Materials
Fluoride-Releasing Restorative Materials and Secondary Caries
John Hicks, DDS, MS, PhD, MD; Franklin Garcia-Godoy, DDS, MS; Kevin Donly, DDS, MS; and Catherine Flaitz, DDS, MS
John Hicks, DDS, MS, PhD, MD, is in the Department of Pathology, Texas Children’s Hospital and Baylor College of Medicine, and Department of Pediatric Dentistry, University of Texas-Houston Health Science Center, Dental Branch, Houston.
Franklin Garcia-Godoy, DDS, MS, is in the Division of Biomaterials, Department of Restorative Dentistry, and Clinical Research Center, Tufts University, School of Dental Medicine, Boston.
Kevin Donly, DDS, MS, is in the Department of Pediatric Dentistry, University of Texas-San Antonio Health Science Center, Dental School, San Antonio, Texas.
Catherine Flaitz, DDS, MS, is in the Division of Oral and Maxillofacial Pathology, and Department of Pediatric Dentistry, University of Texas-Houston Health Science Center, Dental Branch, Houston.
Copyright 2003 Journal of the California Dental Association.
Acknowledgment
Sections of this paper have been published previously in Dental Clinics of North America and have been included in this article with permission of that periodical.
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Secondary caries is responsible for 60 percent of all replacement restorations in the typical dental practice. Risk factors for secondary caries are similar to those for primary caries development. Unfortunately, it is not possible to accurately predict which patients are at risk for restoration failure. During the past several decades, fluoride-releasing dental materials have become a part of the dentist’s armamentarium. Considerable fluoride is released during the setting reaction and for periods up to eight years following restoration placement. This released fluoride is readily taken up by the cavosurface tooth structure, as well as the enamel and root surfaces adjacent to the restoration. Resistance against caries along the cavosurface and the adjacent smooth surface has been show in both in vitro and in vivo studies. Fluoride-releasing dental materials provides for improved resistance against primary and secondary caries in coronal and root surfaces. Plaque and salivary fluoride levels are elevated to a level that facilitates remineralization. In addition, the fluoride released to dental plaque adversely affects the growth of lactobacilli and mutans streptococci by interference with bacterial enzyme systems. Fluoride recharging of these dental materials is readily achieved with fluoridated toothpastes, fluoride mouthrinses, and other sources of topical fluoride. This allows fluoride-releasing dental materials to act as intraoral fluoride reservoirs. The improvement in the properties of dental materials with the ability to release fluoride has improved dramatically in the past decade, and it is anticipated that in the near future the vast majority of restorative procedures will employ fluoride-releasing dental materials as bonding agents, cavity liners, luting agents, adhesives for orthodontic brackets, and definitive restoratives.
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Dental caries is one of the most common diseases occurring in man and is prevalent in developed, developing, and underdeveloped countries.7,33,35,36,51,60,69,106,111,113,117 Within the United States and Western Europe, there has been an overemphasis on national surveys that indicate that more than 50 percent of children and adolescents are caries-free. In reality, a small percentage of late adolescents and young adults are caries-free. Only about one in six 17-year-olds are caries-free. In fact, 94 percent of all dentate adults in the United States have experienced dental caries. Caries affects some children, adolescents, and adults to a much greater degree than others (Figure 1). One-fourth of 5- to 17-year-olds account for 80 percent of the caries experience. At age 17 years, 80 percent of caries occurs in 40 percent of these late adolescents. A similar trend is noted with older adults.69,111 In an ambulatory New England population, 11 percent of elders older than 70 accounted for 70 percent of caries. It was noted that these New England elders had a higher caries prevalence rate than New England children.69 The continuing caries experience throughout adulthood and into the elderly period points out that dental caries is not a disease restricted to children and adolescents. The demand for ongoing preventive and restorative care in adulthood is emphasized in a national survey that found 40 percent of adults between 18 and 74 years of age were in need of immediate dental care.113,117
Once a cavity forms, there are several options for restoration of the carious tooth surface. A U.S. Navy Dental Corps study120 reported the dental materials placed in 4,633 restorations in 17- to 84-year-olds. The following restorations were placed during a two-week period:
* Amalgams (78 percent);
* Composite resins (16 percent);
* Sealants (5 percent);
* Glass ionomers (1 percent); and
* Gold restorations (0.2 percent).
It was noted that 67 percent of amalgams and 50 percent of composite resins were placed because of primary caries. The remaining restorations were replacements of existing restorations. A comprehensive survey8 of more than 9,000 restorations performed in the United Kingdom found the following types of restoration placement:
* Amalgams (54 percent);
* Composite resins (30 percent); and
* Glass ionomers (16 percent).
