From Volume 16, Issue 1, January 2023 | Pages 33-38
Many studies have been undertaken to understand the limitations and benefits of aligners, including systematic reviews with meta-analyses. However, there are very few comprehensive reviews of the current evidence. This literature review provides an up-to-date summary of various aspects of clear aligner therapy, including patient compliance, clinical effectiveness, social interactions and iatrogenic effects.
Clear aligners are an aesthetic alternative to fixed appliances and have come a long way since their conception as a ‘tooth positioner,’ in the 1940s. In 1946, Kesling stated that a series of these positioners could be used to achieve more ambitious tooth movements when used as an adjunct to conventional treatment.1 Many aligner systems were created, but often their scope was limited and required clinicians to take multiple impressions throughout the course of treatment. In 1999, Align technology introduced Invisalign, which used computer aided manufacturing and design principles to mitigate the need for multiple impressions.2 This encouraged more clinicians to engage with this mode of orthodontics, thus resulting in its increase in popularity. Currently, Invisalign and other major clear aligner systems can use specific force delivery via composite precision attachments, generating force vectors to permit a range of specific tooth movements. Once thought to be limited to tipping only, aligners have developed into a system that can attempt most movements achieved with fixed appliances.
Many studies have been undertaken to understand the limitations and benefits of aligners, including systematic reviews with meta-analyses. However, there are very few comprehensive reviews of the current evidence. This literature review provides an up-to-date summary of various aspects of clear aligner therapy, including patient compliance, clinical effectiveness, social interactions and iatrogenic effects.
The accumulation of dental plaque is the major factor that leads to gingival inflammation and periodontal disease. This is of particular concern in orthodontic treatment owing to appliances hindering oral hygiene. Appliances have differing levels of effect on plaque build-up, but there are few studies that directly compare clear aligners to fixed appliances.
Chhibber et al carried out a prospective randomized clinical trial over 18 months that compared oral hygiene at regular intervals with clear aligners and pre-adjusted edgewise fixed appliances with both self-ligated brackets and elastomeric ligated brackets.3 The study measured effects using three different indices:
They found that there was no significant difference in the oral hygiene levels among the different orthodontic appliances after 18 months.
Zhao et al monitored 25 adult patients wearing clear aligners over 6 months and found that there were no significant differences in the oral microbiome before and after treatment.4 They also found that participants had reduced plaque, increased daily brushing frequency and lower sugar intake post orthodontic treatment. It is important to note that this study had a limited sample size with a majority of females and lacked any long-term observation.
It is well documented that, as well as oral hygiene, periodontal health can be affected by orthodontic appliances, and so it is imperative to assess a patient's periodontal status prior to offering orthodontic treatment. With the removable nature of clear aligners, this prompts questions on whether they have an advantage over conventional fixed braces concerning their effects on the periodontium.
Rossini et al carried out a systematic review of five studies and concluded that clear aligners were not only a clinically safe option for the periodontium, but fared better than fixed appliances.5 This may be due to their removability, thus allowing for better oral hygiene measures. Four out of the five studies, however, were not randomized and most had methodological issues, such as bias, confounding variables and lack of blinding. Two papers concluded that clear aligners had superior periodontal outcomes regarding probing pocket depths and plaque and gingival indices. Again the level of evidence was only moderate.6,7
A recent prospective study8 suggested that as long as sufficient oral hygiene instructions were provided to patients, the type of orthodontic appliance had no effect on periodontal health, which is in accordance with other recent literature.3 Nonetheless, there were limitations to this study including baseline characteristics, such as smoking not being recorded, follow-up was only 3 months, and there was significant heterogeneity in the ages between groups (mean age difference of 14.1 years).
A common issue with orthodontic appliances, fixed or removable, is enamel demineralization. White spot lesions lead to poorer aesthetic outcomes, patient dissatisfaction and unwanted long-term colour discrepancy, some of which do not resolve over time.9 Comparing enamel demineralization after clear aligner therapy with fixed appliances has become increasingly more important given the burgeoning increase in aligner therapy in our cosmetic-driven populations.
