There has been increased use of self-ligating. Friction between various self-ligating brackets and archwire couples during sliding mechanics

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1 ORIGINAL ARTICLE Friction between various self-ligating brackets and archwire couples during sliding mechanics Sennay Stefanos, a Antonino G. Secchi, b Guy Coby, c Nipul Tanna, a and Francis K. Mante d Philadelphia, Pa Introduction: The aim of this study was to evaluate the frictional resistance between active and passive selfligating brackets and in stainless steel archwire during sliding mechanics by using an orthodontic sliding simulation device. Methods: Maxillary right first premolar active self-ligating brackets In-Ovation R, In-Ovation C (both, GAC International, Bohemia, NY), and SPEED (Strite Industries, Cambridge, Ontario, Canada), and passive self-ligating brackets SmartClip (3M Unitek, Monrovia, Calif), Synergy R (Rocky Mountain Orthodontics, Denver, Colo), and Damon 3mx (Ormco, Orange, Calif) with in slots were used. Frictional force was measured by using an orthodontic sliding simulation device attached to a universal testing machine. Each bracket-archwire combination was tested 30 times at 0 angulation relative to the sliding direction. Statistical comparisons were performed with 1-way analysis of variance (ANOVA) followed by Dunn multiple comparisons. The level of statistical significance was set at P \0.05. Results: The Damon 3mx brackets had significantly the lowest mean static frictional force (8.6 g). The highest mean static frictional force was shown by the SPEED brackets (83.1 g). The other brackets were ranked as follows, from highest to lowest, In-Ovation R, In-Ovation C, SmartClip, and Synergy R. The mean static frictional forces were all statistically different. The ranking of the kinetic frictional forces of bracket-archwire combinations was the same as that for static frictional forces. All bracket-archwire combinations showed significantly different kinetic frictional forces except SmartClip and In-Ovation C, which were not significantly different from each other. Conclusions: Passive self-ligating brackets have lower static and kinetic frictional resistance than do active self-ligating brackets with in stainless steel wire. (Am J Orthod Dentofacial Orthop 2010;138:463-7) There has been increased use of self-ligating brackets in recent years, and several claims have been made regarding their performance compared with conventional edgewise brackets. 1 Recent manufacturing modifications of brackets aimed at reducing friction between the archwire and the bracket slot are based on improved surface quality, slot geometry modifications, varied bracket width, and integrated ligation systems. 1 It has been reported that friction is determined largely by the nature of the ligation. 2,3 Therefore, various ligation systems have been introduced with the edgewise bracket to secure From the School of Dental Medicine, University of Pennsylvania, Philadelphia. a Postgraduate student, Department of Orthodontics. b Assistant professor, Department of Orthodontics. c Clinical assistant professor, Department of Orthodontics. d Associate professor, Department of Restorative and Preventive Sciences. The first author worked on this research while a resident at the Department of Orthodontics of the University of Pennsylvania. The views expressed in this article are those of the author and do not reflect the official policy or position of the Department of the Navy, the Department of Defense, or the United States government. Reprint requests to: Antonino G. Secchi, 240 S 40th St, Evans Building, room E-9, Philadelphia, PA ; , antonino@dental.upenn.edu. Submitted, August 2008; revised and accepted, November /$36.00 Copyright Ó 2010 by the American Association of Orthodontists. doi: /j.ajodo the wire in the bracket. Some self-ligating brackets have a spring clip that presses against the archwire, such as In-Ovation R and In-Ovation C (GAC International, Bohemia, NY) and SPEED (Strite Industries, Cambridge, Ontario, Canada). These brackets have been described as active self-ligating because there might be a ligation force during sliding mechanics. 4 Other self-ligating brackets such as Damon 3mx (Ormco, Orange, Calif), SmartClip (3M Unitek, Monrovia, Calif), 5,6 and Synergy R (Rocky Mountain Orthodontics, Denver, Colo) have a clip-gate that does not press against the wire during sliding mechanics. 7 These are described as passive self-ligating brackets. Self-ligating brackets have been reported to have lower frictional forces during sliding mechanics and require less chair-side assistance. 4,8 They are generally smoother, more comfortable for the patient, and easier to clean. 9 According to Thorstenson and Kusy, 10,11 the lower friction of self-ligating brackets might be partly explained by a greater critical contact angle with the wire. There are conflicting claims of superior performance regarding friction for both passive and active self-ligating brackets. 8,12,13 This conflict might be due to the wide variations in data reported in previous studies. Some studies show wide ranges between 463

