From Volume 42, Issue 1, January 2015 | Pages 24-34
The need for long-term retention following orthodontic treatment is now considered essential to prevent post-orthodontic treatment relapse. The vacuum-formed retainer (VFR) has gained popularity in recent times. This paper describes briefly the characteristics of VFRs and summarizes the evidence in relation to their effectiveness.
Retention is the phase of orthodontic treatment that attempts to maintain teeth in their corrected positions after active tooth movement.1 It aims, therefore, to prevent relapse which is defined as the return, following correction, of the features of the original malocclusion.2 Although, rather erroneously, patients may consider relapse to include age changes unrelated to orthodontic treatment,1,3 this is not what is regarded here for the purposes of retention.
The rationale for maintaining the teeth in their treated position is to:
The principal method of retention involves the use of an orthodontic retainer. The choice of retainer depends on many factors, such as:
Research has shown that relapse occurs in patients with and without orthodontic retainers.9,10 There appears to be agreement that lifetime retention is necessary if relapse is to be minimal.7,11 Wide variation exists, however, in the retention regimes prescribed by orthodontists.7,11,12
Angle described the first retainer type, based on banded fixed appliances.13 Removable retainers, such as the Hawley retainer14 constructed from acrylic and stainless steel wire, were subsequently developed.3 With the advent of the acid-etch technique in dentistry, bonded fixed retainers (BRs) became another method used to minimize relapse.15 Most recently, the vacuum-formed retainer (VFR) has been introduced16 (Figures 1–3).
The aims of this article are to:
In the literature, the terms ‘drawn down retainer’,3 ‘vacuum retainer’,11 ‘vacuum-formed retainer,6,17,18,19,20,21,22,23,24,25 ‘Essix retainer’,16,26,27,28,29,30,31,32,33,34 ‘thermoplastic retainer’,12,35,36 ‘(clear) overlay retainer’,37,38,39 ‘invisible retainer’40,41 and ‘(clear) slipover’42 appear to be interchangeable. For the purposes of this article, the term ‘VFR’ will be used.
The VFR is a removable thermoplastic retainer and was introduced by Ponitz40 in 1971 and further developed by Sheridan.16 Although minor tooth movement can be achieved by modifying VFR design,27 this article will focus on its use as a retainer. The VFR has increased in popularity and is the most commonly chosen retainer by orthodontists in the Republic of Ireland,7 Australia41 and in the National Health Service (NHS) and hospital practices in the United Kingdom.11
A VFR may be manufactured on two types of plastic thermoforming machine:
Examples of material types used in its manufacture include:
Rectangular and round blanks are available (Figure 6). ‘Blank thicknesses’ ranging from 0.63 to 2.0 mm have been described.17,21,22,37,38 Processing of the blanks reportedly reduces the thickness by half,16 although a recent study suggested that thickness reduction may not be uniform.35Figure 7 outlines the main stages in manufacture of a VFR in the Biostar™ machine.
Two main retainer designs have been described. The original ‘Essix’ retainer was canine-to-canine coverage only16,22,28,35 and is still popular in the United States, whereas the VFR tends to describe full occlusal coverage6,17,21,30 or coverage extending to halfway across the most terminal molar37,43 (Figures 8–10). The marginal extension in each design is commonly 1–2 mm buccal and 3–4 mm lingual to the gingival margins.16 Additional designs have been described.38,43,44
A VFR must be a precision fit. It clicks into place and is not easily removed.16 The appliance is retained in the mouth by engagement in the undercuts gingival to the contact points of the teeth. The fit may be adjusted at chairside by increasing or reducing the depth of the undercuts in the plastic. Blanching will be evident if the retainer is engaging too tightly on the gingiva. Adjustment with scissors and/or a greenstone in a slow speed handpiece may be required.
Patients are advised to refrain from using toothpaste to clean a VFR as this will ‘dull’ the appearance. Regular cleaning with a soapy cotton-tip swab and/or a proprietary cleaning agent (eg Retainer BriteTM*) has been recommended.16
Proposed advantages of the VFR include:
Proposed disadvantages include:
Several studies have compared the effectiveness of VFRs with other retainer types.6,17,18,19,20,21,22,23,25,26,28,31,33,34,37,38,39,47 Prospective randomized controlled trials (RCTs) are considered the gold standard for study design18,48 and will provide the focus in this article's assessment of the evidence available in relation to the effectiveness of this retainer type.
