## 4 Discussion  
In the present work, a possible advantage correlated to the synergic use of Elm-EP and GC-TM, two commonly used home-devices acting in different mode on eroded tooth surface, was investigated. In order to mimic conditions of patients afflicted by diseases such as GERD and BN, the specimens were retrieved from frontal dental elements, as they represent the most affected elements by erosion. Furthermore, the acid experimental test was extended to 60 min to reach partially exposed dentine tubules, reproducing the situation of critically eroded tooth surfaces. The specimens were stored in artificial saliva both after preparation and during the entire treatment period reproducing the natural environment of the mouth. As negative reference group (G0), the specimens where stored in remineralizing saliva for 15 days, while as positive reference (G1) the specimens were treated with the sole toothpaste Elm-EP, which efficacy as anti-erosion agent has been largely assessed and reported in literature [8; 9; 11]. The two products were applied on the specimen surfaces following the methodologies suggested by the manufacturer. The results obtained in this study highlight that the combined use of Elm-EP and GC-TM for 15 days on critically eroded specimens, allow to greatly decrease the surface roughness. Even though, the Sa values of the specimens treated with Elm-EP or its combination with GC-TM were not statistically different, the calculated percentage of decrease in roughness at T2 compared to T1, resulted 42% and 50% for G1 and G2 respectively, confirming the promising action of the combined agents. Together with the morphologic analysis obtained by SEM evaluation, the profilometric analysis allows to speculate about the formation of a protective layer obtained by the Elm-EP active components in which some CPP-ACP clusters inserted, contributing to the increase of the surface uniformity. Considering the severe erosion conditions applied to the specimens, not surprisingly, 15 days of treatment with the studied agents proved to be insufficient to restore the baseline Sa value, but as the action of both Elm-EP and GC-TM is time-dependent, it is possible to hypothesize a gradual decrease of the roughness by increasing the treatment period. The evaluation and comparison of the released calcium concentration during the two acid attacks gave some information about the erosion-protecting attitude of the considered agents: the solubilized calcium titrated at T3, in case of the positive reference group G1, was greatly lower (44%) compared to that at T1, confirming the well-known ability of Elm-EP as anti-erosive system [9]. However, G1 specimen surfaces appeared less homogeneous at T3 in comparison to the baseline and T2. Differently, in case of G2, the concentration of Ca\({}^{2+}\) lost during the second erosion was only 22%, lower than that titrated at T1. It is possible to speculate about this result comparing the SEM imagines regarding G2 at T2 and T3: after the second erosion, the most part of the CPP-ACP clusters, well visible at T2, disappeared from the specimen surfaces, suggesting their dissolution in the acid media. Bearing in mind that the CPP-ACP nanocomplexes are meaning to act as Ca\({}^{2+}\) and PO\({}_{4}\)\({}^{3-}\) storage-release systems, it is plausible to consider that the most percentage of the calcium released and titrated at T3 originated from their dissolution instead of that of the tooth surface. Nevertheless, G2 specimens showed still occluded dentine tubule after the second drastic acid erosion, confirming, ones more, the remineralization process of the two combining agents.