Vacuum-formed Retainer Versus Bonded Retainer to Prevent Relapse After Orthodontic Treatment - A Randomized Controlled Trial About Stability and Patients' Perceptions
Phase: NA
Conditions: Orthodontics, Retention
Sponsor: Region Gävleborg

Research program 
 
Vacuum-formed retainer versus bonded cuspid-to-cuspid retainer to 
prevent relapse after orthodontic treatment– A randomized controlled trial 
about stability and patients’ perceptions 
 
NCT03070444 
 
PhD student Anke Krämer, Senior consultant, Department of orthodontics, Gävle 
Supervisor: Mats Hallman, Associate professor, PhD, Department of oral and maxillofacial 
surgery, Umeå University 
Assistant supervisor: Ingalill Feldmann, PhD, Senior consultant, Department of orthodontics, 
Gävle 
Assistant supervisor: Mats Sjöström, PhD, Associate professor, Department of oral and 
maxillofacial surgery, Umeå University 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

2009-07-13 
 
Aim 
The overall objective of this randomized trial is to evaluate and compare stability after 
orthodontic treatment with an Essix retainer and a bonded cuspid-to-cuspid retainer (CTC) 
respectively in the mandibular arch and longitudinally follow these patients over time up to 5 
years. The patients’ perceptions of the two methods are also evaluated with questionnaires 
during the follow-up period. 
A further aim of this study is to analyze the relationship between diagnosis, treatment outcome, 
treatment time, age at start of treatment and stability with an Essix-retainer in the maxilla.   
 
Introduction 
For successful orthodontic treatment the teeth need to be maintained in their corrected position 
after removal of orthodontic appliances. According to Proffit (2012) orthodontic treatment 
results are potentially unstable and therefore retention is necessary for three major reasons: the 
gingival and periodontal tissues require time for reorganization, growth changes can cause 
relapse and pressure from soft tissues i.e tongue, lips and cheeks can cause the teeth to move or 
rotate in an unwanted direction. 
 
To prevent relapse patients are provided with fixed or removable retainers. The advantage of 
fixed retainers is that they are bonded to the palatal or lingual surfaces and do not depend on 
the patient’s compliance. On the other hand these retainers demand good oral hygiene 
procedures to avoid plaque and calculus accumulation and the bonding procedure is time 
consuming and technique sensitive (Dahl and Zachrisson, 1991).  
Hawley retainers are the most commonly used removable appliance since it was introduced in 
1919, almost a century ago (Atack et al, 2007). However, patients have difficulties with speech, 
are embarrassed over the esthetics and cooperate less the first six months in retention (the 
critical phase) with the Hawley retainer (Hichens et al, 2007) (Bennett and Tulloch, 1999).   
In the early 1990s thermoplastic vacuum-formed retainers were introduced and became very 
popular (Sheridan et al, 1993). Essix retainers are aesthetically well accepted, comfortable and 
less expensive than bonded retainers or Hawley retainers.  Lindauer and Shoff (1998) and 
Rowland et al (2007) showed that vacuum-formed retainers are more effective than Hawley 
devices to maintain teeth in their new position after 6 months of retention.  
In the literature the wear regimen of Essix retainers is not standardized. Lindauer and Shoff 
recommended 3 months full time wear and thereafter at night while Rowland et al 
recommended only one week full time wear. Jäderberg et al (2012), however, found no 
significant difference between full time wear for 3 months and 1 week, respectively, followed 
by night time. Their conclusions agree with the finding of Thickett and Power (2010) that there 
is no difference in the full time vs. part time wear regimen. 
Transection of fibrous tissue (supra-alveolar fibers) to prevent relapse has been described 
already in the 1960s. Reitan (1967) observed that orthodontically derotated teeth are more 

unstable and due to supra-alveolar tissue contraction rotational relapse may occur even after a 
retention period of more than 230 days. A long-term prospective study by Edwards (1988) 
showed that circumferential supracrestal fiberotomy (CSF) could alleviate dental relapse, 
especially of rotated teeth. Side effects such as clinically significant increase of periodontal 
sulcus depth or decrease in the labially attached gingiva were not observed. 
 
