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INTRODUCTION
Due to the growing demand for aesthetic treatments, clear aligners (CAs) are one of the patients’ preferred treatment options.1 The number of patients receiving this type of treatment has significantly increased over the past decade, and this tendency is expected to continue in the future.2
Manufacturers market CAs as a treatment that provides a better patient experience than conventional brackets in terms of discomfort, treatment duration, aesthetics, and quality of life (QoL).3,4 According to some authors, CAs treatment is associated with increased comfort and lower analgesic consumption,5,6 shorter chair time and fewer emergency visits compared to conventional braces treatment.7,8 The clinical performance of the current CA systems seem comparable to that of fixed orthodontics (FAs) in treating most malocclusions.9
Fixed appliances have been reported to affect oral hygiene, favoring plaque retention.7 CAs, being removable, could allow better oral hygiene and could be related to better periodontal conditions.10,11 Recent evidence from systematic reviews suggests that patients in treatment with CAs show better periodontal health indicators than those wearing traditional braces.11-13 However, the level of evidence of the articles included in the qualitative or quantitative synthesis was low due to the risk of bias and heterogeneity in the selected studies.11
White spot lesions (WSLs) are a major concern when dealing with orthodontic patients. However, their prevalence in patients wearing CAs has been the object of a few reports, and only one systematic review reported on this topic;14 however, no meta-regression was performed.
Orthodontic appliances can promote plaque retention and a change in the composition of the oral microbiota, with possible effects on periodontal health and risk of caries. Some authors reported a different microbiota composition in plaque or saliva in aligner patients compared to patients wearing FAs.15,16
All these findings suggest that aligner treatment tends to have a lower impact on oral health and QoL. Few systematic reviews reported on this topic; only one addressed the impact on QoL, but no meta-analysis was performed.17 This systematic review aimed to perform a quantitative assessment of the oral health indicators and QoL in patients treated with CAs compared to patients wearing classic braces.
The aim of this systematic review can be resumed by the following PICO question: (P) in orthodontic patients (I) treated with CAs (C) or conventional fixed appliances (O) is there a difference in variables related to periodontal health such as gingival index (GI), plaque index (PI), probing depth (PD), bleeding on probing (BOP), WSLs, bacterial count in saliva or plaque and their impact on oral health-related QoL (OHRQoL)? The Null Hypothesis was that no correlation exists between these indicators and the modality of orthodontic treatment.
MATERIALS AND METHODS
This systematic review and meta-analysis was conducted according to the statement of Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA).18 The review protocol was registered in PROSPERO (registration number CRD42022314530; https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=314530).
Eligibility criteria
The inclusion and exclusion criteria were developed in accordance with the previously reported PICO question.
The included studies were as follows: controlled clinical trials; randomized clinical trials (RCTs); cohort studies and observational studies, both retrospective and prospective; published or in press; published in English; comparing the variables included in the PICO questions in patients treated with CAs or traditional fixed appliances.
Exclusion criteria: clinical studies not comparing the 2 treatment modalities.
Information sources
An electronic search was performed on the following databases: Web of Science (including Core Collection, Medline, KCI Korean Journal Database, and Scielo Citation Index), Scopus, Embase, Opengrey, and Google Scholar. The search was conducted on December 31, 2021. The search was not restricted in time. A manual search of additional articles related to the topic was carried out on the references of the selected articles.
Search strategy
The search was performed with a combination of keywords and the corresponding Boolean operators, as reported in Table 1.
Study selection
Two authors (A.S.L. and J.M.M.) performed the search process following the guidelines established in PRISMA Protocol.18 The authors were calibrated first and presented a high inter-operator reliability (kappa = 0.87). The retrieved articles were first selected by titles and abstracts, selecting any potentially eligible studies. Subsequently, the same researchers performed a second screening of the full text of the articles according to the defined inclusion and exclusion criteria. A third reviewer (M.A.) was consulted in case of any disagreement.
Data collection process
Data from the selected articles were extracted by one reviewer and exported to an Excel datasheet (Microsoft Office for Mac 2011 package; Microsoft, Redmond, WA, USA) organized according to the Cochrane Consumers and Communication Review Group data extraction template. A second reviewer checked the extracted data, and disagreements were resolved by consensus. No author was contacted in this phase since all numerical data were provided in the published papers.
