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The effect of ligation method on friction in sliding mechanics
ORIGINAL ARTICLE
The effect of ligation method on friction in sliding mechanics Max Hain, BDS, MFDS RCS, BMSc,a Ashish Dhopatkar, BDS, MSc, FDS RCS, MOrth RCS,b and Peter Rock, BDS, DDS, FDS RCS, DOrth RCSc Birmingham, United Kingdom During orthodontic tooth movement with the preadjusted edgewise system, friction generated at the bracket/archwire interface tends to impede the desired movement. The method of ligation is an important contributor to this frictional force. This in vitro study investigated the effect of ligation method on friction and evaluated the efficacy of the new slick elastomeric modules from TP Orthodontics (La Porte, Ind), which are claimed to reduce friction at the module/wire interface. Slick modules were compared with regular nonslick modules, stainless steel ligatures, and the SPEED self-ligating bracket system (Strite Industries, Cambridge, Ontario, Canada). The effect of using slick modules with metal-reinforced ceramic (Clarity, 3M Unitek, Monrovia, Calif) and miniature brackets (Minitwin, 3M Unitek) was also examined. Results showed that, when considering tooth movement along a 0.019 ⫻ 0.025-in stainless steel archwire, saliva-lubricated slick modules can reduce static friction at the module/archwire interface by up to 60%, regardless of the bracket system. The SPEED brackets produced the lowest friction compared with the 3 other tested bracket systems when regular modules were used. The use of slick modules, however, with all of the ligated bracket types tested significantly reduced friction to below the values recorded in the SPEED groups. Loosely tied stainless steel ligatures were found to generate the least friction. (Am J Orthod Dentofacial Orthop 2003;123:416-22)
D
uring orthodontic space closure with sliding mechanics, a frictional force generated at the bracket/archwire interface tends to impede the desired movement. In clinical terms, the force applied must overcome this unknown frictional component and achieve the desired tooth movement. The “loss of applied force” has been commented on by a number of authors1,2 because it places additional strain on anchorage demands and leads to a reduction in the speed of tooth movement. A number of factors have been implicated in influencing frictional forces during orthodontic tooth movement. The effects of archwire material,3 dimensions,4,5 and bracket material6 have been investigated. The method of archwire ligation would appear to be an important determinant in the generation of friction, yet relatively few studies2 have looked at this interacFrom the Orthodontic Unit, School of Dentistry, University of Birmingham, Birmingham, United Kingdom. a Specialist registrar. b Lecturer. c Head of department. This study was supported by a research grant from TP Orthodontics. Reprint requests to: Dr A. A. Dhopatkar, Orthodontic Unit, School of Dentistry, St Chad’s Queensway, Birmingham B4 6NN, United Kingdom; e-mail, [email protected]. Submitted, April 2002; revised and accepted, August 2002. Copyright © 2003 by the American Association of Orthodontists. 0889-5406/2003/$30.00 ⫹ 0 doi:10.1067/mod.2003.14
416
tion. Loosely tied stainless steel ligatures are generally thought to generate less friction than standard elastomeric ligatures,7 although the increase in chairside time required to manipulate stainless steel ligatures has meant that they are still less popular in the clinical situation than elastomers. A new slick elastomeric module system (TP Orthodontics, La Porte, Ind) has recently been introduced; it claims to combine ease of use with low friction. An alternative approach to reducing friction has been to avoid using any form of ligature. This has been achieved by the self-ligating bracket systems,8,9 which have been shown to reduce friction in certain conditions.10 Friction can be defined as the force that acts at the surface between 2 objects when 1 object slides relative to the other. Its magnitude depends on the amount of normal force pushing the 2 surfaces together, the surface roughness, and the nature of the materials from which the surfaces are made.11 A number of components have been linked to this normal force, including engagement of archwires in brackets that are out of alignment, ligatures pressing the wire against the slot, and active torque in a rectangular wire when the tipping tendency is resisted by a 2-point contact between the bracket and the archwire. The relative contribution of these components varies according to the clinical situation.12
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The friction encountered during tooth movement can be considered in 2 distinct phases. Static friction is defined as the resistance that prevents initial tooth movement and is then replaced by dynamic or kinetic friction, which acts during the period of motion itself. Because tooth movement along an archwire occurs in very short steps rather than as continuous motion, static friction is considered to have a greater effect on preadjusted mechanics than dynamic friction.13 Shivapuja and Berger14 found that self-ligating brackets generated less friction than conventional brackets. Hanson15 found similar results and concluded that this might reduce treatment time. Voudouris16 found that self-ligating brackets, both with passive and active labial arms, produced less friction than conventional brackets in association with steel ligatures. ReadWard et al,10 however, found that self-ligating brackets produced less friction only under certain conditions. SPEED brackets (Strite Industries, Cambridge, Ontario, Canada) in particular, produced low friction in round wires, but friction increased greatly with rectangular wires. The effects of saliva on friction are controversial, because investigations carried out under dry conditions or with the addition of human or artificial saliva or water have produced conflicting results.10 Kusy17 stated that experiments conducted in artificial saliva were invalid because it is no substitute for human saliva. Bracket material has been shown to influence friction at the bracket/archwire interface, with ceramic brackets having been found to produce significantly more friction than stainless steel ones.13 Metal-reinforced ceramic brackets, however, are reported to function comparably with conventional stainless steel brackets in their frictional properties.18 Bracket size has also been identified as a contributory factor,19 and larger brackets are prone to increased friction because of the greater normal force exerted by the modules as a result of the additional stretching required for engagement. Some authors20 have questioned the value of experiments that do not consider occlusal forces; however, it is unlikely that these forces would simultaneously affect all bracket/archwire interfaces. In addition, the elastic nature of modules is likely to promote maintenance of wire/module contact during such disturbances. The aims of this study were to investigate the effect of ligation method on friction and to evaluate the manufacturer’s claim that the new slick elastomeric modules reduce friction at the wire/module interface.
Table I.
