Prevention and treatment of secondary caries 41 Prevention of secondary caries

As secondary caries is one of the major reasons for restoration replacement, a large number of clinical dentists and scientists have placed great emphasis on preventing or slowing down the procession of secondary caries lesion from many aspects, so as to increase clinical restoration durability. Secondary caries, the same as other types of dental caries, is determined by the dynamic balance between pathological factors that lead to demineralization and protective factors that lead to remineralization. It is also considered that bacteria are an important etiologic factor leading to demineralization for secondary caries. Generally, the rationales of all the modification of restorative material or prevention of secondary caries normally include two fundamental points: one is the decrease of demineralization and/or increase of remineralization of the hard tooth tissues; the other is to interfere the metabolism of caries-related bacteria and/ or to decrease the amount of bacteria/inhibit bacteria growth in the plaque or /and the carious dentin under restorations. Thus, in all the past years, most scientists and clinical dentists focused on adding anticaries substance into restorative materials.

It has been well-known such restorative materials can release copper, Ag-Cu alloy, zinc, calcium, aluminum and fluoride, which are able to inhibit bacteria growth or decrease colonization and acidogenicity of oral plaque, play antibacterial activities and reduce the rate of restoration replacement. The followings are several basic fillings used and researched by clinical dentists and scientists throughout the world: amalgam restorations, zinc oxide eugenol cement; common composite resin (CR); common glass ionomer cement (GIC); and different ion-released restorative materials containing fluoride-containing materials.

Clemens Boeckh et al. investigated the antimicrobial effects of five restorative materials and showed that the most remarkable inhibitory activity was observed with ZOE [Boeckh et al., 2002]. The antimicrobial effects of zinc oxide and ZOE are well recognized [Podbielski et al., 2000; Yap et al., 1999]. Unfortunately, ZOE cannot be widely used except for temporary filling due to its high solubility and insufficient mechanical properties. Different types of amalgam may have different effects on S. mutans growth and bacterial penetration [Fayyad and Ball, 1987]. For instance, a low-copper amalgam can decrease the lesion size significantly [Grossman and Matejka, 1995], non-gamma-2 amalgam can inhibit the metabolic activity of microorganisms due to the release of copper [Wallman-Bjorklund et al., 1987].

It has been widely shown in long term studies that CR have higher rate of restorations replacement than GIC and amalgam [Leinfelder,et al., 1987; Collins et al., 1998]. An in situ study showed that the percentage of streptococci in plaque on different materials was to be 13.7% on composite, 4.3% on amalgam and 1.1% on glass ionomer cement [Svanberg et al., 1990]. Another study showed up to eight times more microbes beneath composite restorations compared to amalgam and suggested the type of restorative material may have influence on the composition of the microflora [Splieth et al., 2003]. The high rate replacement of CR occurred might be due to its shrinkage and non-fluoride release [Savarino et al., 2004]. Thus, scientists have been making great efforts to improve CR, such as reducing the polymerization shrinkage of composite, increasing the adhesion stress. Remarkably, a modified ion-releasing resin composite (IRCR) has been invented, which can release hydroxyl, calcium, and fluoride ions at low pH [Boeckh et al., 2002]. The rationales of anticaries effect of IRCR include: OH ions can neutralize the organic acids produced by the plaque bacteria [Heintze, 1999], and the calcium and fluoride ions released from IRCR can prevent demineralization and promote remineralization [ten Cate, 1990; ten Cate and van Duinen, 1995; Featherstone, 1994; Forss and Seppa, 1990; Kraft and Hoyer, 1999]. Thus, the restoration can perform anticaries activities. Studies have shown the release of hydroxyl and calcium can exist for a long period [Heintze, 1999]. Persson and his colleagues showed that IRCR could counter the plaque pH fall and maintain it at levels where less enamel and dentin demineralization can occur [Persson et al., 2004], however, the precious research did not consider IRCR could present significant antibacterial effects [Boeckh et al., 2002].

