From the initial 184 articles, 8 were selected [10,35-41].
3.1 Quality of evidence
According to the SBU criteria, the evidence of the selected sample is of medium-high level due to the lack of error of measurements analysis for the collection of material from oral sites. Four of the eight articles chosen presented a moderate methodological quality [10,37,38,40]: the major concern regarding these studies is the lack of blinded outcome assessment, diagnostic reliability tests and reproducibility tests. According to the PRISMA method, two articles had a high quality [35,36] and the remaining two were classified as having a low quality [39,41]. Due to the absence of homogeneity in the study formulation, a meta-analysis could not be performed and a systematic review was realized instead.
When organizing the data according to pathogens, the following results were obtained:
1.2 Candida
All studies detected an increase of Candida spp. concentration during therapy with removable orthodontic therapy [10,36-39]. According to Jabur et al. study, removable orthodontic appliances induced an increase of Candida level up to 13.3% after an average of 5 weeks and 20% after a period of 4 months [10].
On the contrary the increase in Candida was very low after a period of 3 weeks [36] and 6 months [37].
In Addy’s study the Candida prevalence after three weeks from the beginning of treatment resulted to be 46% in the control group and 52% of removable appliance wearers [36].
In Arendorf et al. study, noted a prevalence of Candida of 57.6% for all study subjects, but the 39.4% of the sample was a prior Candida-carrier, so only 18.2% became carriers 5 months after starting the therapy. Results of Mc-Nemar test shows highly significant overall increase in Candida prevalence while patients were wearing the appliances (p < 0.001), especially in posterior and anterior palatal sites, respectively. However, it was followed by a highly significant fall in number after removal of the appliance (p < 0.001), in fact after 5 months form the end of the therapy, only 42.4% reported Candida colonies. This means that removable orthodontic appliances induced an increase of Candida colonies only of 3% [38].
An increasing number of microbiological counts of Candida albicans from baseline to one month, three months and six months after the beginning of therapy was observed, with a sudden increase in the number of Candida counts between the baseline and the end of the first month, which was found to be statistically significant (p < 0.001) [39].
According with Khanpayeh et al. study, the average number of Candida colonies isolated from saliva after 6 months from the beginning of the therapy with removable appliance (p = 0.001) was: Candida albicans 25%, Candida tropicalis 3%, Candida parapsilosis 2%, Candida krusei 1% and Candida kefyr 0%. Though, salivary carrier of Candida species decreased with increasing duration of orthodontic treatment [37].
1.3 Streptococcus mutans
All three articles [35,39,41] which analysed S. mutans colonization of the mouth agreed that removable orthodontic appliances represent a promoting factor for the colonization of the oral cavity by this microorganism.
In Kundu’s et al. article a statistically significant increase of S. mutans was recorded during orthodontic therapy with removable appliances, from the baseline to six months (p < 0.001). Furthermore S. mutans bacterial counts were significantly higher than those of Lactobacillus spp. and Candida albicans at all-time intervals (1 - 3 - 6 months) [39].
The study that analyzed different interceptive removable appliances [41], demonstrated a constant increase of Lactobacillus and an increase of S. mutans after 15 days, followed by a progressive decrease after 30 and 60 days.
The numbers of S. mutans colonies showed a continuous increase during therapy from baseline to one month with statistical significance (p < 0.05) [35].
1.4 Lactobacillus
All the studies [10,39,41] which quantitatively and qualitatively evaluated the difference of frequency in Lactobacillus spp. demonstrated an increase of the microbiological counts.
Kundu et al. and Topaloglu et al. studies both suggested that the microscopic counts of Lactobacillus spp. increased significantly during orthodontic treatment with removable appliances from the baseline (before appliance placement), to follow-up visits at 1 month, 3 months, and 6 months, these were found to be statistically significant (p < 0.05) [39,41].
Jabur et al. noted an increase (6.66%) in Lactobacillus spp. after 4 months of therapy, too [10].
1.5 Moraxella catharralis
According to Jabur et al. study, this pathogen was found in all the included patients, furthermore its oral colonization incredibly increased with removable orthodontic appliances. After a mean of 5 weeks from the appliance use, Moraxella prevalence was of 73.33% and after 5 months it arrived to 100% [10].
1.6 Staphylococcus epidermidis
S. epidermidis colonization of the mouth also appears to be influenced by the use of removable appliances. As Jabur et al. stated, in patients using these devices the percentage increased up to 40% after an average of 5 weeks and arrived to 60% after 4 months [10].
1.7 Streptococcus viridans
According to Jabur et al. the frequency of S. viridans was of 73.33% after an average of 5 weeks of therapy with removable orthodontic appliances. The frequency increased up to 80% after 4 months of treatment [10].
1.8 Others
The following results revealed that the changes in oral microbiota during treatment with removable orthodontic devices, involved also other bacterial species.
Petti et al. revealed that in supragingival and in subgingival plaque G+ cocci decreased after 6 - 8 weeks and increased at 6-7 months, with final values higher than baseline values. Gingivitis risk indices (bacterial count and G-rods) significantly increased progressively after 6-8 weeks. Among periodontitis risk indices, only supragingival m rods and subgingival Spirochetes significantly increased at 6-7 months. Aggregatibacter actinomycetemcomitans (Aa) prevalence was near zero [40].
The presence of anaerobic bacteria in the subgingival dental plaque was detected with the same density (n = 15.75%) at baseline (T0) and at three months (T1), while it increased at nine months (T2) (n = 17.85%), but not significantly (p > 0.05). The most important bacteria that cause periodontal tissue loss – Aggregatibacter actinomycetemcomitans (Aa), Porphyromonas gingivalis (Pg), Tannerella forsythia (Tf) and Prevotella nigrescens – were not detected in any patients [40].