Objective: To test if there are significant evidence-based differences in effectiveness between self-ligation (SL) and conventional-ligation (CL) brackets, including observation of eight daily, self-ligation clinical techniques. The clinical SL advantages are: 1) using colour elastomers upper 2-2, but converting to a completely ligature-free appliance when poor oral hygiene develops, 2) simple clip-click for coil spring insertions, 3) easy clip-clicks for moderate lingual displacements, 4) simple floss and click technique for severe rotations, 5) extraoral cinch bends at distal of SL first and second molars, reducing the stress of placing intraoral posterior tie-backs, 6) step-bends are possible between first and under-erupted second molars because both are SL, 7) Kobi-hook elimination since mini SL tie-wings are open for vertical elastics and 8) lock-in tripod initially of archwires. Materials and Methods: Popular clinical claims of SL were identified through a literature overview of PubMed, EMBASE, Cochrane Library, and Web of Science for the period 1965-2020. Additional hand searching of the references from retrieved articles was completed. The articles containing the inclusion criteria were qualitatively analyzed using the Cochrane risk of bias tool, and one other scale. Applicable RCTs were statistically analyzed with weighted means calculations and forest plots. RCT data that could not be synthesized with one other RCT at this time were reserved for discussion.
Results: The inclusion criteria were satisfied by a total of ten RCT studies, six of which were matched for meta-analysis of three popular clinical claims. Space closure rate, reduced incisor proclination, and the rate of mandibular alignment for SL compared to CL were not statistically significant with confidence intervals of 95%. The remaining four RCTs were collectively analyzed and found no statistically significant difference in discomfort between SL and CL. Conclusion: SL chair time savings was more than 1.5 hrs/day for 40 patients treated/day because the time savings were all cumulative, and found consistently higher versus CL in all seven studies. The null hypothesis that there are no differences between SL and CL, was not rejected due to statistically insignificant results. Additional active SL studies, and well-designed RCTs for MA are needed that includes overall treatment time.
In view of at least a 7x greater number of brackets being applied in children for compliance compared with removable aligners, Randy Lang asked fairly, if self-ligation (SL) had clinical and scientific advantages and was there scientific verification? Afterall, (SL) proponents and manufacturers of SL systems have made clinical claims of their effectiveness and efficiency over conventional ligation (CL), but the scientific evidence for several clinically relevant assertions as Lang states, has remained inconclusive. A few published systematic reviews (SRs), including one SR of in vitro studies, tried to address many of these questions, but there was a lack of high quality evidence available at the time of their publication. Since then, there has been a significant five-fold increase in the number of randomized controlled trials (RCTs) on SL compared to CL, that were suggested to be employed exclusively for meta-analysis (MA), and to prevent statistical errors.1
Clinically, it may have been in 1935 that the first SL, single-wing bracket design was introduced, but SL for the most commonly used type of orthodontic brackets, i.e. the siamese twin bracket design, was only developed recently within the last 20 years. This appears to be a central inflection point because it coincides with an increased frequency of use of active twin SL, especially with the significant reduction in bracket thickness or “Low Profile” twin LP Self-ligation that includes twin NiTi clips (SIA Orthodontic Manufacturer, ZIP Low Profile Siamese Twin Interactive Self-ligating brackets with NiTi clips, and iarch wires, Roca d’Evandro, Caserta, Italy) and the associated claim of chair time efficiency (shorter chair time, Fig. 1A-F, Fig 2A-D, Fig 3A-H).
A patient presents with a history of prior orthodontic treatment and relapse of the Class III anterior open bite of -2mm at the laterals complicated by nasopharyngeal obstruction associated with maxillary constriction and dental interferences, anterior tongue positioning, and TMJ dysfunction. ENT+ Allergist consultation was recommended followed by a miniaturized interactive self-ligating bracket system with NiTi clips for light, continuous and synergistic forces with similar NiTi iarch* wires (.016x.014” CuNiTi, .018x.014” NiTi, followed by .018x.018” ß-tiitanium, .020x.020” STS). The maxillary dental arch was re-expanded and the open bite was closed with a 5/16”, 4.5 oz box-pattern elastic from the upper laterals to the lower canines because there was a reverse dental smile. A Class I occlusion was reestablished using good intercuspation and coordinated arch forms within 11 visits efficiently with shortened chair time of 10 min/visit. Fixed retainers were employed canine-to-canine to prevent return of the open bite. In addition, good facial harmony and balance was established with dental aesthetics. This included full display of upper incisors, refilling of the buccal corridors, good overbite of 25% for anterior guidance and midline coordination with the facial midline.
