Efficiency of self-ligating vs conventionally ligated brackets during initial alignment

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Efficiency of self-ligating vs conventionallyligated brackets during initial alignment

Emily Ong,a Hugh McCallum,b Mark P. Griffin,c and Christopher Hod

Brisbane and Herston, Queensland, Australia

Introduction: The aim of this study was to compare the efficiency of self-ligating (SL) and conventionallyligated (CL) brackets during the first 20 weeks of extraction treatment. Methods: Study models of 50 consec-utive patients who had premolar extractions in the maxillary and/or mandibular arch, 0.022 3 0.028-in slotbrackets, and similar archwire sequences were examined. Forty-four arches received SL Damon 3MXbrackets (Ormco, Glendora, Calif), and 40 arches received either CL Victory Series (3M Unitek, Monrovia,Calif) or Mini-Diamond (Ormco) brackets. The models were evaluated for anterior arch alignment, extractionspaces, and arch dimensions at pretreatment (T0), 10 weeks (T1), and 20 weeks (T2). Results: There were nosignificant differences between the SL and CL groups at 20 weeks in irregularity scores (mandibular arch,P 5 0.54; maxillary arch, P 5 0.81). There were no significant differences in passive extraction space closuresbetween the SL and CL groups (mandibular arch, T0-T2, P 5 0.85; maxillary arch, T0-T2, P 5 0.33). Mandibularintercanine widths increased from T0 to T2: 1.96 and 2.86 mm in the SL and CL groups, respectively. This was notsignificant between the groups (P 5 0.31). Logistic regression did not show a difference between the SL and CLbracket groups. Conclusions: SL brackets were no more efficient than CL brackets in anterior alignment orpassive extraction space closure during the first 20 weeks of treatment. Ligation technique is only one ofmany factors that can influence the efficiency of treatment. Similar changes in arch dimensions occurred,irrespective of bracket type, that might be attributed to the archform of the archwires. (Am J OrthodDentofacial Orthop 2010;138:138.e1-138.e7)

Bracket designs have undergone continual modifi-cations since fixed appliances were first used inorthodontics. The quest to improve treatment

efficiency has culminated in many modern edgewiseappliances. Recently, the promotion of self-ligating(SL) brackets has incited much controversy. Advocatesclaim that low-friction SL brackets coupled with lightforces enhance the rate of tooth movement and decreasetreatment time. Other advantages include decreasedappointment times, improved oral hygiene, increasedpatient acceptance, and superior treatment results.1,2

Most claims of SL brackets have been extrapolatedfrom in-vitro studies. A recent systematic review

aPostgraduate student, Discipline of Orthodontics, School of Dentistry,

University of Queensland, Brisbane, Queensland, Australia.bSenior Dental Specialist (Orthodontics), Royal Children’s Hospital, Herston,

Queensland, Australia.cResearch fellow, School of Population Health, University of Queensland,

Herston, Queensland, Australia.dAssociate professor, Discipline of Orthodontics, School of Dentistry, Univer-

sity of Queensland, Brisbane, Queensland, Australia.

The authors report no commercial, proprietary, or financial interest in the prod-

ucts or companies described in this article.

Reprint requests to: Christopher Ho, University of Queensland, School of Den-

tistry, 200 Turbot St, Brisbane, QLD 4000, Australia; e-mail, Christopher_Ho@

health.qld.gov.au.

Submitted, October 2009; revised and accepted, January 2010.

0889-5406/$36.00

Copyright � 2010 by the American Association of Orthodontists.

doi:10.1016/j.ajodo.2010.03.020

highlighted the limitations of in-vitro studies.3 In partic-ular, studies that demonstrate reduced friction in SLbrackets compared with conventionally ligated (CL)brackets have been coupled with small-diameter wiresin well-aligned arches with no tip and torque.4-9 In-vitro studies are limited because they cannot compre-hensively simulate a clinical scenario. Many variablescan influence the amount of friction generated in a fixedappliance system. These include archwire and bracketcomposition,10 archwire dimension,11 bracket slotdimension and design, interbracket distance, deflectionof the archwire,12 and biologic factors such as saliva11

and perturbations.13 Therefore, it is questionablewhether the use of SL brackets translates into clinicalbenefits such as decreased resistance to sliding, fastertooth movement, and increased treatment efficiency.