Initial restoration placements (49 percent) and replacements (51 percent) were quite evenly divided. More recent studies have indicated that approximately 60 percent of adult restorative care is dedicated to replacing restorations.7,8,33,35,36,38,57,59-62,69-75,81,84-86, 90,106,11-113,115-117,120 With Swedish children and adolescents in 1995,112 the choice of restorative material was quite different from adult populations. In 3- to 8-year-olds, the restorative materials chosen for primary teeth were:
* Compomers (32 percent);
* Glass ionomer (26 percent);
* Zinc oxide-eugenol (23 percent);
* Amalgams (4 percent); and
* Composite resins (3 percent).
With 6 to 19 year-olds, caries in permanent teeth were restored with:
* Composite resins (56 percent);
* Amalgams (31 percent);
* Glass ionomers (9 percent); and
* Compomers (4 percent).
The choice of materials in this Swedish study may reflect the reluctancy toward amalgam usage. Similar studies have not been completed in the United States.
Reasons for Restoration Placement and Replacement
The principal reason for restoration failure is secondary caries in both the permanent and primary dentitions (Table 1).7,8,38,57,59-62,70-75,81,84,85,106,112-113,115-117,120 Secondary caries accounts for approximately 60 percent of all reasons for restoration replacement, regardless of restorative material type.8,70-75,120 Other reasons include material failure, tooth fracture or defect, endodontic involvement, prosthetic abutment utilization, technical errors, and deterioration of esthetic quality with tooth-colored restoratives.8,120 With pediatric patients, secondary caries is responsible for replacement of restorations in 70 percent of cases. Fracture of either the restoration or the tooth is a less frequent occurrence in children and adolescents.
The longevity of failed restorations is variable and dependent upon the restorative material (Table 1).7,8,120 Amalgams tend to have the greatest median and mean survival times when compared with composite resins and glass ionomers. It must be realized that amalgam restorative materials have been available for well more than 100 years; and these materials have been refined for posterior tooth restoration. In contrast, the terms "composite resin" and "glass ionomer" in most clinical studies encompass many different formulations with variable strengths and weaknesses. In such studies of restoration failure and longevity, subtypes of composite resins and glass ionomers were not taken into account.
A sequela of secondary caries is the effect on the tooth requiring restoration replacement. With removal and replacement, the size of the restoration changes considerably.38,56,59-62,74 When secondary caries is present, the original cavity margin is extended by 0.52 mm. When no caries is present, the margin is extended by 0.25 mm. This implies that the replaced restoration width will be larger by 0.5 to 1.04 mm. No doubt after several replacements, the affected tooth will become weakened and may require full coverage.
Clinical and Histopathologic Features of Secondary Caries
Although secondary caries is the etiology of failure in 50 to 60 percent of restorations (Table 1), there is confusion regarding the definition of secondary or recurrent caries.7,8,38,57,59-62,70-75,81,84,85,106,111-113,115-117,120 Often, marginal gaps and ditching around restorations may be ascribed to secondary caries. Only when marginal gaps are ≥ 250 μm can secondary caries be identified consistently by clinical and microscopic criteria. Some clinicians equate a marginal defect of ≥ 50 μm with an increased prevalence of secondary caries. With occlusal amalgams, macroscopic caries has been detected in only 20 percent of ditched margins and 4 percent of nonditched margins. Microscopic examination of these restorations showed histologic caries in 47 percent of nonditched margins and 59 percent of ditched margins. Margin defects and staining are not sufficient to predict the presence or absence of secondary caries and do not allow for treatment decisions.
Secondary (recurrent) caries may be defined most simply as caries detected at the margins of an existing restoration. Similar to primary caries, the enamel or root surface adjacent to the restorative material may possess an inactive arrested lesion, an active incipient lesion, or a frankly cavitated lesion (Figure 2). Clinically, secondary caries has certain features.59-62,70-75 A high proportion of secondary caries is located along the cervical and interproximal margins (>90 percent of failed amalgams, >60 percent of failed composite resins). With enamel surfaces, recurrent caries may be seen as a white spot (active), or a brown spot lesion (inactive). The surface may undergo a certain degree of softening prior to frank cavitation. The enamel lesion color varies depending upon the adjacent restorative material. When the cavosurface is involved and undermined by caries, the adjacent enamel surface takes on a brown to gray to blue hue; however, amalgam restorations impart such color changes due to corrosion. Transillumination may be helpful with tooth-colored restorative materials. Radiographs can detect interproximal caries, especially along gingival margins. The interface between the tooth and restoration needs to be evaluated with an explorer; however, care should be taken to avoid creating an iatrogenic defect along the cavosurface margin or cavitating the lesion’s surface.
