Biomechanics of Aligners: Literature Review

best fit the needs of their patients, as well as a knowledge of the factors that influence the clinical mechanical behavior of aligners.


Introduction
Aligners treatment has been part of orthodontic practice for decades, especially since the introduction of Invisalign devices (Align Technology) in 1998. At first, aligners were used for mild to moderate orthodontic dental movements. Technological developments in aligner materials and production techniques, especially 3D technology to planning dental movements and the incorporation of attachments, allowed the improvement of the control of the position of the teeth in the three planes of space. This made possible the realization, by the aligners, of movements considered until then difficult or even impossible. However, it is recognized that many parameters influence the biomechanical characteristics of aligners such as material properties, material thickness, amount of activation, and use of attachments. Until 2003, few studies, adequately evaluated the biomechanics of aligners, have been published, leaving doubt among clinicians about the effectiveness of the appliance. Published articles mainly case reports. This literature review aimed to identify and review the orthodontic literature, and taking stock of the current state of knowledge regarding the biomechanics of aligners, to provide elements allowing a better understanding of the functioning of the technique of aligners, to improve clinical results and optimize dental movements.

Search strategies
Data collection for this literature review was carried out through: i.
An electronic search of the articles that are the subject of this literature review through the databases: PUB MED, GOOGLE SCHOLAR, SCIENCE DIRECT.
On the various search engines, we have used the following keywords to identify the articles: clear aligner-orthodonticbiomechanic -tooth movement.
For PUBMED, we used the keywords found in MeSH for English articles, according to the following search equations: Clear aligners AND orthodontic, Clear aligners AND biomechanic.

ii.
A manual search to identify other references that could be included in the review. At the end of this research, we have selected 4 books.
We have respected the guidelines of the PRISMA charter in the treatment of references collected from the bibliographic research carried out.

Eligibility criteria a) Inclusion criteria
i.
ii. Studies in humans.
iii. Literature reviews, Systematic reviews and metaanalysis responding to determined objectives. iv.
Studies of orthodontic treatment with aligners in adults.
v. Clinical tests.
vi. Review and review articles.

vii.
In vitro experimental studies.
viii. Case reports.

b) Exclusion criteria
i. Animal studies.
ii. Studies published before 2003.

Results
A preselection of articles was carried out by searching the keywords through the databases: During this step, 2795 articles were preselected. After applying the 2003-2020 Periodic Restriction filters + "humans" filter: we obtained 1192 articles. After reading headlines and eliminating duplicates, 140 articles were left. After reading the titles and abstracts, and considering the eligibility criteria, we kept 66 references. The full text of the articles was read to eliminate articles that did not meet the inclusion criteria. At the end of this step, the number of references obtained is 28. Then we carried out a critical reading of the selected articles, during which we evaluated the validity of the articles to extract the necessary data ( Figure 1).

Force and displacement
There are several types of forces that can be applied to the teeth: translation, torque, rotation, and extrusion require heavy forces of 50-150g, rocking motion requires 50-75g, intrusion requires light forces of 10-25g. Rocking motions require a single force, while translational movements require two or more force systems [1]. Barbagallo et al. [2] concluded that thermoplastic aligners have similar effects on root cement as light fixed orthodontic forces (25g). This is explained by the discontinuous and removable nature of the force [2]. Castroflorio et al. [3] showed that Invisalign aligners produce an initial force of 1N when distalizing a maxillary molar. To this end, aligners can induce the same biological response described for other fixed appliances, at least during the early stages of orthodontic treatment [3].

Types of movements The translational movement
Translational movement is required in three main situations:

Molar distalization
A retrospective study was performed by Ravera et al. [4] the results of which showed that the translational movement of the maxillary molars was achievable with the use of aligners in combination with composite attachments and class II elastics [4]. The aligners allowed the maxillary second molars to be distalized by 2.25mm without version movement or significant vertical crown movements. Another study by Simon et al. [5] in 2014, focused on evaluating the effectiveness of aligners regarding incisal torque movement, premolar derotation, and molar distalization. Molar distalization marked the highest rate of accuracy, approximately 87% without the use of Class II elastics during treatment [5] ( Figures 2-4). .

b) Displacement of teeth adjacent to extraction sites
Closure of space at the extraction sites requires "root control" attachments on the teeth adjacent to the extraction sites. The study by Li et al. [6] whose aim to evaluate the results of Invisalign treatment compared to the fixed multi-brace treatment [6] showed no significant difference between the two groups in the translation of the roots and the achievement of good root angulation. The results of this study could be due to the correct use of attachments to control the movement of the root [7]. This study confirmed that Invisalign can give good results in extraction cases ( Figure 5).