Several in-vivo studies have compared theefficiency of SL and CL brackets during various stagesof treatment with conflicting results. These studiesmeasured treatment efficiency in terms of total treat-ment times, numbers of appointments, and tooth move-ment during initial alignment and active space closure.Early retrospective studies reported up to 6 months’reduction in total treatment time and 7 fewer appoint-ments with SL brackets.2,14 Subsequent well-designedretrospective and prospective studies reported no signif-icant differences during initial alignment or active spaceclosure with various SL and CL brackets.15-20

138.e1

mailto:[email protected]

mailto:[email protected]

138.e2 Ong et al American Journal of Orthodontics and Dentofacial Orthopedics

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Miles et al15 and Miles17 postulated that SL bracketsmight provide a measurable benefit in extractionpatients. Additionally, Scott et al19 suggested that SLbrackets might encourage passive space closure duringinitial alignment. There is a relative lack of evidencecomparing the efficiency of SL and CL brackets inextraction patients because most studies have investi-gated mixed samples. Only 2 clinical trials havecompared SL and CL brackets solely in extractionpatients.16,19 One study investigated the initialalignment phase and reported no difference betweenSL and CL brackets.19 Neither clinical trial investigatedthe efficiency of passive space closure during alignment.If there is a measurable advantage of SL brackets, then itshould be most apparent during alignment and spaceclosure when the bracket slides along the archwire. Inthis study, passive space closure was defined as the ex-traction space closure during alignment without activespace-closing mechanics. The amount of passive spaceclosure varies greatly between patients, but this param-eter has not been investigated before. If the use of SLbrackets could achieve greater passive space closure,there would be less extraction space to close actively.This could reduce the overall treatment time. Further-more, this might minimize the detrimental effects of ac-tive force application such as root resorption.

In this study, we aimed to determine whether thereare significant differences in the efficiency of anteriortooth alignment and the amount of passive space closurebetween SL and CL brackets. Concomitant changes inarch dimensions were also compared between the SLand CL bracket groups.

MATERIAL AND METHODS

Ethical approval was obtained from the Dental Sci-ences and Research Ethics Committee of the Universityof Queensland School of Dentistry and the Royal Child-ren’s Hospital and Health Services District Ethics Com-mittee. Study models of 50 consecutive patients whoreceived comprehensive full fixed appliance treatmentwith 0.022 3 0.028-in slot brackets at the School ofDentistry, University of Queensland and the RoyalChildren’s Hospital were examined.

The dental school patients were treated by postgrad-uate students under the supervision of an experiencedorthodontist. Royal Children’s Hospital patients weretreated by an experienced orthodontist (H.M. or C.H.).

Patient records were included if they satisfied thefollowing inclusion criteria: (1) treatment beganbetween 10 and 18 years of age; (2) treatment includedbilateral mandibular or maxillary extractions followedby fixed appliance therapy; (3) intraoral photos and

study models were available at pretreatment (T0), 10weeks (T1), and 20 weeks (T2) postbonding; (4)treatment included 0.022 3 0.028-in slot brackets (SLbrackets, Damon 3MX, Ormco, Glendora, Calif; orCL brackets, Victory Series, 3M Unitek, Monrovia,Calif, or Mini-Diamond, Ormco); (5) treatment beganwith an initial archwire of 0.014-in copper-nickel-titanium (Damon archform, Ormco), followed by0.014 3 0.025-in copper-nickel-titanium (Damon arch-form, Ormco)21; (6) the patients were reviewed every 5weeks; and (7) the first archwire was left in place untilthe teeth were passively engaged in all bracket slotsbefore proceeding to the second archwire.

The following exclusion criteria were applied: treat-ment with nonsymmetrical extractions, no impacted orunerupted permanent teeth anterior to the first molarsin the arch that received extractions, treatment withremovable appliances or rapid maxillary expansionappliances, and incomplete records at a time point.

Fifty patients (20 male, 30 female) fulfilled theinclusion criteria.

Pretreatment characteristics were recorded includ-ing the patient’s age bonding, sex, mandibular and max-illary crowding, irregularity index, extraction space,intercanine width, intermolar width, and arch depth.

