- Email: [email protected]
Forces exerted by conventional and self-ligating brackets during simulated first- and second-order corrections
SHORT COMMUNICATION
Forces exerted by conventional and self-ligating brackets during simulated first- and second-order corrections Nikolaos Pandis,a Theodore Eliades,b Samira Partowi,a and Christoph Bourauelc Bonn, Germany, and Thessaloniki, Greece Introduction: Our aim in this study was to comparatively assess the forces generated from conventional and self-ligating bracket systems during the late leveling and alignment stage, specifically for first- and second-order movement. Methods: Three types of brackets were selected: Orthos2 (Ormco, Glendora, Calif), Damon2 (Ormco), and In Ovation-R (GAC, Bohemia, NY). The brackets were bonded on resin replicas constructed from a model of an aligned mandibular arch, and a 0.014 ⫻ 0.025-in copper-nickel-titanium wire (Ormco) was placed. First- and second-order corrections— buccolingual and intrusion-extrusion movements—were simulated on the orthodontic measurement and simulation system. A 2-mm displacement was applied on the x-axis and a 1-mm displacement on the z-axis, both in 0.1-mm intervals; 5 repetitions were performed for each wire-bracket-interval combination, and new brackets and archwires were used for each trial. The forces generated by manipulation of the bracket in the 2 directions were recorded directly with the orthodontic measurement and simulation system software and were statistically analyzed with 2-way ANOVA, with bracket and displacement as the discriminating variables. Group differences were further analyzed with the Tukey post-hoc comparisons test with the family error rate set at the 0.05 level. Results: In the first-order correction, the direction showed a significant effect on force magnitude, with inward (lingual) movement having lower force levels for the In Ovation-R. No significant difference was found between the Damon2 and the conventional appliance for this movement. In the second-order model, no difference was noted between the 2 self-ligating brackets in magnitude of force, but the conventional bracket showed higher force levels, which accounted for 20%, or 1 N, of the increase in magnitude. The effect of the direction of displacement (intrusion vs extrusion) on force variation did not produce a significant effect. Conclusions: The forces generated by first- and second-order corrections in self-ligating appliances do not show a consistent pattern and depend on the wire, the direction of movement, and the design of the ligating component. (Am J Orthod Dentofacial Orthop 2008;133:738-42)
S
elf-ligating brackets have been suggested to replace the conventional ligation methods of elastomeric and stainless steel ligatures to increase clinical efficiency. Consistent archwire engagement throughout orthodontic treatment and elimination of the need for frequent visits for the replacement of ligatures are the main advantages of this new ligation mode.1-5 Additionally, due to the unique bracket-archwire engagement methods and the resulting increased intraslot wire play, a reduca
Doctoral candidate, School of Dentistry, University of Bonn, Bonn, Germany. Associate professor, Department of Orthodontics, School of Dentistry, Aristotle University of Thessaloniki, Thessaloniki, Greece. c Cedres ⫹ Metaux-endowed professor and chair, Oral Technology, School of Dentistry, University of Bonn, Bonn, Germany. Reprint requests to: Theodore Eliades, 57 Agnoston Hiroon St, Nea Ionia 14231, Greece; e-mail, [email protected]. Submitted, November 2007; revised and accepted, January 2008. 0889-5406/$34.00 Copyright © 2008 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2008.01.001 b
738
tion in the magnitude of the generated forces may be achieved. However, limited evidence is available on the potential forces applied to the teeth by the various combinations of archwire and self-ligating mechanisms; most of the force derives from simple experimental configurations that do not register force variation as a function of the direction of displacement—ie, first-, second-, or third-order movement.6,7 Additionally, other factors, eg, archwire cross-section, bracket slot dimensions, variation in the extent and type of crowding (widespread or localized), and relaxation of the clip in active self-ligating brackets,8 might modulate the forces transmitted to teeth.9 The hypothesis tested in this study was that both bracket and order of correction (first or second) affect the force levels developed and transmitted to the tooth during engagement. Therefore, our purpose was to comparatively investigate the forces produced by various brackets in first- and second-order corrections.
