Let’s explore one of the most common treatment goals in orthodontics – arch expansion. With the growing popularity of clear aligner therapy, it is crucial to understand how we can achieve predictable expansion results using this treatment modality. Let’s dive into the key elements for success, limitations, opportunities, and critical force systems regarding arch expansion with clear aligners.
Key Elements for Predictable Arch Expansion
Proper case selection forms the foundation of successful expansion with clear aligners. Mild to moderate crowding cases with transverse discrepancies of up to 6mm tend to be more predictable[1]. We should be cautious with severe crowding or skeletal transverse deficiencies, as these may require additional expansion methods or surgical intervention.
Accurate digital treatment planning is essential for achieving desired results. Utilizing ClinCheck or other aligner planning software lets us plan the expansion carefully. However, it’s important to remember that the software overestimates the amount of expansion achievable, especially in the posterior region. Studies have shown that actual expansion is often 70-80% of what is planned digitally[2,3].
Strategic attachment placement plays a crucial role in enhancing expansion control. Optimized attachments, especially on premolars and molars, can help counteract undesired tipping moments. Horizontal rectangular or buccal beveled attachments can be effective in this regard.
Planning for slight overcorrection is often necessary, especially in the posterior region. Research has shown that expansion is less predictable, moving from anterior to posterior, with second molars being the least predictable[2]. Implementing a sequential staging protocol, expanding molars first, followed by premolars and canines, can help maintain aligner fit and force delivery throughout treatment.
Limitations and Challenges
While clear aligners have improved arch expansion, orthodontists must consider several limitations. Achieving bodily expansion of posterior teeth is challenging, with studies showing a tendency for buccal crown tipping rather than true bodily movement, especially in molars[4]. Clear aligners also struggle to deliver adequate buccal root torque during expansion, particularly for molars. Deficiency in controlling buccolingual inclination can lead to excessive buccal crown tipping and potential issues.
Another challenge is controlling second molars, which are often the least predictable teeth in expansion, showing more expansion than planned or inadequate movement[5]. Research has also shown that clear aligners tend to over-flatten the mandibular Curve of Wilson compared to predictions, mainly due to a lack of buccal root torque in mandibular first molars during expansion[5].
The flexibility of aligner material, especially at the posterior ends, can limit force delivery for expansion and torque control. This material limitation contributes to the challenges in achieving predictable posterior expansion.
Opportunities for Improvement
Despite these challenges, clear aligner therapy has exciting opportunities for improvement. Developing aligner materials with variable stiffness or leveraging the corrugation effect on the aligner shape can provide more rigidity in posterior segments and improve expansion predictability. Implementing smart force features, like Invisalign G8’s optimized expansion support attachments, can potentially enhance buccal root torque control during expansion.
Tailoring aligner design to deliver specific force systems for individual teeth could enhance predictability. For example, incorporating features to increase buccal root torque for molars during expansion could address current limitations.
In some cases, combining clear aligner therapy with traditional expansion methods (e.g., rapid palatal expander followed by aligners) may yield more predictable results for severe cases. This integration of techniques allows us to leverage the strengths of both approaches. The newly introduced Invisalign Palatal Expander promises to add another option for greater expansion and transverse control.
Force Systems Critical for Success
Understanding the force systems critical for success is crucial in achieving predictable expansion. Clear aligners must create a force couple that produces equal and opposite forces on the buccal and lingual surfaces to achieve bodily expansion.
Incorporating a moment to move roots buccally is crucial, especially for posterior teeth. This force system counteracts the natural tendency for buccal crown tipping during expansion. Adding a slight extrusive force, particularly on buccal cusps, can aid in maintaining proper occlusal relationships during expansion.
A sequential force application from posterior to anterior helps maintain aligner fit and ensures consistent force delivery throughout treatment. This progressive force application is key to achieving predictable results across the entire arch.
Conclusion
In conclusion, while clear aligner therapy has shown promise in achieving arch expansion, it’s essential to understand its limitations and plan accordingly. By implementing strategic treatment planning, utilizing appropriate attachments, and considering the biomechanics involved, we can improve the predictability of expansion results. As technology and materials continue to advance, we can look forward to even more precise and effective clear aligner treatments.
References:
1. Houle, Jean-Philippe, et al. “The predictability of transverse changes with Invisalign.” The Angle Orthodontist 87.1 (2017): 19-24.
2. 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.
3. Morales-Burruezo, Ignacio, et al. “Arch expansion with the Invisalign system: Efficacy and predictability.” PLoS One 15.12 (2020): e0242979.
4. Bruni, Alessandro, et al. “Efficacy of clear aligners vs rapid palatal expanders on palatal volume and surface area in mixed dentition patients: A randomized controlled trial.” American Journal of Orthodontics and Dentofacial Orthopedics(2024).
5. Goh, Shaun, Craig Dreyer, and Tony Weir. “The predictability of the mandibular curve of Wilson, buccolingual crown inclination, and transverse expansion expression with Invisalign treatment.” American Journal of Orthodontics and Dentofacial Orthopedics 163.1 (2023): 109-116.
