Introduction
The aim of this study was to compare the efficiency of self-ligating (SL) and conventionally ligated (CL) brackets during the first 20 weeks of extraction treatment.
Methods
Study models of 50 consecutive patients who had premolar extractions in the maxillary and/or mandibular arch, 0.022 × 0.028-in slot brackets, and similar archwire sequences were examined. Forty-four arches received SL Damon 3MX brackets (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, extraction spaces, and arch dimensions at pretreatment (T0), 10 weeks (T1), and 20 weeks (T2).
Results
There were no significant differences between the SL and CL groups at 20 weeks in irregularity scores (mandibular arch, P = 0.54; maxillary arch, P = 0.81). There were no significant differences in passive extraction space closures between the SL and CL groups (mandibular arch, T0-T2, P = 0.85; maxillary arch, T0-T2, P = 0.33). Mandibular intercanine widths increased from T0 to T2: 1.96 and 2.86 mm in the SL and CL groups, respectively. This was not significant between the groups ( P = 0.31). Logistic regression did not show a difference between the SL and CL bracket groups.
Conclusions
SL brackets were no more efficient than CL brackets in anterior alignment or passive extraction space closure during the first 20 weeks of treatment. Ligation technique is only one of many 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.
Bracket designs have undergone continual modifications since fixed appliances were first used in orthodontics. The quest to improve treatment efficiency has culminated in many modern edgewise appliances. Recently, the promotion of self-ligating (SL) brackets has incited much controversy. Advocates claim that low-friction SL brackets coupled with light forces enhance the rate of tooth movement and decrease treatment time. Other advantages include decreased appointment times, improved oral hygiene, increased patient acceptance, and superior treatment results.
Most claims of SL brackets have been extrapolated from in-vitro studies. A recent systematic review highlighted the limitations of in-vitro studies. In particular, studies that demonstrate reduced friction in SL brackets compared with conventionally ligated (CL) brackets have been coupled with small-diameter wires in well-aligned arches with no tip and torque. In-vitro studies are limited because they cannot comprehensively simulate a clinical scenario. Many variables can influence the amount of friction generated in a fixed appliance system. These include archwire and bracket composition, archwire dimension, bracket slot dimension and design, interbracket distance, deflection of the archwire, and biologic factors such as saliva and perturbations. Therefore, it is questionable whether the use of SL brackets translates into clinical benefits such as decreased resistance to sliding, faster tooth movement, and increased treatment efficiency.
Several in-vivo studies have compared the efficiency of SL and CL brackets during various stages of treatment with conflicting results. These studies measured treatment efficiency in terms of total treatment times, numbers of appointments, and tooth movement during initial alignment and active space closure. Early retrospective studies reported up to 6 months’ reduction in total treatment time and 7 fewer appointments with SL brackets. Subsequent well-designed retrospective and prospective studies reported no significant differences during initial alignment or active space closure with various SL and CL brackets.
Miles et al and Miles postulated that SL brackets might provide a measurable benefit in extraction patients. Additionally, Scott et al suggested that SL brackets might encourage passive space closure during initial alignment. There is a relative lack of evidence comparing the efficiency of SL and CL brackets in extraction patients because most studies have investigated mixed samples. Only 2 clinical trials have compared SL and CL brackets solely in extraction patients. One study investigated the initial alignment phase and reported no difference between SL and CL brackets. Neither clinical trial investigated the efficiency of passive space closure during alignment. If there is a measurable advantage of SL brackets, then it should be most apparent during alignment and space closure when the bracket slides along the archwire. In this study, passive space closure was defined as the extraction space closure during alignment without active space-closing mechanics. The amount of passive space closure varies greatly between patients, but this parameter has not been investigated before. If the use of SL brackets could achieve greater passive space closure, there would be less extraction space to close actively. This could reduce the overall treatment time. Furthermore, this might minimize the detrimental effects of active force application such as root resorption.
In this study, we aimed to determine whether there are significant differences in the efficiency of anterior tooth alignment and the amount of passive space closure between SL and CL brackets. Concomitant changes in arch dimensions were also compared between the SL and CL bracket groups.
Material and methods
Ethical approval was obtained from the Dental Sciences and Research Ethics Committee of the University of Queensland School of Dentistry and the Royal Children’s Hospital and Health Services District Ethics Committee. Study models of 50 consecutive patients who received comprehensive full fixed appliance treatment with 0.022 × 0.028-in slot brackets at the School of Dentistry, University of Queensland and the Royal Children’s Hospital were examined.
