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Abstract

The present study investigated the nanomechanical properties, surface topography, and color alterations of fiber-reinforced composite (FRC) orthodontic retainers. Two FRC orthodontic retainer materials (glass fiber reinforced (group 1) and polyethylene fiber reinforced (group 2)) were evaluated. Nanomechanical properties (nano-hardness and elastic modulus), surface roughness (Ra), and color stability of FRC retainers at baseline and after 2 years of simulated aging were evaluated. The data collected were analyzed using analyses of variance, followed by Tukey’s honestly significant difference test for post hoc comparisons at the significance level of p < 0.05. Group 1 specimens exhibited highest hardness (0.16 ± 0.06) and elastic modulus (2.68 ± 0.17) at baseline, and group 2 specimens showed lowest hardness (0.09 ± 0.16) and elastic modulus (2.19 ± 0.36) after aging. Significant difference in hardness and elastic modulus was observed between the groups at baseline and after aging (p < 0.05).The aged specimens demonstrated decreased hardness and elastic modulus values compared to baseline specimens, but the differences were not significant (p > 0.05). Group 2 specimens exhibited increased Ra at baseline (0.46 ± 0.79) and aging (0.84 ± 0.94), and the least Ra values were observed with group 1 specimens at baseline (0.31 ± 0.87) and aging (0.59 ± 0.65). There was a significant color alteration of the specimens from baseline to aging in both the groups (p < 0.05). However, group 1 specimens showed noticeable color change, and group 2 showed appreciable color change. Glass fiber-reinforced FRC orthodontic retainer exhibited superior properties compared to polyethylene fiber-reinforced FRC. Accelerated aging decreases surface hardness of the polymer matrix and increases Ra and discoloration of the FRC retainer materials.

Keywords

Fiber-reinforced composite orthodontic retainers glass fibers polyethylene fibers nanomechanical properties surface roughness

Introduction

In contemporary orthodontic practice, the demand for aesthetic or tooth-colored orthodontic components such as orthodontic brackets, arch wires, and retainers has received lot of consideration in recent years.¹ Although aesthetic brackets and arch wires have undergone abundant research in past years, the concept of aesthetic orthodontic retainers still remains a looming topic in modern-day orthodontics. Orthodontic retainers (retention appliances) are used to stabilize the teeth in their newly corrected position after active orthodontic treatment.² Numerous types of retention appliances are used for the stabilization of posttreatment tooth position, both fixed and removable.^3,4 Removable retention appliances are bulky, unaesthetic, and require patient’s cooperation which is the main reason for poor acceptance by most patients. On contrary, fixed retainers are lightweight and do not require patient cooperation.⁴ Furthermore, patients with fixed retention appliance exhibit consistently better alignment at 5 and 10 years after treatment compared to those with removable retainers.⁵

In routine orthodontic practice, fixed retainers are constructed of solid and braided flat or rounded metallic wires and is bonded using composite resin⁶ either only in the canine region or from canine to canine region.^7,8 The limitations, however, include aesthetics concerns, breakage of wires, debonding of the wires from composite matrix, and also the fact that they cannot be used in patients with nickel allergy.^3,9 As an alternative to metallic wires, fiber-reinforced composites (FRCs) retainers in varying forms and widths have been developed.¹⁰ These FRC retainers present with numerous advantages including noncorrosiveness, translucency, low cost, easy chair side fabrication, high aesthetics, good bonding and repair property, and high strength-to-weight ratios compared to most alloys.^11,12

The conventional FRC structures are made of polymer matrix (light cure monomers) reinforced with fine fibers. The polymer matrix holds the fibers together which serves as the reinforcing part providing stability and rigidity.¹³ However, the effectiveness of fiber reinforcement in FRC structure depends on several factors such as type of composite resin used; the type, length, quantity, and orientation of fibers; and adhesion of the fibers to the polymer matrix and impregnation of fibers with the resin matrix.^14,15 The commonly used fibers in dental applications are either glass or polyethylene. Glass fibers have high tensile strength, low extensibility, and are very well suited for dental application with high aesthetic demand because of their excellent translucent property. On contrary, polyethylene fibers are more durable with low modulus and density and good impact resistance. They are also used in aesthetic dental applications because of their white color.¹⁶ The reinforcement of composite with different types of fibers should afford good mechanical and physical properties.¹⁷ However, the use of each type of fiber within FRC matrix presents with its own properties and advantages over each other.¹⁶ Accordingly, evaluation of FRC retainer properties will provide the clinician with proper knowledge in selecting appropriate orthodontic retainer for different clinical situations.

