Factors Inﬂuencing Appliance Wearing Time during Orthodontic Treatments: A Literature Review

: This review aims to analyze the multiple factors affecting patients’ level of compliance (how much they wear removable devices/clear aligners) during orthodontic treatments and to investigate the available methods and devices to monitor the appliance wearing time and to improve it. A literature search was conducted on electronic databases (Pubmed, Scopus, and Google Scholar). The results of the present study suggest that compliance indicators may misestimate the intraoral wearing time. Compliance is affected by patient-related factors (age, personality traits, the importance of personal appearance, self-perception of malocclusion), appliance-related factors (visibility, pain/discomfort), and clinician-related factors (trust, clear and complete communication, motivation). The motivation of pre-pubertal patients is mostly external, while adolescents/young adults have intrinsic motivation for orthodontic treatment. Patients’ self-reports tend to overestimate the appliance wearing time. Even if there is a lack of evidence, teledentistry might improve patients’ awareness of being monitored, thus increasing the time for which orthodontic devices are worn. Compliance is a key factor for clear aligner treatments, but high-quality studies focusing on this aspect are missing. Further studies should focus on how to handle the lack of cooperation and how to increase compliance in order to maximize the treatment’s results.


Introduction
Awareness of the importance of facial and dental appearance has led to an increased perception among laypeople of the malocclusions and irregularities of their teeth. The demand for orthodontic treatment for "aesthetic reasons" has been widespread, from both young and adult patients [1].
In order to achieve successful treatment outcomes, clinicians have to obtain patients' compliance [2].
Compliance (or adherence) is defined as "the extent to which a person's behavior coincides with medical or health advice" [3,4].
In orthodontics, patients' cooperation is required throughout every phase of the therapy [5,6]; patients' adherence to treatment is composed of multiple variables strictly correlated with their motivation: respecting appointments, maintaining a good oral hygiene, and wearing orthodontic appliances for as long as required [5].

Aim and Objectives
The aim of this review was to identify the multiple factors affecting compliance in orthodontic treatments with removable devices, that is, how much patients wear them (wearing time), since an in-depth awareness of these aspects might be helpful to understand the reasons behind a poor motivation and to handle a lack of compliance. A secondary aim was to analyze the available indicators that could provide an objective assessment of the appliance wearing time. Finally, solutions to improve the adherence of patients to the orthodontic treatment were considered.

Methods
A literature search of relevant articles was conducted on electronic databases (Pubmed, Scopus, and Google Scholar) from 1990 up to December 2021. The following keywords were selected for the search string: "patient compliance", "orthodontic appliances, removable", "cooperation", "clear aligners", and "compliance indicators". The eligibility criteria were the following: studies (case reports, case-series, longitudinal studies, randomized clinical trials) on adults and adolescents without any health or mental disease undergoing orthodontic treatment (functional, clear aligners) which discussed the level of compliance during the therapy.
The methodological characteristics of the search strategy were summarized according to the PICO approach: The following search strings were developed for Scopus: (("compliance indicator") OR ("cooperation") OR ("adherence") OR ("wear")) AND (("orthodontics") OR ("clear AND aligners") OR ("removable AND appliance") OR ("orthodontic AND appliance")) The following search strings were developed for Pubmed and adapted for Google Scholar: (((("patient compliance") OR (Treatment Adherence and Compliance)) OR ("compliance indicator") OR ("cooperation"))) AND ((Orthodontic Appliances, Removable) OR (Orthodontics) OR (Orthodontic Appliances)) The titles and abstracts of the papers were pre-screened to identify their consistency with the topic of the present review; then, relevant papers were retrieved for full-text analysis. Additionally, the reference list of the included studies was assessed for potentially eligible articles.

Assessment of Relevance, Validity, and Data Extraction
An overall assessment of methodological quality was undertaken for each included trial using six key methodological criteria: parameters such as setting (I), participant characteristics (II), sample size (III), dependent and independent variables (IV), outcome data (V), and statistical analyses (VI). For each of these parameters, there was a dichotomous issue: positive (+) or negative (−). Relying on the assessment of these six parameters, the available score for each of the included studies varied from 0 to 6.