The dental school patients were treated by postgraduate students under the supervision of an experienced orthodontist. Royal Children’s Hospital patients were treated by an experienced orthodontist (H.M. or C.H.).
Patient records were included if they satisfied the following inclusion criteria: (1) treatment began between 10 and 18 years of age; (2) treatment included bilateral mandibular or maxillary extractions followed by fixed appliance therapy; (3) intraoral photos and study models were available at pretreatment (T0), 10 weeks (T1), and 20 weeks (T2) postbonding; (4) treatment included 0.022 × 0.028-in slot brackets (SL brackets, Damon 3MX, Ormco, Glendora, Calif; or CL brackets, Victory Series, 3M Unitek, Monrovia, Calif, or Mini-Diamond, Ormco); (5) treatment began with an initial archwire of 0.014-in copper-nickel-titanium (Damon archform, Ormco), followed by 0.014 × 0.025-in copper-nickel-titanium (Damon archform, Ormco) ; (6) the patients were reviewed every 5 weeks; and (7) the first archwire was left in place until the teeth were passively engaged in all bracket slots before proceeding to the second archwire.
The following exclusion criteria were applied: treatment with nonsymmetrical extractions, no impacted or unerupted permanent teeth anterior to the first molars in the arch that received extractions, treatment with removable appliances or rapid maxillary expansion appliances, and incomplete records at a time point.
Fifty patients (20 male, 30 female) fulfilled the inclusion criteria.
Pretreatment characteristics were recorded including the patient’s age bonding, sex, mandibular and maxillary crowding, irregularity index, extraction space, intercanine width, intermolar width, and arch depth.
All study models were evaluated by using Little’s irregularity index to quantify the alignment of the 6 anterior teeth. Crowding was calculated as the difference between the sum of tooth widths and arch circumference taken from the line of best fit, through the contact points mesial to the first molars, on a photocopy of the patient’s occlusal archform.
Extraction space was measured from the closest points on the adjacent teeth before extraction. The mesiodistal widths of the teeth to be extracted were not used because they were often displaced from the archform; this decreased the extraction space to be closed. Similarly, extraction spaces at T1 and T2 were measured from the closest points on the crowns of the teeth on either side of the extraction space. The contact points were not used because many teeth were rotated.
Intercanine widths were measured from the cusp tips of the canines. Measurements were not taken from the gingival margin because the quality of the gingival impression was inconsistent. Intermolar widths were measured from the central and mesial occlusal pits of the mandibular and maxillary first molars because this area of the impression was clearer than the cusps. Arch depth was measured as the perpendicular distance from a line drawn through the mesial contact points of the first molars to the labial surfaces of the central incisors.
Wax was applied to cover the brackets on each model before measurement. An identification number was assigned to each model. Therefore, the researcher (E.O.) was blinded to patient name, time point, and bracket type during data collection to minimize systematic error. The study models were measured with electronic calipers with sharpened tips that were accurate to 0.01 mm (Mitutoyo, Tokyo, Japan). All model measurements were made by the principal researcher (E.O.).
Statistical analysis
The difference in irregularity scores was used to determine the sample size. Based on a previous study, a clinically significant difference of 0.98 mm in irregularity score, at a power of 80% and a level of significance of 0.05, would require a minimum of 17 patients per treatment group. In the final sample of 50 patients, 44 arches were treated with SL brackets and 40 arches with CL brackets.
Statistical analysis was performed by using Minitab software (release 15, Minitab, State College, Pa) and SAS software (version 9.2, SAS Institute, Cary, NC). The mandibular and maxillary arches were analyzed separately. Descriptive statistics were calculated, and the data were checked for normality. Two-sample t tests were performed at T0, T1, and T2 to compare the bracket groups for irregularity scores, residual extraction spaces, intercanine widths, intermolar widths, and arch depths. The amounts of passive extraction space closure from T0 to T1, T1 to T2, and T0 to T2 for each bracket group were also calculated and compared by using 2-sample t tests. A chi-square goodness-of-fit test was used to determine whether the male-to-female ratio was significantly different between the bracket groups. Logistic regression was also used to determine whether there was a difference between the SL and CL bracket groups. Regression coefficients and confidence intervals were calculated for each variable (age, sex, irregularity index, intercanine width, intermolar width, and arch depth) for both arches. Multiple imputation was used to account for missing data.
Intraexaminer reliability was assessed by remeasuring 20 subjects at least 4 weeks after the original measurements. A t test was performed to compare the first and second measurements.