Composition of test groups
Group/code 1. VRD 2. VSD 3. VRW 4. VSW 5. VStD 6. VStW 7. VR8D 8. VS8D 9. VR8W 10. VS8W 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
SPD SPW CRD CSD CRW CSW MRD MSD MRW MSW
Specimen Victory brackets with regular modules in dry state Victory brackets with slick modules in dry state Victory brackets with regular modules soaked in saliva Victory brackets with slick modules soaked in saliva Victory brackets with stainless steel ligatures in dry state Victory brackets with stainless steel ligatures in saliva Victory brackets with regular modules tied is figure-8 pattern in dry state Victory brackets with slick modules tied in figure8 pattern in dry state Victory brackets with saliva-lubricated regular modules tied in figure-8 pattern Victory brackets with saliva lubricated slick modules tied in figure-8 pattern SPEED brackets dry SPEED brackets soaked in saliva Clarity brackets with regular modules dry Clarity brackets with slick modules dry Clarity brackets with regular modules in saliva Clarity brackets with slick modules in saliva Minitwin brackets with regular modules dry Minitwin brackets with slick modules dry Minitwin brackets with regular modules in saliva Minitwin brackets with slick modules in saliva
MATERIAL AND METHODS
A custom-made apparatus was constructed to record the resistance to movement of a stainless steel 0.019 ⫻ 0.025-in working archwire through test brackets. This archwire dimension was chosen because it is the recommended size for sliding mechanics with the 0.022-in system brackets used in the investigation.21 Straight lengths of 0.019 ⫻ 0.025-in stainless steel wire (3M Unitek, Monrovia, Calif), each 7 cm long, were used. Four types of maxillary premolar brackets were used, each incorporating ⫺7° torque and zero angulation: standard stainless steel (Victory Twin series, 3M Unitek), miniature twin (Minitwin, 3M Unitek), metalreinforced ceramic brackets (Clarity Twin, 3M Unitek), and SPEED. The elastic modules compared were regular grey and the new super-slick modules incorporating metafasix technology (TP Orthodontics). The test groups are listed in Table I. The brackets and archwires were cleaned with an alcohol wipe before the modules or ligatures were tied with mosquito forceps, 25 mm from the lower end of the archwire, to form a test unit. All units in the saliva
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Fig 1. Test apparatus.
groups were soaked in human saliva for 1 hour before testing. The dry groups were also tied 60 minutes before testing to minimize differences in elastic tension between the samples. The stainless steel ligatures were initially fully tightened and then unwound by 3 turns. Loose ligation was checked by rocking the ligature to confirm that there was a little play between both spans of the ligature and the archwire. The end of the ligature was then tucked in over the archwire. Testing was performed on an Instron 5544 machine (Instron Ltd, High Wycombe, Buckinghamshire, United Kingdom), with a crosshead speed of 20 mm/ min over an 8-mm stretch of archwire (Fig 1). The lower end of each test unit was attached to a heavy base block on the lower crosshead of the testing machine through a hollow screw. A 2-mm right-angle bend in the archwire allowed for freedom of rotational movement. This ensured that the bracket and wire specimens could self-align during a test run, allowing tip and torque to be effectively eliminated as variables so that the effect of ligation method on friction could be studied in isolation. Care was taken to align the archwire so that the sample was parallel with the vertical framework of the machine. The base block was positioned on the machine with a permanent guide mark to aid alignment. The bracket was pulled in a vertical direction by a loop of 0.018-in stainless steel wire (Fig 2), and the force required to initiate and maintain movement of the bracket over the 8-mm test
Fig 2. Close-up of test apparatus.
distance was measured. Figure 3 gives an example of the typical output for a single test. The program was set to highlight the maximum frictional force at initial movement, which was taken to represent the peak static frictional resistance (Fig 3). The initial measurements for groups 1 and 4 were repeated after 1 week to assess the reproducibility of the measurements.
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Results of general linear model ANOVA investigating 4 main variables
Table II.
Variable Module type Wet or dry Normal/F8 Bracket type
Fig 3. Typical output from testing machine for single test run plotted as load (N) vs distance (mm).
DF
Sum of squares
Mean square
F
P
1 1 1 3
26.45 72.88 93.46 6.21
26.45 72.88 93.46 2.07
53.93 148.61 190.56 4.76
.00 .00 .00 .00
Data set used to look at module type, effect of lubrication, and effect of tie configuration excluded values for stainless steel ligatures and SPEED brackets. Data set used to examine effect of bracket type excluded stainless steel and figure-8 ligated subsets.
Post hoc multiple comparisons examining effect of bracket type in more detail with Tukey tests
Table III.
SPEED Clarity MiniTwin
Standard straight wire
SPEED
Clarity
⫺0.097 0.67 ⫺0.55 0.07 ⫺0.24 0.38
⫺0.91* ⫺0.14* ⫺0.60 0.17
0.00* 0.62*
* Significant at 95% level. Family error rate ⫽ 0.05; individual error rate ⫽ 0.01; critical value ⫽ 3.66.
Fig 4. Histogram summarizing group comparison results.
Ten each of the regular and the slick modules were stretched by 1-mm and 2-mm amounts with the testing machine to determine the effect of the 2 module types on ligation force, and the force generated was recorded. The effect on friction of the 4 main variables—type of module, state of wetness, bracket type, and tie configuration—was investigated with analysis of variance (ANOVA) using the general linear model in Minitab (State College, Pa). Post hoc Tukey pairwise comparison tests were performed to determine the significance of these variances with respect to bracket type. Further comparison between each pair of test groups was then carried out with t tests. To control for type I error, the Bonferroni correction was used; it is derived by dividing the chosen alpha level of 0.05 by the number of tests. The corrected alpha level, 0.005, was used to determine significance. RESULTS
Results are presented in Figure 4 and Tables II through VI.
Table II shows the effects of module type, lubrication, tie configuration, and bracket type on static friction by means of ANOVA using the general linear model. The results indicate that each variable has an effect on the friction generated. Table III shows the effect of specific bracket types with post hoc Tukey comparison tests. Table IV shows general descriptive statistics for all variables and the comparison between dry and wet states using t tests. Figure 4 summarizes these results. The overall finding was that slick modules reduced friction by up to 60% compared with their regular counterparts; the reductions were greatest in association with saliva lubrication. SPEED brackets generated less friction than the other test brackets using regular modules. Stainless steel ligatures produced the least friction overall. Table V shows the force recorded during tensile stretching of regular and slick modules by 1-mm and 2-mm amounts. No statistically significant difference was found with t tests between the 2 module types. Table VI shows the results of the reproducibility study in the wet and dry states. There was no significant
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Table IV.