Pandis et al 739
American Journal of Orthodontics and Dentofacial Orthopedics Volume 133, Number 5
MATERIAL AND METHODS
RESULTS
Resin replicas (Palavit G, Heraeus Kulzer GmbH, Hanau, Germany) were constructed from the original mandibular model of a patient under treatment, who was at the aligned midtreatment stage and about to receive a 0.014 ⫻ 0.24-in nickel-titanium (Ni-Ti) archwire. The following bracket systems were used: conventional (Orthos2, Ormco, Glendora, Calif), passive self-ligating (Damon2, Ormco), and active selfligating (In Ovation-R, GAC, Bohemia, NY), all with a 0.022-in slot and the identical prescription. The left first premolar was removed from the acrylic model to allow for placement of a sensor. Brackets were bonded from second premolar to second premolar. A 0.014 ⫻ 0.025-in copper-Ni-Ti Damon archform wire (Ormco), was placed into the brackets of the constructed model, which was mounted sequentially on the orthodontic measurement and simulation system (OMMS) table; the corresponding bracket of the removed premolar was mounted on a special adaptor fitted on the OMSS sensor. The major components of the OMSS system are 2 force-moment sensors capable of measuring forces and moments in all 3 planes of space simultaneously.10 These sensors are mounted on a motor-driven positioning table with full 3-dimensional mobility, whereas all mechanical components are built in a temperature-controlled chamber, interfaced with a computer. This system can perform various measurements, and the resultant force-deflection curves are recorded, thus facilitating a means to study the loads from mock orthodontic tooth movements. Once the constructed model was mounted on the OMSS table and the corresponding bracket was mounted on the OMSS sensor, the model and the bracket were positioned so that they duplicated the arch form of the original aligned dental arch. A simulated intrusion-extrusion movement of 2 mm in 0.1-mm increments and a buccolingual movement of 1 mm in 0.1-mm increments were performed. Each simulation was repeated 5 times for each bracket group, and in all tests a new archwire and elastomeric ring were used. The force/displacement values in the simulated movement were recorded by the OMSS proprietary software. The maximum absolute values of the forces generated by the manipulation in the first and second orders on the OMSS were statistically analyzed by using 2-way ANOVA with bracket and direction of displacement as the predictors. Group differences were further analyzed with the Tukey post-hoc comparisons test with the family error rate set at .05.
The results showed that the force/deflection curves were determined by the characteristics of the wire, resulting in curves that were almost identical or symmetrical on buccolingual and intrusion-extrusion displacement. For the first-order model, the direction was found to have an effect on the force only for the In Ovation-R bracket. This is depicted in Figure 1; the dependence of ranking of force exerted by the In Ovation-R bracket depends on the displacement direction and interval—ie, increases with increasing displacement and with outward (buccal) movement. In Table I, the means of maximum values for the 2 directions and 3 bracket types are shown. The In Ovation-R appliance showed a significant reduction in force relative to the other bracket types in lingual movement, whereas no difference was found for the Damon2 and the conventional bracket for this model. For the second-order model, the ANOVA showed that the variable direction had no significant effect on the variation of force in the model. This is illustrated in Figure 2; the force level per increment of displacement throughout the total path (intrusion-extrusion) of movement of the self-ligating brackets showed a 20% decrease in force relative to their conventional counterparts, accounting for a 1 N difference, whereas no difference was found between the active and passive ligation mode (Table II). Similarly, no difference was noted for direction of movement among the 3 bracket types, and similar values were recorded for maximum intrusion and extrusion movements.
