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.
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.
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.
Professor of Orthodontics, Trinity College Dublin, The University of Dublin, Ireland