All study models were evaluated by using Little’sirregularity index22 to quantify the alignment of the 6anterior teeth. Crowding was calculated as the differ-ence between the sum of tooth widths and arch circum-ference taken from the line of best fit, through thecontact points mesial to the first molars, on a photocopyof the patient’s occlusal archform.

Extraction space was measured from the closestpoints on the adjacent teeth before extraction. Themesiodistal widths of the teeth to be extracted werenot used because they were often displaced from thearchform; this decreased the extraction space to beclosed. Similarly, extraction spaces at T1 and T2were measured from the closest points on the crownsof the teeth on either side of the extraction space. Thecontact points were not used because many teeth wererotated.

Intercanine widths were measured from the cusptips of the canines. Measurements were not takenfrom the gingival margin because the quality of the gin-gival impression was inconsistent. Intermolar widthswere measured from the central and mesial occlusalpits of the mandibular and maxillary first molarsbecause this area of the impression was clearer thanthe cusps. Arch depth was measured as the perpendicu-lar distance from a line drawn through the mesialcontact points of the first molars to the labial surfacesof the central incisors.

Table I. Sample sizes at T0, T1, and T2

Mandibular arch Maxillary arch

SL CL SL CL

T0 19 18 25 22

T1 19 18 25 22

T2 19 14 25 16

American Journal of Orthodontics and Dentofacial Orthopedics Ong et al 138.e3Volume 138, Number 2

Wax was applied to cover the brackets on eachmodel before measurement. An identification numberwas assigned to each model. Therefore, the researcher(E.O.) was blinded to patient name, time point, andbracket type during data collection to minimize system-atic error. The study models were measured with elec-tronic calipers with sharpened tips that were accurateto 0.01 mm (Mitutoyo, Tokyo, Japan). All model mea-surements were made by the principal researcher (E.O.).

Statistical analysis

The difference in irregularity scores was used todetermine the sample size. Based on a previous study,a clinically significant difference of 0.98 mm in irregu-larity score, at a power of 80% and a level of signifi-cance of 0.05, would require a minimum of 17patients per treatment group.19 In the final sample of50 patients, 44 arches were treated with SL bracketsand 40 arches with CL brackets.

Statistical analysis was performed by using Mini-tab software (release 15, Minitab, State College, Pa)and SAS software (version 9.2, SAS Institute, Cary,NC). The mandibular and maxillary arches wereanalyzed separately. Descriptive statistics were calcu-lated, and the data were checked for normality.Two-sample t tests were performed at T0, T1, andT2 to compare the bracket groups for irregularityscores, residual extraction spaces, intercanine widths,intermolar widths, and arch depths. The amounts ofpassive extraction space closure from T0 to T1, T1to T2, and T0 to T2 for each bracket group werealso calculated and compared by using 2-samplet tests. A chi-square goodness-of-fit test was used todetermine whether the male-to-female ratio wassignificantly different between the bracket groups.Logistic regression was also used to determinewhether there was a difference between the SL andCL bracket groups. Regression coefficients and confi-dence intervals were calculated for each variable (age,sex, irregularity index, intercanine width, intermolarwidth, and arch depth) for both arches. Multiple impu-tation was used to account for missing data.

Intraexaminer reliability was assessed by remeasur-ing 20 subjects at least 4 weeks after the originalmeasurements. A t test was performed to compare thefirst and second measurements.

RESULTS

Intraexaminer reliability was high. There were nostatistically significant differences between the firstand second measurements for irregularity index(P 5 0.51) and extraction space closure (P 5 0.38).

The average differences between the measurementswere 0.07 6 0.52 mm for the irregularity index and0.06 6 0.33 mm for extraction space.

Fifty patients (20 male, 30 female) fulfilled the in-clusion criteria. This gave a total of 84 arches; 44 archeswere treated with SL brackets and 40 arches with CLbrackets. In the CL sample, 18 arches received Mini-Diamond brackets. The numbers of arches included inthe statistical analysis for each bracket group at T0,T1, and T2 are summarized in Table I. No SL archeswere excluded. Six CL maxillary arches were excludedfrom analysis at T2 because 5 models were missing, and1 patient received a different archwire sequence. Fourmandibular arches were also excluded at T2 becauseof missing models.

The mean irregularity index scores decreased inboth bracket groups over time (Table II). Both groupshad greater decreases in irregularity during the first 10weeks of treatment compared with the subsequent 10weeks.