American Journal of Orthodontics and Dentofacial Orthopedics April 2003
Values for each variable and comparison between test specimens in dry and wet states (see Table I)
Group Code
Mean load (n)
SD
SE
1. VRD 3. VRW 5. VStD 7. VR8D 9. VR8W 11. SPD 13. CRD 15. CRW 17. MRD 19. MRW
2.78 2.06 0.00 4.56 3.61 1.63 2.71 2.09 2.47 2.19
0.28 0.28 0.02 0.65 0.56 0.28 0.31 0.23 0.24 0.20
0.07 0.07 0.00 0.17 0.14 0.07 0.08 0.06 0.06 0.05
Group Code 2. 4. 6. 8. 10. 12. 14. 16. 18. 20.
VSD VSW VStW VS8D VS8W SPW CSD CSW MSD MSW
Mean load (n)
SD
SE
t
P
1.92 0.83 0.01 4.98 1.51 1.61 2.49 1.29 1.96 0.71
0.29 0.22 0.02 0.56 0.25 0.23 0.54 0.27 0.25 0.13
0.07 0.06 0.00 0.15 0.06 0.06 0.14 0.07 0.07 0.03
8.11 13.50 ⫺0.59 ⫺1.89 13.29 0.21 1.31 8.62 5.76 24.16
.0000* .0000* .5600 .0700 .0000* .8400 .2000 .0000* .0000* .0000*
*
Significant at 95% level.
Table V.
Comparison of tensile properties between regular and slick modules
Variable
Mean load (n)
SD
SE
Regular modules stretched 1 mm Regular modules stretched 2 mm
1.12 2.37
0.14 0.15
0.05 0.05
Table VI.
Variable
Mean load (n)
SD
SE
t
P
Slick modules stretched 1 mm Slick modules stretched 2 mm
1.12 2.42
0.19 0.26
0.06 0.08
⫺0.04 ⫺0.58
.97 .57
Results of reproducibility study
Variable
Mean load (n)
SD
SE
Group 1 original Group 4 original
2.78 0.84
0.29 0.22
0.07 0.06
Variable
Mean load (n)
SD
SE
t
P
Group 1 repeated Group 4 repeated
2.63 0.94
0.20 0.19
0.05 0.05
1.68 ⫺1.35
.11 .19
difference between the original friction values and those recorded a week later. DISCUSSION
The variety of experimental methods used in the literature makes it difficult to compare the results of different studies of this type. The amount of freedom of movement of the bracket relative to the archwire appears to greatly affect results. Differences in the finish of the same materials (eg, surface smoothness of nickel-titanium archwires from different manufactures) also make it difficult to compare findings and to isolate individual contributing factors. For example, brackets differ not only in type and material but also in width and therefore in the amount of force exerted by the elastic modules. In addition, variation in timing measurements could affect the recorded values as a result of differing amounts of stress relaxation of the elastic modules. High variability of friction measurements has been commented on previously.22
Frictional forces measured in this study were comparable in magnitude and range with those reported by other investigators.12,23 The choice of crosshead speed was based on previous work,18 which showed that, from 10 to 0.0005 mm/min, the coefficient of friction for stainless steel archwires was unaffected. It has been shown previously that multiple testing has no adverse effects on wire/bracket couples24 in the absence of high angulation. The present results show that, when archwire/ bracket alignment was carefully controlled, the degree of friction generated at the module/wire interface was affected by type of module, state of wetness, bracket type, and tie configuration (Table II). The results also show that static frictional force is greater, in both the dry and lubricated states, with regular modules than with the new slick modules (Table IV). Soaking in saliva led to a reduction in friction for both types of test module, regardless of the bracket system. The reduction was greater for slick
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(50% to 60%) than for regular (10% to 30%) modules. Overall, under the test conditions, the slick modules produced up to 60% less friction than their regular counterparts. The fact that both module types exhibited similar physical properties on stretching (Table V) suggests that the normal force generated by both would be comparable. It is likely, therefore, that the differences in frictional properties shown here are related to variations in surface characteristics. Loosely tied stainless steel ligatures generally produced negligible friction in both dry and wet situations, and there was no effect on frictional force attributable to lubrication (Table IV). It is clear, therefore, that loose stainless steel ligatures produce less friction than both types of test module. However, the convenience and speed of application of elastomeric modules are likely to ensure their continued popularity in the clinical setting. In addition, although the low normal force exerted by steel ligatures aids in minimizing friction, torque expression might be impaired by their use because of incomplete location of the archwire in the bracket slot. Tying modules in a figure-8 pattern is a useful procedure to ensure full archwire engagement. These results show that this procedure produces increased friction with either module type (Table IV), although there was a significant reduction in friction when lubrication was added. The magnitude of this improvement was dramatically greater with the slick modules (70% compared with 21% for regular modules), and the overall reduction was again greater than 50%. These results showed no clear difference between friction recorded for test units incorporating standard straight wires and SPEED, Clarity, or Minitwin brackets (Table III). There was, however, a statistically significant difference in performance between the SPEED and the ceramic test groups. The SPEED brackets were found to produce significantly less friction than the metal-reinforced ceramic brackets. But when the ceramic brackets were ligated with lubricated slick modules, the mean friction values fell below the levels recorded for the SPEED groups (Table IV). In general, the ceramic brackets were also found to produce more friction than the miniature brackets (Tables III and IV). This is probably a reflection of the larger size of the ceramic brackets around which the stretched modules exert a greater normal force. Berger,25 using a combination of round and 0.016 ⫻ 0.022-in stainless steel archwires, found that SPEED brackets consistently produced lower friction compared with stainless steel brackets with steel or elastomeric ligation. The present results suggest, however, that when SPEED brackets are used in combina-
tion with 0.019 ⫻ 0.025-in stainless steel archwires, their frictional properties might be less favorable. This agrees with the results of Read-Ward et al10 and could be related to the spring-clip design feature, which exerts a normal force proportional to the size of the wire being engaged. Special SPEED bevelled archwires have been recommended as finishing wires for enhanced torque expression by interaction of the spring clip and bevel.26 It would be interesting to know if these wires also have an effect on frictional properties. It is clear from the present results that the new slick elastomeric ties significantly reduce static friction at the wire/module interface. This reduction could be of benefit clinically. However, when considering the total friction generated at the bracket/archwire interface in a clinical situation, factors other than ligation are also involved, including binding between archwire and bracket as teeth are moved through a series of tipping and uprighting phases.27 It has been shown that the effect of the ligature remains significant irrespective of the bracket/archwire angulation,28 although the level of influence of the ligature force diminishes as the bracket/ archwire angulation increases.4 CONCLUSIONS
1. The new slick elastomeric modules from TP Orthodontics generate significantly less static friction at the module/archwire interface than do regular modules when tied normally. 2. The reduction in frictional resistance was considerably greater when the modules were lubricated with human saliva. 3. A figure-8 tie configuration significantly increases frictional resistance, but lubrication with human saliva produced a greater reduction in static friction with the slick modules than with regular modules tied in this way. 4. SPEED brackets generated less friction in general than did any other bracket type tested with regular modules in a normal tie configuration. 5. The use of lubricated, slick modules with any of the tested nonself-ligating bracket types resulted in a reduction of the friction to below SPEED values. 6. Loosely tied stainless steel ligatures offer the lowest frictional resistance of all the ligation methods tested. We thank Mr E. Harrington for his technical expertise in manufacturing the test apparatus. REFERENCES 1. Stoner MM. Force control in clinical practice. Am J Orthod 1960;46:163-8.