Active root surface secondary caries appears as a yellow discoloration and frequently has undergone surface softening.31,55,81,87-89,91,105,117 In contrast, inactive secondary caries in a root surface may become sclerotic and ebonized, with a hardness level similar to that for sound enamel. With both enamel and root-surface secondary caries, the responsible microorganisms remain the same as those for primary caries. The diagnosis of secondary caries is dependent upon visual inspection, tactile sensation with judicious explorer usage, and radiographic interpretation.
During the past three decades, naturally occurring and artificially induced secondary caries (Figure 2) around restorative materials have been characterized microscopically as two separate, but interrelated lesions.1-3,8,12,13,16-24, 26,27, 29-31, 43,44,46,48-50,52-55,59-62,64,67,79,80,82-84,93-96,100,102,103,105,107 The primary (outer) surface lesion develops in the enamel or root surface adjacent to the restoration; while the wall lesion forms in the cavosurface tooth structure along the restorative interface. The outer surface lesion may be readily visualized in the enamel or root surface adjacent to the restoration. The wall lesion occurs due to microleakage of oral fluids, percolation of hydrogen ions and lytic enzymes from plaque, and bacterial colonization along the cavosurface wall. Whenever a restorative material is placed, there is a possibility for a microspace (gap) to be formed between the restorative material and the cavosurface enamel, dentin, and cementum. The ability of a material to resist secondary caries development along the cavosurface is dependent upon complete removal of carious tissue leaving no residual caries, formation of an intimate cavosurface-restorative interface with minimal to no microspace, and release of caries-protective agents (fluoride, metal ions, antimicrobials, acidic ions) to the adjacent cavosurface and outer tooth surface.
Risk factors for development of secondary caries are identical to those for primary caries (Table 2).31,35,36,51,59-62,70-75,81,90,101,104,106 The most reliable predictor of caries risk is the prior caries experience of the patient. One factor that may result in a considerable increase in root surface caries is the larger proportion of dentate elderly in the population with more retained teeth than in the past.87-89,91,113,117 The increased number of retained teeth leads to a greater risk for periodontal disease development. With periodontal disease onset, gingival recession occurs; and this leads to exposure of caries prone root surfaces.87-89 It has been shown that four years following root surface exposure, due to periodontal therapy, almost two-thirds of patients develop root caries. These patients had an average of 6.9 new root caries lesions. Twelve years after periodontal treatment, 80 to 90 percent have root caries with an average of 17.2 lesions per person.
Prevention of secondary caries31,35,36,59-62,70-75 begins at the time of restoration placement with patient education in proper dental hygiene; fluoride regimen implementation (rinses, gels, fluoridated toothpastes); antimicrobials (chlorhexidine); fluoride-releasing restorative material; salivary cariogenic microorganism assessment; salivary flow rate determination; current medication inventory; dietary review; and medical evaluation if necessary. More frequent dental examinations with topical fluoride application may be indicated for especially caries-prone patients.
Various laboratory methods have been developed to investigate microleakage and caries formation along the restorative-cavosurface interface.1,59-62 Techniques employed include: 1) artificial secondary caries systems evaluated by polarized light microscopy, scanning electron microscopy, and microradiography; and 2) microleakage determination with organic and fluorescent dyes, radioisotopes, bacterial cultures, pigmented chemical tracers, air pressure, neutron activation analysis, and electrical conductivity. Each of these methods has certain advantages and disadvantages. The most often used laboratory techniques are artificial caries systems and microleakage assessment with organic dyes. Studies using these techniques have allowed for rapid evaluation of the effects of restorative materials, bonding agents, remineralizing agents, innovative fluoride delivery systems and fluoride-releasing products on microleakage and secondary caries formation.
Fluoride-Releasing Dental Materials
There are numerous dental materials from many different manufacturers that have the ability to release fluoride to adjacent tooth structure and into the oral environment. A brief review of the major categories of fluoride-releasing dental materials is in order.21,30,34,66,76,78 Several decades ago, silicate cements composed of a basic glass and phosphoric acid solution were used as tooth-colored restorative materials. Although these materials were not retained well, it was noted that secondary caries was reduced significantly. This was due to the substantial fluoride release generated by this restorative material. Glass ionomers were developed from aluminosilicate glass with calcium and a fluoride flux. The material requires an acidic polymer to induce an acid-base setting reaction. Considerable quantities of fluoride are released initially with the setting reaction, and continuous release of lower levels of fluoride is detected for long periods. Silver particles have been added to some glass ionomers to increase their physical strength, and these materials are known as glass ionomer cermets. Resin-modified glass ionomer (polyalkenoate) represents a hybrid material with a greater amount of glass ionomer than conventional resin in its makeup. This material uses an acid-base reaction, light- and/or chemical-activated polymerization, and self-curing for its setting reaction (triple-cure). Fluoride is released from this material but to a lesser extent than conventional glass ionomers. Compomer (polyacid modified composite resin) contains a higher content of composite resin with a lessened amount of ionomer material and polymerizable acidified monomer. This material is light-activated for its setting reaction. Fluoride is released primarily during the setting reaction and to a lesser extent over time. Fluoride-releasing composite resins are also available, and these contain some filler particles with releasable fluoride. Long-term fluoride release is quite low. Conventional composite resins lack fluoride-releasing abilities. In summary, there is a continuum of tooth-colored restorative products that range from high fluoride release (glass ionomer) to intermediate fluoride release (resin-modified glass ionomer) to low fluoride release (compomer and fluoride-releasing composite resin) to no fluoride release (conventional composite resin). Physical properties vary with the degree of glass ionomer and composite resin content. In general, decreased physical properties are associated with increased fluoride release.
