Patient Compliance with Removable Orthodontic Retainers During Retention Phase: A Systematic Review

ABSTRACT

Background/Objective

Patient compliance post orthodontic treatment is one of the most common factors that affect long-term success in alignment and occlusal positioning of teeth. The use of removable orthodontic retainers and the monitoring of its use are essential to assess the collaboration of the patient. The aim of this systematic review is to assess different factors that may influence patient compliance and possible interventions for enhancing post-treatment compliance of removable retainers.

Study Design

The author searched PubMed, Web of Science, and Embase databases to identify original articles from incipient to May 2022 containing keywords for patient compliance with removable orthodontic retainer during retention phase. Prospective and retrospective studies, including observational, cross-sectional, randomized, and non-randomized clinical trials, cohort studies, and case-control studies, were reviewed. The review was conducted in accordance with PRISMA guidelines. The risk of bias was assessed in studies selected for full-text review.

Results

The search strategy yielded 842 publications. After initial screening and application of exclusion criteria, 12 studies were included for final review.

Conclusions

Significant differences in patient compliance with removable orthodontic retainers were observed in the following factors: type of removable retainer, parental intervention, age of the patient, location of treatment, and the use of text message reminder. No significant difference in compliance was observed in gender. The overall risk of bias among studies was moderate. There are insufficient high-quality randomized clinical trials (RCTs) to provide additional evidence, and further research is needed.

KEYWORDS:

• Removable orthodontic retainer
• patient compliance
• retention

Introduction

Post-retention phase orthodontic compliance, defined as the adherence to a prescribed wearing schedule correctly or consistently, is essential for long-term successful alignment and occlusal positioning of the teeth. Once orthodontic treatment is completed, the teeth have a proclivity to move toward their initial positions, which is commonly referred as orthodontic relapse. It could occur due to various reasons, such as the inherent memory of periodontal ligament, changes in skeletal structures surrounding teeth, occlusal forces, labial and lingual pressures, and the normal aging process.^1,2 These factors affect the rate and degree of relapse; hence, wearing an appliance post-treatment is essential in maintaining stable alignment of the dentition.^3–6

Clinicians may offer fixed or removable options, and among the most prescribed are the removable Hawley retainers (HR) and removable Vacuum-formed retainers (VFR).^4,7 Post-retention compliance has remained one of the most prevalent challenges in orthodontics, especially in removable appliances, in which the patient is solely responsible for wearing them.^4,8 Several studies have aimed to compare compliance among patients wearing HRs and VFRs, however the results are inconclusive.^7,9–11 A wealth of research has pointed to suboptimal compliance levels among orthodontic patients; moreover, candid patient reports of wear duration are typically not forthcoming.¹² Consequently, embedded microelectronic sensors have gained traction, primarily as a research tool, to corroborate reported estimates of the appliance wear.¹³

The problem of suboptimal compliance with removable retainers has been exposed in several contexts with reasons including but not limited to a lack of motivation to maintain occlusal outcomes, a loss of the removable appliance, discomfort in wear, inability to eat while wearing it, and difficulty while speaking.^14–16 However, relatively few interventions aiming to optimize appliance wear time have been studied. There is a lack of consensus regarding the anticipated wear levels associated with removable retainers and how this related to reported and objectively measured wear durations. The main objective of this systematic review is to assess compliance with removable retainers during the post-retention phase and to identify the factors that relate to continued, regular wear and usage. Factors affecting compliance including type of retainers, age and gender of the patient, parental intervention, location of treatment, and intervention strategies were evaluated.

Materials and Methods

Protocol and Registration

A systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) recommendations.¹⁷

Eligibility Criteria

The following inclusion and exclusion criteria were applied.

1. Study design. Quantitative studies including randomized, nonrandomized controlled clinical trials, prospective cohort studies, and case series (minimum sample size, 20 patients) incorporating objective data on compliance levels were eligible. Qualitative studies exploring patients’ views and experiences of removable orthodontic retainers and the interventions used to improve compliance levels (including barriers and facilitators affecting wear of the appliance) were included. Mixed-method studies in which quantitative or qualitative components met the above criteria were also included.

