Do commercial whitening dentifrices increase enamel erosive tooth wear?

Abstract Objective This in vitro study evaluated the effect of commercial whitening dentifrices on erosive tooth wear (ETW) of bovine enamel samples, in comparison with commercial regular dentifrices. Methodology Sixty bovine crowns were embedded in acrylic resin, polished and then had their baseline profile determined. They were randomly assigned to 5 groups (n=12/group), according to the type of commercial dentifrice to be tested: GI – Crest Anti-cavity Regular; GII – Crest 3D White; GIII – Colgate Total 12 Clean Mint; GIV – Colgate Optic White; GV – Placebo (negative control, fluoride-free dentifrice). The samples were submitted to daily erosive and abrasive challenges for 3 days. The erosive challenges were performed 3 times a day by immersing the specimens in 0.1% citric acid solution (pH 2.5) for 90 s. Each day after the first and last erosive challenges, the specimens were subjected to the abrasive challenge for 15 s, using a toothbrushing machine (Biopdi, São Carlos, SP, Brazil), soft toothbrushes and slurry (1:3 g/ml) of the tested toothpastes (1.5 N). The specimens were kept in artificial saliva between the challenges. The final profile was obtained and the ETW (µm) was calculated. Data were analyzed by Kruskal-Wallis and Dunn’s tests (p<0.05). Results All dentifrices tested significantly reduced the enamel wear in comparison with the Placebo, except GIII. The median (95% CI) ETW was 1.35 (1.25-1.46)bc for GI, 1.17 (1.01-1.34)cd for GII, 1.36 (1.28-1.45)ab for GIII, 1.08 (1.04-1.14)d for GIV and 2.28 (2.18-2.39)a for GV. Conclusion When dentifrices from the same manufacturer were compared, the whitening dentifrices led to similar or less wear than the regular ones.


Introduction
Erosive tooth wear (ETW) is the loss of dental hard tissue caused by the interplay between the exposure to nonbacterial acids and abrasive forces, and the action of these acids is its primary etiological factor. 1 Due to its increasing prevalence 2 and adverse consequences, ETW has been a matter of concern in the dental community in the last decades, 3,4 and appropriate preventive measures must be implemented for high risk patients. 5 The evidence on the efficacy of fluoridated dentifrices to prevent ETW is much less clear 6 than it is for caries prevention. 7 This may be related to the lack of clinical studies on the topic but also to the fact that the erosive challenge is stronger than the carious one due to the lower pH of the dietary and intrinsic acids compared with the bacterial acids. 8 Moreover, dentifrices are used during brushing, which means that depending on how toothbrushing is performed and on type of dentifrice and toothbrush, dentifrices may either increase or decrease the wear degree. 9,10 Considering that abrasion of eroded enamel increases with increasing abrasivity of the dentifrice, 11,12 dentifrices with high abrasivity should not be used by patients at high risk for ETW. 9,13 From the practical point of view, following this recommendation is very difficult for the patients, since information on the abrasivity of the dentifrices is not available on the labels of the product. The general advice is to avoid whitening dentifrices, 9 which might have higher abrasivity in order to optimize the removal of extrinsic stains. 14 However, information on the association between the use of whitening dentifrices

Methodology
Preparation of enamel specimens Sixty enamel specimens were prepared from freshly extracted bovine incisors that had been stored in 0.1% thymol solution (pH 7). The crown and root were separated using a cutting machine (Maruto, Kasuga, Tokyo, Japan) and a diamond disc (Maruto, Kasuga, Tokyo, Japan). The crowns were coupled to a prefabricated silicone mold (Biopdi, São Carlos, São Paulo, Brazil) and embedded in auto polymerizing acrylic resin with the labial surface exposed. After The composition of the dentifrices is described in Figure 1. Briefly, the dentifrices in groups I to IV are fluoridated and the dentifrice in group V is fluoride-free (negative control). In groups I and II both dentifrices were manufactured by Procter & Gamble, being I and II regular and whitening dentifrices, respectively. Considering the mean and SDs of placebo and 1100 ppm fluoride dentifrices, a sample size of 11 specimens would be required to obtain an α=5% and β=80%.

Erosive-abrasive cycles and treatments
The specimens were subjected to daily erosive and abrasive challenges for 3 days. The erosive challenges were performed 3 times a day by immersing the specimens in 0.1% citric acid solution (pH 2.5, 30 mL/ sample) for 90 s min at 25°C under gentle agitation. 22 The samples were then washed with deionized water (10 s) and immersed for 2 h in artificial saliva 23 (pH 6.8, 30 mL/sample) at 25°C for 2 h between the erosive challenges. The specimens were maintained in artificial saliva overnight to complete a 24 h cycle. The citric acid was renewed at each erosive challenge and the artificial saliva was daily replaced by a new one.
After 3 days, the nail varnish was removed with commercial acetone and the final profile was obtained to provide the ETW calculation.

Contact profilometry
The ETW was measured using a contact profilometer and two scratches delimitating the exposed area.
They were inserted into a metal device (x and y axes determination, reproducibility 0.08 µm), to allow the accurate stylus repositioning at each measurement.
The baseline profile was compared with the final one using the software Marh Surf XCR20 for the enamel loss calculation. The scans were superposed, and the average depth of the under-the-curve area was calculated (μm).

