This blog explores Clear Plastic Retainers (VFRs), the most common choice of orthodontic retention, favoured by clinicians for their ease of fabrication and by patients for their comfort and aesthetics. However, as much as we might wish these appliances are foolproof, recent long-term clinical research show the more nuanced reality.
In this Part-1 of retention series, our focus on Clear Plastic Retainers, we look at the reality of VFRs, how well and for how long they are effective retainers, we explain why their effectiveness reduces over time. We will explore the impact of material changes, the reality of patient adherence , and specifically where and why failures and fractures occur.
Part 2, where we will look bonded retainers, followed by Part 3, focusing on failures of bonded retainers.
Clear plastic retainers are made from three main materials, Polypropylene, Polyethylene co-polymers (PET-G) and Polyurethane (PTU). The three materials offer similar properties on paper, in terms of wear resistance, modulus of elasticity and mechanical properties, but it is worth noting there are distinct differences when the three materials go head-to-head, as summarised in the table below:
The below section explores the differences of the materials, but do the differences matter, or is it just something to excite Farooq’s nerd lobe? Well, yes the properties do matter, Moreso the degradation of the material properties with clinical use, which help explain the later section on long-term changes with VFRs. There are two key changes:
Water absorption: Polypropylene (Essix C+) has the highest levels of water absorption of 12.5% at 6 months when compared to PET-G and PTU, 5-7.5%. Polypropylene was also the only material which underwent noticeable structural chemical changes (plasticization) through stretching of the carbonyl group, which was not noticed with either PET-G or PTU over a 6-month period
Wear: Of all the variables, wear appears to be an overriding factor in terms of long-term changes to the retainer material, from the excellent paper by Lina Alfadil 2024, she showed after 2,000 cycles of wear (roughly equivalent to 6 months of wear) all three materials, they had worn by either 30%-60% (0.3 mm-0.6 mm) of a 1mm retainer
Wear AND water absorption: both acting synergistically to further change the material properties. 15% absorbed over an estimated 6 months was observed with PTU (Zendura) with wear, compared to 7.5% absorbed of the same material in the absence of wear (a 200% difference), indicating wear of the retainer can induce greater changes to the material’s properties.
Manufacturing: One of the unfortunate truths from the research does muddy of the water in terms of understanding material properties, as the material itself does not seem to be the only determining factor, the somewhat unknown and commercially sensitive manufacturing process of the material playing a significant factor. Take PTU brands Zendura and Vivera for example, they show significantly different wear, stiffness and water absorption rates, whilst being the same material.
Conclusion: All materials undergo permanent deformation, Polypropylene > Polyethylene > Polyurethane
Wear of the retainer is the biggest influencing factor of material deformation
Manufacturing of the material significantly changes material properties
Although trimlines for retainers have been classically straight cut with gingival extension (2-4 mm), however an alternative design of a scalloped gingival margin has been more recently been available, such as Align Technology’s Vivera retainers. The theory of extended trimlines is that they offer greater stiffness and therefore increase retention, however a recent randomised control trial from Edoardo Staderini in Italy showed no difference in relapse between both retainer design types (P<0.05) over a 6-month period. A side note from Edoardo’s study regarding the degradation of the retainer with time, at 3-months slight expansion had occurred (0.25-0.3 mm), followed by slight constriction at 6-months (0.02-0.4 mm), indicating the conflicting factors of creep and plasticization.
Conclusion: Straight cut (gingival extension 2-4 mm) and gingivally bevelled VFRs are equally effective
The story of VFR effectiveness is one told best through time, showing the combined effect of the two main factors influencing VFR retention: material changes and adherence.
Slight increases in Little’s Irregularity Index by approximately 0.52 mm, most of the changes occur during the first six months of retention (Krämer, 2020). Occlusal changes are the greatest in this period, with a reduction in intercanine width (-0.46 mm), Intermolar width (-0.41 mm), Arch-length (-0.42 mm) (O’Rourke 2016). Although there are changes in this period, they are not considered clinically significant. It is worth considering the patients perception of significance, with research suggesting patients notice posttreatment changes if Little’s irregularity Index is beyond 1 mm (Karsli 2023).
A further increase in irregularity occurs during this period of approximately 1.03 mm (borderline noticeable for patients) (Kalaydzhieva 2023). Occlusal changes are more stable during this period, with small changes in the reduction of intercanine width (-0.26 mm), intermolar width (-0.25 mm), arch-length (-0.19 mm) (O’Rourke 2016).