Data items
The following data were extracted: a) name of the authors; b) year of publication; c) study type; d) sample size of the group treated with FAs; e) sample size of the group treated with CAs; f) sex of the participants; g) mean age, and standard deviation; h) treatment/observation time; i) outcome variables analyzed; and j) conclusions. Unless the same data were gathered from at least 2 selected studies, the relevant data could only be described but not synthesized. The following outcome variables for OHRQoL assessment were considered: World Health Organization Quality of Life (WHOQOL)-BREF, Oral Health Impact Profile (OHIP)-14 and OHIP-SF19, and for periodontal health assessment: PI, BOP, GI, and periodontal probing depth. Biofilm mass, Streptococcus mutans (S. mutans), and Lactobacillus counts were considered for oral microbiome assessment. The percentage of patients who developed WSLs was the variable observed for non-cavitated caries lesions. No new data was generated or analyzed in support of this research.
Quality assessment
The quality assessment of the included studies was performed independently by 2 reviewers (A.S.L., J.M.M.), and if necessary, any disagreement was discussed and resolved by consensus. The quality assessment for RCTs was performed using the Cochrane Risk of Bias 1 tool (RoB-1).19
The tool assesses the following items: 1) random sequence generation, 2) allocation concealment; 3) blinding of participants and personnel; 4) blinding of outcome assessment; 5) incomplete outcome data; 6) selective reporting bias; and 7) other bias. Every single item can be considered as having a low risk, unclear, high risk or no information.
The Cochrane Risk Of Bias in Non-randomised Studies of Interventions (ROBINS-I) tool was used to assess non-randomized studies of interventions and observational studies.20
The tool assesses the following items: 1) bias due to confounding; 2) bias in selection of participants into the study; 3) bias in classification of interventions; 4) bias due to deviations from intended interventions; 5) bias due to missing data; 6) bias in measurement of the outcome; and 7) bias in selection of the reported result. Every single item can have a low, moderate, serious or critical risk of bias.
Statistical analysis
Data were combined through a random effect model (inverse variance method) for quantitative synthesis. To combine data from studies conducted with different measurement scales, the standardized mean difference was used as a measure of effect size. According to Cohen interpretation, a d = 0.2 was considered as a ‘small’ effect size, 0.5 as a ‘medium’ effect size and greater than 0.8 as a ‘large’ effect size. Other effect sizes used were no standardized mean difference, odds ratio (OR) and relative risk.
The level of significance was set at P < 0.05 for the Z test, and the Cochrane Q test and the I2 were employed to quantify the heterogeneity between studies. The heterogeneity was considered high if the Q test presented a P value < 0.1 and I2 was > 75%. A meta-regression was performed to assess the influence of time on the effect size. Forest plots were constructed to visually represent the meta-analysis. Publication bias was estimated using Duval and Tweedie’s trim and fill, which aims at estimating potentially missing studies to gather the funnel plot’s symmetry and compares the change in effect estimate.21 Moreover, the classic fail-safe number was calculated to quantify the number of missing non-significant studies that should be added to make the combined effect size statistically insignificant. Finally, the Egger equation intercept deviation to 0 was observed, as well as statistical significance.22 Meta-analysis and meta-regression were performed using Comprehensive Meta-Analysis Software V3.0 (Biostat Inc., Englewood, NJ, US).
RESULTS
The electronic and manual search allowed us to identify 285 published items; after removing duplicates, 207 records were screened using their title and abstracts. After this first screening, 33 studies were assessed for eligibility by screening the full text. After applying the inclusion and exclusion criteria, 2 studies were removed, and the remaining papers were included in the qualitative synthesis. The key features of the included studies are shown in Table 2. The studies were published between 2005 and 2021. A total of 31 records were included in the qualitative synthesis: 6 studies were RCTs,23-28 3 non-randomized clinical trials (non-RCTs),29-31 14 prospective or cohort studies,15,16,32-43 4 cross-sectional studies,44-47 4 retrospective studies.48-51 The flowchart of the screening process according to the PRISMA statement is displayed in Figure 1. Finally, 17 studies were included in the quantitative synthesis.
