Aligners represent a widely embraced innovation that has revolutionized the traditional approach to seeking and receiving orthodontic treatment. ¹ The concept of clear aligner treatment (CAT) was initially introduced by Kesling in 1946 as a solution for correcting misaligned teeth. ² These aligners entered the market with a promise of delivering the advantages of orthodontic adjustments while being esthetically pleasing ³ and more comfortable compared to the traditional fixed appliance treatment (FAT). ⁴ , ⁵ Recently, patients have shown a strong interest, held high expectations, and emphasized the importance of maintaining a good quality of life throughout the treatment process, even if it means incurring higher costs. ⁵ , ⁶

An orthodontic treatment inevitably companies with several adverse outcomes, including pain, anxiety, and a decline in oral health-related quality of life (OHRQoL). ⁷ OHRQoL is a multifaceted concept encompassing the interplay of general well-being, socioeconomic status, oral health status, and contextual factors. ⁸ , ⁹ Given their reduced bulkiness and improved invisibility, it is plausible to suggest that clear aligners (CAs) could alleviate the aforementioned negative effects compared to conventional FAT. ¹⁰ Studies have indicated that patients undergoing clear CAT tend to experience less pain in contrast to those with FAT. ⁵ , ¹¹ , ¹² Those treated with CAs also seem more tolerable to the initial discomforts with higher OHRQoL ^{. 13}, ¹⁴ , ¹⁵ However, some conflicting reports suggest that pain levels were significantly higher in patients with CAT when compared to the same people with FAT. ⁵ , ⁶ , ¹¹ , ¹² , ¹⁶

Numerous studies have delved into the assessment of OHRQoL indicators in orthodontic patients. The majority of these studies focused on gauging the extent of OHRQoL enhancement following orthodontic treatment, ¹⁷ , ¹⁸ , ¹⁹ , ²⁰ while a few, such as Healey et al., ²¹ ventured into the longer-term perspective by evaluating OHRQoL 21 months after the completion of treatment.

There is a notable absence of evidence-based literature addressing the impact of CAT on OHRQoL. ²² Multiple studies have assessed different aspects of OHRQoL in CAT patients over time. However, the knowledge has not been completely synthesized yet. Therefore, we aimed to systematically review these studies to understand the oral impacts of CAT over time.

This systematic review adheres to the preferred reporting items for systematic reviews and meta-analysis protocol, and the protocol was registered in PROSPERO (ID: CRD 42023389836).

Review question, inclusion, and exclusion criteria

The review question was formulated in population, intervention, comparison, outcome, type of research format as follows:

(P):Adult individuals (>18 years old) undergoing orthodontic treatment regardless of their gender or malocclusion types.

(I):CAT regardless of the treatment accompanied extraction.

(C):FAT regardless of the treatment accompanied extraction.

(O):OHRQoL.

(T):Baseline (T0), 1 week after the start of the treatment (T1), 1 month after the start of the treatment (T2), 6 months after the start of the treatment (T3), and long-term follow-up (T4).

Inclusion criteria: Full-text-available original publications written in English investigating the impact of CAT compared to FAT on OHRQoL.

Exclusion criteria were as follows

Case reports, editorial letters, pilot studies, historical reviews, and studies in languages other than English

Search protocol

Four databases, namely Scopus, Web of Science, Cochrane, and MEDLINE, were thoroughly searched for the studies published until the end of February 2023. Search queries comprised Medical Subject Headings (MeSH) keywords, MeSH synonyms, and general phrases to identify the relevant articles. To combine terms, the “AND” and “OR” Boolean operators were used in the advanced search setting of the mentioned databases. The principal search strings included “aligner,” “Invisalign,” “quality of life,” and “oral health.” The identified references were evaluated based on the eligibility criteria. The reference lists of the shortlisted papers also were investigated for any missing paper.

Study selection

Two authors (SH, PR) independently reviewed the titles and abstracts of the retrieved papers, deduplicated, and shortlisted the eligible studies. The Kappa correlation coefficients ²³ between the two reviewers were 0.92 and 0.99 for the abstract and full-text search, respectively.

Using a structured data extraction form, two authors (MR, QP) extracted the data on the studies' author name, year and type of study, sample size, patients' age and gender, malocclusion type, treatment type, and OHRQoL factors.

Data that could not undergo meta-analysis were subjected to qualitative analysis and subsequently summarized. A meta-analysis was scheduled for quantitative synthesis in instances where treatment comparisons and follow-up methodologies exhibited sufficient similarity, alongside the utilization of identical instruments for assessing the OHRQoL.