Over 20 weeks, the mean irregularity scores in theSL group decreased from 10.88 to 2.84 mm in the man-dibular arch, and from 11.98 to 4.37 mm in the maxil-lary arch. Scores in the CL group decreased from12.52 to 2.45 mm in the mandibular arch, and from12.53 to 4.16 mm in the maxillary arch.

There were no statistically significant differencesbetween the treatment groups at T1 or T2 in the mandi-ble (T1, P 5 0.81; T2, P 5 0.54) or the maxilla (T1,P 5 0.87; T2, P 5 0.81).

For passive extraction space closure, the residual ex-traction spaces were measured, and the left and rightsides were averaged for each patient. The mean residualextraction spaces for each bracket group at T1 and T2were then calculated (Table II). There were no significantdifferences in residual extraction spaces between thegroups at T1 or T2 in the mandible (T1, P 5 0.35; T2,P 5 0.99) or the maxilla (T1, P 5 0.37; T2, P 5 0.44).

Overall space closure from T0 to T2 was similar inboth arches. The differences were less than 1 mm andnot statistically significant.

The mean changes in arch dimensions from T0 to T2were calculated for each arch (Table III). There were nostatistically significant differences between the groups

Table II. Descriptive statistics for the irregularity index and mean extraction space (2-sample t test)


SL mean (SD) CL mean (SD) P value SL mean (SD) CL mean (SD) P value

Irregularity index, T0 10.88 (4.72) 12.52 (5.26) 0.33 11.98 (5.55) 12.53 (7.2) 0.78

Mean extraction space, T0 7.74 (0.75) 7.78 (0.96) 0.84 7.98 (1.92) 8.12 (1.17) 0.67





Table III. Mean changes in arch dimensions (T0-T2) in millimeters (2-sample t test)


SL mean (SD) CL mean (SD) P value SL mean (SD) CL mean (SD) P value

Intercanine width 1.96 (1.78) 2.86 (2.80) 0.31 2.83 (2.49) 3.40 (4.12) 0.63

Intermolar width �1.44 (1.54) �1.34 (2.10) 0.88 0.25 (2.08) 0.14 (1.87) 0.87

Arch depth �1.69 (2.63) �1.08 (1.03) 0.61 �2.42 (3.99) �1.37 (2.87) 0.33


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for any changes in arch dimensions in either arch. Inboth groups, mandibular intercanine widths increased(P 5 0.31), intermolar widths decreased (P 5 0.88),and arch depths decreased (P 5 0.61). In the maxillaryarch, intercanine widths increased (P 5 0.63), intermo-lar widths increased (P 5 0.87), and arch depthsdecreased (P 5 0.33).

The changes were greatest in mandibular and max-illary intercanine widths. Mandibular intercaninewidths increased from T0 to T2: 1.96 and 2.86 mm inthe SL and CL groups, respectively. Maxillary interca-nine widths increased from T0 to T2: 2.83 and 3.4mm in the SL and CL groups, respectively.

Logistic regression was used to determine whetherthere was a difference between the SL and CL bracketgroups. Multiple imputation was used to account forthe small amount of missing data at T2. The followingvariables were tested at 0, 10, and 20 weeks: age, sex,irregularity index, intercanine width, intermolar width,and arch depth.

Regression coefficients and confidence intervalswere calculated for each variable in both arches(Tables IV and V). The confidence intervals alsoincluded the variance because of missing data.

All regression coefficients were close to zero, exceptsex and intermolar width at T2, which also had largeconfidence intervals. No variable was significantly asso-ciated with the probability of reduction in irregularity ineither arch. Arch depth at T2 had borderline significancefor the maxillary arch but was not considered highlysignificant (particularly because of the multiple compar-isons in this study).

DISCUSSION

Studies have demonstrated that SL brackets gener-ate significantly lower levels of in-vitro friction thando CL brackets.4-9,23 This has led to the promotion ofSL brackets on the assumption that decreased frictionleads to enhanced clinical efficiency. However, ourstudy concurs with the growing body of evidence thatthere is no statistically significant difference intreatment efficiency between SL and CL bracketsduring initial alignment. Our study demonstrated thatDamon 3MX SL brackets were no more efficient thanVictory Series and Mini-Diamond CL brackets inanterior alignment or passive extraction space closureduring the first 20 weeks of orthodontic treatment.

