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Journal of the World Federation of Orthodontists

Contents lists available at ScienceDirect Journal of the World Federation of Orthodontists journal homepage:  www.ejwf.org

Research Article

The mechanical testing and performance analysis of

three-dimensionally produced lingual retainers

Sertac Aksakalli a,  Ufuk Ok a, Cagri Temel b,  Demet Sezgin Mansuroglu c,  

Yesim Muge Sahin d,

a Private Practice, Istanbul, Turkey

b Master of Science in Computer Science, Colorado Technical University, Colorado, United States

c Istanbul University, Cerrahpasa Faculty of Engineering, Department of Chemistry, Istanbul, Turkey

d Istanbul Arel University, ArelPOTKAM (Polymer Technology and Composite Application and Research Center), Faculty of Engineering and Architecture,

Biomedical Engineering Department, Istanbul

a r t i c l e i n f o

Article history:

Received 9 November 2022

Revised 26 December 2022

Accepted 26 December 2022

Available online xxx

Keywords:

3-D printing

Adhesion

Biocomposite

Failure

Fiber deformation

Mechanical testing

a b s t r a c t

Background: Introducing three-dimensional (3D) printing has opened new visions in the orthodontic field.

This research evaluates three-dimensionally produced orthodontic retainers and their future possible uses.

For this purpose, in vitro tests were performed for these groups, including bond strength, failure analysis,

discoloration, and biodegradation.

Methods: A total of 30 specimens (n = 30), lower incisor human teeth, were randomly divided into three

groups for a bond strength failure analysis (for each group n = 10). In the experimental groups, lingual

retainers were fabricated using 3D systems (group 1 with 3D dental pen and group 2 with 3D-printed).

In the control group (group 3), the retainer system was a combination of a wire and composite, which

is being used worldwide. A total of 30 specimens (n = 30) from the 3D dental pen and 3D-printed for

discoloration and biodegradation tests were divided into three groups (water, tea, and coffee). Data were

analyzed using the Mann-Whitney U test, ANOVA, and chi-square test.

Results: For all parameters tested, significant differences were determined among groups. The 3D pen

group had the highest score for bond strength, whereas discoloration differed significantly.

Conclusions: According to the limitations of this research, 3D-printed retainers have the potential for clin-

ical use in the near future.

© 2023 World Federation of Orthodontists. Published by Elsevier Inc. All rights reserved.

oped in late 1980s. Customized implants [1] , dental models [2] , and  prosthetic treatments [3] are the widely used 3D applications in

health care. 3D printers can be assumed as simple robotic devices needing computer-aided design software to design and edit prints [4] . There

Funding: The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

Competing interests: Authors have completed and submitted the ICMJE Form for Disclosure of potential conflicts of interest. None declared.

Provenance and peer review: Non-commissioned and externally peer reviewed.

∗ Corresponding author: Department of Orthodontics, Private Clinic, Gokturk,34077, Istanbul, Turkey.

E-mail address: dtufukok@hotmail.com (U. Ok) .

are different 3D printers working with different technologies. Fused deposition modeling is one of them and this printer is also called

as “low-cost home 3D printer. Less complex models can be created easily with this printer by using filaments [5] .

Different 3D printing technologies were investigated in the literature and the current research utilized fused filament fabrication (FFF),

(heated nozzle) to create an object. Once the first layer is added, the building platform is moved into a layer distance to create the

second layer. This process continues through layer by layer addition until the final fabrication of the printed object. The most common

thermoplastic materials that have been used in FFF printing technology are polylactic acid (PLA) and acrylonitrile butadiene styrene.

The first one is known for its excellence in detailing and the second one, for its durability [6] . Recently, 3D pens also have been popular

mostly for hobby or entertainment purposes. 3D pens work like a 2212-4438/$ – see front matter © 2023 World Federation of Orthodontists. Published by Elsevier Inc. All rights reserved.

https://doi.org/10.1016/j.ejwf.2022.12.003

Please cite this article as: S. Aksakalli et al, The mechanical testing and performance analysis of three-dimensionally produced lingual

retainers, Journal of the World Federation of Orthodontists, https://doi.org/10.1016/j.ejwf.2022.12.003

2 S. Aksakalli et al / Journal of the World Federation of Orthodontists xxx (xxxx) xxx

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Table 1

Group specimens

Groups

Group 1 (n = 10) Test materials produced with 3D pen

Group 2 (n = 10) Test materials produced with 3D printer

Group 3 (n = 10) Test materials produced with PentaOne

3D, three-dimensional;

hot glue gun using extruding filaments [7] . PLA is selected usually as filamentsfor 3D pen and printersbecause of their biocompatibility [8] .

