Evaluating orthodontic outcomes with clear aligners and fixed appliances?
Despite their popularity, the effectiveness of clear aligners in complex cases remains uncertain. This is because there has been little research conducted into this type of treatment. However, there have been some systematic reviews, but most of them appear to include large numbers of retrospective studies. The authors of this new paper aimed to conduct a rigorous evaluation of the quality of the outcomes we have achieved with clear aligners.
Regular readers of this blog will be aware that I do not write about systematic reviews due to the problems with their methodology, as I feel that they often include too many outcomes derived from retrospective studies. However, when I came across this review, I had high hopes because it was published in one of our higher-impact journals.
A team from Syria and Saudi Arabia did this systematic review. The European Journal of Orthodontics published their paper.
Ziad Mohamad Alhafi et al
European Journal of Orthodontics on line.
What did they ask?
They did this study to find out;
‘If there was a difference in the quality and stability of orthodontic treatment outcomes achieved with clear aligners compared with traditional fixed appliances?’
What did they do?
They conducted a standard systematic review following Cochrane guidelines. This involved electronic searches, identifying relevant studies, excluding irrelevant ones, data extraction, and finally, analysis of the evidence.
They evaluated the risk of bias for randomised trials using the Cochrane risk of bias tool. For non-randomised trials, they used the Robins-I risk of bias in non-randomised studies of interventions tool. Finally, they assessed the level of evidence with the GRADE framework
The PICO was
Participants.
Individuals of both genders who are undergoing orthodontic treatment.
Intervention
orthodontic treatment with clear aligners.
Comparison.
Orthodontic treatment with any type of fixed orthodontic compliance.
Outcomes.
The primary outcome was the quality of orthodontic treatment judged through objective and reliable evaluation methods. Secondary outcomes included treatment duration.
They included randomised control trials and non-randomised clinical trials that performed baseline assessments of the malocclusion.
What did they find?
At the end of the literature searches, they identified 15 trials to include in the review. Five of these were randomised trials, and the other 10 were non-randomised trials. They were published between 2005 and 2023, with data on a total of 1092 patients, aged between 20 to 35 years.
They presented a large amount of data. I am going to concentrate on the overall quality of treatment.
When they assessed the risk of bias within the studies, they found that one randomised controlled trial was at high risk of bias, while the remaining four studies presented some concerns. For the non-randomised trials, they rated two rated as having a low risk of bias, three as moderate risk, and the other five studies as having a high risk of bias.
When I examined the findings, I noticed that they had combined the randomised trials and retrospective studies in the meta-analysis. I have some concerns about this, which I will address later in my discussion. They presented a large amount of complex data, but overall, the significant findings were.
In non-extraction cases, there is no difference in occlusal results between clear aligners and fixed appliances, as measured by the ABO index. However, they did suggest that when examining PAR index scores, a difference was observed, with a significantly smaller reduction in the clear liner group. I could not find any data on this in the paper and they did not carry out a meta-analysis on PAR scores.
When they examined more complex extraction cases, they discovered that the overall ABO scores were higher in the fixed appliance group. The mean difference was 2.76 (95% CI: 0.41, 5.12; P = 0.02). Again, the quality of evidence was low. Furthermore, I doubt that this difference is clinically significant.
The final conclusions were:
“Clear aligners may achieve comparable treatment outcomes to fixed appliances in mild to moderate non-extraction cases. However, fixed appliances demonstrate superior control for complex cases and have a higher standard of finish”.
They also concluded that we needed well-designed RCTs and that the current level of evidence was low!
What did I think?
As regular readers of the blog will know, my enthusiasm for systematic reviews has greatly declined over the past few years. This is because we have too many systematic reviews that aim to answer critically important questions, but all reach the same conclusion that we need more research.
Furthermore, there is a current trend in orthodontic systematic reviews to include both randomised and non-randomised trials and then including all the data from these studies into a single meta-analysis. My concern here is that including retrospective studies weakens the overall strength of evidence. Unfortunately, this was the case with this systematic review, and it would have been helpful to see an analysis of the data from the RCTs only.
However, we should also be aware that this would have resulted in a small number of included studies and participants, once again reducing the overall quality of the evidence.
Final comments
I have considered this review, and while it has been conducted well, I am disappointed that it does not really help us evaluate the value of aligner treatment. Paradoxically, this treatment is very popular, yet its effectiveness remains uncertain.
Recently, two publications have questioned the quality and necessity of orthodontic systematic reviews. Millett et al and Eliades pointed out that there are more systematic reviews than trials. What we need is primary studies, not more reviews that conclude “we need more research”. I strongly support this suggestion.