Results and Discussion
A total of 253 papers were found in the electronic databases. A total of 18 papers were found by a hand search taking other records from the list of references of selected full texts.
Literature screening was performed by two researchers (G.D.A. and E.S.); in case of divergence, a third author was consulted (F.T.).
Duplicate and irrelevant papers were left out, and then a title-abstract screening was conducted: the titles and abstracts of 165 papers were assessed; 39 papers were selected for full text examination after removing studies that were not focused on the topics of the present review. Studies that presented a quality score < 3 were also excluded (four papers). 35 papers were finally included in the review.
The flowchart reported in Figure 1 shows the entire study selection process.
Appl. Sci. 2022, 12, x FOR PEER REVIEW 3 of 16 data (V), and statistical analyses (VI). For each of these parameters, there was a dichotomous issue: positive (+) or negative (−). Relying on the assessment of these six parameters, the available score for each of the included studies varied from 0 to 6.

Results and Discussion
A total of 253 papers were found in the electronic databases. A total of 18 papers were found by a hand search taking other records from the list of references of selected full texts.
Literature screening was performed by two researchers (G.D.A. and E.S.); in case of divergence, a third author was consulted (F.T.).
Duplicate and irrelevant papers were left out, and then a title-abstract screening was conducted: the titles and abstracts of 165 papers were assessed; 39 papers were selected for full text examination after removing studies that were not focused on the topics of the present review. Studies that presented a quality score < 3 were also excluded (four papers). 35 papers were finally included in the review.
The flowchart reported in Figure 1 shows the entire study selection process. Following a detailed assessment, 11 included studies satisfied at least 3 methodological criteria, 9 satisfied 4 criteria, and 15 fulfilled 5-6 criteria (Tables 1 and 2). Following a detailed assessment, 11 included studies satisfied at least 3 methodological criteria, 9 satisfied 4 criteria, and 15 fulfilled 5-6 criteria (Tables 1 and 2).  If the indication has been correctly established, Invisalign therapy can be a source of great satisfaction for both the patient and physician.
Oliver et al. [32] 1985 Wales Prospective cross-sectional study 100 patients Pain from the appliances and its appearance are the main discouraging features during orthodontic treatment. The color compliance indicator has considerable promise for improving the efficiency and effectiveness of orthodontic treatment with clear aligners.
Zotti et al. [42] 2014 Italy Prospective cohort study 80 patients The weekly sharing of selfies of patients' smiles in a WhatsApp-based chat room contest is an effective and long-lasting way to improve oral hygiene compliance among adolescent orthodontic patients.
The literature reported many factors that may have an influence on how much patients wear orthodontic removable appliances, and they are commonly thought to be exclusively patient-dependent [40,43]. However, compliance seems to be a multifactorial concept: the type of appliance selected for the treatment and the relationship with the orthodontist play a significant role in determining the level of compliance [2,40]. In order to simplify the discussion on factors affecting patients' compliance, the authors arbitrarily divided them in three groups: "appliance-related", "patient-related", and "clinician-patient relationshiprelated" factors (Table 3). Table 3. Factors influencing compliance.

