Relationship between the surface defects and the manufacturing process of orthodontic Ni-Ti archwires

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ Esta es la versión aceptada del artículo publicado en: This is an accepted manuscript of a paper published in: Materials letters: Diciembrer (2011)  DOI: https://doi.org/10.1016/j.matlet.2011.07.088 Copyright: Copyright © 2011 Elsevier B.V. All rights reserved El acceso a la versión publicada del artículo puede requerir la suscripción de la revista. Access to the published version may require subscription. This is an Accepted Manuscript of an article published by Elsevier in Materials letters 2011, available at:  https://doi.org/10.1016/j.matlet.2011.07.088 1 Relationship between the surface defects and the manufacturing process of orthodontic Ni-Ti archwires A. Paúla , C. Ábalosb, A. Mendozac, E. Solanod and F.J. Gile aDepartment of Mechanical and Materials Engineering, School of Engineering, Seville, Spain bDepartment of Dental Materials, School of Dentistry, University of Seville, Spain cDepartment of Pediatric Dentistry, School of Dentistry, University of Seville, Spain dDepartment of Orthodontics, School of Dentistry, University of Seville, Spain eDepartment of Materials Science and Metallurgical Engineering, Polytechnic University of Catalonia, Barcelona, Spain Corresponding author: A. Paúl. Escuela Técnica Superior de Ingeniería, Avda. de los Descubrimientos s/n, 42092 - Sevilla, Spain. Telephone: +34954482277. Fax: +34954460475. E-mail: [email protected] Ábalos C., Mendoza A., and Solano E.: Facultad de Odontología, c/ Avicena s/n, 41009-Sevilla, Spain. Telephone: +34954481137. Fax: +34954481157. E-mail: c[email protected], [email protected], [email protected] Gil, F.J.: Universidad Politécnica de Cataluña, Diagonal 647, 08028 Barcelona, Spain. Telephone: +34934016111. Fax: +34934016210. E-mail: [email protected]. Abstract The surface defects and roughness of orthodontic Ni-Ti archwires are related to the manufacturing and surface finishing processes employed. On grouping archwires according to their surface characteristics instead the commercial brand, comparisons can be made of the influence of different surface finishings upon the corrosion resistance of Ni-Ti archwires. This novel approach shows that orthodontic Ni-Ti archwires with can be grouped into 5 surface patterns according to a dominant surface defect. Each pattern is derived from a given manufacturing process and shows a different response to corrosion in several media. Keywords: NiTi, orthodontic, corrosion, surface defect. 1. Introduction Nickel–titanium (NiTi) alloys are the most common orthodontic archwires used in clinical practice, thanks to their good clinical and mechanical properties [1]. It is known that the 2 biocompatibility of a metal is related mainly to the character of the surface-passive film [2]. The surface oxides can degrade in acidic saliva [3] and chlorine [4] or fluoride solutions [5,6]. When this happens, corrosion results in the release of metallic ions [5,7], morphological surface changes [5,8] and increases in the roughness [5] and friction coefficients [9], among other effects. The surface defects and roughness are related to the manufacturing processes and different surface finishings involved [5, 10]. Previous studies [7] point to surface defects as preferential corrosion areas, while other studies observe no such association [3]. Some authors [11] consider the roughness of the archwire to be an indicator of the tendency towards corrosion, although other investigators [3,5,6] have observed no such relationship. Nevertheless, it is widely accepted that the archwire manufacturer is a variable in itself [3,5]. In previous corrosion studies [3-8], the surface finish of 1 to 4 archwires from different manufacturers was related to a specific commercial brand, though the influence of the type of surface defect was not examined. If we group archwires according to their surface characteristics as received from the manufacturer instead of according to the commercial brand, comparisons can be made of the influence of different surface finishings on the corrosion resistance of Ni-Ti archwires. Each group comprises wires from different manufacturers but with the same surface condition. This approach has not been studied before, and can yield information on the influence of process parameters upon corrosion resistance, since each manufacturing process leaves its own tracks on the surface. The present study characterizes archwire surface using scanning electron microscopy (SEM) and the Laser Scanning Confocal Microscope System (LSCM) to establish criteria for classifying the Ni-Ti orthodontic archwires according to their surface condition. 