Results
Intraexaminer reliability was high. There were no statistically significant differences between the first and second measurements for irregularity index ( P = 0.51) and extraction space closure ( P = 0.38). The average differences between the measurements were 0.07 ± 0.52 mm for the irregularity index and 0.06 ± 0.33 mm for extraction space.
Fifty patients (20 male, 30 female) fulfilled the inclusion criteria. This gave a total of 84 arches; 44 arches were treated with SL brackets and 40 arches with CL brackets. In the CL sample, 18 arches received Mini-Diamond brackets. The numbers of arches included in the statistical analysis for each bracket group at T0, T1, and T2 are summarized in Table I . No SL arches were excluded. Six CL maxillary arches were excluded from analysis at T2 because 5 models were missing, and 1 patient received a different archwire sequence. Four mandibular arches were also excluded at T2 because of missing models.
| Mandibular arch | Maxillary arch | |||
|---|---|---|---|---|
| SL | CL | SL | CL | |
| T0 | 19 | 18 | 25 | 22 |
| T1 | 19 | 18 | 25 | 22 |
| T2 | 19 | 14 | 25 | 16 |
The mean irregularity index scores decreased in both bracket groups over time ( Table II ). Both groups had greater decreases in irregularity during the first 10 weeks of treatment compared with the subsequent 10 weeks.
| Mandibular arch | Maxillary arch | |||||
|---|---|---|---|---|---|---|
| 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 |
| Irregularity index, T1 | 4.38 (3.63) |
4.12 (2.87) |
0.81 | 5.44 (3.72) |
5.64 (4.46) |
0.87 |
| Mean extraction space, T1 | 5.45 (1.43) |
4.98 (1.56) |
0.35 | 5.51 (1.74) |
5.04 (1.73) |
0.37 |
| Irregularity index, T2 | 2.84 (1.86) |
2.45 (1.72) |
0.54 | 4.37 (2.69) |
4.16 (2.59) |
0.81 |
| Mean extraction space, T2 | 4.02 (1.78) |
4.03 (1.65) |
0.99 | 4.30 (2.13) |
3.83 (1.76) |
0.44 |
Over 20 weeks, the mean irregularity scores in the SL group decreased from 10.88 to 2.84 mm in the mandibular arch, and from 11.98 to 4.37 mm in the maxillary arch. Scores in the CL group decreased from 12.52 to 2.45 mm in the mandibular arch, and from 12.53 to 4.16 mm in the maxillary arch.
There were no statistically significant differences between the treatment groups at T1 or T2 in the mandible (T1, P = 0.81; T2, P = 0.54) or the maxilla (T1, P = 0.87; T2, P = 0.81).
For passive extraction space closure, the residual extraction spaces were measured, and the left and right sides were averaged for each patient. The mean residual extraction spaces for each bracket group at T1 and T2 were then calculated ( Table II ). There were no significant differences in residual extraction spaces between the groups at T1 or T2 in the mandible (T1, P = 0.35; T2, P = 0.99) or the maxilla (T1, P = 0.37; T2, P = 0.44).
Overall space closure from T0 to T2 was similar in both arches. The differences were less than 1 mm and not statistically significant.
The mean changes in arch dimensions from T0 to T2 were calculated for each arch ( Table III ). There were no statistically significant differences between the groups for any changes in arch dimensions in either arch. In both groups, mandibular intercanine widths increased ( P = 0.31), intermolar widths decreased ( P = 0.88), and arch depths decreased ( P = 0.61). In the maxillary arch, intercanine widths increased ( P = 0.63), intermolar widths increased ( P = 0.87), and arch depths decreased ( P = 0.33).
| Mandibular arch | Maxillary arch | |||||
|---|---|---|---|---|---|---|
| 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 |
The changes were greatest in mandibular and maxillary intercanine widths. Mandibular intercanine widths increased from T0 to T2: 1.96 and 2.86 mm in the SL and CL groups, respectively. Maxillary intercanine widths increased from T0 to T2: 2.83 and 3.4 mm in the SL and CL groups, respectively.
Logistic regression was used to determine whether there was a difference between the SL and CL bracket groups. Multiple imputation was used to account for the small amount of missing data at T2. The following variables were tested at 0, 10, and 20 weeks: age, sex, irregularity index, intercanine width, intermolar width, and arch depth.
Regression coefficients and confidence intervals were calculated for each variable in both arches ( Tables IV and V ). The confidence intervals also included the variance because of missing data.
