Effects of intraoral aging on surface properties of coated nickel-titanium archwires
ABSTRACT
Objective:
To evaluate the effects of intraoral aging on surface properties of esthetic and conventional nickel-titanium (NiTi) archwires.
Materials and Methods:
Five NiTi wires were considered for this study (Sentalloy, Sentalloy High Aesthetic, Superelastic Titanium Memory Wire, Esthetic Superelastic Titanium Memory Wire, and EverWhite). For each type of wire, four samples were analyzed as received and after 1 month of clinical use by an atomic force microscope (AFM) and a scanning electronic microscope (SEM). To evaluate sliding resistance, two stainless steel plates with three metallic or three monocrystalline brackets, bonded in passive configuration, were manufactured; four as-received and retrieved samples for every wire were pulled five times at 5 mm/min for 1 minute by means of an Instron 5566, recording the greatest friction value (N). Data were analyzed by one-way analysis of variance and by Student's t-test.
Results:
After clinical use, surface roughness increased considerably. The SEM images showed homogeneity for the as-received control wires; however, after clinical use esthetic wires exhibited a heterogeneous surface with craters and bumps. The lowest levels of friction were observed with the as-received Superelastic Titanium Memory Wire on metallic brackets. When tested on ceramic brackets, all the wires exhibited an increase in friction (t-test; P < .05). Furthermore, all the wires, except Sentalloy, showed a statistically significant increase in friction between the as-received and retrieved groups (t-test; P < .05).
Conclusion:
Clinical use of the orthodontic wires increases their surface roughness and the level of friction.
INTRODUCTION
In recent decades, progress in the technology of orthodontic materials has resulted in a large variety of wires with a wide range of properties. Nickel-titanium (NiTi) is one of the most commonly used alloys to manufacture archwires because of its good mechanical and clinical properties.1,2 The most important advantages of NiTi wires are their springback and pseudoelasticity, which allows a wide deflection and activation range by delivering low forces.3
The growing demand of esthetic appliances led to the introduction of coated NiTi archwires into the orthodontic market. There are two main techniques to modify the wire's surface: ion implantation and coating with polymeric resins composed mainly of polytetrafluoroethylyene. Ion implantation is a permanent modification of the surface composition by inserting ionized atoms.4 In contrast, tooth-colored coatings, with a 20–25 µm thick layer, are usually applied in an atomizing process by using purpose-cleaned compressed air as a transport medium for the atomized particles.4 Coating or refining the wire's surface influences the esthetic, mechanical, and biological properties of the wires.5 Furthermore, many studies have shown that surface characteristics may directly influence the efficiency of archwire-guided tooth movement.6,7
Recently, there has been growing interest in the evaluation of esthetic, mechanical, structural, and surface properties of tooth-colored archwires. It has been reported that coating may or may not increase unloading forces and surface roughness of as-received wires, depending on the technique used for surface treatment.5,8–10 Loss of a significant amount of coating,8,11 poor color stability,12 change of mechanical behavior and force values,11 and increase in surface roughness8,13 have all been reported after clinical use.
Therefore, the hypothesis tested in this study is that oral environment exposure affects the surface of ion-implanted and polymer-coated esthetic archwires. Furthermore, we evaluated how these surface modifications affect friction between archwires and brackets in passive configuration, which is the component of sliding resistance that may be directly affected by an increase in surface roughness.
MATERIALS AND METHODS
Five superelastic round 0.016-inch NiTi archwires were used in this study: Esthetic Superelastic Titanium Memory Wire (Polymer Coated), EverWhite (Polymer Coated), and the noncoated control Superelastic Titanium Memory Wire (American Orthodontics, Sheboygan, Wis); Sentalloy High Aesthetic (Ion Implanted) and the noncoated control Sentalloy (GAC International, Islandia, NY). The archwires were divided in two groups: the first group (as-received) was formed by the as-received wires; the second group (retrieved) consisted of archwires used for 1 month of treatment on Radiance brackets, slot size 0.022×0.028 inch (American Orthodontics), and ligated with elastomeric modules (colored ligatures, American Orthodontics). These NiTi wires were retrieved during the regular treatment visits of patients. The NiTi archwire insertion and retrieval were performed according to the procedure reported by Eliades et al.14 Ethical approval was obtained by the Ethical Committee of Bambino Gesù Hospital, Rome, Italy. All the retrieved samples were cleaned with 95% ethanol to remove any precipitation.
