Effectiveness and predictability of clear aligner therapy

Over the past two decades, the demand for orthodontic treatment among prospective adult patients has dramatically increased.¹ Clear aligner therapy has emerged as the primary esthetic alternative to fixed appliance therapy, especially among adults.^2–5 Consequently, a key area of research has focused on the effectiveness of aligners in achieving the desired orthodontic tooth movement.^6,7 It has been reported that between 70-80% of aligner cases either require refinements or are completed with fixed appliances.⁸ This paper provides a narrative review of the current literature on the effectiveness and predictability of clear aligner therapy, addressing tooth movements in the horizontal, vertical, and transverse directions.

Horizontal Movements

Rotations are unanimously reported as the least predictable tooth movement achieved by clear aligner therapy.^6,9–11 Many studies have indicated low degrees of predictability and control when correcting rotated teeth, particularly those that are cylindrically shaped, such as mandibular canines.^12–14 Even after achieving the desired correction, long-term retention and prevention of rotational relapse can be challenging.¹⁵ It has been hypothesized that the difficulty in rotating these cylindrically shaped teeth arises from their lack of surface area and undercuts, which are necessary for the aligners to grip and impart the required forces.¹⁶ As such, the rotation of canines, especially mandibular canines, is the least predictable, while the rotations of incisors are considerably higher.^13,17 Simon et al¹⁸ has shown that staging can influence the predictability of rotations, with higher accuracy achieved when the staging is less than 1.5 degrees per aligner. Other studies suggest using built-in overcorrection to improve predictability.¹⁹

Tipping is generally found to be a more predictable movement to achieve with clear aligners, with the mesial-distal direction being more predictable than the buccal-lingual direction.^10,13 Incisors are easier to move than canines, likely due to the larger root surface areas of canines, which may experience greater difficulty in achieving mesial and distal movements.¹³

Another challenging movement with clear aligner therapy is root movement, particularly in the buccal-lingual direction.²⁰ While clear aligners can easily tip crowns, they cannot adequately move roots, especially when translating through edentulous spaces.²¹ In cases requiring buccal root torque, the difficulty is attributed to the force couple generated by clear aligners, consisting of a tipping force near the gingival margin and a resulting force produced by crown movement against the aligner tray’s inner surface.²² The elastic nature of the aligner makes expression of the force near the gingival margin region difficult; the reversible deformation of the aligner at gingival margin can prevent formation of an effective force couple.^23–25 Studies suggest that fixed appliances are superior to aligners for controlling root movements.²⁵ However, for non-extraction cases of mild-to-moderate complexity, aligners can still achieve statistically significant corrections in root movements.²⁶

Transverse Movements

Expansions of 2-4 mm have been reported with clear aligner therapy, with some cases achieving up to 6 mm.²⁷ Studies suggest that fixed appliance therapy with self-ligating systems is still superior to clear aligner therapy in producing the desired expansion and buccal root torque in the posterior region.^21,28 Expansion with aligners is primarily achieved through buccal crown tipping rather than bodily movement with buccal root torque.^14,29 The resultant tipping may be due to the flexibility of the aligner material, which does not fully express the specified torque.¹⁴ Biological limitations, such as the proximity of molar roots to the cortical plates, may also play a role.^14,25 The reported accuracy for expansion ranges from 73-78% for the maxillary arch and 88-97% for the mandibular arch, with higher overall accuracy in the lower arch, possibly due to the decreased amount of planned expansion and reduced resistance as the upper arch is expanded simultaneously.^29,30

Vertical Movements

Earlier studies reported low predictability of incisor extrusion with clear aligners, but more recent studies have shown improved results, especially when refinements are used.^13,30 Absolute extrusion of incisors is difficult due to the insufficient surface area for the aligner to grasp and apply the necessary forces.¹³ However, extrusion of incisors relative to molar intrusion is readily observed, attributed to the bite-block effect in clear aligner therapy. This suggests that clear aligner therapy may be suitable for anterior open bite cases where the etiology is due to intruded anterior teeth.³⁰ Improvements of 1.5 mm in non-surgical anterior open bite cases have been reported, primarily achieved through incisor extrusion.³¹

Following the initial introduction of the Invisalign system, practitioners often noticed the deepening of overbites in many patients.³² For patients with ideal pre-treatment overbites, Clements et al³³ reported that 16% showed worsening of the overbite. As knowledge of the appliance improved, it became clear that the same bite-block effect could make deepbite corrections more difficult.³⁰ This was a direct result of unintended intrusions of molars, leading to the consequent deepening of the overbite. Early strategies to control the vertical dimension and maintain the existing overbite included removing occlusal coverage over the second molars.³¹ The evolution of clear aligner therapy has led to new tools for improved management of the vertical dimension, such as virtual bite ramps, optimized attachments, and support for auxiliary elements like elastics.^16,34

Studies show that aligners are less effective than fixed appliances in producing adequate occlusal contacts.^21,35 Some reports have shown that occlusal contacts worsened following clear aligner treatment.²⁶ This has been thought to be partly due to difficulties of aligners in producing the intended movements for establishing the necessary occlusal contacts.¹³ Additionally, the thickness of the aligner material can interfere with occlusal settling.¹⁰

Conclusion

Clear aligner therapy has established itself as a primary alternative to fixed appliance therapy, particularly for adults seeking a more esthetic option. However, its effectiveness in achieving precise orthodontic tooth movements varies depending on the direction of tooth movement, tooth morphology, and arch. While earlier studies reported lower accuracies for all combined tooth movements with clear aligners, more recent studies, especially those incorporating refinements, show significant improvements in outcomes. This reflects advancements in development, materials, technologies, and treatment protocols in clear aligner therapy over the past two decades. With appropriate case selection, clear aligner therapy can produce clinically acceptable outcomes.^10,36,37 Continuous advancements in aligner technology and techniques hold promise for enhancing their effectiveness and predictability in comprehensive orthodontic treatment. 

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About the Author

Dr. Tiantong (Tim) Lou earned his dental degree from University of Manitoba and his specialty training in Orthodontics at the University of Toronto. During this time, he completed a Master’s research project on Invisalign and have published numerous articles and book chapters in this subject. In his spare time, Dr. Lou enjoys travelling, trying out new restaurants, spending time with family and friends.