The European Journal of Orthodontics Advance Access published online on November 21, 2008
The European Journal of Orthodontics, doi:10.1093/ejo/cjn053
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Conventionally ligated versus self-ligating metal brackets—a comparative study
* Department of Orthodontics, Regensburg University Medical Center
** Department of Orthodontics, Preventive and Pediatric Dentistry, Medical Center, Ernst Moritz Arndt University of Greifswald
*** Private Practice, Munich, Germany
**** Private Practice, Linz, Austria
Address for correspondence, C. A. Reicheneder, Department of Orthodontics, Medical Center, University of Regensburg, Franz-Josef-Strauss-Allee, 93053 Regensburg, Germany, E-mail:claudia.reicheneder{at}gmx.net
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Summary |
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The purpose of this study was to compare the frictional properties of four self-ligating metal brackets, Speed, Damon 2, In-Ovation, and Time, with those of three conventionally ligated metal brackets, Time, Victory Twin, and Discovery. The self-ligating Time bracket can also be used as a conventionally ligated bracket. Friction was tested 20 times for each bracket/wire combination using a Zwick testing machine with stainless steel wires in three different wire dimensions (0.017 x 0.025, 0.018 x 0.025, and 0.019 x 0.025 inches). All brackets had a 0.022 inch slot and the prescription of an upper first premolar. The data were statistically analysed with unsigned comparisons of all bracket/wire combinations using the Mann–Whitney U-test and the Games–Howell post hoc test.
The results showed almost all brackets to have the lowest frictional force with a wire dimension of 0.018 x 0.025 inch. Friction of the self-ligating brackets using wire with a dimension of 0.018 x 0.025 inches was 45–48 per cent lower than with 0.017 x 0.025 and 0.019 x 0.025 inch wires. Friction of the conventionally ligated brackets showed a 14 per cent or less reduced friction with 0.018 x 0.025 inch wire compared with 0.017 x 0.025 and 0.019 x 0.025 inch wires. The self-ligating metal brackets showed lower frictional forces with a 0.018 x 0.025 inch wire than conventionally ligated brackets, whereas conventionally ligated brackets showed lower friction with 0.017 x 0.025 and 0.019 x 0.025 inch wire.
Friction values vary with different bracket/archwire combinations and, therefore, the choice of a bracket system for treatment should consider the correct wire dimension to produce the lowest possible frictional forces.
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Introduction |
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Friction between the bracket and archwire has gained importance since the increased use of sliding mechanics that followed the development of the pre-adjusted edgewise systems. Since friction reduces the effectiveness of tooth movement along the wire, significant efforts were made to lower friction in orthodontics.
Frictional resistance between archwire and brackets is determined by many factors and varies with wire to bracket angulation (Andreasen and Quevedo, 1970
; Dickson et al., 1994), archwire size and material (Angolkar et al., 1990; Ireland et al., 1991), mode of ligation (Bednar et al., 1991; Sims et al., 1993; Taylor and Ison, 1996), biological resistance (Drescher et al., 1989), saliva (Kusy et al., 1991; Downing et al., 1995), and bracket width (Frank and Nikolai, 1980; Drescher et al., 1989). Drescher et al. (1989) considered bracket width to play an inferior role in frictional forces. Since the mid-1970s, the search for a bracket system with a low frictional resistance resulted in a renewed interest in the development of self-ligating brackets. Two different types of self-ligating brackets were produced: active brackets, that feature a spring clip actively pressing against the archwire, e.g. the Speed bracket, and passive brackets, whose self-ligating clip does not press against the wire, e.g. the Damon bracket.
According to the manufacturers, these self-ligating brackets should not only be easier to handle in the ligation process but also show lower frictional forces than conventionally ligated brackets. Thus, the following question arises: are the frictional properties of self-ligating brackets better than those of conventionally ligated brackets with all wire dimensions, or do their frictional properties vary with different bracket/archwire combinations.
Therefore, the purpose of this study was to compare the frictional properties of four self-ligating and three conventionally ligated metal brackets by determining the force required to pass three standard clinical archwires through these brackets in vitro.
