A range of novel caries-preventive and arresting strategies based on both inhibition of demineralization and facilitation of remineralization are available. Especially in younger children and primary teeth, the availability of efficacious alternatives to fluoride are of relevance given concerns of acute and chronic toxicity of fluoride and the associated negative public sense . SAP have been proposed as one such alternative. We compared a range of novel strategies suggested to prevent caries and inhibit lesion progression against each other and tested SAP for their effects on primary tooth enamel in vitro. For preventing caries lesions, we found established means like FV and FMW to be effective to reduce mineral loss, while novel strategies like SAP, CPP-ACP, CPP-ACPF and nHA did not have any significant preventive effect in vitro. For inhibition of lesion progression, we found RI to most effectively inhibit the lesions from progressing, followed by FV, while again novel strategies like CPP-ACPF and SAP did not have any significant inhibition of lesion progression effect in vitro. We hence reject our hypotheses.
This study has a number of strengths and limitations. First, data on novel strategies, especially SAP, are scarce, and so far, no study tested SAP to prevent caries or inhibit lesion progression in primary teeth. Previous studies on SAP focused on human enamel from permanent teeth or bovine enamel. Both show different mineralization and maturation potential than human primary tooth enamel. Second, SAP have largely been tested for lesion arrest (i.e. facilitation of remineralization), not prevention , and this study is one of few testing this novel strategy for preventive applications as well as caries arrest. Third, the assessment of mineral loss using transverse microradiography is highly sensitive and a valid method that was not employed in previous studies on SAP. Instead, scanning electron microscopy (SEM), surface microhardness or laser fluorescence, which are all only determining proxies for mineral loss [26, 27] had been used. Fourth, and as a limitation, the employed in vitro protocol may have biased our findings to some degree. Notably, grinding and polishing the enamel surface has removed the aprismatic enamel surface layer which is hypothesized to be required for SAP and SAPR action: On prismatic enamel, columnar calcium of the hydroxyapatite crystal is not available any longer; it is assumed that peptide matrix development may be impeded to some degree in prismatic compared with aprismatic enamel. We nevertheless used the described setup, as only then valid mineral loss measurements in TMR are possible. Moreover, and notable, the aprismatic outermost layer of enamel is usually gradually worn off in a clinical setting, too, at least occlusally, and any clinical efficacy of self-assembling peptides might be reduced if strongly relying on this aprismatic enamel being available. In addition, previous studies had by large employed polished specimens, which is why we aimed to retain this concept for reasons of comparability [26, 27, 25]. Fifth, we applied SAP only once, i.e. before the 14d pH cycling period for reasons for standardization (other materials, e.g. FV, were also only applied once). The manufacturer recommends applying it 1–2 times per week. Also, we neither employed human nor artificial saliva, which has been suggested to be required for full action of SAP, as the 3-dimensional matrix formed by the peptides increases the surface area for calcium and phosphate deposition, present in the saliva, thereby allowing the formation of de novo hydroxyapatite crystals [13, 14, 12]. Application frequency and the availability of saliva might explain the differences between our findings and those yielded in situ and in vivo (8), with saliva also being a relevant source of mineralization related to fluoride applications, for instance. Furthermore, we employed an etching step when testing lesion arrest using SAP to mimic its clinical application. Etching of our polished samples might have removed the remaining pseudo-intact surface layer of the lesion claimed to be needed for SAP to effectively remineralize the enamel. The etching step was suggested in vivo to clean the pseudo-intact surface of enamel from pellicle and remove mineral debris; both are not present in artificially induced enamel lesions.
A range of findings needs to be discussed. SAP were not effective to prevent caries or inhibit lesion progression. A limited body of evidence on SAP is available, as discussed, and our findings align with some, but not all of the reported studies. This might be partially due to the mentioned methodological reasons (application time and frequency, absence of aprismatic enamel, lack of saliva and pellicle formation). However, especially for SAP, there remain a number of questions towards its hypothesized preventive mechanism: SAP are designed to assemble in the acidic environment of an active caries lesion and then attract minerals present in human saliva. It is unclear how this mechanism should apply to prevent lesions. In our study, it cannot be excluded that the material was washed away during the first demineralization cycles, or that only very thin peptide layers formed on the sound enamel surfaces, possibly insufficient to protect the enamel from subsequent demineralization.
Topical fluoride is considered the gold standard for caries prevention and lesion arrest (9). This was confirmed by our study. There is evidence suggesting that SAP should be combined with fluoride to harness their complementary mechanism and location of action when it comes to inhibiting lesion progression (fluoride mainly acts on the pseudo-intact surface of initial lesions (9), while SAP is suggested to diffuse into the subsurface body of the lesion (1)). In our study, a possible advantage of the used FV was its consistency and stickiness. It is possible that FV not only had a chemical but also mechanical effect by “sealing” the surface, thereby protecting it from demineralization. Such sealing effect has been described for FV in studies on root caries prevention . Given that we also found FMW to be efficacious, this explanation may not fully apply, though (notably, however, FMW was provided daily in contrast to most other alternatives).
RI is well known for its lesion progression inhibition in non-cavitated lesions by infiltrating carious enamel porosities and thereby occluding the diffusion pathways leading to caries arrest . RI has been demonstrated to be superior to fluoride-based alternatives for lesion arrest by a range of clinical studies , also in the primary dentition , and our data also point into this direction. Notably, the application of RI is technique-sensitive, something which may be relevant especially in the primary dentition and in children.
CPP-ACP and CPP-ACPF did not show any caries preventive or lesion progression inhibiting effect in our study, which may be attributed to the fact that their effect is thought to be enhanced by the presence of a biofilm, which acts as a reservoir for the delivered calcium and phosphate ions and hence prevents mineral loss in intermittent periods of demineralization. Moreover, and as discussed for SAP, the application frequency of CPP-ACP and CPP-ACPF might have been insufficient.
Based on our results, a range of future directions can be derived. First, future in vitro studies assessing SAP should aim to mimic the aprismatic enamel layer even when using ground specimens. Second, the application of saliva prior to remineralization may be recommended in an in vitro setting to allow pellicle formation and simulate clinical conditions as far as possible. Alternative, in situ designs may be employed. Third, mineral loss measurement should not focus on artifact-prone methods (like laser-fluorescence) or unsuitable proxies (like SEM evaluation), but strive to truly determine the mineralization effects, for example using TMR, transverse wavelength-independent microradiography or micro-CT. Last, clinical studies should be employed before translating our findings into any clinical recommendations; so far, clinical data largely focused on lesion remineralization, as described.