"Appliance-Related" Factors
Orthodontic treatment certainly improves both dental and facial appearances, but there are annoying and uncomfortable side effects that may lead to a decrease in patients' compliance [19,20]. This regards therapies with both fixed and removable appliances on aesthetic and functional levels.
For example, fixed appliances are associated with poor aesthetics (such as metallic brackets), and they are therefore less accepted than more aesthetic solutions [19]. These types of appliances can be a source of pain and discomfort, but patients cannot remove them. In the case of fixed appliances, a decrease in cooperation is usually associated with oral hygiene and with the use of additional auxiliaries such as elastics and headgear [20,32].
Removable appliances are generally considered more comfortable, as, in rare cases of pain or discomfort, patients may remove them. Unfortunately, studies show that removable appliances are worn less than required [45]. Some reports show that compliance with removable appliances is suboptimal: patients wear the appliances five hours less than indicated by the clinician [2].
Pain, discomfort, and functional restrictions that are caused by orthodontic appliances seem to be the most annoying side effects of the treatment. According to the literature, patients' discomfort represents one of patients' main reasons for not cooperating with the orthodontic treatment [2,5,19].
Clear aligners (CA) are well-tolerated removable appliances, and they are thus accepted by patients since they are invisible and do not compromise the facial appearance. They cause minimal trauma or irritation thanks to their smooth surface. Patients experience slight alterations of pronunciation, especially in the first period, until they get used to it. This issue is generally considered as the most annoying part of the aligners' therapy, and it could influence compliance levels. However, patients become accustomed to CA very quickly and do not suffer much impairment, as a study on 54 patients showed that 83% of patients got used to the appliance within 1 week and 93% of them felt so secure wearing CA that they felt no inhibition while talking [31].
Clear aligners are designed to exert appropriate, precise, and calibrated forces on the teeth according to the type of malocclusion. Thus, patients are not subjected to heavy forces that may cause "useless" and deleterious aches and pains. Patients wearing CA report lower levels of pain, less stress and social anxiety, and higher Oral Health Related Quality of Life (OHRQoL) measurements. As a consequence, patients wearing clear aligners demonstrate a superior attitude during the treatment [22,46].
A comparison between clear aligners and fixed appliances after the completion of orthodontic treatment assessed patients' satisfaction and OHRQoL. Similar satisfaction levels were found for the two treatment modalities, except for eating and chewing. CA significantly performed better for these activities compared to fixed appliances [21].
Compliance for CAT is required to be high, since aligners are meant to be changed regularly, and they need to be worn almost all day [39].
Timm et al. [39] investigated several factors influencing compliance in clear aligner therapy in a large sample of 2644 individuals using a mobile application for the selfreporting of aligner wear. The results showed that 36% of the patients were fully compliant, 38.3% of them showed fair compliance, and 25.7% showed poor compliance. These findings are particularly relevant as one out of four patients demonstrated insufficient collaboration, which may jeopardize the therapeutic outcome.
The effectiveness of clear aligner therapy, compared to the conventional fixed appliances, is widely discussed in the literature. The improvements introduced over the past few years regarding clear aligner therapy have made this approach comparable with fixed conventional treatments in selected cases (such as the intrusion and rotation of incisors, lingual crown tip, the distalization of upper molars, dental cross-bite, etc.) [47][48][49]. It has been recently reported that complex and even surgical treatments can be successfully completed with clear aligners, but more studies are needed to retrieve solid conclusions [50].
A very good compliance is necessary to obtain successful results with clear aligner therapy [36,41], as the appliances should be worn 22 h per day and changed sequentially every 7, 10, 14 days [51]. An insufficient compliance may increase the treatment time and therefore may compromise the treatment outcome [35].
In order to improve the efficiency of the treatment, it is important for the clinician to understand the optimal aligner wear protocol: Al-Nadawi et al. [51], in 2021, evaluated the efficacy of tooth movement with aligner wear protocols of 7, 10, and 14 days. They found out that a 7-day protocol reduces the treatment duration and is generally enough, as there is not a clinically significant difference compared with a 10-day or 14-day protocol. A 14-day protocol showed a statistically greater accuracy for complex posterior movements such as maxillary intrusion, distal-crown tip, buccal-crown torque, and mandibular intrusion and extrusion [51].
The compliance issue is critical to obtain results and to select accelerated wearing schedules. It should be carefully evaluated by the clinician and thoroughly discussed with the patient and/or with the parents when talking about kids' treatment. If the patient shows a good approach and the adherence to the treatment is foreseen, the clear aligners are an excellent treatment option with effective results, even in difficult cases. A fair compliance could be sufficient in mild malocclusions, but certainly not in complex cases. A poor compliance may cause the failure of any treatment.