Quality assessment

The studies' quality was appraised by two independent reviewers (M. R, Q. P).

The risk of bias 2 (RoB 2) tool ²⁴ , ²⁵ was used for quality appraisal of the retrieved randomized controlled trials (RCTs). Each study was evaluated as low, high, or unclear RoB according to the randomization, allocation concealment, blinding, completeness of outcome data, selective outcome reporting, and other potential biases.

The RoB in nonrandomized studies of interventions I tool was applied to judge the RoB of nonrandomized studies. ²⁶ This tool scrutinizes trials across seven specific domains, assigning them grades of “low risk,” “moderate risk,” “serious risk,” “critical risk,” or “no information.” Subsequently, an overall evaluation of each trial was determined by considering the grades assigned across these seven domains.

For cross-sectional studies, an evaluation was conducted using the modified Newcastle–Ottawa Scale, incorporating a “star system” as well. ²⁷ Studies with a score below 3 were categorized as low quality, those scoring between 3 and 8 were deemed medium quality, and studies achieving scores above 8 were classified as high quality. To assess the potential bias across studies, the Grading of Recommendations Assessment, Development, and Evaluation framework was employed. ²⁸

Statistical methods

The meta-analysis was conducted in support of outcome parameters to find out the associated intervention effect between case and control groups. We considered mean (median) differences of questionnaire's total and sub-scale scores in both case and control groups. Some studies presented their oral health impact profile (OHIP) 14 data as box plots only. Data from these studies were extracted using WebPlotDigitizer version 4.1 (WebPlotDigitizer, Pacifia, CA, USA). ²⁹

The effect sizes are displayed as mean differences, accompanied by 95% confidence intervals (CIs), with statistical significance set at P < 0.05. To account for variations between studies, the meta-analysis employed a random-effects model, which was chosen based on the observed heterogeneity among the studies. ³⁰ Heterogeneity was assessed using the Q test and quantitatively with I-square statistics (I ²). When no statistical heterogeneity was detected among the studies, a fixed-effects model was utilized for the analysis.

For the primary analysis, each study's outcomes were compared between the intervention and control groups at different time points, leading to subgroup analyses. To illustrate each specific effect size, forest plots were generated, displaying the standardized mean difference (SMD) with a 95% CI. Results were deemed statistically significant if the 95% CI did not intersect the zero-point estimate line and if P < 0.05.

To assess publication bias in each included trial, Begg's rank correlation test and Egger's regression intercept test were applied. Once again, statistical significance was determined at P < 0.05. The meta-analysis was conducted using comprehensive meta-analysis version V2. ³¹

Selection of studies

Initially, a total of 94 papers were identified. Following the removal of duplicate entries, 43 papers were subjected to evaluation based on the established eligibility criteria. After a thorough review of titles and abstracts, 10 papers were subsequently excluded. This left a total of 19 papers that met the criteria for inclusion in the systematic review. However, on further examination, 10 of these papers were subsequently excluded from the analysis. A flow diagram of the search strategy and the reason for exclusions is illustrated in Figure 1. Figure 1

Flow diagram showing the selection of studies included in the review.

OHRQoL is a comprehensive concept that encompasses an individual's assessment of their oral health, considering physical, psychological, and social aspects. This concept plays a crucial role in evaluating the oral health status of patients seeking orthodontic treatment. ⁴⁰ In the current study, we conduct a systematic review and time-response meta-analysis to compare OHRQoL in adult patients who underwent treatment with CAT versus FAT at five different time points. It is worth noting that in 2019, a systematic review, comprising only two articles, was published on this subject; ²² however, due to the limited number of studies available, the results were not deemed reliable, and no meta-analysis was conducted.

The current study revealed that initially, both the CAT and FAT groups exhibited similar OHRQoL. However, as time progressed, the CAT group reported significantly higher OHRQoL compared to the FAT group. Interestingly, after an extended period (1 year) or the completion of treatment, both groups displayed a comparable OHRQoL. In this meta-analysis, we employed the OHIP-14 questionnaire, a recognized and reliable tool for assessing OHRQoL among orthodontic patients. ⁴¹ This questionnaire comprises 14 items distributed across seven domains, with each domain encompassing two questions. These items are rated on a 5-point Likert scale: 0 (never), 1 (hardly ever), 2 (occasionally), 3 (fairly often), and 4 (very often or every day). Scores within each domain range from zero to eight, and the overall OHIP-14 scores span from 0 to 56. A higher OHIP-14 score indicates poorer OHRQoL.