2. Participants. Patients of any age treated with removable retainers were eligible.

3. Interventions and comparators. Orthodontic interventions with removable retainers, including Hawley, Essix retainers were included. The use of improving compliance was able to be assessed. Other orthodontic interventions including headgear, protraction facemask, chin cup, removable appliances, fixed appliances and retainers were excluded.

4. Outcome measures. Primary outcomes included compliance levels with orthodontic regimens (e.g., hours per day of wear or percentage of compliance) in relation to both stipulated and patient-reported levels of wear. Secondary outcomes were the impact of the interventions used to improve compliance levels and delineation of patient experiences.

Search Strategies and Data Sources

Various sources have been examined, summarized, and referenced. A comprehensive search of PubMed, EMBASE, and Web of Science databases were conducted for studies published from inception to August 1, 2022, in all languages. In Embase, the search was limited to articles or articles in press. A combination of MeSH/Emtree thesaurus terms and keywords were used to identify studies relating to the compliance of removable orthodontic retainers. The search strategy was developed around three main concepts: patient compliance, removable orthodontic retainers, and retention. Full search strategies are listed in Appendix A. Citation tracking and searching of reference lists of the included studies were performed to identify relevant research. The final searches were completed on August 1, 2022. Additional studies were identified by hand-searching the bibliographies of the articles assessed for the review. The authors of the included studies were contacted via e-mail if additional information was required.

Screening and Data Extraction

Two independent reviewers screened all titles and abstracts to arrive at a bank of relevant articles. Upon discrepancy in the decision to include an article, a third reviewer was consulted. A full-text review was then conducted by two reviewers to further screen the articles for inclusion. Data extracted and charted from articles included the type of study, the type of removal orthodontic retainer, and whether the study discussed the level of wear (stipulated, self-reported, or measured), the use of means to improve compliance.

Risk Assessment

A risk assessment was performed for all included studies using the Risk of Bias in Non-randomized Studies of Interventions (ROBINS-I) tool for the non-randomized studies or the Cochrane Risk of Bias (RoB 2) tool for randomized trials. Risk assessment using the ROBINS-I tool was divided into seven sections: (1) bias due to confounding, (2) bias due to selection of participants into the study, (3) bias of classification of interventions, (4) bias due to deviations from intended interventions, (5) bias due to missing data, (6) bias in measurement of outcomes, and (7) bias in selection of the reported result; this was ultimately followed by an overall risk of bias determined by the previous seven bias domains.¹⁸ Risk for the ROBINS-I tool was assigned as low, moderate, serious, or critical.

Several studies included in this systematic review were attributed a bias due to confounding as elaborated upon in the Discussion section. Confounding bias, which is often prevalent in non-randomized trials, is a systematic distortion in the measure of association between an exposure and health outcome by an extraneous, third variable called a confounder. To be considered a potential confounder or confounding variable, the risk factor must be a known risk factor and not an intermediate causal link between the exposure and outcome.

The RoB 2 tool divided risk assessment into five sections: (1) bias arising from the randomization process, (2) bias due to deviations from intended interventions, (3) bias due to missing outcome data, (4) bias in the measurement of outcomes, and (5) bias in the selection of the reported result.¹⁹ Risk for the RoB 2 tool was assigned as low, some, or high. Additionally, each randomized trial was assigned an overall risk of bias as determined by the previous five bias domains. Within both risk assessment tools, the overall risk of bias was derived from the highest risk level found among all the domains, even if all other domains were deemed lower risk.

Results

Selection of the Studies

The search strategy yielded a total of 842 records from PubMed, Web of Science, Embase and hand search (Figure 1), 240 of which were excluded as duplicates. Title and abstract screening were performed for the remaining 602 records. Search filters provided on each database were used to limit the search to the types of study design specified in the inclusion criteria, resulting in the removal of 536 records and leaving 66 records that were assessed for eligibility. After a review of the abstract and full articles, 54 additional records were excluded, resulting in a final total of 12 articles in the review. These reasons for exclusion are further defined in Table 1.

Figure 1. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) flowchart describes the number of publications identified, screened, and included.

Table 1. Definitions of terms used as reasons for exclusion.