A 3D printing pen is a device that uses a PLA filament and softens it to construct solid 3D objects. PLA is a thermoplastic polymer

with a melting range of 170 °C to 210 °C. It is a biodegradable polymer that has recently gained popularity in the field of medicine because of its biocompatiblity [9] . PLA, because of its rigid nature is ideal for the fabrication of the transfer tray for fabrication lingual retainer . It can be fabricated for either an individual tooth or a group of teeth as per the requirement of the operator and anatomical constraints (undercuts, additional cusp tip, etc.) Through this research, a new method of fabricating lingual retainer by modifying the design of the 3D pen was attempted for better anatomical compatibility and less chair time.

By using both 3D-printed and 3D pen-produced retainers in orthodontics, several advantages can be obtained, such as less or no

technician help, time saving, and decreased costs compared with the conventional method. To the best of our knowledge, this is the

first study evaluating the characteristics of these 3D methods.

In this research, the laboratory tests were performed on 3D printed and 3D-pen-produced materials to understand their possible applications as orthodontic retainers. Further this study evaluaed the color changes observed after immersing into various solutions, the biodegradation analysis and morphological characterization of 3D-printed and 3D-pen-produced materials.

tracted. Teeth with cracks, caries, and defects were excluded. The ssoft tissue remnants over the tooth surfaces were removed by

scalar, and the teeth were stored in a 0.1% thymol solution. The solution was changed weekly to avoid bacterial growth [ 10 , 11 ].

For debonding procedure and failure analysis, two teeth were used for each specimen. Pairs of teeth were used to make a contact

area to mimic the intraoral situation in chemically cured acrylic resin, and the teeth roots were embedded in acrylic. The lingual

parts of the teeth were polished with fluoride-free pumice using a brush for 20 seconds and were then rinsed with water and air-

dried. Retainers were applied on the lingual surface for bonding. A total of 30 specimens were randomly divided into three

groups (n = 10). The groups and their explanations are shown in Table 1 . The lingual surfaces of all teeth were phosphoric acid-

etched (37% for 20 seconds per specimen), rinsed for 30 seconds, and then dried for 15 seconds. For all groups, Transbond XT adhe-

sive primer and Transbond LR (3M Unitek, Monrovia, CA) composite were used for the bonding procedure ( Fig. 1 ).

Group 1 was prepared with the 3D pen (3D Pen 2, Myriwell Ltd., Jiangsu, China)

applying PLA filament on lingual surfaces when activated. The same bonding procedures were applied to group 1 ( Fig. 2 ).

Specimens in group 2 were scanned with CEREC Intraoral Scanner (Sirona Dental Systems, NC, United States) and STL file were

sent to the clinic computer connected to the 3D printer (Anycubic i3 Mega, China) ( Fig. 3 ). Lingual retainers were prepared in Tin-

kerCad (Autodesk Inc., CA, United States). Later, the “print” command was sent to 3D printer through the Cura software (Ultimaker

Inc., Utrecht, Netherlands). Anycubic i3 Mega printer gets ready for 3D printing after preparing the calibration and control system. The

print volume is 210 mm × 210 mm × 205 millimeters. The diameter of the filament used is 1.75 millimeters. It uses the PLA material

to print at 190 °C and the heating plate at 60 °C. The print speed is 60 millimeter per second and the layer resolution is 0.05 to 0.3

millimeter. Also, the printing model produces a 100% filling rate. The printed retainers were adhered to the teeth with same bonding procedure ( Fig. 4 ).

In group 3, PentaOne (0.0215-inch round wire) ( Fig. 5 ) was used with the same bonding procedure as the control group to compare

with the 3D pen group (group 1) and 3D printed group (group 2). All materials were used according to the manufacturer’s instructions.