DISCUSSION
Self-ligating brackets have been advocated mainly for the lower friction variants and the feature of “light forces” attributed to the increased archwire-slot clearance because of the lack of contact points of ligature and wire. Our study indicates that design and material parameters of the ligation scheme of the various selfligating brackets can influence the force exerted by an activated archwire, and, even though moments were generated during the first- and second-order correction simulations, those variables were not investigated and thus were not included in this article. The model used in this study simulates the movement of teeth in a leveled and aligned arch, accompanying the placement of a rectangular Ni-Ti archwire. The choice of the specific size and alloy of the archwire simulates the mechanotherapy of the Damon bracket system manual.11 Although conventional and active
740 Pandis et al
American Journal of Orthodontics and Dentofacial Orthopedics May 2008
Fig 1. Variation of mean force per displacement increment in the first-order correction model; outward or buccal direction is denoted by negative signs. Results represent a complete loadingunloading cycle for each direction. Note the decreased force exerted by the In Ovation-R bracket for the same displacement in the reverse direction arising from the compliance of the closing mechanism.
Table I.
Means of maximum values of forces recorded by the brackets in first-order correction (buccolingual movement)
Bracket Damon2 lingual In Ovation-R buccal Damon2 buccal Orthos2 buccal Orthos2 lingual In Ovation-R lingual
Force (N), mean (SD)
Tukey grouping*
5.7 (0.2) 5.3 (0.2) 4.9 (0.4) 4.7 (0.6) 4.7 (0.8) 2.9 (0.9)
A A A A A B
*Means with the same letter are not significantly different at the .05 level of significance.
self-ligating brackets do not have similar mechanotherapeutical guidelines, the 0.014 ⫻ 0.25-in Ni-Ti archwire in a 0.022-in slot is a feasible alternative for standard wire sequencing. In addition, the specific range of displacement in the experiment was confined to 2 mm in the vertical dimension and 1 mm in the first-order direction because, at the time of wire placement, it is expected that initial leveling and aligning would have probably eliminated variation in crown spatial orientation relative to the arch form. The values reported in this study apply only to this experimental configuration and are valid only for the tooth/arch form/interbracket distances tested; thus, no extrapola-
tion should be made to clinical conditions and no generalization to other types of teeth or arch forms. Overall, the results show similar force/deflection curves, which seem to be determined by the characteristics of the wire. Additionally, these data indicate that, in the intrusion-extrusion movement, the direction of displacement does not affect the force level exerted by any bracket tested, probably because of the irrelevance of the bracket design with the forces generated; these are applied on the incisal and gingival walls of the slot, which did not have remarkable variations among the 3 brackets tested. In this model, self-ligating brackets (5.7, 5.8 N) seem to exert lower forces compared with conventionally ligated brackets (6.7 N). This reduction in force levels might be attributed to the increased play of wires in the slot and the lack of obstacles arising from the contact of an elastomeric ligature outside the wings. The difference, however, accounts for 1 N, or 20%, of that observed with self-ligating brackets; therefore, the clinical significance of this observation requires further investigation. In the first-order model, additional factors come into play. Variations in the design of the closing mechanism of the 2 self-ligating brackets in this study might affect the forces generated by the displaced bracket, because force direction on the lingual movement of the bracket coincides with the compliant section of the bracket slot. The stiffness and rigidity of
Pandis et al 741
American Journal of Orthodontics and Dentofacial Orthopedics Volume 133, Number 5
Fig 2. Variation of mean force per displacement increment in the second-order correction model; intrusion direction is denoted by negative values. Results represent a complete loading-unloading cycle for each direction. Note the consistently increased level for conventional Orthos2 brackets relative to the self-ligating appliances; this is independent of direction. Table II. Means of maximum forces recorded by the brackets in second-order correction (intrusion-extrusion movement) Bracket Orthos2 Damon2 In Ovation-R
Force (N), mean (SD)
Tukey grouping*
6.7 (0.3) 5.7 (0.8) 5.8 (0.6)
A B B
*Means with the same letter are not significantly different at the .05 level of significance.