To investigate wire surface morphology on the micrometer scale, a scanning electronic microscope (SEM) was used. Four samples of each wire were observed. For this purpose the polymer-coated Esthetic Superelastic Titanium Memory Wire and EverWhite wire specimens were vacuum-coated with a thin layer of gold-platinum. The samples were attached to a metal holder using rapid-drying cyanoacrylate glue. The morphology surface analysis was performed by an SEM (FEI, city, The Netherlands); the images were recorded at ×200, ×500, and ×1000 magnification.
To analyze approximately straight specimens, four samples of each wire product (5 mm) were cut from the end of four different preformed archwires and were observed by means of an atomic force microscope (AFM Perception, Assing, Italy) operating in contact mode under ambient conditions, as previously reported.9,15 A standard statistical software package (SPSS version 20.0, SPSS IBM, New York, NY) was used for data analysis. The Kolmogorov-Smirnov test was applied to verify the normality of the data and one-way analysis of variance (ANOVA) followed by Tukey's post hoc test or t-test for unpaired data were performed. The level of significance was set at P < .05.
In addition, the frictional properties of the archwires were evaluated. Two stainless steel plates were manufactured to obtain the best adaptation with an Instron 5566 (Instron Corporation, Canton, Mass), and on these plates two kinds of brackets were bonded. Every archwire was coupled with each types of bracket, a metallic bracket Mini Master Series (American Orthodontics) and an esthetic bracket Radiance (American Orthodontics). Three metallic and three esthetic brackets (upper right lateral incisor, canine, and first premolar brackets) were bonded on the two different stainless plates with an orthodontic composite (Transbond XT Light Cure Adhesive, 3M Unitek, city, state). To avoid binding, the brackets were positioned aligned, in a perfect passive configuration, at an interbracket distance of 7.5 mm. Bracket alignment was tested using a straight stainless steel arch 0.021×0.025 inch. The archwires were settled with elastomeric modules (colored ligatures, American Orthodontics) to avoid the strength variability of metallic ligatures. The experiment was conducted in a dry state. The base was fixed to the machine through a vise, and four samples for each wire typology were subjected to tensile tests with a dynamometer Instron 5566 with a load cell of 50N. The friction produced by the archwires was recorded at a rate of 5 mm/minute for 1 minute. Each sample was assessed five times for types of bracket, and the friction for every couple archwire-bracket was recorded with a total six hundreds value for a minute. For each test the greatest friction value expressed in Newton (N) (Figure 1) was recorded. The Kolmogorov-Smirnov test was applied to verify the normality of the data, and ANOVA followed by Tukey's post hoc test or t-test were performed. The level of significance was set at P < .05.
Citation: The Angle Orthodontist 84, 4; 10.2319/081213-593.1
RESULTS
The SEM micrographs of as-received noncoated superelastic wires showed a regular surface (Figure 2, left). Some anomalies due to inconsistent coating distribution were present on polymer-coated wires, and small depressions and bumps were found along the entire surface of the ion-implanted Sentalloy High Aesthetic (Figure 2C). For all the wires considered in this study, SEM images showed a certain amount of surface modifications due to clinical use. Although the retrieved samples showed an extremely variable surface, with holes, ridges, and cracks (Figure 2, right), the two noncoated archwires revealed less surface modification due to intraoral aging. The Esthetic Superelastic Titanium Memory Wire and the EverWhite suffered from coating delamination; the metal below that appeared smooth (Figure 2H through J). The Sentalloy High Aesthetic exhibited deep cracks and loss of its normal morphology (Figure 2, right).