Interestingly, this meta-analysis did not reveal any differences between the groups at the baseline (T0) in any of the domains of the total OHIP-14 score. Most of the included studies ¹³ , ³² , ³³ reported similar OHRQoL levels for both the CAT and FAT groups at T0. However, one study identified a higher OHRQoL for the CA group at T0. This variation could be attributed to the fact that, unlike other studies with well-matched CAT and FAT groups, this particular study, ¹⁵ possibly treated milder cases with CAT and more severe cases with FAT.

In the present meta-analysis, no differences were observed regarding functional limitations across any of the time intervals. This result aligns with the findings of Alfawal et al. ³³ and Antonio-Zancajo et al., ³⁴ However, Gao et al. ¹³ and Jaber et al. ³² reported the domain of functional limitations to be significant during the early stages of treatment. This outcome suggests that individuals using CAT and FAT did not significantly differ in terms of factors such as pronunciation or a diminished sense of taste.

Physical pain is a substantial factor impacting the OHRQoL of orthodontic patients. ¹³ In terms of physical pain experiences, our study revealed that after the initial assessment (T0), pain consistently tended to be higher in the fixed appliance (FA) group. This finding is consistent with the majority of studies. ¹³ , ¹⁵ , ³² , ³⁴ , ³⁶ However, one study ³³ pointed out that there was no significant difference in pain between the two groups after 6 months. Similarly, studies conducted by Shalish et al. ⁶ and Alajmi et al. ¹⁴ did not find a significant difference in pain levels between the two groups. This observation may be explained by the fact that fixed orthodontic appliances exert continuous force, which can lead to greater tension, pressure, pain, and tooth sensitivity due to the constant pressure exerted by the appliance components. In contrast, removable appliances apply intermittent force, allowing tissues to rest and recover before resuming compressive forces. ⁴²

Regarding psychological problems, the present meta-analysis showed type-specific effects. Patients of both groups had the same experience regarding self-consciousness or a feeling of tension, but when it comes to feeling difficult to relax or feeling embarrassed, FAT showed significantly more problems until the 6 ^thmonth. However, after a long period, this difference disappeared. This finding is not surprising since CAs are invisible and more esthetic than FAs, resulting in better treatment acceptance and improvement in the self-esteem of patients in the CAT group. However, over time, FAT patients get used to the appliance and accept it. This result agreed with previous studies. ¹⁵ , ³³

Physical disability

The present meta-analysis showed a significant difference in physical disability only in the 1 ^stmonth and the 6 ^thmonth.

In the aspect of physical disability, the result of this study was not surprising since aligner patients had no eating limitations after taking off their appliances, whereas patients in the FAT group had chewing difficulties. The present meta-analysis showed a significant difference in physical disability in the 1 ^stmonth and the 6 ^thmonth. Alfawal et al. ³³ observed that after a long time (T4), the difference between the two groups disappears in this regard. This could be attributed to the fact that, with time, FAT patients became accustomed to eating with braces and no longer felt dissatisfied during meals.

In terms of psychological disability, the present meta-analysis revealed a significant difference from the 1 ^stweek to the 6 ^thmonth, which, however, was not significant in the long term. This finding was expected since CAs are invisible and more esthetically pleasing compared to FAs, leading to better acceptance of treatment and an improvement in the self-esteem of patients in the CA group. Nevertheless, over an extended period, patients with FAs also adapted to their devices, aligning with the results of previous studies. ¹⁵ , ³³

One study indicated that orthodontic aligners initially caused more speech difficulties than FAs, with no significant differences after 30 days of treatment. ⁴³ However, in the present meta-analysis, social disability was higher among patients with FAs after 1 week and at the 6-month point from the start of treatment. It can be concluded that FAs could be the primary cause of speech distortions, particularly in the initial stages of treatment. ⁴⁴ On the other hand, CAs can be temporarily removed from the mouth during social situations, potentially reducing pronunciation disturbances. Nevertheless, based on the findings of this study, a 6-month period of adaptation for patients using either aligners or FAs may be reflected in the questionnaire results.