Continuing research into the development of fluoride-releasing composite resins is ongoing in the hope of maintaining the physical properties of these materials and providing long-term fluoride release.46 Incorporation of inorganic fluoride (NaF) has resulted in increased fluoride release but with creation of voids in the matrix as the inorganic fluoride leaches out of the material. Dispersion of leachable glass or soluble fluoride salts (YbF3) into the polymer matrix allows for a water-soluble diffusion of fluoride from the composite resin into the local environment. Most of the fluoride is released during the setting reaction, with a smaller amount of long-term fluoride release. The addition of organic fluorides to the polymer matrix has been attempted to increase fluoride release. These organic fluorides include MF-MMA (methacryloyl fluoride-methyl methacrylate), acrylic amine-HF salt, t-BAEM/HF (t-butylamino ethyl methacrylate hydrogen fluoride), MEM/HF (morpholinoethyl methacrylate hydrofluoride) and most recently TBATFB (tetrabutylammonium tetrafluoroborate). These agents hold promise for increasing fluoride delivery to the adjacent tooth structure while maintaining the physicochemical properties of composite resins.
In addition to the "more traditional" fluoride-releasing restorative materials, other methods for fluoride release are available.4,19,20,23,24,26,29,32,42-44,47,79,93 Glass ionomer, resin-modified glass ionomers, and compomers have been formulated as luting agents and cavity liners. Many bonding agents, total-etch dentin adhesives, and one-step adhesives for various dental materials contain releasable fluoride and protect against secondary caries. Several fluoride-releasing bonding agents for amalgams, as well as fluoridated amalgams, have become available.95,96,107 Other clinical investigators have proposed exposing the prepared cavity to topical fluoride agents to allow rapid fluoride uptake by dentin, enamel, and cementum prior to restoration.29,42,43,67
Fluoride content in a dental material varies considerably ranging from 7 percent to 26 percent in glass ionomers, resin-modified glass ionomers, and compomers.2,3,5,6,11,14-17,21,30,32,34,39-42,58,66,67,77,78,86,97,108,109,114,119 However, the amount of fluoride made available to the oral cavity is not related to the fluoride content of the material, but to the ability for fluoride to leach from the material or to be exchanged for other ions in the oral environment. With all fluoridated dental materials, there is a burst of fluoride release during the setting reaction and this is followed by a gradual decline in the amount of fluoride leached into the oral environment. The dental material provides a low level of fluoride for a considerable period (Table 3). Several studies have shown well-documented fluoride availability for 2.7 to eight years from glass ionomer-based materials and up to five years from composite resins.2,3,17,30,32,39,40 The ability to continue to release fluoride in vitro over extended periods is remarkable considering the fact that the materials are constantly exposed to an aqueous environment. The quantity of fluoride available for uptake is dependent upon the media into which the fluoride-containing restorative is placed.11,58,108,114 Many laboratory studies report the daily or accumulated fluoride released into water. Artificial saliva tends to decrease the release of fluoride, most likely due to precipitation of calcium, phosphate and fluoride complexes on the surface of the restorative material. Exposure of the restorative material to demineralizing solutions increases the available fluoride; while remineralizing fluids decrease the amount of fluoride released.11,58,108,114 This tends to hold true for all glass ionomer-based materials. It is well-known that during acid challenges glass ionomers mobilize and release increased amounts of fluoride into the environment.76 This is an important feature for facilitating reprecipitation of mineral into demineralized enamel and root surfaces; thereby enhancing remineralization.
