Impact of transparent tray-based application of bioactive glasses desensitizer on the permeability of enamel and dentin to hydrogen peroxide: an in vitro study

Tooth whitening is any process that lightens the color of teeth. Whitening could be achieved either by physically removing discolored dental hard tissues or chemically bleaching stains. Compared with resin or ceramic veneers and all-ceramic crowns which could physically replace discolored hard tissues, chemically tooth bleaching is considered to be the simplest, most cost-effective and least invasive method of treating colored teeth [1‐4]. Therefore, it is now widely applied.

The active ingredient in most tooth bleaching products is peroxide in the form of hydrogen peroxide (H₂O₂) or carbamide peroxide. For example, in-office used bleaching products are highly concentrated (35% or 40%) H₂O₂ solutions or bleaching gels containing 5.9% of H₂O₂. Over-the-counter products are usually liquid gels containing a low concentration (10%) of H₂O₂ [1, 3, 4]. Peroxides are highly reactive and could convert to superoxide (O²⁻), hydroxyl (HO-), peroxyl (ROO-) or alkoxyl (RO-) radicals in physical environments. These oxidative oxygen species can freely penetrate dental enamel, react with discolored substances and exert their bleaching or whitening effect. However, side effects like pulp damage would occur when H₂O₂ and oxygen species are present at high concentrations or when oral tissues are exposed to them for a long period of time [5]. H₂O₂ penetrates dentinal tubules and damages the pulp based on its low molecular weight and the ability to induce protein denaturation [6]. Similarly, the presence of reactive oxygen species in the process of tooth bleaching can lead to temporary inflammations of the pulp and even irreversible changes [7‐11].

Since H₂O₂ and other reactive oxygen species come into contact with the pulp after penetrating dentinal tubules, attempts to occlude dentinal tubules prior to the application of tooth bleaching products might prevent post-treatment complications from occurring. Hydroxyapatite, the main component of dental hard tissues, is considered as the most ideal material for occluding dentinal tubules. As have been demonstrated in other studies, dentin desensitizers containing bioactive glasses (BGs) could release calcium and phosphorus ions in physiological environments and induce the formation of hydroxyapatite deposit within dentinal tubules [12‐14], which could prevent H₂O₂ and other reactive oxygen species from penetrating dentinal tubules and reaching the pulp. Hydroxyapatite is considered to have the potential to restore enamel surface, which might also keep the reactive bleaching oxygen species from penetrating the enamel [6]. Moreover, nanoscaled hydroxyapatite contained in a desensitizing paste was demonstrated not to jeopardize its whitening effect [15]. Thus, the application of dentin desensitizers containing BGs prior to tooth bleaching is an ideal method for dealing with post-bleaching complications. However, most dentin desensitizers are administrated by simply smearing on teeth. Due to poor patient compliance and short exposure times, their dentinal tubule occluding effects are limited. Therefore, it is expected that a method capable of improving the dentinal tubule occluding effect of dentin desensitizing agents, particularly before the application of dental bleaching products, can be developed.

The aim of this in vitro study is to investigate the impact of BGs desensitizer on the permeability of enamel and dentin to H₂O₂. In the present study, BGs desensitizer was dispensed onto tooth specimens with custom made transparent trays which were commonly used clinically as orthodontic retainers or tooth whitening products containers. Transparent trays here were used in order to obtain enough contact time and an effective concentration of the BGs desensitizer. The null hypotheses were (1) BGs desensitizer had no significant effect on the permeability of dentin and enamel to H₂O₂ and (2) customized transparent trays had no significant effect on improving the efficiency of the BGs desensitizer on reducing the amount of H₂O₂ infiltrating into pulp chambers.

As is shown in Table 1, after tooth specimens were immersed in DW for 30 min at 37 °C, regardless of what kind of surface treatment was applied, hydrogen peroxide was not detected in pulp chambers in control groups (Group A, E, and F). In experimental groups (Group B, C, and D), on the other hand, pulpal H₂O₂ was found. The amount of pulpal H₂O₂ in Group B, Group C, and Group D are shown in Fig. 3. The results one-way ANOVA analysis and LSC post hoc tests indicated that significant differences existed between these groups (F = 459.748, p < 0.05).

Peroxides such as hydrogen peroxide and carbamide peroxide used to bleach discolored dental hard tissues have been shown to have the ability to penetrate enamel and dentin, which is determined by several factors including the concentration of peroxides, the exposure time of teeth to peroxides, cracks and restorative materials existing on teeth, and etc. [18‐23]. As was shown in previous studies, more than 66% of patients suffered from dentin hypersensitivity or gingival irritation after tooth bleaching [9]. Other studies have also reported that the infiltration of peroxides into the pulp could lead to pulpal inflammation and necrosis [24, 25]. Although at present, no literature has given the concentration of H₂O₂ in the pulp that can lead to dentin hypersensitivity of post-bleaching pain of patients, in a study using bovine teeth, bleaching agents using H₂O₂, the concentrations of which were 20 and 35%, as their main components were shown to lead to moderate or severe inflammation in the upper thirds of the coronal pulp. The authors concluded that in-office bleaching protocols using lower concentrations of H₂O₂ should be preferred [26]. Since tooth bleaching agents are thought to infiltrate into the pulp through dentinal tubules, the application of dentin desensitizers which could occlude dentinal tubules is, therefore, an effective option to help dentists deal with dentin hypersensitivity and pulpal damage emerging during or after tooth bleaching [27‐29]. Dentin desensitizers containing potassium nitrate or sodium fluoride are widely used to solve dentin hypersensitivity, especially before or after tooth bleaching. They are also added into bleaching agents to alleviate pulpal responses without jeopardizing the tooth bleaching effect. However, desensitizing agents which contain potassium nitrate relief pulpal symptoms primarily by reducing the activity of dentinal sensory nerves, which is the main effect of potassium ions [30]. As a result, they cannot help us prevent the infiltration of peroxides into pulp tissues from occurring or reduce the amount of pulpal peroxides. Thus, they are not ideal for us to solve post-bleaching pulpal damage. Sodium fluoride, on the other hand, alleviates dentin hypersensitivity by inducing a calcium fluoride layer which covers enamel and exposed dentin [30]. However, as this layer could be easily removed by tooth brushing, the utilization of sodium fluoride is limited. Therefore, it is urgent that better products or methods be developed.

