Correlation Between Frictional Force and Surface Roughness of Orthodontic Archwires

Transcription

1 SCANNING VOL. 37, (2015) Wiley Periodicals, Inc. Correlation Between Frictional Force and Surface Roughness of Orthodontic Archwires SAMJIN CHOI, 1 EUN-YOUNG HWANG, 2 HUN-KUK PARK, 1 AND YOUNG-GUK PARK 2 1 Department of Biomedical Engineering, College of Medicine, Kyung Hee University, Seoul, Korea 2 Department of Orthodontics, College of Dentistry, Kyung Hee University, Seoul, Korea Summary: Lateral force microscopy measures the lateral bending of the cantilever depending on the frictional force acting between the tip and surface. The aim of this study was to investigate and compare the relationship between the surface roughness and frictional resistance of four archwire and bracket combinations consisting of the inch NiTi and inch stainless steel archwires interacting clinically with two representative self-ligating brackets, active-type Clippy-C 1 ceramic self-ligating brackets, and passive-type Damon 1 stainless steel self-ligating brackets, using the lateral force microscopy technique. A inch NiTi archwire interacting with passivetype Damon 1 stainless steel self-ligating brackets showed the smoothest surface roughness and the lowest frictional resistance compared to other combinations. The archwires interacting with passive-type Damon 1 stainless steel self-ligating brackets showed significantly lower surface roughness and frictional resistance than those interacting with active-type Clippy-C 1 ceramic self-ligating brackets. The frictional force in the in vivo archwire and bracket system increased with increasing surface roughness of the archwire. This positive correlation suggests that surface roughness can be used as an evaluating marker for estimating the efficiency of orthodontic treatment, rather than the direct measurement of frictional force. SCANNING 37: , Wiley Periodicals, Inc. Contract grant sponsor: Korean Health Technology Research & Development Project by the Ministry of Health & Welfare. Contract grant sponsor: Republic of Korea; Contract grant number: HI14C2241. Conflict of interest: None Address for reprints: Young-Guk Park, D.M.D., Ph.D., Department of Orthodontics, College of Dentistry, Kyung Hee University, 26, Kyungheedae-ro, Dongdaemun-gu, Seoul , Republic of Korea. medchoi@khu.ac.kr Received 24 February 2015; revised 20 April 2015; Accepted with revision 29 April 2015 DOI: /sca Published online 27 May 2015 in Wiley Online Library (wileyonlinelibrary.com). Key words: lateral force microscopy, frictional resistance, surface roughness, archwire-bracket system, self-ligating bracket Introduction Lateral force microscopy (LFM) is an atomic force microscopy (AFM) technique that identifies relative differences in surface friction. It is one of several methods designed as an extension to the basic morphological mapping capabilities of scanning force microscopy. In particular, LFM is useful for classifying surface materials (Choi et al., 2007; Reitsma, 2007; Wang and Zhao, 2007; Prunici and Hess, 2008; Karhu et al., 2009). During scanning in contact mode, the cantilever bends not only vertically along the surface as a result of repulsive Van der Waals interactions, but also undergoes lateral deformation. Since this technique measures the lateral bending of the cantilever depending on the frictional force acting on the tip, this is also known as friction force microscopy. This technique has been used to identify the transitions between different components in polymer blends, composites, and other mixtures, identifying organic, and other contaminants on surfaces, delineating coverage by coatings, and other surface layers, and for chemical force microscopy using functionalized tips (Karhu et al., 2009). In orthodontics, tooth movement can be controlled by the friction between the wire and bracket. This friction results from the mechanical force applied to the tooth. The teeth and their surrounding structures respond to the force by a complex biologic cascade, and tooth movement takes places through the alveolar bone (Choi et al., 2012a). Friction is the force resisting the motion of solid surfaces or motional elements sliding against each other (Geminard and Bertin, 2010). Tooth movement comes about as the applied force goes beyond the friction induced by the interaction of orthodontic appliances. Frictional force between the wire and bracket increases with decreasing actual bone application to the tooth. This negative correlation decreases the efficiency of orthodontic treatments (Choi et al., 2012a).