2. Materials and methods Seventeen NiTi archwires (0.016 x 0.022 inches) from 9 different manufacturers (Dentaurum, Forestadent, GAC-International, G&H-wire, Leone, Ormco, RMO, TPO and 3M-Unitek) were studied in the as-received condition. Two-cm segments were extracted from three different 3 samples from each batch of archwires, each sample coming from a different archwire within the batch. The archwires were classified according to their surface using 18 variables, of which 7 were used in this study (Table 1). The surface topography of the NiTi archwires was examined by SEM (Philips XL-30). Five different surface defects were identified and quantified according to the percentage area occupied: Crack is a long, deep fissure with irregular edges and of variable width (Fig. 1e); Scratch is a long fissure with smooth edges and constant width, in which the bottom is clearly visible (Fig. 1b); Dimple is a small hole, an elliptical or rounded depression on the surface similar to the dimples on a golf ball (Fig. 1c); and Pore is a deep, usually rounded defect (Fig. 1d). The surface pattern indicates the predominant defect in the archwire surface. We used 400x SEM images to measure each variable in three different areas each measuring 10,000μm2. The three individual measurements were then averaged. When micropores appeared on the surface, we used the 1600x images with an area of 400μm2. The surface roughness of the NiTi archwires was measured using LSCM (Leica TCS-SP2). The microscope was previously calibrated using a Mitutoyo precision reference specimen (Code no.178-601) with Ra=3.1μm, and an error coefficient of under 0.3%. The images were processed using the Leica Confocal software. Micrographs corresponding to 750 linear μm were taken. We randomly selected 5 areas measuring 10,000μm2 from each topographic image, and measured the roughness, Ra - the individual values for each area being averaged. Multivariate regression analysis was used to evaluate the existence of correlations between the variables, using the SPSS version 14.0 statistical package. 3. Results and discussion We have been able to group the wires according to their mechanical finishing thanks to the correlation between the surface pattern and a predominant defect (Table 1). Due to the manufacturing process involved, the pattern was the same for each batch [12]. Although we studied the most common archwires, other patterns can also exist; nevertheless, our review of the literature yielded no other surface defects [6,7,13]. The importance of this classification may be merely theoretical, though we have observed different responses to corrosion depending on 4 the pattern involved. The patterns range as follows, from more to less corrosion resistance: smooth, dimple, pore, scratch and crack [14]. Future corrosion studies should take into account the surface pattern involved. The surface conditions of the archwires in our study were variable, and good quality was observed in few of them – in coincidence with the findings of other authors [13,15]. In our study the defects were aligned in the direction of the archwire axis, as expected from the manufacturing process used [15]. Traditional surface treatments for biomaterials include mechanical polishing (Mp) [16], electropolishing (Ep), chemical etching in acid solutions (Ce), heat treatments under various conditions (Ht) [17], sandblasting, and shot pinning. Archwires get their final form after drawing or rolling [18]. The heat treatments of Nitinol are also an essential part of alloy processing and device manufacturing, though they enrich the surface with Ni [19]. Temperatures about 700ºC are employed for wire annealing during drawing; temperatures in the 450-550ºC interval are used to design optimum shape memory and superelasticity, and for the shape setting procedure. Ht at temperatures <300ºC are customarily associated with Nitinol surface modification processes [17]. Since the production techniques are generally similar for all types of wires corresponding to one same manufacturer [12], we found a correlation between roughness and manufacturer. Nevertheless, only half of the manufacturers showed a similar surface pattern. These differences can be attributed to the influence of the final polishing process of the wires [12]. The five surface patterns found in our study are described below. The smooth pattern (Fig. 1a) is not common (11.8%); it has few defects, the absence of cracks and exhibits medium-low roughness (Table 1). Most uniform surfaces are obtained by drawing [15] followed by electropolishing. Mechanical polishing leave some scratches. These techniques lead to enhanced corrosion resistance [16]. Mp is more heterogeneous, while Ce yields a Ni-rich surface. This can be compensated by water boiling (Wb) and autoclaving in steam and water under pressure [17]. The scratch pattern (Fig. 1b) is the most frequent pattern (47.1%). Besides scratches, all have some pores and cracks. Roughness is medium-low (Table 1). The corrosion resistance is medium [14]. Drawing and rolling are processes that produce surface scratches [18]. 