Buschang et al studied white spot lesions in both fixed appliances and clear aligner therapy using 450 cases (244 on clear aligner therapy and 206 on fixed appliance therapy). A reported incidence of 25.7% was found with fixed appliances, while clear aligner therapy was shown to have an incidence of 1.2%.10 According to the study, this was largely linked to differences in oral hygiene and plaque control. It has been established previously that there is a close correlation between poor oral hygiene and white spot lesions.11,12 Poor pre-treatment oral hygiene doubles the risk of developing lesions, while worsening of oral hygiene during treatment has been shown to triple the risk.13 Clear aligner therapy, although making it easier to clean effectively, does not completely mitigate the chances of developing white spot lesions. Lack of analysis via quantitative light fluorescence (QLF) and differences in treatment time between the two groups could have confounded these results.
A prospective randomized clinical control trial by Albhaisi et al used QLF to evaluate enamel demineralization during fixed appliance and clear aligner therapy. In total, 113 patients were examined, of whom 49 were deemed suitable. Like Buschang,10 Albhaisi et al concluded that fixed appliance therapy resulted in a higher incidence of white spot lesions.14 Analysis of the white spot lesions found that clear aligner therapy, although having a lower incidence and less mineral loss, resulted in significantly wider lesions when compared to fixed appliances.14 Conversely, fixed appliances encouraged deeper white spot lesions with more mineral loss. This can be accounted for by the fact that clear aligners use attachments that take up a significant area on the tooth surface. Additionally, saliva flow is reduced when aligners are worn resulting in decreased ability to buffer acids and provide minerals to tip the balance in favour of remineralization.11 As stated by Albhaisi et al, the study was limited by its duration (3 months) and most participants were female, and therefore does not allow for a comprehensive gender-based comparison.
Quite clearly there is need for a comparison of fixed appliances and clear aligners and incidence of white spot lesions with QLF, but it is evident that oral hygiene pre-treatment and mid-course significantly correlates to the formation of such demineralization.
The recent surge in popularity of clear aligners has led to widespread use in both adults and children. Evidence on what they can accurately and predictably achieve is still in its early stages. One online survey found that 45% of orthodontists believed that aligners limit orthodontic treatment outcomes, whereas only 5% of general dentists thought the same,15 highlighting a disparity within the profession over whether clear aligners can produce high-quality results.
A prospective clinical study by Haouili et al followed up Kravitz et al's study, which assessed movements of all teeth using Invisalign, and found the overall mean accuracy of tooth movement to be 50%.16,17 Although far from ideal, this is a clinical improvement from the previous 41%.17 Haouili et al measured all movements apart from torquing, as radiographs would have been required to assess root movement.16 The most accurate movement was buccal-lingual crown tip (56%) and the least was rotation (46%). Jiang et al used radiographs to measure accuracy of torquing movements and found them to be worse than rotations (Table 1).18 Intrusion was found to be challenging, and results suggested that Invisalign favours bite closure over bite opening, thus these cases may benefit from the ‘hybrid’ approach of Invisalign in the maxillary arch and fixed appliances in the mandibular. The study was limited because it used ClinCheck digital models to assess accuracy. However, ClinCheck does not necessarily represent the actual clinical end result, but is merely a cartoon of a computer-generated model representing a desired result.