2 400 SCANNING VOL. 37, 6 (2015) The main factor associated with the frictional force that determines the efficiency of orthodontic treatment (Table S1, Supporting Information) is the material of orthodontic appliances. In particular, the surface roughness of orthodontic appliance materials determines the superficial contact interaction and affects corrosion and biocompatibility. The frictional resistance between the wire and bracket has been indirectly measured by the surface roughness of orthodontic appliances using various tools. Profilometry is a fundamental technique for measuring surface roughness by scanning the topography in a line of a preselected area (Wichelhaus et al., 2005; Zinelis et al., 2005). However, one drawback is that it cannot measure the surface defects adjacent to the scanning line. In addition, this method may damage the surface of specimens during scanning (Choi et al., 2012a). Alternative methods for indirectly estimating surface roughness are scanning electron microscopy (SEM) and index scoring methods (Marques et al., 2010; Hosseinzadeh et al., 2013; Chng et al., 2014); however these techniques do not elucidate real-time changes and provide no quantitative information about the sample surfaces. On the other hand, AFM provides information regarding surface roughness and the overall surface of target materials. Our research group has reported that frictional resistance can be indirectly evaluated through the surface roughness calculated from the AFM image of clinically relevant orthodontic appliances (Lee et al., 2010a; Park et al., 2010; Choi et al., 2011; 2012a, 2012b). To the best of our knowledge, there have been no studies measuring the frictional resistance of clinically used orthodontic wires using the LFM technique. The objective of this study was to quantitatively investigate the in vivo frictional resistance on the wire surface according to four wire and bracket combinations with bicuspid-extraction treatment and anterior alignment, and to compare the indirect approach for measuring friction through contact-mode AFM topographical images surface roughness and a direct approach through contact-mode LFM frictional images lateral force (Scheme 1). collected from patients with upper anterior crowding, and the upper arch was used. The wire sections engaged with the brackets of teeth located on the lingual side about 1 2 mm from the adjacent teeth were included in this study. The inch NiTi wires were applied for four weeks in the aligning stage. The inch SS archwires (3M, Monrovia, CA) were collected from patients with first premolar-extraction treatment. Wire sections engaged with the brackets of the second premolars were cut. Orthodontic appliances were reactivated four times at 4-week intervals during enmasse retraction. All archwires were collected from patients seen in the Department of Orthodontics, Kyung Hee University School of Dentistry, Seoul, Korea. Experimental Group There were four experimental groups according to the combination of two archwires and two SLB systems (Table I): Clippy-C 1 CE-SLB þ inch NiTi archwire, Clippy-C 1 CE-SLB þ inch SS archwires, Damon 1 SS-SLB þ inch NiTi archwire, and Damon 1 SS-SLB þ inch SS archwire. To expose the surface of the wires corresponding to the second premolar region for AFM-LFM analysis, each archwire was cut at the marked three sites with a precise cutter. Each of the cut wires was rinsed with physiologic saline and dried with an air syringe. The wires were then immobilized on mica with double-sided tape and fixed to face lingual-side up to undergo frictional interaction with the bracket slot (Fig. S2, Supporting Information). AFM-LFM Measurements Contact-mode AFM topographical-deflection and LFM frictional images of the four wire groups were Methods Wire Preparation Orthodontic inch nickel-titanium (NiTi) archwires (n ¼ 15) and inch stainless steel (SS) archwires (n ¼ 15) after orthodontic use were employed. The wires (n ¼ 5 for each type) were interacted with active-type Clippy-C 1 ceramic (CE)- SLBs (Tomy, Tokyo, Japan) and passive-type Damon 1 SS-SLBs (Ormco, Orange County, CA), as shown in Table I and Fig. S1 (Supporting Information). Asreceived NiTi and SS archwires were employed as the control group (n ¼ 5 for each type). All archwires were Scheme 1. AFM topographical (Fig. 1A), LFM frictional (Fig. 1B), and AFM deflection (Fig. 1C) signals of the cantilever and the corresponding obtainable information.