There is no or limited studies on FRC properties comparing each fiber type within the FRC structure to be used as orthodontic retainers. Consequently, the aim of the present study is to evaluate the nanomechanical properties, surface topography, and color stability of glass fiber-reinforced and polyethylene FRC orthodontic retainers before and after artificial aging. The null hypothesis of the present study was that there are no significant differences among the two FRC orthodontic retainers with regard to tested parameters.

Materials and methods

Specimen preparation

Two different FRC orthodontic retainers, namely, everStick ORTHO (Stick Tech Ltd, Finland) and Ribbond-THM Ortho (Ribbond Inc., Seattle, Washington, USA) were evaluated in this study. The FRC specimens were cut to 15 mm in length (n = 5) and handled according to the manufacturer’s instructions.

everStick ORTHO

The everStick ORTHO available as preimpregnated FRC was polymerized according to the manufacturer’s instructions using a light curing unit (Elipar free light, 3M ESPE, Germany) with a radiance power of 1505 mW/cm² and a wavelength between 420 nm and 540 nm. The output of the curing unit was monitored by MARC device (BlueLight Analytics Inc., Halifax, Canada).

Ribbond-THM Ortho

The Ribbond-THM Ortho FRC was in non-impregnated form and was impregnated with unfilled adhesive resin (Heliobond, Ivoclar Vivadent AG, Schaan, Liechtenstein) and was light cured according to the manufacturer’s instructions using a light curing unit (Elipar free light, 3M ESPE, Germany). All specimens from both the groups were stored in deionized water for 24 h before testing.

Nanoindentation measurements

The specimens for nanoindentation measurement were prepared by embedding FRC materials (15 mm) in self-curing acrylic resin (Rapid Repair, DeguDent GmbH, Hanau, Germany) using a stainless steel mold. The FRC material was placed slightly above the acrylic surface and cleaned of any surface impurities. The nanomechancial properties (hardness and modulus of elasticity) of the polymer matrix of the FRC specimens were performed with a nanoindenter (Bruker, Santa Barbara, California, USA) equipped with a Berkovich diamond indenter tip with nominal radius of nearly 100 nm. The experiments were performed at a controlled temperature of 23°C ± 1°C, with loading and unloading rates of 0.05 mN/s, a 10-s dwell time and maximal load set at 0.5 mN (Figure 1). The diamond indenter tip was pressed randomly onto the specimen’s polymer matrix surface (away from fibers). Five indents were made on each specimen at least 50 µm apart to avoid overlapping indentations.

Figure 1.

Load-penetration depth for nanoindentation measurement of the polymer matrix of FRC.

Surface topography assessment

The surface topography assessment was done by measuring the surface roughness (Ra) of the specimens using a noncontact surface profilometer (Bruker Contour GT, Tucson, Arizona, USA) with a 3-D optical (Nanolens atomic force microscopy) model.¹⁸ Five specimens from each FRC material were cleaned with distilled water to remove any surface impurities and dried using a tissue paper before subjecting to roughness measurement. The specimens were placed on the movable turret and scanned with white light interferometry at five random surfaces at least 30 μm apart to avoid overlapping of the measurements. The average of the five measurements was considered as Ra of that particular specimen.

Color stability assessment

The color analysis of the FRC retainers was assessed using Color Eye 7000 spectrophotometer (GretagMacbeth, New Windsor, New York, USA) calibrated against white background according to the manufacturer’s guidelines. The color changes were determined using Commission Internationale de l’Eclairage L*a*b* color space system. The total color differences (ΔE) was calculated using the formula

Δ E = {[{(Δ L *)}^{2} + {(Δ a *)}^{2} + {(Δ b *)}^{2}]}^{1 / 2}

In relating the ΔE values to clinical environment, the values were converted to National Bureau of Standards (NBS) units (Table 1) as follows¹⁹

NBS unit = 0.92 \times (Δ E)

Table 1.

NBS interpretation of color changes.¹⁹

NBS unit	Color change	Clinical interpretation
0.0–0.5	Trace	Extremely slight change
0.5–1.5	Slight	Slight change
1.5–3.0	Noticeable	Perceivable
3.0–6.0	Appreciable	Marked change
6.0–12.0	Much	Extremely marked change
>12.0	Very much	Change to other color

NBS: National Bureau of Standards.