The main conclusion from this review, and others, is that we do not know much about the outcomes of clear aligner treatment. All we have is a few low-number trials, clinical opinion and social media posts by highly paid Key Opinion Leaders.
In many ways, it takes me back to an editorial I wrote with Jonathan Sandler in 2010 titled ‘In the land of no evidence, is the salesman King”?This is where we still are with aligner treatment.
I would really like to see future researchers carrying out some high-quality studies in this area. This may be a hopeless pipe dream.
Emeritus Professor of Orthodontics, University of Manchester, UK.
One would have thought that a certain aligner company with disposable income would have invested in research to show how good their aligner treatment can be. The lack of investment in this direction speaks volumes.
With respect, It does not take much research to learn that Align Technology’s research and development expenses were $373.7 million for the twelve months ending in June 2025. It has increased each year from $175.3 million in 2020 to $364.2 million in 2024. The company also supports external research through its annual Research Award Program, which has provided approximately $2.7 million in grants to universities since 2010.
The amount invested by Align Technology into research is more than all other aligner companies combined (and probably orthodontic companies in general) – that have produced either zero or low level data, but no one seems to notice. They rely on the significant evidence base (arguably the largest for any individual orthodontic mechanical system including individual bracket prescription and wire sequence ) largely supported by Align Technology to blur the boundaries of “aligner research”. So please, let’s be accurate and impartial as possible in this blog with its quest for evidence. The above fiscal data is public knowledge and indisputable. As for evidence in the literature, a simple search of PubMed or the like will corroborate the above. I applaud the continual R&D effort of Align Technology to further our specialty and patient wellbeing, at a massive cost.
The predicament being, not only do individuals such as yourself chose to ignore the significant fiscal investment you deny exists, but when data is collected and presented within our top tier literature, it is either ignored or deemed biased if supported by the company in question. Yet they persist.
Any clinician using an aligner system, treating a significant adult population or using any form of Digital Orthodontics, like it or not, we owe a debt of gratitude to Align Technology; along with the millions of patients who have benefited in terms of oral health and quality of life.
* VV provides sponsored lectures for Align Technology, Pioneer of Digital Orthodontics
I followed this discussion with interest, and I was wondering if there is a breakdown of the 373.3 million spent on R&D to see how this translates to better patient care. In addition, the 2.7 million over 15 years for external funding seems quite low and disproportonate to the internal R&D spending (180K vs 373.3 million per year). Shouldn’t there be more balance between internal and external funding, in order to allow external and independent sources to assess the efficiency of the product?
Thank you
I declare no conflicts of interest related to this message.
Just a couple of points worth a thought:
1. The key words being ‘invested in research to show how good their aligner treatment can be. The lack of investment in this direction…’ largely remain unanswered. The percentage working out by Nikolaos using your own figures speak for themselves.
2. Ignoring research more than 10 years old is illogical. There are advances in health in all parameters and selectively ignoring history on a date of convenience looks unbalanced. Might seem appropriate to compare with evidence the new with the old. Thats assuming we have any new ‘suitable’ evidence.
Well stated. Did you retitle your 2010 editorial into a statement rather than a question? I really liked the original title!
Thank you for your post reviewing this systematic review Kevin, despite your claim that you “do not write about systematic reviews!”; and that as you pointed out, the methodology suffered from the same weakness of other such efforts. Not to mention including 2005 outcomes included…20 years old technology, light years in terms of digital orthodontic evolution.
Motivated by your blog, I clicked on the link to the paper that you provided and was surprised to find a rather different and correct conclusion was published by the authors, considering the strength of any differences, from the one that you have blogged above
“Conclusions:
Both clear aligners and fixed appliances effectively achieve orthodontic treatment goals. Case complexity and patient compliance should guide appliance selection. Further large-scale, well-designed trials with long-term follow-up are necessary to define comparative outcomes.”
Rather favorable compared to yours, and inconclusive at worst. No mention of superiority of any appliance system; as you know could not be validated via the sample.
Results
Fifteen trials involving 1084 patients were included. The risk of bias varied across studies. No significant differences were found in treatment quality or duration between clear aligners and fixed appliances in non-extraction cases, although sensitivity analyses suggested shorter treatment duration with clear aligners. In extraction cases, fixed appliances provided superior treatment quality, attributed to enhanced control of tooth movements. Limited evidence indicated similar post-treatment stability between the two interventions. However, study heterogeneity and short follow-up durations limited the strength of conclusions.