"Patient-Related" Factors
Psychological studies have shown how individuals cope with any general discomfort, including pain caused during orthodontic treatments, as influenced by personal values, personality, and the attitude towards the therapy's phases [5,35].
Therefore, it is essential, helpful, and interesting to discuss the psychological dimension behind the patients and the related factors that may influence the adherence to the treatment and how much they wear the devices. Orthodontic treatments usually last several years and are often undertaken in the middle of skeletal growth; thus, orthodontic patients are generally people who are in the pubertal phase. This implies that cooperation is influenced by developmental factors [3], including the patient's age and personality. In the past, it was believed that gender could influence cooperation [17,52], but more recent studies have not recognized it as a significant influential factor [13,40].
Patients with different ages can face the treatment in different ways: it seems that there is a compliance reduction with the increase in age-from middle childhood to early adulthood [40]. It has been observed that patients in middle childhood (6-8 years) are more compliant than adolescents (12-15 years) and early adults [12]. Compliance is influenced by different factors in each of these life periods: during middle childhood, the "parentchild" bond is central and has a strong impact on treatment motivation [1]. Since a child is not aware enough of his/her appearance, parents and caregivers pay attention to the patients' needs, including orthodontic treatment [1]; during adolescence and adulthood, self-perception, self-esteem, and social feedback deeply influence motivation and adherence to the treatment [53].
It has been observed how the psychological dynamics underlying the level of collaboration and adherence to the rules in any situation of everyday life are regulated by the "effect" that they have on the quality of people's life: if positive, the level of compliance would be high [45]; on the contrary, if negative, it would be low. People are strongly motivated to take medications that relieve pain and promote healing because they have a positive effect on their quality of life. In orthodontics, the treatment represents a transitory phase in which it is possible to experience pain or discomfort, negatively affecting patients' compliance [37,45]; however, in the long term, the treatment would lead to an improvement in aesthetics and quality of life.
Psychological "patient-related" factors include the self-perception of facial appearance, attitude towards the treatment, pain, and discomfort threshold. Facial appearance determines how a person feels about himself/herself and how society feels about people's appearances [44]. If someone is particularly concerned about their physical appearance and has a high self-esteem, when malocclusions or irregularities of the teeth are noticed, the person is strongly motivated to resolve them to gain or maintain acceptance and compliments from other people [23]. Subjects with a good self-esteem who are emotionally stable and have a tendency to observe the rules and understand the importance of constantly wearing the orthodontic device will be highly collaborative. On the contrary, subjects with a confused lifestyle or an attitude of distrust towards their own therapy management skills tend to be unpredictable in terms of collaboration, as their personality makes them nervous and whiny about pain and discomfort when experiencing the treatment [38].
The perception of malocclusions is another important factor that influence the wearing time of orthodontic devices. Masood et al. showed that dental irregularities are perceived in a different way based on two opposite patient personalities: subjects with slight defects are excessively alarmed, while subjects with severe malocclusions are fine with their appearance. The reaction is not always in line with reality, as it is strictly related to the self-awareness of dental and facial appearance [29,30]. Generally, the more severe the malocclusion is perceived to be, the higher the level of compliance will be. However, if the patient is not fully aware of how serious the problem is, he/she will tend to underestimate the situation and not accurately comply with the treatment.
Therefore, the patient's perception of his/her own malocclusion can be a predictor of cooperation [12]. As patients' self-esteem is considered a major predictor of adherence to treatment, the clinicians should illustrate the treatment plan with the support of the patient's documentation (pictures, radiographs, 3D facial scans, dental models).

