# Comparative Study of the Expansive Behaviour of Different Internal Bleaching Agents
## 4 Discussion
While considered to be a safe treatment, the mechanism by which a tooth is bleached is not fully understood [10] and although it has many advantages, the procedure is not free from possible drawbacks or complications described as resulting from the oxidising behaviour of HP--such as the appearance of an ECR or adverse effects upon the cavity seal [11]. The reaction observed in this experimental model creates an increase in intracameral pressure and would be responsible for the displacement of the temporary restoration that has been observed in other studies over the years, which can leave a microscopic gap between the restoration and the tooth [21; 23; 28; 29]. Displacement of the temporary restoration could result in prolonged treatment with more sessions due to failure to obtain the desired results, bacterial infiltration and even failure of the endodontic treatment, as well as irritation of the tissues with which the bleaching agent comes into contact as it seeps into the oral cavity through the open space [11; 24; 25; 30; 31]. Based on the results obtained in this study, HP 30% would be by far the bleaching agent with the greatest capacity to produce such a displacement. In turn, the expansion observed in this study also agrees with the whitening capacity reported in other publications, with better results using HP 30% after a few days, coinciding with the days of greatest expansion observed, and whitening the teeth more quickly and effectively than with other agents [31; 32; 33; 34]. These results also apply to SP, as many studies suggest that it has a more stable behaviour but with a lower bleaching capacity, as our findings also suggest [34]. On the other hand, it has been postulated that oxidising agents from internal bleaching can cause cement necrosis, an inflammation of the periodontal ligament and may contribute to ECR from the pulp chamber through the dentinal tubules [35]. In a study on the ultrastructural changes in dentin induced via intracoronal bleaching agents, [PERSON] et al. (2012) found that HP 35%, CP 45% and HP 30% + SP produced an increase in the diameter of dentinal tubules, with the exception of SP with distilled water, probably as a result of protein oxidation [36]. Similarly, [PERSON] et al. also studied in vitro the possible changes in enamel and dentine produced by SP for 7 days, and the results showed that it did not produce microstructural changes in enamel and dentine [37]. In addition, [PERSON] et al. [38] and [PERSON] et al. [39] also observed in their in vitro studies changes in MTA produced by intracoronal bleaching agents, in surface morphology and chemical composition that could negatively affect the subsequent restoration. These results are consistent with those of our study, which shows a greater reaction with these agents, while the reaction with SP would not have sufficient capacity to alter dentine in the same way. Furthermore, they also agree with the findings of [PERSON] et al. (1995), who observed the ability of _Streptococcus faecalis_ to penetrate through the dentinal tubules of teeth bleached with HP 30% and HP 30% + SP, but not of teeth bleached with SP [40]. This finding was relevant because it has been observed that bacteria in the gingival or pulp chamber sulci can subsequently colonise the empty dentinal tubules, cause the persistent inflammation of the surrounding tissues and may contribute to the development of ECR [19; 36; 41; 42]. There are published studies on the relationship between the reduction in the organic content of enamel and dentine with changes in biomechanical properties during bleaching procedures. Since the dentine structure constitutes an important part of endodontically treated teeth, any alteration in the biomechanical properties of dentine may have an impact on the overall strength of the tooth [22; 42; 43]. Using extracted human teeth, [PERSON] et al. examined the effect of HP 30% and SP on dentine and enamel microhardness [43].