Introduction

The emergence of aligner therapy has offered an alternative to the use of fixed appliances within contemporary orthodontics. Nevertheless, while some improvement in the predictability of aligner therapy may have occurred in recent years, fixed appliances continue to form the mainstay of active orthodontic treatment, with continued uncertainty regarding the predictability of aligners in the correction of a range of presentations, including rotational control, transverse correction and overbite reduction.1,2 A previous paper by Mandall and Malik (2007)3 has catalogued the evidence underpinning the use of pre-adjusted edgewise attachments and mechanics, including:

  • Archwire sequences

  • Choice of initial aligning archwire

  • Use of lacebacks

  • Levelling the curve of spee

  • Space closure mechanics

  • Customisation of orthodontic appliances.

In view of the wealth of recent research concerning fixed appliance design and mechanics, allied to the continued refinement of fixed appliance systems, we aimed to update this review on the basis of contemporary research, including randomised and other controlled clinical trials, in order to better inform the selection of fixed appliance design and mechanics.

Method

An electronic search of Medline, Web of Science and the Cochrane Clinical Trials Register from June 2000 to June 2022 was undertaken using pre-defined search terms (Appendix 1). Randomised controlled trials (RCTs) and clinical controlled trials were included, with retrospective studies, case reports and case series excluded. Interventions assessed included any variation on the use of fixed appliance attachments or mechanics. Outcomes assessed included treatment outcome using a validated scale or a measure of treatment efficiency with assessments both during and after treatment.

The following search terms were used:

  • Archwire sequence

  • Initial aligning archwire

  • Bracket variations

  • Customisation of orthodontic appliances

  • Local bracket variations

  • Levelling the curve of Spee

  • Lacebacks

  • Space closure

  • Rate of space closure

  • Nickel titanium (NiTi) closing coils

  • Powerchain

  • Elastomeric ligatures

  • Pre-adjusted edgewise mechanics

  • Straightwire appliance.

Results

Levelling the curve of Spee

Clinical trials have been performed to evaluate the relative merits of the use of fixed anterior bite planes and reverse curve of Spee NiTi (0.016 x 0.022 inch) wires,4 in addition to the impact of the timing of inclusion of second molars within the appliance.5 The efficiency of overbite reduction was found to be independent of the latter.5 However, overbite reduction appeared to be more efficient with the use of bite turbos, while also culminating in less lower incisal advancement (Table 1).4

Table 1 Levelling the curve of Spee
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Archwire sequence

Ong et al. (2011) evaluated the efficiency of orthodontic archwire sequences produced by three manufacturers. In total, 132 patients completed the study.6 There were no differences between the groups either in relation to the time to reach the working archwire or in the alleviation of irregularity in the lower anterior segment.

Similarly, Mandall et al. (2006)7 compared different archwire sequences, highlighting no statistically significant differences in the level of discomfort or risk of root resorption with either sequence. The time to reach the working (0.019 x 0.025 inch stainless steel [SS]) archwire was significantly longer with one of the three sequences (Table 2).

Table 2 Archwire sequence
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Effect of bracket slot and prescription

Slot size

Yassir et al. (2019)8,9 and El-Angbawi et al. (2019)10 in a series of articles considered the effectiveness of an 0.018 inch and an 0.022 inch slot MBT orthodontic bracket system in terms of the duration, outcomes and impacts, noting no difference between the variants concerning the quality of occlusal outcomes (Peer Assessment Rating score reduction), incisor inclination change, patient perception of treatment, or the development of orthodontically induced inflammatory root resorption (OIIRR) (Table 3).

Table 3 Effect of brackets slot and prescription
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Bracket prescription

Mitall et al. in an RCT investigated the effect of bracket prescription on the outcome of treatment.11 Overall, 40 sets of post-treatment study models were used (20 Roth and 20 MBT). The crown inclinations of the 21, 13 and 41 were assessed from the digital images. The Roth and MBT bracket prescriptions were compared in terms of the Incisor and Canine Aesthetic Torque and Tip score. No statistically significant difference in the final inclination of the anterior teeth was observed (p = 0.132). This mirrors the findings elucidated within an allied subjective evaluation of incisor inclination change with MBT and Roth.12 However, the evaluation was undertaken in subjects with crowding being treated on a non-extraction basis. It is therefore conceivable that the effect of bracket prescription was not fully elucidated.