The specimens were tested with a Standard Testing Machine(Esetron Smart Robotechnologies, Mod Dental, Ankara, Turkey). A

hook was mounted in the testing machine. The hook was positioned midpoint of the wire or the 3D-produced material. The

crosshead speed was set at 1 millimeter per minute. The maximum force to debond the retainer was recorded. The tensile bond

strengths were calculated.

The fracture mode was evaluated on the side where the initial bond failure occurred by using an optical stereomicroscope (SZ40,

Olympus, Tokyo, JAPAN) at × 2 magnification. The remnant adhesive on the enamel surface was recorded by one researcher blinded

to the study groups. According to the adhesive remnant index, the fractures were coded and ranked from 0 to 3, based on the amount

of adhesive remaining on the bracket removal [12] . To measure color changes and biodegradation analysis, 30 specimens for each group with a diameter of 18 millimeters and aheight of 2 millimeters were prepared. In the 3D pen group, PLA was placed in cast models having the same diameter. Later, discs were taken from the casts. In 3D print group, the discs were prepared in TinkerCad software, and later, 3D-printed using the Cura software.

A total of 15 specimens from group 3D pen and 3D print were divided into three subgroups (n = 5) and put in cups, including tea,

coffee, and distilled water. Measurements were repeated three times for each specimen, and the mean values of L ∗, a ∗, and b ∗ ( L ∗ indicates lightness, a∗ is the red/green coordinate, and b ∗ is the yellow/blue coordinate) were calculated. The colors of all specimen groups were measured before exposure (baseline) with a colorimeter (Vita Easyshade, Vita Inc., Germany) using the Commission Internationale d’Eclairage L ∗ a ∗ b ∗ relative to the standard illuminant. According to the manufacturer’s recommendations, before the measurement of the specimens, the colorimeter was calibrated.

Please cite this article as: S. Aksakalli et al, The mechanical testing and performance analysis of three-dimensionally produced lingual

retainers, Journal of the World Federation of Orthodontists, https://doi.org/10.1016/j.ejwf.2022.12.003

S. Aksakalli et al / Journal of the World Federation of Orthodontists xxx (xxxx) xxx 3

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Fig. 1. Prepared Specimens.

Fig. 2. Scanning specimens with CEREC Intraoral Scanner.

Saliva contains many biomarkers, including immunoglobulins, glycoproteins, enzymes, blood type substances, hormones, and

electrolytes [13] . Artificial saliva, mimicking salivary fluid composition was prepared for this study. The effect of saliva substances on

PLA surface was investigated. A few common ingredients in human saliva, including lysozyme and α-amylase were spiked into an arti-

ficial saliva [14] . The concentrations of lysozyme (0–600 mg/L) and amylase (0–10 0 0 U/mL) were spiked into deionized water, with pH

7.2 to 7.4 [15] . Water absorption studies of the polymer composites were carried out according to the following test method. The specimens

Please cite this article as: S. Aksakalli et al, The mechanical testing and performance analysis of three-dimensionally produced lingual

retainers, Journal of the World Federation of Orthodontists, https://doi.org/10.1016/j.ejwf.2022.12.003

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Fig. 3. PLA applied with 3D dental pen.

Fig. 4. Printing lingual retainer with Anycubic.

were immersed in artificial saliva for 1 month and the weight gain was calculated using the precision balance (OHAUS, Adventurer,

Wildwood Crest, NJ, United States). The initial and after exposure weight of the specimens after subjecting to water absorption test

were recorded.

The observations have been performed with a FEI QUANTA FEGSEM 450. During the imaging process, we used a large field de-

tector for the 3D dental Pen and Transbond LR. Nonconductive samples were gold-coated before scanningand, the samples were

placed on the aluminum setup coated with carbon conductive tape (Ted Pella, double-coated, 8 millimeters W × 20 mL). Images were

taken at 10 0 0 × and 50 0 0 × magnifications, at 5- to 10-millimeter working distance, with 100-Pa pressure, and a voltage of 7 kilo-

volts, under low vacuum [16] .

