# Microbiological Changes during Orthodontic Aligner Therapy: A Prospective Clinical Trial
## 4 Discussion
In accordance with this result, the increase in PPD from T\({}_{0}\) to T\({}_{1}\) in the trial group (from 1.89 to 1.99) was not even significant in our study. The second periodontal index considered in our research protocol was Bleeding on Probing (BOP). A study conducted by [PERSON] et al. [11] evaluated BOP in two groups of orthodontic patients (treated with fixed appliances or clear aligners) along three periodic follow ups: in both groups, BOP was similar after 6 weeks from the beginning of the treatment, but, subsequently, the former group started to show increased values of bleeding. At the 6-month follow up, the differences between the two groups increased, with a significant difference even higher at the 1-year follow up. Moreover, the latter group reported a progressive improvement for BOP. Similarly, [PERSON] et al. [12] reported a significant improvement of BOP after a year of treatment with clear aligners (probably due to the attention to oral hygiene and to the instructions received), differently from the one with fixed appliances. [PERSON] et al. [23] also assessed significantly higher BOP values in patients treated with fixed therapy, despite no significant improvement of these values after the treatment with clear aligners. This last result is in accordance with our data: despite the fact that the BOP values in the trial group decreased from 4.55 to 4.08, the reduction was not statistically significant. The last periodontal index considered in our study is related to plaque accumulation, considering that this event is generally linked to orthodontic therapy. Most authors have reported a significant difference between Plaque Index (PI) in orthodontic patients treated with fixed or removable appliances. [PERSON] and [PERSON] [22] found that the plaque values were almost double in the patients treated with lingual fixed attachments compared to those using Invisalign(r), in addition to that, during the subsequent evaluations, a reduction in PI was reported for the latter. Comparable results emerged from further studies [11, 12], despite the fact that some authors did not confirm a significant PI reduction in clear aligners-treated patients at the end of the follow up [24]. In our study, a certain reduction for this index was assessed in the trial group (0.39 at baseline and 0.30 after two months), but this variation did not turn out to be significant. Anyway, this difference might be attributed to easier oral hygiene procedures for the patients with removable aligners and to reduced plaque retention sites compared to fixed appliances [10]. According to the results obtained in our study concerning periodontal clinical indexes, treating orthodontic patients with CAT does not predispose a worsening of the periodontal condition, at least until the initial phase of the therapy. Furthermore, the periodontal status of these patients is similar to that of the control patients not under treatment. Therefore, the first null hypothesis of this study could not be rejected. The second aim of this report was to verify the impact of clear aligners on the total bacterial load of the oral cavity and, in particular, on specific periodontal pathogens. Even in this case, the null hypothesis was accepted, since for all the bacteria tested, no significant increase in copies/microlitre or percentage was reported, neither between the trial and the control group nor within each one. This result is in accordance with the evidence reported by [PERSON] et al. [10], stating no significant alterations in the total bacterial load due to clear aligners and, consequently, a better impact of these appliances on periodontal health with respect to the traditional therapy. In addition, a literature review by [PERSON] et al. [25], confirmed that minimal quantitative and qualitative variations of the microbiota derive from the treatment with aligners. A subsequent study [26] analysed bacterial DNA from the plaque using 16S rRNA gene sequencing, both before the treatment, as well as one and three months after.