January 02, 2023

Going solo with Invisalign?

Happy New Year to all our readers.  Padhraig has written the first post of the year. This is on the need to change from aligners to fixed appliances, in order to finish treatment.

Aligner therapy continues to gain traction among the dental and orthodontic community. Aligner treatment now accounts for 10% or more of the caseload of orthodontists in the United Kingdom. Their popularity primarily relates to pleasing aesthetics, patient experiences and good tolerance (at least among compliant patients). However, numerous studies have shown that the predictability of tooth movement is limited. For example, severe rotations and deep overbite correction remain challenging. As such, some do not see aligners as a standalone solution, and most orthodontists offer both fixed appliances and aligners.

Moreover, it is not uncommon for fixed appliances to either precede or follow aligner therapy as a ‘hybrid’ approach to achieve optimal outcomes and enhance efficiency. There is, however, little information regarding the requirement to transition to fixed appliances during Invisalign treatment. Therefore, the authors attempted to study this in this paper published in the American Journal of Orthodontics and Dentofacial Orthopedics.

What percentage of patients switch from Invisalign to braces? A retrospective study evaluating the conversion rate, number of refinement scans, and length of treatment.

American Journal of Orthodontics and Dentofacial Orthopedics, 18th December 2022.


Neal D Kravitz, Dalloul B, Zaid YA, Shah C, Vaid NR.

What did they do?

They conducted a retrospective study evaluating 500 patients who had commenced treatment with either Invisalign Full or Invisalign Teen.


Participants were 14 or older, with an average age of 33.6 years. Those commencing treatment with sectional fixed appliances, requiring extractions, surgical exposure or restorative work during treatment were excluded.


Invisalign Full or Invisalign Teen.


The need for fixed appliances to detail the outcome with Invisalign, number of refinements and treatment duration

What did they find?

Overall, 1 in 6 patients (17.2%) required a hybrid approach with fixed appliances to detail the outcome. An average of 2.5 refinement scans were needed in each case. Interestingly, only 6% completed treatment without a refinement stage. This group required an average of 22 aligners. Conversely, 41% required 3 or more refinements overall. The overall mean number of aligners was 64, and the average length of Invisalign treatment was 22.8 months, which exceeded the projected treatment length by 5 months.

For those requiring a hybrid approach, the average number of aligners was 81, and the mean duration of the fixed appliance phase was 7 months. More refinement scans did correspond with a higher likelihood of the need for fixed appliances.

What did I think?

I liked this study because it provided us with new and helpful information. It gives us ‘fly on the wall’ insight into how aligners are used by one world-leading orthodontist. As clinicians, we know that many patients are aware of and are knowledgeable about aligners before seeing us. Equally, experiences among adults using aligners tend to be positive. However, as Peter Greco suggested in a recent article in the AJO-DO, ‘Ideal treatment objectives should be approached without bias toward the appliance used to meet those objectives’. The frequency with which fixed appliances were used as an adjunct to aligners in this study supports the idea that ‘going solo’ risks unnecessary compromise in many patients.

From a methodological perspective, the study was limited, being retrospective and confined to just one practitioner’s office. There is, therefore, a question mark around external validity. There is also an increased risk of selection bias, whereby the researchers may have been tempted to select those performing better. Notwithstanding, as the study was directed at identifying patients who may have fared less well, I think this risk is relatively low. Moreover, given the provider’s experience, the relatively high frequency of transition to fixed appliances (17%) and near routine requirement for refinement are interesting. In particular, the practitioner is experienced and, therefore, likely to have carefully selected patients and planned cases (thus minimising the likelihood of refinement). However, this is likely to be balanced by his high standards and a low tolerance for imperfection prompting the decision to either undergo further refinement or to transition to fixed appliances.

Final thoughts

Finally, it would be helpful to understand the specific reasons for additional refinements or the need for fixed appliances. As such, a more granular evaluation of these cases may help assist our treatment planning. This information may, in turn, also be helpful in better informing our consent processes.

What can we conclude?

Aligners are a tool. Fixed appliances are another. Both offer advantages. But both subordinate to trained clinicians. Some (like me) are more comfortable using fixed appliances in most scenarios and particularly to optimise finishing. However, if this study proves anything, it may be that the ability to use both approaches well is becoming increasingly indispensable.

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Have your say!

  1. HNY Kevin all all fellow bloggers!
    Thanks for the study detailing how one individual best finishes their occlusal result. My thoughts on this blog are that “finishing” is the treatment phase requiring additional attention and is required to both accomplish movements that are not fully expressed or to compensate for movements created by applied mechanics that are unfavorable to our occlusal goal – both with Fixed appliances or Computer programmed aligners. When using Fixed appliances, we reactively need to consider re-bonding, wire bending, elastics or even for some….positioners; when using CPA we may “finish” proactively in determining final occlusal goal and consider either proactively or reactively additional aligner orders- the traditional and ubiquitous CPA finishing tool – for the same 2 reasons noted above relating to both mechanical systems, and additional reasons such as compliance (is it mentioned/ measured?) and the fact that we never get 100% of programmed movement – be it when bending a wire or working with a system that involves many engineering steps. For me, bottom line is it is necessary for both systems, takes additional effort and no appliance system – digital brackets using pre-bent wires or CPA – is or ever will be capable of being programmed in 1 step start to finish for every patient due to the most significant variable – those of the patients. Its the 21st century – to me the more accurate and measurable solution to “finishing” is / will be digital. The beautiful thing is we can all choose our tools, albeit wisely.

