange). The crack stops at or just past the Shear strength 90 MPa 138 MPa interface. This image also shows that the DEJ is scal- Flexural strength 60-90 MPa 245-280 MPa loped with its concavity directed toward the enamel. This means that most cracks approach the DEJ at Hardness 3-6 GPa 0.13-.51 GPa an angle, and this may lead to arrest of many of the cracks. The scalloped structure actually has three lev- els: scallops, microscallops within the scallops, and will become clearer when we define these terms in a finer structure. Figures 2-13, C, and 2-13, D, show Chapter 4. In addition, dentin is viscoelastic, which images of larger scallops in molars (~24 μm across) means that its mechanical deformation characteris- and smaller scallops (~15 μm across) in anterior teeth tics are time dependent, and elastic recovery is not after the removal of the enamel. Finite element mod- instantaneous. Thus dentin may be sensitive to how els suggest that the scallops reduce stress concentra- rapidly it is strained, a phenomenon called strain tions at the interface, but it is not known whether rate sensitivity. Strain rate sensitivity is characteristic the larger scallop size in posterior teeth is an adap- of polymeric materials; the collagen matrix imparts tion to higher masticatory loads or a developmental this property to tissues such as dentin. Under normal variation. In Figure 2-13, E, the crystals of dentin are circumstances, enamel and other ceramic materials almost in contact with those of the enamel, so that do not show this characteristic in their mechanical the anatomical DEJ is said to be optically thin. How- properties. ever, measurements of property variations across the DEJ show that this is a graded interface with proper- ties varying from those of the enamel to the adjacent The Dentin-Enamel Junction mantle dentin over a considerable distance. This gra- The dentin-enamel junction (DEJ) is much more dient, which is due in part to the scalloped nature of than the boundary between enamel and dentin. the DEJ, makes the functional width of the DEJ much Because enamel is very hard and dentin is much larger than its anatomical appearance and further  2. THE ORAL ENVIRONMENT 17 Enamel DEJ Dentin Cracks 50 m A B 50 m C D Apatite crystals Enamel DEJ E Dentin FIGURE 2.13 Cracks in enamel appear to stop at the dentin-enamel junction (DEJ). A, Low-magnification view of cracks in enamel. B, Indentation-generated cracks stop near or at the scalloped DEJ (orange). C, Large scallops in molars. D, Smaller scallops in anterior teeth. E, Crystals of the enamel are nearly in contact with dentin crystals at the DEJ forming an optically thin but functionally wide union. (A, C-E from Marshall SJ, et al: J. European Ceram. Soc. 23, 2897-2904, 2003; B from Imbeni V, et al: Nature Mater. 4, 229-232, 2005.) 18 CRAIG’S RESTORATIVE DENTAL MATERIALS reduces stresses. In addition, although collagen is absent from enamel, collagen fibers cross the DEJ from dentin into enamel to further integrate the two tissues. At this point, no unique components, such as proteins, have been identified that could serve as a special adhesive that bonds the enamel to the dentin. ORAL BIOFILMS AND RESTORATIVE DENTAL MATERIALS Biofilms are complex, surface-adherent, spatially organized polymicrobial communities containing bacteria surrounded by a polysaccharide matrix. Oral biofilms that form on the surfaces of teeth and biomaterials in the oral cavity are also known as den- tal plaque. When the human diet is rich in fermentable carbohydrates, the most prevalent organisms shown to be present in dental plaque are adherent acido- genic and aciduric bacteria such as streptococci and lactobacilli that are primarily responsible for dental caries. Other consequences of long-term oral biofilm accumulation can also include periodontal diseases and peri-implantitis (inflammation of the soft and hard tissues surrounding an implant), depending on the location of attachment of the biofilm. FIGURE 2.14 Spatiotemporal model of oral bacterial colonization, showing recognition of salivary pellicle Biofilm formation on hard surfaces in the oral receptors by early colonizing bacteria and co-­aggregations cavity is a sequential process. A conditioning film between early colonizers, fusobacteria, and late coloniz- from saliva (known as pellicle) containing adsorbed ers of the tooth surface. Starting at the bottom, primary macromolecules such as phosphoproteins and gly- colonizers bind via adhesins (round-tipped black line symbols) coproteins is deposited on tooth structure and bio- to complementary salivary receptors (blue-green vertical