Bioactive glasses (BGs), the main component of which is CaO-Na₂O-P₂O₅-SiO₂, were developed in Professor Hench’s laboratory at the University of Florida in 1969 and were later known as 45S5 or Bioglass® [31]. Due to the ability in inducing the formation of hydroxycarbonate apatite (HCA) in physiological conditions, BGs are comprehensively studied in the fields of tissue engineering, bone regeneration and dentin remineralization [31‐33]. BGs have been demonstrated to occlude dentinal tubules by inducing hierarchical mineralization via the precipitation of apatite composed of calciumsilicate in dentinal tubules [34]. Our previous study showed that dentinal tubules, which were in tooth specimens treated twice a day with BGs desensitizing paste and immersed in artificial saliva for 7 days, could be completely occluded with calcium-deficient hydroxyapatite deposits to a depth of 5.5–8.1 μm [14]. Therefore, BGs desensitizers would be ideal to alleviate dentin hypersensitivity, especially when it happens during or after the process of tooth bleaching.

Although BGs desensitizer is considered to be an ideal material to treat dentin hypersensitivity, in clinical practice, most dentin desensitizers including BGs desensitizing paste are administrated by patients themselves according to the manufacturer’s instructions after the first application carried out by dental practitioners. Poor patient compliance and short therapeutic exposure will lead to relatively limited dentinal desensitization and dentinal tubule occlusion effects. In order to deal with this condition, a transparent tray which could hold the desensitizer for a while in patients’ oral cavities is considered. In our previous research, better dentinal tubules occluding effect could be guaranteed by using BGs desensitizer dispensed in a transparent tray [14]. Therefore, a more effective approach to deal with dentin hypersensitivity and pulpal damage during or after tooth bleaching is expected. In the present study, the peroxides penetration-reducing effect of BGs desensitizer in the existence of transparent trays was explored by measuring the amount of pulpal H₂O₂ through UV-Vis spectrophotometry after pretreated tooth samples were immersed in 30% H₂O₂.

As is shown in Table 1, no pulpal H₂O₂ was detected in teeth in group A, E, and F. The concentration of pulpal H₂O₂ in teeth without BGs desensitizer treatment (group B) was the highest among all experimental groups. This fact, which was very similar to that reported by other researchers, indicated that H₂O₂ could penetrate enamel and dentin, as was assumed before. Concentrations of pulpal H₂O₂ in tooth specimens treated with BGs desensitizer, regardless of whether transparent trays were employed or not, were significantly lower than that in untreated tooth specimens, indicating that BGs desensitizer had the ability to keep H₂O₂ from infiltrating into pulp chambers. This could attribute to the dentinal tubule occluding effect of HCA deposits which were induced by BGs in artificial saliva. A series of stages were thought to be responsible for HCA formation in physiological fluids including artificial saliva. Firstly, rapid ion exchange happens between Na⁺ and Ca²⁺ from BGs and H⁺ from the solution, leading to the formation of silanol bonds and a silica-rich layer on the surface of BGs. Thereafter, an amorphous calcium phosphate layer on the surface of this silica-rich layer appears by sequestering Ca²⁺ and phosphorous and further crystallizes into HCA layers, consuming OH⁻ and CO₃²⁻. HCA layers then combine with each other and with dentinal tubule walls, leading to their dentinal tubule occluding effect [35‐37]. HCA layers are also believed to restore cracks within dental enamel.

In this study, transparent trays were demonstrated to help improve the effect of BGs desensitizer as the concentration of pulpal H₂O₂ was significantly decreased in BGs desensitizer treating groups after transparent trays were employed. Customized trays are widely used as removable retainers in orthodontics and also as agent-holding devices using in tooth bleaching [38]. In the latter condition, customized trays have been found to reduce the penetration of peroxide into the pulp chamber [39]. Moreover, clinical trials have demonstrated that the level of sensitivity during the in-office tooth bleaching procedure could be reduced when bleaching agents were covered with customized trays [40, 41]. Here, custom-fitted transparent trays were used as the supporter of BGs desensitizer in order to prolong the treatment duration by putting BGs desensitizer into the storage space [32, 42, 43]. By doing so, the dilution of BGs desensitizer by saliva in patients’ oral cavities would be decelerated and the exposure duration of tooth to desensitizer would be extended.

With the limits of this study, the null hypothesis has to be rejected. The additional use of transparent trays, combined with a simple coating of BGs containing dentin desensitizer, could significantly reduce the infiltration of H₂O₂ into pulp chambers. In conclusion, application of BGs desensitizer with transparent trays as its container was considered to be more effective in keeping peroxides in tooth bleaching agents from penetrating enamel and dentin and damaging pulp tissues during or after tooth bleaching treatment. This could open up new possible applications in future clinical dentistry.