You state above “The main conclusion from this review, and others, is that we do not know much about the outcomes of clear aligner treatment. All we have is a few low-number trials, clinical opinion and social media posts by highly paid Key Opinion Leaders.” I respectfully ask, once again, please evidence to me that this statement is not the same for any individual bracket prescription and wire sequence used. I know – albeit anecdotally and from few weak powered published studies – about outcomes of fixed appliance treatment after working as a specialist orthodontist for over 25 years, wincing at the relapse, divergent root position, anchorage loss, recession and white spot lesions in most patients presenting for re-treatment.
Once again, I ask you to reference the massive bank of research that is assumed conveniently to exist, that provides definitive treatment outcomes using specific fixed appliance systems in large prospective samples. The Richmond / Shaw / O’Brien studies come close (retrospective, combine all FA systems), unfortunately the outcomes were proven to be poor. Let’s call a watermelon a watermelon – Invisalign (because “aligner research” is 99% Invisalign) is one of, if not the most investigated orthodontic appliance systems. Period. Much “fixed appliance” research is poor, combines many different clinicians, mechanical protocols and equipment and identifies variable treatment outcomes. (Also ignores largely iatrogenic and quality of life variables). Yet we may obtain a high standard of outcome with various fixed appliance systems if the principles of diagnosis, treatment planning and adherence to biomechanical principles are respected.
We should be asking the same questions, demanding the same evidence of all orthodontic appliances sold, including every individual aligner system. From your review above, if you are so despondent about the quality of aligner treatment outcome, should you not be equally despondent about the equivalent or statistically insignificant outcome difference resulting from the fixed appliance sample, considering your invalidated perception that fixed appliances are truly superior?
*VV provides sponsored lectures for Align Technology and is not a highly paid KOL!- please define!
Thanks for the message and there is a lot to discuss here. I shall try and be brief.
1. I have had a good look at may post and I cannot find anywhere where I stated that “fixed appliances are superior to aligners”. I have found this statement in the post but this was the opinion of the authors of the paper. It is certainly not my opinion that fixed appliances are superior. My opinion is that we do not know either way, because the research is lacking.
2. This then brings me to my central point. This is that the evidence base for the effectivness of aligners is very low. This was supported by this review and the Delphi consensus papers that have recently been published.
3. You suggest that there is not sufficient evidence to support the effects of fixed appliances.You also draw attention to our early work using PAR on retrospective samples. At this time the main form of study was a retrospective cohort and we did not really know much about bias and the advantages of trials. So we did what were simple studies. These served their purpose and Steve Richmond highlighted deficiencies in the quality of UK treatment. This then led to changes in provision and improvements. Since then there have been many randomised trials of the effects of fixed appliances, for example, the trials of functional appliances followed by fixed, self ligating brackets, methods to speed up treatment and studies looking at bracket design. These all show that fixed appliances provide high quality treatment.
4. So lets move to aligners. I may have missed a large trial on aligners but I am pretty certain that I would have seen it. But if I have, can you let me have a reference for a high quality RCT with good numbers that has been published in a major orthodontic journal.
5. A KOL is anyone who receives money from a company to lecture about or promote their products. If they are a KOL, people may consider that their opinion is not independent of the company.
Again, thanks for your comments. Best wishes: Kevin
Hi Kevin, and many thanks for taking time to reply to my post. I will also try to be brief, not my forte!
1. Thank you for clarifying that you do not believe that fixed appliances (FA) are superior to Invisalign, as we do not know either way.
I contend that we should ask the same questions to and hold the same rigorous standards toward every individual orthodontic appliance manufacturer and cease to negligently accept that all fixed appliance bracket and wire systems and all aligner systems are equivalent.
Regarding the opinion of the authors of the paper discussed, my interpretation of that statement made in the results (and posted by myself), in context of the statistical analysis is different; that they subsequently did not have the power to support that hypothesis, nor support any “attribute” of enhanced tooth movement control – and that’s why it was not a published, nor valid conclusion. Perhaps the authors may step in?
You mention that the Delphi publication that I took part in: Am J Orthod Dentofacial Orthop 2025;168:620-38) Arveda et al. corroborated your assertion that the evidence for the effectiveness of aligners was low; in fact if that were a conclusion, it would not be supported by the M&M of that Delphi, that as you know looks for consensus amongst experts, in this case on the clinical; ability of aligners (in general- heterogenous sample).
Conclusions: “The study, based on the modified Delphi method, collected the opinion of experts, comparing it with the scientific literature to evaluate the potential and limitations of orthodontic aligners, obtaining 47 consensus statements related to biomechanics and extra clinical factors.” I am intrigued that there are actually 47 areas of consensus regarding the ability of aligners!
