Introduction
The use of removable retention following orthodontic treatment is commonplace in order to mitigate against relapse related to treatment allied to maturational changes. Essix-type retainers are clear thermoplastic removable retainers first introduced in 1971 [
1]. They were refined and popularized by Sheridan in 1993 [
2] and are increasingly popular among orthodontists being the removable retainer of choice in the USA, UK, Ireland and Australia [
3‐
7]. Their widespread adoption relates primarily to acceptable aesthetics, low cost and ease of fabrication.
Essix-type retainers are made from thermoplastic polymers that can be divided into two types: amorphous and semi-crystalline. Polypropylene (PP) is the most common semi-crystalline material used for Essix-type retainers. Amorphous polymers include polyethylene co-polymer (PETG), and more recently polyurethane polymer (PU). When these materials are tested under high temperatures exceeding their glass-transition temperature (Tg), the polymer chains relax, separate and become mobile, making the material highly viscoelastic, which permits moulding into the shape required. As the material cools below that temperature threshold, hardening occurs. During the fabrication process, the retainers are formed through either a vacuumed or pressured heating cycle using blanks varying in thickness from 0.4 to 2mm.
The longevity of Essix-type retainers is known to be limited with a reported failure rate of 10% over a 2-year period [
8] and minor fractures as well as loss also commonplace contributing to a lifespan of as little as 6 months based on one prospective study [
9]. Thermoplastic materials are exposed to temperature variation in the intra-oral environment. This makes them susceptible to hydrolytic degradation, a process that affects polymers that are more water-absorbent in high-temperature states. The process of degradation is influenced by hydrophobic/hydrophilic properties, level of crystallinity, molecular weight, glass transition temperature (
Tg) and manufacturing procedure. Hence, different types of Essix-type retainer materials demonstrate characteristic mechanical properties and are vary in their propensity to degradation, wear and fracture. In view of the relative flexibility of Essix-type materials, alternatives including the use of metal-reinforced Essix-type retainers and substitution of Essix-type retainers for more rigid Hawley-type retainers have been advocated in order to maintain significant transverse change, particularly following active transverse expansion [
10].
Previous studies have compared water absorption, wear resistance and post-fabrication morphology associated with Essix-type retainers. However, the mechanical properties of novel amorphous and semi-crystalline retainers are unclear. Moreover, the effect of varying retainer thickness on stiffness is yet to be investigated.
Discussion
The VFR brands were selected to represent the three most popular chemical compositions (PP, PETG and PU), while also including PU variants given their novelty and the relative lack of associated research. A conventional design was tested in this study which did not include the palatal coverage in order to offer the most representative retainer design. The findings expose significant differences associated with VFR materials in terms of key physical properties. This knowledge can be utilized to tailor retention regimes where higher stiffness might be required; for example, to resist the tendency to maxillary arch constriction following active expansion during treatment. Similarly, designs less susceptible to wear could be considered in patients with parafunctional habits.
Previous studies used different methods and customized jigs to produce surface wear on thermoplastic materials. Raja et al
. [
11] investigated the wear resistance of PP and PETG retainers after thermoforming onto a template block and used metal rods with steatite balls attached to a wear machine with a load of 460 g for 1000 cycles. Gardner et al
. [
12] tested PP and PETG in a thermoformed state using a stone block (76 × 50 × 38 mm) followed by use of a two-body wear machine with steatite balls under a load of 25 kg for 1000 cycles of wear. Bratu et al. [
13] used a custom jig, and upper and lower stone study models with retainers in place fixated on a metal plate with screws under a load of 61.2 kg for 10,000 cycles. The steatite ball possesses a hardness similar to tooth enamel (Mohs scale, 7.5) and is therefore more likely to induce a representative amount of wear on the thermoplastic material better simulating the intra-oral environment and related cycling. Furthermore, the use of block models for fabrication means that the thickness of the samples is uniform, while retainers tend to vary in thickness intra-orally; hence, breakages and perforation are seen in specific areas in the retainers with long-term wear [
9,
14]. As such, a bespoke attachment with 10 mm steatite balls attached to the wear machine was used in the present study.
The samples used were thermoformed onto a 3D printed model based on our typodont model and then flattened to maintain the thickness variation of the materials in the molar-premolar region with the same sample dimensions.