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Materials and methods |
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Bracket systems and wires
Four types of self-ligating metal brackets, Speed (Strite Industries Ltd, Cambridge, Ontario, Canada), Damon 2 (Ormco, Orange, California, USA), In-Ovation (GAC Int., Bohemia, New York, USA), and Time (Adenta, Gilching, Germany), as well as the three different types of conventionally ligated metal brackets Time (Adenta), Victory Twin (3M Unitek, Monrovia, California, USA), and Discovery (Dentaurum J. P. Winkelstroeter KG, Ispringen, Germany) were tested. While the self-ligating brackets, Speed, In-Ovation, and Time, are interactive brackets, the Damon 2 is a non-interactive so-called passive self-ligating bracket. The Time bracket can be used either as self-ligating or conventionally ligated bracket. The bracket specifications are summarized in Table 1.
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Ten brackets of each type were ligated to rectangular stainless steel (SS) archwires (Dentaurum) that had three different dimensions 0.017 x 0.025, 0.018 x 0.025, and 0.019 x 0.025 inches and came from plain strands of wire. The conventional brackets were ligated with elastic modules (Dentalastics, Dentaurum) in order to prevent individual differences in forces resulting from ligature wires. All brackets, except Damon 2, used in this study had a 0.022 x 0.028 inch slot and the prescription of an upper first premolar of the Roth system (tip = 0 degree; torque = –7 degrees). The Damon 2 bracket for an upper first premolar had a 0.022 x 0.027 inch slot, a torque of –7 degrees and tip of +2 degrees.
The tolerance of slot and wire sizes is usually the responsibility of the manufacturers’ quality control management. Random testing of the slot and wire sizes indicated by the manufacturers failed to reveal serious deviations.
Experimental set-up
The brackets were bonded centrally onto a round metal base that had been sandblasted to improve retention. To ensure correct positioning of the wire–bracket couples on the metal base, the following technique was used: two identical standard edgewise 0.022 inch brackets (tip = 0 degree; torque = 0 degree) were bonded onto a flat aluminium plate in a straight line. This position was secured by ligating these brackets to a straight piece of a 0.022 x 0.025 inch SS wire before bonding. The plate was mounted centrally to the model table of the milling machine ‘Degussa F2’ (Degudent, Hanau, Germany). Each round metal base was fixed in the milling machine and adjusted in such a way that its centre corresponded with the centre of the tested bracket. Bracket bases were supplied with the primer Exite (Ivoclar Vivadent, Ellwangen, Germany) and the light curing composite Tetric Flow (Ivoclar Vivadent). The metal base-holding part of the milling machine was then lowered vertically towards the bracket base ensuring parallelism between the metal base surface and bracket–wire couple. Finally, the composite was cured for 60 seconds with a halogen light, Ortholux XT (3M Unitek).
Friction was tested with a universal test machine (Model 1446, Zwick, Ulm, Germany). This experimental set-up consisted of a metal framework (Figure 1), which allowed rotation of the metal base bonded to the bracket/wire unit in order to simulate the continuous tipping–uprighting sliding movement of bonded teeth. Two guide rollers were placed above and below the metal base to guide the movement of the wire. A 12 mm piece of metal was attached to this metal base, from which a 250 g weight was suspended to increase wire binding at the edges of the bracket during sliding and therefore simulate the clinical situation of an angulation effect.
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Each rectangular SS wire was pulled through twice with a constant velocity of 12.7 mm/minute (Bednar et al., 1991
). Other studies using different speeds ranging from 0.5 to 50 mm/minute showed no significant differences in friction measurements (Ireland et al., 1991; Taylor and Ison, 1996). Each bracket was pulled 10 mm along the wire and the maximum frictional forces were measured. Altogether, 60 tests per bracket type were carried out.
Statistical analysis
Statistical analysis was undertaken with the Statistical Package for Social Sciences, Version 12.0 for Windows (SPSS Inc., Chicago, Illinois, USA), and the results were considered as significant at P 
0.05. With the chosen sample size of n = 20 per unit of analysis (bracket/archwire combination), a minimum statistical power of 0.80 was estimated for each two-sided comparison. The data were presented graphically by box and whisker plots using SigmaPlot 10.0 (Systat Software GmbH, Erkrath, Germany). Normal distribution of the data was tested using the Kolmogorov–Smirnov test, and the homogeneity of variance with Levene's test. Since the data showed normal distribution but no homogeneity of variance, frictional forces were evaluated with unsigned comparisons of all bracket-wire combinations using Mann–Whitney's U-test and analysed post hoc using the procedure of Games–Howell for control of the multiple comparisons.