Vestibulo-lingual / mesio-distal tooth displacement
An in vitro study was performed by Elkholy et al. [8] in 2016 to study the forces and moments delivered by the new thinner PETG aligners during the labio-palatal translational movement of a maxillary central incisor led to the conclusion that for aligners without specific modifications such as pressure points specific or more rigid cervical regions to create a counter moment, the translational movement of the maxillary central incisors in the vestibular or palatal direction is impossible to achieve. According to Zhang et al. [9] the coronary movement produced (~ 2.5mm) was relatively greater compared to the root movement (~ 0.4mm). These results indicate that the aligners cannot perform translational movements, which explains the lower quality of treatment and the high recurrence rate compared to the fixed multi-attachment treatment. According to the authors, the aligners moved the teeth mainly with version movements inducing an increase in stress in the cervical and apical regions. In a study by Li et al. [10], the orthodontic force in the lingual direction generated by the aligners increases as the alignment of the aligner increases. Except that the difference in the orthodontic forces produced between activation of 0.5mm and 0.6mm was not statistically significant. These results indicate that the activation of the aligner should not exceed 0.5mm for the lingual translational movement of the maxillary central incisors. [10] In a study by Gomez et al. [11] the initial force system generated during translational movement of upper canines with aligners, with and without composite attachments was described [11].

The extrusion movement
The extrusion movement seems to be difficult to obtain. Indeed, in a prospective clinical study carried out by Kravitz et al. [12] the efficiency of tooth movement with removable polyurethane aligners was evaluated and the least accurate movement was the extrusion (29.6%) -specifically, extrusion of the maxillary (18.3%) and mandibular (24.5%) central incisors. Also, an observational study by Tuncay et al. [13] examining tooth movement during upper lateral incisor eruption by aligners found that pure extrusion along the long axis is extremely difficult. This movement takes place in a non-linear movement for both the root and the crown, referred to as "wobbling". Kravitz et al. [14] conduct a new study to provide an update on the accuracy of tooth movement with Invisalign. One of the most promising findings of this study was the improvement in the accuracy of maxillary incisor extrusion (55%). That may be from the use of optimized extrusion attachments. It is then necessary to use attachments that have a so-called active flat surface, on which the aligner can exert force. The active part is the essential element of the attachment since it is the part that is engaged by the aligner. The rest of the attachment serves as a bond and a reinforcement [15]. These attachments should be beveled on the gum side. These attachments can be placed on a single tooth or group of teeth when multiple extrusions are required [16]. Beveled attachments are most often used when extruding a tooth. They can be 3, 4, or 5mm wide, 2mm high, and 0.25 to 1.25mm thick [7]. In open bite cases requiring absolute extrusion of the incisors, the optimized extrusion attachments are automatically placed on the incisors by the software when a pure extrusion of 0.5mm or more is detected (Figures 6-8) [17].  Another method is to use a Power grip bonded to the buccal surface of the tooth affected by the extrusion, two buttons on the aligner at the adjacent teeth, and intra-jaw elastics. Extrusion is activated by changing the elastics 3-4 times a day. When the necessary extrusion is complete, the aligner locks the tooth in the desired position ( Figure 9) [15]. In cases of open bite requiring 2 to 3 mm extrusions, intermaxillary elastics can be used. The extrusion teeth are provided with buttons and are attached to the opposing arch by rubber bands on the buttons as well. When the desired extrusion is complete, the target teeth will be in contact with the inner surface of the aligner and no further extrusion will occur ( Figure 10). This method can be modified by placing the buttons on the lingual surface of the aligner in the same arch with the use of intra-arch elastics ( Figure 11) [18].

The intrusion movement
The best aligner treatment protocol to accomplish intrusion in a shorter period of time is to exert optimal intrusion force on the target teeth while maintaining stability of the anchorage teeth. The aligners have advantages because they cover all the teeth thus preventing the extrusion of the posterior teeth [19]. Intrusion normally does not require composite attachments on the affected teeth. However, the anchor teeth should be fitted with horizontal rectangular attachments for anchoring and relative extrusion of these premolars [20] (Figure 12). Another method is to position two buttons on the aligner buccally and lingually at the gum level of the target tooth. Two 1.5 to 2 mm deep grooves are then made on the incisal edge with a 1 mm diameter to hook the elastics in case of intrusion of the anterior tooth ( Figure 13); in the case of a posterior tooth, the occlusal part of the aligner is cut [21].  The biomechanics of an ingressive force take into account the point of application of the force, but also the vestibulo-palatal / lingual inclination of the tooth. So, to intrude the teeth, it is necessary to correct the torque then the intrusion can be done in the cancellous bone by exerting a force on the tooth surface. Maintaining control of bucco-lingual tilt (torque) during intrusion is very important, because if the root apex is in contact with the buccal or lingual cortical bone, root resorption may occur [1]. A study was carried out by Saxena et al. [22] in 2010, showed an average tooth displacement rate of 0.9mm per month for canines and 0.7 mm per month for incisors exposed to the same ingressive force. However, it has been observed clinically that the canines are more difficult to intrude than the incisors due to the length of the roots and their larger periodontal surface [22]. In an in vitro study by Yang Liu et al. [19] in 2018, the actual activation of the aligner on each tooth, the shape and position of the attachment, and the relative movement of adjacent teeth influenced the final intrusion force [19]. This study also showed that, for the same activation, the canines exhibit the greatest force of intrusion when they are intruded alone. To optimize the intrusive movement, the use of attachments on the affected teeth themselves may prove to be in favor of the predictability of such movement.