Another recent Delphi study; D’Anto et al. Prog Orthod 2025 Aug 4;26(1):28. doi: 10.1186/s40510-025-00575-1 also did not test nor support your statement that the evidence base for the – effectiveness – of aligners is low.
(Even if it were stated, the ubiquitous suggestion that more research is required does not mean that it was in fact a tested variable.)
2. I am forced to agree that the evidence base for “aligner- s” is “very low”; in fact, non-existent for most aligner companies.
Contrary to this, Invisalign, (~98% of the research base) has a significant evidence base, all the way to the top of the pyramid. As earlier stated, the largest evidence base of any single orthodontic appliance considering the heterogeneity of brackets and wires. Conducting a literature search will verify this- search for an individual bracket type PRCCT for occlusal and / or quality of life outcome. Kevin, you may take umbrage from the quality of many published studies – I agree. Additionally, I do not consider outcomes from data older than 10 years due to the significant material, mechanics and programming changes.
3. I also agree that we have become more rigorous in our requirements for high quality research, largely thanks to the introduction of digital orthodontics and the increased ability to document and measure -thanks to you know who?!
I’m skeptical of ” …there have been many randomized trials of the effects of fixed appliances, for example, the trials of functional appliances followed by fixed, self-ligating brackets, methods to speed up treatment and studies looking at bracket design. These all show that fixed appliances provide high quality treatment.”
At best, there may be publications that may show that clinicians using a combination of various fixed appliances may result in high quality of occlusion and alignment- using the rulers chosen by the investigators. “High Quality Treatment” may not take into account many variables associated with orthodontic treatment outcome evaluation.
Is there a single reference from the above “many” publications that unequivocally shows the high quality of treatment outcome obtained by FA, so that we may emulate with an aligner product?
Kevin, I cannot find one single prospective RCT FA study, – 120 years of modern orthodontics and current benchmark -with a “large” sample, published in a “major” orthodontic journal that would pass the KOB test for aligner pubs (parameters you list, below); segway into your 4th point.
.
4. When you ask for a “large trial” is that one that reaches statistical significance for what it is measuring, or you have a particular number in mind? Meantime, here are some outcome studies in “major” (There are others as you know, but I’m looking for the quality journals), most prospective. As mentioned, I do not consider occlusal outcome studies older than 10 years valid; quality of life and oral health outcomes exempted.
Outcome assessment of orthodontic clear aligner vs fixed appliance treatment in a teenage population with mild malocclusions Bordaa et al. Angle Orthod. 2020;90:485–490.)
Comparison of clinical outcomes between Invisalign® and conventional fixed appliance therapies in adult patients
with severe deep overbite treated with nonextraction Fujiyama K, Kera Y, Yujin S, et al. Am J Orthod Dentofacial Orthop. 2022 Apr;161(4):542–547.
Management of overbite with the Invisalign® appliance Khosravi et al. Am J Orthod Dentofacial Orthop. 2017 Apr;151(4):691–699
Discomfort associated with Invisalign and traditional brackets: A randomized, prospective trial
David W. Whitea; Katie C. Julienb; Helder Jacobc; Phillip M. Campbelld; Peter H. Buschange Angle Orthod. 2017;87:801–808.
Braces versus Invisalign®: gingival parameters and patients’ satisfaction during treatment: a cross-sectional study Azaripour et al. BMC Oral Health (2015) 15:69 Periodontal status of adult patients treated with fixed buccal appliances and removable aligners over one year of active orthodontic therapy Karkhanechia et al. Orthod. 2013;83:146–151.)
A comparison of treatment impacts between Invisalign® aligner and fixed appliance therapy during the first week of treatment Miller KB, McGorray SP, Womack R, et al. Am J Orthod Dentofacial Orthop. 2007;1(3):302
)….and coming – the multicenter outcome study that I sent you the protocol for – )
Finally, thank you for your definition of a “KOL”. I was specifically looking for the definition for the term that you used and use frequently “highly paid KOL”. Yes, I am paid to provide lectures for a company that does manufacture orthodontic appliances. My 2025 contract provides for $1.291.22 GBP for a 1-hour presentation. No travel time reimbursement, no preparation fee; no free cases; clearly, I make more money staying in my practice and treating my patients. Why then? Because I love teaching, I am passionate about “I” and the increased, noninvasive clinical ability I can offer patients, and I learn about orthodontics as a whole each time I prepare a lecture. We have all learned from KOL’s , using our own discretion, and I am following in Angle’s footprints.
Thanks for taking the time to share your perspective Kevin,
VV* provides lectures for Invisalign