the Damon2 buccal slot wall might be a limiting factor that does not allow movement of the wire as the bracket is forced lingually and the wire comes in contact with the outer slot wall. In contrast, the elastically deformed clip of the In Ovation-R bracket provides flexibility as the wire is pressed against the buccal segment of the slot. Although conventional brackets do not possess this fourth wall, the use of a new elastomeric ligature might also restrict the movement of the achwire. In this model, the advantage of the active self-ligating bracket results in an almost 40% reduction of the force magnitude. This favorable response is eliminated when the direction of displacement is reversed to the buccal (outward) movement. In this direction, the role of the closing segment of the slot in self-ligating brackets has no importance because the wire is pressed against the rigid lingual slot wall. The same pattern has been recorded in correction of rotations, imply-
ing that the exertion of force or moment by brackets with different ligating mechanisms is multifactorial and complex.12 The magnitude of force developed during engagement can also vary depending on the number of teeth ligated in the proximal and distal segments of the arch.13 This effect arises from the increased stiffness of the wire-bracket complex associated with the many dental units incorporated into the mechanotherapy.
CONCLUSIONS
First- and second-order corrections with different bracket types result in the following effects. 1. In the first-order correction model, the active selfligating bracket In Ovation-R showed a 40% force reduction in the lingual direction, attributed to its compliant closing mechanism; no difference was observed between the Damon2 and the conventional bracket with respect to force generation during buccolingual movement. 2. In the second-order correction, self-ligating brackets exert 20% (or 1 N) less force than their conventional counterpart tested in this study. No effect of the direction of movement (instrusion or extrusion) on the force variants was observed.
742 Pandis et al
American Journal of Orthodontics and Dentofacial Orthopedics May 2008
3. The identical pattern of force-deflection curves among the bracket types tested seems to be dictated by the characteristics of the wire rather than the ligating mechanism of brackets.
design on force levels in tooth movement: a comparative in vitro study. Am J Orthod Dentofacial Orthop 1990;97:219-28. Berger JL. The SPEED appliance: a 14-year update on this unique self-ligating orthodontic mechanism. Am J Orthod Dentofacial Orthop 1994;105:217-23. Pandis N, Bourauel C, Eliades T. Changes in the stiffness of the ligating mechanism in retrieved active self-ligating brackets. Am J Orthod Dentofacial Orthop 2007;132:834-7. Iwasaki LR, Beatty MW, Randall CJ, Nickel JC. Clinical ligation forces and intraoral friction during sliding on a stainless steel archwire. Am J Orthod Dentofacial Orthop 2003;123:408-15. Bourauel C, Drescher D, Thier M. An experimental apparatus for the simulation of three-dimensional movements in orthodontics. J Biomed Eng 1992;14:371-8. System archwire sequence in Damon system. The workbook. 2nd ed. Glendora, Calif: Ormco; 2002. p. 7-8. Pandis N, Eliades T, Partowi S, Bourauel C. Moments generated during simulated rotational correction with self-ligating and conventional brackets. Angle Orthod 2008 (in press). Drenker E. Calculating continuous archwire forces. Angle Orthod 1988;58:59-70.
7.
8. REFERENCES 1. Hanson H. The SPEED system: a report on the development of a new edgewise appliance. Am J Orthod 1980;78:243-65. 2. Voudouris JC, Kuftinec MM. Excellence and efficiency. Interactive twin self-ligation. Toronto: Self-ligating technology publications; 2006. 3. Damon DH. The Damon low friction bracket: a biologically compatible straight-wire system. J Clin Orthod 1998;32:670-80. 4. Shivapuja PK, Berger J. A comparative study of conventional ligation and self-ligation bracket systems. Am J Orthod Dentofacial Orthop 1994;106:472-80. 5. Harradine NW. Self-ligating brackets: where are we now? J Orthod 2003;30:262-73. 6. Berger JL. The influence of the SPEED bracket’s self-ligating
9.