The tipping movement
This is the easiest move to obtain when a force is applied against the crown.

a)
The uncontrolled tipping A study was carried out by Baldwin et al. [23] in 2006 to describe the movement of teeth adjacent to premolar extraction spaces during space closure with aligner appliances and then fixed appliances. In this study, treatment with aligners resulted in significant tipping of the teeth adjacent to premolar extraction sites [23].

b)
The controlled tipping As described in the literature, tipping is predictable with thermoplastic aligners, but it remains difficult to establish comparable root control [3,24]. However, there is a side effect of tooth movement with thermoplastic aligners called the 'water melon seed' effect, which refers to the unintentional intrusion of the displaced tooth triggered by an intrusive force caused by the distortion of the appliance [25]. A study was performed by Gao & Wichelhaus [26] in 2017 to evaluate the effect of material thickness on the forces and moments delivered by aligners to a maxillary incisor during tipping and intrusion [26]. The calculated mean moment/force (M / F) ratio was between 8 and 9 (8.2) for tipping. This confirms the results of previous studies.

The torque movement
Torque movement with aligners can be done using Power -Ridges. They are supposed to apply a lingual or palatal force on the cervical part of the crown, which, when constrained by the plastic covering the incisal edge of the same tooth, creates a couple of forces, this couple is supposed to produce the lingual torque. Another method used in the Essix system is also effective: using Hilliard thermoplier or one layer of composite, two forces are applied simultaneously buccally and lingually of the target tooth [21]. Controlling the torque of an upper central incisor requires the creation of effective couple of forces: [24,25] (Figures 14 & 15).

i.
A tipping force, caused by a reversible deformation of the device near the gingival line

ii.
A resultant force in the opposite direction, produced by the movement of the tooth against the opposing internal surface of the appliance near the incisal edge.
A study was carried out by Li et al. [6] in which they compare the results of the two treatments, one with an aligner and the other with fixed appliances. The results showed no significant difference between the two groups [6]. Although, the results showed that Invisalign might not produce enough radicular torque, especially in the posterior region.

The rotation movement
Hahn et al. [27] found that only a slight activation of ± 0.17mm or 0.5 ° per step during rotation could produce ideal forces estimated between 0.35 and 0.6 N. 22 [27].
There are 2 methods to perform the rotation: i.
A couple of forces can be created to rotate a tooth by bonding buttons to the buccal and palatal surface of the rotated tooth and use elastic chains between them before or during the aligner treatment. The chains can be changed every 3 weeks until the rotation is corrected [28]. ii.
It is also possible to perform canine and bicuspid rotations in a more predictable way using optimized rotation attachements.
These attachments have different shapes depending on the direction of force required to rotate the affected tooth (Figures 16  & 17). The rotation of the 21, 22 was performed by the aligner himself, while an optimized attachment positioned distal of the 23 allowed for correction of the mesio-palatal rotation.
A study by Hahn et al. [27] in 2010, to evaluate the forces and moments exerted by removable thermo-plastic appliances produced by three different, hard thermoplastic materials, with the same thickness, on a maxillary central incisor during rotation in vitro, reported that it is also possible to measure an intrusive force, a general phenomenon associated with thermoplastic appliances called the "watermelon seed effect [27]. These intrusive forces can be explained by local deformation of the inclined inner surfaces of the appliance and with a reduced fit during progressive axial rotation, which tends to deform the appliance like a bow, by raising it in the anterior region [27]. Hahn et al. [28], Mencattelli et al. [29], Elkholy et al. [30] made the same observation. So, there is no pure rotation, but intrusion and tipping also occur. A general phenomenon was also observed, the forces and moments for anticlockwise activation were larger than for clockwise rotation.

Conclusion
Orthodontic tooth movement is the result of the complex interaction of forces generated during the simultaneous movement of several dental units and biological events (apposition and resorption) that cannot be simulated in vitro, in addition to the complex chemical composition of the oral environment. In fact, additional studies are therefore needed to explore the biomechanical behavior of the aligner in vivo, allowing a better understanding of clinical performance within different aligner treatment systems.