materials within minutes after a thorough cleaning. round-topped columns) in the acquired pellicle coating the This stage is followed by the attachment of plank- tooth surface. Secondary colonizers bind to previously tonic (free-floating) bacteria to the pellicle. Division bound bacteria. Sequential binding results in the appear- of the attached initial colonizing bacterial species ance of nascent surfaces that bridge with the next co-­ produces microcolonies, and subsequent attachment aggregating partner cell. The bacterial strains shown are Actinobacillus actinomycetemcomitans, Actinomyces israelii, of later colonizing species results in the formation of Actinomyces naeslundii, Capnocytophaga gingivalis, Capnocyto- matrix-embedded multispecies biofilms. These bio- phaga ochracea, Capnocytophaga sputigena, Eikenella corrodens, films can mature over time if they are not detached Eubacterium spp., Fusobacterium nucleatum, Haemophilus by mechanical removal or intrinsic factors. parainfluenzae, Porphyromonas gingivalis, Prevotella denticola, Biofilm formation occurs via complicated physical Prevotella intermedia, Prevotella loescheii, Propionibacterium and cellular interactions between the substrate, pel- acnes, Selenomonas flueggei, Streptococcus gordonii, Streptococ- licle, and bacteria. These interactions occur at several cus mitis, Streptococcus oralis, Streptococcus sanguis, ­Treponema levels and can include physical proximity, metabolic spp., and Veillonella atypica. (From Kolenbrander PE, et al: exchange, signal molecule-mediated communica- Microbiol. Mol. Biol. Rev. 66, 486-505, 2002.) tion, exchange of genetic material, production of inhibitory factors, and co-aggregation (“specific cell- 60% to 90% of the initial bacterial flora on enamel in to-cell recognition between genetically distinct cell situ. Furthermore, the streptococci are less sensitive types,” as defined by Kolenbrander et al., 2006). to exposure to air than most oral bacteria because The pellicle contains a variety of receptor mol- they are facultatively anaerobic and can participate ecules that are recognized primarily by streptococci in modifying the biofilm environment to a more (Figure 2-14). This is evident in healthy individu- reduced state, a condition often considered to favor als, who typically have biofilms containing a thin an ecological shift towards gram-negative anaerobes. layer of adherent gram-positive cocci. The ability to Interactions among human oral bacteria are piv- bind to nonshedding surfaces such as enamel gives otal to the development of oral biofilms (see Figure streptococci a tremendous advantage and is consis- 2-14). In the first 4 hours of biofilm formation, gram- tent with the observation that streptococci constitute positive cocci appear to predominate, particularly  2. THE ORAL ENVIRONMENT 19 mitis group streptococci. After 8 hours of growth, the fabrication of a restoration, has also been associated majority of the bacterial population continues to be with biofilm formation. largely coccoid, but rod-shaped organisms are also Bacterial adhesion in vivo is considerably reduced observed. By 24 to 48 hours, thick deposits of cells by the formation of a pellicle, regardless of the com- with various morphologies can be detected, includ- position of the underlying substrate. Pellicle forma- ing coccoid, coccobacilliary, rod-shaped, and fila- tion has also been shown to have a masking effect mentous bacteria. Within 4 days of biofilm growth, on specific surface characteristics of biomaterials an increase in the numbers of gram-negative anaer- to a certain extent. Surfaces having a low surface obes is observed, and particularly of Fusobacterium energy were observed to retain the smallest amount nucleatum. The latter organism has the unique ability of adherent biofilm due to the lower binding forces to co-aggregate with a wide variety of bacteria and between bacteria and substrata even after several is believed to play a pivotal role in the maturation days of exposure in the human oral cavity. Recipro- of biofilm because it forms co-aggregation bridges cally, the higher surface energy of many restorative with both early and late colonizers. As the biofilm materials compared with that of the tooth surface matures, a shift is observed toward a composition could result in a greater tendency for the surface of largely gram-negative morphotypes, including and margins of the restoration to accumulate debris, rods, filamentous organisms, vibrios, and spiro- saliva, and bacteria. This may in part account for the chetes. These shifts in the microbial composition of relatively high