Effect on oral health-related quality of life
To explore the effect of CAs versus FAs on OHRQoL between the first week and the first 6 months of treatment, 4 studies were selected,32,33,44,47 and the overall questionnaire scores were combined. A Mantel-Haenszel method for random effects was employed. The heterogeneity could be considered high Q = 95.3; P < 0.001 and an I2 = 96.8%. The standard difference in means was –1.57 (95% confidence interval [CI], –3.04 to –0.10; P = 0.037), indicating a lower and better impact of CAs on the overall QoL (Figure 2). According to the Cohen classification, the effect size could be considered “large”.
A meta-regression was carried out with a random effect model to explore the effect of time on the OHRQoL. The variable time did not appear to significantly affect the model (Q = 0.04; P = 0.846).
Effect on plaque index
The differences in plaque index were analyzed at 1 and 3 months into treatment and between 6 and 12 months into treatment. To estimate the difference of standardized means at 1 month into treatment, 3 studies15,28,29 were combined through a fixed effect model since no heterogeneity was detected (Q = 0.292; P = 0.864; I2 = 0%). A statistically significant difference of –0.62 (95% CI, –0.98 to –0.25; P = 0.001) was obtained, indicating how CA patients display a lower plaque index than the ones in FAs 1 month into treatment. According to Cohen’s classification, the size of the effect could be considered “medium” (Figure 3).
The study of the effect at 3 months into treatment in 3 studies28,29,41 were combined through a random effect model due to the high heterogeneity of the meta-analyses (Q = 12.1; P = 0.002; I2 = 83.4%). A statistically significant difference in standardized means of –1.16 (95% CI, –2.05 to –0.27; P = 0.011) was detected. CA patients display a lower plaque index than the ones in FAs at 3 months into treatment. According to Cohen’s classification, the effect size could be considered “large” (Figure 3).
At 6–12 months into treatment23,41,46,49,51 the tendency was maintained with a statistically significant difference of –3.05 between the 2 groups (95% CI, –5.20 to –0.90; P = 0.005), a random effect model was applied due to the high heterogeneity (Q = 164.7; P < 0.001; I2 = 97.5%), the effect size could be considered “large” (Figure 3).
The values extracted by 8 studies15,23,28,29,41,46,49,51 were combined through a random effect meta-regression model to explore the influence of time on PI. A statistically significant standardized difference in means of –0.25 (95% CI, 0.48 to –0.02; P = 0.035) was detected for each month into treatment favorable to the CAs group. The effect size is time-dependent and can be considered “medium” at 1 to 3 months into treatment and large 3 to 12 months into treatment (Figure 4).
Effect on gingival bleeding
The differences in gingival bleeding index were analysed at 1 and 3 months into treatment and between 6 and 12 months into treatment. To estimate the difference of standardized means at 1 month into treatment, 2 studies28,29 were combined through a fixed effect model, since no heterogeneity was detected (Q = 0.001; P = 0.974; I2 = 0%). A difference of –0.39 (95% CI, –0.83 to –0.06; P = 0.086) was obtained, indicating how CAs patients display lower gingival bleeding than the ones in FAs at 1 month into treatment, although the difference was non-significant (Figure 5).
To explore the effect at 3 months into treatment, 2 studies28,29 were combined through a random effect model (Q = 3.552; P = 0.0522; I2 = 71.9%). A difference was detected in standardized means of –0.71 (95% CI, –1.72 to –0.31; P = 0.174). Clear aligner patients at 3 months into treatment display a lower gingival bleeding score than the ones in FAs, but according to what was detected at 1 month, the difference was not statistically significant (Figure 5).
To assess gingival bleeding at 6 to 12 months into treatment, the data from 4 studies23,25,46,49 were combined through a random model due to the high heterogeneity (Q = 92.8; P < 0.001; I2 = 96.8%). A statistically significant difference of –3.96 between standardized means was obtained (95% CI, –6.13 to –1.78; P < 0.001). The effect size could be considered “large” (Figure 5).
The values extracted from 6 studies23,25,28,29,46,49 were combined through a random effect meta-regression model to explore the influence of time on Gingival bleeding. A statistically significant standardized difference in means of –0.22 (95% CI, –0.42 to –0.01; P = 0.031) was detected for each month of treatment in CAs patients. The meta-regression model indicates how gingival bleeding decreases with time in CA patients (Figure 6).