The inhibition of fluoride at acidic pH is due to the effect of hydrogen fluoride, which can penetrate into the bacteria cell membrane [Nakajo et al., 2009]. The HF can dissociate into hydrogen ion and fluoride ion. Some researchers confirmed that the fluoride can reduce the activities of enolase and proton-extruding ATPase, which are very important for metabolism of bacteria [Hayacibara et al., 2003]. Besides, the hydrogen ion can promote cytoplasmic acidification, which is critical for enzymes of the glycolytic pathway [Huther et al., 1990]. So the combination effects of hydrogen ion and fluoride can have negative influence on the glycolytic acid production and the metabolism of caries-related bacteria. As a result, fluoride-containing restorative material can inhibit bacteria growth and decrease the demineralization of tooth tissue and the occurrence of secondary caries around restorations. However, these antimicrobial effects in caries prevention are often regarded as little or of no importance as compared to the direct interactions of fluoride with the hard tissue during caries development and progression [Wiegand et al., 2007]. Recent observations have found fluoride in the aqueous phase surrounding the carbonated apatite crystals is much more effective in inhibiting demineralization than fluoride incorporated into the crystal [R0lla and Ekstand, 1996]. Fluoride may precipitate onto tooth surfaces as calcium fluoride-like layer, which serves as a reservoir for fluoride when the pH drops [R0lla et al., 1993]. While amount of in vitro studies showed the inhibitive effect of fluoride on the demineralization of both enamel and dentin around the restoration in primary and permanent teeth[Attar and Onen, 2002; Donly and Gomez, 1994; Francci et al., 1999; Hicks et al., 2002; Tam et al., 1997; Yaman, 2004], the results from some in situ studies are not consistent with that in vitro, which have not confirmed the preventive ability of fluoride to secondary caries[Kielbassa et al., 1999, 2003; Papagiannouslis et al., 2002]. And this conflict is also showed among varieties of clinical studies. In a six-year follow-up assessment, class I restorations in permanent molars exhibited significantly less secondary caries for glass-ionomers(2%) compared to amalgam(10%) [Mandari et al., 2003]. Another five-year evaluation showed the glassionomer had a lower survival time and greater loss of anatomic form and marginal integrity, but less secondary caries compared to amalgam [Welbury et al., 1991]. Instead, Van Dijken found no difference in secondary caries development of class III cavities restored with two fluoride-containing materials and one composite in a three-year observation period [van Dijken, 1996]. Moreover, there is no clear evidence for inhibition of secondary caries by glass-ionomer cements as shown in a recent review summarizing extensive literature research [Randall and Wilson, 1999].

The above conflicts might be explained by the following factors: 1) the intrinsic formulation of fluoride-containing materials, the duration of fluoride releasing may influence its effect. The antibacterial effect of dentin is obtained only when the fluoride release is very large

[Kawai et al., 1998]. 2) The ability of fluoride uptake by different dental hard tissue. Due to differences in micro-structure and porosities, the amount of fluoride uptake from restorations and the depth of fluoride penetration are higher for both dentin and cementum than for enamel [Souganidis et al., 1981; Retief et al., 1984]. 3) Other environmental conditions. A decrease in pH increases the dissolution of the material leading to a higher fluoride level and the highest fluoride release is found in acidic and demineralizing-remineralizing regimes and lowest in saliva [Karantakis et al., 2000]. A pellicle forming by the components from saliva on the surface of the restorative material and various bleaching agents can impede fluoride ion release [Behrend and Geurtsen, 2001; Bell at al., 1999; Levallois et al., 1998]. 4) Extra application of fluoride. In a recent research, Cenci et al. found that the fluoride provided either by GI or fluoride dentifrice might be important to decrease demineralization adjacent to fillings, and by using fluoride dentifrice, demineralization adjacent to the restorations were similar [Cenci et al., 2008], which was testified by the previous study [Hara et al., 2006]. 5) In some cases, cavity type may play a significant role in the development of secondary caries [da Rosa Rodolpho et al., 2006], for example class II restorations involve the marginal ridges and significantly reduce the tooth resistance to fracture [Mondelli et al., 1980], and is susceptible to the biodegradation of saliva esterase activity [Finer and Santerre, 2004].

Since the final formation of caries is influenced by multiple factors, the prevention of secondary caries beginning at the time of restoration replacement contains a variety of aspects including the excavation of carious tissue; wise choice of restorative materials; fluoride regimen implementation(rinses, gels, fluoridated toothpastes); salivary flow rate assessment; healthy dietary; oral health or medical education and so on. Therefore, the prevention of secondary cares not only depends on the clinical operation by dentists, but also is influenced by other significant aspects from patients themselves.

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