All data for each measurement were distributed normally, and one-way ANOVA and Tukey post hoc test were performed using

the statistical software (SPSS version 20, SPSS Inc., Chicago, United States). For color changes, the two-way ANOVA was used. Fracture

modes were analyzed with the Pearson chi-square test ( P = 0.05).

Tensile bond strength values obtained from all three evalaution groups are provided in Table 2 . Group 1 had the highest score;

although, there was no statistical significant difference between group 3 and 1. Group 2 had lower tensile strength values, which

is statistically significant when compared to other two groups. Adhesive reminant index from all three evaluation groups scores

are shown in Table 3 . There observed significant difference among  Please cite this article as: S. Aksakalli et al, The mechanical testing and performance analysis of three-dimensionally produced lingual retainers, Journal of the World Federation of Orthodontists, https://doi.org/10.1016/j.ejwf.2022.12.003

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Fig. 5. PentaOne (0.0215-inch round wire) for control group.

Table 2

Tensile bond strength

Groups Sample size Minimum Maximum Mean SD

Group 1 10 60.7 136.2 91.6 19.3

Group 2 10 15 55.9 39.5 13.1

Group 3 10 45.6 129.6 82.3 31.3

Table 3

ARI scores

ARI scores

Groups Sample size 0 1 2 3 Pearson chi-square

Group 1 10 0 1 0 9

Group 2 10 0 0 0 10

Group 3 10 0 1 8 1 P = 0.00

ARI, Adhesive Remnant Index.

Table 4

Comparison of color changes

Solution Mean Delta E ( SD )

N = 30 Group 3D Pen (n = 15) Group 3D Print (n = 15)

Distilled Water (n = 5) 2.1 (1.1) 4.5 (1.8)

Tea (n = 5) 3.3 (1) 10.6 (3.1)

Coffee (n = 5) 3.7 (1.1) 11.3 (2.9)

groups,with group 3 exhibiting mixed failure types, whereas the other groups had mostly a score 3.

The color changes were much greater in group 3 than in group 2 ( Table 4 ). The coffee solution made more

discoloration than the other solutions for both groups (tea and distilled water). The highest Delta E value

was obtained in group 3, which is exposed to coffee solution.

The water absorption results are shown in Table 5 . There is a statistically significant difference between the group 1 T0 and T1

values. (t: −5.05 , P = 0.001 < 0.05) In both the groups evaluated, it is seen that the T1 values are higher than the T0 values .

There is no statistically significant difference between the group 2 T0 and T1 values (t: −1.18 , P = 0.305 > 0.05).

Morphological characterization with SEM revealed new crystal formations, channel-shaped pits, and pits on the surface of the PLA

material . These formations can be considered causative for plaque adherence in vivo because of its effect on surface smoothness

( Figs. 6 and 7 ) Likewise, in the surface examination for Transbond LR material, it was observed that pit-like formations increased as time pro-

gresses. PLA has been noted to have less pit-like formation on the PLA surface than the Transbond LR but less stable in mass.

In dentistry, color changes are assessed using colorimeters, although, visual examination is a suitable method [17] . In the color

measurementby using the CIELAB color system, a method advanced by the Commission Internationale d’Eclairage for characterizing colors , color change is symbolized with Delta E. This system is very popular in most of the studies about discoloration. Delta E value

of 3.7 is considered clinically acceptable and higher values are not desirable. In the current study, the discoloration of group 2 was >

3.7, whereas group 1 had lower scores than 3.7. Some conditions can affect the color measurement, such as lightning in the room [18] . To annihilate the edge loss effect on color, the diameter of the prepared specimens was higher than

the aperture size of the instrument. For both groups tested, coffee changed color more than tea and distilled water. This result is in

accordance with a previous study; although, they additionally used red wine [18] . Similarly, Erta s¸ et al. [19] found coffee as the most

powerful discolorative after red wine. Both tea and coffee have yellow colorants that can cause discoloration; but yellow colorants in

these drinks have different polarities, which changes discoloration mechanism for both drinks.

