This parallel-design randomized clinical trial directly evaluated the effects of the assigned tooth brushing and dietary advice on reducing cariogenic dental biofilm and cariogenic food consumption. This study design can make causal inferences, provide evidence of an intervention’s efficacy, and minimize bias. The strengths of our study include its relatively large sample size (> 30 patients), high retention rate, and good intra-examiner calibration. We adopted several strategies for reducing possible bias, including randomization, allocation concealment, and intention-to treat analysis, which are recommended in the CONSORT guidelines for reporting on randomized clinical trials [24, 36]. Furthermore, the CONSORT statement recommends a table demonstrating the baseline characteristics of the patients in each group, without testing for significant differences and p values [37, 38]. Therefore, significant differences in the baseline data between the SSL and El groups were not presented in this study. However, most baseline data had a non-normal distribution. Therefore, we could not use an absolute standardized difference to indicate an imbalance in randomization [39]. The quality of randomization and the imbalance of this study were assessed as described by Chen et al. [40]. First, we described the participant characteristics in detail. The baseline Tables 1 and 2 allow readers, especially clinicians, to judge how relevant the results of this trial might be to a particular patient. Next, to establish compliance with the study protocols: the inconsistencies between our trial protocols and the final report have been documented. For example, the CONSORT flow diagram of the participants provides the participant eligibility, trial sample size, and missing data. Furthermore, we provided the baseline information necessary for replicating this trial and for comparing it with other similar randomized control trials. Lastly, the baseline demographic and clinical characteristics of the trial are useful for evaluating this trial’s generalizability. Our eligibility criteria were not very rigorous and we recruited participants from patients currently undergoing fixed orthodontic therapy.
The baseline characteristics observed in this study were similar to those reported in previous studies. Generally, the oral hygiene practices of the orthodontic patients in our study were good. This finding was expected because patients are repeatedly advised to maintain good oral hygiene while receiving any orthodontic treatment. Our results suggest that regular visits to the orthodontist is a key factor in improving oral hygiene and gingival health [41]. Many studies reported that a high percentage (> 90%) of orthodontic patients brushed their teeth at least twice a day [42, 43]. Active orthodontic patients had a higher likelihood of brushing their teeth for 2 min or more compared with the control group [44]. Horizontal, vertical, or circular scrubbing techniques were performed by the patients in the current study. The combination method was the most common brushing technique in the SSL and EL groups and our findings are in line with those of a previous report [45]. However, our brushing technique findings did not concur with those of Atassi and Awartani [46], who revealed that the horizontal method was the most commonly used brushing method.
During orthodontic treatment, ideal oral hygiene may not be achieved solely by tooth brushing. Interdental tools and daily rinsing with fluoridated mouthwash can significantly improve dental health. The use of dental floss, interdental brushes, and mouthwashes was commonly observed in our patients, which agrees with previous studies [43, 47]. The possible explanation for our findings might be that the use of mouth rinse and interdental tools was reinforced in our patients. However, some studies reported that inappropriate oral hygiene was observed in orthodontic patients. Most of their orthodontic patients did not use dental floss or interdental brushes [45, 46]. These disparate findings could be attributed to several reasons, such as differences in demographic factors or oral hygiene advice protocols implemented in different countries.
Of the 70 patients who underwent fixed orthodontic treatment for 0.3–7.0 years, 0–3.58% of the examined teeth had WSLs. The reported proportion of patients with WSLs after fixed orthodontic treatment varies widely from 2–96%. This variation can be attributed to differences in the standardization of clinical examinations and the use of various detection tools with different sensitivities and specificities in diagnosing demineralization [48]. Conversely, our results were in line with those of Gorelick et al. who found that 3.6% of teeth had WSLs after placing fixed appliances for ~ 1 year. Similar to our findings, no excessive white spots or cavitation were found in their patients. According to the WSL results in the SSL and EL groups, it was assumed that the two groups had a similar caries risk status [13, 49].
In this study, we investigated the influence of SSL and EL on dental biofilm cariogenicity. To assess caries activity based on the ecological plaque hypothesis, we used the 3-tone disclosing gel to determine plaque maturity. The ecological plaque hypothesis focuses on acidogenic bacterial species and the factors that result in the environmental change of dental biofilm. Based on the determinants-confounders’ model in dental caries, socio-demographic characteristics, dental health behaviors, diet habits, and caries risk status are confounders of dental biofilm and caries formation [50]. Therefore, these confounding factors should be controlled in a randomized controlled trial. Notably, in this study, no meaningful differences in sociodemographic characteristics, dental health, behaviors, caries risk status (WSL prevalence), frequency of sugary and acidic food exposure, or duration of orthodontic treatment were observed between the SSL and EL groups at T1. These results indicated that in the present study, the above-mentioned confounders were well controlled between the two study groups.
Fixed orthodontic therapy typically causes retentive site-associated dental plaque accumulation [51]. Elastomeric rings and ligature wires are the two most commonly used techniques for tying archwires. Forsberg et al. [6] discovered higher levels of acid-producing bacteria, particularly S. mutans and Lactobacilli, on EL compared with SSL in 12 orthodontic patients. In contrast, Tukkahraman et al. [7] found no significant difference in the number of S. mutans and Lactobacilli from teeth ligated with either SSL or EL. Our findings are in line with Forsberg et al. [6] because we also found a higher percentage of cariogenic dental biofilm in the EL group compared with the SSL group. This difference might be due to an increase in the retentive area of elastomeric rings compared with stainless steel ligatures [51]. Our results, however, differ from the results of Rodrigues et al. [52] who reported no difference in the dental biofilm formation between the surfaces tied with either SSL or EL when using a plaque staining method that evaluated dental biofilm accumulation, but not its cariogenicity.
