The Effect of Whitening on Restorative Materials
INTRODUCTION
The purpose of this chapter is to summarize and discuss the available information concerning the effects of peroxide-releasing whitening agents on composite restorative materials and restorations. All original scientific full papers or reviews listed in PubMed or ISI Web of Science (search terms included bleaching, plus composite or resin or compomer or adhesive) were screened and included.
The influence of various whitening agents on physical properties, surface morphology, and color of composite restorative materials has been investigated in several in vitro studies simulating the clinical situation as closely as possible. In those studies, home whitening products (10–16% carbamide peroxide) were generally used within a 2- to 4-week whitening simulation with application intervals of 4–8 hours per day. Tooth whiteners designed for in-office application (30–35% hydrogen or carbamide peroxide) were applied in treatment intervals of 15–60 minutes (as recommended by the manufacturers). These different whitening regimens were preferably used in the studies reviewed and are therefore not repeatedly mentioned in describing and discussing the results of the respective studies. Most of the studies did not show differences among these different whitening regimens with regard to their influence on dental restorative materials. However, it should be borne in mind that the total time period of application was much higher for the low-concentration regimens than for the high-concentration ones.
In the literature, the greatest focus has been on the impact of whitening agents on composite restorations and dental adhesives. Therefore this chapter will deal preferentially with these interactions, although it should also be noted that whitening agents may also exert some effects on other restorative materials, such as glass ionomer cements, metal alloys, amalgam, or ceramics (Attin et al. 2004, El-Murr et al. 2011). Because the chemistry of polyacid-modified composite resin materials (i.e., compomers) is very close to that of composites, the influence of whitening regimens on this type of restorative material is also included in this chapter.
INFLUENCE OF WHITENING AGENTS ON PHYSICAL AND OPTICAL PROPERTIES OF COMPOSITE MATERIALS
SURFACE ROUGHNESS
Some studies that did not simulate a salivary effect have shown that application of whitening agents might lead to a slight increase in surface roughness and amount of porosities of microfilled and hybrid composite resins (Cehreli et al. 2003, Turker and Biskin T 2003, Basting et al. 2005, Rosentritt et al. 2005, Moraes et al. 2006, Dutra et al. 2009, Gurgan and Yalcin 2007, Hafez et al. 2010, Martin et al. 2010, de Andrade et al. 2011, Wang et al. 2011). This increase in surface roughness corresponds to the fact that surface energy of composites is enhanced after whitening, thus increasing surface wettability (Buzoglu et al. 2009). Other studies have reported only minor or negligible effects of whitening gels in terms of resin composite surface roughness (Kim et al. 2004, Wattanapayungkul et al. 2004, Polydorou et al. 2006, Zavanelli et al. 2011). Interestingly, it should be noted that no effect of whitening on composite surface roughness was observed when hydrogen peroxide gels were applied on composites in a cycling protocol with intermittent storage of the samples in pooled human saliva or in a 14-day in situ study (Schemehorn et al. 2004, de A Silva et al. 2006). It was suggested that contact with saliva might have modified or attenuated the hydrogen peroxide impact by formation of a surface-protective salivary layer on the restorative material.
SURFACE HARDNESS
Previous studies had yielded controversial results with regard to surface hardness changes of composite resins after application of different home whitening regimens ranging from significant softening, through unchanged hardness, to even increased surface hardness (Garcia-Godoy et al. 2002, Turker and Biskin 2002, Mujdeci and Gökay 2005, Mujdeci and Gökay 2006, Okte et al. 2006, Silva Costa et al. 2009, Polydorou et al. 2007a, Polydorou et al. 2007b, Lima et al. 2008, Yu et al. 2008, Briso et al. 2010b, Kwon et al. 2010, Malkondu et al. 2011). It seems that heated whitening agents lead to more pronounced hardness reduction than unheated agents (Yu et al. 2011). However, in all of these studies, salivary impact was not simulated. In contrast, studies with simulation of salivary exposure between the whitening applications did not reveal surface softening of composite restorative materials using different whitening gels and application forms (Campos et al. 2003, White et al. 2003, Duschner et al. 2004, Duschner et al. 2006).
