Vacuum-formed versus fixed orthodontic retainers

What is the risk of orthodontic relapse?

As discussed by Johnston et al in a 2015 British Dental Journal article, the risk of orthodontic relapse due to the age-related changes in the skeletal and soft tissues surrounding the teeth means relapse is unpredictable but likely.³ One study has shown that between 40% and 90% of patients have experienced unacceptable dental alignment 10 years post-treatment.⁴ Similarly, Little et al found in their study that only 10% of those assessed maintained mandibular incisor alignment 20 years post-retention.⁵ As a result of the unpredictable nature and the difficulties in differentiating which patient would and which would not experience relapse, we must implement universal retention protocols. To try to differentiate between these patient groups would be a futile task. Therefore it is of the utmost importance that as a part of any consent process the patient is aware of the life-long commitment to retention, and how this responsibility remains with the patient following the completion of active treatment.

What is the best practice for retention protocol?

First, one must clarify the goal of orthodontic retention. One review describes this as the phase of orthodontic treatment, which attempts to keep teeth in the corrected positions after orthodontic braces.⁶ There is no clear consensus among orthodontists when surveyed, on what protocol is considered best practice. Neither is there agreement concerning the timings of retention protocols. This same survey did conclude that more clinicians are now opting for a dual bonded and removable retainer approach, but there was a desire for the development of a common retention protocol.⁷ Many retention protocols are based around the knowledge that an average minimum of 232 days is what is needed for the supporting fibres around a tooth to remodel to the new tooth position.⁸ Studies have also demonstrated that wearing a vacuum-formed retainer (VFR) nightly or full time in the initial 6–12 months appears to be as effective.^9,10 This result allows for comparison between studies that use these different VFR protocols, knowing that they are both as effective, but there are many other variations on these protocols that have not yet been compared for effectiveness, which emphasizes the importance of a standardized approach to the use of VFRs.

Orthodontic retention treatment options

Below are some of the most common passive retention options. There are active appliances that once the correction has been achieved, they also act as a retainer, but for this clinical scenario and literature review, these are not considered. Examples of such active turn retentive appliances are functional appliances, Barrer appliances and headgear.

The Hawley appliance: this is the most traditional of the retentive appliances. It is durable and can be manufactured for both the maxillary and mandibular arch. It allows for modifications that aid in maintaining certain corrections. One significant drawback of these appliances is the lower patient acceptance compared to an ‘Essix’ or a bonded retainer. A Begg retainer is one such example of a modified Hawley appliance, with the removal of the Adams clasp component.

The vacuum-formed retainer (VFR): there are many branded and non-branded versions of these retainers, with some claiming greater fit and improved durability over others. Some of the more well-known brand names are Essix or Vivera retainers (by Invisalign). Patients are accepting of this type of retainer due to its comfort and great aesthetics, more so than the traditional Hawley retainer.¹¹ VFRs have also been shown to provide improved lower labial segment retention in the initial 6 months when compared to the Hawley retainer, making them popular with clinicians and patients alike.¹² They also benefit from having a low cost of replacement and no effect on a person's speech.

The bonded retainer (BR): this type of retention is often used in conjunction with a vacuum-formed retainer in case of a breakage of one. The advantage of these is they do not rely on patient compliance. They can be constructed either chairside or by a lab, depending on clinician preference, as neither has been shown to be more successful.¹³ They can be constructed from multi-strand wire such as ‘twistflex’ or braided wires, such as ‘Orthoflex wire’ (Reliance Orthodontic Products, Itasca, IL, USA). More recently these have been placed using a direct fibre reinforced composite method, although this has been linked with a more detrimental periodontal response when compared to the wire alternatives.¹⁴ The main disadvantage with these is that some patients may experience difficulty with maintaining excellent oral hygiene around the retainer and close monitoring is required. When using these alone, the clinician relies on the interdigitation achieved posteriorly to maintain the position of the posterior teeth. Patients receiving these types of retainers should be educated on the use of oral hygiene adjuncts, for example Superfloss (Oral-B, Cincinnati, OH, USA), TePe brushes or EasyPicks (TePe, Malmö, Sweden).