When assessing stability after orthodontic treatment, most retention studies have focused on changes in lower incisor alignment, as this is a highly noticeable site of relapse and often complained of by patients.18 Little's Irregularity Index (LII)49 is most commonly used to assess incisor irregularity; it is usually measured directly from plaster models but may also be applied to digital models (Figure 13). Additional measurements assessed in retention studies include intercanine width, intermolar width, overjet, overbite, arch length, the PAR Index50 score and the American Board of Orthodontics discrepancy index and cast and radiograph evaluation.51 Measurements at the time of fixed appliance removal are compared with those taken after a period of retainer wear. Patients who require orthognathic surgery and/or have craniofacial anomalies, intellectual impairment, hypodontia, single arch treatment or require a specific retainer type because of periodontal consideration or arch expansion, tend to be among the exclusion criteria in retention studies.6,17,22,30,35,36
Table 1 shows the mean changes in LII recorded in recent RCTs comparing the effectiveness of VFRs with other retainer types. In one study,31 all patients were also fitted with BRs. Outcomes were measured between 6 and 24 months after active orthodontic treatment.
| RCT | Intervention | Sample Characteristics | VFR Characteristics and Wear Protocol | Mean Change in LII (mm) | ||
|---|---|---|---|---|---|---|
| Rowland et al17 | VFR vs Hawley | N: 397 Mean age: 14.7–15 (SD 1.7–1.4) years | Full occlusal coverage – ‘all occlusal surfaces’ 1.5 mm thickness F/T followed by P/T | VFR | Hawley | |
| Maxilla | 0.26 | 0.51 | ||||
| Mandible* | 0.46 | 1.02 | ||||
| McDermott et al21 | VFR vs BR | N: 85 Mean age: Unreported | Full occlusal coverage 1.0 mm thickness F/T | VFR | BR | |
| Mandible* | 0.61 | 0.03 | ||||
| Kumar & Bansal31 | VFR vs Begg | N: 224 Mean age: Unreported | Full occlusal coverage – ‘occlusal surfaces up to and including the most distal molars’ 1.5 mm thickness F/T followed by P/T | VFR | Begg | |
| Mandible* | 0.12 | 0.37 | ||||
| Tynelius et al22 | VFR vs pre-fabricated positioner | N: 75 Mean age: 14.4 (SD 1.5) years | Canine-to-canine coverage 2.0 mm thickness F/T followed by P/T | VFR | pre-fabricated positioner | |
| Maxilla | 0.5–0.8 | 1.1 | ||||
KEY:
: statistically significant;
BR: bonded retainer; F/T: full-time; N: number of subjects; P/T: part-time; VFR: vacuum-formed retainer; vs: versus.
All subjects had completed a course of upper and lower fixed appliance treatment with one study17 including patients who had received upper removable appliance and/or functional appliance treatment prior to fixed appliances, with or without extractions. Two studies detailed the ‘pre-orthodontic treatment’ clinical characteristics of their subjects. One22 included patients who had ‘normal skeletal and dento-alveolar sagittal, vertical and transverse relationships’ and required extraction of four premolar teeth as part of their orthodontic treatment. Another17 included patients from all incisor2 and skeletal classifications, with or without missing incisor or premolar teeth.
The prescribed VFR wear protocol differed in all studies. Two adopted a full-time wear protocol; one21 required removal for eating while the other31 did not. Another study17 required full-time wear (except eating and toothbrushing) for the first week after provision of the retainer and 12-hour daily wear thereafter. Patients in a Swedish study22 were instructed to wear their VFRs 22–24 hours per day for 2 days and nights, followed by ‘during the night’ for 12 months, and then alternate nights for the next 12 months.
Significant differences were noted between the retainers in LII, but not in any of the additional measurements assessed. Some relapse occurred in all measurements. The evidence suggests that VFRs were slightly more effective than Hawley retainers in maintaining incisor alignment in the mandible but less effective than BRs. No significant differences appear to exist between the VFR, Hawley retainer and pre-fabricated positioner in maintaining incisor alignment in the maxilla.
