Understanding Torque in Clear Aligner Therapy
Understanding Torque in Clear Aligner Therapy
Clear aligner therapy is constantly advancing, yet achieving anterior tooth rotation remains a significant challenge. This article explores the current research on this topic, examining limitations, potential solutions, and crucial factors for successful treatment planning when anterior torque is required.
Understanding the Impact of Torque in Clear Aligner
Tooth torque in clear aligner therapy significantly impacts orthodontic treatment’s results. While clear aligner therapy offers numerous advantages, achieving precise torque control can be challenging due to the inherent flexibility of the aligner material and the complex biomechanics involved.
As Burstone [1] emphasized, the moment-to-force (M/F) ratio plays a crucial role in dictating the type of tooth movement. An M/F ratio of 5:1 typically results in uncontrolled tipping, 7:1 in controlled tipping, 10:1 in bodily translation, and 12:1 or higher is generally required for root movement. To achieve the desired clinical outcome, proper biomechanics and force systems must be carefully considered within the clear aligner appliance design, ensuring that the treatment plan translates effectively from the digital simulation to the actual clinical result.
Limitations in Controlling Torque in Clear Aligner
Multiple studies have underscored the limitations in achieving precise torque in clear aligner therapy.
Jiang et al. [2] observed that the mean accuracy of incisor movements in the sagittal plane with Invisalign was only 55.58%, with lingual root movement proving significantly more challenging than labial root movement.
Gaddam et al. [3] reported that torque expression is often underestimated when incisors are programmed for labial movement. The average discrepancy between predicted and achieved torque for upper central incisors reached 6.43°.
Hahn et al. [4] noted that applying torque can lead to aligner “lift-off” near the gingival margin, compromising fit and reducing the effectiveness of force delivery. Furthermore, studies by Elkholy et al. [7] and Hahn et al. [4] suggest a tendency towards tipping movements rather than desired bodily movements, torque control, or root movements. This tipping effect is attributed to limitations in generating an effective force couple at the cervical and incisal regions of the aligners.
Improving Torque Control
While limitations exist, several strategies can enhance torque control in clear aligner therapy. Castroflorio et al. [5] demonstrated that Power Ridges offer greater control over upper incisors compared to traditional pre-adjusted brackets, particularly for torque corrections around 10°.
Similarly, Simon et al. [6] found Power Ridges to generate significant torque compensation during upper central incisor movement. Optimized attachments, especially on canines and lateral incisors, can significantly improve torque expression and overall control of the anterior teeth during retraction.
A key challenge identified in the research is aligner “lift-off” near the gum line, leading to ineffective force delivery [4]. Careful treatment planning and appliance design are crucial to address this issue. Techniques like incorporating intrusion with retraction (as suggested by Jiang et al. [2]), adding attachments near Power Ridges, and strategically staging intrusion with torque can help mitigate this tipping tendency and achieve desired tooth movements.
Liu et al. [8] further emphasized the importance of appropriate aligner overtreatment with canine attachments, particularly in extraction cases, to ensure bodily retraction of incisors and counteract the tipping bias of aligners.
By implementing these strategies, orthodontists can improve the effectiveness of clear aligner therapy in achieving precise torque control.
Success Elements for Achieving Predictable Torque in Clear Aligner
Success in achieving predictable anterior torque with clear aligners hinges on several key elements.
Overcorrection during treatment planning
is often necessary to compensate for the inherent limitations of clear aligners in achieving full torque expression. The degree of overcorrection will vary depending on the initial inclination of the teeth.
A comprehensive force system
is vital for effective torque control. This includes utilizing Power Ridges on central incisors, incorporating optimized attachments on canines for root control, and employing optimized attachments on lateral incisors to enhance aligner retention and distribute forces effectively. Integrating an intrusion component during incisor retraction can promote bodily movement and improve torque control.
Regular monitoring and the flexibility to refine the treatment plan are crucial
Clinicians must understand the properties of the aligner material and their impact on force delivery and torque expression. It’s essential to recognize that tooth morphology and individual patient factors significantly influence torque outcomes, necessitating personalized treatment approaches.
Cheng et al.’s [9] finite element study provides valuable insights into optimizing Power Ridge height for different clinical scenarios. For teeth with normal inclination (U1-SN = 105°), a Power Ridge height of 0.7 mm was found to be optimal. For over-inclined incisors (U1-SN = 110°), a Power Ridge height of 0.4 mm was sufficient to achieve the desired torque change.
Other Elements that Affect Achieving Predictable Torque in Clear Aligner Therapy
Clinicians must also be mindful of the sensitivity of tooth movement. Small adjustments to the moment-to-force ratio can significantly impact the center of rotation as the tooth approaches bodily movement. This factor requires careful consideration in clinical practice to prevent unintended movements.
Clear Aligners and The Challenge of Torque in Clear Aligners
Despite the inherent challenges of achieving predictable anterior torque with clear aligners, a comprehensive understanding of biomechanics, meticulous treatment planning, and the utilization of advanced features such as Power Ridges in conjunction with optimized attachments can significantly enhance treatment outcomes. The predictability of incisor proclination with clear aligners varies widely, ranging from approximately 37% to 72%.
Intrusion accuracy typically falls within a range of 30% to 55%. While labial root movement appears to be more accurately achieved, lingual root movement remains more challenging. Additionally, mandibular incisors may exhibit greater responsiveness to labial movement compared to maxillary incisors.
Importance of understanding The Biomechanics
While achieving predictable anterior torque with clear aligners presents challenges, a robust understanding of biomechanics, meticulous treatment planning, and the strategic utilization of advanced features like Power Ridges and optimized attachments can significantly enhance treatment outcomes. Studies have demonstrated that the predictability of incisor proclination with clear aligners varies widely, ranging from approximately 37% to 72%, while intrusion accuracy typically falls within a range of 30% to 55% [10,11].
Jiang et al. [2] observed that labial root movement appears to be more accurately achieved than lingual root movement, and mandibular incisors may exhibit greater responsiveness to labial movement compared to maxillary incisors.
It’s crucial to recognize that the actual tooth movement achieved may deviate from the intended movements depicted in the initial digital setup. For instance, controlled tipping movements may result in pure tipping, and translation movements may only be partially realized. This underscores the importance of vigilant monitoring and the flexibility to refine the treatment plan as needed.
Benefits of Comprehending Torque in Clea Aligner Biomechanics
By comprehending these biomechanical principles and limitations, orthodontists can design more effective clear aligner treatment plans and effectively manage patient expectations. As technology and materials continue to evolve, we can anticipate further advancements in torque control within the realm of clear aligner therapy. Ongoing research will likely lead to refined treatment protocols and improved aligner designs, ultimately enhancing torque control and overall treatment efficacy.
References and Research Papers
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.
