HA discs have already been used to replace enamel surfaces in past studies (10–14). They are produced in a standardized manner, all have an identical surface texture, and are unaffected by external influences as they exist in the oral cavity prior to extraction. In the study by Imthiaz et al. 2008 (15), HA discs and human enamel were compared, and it was concluded that HA discs can be used as an alternative to enamel in comparative laboratory studies. Each disc was bonded with a conventional metal bracket and a piece of arch-wire was ligated in order to simulate a daily clinical situation.
The superiority of electric toothbrushes over manual toothbrushes in patients undergoing orthodontic treatment has been demonstrated several times (16, 17). Erbe et al. (16) found that electric toothbrushes were significantly superior to manual toothbrushes in terms of plaque removal in patients with fixed orthodontic appliances. In 2010, Silvestrini et al. (17) declared that patients with MBA who used an electric oscillating-rotating toothbrush displayed a more positive impact on plaque levels and gingival bleeding than those using a manual toothbrush.
To ensure that all specimens were brushed uniformly and comparably, the brushing intervals were performed by a toothbrush simulator. The use of a toothbrush simulator is standard in study models such as this one and is repeatedly described in the literature (18–21). The American Dental Association® (ADA) (22) recommends brushing twice a day for two minutes. In this study, the brushing time was selected based on the time module established by Deckers et al. (9). In calculating this time module, an average of two minutes of brushing is assumed. This is consistent with the recommendation of the ADA (22) and the studies of Cronin et al. 2002 (23) and Hickman et al. 2002 (24). The time module provides for a brushing duration of 126s for the simulation of toothbrushing over six weeks and a brushing duration of 504s for six months.
One toothpaste was used for all brushing intervals, therefore, the abrasiveness can be neglected when comparing the change in layer thickness of the sealants investigated. No toothpaste slurry was prepared, following Behnan et al. 2010 (25) and Deckers et al. (9). Between 0.3 and 0.4 g of toothpaste were used for each brushing session. Since the amount of toothpaste was not exactly the same for each cleaning interval, there may be minimal differences in the abrasiveness of the cleaning process on the sealant.
MP and OP are measurement methods widely used in the literature, and are frequently reported techniques in the study of tooth surfaces (26–29) (25, 30–32). The measuring positions of the profilometric investigations on the HA disc (transition from sealed and unsealed area) were individually selected by the examiner for each specimen and were not identical for MP and OP. Thus, possible changes of the sealant caused by the contact of the stylus during the MP measurement cannot influence the optical measurement. Overall, there are significant differences between the sealer thickness values when comparing both measurement methods. However, when the change of the sealer thickness is considered, as is sufficient for the purpose of this study, this method effect does not apply. The significantly different level of coating thickness is largely cancelled out by the difference. Therefore, one measurement method is sufficient for studies of this type.
There are a wide range of sealants available on the dental market. The literature contains numerous in vitro as well as in vivo studies due to sealing the area around the bracket base. In several studies Tetric EvoFlow® has shown a good protective effect against abrasive and erosive influences, as well as longevity (9, 33–35) and has therefore been selected as the positive control group/ material in this study.
The lowest material loss of the sealants tested in this study was recorded with Pro SealTM. It was 2.0% (MP) and 12.1% (OP) after six weeks and 8.7% (MP) and 22.0% (OP) after six months. These values testify to a high mechanical load-bearing capacity of the material used. The manufacturer of Pro SealTM promises "protection over the entire treatment period", which, according to the available results, is very likely. Many studies describe comparable results and list Pro SealTM as the best of all the materials tested. In 2011, Shinaishin et al. (36) conducted an in vivo study on the effectiveness of light-activated sealants in regards to protection against enamel demineralization. The Pro SealTM group achieved the lowest degree of roughness and the lowest total surface area. The characteristics of the sealant surface structures were comparable to those of ordinary human enamel. Based on these results, the use of Pro SealTM was concluded to be an efficient prophylactic measure to reduce enamel demineralization. The results of the in vitro study by Buren et al. 2008 (37) also confirm that Pro SealTM can reduce the average lesion depth by up to 97% compared to the control group. Hu and Featherstone 2005 (38) demonstrate that after mechanical and chemical loading, Pro SealTM records significantly less material loss than the other sealants.
Deckers et al. (9) investigated similar sealants to those tested in this study. They were applied to extracted bovine teeth in the bracket environment and subjected to mechanical, chemical and thermal stress. The material defects of Pro SealTM were the lowest of all the materials tested, being comparable to the control group Tetric EvoFlow®.
