Changes in oral microbiota due to orthodontic appliances: a systematic review

ABSTRACT Background: Oral microbiota has been at the center of cultural attention in recent years. In daily clinical practice, orthodontic appliances may be associated with an increased cariogenic risk and a worsening of preexisting periodontal diseases. Objective: The purpose of this review is to investigate the available evidence regarding the association between orthodontic appliances and changes in the quality and quantity of the oral microbiota. Design: The research included every article published up to October 2017 featuring the keywords ‘Orthodontic appliance* AND (microbiological colonization OR periodontal pathogen* OR Streptococcus mutans OR Lactobacillus spp. OR Candida OR Tannerella forsythia OR Treponema denticola OR Fusobacterium nucleatum OR Aggregatibacter actinomycetemcomitans OR Prevotella intermedia OR Prevotella nigrescens OR Porphyromonas gingivalis)’ and was conducted in the major medical databases. The methodological quality of selected papers was scored using the ‘Swedish Council on Technology Assessment in Health Care Criteria for Grading Assessed Studies’ (SBU) method. Results: Orthodontic appliances influence the oral microbiota with an increase in the counts of S. mutans and Lactobacillus spp. and in the percentage of potentially pathogenic gram-negative bacteria. Conclusions: There is moderate/high evidence regarding the association between orthodontic appliances and changes in the oral microbiota. PROSPERO registration number CRD42018091589.


Introduction
Periodontal health is crucial and requires special attention when performing an orthodontic treatment plan, both in adult and pediatric patients [1]. Preserving the integrity of periodontal tissues is one of the main concerns of orthodontics specialists, which has led to the definition of specific hygiene protocols for orthodontic patients [2]. Since 1985, the scientific community has been very concerned about the interaction between orthodontic devices and oral bacteria [3,4]; in fact, the first studies to analyze the oral microbiota and conventional braces (CB) took place in this period. In 2012, Freitas et al. published a systematic review regarding the alteration of the oral microbiota caused by fixed appliances [5]. The authors concluded that 'The literature revealed moderate evidence that the presence of fixed appliances influences the quantity and quality of oral microbiota'. However, the authors included papers that analyzed bacteria from appliance surfaces and from oral mucosa, without distinction. Furthermore, a significant number of studies have been published since 2012. Our review aims to update the research of Freitas et al., focusing on studies that have analyzed the microbiota collected from oral sites and not directly from appliances, and including all appliance types (self-ligating braces, invisalign aligners, sports-mouthguards, and other removable appliances) and not only fixed appliances.
Thus, the clinical research questions were as follows: • Do orthodontic appliances influence the quality and quantity of the oral microbiota? • What are the effects of orthodontic devices on the different bacterial species in the oral cavity?

Materials and methods
A search of the keywords Orthodontic appliance* AND (microbiological colonization OR periodontal pathogen* OR Streptococcus mutans OR Lactobacillus spp. OR Candida OR Tannerella forsythia OR Treponema denticola OR Fusobacterium nucleatum OR Aggregatibacter actinomycetemcomitans OR Prevotella intermedia OR Prevotella nigrescens OR Porphyromonas gingivalis) was conducted in PubMed, PMC, Scopus, Lilacs, Scielo, Cochrane Trial Library, Web of Science. All articles published up to October 2017 were included. The Preferred Reporting Items for Reporting Systematic reviews and the Meta Analyses protocol were adopted for this systematic review [6]. During the first phase, all the articles were selected by title and abstract by two of the authors and duplicate exclusion was performed. In the next phase, the full texts of potentially relevant papers were evaluated to determine if they met the eligibility criteria. Articles were selected on the basis of the criteria listed in Table 1. The article selection process is illustrated in Figure 1. Discussions were held to resolve any disagreements; when a resolution could not be found, a third review was consulted. Data extraction from the selected papers was performed independently by two review authors who adopted a template similar to that of Freitas et al. [5]. The template was adapted to the necessities of our study and is shown in Table 2 [5].
Extracted data included first author, year of publication, study design, sample size, age of the patients, type of appliance analyzed, collection time of the study, collection methods, microbial analysis methods, and quality of the study.

Quality analysis
The methodological quality is 'the extent to which the design and conduct of a study are likely to have prevented systematic errors (bias)'. Variation in quality can explain variation in the results of studies included in a systematic review. More rigorously designed (better 'quality') trials are more likely to yield results that are closer to the 'truth' [7].
The methodological quality of selected papers was scored using the 'Swedish Council on Technology Assessment in Health Care Criteria for Grading Assessed Studies' (SBU) method, which was also used to assess the level of evidence for the conclusions of this review. The SBU method divided the methodological quality of the articles into three grades: grade Ahigh value of evidence, grade Bmoderate value of evidence, and grade Clow value of evidence; once a score had been assigned to each study, the review's level of evidence was stated in four grades: grade 1strong scientific evidence (at least two studies assessed at level A), grade 2moderate scientific evidence (one level A study and at least two studies at level B), grade 3limited scientific evidence (at least two studies at level B), and grade 4insufficient scientific evidence (fewer than two studies at level B) (Table 3-4) [8].
The invisalign study, conducted by Levrini et al. [31], revealed lower values of PI, BOP, and bacterial component at 3 months for the invisalign group.
In the two studies with thermoplastic retainers, Türköz et al. [52] showed an increase of S. mutans and Lactobacillus spp. while Farhadian et al. [18] observed that the addition of silver nanoparticles reduced the levels of S. mutans after 7 weeks.
In one study [11], the use of space maintainers defined an increase in BOP in the number of bacteria and in Candida. Furthermore, D'Ercole et al. [17] pointed out that the use of sports mouthguards produced an increase in BOP and PI (Table 8).

