Clinical Oral Investigations (2024) 28:185
`https://doi.org/10.1007/s00784-024-05503-x
`
`RESEARCH
`
`Assessment of wear characteristics, longevity and stiffness
`of Essix‑type retainers
`
`Lina Alfadil1 · Mangala Patel2 · Nikolaos Pandis3 · Padhraig S. Fleming4
`
`Received: 29 July 2023 / Accepted: 8 January 2024 / Published online: 2 March 2024
`© The Author(s) 2024
`
`Abstract
`Objective To compare four commercially available Essix-type retainers in terms of longevity, wear characteristics, stiffness
`and their range of rigidity.
`Materials and methods An in vitro study was conducted at Queen Mary University of London. Four groups of thermoplastic
`materials were included: Duran (PETG), Essix C + (Polypropylene), Vivera and Zendura (Polyurethane). A working typodont
`was fabricated to evaluate surface wear characteristics using a wear machine with a customized jig. Retainers were measured
`for tensile test, and water absorption was measured at five different time points up to 6 months after initial immersion in two
`different physical states and two different solutions. Hydrolytic degradation was also evaluated using FTIR spectroscopy.
`Results Essix C + was the most flexible retainer with Vivera the stiffest material. Zendura and Essix C + had the most surface
`wear (413 μm ± 80 and 652 μm ± 12, respectively) with absorption rates of up to 15 wt% in artificial saliva occurring with
`Zendura. Only Essix C + displayed signs of degradation following water absorption.
`Conclusions All materials had characteristic levels of flexibility and were susceptible to water absorption. Duran 1.5 mm
`performed similarly to Vivera in relation to stiffness and wear properties. While Zendura and Vivera have similar chemical
`structures, they exhibited differences concerning wear resistance and water absorption. Further clinical research evaluating
`the clinical relevance of these laboratory findings is required.
`Clinical relevance Characteristic patterns of wear and rigidity of four commercially available Essix-type retainers were
`observed. This information should help in the tailoring of retainer material on a case-by-case basis considering treatment-
`related factors and patient characteristics including parafunctional habits.
`
`Keywords VFRs · Retainers · Essix · Orthodontic · Retention · Thermoplastic
`
`Introduction
`
`The use of removable retention following orthodontic treat-
`ment is commonplace in order to mitigate against relapse
`related to treatment allied to maturational changes. Essix-
`type retainers are clear thermoplastic removable retainers
`
` * Lina Alfadil
`
`dr.linaalfadil@gmail.com
`1 Queen Mary University of London, London E1 4NS, UK
`2 Centre Lead for Oral Bioengineering, Queen Mary
`University of London, Mile End Road, London E1 4NS, UK
`3 Universität Bern, Bern, Switzerland
`4 Chair/Professor of Orthodontics, Division of Public
`and Child Dental at Trinity College Dublin, Dublin Dental
`University Hospital, Lincoln Place, Dublin 2 D02 F859,
`Ireland
`
`first introduced in 1971 [1]. They were refined and popular-
`ized 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 wide-
`spread adoption relates primarily to acceptable aesthetics,
`low cost and ease of fabrication.
`Essix-type retainers are made from thermoplastic poly-
`mers 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 poly-
`mer chains relax, separate and become mobile, making
`the material highly viscoelastic, which permits moulding
`into the shape required. As the material cools below that
`
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`Clinical Oral Investigations (2024) 28:185
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`temperature threshold, hardening occurs. During the fab-
`rication 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 com-
`monplace contributing to a lifespan of as little as 6 months
`based on one prospective study [9]. Thermoplastic materi-
`als 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 prop-
`erties, level of crystallinity, molecular weight, glass transi-
`tion 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 Haw-
`ley-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 stiff-
`ness is yet to be investigated.
`
`Aim and hypothesis
`
`To compare in vitro four commercially available Essix-type
`retainers in terms of longevity, wear characteristics and
`stiffness. Our null hypothesis dictates no difference exists
`
`between the types of materials with respect to longevity
`based on susceptibility to wear and degradation.
`
`Materials and methods
`
`Study design
`
`A controlled laboratory-based investigation was undertaken
`within the Dental Physical Sciences Unit, on the Mile End
`Campus at Queen Mary University of London.