incidence of secondary (recurrent) biofilm are important because they correlate with the carious lesions seen in enamel at the margins of resin development of gingivitis (inflammation of gingival composite and amalgam restorations. tissues). Investigations of oral biofilms on restorative Even though biofilms accumulate on restorative, materials can generally be divided into in vivo, in orthodontic, endodontic, and implant biomaterials, situ, and in vitro studies, with the latter comprising the remainder of this section focuses on biofilms monospecies or multispecies investigations. Biofilms that accumulate on the surfaces of restorative and that are formed on restorative materials can vary in implant materials only. The precise mechanisms of thickness and viability. In vivo and in situ studies of bacterial adhesion and biofilm formation on the sur- biofilm formation on dental materials have produced faces of dental materials have not yet been identified inconsistent results, and a trend for accumulation on in spite of decades of research effort but are accepted materials has not been determined so far. to be complex processes that depend on a large num- Levels of cariogenic organisms (capable of pro- ber of factors. In vitro studies have shown that the ducing or promoting caries) such as Streptococcus adhesion of salivary proteins and bacteria at small mutans have been shown to be higher in biofilms distances (5-100 nm) from the surfaces of biomateri- adjacent to posterior resin restorations than in bio- als is influenced by a combination of Lifshitz-van der films adjacent to amalgam or glass ionomer res- Waals forces, electrostatic interactions, and acid-base torations. The formation of oral biofilms has been bonding. Other properties such as substrate hydro- associated with an increase in the surface roughness phobicity, surface free energy, surface charge, and of resin composites, degradation of the material due surface roughness have commonly been investigated to acid production by cariogenic organisms, hydroly- in vitro for correlation with the number of adhering sis of the resin matrix, and a decrease in microhard- bacteria. Many of the above-mentioned surface prop- ness of the restoration’s surface. Additionally, it has erties are described in later chapters. been theorized that planktonic bacteria can enter The role of surface roughness in biofilm formation the adhesive interface between the restorative mate- has been widely investigated. Smooth surfaces have rial and the tooth, leading to secondary caries and been shown to attract less biofilm in vivo than rough pulp pathology. On the other hand, trace amounts surfaces. It has also been observed that hydropho- of unpolymerized resin, resin monomers, and the bic surfaces that are located supragingivally attract products of resin biodegradation have been shown to less biofilm in vivo than more hydrophilic surfaces modulate the growth of oral bacteria in the vicinity over a 9-day period. An increase in the mean surface of resin restorations. All of these factors create a cycle roughness parameter (Ra) above a threshold value of bacteria-surface interaction that further increases of 0.2 μm or an increase in surface free energy were surface roughness and encourages bacterial attach- both found to result in more biofilm accumulation on ment to the surface, thereby placing the adjacent dental materials. When both of those surface proper- enamel at greater risk for secondary caries. ties interact with each other, surface roughness was Bacterial adhesion to casting alloys and dental observed to have a greater effect on biofilm accumu- amalgams has received limited attention in recent lation. The creation of a rough restoration surface times. Biofilms on gold-based casting alloys are caused by abrasion, erosion, air polishing or ultra- reported to be of low viability, possibly due to the sonic instrumentation, or a lack of polishing after the bacteriostatic effect of gold. Biofilms on amalgam 20 CRAIG’S RESTORATIVE DENTAL MATERIALS are also reported to have low viability, which could in bacterial adhesion to dental implant materials. In be attributed to the presence of the Hg(II) form of addition, the surface characteristics of the bacteria, mercury in dental amalgam. Interestingly, amalgam the design of the implant and the abutment, and the restorations have been shown to promote the lev- micro-gap between the implant and abutment have els of mercury (Hg)-resistant bacteria in vitro and also been shown to influence microbial colonization in vivo. Resistance to antibiotics, and specifically on dental implants. tetracycline, was