The alternative hypothesis of this study was supported as charcoal toothpaste increased enamel surface roughness compared to the conventional toothpaste and the negative control, especially when the bristles’ stiffness was hard. The enamel surface roughness change following the application of the charcoal toothpaste was comparable to the whitening toothpaste, suggesting that both may induce harmful effects against the enamel surface. The results of our findings suggest that people should be cautious when using dental products containing charcoal or whitening ingredients for an elongated period as they may abrade the enamel.

The number of toothbrushing cycles applied in this study served as a prolonged representation of toothbrushing. In real-life situations, toothbrushing usually occurs at a rate of 4.5 strokes per second [17]. Since 2 min is the suggested brushing time to eliminate plaque [17], it is expected that around 90 strokes are needed in each sextant. This equates to 15 brushing strokes per tooth or 5 brushing strokes per surface (buccal, lingual, occlusal) [17]. Consequently, if patients brush their teeth on three occasions a day, a total of 5475 strokes would be accomplished on a particular surface in a year. For this study, simulating approximately three and six months, the number of brushing cycles was set at 1,250 and 2,500, respectively. These durations are expected to induce initial changes in the enamel surface. However, it is important to note that the parameters used in this study, as well as the number of cycles, are limited by the absence of saliva, which serves as a potential remineralizing reservoir, and the salivary pellicle, which acts as a protective layer.

In several in vitro studies, the brushing force used was between 0.2 and 4.2 N, with an average between 2 and 3 N [3, 8, 14,15,16, 20,21,22,23]. In this study, we applied a force equivalent to 2 N to be convenient with the current literature. The Charcoal and whitening toothpastes experienced the highest increase in the Ra value, which is a measure of surface roughness. This increase was mainly attributed to the presence of abrasive materials in the toothpaste. Apart from silica and hydrated silica, Charcoal Formula toothpaste also contains activated carbon or charcoal. Charcoal particles in the toothpaste have a star-shaped or fractal shape, which may contribute to its abrasivity [24].

We intended in this study to use toothpastes with comparable ingredients to emphasize the possible impact of the charcoal powder in the charcoal toothpaste. Statistical analysis comparing the conventional toothpaste (positive control) and the charcoal toothpaste, which have similar types of abrasive materials, revealed significant differences in the changes of Ra values (surface roughness) after three months of brushing. This suggests that the charcoal component in the toothpaste may have played a role in altering the surface roughness of the tooth enamel despite the similar types of abrasive materials used in both toothpastes. It is also possible that the size and quantity of silica particles differ between the two toothpastes. Larger and more abundant silica particles may induce greater surface changes [25]. This is more likely when comparing the ingredients of the conventional and whitening toothpastes, as the only difference between them is the number of pigments, which probably have little to do with the abrasivity of the toothpastes. The relative dentin abrasion values (RDA) of the conventional and whitening toothpastes used in this study were found to be 70 and 124, respectively [26]. This suggests that the conventional toothpaste used here has low abrasivity, while the whitening toothpaste has high abrasivity, mainly related to the size and number of silicas. No available information concerning the RDA value of the charcoal toothpaste used in this study, but it is expected to be close to the whitening toothpaste based on the obtained results.

This study also suggests that the use of medium and hard bristles may lead to greater enamel wear compared to soft bristles. While dental practitioners typically recommend soft-bristled toothbrushes to their patients, it is essential to investigate the extent of enamel wear associated with medium and hard bristles. Additionally, the potential interactions between these bristles and toothpastes with varying ingredients and abrasivity warrant exploration. Interestingly, the effects of medium and hard bristles may be comparable to those of soft bristles when used with low-abrasive toothpaste, which was also demonstrated in the study by Turssi et al. [17]. Therefore, a key objective of this article was to examine the interactions between the various toothpastes investigated and the stiffness of different bristles.