10.
11. 12.
13.
Forces exerted by conventional and self-ligating brackets during simulated first- and second-order corrections Nikolaos Pandis,a Theodore Eliades,b Samira Partowi,a and Christoph Bourauelc Bonn, Germany, and Thessaloniki, Greece Introduction: Our aim in this study was to comparatively assess the forces generated from conventional and self-ligating bracket systems during the late leveling and alignment stage, specifically for first- and second-order movement. Methods: Three types of brackets were selected: Orthos2 (Ormco, Glendora, Calif), Damon2 (Ormco), and In Ovation-R (GAC, Bohemia, NY). The brackets were bonded on resin replicas constructed from a model of an aligned mandibular arch, and a 0.014 ⫻ 0.025-in copper-nickel-titanium wire (Ormco) was placed. First- and second-order corrections— buccolingual and intrusion-extrusion movements—were simulated on the orthodontic measurement and simulation system. A 2-mm displacement was applied on the x-axis and a 1-mm displacement on the z-axis, both in 0.1-mm intervals; 5 repetitions were performed for each wire-bracket-interval combination, and new brackets and archwires were used for each trial. The forces generated by manipulation of the bracket in the 2 directions were recorded directly with the orthodontic measurement and simulation system software and were statistically analyzed with 2-way ANOVA, with bracket and displacement as the discriminating variables. Group differences were further analyzed with the Tukey post-hoc comparisons test with the family error rate set at the 0.05 level. Results: In the first-order correction, the direction showed a significant effect on force magnitude, with inward (lingual) movement having lower force levels for the In Ovation-R. No significant difference was found between the Damon2 and the conventional appliance for this movement. In the second-order model, no difference was noted between the 2 self-ligating brackets in magnitude of force, but the conventional bracket showed higher force levels, which accounted for 20%, or 1 N, of the increase in magnitude. The effect of the direction of displacement (intrusion vs extrusion) on force variation did not produce a significant effect. Conclusions: The forces generated by first- and second-order corrections in self-ligating appliances do not show a consistent pattern and depend on the wire, the direction of movement, and the design of the ligating component. (Am J Orthod Dentofacial Orthop 2008;133:738-42)
S
elf-ligating brackets have been suggested to replace the conventional ligation methods of elastomeric and stainless steel ligatures to increase clinical efficiency. Consistent archwire engagement throughout orthodontic treatment and elimination of the need for frequent visits for the replacement of ligatures are the main advantages of this new ligation mode.1-5 Additionally, due to the unique bracket-archwire engagement methods and the resulting increased intraslot wire play, a reduca
Doctoral candidate, School of Dentistry, University of Bonn, Bonn, Germany. Associate professor, Department of Orthodontics, School of Dentistry, Aristotle University of Thessaloniki, Thessaloniki, Greece. c Cedres ⫹ Metaux-endowed professor and chair, Oral Technology, School of Dentistry, University of Bonn, Bonn, Germany. Reprint requests to: Theodore Eliades, 57 Agnoston Hiroon St, Nea Ionia 14231, Greece; e-mail, [email protected]. Submitted, November 2007; revised and accepted, January 2008. 0889-5406/$34.00 Copyright © 2008 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2008.01.001 b
738
tion in the magnitude of the generated forces may be achieved. However, limited evidence is available on the potential forces applied to the teeth by the various combinations of archwire and self-ligating mechanisms; most of the force derives from simple experimental configurations that do not register force variation as a function of the direction of displacement—ie, first-, second-, or third-order movement.6,7 Additionally, other factors, eg, archwire cross-section, bracket slot dimensions, variation in the extent and type of crowding (widespread or localized), and relaxation of the clip in active self-ligating brackets,8 might modulate the forces transmitted to teeth.9 The hypothesis tested in this study was that both bracket and order of correction (first or second) affect the force levels developed and transmitted to the tooth during engagement. Therefore, our purpose was to comparatively investigate the forces produced by various brackets in first- and second-order corrections.