| Tooth movement | Percentage accuracy (%) | |
|---|---|---|
| Maxilla | Mandible | |
| Mesial crown tip | 52.7 | 48.8 |
| Distal crown tip | 53.4 | 53.4 |
| Buccal crown tip | 57.6 | 57.6 |
| Lingual crown tip | 55.2 | 54.3 |
| Intrusion | 44.4 | 47.7 |
| Extrusion | 45.9 | 45.9 |
| Mesial rotation | 49 | 45 |
| Distal rotation | 45.6 | 42.5 |
| Torquing | 35.21* | |
ClinCheck, developed by Align Technology, is the algorithmic software in which treatment plans and prediction of tooth movement are conveyed to the clinician. The accuracy of this software and actual outcomes have been studied by many clinicians with varying outcomes. Sheridan reported 70–80% of patients treated with aligners required some form of refinement or mid-course adjustment.19
Kravitz et al17 showed discrepancies between predictions of tooth movement and outcomes following clear aligner therapy. Overall, no significant difference was found between mandibular and maxillary movement accuracy between teeth of the same type.17 This study did not evaluate posterior teeth, unlike the study by Robertson et al,20 and only mild cases were used with few bodily movements exceeding 2 mm. Additionally, overcorrections were not accounted for and confounding factors, such as age, periodontal condition, bone density and root morphology, were not mitigated.
Charalampakis et al and Grünheid et al had similar conclusions on accuracy of movement when compared to predictions.21,22 Robertson et al20 also agreed that rotational movements were inaccurate, demonstrating statistical differences between predicted and achieved tooth movements. They found that posterior tooth movements and anterior extrusion predictions were the most inaccurate.20
Many of the studies carried out were retrospective and of non-extraction cases. A difficulty in assessing the accuracy of specific tooth movement with ClinCheck is that with aligners, teeth are moved in a compound manner and so specific movements are hard to analyse and compare. On the other hand, prospective studies16,17 provide good evidence that predictions of tooth movement with ClinCheck have improved significantly in recent times, but refinement is often needed to compensate for lack of predictability of tooth movement. Another pertinent observation described was that the ClinCheck software fails to show roots of teeth and root movement and does not accurately reflect the patient's final outcome.23 Clinicians should understand that ClinCheck predictions are not extremely accurate, but are simply estimations of where teeth are likely to move to. It is clear from the literature that aligner therapy does require refinement most of the time; however, the severity of the malocclusion, as well as other factors outlined, determines how much refinement is needed.
A recent randomized clinical trial evaluated aligner wear protocol and its effect on treatment outcomes in mild malocclusions. They compared 7-, 10- and 14-day wear patterns and observed statistically significant improvements in the 14-day group for maxillary posterior teeth, which suggests that for complex posterior segment movements, 14-day wear may be beneficial.24 The study found no clinically significant differences in achieved tooth positions. However, that the mean treatment duration for the 7-day group was 4 months faster than the 14-day group, which is clinically significant.
Attachments can be a useful adjunct for clear aligner treatment and with the more recent development of optimised attachments, which are engineered to provide a bespoke force to a tooth, improvements in the accuracy of tooth movements would be expected. Karras et al compared the use of conventional and optimised attachments. They found that there were no clinical differences in accuracy of tooth movement between conventional and optimised attachments.25 This was a retrospective study with significant heterogeneity in ages of patients and did not cover the use of elastics or use of attachments in more complex cases.
One retrospective study evaluated the treatment outcomes of Invisalign in conjunction with Dental Monitoring, a form of teleorthodontics that allows patients to digitally scan their mouth from home using their smartphone and grants the orthodontist remote access to review progress.26 The study found no differences in treatment duration, emergency visits, number of refinements or number of refinement aligners. The Dental Monitoring group had a mean of 3.52 fewer visits than the control group and therefore this may be a useful tool to help increase patient satisfaction.
Pain is a common patient factor in orthodontics, which is subjective in nature and depends on a range of factors, such as age, sex, pain threshold, stress levels and cultural differences.27 There are few studies, however, directly comparing pain levels in clear aligners and fixed appliances. One systematic review compared the treatment modalities in mild malocclusion cases and found that there were no significant differences in pain levels after 3 months, but clear aligners performed better in the first few days.28 There were significant limitations in the review, including heterogeneity in study types, lack of randomization of studies analysed, presence of potential financial bias, insufficient descriptions of types of malocclusions with which patients presented and lack of reporting on the use of analgesics. Analgesia can significantly reduce pain levels and most certainly would affect results: the fixed appliance group had a higher pain medication intake, which may underestimate the level of pain experienced.
