INTRODUCTION

Friction is defined as the resistance to motion when one object moves tangentially against another.1 During mechanotherapy involving movement of the bracket relative to the wire, friction at the bracket-wire interface may prevent the attainment of optimal force levels in the supporting tissues.2 Therefore, a decrease in frictional resistance tends to benefit the hard and soft tissue response.3 It has been proposed that approximately 50% of the force applied to slide a tooth is used to overcome friction.4 Other factors that affect frictional resistance include saliva,5 archwire dimension and material,6–8 angulation of the wire to the bracket,9 and mode of ligation.1011

The term self-ligation in orthodontics implies that the orthodontic bracket has the ability to engage itself to the archwire12 and is therefore assumed to reduce friction by eliminating the ligation force. These bracket systems have a mechanical device built into the bracket to close off the edgewise slot. Two types of self-ligating (SL) brackets have been developed: those that have a spring clip that presses against the archwire and those in which the SL clip just closes the slot, creating a tube, and does not actively press against the wire. With every SL bracket, whether active or passive, the movable fourth part of the bracket is used to convert the slot into a tube.13

SL brackets are not new to orthodontics; in the mid-1930s, the first SL bracket, the Russell attachment, was introduced in an attempt to enhance clinical efficiency by reducing ligation time.14 However, there has recently been resurgence in the use of SL brackets.15

Although reduced friction has been reported to be one of the advantages of SL,1617 the issue of friction and SL brackets is still controversial,16 as some studies have reported the reduction in friction with SL brackets to be significant while others claim that SL brackets produce similar or higher friction compared with conventional brackets.1018

Practitioners need to decide whether self-ligation will be beneficial to their specific treatment plan for each individual patient. To make this decision, they need to know if friction between brackets and archwires is significantly reduced by self-ligation in a clinically meaningful quantity and also if this reduction is limited to certain circumstances. Only a conventional review19 has been previously published, which presented an overview of the status of self-ligation in the early 2000s. The aim of this systematic review was to compare in an in vitro setting the amount of expressed frictional resistance that orthodontic SL brackets produce compared with conventional brackets.

MATERIALS AND METHODS

A systematic computerized search of electronic databases was undertaken in Medline, PubMed, Embase, Cochrane Library, and Web of Science until April 2008. The following search strategy (with the use of Boolean operators) was used in Medline: ((Orthodontic bracket*) OR (exp Orthodontic Brackets) AND (self ligat* OR self ligation)). A similar strategy was adapted for use in PubMed, Embase, Cochrane Library, and the Web of Science database. Specific search dates and the search strategies are illustrated in Table 1. No limits were applied to the electronic searches. Duplicate results were identified and removed.

Table 1.  Search Dates, Search Strategies, and Number of Results for Each Database

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Abstracts of the retrieved results were scrutinized, and papers that seemed to meet our initial selection criteria defined (in vitro studies that addressed friction of SL brackets) were identified. Papers were excluded at this stage if they were descriptive, editorial, letter, in vivo, not investigating SL brackets, or were studying other properties of SL brackets rather than friction. For papers that did not have any abstract except the title available on Medline/PubMed, the full text was retrieved.

Full articles were obtained from the abstracts/titles that met the initial selection criteria. Papers were then evaluated according to the methodological score outlined in Figure 1 to identify papers of acceptable quality (final selection criteria). Minimal quality consisted of a minimal sample size of five per subgroup analyzed and statement of P value and confidence interval for the results. Table 2 shows the methodological scores for the finally selected papers.

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Table 2.  Methodological Scores for the Selected Papers^a

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Selection of the articles at each stage was performed by two researchers. The selections were then discussed, and discrepancies were resolved so that final selections were agreed on by both researchers. Furthermore, a secondary (manual) search was then performed by going through the reference lists of the selected articles to identify any paper that met the initial inclusion criteria but was missed by the electronic searches.

RESULTS

Search Process

Sixty hits were obtained from Medline and PubMed. From the 40 hits retrieved from Web of Science, 30 also appeared in Medline/PubMed. All of the four hits retrieved in the Cochrane Database were also found among Medline/PubMed results. Embase returned no results. After removing the duplicates, the total number of hits (from all the electronic searches together) was 70 (Table 1).