`
`Sample selection
`
`Four different materials were compared: Essix C + (Raintree
`Essix, Inc., LA, USA), Vivera (Align Technology Inc., CA,
`USA), Zendura (BayMaterials LLC, Fremont, CA, USA)
`and Duran (SCHEU-Dental GmbH, Iserlohn, Germany) in
`two different thicknesses (1 mm and 1.5 mm). Vivera and
`Zendura are both polyurethane materials (PU), while Duran
`is a polyethylene co-polymer (PETG), and Essix C + is com-
`posed of polypropylene (Table 1). Five retainers were used
`in each group with a total of 25 retainers tested in this study.
`
`Retainer fabrication
`
`An intra-oral scanner (7 Series, Straumann Group, Switzer-
`land) was used to scan a typodont model (aligned U-shaped
`arch form) creating a 3D printed model to aid with the fab-
`rication of three of the Essix-type retainers. To fabricate the
`Vivera retainers, an iTero intra-oral scanner (Align Technol-
`ogy Inc., CA, USA) was used. By following manufacturer
`guidelines, the Essix-type retainers were pressure-formed
`on the 3D printed models using a universal pressure-therm-
`oforming unit (Dreve-Drufomat- TE/-SQ, Dreve-Dentamid,
`Germany). The Vivera retainers were fabricated separately
`by Align Technology.
`
`Table 1 Thermoplastic
`materials and dimensions used
`in the study
`
`Product
`Essix C +
`Vivera
`Zendura
`Duran 1
`
`Dimensions (thickness)
`125 mm × 125 mm × 1 mm
`125 mm × 125 mm × 1 mm
`125 mm × 125 mm × 1 mm
`125 mm × 125 mm × 1 mm
`
`Manufacturer
`Dentsply Raintree Essix, LA, USA
`Align Technology Inc., CA, USA
`Bay Materials LLC, Fremont, CA, USA
`SCHEU-Dental GmbH, Iserlohn, Germany
`
`Duran 1.5
`
`125 mm × 125 mm × 1.5 mm SCHEU-Dental GmbH, Iserlohn, Germany
`
`Composition
`Polypropylene
`Polyurethane
`Polyurethane
`Polyethylene
`terephtha-
`late glycol
`(PETG)
`Polyethylene
`terephtha-
`late glycol
`(PETG)
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`Mechanical testing procedures
`
`Wear test
`
`The retainers were cut into 25 samples (30 mm × 20 mm)
`using a digital calibrator targeting the second premolar-first
`molar region standardized on each sample, to fit into the
`steel plates housed in the wear testing machine. The retain-
`ers were cut into 25 samples (30 mm × 20 mm from each
`retainer sample), to fit into the steel plates housed in the
`wear testing machine. The cut samples were then flattened
`by oven heating at a temperature below the Tg of the mate-
`rials (80 °C for 30 s) before being pressed for 10 s under
`a load of 2 kg. The post-thermoforming thickness of the
`Essix-type retainers may vary depending on the tooth sur-
`face (i.e. with greater thickness on the occlusal surfaces of
`the molars and canine region versus the labial surfaces of
`the teeth). Allowance was made for this with the average
`thickness for each sample recorded. The pre-cut specimens
`were placed on rectangular steel plates attached to the base
`of the wear testing machine (Boston Gear, Braintree, MA).
`A custom-made attachment was fabricated and attached to
`the extending metal rods of the wear machine with a load of
`470 g, which consisted of 10 mm steatite balls embedded in
`light-cured acrylic.
`A full cycle was represented by the movement of the
`attachments in a horizontal motion by 40 mm to the left
`followed by 40 mm to the right and ending in the starting
`position. Two thousand cycles were performed per speci-
`men, requiring approx. 14 h in total. Between testing of each
`specimen, the machine and samples were cleaned with dis-
`tilled water and air.
`A three-dimensional, non-contact optic profilometry
`scanner (Proscan 2000; Scantron, Taunton, UK) with a
`resolution of 0.01 to 4 μm was used. Samples were scanned
`in an unworn state initially in order to account for initial
`surface irregularities. Scans were repeated after the wear
`
`Fig. 1 Sample distribution for
`hydrolysis test
`
`Page 3 of 10 185
`
`process to permit assessment of the wear characteristics of
`the materials. An average of two readings was taken using
`the same reference points for all samples with surface wear
`measured in micrometres (μm). Each scan required a mini-
`mum of 45 min.