COLOR STABILITY
It has been suggested that under clinical conditions in the mouth, ΔE* color differences, as spectroscopically evaluated with the CIE-L*a*b* method, have been reported to be relevant and perceptible only when higher than 3.3 (Ruyter et al. 1987) or 3.6 (Johnston and Kao 1989). Previous studies had shown that application of 10% hydrogen peroxide or heated and unheated 30% (or 35%) hydrogen peroxide resulted in composite color changes that were presumably clinically detectable, with ΔE* ranging from 2 to 11 for the different materials and shades tested (Monaghan et al. 1992a, Monaghan et al. 1992b, Canay and Cehreli 2003, Hubbezoglu et al. 2008, Rao et al. 2009, Hafez et al. 2010, de Andrade et al. 2011). Also, gloss of composites seems to be reduced after whitening (Anagnostou et al. 2010, Yalcin and Gurgan 2005b). Application of a film type of whitening regimen demonstrated higher color change of composites than a 10% carbamide peroxide control gel (Anagnostou et al. 2010). Only a slight or even negligible color change of composites was reported for a 10% carbamide peroxide gel (Monaghan et al. 1992b, Canay and Cehreli 2003, Li et al. 2009, Silva Costa et al. 2009, Kwon et al. 2010). It was also proved that color change of composite restorative materials resulting from whitening is significantly lower compared with color change of (bovine) enamel (Rosentritt et al. 2005). Moreover, in this study no close correlation could be observed between hydrogen concentration of whitening agents (based on different compositions) from different manufacturers and the changes in ΔE*.
It should be noted that whitening might increase the propensity of resin composites to staining induced by contact with discoloring solutions (tea, coffee) (Celik et al. 2009, Yu et al. 2009). On the other hand, whitening is also effective in removal of superficial and also intrinsic staining of composites (Villalta et al. 2006, Türkün and Türkün 2004a, Abd Elhamid and Mosallam 2010, Pruthi et al. 2010).
The alterations in color of the restorative materials have been attributed to oxidation of surface pigment texture and amine compounds, with different color changes occurring in different materials as a result of different amounts of resin and different degrees of conversion of the resin matrix (Monaghan et al. 1992b). These negative influences of the oxidizing agents on the resin matrix are also responsible for water uptake of the restorative materials. This may lead to complete or partial debonding of fillers, causing reduced surface integrity and loss of hardness of the materials (Wattanapayungkul and Yap 2003).
OTHER PHYSICAL PROPERTIES
Some studies have shown that fracture toughness of composites seems to be negatively affected by whitening agents, although this has not been proved in all studies dealing with this topic (Yu et al. 2010, Firoozmand and Pagani 2009, Cho et al. 2009).
In addition, it was recently proved that whitening with hydrogen peroxide has an effect on the three- dimensional polymer network in polymerized composites, leading to an increase in the release of unpolymerized monomers, additives, and unspecific oxidative products (Durner et al. 2011). It might be speculated that this effect is material dependent, because another study revealed less leaching of monomers from whitened composites than from unwhitened ones (Polydorou et al. 2009).
Brushing resistance of composite resins seems not be reduced as a result of whitening, as shown in a whitening/brushing cycle performed over a 21-day period (Faraoni-Romano et al. 2009).