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Results |
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Table 2 summarizes friction data and mean frictional forces of the four self-ligating and three conventionally ligated metal brackets for the three different wire dimensions. Pairwise comparisons between all wire dimensions and bracket types were calculated using the Mann–Whitney U-test (Table 3). The results showed that the frictional properties of the different brackets depended mainly on the different wire dimensions. As visualized by the boxplots (Figure 2), the self-ligating brackets showed lower frictional values than the conventionally ligated brackets only with 0.018 x 0.025 inch SS wire. With the wire dimensions 0.017 x 0.025 and 0.019 x 0.025 inch, the conventionally ligated brackets demonstrated lower frictional forces than the self-ligating brackets.
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Almost all brackets showed the lowest frictional values with 0.018 x 0.025 inch wire. The only exception was the conventionally ligated Time bracket. The lowest friction for this bracket was found in combination with 0.019 x 0.025 inch wire.
A comparison of the pooled group of self-ligating brackets with the pooled group of conventionally ligated brackets at the three different wire dimensions revealed that friction was higher in the self-ligating group with 0.017 x 0.025 and 0.019 x 0.025 inch SS wires than in the conventionally ligated group (Table 4). Self-ligating brackets had the lowest frictional values with 0.018 x 0.025 inch wire, with friction being lower than with the conventionally ligated brackets.
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Self-ligating brackets in combination with 0.018 x 0.025 inch wire showed a 45–48 per cent reduction in friction (calculated from the mean; Table 4) in comparison with wire dimensions of 0.017 x 0.025 and 0.019 x 0.025 inches. With the conventionally ligated brackets with 0.017 x 0.025 and 0.019 x 0.025 inch wires, differences in frictional forces amounted to only about 14 per cent or less in comparison with the 0.018 x 0.025 inch wire (calculated from the mean; Table 4).
In the group with 0.018 x 0.025 inch wire, the self-ligating brackets showed a median frictional force of 6.1 N, a maximum of 13.8 N, and a minimum of 2.1 N. For the conventionally ligated brackets with the same wire dimension, the frictional forces were 7.1, 20.0, and 2.5 N, respectively. With 0.019 x 0.025 inch wire, the median of the self-ligating brackets was 11.9 N while that of the conventionally ligated brackets amounted only to 7.9 N. With 0.017 x 0.025 inch wire, the median of the self-ligating brackets was 11.1 N and that of the conventionally ligated brackets 9.4 N.
The Time bracket can be used as a self-ligating bracket with a clip, if for any reasons the clip is lost, as a conventionally ligated bracket and therefore allows direct comparison of friction. Figure 3 shows the frictional forces of the conventionally ligated compared with the self-ligating Time bracket at the different wire dimensions. The self-ligating Time bracket showed the largest frictional differences of all self-ligating brackets with the three different wire dimensions. The medians were 9.8 N for 0.017 x 0.025, 5.1 N for 0.018 x 0.025, and 13.6 N for 0.019 x 0.025 inch wire. In contrast, using the Time bracket as a conventionally ligated bracket with an elastic ligature yielded a markedly smaller range of mean frictional forces at the different wire dimensions varying between 5.7 N with a 0.019 x 0.025 inch, 7.1 N with 0.018 x 0.025 inch, and 7.6 N with 0.017 x 0.025 inch. However, the standard deviations in general were higher than those of the self-ligating Time bracket. The difference in frictional force between the self-ligating and conventionally ligated Time bracket was significant with wires with a dimension of 0.018 x 0.025 (P = 0.0003) and 0.019 x 0.025 inches (P = 0.0000).
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Discussion |
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This laboratory study was designed to compare the friction produced by self-ligating and conventionally ligated metal brackets at different wire dimensions.
Certain wire grades and sizes are recommended by the manufacturers for various reasons.