Pandis et al 739
American Journal of Orthodontics and Dentofacial Orthopedics Volume 133, Number 5
MATERIAL AND METHODS
RESULTS
Resin replicas (Palavit G, Heraeus Kulzer GmbH, Hanau, Germany) were constructed from the original mandibular model of a patient under treatment, who was at the aligned midtreatment stage and about to receive a 0.014 ⫻ 0.24-in nickel-titanium (Ni-Ti) archwire. The following bracket systems were used: conventional (Orthos2, Ormco, Glendora, Calif), passive self-ligating (Damon2, Ormco), and active selfligating (In Ovation-R, GAC, Bohemia, NY), all with a 0.022-in slot and the identical prescription. The left first premolar was removed from the acrylic model to allow for placement of a sensor. Brackets were bonded from second premolar to second premolar. A 0.014 ⫻ 0.025-in copper-Ni-Ti Damon archform wire (Ormco), was placed into the brackets of the constructed model, which was mounted sequentially on the orthodontic measurement and simulation system (OMMS) table; the corresponding bracket of the removed premolar was mounted on a special adaptor fitted on the OMSS sensor. The major components of the OMSS system are 2 force-moment sensors capable of measuring forces and moments in all 3 planes of space simultaneously.10 These sensors are mounted on a motor-driven positioning table with full 3-dimensional mobility, whereas all mechanical components are built in a temperature-controlled chamber, interfaced with a computer. This system can perform various measurements, and the resultant force-deflection curves are recorded, thus facilitating a means to study the loads from mock orthodontic tooth movements. Once the constructed model was mounted on the OMSS table and the corresponding bracket was mounted on the OMSS sensor, the model and the bracket were positioned so that they duplicated the arch form of the original aligned dental arch. A simulated intrusion-extrusion movement of 2 mm in 0.1-mm increments and a buccolingual movement of 1 mm in 0.1-mm increments were performed. Each simulation was repeated 5 times for each bracket group, and in all tests a new archwire and elastomeric ring were used. The force/displacement values in the simulated movement were recorded by the OMSS proprietary software. The maximum absolute values of the forces generated by the manipulation in the first and second orders on the OMSS were statistically analyzed by using 2-way ANOVA with bracket and direction of displacement as the predictors. Group differences were further analyzed with the Tukey post-hoc comparisons test with the family error rate set at .05.
The results showed that the force/deflection curves were determined by the characteristics of the wire, resulting in curves that were almost identical or symmetrical on buccolingual and intrusion-extrusion displacement. For the first-order model, the direction was found to have an effect on the force only for the In Ovation-R bracket. This is depicted in Figure 1; the dependence of ranking of force exerted by the In Ovation-R bracket depends on the displacement direction and interval—ie, increases with increasing displacement and with outward (buccal) movement. In Table I, the means of maximum values for the 2 directions and 3 bracket types are shown. The In Ovation-R appliance showed a significant reduction in force relative to the other bracket types in lingual movement, whereas no difference was found for the Damon2 and the conventional bracket for this model. For the second-order model, the ANOVA showed that the variable direction had no significant effect on the variation of force in the model. This is illustrated in Figure 2; the force level per increment of displacement throughout the total path (intrusion-extrusion) of movement of the self-ligating brackets showed a 20% decrease in force relative to their conventional counterparts, accounting for a 1 N difference, whereas no difference was found between the active and passive ligation mode (Table II). Similarly, no difference was noted for direction of movement among the 3 bracket types, and similar values were recorded for maximum intrusion and extrusion movements.