The results of our study agree with previous investigations showing that charcoal toothpaste may induce enamel surface roughness [20, 22, 27, 28]. However, our results contraindicate other findings, revealing no difference in enamel wear subjected to conventional and charcoal toothpastes [15, 16]. The conflicting results in the existing literature may arise from several methodological differences, including variations in brushing force applied during the experiments, which can significantly influence enamel wear. Additionally, the number of brushing cycles used in each study varies, potentially leading to differing levels of abrasion severities. Furthermore, the types of dentifrices utilized, each with distinct formulations, active ingredients, and abrasive properties, can also contribute to the discrepancies observed in the outcomes. Besides, this controversy could be attributed to the use of one type of bristles stiffness, which could be an important modulator in the wear of enamel [25, 29]. These factors underscore the need for standardized methodologies to allow for more reliable comparisons across studies.

Therefore, we intended to use different bristles’ stiffness in this study with clinically relevant parameters to investigate the interaction between the charcoal toothpaste and the stiffness of the bristles. This study found a significant difference in enamel surface roughness between conventional toothpaste versus charcoal and whitening toothpastes when used with hard and medium bristle toothbrushes. However, only a slight difference was observed when using soft bristles. These findings suggest that the risk of enamel wear may be greater when using charcoal and whitening toothpastes in combination with hard or medium bristle toothbrushes, especially with long-term use. While some may wonder if it is safe to use charcoal toothpaste even for a short period, it should be noted that despite the abrasiveness of charcoal toothpaste, which can remove extrinsic stains, it does not provide any intrinsic whitening benefits and can still harm the enamel surface, as indicated by the results of this study. Additionally, there is growing evidence that charcoal toothpastes are not particularly effective at improving teeth color [14, 15, 21, 22]. Therefore, it may be advisable to avoid the use of charcoal-based toothpastes and instead opt for home or professional bleaching/whitening treatments for safe and effective teeth whitening.

Our results suggest that individuals should be cautious when using highly abrasive toothpastes and stiff-bristled toothbrushes, as they may have significant adverse effects on an individual’s oral health. One of the primary concerns is the gradual wear down of the tooth enamel over time. Tooth enamel is the outermost, hardest layer of the tooth, and it plays a crucial role in protecting the underlying dentin and pulp. However, the aggressive abrasive action of these dental products can gradually abrade the enamel, leading to increased sensitivity [30, 31]. This increased sensitivity can make everyday tasks like eating and drinking a challenging and unpleasant experience. Furthermore, the loss of enamel can also compromise the aesthetic appearance of the teeth, potentially leading to an aged or unattractive look. Another significant consequence of using abrasive toothpastes and stiff-bristled toothbrushes is the increased risk of gingival recession [32]. To mitigate these issues, it is recommended that individuals use a soft-bristled toothbrush and a non-abrasive, fluoride-containing toothpaste with proper brushing technique. Besides, seeking professional advice from dental practitioners can ensure using the most effective tools for oral hygiene practice.

This laboratory study yielded significant findings regarding the impact of charcoal toothpaste and toothbrushes with varying bristle stiffness on enamel surface topography. However, there are some limitations that should be acknowledged. While efforts were made to select and polish the teeth within specific criteria, it is impossible to perfectly standardize the baseline surface roughness and mineral content of the enamel samples [33, 34]. This variability in the starting conditions of the samples could have introduced some inherent differences. Furthermore, various patient-related factors, such as individual differences in oral health status, overall physiological conditions, and general health, can significantly influence the enamel’s response to the wear challenge. These clinical variables are not easily replicated in a controlled laboratory setting. Besides, in the oral environment, the formation of salivary pellicles over the tooth surface may potentially reduce the degree of wear induced by toothbrushing [4, 5]. Putting all these factors into consideration in addition to the need to investigate other commercially available charcoal-based toothpastes, the results obtained here can not be generalized. It is crucial to validate the findings of this in vitro study through well-designed in-vivo models. Despite these limitations, the current study provides valuable insights into the potential abrasive effects of charcoal toothpaste and different bristle stiffnesses on enamel topography. Further clinical research is warranted to fully understand the implications for dental health and to establish safe and effective oral hygiene practices.