Data for self-ligating and conventional brackets were pooled. As most self-ligating brackets were somewhat narrower compared with the conventional brackets, precautions were taken to minimize confounding bracket type and width. Therefore, the In-Ovation bracket was chosen in its regular form instead of its narrower form (In-Ovation R). Similarly, the Time system was examined both in self-ligating and conventional mode. However, the significance of bracket width for friction is controversial and was considered of secondary importance (Frank and Nikolai, 1980; Drescher et al., 1989).
Pooling conventional and self-ligating brackets also resulted in a mixture of active and passive (Damon 2) clip mechanism in the latter group. Data inspection shows, however, that dropping Damon 2 from this group would increase the mean differences from the conventional group.
To take account of the above-mentioned limitations of pooling groups, significance testing was restricted to pairwise inter-bracket comparisons.
The results showed that the self-ligating brackets had lower frictional values only with 0.018 x 0.025 inch SS wire. With wire dimensions of 0.017 x 0.025 and 0.019 x 0.025 inches, the conventionally ligated brackets showed lower frictional forces than the self-ligating brackets. A number of authors have demonstrated that self-ligating brackets show reduced frictional forces compared with conventionally ligated brackets (Sims et al., 1993; Shivapuja and Berger, 1994; Kapur et al., 1998; Pizzoni et al., 1998; Thomas et al., 1998; Thorstenson and Kusy, 2001; Taylor and Ison, 1996). The present study demonstrates that friction values of self-ligating and conventionally ligated brackets depend mainly on the different wire dimensions. Therefore, information concerning the frictional values of brackets should always consider wire dimensions and testing conditions (e.g. angulation).
Similar experiments (Henao and Kusy, 2004, 2005) demonstrated the effects of alternative wire sizes recommended by the manufacturers. However, compared with the present study those authors used a different experimental method, testing friction in quadrants of typodont models.
Tipping is a constant phenomenon during sliding tooth movements. Thus, teeth will tip until contact is established between the archwire and the diagonally opposite corners of the bracket wings. According to the previously described model for measuring friction, this study, like that of Bednar et al. (1991), permitted rotation of the bracket and wire in order to simulate this clinical condition.
Similar to the present experimental set-up, a number of trials included a second-order angulation (Bednar et al., 1991; Sims et al., 1994; Read-Ward et al., 1997; Pizzoni et al., 1998; Thorstenson and Kusy, 2001). According to Pizzoni et al. (1998), friction increases with angulation for all bracket/wire combinations. For this reason, the frictional forces found in this study are much higher than, for instance, those reported by Kapur et al. (1998), who measured friction without angulation. In the present investigation, torque effects (third-order angulation) were not simulated. Even though torque increases friction in clinical situations, only a few studies simulating this effect are found in literature (Drescher et al., 1991; Bourauel et al., 1992).
The results found by Bednar et al. (1991) were mostly confirmed under similar testing conditions. Simulating the clinical situation in which teeth tip slightly while they slide along the archwire, they found that self-ligating steel brackets did not demonstrate less friction than elastic or SS ligated brackets.
The variability of friction among self-ligating brackets is probably due to the different clip mechanisms. While the Speed, In-Ovation, and self-ligating Time bracket feature an active clip mechanism, the Damon 2 bracket has a passive mechanism.
The Time bracket can be used as a self-ligating bracket or as a conventionally ligated bracket, thereby allowing direct comparison of friction. The self-ligating Time bracket showed the largest frictional differences of all self-ligating brackets between the three different wire dimensions. This might be due to the effect of tilting, which is much lower with self-ligating brackets and with smaller wire dimensions. Using the Time bracket as a conventionally ligated bracket with an elastic ligature yielded different frictional forces, showing a smaller range among the different wire dimensions.
In this study, friction was tested under dry conditions. The effect of lubrication by saliva on friction is controversial. Andreasen and Quevedo (1970) claimed that saliva played an insignificant role, while Read-Ward et al. (1997) concluded that the presence of human saliva had an inconsistent effect on static friction and sliding mechanics. Baker et al. (1987) found that saliva acted as a lubricant, while Stannard et al. (1986) and Downing et al. (1995) reported that saliva increased friction. Kusy et al. (1991) regarded artificial saliva as inadequate for replacement of human saliva and hence such experiments as invalid. Thus, in the present investigation, the wire–bracket couples were tested under dry conditions.