  2. A great post to start the year. I see nothing wrong with a hybrid approach with aligners and fixed appliances. We have certainly used a combined removable and fixed appliance approach for decades in orthodontics. The combined appliance approach helps address some of the esthetic concerns of patients while allowing the clinician to have a final judgement on the detailing and refinement of the occlusal finish.
    As long as the financial considerations of hybrid appliances are not burdensome on the patient or provider, and we initially acknowledge the possible conversion to fixed appliances to detail the result, a hybrid approach provides a very acceptable treatment mechanic. It also can reduce the tendency to practice “squintodontics”: the tendency to squint just enough to feel that the result looks good enough to be acceptable!

  3. I think this article sheds light on the significant weakness of the aligners, especially in the finishing stage of orthodontic treatment. But the finishing is not only the last step in the treatment. It is the primary goal of the treatment; therefore, it illuminates the major weakness of the aligners. Orthodontics is a profession based on the laws of biomechanics, which are based on the laws of mechanics. The mechanical capabilities, and therefore the biomechanical capabilities of the aligners, are limited to only one force: the tipping. Alternatively, as we call it professionally: one point force application, where the force vector is directed away from the center of resistance of the tooth. No other movement, but tipping, can be developed by aligners since aligners belong to the removable appliances group. How many of you still remember the Hawley appliance as a tool to move teeth and its weaknesses? Aligners, sorry to say it, belong to this group – and own all its weaknesses and traits. Most orthodontic movements cannot be developed and further executed by aligners. For example, true intrusion and extrusion movements should be along the longitudinal axes of the tooth and not as a result of tipping movements of the crowns, as is performed by the aligners. By tipping, you change the vertical location of the edge of the tooth, but one definitely cannot define it as intrusion or extrusion.
    Further, the unfeasibility of developing a couple on the crowns of the teeth, a force that only fixed appliances can develop, as Brody said in his 1933 article – the greatest weakness of removable appliances is their being removable. There is no possibility of developing a true long-term couple (force), which only the fixed appliance can develop, and, by that, the most complex movement in orthodontics – the torque. Only fixed appliances can deliver long enough couples on the teeth’s crown to develop root movement of the upper incisors, for example, when needed, according to Andrews’ 1972 publication, as well as allow us to develop root parallelism in all areas we need, especially in extraction cases. As I see it, those reasons will make the fixed appliances alive, unless the goals of the finishing cases, as determined by the ABO, for example, will be changed to the weakness of the aligner’s goals.

  4. Dear Sir,

    Thank you for this very interesting article – more akin to a revealtion (as you point out) rather than a rigorous piece of research.

    I would like to add that nothing succeeds like success. A popular and capable clinician would be inclined to help their patient in the best way the patient wants and therefore might even push themselves to the limit of what a particular treatment modality might be able to achieve. 80+ aligners is not something that any patient would be keen on to begin with, no matter how difficult the case is. The patients are more likely to agree with a hybrid approach if advised at the start about the need for refinement(s).

    As more and more children are treated with conventional fixed orthodontic appliances, their acceptance is increasing (albeit the cosmetic ceramic versions) among the parents and older family members.

    One size fits all never works in medicine, least of all in dentistry. This study is another reminder that it is best practice to involve a Specialist Orthodontist at the treatment planning and consenting stage (at the very least, regular supervision being ideal) for better and predictable treatment outcomes.

    Thank you.

    Yours sincerely,

    Mr Karun Sagar, BDS, MJDF
    Clinical Assistant in Orthodontics

  5. Interestingly, 35 years ago it was not uncommon to use aligner “finishing retainers” to finalize aesthetics and occlusion after the fixed appliance. Now we often see the fixed appliance needed after aligner therapy to finalize aesthetics and occlusion.

  6. Could not agree more with Dr. Naphtali Brezniak.
    Persistent KOL protestations notwithstanding ( did not see a disclosure statement from the Align KOL by the way), aligners simply do not (and cannot) predictably generate the M:F ratios needed for complete leveling, translation, arch form control, and root paralleling.

    Are we really arguing that a removable appliance will deliver couples of the magnitude, direction and duration (and in all 3 planes of space) like a fixed appliance would, during space closure? That would be like comparing a Maserati to an Oxcart.