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2. Edwards GD, Davies EH, Jones SP. The ex vivo effect of ligation technique on the static frictional resistance of stainless steel brackets and archwires. Br J Orthod 1995;22:145-53. 3. Kusy RP, Whitley JQ. Coefficients of friction for arch wires in stainless steel and polycrystalline alumina bracket slots. I. The dry state. Am J Orthod Dentofacial Orthop 1990;98:300-12. 4. Frank CA, Nikolai RJ. A comparative study of frictional resistances between orthodontic bracket and archwire. Am J Orthod 1980;78:593-609. 5. Peterson L, Spencer R, Andreasen GA. Comparison of friction resistance for Nitinol and stainless steel wire in edgewise brackets. Quintessence Int 1982;13:563-71. 6. Keith O, Jones SP, Davies EH. The influence of bracket material, ligation force and wear on frictional resistance of orthodontic brackets. Br J Orthod 1993;20:109-15. 7. Bednar JR, Gruendeman GW, Sandrik JL. A comparative study of frictional forces between orthodontic brackets and archwires. Am J Orthod Dentofacial Orthop 1991;97:219-28. 8. Sims APT, 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. Eur J Orthod 1993;15:377-85. 9. Berger J. Self-ligation in the year 2000. J Clin Orthod 2000;2: 74-81. 10. Read-Ward GE, Jones SP, Davies EH. A comparison of selfligating and conventional orthodontic bracket systems. Br J Orthod 1997;24:309-17. 11. Rabinowicz E. Friction and wear of materials. New York: John Wiley and Sons; 1965. 12. Tidy DC. Frictional forces in fixed appliances. Am J Orthod Dentofacial Orthop 1989;96:249-54. 13. Omana H, Moore R, Bagby M, Erickson L. Frictional properties of ceramic brackets during simulated cuspid retraction [abstract]. J Dent Res 1992;71:A500. 14. Shivapuja PK, Berger J. A comparative study of conventional ligation and self-ligation bracket systems. Am J Orthod Dentofacial Orthop 1994;106:472-80.
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15. Hanson H. The SPEED system: a report on the development of a new Edgewise appliance. Am J Orthod 1980;78:243-65. 16. Voudouris JC. Interactive edgewise mechanisms: form and function comparison with conventional edgewise brackets. Am J Orthod Dentofacial Orthop 1997;111:119-40. 17. Kusy RP. Materials and appliances in orthodontics: brackets, arch wires, and friction. Curr Opin Dent 1991;1: 634-44. 18. Kusy RP, Whitley JQ. Frictional resistance of metal-lined ceramic brackets versus conventional stainless steel brackets and development of 3-D friction maps. Angle Orthod 2001;71:36474. 19. Kapila S, Angolkar PV, Duncanson MG Jr, Nanda RS. Evaluation of friction between edgewise stainless steel brackets and orthodontic wires for four alloys. Am J Orthod Dentofacial Orthop 1990;97:117-26. 20. Braun S, Bluesstein M, Moore BK, Benson G. Friction in perspective. Am J Orthod Dentofacial Orthop 1999:115;619-27. 21. McLaughlin RP, Bennett JC, Trevisi HJ. Systemized orthodontic treatment mechanics. St Louis: Mosby; 2001. 22. Loftus BP, Artun J. A model for evaluating friction during orthodontic tooth movement. Eur J Orthod 2001;23:253-61. 23. Tselepis M, Brockhurst P, West VC. The dynamic frictional resistance between orthodontic brackets and archwires. Am J Orthod Dentofacial Orthop 1994;106:131-8. 24. Saunders CR, Kusy RP. Surface topography and frictional characteristics of ceramic brackets. Am J Orthod Dentofacial Orthop 1994;106:76-87. 25. Berger J. The influence of the SPEED bracket’s self ligating design on force levels in tooth movement: a comparative in vitro study. Am J Orthod Dentofacial Orthop 1990;97:219-28. 26. Berger JL. SPEED appliance: a 14-year update on this unique self-ligating orthodontic mechanism. Am J Orthod Dentofacial Orthop 1994;105:217-23. 27. Nanda R. Biomechanics in orthodontics. Philadelphia: W. B. Saunders; 1996. 28. De Franco DJ, Spiller RE Jr, Von Fraunhofer JA. Frictional resistances using Teflon-coated ligatures with various bracketarchwire combinations. Angle Orthod 1995;65:63-74.