Citation: The Angle Orthodontist 84, 4; 10.2319/081213-593.1
Three-dimensional and two-dimensional images of archwire surfaces, obtained by means of AFM, are shown in Figure 3. The roughness average, root mean square, and maximum height values were used to quantitatively evaluate the surface topography of each archwire and are shown in Table 1 as mean ± standard deviation in nanometers. Among as-received materials, EverWhite showed the lowest values of surface roughness, followed by the Superelastic Titanium Memory Wire and the Esthetic Superelastic Titanium Memory Wire. Furthermore, all the wires suffered a statistically significant increase in surface roughness due to intraoral aging (Tables 1 and 2). The Esthetic Superelastic Titanium Memory Wire showed the greatest increase in roughness after clinical use. The roughest wire, in both the as-received and retrieved groups, was the Sentalloy High Aesthetic (ANOVA, P < .05).
Citation: The Angle Orthodontist 84, 4; 10.2319/081213-593.1
The sliding resistance (classical friction), in passive configuration, for each bracket-archwire combination is reported in Tables 3 and 4. The Superelastic Titanium Memory Wire (ANOVA, P < .05) showed the lowest friction on metallic and esthetic brackets, both in the as-received and retrieved groups (Tables 3 and 4). For all the wires, an increase in friction due to the use of esthetic brackets and retrieved archwires was recorded (Table 3). Compared with Superelastic Titanium Memory Wire, all the esthetic wires showed a significant increase in friction, especially when used on clear brackets after intraoral aging. The Sentalloy was the only archwire that did not show a statistically significant increase in friction between the as-received group and the retrieved group on both bracket types.
DISCUSSION
The present study focuses on the evaluation of intraoral aging of five different NiTi archwires; the aim was to compare the behavior of esthetic and noncoated superelastic wires. There is still a lack of studies concerning the intraoral aging of wires and the associated phenomena, such as variation of mechanical properties and surface alterations.16 Therefore, retrieval analyses conducted on dental materials are receiving growing interest because of the critical information provided.14,17 Recent ex vivo studies showed that intraoral aging might compromise the esthetic properties of tooth-colored wires.8,11,13 Mechanical properties, with a decrease of delivered forces, are affected as well.8,11 Moreover, it has been proposed that intraoral aging influences surface roughness of polymer-coated wires.8,13
In this study, the AFM and SEM were used to establish the topographic alterations of the archwires, evaluating polymer-coated and ion-implanted materials and their noncoated counterparts. The AFM was found to be an excellent tool to determine numeric values that describe the surface roughness, even if it presents some drawbacks, such as small scan size.9,18,19 Therefore, the SEM was used for qualitative evaluation of the surface at a micrometer scale.
The SEM analysis of as-received wires presented different surface images depending on the manufacturing process. The noncoated samples showed the typical characteristics of the superelastic archwires and had a regular surface; the polymer coated wires exhibited some anomalies, mainly regarding coating layers, and the rhodium-implanted Sentalloy High Aesthetic showed a peculiar highly rough surface. In the retrieved group, a certain degree of corrosion and a large amount of debris were present on the noncoated wires, as might be expected.20 Polymer-coated archwires revealed a considerable amount of coating delamination, consistent with findings of previous studies.8,13 This deterioration may impair the esthetic properties, thereby affecting patient satisfaction.11 Even if multiple areas of the polymer layer peeled off, no defects were found on the naked surface, probably because of the manufacturing process, as suggested by previous studies that found controversial results on polymer-coated wires of different manufacturers.8,13 A relation between bracket imprints and delamination areas was notable in several specimens. It must be emphasized that the irregular surfaces may lead to plaque accumulation, and tooth movement may be affected because of the entrapment of bracket edges inside these defects.7,21,22
After clinical use, the ion-implanted Sentalloy High Aesthetic also lost its typical surface characteristics. This study was the first to assess changes in surface roughness due to intraoral aging of an ion-implanted wire. In a previous study, the corrosion resistance of nitrogen-implanted Neo Sentalloy Ionguard in fluoride mouth rinse solutions was tested. These results were similar to classic Neo Sentalloy23 but are not comparable to our results because of the different materials and conditions of the experiment.