It has not been unusual in clinical applications, that obvious, significant developments have occurred prior to in-depth scientific studies. Three examples have been the discovery of the titanium implant in a rabbit bone fracture by Per-Ingvar Branemark, penicillin from mold colonized on bacterial petri dishes by Sir Alexander Fleming, and the edgewise appliance by Dr. Edward H. Angle in 1929. Although it is critical that scientific, evidence-based data be used here to ultimately determine differences between conventional and SL brackets, there have been several similarly obvious, clinical procedures over the last 35 years that are relevant to shortened chair time (Figs. 2A-B, 3A-C, 3D-F, 3G-H).
However, the number of other SL advantages began to escalate dramatically, with counter-arguments that refuted all advantages of SL. Significantly, there have also been two SL bracket mechanisms and schools of siamese twin philosophies: the interactive (ISL) or active, and the passive (PSL) as 2 different groups for SL investigations.
Few differences were found for dental-occlusal dimensions such as molar and intercanine width comparing SL with CL. No adequate, evidence-based data were available that showed SL increased maxillary and mandibular buccal bone growth, reduced root resorption, improved torque, or increased bond failure rate. Since the publication of past SRs, many RCTs comparing SL with CL have been published in the peer-reviewed literature and inclusion of these recent RCTs for MA is indicated.
The aim of this SR with MA is to evaluate whether SL brackets are more effective than CL brackets by testing hypotheses generated from several popular clinical claims including that SL shows: 1) earlier in vivo space closure, 2) reduced lower incisor proclination, 3) earlier initial mandibular incisor alignment, 4) reduced initial discomfort and 5) reduced halitosis. The additional objective is to test if SL brackets are more efficient, defined as maximizing outputs of patient-doctor time, compared to CL in terms of 6) reduced overall treatment time, and 7) shortened chair time. By applying a search strategy to identify RCTs, the objective is to test if SL compared to CL has any treatment advantages. The higher filter for the data uses at least two RCTs for MA to determine statistical significance.1 The null hypothesis is that high quality evidence-based data do not substantiate a significant difference between SL and CL.
METHODS AND MATERIALS
This review applied RCTs for MA of the clinically relevant questions. The treatment outcomes that could be studied statistically were determined by the number of RCTs matched for MA and followed the PICOS format. Inclusion and exclusion criteria were applied a priori to select articles.
- RCTs were assessed with other closely comparable RCTs using the Cochrane risk of bias, for MA (one other tool was used for the characterization of the good quality studies with a scale for low, moderate, and high risk bias).
- Studies including SL and CL orthodontic treatment that did not discriminate for age or gender.
- Studies that compared SL with CL appliances concerning effectiveness (i.e. chair time efficiency).
- Studies that did not have a comparison cohort.
- Animal studies, in vitro, or ex-vivo studies.
- Editorials, opinions without age, gender, language discrimination or analytical outline.
Four main electronic databases, including PubMed, EMBASE, Cochrane Library, and Web of Science, were reviewed comparing SL to CL studies from 1965 until 2020. Search strategies and keywords were employed for each database (Appendix 1). The literature overview was used to identify the prevalence of SL clinical claims qualitatively by listing them in customized forms by two reviewers (U.V., W.X.).
Articles to be included were independently read, reviewed and discussed without language discrimination. Inclusion was based on consensus agreement by at least 2 of 3 investigators (U.V., W.X., Y.Z.) using independent search and custom data abstraction forms piloted on RCT, cohort and cross-sectional studies. Article references were also hand-searched. Quality investigations were included that had well-conducted statistical analysis (independent of a low, moderate or high risk of bias) and a statistics expert was consulted during statistical analysis. If there was disagreement, a third reviewer (U.V.) was used to reach consensus. Grey literature was evaluated for an overview but in the end peer-reviewed articles were included.