The effect of ligation method on friction in sliding mechanics Max Hain, BDS, MFDS RCS, BMSc,a Ashish Dhopatkar, BDS, MSc, FDS RCS, MOrth RCS,b and Peter Rock, BDS, DDS, FDS RCS, DOrth RCSc Birmingham, United Kingdom During orthodontic tooth movement with the preadjusted edgewise system, friction generated at the bracket/archwire interface tends to impede the desired movement. The method of ligation is an important contributor to this frictional force. This in vitro study investigated the effect of ligation method on friction and evaluated the efficacy of the new slick elastomeric modules from TP Orthodontics (La Porte, Ind), which are claimed to reduce friction at the module/wire interface. Slick modules were compared with regular nonslick modules, stainless steel ligatures, and the SPEED self-ligating bracket system (Strite Industries, Cambridge, Ontario, Canada). The effect of using slick modules with metal-reinforced ceramic (Clarity, 3M Unitek, Monrovia, Calif) and miniature brackets (Minitwin, 3M Unitek) was also examined. Results showed that, when considering tooth movement along a 0.019 ⫻ 0.025-in stainless steel archwire, saliva-lubricated slick modules can reduce static friction at the module/archwire interface by up to 60%, regardless of the bracket system. The SPEED brackets produced the lowest friction compared with the 3 other tested bracket systems when regular modules were used. The use of slick modules, however, with all of the ligated bracket types tested significantly reduced friction to below the values recorded in the SPEED groups. Loosely tied stainless steel ligatures were found to generate the least friction. (Am J Orthod Dentofacial Orthop 2003;123:416-22)
D
uring orthodontic space closure with sliding mechanics, a frictional force generated at the bracket/archwire interface tends to impede the desired movement. In clinical terms, the force applied must overcome this unknown frictional component and achieve the desired tooth movement. The “loss of applied force” has been commented on by a number of authors1,2 because it places additional strain on anchorage demands and leads to a reduction in the speed of tooth movement. A number of factors have been implicated in influencing frictional forces during orthodontic tooth movement. The effects of archwire material,3 dimensions,4,5 and bracket material6 have been investigated. The method of archwire ligation would appear to be an important determinant in the generation of friction, yet relatively few studies2 have looked at this interacFrom the Orthodontic Unit, School of Dentistry, University of Birmingham, Birmingham, United Kingdom. a Specialist registrar. b Lecturer. c Head of department. This study was supported by a research grant from TP Orthodontics. Reprint requests to: Dr A. A. Dhopatkar, Orthodontic Unit, School of Dentistry, St Chad’s Queensway, Birmingham B4 6NN, United Kingdom; e-mail, [email protected]. Submitted, April 2002; revised and accepted, August 2002. Copyright © 2003 by the American Association of Orthodontists. 0889-5406/2003/$30.00 ⫹ 0 doi:10.1067/mod.2003.14
416
tion. Loosely tied stainless steel ligatures are generally thought to generate less friction than standard elastomeric ligatures,7 although the increase in chairside time required to manipulate stainless steel ligatures has meant that they are still less popular in the clinical situation than elastomers. A new slick elastomeric module system (TP Orthodontics, La Porte, Ind) has recently been introduced; it claims to combine ease of use with low friction. An alternative approach to reducing friction has been to avoid using any form of ligature. This has been achieved by the self-ligating bracket systems,8,9 which have been shown to reduce friction in certain conditions.10 Friction can be defined as the force that acts at the surface between 2 objects when 1 object slides relative to the other. Its magnitude depends on the amount of normal force pushing the 2 surfaces together, the surface roughness, and the nature of the materials from which the surfaces are made.11 A number of components have been linked to this normal force, including engagement of archwires in brackets that are out of alignment, ligatures pressing the wire against the slot, and active torque in a rectangular wire when the tipping tendency is resisted by a 2-point contact between the bracket and the archwire. The relative contribution of these components varies according to the clinical situation.12
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The friction encountered during tooth movement can be considered in 2 distinct phases. Static friction is defined as the resistance that prevents initial tooth movement and is then replaced by dynamic or kinetic friction, which acts during the period of motion itself. Because tooth movement along an archwire occurs in very short steps rather than as continuous motion, static friction is considered to have a greater effect on preadjusted mechanics than dynamic friction.13 Shivapuja and Berger14 found that self-ligating brackets generated less friction than conventional brackets. Hanson15 found similar results and concluded that this might reduce treatment time. Voudouris16 found that self-ligating brackets, both with passive and active labial arms, produced less friction than conventional brackets in association with steel ligatures. ReadWard et al,10 however, found that self-ligating brackets produced less friction only under certain conditions. SPEED brackets (Strite Industries, Cambridge, Ontario, Canada) in particular, produced low friction in round wires, but friction increased greatly with rectangular wires. The effects of saliva on friction are controversial, because investigations carried out under dry conditions or with the addition of human or artificial saliva or water have produced conflicting results.10 Kusy17 stated that experiments conducted in artificial saliva were invalid because it is no substitute for human saliva. Bracket material has been shown to influence friction at the bracket/archwire interface, with ceramic brackets having been found to produce significantly more friction than stainless steel ones.13 Metal-reinforced ceramic brackets, however, are reported to function comparably with conventional stainless steel brackets in their frictional properties.18 Bracket size has also been identified as a contributory factor,19 and larger brackets are prone to increased friction because of the greater normal force exerted by the modules as a result of the additional stretching required for engagement. Some authors20 have questioned the value of experiments that do not consider occlusal forces; however, it is unlikely that these forces would simultaneously affect all bracket/archwire interfaces. In addition, the elastic nature of modules is likely to promote maintenance of wire/module contact during such disturbances. The aims of this study were to investigate the effect of ligation method on friction and to evaluate the manufacturer’s claim that the new slick elastomeric modules reduce friction at the wire/module interface.
Table I.