`
`Water uptake and hydrolytic degradation
`
`For the hydrolytic uptake and degradation test, the worn
`samples were cut into even halves, producing 50 samples
`(15 mm × 10 mm) with uniform thickness (with the excep-
`tion of Duran 1.5 mm). The thermoformed-only group
`also involved a digital calibrator to ensure similar loca-
`tion and dimension to those in the thermoformed and
`worn group. Thereafter, the samples were divided into two
`main groups—a control group, and an experimental group
`(Fig. 1). The control group consisted of retainers that had
`been thermoformed only, while the experimental group con-
`sisted of worn retainers evaluated after being subject to wear
`cycling. Each of the two groups was further divided as fol-
`lows: Group 1 was immersed in 37 °C de-ionized water (pH
`level of 7.4), and Group 2 was immersed in artificial saliva
`at 37 °C. Proprietary artificial saliva was used (A.S. Saliva
`Orthana, CCMed, UK). Both groups were immersed and
`evaluated for water uptake at five intervals (T0: Baseline;
`T1: 12 h; T2: 24 h; T3: 720 h, i.e. 1 month; T4: 2160 h, i.e.
`3 months; T5: 4320 h, i.e. 6 months).
`Percentage water uptake was calculated using the
`equation:
`
`o × 100
`o ∕w
`(wt%) = w
`i − w
`
`wi and wo are the weight of the specimen before and
`after uptake, respectively. For each reading, the specimen
`was blotted with filter paper to absorb water from the sur-
`face and then weighed using an electronic balance at room
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`Clinical Oral Investigations (2024) 28:185
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`temperature (21 ± 1 °C). Reading accuracy was 0.0001 g,
`and variation in specimen weight was less than 0.1%.
`For the degradation progress, Fourier transform infrared
`spectroscopy (FTIR) was used (PerkinElmer Frontier IR/
`FIR, PerkinElmer, UK) pre-testing (in the thermoformed
`state) and following cycling (T5) to assess the composition
`of the materials, degradation and changes in their chemical
`composition. Two samples were scanned twice to ensure
`homogeneity of the results with samples then dried for 1
`week in a drying oven at 37 °C ± 1 °C, then re-scanned to
`confirm the results.
`
`Tensile test
`
`Thermoformed only retainer samples were cut into a dog-
`bone shape (70 mm × 7mm × 14 mm from each retainer sam-
`ple, measured using a digital calibrator and cut with a surgi-
`cal blade). The tensile strength test was performed using a
`universal mechanical testing instrument (Instron Co., Nor-
`wood, MA, USA) with a load cell of 3 kN at 37 °C. The dis-
`tance between points was defined as 10 mm, and the cross-
`head speed was 0.2 mm/s in order to obtain stress–strain
`curves. Young’s elastic modulus (MPa) and tensile yield
`stress (MPa) were calculated from the obtained stress–strain
`curves.
`
`Statistical analysis
`
`Descriptive analysis is presented for all experimental groups
`as mean values and standard deviations. To examine the
`effect of material on the yield and the Young’s module of
`elasticity linear regression was used and Scheffe’s method
`was applied for post hoc pairwise comparisons. For the effect
`of brand and time both on wear and water uptake adjusted
`for wear and solution a generalized estimating equation
`(GEE) model was used with robust standard errors. Linear
`
`regression analysis was used to assess the effect of brand on
`yield and Young’s modulus of elasticity. All analyses were
`conducted using Stata 17 (Stata Corp, TX, USA) and the R
`Software version 4.0.3 (R Foundation for Statistical Com-
`puting, Vienna, Austria). A P value of < 0.05 was considered
`statistically significant.
`
`Results
`
`Degree of surface wear on the materials
`
`Twenty-five thermoformed samples were scanned prior to
`and following wear cycling. Essix C + and Zendura exhib-
`ited the highest surface wear, averaging 413 μm ± 80 and
`652 μm ± 12, respectively. Similar levels of wear were
`observed with Duran 1 mm and 1.5 mm (P = 0.9; Fig. 2).
