From Volume 5, Issue 1, January 2012 | Pages 15-19
This article investigates the treatment efficiency of self-ligating brackets in terms of overall treatment times and the number of appointments required compared with conventional ligation bracket systems. Two retrospective studies are reported which may shed further light on the subject.
Manufacturers of self-ligating bracket systems claim that their brackets produce faster treatment.1 They claim that the low friction of self-ligating brackets will allow more rapid tooth alignment. This assumption needs careful investigation.
Conventional ligation is considered to be the use of an elastic module to hold the archwire into a Siamese twin bracket. Figure 1 shows two types of ligation with elastic modules; ‘conventional circle’ and ‘figure of eight’ (Figures b and d), plus examples of wire and self-ligation (Figures a and c).
Many articles2,3,4,5,6 have shown that friction is considerably lower in self-ligating systems than with conventional ligation. In fact, the orthodontic specialty is oversaturated with self-ligation frictional studies and the American Journal of Orthodontics has refused to accept any further papers on the topic.
Conventional circle ligation has high frictional levels and figure of eight ligation has even greater friction. Force figures vary considerably with archwire size and type.6 Self-ligation has much lower levels of friction and it is normally accepted that wire ligation has equally low levels of friction.3 Some authors, however, report high friction with wire ligation and others a difficulty in producing consistent ligation tightness.4 Common sense would suggest that differences in the tightness of wire ligation can produce different levels of friction. To summarize, the main features of these ligation methods are as follows:
It is worth mentioning that, in the early 1980s, there was a general change from wire ligation to elastic modular ligation, that is from a normally low friction system to a high friction system. Thurrow7 noted the friction change and drew attention to the adverse effect on anchorage. There were no reports in the literature which showed increased treatment times associated with the use of elastic modules. This could be the first indication of a lack of a relationship between friction and treatment duration.
The evidence for a reduced treatment time with self-ligating brackets is not robust. The strongest evidence was produced by Harradine8 in which 30 matched cases treated with Damon self-ligating brackets compared with conventional brackets showed an average reduction of active treatment time of 4 months, from 23.5 to 19.4 months. Eberting et al9 carried out a larger retrospective study of 215 cases and found an average reduction of 5 months from 30 to 25 months. This study was multi-centred with several different operators, including trainees, so that it is difficult to exclude other variables from the results. The lengthy initial treatment time compared with Harradine's study, for example, suggests that there was scope for other treatment modifications that might improve efficiency and reduce duration of the average treatment. Robert Fry10 reported a 4 month difference between treatment times in his two practices. The practice with shorter treatment time had been converted to self-ligating brackets, while the other remained with conventional ligation. Once again, several other variables between practices and operators could have played a part in the differences.
A growing number of publications have the view that self-ligating systems do not provide shorter treatment. Miles et al11,12 and Pandis et al13 examined the speed of alignment of lower incisors and found no differences between the two systems. Fleming et al14 also confirmed no improvement in efficiency of mandibular arch alignment with self-ligating brackets. Scott et al15 reported Damon brackets were no more efficient than conventional ligated brackets in initial or overall rate of mandibular incisor alignment. Hamiliton et al16 carried out a large retrospective study of 762 consecutively treated patients, half of whom were treated with self-ligating brackets and half with conventional brackets. He found no measurable differences in treatment time or number of visits between the groups. AT Dibiase and R Spencer both presented separate papers of randomized controlled trials at IOC Sydney 2010 showing no reduction in overall treatment times with self-ligating brackets. D Bearn has a paper in press with the same conclusion. Finally, Fleming and Johal17 and Chen et al18 conducted systematic reviews and neither found evidence as yet to suggest that orthodontic treatment was more or less efficient with self-ligating brackets.
In May 2002, the author's practice was converted to sole use of self-ligating brackets (GAC innovation). This specialist orthodontic practice has a single operator and treats National Health Service (NHS) and private patients. In 2005, an initial retrospective study on treatment efficiency was carried out. A second study followed in 2009.
The surgery daybook was examined for fixed appliance patients consecutively started immediately prior to May 2002. Records for these patients were retrieved and those with four premolar extractions were included in the study. The examination continued working backwards until 50 consecutively started premolar extraction cases had been achieved. These formed the cohort of cases where conventional ligation had been used. In a similar manner, the daybook was consulted and 50 consecutively started premolar extraction cases post May 2002 were identified. These formed the cohort of self-ligating cases.
The patients' notes were checked for the date of bonding brackets and date of removal from both archs. The time interval between dates was recorded, together with the number of visits required. Note that this number of visits excluded bonding, banding and debonding appointments. The average active treatment time and average number of visits for each cohort were calculated. The average time interval between visits was also worked out.