3 Choi et al.: LFM analysis of AWs 401 TABLE I Specifications of the orthodontic appliances used in this study.* Property Clippy-C 1 SLB Damon 1 SLB Circular wire Rectangular wire Size (inch) Width (mm) Composition Ceramic Stainless steel NiTi Stainless steel Ligature Active Passive *Bracket indicates the dimension and composition of a slot. obtained using an NANOS N8 NEOS (Bruker, Herzogenrath, Germany) equipped with a mm 3 XYZ scanner and two Zeiss optical microscopes Epiplan 200 /500 (Carl Zeiss Inc., Standort G ottingen- Vertrieb, Germany). External noise was eliminated by placing the AFM machine on an active vibration isolation table (Table Stable Ltd., Surface Imaging Systems, Herzogenrath, Germany) inside a passive vibration isolation table (Pucotech, Seoul, Korea). The wire surface was scanned in air with a size of mm 2, a resolution of pixels, and a scan speed of 0.4 lines/s. LFM contact-mode frictional imaging was performed at room temperature and 35% relative humidity using a cantilever PPP-LFMR (NANOSEN- SORS TM, Neuchatel, Switzerland; Table S2, Supporting Information). The planification process was performed on all images. To identify the frictional and morphological changes in the surfaces of each archwire, four parameters (Table S3, Supporting Information) including mean values (Eq. S1), root-mean-square values (Eq. S2), peak-to-peak values (Eq. S3), and ten-point values (Eq. S4) were calculated using Scanning Probe Image Processor SPIP TM ver (Image Metrology, Hørsholm, Denmark). These values were represented as the average of three images per wire specimen. Statistics The quantitative data were expressed as the mean standard deviation. Statistical analysis was performed using a two-tailed Student s t-test to compare relative differences in surface roughness and frictional resistance between two groups. P-values less than 0.05 were considered statistically significant. Results Figure 1 shows representative mm 2 contactmode AFM topographical images, LFM fractional images, and AFM deflection images of the surfaces of a inch NiTi wire interacting with the Clippy-C 1 CE-SLBs. Each image clearly shows distinctive features according to operation principles. After clinical orthodontic treatment, the contact-mode AFM topographical images (Fig. 1(A)) and deflection images (Fig. 1(C)) clearly showed the nanostructure and edge of the wire surface interacting with the Clippy-C 1 CE-SLB, while the contact-mode LFM frictional image (Fig. 1(B)) showed the nanostructure as well as the presence of frictional resistance in the wire surface interacting with the Clippy-C 1 CE-SLBs. Figures 2 and 3 show representative contact-mode AFM topographical and LFM frictional images and their line profiles for a inch NiTi wire surface interacting with the Damon 1 SS-SLBs and the Clippy-C 1 CE-SLBs, respectively. The nanostructural findings of the contactmode AFM topographical images showed a different pattern compared to the LFM frictional images. The wires with sliding movement showed severe scratches caused by the fictional force between the bracket slots and wires. Contact-mode AFM topographical images showed only the morphological changes in the wire surface, while Fig. 1. Comparison of the contact-mode AFM topographical image, LFM frictional image, and AFM deflection image for the inch NiTi wire and Clippy-C 1 CE-SLB combination. au; arbitrary unit. Sz, Fz, and Dz indicate the peak-to-peak values for the topographical, frictional, and deflection images, respectively. Scale bar¼5 mm.