`Vivera underwent less wear than Duran 1 (324 μm ± 71),
`while no significant difference was observed between Duran
`1 mm, Duran 1.5 mm and Vivera in terms of wear rates
`(Table 2, Fig. 2). The results from the GEE model are shown
`in Table 3 with the Wald test for the main effects confirm-
`ing that retainer material (P < 0.001) and time (P < 0.001)
`were significant wear predictors. A graphical display of the
`predicted effects is shown in Fig. 3.
`
`Water absorption and degradation properties
`of the materials
`
`The amount of water absorbed is presented in Table 4 and
`5 and shown graphically in Fig. 4. All samples experienced
`water uptake and reached a plateau (equilibrium) during the
`experiment at the 3-month time-point (T4).
`Overall, the worn (experimental) group absorbed more
`water compared to the thermoformed (control) group and
`samples immersed in artificial saliva absorbed more water in
`
`Fig. 2 Boxplot of the surface
`irregularity in the unworn state
`and following wear cycling (in
`μm) for each retainer type
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`Table 2 Descriptive data of surface irregularities in the unworn state
`and following wear cycling
`Essix-type
`Mean (μm)
`retainer
`Duran 1
`  Pre
`  Post
`Duran 1.5
`  Pre
`  Post
`Essix C
`  Pre
`  Post
`Vivera
`  Pre
`  Post
`Zendura
`  Pre
`  Post
`
`211.19
`346.81
`
`248.12
`308.47
`
`321.08
`413.88
`
`250.86
`324.66
`
`508.57
`652.89
`
`80.40 205.80
`113.71 391.90
`
`54.34 226.20
`99.95 357.60
`
`52.62 321.60
`79.45 423.35
`
`106.72 215.65
`71.67 332.25
`
`89.63 509.20
`121.06 657.95
`
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`Page 5 of 10 185
`
` ± SD Median (μm; p50)
`
`IQR
`
`69.80
`68.00
`
`51.30
`187.00
`
`68.40
`26.90
`
`115.75
`69.30
`
`152.35
`205.95
`
`comparison to those immersed in de-ionized water (Table 4,
`Fig. 4). However, no significant difference in the absorption
`was noted based on wear cycling (P = 0.26). Essix C + group
`absorbed an average of 6 wt% in de-ionized water and up
`to 15 wt% in artificial saliva, for both the control (thermo-
`formed only) and experimental (thermoformed and worn)
`groups (P < 0.01).
`Zendura in the control group absorbed ~ 3 wt% de-ion-
`ized water, and more than double the amount was absorbed
`in artificial saliva (~ 8 wt%). A similar absorption pattern
`was seen with Zendura in the experimental group, with an
`increase to 13 wt% in de-ionized water and 15 wt% in arti-
`ficial saliva. Vivera and Duran 1 mm performed in a similar
`manner throughout the different groups and solutions, aver-
`aging ~ 8% for maximum absorption. Meanwhile, the Duran
`1.5 mm group had the lowest absorption values in all groups
`and solutions, reaching a peak of 6% in the worn state. The
`results of the adjusted GEE model are shown in Table 5. The
`overall Wald tests after fitting the GEE model showed that
`time and brand were significant predictors for water uptake
`(< 0.001).
`With the exception of Essix C + (polypropylene), there
`was no difference in the FTIR spectra of the samples when
`compared between post-thermoforming, 6 months immer-
`sion, and 1 week of drying. Both Zendura and Vivera
`(polyurethane) displayed similar FTIR spectra confirming
`the polyurethane structure with the characteristic carbonyl
`absorption band of the ester bond located at 1750  cm−1 and
`a shoulder at 1656  cm−1 indicating a stretching vibration
`of carbonyl (C = O) group. The absorbance at 3305  cm–1
`represents the stretching of the NH bond which is typically
`noted in urethane and urea groups. These bonds remained
`consistent throughout all timelines. The spectra for Duran
`(polyethylene terephthalate glycol) showed the characteristic
`bands of C–H stretching at 2906  cm−1 and 2866  cm−1, C = O
`at 1711  cm−1, and two peaks at 1410  cm−1 and 1240  cm−1
`ascribed to –CH2– and C(O)–O stretching of ester groups,
`respectively.