observed to be concurrent with The most common reason for the replacement of Hg-resistance in oral bacteria. However, it is worth dental restorations is secondary caries at the gingi- noting that Hg-resistant bacteria were also found in val tooth-restoration margin. It is estimated that 50% children without amalgam fillings or previous expo- to 80% of resin restorations are replaced annually in sure to amalgam. the United States alone. The cost of replacing resto- Information regarding the morphology of bio- rations is estimated to be in the billions of dollars films on ceramic restorations is limited, although it worldwide, and the number and cost of replacing is generally accepted that ceramic crowns accumu- restorations is increasing annually. Although bacte- late less biofilm than adjacent tooth structure. The riological studies of secondary caries indicate that recent demonstration of increased surface roughness its etiology is similar to that of primary caries, the of zirconia surfaces in vitro after the use of hand and mechanisms by which secondary caries occur are a ultrasonic scaling instruments could be theorized focus of ongoing investigations. to produce greater biofilm accumulation on zirco- The removal of tenaciously adherent oral bio- nia restorations subsequent to dental prophylaxis films from hard surfaces is crucial to caries control procedures. and is most effectively accomplished by mechanical Biofilms that adhere to denture base resins pre- brushing with toothpaste, especially in interproxi- dominantly contain the Candida species of yeast. mal regions and posterior teeth along with the use of However, initial adhesion of bacteria such as strep- adjunctive chemical agents. Although tooth brushing tococci to the denture base may have to occur before has been associated with increased surface roughness Candida species can form biofilms. This is attributed of restorations over time due to the process of wear, to the observation of bacteria on dentures within which could permit additional bacterial attachment hours and Candida species after days, and to the on the surface, mechanical removal has been shown ability of Candida species to bind to the cell wall to be more effective than chemical intervention. This receptors in streptococci. Biofilms on dentures have is because bacteria in biofilms are typically well pro- commonly been associated with denture stomatitis tected from the host immune response, antibiotics, (chronic inflammation of the oral mucosa) in elderly and antibacterials when embedded within a com- and immunocompromised patients. Removal of bio- plex biofilm matrix. Furthermore, most antimicrobial films from dentures typically requires mechanical agents have commonly been tested against plank- and/or chemical means and is a significant clinical tonic bacteria, which are killed by much lower con- problem because of biofilm adherence to the denture centrations of antimicrobials than biofilm bacteria. base resins. Chemical control of biofilms has also been limited The accumulation of biofilms on titanium and by concerns regarding the development of resistant titanium alloys that are used in dental implants has microorganisms resulting from the prolonged use of received much attention because biofilms play a sig- antimicrobials, and acceptance of the hypothesis that nificant role in determining the success of an implant. the microflora should not be eliminated but should The sequence of microbial colonization and biofilm instead be prevented from shifting from a favorable formation on dental implants has been shown to be ecology to an ecology favoring oral disease. similar to that on teeth, but differs in early coloniza- The accumulation of biofilms on glass ionomer tion patterns. Several in vivo studies have confirmed and resin-modified glass ionomer biomaterials is a that a reduction in mean surface roughness (Ra) factor that has been associated with an increase in of implant materials below the threshold value of the surface roughness of those biomaterials. Fluo- 0.2 μm has no major effect on adhesion, colonization, ride-releasing materials, and glass-ionomers and or microbial composition. Compared to polished compomers in particular, can neutralize acids pro- titanium surfaces, titanium implant surfaces that duced by bacteria in biofilms. Fluoride can provide were modified with titanium nitride (TiN) showed cariostatic benefits and may affect bacterial metabo- significantly less bacterial adhesion and biofilm for- lism under simulated cariogenic conditions in vitro. mation in vivo, thereby potentially minimizing bio- Although