Research question

For the scenario outlined in Box 1, a literature search based on the following question was undertaken: with regard to bonded and vacuum-formed retainers, does one provide greater stability in the labial segments post-orthodontic treatment than the other?

Clinical scenario

A patient in their mid twenties presents complaining of crowding, particularly identifying her lower incisors. After thorough assessment and planning, she undergoes clear aligner treatment to align these teeth. Once the patient is finished active treatment she is presented with the options of a vacuum-formed retainer versus a bonded retainer. The patient asks which of these options offers predictability concerning stability over time, she has not expressed a preference for either a removable or a fixed appliance.

Summary and discussion

The five papers included for analysis consisted of two RCTs (one trial had two publications), and two review articles. All of the studies had small sample sizes, although each stated that the sample size was sufficient to allow the primary outcome to be statistically significant. The two RCTs^15,17 were prospective hospital-based RCTs. Neither was blinded owing to the nature of the interventions. Both studies' primary outcome was to measure stability using the Little's irregularity index. Secondary outcomes varied in each study and included periodontal health, intercanine width, intermolar width, arch length or extraction site opening, and patient satisfaction.

Two recent Cochrane reviews were also included^6,18 because they both provided well-researched in-depth views of the topic at their dates of publication. The first of these reviews focused on interventions for managing relapse specifically in the lower front teeth, and at the date of publication, concluded that no RCT met their inclusion criteria.¹⁸ This pointed to a lack of evidence and encouraged further research in this area. The second of the two reviews that was included was broader in its scope and looked at the evidence in relation to stabilizing of tooth position post orthodontic treatment.⁶ This looked into various comparisons, although the present article focused on the findings concerning bonded versus fixed retainers. One of the three studies they identified supported the view that VFRs provided slightly poorer stability in the lower arch than BRs using Little's irregularity index of 0.6 mm; however, it was noted that the evidence was low quality with a high risk of bias.

O'Rourke et al were the first to carry out a prospective long-term RCT comparing bonded and vacuum-formed retainers.¹⁵ As in the other papers reviewed, O'Rourke's team used Little's irregularity index to assess the degree of relapse that had occurred. One concern with using this index as an indication for the degree of relapse is that it does not take into consideration the relapse that may have occurred elsewhere in the dentition. It also disregards spacing, vertical displacements or mutual rotations where the contacts remain intact. The index is also accumulative, which could mean that a score of 1.9 mm may be evenly distributed and clinically acceptable to the patient, or in one location, for example between the central incisors, and could cause the patient great concern. Despite these concerns, the author of the present article accepts that using this easily applied and quick index is of benefit because it allows studies to be easily compared.

Al-Moghrabi et al¹⁶ published a follow-up paper to O'Rourke et al's original study and assessed the 4-year results. This paper is arguably the most clinically relevant of those reviewed because orthodontic relapse is often slow, and the true benefits of having a bonded retainer, which does not rely on patient compliance, may not be observed in the shorter assessment timeframes of other studies. The prescribed VFR protocol for this trial was 6 months full time, 6 months nightly, alternative nights for 12 to 18 months, and 1 to 2 nights weekly after this. This protocol differs from that of Littlewood et al who prescribed nightly wear throughout.⁶ These inconsistencies between trials leave room for debate around whether the results would have differed with an alternative routine. Al-Moghrabi et al concluded that BRs appeared to offer greater stability at 4 years when compared to VFRs, with 1.6 mm less irregularity developing with the BR group. However, both groups demonstrated a deterioration in the alignment after the debond.