Independent quality assessment for bias (Appendix 2A,B), was conducted for the included, peer-reviewed studies using the Cochrane risk of bias assessment tool specific for RCTs (and the modified Newcastle-Ottawa scale for also NRTs), by 2 researchers (W.X., Y.Z.). A third investigator (U.V.) was consulted in areas of dispute to form a consensus. Seven criteria details in the Cochrane tool were applied separately for the RCTs. Low (+), high (-) or unclear (?) risk of bias was given to each criterion to determine overall level of bias. Qualitative assessment of each peer-reviewed study not only encompassed risk of bias, but also contained a detailed characteristics evaluation for each of the included investigations.
Quantitative assessment was then made using the RCTs that could be matched with at minimum, another RCT study for a MA to be performed. A threshold level of a low or moderate level of bias for each quality RCT was deemed to be acceptable for MA. If a MA could not be conducted using the articles included in the SR, it was not considered quantitative evidence. Additionally, a sensitivity analysis was performed, where necessary, with regard to qualitative analysis, risk of bias, and publication status.
Single good quality RCTs that met the inclusion criteria, but that could not be matched, and currently could not undergo MA were not included in the statistical results. Rather, they were reserved for discussion because of their potential to possibly guide the designs of future RCTs needed for MA.1
A MA was performed to synthesize comparable RCT data for each clinical claim of interest using the metafor2 package in R software (MATLAB version 8.2, Mathworks, Natick, Massachusetts, United States). Heterogeneity was assessed for the included investigations. A fixed-effects model was used with less heterogeneous results (I2 statistics < 75%). A random effects model was used for more heterogeneous results (I2 statistics > 75%). Forest plots were constructed using weighted mean differences only from RCTs. If high quality RCT investigations with lowest possible bias could not be found for a specific claim, forest plots were not constructed. Dichotomous data was assembled with the use of odds ratios. Funnel plots were used to assess publication bias.
The electronic search and overview of the orthodontic literature identified 236 studies for prevalence of clinical claims. The claims were divided into two groups, primary questions 1-7 and secondary questions 8-19 (Table I), toward the preparation of the MA. From the 236 articles, 35 met the inclusion criteria (Table 2, 22 RCTs and 13 NRTs), including one from hand searching (Fig. 4). Table 2 outlined the characteristics and details of each of the 35 studies including their risk of bias. The 13 NRTs were characterized and reserved for discussion.
Ultimately, 10 RCTs were found. For three relevant SL claims (in vivo space closure, incisor proclination and rate of mandibular incisor alignment in days,) a MA was performed using calculated weighted mean differences. There were two RCTs for each of three claims producing six matched RCTs and forest plots were constructed for the three relevant claims (Figs. 5-7). In addition, the data of four RCTs on discomfort were synthesized for MA. The excluded studies did not provide useful data.
Rate of in vivo Space Closure
There were two in vivo studies on distal movement of canines for space closure on round wire that applied retraction forces directly on the canine hooks (Fig. 5). The forest plot showed a mean difference of 0.17 mm for CL in the study by Burrow,3 compared to 0.06mm for PSL in the investigation by Mezomo et al,4 with both studies using 28-day intervals of canine retraction. However, this small effect made no practical, clinical difference to a patient’s treatment5 and the MA also demonstrated no statistically significant difference. There was significant heterogeneity in studies on space closure. One RCT6 investigated used large rectangular archwire rather than round archwire3,4 and skeletal anchorage of TADs, and another RCT7 used en masse movement of incisors and canines with a continuous arch where both RCTs did not allow inclusion of this data for MA.
Reduced Incisor proclination
The two RCTs, by Pandis et al8 and Fleming et al9 were included since they used non-extraction to assess incisor proclination (Fig. 6), but the differences were not found to be statistically significant. Another RCT10 was not included because it studied incisor proclination with extraction treatment and moderate dental crowding.