Composition of test groups
Group/code 1. VRD 2. VSD 3. VRW 4. VSW 5. VStD 6. VStW 7. VR8D 8. VS8D 9. VR8W 10. VS8W 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
SPD SPW CRD CSD CRW CSW MRD MSD MRW MSW
Specimen Victory brackets with regular modules in dry state Victory brackets with slick modules in dry state Victory brackets with regular modules soaked in saliva Victory brackets with slick modules soaked in saliva Victory brackets with stainless steel ligatures in dry state Victory brackets with stainless steel ligatures in saliva Victory brackets with regular modules tied is figure-8 pattern in dry state Victory brackets with slick modules tied in figure8 pattern in dry state Victory brackets with saliva-lubricated regular modules tied in figure-8 pattern Victory brackets with saliva lubricated slick modules tied in figure-8 pattern SPEED brackets dry SPEED brackets soaked in saliva Clarity brackets with regular modules dry Clarity brackets with slick modules dry Clarity brackets with regular modules in saliva Clarity brackets with slick modules in saliva Minitwin brackets with regular modules dry Minitwin brackets with slick modules dry Minitwin brackets with regular modules in saliva Minitwin brackets with slick modules in saliva
MATERIAL AND METHODS
A custom-made apparatus was constructed to record the resistance to movement of a stainless steel 0.019 ⫻ 0.025-in working archwire through test brackets. This archwire dimension was chosen because it is the recommended size for sliding mechanics with the 0.022-in system brackets used in the investigation.21 Straight lengths of 0.019 ⫻ 0.025-in stainless steel wire (3M Unitek, Monrovia, Calif), each 7 cm long, were used. Four types of maxillary premolar brackets were used, each incorporating ⫺7° torque and zero angulation: standard stainless steel (Victory Twin series, 3M Unitek), miniature twin (Minitwin, 3M Unitek), metalreinforced ceramic brackets (Clarity Twin, 3M Unitek), and SPEED. The elastic modules compared were regular grey and the new super-slick modules incorporating metafasix technology (TP Orthodontics). The test groups are listed in Table I. The brackets and archwires were cleaned with an alcohol wipe before the modules or ligatures were tied with mosquito forceps, 25 mm from the lower end of the archwire, to form a test unit. All units in the saliva
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Fig 1. Test apparatus.
groups were soaked in human saliva for 1 hour before testing. The dry groups were also tied 60 minutes before testing to minimize differences in elastic tension between the samples. The stainless steel ligatures were initially fully tightened and then unwound by 3 turns. Loose ligation was checked by rocking the ligature to confirm that there was a little play between both spans of the ligature and the archwire. The end of the ligature was then tucked in over the archwire. Testing was performed on an Instron 5544 machine (Instron Ltd, High Wycombe, Buckinghamshire, United Kingdom), with a crosshead speed of 20 mm/ min over an 8-mm stretch of archwire (Fig 1). The lower end of each test unit was attached to a heavy base block on the lower crosshead of the testing machine through a hollow screw. A 2-mm right-angle bend in the archwire allowed for freedom of rotational movement. This ensured that the bracket and wire specimens could self-align during a test run, allowing tip and torque to be effectively eliminated as variables so that the effect of ligation method on friction could be studied in isolation. Care was taken to align the archwire so that the sample was parallel with the vertical framework of the machine. The base block was positioned on the machine with a permanent guide mark to aid alignment. The bracket was pulled in a vertical direction by a loop of 0.018-in stainless steel wire (Fig 2), and the force required to initiate and maintain movement of the bracket over the 8-mm test
Fig 2. Close-up of test apparatus.
distance was measured. Figure 3 gives an example of the typical output for a single test. The program was set to highlight the maximum frictional force at initial movement, which was taken to represent the peak static frictional resistance (Fig 3). The initial measurements for groups 1 and 4 were repeated after 1 week to assess the reproducibility of the measurements.
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Results of general linear model ANOVA investigating 4 main variables
Table II.
Variable Module type Wet or dry Normal/F8 Bracket type
Fig 3. Typical output from testing machine for single test run plotted as load (N) vs distance (mm).
DF
Sum of squares
Mean square
F
P
1 1 1 3
26.45 72.88 93.46 6.21
26.45 72.88 93.46 2.07
53.93 148.61 190.56 4.76
.00 .00 .00 .00
Data set used to look at module type, effect of lubrication, and effect of tie configuration excluded values for stainless steel ligatures and SPEED brackets. Data set used to examine effect of bracket type excluded stainless steel and figure-8 ligated subsets.
Post hoc multiple comparisons examining effect of bracket type in more detail with Tukey tests
Table III.
SPEED Clarity MiniTwin
Standard straight wire
SPEED
Clarity
⫺0.097 0.67 ⫺0.55 0.07 ⫺0.24 0.38
⫺0.91* ⫺0.14* ⫺0.60 0.17
0.00* 0.62*
* Significant at 95% level. Family error rate ⫽ 0.05; individual error rate ⫽ 0.01; critical value ⫽ 3.66.
Fig 4. Histogram summarizing group comparison results.
Ten each of the regular and the slick modules were stretched by 1-mm and 2-mm amounts with the testing machine to determine the effect of the 2 module types on ligation force, and the force generated was recorded. The effect on friction of the 4 main variables—type of module, state of wetness, bracket type, and tie configuration—was investigated with analysis of variance (ANOVA) using the general linear model in Minitab (State College, Pa). Post hoc Tukey pairwise comparison tests were performed to determine the significance of these variances with respect to bracket type. Further comparison between each pair of test groups was then carried out with t tests. To control for type I error, the Bonferroni correction was used; it is derived by dividing the chosen alpha level of 0.05 by the number of tests. The corrected alpha level, 0.005, was used to determine significance. RESULTS
Results are presented in Figure 4 and Tables II through VI.
Table II shows the effects of module type, lubrication, tie configuration, and bracket type on static friction by means of ANOVA using the general linear model. The results indicate that each variable has an effect on the friction generated. Table III shows the effect of specific bracket types with post hoc Tukey comparison tests. Table IV shows general descriptive statistics for all variables and the comparison between dry and wet states using t tests. Figure 4 summarizes these results. The overall finding was that slick modules reduced friction by up to 60% compared with their regular counterparts; the reductions were greatest in association with saliva lubrication. SPEED brackets generated less friction than the other test brackets using regular modules. Stainless steel ligatures produced the least friction overall. Table V shows the force recorded during tensile stretching of regular and slick modules by 1-mm and 2-mm amounts. No statistically significant difference was found with t tests between the 2 module types. Table VI shows the results of the reproducibility study in the wet and dry states. There was no significant
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Table IV.
