Achieving Predictable Rotations with Clear Aligner Therapy

While aligners have revolutionized orthodontic treatment, rotational movements—particularly of rounded teeth like canines and premolars—remain one of the most challenging aspects. Let’s explore the key elements for success, limitations, and opportunities regarding rotational movements with clear aligners.

Understanding the Challenge

Rotational movements with clear aligners face several inherent challenges. The rounded morphology of canines and premolars provides limited undercuts for aligners to grip and rotate effectively. Attachments are added to help with rotations but sometimes leave clinicians frustrated with insufficient correction, as tracking and aligner fit can be issues with significant rotational movements.

Key Elements for Success

Despite these challenges, several strategies can improve the predictability of rotations. The use of attachments significantly improves rotational control. Studies suggest that optimized rotation attachments or vertical rectangular attachments have shown better results than horizontal or ellipsoid designs. For mandibular canines, optimized rotation attachments with 1-week aligner wear may be most effective [2].

Careful staging and velocity control are crucial. Limiting rotational movements to 2° or less per aligner and considering slower staging for more difficult rotations, especially of rounded teeth, can improve outcomes. Strategic interproximal reduction (IPR) can create space to facilitate rotation, particularly for severely rotated teeth. Studies have shown enhanced rotational accuracy when IPR is incorporated [3]. Consider making space in the treatment plan adjacent to the rotations to gain more predictability whenever significant rotations occur.

Building overcorrection into your treatment plan, especially for canines and premolars, can help achieve desired end positions. Consider programming an additional 20-30% rotation beyond the desired end position. Newer aligner materials like SmartTrack have shown improved force delivery and maintenance compared to earlier materials [1].

While some studies show no significant difference between 1-week and 2-week wear protocols for rotations, others suggest that 1-week changes benefit specific movements. Consistent wear (20-22 hours per day) is crucial regardless of change interval. Regular monitoring to assess tracking is essential, and practitioners should consider additional aligner orders for rotations that aren’t tracking as planned [4].

Force Systems for Rotational Success

Understanding the biomechanics of rotation is crucial for success. Aligners need to create a couple (two equal and opposite forces) to generate the moment necessary for rotation. We can create a couple with bucco-lingual force features, buttons, and a power chain. Hinge rotations can also help certain teeth to gain leverage on the rotation. The further from the center of resistance the force is applied, the greater the rotational moment. It’s crucial to design attachments and aligner features to resist undesired tipping or other movements that may occur during rotation. Consider a progressive force system where the aligner geometry changes slightly with each stage to maintain optimal force levels throughout treatment [5].

Limitations and Opportunities

While clear aligner therapy has made great strides in managing rotations, some limitations persist. Studies show that achieved rotations often fall short of predicted movements, with accuracies ranging from 60% to 80% for most teeth. Significant individual variation in response to treatment makes precise prediction challenging. Second molars and canines tend to show the lowest rotational accuracy [1,4].

However, these limitations also present opportunities for innovation. Research is ongoing to develop new aligner materials with improved force delivery and resilience. AI-driven treatment planning, using machine learning algorithms to optimize attachment design and movement staging based on individual tooth morphology and treatment response, holds promise for the future. Some clinicians use hybrid techniques, strategically using fixed appliances or auxiliaries with aligners for difficult rotations.

Conclusion

Achieving predictable rotations with clear aligners requires a thorough understanding of biomechanics, careful treatment planning, and strategic use of attachments, space creation, and IPR. While challenges remain, continued research and technological advancements steadily improve our ability to manage these complex movements. By incorporating these principles and staying abreast of the latest developments, we can confidently tackle rotational corrections and deliver excellent patient results.

References:

1. Grünheid, Thorsten, Charlene Loh, and Brent E. Larson. “How accurate is Invisalign in nonextraction cases? Are predicted tooth positions achieved?.” The Angle Orthodontist 87.6 (2017): 809-815.

2. Stephens, Caitlin, et al. “Clinical expression of programmed mandibular canine rotation using various attachment protocols and 1-vs 2-week wear protocols with Invisalign SmartTrack aligners: a retrospective cohort study.” American Journal of Orthodontics and Dentofacial Orthopedics 162.3 (2022): e103-e115

3. Xie, Jinchen, et al. “Factors affecting the efficacy of Invisalign in anterior tooth rotation.” American Journal of Orthodontics and Dentofacial Orthopedics 163.4 (2023): 540-552.

4. D’Antò, Vincenzo, et al. “Predictability of tooth rotations in patients treated with clear aligners.” Scientific Reports 14.1 (2024): 11348.

5. Maree, Amesha, et al. “Clinical expression of programmed rotation and uprighting of bilateral winged maxillary central incisors with the Invisalign appliance: a retrospective study.” American Journal of Orthodontics and Dentofacial Orthopedics161.1 (2022): 74-83.