Initial aligning archwires

Comparison between different types of NiTi wires

Four trials have investigated the efficiency of initial alignment using heat-activated copper NiTi (CuNiTi), superelastic NiTi and/or conventional NiTi archwires (Table 4). Atik et al. (2019), in a trial involving 50 patients, found no difference between CuNiTi and superelastic NiTi archwires in relation to the efficiency of alignment, arch width changes and incisor inclination.13 Similarly, Azizi et al., identified no difference between CuNiTi archwires and conventional NiTi archwires in terms of both efficiency of alignment and pain perception.14 Abdelrahman et al. also observed no difference between CuNiTi, superelastic NiTi and/or conventional NiTi archwires concerning either the efficiency of alignment or pain perception.15

Table 4 Initial aligning archwires
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NiTi archwires versus other non-NiTi wires

Sandhu et al. (2013) investigated the differences in pain perception among 96 patients having initial alignment with either superelastic NiTi or multistranded SS wires.16 There was no difference between the two groups in terms of pain perception. Nordstrom et al. (2018) in an investigation of the efficiency of niobium-titanium-tantalum-zirconium and NiTi archwires also found comparable results between both groups, implying that niobium-titanium-tantalum-zirconium archwires may offer an alternative in patients with nickel hypersensitivity.17

Different configurations of NiTi wire

Sebastian et al. (2019) investigated the efficiency in alignment of mandibular anterior crowding in both extraction (40 patients) and non-extraction cases (24 patients), using coaxial NiTi archwires and single strand NiTi archwires. Co-axial forms did not demonstrate clinically relevant superiority in efficiency of alignment over the single strand NiTi archwires.18

Preformed versus customised CuNiTi wire

Phermsang-Ngarm et al. (2018) investigated the effect of the use of preformed CuNiTi and customised CuNiTi archwires in 32 patients.19 Preformed CuNiTi archwires were more efficient in terms of alignment, while the customised version resulted in less root resorption and bone loss. However, the overall differences were not considered to be clinically relevant.

Use of lacebacks

Usmani et al. (2002), in a RCT involving 35 patients, found that the use of lacebacks had a statistically significant effect on incisor inclination with no meaningful effect on anchorage loss.20 However, Irvine et al. (2004) found that the use of lacebacks had no tangible effect on incisor inclination or vertical positioning of incisors, with significantly more molar mesial movement identified (Table 5).21

Table 5 Use of lacebacks
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Space closure

NiTi closing coils versus elastomeric chains - rate of space closure

The efficiency of a range of space closing mechanics has been assessed in high-level primary studies. The most common modalities tested were NiTi closing coils and elastomeric power chains. Dixon et al. (2002) investigated the rate of space closure between active ligatures, elastomeric power chains and NiTi closing coil in 33 patients over a four-month period.22 NiTi closing coil were superior to active ligatures but they performed similar to elastomeric chains. Bokas et al. (2006) corroborated these findings in an RCT of 12 patients, assessing the rate of canine retraction between NiTi closing coils and elastomeric chains.23 Both Bokas et al. (2006) and Hashemzadeh et al. (2022) observed marginally more efficient closure with NiTi closing coils with potential deleterious effect associated with NiTi closing coils (such as canine rotation, tipping and molar tipping) being minimal and not clinically relevant (Table 6).23,24

Table 6 Space closure
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NiTi closing coils versus elastomeric chains - force decay

Nightingale et al. (2003), in a trial involving 22 patients, found that at the 15-week period, 59% of elastomeric power chains retained half their initial force compared to 49% of NiTi closing coils, with no associated difference in the rate space closure.25 Hence, force decay rates may be similar with the two modalities, with force decay having an inconsistent effect on the rate of space closure.