All three conventional metal brackets were ligated with elastomeric modules in this study. Tying with SS ligatures was found to vary both inter- and intraindividually. Since Schumacher et al. (1990) reported a considerable variation of pressure between 2 and 8 N with 0.011 inch SS ligatures, elastic ligatures were used in the present study in order to minimize variations and to standardize the ligation process.
Generally, friction appears to increase with archwire diameter (Angolkar et al., 1990), a finding that could not be supported by the results of the present investigation. For nearly all bracket types, the 0.018 x 0.025 inch SS wire produced lower friction than the 0.017 x 0.025 and 0.019 x 0.025 inch wire. Self-ligating brackets combined with a 0.018 x 0.025 inch wire showed a 45–47 per cent reduction in friction compared with wire dimensions of 0.017 x 0.025 and 0.019 x 0.025 inches. With the conventionally ligated brackets, the difference in frictional force between 0.017 x 0.025 and 0.019 x 0.025 inch wires in comparison with 0.018 x 0.025 inch wire was approximately 14 per cent or less. Wire dimension had a lower influence on friction with conventionally ligated brackets than with self-ligating brackets. Therefore, the choice of a bracket system for patient treatment should always consider the correct wire dimension to produce the lowest possible frictional force.
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Conclusions |
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TopSummaryIntroductionMaterials and methodsResultsDiscussionConclusionsReferences |
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- Nearly all brackets showed the lowest frictional forces with SS wire with a dimension of 0.018 x 0.025 inches, except the conventionally ligated Time bracket.
- Friction of the self-ligating brackets was 45–48 per cent lower with 0.018 x 0.025 inch wire compared with 0.017 x 0.025 and 0.019 x 0.025 inch wire. With conventionally ligated brackets, there was a 14 per cent or less reduction of friction using 0.018 x 0.025 inch wire.
- Friction of the self-ligating brackets was lower only with 0.018 x 0.025 inch wire. With wire dimensions of 0.017 x 0.025 and 0.019 x 0.025 inches, friction of the conventionally ligated brackets was lower. There was a significant difference (P < 0.001) in friction between the self-ligating and conventionally ligated brackets with all wire dimensions. Wire dimension had a strong influence on the friction of self-ligating brackets. With conventionally ligated brackets, wire dimension had a much lower influence.
- The highest frictional differences were observed with the Time bracket. Friction with the 0.018 x 0.025 inch wire was significantly less (P < 0.001) and with 0.019 x 0.025 inch wire significantly greater (P < 0.001) than in conventionally ligated brackets.
- Wire dimension and testing apparatus had an influence on friction measured in vitro.
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Acknowledgement |
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The authors are grateful to M. Rosentritt, Department of Prosthodontics, Regensburg University Medical Center, for his support in handling the different machines during the tests. We also wish to thank all mentioned companies for supplying the test brackets and materials used.
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References |
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Andreasen GF, Quevedo FR. Evaluation of friction forces in the 0.022 x 0.028 edgewise bracket in vitro. Journal of Biomechanics (1970) 3:151–160.[CrossRef][Web of Science][Medline]
Angolkar PV, Kapila S, Duncanson Jr MG, Nanda RS. Evaluation of friction between ceramic brackets and orthodontic wires of four alloys. American Journal of Orthodontics and Dentofacial Orthopedics (1990) 98:499–506.[Web of Science][Medline]
Baker KL, Nieberg LG, Weimer AD, Hanna M. Frictional changes in force values caused by saliva substitution. American Journal of Orthodontics and Dentofacial Orthopedics (1987) 91:316–320.[CrossRef][Web of Science][Medline]
Bednar JR, Gruendeman GW, Sandrik JL. A comparative study of frictional forces between orthodontic brackets and arch wires. American Journal of Orthodontics and Dentofacial Orthopedics (1991) 100:513–522.[Web of Science][Medline]
Bourauel C, Drescher D, Thier M. An experimental apparatus for the simulation of three-dimensional movements in orthodontics. Journal of Biomedical Engineering (1992) 14:371–378.[CrossRef][Web of Science][Medline]
Dickson JA, Jones SP, Davies EH. A comparison of the frictional characteristics of five initial alignment wires and stainless steel brackets at three bracket to wire angulations—an in vitro study. British Journal of Orthodontics (1994) 21:15–22.[Abstract]
Downing A, McCabe JF, Gordon PH. The effect of artificial saliva on the frictional forces between orthodontic brackets and archwires. British Journal of Orthodontics (1995) 22:41–46.[Abstract]
Drescher D, Bourauel C, Schumacher HA. Frictional forces between bracket and arch wire. American Journal of Orthodontics and Dentofacial Orthopedics (1989) 96:397–404.[CrossRef][Web of Science][Medline]
Drescher D, Bourauel C, Thier M. Application of the orthodontic measurement and simulation system (OMSS) in orthodontics. European Journal of Orthodontics (1991) 13:169–178.