4 402 SCANNING VOL. 37, 6 (2015) Fig. 2. Representative contact-mode AFM topographical image (A), LFM frictional image (B), and corresponding line profiles for the inch NiTi wire and Damon 1 SS-SLB combination. Scale bar¼5 mm. contact-mode LFM frictional images showed the morphological changes as well as the presence of frictional resistance of the wire surfaces. Therefore, we could estimate position-to-position frictional information from the contact-mode LFM frictional images. From Table II, the mean surface roughness of a inch wire interacting with the Damon 1 SS-SLBs was smaller than that with the Clippy-C 1 CE-SLBs. The mean frictional resistance of a inch wire interacting with the Damon 1 SS-SLBs was smaller than that of the Clippy- C 1 CE-SLBs. This change in the inch NiTi wire showed a similar pattern to that of the inch archwire; both mean surface roughness and frictional resistance of the inch archwire interacting with the Damon 1 SS-SLBs were smaller than those of the Clippy-C 1 CE-SLBs. The change in the LFM-acquired Fig. 3. Representative contact-mode AFM topographical image (A), LFM frictional image (B), and corresponding line profiles for the inch NiTi wire and the Clippy-C 1 CE-SLB combination. Scale bar¼5 mm.

5 Quantitative analysis of frictional resistance and surface roughness according to the inch NiTi wire and inch SS wire surfaces interactions with the Damon 1 SS-SLBs and the Clippy-C 1 CE-SLBs. TABLE II Parameter inch NiTi archwires inches SS archwires Damon 1 SS-SLBs Clippy-C 1 CE-SLBs P-value Damon 1 SS-SLBs Choi et al.: LFM analysis of AWs 403 Clippy-C 1 CE-SLBs P-value AFM-Sa (nm) N.S <0.01 AFM-Sq (nm) N.S <0.01 AFM-Sz (nm) < <0.001 AFM-S10z < <0.005 (nm) LFM-Fa (au) N.S < LFM-Fq (au) < < LFM-Fz (au) N.S <0.01 LFM-F10z (au) N.S <0.01 frictional resistance was similar with that of the AFMacquired surface roughness. Overall, this result was more prominent in the inch SS archwires than in the inch NiTi archwires, and in the Clippy-C 1 CE- SLBs as compared to the Damon 1 SS-SLBs. Discussion The study first investigated the relationship between the frictional force and surface roughness of inch NiTi and inch SS archwires after orthodontic use through contact-mode AFM and LFM measurements. Since the inch NiTi (circulartype) and inch SS archwires (rectangulartype) were taken from patients treated clinically with bicuspid extraction, this study might be more clinically applicable. From the AFM-LFM findings, it is obvious that the surface of the archwires before treatment was relatively smooth (not shown), but after orthodontic treatment they showed severe scratches caused by the sliding movement of the wires. The interactions of the circular NiTi and rectangular SS archwires with the Clippy-C 1 CE-SLBs not only led to higher morphological changes but also higher frictional changes compared to the Damon 1 SS-SLBs. AFM imaging provides height information for the specimens. Therefore, the AFM provides detailed and quantitative three-dimensional structural information regarding the surface morphology with minimal sample preparation (Marques et al., 2010). Some studies have reported a difference in surface roughness according to the types and materials of orthodontic appliances using AFM investigation. In bracket systems, Lin et al. (2006) examined the surface analysis and corrosion resistance of different SS orthodontic brackets in artificial saliva. Park et al. (2010) investigated the AFM-based surface roughness of the slot surfaces for different intact brackets with SS materials. Lee et al. (2010b) examined the changes in surface roughness for some orthodontic appliances after sliding mechanics. Choi et al. (2011) investigated the surface roughness and mechanical properties of SS brackets in orthodontic treatments with bicuspid extraction. Bourauel et al. ( 98) compared the surface roughness of archwires using laser specular reflectance, profilometry, and AFM. Widu et al. (1999) examined the corrosion and biocompatibility of archwires. Choi et al. (2012a) reported on the effects of SLBs on the surfaces of SS wires after clinical use. Most studies have indirectly evaluated the relative difference in the frictional resistance of in vitro orthodontic specimens using AFM-based surface roughness findings. To the best of our knowledge, there have been no studies directly measuring the frictional resistance of orthodontic appliances used clinically applicable LFM. It is significant that this study used LFM to measure the frictional resistance of wires according to the ligation method and material of the brackets, as well as the size and type of the wire. There are two types of brackets used in SLBs: passive and active. Active-type SLBs contact the archwire and apply a force to fully seat the archwire in the