Scanning electron microscopic analysis

Scanning electron microscopic (SEM; Jeol JSM-5900 LV SEM, Tokyo, Japan) analysis was performed on the specimens from both groups at baseline and after artificial aging. The SEM device was operated at 15 kV, in vacuum and at 500× magnification.

Artificial aging

Following baseline roughness measurements and color analysis, the specimens from both groups were subjected to artificial aging. The artificial aging was carried out by thermocycling (Huber 1100, SD Mechatronik, Feldkirchen-Westerham, Germany) with 5000 cycles at 5–55°C, dwell time of 20 s, and transfer time of 10 s. After thermocycling, the roughness and color changes were recorded again.

Statistical analysis

The data collected were analyzed with one-way analysis of variance and Tukey’s honestly significant difference post hoc at the significance level of 0.05. All statistical analyses were performed using Statistical Package for Social Sciences (SPSS; SPSS Inc., Chicago, Illinois, USA) version 20.0 statistical analysis software.

Results

Nanomechanical properties

The nanomechanical properties of the FRC retainers before and after aging are presented in Table 2. The highest hardness (0.16 ± 0.06) and elastic modulus (2.68 ± 0.17) was observed in group 1 specimens at baseline, and the lowest hardness (0.09 ± 0.16) and elastic modulus (2.19 ± 0.36) was observed in group 2 specimens following aging. The results showed a nonsignificant (p > 0.05) decrease in hardness and elastic modulus values in both groups from baseline to aging after 2 years of simulated aging. Significant difference in hardness and elastic modulus was observed between the groups irrespective of measurement at baseline or aging (p < 0.05).

Table 2.

Nanomechanical properties of the study groups before and after aging test.^a

Nanomechanical properties	Measurement	Group 1/everStick ORTHO	Group 2/Ribbond
Nanohardness	Baseline	0.16 ± 0.06^b,A	0.11 ± 0.04^c,B
Nanohardness	Aging	0.12 ± 0.13^c,A	0.09 ± 0.16^d,B
Elastic modulus	Baseline	2.68 ± 0.17^b,A	2.39 ± 0.23^c,B
Elastic modulus	Aging	2.54 ± 0.29^c,A	2.19 ± 0.36^d,B

^a The values are expressed as mean ± SD. Different lower case letters in a row and different upper case letters in a column imply statistically significant difference (p < 0.05).

Surface topography

The increased Ra values (0.31 ± 0.87) at baseline and after aging were observed for group 2 specimens and least Ra values (0.46 ± 0.79) were observed for group 2 specimens. Ra increased significantly from baseline to aging (p < 0.05) in both groups. Also significant differences in Ra were seen between the groups (p < 0.05; Table 3). Figure 2 shows the profilometric images of histograms and Ra changes of the specimens at baseline and after thermocycling. The histogram curve for group 1 specimens presented with lesser elevations and depressions compared to group 2 specimens reflecting the Ra values. However, following thermocyling, the curves in group 2 specimens presented with a marked elevation and depression compared to group 1 specimens.

Table 3.

Surface roughness of the study groups before and after aging test.^a

Groups	Surface roughness (Ra) measurement
Groups	Baseline	Aging
Group 1/everStick ORTHO	0.31 ± 0.87^b,A	0.59 ± 0.65^c,B
Group 2/Ribbond	0.46 ± 0.79^c,B	0.84 ± 0.94^d,C

^a The values are expressed as mean ± SD. Different lower case letters in a row and different upper case letters in a column imply statistically significant difference (p < 0.05).

Figure 2.

Profilometric images of surface roughness changes of the specimens at baseline ((a) and (c)) and after aging ((b) and (d)). (a) everStick ORTHO and (c) Ribbond THM.

Color analysis

Table 4 presents the color measurement of the study groups. ΔE of the group 2 (1.59 ± 0.87) was higher than group 1 (1.21 ± 0.87). There was a significant alteration in the color of the specimens from baseline to aging in both groups (p < 0.05). Group 1 specimens showed noticeable color change; on the contrary, group 2 specimens showed appreciable color change.

Table 4.

