Invisalign Aligners Fall Short on Extrusion and Tipping, Study Finds

Invisalign clear aligners deliver highly predictable expansion and rotation of the upper teeth but struggle with extrusion and labial crown inclination, according to a retrospective study published in the journal Digital and Aligner Orthodontics. Researchers from the University of Montreal and private orthodontic practices in Quebec analyzed 447 upper teeth from 61 patients and found that the overall predictability of movements across all attachment types was 29.5 percent for extrusion, 33 percent for labial crown inclination, 70 percent for rotation, and 86 percent for expansion. The median predictability of all movements was 65.55 percent, meaning that roughly one third of the planned tooth movement did not materialize as predicted in the ClinCheck virtual treatment plan.

The study, led by Gabrielle Paquet, Clarice Nishio, Pierre Rompré, and Louis Dorval, set out to evaluate how closely the movements achieved with Invisalign aligners match the movements simulated in the ClinCheck virtual treatment plan, and to compare the efficiency of two different types of attachments, conventional and optimized, for rotational and extrusive movements on the upper dentition. Attachments are small composite shapes bonded to the teeth that give the aligner a better grip and allow it to apply force more precisely. Conventional attachments are non-specific and can be oriented according to the desired movement and force direction, while optimized attachments are automatically generated in the virtual treatment plan when a specific tooth requires more than 0.5 millimeters of extrusion or more than 5 degrees of rotation.

To measure what actually happened to the teeth, the researchers superimposed pre- and post-treatment digital models on the entire maxillary model, including the palate, using the 3D Slicer platform and the iterative closest point algorithm. This method aligns surface models by minimizing the distances between their respective vertex sets, and it allowed the team to use the largest area of the gingivae and palate as a stable anatomical reference rather than relying on posterior teeth that might themselves have moved during treatment. The researchers then measured extrusion and labial crown inclination of upper incisors, rotation of upper canines and premolars, and expansion of the first molars and premolars, and compared those measurements with the tooth movement table of the ClinCheck plan.

The predictability in percentage was calculated from a formula that subtracted the absolute difference between predicted and achieved values, expressed as a proportion of the predicted value, from 100 percent. When achieved movements occurred in the opposite direction of the planned movement, achieved values were set to zero for the primary analysis to reflect a clinically meaningful definition of predictability. This situation occurred for only 6 teeth out of the 340 evaluated, or 1.8 percent of the sample. To ensure intra-examiner validity, the examiner twice measured all four movement types on 10 randomly selected models, with an interval of several days between measurements, obtaining intraclass correlation coefficients ranging from 0.915 for rotation to 0.992 for extrusion.

The results revealed a clear hierarchy of predictability. Expansion was the most reliable movement, with a median predictability of 86.25 percent, broken down into 81.34 percent for first molars, 86.36 percent for second premolars, and 89.6 percent for first premolars, with no significant differences among the three groups of teeth. Rotation was also reasonably predictable, with canines achieving 66.36 percent and premolars 74.96 percent, for a total median predictability of 69.96 percent. Premolars showed a smaller discrepancy between achieved and predicted rotation than did the canines, a difference that was statistically significant at p less than 0.001.

Extrusion and labial crown inclination told a very different story. For extrusion of central and lateral incisors, predictability was 27.03 percent for the former and 32.17 percent for the latter, resulting in a 29.43 percent median predictability for both teeth. Central incisors achieved greater extrusion than laterals when comparing the predicted and achieved difference, a result that was statistically significant at p equal to 0.011. For labial crown inclination of upper anteriors, the median predictability was 33.23 percent, resulting from 25.75 percent for centrals and 42.8 percent for laterals. Only the laterals demonstrated significantly greater labial crown inclination relative to predicted values when compared to central incisors, at p equal to 0.023.

The comparison between attachment types produced a notable asymmetry. When conventional attachments were compared to optimized ones for extrusion, no statistically significant difference was observed, with a 0.06 millimeter difference between achieved and predicted measures between both types of attachments. In contrast, the difference in rotation between the two attachment types was significant, with conventional attachments achieving 1.95 degrees more rotation than optimized attachments, a result that was statistically significant at p equal to 0.002. Conventional attachments also showed a smaller difference between achieved and predicted values than optimized attachments, as well as a difference in predictability of 10.75 percent. Overall predictability for conventional rotation was 81.24 percent, optimized rotation 70.15 percent, conventional extrusion 27.32 percent, and optimized extrusion 31.89 percent.

Despite the statistical significance, the researchers caution that the 1.95 degree and 10.75 percent differences between attachment types are likely not clinically relevant. This means that clinicians might use their personal attachment preferences in treatment planning, with respect to size and ease of placement, rather than expecting one type to outperform the other in a way that would change the clinical result. The finding that optimized extrusion attachments are as efficient as conventional ones for extrusive movements, while rotational movements showed better efficiency with conventional attachments, adds to a literature that the authors describe as inconclusive, since very few studies have been published on the subject.

The results are consistent with previous reports in the literature, which have demonstrated movement predictability ranging from 41 to 59 percent. Extrusion seems to be a particularly difficult movement to achieve, and the authors suggest this may be explained by the fact that extrusion requires the aligner to effectively grip the tooth in order to generate an extrusive force, which is challenging given the elastic properties of the aligner material, the absence of significant undercuts on the tooth, and the smaller size of the upper lateral incisor compared with the central incisor. Additionally, labial tipping of a tooth may cause relative intrusion, which could contribute to the low predictability observed for extrusion. To improve extrusion predictability, clinicians may consider using auxiliaries like elastics on teeth in cases requiring greater amounts of extrusion.

The study had several limitations. Patient cooperation and aligner wear time are difficult to control, and although cooperation was monitored with follow-up charts at each recall appointment and no loss of tracking was observed for the measured teeth, actual wear time is probably a little less than the prescribed 22 hours per day even for compliant patients. Teeth were analyzed as independent observations even though multiple teeth originated from the same patient, a clustering that may result in an underestimation of variance and an overestimation of statistical significance. The patients were treated in three distinct orthodontic clinics, two private practices and one university orthodontic clinic, and all clinicians have their own treatment preferences and level of experience in planning the virtual treatment plan. The authors also note that the study evaluated only the initial phase of treatment, corresponding to the early stages of tooth alignment, and that a second series of aligners may be necessary to further improve alignment. The ClinCheck virtual treatment plan is a representation of a force system and not a final accurate outcome, and the authors recommend that clinicians apply additional overcorrection for extrusion and labial crown inclination movements in order to achieve the expected clinical result.

Subject of Research: Predictability of tooth movements achieved with Invisalign clear aligners and the efficiency of conventional versus optimized attachments

Article Title: Predictability of movements achieved with Invisalign® clear aligners: a retrospective study

Article References: Paquet, G., Nishio, C., Rompré, P., & Dorval, L. (2026). Predictability of movements achieved with Invisalign® clear aligners: a retrospective study. Digital and Aligner Orthodontics, 1(1), Article 10. https://doi.org/10.1007/s44525-026-00011-7

Image Credits: AI Generated

DOI: 10.1007/s44525-026-00011-7

Keywords: Invisalign, clear aligners, orthodontics, tooth movement predictability, ClinCheck, attachments, extrusion, rotation, expansion, labial crown inclination, retrospective study, 3D superimposition