5 The dimple pattern (Fig. 1c) was found in 17.6% of the samples. It is characterized by the presence of dimples and some pores, together with the absence of scratches, deposits and inclusions. It is associated with important roughness (Table 1). During archwire manufacture, the surface is subjected to Ce in HF/HNO3 solution to eliminate surface scratches, to remove highly deformed defect material, to oxidize the surface, and to leach nickel produced after Ht. Ce induces the formation of dimples and pores with a chemically stable surface [19]. Ce and Wb successfully prevent surface enrichment in Ni. The pore pattern (Fig. 1d) was only found in one type of archwire. It presents no scratches or dimples. Roughness is medium (Table 1). Pores are due to air trapping during metal cooling. The crack pattern (Fig. 1e) was found in 17.6% of the samples. It always presents pores and is the only pattern showing inclusions. It presents the lowest corrosion resistance [14]. The surface is obtained by Mp with large particles, this being the cause of the chemical heterogeneity observed after heat treatment, inducing non-uniform deformation of surface sub-layers associated with grinding. [19] Cracks are left by hard inclusions emerging to the Nitinol surface removed by the Mp process. In good agreement with other studies, we found small differences in the chemical composition of Ni-Ti. Archwire characteristics may result mainly from thermomechanical treatment [19]. Thermo-Active Copper NiTi contains 6.3 wt% Cu, and shows a different response to corrosion compared with stoichiometric NiTi [8]. This archwire exhibits a dimple pattern. To evaluate the true influence of Cu, we should compare Ni-Ti and Ni-Ti-Cu archwires with the same surface pattern. There is a clear correlation between manufacturer and surface roughness, indicating uniformity in the manufacturing process [9,12]. The surface roughness for the NiTi wires subjected to mechanical treatment (Fig. 2) ranged from 0.22 to 1.32μm (Table 1), in good agreement with other authors [12], though some studies report higher values [13]. This cannot be attributed to LSMC, as it tends to overestimate measurements made with AFM and profilometry [20]. Roughness is independent from the surface defects. This is due to the fact that the surface free of defects is greater than the surface occupied by defects. These have been defined as microroughness and macroroughness [21]. 6 4. Conclusions Orthodontic Ni-Ti archwires subjected to mechanical treatment can be grouped into 5 surface patterns according to the existing dominant defect. Each pattern is derived from a given manufacturing process and shows different responses to corrosion in several media. In future corrosion or surface studies the pattern must be taken into account. In addition, manufacturing standards should be developed to avoid patterns that are less resistant to corrosion. 5. Acknowledgments We appreciate the cooperation in this study of Dr. Juan Luis Ribas (LSCM expert) and the Center for Technology and Innovation Research, University of Seville (Spain). 6. References [1] Torrisi L. Biomed Mater Eng 1999;9:39-47. [2] Wever DJ, Veldhuizen AG, de Vries J, Busscher HJ, Uges DR, van Horn JR. Biomaterials 1998;19:761-769. [3] Huang HH. Angle Orthod 2005;75:661-665. [4] Li X, Wang J, Han EH, Ke W. Acta Biomater 2007;3:807-815. [5] Huang HH. Dent Mater 2007;23:24-33. [6] Lee TH, Huang TK, Lin SY, Chen LK, Chou MY, Huang HH. Angle Orthod 2010;80:547- 553. [7] Es-Souni M, Es-Souni M, Fischer-Brandies H. Biomaterials 2002;23:2887-2894. [8] Walker MP, White RJ, Kula KS. Am J Orthod Dentofacial Orthop 2005;127:662-669. [9] Wichelhaus A, Geserick M, Hibst R, Sander FG. Dent Mater 2005;21:938-945. [10] Es-Souni,M. Fischer-Brandies,H. Kock,N. Bock,O. Rätzke,K. IOK 2001;33:87-106. [11] Widu F, Drescher D, Junker R, Bourauel C. J Mater Sci Mater Med 1999;10:275-281. [12] Bourauel C, Fries T, Drescher D, Plietsch R. Eur J Orthod 1998;20:79-92. [13] Fischer-Brandies H, Es-Souni M, Kock N, Raetzke K, Bock O. J Orofac Orthop 2003;64:88-99. [14] Palazón C. 2010;PhD thesis[University of Seville]:90-261. 7 [15] Grimsdottir MR, Hensten-Pettersen A. Dent Mater 1997;13:163-167. [16] Wu W, Liu X, Han H, Yang D, Lu S. J Mater Sci Tech 2008;24:926-930. [17] Shabalovskaya S, Anderegg J, Van Humbeeck J. Acta Biomater 2008;4:447-467. [18] Prososki RR, Bagby MD, Erickson LC. Am J Orthod Dentofacial Orthop 1991;100:341- 348. [19] Shabalovskaya SA, Anderegg J, Laab F, Thiel PA, Rondelli G. J Biomed Mater Res B Appl Biomater 2003;65:193-203. [20] Al-Shammery HA, Bubb NL, Youngson CC, Fasbinder DJ, Wood DJ. Dent Mater 2007;23:736-741. [21] Fries T, Bourauel C, Drescher D, Plietsch R. Anal Bioanal Chem 1995;353:427-432. Figure captions Figure 1. Secondary electron images of the 5 surface patterns described in the study. a) smooth, b) scratch, c) dimple, d) pore, and e) crack. Figure 2. Laser scanning confocal microscopic (topographic) images of the 5 surface patterns described in the study. a) smooth, b) scratch, c) dimple, d) pore, and e) crack. Table 1. Surface defects and roughness of Ni-Ti orthodontic archwires. Pores* Micropores* Cracks* Scratches † Dimples* Roughness ( μ m) Surface pattern 0 0.01 0 2.9 0 0.22 Smooth 0.12 0.02 0 0 0.18 0.46 Smooth 0.19 0.02 0.06 8.1 0 0.29 Scratch 0.04 0 0.66 4.5 0 0.35 Scratch 0.49 0.02 0.45 18.1 0 0.41 Scratch 0.22 0.06 0.12 14 4.7 0.30 Scratch 0.23 0.01 0.06 9.5 0.87 0.43 Scratch 0.29 0.11 0.04 14.5 0.13 0.53 Scratch 0.44 0.06 0.20 13.5 0 0.27 Scratch 0.53 0.04 0.16 10.2 5.6 0.27 Scratch 0.85 0.07 0.05 0 13.02 0.70 Dimple 1.24 0.05 0 0 24.29 0.82 Dimple 0.62 0.04 0 0 64.44 1.32 Dimple 3.86 0.32 0.17 0 0 0.49 Porous 0.38 0.16 6.35 0 0 0.26 Crack 0.27 0.05 3.89 0 0 0.23 Crack 0.13 0.01 7.38 4.2 0.11 0.44 Crack (*) % area of defects in the sample (†) μm per 100 μm in linear scratches 8 Fig.1 Fig.2