The use of the proposed minimally invasive technique can not only induce recovery of the natural tooth appearance but also promote enamel remineralization in depth, so it may be considered a potential alternative to conventional operative treatment. The proposed minimally invasive technique mainly utilizes a combined approach of microabrasion and enamel remineralization [36]. Fluoride therapy has long been considered as the base non-invasive treatment for early carious lesions, although many defects exist in the use of fluoride. The low permeability of fluoride hinders elimination of the opaque whitish aspect, thus compromising esthetics [12]. Biotoxicity from the inappropriate use of fluoride may have adverse effects such as fluorosis [14]. CPP-ACP, a nanocomplex derived from milk, can restrict calcium phosphate growth to the critical size required for nucleation and subsequent precipitation [37]. In addition to its high safety level, CPP-ACP has demonstrated anticariogenic potential in the laboratory and human in situ experiments [38-40]. In conclusion, CPP-ACP has the advantages of maximum tooth substance conservation and excellent acceptance by patients. Thus, the aim of this study is to comprehensively evaluate the biological remediation effects of CPP-ACP on patients with WSLs in clinical and on artificial demineralized models in vitro. Several measurement indicators, including QLF detection, average surface roughness, and surface microhardness, can provide a comprehensive assessment of CPP-ACP in terms of form, aesthetics, and function restoration, respectively, which are important requirements for minimally invasive dentistry [8].
Experimental results in clinical [26, 27] indicate that, compared with placebo paste without CPP-ACP, CPP-ACP paste showed no significant advantage for the prevention of enamel demineralization. This can be attributed to the intervention of fluoride in the control group, which can facilitate calcium and phosphate diffusion into the WSLs to partially remineralize the crystalline structures. In order to remove this interfering factor, supplementary analysis without fluoride is necessary. Then the supplementary comparison between the CPP-ACP and baseline groups was performed. As shown in Fig. 3D, after the data from the two studies were pooled, no significant heterogeneity was found (Chi2 = 0.16, df = 1, P = 0.69, I2 = 0%). Meta-analysis demonstrated a significant difference between the CPP-ACP and baseline groups, as assessed by QLF values (SMD = −0.43, 95% CI: [−0.79, −0.07], P = 0.02). QLF meets the basic requirement for detection, quantification, and monitoring of carious lesions and is widely used in clinical trials to monitor WSLs as well as to investigate the efficacy of bioremediation. Based on the principal of detecting changes in fluorescence correlating to mineral loss, caused by caries destruction of enamel, QLF measurement can reflect changes in tooth enamel form [41]. Our results showed a significant improvement in WSLs regression by using CPP-ACP as assessed with QLF. In conclusion, this meta-analysis showed a noticeable improvement of WSLs remineralization as assessed by QLF, which means that CPP-ACP can accomplish enamel form recovery.
The analysis of average surface roughness [15, 24, 31] indicated that, with the help of CPP-ACP, the roughness of the enamel surface was decreased to a statistically significant degree. Atomic force microscopy (AFM) permits observation of the nanoscale appearance of softened enamel surfaces. The average surface roughness not only refers to aesthetic properties, but also reflects bacterial adhesion and plaque formation potential in the oral environment [42]. The enamel surface roughness measurement results confirmed that the center areas of enamel prisms were restored gradually by CPP-ACP induction, still according to the orientations of mineralized fibrils, until the enamel surface became flat and smooth [15]. This analysis of average surface roughness performed by AFM suggests that CPP-ACP has excellent ability to repair and smooth the surface of enamel, ultimately acquiring desirable aesthetic effects.
Regarding the assessment of function restoration, the analysis of SMH in vitro was performed. The SMH test offers a relatively simple, rapid, and non-destructive approach in demineralization and remineralization studies [43]. Different description units of SMH (Vickers hardness and Knoop hardness) could all induce heterogeneity within one study when comparing outcomes; therefore, we conducted subgroup analysis according to different testing methods, such that the heterogeneity reduced I2 from 77% to 16% and 0%, respectively (Fig. 3C). This indicated that the different testing methods were the main factors inducing heterogeneity, so the subgroup analysis permitted comparison of the outcomes. After CPP-ACP treatment and remineralization, the mean SMH values increased significantly compared to those of the control group, whether measured with a Vickers microhardness tester or Knoop hardness tester. This result can be attributed to the mineralization induction of CPP-ACP: after the localization of ACP at the enamel surface, free calcium and phosphate ions were buffered, thereby helping to maintain a state of supersaturation with respect to tooth minerals, depressing enamel demineralization, and promoting remineralization [37, 44].
Some potential limitations of the study should be addressed. Our meta-analysis, including average surface roughness and surface microhardness, was based on in vitro environments, which was not reproduced in clinical; therefore, many limitations were unavoidable. For instance, the effects of salivary enzymes, proteins, pellicle, dental plaque, and additional fluoride sources on demineralization and remineralization cycles in the oral environment were not included [45]. However, from another perspective, considering the good control of interfering factors for in vitro studies, the results may be more stable and convincing. Furthermore, there is considerable risk of lowering the quality of the evidence in surface microhardness analysis. The CPP-ACP group included both “GC Tooth Mousse Plus” and “MI Paste Plus,” which contains a small amount of fluoride, as expounded in Table 2 [37, 44]. However, certain studies [46, 47] have demonstrated that CPP-ACP combined with fluorides achieved no clinical advantage, so we combined the three experiments into our meta-analysis. Despite these limitations, the ideal treatment effect of CPP-ACP for WSLs remained evident.
Some guidelines may be helpful in clinical operation and future studies. When treating patients with WSLs, dentists are recommended to prioritize CPP-ACP, especially in children whose risk factors can be controlled adequately. Considering its preferable aesthetics effect, CPP-ACP is highly recommended to those who, with high aesthetic requirements, developed WSLs after orthodontic treatment. But one thing still needs to be pointed out, CPP-ACP is not of zero risk and absolute security. There have been reports of patient deaths from allergic reaction. When applied in clinic,we need to pay special attention to their system history and allergies. To evaluate the effect of remineralization in a more comprehensive way, further studies of CPP-ACP, especially in combination with new detection indexes both in clinical and in vitro, remain necessary. Considering the good advantage of CPP-ACP over traditional fluoride, the subsequent comparison between CPP-ACP and fluoride must be inevitable.