INFLUENCE OF WHITENING AGENTS ON PHYSICAL AND OPTICAL PROPERTIES OF POLYACID-MODIFIED COMPOSITE RESINS
Simulated overuse of highly concentrated whitening regimens for 1–5 days showed detrimental effects on surface texture and mechanical properties of polyacid-modified resin-based composites (compomers) (Jung et al. 2002, Lee et al. 2002, Taher 2005). In this sense, a reduction of surface microhardness was also detected in an in situ experiment using 15% carbamide peroxide over a period of 28 days (Yu et al. 2008). In contrast, a total of three 30-minute whitening sessions with highly concentrated whitening agents conducted at 1-week intervals did not result in detrimental effects of the surface finish of compomers (Wattanapayungkul and Yap 2003). Also, with use of highly concentrated whitening regimens, no surface microhardness changes were observed in polyacid-modified resin-based composites (Yap and Wattanapayungkul 2002). Treatment with 10–16% carbamide peroxide whitening gels resulted in either an increased surface roughness or a decreased surface roughness, suggesting that the effects of the gels seem to be material dependent (Cehreli et al. 2003, Turker and Biskin 2003).
The occurrence of visible color changes in compomers treated with 10–16% carbamide peroxide were, as with the changes in surface texture, dependent on the compomer brand tested, whereas treatment with either film type (6.5% H2O2) whitening regimens or 10% and 30% hydrogen peroxide resulted in noticeable color change irrespective of the compomer material evaluated (Canay and Cehreli 2003, Kwon et al. 2003, Rosentritt et al. 2005). It should be noted that color changes resulting from whitening seem to be more pronounced in compomers than in composite materials (Yalcin and Gurgan 2005a, Li et al. 2009, Yu et al. 2009).
CLINICAL RELEVANCE OF IMPACT OF WHITENING AGENTS ON PHYSICAL AND OPTICAL PROPERTIES OF RESIN-BASED RESTORATIVES
Unfortunately, none of the aforementioned studies investigated how far the induced changes in surface texture and hardness led to recommendable need for repolishing or even replacement of existing restorations after whitening to ensure longevity of the restorations. Only a single study examined subsurface extension of the impact of bleaching agents in composite and polyacid-modified composites (Hannig et al. 2007). This study revealed significant subsurface softening to an extent of up to 0.4 mm depth as a result of contact with whitening agents. Nevertheless, it remains speculative whether the mentioned changes in surface texture and hardness are relevant under clinical conditions or if they are mainly a surface phenomenon that could be removed by simple polishing of restorations.
Bond Strength of Composite Resin to Whitened Enamel and Dentin
There is a long list of in vitro studies showing that bond strength of composite restorative and composite-based materials is significantly reduced when composite application to dentin or enamel surfaces (including acid- etching pretreatment) is performed immediately—that is, within 1 day—after completion of a whitening regimen with both high- and low-concentration agents (Titley et al. 1988, Torneck et al. 1991, Stokes et al. 1992, Titley et al. 1992, Bishara et al. 1993, Dishman et al. 1994, van der Vyver et al. 1997, Demarco et al. 1998, Spyrides et al. 2000, Far and Ruse 2003, Uysal et al. 2003, Miguel et al. 2004, Miyazaki et al. 2004, Shinohara et al. 2004, Teixeira et al. 2004, Cavalli et al. 2005, Shinohara et al. 2005, Cacciafesta et al. 2006, Montalvan et al. 2006, Adanir et al. 2007, Dadoun and Bartlett 2007, Shinkai et al. 2007, Turkkahraman et al. 2007, Barbosa et al. 2008, Mullins et al. 2009, Uysal et al. 2009, Wilson et al. 2009, Barcellos et al. 2010, Briso et al. 2010a, Can-Karabulut et al. 2010, Dietrich et al. 2010, Abe et al. 2011).
In contrast, only a few studies did not reveal an impact on composite bond strength after enamel or dentin whitening (Basting et al. 2004, Loretto et al. 2004, Arcari et al. 2007, Sasaki et al. 2007, Amaral et al. 2008, Mishima et al. 2009, Lima et al. 2011b). Use of a carbamide peroxide– containing dentifrice before an adhesive restoration does not seem to influence enamel and dentin bond strength of resin composite (da Silva et al. 2007). In terms of bond strength to cavities, a single study was able to show that whitening does not affect bond strength to enamel-dentin cavities in bovine teeth restored with siloraneand dimethacrylate-based restoratives (Lima et al. 2011b).
