Clear aligner (CA) therapy has undergone remarkable developments over the past 2 decades, transforming from a simple alignment modality into a mainstream orthodontic treatment capable of addressing complex malocclusions. Despite these innovations, the predictability of aligner tooth movements remains one of the most challenging aspects of clinical practice. This review explains the biomechanical considerations and clinical determinant factors that underline the variability in tooth movement predictability with CAs. From a biomechanical standpoint, 3 fundamental conditions—anchorage adequacy, stress continuity, and sufficient aligner-to-tooth contact—collectively determine the accuracy with which aligner biomechanics are translated into clinical reality. This proposed anchorage-stress-contact triad provides the biomechanical rationale for evaluating aligner predictability. The predictability of aligner tooth movements is clinically governed by a multifactorial interplay among aligner-related factors (material properties and shape design), practitioner-related factors (digital setup precision, attachment design, aligner staging, and clinical monitoring), and patient-related factors (compliance and biological variability). Understanding how these determinants interact provides a comprehensive basis for interpreting inconsistencies between digitally programmed and clinically achieved outcomes, offering insights to optimize aligner tooth movements and enhance aligner treatment outcomes.
Highlights
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Anchorage-stress-contact biomechanical triad is proposed for assessing aligner predictability.
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Aligner predictability is determined by aligner, practitioner and patient-related factors.
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Aligner outcomes can be optimized by addressing biomechanical principles and key determinants.
The past 2 decades have witnessed a dramatic evolution and rapid development of clear aligner (CA) therapy. Initially confined to simple malocclusions, CA therapy is evolving and has steadily expanded its indications to manage increasingly complex cases. The advances in digital technology, aligner materials, and biomechanics have collectively expanded the clinical scope of CA therapy. Unlike fixed appliances, CAs rely on programmed digital setups, material properties, distinct biomechanical designs, and patient compliance. This reliance can explain individual variabilities between predicted treatment outcomes and actual clinical results. Cogent evidence indicates that the predictability of individual tooth movements with CAs varies widely, underscoring the need to explore relevant biomechanics and determinants.
This special article aims to decode the biomechanical cues and clinical factors that explain the variable predictability of tooth movements with CAs, offering insights for optimizing clinical outcomes with CAs.
Current evidence regarding aligner tooth movements
Collective findings demonstrate that CAs are able to produce clinically desired treatment outcomes, but they fail to achieve all planned tooth movements as predicted. , A detailed analysis of the evidence reveals a distinct hierarchy of predictability for different types of tooth movements, which can be broadly categorized into 3 tiers: high predictability (>70%), moderate predictability (40%-70%), and low predictability (<40%) ( Fig 1 ). ,,,,,, Tooth movements with high predictability are exemplified by tipping movements that require simple aligner biomechanics. Those with moderate predictability include incisor derotation and intrusion, whereas it is difficult to predict biomechanically complex movements, such as derotation of ovoid teeth, extrusion, and translation. Specifically, it’s not easy to derotate ovoid teeth because of the round anatomy of crowns, limiting surface area for the aligner to grip and generate a rotational moment. Extrusion is very difficult because it requires purely extrusive forces, whereas translation is challenging because it demands sophisticated biomechanics to move the roots and crowns together.
Different predictabilities of tooth movements with CAs.
Biomechanical considerations for differing predictability
The distinct biomechanical features of CAs can largely explain the discrepancies between achieved and planned tooth movements. Compared with fixed appliances, which can deliver both pulling and pushing forces, CAs are essentially limited to pushing forces. From a biomechanical perspective, successful expression of planned aligner tooth movements depends on 3 essential factors: anchorage, stress, and contact (ASC) ( Fig 2 ).
ASC triad of aligner biomechanics.
Presence of anchorage units on the opposite side of the planned movement
A prerequisite for efficient orthodontic force application is a stable anchorage system. CAs deliver forces primarily through pushing vectors. This is why predictability increases substantially when a tooth to be moved has anchorage teeth on the opposite side. For example, as seen in Figure 3 , A , during molar distalization, the anterior teeth and premolars act as stable anchorage on the opposite side of the distalizing force, ensuring high predictability of molar distalization. In contrast, for incisor retraction, the anchorage teeth (premolars and molars) lie on the same side of the retraction force and offer pulling force rather than pushing force for the anterior teeth, rendering anterior retraction less predictable ( Fig 3 , B ). Thus, to improve the accuracy of tooth movements, auxiliary biomechanical systems (eg, elastics and miniscrews) are frequently incorporated to complement aligner biomechanics ( Fig 3 , C and D ).
