September 04, 2023

Lateral incisor attachments: Out with the old, in with the new.

The lack of evidence underpinning aligner therapy is gradually being addressed. However, most recent studies have been retrospective, typically involving performance evaluation relative to the ClinCheck (which incidentally is a force representation system, NOT a simulated outcome as so many seem to suggest). However, we remain in the dark concerning the relative merits of various staging protocols and attachment designs. 

Lost tracking of the maxillary lateral incisor is a problem we see all too often, with recovery not predictable. A range of factors may be at play with planning, staging, inadequate space and failure to control neighbouring teeth with higher anchorage values, often the root cause. However, the lateral incisor’s attachment design is worth consideration.

So, what are the possible options in terms of lateral incisor attachments? Many see This as a ‘dark art’, and I think the authors explain this clearly. I have directly quoted from the Introduction: ‘To improve the predictability of various tooth movements with aligners, practitioners prescribe attachments to be bonded to those teeth requiring more control during treatment … The first attachments introduced were ellipsoid in configuration, and they were largely replaced by more sharply defined and bulkier conventional attachments: horizontal, vertical, and beveled attachments, to improve control. Align Technology introduced optimised attachments as a SmartForce feature to facilitate larger movements such as rotation .5 degrees or extrusion .0.5 mm on certain teeth. Optimised attachments are typically smaller than horizontal attachments but are specifically designed to include a gap in the aligner’.

The authors of this study give attachment design due consideration in an interesting clinical trial comparing Optimised attachments and three horizontal attachment designs: horizontal nonbeveled (H), horizontal incisally-beveled (HIB), and HGB attachments. 

Effect of clear aligner attachment design on extrusion of maxillary lateral incisors: A multicenter, single-blind randomised clinical trial.

Groody JT, Lindauer SJ, Kravitz ND, Carrico CK, Madurantakam P, Shroff B, Darkazanli M, Gardner WG.

Am J Orthod Dentofacial Orthop. 2023 Aug 21:S0889-5406(23)00422-5. doi: 10.1016/j.ajodo.2023.07.011.

What did they do?

They conducted a multi-centre, four-group, randomised controlled trial involving 40 participants aged 16 and over. Randomised using a split-mouth design was undertaken at the tooth (rather than patient) level.


Participants were recruited from three orthodontic practices. They all needed a minimum of 0.3 mm of extrusion during the first 20-25 Invisalign therapy series. Either optimised, rectangular horizontal nonbeveled, rectangular horizontal incisally-beveled, or rectangular horizontal gingivally-beveled (HGB) attachments were randomised for placement on the upper lateral incisors. There was a maximum of 6mm of upper arch crowding or spacing with all teeth present and erupted. Patients with severely rotated (>15 degrees) anterior and anterior crossbites were excluded.

Intervention Groups: 

One of the four attachment designs was placed on the maxillary lateral incisor with overcorrections permitted during the ClinCheck. All horizontal attachments were a minimum of 4mm in width. A movement limit of 0.25mm per aligner was set with flexibility regarding planning and IPR prescription. Participants were asked to wear each aligner for up to 22 hours daily for 7 days each. Midcourse procedures to improve tracking, including rescanning or the use of auxiliary appliances, were recorded but were not analysed as part of the corresponding group. 

After the first series, a blinded evaluator measured extrusion using superimpositions between initial and predicted models using STL files using a best-fit model on the posterior teeth. The discrepancy between the predicted extrusion and that obtained was assessed with the effect of patient age, gender, number of trays, and self-reported compliance being considered.

What did they find?

Data were obtained from almost all participants (38 overall), with two patients having HGB attachments lost to follow-up due to poor compliance. The number of attachments with each design varied (from 23 with Optimised to 14 with HGB). The prescribed extrusion was similar in all four groups, ranging from 0.31 mm to 2.46 mm, with an average of 0.84 mm. The amount of extrusion achieved was significantly less than that predicted (0.21 mm less), reflecting 73% of that planned.  

Importantly, attachment type was a predictor of success, with the optimised attachment faring least well and little to choose between the three horizontal alternatives. Regarding the proportion of extrusion expressed, 62% occurred with the optimised attachments, while a mean of 79% was affected with the horizontal designs. A further statistical model accounting for the effects of clustering and potential confounding factors had little impact on the trends, with an estimated 22% improvement in the predictability of extrusion using a horizontal attachment compared with an optimised design.

What did I think?

I think that this was an excellent study. There have been many retrospective studies involving aligners and, sadly, many systematic reviews based on these limited data. It is, therefore, excellent to see well-designed prospective research in this area. 