All cases were NHS patients. They were consecutively started and all had four premolar extractions. Non-extraction cases, single arch treatments, canine exposure cases, orthognathic patients and other extraction patterns were excluded. It was felt that choosing premolar extraction cases standardized the quantity and type of orthodontic work required. This would allow a more equitable comparison of the treatment efficiency of the two bracket systems. The group characteristics are given in Table 1. It was noted that the groups were well matched for age, sex, incisor classification and extraction pattern.
| Characteristics | Conventional ligation | Self-ligation |
|---|---|---|
| Age | 13½ | 13 |
| Male | 18 | 22 |
| Female | 32 | 28 |
| Class I | 7 | 9 |
| Class II div 1 | 30 | 26 |
| Class II div 2 | 10 | 9 |
| Class III | 3 | 6 |
| Ext 4s | 29 | 29 |
| Ext 5s | 11 | 15 |
| Ext up4s/Low5s | 10 | 4 |
| Ext 5s R 4s L | - | 2 |
The results are given in Table 2. Both the self-ligating patients and the conventional ligation cases took a similar time and number of appointments. The self-ligating average treatment time was a third of a month shorter and required half an appointment less. Neither of these differences was significant.
| Brackets | Treatment time | No. Appts | Interval |
|---|---|---|---|
| Conventional | 20.68 months | 11.86 | 9 weeks |
| Self-ligating | 20.32 months | 11.32 | 9 weeks |
It is important to note a difference from the patient and practitioner's perspective. Treatment efficiency for the practitioner primarily focuses on the number of visits required. The more visits required then the less efficient treatment is and the greater the overall cost. For the patient, treatment efficiency focuses on shorter treatment times. Self-ligating brackets did not appear to have any advantage in treatment efficiency, either in treatment duration or in number of visits.
In this initial study, no significant change had been made to the treatment philosophy or to treatment mechanics. Self-ligating brackets have different mechanical characteristics and, to take advantage, these various suggestions have been put forward. The most well known is Dwight Damon's philosophy of encouraging non-extraction and expansion treatments. This is ostensibly to take advantage of the lighter forces generated through lower friction in the brackets. The philosophy remains controversial. Other eminent clinicians, such as Nigel Harradine, Ron Rocone, and Gerald Samson, have made suggestions on altering treatment mechanics. Having listened to these individuals, a decision was made to implement the following changes in the author's practice from August 2005:
Starting retraction of canines earlier on lighter wires is feasible with self-ligating brackets because of the secure wire engagement. The archwire does not escape from the bracket slot. Even if initially the archwire is overpowered and distorts, with time it recovers and corrects the tooth position. Teeth do not rotate out of the archwire during retraction when using self-ligating brackets.
Modern super-elastic archwires and secure full ligation allow longer working periods between appointments. This benefits the patient with less frequent visits and allows the orthodontist to carry a greater workload (number)of patients under treatment.
It is not always possible to position brackets correctly at initial bond-up because of rotations or deceptive angulations. Roncone recommends early repositioning of these brackets to avoid time consuming corrections at a later stage. A longer second appointment should be reserved to accommodate this procedure. It is noted that this action is not necessarily limited to self-ligating systems but can be applied universally.
Archwire sequence was changed to take advantage of secure engagement and low friction (Table 3).
| The old archwire sequence | The new 2005 archwire sequence |
|---|---|
| 0.014″ NiTi | 0.016″ sentalloy (HANT) |
| 0.018″ NiTi | 20 × 20″ neo-sentalloy |
| 0.018″ steel | 19 × 25″ steel |
| 19 × 25″ steel | occasional 21 × 25″ TMA |
| occasional 21 × 25″ TMA |
These sequences are generalizations and differing patterns may be dictated by individual cases. The new sequence allows for one less archwire change thus, theoretically, saving one appointment. The changes were implemented in August 2005.