Anchorage requirements for aligner biomechanics: A, During molar distalization, the moving unit is displaced away from the anchorage unit that is on the opposite side of the planned movement, so that the pushing force ( blue ) can be executed; B, During space closure, the moving unit is displaced toward the anchorage unit on the same side of the planned movement. The force ( blue ) applied onto the anterior teeth by the posterior teeth is a pulling force rather than a pushing force; C, Elastic traction is incorporated to create a pulling force; D, Elastic traction facilitates the extrusion of the canine that requires a pulling force.
Stress continuity within the aligner
CAs act as force-transmitting shells that distribute force stress between anchorage teeth and moving teeth. However, when the spatial distance between the anchorage and the moving teeth becomes excessive, the continuity of stress transfer is disrupted, a phenomenon known as stress discontinuity. In such patients, the adequacy of force application is decreased, and the intended tooth movement may not be achieved, resulting in low predictability.
A clinical example of this stress discontinuity is anterior retraction ( Fig 4 ). If a conventional single-phase retraction is used, the large distance between the anterior teeth (moving unit) and the posterior teeth (anchorage unit) reduces the aligners’ ability to deliver effective force ( Fig 4 , A ). Thus, a practical strategy to overcome this biomechanical limitation is to adopt a multi-phase retraction protocol. Canines are distalized to close the space partially (phase 1), then 4 incisors are moved and retracted to close the space between the canines and lateral incisors (phase 2), followed by en masse anterior retraction (phase 3) ( Fig 4 , B ). This strategy reduces the distances between the anchorage unit and the moving unit, exhibiting superior anterior retraction and molar anchorage protection.
Stress continuity within the aligner: A, En-masse retraction. The 6 anterior teeth were retracted throughout all the 3 phases. Because of the large space between the anterior teeth and the posterior teeth, a stress discontinuity exists. A bowing effect was observed; B, Alternate retraction strategy. Canines were distalized first to decrease premolar-extraction space (phase 1), followed by incisor retraction (phase 2), and then the 6 anterior teeth were retracted together (phase 3). This reduced stress discontinuity within the aligner and facilitated force application.
Adequate, effective aligner-tooth contact area
Because aligners primarily exert pushing forces that act perpendicular to the aligner-tooth interface, certain tooth morphologies and types of movement inherently limit their ability to generate adequate forces or moments. A classic example of this is premolar derotation, which requires tangential forces acting in concert to generate a rotational moment ( Fig 5 ). Owing to the premolar crown’s round or elliptical morphology, the effective contact area perpendicular to the desired rotational vector is minimal, resulting in low predictability ( Fig 5 , A ). To overcome this biomechanical limitation, attachments might be useful, with no differences between conventional and optimized attachments. ,, They increase the effective contact surface perpendicular to the intended direction of movement, enhance the aligner’s capacity to deliver the rotational moment, and may improve the predictability of premolar derotation ( Fig 5 , B and C ).
Premolar derotation: A, No attachment is designed, and the applied forces ( yellow ) are offset with each other, resulting in no effective tangent force; B, An attachment is designed, and an effective force ( blue ) can be applied, facilitating the derotation; C, An effective attachment-aligner contact area is present and receives effective force applications ( blue ) that facilitate derotation.
Determinant factors for aligner tooth movements
The predictability of aligner tooth movements is governed by a complex interplay among aligners, practitioners, and patients. The material and biomechanical properties of aligners lay the foundation for tooth movements, whereas pivotal clinical decisions are made during aligner treatment, including programmed movement design, attachment configuration, staging, and clinical monitoring. Equally critical are patient-related factors, including compliance with wear protocols and individual biological variability in tissue response. A comprehensive understanding of these multifactorial determinants is therefore essential for achieving predictable tooth movement ( Fig 6 ).
Determinant factors (aligner-practitioner-patient triad) of aligner tooth movements.
Aligner-related factors
Aligner tooth movements are primarily achieved by shape-driven elastic CA changes that exert force on dentition. Thus, aligner tooth movements are influenced by various factors relative to the material and mechanical properties of aligners.
Mechanical properties
The force magnitude generated by CAs in response to aligner shape change is governed by the inherent mechanical property of the aligner materials (elastic modulus). Materials with higher elastic moduli provide higher forces and are perceived as stiffer. Likewise, those with lower elastic moduli are more flexible. Thus, aligner materials with different elastic moduli may influence the initial aligner force magnitudes ( Fig 7 , A ). Clinically, aligner materials with desired elastic moduli should be selected for designated tooth movements. For example, low-elastic-modulus (flexible) aligner materials may not provide sufficient force to achieve designated tooth movements, such as root torque, and may result in aligner off-tracking. In contrast, high-elastic-modulus (stiff) materials may be detrimental to periodontal tissues.