Statistical analysis
The GraphPad Instat software for Windows version 3.0 (GraphPad software Inc., La Jolla, CA, USA) was used. The data were analyzed by Kruskall-Wallis and Dunn's test after testing the equality of variances and normal errors distribution. The significance level was set at 5%.

Groups
Commercial

Results
All tested dentifrices, except Colgate Total 12 Clean Mint, significantly reduced the enamel wear in comparison with the Placebo ( Table 1). The lowest wear was found for the Colgate Optic White that performed significantly better than the other dentifrices, except for the Crest 3D White. When dentifrices from the same manufacturer were compared, the whitening dentifrices led to similar or less wear than the regular ones.

Discussion
This study was developed based on the increased concern regarding ETW 4 , the recommendation to at-risk patients on avoiding the use of whitening dentifrices due to their potentially increased abrasivity, 9,13 and the contradictory results found in the literature regarding the abrasive potential of these dentifrices. [15][16][17][18][19] According to the results of this study, the null hypothesis was accepted, since brushing with the evaluated commercial whitening dentifrices did not increase the enamel wear degree in comparison with the regular ones. In fact, the commercial whitening dentifrices led to similar or less wear when compared with the regular commercial dentifrices.   (NaF) or sodium monofluorophosphate (MFP). In fact, the effect of monovalent fluoride compounds in the dentifrices to reduce erosion and abrasion is limited, with more promising results obtained for SnF 2containing dentifrices. 9,33 Interestingly, the lowest wear was found for the MFP-containing dentifrice. However, this effect was not related to MFP, since this study did not use any technique to break the covalent bond between fluoride and phosphate, which means that the amount of available fluoride was negligible. 30 Thus, the presence of fluoride in the tested dentifrices in this study might not explain the differences found between the distinct formulations. Regarding the commercial non-whitening dentifrices, the inclusion criteria were to have dentifrices most commonly used from both manufacturers. In addition, since this study had, from the same manufacturer, one whitening and one nonwhitening dentifrice, as a negative control a fluoridefree dentifrice was included, despite the role of fluoride against ETW is not as well established as that against caries. 8 Other options for negative controls would be not to brush (erosion only) 19 or brushing with water (to reveal the effect of toothbrush filament stiffness).

The model of this study involved an in vitro erosive-
However, it has been shown that the abrasivity of the dentifrice is more important than the toothbrush filament stiffness, at least for the enamel. 10,11 Whitening dentifrices contain abrasive and whitening agents to remove extrinsic stains from the tooth surface. All the dentifrices evaluated, including placebo, had silica as abrasive agent. However, the number of abrasive agents, as well as the size, hardness and shape of the particles, are unknown since information regarding RDA/REA is not displayed on the labels. These elements are determining factors in the degree of abrasivity 34 , and not knowing this information became a limitation of this study. No linear relationship is observed between the amount of silica present in the dentifrices and the degree of enamel loss under erosive and abrasive conditions. 35 It was recently shown that for dentifrices containing fluoride and tin, tissue loss increases up to a silica content of 10% but decreases significantly with higher amounts (20% silica is similar to the silica-free formulation). 35 The whitening agent present in the commercial whitening dentifrices is pyrophosphate, but Colgate Optic White also contains hydrogen peroxide. While some studies report that dentifrices containing silica and pyrophosphate lead to greater enamel wear in comparison with silica-only dentifrices, 15,16,19 the results of other studies 17,29,36 agree with this one.
The contradictory results might be explained by the distinct protocols employed in different studies.
Regarding the studies that found a higher degree of wear for the dentifrices containing silica and pyrophosphate in comparison with those containing silica-only: one employed more concentrated slurry (1 part of dentifrice for 2 parts of water), 16 the other employed longer erosion and abrasion cycles 19 and another one was conducted in situ testing both sound and softened enamel. 15 Interestingly, there was no difference between regular and whitening toothpastes for eroded enamel in the latter study, while for sound enamel the whitening dentifrice significantly increased the enamel wear. 15 The study by Mosquim,et al. 19 (2017) had a very similar protocol to that of this study, despite the erosive cycles, which were performed for 7 days. Interestingly, in the study by Mosquim,et al. 19 (2017), whitening dentifrices containing silica and pyrophosphate led to higher ETW than the dentifrices containing silica-only. One could speculate that the absence of difference in the degree of wear between whitening and conventional commercial dentifrices in this study is due to the shortest period of erosive and abrasive challenges. However, in the in situ study by Joiner, et al. 36 (2008) the higher degree of ETW found for whitening dentifrices in comparison with conventional ones at 4 weeks was not observed at the 12-week evaluation.

Conclusion
In conclusion, the results of this study do not support the assumption that brushing with whitening dentifrices increases the degree of ETW in comparison with regular dentifrices. Thus, the recommendation that patients at high risk for ETW should avoid the use of whitening dentifrices lacks support in the current literature. Additional studies using methodologies that more closely resemble the clinical condition should be conducted to add evidence on this matter, considering the increasing concern regarding ETW.