At 4 years irregularity continues to increase to approximately 3.16 mm, noticeable clinically and to patients (Al-Moghrabi 2018). Contrasting irregularity changes and occlusal changes are the most stable during this period, (Moghrabi 2018).
Conclusion: Incisor Irregularity increases each year, and noticeable at year 4
Occlusal changes occur mainly within the first 6 months, and reduce each year
Non-adherence: Year one 19% > Year two 52% > Year four 67%
One of the debates in orthodontic retention is about the optimal protocol for removable retainers. Randomised clinical trials have compared full-time wear protocols with part-time use (8-12 hours), the results demonstrate no clinical or statistical difference (P>0.05) (Thickett & Power, 2010; Krämer, 2020).
How much do patient’s actually wear their VFRs? Well a systematic review found patients reported wear of 5.8 hours per day (95% CI, 4.98, 6.64), significantly lower than the prescribed 8-12 hours or full time Al-Moghrabi 2017.
Conclusion: Effectiveness is the same for part-time & full time VFR wear
Patient’s adherence of VFRs is 50% of the prescribed time from their clinician
Hope is not lost, patient adherence can be increased through different methods of education and reminders. A simple yet effective method is showing patients and parents images of the consequences of poor retainer adherence , increasing adherence by 3 hours per day (P>0.05) when compared to verbal and written instructions only in a randomised control trial over 3 months (Lin, 2015). Interestingly the same study showed when only the patient was shown images (i.e. the parent was not shown), adherence was 1.2 hours less than the group where the patient and parent were shown the images.
Educational videos and weekly reminder messages have been shown to improve retainer adherence by 2.8 hours at 6 months compared with verbal instructions alone (p = 0.033) (Wafaie, 2023).
Conclusion: Images of relapse to the patient and parent increase adherence by 3 hours
Videos and weekly messages to patients increase adherence by 2.8 hours
Fracture of the retainer are the most common cause of VFR failure (43% Jin 2018). Fractures occur most commonly at the canine – premolar region and midline ( Bilińska (2025). The canine – premolar region is thought to be due to the geometric change in the arch width between canine and premolars creating a ‘step’, causing a stress point in the material (Williams 2011, Bilińska (2025). Midline fractures occur due to it being at the greatest curvature of the VFR, as the retainer undergoes elastic deformation labially / palatally to flex over the undercuts, tension occurs across the material, with the greatest tension at this area of maximum curvature. Improper removal, with ‘one sided’ elevation is also thought to increase midline fracture rates (Meade 2015). The thickness of the material does change the fracture rates, with thicker 1.5 mm VFRs having 8% less fractures at 6 months Moslemzanzadeh 2018.
Occlusal wear of VFRs occurs commonly with failures due to wear contributing to 28% of VFR failures Dermott 2008.
As annoying as loss of retainers can be, they seem to occur relatively infrequently at 7-15%, and usually within the first 6 months (Forde 2018, Hichens 2007). This may offer the clinician an opportune time to revisit the retention regime, and consider non-compliant retention with a fixed bonded retainer.
Conclusion: fractures are the most common cause of VFR failure
Patients can reduce midline fractures by avoiding one sided removal
Failure: Fracture 43% > Occlusal wear 28% > Loss of VFR 7-15%
If there is one major takeaway from the data, it is that VFRs are not lifelong. They are highly effective for a period, but they are fundamentally subject to material fatigue, and human nature.
VFRs are, and will remain the most common retainer, however they require regular inspection and replacement to achieve retention, as well as regular motivation for our patients.
Join us for Part 2, where we leave the plastic behind and dive into the design, success rates, and headaches (Farooq’s words) of Bonded Retainers.
Contents: Kerolos AlHakeem 
Contents: James Andrews
Edited/contents: Farooq Ahmed
A special thanks to Lina Alfadil for editing the blog, and contributing to the retention literature with her research: Assessment of wear characteristics, longevity and stiffness of Essix-type retainers
We hope you have enjoyed this blog, This blog started with a conversation between Kerolos AlHakeem, Farooq Ahmed and James Andrews, to follow more conversations, clinical tips and commentaries on orthodontics please check us out on Instagram / facebook