The quantitative surface analysis performed by means of the AFM showed significant differences among the tested wires. Our findings suggest a different effect of the surface treatments of the two esthetic wires. EverWhite and Superelastic Titanium Memory Wire were the least rough wires in the as-received group; Sentalloy High Aesthetic was the roughest. This increase in surface roughness due to ion implantation was confirmed by a previous study.11
After clinical use there was an increase in the roughness parameters for all the wires. Among retrieved samples, the Sentalloy High Aesthetic was the roughest wire, and the greatest increase in roughness was observed on the Teflon-coated Esthetic Superelastic Titanium Memory Wire. These data are consistent with the results of other studies that report a surface roughness increase after clinical use9 and should be evaluated in light of the key role of surface roughness on wire performance, in terms of corrosion aging, plaque accumulation, biocompatibility, and sliding resistance.5,7,24
Sliding resistance is the result of the additive effects of three phenomena: classical friction, binding, and notching.25 In our study the frictional behavior of five different NiTi wires were assessed in vitro, in a passive bracket configuration. The decision to use the brackets in passive configuration was determined by the necessity to evaluate only simple friction, avoiding binding and notching components of the sliding resistance.25 In all four tested scenarios, the wire that exhibited the least amount of friction was the Superelastic Titanium Memory Wire. In the as-received group, esthetic wires showed a higher level of friction than noncoated controls. Previous studies have shown that polymer coating may decrease the friction produced by wires, in disagreement with our results. Some variables, like the use of different brackets, different dimensions of wires, and different wire brands may explain these differences.4,26 Nevertheless, in the as-received group it was not possible to establish a clear correlation between AFM results and friction analysis. The increase in friction due to surface roughness is a controversial topic discussed in relevant literature.6,27,28 Some authors confirm the existence of a close correlation between surface roughness and friction,27 but other studies state that a wire's low surface roughness is not a sufficient condition for low frictional coefficients.6,28
As for surface parameters, all the wires suffered an increase in friction due to intraoral aging. The esthetic wires were more affected by clinical use in the friction test. In the retrieved group, the Esthetic Superelastic Titanium Memory Wire produced the highest friction on the metallic brackets, and the EverWhite produced the highest values on the ceramic brackets. These results may be due to coating degradation.
The clinical relevance of this study is that, despite the vast improvements in manufacturing esthetic brackets, an adequate esthetic wire has not been produced yet. Despite the unsatisfying coating durability, coated wires continue to be marketed and used in clinical practices. Orthodontists should be aware that the exposure to the oral environment significantly affects the performances of esthetic archwires.
A limitation of this study was the examination of the wires at only two time intervals. Differences in oral hygiene protocols, pH, fluoride use, and degree of teeth irregularity in the in vivo part of the trial could also have influenced the outcome .
CONCLUSIONS
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The clinical use of wires altered their surface properties and increased surface roughness and level of friction.
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The SEM images confirmed the heterogeneous surface of the coated wires after clinical use. Even if EverWhite wire overcame some of the disadvantages of the Esthetic Superelastic Titanium Memory Wire, exhibiting a lower and more stable surface roughness, it still suffers coating delamination.
Representative time/friction graphs. (Top) Sentalloy High Aesthetic on metallic brackets; (Bottom) Sentalloy High Aesthetic on esthetic brackets.
Representative scanning electron microscope images (×1000) of the as-received group, (A) Sentalloy, (C) Sentalloy High Aesthetic, (E) Superelastic Titanium Memory Wire, (G) Esthetic Superelastic Titanium Memory Wire, and (I) EverWhite, and the retrieved group, (B) Sentalloy, (D) Sentalloy High Aesthetic, (F) Superelastic Titanium Memory Wire, (H) Esthetic Superelastic Titanium Memory Wire, and (J) EverWhite.
Representative two-dimensional and three-dimensional atomic force miscroscope topography images (15 × 15 µm) of the as-received group, (A) Sentalloy, (C) Sentalloy High Aesthetic, (E) Superelastic Titanium Memory Wire, (G) Esthetic Superelastic Titanium Memory Wire, and (I) EverWhite, and the retrieved group, (B) Sentalloy, (D) Sentalloy High Aesthetic, (F) Superelastic Titanium Memory Wire, (H) Esthetic Superelastic Titanium Memory Wire, and (J) EverWhite.
Contributor Notes