American Journal of Orthodontics and Dentofacial Orthopedics April 2003
Values for each variable and comparison between test specimens in dry and wet states (see Table I)
Group Code
Mean load (n)
SD
SE
1. VRD 3. VRW 5. VStD 7. VR8D 9. VR8W 11. SPD 13. CRD 15. CRW 17. MRD 19. MRW
2.78 2.06 0.00 4.56 3.61 1.63 2.71 2.09 2.47 2.19
0.28 0.28 0.02 0.65 0.56 0.28 0.31 0.23 0.24 0.20
0.07 0.07 0.00 0.17 0.14 0.07 0.08 0.06 0.06 0.05
Group Code 2. 4. 6. 8. 10. 12. 14. 16. 18. 20.
VSD VSW VStW VS8D VS8W SPW CSD CSW MSD MSW
Mean load (n)
SD
SE
t
P
1.92 0.83 0.01 4.98 1.51 1.61 2.49 1.29 1.96 0.71
0.29 0.22 0.02 0.56 0.25 0.23 0.54 0.27 0.25 0.13
0.07 0.06 0.00 0.15 0.06 0.06 0.14 0.07 0.07 0.03
8.11 13.50 ⫺0.59 ⫺1.89 13.29 0.21 1.31 8.62 5.76 24.16
.0000* .0000* .5600 .0700 .0000* .8400 .2000 .0000* .0000* .0000*
*
Significant at 95% level.
Table V.
Comparison of tensile properties between regular and slick modules
Variable
Mean load (n)
SD
SE
Regular modules stretched 1 mm Regular modules stretched 2 mm
1.12 2.37
0.14 0.15
0.05 0.05
Table VI.
Variable
Mean load (n)
SD
SE
t
P
Slick modules stretched 1 mm Slick modules stretched 2 mm
1.12 2.42
0.19 0.26
0.06 0.08
⫺0.04 ⫺0.58
.97 .57
Results of reproducibility study
Variable
Mean load (n)
SD
SE
Group 1 original Group 4 original
2.78 0.84
0.29 0.22
0.07 0.06
Variable
Mean load (n)
SD
SE
t
P
Group 1 repeated Group 4 repeated
2.63 0.94
0.20 0.19
0.05 0.05
1.68 ⫺1.35
.11 .19
difference between the original friction values and those recorded a week later. DISCUSSION
The variety of experimental methods used in the literature makes it difficult to compare the results of different studies of this type. The amount of freedom of movement of the bracket relative to the archwire appears to greatly affect results. Differences in the finish of the same materials (eg, surface smoothness of nickel-titanium archwires from different manufactures) also make it difficult to compare findings and to isolate individual contributing factors. For example, brackets differ not only in type and material but also in width and therefore in the amount of force exerted by the elastic modules. In addition, variation in timing measurements could affect the recorded values as a result of differing amounts of stress relaxation of the elastic modules. High variability of friction measurements has been commented on previously.22
Frictional forces measured in this study were comparable in magnitude and range with those reported by other investigators.12,23 The choice of crosshead speed was based on previous work,18 which showed that, from 10 to 0.0005 mm/min, the coefficient of friction for stainless steel archwires was unaffected. It has been shown previously that multiple testing has no adverse effects on wire/bracket couples24 in the absence of high angulation. The present results show that, when archwire/ bracket alignment was carefully controlled, the degree of friction generated at the module/wire interface was affected by type of module, state of wetness, bracket type, and tie configuration (Table II). The results also show that static frictional force is greater, in both the dry and lubricated states, with regular modules than with the new slick modules (Table IV). Soaking in saliva led to a reduction in friction for both types of test module, regardless of the bracket system. The reduction was greater for slick
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(50% to 60%) than for regular (10% to 30%) modules. Overall, under the test conditions, the slick modules produced up to 60% less friction than their regular counterparts. The fact that both module types exhibited similar physical properties on stretching (Table V) suggests that the normal force generated by both would be comparable. It is likely, therefore, that the differences in frictional properties shown here are related to variations in surface characteristics. Loosely tied stainless steel ligatures generally produced negligible friction in both dry and wet situations, and there was no effect on frictional force attributable to lubrication (Table IV). It is clear, therefore, that loose stainless steel ligatures produce less friction than both types of test module. However, the convenience and speed of application of elastomeric modules are likely to ensure their continued popularity in the clinical setting. In addition, although the low normal force exerted by steel ligatures aids in minimizing friction, torque expression might be impaired by their use because of incomplete location of the archwire in the bracket slot. Tying modules in a figure-8 pattern is a useful procedure to ensure full archwire engagement. These results show that this procedure produces increased friction with either module type (Table IV), although there was a significant reduction in friction when lubrication was added. The magnitude of this improvement was dramatically greater with the slick modules (70% compared with 21% for regular modules), and the overall reduction was again greater than 50%. These results showed no clear difference between friction recorded for test units incorporating standard straight wires and SPEED, Clarity, or Minitwin brackets (Table III). There was, however, a statistically significant difference in performance between the SPEED and the ceramic test groups. The SPEED brackets were found to produce significantly less friction than the metal-reinforced ceramic brackets. But when the ceramic brackets were ligated with lubricated slick modules, the mean friction values fell below the levels recorded for the SPEED groups (Table IV). In general, the ceramic brackets were also found to produce more friction than the miniature brackets (Tables III and IV). This is probably a reflection of the larger size of the ceramic brackets around which the stretched modules exert a greater normal force. Berger,25 using a combination of round and 0.016 ⫻ 0.022-in stainless steel archwires, found that SPEED brackets consistently produced lower friction compared with stainless steel brackets with steel or elastomeric ligation. The present results suggest, however, that when SPEED brackets are used in combina-
tion with 0.019 ⫻ 0.025-in stainless steel archwires, their frictional properties might be less favorable. This agrees with the results of Read-Ward et al10 and could be related to the spring-clip design feature, which exerts a normal force proportional to the size of the wire being engaged. Special SPEED bevelled archwires have been recommended as finishing wires for enhanced torque expression by interaction of the spring clip and bevel.26 It would be interesting to know if these wires also have an effect on frictional properties. It is clear from the present results that the new slick elastomeric ties significantly reduce static friction at the wire/module interface. This reduction could be of benefit clinically. However, when considering the total friction generated at the bracket/archwire interface in a clinical situation, factors other than ligation are also involved, including binding between archwire and bracket as teeth are moved through a series of tipping and uprighting phases.27 It has been shown that the effect of the ligature remains significant irrespective of the bracket/archwire angulation,28 although the level of influence of the ligature force diminishes as the bracket/ archwire angulation increases.4 CONCLUSIONS
1. The new slick elastomeric modules from TP Orthodontics generate significantly less static friction at the module/archwire interface than do regular modules when tied normally. 2. The reduction in frictional resistance was considerably greater when the modules were lubricated with human saliva. 3. A figure-8 tie configuration significantly increases frictional resistance, but lubrication with human saliva produced a greater reduction in static friction with the slick modules than with regular modules tied in this way. 4. SPEED brackets generated less friction in general than did any other bracket type tested with regular modules in a normal tie configuration. 5. The use of lubricated, slick modules with any of the tested nonself-ligating bracket types resulted in a reduction of the friction to below SPEED values. 6. Loosely tied stainless steel ligatures offer the lowest frictional resistance of all the ligation methods tested. We thank Mr E. Harrington for his technical expertise in manufacturing the test apparatus. REFERENCES 1. Stoner MM. Force control in clinical practice. Am J Orthod 1960;46:163-8.