Glass ionomers and other fluoride-releasing restorative materials increase the fluoride composition of adjacent tooth structure (Table 3).4,6,30,32,37,42,45,92,94,114,118,119 The amount of acquired fluoride in sound enamel and root surfaces adjacent to glass ionomer restorations is substantial and may be appreciated for long periods. In addition, a dramatic increase in fluoride content in enamel and dentinal cavosurfaces, as well as the dentinal axial wall, has been shown in a recent electron probe analysis.118 Both wavelength and energy dispersive spectrometry studies also found that fluoride is incorporated into the hybrid layer formed by fluoride-containing dentin adhesives.37 Fourier transform infrared spectroscopy of the intermediate layer between glass ionomer and dentin indicates that the primary component of this layer is fluoridated carbonatoapatite.45 This substance is known to have a low solubility and increased acid resistance. No doubt readily available fluoride from glass ionomers will enhance the in vivo caries resistance of the tooth structure composing the cavosurfaces and tooth surfaces adjacent to such restorations.
In Vitro Secondary Caries and Fluoride-Releasing Dental Materials
The ability of a fluoride-releasing material to affect in vitro secondary caries formation is illustrated by the reduction in the occurrence of wall lesions along the tooth-restorative material interface (Table 4, Figures 3 and 4).2-4,12,16-23,26-28,41,43,44,48,50,52-55,67,79,80,82-84,93-96,102,103,105,107 Many different fluoride-releasing materials reduce wall lesion frequency considerably (40 percent to almost 80 percent). Typically, restorative materials with a higher fluoride content tend to provide the greatest degree of protection along cavosurfaces. Not only is wall lesion development affected, but also wall lesion depth and length. Reductions in cavity wall depth and length range from 10 percent to 25 percent for fluoride-releasing composite resins to 35 percent to 41 percent for fluoridated amalgams to 70 percent to 74 percent for conventional glass ionomers. In addition, the higher the fluoride release from the dental material, the greater the chance that wall lesion formation will be inhibited and create caries inhibition zones in the cavosurface tooth structure. Typically, glass ionomers and resin-modified glass ionomers produce inhibition zones in the cavosurface, while compomers and fluoride-releasing composite resins rarely develop such inhibition zones. Outer (primary) surface lesions that form in enamel and root surfaces next to the restorations are also affected by fluoride-releasing dental materials. Reductions in outer lesion depths range from less than 10 percent for fluoridated composite resins to almost 75 percent for conventional glass ionomers. Cavity liners, desensitizers and topical fluoride application decrease substantially both outer and wall lesion depths.
The retention of greater amounts of mineral in secondary caries lesions is also apparent from microhardness studies.4,64,94 These studies have found that the outer lesion adjacent to a glass ionomer had only a 7 percent reduction in microhardness compared with sound enamel; however, a nonfluoride-releasing composite resin resulted in a 44 percent reduction in microhardness in the outer lesion compared with sound enamel. The availability of fluoride from the adjacent restoration results in a reduction in mineral loss from the outer lesion.
Both lesion depth and mineral loss are related in a linear fashion to the amount of fluoride released over time.94,105 In fact, under plaque conditions, complete inhibition of secondary caries may be realized if 200 to 300 μg of fluoride are released per cm2 of the dental material during a one-month period.2,3,17
Remote Effect of Fluoride-Releasing Dental Materials
The local environment for fluoride release is relatively extensive and is not limited to the immediately adjacent cavosurface or surface enamel (Table 5).17,48,49,92,100 Fluoride uptake in vitro by enamel and root surfaces from conventional glass ionomers is substantial and maintained for at least six months.92 Enamel located 1.5 to 7.5 mm from glass ionomers increases its fluoride content by more than 2,000 ppm. Root surfaces up to 7.5 mm from glass ionomers have a greater ability to absorb fluoride than enamel (more than 5,000 ppm). Perhaps even more remarkable is that the glass ionomer-restored teeth were stored in artificial saliva, which is known to reduce the amount of fluoride available for uptake.
The amount of mineral loss from in vitro lesions adjacent to and up to 7 mm from glass ionomer restorations is reduced significantly (Table 5).100 When compared with nonfluoride-releasing restorations, the placement of a glass ionomer reduces mineral loss by almost 80 percent at 0.2 mm from the restoration to 37 percent at 7 mm from the restoration margin. Similarly in polarized light microscopic studies, mean lesion depth for primary tooth enamel and permanent tooth root surfaces increased gradually the farther the lesion was located from the glass ionomer restoration.48,49 In contrast, in vitro microradiographic studies found a lessened effect for fluoride-releasing composite resins.17 The lesion depth and mineral loss was reduced in close proximity to the fluoridated resin; whereas tooth surfaces positioned 3 to 4 mm from the fluoridated restoration did not receive the benefits of fluoride release.