Frank CA, Nikolai RJ. A comparative study of frictional resistances between orthodontic bracket and arch wire. American Journal of Orthodontics (1980) 78:593–609.[CrossRef][Web of Science][Medline]
Henao SP, Kusy RP. Evaluation of the frictional resistance of conventional and self-ligating bracket designs using standardized archwires and dental typodonts. Angle Orthodontist (2004) 74:202–211.[Web of Science][Medline]
Henao SP, Kusy RP. Frictional evaluations of dental typodont models using four self-ligating designs and a conventional design. Angle Orthodontist (2005) 75:75–85.[Web of Science][Medline]
Ireland AJ, Sherriff M, McDonald F. Effect of bracket and wire composition on frictional forces. European Journal of Orthodontics (1991) 13:322–328.
Kapur R, Sinha PK, Nanda RS. Frictional resistance of the Damon SL bracket. Journal of Clinical Orthodontics (1998) 32:485–489.[Medline]
Kusy RP, Whitley JQ, Prewitt MJ. Comparison of the frictional coefficients for selected archwire-bracket slot combinations in the dry and wet states. Angle Orthodontist (1991) 61:293–302.[Web of Science][Medline]
Pizzoni L, Ravnholt G, Melsen B. Frictional forces related to self-ligating brackets. European Journal of Orthodontics (1998) 20:283–291.
Read-Ward GE, Jones SP, Davies EH. A comparison of self-ligating and conventional orthodontic bracket systems. British Journal of Orthodontics (1997) 24:309–317.[Abstract]
Schumacher HA, Bourauel C, Drescher D. Das Friktionsverhalten von Keramikbrackets bei der bogengeführten Zahnbewegung. Fortschritte der Kieferorthopädie (1990) 51:259–265.[CrossRef][Medline]
Shivapuja PK, Berger J. A comparative study of conventional ligation and self-ligation bracket systems. American Journal of Orthodontics and Dentofacial Orthopedics (1994) 106:472–480.[Web of Science][Medline]
Sims AP, Waters NE, Birnie DJ, Pethybridge RJ. A comparison of the forces required to produce tooth movement in vitro using two self-ligating brackets and a pre-adjusted bracket employing two types of ligation. European Journal of Orthodontics (1993) 15:377–385.
Sims AP, Waters NE, Birnie DJ. A comparison of the forces required to produce tooth movement ex vivo through three types of pre-adjusted brackets when subjected to determined tip or torque values. British Journal of Orthodontics (1994) 21:367–373.[Abstract]
Stannard JG, Gau JM, Hanna MA. Comparative friction of orthodontic wires under dry and wet conditions. American Journal of Orthodontics (1986) 89:485–491.[Medline]
Taylor NG, Ison K. Frictional resistance between orthodontic brackets and archwires in the buccal segments. Angle Orthodontist (1996) 66:215–222.[Web of Science][Medline]
Thomas S, Sherriff M, Birnie D. A comparative in vitro study of the frictional characteristics of two types of self-ligating brackets and two types of pre-adjusted edgewise brackets tied with elastomeric ligatures. European Journal of Orthodontics (1998) 20:589–596.
Thorstenson GA, Kusy RP. Resistance to sliding of self-ligating brackets versus conventional stainless steel twin brackets with second-order angulation in the dry and wet (saliva) states. American Journal of Orthodontics and Dentofacial Orthopedics (2001) 120:361–370.[CrossRef][Web of Science][Medline]
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