There are several studies published investigating the effectiveness of bonded retainers and 
removable appliances to maintain tooth position after orthodontic treatment. Nevertheless, a 
systematic review by Littlewood et al (2006) stated that there is no evidence concerning the 
most appropriate retention strategy. They concluded that further research with well-designed 
randomized controlled trials to compare the effectiveness of different retention appliances is 
needed.  
Only a few studies have analyzed patients’ perception of orthodontic treatment (Bennett et al 
1999, Hichens 2007, Feldmann et al 2012). Very few studies have analyzed patients’ perception 
of retention appliances. Kumar and Bansal (2011) compared in a prospective study the 
effectiveness and acceptability of Essix and Begg retainers. They found that both appliances 
allowed some relapse but the differences were small and not clinically significant. The patients 
liked the comfort and appearance of the Essix retainer and preferred the Begg retainer for 
chewing and biting. 
Although retention is a decisive factor for successful orthodontic treatment there are no 
evidence-based recommendations in the literature for retention appliances considering 
diagnoses before treatment such as vertical and skeletal deviations, the type of orthodontic 
movement (e.g. expansion or extraction cases) or for patients treated with fixed appliances and 
orthognatic surgery. Especially for patients with orthognatic surgery the choice of retention 
method and prevention of relapse are crucial since both skeletal and dental changes occur during 
treatment.  
Materials and methods  
 
The study is conducted at the Orthodontic Clinic, Gävle, Sweden and consists of 104 patients. 
All patients are treated with fixed appliances in the upper and lower jaw (standard .022 straight 
wire), both with and without extractions.  
Patients treated with Rapid Maxillary Expansion (RME), orthognathic surgery, segmented 
appliance and patients with syndromes or agenesis of a mandibular incisor were excluded from 
the trial. 
All patients who meet the inclusion criteria are invited to enter the trial. One experienced 
orthodontist (AK) provides the patients and parents with both oral and written information 
about the study protocol. After written consent is obtained from both the patient and parent the 
patients are randomized in blocks and stratified by gender into two groups. The allocation 
sequence is computer generated by a statistician at Gävleborg Count Hospital and concealed 
in opaque envelopes until randomization. 
The patients are randomized into two groups with 52 patients in each group. All patients have 
Essix retainers in the maxilla covering all erupted teeth. The extension of the Essix retainers in 
the mandible covers only the dentition from first premolar to first premolar. The CTC-retainer 
is bonded to lingual surfaces of the mandibular canines. 

Patients with rotated lower incisors (more than 30 degree) are treated with circumferential 
supracrestal fiberotomy 2 weeks prior to retention. 
 
Group A: 
Retention with a CTC in the mandible and Essix retainer in the maxilla. 
Two weeks before debonding an impression of the mandibular jaw is taken in 
order to manufacture the CTC retainer. The CTC-retainer is bonded to the canines 
directly after debonding. The Essix retainer is manufactured on dental casts after 
debonding and handed out to the patients the same day. The patients are instructed 
to wear the Essix retainer full-time the first week and thereafter night only. 
Group B: 
Retention with Essix retainers in the maxilla and mandible. 
The Essix retainers are manufactured on dental casts after debonding and handed 
out to the patients the same day. The patients are instructed to wear the Essix 
retainers full-time the first week and thereafter night only. 
 
 
Dental cast are obtained at the debond appointment (T1) and at the follow-up controls after six 
months (T2), 18 months (T3) and 5 years (T4) for both groups. All measurements on casts are 
made with an electronic digital calliper to the nearest 0.25 mm. Comparisons within and 
between groups are performed. 
Stability after retention is also analyzed depending on diagnoses before treatment and treatment 
outcome. 
 
 
 
 
Outcome measures on the dental casts are as follow: 
 Little’s irregularity index 
 Intercanine width  
 Intermolar width 
 Arch length 
 Overjet 
 Overbite 
 
To assess measurement precision and reliability, 30 randomly selected dental casts are 
measured by the same examiner at a five weeks interval. 
 
At the visit two weeks in retention and at the follow-up visits after 6, 18 and 60 months the 
patients assessed questionnaires in order to evaluate their experience of retention with an Essix 
retainer and a CTC respectively. The questionnaires include self-report questions from a 
previous study where reliability and face validity were found to be acceptable (Feldmann et al 
2007). In addition, a few questions modified for this study are included. The fit of the retainers 
and the CTC are checked at each visit. All questionnaires are evaluated and compared between 
the two groups 

Statistical analysis 
 
Descriptive statistics are calculated for both groups (median, interquartile ranges, and test for 
normal distribution). The post-treatment changes are described by comparing measurements at 
debond (T1), after 6 months (T2), after 18 months (T3), and after 5 years (T4). Generalized 
estimating equations (GEE) are used to estimate the effect of the two treatment groups over 
time for the outcome variables. In the GEE analysis, all digital measurements, extraction 
therapy, sex, retention group, and time and the interaction of time with retention group are 
tested as explanatory variables (predictors). The GEE correlation structure is set to 
autoregressive, a convenient option for models with time-dependent observations.  
Mann-Whitney U tests are used to analyse and compare the answers on questionnaires for the 
VFR/VFR and VFR/CTC group. Spearman’s correlation coefficient is calculated to determine 
the relationship between patients’ perceptions, self-reported VFR compliance, and LII. 
Demographic data are analysed with cross tabulations. P-values less than 0.05 are considered 
statistically significant. 
 