The influence of whitening agents on enamel and dentin bond strength seems to be more pronounced for highly concentrated agents (Patusco et al. 2009). With respect to intracoronal whitening of nonvital teeth, the application of carbamide peroxide gels or mixtures of sodium perborate mixed with water are less negative than applications containing hydrogen peroxide (Teixeira et al. 2004, Timpawat et al. 2005). Controversy still exists regarding whether fluoride-enriched carbamide peroxide gels exert the same reduction in enamel bond strength as nonfluoridated agents (Metz et al. 2007, Chuang et al. 2009). On the other side, for bond strength to whitened enamel it was shown that etch-and-rinse adhesives seem to perform better than nonrinse, self-etching products (Moule et al. 2007, Gurgan et al. 2009). The kind of light-curing device used (light-emitting diode [LED], quartz-tungsten, or plasma arc) does not affect bond strength to whitened enamel and dentin (Bulucu and Ozsezer 2007, Bulucu et al. 2008).
In vitro studies investigating the appropriate time point for bonding of composites after termination of whitening showed that a delay of at least 1–3 weeks is necessary to allow for optimal bonding to both enamel and dentin (Torneck et al. 1991, Adibfar et al. 1992, Titley et al. 1993, van der Vyver et al. 1997, Cavalli et al. 2001, Basting et al. 2004, Cavalli et al. 2004, Türkün and Kaya 2004, Bishara et al. 2005, Bulut et al. 2006, Da Silva Machado et al. 2007, Barbosa et al. 2008, Can-Karabulut and Karabulut 2011, Souza-Gabriel et al. 2011, Oztaş et al. 2012). A single study showed that a time lapse of only 24 hours should be enough to recover enamel bond strength after whitening (Unlu et al. 2008).
Under simulation of intraoral conditions, conflicting results are given for testing bond strength to whitened dental hard tissue. It was shown that no delay is necessary for establishing common bond strength to enamel using 16% carbamide peroxide for whitening; however, for root dentin a delay of 21 days was suggested to achieve proper bond strength under in situ conditions (Miguel et al. 2004, Barbosa et al. 2009). When a 35% hydrogen peroxide agent was used for enamel whitening in situ, a delay of 7 days was recommended (Bittencourt et al. 2010).
REASONS FOR REDUCED BOND STRENGTH TO WHITENED DENTAL HARD TISSUE
Several factors are responsible for the reduction in composite bond strength to whitened enamel and dentin. It was shown that resin tags in whitened enamel subsequently acid etched with 37% phosphoric were less defined, were more fragmented, and penetrated to a lesser depth than in unwhitened enamel controls (Titley et al. 1991, Sundfeld et al. 2005, Nour El-din et al. 2006). Also, tag formation in whitened dentin is compromised (Ferreir et al. 2011). Bond strength to dentin was even reduced when the whitening agent was applied on the surface enamel covering underlying dentin (Lima et al. 2010).
Moreover, whitening with hydrogen peroxide or hydrogen peroxide–releasing agents may result in significant decrease of enamel calcium and phosphate content and in morphologic alterations in the most superficial enamel crystallites (Ruse et al. 1990, Perdigão et al. 1998). Besides this, acid etching of whitened enamel surface produced loss of prismatic form, resulting in an enamel surface that appeared to be overetched (Josey et al. 1996). In addition, it was suggested that the enamel and dentin organic matrix was altered by the oxidizing effect of hydrogen peroxide (Kodaka et al. 1992, Hegedüs et al. 1999). These factors may have led to enamel and dentin surfaces that did not allow for formation of a strong and stable bond and interaction between the composite applied and the superficial etched enamel layer. Furthermore, reduction in bond strength in hydrogen peroxide–treated enamel and dentin could be caused by the presence of residual oxygen in enamel and dentin pores after completion of the whitening treatment. Liberation of the oxygen could either interfere with resin infiltration into enamel and dentin (Torneck et al. 1990, McGuckin et al. 1992) or inhibit polymerization of resins that cure via a free-radical mechanism (Rueggeberg and Margeson 1990). The latter aspect might result in oxygen-inhibited polymerization of the composite components directly in contact with the dental hard tissues, leading to a soft interface not able to withstand debonding forces sufficiently. The recommendations for a 1- to 3-week delay before placement of composite restorations after termination of whitening therapy are made under the assumption that the residual oxygen may have sufficient time to leach from the dental hard tissues.