Color measurement of the study groups before and after aging test.^a

	Color measurement and interpretation
Groups	Baseline		Aging		Color change	Clinical interpretation
Groups	ΔE	NBS	ΔE	NBS	Color change	Clinical interpretation
Group 1/everStick ORTHO	1.21 ± 0.87^b,A	1.11	2.67 ± 0.87^c,B	2.45	Noticeable	Perceivable
Group 2/Ribbond	1.59 ± 0.87^c,B	1.46	3.72 ± 0.87^d,C	3.42	Appreciable	Marked change

NBS: National Bureau of Standards.

^a The values are expressed as mean ± SD. Different lower case letters in a row and different upper case letters in a column imply statistically significant difference (p < 0.05).

SEM image analysis

Figure 3 shows the SEM micrographs of the specimens in cross section before and after aging. The images revealed close integration of the fibers with the polymer matrix in both specimen groups at baseline. However, following aging, group 2 specimens showed disintegration of fibers at large caused due to dissolution of polymer matrices as compared to group 1 specimens.

Figure 3.

Scanning electron microscopic image (×500 original magnification) of the specimens at baseline ((a) and (c)) and after aging ((b) and (d)); (a) everStick ORTHO and (c) Ribbond THM. (The circled area shows good integration of fibers to polymer matrix; the arrow shows disintegration of fiber–polymer matrix following aging.)

Discussion

The present study evaluated nanomechanical properties, Ra, and color stability of glass and polyethylene FRC orthodontic retainers. The null hypothesis of the present study was rejected. Significant differences were observed between glass and polyethylene FRC retainers with regard to the tested parameters.

Bonded orthodontic retainers remain in the mouth for long duration depending on the severity of the cases, and thus, it becomes necessary to study the behavior of such materials in clinical environment.²⁰ In in vitro testing of dental materials, clinical simulations are frequently carried out by artificial aging protocols such as thermocycling, immersion in liquid media, brushing simulation, and light aging.^21,22 In the present study, artificial aging was performed by thermocycling with 5000 cycles which is equivalent to 2 years of aging (representing duration of retainer wear by patients).²³ The FRC specimens in both groups behaved differently against aging stimuli.

The polymer structure and cross-linking density of the FRC retainers were evaluated by nanomechanical analysis of the polymer matrix in the current study. Nanoindentation is an effective technique in measuring the indented surface area at nanoscale.²⁴ Retainers in oral environment are exposed to various stimuli, and this could initiate rapid wear of the polymer matrix from the FRC structure which could lead to thinning of the structure and subsequent failure of the retainer. Thus, the FRC structure with superior hardness values might be favorable.^25,26 In contrast, FRC retainer materials with low rigidity are beneficial to allow physiological movement of teeth and to prevent tooth ankylosis.²⁷ In the present study, nanoindentation was performed on the polymer matrix to hinder any possible influence of the fibers on the measurements. The possible evaporation of monomers and activator prepregs and initiators has limited the shelf life of FRC-based materials.²⁸ Following aging, both materials in the present study demonstrated decreased hardness and elastic modulus properties. However, the difference was not significant from baseline to aging. The difference in the nanomechanical properties of the FRC materials from baseline to aging could be attributed to the presence of polymethyl methacrylate in the cross-linked polymer network which could have reduced the cross-linking density, and thus, the decreased hardness and elastic modulus values of the retainer materials were observed.²⁸ Previous studies^25,26 have demonstrated an increased hardness of the composite resin and orthodontic retainer adhesive following aging which was in disagreement with the outcome of our study. This could be due to the different aging protocols followed in different studies.

The nanomechanical properties also support the findings of the scanning electron microscopy. The aged specimens showed disintegration of the FRC retainers following aging. The disintegration of the resin matrix could be due to limited stability of polymer components of this particular FRC structure. Also, the fibers act as voids due to inadequate adhesion between the resin matrix and the reinforcing fibers, thereby weakening the entire FRC structure.²⁸ Although both materials in this study demonstrated decreased adhesion between reinforcing fibers and composite resin matrix from baseline to aging, the changes were more apparent with polyethylene-reinforced fibers compared to glass fiber-reinforced resin composites. The decreased adhesion of resin composite to polyethylene fibers could be related to the problems associated with plasma coating, impregnation, and silanization of the fibers.^29,30 Previous studies have demonstrated increased adhesion between the fibers and the organic resin matrix with silanization of glass fibers.^31,32 Foek et al.²⁹ evaluated the debonding force and failure type of glass and polyethylene FRC after 1,000,000 fatigue loading cycles. Glass FRC demonstrated higher debonding force (830 N) than polyethylene FRC (731 N), but the difference was not significant. Similarly, glass FRC’s presented with partial adhesive debonding (failures) from one of the teeth in 80% and with complete adhesive debonding in 20%. On the contrary, polyethylene FRC specimens showed complete adhesive debonding and material fracture in 50% and 40%, respectively.