Rate of initial mandibular incisor alignment (in days) in standardized mean difference
The RCT by Songra et al11 and the data from Scott10 including mandibular premolar extractions were used for MA, toward the construction of a forest plot (Fig. 7). The results showed in the early alignment stage the difference was not statistically significant. The overall effect must be interpreted with reservation since the I2 value was high, due to the heterogeneous methodology, where age ranges and time intervals were different.11 However, the two RCTs were the highest quality studies currently available with continuity because both investigated mandibular incisor alignment with mandibular premolar extractions, and SL brackets with a control. One other RCT12 used upper premolar extractions and had a high risk of bias, while another RCT13 included non-extraction patients precluding MA.
Four RCTs14-17 were found which had an acceptable risk of low to moderate bias and studied discomfort after 4 hours, 24 hours, 3 days and 7 days following archwire insertion and had reported a MA. They employed the VAS (Visual Analog Scale) assessment and found no differences in discomfort between PSL and CL. Another multi-centered RCT18 compared PSL and CL at 24 hours and 3 days but this data was based on the VRS (Verbal Rating Scale) that produced inaccuracies since it relied on verbal responses compared to the tangible, visual analog scale, that precluded MA with the other 4 RCTs.
Overall Treatment Time
Three RCTs19-21 on total treatment time with low risk of bias have been reported. None of these revealed any statistically significant differences in total treatment time between SL and CL. Other retrospective studies5,22,23 investigated treatment time (and also used occlusal indices) that may have suffered from a higher risk of bias.1 The significant, heterogeneous designs of these 3 RCTs above also required the use of standardized mean differences to minimize methodological differences among several investigations of overall treatment time.9,24-27 In terms of assessing potential reduction in overall treatment time, results could not be synthesized with the three RCTs above for MA because of the different statistical analyses and ratings systems used (PAR, ICON, SPSS). Although PAR and ICON scores are not measures of treatment time, DiBiase et al19 used PAR (Peer Assessment Rating) scores, O’Dwyer et al20 used SPSS (statistical package for the social sciences, IBM Corporation) software with a frequency histogram and a different rating system, while Johansson and Lundstrom21 used ICON (Index of Complexity, Outcome, and Need) evaluation. Consequently, a reduction in overall treatment time could not be reliably determined.
Halitosis and periodontal indices
One RCT by Nalcaci et al28 found halitosis and periodontal indices were significantly increased for CL with elastomers compared to PSL. Another RCT29 used CL with metal ligatures studying similar parameters and found no differences. However, the two different CL methods, did not permit MA. The data from a third RCT30 studied periodontal indices and bacterial levels and this data also could not be synthesized for MA.
Shortened Chair Time
One RCT by Miles and Weyant17 found shortened chair time that was statistically and clinically significant of 130.2 secs/arch saved compared to metal CL, using six anterior esthetic brackets. Daily clinical experiences with active self-ligation compared to conventional ligation Figs 1, 2A-D and Figs 3A-H have verified the scientific data that active self-ligation significantly shortens chair time in eight ways during SL archwire engagement with 1) moderate rotations, 2) lingual displacements, 3) coil spring insertion and rotation prevention by active clip seating, 4) floss-and-click for severe rotations, 5) cinch-backs, 6) step-down bends at 6s and 7s, 7) eliminating Kobi-hooks for vertical elastics and 8) secure central incisor lock-ins on initial insertion. In the middle and finishing stages there is significant shortened chair time in engaging straight wires actively for torque control with 24 repeated clip closures upper and lower 6-6 for each patient using finger pressure for active self-ligation systems.
This SR applied additional, high quality RCTs available for MA.1 The synthesis of data from RCTs was possible for several clinical claims regarding SL in this SR. The electronic search led to the selection of 236 articles. All were published in English except one, which was in Chinese31 that were translated for use in this review. The final 35 SL studies expanded the qualitative analysis.3-26,28-38 Due to the low number of RCT studies for each outcome, implications could not be derived from funnel plots to assess publication bias.