Effect on gingival index
The effect on gingival index could not be quantitatively synthesized at 1 and 3 months into treatment because only 1 study reported the full data set was available. However, the studies were included in the meta-regression model.
At 6 to 12 months into treatment, the results of 4 studies25,46,49,51 were combined through a random model meta-analysis due to the high heterogeneity (Q = 21.44; P < 0.001; I2 = 86%).
A statistically significant difference of –1.18 between standardized means was obtained (95% CI, –1.99 to –0.36; P < 0.001). The effect size could be considered “large” (Figure 7).
The meta-regression model used to detect the influence of time on the outcome included the data from 5 studies25,29,46,49,51 gathered at 7 different time points. The effect of time on the gingival index was insignificant (Q = 0.25; P = 0.616), as the standardized difference means –0.02 (95% CI, –0.09 to 0.06) for each month into treatment.
Effect on probing depth
The effect on probing depth was explored at 1 and 3 months into treatment. The data between 6 and 12 months could not be quantitatively assessed since only 1 study reporting the full data set was available. However, the study could be included in the meta-regression model. At 1 month28,29 into treatment, the difference in means between the 2 groups was statistically significant and equal to –0.26 (95% CI, –0.51 to –0.02 mm; P = 0.030). Due to the low heterogeneity, a fixed model was used (Q = 0.304; P = 0.581; I2 = 0%).
The data from 2 studies28,29 were analyzed through a random effect model due to the heterogeneity magnitude (Q = 2.082; P = 0.149; I2 = 51.9%) to study the effect at 3 months into treatment. The difference in means between the 2 treatment groups was equal to –0.04 mm and statistically significant (95% CI, –0.80 to –0.01; P = 0.045) (Figure 8).
Effect on biofilm mass
To estimate the differences in biofilm mass 1 month into treatment the data proceeding from 2 studies16,28 could be quantitatively synthetized. The standardized difference in means was statistically significant and equal to –0.60 (95% CI, –1.17 to –0.03; P = 0.04). Due to the low heterogeneity a fixed effect model was applied test (Q = 1.09; P = 0.296; I2 = 8.3%) (Figure 9).
To quantify the effect of time on the outcome in CAs and FA patients, a meta-regression model was applied to the data provided by 2 studies at 3 different time points. The effect of time on biofilm mass was not significant (Q = 0.57; P = 0.448).
Effect on S. mutans concentration
Two studies16,41 reported a high concentration of S. mutans between 4 and 12 weeks into treatment; the OR was used to quantify the effect size (Figure 10). The meta-analysis was characterized by no heterogeneity (Q = 0.155; P = 0.694; I2 = 0%). An OR of 0.21 could be estimated as non-statistically significant (95% CI, 0.03–1.37; P = 0.104).
Effect on Lactobacillus concentration
As for S. mutans concentration, 2 studies16,41 reporting on the high concentration of Lactobacillus between 4 and 12 weeks into treatment were included in the meta-analyses, and the OR was used to quantify the effect size (Figure 11). No heterogeneity characterized the meta-analysis (Q = 0.155; P = 0.694; I2 = 0%). An OR of 0.32 could be estimated as being non-statistically significant (95% CI, 0.03–3.18; P = 0.330).
Effect on the onset of white spot lesions
Two studies34,50 could be included in the quantitative synthesis; the onset of new WSL was assessed between 9 months into treatment and after treatment. The meta-analysis presented a moderate heterogeneity (Q = 2.652; P = 0.103; I2 = 62.3%).
A Risk Ratio of 0.10 could be estimated (95% CI, 0.02– 0.42; P = 0.002). The risk of presenting new WSLs in the clear aligner group was 0.1 the risk than the conventional appliances group. The patients in treatment with CAs presented a tenfold lower risk of developing new lesions when compared to the individuals treated with FAs (Figure 12).
Synthesis of results
Table 3 summarizes the estimates and effect sizes of the 14 meta-analyses and 8 meta-regressions for the 9 variables studied.
Publication bias analyses
Publication bias could not be detected by means of Duval and Tweedie’s trim and fill methods, no missing studies could be estimated to get a symmetric image that modifies the estimate obtained by the meta-analysis.