Reason for Exclusion	Definition
Wrong Outcome Measure	Does not measure patient compliance as the main study outcome
Wrong Intervention	Does not discuss removable retainers as the orthodontic intervention Do not use intervention during the relapse phase
Foreign language	Not written nor transcribed in English language
Wrong population	Not written for orthodontic or general dental professionals

The 12 articles in this review were published between 2011 and 2022. Nine were prospective studies,^12,20–27 one was retrospective study,⁶ and two were cross-sectional cohort studies.^28,29 Seven of the prospective studies were controlled,^21–27 and two were not controlled.^12,20 The number of subjects in the research ranged from 18 to 326. The mean age of the participants varied from 12.7 to 17.5 years, and the range of ages extended from 6 to 40 years.

In terms of the removable orthodontic retainers prescribed, Hawley retainers were discussed in ten articles, Essix retainers or vacuum formed retainers were discussed in six articles, with four articles utilized both types of retainers in their studies. The range of study duration varied from 1 month to 1.5 years. To assess patient compliance with removable retainers, most studies utilized either the objective or subjective recording of wear. Regarding the objective measure of wear time, TheraMon® micro-electronic sensor was utilized in six studies^{12,20,24,25,27,28} and a SMART Microsensor with the ability to monitor temperature was used in one study.²² Subjective recording of wear was captured by self-reporting in the form of logs or surveys in six studies.^{6,12,21,24,25,29} Two studies evaluated the level of patient compliance through comparing the change in intercanine width or opening of extraction space during retention phase.^23,26 The characteristics of the studies included are summarized in Table 2.

Table 2. Baseline characteristics of the included studies.

Study	Design	Participants and setting	Appliance	Objective measure	Intervention to improve compliance	Stipulated wear time (h/d)	Study duration	Subjective recording of wear	Awareness of being monitored
Pratt et al.^Citation6	Retrospective study	1085 patients were surveyed, 280 (25.8%) questionnaires returned. Orthodontic clinic at University of Kentucky	Hawley retainers and vacuum formed retainers (VFRs)	N/A	N/A	N/A	6 months to 6 years	Yes	No
Schott et al.^Citation20	Prospective cohort study	N = 100 (52M, 48F); mean age = 15.46, age range = 13–20 University Hospital of Tubingen, Germany, and 4 private practices in Germany	Hawley retainers, functional appliance retainers	TheraMon® micro-electronic sensor	N/A	8	450 days with follow-up each 100 days	No	Yes
Tsomos et al.^Citation28	Cross-sectional cohort study	N = 45 (25M, 20F) mean age = 12.7, age range = 7.2–21.5 Private orthodontic practice in Germany	Frankel II, III, Sander II, Cow Catch, Hawley, Essix	TheraMon® micro-electronic sensor	N/A	8	Mean = 177d (range 55–293 days)	No	Yes
Lin et al.^Citation21	Randomized controlled trial	N = 326; age range = 11–18 School of Stomatology, Wenzhou Medical University, China	Hawley retainers	N/A	Images illustrating the severe consequences of poor compliance with Hawley retainer use	22*(full-time)(full-time) (full-time)	3 months	Yes	Yes
Hyun et al.^Citation22	Randomized controlled trial	N = 18 (7M,11F) mean age, 15.44±1.38 University of Buffalo, NY, Orthodontic clinic	Maxillary Hawley retainers	SMART Microsensor, Great Lakes Orthodontics, Buffalo, New York	Group A: aware of monitoring from start of study (n = 8) Group B: aware of monitoring at T1 (n = 10)	19	12 wk	No	Yes at T1
Mirzakouchaki et al.^Citation29	Cross-sectional study	N = 77 (38M, 39F); age range = 7–11 Department of Orthodontics at the Tabriz University of Medical Sciences and private practice, Iran	Hawley retainers and vacuum formed retainers (VFRs)	N/A	N/A	20	3 months	Yes	Yes
Schott et al.^Citation12	Prospective cohort study	N = 42 (5.5% F); Mean age = 14.8±2.1, 12.3; age range = 6–20; Eberhard Karls University, Department of Orthodontics, private orthodontic practice, Germany	Retention plates	TheraMon® micro-electronic sensor	N/A	9.4±3.4	102.8±65.8 days	Yes	Yes
Zotti et al.^Citation23	Randomized controlled trial	N = 60 (24M, 36F); mean age = 17.5, age range = 16–19 University of Verona, Italy	Hawley retainers	Mandibular intercanine width	WhatsApp	8	1 year	No	Yes
Vagdouti et al.^Citation24	Randomized controlled trial	N = 77; age range = 12–18 National and Kapodistrian University of Athens, and a private practice, Volos, Greece	Hawley retainers and vacuum formed retainers (VFRs)	TheraMon® micro-electronic sensor	N/A	22* (full-time)	1 month & 3 months	Yes	Yes
Al-Moghrabi et al.^Citation25	Randomized controlled trial	N = 84; age range = 12–21 London School of Medicine and Dentistry, UK	Thermoplastic retainers (TPRs)	TheraMon® micro-electronic sensor	‘My Retainers’ mobile application		3 months	Yes	Yes
Sharma et al.^Citation26	Randomized controlled trial	N = 54 (24M, 30F); age range = 18–25 University Orthodontic clinics, India	Begg’s retainers	Canine-canine width and opening of extraction space	Text messages (3 times/week)	N/A	6 months	No	Yes
Zhang et al.^Citation27	Randomized controlled trial	N = 84 (42M, 42F) Mean age = 17.24±2.00 Royal London Hospital Barts Health NHS Trust, UK	Maxillary and mandibular Essix-type pressure-formed retainers	TheraMon® micro-electronic sensor	‘My Retainers’ mobile application	22h for first 6 months, 8h for an additional 6 months	12 months	No	Yes