Tooth surfaces opposing adjacent teeth restored with glass ionomers receive a certain degree of protection against caries formation in vivo (Figure 5).4,12,25,28,68,86,98,99,110 In a three-year longitudinal study,98,99 about 25 percent of interproximal tooth surfaces adjacent to teeth restored with glass ionomer tunnel restorations had developed caries. In marked contrast, slightly more than 80 percent of interproximal surfaces in teeth adjacent to amalgam-restored teeth succumbed to caries. A similar three-year clinical study86 with primary teeth found an almost twofold increase in interproximal caries in teeth adjacent to amalgams when compared with those next to glass ionomers. Another study found that the use of a resin-modified glass ionomer base under a resin decreased the risk of caries development in the opposing interproximal surface to a similar degree.110
Remineralization of existing lesions may also occur when these lesions are in close proximity to a fluoride-releasing dental material.10,28 Placement of amalgam, nonfluoride-releasing composite resin and conventional glass ionomer Class II restorations in extracted teeth in contact with other teeth possessing well-defined proximal lesions provided insight into the effects of these restorative materials on adjacent interproximal surfaces. Following a two-week exposure to a cyclic demineralizing-remineralizing artificial caries system, differences were identified among the lesions adjacent to glass ionomer restorations compared with those in contact with fluoride-releasing composite resins and nonfluoridated amalgams. The lesions next to glass ionomers had regressed slightly in area (-2 percent); while the lesions adjacent to amalgams and fluoride-releasing resins had increased by 64 percent and 28 percent, respectively. Fluoride release into the local environment by the glass ionomer restorations resulted in no caries progression with the lesion in the opposing tooth surface.
Similarly, resin-modified glass ionomer has been shown to provide protection against in vitro lesion progression and to induce remineralization to a similar extent as that found with fluoride dentifrices, but less than that for a low-concentration (0.05 percent) sodium fluoride rinse.68 Lesional areas of artificial caries have been noted to be reduced by 2.45-fold when placed adjacent to resin-modified glass ionomers, by 2.23-fold when exposed to a fluoridated dentifrice, and by 3.74-fold when exposed to a 0.05 percent sodium fluoride mouthrinse.
The ability of glass ionomers to release fluoride to adjacent tooth surfaces accounts for the hypermineralization of enamel and dentinal lesions seen in microradiographic investigations and inhibition zones with polarized light microscopy.102,103 With an in vitro pH-cycling demineralizing-remineralizing system and an in vivo intraoral partial denture model, it has been noted that enamel and dentinal lesions in contact with glass ionomers possess increased calcium content and mineral volume percentage compared with enamel and dentinal lesions adjacent to nonfluoridated amalgams and composite resins. The mineral content of the glass ionomer-associated hypermineralized layer was reported to be more than threefold greater than those for the lesions adjacent to amalgams and resins. In addition, the hypermineralized area extended up to 300 μm into the underlying tooth structure. In contrast, the carious lesions adjacent to the amalgams and resins progressed and increased in depth by fourfold and threefold, respectively. The glass ionomer material induced remineralization and hypermineralization of adjacent enamel and dentinal lesions, while the nonfluoridated amalgam and resin restorations were associated with progressive demineralization.
Plaque and Fluoride Releasing Dental Materials
While dental plaque is intimately involved in caries development, this organic film may act as a fluoride reservoir and provide a means to affect the demineralization-remineralization process. Glass ionomer materials release fluoride into the oral environment and are in direct contact with the overlying dental plaque. Clinical studies have shown that plaque adjacent to glass ionomers has increased fluoride concentrations compared with nonfluoridated composite resins.4,47,63,64,99 The plaque fluoride content ranges from 15.0 to 21.2 μg/g for glass ionomers compared with 0.4 to 3.5 μg/g for nonfluoridated resins. Although these levels seem to be relatively low, only small concentrations of fluoride in plaque, saliva, or calcifying fluids are necessary to shift the equilibrium from demineralization to remineralization. In fact, remineralization of enamel lesions begins with only 0.03 ppm fluoride in artificial saliva, plaque, and calcifying fluids.35,36,101,104 Remarkably, optimal remineralization requires a fluoride concentration of only 0.08 ppm. Children in communities either with or without water fluoridation have similar baseline salivary fluoride levels of between 0.02 to 0.04 ppm, which is less than optimal for remineralization. In a four-year longitudinal study,35,36 it has been reported that children with high salivary fluoride levels (≥0.075 ppm) are more frequently caries-free than those with lower salivary fluoride concentrations. A child’s salivary fluoride level, regardless of whether fluoridated drinking water is available, is associated with the child’s caries status. It is apparent that water fluoridation has less of an effect on caries than the availability of other sources of fluoride, such as dietary fluoride, fluoridated dentifrices, and fluoride mouthrinses.