2 464 Stefanos et al American Journal of Orthodontics and Dentofacial Orthopedics October 2010 Fig 2. Force vs displacement plot for static and kinetic friction. Fig 1. The orthodontic sliding simulation device used in this study. minimum and maximum values, 2,12,14 and others have reported mean frictional force values with high standard deviations, making it difficult to determine whether there are significant differences between brackets. 9,15,16 In orthodontics, research efforts to understand the factors that influence frictional resistance when considering sliding mechanics have been focused on bracket width, archwire material, archwire size, second-order angulation, 18,20-23 ligation type and technique, 2,6-8,13-17 effect of saliva, 23,24 and interbracket distance. These factors are critical when considering the clinical application of sliding mechanics. 15 According to Pizzoni et al, 25 experimental setups to determine the effect of the above-mentioned factors on friction can be divided into 4 main groups: (1) archwires sliding through contact flats; (2) archwires sliding through brackets parallel to the brackets slot; (3) archwires sliding through brackets with different second- and third-order angulations; and (4) brackets submitted to a force with a certain degree of tipping allowed. The experimental design of this study provided for archwires sliding through brackets parallel to the bracket slot; it falls into the second group. Our hypothesis was that there is no significant difference in the resistance to sliding of a in stainless steel archwire between active and passive self-ligating brackets. The purpose of this study was to compare the static and kinetic frictional forces generated between active and passive self-ligating brackets by using a sliding simulator coupled with a 10-N load cell. MATERIAL AND METHODS An orthodontic sliding simulation device modified from that reported by Articolo and Kusy 19 and Articolo 26 was used to simulate the clinical use of orthodontic brackets (Fig 1). The simulation device consisted of a special fixture mounted to the base of a mechanical testing machine (model 4206, Instron, Canton, Mass). The fixture held a bracket slot that allowed for reproducible bracket positioning and was attached to an angulation dial. A test archwire was suspended from a collet connected to the force transducer and the transverse beam of the testing machine. The orthodontic device was modified for self-ligating brackets by removing in of ligature wire attached to a piston assembly to transmit a normal force. The weight of the collet holding the archwire was reduced to minimize noise in data collection. The brackets were cemented onto the simulation device bracket slot and initially set to be passive (0 ) in the second order via the adjustable angulation dial. All tested brackets had 7 of torque and 0 of secondorder angulations, with the exception of Damon 3mx, which has 2 of distal offset. The 2 distal offset of the Damon 3mx bracket was compensated for by using the angulation dial. All angulations and torque values of the brackets remained fixed during data collection. During testing, the transverse beam with the collet holding a in stainless steel archwire was lifted up to draw the archwire through the bracket. The drawing force was monitored by the machine s load cell (10 N) and transmitted to computer software (version 2.0, Measure, National Instruments, Austin, Tex) for plotting drawing force vs distance charts. Maxillary right first premolar brackets with in slots and prescriptions as described above were used. Each archwire-bracket couple was cleaned with 95% ethanol and compressed air just before evaluation. All testing was done in the dry state in prevailing air at 21 C. Each test consisted of 1 bracket and 1 archwire at 0 angulation. Two examiners (N.T. and S.S.) verified proper mounting of the brackets under 10-times magnification. The static frictional force was measured as the initial rise or peak force required to initiate movement

3 American Journal of Orthodontics and Dentofacial Orthopedics Stefanos et al 465 Volume 138, Number 4 Table I. Descriptive statistics and statistical comparisons of static frictional forces Table II. Descriptive statistics and statistical comparisons of kinetic frictional forces Bracket Mean SD Median Minimum Maximum Bracket Mean SD Median Minimum Maximum Damon 3mx In-Ovation R In-Ovation C SmartClip SPEED Synergy R All mean static friction values are significantly different from each other (P \0.05). Damon 3mx In-Ovation R In-Ovation C 28.8 a SmartClip 30.1 a SPEED Synergy R Mean kinetic friction values with the same superscript letter are not significantly different (P \0.05). of the wire through the bracket (Fig 2). The peak force was halved and defined as static frictional force. 27 A new bracket-archwire combination was used for each test. Each test was performed 30 times. The drawing force required to maintain movement beyond the point of initial displacement was averaged and then halved and recorded as the kinetic frictional force. The archwire was drawn through the bracket a distance of 20 mm at a speed of 1 cm per minute for determination of kinetic friction. Data were obtained at a rate of 5 scans per second for 2 minutes. The 1-way analysis of variance (ANOVA) and the Dunn multiple comparison tests were performed with statistical software (version 3.5, SigmaStat, Systat Software, Point Richmond, Calif). The level of statistical significance was set at P \0.05. RESULTS A plot of force vs displacement obtained during friction testing is shown in Figure 2. The region for determining static friction was designated static friction, and regions for determination of kinetic friction were called kinetic friction. Table I shows the results of the mean static frictional forces for the brackets investigated. Statistical analysis showed that passive self-ligating brackets ranked as follows from lowest to highest mean static friction: Damon3mx (8.6 g), Synergy R (23.8 g), and SmartClip (30.3 g). The static friction of the passive self-ligating brackets was significantly different from each other. Among the active self-ligating brackets, the ranking from lowest to highest static friction was In-Ovation C (33.4 g), In-Ovation R (38.1 g), and SPEED (83.1g). The active self-ligating brackets showed significantly higher static frictional forces than the passive self-ligating brackets. The kinetic frictional force