    While there may be conflicts of interest galore, how does one ignore Biomechanics 101 when suggesting that appliances delivering a single force at the coronal level can even approach the complex, statically indeterminate force system generated by fixed appliances? And that occlusal finishing with a piece of plastic interspersed between the arches is equal to that obtained without? There is simply no comparison.

    • Hi Dr Raj –
      If you are referring to my comment above, as the “persistently protesting” KOL with non-disclosure; as my response was a general observation on the topic of “finishing” I did not believe that it was necessary to communicate- that I teach my 26 year experience with the Invisalign system both at Align Technology events and independently at Professional meetings, also within graduate University programs, never having owned a share, but yes, receiving an honorarium if speaking at Align events. I respectfully hope that clarifies my potential bias on the topic of “finishing” ). Further Disclosure – I also made a comment on the following topic of “CAT use in Australia” blog – and did not make any disclosure, for the same reason. As for the discussion below, my disclosure is relevant as I would like to challenge some of the above comments using Invisalign as an example.
      Speaking to only some of the assertions made – on the basis that no removable appliance system “ is or will ever be” capable of applying moments and couples “for long enough”, or that extraction spaces may not be closed with parallel roots, simply because the appliance is “removable” or because the material is “plastic”, I beg to disagree. There is ample literature available measuring complex force systems plastics are capable of expressing both within and external to the orthodontic specialty; as well as documented well finished “extraction cases”. Force-driven CAT systems (I am not sure if Invisalign is the only one) are engineered to apply force not by tooth displacement – aligner “ full contact “ and shape determined by tooth shape,- but by modifying the shape of an aligner to contact only in specific sites to generate the force system required to move a tooth into the clinician determined position. The Invisalign Force driven system was mostly designed by John Morton, chief engineer at Align Technology – yes the same John Morton who published many times with the great Charlie Burstone and contributed to SureSmile Technology. While some may wish to question my ” persistently protesting” understanding of 101 Biomechanics, I am fairly certain that no-one would question his? – (notwithstanding his financial interest. ). To me, the unknown is not whether plastic / removable materials may deliver complex force systems , but how they are expressed in vivo, considering biologic variables and super-imposing the removable nature of these appliances. What is the wear threshold required to express any complex force system – I doubt it is , as asserted above 100% (this doesn’t happen with fixed due to inevitable force decay and variation in re-activation times ) and what is the effect of adjacent forces on each other? We are better positioned to model and measure these parameters in a digital system.
      Considering comments as to the inability of an appliance with a “piece of plastic interspersed between the arches” to “finish” occlusion , let’s not forget Physiology 101 – that we have “Freeway Space”. Teeth are contacting around 6-30 minutes a day, depending on your article of choice, during chewing and swallowing. Aligners are removed during mastication and teeth are “free” to move” and interdigitate. Patients with fixed appliances also do not walk around with their teeth contacting, even if box/ other elastics are used. I am supposing that anyone who feels as though an appliance with occlusal coverage cannot finish occlusions well, never dares use Essix-type retainers. Using the same logic, the “passive” occlusal cover would destroy the hard-won occlusal finish?
      Finally, more on the topic of “orthodontic finishing”, Robert Kaczmarski’s comments in the following weeks blog, I found insightful.

  7. This article confirms my experiences.

    It’s taken me 25 years to learn the limits of what aligners can do for patient satisfaction.

    If a patient choses aligners to please their aesthetics, then how on earth does it help to say “We must to include labial braces to start or finish”

    “Oh, and because this is a double-up in treatment modalities, it involves more time and more money”

    This is why I have also developed my skills in lingual braces over the last 25 years.

    Lingual braces have a lot of answers here.

    I have used many systems. I currently use a lingual system of robotically made wires (SureSmile) and stock brackets. My way is just one of many fixed lingual appliance systems. Lingual systems all provide invisible orthodontics that provide stronger mechanical capabilities than aligners. It’s not perfect. Occasionaly we finish with 3 to 6 clear aligners. But this is going from one invisible system to another.

    Much orthodontic discussion about appliances is un-nessarily and unfortunately blinkered, polarised and narrow. About one system “versus” another. This is so sad. It is not one thing versus another.

    We need to be doctors with a broad inventory of therapy agents and use many systems taking the advantages of all systems.

    To provide the best possible, doctors must put all possible tools into their toolbox. No matter the name, the different tools have strengths and weaknesses. A doctor’s duty is to know what tools to access, be well-trained in their use, and know when to use, and when not to use.

    Orthodontists are biomechanical engineers. Each of our tasks is different in every case. Unfortunately, our handbooks are indeterminate on tool selection.

    It is important to be objective and independent of KOLs or any type of super-advocate with biased motives for what is said.

    Publications in independent journals are our guiding light. This new contribution to the literature is helpful and adds to the increasing body if evidence.

    There is still the non-science to consider in this discussio. i.e. the ethics and health economics questions. For example,

    Let’s put this to the patient who came for invisible treatment….

    “Sure you can have invisible aligner treatment”.
    As long as it’s ok to have braces too!. So we don’t compromise your outcome”.

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