As clear aligner therapy continues to evolve, achieving predictable anterior torque remains one of the most challenging aspects. This blog post will review the current literature on this topic, discussing limitations, opportunities, and key elements for success in treatment planning cases where anterior torque is necessary.
Understanding Torque in Clear Aligner Therapy
Torque and the buccolingual inclination of teeth are crucial for aesthetic and functional outcomes in orthodontic treatment. In clear aligner therapy, achieving precise torque control can be challenging due to the flexibility of the aligner material and the complex biomechanics involved. According to Burstone [1], the moment-to-force (M/F) ratio is critical in determining the type of tooth movement. An M/F ratio of 5:1 results in uncontrolled tipping, 7:1 leads to controlled tipping, 10:1 achieves translation, and 12:1 or higher is necessary for root movement. Management of the moment-to-force ratio requires proper biomechanics and force systems in the clear aligner appliance to arrive at a clinical result, not just a digital visualization of the goal.
Limitations in Torque Expression
Several studies have highlighted the limitations in torque expression with clear aligners. Jiang et al. [2] found that the mean accuracy of incisor movements in the sagittal direction with Invisalign was only 55.58%, with lingual root movement being significantly more challenging to achieve than labial root movement. Gaddam et al. [3] reported that incisor torque is under-expressed when incisors are programmed to move labially, with the difference between predicted and achieved torque for upper central incisors averaging 6.43°.
Hahn et al. [4] noted that as torque is applied, aligners tend to “lift up” near the gingival margin, leading to reduced fit and less effective force delivery. Studies by Elkholy et al. [7] and Hahn et al. [4] suggest that clear aligners tend to produce more tipping movement than bodily movement, torque, or root control. The tipping is due to limitations in the force couple generated at the cervical and incisal regions of the aligners.
Opportunities for Improvement
Despite these limitations, several strategies can improve torque control. Castroflorio et al. [5] demonstrated that Power Ridges can better control upper incisors than pre-adjusted bracket systems, especially for torque corrections of about 10°. Simon et al. [6] found that power ridges generated significant torque compensation (7.9 N.mm) during torque movement of upper central incisors.
Optimized attachments, particularly on canines and lateral incisors, can significantly improve torque expression and overall control of the anterior segment during retraction. Jiang et al.’s [2] finite element study showed that incorporating intrusion with retraction led to more bodily movement of central incisors and a tendency for lingual root movement of incisors. The “lift up” seen in studies that led to deficient movements is a critical issue that orthodontists must manage in treatment planning and appliance design. Proper retention in the anterior, adding attachments adjacent to power ridges, and staging intrusion with torque in the plan will help mitigate this common deficiency. If we don’t address this potential issue, the appliance will not lead to the desired results.
Liu et al. [8] demonstrated that appropriate aligner overtreatment with canine attachments should be designed to ensure the bodily retraction of incisors in extraction cases. This approach can help compensate for aligners’ tendency to produce tipping movements.
Key Elements for Success in Treatment Planning
Several factors should be considered to achieve predictable anterior torque with clear aligners. Overcorrection in the setup is often necessary to compensate for incomplete torque expression. The amount of overcorrection needed may vary based on the teeth’s initial inclination.
Implementing a comprehensive force system is crucial. A force system for anterior torque includes using power ridges on central incisors, optimized attachments on canines for root control, and optimized attachments on lateral incisors for improved aligner retention and force distribution. Including an intrusion component during incisor retraction can promote more bodily movement and better torque control.
Regular monitoring and preparation to refine the treatment as needed are essential. Clinicians should understand aligner material properties and their impact on force delivery and torque expression. It’s also important to recognize that tooth morphology and individual patient factors can influence torque expression, necessitating individualized treatment approaches. Orthodontics is not a one-size-fits-all approach.
Cheng et al.’s [9] finite element study provides insights into optimizing power ridge height for different clinical scenarios. For translating teeth with normal inclination (U1-SN = 105°), a power ridge height of 0.7 mm was found to be optimal. For correcting over inclined incisors (U1-SN = 110°), a power ridge height of 0.4 mm was sufficient to achieve the desired torque change.