Moreover, the number of missing studies that should be added to make the combined effect size statistically insignificant (classic fail-safe number) was very high regarding the plaque index (568), bleeding index (264), and gingival Index (116). The values were lower for probing depth (32) and biofilm mass (5). The number was not estimated for S. mutans and Lactobacillus concentration. The funnel plots related to OHRQoL, PI, BI and GI display a coincidence between the white diamond that represents the overall effect size calculated in the meta-analysis and the black one that is estimated by the Duval and Tweedie method.
A slight discrepancy could be observed in the case of probing depth and biofilm mass due to 1 study at the right of the standardized mean difference; in both cases, significance was maintained. In the case of S. mutans and Lactobacillus concentration, the discrepancy, due to one study on the right, was irrelevant since the estimated effect size was insignificant. Funnel plots are available for consultation in Supplementary Figure 1. Publication bias was not estimated regarding WSL since a minimum of 3 studies was required, and only 2 were available.
Quality assessment
The quality assessment performed through the RoB-1 tool on the 6 RCTs included in the qualitative synthesis is displayed in Supplementary Figure 2. The most severe form of bias was the lack of blinding of participants and personnel, which could be observed in all the included studies, followed by the blinding of outcome assessment and the Incomplete outcome data report.
The quality assessment performed through the ROBINSON-I tool on the 25 non-randomized and observational studies included in the qualitative synthesis is displayed in Supplementary Figure 3.
The most prevalent bias in non-randomized studies was due to confounding, followed by the bias in the selection of participants and the bias in the measurement of the outcome. The risk of bias was low due to deviations from the intended intervention or missing data. At a global level, the risk of bias was regarded as moderate in 32% of the studies and severe in 68%.
DISCUSSION
Patients’ aesthetic requirements have favored the use of aligners for orthodontic purposes. Moreover, the patients perceive this treatment as less harmful to their OHRQoL.2,5 Removable appliances CAs make oral hygiene maintenance easier, facilitating brushing and interdental cleaning. This aspect is paramount and related to periodontal health and dental health throughout treatment. Authors such as Azaripour et al.49 conclude that CA patients present better periodontal health and report higher satisfaction during orthodontic treatment than patients with FAs.
Within the limitations of the current investigation, we can state that the studies included in our systematic review, meta-analysis and meta-regression suggest that CAs patients display better periodontal health indicators when compared to those in FAs, being our result consistent with the ones reported by Jiang et al.,11 Rossini et al.,12 and Lu et al.52
In our systematic review, we analyzed 11 studies that assessed OHRQoL between 1 week and 6 months into treatment, of which 4 were included in the meta-analysis. According to our findings, those who underwent CAs treatment reported a lower impact on OHRQoL, with a “significant” effect size, compared to patients treated with FAs. The influence of time on OHRQoL has not been significant. To our knowledge, this is the first systematic review that includes a meta-analysis on the impact of CA treatment on OHRQoL. Zhang et al.17 performed a systematic review on the same topic but included only 2 articles in the qualitative synthesis and did not perform the quantitative analysis. Their results are inconsistent with ours since they stated that there is little evidence of the improvement of OHRQoL in CA patients compared to FAs ones, highlighting differences only regarding the item “problems during chewing”.
At the periodontal level, CA patients presented better indicators than FA patients, with the GI, PI, BI, and PD significantly worse in patients in the FA group. These findings are consistent with what Jiang et al.11 and Oikonomou et al.14 previously reported, highlighting significantly lower plaque index scores in CA patients. An integrative review performed by Partouche et al.13 concluded that CA patients were less subject to plaque accumulation than FA patients. However, among all the indexes evaluated, only PI differed significantly. According to the meta-regression results, the differences between the 2 groups become more noticeable according to the time of treatment and the significant influence of time. In terms of plaque index, the effect size turns from “moderate” to “large” 3 months into treatment, maintaining its magnitude at 12 months.
Bleeding on probing displayed a significant difference between the 2 groups 6 months into the treatment, being non-significant earlier into treatment. Our results agree with those presented by Oikonomou et al.,14 but differ from those of Lu et al.,52 which highlighted a significantly lower bleeding and plaque index in CA patients at the beginning of treatment from the first to 6 months. Of the 5 articles included by Lu et al.,52 only 2 met our inclusion criteria.
