Self Ligating vs Conventional Twin Brackets

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SHORT COMMUNICATION

Self-ligating vs conventional twin bracketsduring en-masse space closure with sliding

mechanicsPeter G. Miles

Caloundra, Queensland, Australia

Introduction: The aim of this study was to compare the rate of en-masse space closure with sliding

mechanics between passive self-ligating SmartClip brackets (3M Unitek, Monrovia, Calif) and conventional

twin brackets ligated with stainless steel ligatures. Methods: Nineteen patients including 20 arches

participated in this prospective trial with 0.018-in slot brackets. All patients had first premolar extractions in

at least 1 arch, with the second premolar and the first molar distal to the extraction site bonded with

SmartClip brackets on 1 side and conventional twin brackets on the other. The sides were alternated with

each consecutive patient. Space closure was achieved on 0.016 0.022-in stainless steel wires with

nickel-titanium coil springs activated 6 to 9 mm. The patients were recalled every 5 weeks until 1 side hadclosed. The distances from the mesial aspect of the canine bracket to the distal aspect of the first molar

bracket were recorded before and after space closure, and an average rate of space closure per month was

calculated. Results: Thirteen patients completed the trial (14 arches); the median rates of tooth movement

for the SmartClip bracket side (1.1 mm per month) and the conventional twin bracket side (1.2 mm per month)

were not significantly different ( P .86). Conclusions: There was no significant difference in the rate of

en-masse space closure between passive SmartClip brackets and conventional twin brackets tied with

stainless steel ligatures. (Am J Orthod Dentofacial Orthop 2007;132:223-5)

During premolar extraction treatment, the orth-

odontist has several options for space closure.

A popular method is en-masse space closure

with sliding mechanics and coil springs. Some self-

ligating brackets are labeled as passive and promoted

on the premise that elimination of ligatures reduces

friction and allows for faster sliding mechanics. If true,

self-ligating brackets could reduce overall treatment

time.1 Two retrospective studies of patients treated with

Damon SL brackets reported reductions in treatment

time of 4 to 6 months and 4 to 7 visits during active

treatment.1,2 However, a recent prospective study com-

paring Damon 2 with conventional twin brackets found

that the passive Damon 2 bracket was no better during

initial alignment than a conventional bracket ligated

with elastomeric modules.3 In a similar study, passiveSmartClip brackets were no more efficient during initial

alignment with identical wire sequences and cross-

sections.4

One can then surmise that, if there is a potential

time saving with passive self-ligating brackets that is

not during initial alignment, it must be at a later stage

of treatment. Perhaps in extraction patients, the reduced

normal force of friction with passive self-ligating

brackets allows faster sliding mechanics during space

closure and therefore reduces treatment time. The aim

of this study was to compare the rates of space closure

between conventional twin brackets ligated with stain-

less steel (SS) ligatures and passive self-ligating Smart-

Clip brackets (3M Unitek, Monrovia, Calif).

MATERIAL AND METHODS

Nineteen consecutive patients who met the selec-

tion criteria were drawn from the author’s private

orthodontic practice to obtain 20 arches for compari-son. The criteria included extraction of the first premo-

lars in at least 1 arch, no missing teeth other than third

molars, and no asymmetric mechanics or asymmetric

elastic wear during space closure.

Eligible subjects were assigned to 1 of 2 groups in

a split-mouth design with sides alternated with each

consecutive subject. All mandibular anterior teeth were

bonded with conventional metal 0.018-in MBT pre-

scription brackets (3M Unitek), and the maxillary

anterior teeth were bonded with Clarity MBT prescrip-

tion brackets (3M Unitek). On 1 side, the second

Senior lecturer, University of Queensland Dental School, Brisbane; and private

practice, Caloundra, Queensland, Australia.

Reprint requests to: Peter G. Miles, 10 Mayes Ave, Caloundra, Queensland

4551, Australia; e-mail, [email protected].

Submitted, revised and accepted, April 2007.

0889-5406/$32.00

Copyright © 2007 by the American Association of Orthodontists.

doi:10.1016/j.ajodo.2007.04.028

223



premolar and the first molar distal to the extraction site

were bonded with 0.018-in SmartClip MBT prescrip-

tion brackets, and the contralateral side was bonded

with conventional metal MBT prescription brackets.

All patients were told the purpose of the study but wereunaware of which side had the SmartClip or the

conventional twin brackets.