Three recent studies have investigated whether the effectiveness of VFRs differs with regard to the amount of daily wear of the retainer.30,35,36Table 2 outlines the median or mean changes in LII recorded at 6–12 months into the retention phase in patients adopting full-time or part-time wear of VFRs. All subjects had completed a course of upper and lower fixed appliance treatment, with a Swedish study35 also including patients who had single arch fixed appliance treatment. All subjects were treated with extraction of four premolars in another study.36 Gill et al30 included subjects who were treated with or without extractions. That study also provided a breakdown of subjects by ‘molar classification’. ‘Pre-orthodontic treatment’ clinical characteristics or the number of subjects who were treated with extractions were not specified in the Swedish study.35
| RCT | Intervention | Sample Characteristics | VFR Characteristics | Median Change in LII (mm) | ||
|---|---|---|---|---|---|---|
| Gill et al30 | F/T vs P/T VFR wear | N: 60 Mean age: 13.4–13.7 (SD 1.4–2.5) years | Full coverage – ‘full-arch coverage’ 1.0 mm thickness | F/T | P/T | |
| Maxilla+** | 0.14 | 0.17 | ||||
| Mandible+** | 0.29 | 0.12 | ||||
| Thickett & Power36 | F/T vs P/T VFR wear | N: 62 Mean age: 13.6–13.8 (SD 1.5) years | Coverage and thickness: unspecifed | F/T | P/T | |
| Maxilla** | 0.63 | 0.73 | ||||
| Mandible** | 0.56 | 0.60 | ||||
| Jäderberg et al35 | F/T vs P/T VFR wear | N: 69 Mean age: 15.7–15.9 (SD 1.9–2.3) years | Full coverage – ‘all teeth’ (maxilla) |
F/T | P/T | |
| Maxilla** | 0.71 | 0.51 | ||||
| Mandible** | 0.35 | 0.35 | ||||
KEY:
: mean change;
: not statistically significant;
F/T: full-time; P/T: part-time; VFR: vacuum-formed retainer; vs: versus.
Prescribed full-time protocols included:
Part-time wear protocols included:
There were no clinically significant differences between the changes in LII or in the additional parameters measured. Part-time wear of a VFR appears to be sufficient in maintaining post-orthodontic treatment stability. Greater pre-treatment crowding than that recorded in these studies, however, may warrant a different VFR protocol.30,36
It is essential that any prescribed retainer is not responsible for adverse health effects.52 Data from one study20 indicated that a VFR may be less harmful to dental and periodontal tissues than a BR. After 12 months in retention, no caries or periodontal pocketing was found related to the lower six anterior teeth in patients wearing either retainer. There was, however, significantly more gingival bleeding in those wearing a BR.
There has been an increasing emphasis on assessment of patient satisfaction in healthcare provision; this is usually conducted via self-reported questionnaires.18 In a study comparing full- and part-time VFR wear, patients reported difficulty with speech (22%), soreness (13%), bad taste (10%) and difficulty with cleaning (10%). Most (97%), however, found their VFRs ‘easy to get used to’.35
Patients also found BRs significantly more ‘acceptable to wear’ than VFRs,19 but VFRs were significantly better than Hawley retainers in terms of speech and appearance (92.8% vs 82.6%).18 In addition, VFRs were significantly more comfortable and aesthetic than Begg retainers but eating/chewing was easier with Begg retainers.31 Retainer wear during eating was instructed for both types of removable retainer in that study which is contrary to the wear protocol for VFRs that is currently recommended.16,17,21
The survival rate of retainers is an important consideration as it may have an impact on cost, patient satisfaction and effectiveness. In the six months following orthodontic treatment, significantly fewer breakages were reported with VFRs (6.6%) compared to Hawley retainers (19.4%), although loss rates were similar.18 At 12 months, however, Sun et al found no significant difference in survival times between VFRs and Hawley retainers.38 In that study, patients were advised to wear their retainers ‘day and night’. VFR thickness was 0.75 mm and fracture (17%) followed by loss (15%) were the most common reasons for VFR ‘breakage’. Mandibular VFR fracture was most common in the midline.38
In the first 12 months post-orthodontic treatment, more problems have been reported with VFRs compared with BRs. Fractures (28%), wear (28%) and loss (14%) were the most common problems with VFRs and composite loss (5%) and de-bond (12%) were most commonly reported with BRs.19
As a trend for advising life-time wear of retainers now exists,7,11,12 the cost implications of prescribing a particular retainer is an important factor in retainer choice. In the UK, VFRs were found to be more cost-effective than Hawley retainers in the six months after orthodontic treatment.18 This was not only from the perspective of the patient (mean difference in cost per patient: €4.71) but particularly the NHS (mean difference in cost per patient to the NHS: €31.34) and the orthodontic practice (mean difference in cost per patient to the practice: €32.60).
