Diamond particles have found their way as abrasives into some novel toothpastes. This has raised some concerns regarding the abrasive effect of those particles on dental enamel. This study was therefore conducted to investigate the abrasive enamel wear caused by three novel toothpastes with diamond particles. The results show that diamond-loaded toothpastes cause higher abrasive enamel wear than traditional toothpastes and also show differences in the abrasivity behaviour between the diamond-loaded toothpastes. The null hypothesis was therefore rejected.
Eighty enamel samples were prepared from bovine teeth for this study. Bovine enamel has similar physical and chemical properties as human enamel, and is considered a suitable alternative in abrasion studies [12, 13]. Furthermore, the large surfaces of bovine crowns allow the preparation of several samples from one tooth. Although bovine enamel was found to abrade similarly as human enamel , some studies reported that both behave differently under other situations (e.g., when subjected to erosion) [14,15,16]. Therefore, it should be mentioned that bovine enamel might not function as a like-for-like replacement for human enamel in every situation. The samples were brushed for 21,600 cycles (60 cycles/min for 6 h). Taking the clinical recommendation to brush teeth for 2 min, this corresponds to 4-year clinical brushing time, when a patient brushes her/his teeth three times a day . The brushing force, at which the samples were brushed (2.5 N), lies within the brushing force applied by most of the abrasion studies (2–3 N) .
All tested toothpastes with diamond particles resulted in a statistically significantly higher abrasive enamel wear than the traditional toothpaste. This finding corresponds to the higher REA values also measured for these toothpastes and is attributed to the higher hardness of the diamond particles compared to dental enamel .
However, the fact that SSDG caused statistically significantly higher enamel wear than the other two toothpastes with diamond particles and the fact that CWD and EFD caused statistically comparable enamel wear in this study cannot be supported by their recently measured REA values. In the study of Hamza et al.  CWD had the highest REA value (244 ± 76) followed by SSDG (177 ± 70) and EFD (51 ± 25). On one side, some discrepancy between the radiotracer and the here used profilometry method has already been reported [4, 8], which is corroborated by the findings of the present study. On the other hand, it could also be argued that the manufacturers might have altered the amount and/or the properties of the diamond particles utilised in their toothpastes during the last two years leading to the current—different—abrasive behaviour. Therefore, the higher abrasive wear caused by SSDG could be attributed to more incorporated diamond particles or different diamond particle sizes. Furthermore, the abovementioned sharp edges observed in the abrasives utilised in SSDG might have also contributed to this observation . The fact that SSDG utilises hydroxyapatite would rather not explain its higher abrasive behaviour compared to the other diamond toothpastes as the latter is a medium-hard abrasive and softer than enamel .
In general, it is safe to assume that the profilometric method of measuring a toothpaste’s abrasivity is much simpler than the radiotracer method (REA/RDA). It does not involve any neutron bombardment of the samples, and thus could be considered environment-friendlier . Furthermore, the profilometric method has the advantage of directly measuring the abrasive wear compared to the indirect—relative—measurement provided by the radiotracer method . However, González-Cabezas et al.  stated that the radiotracer method offers lower variations than the profilometric method (i.e., is more robust) and that the latter still needs to be better developed and refined.
Based on the present results, the diamond toothpaste with the highest enamel abrasivity (SSDG) would cause 4.75 µm enamel loss in one year. The time needed to cause 1 mm enamel loss would hence be more than 200 years. This seems safe (in terms of not completely abrading enamel and exposing dentine in a lifetime) also when taking the relatively thin enamel layer (≤ 1 mm) at the cervical third of human tooth crowns into consideration [21, 22]. However, other factors related to toothbrushing, which could increase the abrasive wear (e.g., the stiffness of the used toothbrush, the applied brushing force, using a sonic toothbrush) should be kept in mind. Another factor to be considered is the mineral quality of the enamel being brushed. Wegehaupt et al.  reported that a diamond-containing toothpaste (CWD) caused higher abrasive enamel wear when the enamel was previously eroded compared to sound enamel.. Regardless, a toothpaste abrasivity should always be kept in line with the cleaning efficacy it offers. A recent study reported similar cleaning efficacy of a diamond toothpaste to traditional toothpastes . In this regard, other factors (e.g., the toothpaste abrasivity towards dentine, effect on the surface gloss and roughness of enamel and composite restorations) should also be kept in mind when advising patients. For instance, a recent study reported that EFD presented better gloss values than CWD when both toothpastes were used to brush composite restorations in vitro . The authors also attributed the finding to possible differences in diamond particle size and concentration.
One of the limitations of the present study is that the sample size was not calculated, which presents a limitation in interpreting the results . Furthermore, the present study only investigated the abrasive enamel wear caused by diamond-containing toothpastes using a medium-bristles toothbrush. Future studies should consider investigating dental plaque cleaning efficacy and the surface roughness offered by diamond-containing toothpastes in comparison to traditional toothpastes. Investigating the interplay between factors that can influence the abrasive enamel wear (e.g., stiffness of the toothbrush, brushing forces, other toothpaste active ingredient, etc.) would also be beneficial.