According to the GI meta-analysis, carried out on 4 studies at 6 and 12 months, a significant difference favorable to CA patients could be observed, while in the meta-regression, where we included 8 measurements from 5 studies, we did not find a significant effect over time.
In the meta-analysis of PD, including 2 studies that assessed this variable at 1 and 3 months, statistically significant differences in favor of CAs could be highlighted, while the meta-regression indicated that the effect was not significantly different over time. These results are consistent with those reported by Rossini et al.,12 who concluded that periodontal health indices improve significantly during CAs treatment. Jiang et al.11 and Oikonomou et al.14 also highlighted how both GI and PD show statistically significantly lower scores in CA patients compared to those wearing FAs. In contrast, authors such as Lu et al.52 could not retrieve any statistically significant differences between the PD and the GI when the 2 treatment modalities were compared.
Regarding the microbiological parameters analyzed, we could not find any significant differences in the OR for the presence of high-concentration S. mutans and Lactobacillus, both at 4 and 12 weeks in the CAs and FAs groups. The effect of time was also not significant on this variable.
In contrast, Oikonomou et al.14 obtained in their meta-analysis for a 3 and 6-month follow-up period that adult patients treated with aligners had a lower risk for the presence of S. mutans and Lactobacilli. Although in the adolescents group, the difference was insignificant between the CAs and FAs groups, in this case, the evaluation was carried out at the beginning of treatment after just 1 month.
In our work we also performed a meta-analysis on the variable mass of biofilm by combining 2 studies that reported the values after 1 month into treatment. Our findings show a statistically significant difference in biofilm mass in patients wearing CAs than in patients with FAs, being lower in those with CAs.
Regarding incipient caries/WSL, in our meta-analysis with 2 included studies, we have found that at the end of treatment (9–24 months), aligners patients have ten times less risk of presenting new WSLs than conventional fixed appliance patients. Our results are consistent with those of Albhaisi et al.,24 who reported that those wearing CAs presented a lower risk of developing white spots, fewer lesions, and less mineral loss, but with a greater extension than the FAs group. In patients in the FAs group, lesions were more prevalent but smaller and had greater mineral loss than in the CAs group.
Our systematic review and meta-analysis analyzed the existing evidence on periodontal health indicators, oral microbiota, and pre-cavitated enamel lesions. We can consider that only a few studies on these topics have been published that could be quantitatively synthesized, and a thorough assessment of the overall impact of CAs on oral health indicators is, therefore, difficult. Since orthodontic treatments last over time, analyzing the influence of treatment modalities on oral health indicators should be considered extremely important. Meta-regression highlighted the behavior of oral health variables over time.
The small number of randomized studies with high-quality designs that were included in the meta-analysis can be regarded as one of the main limitations of the current study. Moreover, most of the included studies were non-randomized and observational. It should be considered that even if high-quality RCTs are regarded as the gold standard among clinical trials, high-quality cohort studies can also be adequate to study this topic. It should be considered that the choice of orthodontic appliances strongly depends on the patients’ socioeconomic background and esthetic requirements. It is challenging to randomly distribute patients into various therapy groups unless the study has external financial funding. Furthermore, the quality assessment underlined how the most prevalent bias was related to “blinding”, unavoidable due to the nature of the appliance being both FAs and CAs visible to the participants and the personnel involved in the study. Moreover, many indexes and a lack of standardization made it difficult to combine them in a quantitative synthesis when assessing periodontal health.
To our knowledge, this is the first meta-regression conducted on this topic. Among the strengths, we can mention the thorough search process among 3 different databases, which allowed us to access most of the published studies that addressed our research question. More than half of the meta-analyses presented slight or no heterogeneity, and no publication bias was detected.
CONCLUSIONS
The findings of this systematic review, meta-analysis and meta-regression suggest that CA patients present better oral health indicators and OHRQoL than FA patients.
The periodontal indicators analyzed show significantly better values in patients treated with CAs, with moderate to large effect sizes. Both the plaque and the bleeding index show a significant tendency to improve throughout treatment with CAs.
The risk of developing WSLs at the end of treatment is ten times lower in the CAs group.
CA patients have a lower biofilm mass compared to those treated with FAs. However, no differences were found in the percentage of patients with high S. mutans and Lactobacillus counts.
Clear aligners treatment shows a lower impact on OHRQoL than the treatment with fixed appliances.
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