A second retrospective study was carried out to determine what effect the new mechanics had on treatment efficiency. A further 50 consecutively treated premolar extraction cases were examined for length of treatment and number of visits. The method of selection and inclusion criteria were identical to the 2005 study, starting with patients treated post August 2005. These cases were treated with self-ligating brackets and the altered mechanics described above. The group characteristics are given in Table 4. After initial enthusiasm, the ‘longer second appointment’ protocol tended to slip from treatment regime. The daily pressure to see as many patients as possible pushed this protocol on to the back foot.
| Characteristics | Conventional ligation | Self-ligation 2005 | Self-ligation 2009 |
|---|---|---|---|
| Age | 13½ | 13 | 13½ |
| Male | 18 | 22 | 20 |
| Female | 32 | 28 | 30 |
| Class I | 7 | 9 | 5 |
| Class II div 1 | 30 | 26 | 24 |
| Class II div2 | 10 | 9 | 15 |
| Class III | 3 | 6 | 6 |
| Ext 4s | 29 | 29 | 35 |
| Ext 5s | 11 | 15 | 7 |
| Ext up4s/low5s | 10 | 4 | 6 |
| Ext 5s R/4s L | - | 2 | 1 |
| Ext 5s L/4s R | - | - | 1 |
| Ext up4s low R4 low L5 | - | - | 1 |
Results are given in Table 5. This time there was a significant change in treatment duration. Unfortunately, treatment took on average a month and a quarter longer than with the conventional bracket system. However, treatment was completed in almost two visits less with new mechanics and self-ligating brackets.
| Brackets | Treatment time months | No. Appts | Interval weeks |
|---|---|---|---|
| Conventional | 20.68 | 11.86 | 9 |
| Self-ligating 2005 | 20.32 | 11.32 | 9 |
| Self-ligating 2009 | 21.56 | 9.6 | 10 |
Treatment efficiency from the patients' perspective was worse in the 2009 study. The overall length of treatment was longer. From the orthodontist's perspective, treatment efficiency improved as two fewer visits were required. The decision to extend the appointment interval to 10 weeks could be an important factor in the results. The secure engagement of self-ligating brackets makes these longer appointment intervals more viable, as opposed to less secure engagement of conventional ligation.
It is reasonable to ask if the quality of treatment result is the same for conventional ligation and self-ligation. A PAR study was commenced to compare the three treatment cohorts. It became obvious that the PAR index was too crude to detect any small differences there might have been between the groups. It was decided to restrict the study to the self-ligating bracket cases but to include all types of cases treated, not just the premolar extraction cases. This would allow easy comparison with other published studies to ensure that the self-ligating bracket system and the operator were producing an equivalent standard of treatment.
Sixty consecutively started fixed appliance cases were scored by two PAR calibrated examiners. The results are given on PAR nomogram Figure 2.
Richmond et al19 state that a reduction in PAR of greater than 70% is regarded as a good standard of treatment. In this study, a mean reduction of 80.8% was achieved. This indicates a good quality of treatment with the self-ligating bracket system. Comparison with other well known published PAR studies is possible in Table 6.
Two other questions arise. Harradine has suggested that self-ligating brackets might be more appropriate for non-extraction cases where there is less tooth translation but more tipping or rotation. Would cases other than extraction cases have produced different results? The studies by Miles et al11,12 and Pandis et al13 have already concluded that self-ligating brackets are not faster at aligning and derotating teeth. Further study is needed but it is unlikely that self-ligating brackets will produce faster treatment in non-extraction cases.
The second question is, if similar changes to treatment mechanics were applied to patients with conventional ligation systems, would they have the same effect on treatment efficiency? In other words, was it the self-ligating brackets or the altered mechanics that were responsible for the changes in treatment efficiency? This question is open to further investigation.
It would seem logical that the factor most likely to determine the rate of tooth movement is the speed of bone metabolism. How fast can bone be resorbed and remineralized? Does the amount of force influence the rate of bone remodelling? The biology of tooth movement is complex and current knowledge suggests that there is a low threshold force above which individual teeth will move.23 There is no suggestion that the rate of tooth movement is linear with the amount of force used. Provided an optimal force range is delivered for individual teeth, continuously, then the maximum rate of tooth movement will be achieved.24 The influence of the amount of friction in a bracket is restricted to varying the resultant amount of force on the tooth. Increased friction can be overcome by increasing the force applied. (This may have adverse implications for anchorage balance.) Lowering the frictional resistance, by using a self-ligating bracket system, means more of the applied force becomes resultant force (ie increases) on to the tooth, but it is not likely to speed up movement.
There would seem to be other benefits to using self-ligating brackets. For example, low friction in buccal segments has been shown to reduce proclination of incisors when aligning displaced canines. The work of Badawi et al25 is particularly exciting in this field. A reduction in anchorage demand seems a feasible goal. The quick and easy ligation is reported to generate operator preference.
These studies indicate that self-ligating bracket systems are not faster than conventional ligation appliances at treating premolar extraction cases.
When self-ligating brackets were used in conjunction with altered treatment mechanics, there was an average saving of two appointments per case. A slight lengthening of overall treatment time occurred.