A Swedish study,23 however, showed that a pre-fabricated positioner was more cost-effective than either an upper VFR and lower BR combined or upper VFR and lower stripping of the incisors and cuspids combined in the two years after orthodontic treatment. The ‘societal costs’ per patient for scheduled appointments/unscheduled appointments were €420/€0 for a pre-fabricated positioner, €497/€807 for an upper VFR and lower BR combined, and €451/€303 for an upper VFR and lower stripping of the incisors and cuspids combined.
Not surprisingly, clinicians have found VFRs significantly easier to fit than BRs. One study19 found that 97% of VFRs took less than 10 minutes to fit compared with ‘up to 20 minutes’ for 96% of BRs.
A limitation with removable appliances is patient compliance.35 If the retainer is not worn as instructed, relapse is inevitable. Patient-reported compliance with instructed VFR wear was found in 35–95% of patients at 6–12 months after orthodontic treatment;18,21,35 one study21 reported that stability of mandibular incisor alignment was proportional to patient reported wear.
In this article, we have described the characteristics of VFRs. In addition, we have provided a summary of the evidence in relation to the effectiveness of VFRs. Although only clinical trials with the best quality research design (RCTs) were considered, these included a variety of VFR designs and retention protocols. The studies also differed in clinical setting, the number, age, ‘pre-orthodontic treatment’ clinical characteristics (such as presenting malocclusion) of subjects, orthodontic treatment carried out and time-points of outcome measurements. In addition, the value at which changes of LII was considered to be statistically significant varied from 0.2 to 2.0 mm. This lack of standardization should be taken into account when interpreting the outcomes assessed in this overview. Also, not all studies provided data on the age of subjects but most appear to have concentrated on adolescents. Conclusions from these studies, therefore, may not be relevant to older patients.
There are, however, some suggestions from these trials that can offer evidence-based guidance to the clinician with regard to retainer choice and wear protocol:
Retainer design, however, must be tailored for the individual patient. The patient's pre-treatment situation and end of orthodontic treatment result should be important factors in retainer choice. Further investigation and evaluation of VFRs is required in those malocclusions which require special consideration. Patients presenting with periodontally compromised or severely rotated and/or displaced teeth prior to orthodontic treatment may not be suitable for a VFR. Without modification, a VFR may not be sufficiently rigid to maintain an expanded maxillary arch.45 In addition, the relatively high fracture and wear rates reported in the trials may suggest that further investigation is required to develop a more robust VFR material. The concern, however, that a VFR may be associated with an anterior open bite does not appear to be borne out in the studies reviewed here.17,30,35,36
As there appears to be agreement among orthodontists that life-time wear of a retainer is necessary to minimize relapse,7,11,12,41 and all RCTs in this review evaluated retainer effectiveness 6–24 months into retention, future research is also required to assess the following over the long-term:
Based on the available evidence, the VFR appears to be a popular orthodontic retainer because of its clinical and cost-effectiveness and patient and clinician acceptability. VFR design characteristics, dependence on patient compliance and other patient-related factors, such as presentation of severely rotated and/or displaced teeth prior to orthodontic treatment, however, may limit the VFR as the optimal retainer choice for all orthodontic patients. Further research is necessary to determine whether the advantages of VFRs are maintained over the long-term.