In addition to the difference in retentive areas for dental biofilm accumulation, elastomeric rings are an organic material that might be more favorable for bacterial colonization compared with stainless steel, which is an inorganic material with an inert material surface [53, 54]. Furthermore, fixed orthodontic appliances can induce oral ecologic changes, such as a low pH environment, which may lead to increased dental biofilm cariogenicity. It was found that early in orthodontic treatment, EL significantly reduced salivary pH compared with SSL, lowering it below the critical pH level [55]. Our findings confirm those of previous studies that ligature type affects dental biofilm accumulation. The results of the present study suggest that ligature type has a significant effect on the ecological environment of the dental plaque deposited on orthodontic appliances. The increased retentive areas and organic nature of elastomeric rings play important roles in increasing dental biofilm cariogenicity.
Due to the increase in dental biofilm accumulation and its ecological changes, excellent oral hygiene practice during fixed orthodontic treatment must be performed. Preventive programs must be emphasized to all orthodontic patients [56]. Mechanical plaque control is a meaningful challenge for orthodontic patients, because they face multiple difficulties, including which brushing technique to use, how to access posterior areas of the dentition, the correct frequency and duration of tooth brushing, and the best method of achieving plaque control in the interdental regions [44, 45, 57]. Fixed appliances, including brackets and archwires, impede access to the tooth surface and increase the complexity of the mechanical cleaning that the patient must perform [58].
Currently, there is a lack of sufficient scientific evidence to support any recommendations on tooth brushing techniques for patients receiving orthodontic treatment with fixed appliances [25]. A few studies compared tooth brushing techniques for orthodontic patients. For patients with fixed appliances, the Bass technique was found to be more effective than the Horizontal technique, followed by the Modified Stillman technique in improving gingival health. However, for removing dental biofilm, the Horizontal technique was found to be the most effective, while the Bass technique was the least effective. This might be because the Bass technique targets biofilm on the gingival margin and in the sulcus, but is unable to clean the areas around fixed appliances [59]. In contrast, Hussein et al. [60] reported that the roll technique had a superior cleansing effect compared with the Horizontal method. The authors suggested that although orthodontic patients practiced the Horizontal brushing technique, which is the most used technique, patients practiced it with some technical errors, especially with sulcular brushing or gingival cleansing. Moreover, patient discomfort from Horizontal scrubbing was alleviated when brushing with the roll technique, resulting in them preferring the roll technique rather than the Horizontal technique. However, a study on the effect of tooth brushing on S. mutans and Candida albicans counts suggests that the modified Bass technique is an important tool in maintaining good oral hygiene [61]. Based on the results of the present study, we recommend the Horizontal technique combined with the Charter’s and modified Bass techniques to ensure the complete removal of dental biofilm around the bracket and archwire areas, the undercut below and above the wing of brackets, occlusal and palatal surfaces, and intrasulcular areas. The intended effect of the combined brushing protocol was observed; the cariogenic dental biofilm in the SSL and El groups was significantly reduced at the follow-up appointment. However, the combined tooth brushing reduced the percentage of mature dental biofilm in the SSL, but not the EL group. This discrepancy in these results might be due to the differences in new retentive areas and physical properties of the two ligatures [51, 53, 54].
A specific 3-tone plaque disclosing dye can help orthodontists quickly identify where patients are struggling with mechanical cleaning. Being able to see the metabolic activity of the dental biofilm at individual sites empowers orthodontists and motivates orthodontic patients, because both can discuss the issue with real-time assessment of bacterial acid production. Furthermore, orthodontists can demonstrate appropriate dental biofilm removal methods. In addition, applying 3-tone plaque staining is useful for monitoring diet compliance of the patients. A suitable recall period can be set to allow timely monitoring of the patients [17, 62].
This study also explored whether receiving dietary counseling from an orthodontist could improve the patient’s dietary habits for preventing caries. A high frequency of sugar (> 1 time/day) and acidic food (> 2 times/day) between meals is considered a high caries risk behavior [28]. Most of our patients were exposed to sugary and acidic food between meals at a frequency of less than 1 time/day and thus were not caries risk subjects. Furthermore, there was no decrease in the frequency of sugar and acid intake after dietary counseling in the SSL and EL groups. This implies that although tooth brushing and dietary counseling were provided to our patients, dietary advice did not have a meaningful impact on oral hygiene maintenance and improvement [63, 64]. However, it is possible that the baseline exposure level of cariogenic foods in this study was too low to allow for detecting the effect of diet advice.
This study has some limitations. First, blinding of the investigator and participants was not feasible. Second, short-term follow-up was not conducted at the same period for all patients because the follow-up appointment depended on the patient’s availability. The average follow-up duration was 4 weeks and our findings might be affected by habits established during their orthodontic treatment before enrolling in the study. Third, our patients were not a homogenous group with a wide age range (13–53 years old). This may affect the tooth brushing ability of the subjects. However, several adolescents participated in the study. Two patients (13 and 18 years old) and two patients (15 and 18 years old) were enrolled in the SSL and EL groups, respectively. The patients were instructed to use the tooth brushing methods that were previously recommended to fixed orthodontic appliance patients [25]. Furthermore, the tooth brushing performance between the adolescent and adult subjects in our study was similar. Finally, dental biofilm maturity was not measured at T0. This evaluation should be performed in future studies to determine the effect of ligature type on dental biofilm maturity.