5: Theoretical and practical considerations in planning an orthodontic treatment with clear aligners
Tommaso Castroflorio, Gabriele Rossini, Simone Parrini
Introduction
After the Stone Age, the Iron Age, and the Bronze Age, are we switching to the Polymer Age? This question is legitimate when examining the increase of plastic materials production during the last half-century.
In the last decades, plastics have permeated industrial technology. Plastic materials have replaced many materials used in the past, and they have made possible industrial and medical applications that would not have been possible with older technologies. The key to the widespread dissemination of these materials is their incredible versatility.1
Furthermore, we are living in the personalized medicine era. Personalized medicine represents the natural evolution of health care. When medicine is informed solely by clinical practice guidelines, the patient is not treated as an individual but as a member of a group. Personalized or precision medicine characterizes unique biologic characteristics of the individual to tailor diagnostics and therapeutics to a specific patient. Personalized medicine uses additional information about the individual derived from knowing the patient as a person.2
Orthodontists have always been educated in collecting and analyzing patients’ individual characteristics to perform a diagnosis and define a personalized treatment plan. In this view, orthodontics will be the pioneer in guiding dentistry into the personalized medicine process. What is still missing is the integration of biologic markers into the diagnostic process and treatment planning, but researchers are going to fill the gap.3–5
In the last century, orthodontics was mostly a matter of metals and predefined prescriptions. In the last decades, the introduction of clear aligners moved the attention toward thermoplastic materials and their possible applications and personalized prescriptions. In clear aligner therapy (CAT), every aligner is built for a specific stage of orthodontic tooth movement (OTM) of a specific patient. Aligners are comfortable, less visible, and more aesthetically pleasant compared with buccal fixed appliances; they can be removed for eating and oral hygiene procedures, reducing the occurrence of emergencies. Despite those advantages making clear aligner increasingly requested by patients in our beauty-conscious society, there was always a great debate involving efficacy and efficiency of this appliance in controlling OTM. For instance, questions have been raised regarding the extent to which aligners can control extrusion, rotation, bodily movement, and torque.
As stated by Proffit in 2013, effectiveness, efficiency, and predictability are the three things orthodontists need to know about the treatment they are providing.6 A recent review7 stated that CAT can control complex movements as maxillary molars bodily distalization and extraction spaces close and that the buccolingual inclination of incisors is well controlled in mild to moderate malocclusions. Furthermore, in a recent research paper, Grünheid et al.8 analyzed the differences between predicted and achieved tooth positions and found statistically significant differences for all teeth except maxillary lateral incisors, canines, and first premolars. In general, anterior teeth were positioned more occlusally than predicted, rotation of rounded teeth was incomplete, and movement of posterior teeth in all dimensions was not fully achieved. However, except for excessive posttreatment of buccal crown torque of maxillary second molars, these differences were not large enough to be clinically relevant.
Therefore, with respect to what was possible a few years ago when the recommendation was to treat only simple malocclusions with aligners, the growing base of common knowledge regarding the control of OTM made it possible to use this technique even in more complex cases with good results when compared to conventional fixed orthodontics. Those results were made possible thanks to orthodontists who started to consider the virtual setup not only to visualize moving teeth but as an instrument to design the proper biomechanics, starting to transfer well-known concepts in this field.
As stated by Burstone9 during a JCO interview:
The nice thing about scientific biomechanics is that it is not dependent on any given appliance or technique. No matter what appliance you use, it allows you to use it better with more predictable results. Today, we have much too much commercialism in orthodontics; a healthy dose of science in understanding appliances and how they work is a good antidote. It is interesting to note that many of the new appliances that are suggested are nothing more than reinventions of old appliances.
Theoretical and practical considerations in CAT
Based on these assumptions and on clinical and laboratory research,10–13 the biomechanics of clear aligners could be described as a sequence of crown tipping and root uprighting. The first part of movement occurs in the occlusal part of the tooth because the aligner envelopes the entire tooth crown, while the interactions between aligner and attachments determine root movement. Therefore, when designing a virtual treatment plan, we must always remember which is the interaction surface between the aligner and the tooth, which is the effect of the force application at the crown level, and which is the anchorage unit required to avoid undesired movements.
The analysis of a virtual treatment plan using dedicated software should be based on the following steps:
Analysis of the final position
According to Sarver et al.14,15 it may be inappropriate to place everyone in the same esthetic framework and even more problematic to attempt this based solely on hard tissue relationships since the soft tissues often fail to respond predictably to hard tissue changes. Nevertheless, it is accepted that esthetic considerations are paramount in planning appropriate treatment but that rigid rules cannot be applied to this process. In view of our inability to apply rules defining optimal esthetics, the use of scientific methods to plan the most esthetic treatment may therefore be complicated. Nevertheless, it is clear that laypeople can identify various factors affecting smile esthetics. Thus clinicians can expect their patients to be more attentive to some dental esthetic factors than they are to others.16
A recent review was conducted to define the minimum level of esthetic harmony that can be approved as pleasurable by an external observer.17 The indications provided in Fig. 5.1 represent the threshold of acceptance of smile esthetics provided by laypeople that should be considered when analyzing the final position of front teeth.
Regarding the final position of upper maxillary molars, it is recommendable to refer to the position indicated by Ricketts in 1974 in which the line connecting the distobuccal and the mesiolingual cusps of the upper first molar is passing through the cusp of the opposite canine at the end of treatment.18 This final position is based on precise anatomic landmarks and can prevent misunderstandings between the prescribing clinician and the technician transferring the information in the virtual treatment plan.