Early clinical studies by Eberting et al2 and Harra-dine14 reported decreased total treatment times andfewer appointments for patients treated with DamonSL brackets. However, these retrospective studieswere both potentially subject to bias. The effect of con-founding factors might have been considerable becausethe selection criteria were not well detailed, the pretreat-ment characteristics of the sample were not tested forequivalence,2 and clinical variables such as archwire se-quences were different in each bracket group.14

Subsequent well-designed retrospective andprospective clinical studies reported no significantdifferences in treatment efficiency between SL andCL brackets during initial alignment15,17-20 and activespace closure.16 Most of these studies evaluated thealignment efficiency of the mandibular anterior archbecause rotations, irregularity, and small interbracketdistances are typically encountered in this region.

Table IV. Logistic regression for the mandibular arch

Parameter Estimate of regression coefficient 95% CI P value

Age �0.09 �0.80 0.61 0.80

Sex �1.55 �4.18 1.07 0.25

Irregularity index, T0 0.20 �0.27 0.68 0.40

Intercanine width, T0 �0.71 �1.67 0.24 0.14

Intermolar width, T0 0.74 �0.38 1.85 0.19

Arch depth, T0 0.35 �0.24 0.93 0.25

Extraction space, T0 �0.17 �1.98 1.63 0.85

Irregularity index, T1 �0.20 �0.90 0.49 0.56


Irregularity index, T2 �0.35 �1.44 0.73 0.52

Intercanine width, T2 0.53 �0.53 1.60 0.32

Intermolar width, T2 �1.10 �2.70 0.49 0.17

Arch depth, T2 0.09 �0.73 0.90 0.83

Extraction space, T2 0.49 �0.65 1.63 0.40

Table V. Logistic regression for the maxillary arch

Parameter Estimate of regression coefficient 95% CI P value

Age �0.30 �0.92 0.32 0.34

Sex 0.35 �2.09 2.79 0.78


Intercanine width, T0 0.08 �0.29 0.46 0.66

Intermolar width, T0 �0.13 �0.64 0.39 0.63

Arch depth, T0 0.19 �0.27 0.65 0.40





Intercanine width, T2 �0.14 �0.70 0.41 0.60

Intermolar width, T2 0.23 �0.58 1.05 0.56

Arch depth, T2 �0.78 �1.54 �0.03 0.04



We investigated the anterior alignment of both archeswith the irregularity index.

Various authors have drawn the same conclusionsirrespective of the particular brand of SL bracket. Mileset al15 and Miles17 did not find any significant differ-ences when they prospectively compared Damon SLand SmartClip (3M Unitek) SL with Victory SeriesCL brackets during initial alignment. Pandis et al18

also found no difference when comparing the time toalignment in the mandibular arch between Damon 2(Ormco) SL and Microarch (GAC International, Bohe-mia, NY) CL brackets. A recent large retrospectivestudy concluded that InOvation (GAC) SL bracketshad no measurable advantages over Victory Series CLbrackets in initial alignment time, total treatment time,or number of appointments.20

Despite these findings, it has been suggested that SLbrackets might provide benefits for extraction patientsduring initial alignment or space closure.15,17

Correspondingly, spontaneous tooth movement into

extraction spaces because of reduced friction duringalignment could be a time-saving benefit.19 Therefore,we exclusively investigated extraction subjects duringinitial orthodontic treatment to assess the efficiency ofalignment and passive extraction space closure.

Two previous studies compared SL and CL bracketssolely in extraction patients. Miles16 prospectively com-pared 0.018-in Victory series CL brackets and Smart-Clip SL brackets during active space closure. Hefound no differences in the rate of tooth movementbetween the bracket types: 1.1 mm per month with SLbrackets and 1.2 mm per month with CL brackets. Theserates are not comparable with our results because of dif-ferent biomechanics and types of tooth movement dur-ing these stages.

Scott et al19 conducted a randomized controlled trialof patients having mandibular first premolar extractions.He concluded that Damon 3MX brackets were no moreefficient during mandibular alignment than Synthesis(Ormco) CL brackets. Those authors did not investigate


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the maxillary arch or passive extraction space closure.Our study is the first to quantify the amount of toothmovement during passive extraction space closure. Wefound no difference in the amount of passive spaceclosure during initial alignment, in either arch, betweenthe bracket types.