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2. Edwards GD, Davies EH, Jones SP. The ex vivo effect of ligation technique on the static frictional resistance of stainless steel brackets and archwires. Br J Orthod 1995;22:145-53. 3. Kusy RP, Whitley JQ. Coefficients of friction for arch wires in stainless steel and polycrystalline alumina bracket slots. I. The dry state. Am J Orthod Dentofacial Orthop 1990;98:300-12. 4. Frank CA, Nikolai RJ. A comparative study of frictional resistances between orthodontic bracket and archwire. Am J Orthod 1980;78:593-609. 5. Peterson L, Spencer R, Andreasen GA. Comparison of friction resistance for Nitinol and stainless steel wire in edgewise brackets. Quintessence Int 1982;13:563-71. 6. Keith O, Jones SP, Davies EH. The influence of bracket material, ligation force and wear on frictional resistance of orthodontic brackets. Br J Orthod 1993;20:109-15. 7. Bednar JR, Gruendeman GW, Sandrik JL. A comparative study of frictional forces between orthodontic brackets and archwires. Am J Orthod Dentofacial Orthop 1991;97:219-28. 8. Sims APT, 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. Eur J Orthod 1993;15:377-85. 9. Berger J. Self-ligation in the year 2000. J Clin Orthod 2000;2: 74-81. 10. Read-Ward GE, Jones SP, Davies EH. A comparison of selfligating and conventional orthodontic bracket systems. Br J Orthod 1997;24:309-17. 11. Rabinowicz E. Friction and wear of materials. New York: John Wiley and Sons; 1965. 12. Tidy DC. Frictional forces in fixed appliances. Am J Orthod Dentofacial Orthop 1989;96:249-54. 13. Omana H, Moore R, Bagby M, Erickson L. Frictional properties of ceramic brackets during simulated cuspid retraction [abstract]. J Dent Res 1992;71:A500. 14. Shivapuja PK, Berger J. A comparative study of conventional ligation and self-ligation bracket systems. Am J Orthod Dentofacial Orthop 1994;106:472-80.
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15. Hanson H. The SPEED system: a report on the development of a new Edgewise appliance. Am J Orthod 1980;78:243-65. 16. Voudouris JC. Interactive edgewise mechanisms: form and function comparison with conventional edgewise brackets. Am J Orthod Dentofacial Orthop 1997;111:119-40. 17. Kusy RP. Materials and appliances in orthodontics: brackets, arch wires, and friction. Curr Opin Dent 1991;1: 634-44. 18. Kusy RP, Whitley JQ. Frictional resistance of metal-lined ceramic brackets versus conventional stainless steel brackets and development of 3-D friction maps. Angle Orthod 2001;71:36474. 19. Kapila S, Angolkar PV, Duncanson MG Jr, Nanda RS. Evaluation of friction between edgewise stainless steel brackets and orthodontic wires for four alloys. Am J Orthod Dentofacial Orthop 1990;97:117-26. 20. Braun S, Bluesstein M, Moore BK, Benson G. Friction in perspective. Am J Orthod Dentofacial Orthop 1999:115;619-27. 21. McLaughlin RP, Bennett JC, Trevisi HJ. Systemized orthodontic treatment mechanics. St Louis: Mosby; 2001. 22. Loftus BP, Artun J. A model for evaluating friction during orthodontic tooth movement. Eur J Orthod 2001;23:253-61. 23. Tselepis M, Brockhurst P, West VC. The dynamic frictional resistance between orthodontic brackets and archwires. Am J Orthod Dentofacial Orthop 1994;106:131-8. 24. Saunders CR, Kusy RP. Surface topography and frictional characteristics of ceramic brackets. Am J Orthod Dentofacial Orthop 1994;106:76-87. 25. Berger J. The influence of the SPEED bracket’s self ligating design on force levels in tooth movement: a comparative in vitro study. Am J Orthod Dentofacial Orthop 1990;97:219-28. 26. Berger JL. SPEED appliance: a 14-year update on this unique self-ligating orthodontic mechanism. Am J Orthod Dentofacial Orthop 1994;105:217-23. 27. Nanda R. Biomechanics in orthodontics. Philadelphia: W. B. Saunders; 1996. 28. De Franco DJ, Spiller RE Jr, Von Fraunhofer JA. Frictional resistances using Teflon-coated ligatures with various bracketarchwire combinations. Angle Orthod 1995;65:63-74.