In the hydrolysis test, previous studies have only tested thermoformed samples in distilled water over 2 weeks [
15‐
17]. However, the water uptake between thermoformed and worn retainer samples had not previously been assessed. Therefore, in this study, a comparison was performed between thermoformed (control) and worn (experimental) groups in two media (de-ionized water and artificial saliva) with more prolonged follow-up incorporating five different time points up to 6 months.
Wear is considered the removal of material from a solid surface when undergoing mechanical interaction; however, clinical wear is a more complex process being influenced by normal function, parafunction and the effects of intra-oral cycling including temperature and pH change [
18]. No material was resistant to wear with both Duran groups having less wear in comparison to Essix C + with a mean of 367 μm for the 1-mm group. These findings mirror those of Raja et al. [
11] who found Duran to be 3.7 times more resistant to wear when compared to Essix C + . Moreover, Bratu et al
. [
13] found Duran to have 549-μm median wear in the lower arch. More significant wear levels in the latter study may relate to the increase in the load used (61.2 kg) and a higher number of cycles (10,000). PETG thermoplastic material was also proven to be more durable in terms of wear loss than PP [
12], which agrees with our findings. This suggests that this retainer type may have lower levels of longevity and may be best avoided in those with known parafunctional activity.
Zendura had the highest amount of wear in the present study (652 μm). This was somewhat surprising as Vivera, which is a polyurethane material, similar to Zendura, had very minimal wear ~ (324 μm) comparatively. However, regardless of the amount of wear of these materials, visual inspection did not reveal obvious perforation of the tested samples. To date, there appear to be no studies that have compared the wear resistance of polyurethane Essix-type retainer materials. The present findings highlight the need to examine the material properties of these relatively novel retainer variants in more detail and in the clinical environment.
Young’s modulus and yield stress were lowest in Essix C + (PP) followed by Zendura (PU) and Duran 1 (PETG), then Duran 1.5 (PETG), and finally Vivera (PU) which had the highest values for both properties. These findings reflect those reported in the literature [
16,
19] with PU having the highest stiffness followed by PETG and then PP with the least stiffness. Furthermore, Tamburrino et al
. [
20] compared Zendura and Duran in thermoformed states and following 7 days of immersion in artificial saliva finding only a 37-MPa difference between the two groups. Although the observed results are higher than those recorded in the present study, this may relate to the differences in sample dimension and the thickness of the blank sheets. The results for tensile strength with Duran were higher than those observed by Ahn et al
. [
21], which may relate to their use of a thinner dimension (0.8 mm vs. 1.5 mm and 1 mm in our study).
In terms of the hydrolytic absorption and degradation, uptake differed from one material to the next as Essix-type retainer materials have varying levels of crystallinity, which ultimately may have an effect on their molecular structure. Essix C + had the highest absorption value in the control group (thermoformed) with 15% in artificial saliva after 6 months, while Zendura had the highest absorption value of 15 wt% in the experimental group, followed closely by Essix C + after 6 months of water absorption. These outcomes differ from Ryokawa et al. who monitored the water absorption of thermoformed samples at different timelines up to 2 weeks of immersion. PU samples absorbed the highest amount at 1.5wt% followed by PETG with 0.85 wt% and PP at 0.12wt%. Similarly, Inoue et al
. observed that PP had the least amount of absorption [
15,
16]. Although propylene is a hydrophobic polymer, variations in the immersion time (2 weeks vs. 6 months) and medium (water vs. artificial saliva) as well as differences in the drying processes (drying with cloth vs. filter paper) may have a bearing on the observed findings.
Based on the FTIR spectra, the degree of absorption seen with Essix C + suggests some alteration to the molecular compounds compared to the initial scan. Polypropylene is a linear hydrocarbon polymer with a chemical structure (CH
2 = CHCH
3). In the FTIR, an additional absorption broad O–H group stretch at 3300 cm
−1 and absorption at 1611 cm
−1 assigned to the stretching vibration of the carbonyl (C = O) group were noted, post-testing and post-drying. These were not detected in other materials including Zendura (PU). Our findings were similar to Ahn et al. [
21] who confirmed no changes were seen in the surface structure of the materials following 6 months of wear in vivo for PETG thermoformed retainers. However, they did detect new elements including silicone (Si), phosphorus (P) and calcium (Ca) after EDX spectroscopy which we have not included in this study. The stability of the chemical structure in both PU groups in this study is in keeping with Bradley et al. [
22] who investigated the chemical and mechanical change of PU aligners after 44 ± 15 days of wear. However, further studies are required to investigate the different types of polymers and their degradation rates with long-term surface changes pertaining to removable orthodontic retention.
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