Rate of in vivo space closure
The great majority of SL brackets in the included RCTs were the PSL type, and additional interactive (active) SL studies were clearly needed. Space closure with narrower PSL brackets3 can result in binding and notching of the archwire with conventional retraction forces that may have been higher for the narrower brackets compared to CL brackets. In the RCTs of this SR, the PSL brackets would have allowed a looser fit of the archwire in the slot compared to CL, greater tipping and consequently greater resistance to sliding. Other factors may have also affected space closure including bone density, occlusion, or dental interferences.
Reduced incisor proclination, rate of initial mandibular incisor alignment and discomfort
The NRT31 for incisor proclination was not used in this SR because it had a high risk of bias. Although it was a non-extraction study, it also used only mild dental crowding (3mm) compared to an RCT10 using extraction with moderate crowding. Due to the statistically insignificant difference found, the clinical relevance of incisor proclination requires further study.1
Initial mandibular incisor alignment may have been limited because PSL brackets do not seat archwires into the base of the slot. More interactive SL studies are needed since they are designed to seat the archwire earlier than PSL, used in one RCT11 although there was a concern for consistency in methods because CL brackets were evaluated every 6 weeks compared to SL’s evaluation every 12 weeks. The RCT by Fleming et al32 measured rate of alignment but was excluded in this MA because it used non-extraction treatment and the irregularity index rather than the measurement in days. Discomfort is a complex outcome to evaluate in patients because it is highly subjective with wide ranges of sensitivities and perceptions of discomfort. RCTs on discomfort can sometimes employ different scales such as the verbal rating scale (VRS)18 rather than a visual analog scales (VAS)14-17 although both can offer imprecise results. Contradictory results on discomfort can also be related to other factors such as age and gender.
Shortened chair time
From the electronic search, the finding by Miles and Weyant17 of shortened chair time for six anterior, esthetic, SL brackets of 130.2 sec, or 2.2 minutes/(1 arch) that was statistically and clinically significant compared to metal CL in this one RCT reported, was also found in the majority of studies examined.5,27,35-37 There was also support for the above RCT in the study by Turnbull and Birnie38 (Appendix 2b). They demonstrated statistically significant SL clip closing (P<.001) and opening (P<.01) compared to elastomeric CL with a total SL chair time savings of 76.8 sec/arch, or 2.5 min/(2 arches) working with more posterior brackets (10 brackets/arch). Although archwire ligation is one of the most repetitive orthodontic procedures and clinical techniques vary, the compounding effect27 of SL chair time savings of the lower 2.5min/patient38 was calculated. Applying 40 patients/8hr-day, 4 days/wk produced an interesting potential savings of 102.4 min/day, Fig. 8.
A rigorous systematic review used RCTs to study SL clinical claims and found:
- Faster in vivo space closure during upper canine retraction into first premolar extraction sites, was not statistically significant, or clinically significant.
- Reduced incisor proclination showed the effective amount was not statistically significant.
- Reduced number of days for mandibular incisor alignment was statistically insignificant, with clinically insignificant differences.
- Initial discomfort was highly subjective and no differences could be found using four RCTs and meta-analysis.
- Total treatment time, similar to halitosis, was not found to be different in three unmatched RCTs that precluded meta-analysis.
- Significantly shortened chair time was found in one RCT, supported by a good prospective, cross-sectional study at this time. This shortened chair time was verified in eight orthodontic clinical techniques of 1) ligature-free with 24 internal clips 2) coil insertion click-in for space opening 3) lingual displacements clicked-ins 4) floss-and-click for severe rotations 5) extraoral cinching of archwire ends 6) step-bends between SL 6s and 7s 7) Kobi-tie elimination with open SL tie-wings 8) efficient initial archwire centrals lock-in for tripod.
More well-conducted RCTs particularly of interactive SL are indicated (Table 3).
Oral Health welcomes this original article.
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About the Author
John C Voudouris, University of Toronto, Faculty of Dentistry, Instructor Graduate Orthodontic Program, New York University, Division of Biological Sciences, American Association of Orthodontists Milo Hellman Research Award Recipient, Member of the Edward H. Angle Society. He is the researcher and developer of interactive self-ligating brackets with several patents and patents pending. John Dean Voudouris, is a research student, Upper Canada College.