`The FTIR spectrum of Essix C + displayed a shoulder
`at 2910  cm−1, asymmetric and symmetric in-plane C–H
`(–CH3) bond at 1446  cm−1, and the shoulder at 1372  cm−1
`confirms that it is polypropylene. The peak at 1376  cm−1 is
`assigned to the –CH3 group. Additional absorption bands
`were found as broad O–H group stretch at 3300  cm−1 and
`1611  cm−1, which can be attributed to stretching vibration
`of carbonyl (C = O) group that was noted following testing
`and drying (Fig. 5).
`
`Stiffness of the materials
`
`Overall, Essic C + had the lowest Young’s modulus of elas-
`ticity and yield stress when compared to the other groups
`with means of 1007.6 MPa and 16 MPa, respectively. The
`
`Table 3 GEE analysis assessing the effect of material on surface wear
`adjusted for time
`Surface wear
`
`Coefficient P value 95% confi-
`dence interval
`
`Reference
`
`Duran 1.5 mm (base com-
`parison)
`0.70
`Duran 1 mm
`89.19
`Essix C +
`9.46
`Vivera
`302.44
`Zendura
`Pre-wear (base comparison) Reference
`Post-wear
`101.38
`
`0.98
`0.01
`0.84
`0.00
`
`0.00
`
` − 79.76 81.17
`21.24
`157.13
` − 80.56 99.49
`204.92
`399.95
`
`65.58
`
`137.18
`
`Fig. 3 Predictive margins of time with 95% confidence intervals on
`the degree of surface wear (in μm) between the groups
`
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`Table 4 Water absorption (wt%) for the worn retainer group in grammes (experimental) in de-ionized water (H2O) and artificial saliva (AS)
`Material
`12 h
`24 h
`1 month
`3 months
`6 months
`Mean wt%
`Mean wt%
`Mean wt%
`Mean wt%
`Mean wt%
`0.19
`0.20
`0.20
`0.20
`0.20
`0.20
`0.21
`0.21
`0.21
`0.20
`0.23
`0.23
`0.24
`0.24
`0.24
`0.23
`0.24
`0.24
`0.24
`0.24
`0.25
`0.26
`0.26
`0.26
`0.26
`0.25
`0.25
`0.26
`0.25
`0.25
`0.39
`0.39
`0.39
`0.40
`0.39
`0.38
`0.39
`0.39
`0.39
`0.38
`0.26
`0.26
`0.26
`0.26
`0.26
`0.26
`0.26
`0.26
`0.26
`0.26
`
` ± S.D
`0.01
`0.02
`0.01
`0.01
`0.02
`0.02
`0.02
`0.03
`0.01
`0.03
`
` ± S.D
`0.01
`0.01
`0.02
`0.01
`0.02
`0.01
`0.02
`0.03
`0.01
`0.03
`
` ± S.D
`0.01
`0.02
`0.02
`0.01
`0.01
`0.01
`0.02
`0.03
`0.01
`0.03
`
`Essix C + H2O
`AS
`Vivera H2O
`AS
`Duran 1 H2O
`AS
`Duran 1.5 H2O
`AS
`Zendura H2O
`AS
`
` ± S.D
`0.01
`0.01
`0.01
`0.01
`0.02
`0.01
`0.02
`0.04
`0.01
`0.03
`
` ± S.D
`0.01
`0.02
`0.01
`0.01
`0.02
`0.02
`0.02
`0.03
`0.01
`0.03
`
`Table 5 Adjust GEE model for
`water uptake based on retainer
`type and time
`
`Covariate
`Time
`0 h* (base comparison)
`12 h
`24 h
`1 m
`3 m
`6 m
`Brand
`Duran 1* (base comparison)
`Duran 1.5
`Essix C +
`Vivera
`Zendura
`State
`Thermoformed* (base comparison)
`Worn
`Solution
`Water* (base comparison)
`Artificial saliva
`
`Coef
`
`95% conf. interval
`
`P value
`
`Reference
`0.010
`0.014
`0.017
`0.018
`0.017
`
`Reference
`0.114
` − 0.061
` − 0.011
`0.022
`
`Reference
` − 0.002
`
`Reference
` − 0.001
`
`(0.009 to 0.011)
`(0.012 to 0.015)
`(0.015 to 0.019)
`(0.017 to 0.020)
`(0.015 to 0.019)
`
`(0.103 to 0.125)
`(− 0.065 to − 0.056)
`(− 0.014 to − 0.008)
`(0.020 to 0.024)
`
`(− 0.006 to 0.002)
`
`(− 0.003 to 0.001)
`
` < 0.001
` < 0.001
` < 0.001
` < 0.001
` < 0.001
`
` < 0.001
` < 0.001
` < 0.001
` < 0.001
`
`0.265
`
`0.394
`
`Vivera group had the highest stiffness (2058 MPa and yield
`stress of 26 MPa), followed by Duran 1.5 (1713 MPa and
`22 MPa). Zendura and Duran 1 mm had very similar out-
`comes with 1342 MPa for both groups and 18 to 16 MPa,
`respectively. The variation between the Vivera and Duran
`1.5 mm in comparison to Essix C + group was found to be
`statistically significant (P < 0.01; Table 6).