Clinicians should be aware of the variation in sensitivity in tooth movement. Small changes in the moment-to-force ratio can lead to large changes in the center of rotation as the tooth approaches bodily movement. This requires careful consideration in clinical settings to avoid unintended movements.
Conclusion
While achieving predictable anterior torque with clear aligners remains challenging, a thorough understanding of biomechanics, careful treatment planning, and advanced features like Power Ridges with optimized attachments can significantly improve outcomes. The predictability of incisor proclination ranges from about 37% to 72%, while intrusion accuracy is typically between 30% and 55%. Labial root movement appears more accurate than lingual root movement, and mandibular incisors may be more responsive to labial movement than maxillary incisors.
While achieving predictable anterior torque with clear aligners remains challenging, a thorough understanding of biomechanics, careful treatment planning, and the use of advanced features like Power Ridges and optimized attachments can significantly improve outcomes. Studies have shown that the predictability of incisor proclination ranges from about 37% to 72%, while intrusion accuracy is typically between 30% and 55% [10,11]. Jiang et al. [2] found that labial root movement appears to be more accurate than lingual root movement, and mandibular incisors may be more responsive to labial movement than maxillary incisors.
Clinicians should know that the type of tooth movement achieved may differ from the planned digital setup. For example, controlled tipping may result in pure tipping, and we may only partially achieve translation movements. This underscores the importance of careful monitoring and potential refinement during treatment.
Understanding these biomechanical principles and limitations can help orthodontists design more effective clear aligner treatments and set appropriate patient expectations. As technology and materials advance, we can expect further improvements in torque control with clear aligner therapy. Ongoing research will likely improve treatment protocols and aligner designs, enhancing torque control and overall treatment efficacy.
References:
1. Burstone, Charles J. “Application of bioengineering to clinical orthodontics.” Orthodontics: current principles and techniques(1985): 154-86.
2. Jiang, Ting, et al. “A cone-beam computed tomographic study evaluating the efficacy of incisor movement with clear aligners: assessment of incisor pure tipping, controlled tipping, translation, and torque.” American Journal of Orthodontics and Dentofacial Orthopedics 159.5 (2021): 635-643.
3. Gaddam, Raj, et al. “Reliability of torque expression by the Invisalign appliance: A retrospective study.” Australasian Orthodontic Journal 37.1 (2021): 3-13.
4. Hahn, W., et al. “Torquing an Upper Central Incisor with Aligners–Acting Forces and Biomechanical Principles.” Informationen aus Orthodontie & Kieferorthopädie 43.02 (2011): 97-104.
5. Castroflorio, Tommaso, et al. “Upper-incisor root control with Invisalign appliances.” Journal of Clinical Orthodontics 47 (2013): 346-351.
6. Simon, Mareike, et al. “Forces and moments generated by removable thermoplastic aligners: incisor torque, premolar derotation, and molar distalization.” American Journal of Orthodontics and Dentofacial Orthopedics 145.6 (2014): 728-736.
7. Elkholy, Fayez, et al. “Forces and moments delivered by PET-G aligners to an upper central incisor for labial and palatal translation.” Journal of Orofacial Orthopedics/Fortschritte der Kieferorthopadie 76.6 (2015).
8. Liu, Lu, et al. “The effects of aligner overtreatment on torque control and intrusion of incisors for anterior retraction with clear aligners: a finite-element study.” American Journal of Orthodontics and Dentofacial Orthopedics 162.1 (2022): 33-41.
9. Cheng, Yuxun, et al. “Torque movement of the upper anterior teeth using a clear aligner in cases of extraction: a finite element study.” Progress in Orthodontics 23.1 (2022): 26.
10. Kravitz, Neal D., et al. “How well does Invisalign work? A prospective clinical study evaluating the efficacy of tooth movement with Invisalign.” American Journal of Orthodontics and Dentofacial Orthopedics 135.1 (2009): 27-35.
11. Simon, Mareike, et al. “Treatment outcome and efficacy of an aligner technique–regarding incisor torque, premolar derotation and molar distalization.” BMC oral health 14 (2014): 1-7.