bracket slot when the clip is active. When the clip is passive, it neither contacts the archwire nor applies a force to the archwire. In passive-type SLBs, the clip does not apply a ligation force to the archwire because the slide covers only the slot and hence restrains the archwire. Among the many advantages of SLBs, the most compelling is the decrease in overall treatment time resulting from a reduced frictional resistance (Damon, 98; Berger, 2008). Previous studies (Tecco et al., 2007; Budd et al., 2008; Franchi et al., 2008; Choi et al., 2012a) have reported the frictional resistances with respect to the bracket types and self-ligation method. Most studies have revealed that SLBs lead to a significant decrease in frictional force compared to conventional brackets. However, some studies (; Tecco et al., 2007; Franchi et al., 2008) reported no significant differences in the frictional force between conventional brackets and SLBs when loosened steel ligatures are used. Budd et al. (2008) showed that passive-type SLBs produced lower frictional resistance than active-type SLBs. This result

6 404 SCANNING VOL. 37, 6 (2015) was consistent with this study s findings of significantly lower surface roughness and frictional resistance in the interactions between NiTi (0.016 inch) and SS archwires ( inches) and the Damon 1 SS-SLBs (passive-type) as compared to interactions with the Clippy-C 1 CE-SLBs (active-type). Although the two SLB systems are made with different materials, this finding suggests that passive-type SLBs cause lower frictional resistance than active-type SLBs, which can reduce the orthodontic treatment time. The orthodontic appliance material is another main factor determining frictional resistance. The measurements of the friction of orthodontic archwire-bracket systems have been carried out using various tools examining surface roughness. Kusy (1988) evaluated the surface roughness and friction of orthodontic materials. SS material showed the lowest surface roughness, followed by cobalt-chrome (Co-Cr), beta-titanium (Beta-Ti), and NiTi. In terms of frictional resistance, SS<Co-Cr<NiTi<Beta-Ti. In particular, SS showed the lowest values for both properties compared to the other materials. Husain and Kumar (2011) showed that Ti brackets produced more friction than SS brackets. Lee et al. (2010b) examined the nanostructure of the bracket slots with SS and CE materials using in vitro experimental sliding tests. They revealed that polycrystalline alumina brackets (Crystalline V 1 ) and monocrystalline alumina brackets (Perfect 1 ) had a rougher surface than SS brackets (Succes 1 ), polycrystalline alumina brackets (Invu 1 ), and monocrystalline alumina brackets (Inspire Ice 1 ). In addition, more changes in surface roughness were observed in wires than in bracket slots. After the sliding test, SS brackets, SS archwires, and Beta-Ti archwires showed increased surface roughness, while CE brackets showed decreased surface roughness. In addition, the sliding test with SS archwires led to no significant changes in surface roughness between the SS and CE bracket combinations. This finding was consistent with the result of this study in that inch NiTi and inch SS wires interacting with the Clippy-C 1 (active CE- SLB) showed a higher change in morphology and frictional resistance as compared to interactions with the Damon 1 (passive SS-SLB). In summary, this study quantitatively investigated the changes in the surface roughness and frictional resistance of a inch NiTi archwire and inch SS archwire engaged with passive-type Damon 1 SLBs (SS material; Ormco, Orange County, CA) and active-type Clippy-C 1 SLBs (CE material; Tomy, Tokyo, Japan) exposed to an intraoral environment for four months of the retraction stage by sliding mechanics utilizing AFM and LFM. The following findings were observed from this study; 1. Since the changes in surface roughness showed a similar pattern to those in frictional force, an indirect method is effective in estimating the fictional force of orthodontic appliances through surface roughness. 2. A inch NiTi archwire interacting with SS- SLBs showed the least surface roughness and the lowest frictional force compared to other combinations. 3. The archwires interacting clinically with passivetype SLBs showed significantly lower surface roughness as well as lower frictional force than interactions with active-type SLBs. However, the frictional force analyzed in this study was indicated by the relative difference in surface friction. Further investigations are needed to measure the relationship between the authorized frictional force and its surface roughness by an in vitro sliding test and for calibrating the lateral force to lateral deflection. 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