Please cite this article as: S. Aksakalli et al, The mechanical testing and performance analysis of three-dimensionally produced lingual

retainers, Journal of the World Federation of Orthodontists, https://doi.org/10.1016/j.ejwf.2022.12.003

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Table 5

Comparison of water absorption

Groups X ± Ss

Average

difference 95% confidence interval of the difference t ∗ P

Lower Upper

PLA T0 35.46 ± 6..58 -0.44 -0.68 -0.20 -5.05 0.001

PLA T1 35.90 ± 6.41

Transbond LR T0 46.58 ± 2.93 -0.06 -0.20 0.08 -1.18 0.305

Transbond LR T1 46.64 ± 2.86

∗ Paired sample t test.

Fig. 6. Surfaces of PLA under standard error of mean (SEM).

To prevent lower arch crowding, bonded retainers are commonly preferred. Patients with fixed retention had stable results

at 5-years follow-up evaluation. Fixed retainers on the lingual side have no negative effects on hard and soft tissues [20] ; so, many

wire and composite combinations are prevalent in orthodontics. According to literature, when a perpendicular force is given to

a bonded retainer, torsion, shear, or tension forces may occur [21] .

Eventhough different methods to obtain bond strength values for retainers are reported in orthodontic literature, we preferred the

same method as that of Baysal et al. [22] for this study. In the present study, we used the combination of coaxial wire (PentaOne)

and a composite (Transbond LR) as a control group. According to Dahl and Zachrisson, the test results of PentaOne is encouraging

[23] . Aldrees et al. [24] found higher shear bond strength values with the wire (PentaOne) than with the solid-chain retainer.

Although Reynolds [25] suggested 60 to 80 kilogram per centimeter squared bonding power for bonded orthodontic attachments, these data are not applicable for fixed retainers as a standard. We found significant difference among groups. Group 2 (those prodcued with 3D printer) had the lowest tensile bond strength values, whereas the other two groups had no difference as far as tensile strength is concerned. Our lowest values were found in group 2 (39.5 N); although, Baysal et al. [22] found similar values in their study for the Respond (Ormco Inc., CA, United States)

group.

A tensile bond strength test was performed to evaluate the adhesion between the adhesive material (composite) and retainer.

Çokakoglu and Kızılda g˘ [26] reported that larger diameter retainer wires with higher enamel surface area require higher forces to

debond the retainers. Our results are not in accordance with this information. Group 2 had a larger diameter than group 3; al-

though, the diameter in group 3 were smaller. However, more clinical studies may be needed to assess the effect of saliva, physiologic movement of teeth, lingual retainer position, functional Please cite this article as: S. Aksakalli et al, The mechanical testing and performance analysis of three-dimensionally produced lingual retainers, Journal of the World Federation of Orthodontists, https://doi.org/10.1016/j.ejwf.2022.12.003

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Fig. 7. Surfaces of Transbond XT under standard error of mean (SEM).

forces of tongue, mastication, and the presence of plaque and calculus.

The literature includes conflicting reports for adhesive remnants. Failures have been found in wire-composite areas [21] and at

the composite-tooth areas [27] . In group 2, all adhesives remained on enamel. In group 1, we had the same result only one excep-

tion of score 1. These groups revealed that there is a weak connection between wire and composite interfaces. Baysal et al. [22] also

found similar result in their soft wire group. In group 3, similarly more than 50% of the composite remained on the enamel.

The water absorption behavior of PLA- and LR-based composites are shown in Figs. 5 and 6 . Although the surface proper-

ties deteriorated less in the PLA group, , the water absorption was high. The water absorption is more for PLA than LR because

PLA is more hydrophilic than Transbond LR [28] . The hydrogen molecules in the water, bond with the hydroxyl groups present

in the cellulosic fibers. This situation paves the way for the diffusion of water molecules into the matrix/fiber interface. Furthermore, the porous tubular structure of fibers in PLA enables the diffusion of water molecules into the PLA through the capillary effect [29] .

1. The tensile bond strength exhbited by both 3D printed retainers are favorable for thier clinical orthodontic use.

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retainers, Journal of the World Federation of Orthodontists, https://doi.org/10.1016/j.ejwf.2022.12.003

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Please cite this article as: S. Aksakalli et al, The mechanical testing and performance analysis of three-dimensionally produced lingual

retainers, Journal of the World Federation of Orthodontists, https://doi.org/10.1016/j.ejwf.2022.12.003