Based on the initial inclusion criteria, 46 of 70 results were excluded based on their abstracts. A total of 24 papers (from the electronic search) met the initial selection criteria.131011131820–37 However, in our second (final) stage of selection, 6 papers were eliminated because of undetermined sample size,232534 very small sample size,1035 or not reporting any standard deviations (or confidence intervals) and P values.21

The secondary (hand) search of the reference lists of the selected papers resulted in three additional papers.383940 All met the initial selection criteria, but two3839 were excluded at the second (final) selection stage because of small sample sizes. A summary of the selection process is illustrated in Figure 2.

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Selected Articles

A summary of key methodological data and the results from the selected studies can be found in Table 3. More specific information of the comparisons made, wires used, methodological process, and so forth can be read in Appendix 1.

Table 3.  Summary of Selected Papers

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DISCUSSION

Regarding the final selected studies, it can be said that the variability of the experimental methods among the selected in vitro studies may explain the inconsistency of the selected study results. Although most of the studies published in orthodontics talk about evaluating friction they actually evaluate resistance to sliding which is not interchangeable with friction. For the purpose of this discussion the term friction will be used although is not correct. This will facilitate reading and understanding of the discussion.

A consistent agreement was found among the reviewed studies that SL brackets produce lower friction compared with conventional brackets when coupled with small round archwires,313202426–323640 whereas only one study18 disagreed. Some discussion of these studies may shed some light in this regard. As for findings of both this study and another conflicting study that was not included in the review,1018 the flexibility of the spring clip of active SL brackets, which can actively engage the wire in the presence of tipping, may explain the controversy. Tipping is a constant phenomenon during sliding tooth movements, and it always occurs when orthodontic force is applied to a tooth.31 For this reason, teeth will tip until contact is established between the archwire and the diagonally opposite corners of the bracket wings.29 Tipping and torquing forces therefore can affect the frictional resistance during space closure.32 The spring clip of both Speed and Time brackets, two types of active SL brackets, is flexible, which also means it is an active clip. Therefore, when it is subjected to a constant force in any one of rotational, tip, and torque spatial planes during the movement of the archwire through the slot, the spring clip will be maintained continuously in a displaced condition while the archwire is pulled past it. In other words, the archwire is actively engaged by the spring clip and pressed into the slot.

Unlike most other studies, in the studies by Bednar10 and Redlich et al,18 the brackets were made to tip relative to the archwire; therefore, the above theory may explain why the findings of these two studies do not agree with the others. Sims et al22 and Reicheneder et al29 also allowed tipping of the brackets relative to the wire in their studies, but both studied only passive SL brackets. This may imply that passive SL brackets may exert less friction than active ones when round wires are used in specific clinical situations. Unlike the studies by Bednar10 and Redlich et al,18 in studies by Shivapuja and Berger3 and Thomas et al,32 the bracket was locked in place so that the slot was parallel to the archwire. This way, the bracket width while rotating (torque) as a factor contributing to friction was eliminated.32 Shivapuja3 argued that without simulating angulation, each system can be compared equally. However, Shivapuja admits that this portion of their study is really a comparative study and in no way was capable of simulating clinical conditions with all the attended variables. Tecco et al31 also admitted that lack of reproducibility of tipping was a limitation of their study. It has been previously reported that as the bracket to archwire angulation increases, so does the frictional resistance of a particular bracket and archwire combination.9

As for large rectangular archwires, there seems to be controversy among the evaluated papers. Some of them reported that with large rectangular archwires, the friction of SL brackets was not lower compared with conventional brackets,126–2831 while others claimed that SL brackets produced lower friction compared with conventional brackets.11132430323740 However, half of the latter group13303240 still confirmed that even though friction of SL brackets was lower compared with conventional brackets, friction increased as the archwire size increased. So in general, these findings are in agreement with several studies that have previously reported that friction increases as wire dimension increases68 and that frictional force is generally greater with rectangular wires than with round wires.735 A reason why rectangular wires produced an increased friction even in SL brackets is that, as the bracket slot is filled, the differences between SL and conventional brackets are minimized. This is related to less tipping allowed before teeth are straightened back by the wire resilience. This cycle occurs at a faster rate with more slot play.

