Despite the growing consumer demand and worldwide use of clear aligners to treat misaligned teeth, several concerns regarding the efficacy of the system in controlling tooth movement remain. The tooth movement achieved upon aligner therapy differs from the tooth movement planned by the virtual setup21. This less predictable movement might be due to the fact that the force-transmission mechanisms of clear aligners are not yet well clarified. In fixed-appliance treatment, the force originating from metal wire and bracket interactions is transmitted to a certain point on the tooth, causing displacement. In contrast, clear aligner therapy outcomes are the result of a predetermined mismatch between the tooth and aligner. The force application point is ever changing, and the direction and amount of the force are very elusive.
Several experimental approaches have been carried out on the biomechanics of possible tooth movement with clear aligners. Compared to displacement measurements and strain gauge measurements, 3D FE analysis has the advantage of taking periodontal tissues into consideration and can calculate the stress, strain and displacement of any part of the tooth and aligner11,22,23,24,25,26,27. However, most FE studies on clear aligners were limited to a single tooth or a segment of teeth11,22,23,24,26. In addition, the loading method was conventionally described as moving the assembled aligner directly, which was inconsistent with clinical reality26. In this study, the incisors were first forced in the reverse direction of retraction to activate the aligner. The forces resulting from the deformation of the aligner were then calculated by FE analysis, which were in turn loaded back on the corresponding teeth. The contact interfaces between the aligner and the crown surface and attachments were set to a friction coefficient of 0.2, thus mimicking the mode of action of a shape-mismatched aligner on the dentition. To our knowledge, this is the first time that a clear aligner FE model with intact maxillary dentition and attachments was constructed, and different protocols of tooth movement were analysed. The results showed that the method was practical and reliable.
Controlling the movement of incisors at will is a challenge for extraction treatment. In most cases, translation or controlled tipping is preferred for the retraction of upper incisors. In fixed appliances, the moment/force ratios at the bracket are the primary determinant of controlling the tooth movement28,29. It can also be achieved by positioning mini-screws and hooks to adjust the “line of force” related to the centre of resistance of the anterior segment in sliding mechanics30,31. However, the biomechanics of en masse retraction of incisors with clear aligners are not well understood.
Previous studies have reported that clear aligners caused tooth movement mainly by tilting motions and that bodily movement for extraction space closure was less successful1,21. Our present study showed that incisors exhibited uncontrolled tipping and undesired extrusion when bodily retraction was designed (Fig. 3). Thus, similar to fixed appliances, a “bowing effect” would occur during incisor retraction with clear aligners. This might be caused by forces acting below the resistance centres of the four incisors, and the aligner was not rigid enough to hold the teeth vertically. It is thus logical to allow for a certain amount of intrusion during the retraction movement. According to Invisalign, tooth movement is limited within 0.25 mm per stage19,20. Therefore, we designed two retraction and intrusion combinations for incisors, and the total movement amount was 0.25 mm (Fig. 2). On the basis of our FE results, intrusion movement helped generate a force system that approximated the bodily movement of incisors. As the amount of intrusion movement increased, the root movement in the lingual direction became more prominent. However, this tendency declined from the centre to the lateral sides, which produced inconsistent types of movement between the central and lateral incisors. Apart from their different positions in the dental arch, the discrepancy between central and lateral incisors might result from the different anatomical configurations of the crowns.
Anchorage is another major concern during space closing in extraction cases. To maximize the anchorage value of the posterior segment, a protocol of sequential retraction of the canines and incisors was recommended15. Our present study adopted this concept so that 0.5 mm space was created mesial to the canines on the FE model to mimic canine distalization. This gap also increased the aligner surface area around the canine and incisor crowns. Meanwhile, the flexibility of the edentulous span of the aligner distal to the incisor segment was effectively reduced by canine positioning15. In our results, the posterior teeth and canines showed similar mesial tipping movement in the three protocols. With the decrease in the amount of retraction in A2 and A3, less mesial migration of the posterior teeth was observed. However, canines revealed more extrusion displacement when more intrusion was planned on the incisors (Figs. 3 and 4). Higher stresses on canine PDLs were detected (Fig. 5). Based on these results, intraoral elastics could be placed on canines to reinforce anchorage. Taking the vertical movement of canines into consideration, class II elastics should be used for protocol A1, and elastics supported by mini-screws on posterior maxillary bone would be beneficial for protocols A2 and A3.
Auxiliaries such as attachments were mandatory in clear aligner therapy to achieve desired results25,26,32. While some reports pointed out that customized attachments could facilitate complex movements, others failed to find significant differences in the shape and position of attachments on tooth movement16,20,22,26. Most likely, the effect of the attachment would depend on the shape of the tooth. In our present study, only conventional rectangular attachments were used for convenience. Much higher stresses on the PDLs and gingival surfaces of the attachments were detected on lateral incisors (Figs. 5 and 6). These results were in agreement with previous findings that upper lateral incisors commonly come off track during space closure, and horizontal bevelled attachments are recommended for better retention33. Higher stresses were also observed on the gingival surfaces of the attachments on the 1st and 2nd molars when more intrusion was placed on incisors (Fig. 6). This emphasized the significance of the attachments on molars to support incisor intrusion.
Considering all the results obtained through the finite element analysis, it could be stated that the incisors underwent a transition from uncontrolled tipping to gradual root-controlled movement after intrusion displacements were planned. It is worth noting that intrusion was always accompanied by various movement types. Thus, assessment before treatment was indispensable from a clinical perspective. For patients with a deep overbite, more intrusion could not only contribute to improving the occlusion but also lead to good control of root lingual movements. Moreover, the different movement types on incisors draw attention to the clinical aspects of biomechanical analysis, showing that different forces might be loaded on incisors, which should be considered by orthodontists when planning clear aligner therapy.
Our present results were based on the fact that the upper incisors were on normal labiolingual inclination. Calibrating the amount of intrusion and retraction movement in detail according to different inclinations of incisors would help clinicians with treatment planning for individual cases. Although the reliability of finite element analysis must be verified, further studies examining different thicknesses and configurations of the aligners, as well as various attachment designs, should be carried out for a better understanding of clear aligner treatment.