One study looked at anxiety and oral health-related quality of life in relation to pain during orthodontic treatment and found patients with clear aligners experienced lower anxiety and pain, and a higher quality of life.29 Reasons behind this could be that clear aligners decrease social anxiety and as pain and anxiety positively correlate with one another, decreased anxiety could lead to less pain. Additionally, it was suggested that patients undergoing clear aligner treatment were more confident in treatment progression and outcome because 3D visuals could be viewed on the aligner software, which could decrease anxiety regarding treatment.
A randomized clinical trial found speech was only affected immediately after insertion of clear aligners, but there were no differences after 30 days between aligners and fixed appliances. Therefore, they concluded temporary changes of speech would be most likely to occur in the first month, but the oral and peri-oral musculature tends to adapt quickly to the changes, regardless of the appliance.30
Root resorption is the permanent loss of the cementum and dentine at the apical region of the root structure via osteoclastic activity, either through physiological or pathological phenomena. This occurrence is multifactorial, and apical root resorption (ARR) secondary to orthodontic treatment is a common unwanted complication, classified as pathological (inflammatory) in origin.31,32 There have been many studies in this area, with conflicting results mainly being due to contrasting study designs.33
A systematic review by Ghandi et al compared external apical root resorption of clear aligner therapy (CAT) and fixed pre-adjusted edgewise appliances (PEA) in non-extraction cases. The review looked at 16 studies, 12 of which were retrospective and four were prospective studies. Following a random effect meta-analysis and subgroup comparison, it was found that there was no clinical significance between clear aligner therapy and fixed pre-adjusted edgewise appliances because both resulted in similar mean root resorption of less than 1 mm. The average ARR of central and lateral incisors was found to be 0.4 9mm when using either PEA or CAT, whereby CAT resulted in an average of 0.44 mm of ARR and PEA resulted in 0.52 mm. The only statistical significance found between the two methods was the ARR of the upper right lateral incisor (0.74 mm with PEA and 0.36 mm with CAT); however, given the minimal difference and confounding factors, this was deemed clinically insignificant. Furthermore, these data cannot be extrapolated to extraction cases and are open to selection bias, with no randomized controlled trials satisfying the inclusion criteria.34
Aman et al collated data from 160 patients using clear aligner therapy. This study also found minimal root resorption associated with comprehensive clear aligner therapy techniques when comparing pre- and post-treatment CBCT scans. Mean ARR of central incisors and lateral incisors was found to be 0.49 ± 0.57 mm and 0.53 ± 0.59 mm, respectively. However, this too, did not include many extraction cases, assessed maxillary incisors only, and provided no control against fixed appliances.33
Further studies investigating the incidence and severity of root resorption with clear aligner therapy, compared to fixed appliances (both self-ligating and ligated systems) should be carried out to mitigate confounding factors. Further, high-quality studies and randomized controlled trials may elucidate which therapy definitively causes less root resorption. However, clinically, there seems to be no major significance. Both techniques may lead to ARR, but when used correctly, the changes to root resorption are usually less than 1 mm and so of little clinical concern.
Relapse following orthodontic treatment has been described both in terms of short-term relapse following orthodontic treatment and long term as part of maturation.35 There is currently very little evidence comparing fixed appliances to clear aligners. According to Zheng et al,36 only one study37 sufficiently studied the differences in stability between the two methods. Patients treated by Invisalign were shown to have relapsed more than those treated with conventional fixed appliances, following a similar retention protocol.36,37
Force magnitude was also a point of contention. Fixed appliances provide measured forces onto teeth to allow for optimal movement without tissue damage. Owing to the design of aligners, force magnitude cannot be measured accurately and so, tooth movement is considered to be distance-rather than force-based.36 The lack of evidence around force magnitude is a potential area that needs further investigation.
Overall, more studies are required to provide evidence on the differences in stability between aligner therapy and fixed appliances.