Five of the included studies were non-randomized,^{6,12,20,28,29} so the ROBINS-I risk assessment tool was used accordingly. Seven of the studies were randomized,^21–27 so the RoB 2 risk assessment was used. All five non-randomized studies were found to be of moderate risk of bias overall (Table 3). Of the seven randomized studies, four studies were found to have a moderate risk of bias, and the rest were found to have a low risk of bias (Table 4).

Table 3. The risk of bias assessment using the ROBINS-I tool for non-randomized studies.

Study	Confounding	Selection of participants	Classification of interventions	Deviation from intervention	Missing data	Measurement of outcomes	Selection of reported result	Overall risk of bias
Pratt, Kluemper, and Lindstrom.^Citation6	Moderate	Low	Low	Low	Low	Low	Low	Moderate
Schott et al.^Citation20	Moderate	Low	Low	Low	Low	Low	Low	Moderate
Tsomos et al.^Citation28	Moderate	Low	Low	Low	Low	Moderate	Low	Moderate
Mirzakouchaki et al.^Citation29	Moderate	Low	Low	Low	Low	Moderate	Low	Moderate
Schott et al.^Citation12	Moderate	Low	Low	Low	Low	Moderate	Low	Moderate

Table 4. The risk of bias assessment using the Risk of Bias 2 (RoB-2) tool for randomized studies.

Study	Randomization process	Deviation from intended interventions	Missing outcome data	Measurement of outcomes	Selection of reported result	Overall risk of bias
Lin et al.^Citation21	Low	Low	Low	Moderate	Low	Moderate
Hyun et al.^Citation22	Low	Moderate	Low	Moderate	Low	Moderate
Zotti et al.^Citation23	Low	Low	Low	Moderate	Low	Moderate
Vagdouti et al.^Citation24	Low	Low	Low	Low	Low	Low
Al-Moghrabi et al.^Citation25	Low	Low	Low	Low	Low	Low
Sharma et al.^Citation26	Low	Low	Low	Moderate	Low	Moderate
Zhang et al.^Citation27	Low	Low	Low	Low	Low	Low

Discussion

Assessment of compliance for clinical or research purposes is difficult because of the wide variety of factors determining patient’s compliance.³⁰ The use of removable retainers means that the responsibility for retention lies with the patient. Monitoring the compliance with removable retainer usage can be difficult for the orthodontist. This can lead to frustration for both practitioners and patients.²⁰ This review attempts to establish the relationship of orthodontic patients’ compliance with various clinical and social parameters as well as assess the effectiveness of currently available interventions in improving compliances.