the large volume of saliva normally pres- film accumulation and subsequent peri-implantitis. ent in the oral cavity is hypothesized to result in Other contributing factors such as the hydrophobic- fluoride concentrations that are too low for cavity- ity, surface chemistry, and surface free energy of the wide antibacterial protection, the amount of fluoride implant material have been found to play vital roles released could theoretically be sufficient to minimize  2. THE ORAL ENVIRONMENT 21 demineralization in the tooth structure adjacent to demonstrated that fluoride-releasing restorative glass ionomer and resin-modified glass ionomer res- materials significantly reduce the incidence of sec- torations. In addition, glass ionomer materials can ondary caries as compared to non-fluoride-releasing be recharged by daily exposure to fluoride-contain- biomaterials. More studies are therefore needed to ing dentifrices, thereby compensating for the sig- determine the impact of fluoride-releasing restora- nificant decrease in fluoride release that occurs over tions on the development and progression of second- time. Interestingly, clinical studies have not clearly ary caries. SELECTED PROBLEMS PROBLEM 1 enamel mineral, and because there is less total min- The enamel microstructure is unique among the eral in dentin than in enamel, the acid attack proceeds dental calcified tissues in that it is the hardest tis- more quickly in dentin. The dentin tubules also pro- sue in the body. Explain why its unique structure is vide pathways for this dissolution. Therefore caries important to its function and how this has been used proceed more quickly once into dentin and gener- in restorative treatments to provide reliable bonding. ally require surgical intervention and restoration. In contrast, early enamel caries can be treated and the Solution enamel remineralized. Enamel has the highest mineral content of the calci- fied tissues providing high hardness and wear resis- tance needed for mastication. The small amount of PROBLEM 3 organic substances coats each crystallite and increases Dentin tubules are an important structural feature toughness in comparison to pure apatite. The very long of dentin and are the pathways taken by odontoblasts crystals are packed into keyhole–shaped enamel rods during formation, starting from the DEJ and proceed- that are separated by regions of higher organic con- ing to the pulp chamber. Why does this result in a dif- tent. This structure etches differently when exposed ferent number of tubules per unit area and difference to acids, such as phosphoric acid, leading to a clean, in moisture level with distance from the pulp? high-energy surface with varying roughness within and between the rods. Solution When infiltrated with resins and subsequently Each odontoblast forms one tubule during den- polymerized, the resins form a tight and reliable bond tinogenesis. Because the DEJ surface is larger than between the enamel structure and the resin. the surface of the pulp chamber, the tubules are more concentrated at deeper levels, resulting in an increased number of tubules per unit area. Because PROBLEM 2 tubules are filled with fluid and there is a positive Apatite is the mineral component of both enamel pressure from the pulp, the higher number of (and and dentin, but there are important differences somewhat larger diameter) tubules in deeper den- between these critical forms of apatite. What are the tin results in more moisture when deep dentin is critical differences and how do these differences affect cut than when superficial dentin is cut. Thus deep restorative dentistry? dentin is inherently wetter than superficial dentin. This has important implications for bonding because Solution moisture may interfere with bonding procedures and Enamel and dentin mineral contain calcium-­ there is less solid dentin in deeper dentin available deficient carbonate-rich hydroxyapatite. However, for bonding. the crystallite size is different, with enamel having much larger crystals that comprise 85 vol% of the structure, compared to about 50 vol% in dentin. The PROBLEM 4 larger surface area of the dentin apatite increases its Cavity preparations in the crown nearly always dissolution susceptibility when exposed to acids. involve both enamel and dentin, and often at least This susceptibility is further increased because of the some portions of the dentin may have been affected by higher carbonate content of the dentin mineral. Thus caries. What difficulties does this situation present for the dentin mineral is dissolved more rapidly than the restorative dental treatments?