values are shown in Table II. The ranking of the kinetic frictional forces of passive and active self-ligating brackets to archwire combinations was the same as that for the static frictional forces. Kinetic friction values for all bracket-archwire combinations were significantly different, except for SmartClip and In-Ovation C. DISCUSSION These results show significant differences in both static and kinetic frictional forces among passive and active self-ligating brackets with a in stainless steel archwire. The exception to this finding was that kinetic frictional forces associated with Smart- Clip, a passive self-ligating bracket, were not statistically different from those of In-Ovation C, an active self-ligating bracket. Among the passive self-ligating brackets investigated, Damon3mx had the lowest and SmartClip the highest static and kinetic frictional forces. The results further showed significant differences between active self-ligating brackets, with SPEED having the highest static and kinetic frictional forces. Our findings agree with those of previous studies, that passive self-ligating brackets generate lower static and kinetic frictional forces than do active self-ligating brackets. 9,25,28 The similarity between the kinetic frictional forces generated by SmartClip and In-Ovation C is an interesting result. SmartClip has been described as a programmed nickel-titanium clip that releases the wire if the force of ligation needed to keep the wire in the slot exceeds a certain limit. 5,29 This phenomenon was not observed in this study. The classification of SmartClip as a passive self-ligating bracket might hold true when the archwire size is smaller than in. 5 At higher archwire sizes, some ligation force might be exerted. 12 Based on these results, it is questionable to classify SmartClip as a passive selfligating bracket for all archwire sizes. The relatively low static and kinetic frictional force values observed for In-Ovation C (an active self-ligating bracket) might be explained by the rhodium coating on the clip, according to the manufacturer s information. However, the

4 466 Stefanos et al American Journal of Orthodontics and Dentofacial Orthopedics October 2010 stability of the coating on In-Ovation C brackets during treatment is unknown. The simulation device coupled with a 10-N load cell provided friction values with relatively low standard deviations and a low spread of values between minimum and maximum friction (Tables I and II) compared with other studies. 2,9,12,14,16 The data obtained in this laboratory study do not describe the complete complex clinical situation. Since the ligation force of self-ligating brackets is predictable and independent of force decay, the comparative data obtained are useful for guiding the selection of wirebracket combinations for sliding mechanics. 1 The Damon3mx bracket showed significantly the lowest frictional force compared with the other brackets tested. This bracket will be expected to show less friction in sliding mechanics with rectangular in stainless steel wire. Conversely, it might be difficult to fully express the bracket s prescription if used with in stainless steel as a finishing wire. The SPEED bracket generated the highest static and kinetic frictional forces. SPEED brackets could more favorably express the bracket prescription if used with in stainless steel wire. On the contrary, SPEED brackets might not favor sliding mechanics. An important aspect to consider when evaluating bracket design is the normal (perpendicular) force of ligation. 2,3 In most studies, the frictional force decreases as the normal ligation force is minimized. 11,14,15 Different methods of ligation that have been introduced with edgewise brackets have resulted in varying normal forces and their corresponding frictional forces. Frictional forces are important to study because a large, variable percentage of the force applied by the orthodontist is lost to overcome friction instead of moving teeth. 26,30 Information about the friction of orthodontic brackets and archwire systems is important for improving the effectiveness of orthodontic treatment. The selection of brackets should be based on the desired clinical outcome. Low frictional forces might be desired during leveling and aligning but could be inappropriate for expressing the torque in the bracket or achieving other objectives of finishing and detailing. Likewise, high frictional forces might be desired for expressing torque in the bracket or finishing and detailing but be inappropriate for the leveling and aligning stages of treatment. CONCLUSIONS This study confirmed that passive self-ligating brackets have lower static and kinetic frictional forces compared with active self-ligating brackets when coupled with in stainless steel wire. The Damon3mx bracket has significantly the lowest static and kinetic frictional forces, and SPEED has the highest frictional force of the brackets investigated. The simulation device and low threshold load cell provided data with lower measurement variations when comparing differences in friction of sliding. We thank 3M Unitek, GAC International, Strite Industries, Ormco, and Rocky Mountain Orthodontics for providing the materials tested in this study; Robert L. Vanarsdall, Jr, Department of Orthodontics, School of Dental Medicine, University of Pennsylvania, for support and advice in designing the research; Alex Radin, Department of Materials Science and Engineering, School of Engineering, University of Pennsylvania, and Laurence Articolo, orthodontist of Blackwood, NJ for their technical expertise in the construction of the sliding simulation device. REFERENCES 1. Gostovich LT. The influence of bracket design on resistance to sliding [thesis]. 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