Tooth color change promoted by different whitening toothpastes under alternate cycles of staining and brushing

In recent years, the science of tooth color has been extensively explored. Many studies with different designs have been conducted to gain a better understanding of the dental hard tissue susceptibility to staining [1], and the efficacy of bleaching products [2]. This increased interest is a reflection of the impact that whiter teeth have on people's appearance and social relationships, with a trend towards a preference for whitened teeth over the color of natural teeth [3].

Tooth discoloration is a frequent complaint in dental offices, and the etiology of stains may differ. Moreover, the location of stains, superficial characteristics, and degree of adherence to the tooth structure can also vary widely [4]. Ideally, the first choice of treatment for discoloration should be tooth whitening performed under professional supervision, because it is a minimally invasive procedure that results in a high level of satisfaction [5,6]. However, tooth sensitivity [7] and minerals loss, which can be related to the gel composition, have been reported to be some of the drawbacks of this therapy [8]. At present, there is an increasing availability of over-the-counter products that claim to perform tooth whitening [9]. These products may be presented in the format of paint-on films, strips, chewing gums, mouthwashes, and toothpastes.

The composition of whitening toothpastes is constantly reformulated by the manufacturers and consists of a wide range of physical and chemical ingredients to obtain a synergistic whitening effect. Physically abrasive agents, such as silica, can remove extrinsic pigments by abrasion. To improve tooth whitening promoted by toothpastes, enzymes as hydrogen peroxide, used for professional tooth whitening, can be added in lower concentrations to the toothpaste formulation. Different types of enzymes have demonstrated the same ability of abrasives in performing stain removal [10].

Chemical cleaning agents, such as sodium tripolyphosphate, are condensed phosphates used to stain removal, to avoid new-stain chromogen adsorption [11], and to prevent the formation of dental calculus [12]. Different cosmetic compounds can also be associated to the abrasives, such as titanium dioxide, blue covarine and other pigments [13]. These optical agents are expected to be deposited on tooth surface during toothbrushing and affect the proportion of the yellow/blue color axis. As consequence, tooth should appear whiter after brushing [14]. Recently, activated charcoal powder has been incorporated into toothpastes, and great focus has been done to better understand the color change promoted by charcoal-based toothpastes [15], [16], [17], and the possible effects on enamel surface [17]. As manufacturers claimed the use of charcoal as a natural option, potential consumers are attracted to this new whitening option [18], but studies show that these products seem to be ineffective to promote tooth color change [19].

Considering the increased popularity of tooth whitening, it is important to test the ability of whitening toothpastes in promoting tooth color change. Although this topic has been widely explored, there is a great variation in the experimental designs among studies, especially regarding the simulation of tooth staining. Usually, in in vitro studies, tooth staining is performed prior to tooth brushing simulation with the whitening toothpastes, while there are studies that did not perform tooth staining, which can sub estimate the effect of the whitening toothpastes [16,20]. However, in clinical conditions, subjects alternate periods of staining and brushing and this should be reproduced in studies that aimed to evaluate tooth color change promoted by whitening toothpastes, as it is a more faithful reproduction of the daily life. Additionally, whitening toothpastes have different whitening ingredients in their composition, and as consequence, different mechanisms of action. Although a recent study showed that different whitening ingredients promote similar tooth color change [21], these have not yet been evaluated under conditions that simulate alternate staining and brushing cycles. To improve the knowledge regarding tooth whitening, these toothpastes should be also compared with the conditions usually found in most previous studies, that is previous staining followed by prolonged toothbrushing. The aim of this study was to compare the effect of whitening toothpastes on tooth color change of discolored teeth subjected to/ or not, to additional staining. The null hypotheses tested were: 1. Tooth color change would be similar under alternate cycles of stanning and brushing (simulating the clinical conditions), when compared with stanning followed by prolonged brushing times; 2 None of the whitening toothpastes would differ from the regular type, in terms of general tooth color change (ΔE).

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