The importance of relatively frequent exposure to low-dose fluoride sources is emphasized by clinical studies that have shown that caries around orthodontic brackets and in xerostomic patients may be eliminated or greatly reduced with fluoridated dentifrice usage and/or daily sodium fluoride (0.05 percent) rinsing.24,35,36,101,104 Such preventive agents increase the fluoride content of saliva and plaque above the level necessary to facilitate remineralization for at least two to six hours. The levels may be prolonged and at higher levels if the individual does not rinse following toothbrushing or fluoride mouthrinsing.
As noted previously, glass ionomers may induce remineralization of carious lesions and also induce hypermineralization in enamel and dentin adjacent to the restorative material. These materials continually release small amounts of fluoride into the local environment. This fluoride is then taken up by plaque and saliva.35,36,76,101,104 In many ways, these materials may be looked at as slow-release fluoride devices. Not only is fluoride available to inhibit demineralization of sound tooth structure and facilitate remineralization of hypomineralized and carious tooth structure, but the released fluoride also affects bacteria within dental plaque. Several clinical studies have shown substantial reductions (46 to 77 percent) in cariogenic bacteria (mutans streptococci, lactobacillus) within plaque adjacent to glass ionomers. This effect has been observed up to six months after restoration placement. Dental plaque fluoride, even in small concentrations, inhibits bacterial metabolism by diffusion of hydrogen fluoride from the plaque into the bacteria. Once inside the bacteria, the hydrogen fluoride acidifies the bacterial cytoplasm and leads to release of fluoride ions. These ions interfere with enzymes essential for bacterial metabolism (enolase, acid phosphatase, pyrophosphatase, pyrophosphorylase, peroxidase, catalase, proton-extruding ATPase). In addition, increased plaque fluoride decreases adherence of bacteria to hydroxyapatite. This results in reduced plaque formation.
Recharging of Fluoride-Releasing Dental Materials
Most of the fluoride-release studies performed with fluoridated dental materials have evaluated the amount of fluoride released during varying lengths of times without the material being exposed to exogenous sources of fluoride. From the information presented previously, it is obvious that certain materials release fluoride for long periods (up to eight years). This is not equivalent to what happens in the oral environment. The vast majority of individuals in developed and developing countries have access to fluoridated toothpastes, over-the-counter low-dose fluoride rinses, and prescription high-dose fluoride rinses and toothpastes. Exposure of fluoride-containing dental materials to exogenous fluoride sources replenishes the fluoride within the dental material and provides a continuing, renewable source of fluoride for the oral environment.5,10,25,28,29,39,40,42,65,68,97,108 This is particularly true for all glass ionomer-based restorative materials and less so for composite resin-based materials. Professionally applied acidulated phosphate fluoride treatment provides a 2.5 to 4.0 increase in fluoride release from fluoride-releasing dental materials. Even with commercially available fluoridated toothpastes, the fluoride uptake and release by fluoride-containing materials is substantial and adequate to increase plaque and saliva fluoride to levels sufficient to inhibit demineralization and facilitate remineralization. Significant increases in fluoride release (twofold) may be achieved when conventional and resin-modified glass ionomers are exposed for short periods to only a 50 ppm fluoride solution.
The benefit of recharging glass ionomers and fluoride-releasing composite resins has been demonstrated in vivo using well-defined artificial lesions placed in the interproximal aspects of crowns and opposing fluoridated restorative materials.28 Changes in the lesional areas due to the fluoride-releasing materials were determined in the absence and presence of fluoridated toothpaste. In a relatively short period in the oral cavity, twice daily exposure to the fluoridated toothpaste for one minute resulted in a reduction in lesional area of about 10 percent for a fluoride-releasing composite resin and about 5 percent for a glass ionomer. In another laboratory study,10 fluoridated toothpaste used in concert with a fluoride-releasing resin and glass ionomer reduced the lesional areas by 18 and 14 percent, respectively. The ability to recharge fluoride-containing restorative materials with fluoridated dentifrices provides continuous low-level fluoride release that may prevent secondary caries in the restored tooth and primary caries in both the restored tooth and neighboring tooth, and remineralize existing caries and hypomineralized tooth structure.