NiTi closing coils versus SS closing coils - rate of space closure

Norman et al. (2016), in a trial of 40 patients, investigated the rate of space closure on a 19 x 25 inch SS archwire using NiTi or SS closing coils. SS closing coils were found to be associated with more efficient space closure.26 Coupled with their reduced cost, this data supported the potential use of SS closing coils as an alternative to NiTi.

NiTi T-Loops versus SS T-loops - rate of space closure

Keng et al. (2012) investigated the efficiency of space closure and potential deleterious effect with beta titanium and NiTi T-loops. No difference regarding either the rate of space closure or concerning deleterious effects was observed.27

Ligation method

Wong et al. (2013) investigated the method of ligation using two different types of elastomeric O-rings and self-ligation in 45 patients (Table 7). The rate of space closure on a 0.019 x 0.025 inch SS wire over three months using NiTi coils was measured periodically. No effect of the method of ligation on the rate of space closure was found.28

Table 7 Litigation method
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Discussion

The present article provides an update on the scientific evidence underpinning the use of the pre-adjusted edgewise appliance and related mechanics, augmenting a previous report published in 2007.3 A noteworthy increase in the volume of high-quality prospective research was obtained with, for example, no prior prospective research concerning either the mechanics used for overbite reduction or in relation to the selection of the optimal archwire sequence.4,5,6 This increased volume of literature underpinning the use of fixed appliances mirrors the orthodontic literature more widely, although concerns in relation to the quality of reporting continue to exist.29 Notwithstanding this, further evidence concerning the use of certain mechanics, for example, lacebacks, has not emerged. This likely relates to the consistency among the pre-existing studies,20,21 suggesting that further research is not necessarily required in order to validate the findings, although replication of previous clinical trials may be regarded as helpful in order to mitigate against the risk of inadequate power associated with single studies.30

Since 2007, there have been numerous mechanical improvements, particularly in relation to fixed appliance auxiliaries, with elastomeric materials used for space closure an example. A number of new primary studies concerning the choice of mechanic on the rate of space closure have been conducted, with no consistent pattern being observed.23,24,31 A number of the more recent studies have involved comparison of proprietary brackets with alternative systems, with self-ligating and customised systems being compared to conventional brackets. This research on marketed products is of value, with much of it highlighting little distinction between newer marketed products and less heralded systems.32 It would therefore be helpful if independent clinical research concerning marketed products is instituted earlier in the development pathway in order to better inform end users concerning the related advantages and limitations.

This review was confined to the selection of attachments and mechanics associated with labial fixed appliance systems. We also did not include anchorage management techniques. Moreover, while the focus of this review was on the evidence underpinning pre-adjusted edgewise labial appliances, aligner therapy has seen considerable growth in recent years.33 While aligners offer aesthetic advantage and may be associated with lower pain experience over the initial three-month period,34 the ability to produce more predictable correction of complex tooth movements, including rotational control, has been questioned.1,2 Using the American Board of Orthodontics Objective Grading System, patients treated with Invisalign were also shown to have deficient scoring in respect of buccolingual inclinations and occlusal contacts compared to those treated with fixed appliances.35 Other related limitations have also been demonstrated on the basis of observational research.1,36,37

A lack of distinction associated with a range of interventions, including the selection of archwires, was observed. This may well reflect a lack of difference or indeed statistical power within the primary studies. However, it is also conceivable that the chief discriminator is the operator and handling of archwires, rather the selection of the specific wire. As such, the importance of optimal education and the practical application of knowledge cannot be overstated. Equally, it is also important to note that significant variability in relation to the choice of outcome measures, as well as the timing of data collection, exists.38 A potential solution is the creation and wider adoption of a core outcome set within orthodontic research to better inform trial design, maximising the yield from research and minimising associated waste of scarce resource.39,40

Conclusion

In recent years, there has been a large increase in the amount of high-quality evidence for pre-adjusted edgewise mechanics. In particular, there is a wealth of evidence to support the selection of fixed appliance attachments, archwires and auxiliaries, including lacebacks and space closing mechanics. This information can be used to routinely make informed decisions in the selection and manipulation of fixed appliances during daily practice.