DISCUSSION
Self-ligating brackets have been advocated mainly for the lower friction variants and the feature of “light forces” attributed to the increased archwire-slot clearance because of the lack of contact points of ligature and wire. Our study indicates that design and material parameters of the ligation scheme of the various selfligating brackets can influence the force exerted by an activated archwire, and, even though moments were generated during the first- and second-order correction simulations, those variables were not investigated and thus were not included in this article. The model used in this study simulates the movement of teeth in a leveled and aligned arch, accompanying the placement of a rectangular Ni-Ti archwire. The choice of the specific size and alloy of the archwire simulates the mechanotherapy of the Damon bracket system manual.11 Although conventional and active
740 Pandis et al
American Journal of Orthodontics and Dentofacial Orthopedics May 2008
Fig 1. Variation of mean force per displacement increment in the first-order correction model; outward or buccal direction is denoted by negative signs. Results represent a complete loadingunloading cycle for each direction. Note the decreased force exerted by the In Ovation-R bracket for the same displacement in the reverse direction arising from the compliance of the closing mechanism.
Table I.
Means of maximum values of forces recorded by the brackets in first-order correction (buccolingual movement)
Bracket Damon2 lingual In Ovation-R buccal Damon2 buccal Orthos2 buccal Orthos2 lingual In Ovation-R lingual
Force (N), mean (SD)
Tukey grouping*
5.7 (0.2) 5.3 (0.2) 4.9 (0.4) 4.7 (0.6) 4.7 (0.8) 2.9 (0.9)
A A A A A B
*Means with the same letter are not significantly different at the .05 level of significance.
self-ligating brackets do not have similar mechanotherapeutical guidelines, the 0.014 ⫻ 0.25-in Ni-Ti archwire in a 0.022-in slot is a feasible alternative for standard wire sequencing. In addition, the specific range of displacement in the experiment was confined to 2 mm in the vertical dimension and 1 mm in the first-order direction because, at the time of wire placement, it is expected that initial leveling and aligning would have probably eliminated variation in crown spatial orientation relative to the arch form. The values reported in this study apply only to this experimental configuration and are valid only for the tooth/arch form/interbracket distances tested; thus, no extrapola-
tion should be made to clinical conditions and no generalization to other types of teeth or arch forms. Overall, the results show similar force/deflection curves, which seem to be determined by the characteristics of the wire. Additionally, these data indicate that, in the intrusion-extrusion movement, the direction of displacement does not affect the force level exerted by any bracket tested, probably because of the irrelevance of the bracket design with the forces generated; these are applied on the incisal and gingival walls of the slot, which did not have remarkable variations among the 3 brackets tested. In this model, self-ligating brackets (5.7, 5.8 N) seem to exert lower forces compared with conventionally ligated brackets (6.7 N). This reduction in force levels might be attributed to the increased play of wires in the slot and the lack of obstacles arising from the contact of an elastomeric ligature outside the wings. The difference, however, accounts for 1 N, or 20%, of that observed with self-ligating brackets; therefore, the clinical significance of this observation requires further investigation. In the first-order model, additional factors come into play. Variations in the design of the closing mechanism of the 2 self-ligating brackets in this study might affect the forces generated by the displaced bracket, because force direction on the lingual movement of the bracket coincides with the compliant section of the bracket slot. The stiffness and rigidity of
Pandis et al 741
American Journal of Orthodontics and Dentofacial Orthopedics Volume 133, Number 5
Fig 2. Variation of mean force per displacement increment in the second-order correction model; intrusion direction is denoted by negative values. Results represent a complete loading-unloading cycle for each direction. Note the consistently increased level for conventional Orthos2 brackets relative to the self-ligating appliances; this is independent of direction. Table II. Means of maximum forces recorded by the brackets in second-order correction (intrusion-extrusion movement) Bracket Orthos2 Damon2 In Ovation-R
Force (N), mean (SD)
Tukey grouping*
6.7 (0.3) 5.7 (0.8) 5.8 (0.6)
A B B
*Means with the same letter are not significantly different at the .05 level of significance.