Regarding differences in friction between passive and active SL brackets, some controversy exists. Six of the 11 studies112425323640 reported that passive brackets generated a lower level of friction compared with the active group, while 2 studies327 reported no differences. The remaining 3 studies283031 did not observe a consistent trend. The difference in friction between passive and active SL brackets have been attributed to the fact that the former group of brackets form a rigid tube when closed, applying no direct force to the wire.11 Other possible explanations for the results could be differences in archwires tested and diverse brackets tested.

All selected articles used a consistent 0.022-in bracket slot size for all studies, and therefore conclusions regarding the influence of bracket slot size change on frictional resistance cannot be drawn. According to Smith et al,40 “bracket slot size may not influence the frictional resistance,” but some of the studies identified by the initial inclusion criteria did suggest that frictional resistance decreases as slot size increases.2 The explanation here is that the friction when related to slot size is more a function of the dimension of the archwire engaged.

Steel SL brackets were consistently reported to show lower friction compared with ceramic and polycarbonate conventional brackets.1313203640 This is probably due to the increased roughness and porosity of ceramic, which leads to a higher coefficient of friction compared with stainless steel.13 These findings are in agreement with previous studies that reported friction to be higher with ceramic brackets (conventional) compared with steel brackets (conventional).741 Esthetic (fiberglass-reinforced composite [FRC] and resin) SL brackets were reported to have lower friction compared with conventionally ligated esthetic (ceramic and FRC) brackets,29 which may be related to the mechanical binding of elastomeric tie and the ceramic bracket surface of conventional brackets.3

As with any in vitro study, none of the evaluated papers included in this review can accurately simulate what really happens in clinical situations because of variables such as masticatory forces20 and oral functions,3031 different degrees of malocclusion,262733 width and compressibility of PDL,1 tooth rotation,2438 torque at the wire-bracket interface,23 bracket/archwire angulation,2528 and temperature and moisture.2931

Clinicians should be cautioned that although in vitro findings are a useful guide to anticipated clinical behavior, the observed clinical performance might be quite different.42 Furthermore, leveling and alignment of malposed teeth, which begins as part of the initial stage of orthodontic treatment, should be accomplished with flexible archwires.33 Therefore, experiments with small round archwires in the absence of malalignment may not accurately reproduce what actually happens in clinical situations. A rectangular archwire is often used to complete the initial leveling and aligning stage, express rotation control, and start torque control; it is also usually recommended for space closure (after teeth are well leveled and aligned),22 retraction (during which teeth may tip or rotate), and finishing.33 Therefore, experiments with rectangular archwires should preferably incorporate tipping and rotation in their testing settings for the conclusions to be applicable to all phases of orthodontic treatment. Only a few of the studies considered malocclusion in their experiments; all agreed that as malocclusion increased, friction increased as well, regardless of the bracket type or wire size.26273336 A novel approach mimicking malocclusions using a three-dimensional setup with nanotechnology transducers appears to have great potential to help us understand the complexity of intra-arch biomechanics and its impact on frictional resistance among other mechanical aspects of orthodontics.43

Some limitations were identified that should be considered in future reviews. Because of the theoretical differences between active and passive self-ligation brackets, a specific analysis considering subgrouping the studies could have added more depth to the discussion. The same would apply to differences in slot size and bracket prescriptions. Because of the limited number of studies finally selected, further subgrouping will not deem information with enough sample size to warrant meaningful conclusions.

CONCLUSIONS

• Compared with conventional brackets, SL brackets maintain lower friction when coupled with small round archwires in the absence of tipping and/or torque in an ideally aligned arch.

• There is not enough evidence to claim that with large rectangular wires, in the presence of tipping and/or torque and in arches with considerable malocclusion, SL brackets produce lower friction compared with conventional brackets.

• Most of the evaluated studies agreed that friction of both self-ligated and conventional brackets increased as the archwire size increased.