"Clinician-Related" Factors
The ways clinicians interact with their patients directly influence how much they cooperate during the treatment [5,35].
Developing a healthy relationship with patients represents a real challenge that requires attention regarding some essential aspects such as the communication and awareness of psychological issues and the dynamics involved with orthodontic treatment [5]. Clinicians should build a mutually beneficial relationship based on trust, honesty, and loyalty. The orthodontist should speak to the patients to introduce them to the treatment, to give advice, or to satisfy a request by using clear and coherent words. The purpose is to make the patients aware of every aspect of the treatment, such as the benefits, risks, costs, and uncomfortable effects. Moreover, the case discussion has the role of reducing the possibility of dissatisfaction or premature "drops-out" [54]. As patients' self-esteem is considered a major predictor of the adherence to treatment, the clinicians should illustrate the treatment plan with the support of the patient's documentation (pictures, radiographs, 3D facial scans, dental models). The use of patients' documentation is of twofold importance: firstly, it demonstrates a successful outcome that can be attained if the patient adheres to the appliance's wearing instruction during every phase of the treatment; secondly, it shows that the therapeutic outcomes would outweigh any experienced discomfort [54].
It has been observed that patients will choose to be treated only if the "positive" motivation (keeping in mind the desired results) is stronger than the "negative" motivation (the pain and discomfort of the treatment) [54]. Furthermore, the level of cooperation is optimal if the "positive" motivation is kept high [7]. The motivation through an orthodontic process can be "internal" or "external" [33], and the role of the orthodontists' communication is crucial in both. The "internal" motivation is influenced by the awareness among patients about their personal orthodontic situation combined with the desire to achieve good results (aspects that should be explained and clarified by the orthodontist). The motivation of younger patients is mostly external; teenagers who seek orthodontic treatment are usually motivated by intrinsic factors. Indeed, the "external" motivation is generally increased by other people such as family, friends, partners, and schoolmates, but also by orthodontists who have to constantly encourage the patients [33]. The influence of both types of motivation makes the clinician-patient relationship mutually beneficial.

Self-Reported Patient Compliance: Myths and Facts
Since "orthodontic compliance" does not involve a single general aspect of compliance [27], (i.e., a patient who keeps high levels of oral hygiene does not necessarily wear the appliance for the required time, and vice versa), conducting accurate measurements of the patients' level of adherence to the treatment represents a very challenging task [27,40]. Compliance can be subjectively assessed with self-reported questionnaires filled by patients or parents; however, many authors suggest that subjective evaluations are usually unreliable [14,15,27,34].
In fact, patients are often unrealistic about the wear-time of the removable appliances: they tend to overestimate their wear-time [15,34] by one-third when they do not know that it is monitored [34].
In the past decades, the introduction of several devices and monitoring tools has helped to overcome these limitations and has provided an objective evaluation of the level of patient compliance [3].