METHODS TO COUNTERACT REDUCED BOND STRENGTH TO DENTAL HARD TISSUES
To dissolve remnants of peroxide, cavities can also be cleaned with antioxidants, such as catalase (Rotstein 1993, Kum et al. 2004), α-tocopherol (Sasaki et al. 2009), grapeseed extract (Vidhy et al. 2011), or 10% sodium ascorbate gel or solution (Lai et al. 2001, Lai et al. 2002, Kum et al. 2004, Türkün and Kaya 2004, Türkün and Türkün 2004b, Bulut et al. 2005, Kimyai and Valizadeh 2006, Muraguchi et al. 2007, Gökçe et al. 2008, Kimyai and Valizadeh 2008, Khoroushi et al. 2009, Türkün et al. 2009, Comlekoglu et al. 2010, May et al. 2010, Uysal et al. 2010, Danesh-Sani and Esmaili 2011, Feiz et al. 2011, Kunt et al. 2011, Mazaheri et al. 2011). Pretreatment with ethanol or calcium hydroxide is not effective for recovering bond strength levels to the level of unwhitened enamel (Kum et al. 2004, Türkün and Kaya 2004, Khoroushi et al. 2009). Most of the recent studies dealt with the application of sodium ascorbate as an antioxidant to reduce waiting time before restoration of whitened teeth. Application of sodium ascorbate is also helpful to restore compromised fracture resistance of whitened enamel and does not affect enamel surface hardness (Khoroushi et al. 2010, Oskoee et al. 2010). Pretreatment of whitened enamel with sodium ascorbate before bonding increases tag formation in enamel (Briso et al. 2012). In most of these studies, the application time of sodium ascorbate amounted to 10 minutes, although Lima et al. (2011a) found a 1-minute period to be sufficient. In contrast to this recommendation, Kaya et al. (2008) revealed a 60-minute application of sodium ascorbate gel to be superior to shorter application times. The length of the application time seems to be positively correlated with the antioxidizing effect and the regaining of bond strength. This was shown in a study in which a 120-minute application of 10% sodium ascorbate was more effective than shorter periods. Moreover, addition of a surfactant to a sodium ascorbate gel might increase its effectivity (Moosavi et al. 2010). In contrast, increasing the concentration of sodium ascorbate from 10% to 20% did not improve effectiveness (Dabas et al. 2011). It was also reported that laser treatment (neodymium:yttrium-aluminum-garnet [Nd:YAG] or erbium:yttrium-aluminum-garnet [Er:YAG]) of the whitened enamel and dentin surface can reverse the bond strength reduction (Lago et al. 2011, Leonetti et al. 2011, Rocha Gomes Torres et al. 2012). However, application of the agents or methods mentioned might be time-consuming or expensive, so further investigations are needed to optimize their use under clinical conditions. It is more feasible to follow the aforementioned recommendations to allow for contact time of at least 7 days with water and saliva to avoid the reduction of adhesion of composites to enamel. Optimum bonding to prewhitened dental hard tissue could be achieved after a period of about 3 weeks (Cavalli et al. 2001, Shinohara et al. 2001). In addition, prolonged polymerization times of up to 60 seconds are advised for better conversion rate of applied adhesives (Cadenaro et al. 2006, Bresch et al. 2007, Hussain and Wang 2010).
IMPACT OF WHITENING ON MARGINAL SEAL OF RESIN-BASED RESTORATIONS
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