Type of Retainers

The two most prescribed removable retainers for patients during the post-treatment phase are Hawley and Essix or vacuum-formed retainers (VFRs), made of metal/acrylic and clear plastic, respectively. Among several studies,^6,24,29 patient compliance was shown to be greater with vacuum-formed retainers compared to Hawley retainers for the initial period post-debonding. The objectively assessed median daily wear time is higher for the group wearing VFRs. These results agreed with a previous study by Hichens et al.³¹ who found that retainer compliance was greater with VFRs than with Hawley retainers at 3- and 6-month intervals after debonding. However, the patient compliance with removable retainers may be time-sensitive, as the compliance with VFRs has shown to decrease at a much faster rate than Hawley retainers.⁶ Pratt et al. concluded that patient compliance was greater with Hawley retainers after 2 years of debonding. A previous study speculated that the initially higher rate of compliance with VFRs could be explained by aesthetic considerations.³² However, there is not sufficient evidence showing the use of VFRs is more beneficial than Essix retainers over extended period of time. Therefore, the reason for greater compliance with VFRs after debonding has yet to be determined.

Factors Affecting Patient Compliance

Age

Age can be a highly relevant factor affecting adherence to removable orthodontic retainers post treatment. Four of the included studies observed that the greater the patient’s age, the shorter the appliance wear time.^6,20,28,29 In other words, older patients had worse compliance than younger patients, with studies ranging from 15 to 40 years old. One study, Mirzakouchaki et al., found that Junior high school children (between 13 to 15 years of age) were the most compliant patients (p < .001).²⁹ Similarly, Schott et al. also found the wear time of patients between 13 and 15 years of age was 1.4 hours longer than that of older patients (16–20 years; p = .206).²⁰ Larger samples are needed in order to investigate in further detail the influence of age on compliance and to provide more specific guidelines for treatment with removable retainers.

Gender

Among the included studies, all but one study showed gender does not affect appliance wear time.^6,33 In most studies, there are no statistically significant differences between females and males.^20,28,29 However, one study by Pratt et al.⁶ concluded that gender was a statistically significant variable, in which females generally had a higher compliance rate than males within 6-year post-orthodontic treatment (p = .0022).

Parental intervention

Two included studies observed that parental attitude and intervention affect compliance with retainer use in adolescents.^21,29 Lin et al.²¹ found that the mean daily wear time was significantly higher when parents and patients were shown images depicting the severe consequences of poor compliance with Hawley retainer than when the images were only shown to the patients. This agreed with previous studies which have also concluded that parental involvement positively affects patients’ compliance.^34–36 One paper revealed that mother’s influence as compared to father’s is more instrumental to treatment compliance.³⁷ These findings indicate the important role of parents on patient’s compliance.

Place of treatment

In comparing post-treatment wear time among patients treated in private practices and in university settings, studies have shown similar findings of increased compliance in private practices.^20,24,29 In patients having treatment in private practice, there was an increased potential for compliance of 2.16 h/d compared to those in a university setting (95% CI 0.34–3.97; p = .02), according to a study by Vagdouti et al.²⁴ In two similar studies, Schott et al.²⁰ and Mirzahouchaki et al.²⁹ also compared the compliance for patients who were treated in private practice compared with patients treated in the university clinic. The wear time for patients who were treated at the university hospital seemed slightly shorter than those from private practices (median wear time was 6.0 h/d for a university hospital, 7.2 h/d for private practice in Schott et al study²⁰; mean wear time was 14.8 h/d for university clinic, 16.4 h/d for private practice in Mirzahouchaki et al study.²⁹ However, no significant differences could be found (p > .05) in these two studies; thus, further investigations are needed to draw a definite conclusion.

Intervention to Improve Compliance

Several included studies investigated different means of intervention to improve patient compliance with the use of text reminders, social media, or mobile applications. One study by Sharma et al. compared compliance among subjects who received text messages versus those who did not. Results indicated that subjects receiving text reminders had a significantly smaller increase in both inter-canine width and extraction spaces in comparison to the control group, revealing that the text message reminders had a positive influence on patient compliance. Zotti et al. also examined the impact of joining a WhatsApp group chat on patient compliance and relapse rate in adolescent patients.²³ The study showed participants in the WhatsApp group featured smaller changes from their inter-canine widths at baseline compared to the control group during all time markers, i.e., 4, 8, and 12 months after brace removal. In addition, the control group had decreased follow-up attendance after the first 8 months from debonding compared to the subject group. Two other studies assessed the effectiveness of a tailored mobile application “My Retainers” in enhancing removable retainer wear. Al-Moghrabi et al.²⁵ showed the receipt of mobile application resulted in higher median wear time (0.91 h/d) than the control. However, this difference was not statistically significant, which may be due in part to the short follow-up period of 3 months and the confined single-center university clinic setting. Similarly, Zhang et al.²⁷ showed the while there was a marginally higher wear time observed for the group with mobile application intervention as compared to the control group, the results were also not statistically significant. Castle et al. proposed a Bluetooth-enabled retainer that enabled wireless data transfer to cloud databases and mobile application.¹³ Patient compliance could be monitored daily via a provider webpage and early intervention was possible with reminder messaging.