After initial alignment, a 0.016 0.022-in SS

archwire was placed with a soldered hook mesially to

the canines and then left in place for 5 weeks. The 6

anterior teeth were consolidated with elastic chain, and

the archwire ended flush with the distal aspect of the

first molar bracket on each side. After 5 weeks of

alignment, 9-mm nickel-titanium (Ni-Ti) medium Sen-

talloy (150 g) coil springs (GAC International, Bohe-

mia, NY) were placed across the extraction sites from

the bracket hook on the first molar; the spring was

activated between 6 and 9 mm and then ligated with an

SS ligature to the archwire hook mesial to the canine.

The space between the mesial aspect of the canine

bracket and the distal aspect of the first molar bracket

was measured intraorally by using a digital caliper (150

mm ECP-015D digiMax caliper, Moore and Wright,

Buchs, Switzerland) to the nearest 0.1 mm. Three

measurements were taken, and, if there was any dis-

crepancy, the 2 closest were recorded and averaged.

The patients were recalled every 5 weeks, excess wire

was clipped distal to the molar brackets, the springs

were checked for 6 to 9 mm activation, and the spaces

were measured until 1 extraction space had closed. Thedifference between the initial and the final measure-

ments was calculated to give the total amount of space

closure, and this was divided by the number of months

to give the rate of space closure in millimeters per

month. Because the small sample size makes an as-

sumption of normality difficult, a nonparametric Wil-

coxon signed rank test was used to assess the data.

RESULTS

Nineteen patients were enrolled in the study; 1

subject had both arches involved, for a total of 20

arches. Six subjects were excluded during the study.Two had at least 1 extraction space closed during the

initial alignment phase, 1 moved away, 1 had a mea-

surement not recorded, 1 had the anterior elastic chain

break resulting in canine drift distally, and 1 had a loose

molar bracket during space closure. The final sample

included 13 subjects and 14 arches (8 female, 5 male;

median age, 13.1 years; range, 11.8-29.4 years; 11

maxillary arches, 3 mandibular arches). Because the

rates of space closure were similar in both arches and

also for the only adult patient in the group, the data

were pooled. The median space to be closed was 4.9

mm for both groups, and the median calculated rates of

movement were 1.1 mm per month for SmartClip and

1.2 mm per month for conventional twin brackets. The

Wilcoxon signed rank test showed no statistically

significant difference (P

.86) between the medianrates of space closure (95% CI, 0.29 to 0.20).

DISCUSSION

Previous in-vitro studies of self-ligating brackets

clearly showed that passive ligation results in less

friction than active ligation.5-7 However, it was as-

sumed that, along with low friction in vitro, come more

rapid space closure and reduced treatment time in vivo.

The results of this study demonstrate that the rates of

space closure were almost identical with the passive

SmartClip bracket and the conventional brackets tied

with SS ligatures distal to the extraction site. Clinicians

can therefore use their preferred bracket type (conven-

tional or passive self-ligating) without it affecting the

rate of space closure in extraction patients. SS ligatures

were tied normally with no attempt to keep them loose,

so this would seem to offer no advantage during

en-masse space closure when the teeth are already

leveled and aligned. However, if a different method

were used such as modules or a chain tied around the

bracket, the higher resistance to sliding might impact

the rate of space closure. A previous split-mouth study

comparing Ni-Ti springs with a stretched elastomeric

module showed that springs were superior to the

module for en-masse space closure.8 However, noattempt was made to equalize the initial forces applied.

The same authors, when comparing 150-g springs (as

used in this study) with 200-g springs, found no clinical

difference in the rate of space closure.9 A randomized

clinical trial of a 22-in slot preadjusted bracket with

0.019 0.025-in SS wires compared active ligatures,

power chain, and Ni-Ti springs during space closure.10

The Ni-Ti coil springs (activated no more than 9 mm)

were found to achieve the most rapid rate of space

closure at 0.81 mm per month. These authors concluded

that intermaxillary elastics were not a factor in the rate

of space closure. In a split-mouth comparison of elas-tomeric chain vs Ni-Ti coil springs with a 22-in slot

system and 0.019 0.025-in SS wires, no statistically

significant difference was found in another study.11 The

elastomeric chain achieved movement of 0.21 mm per

week (about 0.9 mm per month), whereas the 9-mm

Ni-Ti springs (stretched the whole length of the span)

achieved 0.26 mm per week (about 1.1 mm per month);

this is similar to the median rate of space closure

(1.1-1.2 mm per month) in this study. Although these

previous studies used a 22-in slot system, the rates of

space closure were less than or similar to the 18-in slot

American Journal of Orthodontics and Dentofacial Orthopedics

August 2007

224 Miles



system used in this study. It would therefore appear that

the slot size in conjunction with the appropriate wire

size makes minimal clinical difference in the rate of

space closure. In addition, the use of the passive

SmartClip bracket made no difference to the rate of space closure when compared with the conventional

twin bracket ligated with SS ligature. It appears that,

once the initial static friction in either system is

overcome, the residual force with the 150-g spring is

sufficient to produce similar rates of movement. There-

fore, the clinical choice of slot size and bracket ligation

(passive self-ligating vs SS ligature) for sliding me-

chanics can be based on practitioner preference. Al-

though self-ligating brackets save time compared with

conventional brackets when untying and ligating, once

a SS ligature is tied at the initial placement of the SS

archwire for space closure, it can be left for the entire

duration of space closure without retying.2,12 The time

saved for ligation would be greater at wire changes

during initial alignment and in the final detailing stages

of treatment.