the Damon2 buccal slot wall might be a limiting factor that does not allow movement of the wire as the bracket is forced lingually and the wire comes in contact with the outer slot wall. In contrast, the elastically deformed clip of the In Ovation-R bracket provides flexibility as the wire is pressed against the buccal segment of the slot. Although conventional brackets do not possess this fourth wall, the use of a new elastomeric ligature might also restrict the movement of the achwire. In this model, the advantage of the active self-ligating bracket results in an almost 40% reduction of the force magnitude. This favorable response is eliminated when the direction of displacement is reversed to the buccal (outward) movement. In this direction, the role of the closing segment of the slot in self-ligating brackets has no importance because the wire is pressed against the rigid lingual slot wall. The same pattern has been recorded in correction of rotations, imply-
ing that the exertion of force or moment by brackets with different ligating mechanisms is multifactorial and complex.12 The magnitude of force developed during engagement can also vary depending on the number of teeth ligated in the proximal and distal segments of the arch.13 This effect arises from the increased stiffness of the wire-bracket complex associated with the many dental units incorporated into the mechanotherapy.
CONCLUSIONS
First- and second-order corrections with different bracket types result in the following effects. 1. In the first-order correction model, the active selfligating bracket In Ovation-R showed a 40% force reduction in the lingual direction, attributed to its compliant closing mechanism; no difference was observed between the Damon2 and the conventional bracket with respect to force generation during buccolingual movement. 2. In the second-order correction, self-ligating brackets exert 20% (or 1 N) less force than their conventional counterpart tested in this study. No effect of the direction of movement (instrusion or extrusion) on the force variants was observed.
742 Pandis et al
American Journal of Orthodontics and Dentofacial Orthopedics May 2008
3. The identical pattern of force-deflection curves among the bracket types tested seems to be dictated by the characteristics of the wire rather than the ligating mechanism of brackets.
design on force levels in tooth movement: a comparative in vitro study. Am J Orthod Dentofacial Orthop 1990;97:219-28. Berger JL. The SPEED appliance: a 14-year update on this unique self-ligating orthodontic mechanism. Am J Orthod Dentofacial Orthop 1994;105:217-23. Pandis N, Bourauel C, Eliades T. Changes in the stiffness of the ligating mechanism in retrieved active self-ligating brackets. Am J Orthod Dentofacial Orthop 2007;132:834-7. Iwasaki LR, Beatty MW, Randall CJ, Nickel JC. Clinical ligation forces and intraoral friction during sliding on a stainless steel archwire. Am J Orthod Dentofacial Orthop 2003;123:408-15. Bourauel C, Drescher D, Thier M. An experimental apparatus for the simulation of three-dimensional movements in orthodontics. J Biomed Eng 1992;14:371-8. System archwire sequence in Damon system. The workbook. 2nd ed. Glendora, Calif: Ormco; 2002. p. 7-8. Pandis N, Eliades T, Partowi S, Bourauel C. Moments generated during simulated rotational correction with self-ligating and conventional brackets. Angle Orthod 2008 (in press). Drenker E. Calculating continuous archwire forces. Angle Orthod 1988;58:59-70.
7.
8. REFERENCES 1. Hanson H. The SPEED system: a report on the development of a new edgewise appliance. Am J Orthod 1980;78:243-65. 2. Voudouris JC, Kuftinec MM. Excellence and efficiency. Interactive twin self-ligation. Toronto: Self-ligating technology publications; 2006. 3. Damon DH. The Damon low friction bracket: a biologically compatible straight-wire system. J Clin Orthod 1998;32:670-80. 4. Shivapuja PK, Berger J. A comparative study of conventional ligation and self-ligation bracket systems. Am J Orthod Dentofacial Orthop 1994;106:472-80. 5. Harradine NW. Self-ligating brackets: where are we now? J Orthod 2003;30:262-73. 6. Berger JL. The influence of the SPEED bracket’s self-ligating
9.
10.
11. 12.
13.