Caries Preventive Mechanisms of Fluoride-Releasing Materials
Fluoride-releasing dental materials prevent secondary caries by several different mechanisms (Table 6).34-36,50,53,55,66,76,101,104 The release of fluoride into the local environment may inhibit or slow the process of demineralization. As little as 1 ppm fluoride in demineralizing, acidic and plaque fluids reverses the demineralization process. Fluoride released from restorative materials may coat hydroxyapatite crystals that form the mineral substance of enamel, dentin, and cementum. Although fluorapatite has the greatest degree of acid resistance, the acid solubility of hydroxyapatite with a fluoride coating or veneer approaches that for fluorapatite. Such fluoridated hydroxyapatite may be formed in the presence of fluoride-releasing dental materials and provide even greater caries resistance than native tooth structure. Remineralization of lesions and hypomineralized tooth structure is facilitated by very low levels of fluoride (≥0.03 ppm) in saliva and plaque fluid. As noted previously, bacteria in dental plaque have several enzyme systems that are dysregulated by hydrogen fluoride derived from plaque. These enzyme systems are necessary for glycolysis and energy production by the bacteria. Fluoride-releasing materials provide a source for continuous fluoride that elevates salivary and plaque levels and adversely affects plaque bacteria. Fluoride releasing materials may act as continuous low-level fluoride delivery systems, especially when "recharged" by readily available exogenous fluoride sources. Finally, an intimate interface with a minimal to no microspace between the restorative material and cavosurface may be enhanced by physicochemical bonding with glass ionomer-based materials and by mechanical bonding with composite resins.
Appropriate Utilization of Polyacid-Modified Resin-Based Composites
1. Class III restorations in primary and permanent dentition in mild- to moderate-risk patients.
2. Class V restorations in primary and permanent dentition in mild- to moderate-risk patients.
3. Class I restorations in primary teeth in mild- to moderate-risk patients.
4. Class II restorations in primary teeth where preparation remains within the proximal line angles and/or patient is at mild to moderate risk.
Appropriate Utilization of Glass Ionomer or Resin-Modified Glass Ionomer Cements
1. Root caries restorative material.
2. Base/liner in moderate risk patients.
3. Restorative material in high-risk adult patients.
4. Class III restorations in primary and permanent dentition where tooth isolation is not possible and/or patient is at moderate to high risk.
5. Class V restorations in primary and permanent dentition where tooth isolation is not possible and/or patient is at moderate to high risk.
6. Class I restorations in primary teeth where tooth isolation is not possible and/or patient is at moderate risk.
7. Class II restorations in primary teeth where tooth isolation is not possible and the preparation remains within the proximal line angles and/or patient is at moderate risk.
8. Cementation of orthodontic bands.
9. Cementation of crowns in moderate to high-risk patients.
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To request a printed copy of this article, please contact: John Hicks, DDS, MS, PhD, MD, Department of Pathology, MC1-2261, Texas Children’s Hospital, 6621 Fannin St., Houston, TX 77030-2399 or mjhicks@texaschildrenshospital.org.
Figure Legends
Figure 1a. Variability in caries experience with mixed dentition.
Figure 1b. Variability in caries experience in adult. Secondary caries and restoration failure are common experiences in most individuals.
Figures 2a and b. Secondary in vitro caries around an amalgam restoration. a. An artificial white spot lesion (arrow) surrounds the amalgam restoration. b and c. Polarized light microscopic examination of this amalgam-restored tooth demonstrates the two components of secondary caries - the primary outer surface lesion (O, OL) and the wall lesion (arrow, WL). The wall lesion is formed due to percolation of acidic byproducts, lytic enzymes and colonization of plaque along the microspace present between the restorative material (R) and the cavosurface tooth structure. Secondary caries formation adjacent to restorations in coronal and root surfaces appears similar.
Figures 3a through d. In vitro secondary caries formation in root surfaces adjacent to restorations (R) filled with nonfluoride releasing (a) and fluoride-releasing dental materials (b-d). Dramatic reductions in the primary outer root surface lesion (O) depth occur when nonfluoride releasing composite resin (a) restorations are compared with fluoride-releasing composite resin (b) restorations, and compomer restorations (c), and resin-modified glass ionomer restorations (d). (arrow = wall lesion).
Figures 4a and b. Fluoride-releasing amalgam restorative material and in vitro secondary caries formation in root surfaces. Caries formation in the root surface (O) adjacent to a conventional amalgam restoration (a) is quite extensive and considerably greater than that for the primary outer root surface lesion (O) adjacent to a fluoride-releasing amalgam (b). (R = restored cavity where material was lost during the sectioning procedure; arrow = wall lesion).
Figures 5a through c. An intraoral method for testing the effect of a fluoride releasing restorative material (R) employs the temporary placement of a gold crown (G) with a mesial slot (S) for retaining sections of human tooth enamel or root surfaces. Development of the intraoral caries in the tooth sections (b, c) is influenced by the release of fluoride from the adjacent restoration into the oral environment. A nonfluoride-releasing dental material allows for more extensive caries formation (L = body of lesion) in a previously sound root surface (b), compared with a lessened degree of caries formation (L = body of lesion) in a previously sound root surface (c) in close proximity to a fluoride-releasing dental material.