 
Ethical considerations 
 
All patients and parents are given oral and written information and sign a written consent before 
being included in the trial. The participation is voluntary and patients can leave the trial at any 
time without giving reasons. 
Both of the tested retention-appliances are common and used frequently in orthodontic 
practices. The impressions at the follow-up visits are additional for this study. 
The Ethics Committee of Uppsala (Regionala etikprövningsnämnden) approved the study 
protocol (diarienummer: 2009/177). 
 
 
Research projects: 
 
1. Effectiveness of Essix retainer versus bonded CTC in the mandibular arch after 6 
and 18 months. 
Aim: The aim of this project is to evaluate and compare stability in the mandibular arch 
6 and 18 months after retention with an Essix retainer compared to retention with a 
bonded cuspid-to-cuspid retainer (CTC).  
Material and methods: 104 patient are randomized into 2 groups; CTC (group A) and 
Essix retainer (group B). Little´s Irregularity Index, intercanine width, intermolar width, 
arch length, overjet and overbite are measured on dental casts and analysed at the 
debond appointment, after 6 months and after 18 months. 
The null hypothesis is that there is no difference in retention capacity between Essix 
retainer and cuspid-to-cuspid retainer. 
The results from this study will be presented at the Swedish Dental Congress in 2018. 
 

2. Evaluation of patients’ perception of the retention appliances by questionnaires 
after 6 months and 18 months. 
Aim: The aim of this project is to evaluate and compare patients’ perception of their 
retention with an Essix retainer compared to retention with a bonded cuspid-to-cuspid 
retainer (CTC) 6 and 18 months after debonding. 
Material and methods: 104 patients assessed a questionnaire after debonding and at the 
follow-up visit after six and 18 months to evaluate their experience of retention with an 
Essix retainer and a CTC respectively.  
The null hypothesis is that there is no difference in patients’ perception between Essix 
retainer and cuspid-to-cuspid retainer. 
The results from this study will be presented at the Swedish Dental Congress in 2019. 
 
3. Long-term stability in the mandibular arch after 5 years of retention. 
Aim: The aim of this project is to evaluate and compare stability in the mandibular arch 
5 years after retention with an Essix retainer compared to retention with a bonded 
cuspid-to-cuspid retainer (CTC).  
Material and methods: 104 patients are randomized into 2 groups; CTC (group A) and 
Essix retainer (group B). Little´s Irregularity Index, intercanine width, intermolar width, 
arch length, overjet and overbite are measured on dental casts and analysed after 5 years 
of retention. 
The null hypothesis is that there is no difference in retention capacity between Essix 
retainer and cuspid-to-cuspid retainer.  
The results from this study will be presented at the Swedish Dental Congress and the 
European Orthodontic Society Congress in 2021. 
 
4. Retention stability in the maxilla depending on the diagnoses before treatment and 
on the treatment outcome after fixed orthodontic appliances. 
Aim: The aim of this project is to evaluate and compare stability in the maxilla 6 months, 
18 months and 5 years after retention with an Essix retainer. 
Material and methods: Dentals casts of 104 patients are measured and analyzed 
concerning overbite, overjet, intercanine width, intermolar width and arch length after 
6 months, 18 months and 5 years of retention.  
The null hypothesis is that there is no difference in retention capacity for Essix retainer 
in the maxilla concerning diagnosis before treatment, treatment outcomes, treatment 
time and age at start of treatment. 
 
Relevance 
 
CTC and Essix retainer are frequently used after orthodontic treatment. Since this is a 
randomized trial with a large sample size we hope to contribute to increase the knowledge and 

also to clarify if diagnoses before treatment and/or treatment outcome, treatment time and age 
are factors that must be considered when retention is planned.  
Furthermore, there is very little knowledge about patient’s perception of different retention 
strategies which of course is an important factor to consider.  
 
 
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