A disadvantage of a slit-mouth design as in this

study is the potential for the archwire to slide to 1 side,

but it was thought that the soldered posts would prevent

or minimize this. The canine could not move if the

archwire slid because the 6 anterior teeth were consol-

idated with elastic chain. If the archwire was to move,

it would be expected to be toward the side with less

friction; this would affect the measurements if taken

from a fixed point on the archwire. Because themeasurements were taken bracket to bracket, this

should not impact the measured changes. It could be

argued that the use of conventional brackets and chain

in the anterior would increase friction in the system,

but, during first premolar extraction space closure, the

archwire slides distally to the extraction site, which is

where friction would be critical, so this should not

influence the result. The archwires in this study ex-

tended to the first molars during space closure, and, if

extended to the second molars, this might affect the rate

of closure. However, because a tube is normally used

on second molars even in self-ligating bracket systems,it is unlikely to alter the outcome. With a small sample

size, it is possible that a type II error was made and that

the null hypothesis was accepted when it should have

been rejected (no difference was found when it would

have been found with a larger sample with higher

power), so a larger sample would be preferable. Be-

cause the 95% CI for the differences in the medians

(0.29 to 0.20 mm) spans either side of zero (indi-

cating no statistically significant difference) and the

range is small (only 0.2-0.3 mm), it is still quite likely

that a larger sample would still find no clinically

significant difference.

Further prospective research with identical wire

progressions and mechanics is required to determine

whether there is any reduction in overall treatmentduration with self-ligating brackets. The findings of this

and previous research with the same wire sequences

showed no differences between passive self-ligating

brackets and conventional twin brackets during either

initial alignment or space closure.3,4 Therefore, any

treatment time savings might still be possible during the

latter stages of treatment or only in certain types of

patients, or there might be no time saving.

CONCLUSIONS

With Ni-Ti coil springs with the wire sequence and

bracket prescription as used in this study, there was nodifference in the rates of space closure between the

passive self-ligating SmartClip bracket and the conven-

tional twin bracket ligated with SS ligatures.

REFERENCES

1. Eberting JJ, Straja SR, Tuncay OC. Treatment time, outcome,

and patient satisfaction comparisons of Damon and conventional

brackets. Clin Orthod Res 2001;4:228-34.

2. Harradine NWT. Self-ligating brackets and treatment efficiency.

Clin Orthod Res 2001;4:220-7.

3. Miles PG, Weyant RJ, Rustveld L. A clinical trial of Damon 2 vs.

conventional twin brackets during initial alignment. Angle

Orthod 2006;76:480-5.4. Miles PG. SmartClip versus conventional twin brackets for

initial alignment: is there a difference? Aust Orthod J 2005;21:

123-7.

5. Pizzoni L, Raunholt G, Melsen B. Frictional forces related to

self-ligating brackets. Eur J Orthod 1998;20:283-91.

6. 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. Eur J Orthod 1993;15:377-85.

7. Shivapuja PK, Berger J. A comparative study of conventional

ligation and self-ligation bracket systems. Am J Orthod Dento-

facial Orthop 1994;106:472-80.

8. Samuels RH, Rudge SJ, Mair LH. A comparison of the rate of

space closure using nickel-titanium spring and an elastomeric

module: a clinical study. Am J Orthod Dentofacial Orthop1993;103:464-7.

9. Samuels RH, Rudge SJ, Mair LH. A clinical study of space

closure with nickel-titanium coil spring and an elastic module.

Am J Orthod Dentofacial Orthop 1998;114:73-9.

10. Dixon V, Read MJF, O’Brien KD, Worthington HV, Mandall

NA. A randomized clinical trial to compare three methods of

orthodontic space closure. J Orthod 2002;29:31-6.

11. Nightingale C, Jones SP. A clinical investigation of force

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12. Berger J, Byloff FK. The clinical efficiency of self-ligated

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American Journal of Orthodontics and Dentofacial Orthopedics

Volume 132, Number 2

Miles 225

Self Ligating vs Conventional Twin Brackets

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