Surface characteristics, in particular Ra of biomaterials, are of prime importance because of their influence on bacterial adhesion.³³ The rough surface on orthodontic retainers similar to other dental materials could also pose a risk for bacterial plaque accumulation and subsequent demineralization at the bonding surface. Many factors such as matrix characteristics, amount and size of reinforced fibers, exposure of these fibers, and formation of voids during retainer preparation are found to contribute to the roughness of the retainer material.³⁴ It has been hypothesized that Ra of composite resin should be below 0.2 μm to prevent accumulation of plaque and microorganisms.³⁵ The intraoral hard surface can cause discomfort and can be detected by the tip of the tongue if the roughness value exceeds 0.5 μm.³⁶ In the present study, both FRC retainers demonstrated roughness values below 0.5 μm at baseline. Glass fiber-reinforced FRC demonstrated significantly lower Ra than the polyethylene FRC. However, following aging the roughness increased almost twofold from their baseline values. This could be explained by the fact that water sorption of the polymer matrix during aging causes hydrolytic degradation of the composite resin matrix, the filler/matrix interface, or the filler resulting in increased Ra.³⁷

The present study was in agreement with the findings of Tanner et al.³⁴ and Lassila et al.³⁸ Tanner et al. studied the early plaque accumulation and bacterial adhesion of glass and polyethylene FRC materials and found increased plaque accumulation and adhesion of Streptococcus mutans due to increased Ra of polyethylene FRC compared to glass FRC materials. Lassila et al. studied the Ra and bacterial adhesion of FRCs (glass and polyethylene) and conventional restorative materials. The outcome demonstrated that polyethylene FRC specimens with highest Ra value of 2.33 μm, while Ra of glass FRC was almost half the value of polyethylene FRC.

Intraoral discoloration of the dental composite resin is one of the most common reasons for their replacement in aesthetic areas. Fiber reinforcement has shown to produce changes in composite depending on the type of fiber. In the present study, both glass- and polyethylene-reinforced composite resin materials accounted for significant color difference at baseline. After 5000 cycles of aging (equivalent to 2 years of clinical use), the glass FRCs showed clinically noticeable color changes (ΔE = 2.67), whereas polyethylene FRCs showed appreciable color changes (ΔE = 3.72). Also, significant difference was observed between baseline and aged specimens with both FRC retainers. Thus, it was confirmed that reinforcement of fibers did not improve the color stability of composite resins subjected to aging. In clinical interpretation (NBS) of ΔE values, glass FRCs demonstrated perceivable changes (NBS-2.45) and polyethylene FRC’s demonstrated marked color changes (NBS-3.42).

This difference in the color change between glass and polyethylene fiber types could be related to their chemical structure. Glass fibers are an inorganic material consisting of silica tetrahedral bonded together in a random network which is different from that of organic polyethylene fibers. The inorganic glass fibers are reported to have lower water sorption rate and lower discoloration incidence than that of organic polyethylene fiber.³⁹ The present outcome was in agreement with the study of Tuncdemir and Güven³⁹ who studied the aging effect of polyethylene and glass FRCs on the color changes and stability before and after accelerated aging.⁹ The authors confirmed polyethylene FRC with increased color change while the glass FRC showed the least color change.

Conclusion

Glass fiber-reinforced FRC orthodontic retainer exhibited superior properties as compared to polyethylene fiber-reinforced FRC. Accelerated aging decreases surface hardness of polymer matrix and increases Ra and discoloration of the FRC retainer materials.

Footnotes

Acknowledgments

The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Deanship of Scientific Research at King Khalid University through General Research Project under grant number (G.R.P-322-38).

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Nanomechanical properties,surface topography,and color stability of fiber-reinforced composite orthodontic retainers

Abstract

Keywords

Introduction

Materials and methods

Specimen preparation

everStick ORTHO

Ribbond-THM Ortho

Nanoindentation measurements

Surface topography assessment

Color stability assessment

Scanning electron microscopic analysis

Artificial aging

Statistical analysis

Results

Nanomechanical properties

Surface topography

Color analysis

SEM image analysis

Discussion

Conclusion

Footnotes

Acknowledgments

Declaration of conflicting interests

Funding

References