Furthermore, when defining the final position, the clinician should always consider the buccal and the frontal limits of the arches, considering bone and periodontal support and the cephalometric information. Those indications are very important to avoid excessive expansion and/or proclination movements that can result in severe periodontal iatrogenic effects.19,20
Analysis of the movements occurring at each stage for each tooth
The analysis of movements occurring in every stage should consider three different aspects:
Aligner auxiliaries
Since the introduction of orthodontic aligners in early 2000s, several auxiliaries have been adopted from manufacturing companies and from clinicians to prevent anchorage loss and maximize treatment efficiency.
The most commonly adopted auxiliaries could be classified as follows:
Attachments and pressure areas.
Using aligners without attachments is something like orthodontics but not orthodontics. Attachments are useful to guide teeth in a determined direction but are also useful in providing anchorage control depending on the type of planned orthodontic movement. The use of attachments is crucial to achieve effective treatments. Ravera et al.21 and Garino et al.22 demonstrated the importance of using attachments to improve the root control of distalizing molars in class II treatments. In an in vitro study, Simon et al. demonstrated that load transfer from aligners to teeth without the use of attachment is possible only to a limited extent.11
Attachments are divided into two categories:
Conventional attachments (Figs. 5.2, 5.3, and 5.4) can be positioned by the clinician on every tooth (compatibly with tooth dimension) and can be oriented in any direction. Rectangular attachments are usually placed to increase anchorage in posterior teeth or to reinforce the retention of the aligner.
Optimized attachments (Fig. 5.5) are positioned by technicians, and the orthodontist is not able to modify their position, dimension, and orientation. This kind of attachment was introduced to generate a dedicated couple of force during rotations, especially in canines and premolars.
The “play” of aligners on teeth and attachments is another key factor in producing desired outcomes, which is strictly related to attachment application. An in vitro study by Dasy et al.23 demonstrated that attachment shape affects retention: Rectangular attachments are more retentive than ellipsoid ones. Two in vitro studies demonstrated that aligners produced by different companies (Invisalign, Align Technology, San José, CA, USA; CA Clear Aligner, Scheu-Dental, Iserlohn, Germany; F22 Aligner, Sweden & Martina, Due Carrare, Italy) showed excellent fitting on teeth and attachments.24,25 F22 aligners seem to have the best values in terms of fitting on attachments: the values range from 1 to 178 μm. The Invisalign fitting ranges from 5 to 212 μm. The measured values for CA Clear Aligner analysis range from 7 to 298 μm. Dasy et al. demonstrated that edgeless aligners generated significantly lower forces than those with a wider edge. The increased force might be due to the enhanced stiffness caused by material shape. Consequently, the enhanced stiffness may reduce the fitting of the aligner on the attachments. This could be the reason why CA aligners showed the worst results in terms of fitting. However, despite the statistical significance, measured differences might not be clinically relevant. Therefore the play of aligners on teeth and attachments is minimal, resulting in a precise transfer of the mechanical properties of the thermoplastic material to teeth.
From a biomechanical point of view, only a few studies in existing literature have analyzed the interaction between aligners and attachments. An efficient method for studying aligner mechanics is the finite element method (FEM). Applications of FEM on aligner studies will be presented in the next parts of this chapter. Except for the Yokoi et al. study,26 reported FEM results will refer to the initial instance of aligner wearing; thus these results should be considered in terms of initial force systems and displacements, not taking into account such precise measurements of the amount of movement expressed by the aligner on teeth.
Using FEM, Gomez et al. investigated a theoretical 0.15-mm displacement of an isolated upper canine with and without a composite attachment.27 The attachment considered for this analysis was inspired by the “optimized attachments” adopted by Align Technology to increase root control during distalization. The authors observed uncontrolled distal crown tipping without the attachment and a displacement similar to bodily movement with the attachment. Thus the authors highlighted the difficulty to obtain a controlled movement in CAT using only aligners and suggested the use of composite attachments to increase root control.
The biomechanical explanation of attachments usefulness in controlling tooth movement could be related to the role of braces in fixed orthodontics. While in fixed appliance orthodontics the moment is developed in the bracket itself by the engagement of the wire, in CAT it is developed by the interaction of aligner and auxiliaries.28 The aligner without attachments tends to move away from the teeth in its gingival edge. In such eventuality, all force is concentrated only in the occlusal part, and no couple of force could be generated. When recurring to attachments, the interaction between the displacement applied to the aligner and the attachment generates the adequate forces and moments to obtain a more controlled movement.
Yokoi et al. in 2019 published a paper that demonstrated these concepts using FEM to compare upper incisor diastema closure without attachments and with optimized ones. As reported by authors, the initial displacement corresponded to uncontrolled crown tipping for both the simulations; however, after hundreds of iterations that simulated the bone remodeling process, the simulation without attachments resulted in uncontrolled tipping, while bodily movement was observed in the simulation with optimized attachments.26
Regarding pressure areas, the kind of movements in which they are adopted depends on the aligner manufacturer. Usually, pressure areas are adopted to improve efficiency in crown tipping, rotations, and root torquing. Barone et al. in their FEM study from 2016 reported that pressure areas are the most effective auxiliaries in lower incisors tipping, even more than rectangular attachments.12
A study by Castroflorio et al. regarding control of root movement demonstrated the efficacy of pressure areas to improve this type of movement.29 The force couple generated by an aligner torquing a tooth consists of a force near the gingival margin and a resulting force produced by movement of the tooth against the opposite inner surface of the appliance near the incisal edge.30 Since the gingival edge of the aligner is elastic, it is difficult to control the forces applied in this region without an altered geometry.27
Intraoral elastics.
Regarding intraoral elastics, three main variables could influence the right choice for the planned treatment:
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