Furthermore, the use of microsensors as an objective measurement method enhances the communication between patient and clinician, and boosts confidence in the orthodontist-patient relationship.³⁸ There is often a discrepancy between what a patient reports their wear time to be and what the clinical examination shows. In these cases, a smart retainer with a microsensor can provide information to allow the orthodontist to discuss actual-versus-prescribed retainer usage with the patient in order to make data-driven recommendations for future retention.³⁹ However, more random clinical studies are needed to establish to what extent the temperature sensors influence the orthodontic outcome.

Limitations of Studies

Among the limitations of the present review were the great variability of age, gender, and sample size among the studies, compounded by the variability of differing follow-up times, from 1 month to 6 years, as well as the differing intervention times. The subjectivity presented by each individual when considering the adherence hours, makes for an overall heterogeneity of the result. Secondly, as with any study relying on the use of self-reporting from the subjects, there is the potential for inaccurate reporting. Several studies tried to limit the potential for this error by using a micro-electronic sensor to obtain the objective wear time as a more accurate means to measure compliance.

Additionally, being aware of the presence of microsensors in removable retainers may affect a patient’s compliance in a positive way, as seen in two studies included.^22,40 However, in other studies even though the patient was aware of monitoring, there was no direct influence on adherence to the hours pre-established by the professional.^28,41 Furthermore, not all studies were able to obtain the objective measure of wear time. Retrospective studies and some prospective studies evaluated patient compliance through subjectively reported wear time, which were shown in many studies that tend to be overestimated than the actual wearing time.^22,25,33

Conclusions

Compliance in orthodontics is largely a factor affecting long-term success. Based on the results of this systematic review, clinicians prescribing Hawley retainers 2-years post debonding, the use of parental intervention, treatment based in private or hospital practices as opposed to institutions, and text message reminders and Application-based groups have helped with increasing compliance. In addition, studies have shown that older patients tend to have worse compliance than younger, with patients in their teen ages experiencing optimal compliance, however, larger studies are needed to further investigate this. Lastly, gender does not appear to affect compliance, yet one study showed females experience better compliance than their male counterparts, concluding that more research is also needed to evaluate this. Among the studies included, the risk of bias was low to moderate. Overall, further research is needed to determine the best factors for promoting post-treatment compliance for future orthodontic clinicians.

Declaration of Patient Consent

Patients’ consent is not required as no patient’s identity is disclosed or compromised.

Disclosure Statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The author(s) reported there is no funding associated with the work featured in this article.

Notes on contributors

Lara Moyu Fu

Lara Moyu Fu is a dental student at University of California, San Francisco, California, US.

Stephanie Ellman

Stepanie Ellman is a dental student at University of California, San Francisco, California, US.

Jonathan Gao

Jonathan Gao is an orthodontic resident at University of California, San Francisco, California, US.

Peggy Tahir

Peggy Tahir is Research & Copyright librarian at University of California, San Francisco, California, US.

Snehlata Oberoi

Snehlata Oberoi is a professor at Orthodontics Department of University of California, San Francisco, California, US.

Appendix A:

Search strategies

Orthodontic appliances

Final update searches: 8/1/2022

Limits: Embase limit to articles/articles in press

Table

DATABASE	SEARCH STRATEGY
PubMed	(“Orthodontic Retainers”[Mesh] OR removable orthodontic* retainer*) AND (“Patient Compliance”[Mesh] OR “patient compliance“OR compliance OR retention)
Web of Science	(removable orthodontic* retainer*) AND (“patient compliance” OR compliance OR retention)
Embase	(‘orthodontic retainer’/exp OR ‘orthodontic retainer’) AND (‘patient compliance’/exp OR ‘patient compliance’ OR ‘retention’/exp OR retention)