The trial carried out by Forde et al¹⁷ had a shorter follow-up period than that of Al-Moghrabi et al,¹⁶ but offered interesting results for comparison. Forde et al's intervention of choice was an Essix VFR, but they decided on a three-strand twistflex stainless steel wire for the BR. Their primary measurement outcome was also Little's irregularity index. They started with slightly fewer participants, 60, but this was sufficient for the study, which required 22 participants in each of the two groups to achieve the desired clinical relevance. This study did differ from the previously discussed trial in that it also assessed the maxillary arch. It concluded that there was no difference between the two test groups in post-treatment changes in the maxillary arch, but in the mandibular arch the BRs reduced post-treatment changes. One anomaly found in this study when compared to the previous study was the high failure rate of BRs (50%). The authors suggested that this may have been due to the relative inexperience of the two treating clinicians who were still in their orthodontic training: placement of BRs is technique sensitive. This failure rate may have had an influence on the results of the trial, and depending on the delay in time before they were repaired may have shown the BRs to be even better at retaining the teeth. This high failure rate is at the higher end of the published rates which range from 7.8% to 53%.^19,21 This study also failed to record the patients' compliance with the VFR, and the obvious concern here is did the relapse occur because of an inefficient retainer or a lack of compliance? To answer this question, a future study would be required where this was assessed as a secondary outcome. A strength of this study and one that improves its clinical relevance is that it was a multicentre study. There were two regional hospitals and one teaching hospital with two treating clinicians. None of the studies was carried out in a general dental setting or by an orthodontist based in a practice setting, which would be of interest for the clinical scenario (Table 1).

All of the studies have some degree of bias. O'Rourke et al minimized this by obscuring the lingual region with wax so that the assessor could not see what intervention was carried out. This was not possible in that of Forde et al because a digital model method was used for assessment, thus introducing a greater degree of bias here.

Two of the included papers discussed the same trial, although at different times. One discussed the initial findings at the 6-month follow-up,¹⁵ while the second reviewed these patients at 4 years.¹⁶ The increased length of follow-up increase these studies' clinical relevance. Due to the nature of orthodontic relapse, a 6-month follow up offers only a very low level of evidence for the efficacy of a retentive protocol. As we know, the length of time for periodontal fibres to remold is 232 days. The findings from the first paper were that BRs provide better stability within the first 6 months, but that some mild relapse is to be expected regardless of the retention protocol.

Conclusion

It is difficult to quantify the clinical relevance for each of the reviewed papers due to the variation and inconsistencies pointed out above, but one can conclude that the current evidence indicates that in mandibular teeth, the placement of a bonded retainer appears to offer greater stability over a VFR. There is insufficient evidence when considering the maxillary arch, as only one of the included studies compared these and found no clinically significant difference. The caveat here is that we cannot be sure that the VFRs were worn as prescribed, and in each case, the prescription differed. This begs the question that if a VFR was worn as per prescription would there be any difference? It is likely that in a real-life situation the certainty of placing a fixed retainer, and so not relying on compliance, would offer greater stability in the long term. This is a question that needs further research over a longer recall interval to be answered. It should also be mentioned that with technological developments, there are new types of vacuum-formed retainers, some reporting increased rigidity. A trial to compare the types of clear plastic retainers available as a precursor to trials of the VFR versus bonded retainer would be worthwhile to ensure that the best of both systems is compared. Similarly, there needs to be research and guidance on what type of bonded retainer, whether it be braided, twistflex or fibre, offers the best retention with the minimum negative effects on periodontal health. Once this guidance exists, then a true comparison between removable and fixed retainers can be carried out.

When taking the available evidence in to consideration, it could be advised that, at present, a mandibular bonded retainer should be placed for retention of the labial segment. The placement of a maxillary fixed retainer over a VFR cannot be recommended based on current evidence, but placement could be considered if compliance with a VFR would be of concern. In addition to a fixed retainer, many clinicians prefer to provide dual arch VFRs. As the evidence shows the risk of relapse to be relatively high, the provision of this additional retentive tool seems pragmatic to help maintain patient satisfaction. No single vacuum-formed retainer regimen can be recommended over another as a comparison of the evidence regarding these was not in the scope of this review.