Compliance Monitoring: Physical Devices and Chemical Indicators
Pauls et al. reported that, to achieve an orthodontic treatment goal, functional appliances should be worn at least 12.8 h per day, and active plates should be worn for 13.9 h per day [34]. The literature agrees on the fact that compliance never exceeds 7 to 9 h of the prescribed 8 to 15 h per day [11,12,34]. It is very poor at 65% of the 13 h prescribed and is likely to compromise the efficacy of the orthodontic treatment [12].
Patients are used to cheating, declaring that they wear orthodontic appliances for more hours than the real time of wearing [15]. The awareness of being monitored by monitoring devices leads patients to be more realistic about their wear-time. When confronted with the objective and real measurement of the wear-time of the removable appliances, patients start to provide more accurate subjective estimations, with better transparency about their behavior, which may help in understanding possible failures of the treatment [34,55].
Yet, the evidence reported that wear-time monitoring by clinicians and caregivers does not necessarily boost compliance or increase the general amount of time spent wearing removable appliances [12]: if patients are informed that their wear-time is recorded, they tend to be more compliant but still wear the appliances for a shorter period than that prescribed by the given instructions [14,36].
Physical compliance devices have been developed to associate the wear-time with intraoral environmental factors (e.g., pressure, body temperature) [56]. The first timing device was a piece of headgear introduced by Northcutt (1974) which was activated by pressure switches on the neckstrap, but it was easy to cheat by just activating the timer by placing heavy objects on the pressure switch [57]. A timer monitor designed by Savage (1982) used a soluble controlled-release glass timing disc incorporated into the removable appliance [58], and Cureton et al. (1991) developed a device with a small quartz calendar wristwatch [59].
These devices were innovative, but they never spread for clinical purposes, not even for research, because of the inadequate characteristics such as the elevated costs, the complicated use, and the uncomfortable size affecting the acceptance among patients and orthodontists [26,40,56,60]. Low reliability and poor accuracy in the measurements were also reported [40,60].
More recently, environmental microsensors which can be embedded in appliances and automatically measure and record temperature changes between the oral cavity and surrounding area were introduced.
The TheraMon ® System (Handelsagentur Gschladt, Hargelsberg, Austria or Forestadent, Pforzheim, Germany) is a small device (13 × 9 × 4.5 mm) that measures the intraoral wear-time: it works as a recording thermometer by measuring the temperature of the surrounding area of the patient's orthodontic appliance. The system is a useful tool to briefly ascertain the time for which patients wear removable appliances, but it does not allow for the objectification of the effective wear-time due to the several confounders (starting from temperature variations in an individual's mouth) that affect the data recordings [16,61].
The Smart Retainer ® (Scientific Compliance, Atlanta, GA, USA) gathers several pieces of data from the environment (light, vibration, temperature) and consists of a button-size environmental microsensor that can be easily embedded into many types of orthodontic devices. At preset intervals, the microsensors automatically monitor any change in the environmental surroundings and store the data in an encrypted form. When the retainer is plugged in the proprietary USB-powered Smart Reader, the data are analyzed using proprietary algorithms for trends and use pattern periods [26].
Chemical indicators were also introduced to monitor the wear-time of clear aligners by Align Technology [41]. The compliance indicators are made of a colorful food dye (Erioglaucine disodium salt) encapsulated in the aligner. When the clear aligner is in contact with saliva or oral fluids, the dye is released from the appliance; if the aligner is worn over the 300-400 h of recommended wear for 14 days, the color of the compliance indicator turns from dark to clear blue. Indeed, the color-fade indicates of the quantity of dye loss, which will correspond to the amount of time that the patient wore the aligner, referring to how the appliance was in contact with the saliva (there are two different formulations of yeasts to account for patients' individual dissolution rates of saliva) [41].
Evidence of the clinical efficacy of chemical compliance indicators in clear aligners therapy is lacking. It is reported that chemical indicators have several flaws, since patients can easily manipulate the color changes (on purpose or without intention) and affect the results. The color chart can be biased if the patient leaves the aligners in his/her mouth while drinking or uses inappropriate cleaning-tablets containing oxidizing agents or dishwasher [41].