The present meta-analysis has shown a significant difference in terms of handicap within the 1 ^stweek and 6 months after the start of treatment. This indicates that the FAT group felt life less satisfying and totally unable to function in comparison with the CAT group, although this difference is relatively minor. In general, FAT leads to a significant decrease in OHRQoL compared to CAT steadily until the 6 ^thmonth, but in the long run, both groups seem to have a similar perception of their treatment. This can be explained by the fact that during the early stages of treatment, CA may cause less pain, eating disturbance, or esthetic concerns. This is because the CA size was reduced and optimized compared with the traditional attachments. However, in the long run, the practical inconvenience of wearing and removing the aligners and additional aligners sets indicating longer treatment times were more likely to impact patient experience in those patients who required additional aligners. Another reason might be that the neuromuscular adaptation documented after 6 months of treatment remained stable over a 24-month observation period. ⁴⁵

Shalish et al. ⁶ employed another validated QHRQoL questionnaire to assess various aspects during the 1 ^stweek and again on day 14 of treatment, including pain, oral dysfunction, disturbance in eating, oral symptoms, and general activities. In their study, the CAT group consistently experienced significantly lower levels of eating disturbance, encompassing difficulties in eating, reduced enjoyment of food, and changes in taste, compared to the FAT group throughout the 1 ^stweek of treatment and also on day 14 (P < 0.05). ²² They noted that there was no significant difference between the two groups in terms of pain levels during the initial 14 days of treatment (P > 0.05). However, CAT did lead to significantly lower discomfort levels, including oral symptoms on the tongue, cheek, or lip, bad tastes/smells, and food accumulation, compared to FAT in their trial. ²² They reported no significant disparities in general performance related to sleeping, concentration during work or studies, absences from work or studies, and difficulties in daily activities between both groups (P > 0.05).

Baseer et al. ³⁶ employed the same questionnaire and concluded that fixed orthodontic treatment, compared to removable orthodontic treatment, resulted in more severe pain, sleeping difficulties, sores on the tongue and cheeks, and food impaction after 1 week of appliance activation. Alajmi et al. ¹⁴ had patients complete the questionnaire 1 week after their routine orthodontic treatment. They found that patients on CA therapy reported significantly more speech difficulties (P = 0.035), necessitating changes in speech delivery (P = 0.003). However, they reported better chewing ability (P < 0.001), no restrictions on the amounts or types of food they could consume (P = 0.02), and fewer mucosal ulcerations (P = 0.01). Effects on daily routines, the use of analgesics, and overall treatment satisfaction did not significantly differ between the two groups. The present study aligns with the findings of the above studies in terms of reducing the amount of pain experienced by CA patients during the 1 ^stweek. It is worth noting that this questionnaire places more emphasis on chewing and eating problems compared to the OHIP-14 questionnaire. Consequently, although previous studies found the effect of CAs on eating to be significant in the 1 ^stweek, the current meta-analysis of physical disability did not reveal a significant difference in this regard.

Flores-Mir et al. ³⁵ utilized the oral impacts on daily performance (OIDP) and PSQ to evaluate QHRQoL after completing the entire treatment course. The OIDP questionnaire encompassed five dimensions: appearance, pain, comfort, general performance, and eating restriction. This scale allowed participants to respond to the 36 items using a binary format (Yes or No). ⁴⁶ PSQ (part II) focused on patient satisfaction, exploring sub-dimensions related to the doctor–patient relationship, situational aspects, psychosocial factors, dentofacial improvements, and dental function. ⁴⁷ They discovered that a significantly higher percentage of CAT patients reported 100% satisfaction with their eating and chewing condition compared to FAT patients (47% vs. 24%, P = 0.047) after completing treatment. However, there was no significant difference between both groups in terms of pain and discomfort, psychosocial improvement, and social performance (P > 0.05) following treatment.

One limitation of this study was the inclusion of patients with different malocclusion statuses. Nevertheless, it is essential to recognize that the perception of malocclusion varies between professionals and patients, and self-perceived OHRQoL does not always correspond to the severity of malocclusion. ⁴⁸ Individuals with severe malocclusions may not report a negative impact on their quality of life, while others with minor irregularities may report significant negative impacts on their quality of life. ⁴⁸ , ⁴⁹ , ⁵⁰ Therefore, the incorporation of patient-centered measures such as OHRQoL and self-esteem assessments in orthodontics is crucial for studying treatment needs, outcomes, and managing patient expectations.

While initially, the CAT and FAT groups exhibited similar levels of OHRQoL, it became evident that the CAT group had a notably superior OHRQoL compared to the FAT group as time progressed. After an extended period of 1 year or on completing the treatment, both groups ultimately reported similar levels of OHRQoL. However, it is important to note that FAT patients continued to experience more physical pain even after a year had passed.

Financial support and sponsorship

Nil.

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or non-financial in this article.