In the current study (9), the sealant Light BondTM achieved a higher overall material loss. This was 9.2% (MP) and 12.2% (OP) after six weeks and 18.7% (MP) and 16.1% (OP) after six months. The MP measurements in particular showed a significantly greater loss of material than with Pro SealTM. The surface was still sufficiently sealed after six months, which can be assigned to a mechanical load-bearing capacity (wear resistance) of Light BondTM. The more than one year durability of the sealant promised by the manufacturer is very likely. Light BondTM showed good properties against mechanical stress in this study. This is largely consistent with the results in the literature. In their 2009 in vitro study, Tanna et al. (39) investigated the effect of sealants and self-conditioning primers on enamel demineralization. Enamel lesions were observed in 50% of the Light BondTM samples, with an incidence of 100% in the primer and control groups. Heinig and Hartmann (40) confirmed the protective properties of Light BondTM against enamel demineralization in their clinical study on the effectiveness of a sealant. In contrast to the unsealed teeth, significantly fewer areas of shallower depth were affected by demineralization. In 2013, Korbmacher-Steiner et al. (41) investigated the abrasion resistance of four different sealants to toothbrush and toothpaste in vitro. In the case of LightTM Bond, defects in the sealant layer with a diameter of up to 300 µm were observed after two years. Bechtold et al. (42) investigated Light BondTM in a clinical study in 2013. No caries-protective effect was observed after six months. Deckers et al. (9) recorded a significant material loss of 38.4% after six weeks and 39.7% after six months in his study. These values are significantly higher than the results obtained in the present study for the change in layer thickness (i.e. material loss).
A high material loss was observed for ClinproTM XT Varnish. It was only possible to measure two of the eight samples, as the varnish layer was already very brittle during the initial measurement and began to detach from the HA surface. The other six samples could not be measured because an air gap was already initially present between the HA and the sealant. Due to the small number of samples, these values are not representative. The material loss was 36.8% (MP) and 51.1% (OP) after six weeks of simulated brushing time and 75.3% (MP) and 66.6% (OP) after six months of simulated brushing time. This result indicates that the material is susceptible to mechanical stress, even after a short period of time. According to the manufacturer, protection should be expected for six months. This was proven by the two measurable samples.
The technical literature contains numerous studies describing contrary results for ClinproTM XT Varnish. In 2015, Kumar Jena et al. (26) investigated this sealant in patients undergoing early orthodontic treatment with fixed appliances regarding protection against the development of WSL. Compared to the untreated control group, significantly fewer WSL were observed in teeth sealed with ClinproTM XT Varnish. Mehta et al. (43) examined 126 extracted premolars for WSL in their in vivo study from 2015. Except for three teeth, no demineralization lesions were detected on the sealed premolars. The sealant also achieved better results than in our study with regards to mechanical loading capacity. Deckers et al. (9) described a material loss of 1.4% after a toothbrushing simulation of six weeks and a material loss of 1.9% after six months. This material loss is not comparable to the values in the current study. It is conceivable that the storage period between sealing and initial measurement was too long, which results in changes of the material properties.
In the case of Protecto® CaF2 Nano and Fluor Protector, the entire coating layer was removed after a rendering period of only six weeks. The layer thickness was initially not measurable with the applied methods, as it was too low. Even under optimum in vitro conditions and after repeated application of the coating, no sufficient layer was formed to protect the surface adequately. It is conceivable that due to the low viscosity of these sealers, the material is penetrated in the pore-like structure of the HA discs.
The results of Protecto® CaF2 Nano are also repeatedly confirmed in the literature. In their 2015 study, Paschos et al. (6) demonstrated a protective effect against demineralization in contrast to the untreated group, but not as effective as ProTM Seal. In their clinical study from 2013, Bechtold et al. (42) found no difference in the protective abilities against demineralization between Protecto® CaF2 Nano and Light BondTM. The results of Deckers et al. (9) also demonstrate insufficient protection against mechanical stress. In their trials, material loss was 91.7% after six weeks and 93.9% after six months.
The mechanical load-bearing capacity of Fluor Protector was also insufficient in this study. This is partially confirmed in the literature. Van der Linden and Dermaut (44) were unable to demonstrate any significant protection against the development of WSL in combination with the glass ionomer cement in 1998. Bichu et al. (45), on the other hand, showed the lowest average lesion depth in the group treated with Fluor Protector after exposure to the demineralization bath. The results of this study are in favor of demineralization protection by Fluor Protector. The in vivo study by Shafi 2008 (46) also confirms that Fluor Protector reduces the risk of developing WSL during treatment with a fixed orthodontic appliance. Deckers et al. (9) recorded a significant loss of material after mechanical loading in his study. This was 40.3% after six weeks and 64.5% after six months. Thus, the material loss was lower than with Protecto® CaF2 Nano.
The control group Tetric EvoFlow® recorded a material loss of 12.8% (MP) and 15.1% (OP) after 6 weeks and 29.3% (MP) and 20.5% (OP) after 6 months. The material losses of the sealants can be summarized and ranked for abrasion resistance to mechanical loading: Pro Seal™ > Light Bond™ > Clinpro™ XT Varnish The mechanical resistance of Protecto® CaF2 Nano and Fluor Protector cannot be conclusively assessed.