How to Improve Compliance? Teledentistry Could Be an Answer
High levels of compliance (such as better attendance, accurate appliance wear-times, and good oral hygiene) are associated with the increased efficacy and effectiveness of orthodontic treatment. Li et al. reported that messaging apps such as WhatsApp (WhatsApp Inc., Mountain View, CA, USA) and WeChat (Tencent Holdings Limited, Shenzhen, China) have an impact on improving patients' compliance, especially among adolescents, and on reducing the duration of orthodontic treatment (DOT) [28].
Smartphones and tablets have been widely used in health areas (to improve education and to facilitate the patient's management). "Telecommunication" is a method of communication between people (the clinician and the patients) who are physically separated: the distance does not inhibit the clinical process but is empowered by the frequent sharing of photos and information, which allows for an increased access to oral care and for remote but strict monitoring of the evolution of a treatment [62]. Recently, the Association of American Medical Colleges coined the term "Teledentistry"-the use of digital tools (diagnostic imaging, software, and devices) for diagnosis, follow-ups, and telecommunications in dentistry. Sharing digital information, data, graphics, and photos for clinical care using technology (computers, smartphones, apps) represents an alternative and very innovative method to deliver dental care [18,42,62].
Telecommunication in dentistry is a valid tool for the clinician, who can better educate and guide the patient through the therapy with advice, warnings, and reminders for hygiene or time wearing. Additionally, it constantly controls the integrity of appliances, making earlier diagnoses of vestibular cavities (the evidence suggested the effectiveness of this approach in decreasing the incidence of white spot lesions during the first year of treatment with multibracket appliances) [2]. It can also detect irregularities and mistakes in the treatment process. It is also a valid tool for patients, who can solve some doubts by simply sending a WhatsApp message to consult the dentist. Teledentistry is positively judged by both patients and dentists, who consider it as a valid method to increase the accuracy of the treatment [63], and has been an effective aid during the COVID-19 pandemic, which made it possible for a lot of clinicians to monitor the course of orthodontic treatments during the lockdown periods [24,64].
WhatsApp, WeChat, or any other chat platforms represent an effective way to improve the wearing time of orthodontic devices: it is reported that the DOT is shortened by 7.3 weeks on average [28]. In dentistry, the evidence shows that attendance can be incentivized by simply sending a reminder of any type before the appointments [28].
Dental Monitoring TM (DM, Dental Monitoring, Montreal, France) is a further step in Teledentistry. It is a digital system with three integrated platforms (a teeth movementtracking algorithm, an app for the patient, and a web-based Doctor Dashboard) designed to conduct orthodontic follow-ups at a distance [18]. The dedicated app makes the patient's smartphone a "remote intraoral scanner" (the patients themselves can make an intraoral video which is processed by DM) by which orthodontists can control teeth movement and the integrity of the appliances and make assessments about the patient's treatment progress. The DM receives the patient's pretreatment photographs and a 3D model in stereolithography file format (.stl). An artificial intelligence-based algorithm compares pictures taken by the patient's smartphone with the .stl file of the initial scan. With the pictures at the beginning of the therapy, it calculates the tooth positions, the overjet, the overbite, and the interarch relationships, realizing a "multidimensional information map" of the teeth (uploaded to the Doctor Dashboard), which provides the clinician with automatically generated information regarding the course of the treatment [18,63]. Hansa et al., in 2021 [64], compared the effects of clear aligner treatment with and without DM in terms of the duration of the treatment, the number of appointments, refinements, and refinement aligners, and the achievement of predicted tooth positions: the results suggested that DM leads to a reduction of appointments by 3.5 visits (33.1%) over the treatment duration, which obviously allows for an increase in office efficiency. DM seems to require 4-5 h per week to manage 275 active patients: 30 min per week with the doctor, 1 min per patient per week for the staff, and a few seconds per week for the orthodontist. Reduced appointments also represent a benefit for the patients by reducing travel costs and allowing them to not leave work or school; so, generally, DM is well received and appreciated by patients [24]. DM also reduces the first refinement time by 1.7 months when DM refinements are performed earlier (due to the more frequent and proactive monitoring). The treatment's efficacy may also be improved by this system, which offers the possibility of identifying mistakes and problems at an early stage, such as debonded brackets, non-tracking aligners, and broken ligatures [25,65,66].
These promising technologies, however, need sound scientific evaluation and cannot yet be considered validated [67,68]. Based on our knowledge, all these devices may integrate the clinical examination and may reduce the frequency of visits, but they cannot totally replace in-person appointments.

Limitations of the Study
The present review has methodological limitations (the small number of electronic databases, PROSPERO registration) due to its "narrative" purpose: the absence of scientific evidence on the topic of compliance and clear aligners leads to a move away from a rigorous, systematic approach. This study seeks to satisfy the need for "ground knowledge" on this topic and to offer practical hints for developing further investigations.

Conclusions
Compliance is a multifactorial issue: the wearing time of removable appliances is influenced by patient-related factors (age, personality traits, the importance of personal appearance, the self-perception of malocclusions), appliance-related factors (visibility, pain/discomfort), and clinician-related factors (trust, clear and complete communication, motivation).
Compliance is a key factor for clear aligner treatments, but high-quality studies focusing on this aspect are missing. Patients tend to overestimate appliance wearing time, and poor compliance is a very frequent problem (25%) in clear aligner therapies. Evidence is lacking on the clinical efficiency of compliance indicators. Teledentistry (communication by messaging apps such as WeChat (Tencent, Shenzhen, China) and WhatsApp (Meta Platforms, Inc., Cambridge, MA, USA) can improve patients' adherence. More sophisticated monitoring apps based on artificial intelligence might increase patients' compliance, but they still need to be scientifically validated. Further studies should focus on how to handle the lack of cooperation and how to increase compliance in order to maximize the treatment's results.