Clear aligners have been an available orthodontic treatment option for many years and are rapidly increasing in popularity. There is still a shortage of high-quality evidence concerning the treatment modality. Most of the evidence is retrospective and thus, the conclusions drawn are questionable (Table 2). To accurately assess the risks and benefits of aligner therapy, more randomized controlled trials are required.
| Area | Quality of evidence | Research findings | Clinical significance |
|---|---|---|---|
| Oral hygiene | One randomized clinical trial and one prospective cohort study | No significant differences in plaque and bleeding levels in clear aligners and fixed appliances overall, but slightly better oral hygiene in the early stages of treatment in the clear aligner group | Clear aligners may allow for better oral hygiene at the start of treatment, but there is no overall clinical difference between the orthodontic treatment modalities |
| Periodontal effects | RCTs and systematic reviews present, but more recent RCTs required | Significantly better plaque and bleeding indices, probing depths and biofilm mass in clear aligners, however the level of evidence for this is moderate and more recent studies must be carried out to support these findings | Clear aligners may be beneficial for patients with a compromised periodontium |
| Enamel demineralization | Prospective randomized clinical trials reviewed | Fixed appliance showed greater incidence of deep and narrow WSLs. Less WSLs were found in CAT but WSLs were wider and shallower | WSLs are heavily attributed to poor oral hygiene, CAT allows patients to uphold good hygiene but as a result of patient factors CAT does not mitigate the presence of WSLs completely |
| Root resorption (ARR) | Systematic reviews dominate this area of research, however further RCTs are required to provide more information to compare PEAs and CAT | Similar mean root resorption between CAT and PEAs. CAT showed marginally less ARR associated with UR2 according to Ghandi et al. | There is no clinically significant difference between PEAs and CATs with regards to ARR |
| Patient related interests: pain, speech and social perceptions | Pain: systematic reviews present; significant limitations | Overall, no significant differences in pain between clear aligners and fixed appliances, however the use of analgesia was not documented. Lower anxiety in the clear aligner group, which may affect pain levels. There are no significant differences in speech between the different orthodontic treatments | There is no clinical difference in pain levels between clear aligners and fixed appliances, however anxiety levels have been shown to be better in clear aligner patients, which can affect pain perception. There are no clinically relevant differences regarding speech |
| Speech: RCT | |||
| Social perception: cohort studies | |||
| Clinical effectiveness | Only one randomized control trial, majority of papers were retrospective. | Accuracy of tooth movement has increased overall, but rotations and torquing are still inaccurate. |
Overcorrection may still be required to achieve desired tooth positions 7-day wear protocol has been shown to be 4 months faster than 14-day wear |
| ClinCheck accuracy | Prospective and retrospective studies with additional follow up | Accuracy of ClinCheck has improved with time but there is still a limit to its ability to predict accurately, even for movements which have been shown to be the most predictable. More evidence is required to assess this accuracy for extraction cases as well as assessment of movements in the posterior regions | Clinicians should be aware that predictions are simply estimations of likely tooth movement and some form of correction will be necessary when utilising CAT. |
| Relapse and stability | Systematic reviews studied a single reliable cohort study only, further studies are required | CAT was shown to have quicker relapse times than fixed appliances. However, more studies are required to provide evidence in order to draw this conclusion | It is important that patients wear retainers indefinitely and follow retention research protocol |
Clear aligners have no clinically significant differences with fixed appliances regarding oral hygiene, apical root resorption and speech. There are, however, studies to suggest that clear aligners may benefit patients with compromised periodontal health and induce lower pain levels compared to their fixed counterparts. Higher-quality studies are needed to support these conclusions.
The literature indicates that ClinCheck accuracy has improved, but only to 50%. It is important for clinicians to use it only as a guide, and to build in overcorrection, particularly for movements such as torquing and rotations.
There are differences between fixed appliances and clear aligner therapy in terms of relapse and stability, suggesting faster relapse in clear aligner therapy cases. Further research would help to identify why this is the case.
Aligner education should be introduced into orthodontic specialty training programmes to inspire further research, as well as to ensure newly qualified orthodontists are comfortable and confident with aligner orthodontics.
