- Top of page
- Materials and Methods
In children, the deciduous or primary teeth typically begin to erupt at 6–8 months of age. However, from around the age of 6 years, these teeth start to be replaced by permanent teeth. At this stage, the mouth will contain both deciduous and permanent teeth until the last deciduous tooth is lost at around 12 years of age[1, 2].
There are a number of important differences between deciduous and permanent enamel. Specifically, deciduous enamel contains more organic material, is generally less mineralised in the outer enamel layers and has a higher porosity. This makes deciduous enamel more susceptible to acid-mediated dissolution than permanent enamel[6-8]. In addition, deciduous enamel is softer and therefore less mechanically resistant than permanent enamel. Consequently, tooth wear owing to abrasion can be more pronounced in deciduous than in permanent teeth.
Childhood caries is recognised as being a major public health problem among the general population. According to the National Health and Nutrition Examination Survey carried out from 1999 to 2004 in the USA, 42% of children aged 2–11 years have had dental caries in their primary teeth and 21% of children aged 6–11 years have had dental caries in their permanent teeth. A number of comprehensive reviews of caries clinical trial data in children and adolescents concluded that there was strong evidence that daily use of fluoride (F)-containing toothpastes can reduce the incidence of caries compared with a placebo or with non-brushing[11, 12]. These effects were also improved with supervised brushing and increased frequency of brushing.
Mechanistic studies have shown that the anti-caries effect of fluoride is through the prevention of demineralisation and enhancement of remineralisation. In addition, fluoride has been shown to interfere with bacterial metabolism in vitro, which may inhibit plaque acid production.
A series of guidelines have been established by the US Food and Drug Administration (FDA), the American Dental Association (ADA) and FDI World Dental Federation to ensure that marketed toothpastes are safe and effective[15-17]. Caries clinical trials are considered to be the ultimate proof of anti-caries effectiveness, however, because of the long duration and high costs associated with these types of studies, a number of preclinical methods known as bioequivalence studies have been developed to evaluate fluoride efficacy. In these studies, the experimental toothpaste is tested against a clinically proven control toothpaste containing the same active ingredient at the same nominal concentration.
The maximum permitted concentrations of fluoride for toothpastes in different markets are governed by regulatory requirements; however, fluoride concentrations for specific child age ranges are generally set by national guidance via health authorities or dental associations.
A new range of toothpastes have been developed for use by children at different stages of their development (typical ages are 0–2 years, 3–5 years and 6+ years) containing different concentrations of fluoride and for different markets. The aim of the present studies was to evaluate the efficacy of these toothpastes to (1) promote lesion remineralisation under dynamic demineralising/remineralising conditions simulating in vivo caries formation, and (2) promote enamel fluoride uptake. The enamel and dentine abrasivities of these toothpastes were also determined relative to the ADA reference abrasive.
- Top of page
- Materials and Methods
The range of experimental toothpastes evaluated in this study have been developed for use by children at the different stages of their development. The formulations for the three age groups are different; specifically, in the level of surfactant and the type of thickening gums used. In addition, flavour and colour variations are present across the three formulations. The rationale for these differences is discussed by Stovell et al. These toothpastes were designed to maximise fluoride availability, minimise abrasivity and incorporate levels and types of surfactant that will minimise interference with fluoride delivery.
In vitro pH cycling models are frequently used to replicate the dynamics of demineralisation and remineralisation involved in caries lesion formation and daily toothpaste usage. The pH cycling model described in this paper has been previously used to investigate the effect of fluoride containing toothpastes on remineralisation of artificial caries lesions[27, 28]. The cycling studies described here used permanent human teeth because of the difficulty in obtaining sufficient numbers of deciduous teeth. Furthermore, deciduous teeth are smaller and so there is less surface area for experimental manipulation. While there are a number of distinct differences between deciduous and permanent enamel, it has been reported that remineralisation of initial caries lesions is similar in both substrates. This means that permanent enamel can be used as a surrogate for deciduous enamel in these studies. The lesions used in this study represented early stage lesions where the mineral loss was confined to the outer regions of the enamel where fluoride is considered most effective.