The methodology and writing are both excellent, and the paper has been written very well, with a candid discussion of some limitations. The findings are interesting and probably reflect what many of us have felt regarding optimised attachments. How many times have we planned cases on patients who are reticent to have visible, bulky attachments? My default has often been to accede to patient wishes for subtlety (creating more work for myself) primarily because we had ‘no proof’ that large attachments are superior. This study will give us a little more courage in our convictions.

I am pleased to see the partnership between specialist practices and a university setting. Most orthodontic treatment is performed in offices, while research is often confined to academic settings. The inclusion of high-volume users of proprietary systems is therefore valuable for ensuring that the study reflects a ‘real-world’ scenario and best contemporary practice.

The sample size is a little low. While I accept that a sample size calculation is presented, I wonder whether this low sample may have led to limited statistical power (with clinically significant differences not always borne out in the statistical tests). But also, the effect of confounding factors (including crowding) may be amplified. The selection criteria were also quite broad, with up to 6mm of crowding or spacing included. These are, however, relatively minor caveats associated with an excellent, relevant piece of research.

What can we conclude?

Based on this excellent clinical trial, using more traditional, bulkier horizontal attachments may offer more predictable extrusion than optimised Invisalign attachments. All that glitters may not seem to be gold.

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Have your say!

  1. This month marks my 24th year of doing clear aligners specifically Invisalign. I have never adopted the optimized attachments. One clue that they did not work as well as conventional. Is that the company never provided any research to back up? What they were proposing. I felt it was a marketing ploy. I use beveled attachments exclusively. I also use them upside down compared to what is recommended. I do believe the larger attachments work better than the 3 mm attachments. However, I tend to use a lot of 3 mm attachments not only for the aesthetics, but for ease of removal. I only use the 4 mm and greater attachments for very difficult movements. Thank you for reviewing this article and I’m excited to see more of these so that my experience with CA is the same as others. David

  2. Interesting parts for me:

    The biggest was of course that
    “precision attachments” were the least effective in correcting rotations. With “22% improvement in the predictability of extrusion using a horizontal attachment”, any horizontal attachment, “compared with an optimised design”.

    This would seem another nail in the coffin to the concept that there is some magic superiority of a certain corporations algorithms that produces those “force system representations”, precision attachments, or ClinCheck cartoons.

    The average extrusion ranged “from 0.31 mm to 2.46 mm, with an average of 0.84 mm” and greater than 15% rotation was considered severely rotated and grounds for exclusion.

    With those levels of movement being the average and cutoff levels respectively, it is interesting that some are still seemingly claiming that aligners are equal to braces in all regards. I don’t expect that any of us would struggle achieving 100% of those prescribed corrections with braces.

    The use of a multi center high volume practices as part of a randomized controlled trial would seem to have a benefit in addition to the inclusion of real world situations and a well constructed research protocol. That is that the 1st critiques which seem to come up from those who seem completely aligner aligned are of the nature;
    “They weren’t experienced clinicians at the XYZ level with the proprietary company” or
    “The research was done by residents who don’t understand aligner biomechanics” or
    “They weren’t using the latest improvements”.

    Using experienced clinicians and practices would seem to eliminate all of those now standard rhetorical replies when research shows that an companies aligner system doesn’t do exactly what it and their KOLs claim.

    I am reminded of another recent research paper showing that with attachments, bigger is better, producing closer to the desired movements.

    However, if we go bigger with attachments, considering how large anterior attachments look on a patient compared to the appearance of ceramic braces, sometimes the lack of difference is what is most striking.

  3. My friend Dr. Harnick makes some excellent observations. I do believe optimized attachments (OA’s) work better on some teeth than others. . I’ve used everything imaginable on L3’s…5,4 and 3mm HBGA and HIBA, sashes, lingual, double A’s… and have found nothing better than OA’s, and allow the software to choose which one: Optimized extrusion, rotation, deepbite, root control, retention or multiplane. In my practice, if the L3’s and incisors aren’t aligned after 20-30 weeks, the pt. is not committing enough time to each aligner.

    But the topic is U2’s I have long thought that the U2’s movements are more dependent on what is happening with the 1’s. Most cases require intrusion and torque on the 1’s. If the 1’s don’t track as programmed, then the aligners soon won’t fit the A’s on the U2’s no matter their design. Another important consideration if multiple movements are needed for the 2’s, how are they prioritized?

    Anecdotally, I’ve noticed the ease of movement of the U2’s if the 1’s are missing. Also the U3’s…like the L3’s…appear to move well for me with OA’s. Although I’m not a fan of OA’s on U3’s when Class 2 hooks are involved.
    Exciting times…Keep the research coming!

  4. For extrusion of laterals, I always try to place a lingual attachment as well.
    if the buccal attachment has the same or less angle as the lingal surface of the tooth, extrusion is almost impossible of you visualise all the vectors (if there is no undercut by placing a lingual attachment, the tooth is pushed lingual and will fail to track with the aligner).

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