`
`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 associ-
`ated 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 follow-
`ing 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
`
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`Fig. 4 Graph demonstrating
`percentage water absorption in
`different groups
`
`Page 7 of 10 185
`
`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 representa-
`tive 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.
`
`Fig. 5 FTIR scan of Essix C + group at different time points
`
`Table 6 Linear regression analysis comparing Young’s modulus of
`elasticity between the five groups (with Essix C + as a comparator)
`Young’s modulus
`Coefficient P value 95% confidence
`interval
`
`Essix C + (base compari-
`son)
`Duran 1
`Duran 1.5
`Vivera
`Zendura
`
`Reference
`
`334.6
`705.8
`1044.6
`333.9
`
`0.09
` < 0.01
` < 0.01
`0.09
`
` − 64.20 733.34
`307.04
`1104.58
`645.84
`1443.38
` − 64.88 732.66
`
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`Clinical Oral Investigations (2024) 28:185
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`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 mate-
`rial 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, vis-
`ual 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] com-
`pared Zendura and Duran in thermoformed states and fol-
`lowing 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 dimen-
`sion (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 out-
`comes 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 high-
`est 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 propyl-
`ene 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 (dry-
`ing 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
` (CH2 = CHCH3). 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 car-
`bonyl (C = O) group were noted, post-testing and post-dry-
`ing. 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 struc-
`ture 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 sur-
`face changes pertaining to removable orthodontic retention.
`
`Limitations
`
`The in vitro design used in this study cannot fully account
`for intra-oral variables such as variation in saliva composi-
`tion. Furthermore, significant variability in terms of intra-
`oral conditions is known to occur clinically with Gibbs et
`al. [23] reporting that the average occlusal force of posterior
`
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`Page 9 of 10 185
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`teeth upon closure can be elevated in those with parafunc-
`tional activity. The maximal force utilized did not exceed
`4.6N (470 g) being similar to force levels used in previous
`research [10]. Higher loads (245 N/25 kg and 600 N/61.2 kg)
`with similar numbers of wear cycles (2000 and 10,000) were
`used in other studies with no signs of perforation or occur-
`rence of tear points [11, 12]. There is a need for further stud-
`ies investigating the effects of localized excessive surface
`wear on Essix-type retainers. It would be useful to include
`variable force levels and patterns within these experiments.
`
`Conclusion
`
`• Essix C + (PP) was found to be the least stiff material
`with Vivera (PU) having the highest level of stiffness.
`• All materials were susceptible to water absorption; how-
`ever, the chemical structures were stable in all groups
`with the exception of Essix C + (PP).
`• Duran 1.5 mm (PETG) performed similarly to Vivera
`(PU) in terms of stiffness and wear properties.
`• While Zendura and Vivera have similar chemical struc-
`ture (PU), they performed differently in terms of wear
`resistance and water absorption.
`• Further clinical research is required to validate the pre-
`sent findings and to understand the effectiveness and lon-
`gevity of a range of Essix-type retainer materials.
`
`Author contribution All authors contributed to the study's concep-
`tion and design. Material preparation, data collection was performed
`by Lina Alfadil and Padhraig S Fleming. Laboratory design was per-
`formed by Mangala Patel, and data analysis was performed by Nikolaos
`Pandis. The first draft of the manuscript was written by Lina Alfadil
`and all authors commented on previous versions of the manuscript. All
`authors read and approved the final manuscript.
`
`Declarations
`
`Competing interests The authors declare no competing interests.
`
`Ethics approval and consent to participate As this is an in vitro study,
`ethical approval is not required.
`
`Open Access This article is licensed under a Creative Commons Attri-
`bution 4