The changes in specimen hardness following pH cycling were less that those reported by Newby et al. using the same experimental protocol. These differences may be explained by compositional variations in the saliva used as the remineralisation medium. As saliva varies from person to person, and the composition of an individual's saliva can vary depending on the time of collection, the saliva used in these studies was pooled from at least five individuals in an attempt to mitigate this variability.
In this paper, the enamel remineralisation neared a plateau in the range of 250–500 ppm F, however, in most cases the dose response continued to be significant up to higher fluoride concentrations. Similar remineralisation plateaux have been observed by other authors for shallow lesions. In addition, while net remineralisation increased from 10 days to 20 days for all fluoride-containing toothpastes, the changes were small. A plausible explanation could be a surface-zone blocking effect, which reduces the number of diffusion pathways to the lesion body. This effect has been demonstrated during mechanistic studies in vitro[33, 34].
The fluoride source in all experimental toothpastes and the majority of the commercial control toothpastes is NaF. Only the Aquafresh Kids Bubble Fresh and Aquafresh Milk Teeth Toothpastes use SMFP as a fluoride source. To be effective in the mouth, the fluoride ion needs to be freely available. While this is the case for NaF, the SMFP must be initially hydrolysed by salivary or microbial phosphatases in order to release the fluoride ion. As a result, the toothpaste slurries used in the pH cycling studies were prepared in human saliva in order to initiate hydrolysis. It is worth noting that whilst the in vitro pH cycling data has shown that treatment with both experimental toothpastes (US) produced greater lesion remineralisation and a higher enamel fluoride content than the Aquafresh Kids Bubble Fresh toothpaste, a number of caries clinical trials have shown no significant differences in the anti-caries effectiveness of toothpastes containing fluoride as either NaF or SMFP.
Fluoride uptake has long been accepted as a positive indicator of the anti-caries activity of fluoride toothpastes. All experimental toothpastes contain a high level of bioavailable fluoride and were effective at delivering this fluoride to demineralised enamel. However, not all toothpaste containing an equivalent fluoride source and concentration produced the same fluoride uptake. This demonstrates that fluoride uptake can be influenced by different toothpaste excipients. In both EFU studies, the fluoride uptake was higher from NaF than from SMFP toothpastes containing equivalent fluoride ion concentrations. For comparison, Arends et al. and de Rooij et al. reported similar findings using sound enamel. For the US experimental toothpastes, the fluoride uptake was either equivalent to or greater than the USP reference toothpaste and therefore satisfies this part of the monograph testing requirements. However, as the EFU method does not incorporate biological factors that in vivo would promote SMFP hydrolysis, it is not possible to make any inference from this study as to the efficacy of the Aquafresh Kids Bubble Fresh toothpaste relative to the USP reference toothpaste. Although not reported in this paper, the Aquafresh Kids Bubble Fresh toothpaste fulfils all testing requirements listed in the anti-caries monograph. The fluoride uptake results were consistent with those measured in the pH cycling studies.
Abrasives are added to toothpastes to remove plaque and further to remove the stained pellicle on the tooth surface and thus it is important to ensure that the abrasive will not cause mechanical damage to the teeth[39, 40]. In vitro abrasivity tests are routinely performed to provide information on the abrasive potential of toothpastes; however, the protective nature of the pellicle towards toothpaste abrasion means that any extrapolation of in vitro data to levels of in vivo abrasive wear should be treated with caution. While no specific REA and RDA limits have been established for children's toothpaste, ISO 11609:2010 sets toothpaste abrasivity limits of 250 for RDA and 40 for REA. As RDA is not a predictor of REA and vice versa, both abrasivity tests were conducted. In consideration of the differences between primary and permanent enamel with regard to its abrasion resistance, all children's toothpastes have been designed to have low levels of abrasivity. The results show that all toothpastes were below the recommended abrasivity limits and are therefore considered safe for everyday use.