Other appliances
Two studies investigated other kinds of orthodontic appliances [25,56]: one fixed interceptive orthodontic appliance and one esthetic brace. In a study that analyzed fixed interceptive appliances, Ortu et al. [56] demonstrated an increase in S. mutans and Lactobacillus spp. (Table 9).

(Continued )
Previous studies have assessed the role of biomaterials in the regulation of the oral microbiota [58]. As stated by Antonelli et al. [59], the simplest surfaces for bacteria to colonize are hard ones as mucous membranes tend to scale off and, therefore, do not guarantee a stable adhesion. The only exception to this is the tongue, which is highly colonized even if it is a mucosal surface because of the irregular surfaces of papillae [60]. Consequently, the introduction of a biomaterial into this open system creates a further retentive surface on which bacterial species are able to reproduce and where there is an increased difficulty in maintaining oral hygiene [58]. As revealed by the Øilo and Bakken [58] literature review, the presence of biomaterials results in an increase in plaque and alterations in the oral microbiota.
Thus, on the basis of these assessments, it seems reasonable to state that the grade of bacterial  colonization related to orthodontic appliances is affected by the energy and roughness of the appliance surfaces, as well as their design and dimensions. This may be a key factor in efficiently performing hygiene procedures [58].
Another significant variable for microbiota alterations is the amount of time the appliance is worn in the oral cavity, with removable appliances having significantly less impact on oral bacteria than fixed appliances [61].  The quantitative alteration of the oral microbiota is related to an increase in clinical parameters, PI and BOP, which are risk indicators for oral pathologies [62].
Together with the quantitative change, there is also a qualitative variation; indeed, there is an increase in gram-positive and gram-negative more aggressive bacteria, such as: S. mutans and Table 6. Self-ligating braces results. Reference PI BOP Microbiological analysis Al-Anezi [9] P(T0) = 0.001-P(T1) = 0.002 P(T0) = 0.125-P(T1) = 0.508 NS Baka et al. [14] Significance between T0-T1, T1-T2, T0-T1:  SL:  P(T0-T2  Lactobacillus spp. (gram-positive) and P. gingivalis, Tforsythia, and T. denticola (gram-negative); and these bacteria are closely associated with, respectively, enamel and dentin pathologies (e.g. demineralizations or caries) and with periodontal disease [63]. Recent papers have highlighted the complexity of periodontal disease etiology, with a special focus on the identity of bacteria which are responsible for this pathology [64][65][66]. Thus, authors have stated that the presence alone of specific micriobial species seems insufficient in causing gingivitis and periodontal disease, and that the change in biofilm equilibrium is another key factor in the development of these diseases [64][65][66]. Oral microbiota alterations registered in orthodontic patients appear to be consistent with the modifications occurring in patients with poor oral hygiene presenting gingivitis and/or periodontal diseases. In addition, orthodontic devices could represent a direct risk factor for periodontal diseases as they are often related to an increase in periodontopathogenic species [24,34,43,44,48]. However, it seems reasonable to state that the susceptibility of each subject, as well as other factors that may alter the biofilm balance, may play a key role in determining the entity of periodontal sequelae. Even though changes in the microbial system involve all types of orthodontic appliance, more rapid modifications occur during fixed orthodontic treatment. These alterations may be recorded even 1 month after the beginning of treatment and may lead to a decrease in patients' periodontal health perception [41]. Even so, as stated by Perinetti et al. [41], the role of subgingival bacteria in periodontal modifications needs to be evaluated together with the action of enzymes activated in response to the stimuli of orthodontic forces.
If it is true that all appliances increase the bacterial component, it is also the case that mobile devices make minor changes as they are removable and can be completely cleaned, resulting in better oral hygiene minimizing retentive artifacts. It should also be emphasized that, of these appliances, the use of mouthguards is limited to a small population and they are carried only for limited periods of time, involving a less pathogenic effect.
Less devastating results from changes in the oral microbiota emerged from studies on functional appliances and on aligners, which are used up to 22 h a day [61]. So, it seems more  important to be able to remove the appliance and wash both it and the teeth rather than the length of time the device is worn.
In view of the changes in microbiota that occurred with the introduction of biomaterials into the oral cavity, and more specifically of the orthodontic devices, it would be appropriate for patients undergoing dedicated hygiene protocols to keep the oral bacterial charge under control and then to reduce the risk of the carious process and periodontal disease, as evidenced by various authors [2,67,68].