We used the same initial archwire sequence as thatof Scott et al,19 which included 0.014-in and 0.014 3

0.025-in copper-nickel-titanium archwires in the Damonarchform. The SL group of Pandis et al18 also used thesame sequence. Similar changes in arch dimensionswere observed in all patients with these archwiresregardless of the bracket type used. Therefore, thedimensional changes can be attributed to the Damon arch-form. Mean mandibular intercanine widths increased,mandibular intermolar widths decreased, and arch depthsdecreased. Mandibular intercanine widths increased byaverages of 1.96 and 2.86 mm in the SL and CL groups,respectively. Scott et al19 reported similar increases of2.55 and 2.66 mm in SL and CL groups, respectively.

Mean maxillary intercanine widths increased by2.83 and 3.4 mm in the SL and CL groups, respectively.The increases in maxillary intermolar widths in bothbracket groups were negligible. Interestingly, maxillaryand mandibular intercanine widths increased despite theextractions. This might be due to the distal movement ofthe canines into the extraction spaces.19 These findingsdiscredit previous suggestions that premolar extractionsinevitably cause ‘‘shrinking’’ of the dental arch, in-creased buccal corridors, and damage to smile es-thetics.24 Furthermore, studies have shown that buccalcorridors do not influence smile esthetics.25,26

The results from this study concur with previousstudies that found no difference in the alignment of man-dibular teeth in extraction patients with severeirregularity.18,19 The mean irregularity scores in thestudy of Scott et al19 were 12.44 mm in the CL groupand 11.23 mm in the SL group. Similarly, the patientswe investigated had severe irregularity scores. Pandiset al18 investigated moderate and severe irregularity. Theyreported no significant difference in subjects with severeirregularity scores greater than five. Interestingly, theyfound that patients with moderate irregularity, with irreg-ularity scores between 2 and 5, were 2.7 times more likelyto align faster in the SL bracket treatment group. Differ-ent archwire sequences were used in each bracket groupin the study of Pandis et al18; this might have been a con-founding factor contributing to the hazard ratio.

A strength of this study was the inclusion and exclu-sion criteria that enabled control over certain clinicalvariables such as bracket composition and dimension,archwire type and sequence, and interappointment in-terval. Consecutive eligible patients were included to

minimize confounding factors. Therefore, the criticaldifference between the treatment groups was themethod of ligation.

This study had several limitations. There was nocontrol group available to measure the natural drift ofthe teeth into the extraction spaces without orthodonticappliances. It could also be argued that extraction ther-apy is not routinely performed with the SL bracket phi-losophy and is not intended for this sample of patients.Despite the strict inclusion and exclusion criteria, thepossibility of sampling bias because of the retrospectivenature of the study cannot be dismissed. Several patientswere excluded because of missing models at the varioustime points, or they received an alternate archwiresequence that reduced the power of the study at T2.The patients were also treated by several clinicianswith various levels of experience. The 3 bracket typesused in this study had different prescription values.

It might be postulated that, as irregularity increases,other factors negate any benefit of the reduced frictionof SL brackets. For example, the narrower bracketdesign of the Damon 3MX might have increased thecontact angle and contributed to elastic binding andnotching. Future research could explore these hypothe-ses in extraction patients, with larger sample sizes ina prospective clinical trial.

It is not surprising that bracket type does not appear tohave a significant influence on treatment efficiency. Treat-ment efficiency is the product of many mechanical and bi-ologic factors. It is unlikely that any 1 factor is responsiblefor the rate of tooth movement. The biology of toothmovement is a complex and highly coordinated processat the cellular, molecular, and genetic levels. Individualvariation undoubtedly has a fundamental underlying rolein tooth movement and treatment efficiency.

CONCLUSIONS

1. SL brackets were no more efficient than CLbrackets in anterior alignment and passive extrac-tion space closure during the first 20 weeks oforthodontic treatment.

2. Changes in arch dimensions were similar in the SLand CL groups.

We thank the Australian Society of Orthodontists’Foundation for Research and Education for providinga grant to fund this research project.

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Efficiency of self-ligating vs conventionally ligated brackets during initial alignment