The straight wire technique was designed to reduce the number of bends required in the archwire by incorporating information directly into the brackets [18]. However, the results of this treatment technique depend on several factors, notably the accurate positioning of brackets [3].

For Gianelly, the main advantage of the indirect technique would be better bracket positioning accuracy [19]. However, few studies have been able to validate this [9,10,11,12].

Recent technological advances have pushed the limits of indirect bonding even farther by allowing the orthodontist to do a virtual bracket placement with a visual 3D access to the roots using software applications such as OrthoAnalyzer™ [15]. To the best of our knowledge, no studies compared direct bracket positioning to the new aforementioned digital indirect technique.

The aim of this study was to assess whether there are any statistically significant differences between direct and digital indirect placement techniques of two bracket types. Three parameters were studied: height, angulation and mesio-distal positioning of the brackets. The influence of the bracket type on the positioning accuracy was also evaluated as a secondary objective.

In this study, the “best fit matching” option in the Exocad™ software was used for the superimpositions: The user is asked to place as many reference points as needed on the two models. The software then performs an accurate and precise alignment. Other software applications, such as the OrthoAnalyzer™ software, only allow the user to place a total of three alignment points, which may hinder accurate superimposition.

Measurements were taken on 2D cross sections rather than using the 3D option. This was done because we noticed a larger inter-operator variation using the 3D option, since the distance was evaluated between two points that can be moved in all three dimensions, whereas the 2D cross sections allowed for precise and reproductible measurements that were automatically calculated by the software.

In terms of height, the difference between the direct and indirect method was less than, or equal to 0.5 mm for both Damon and MBT brackets on all of the maxillary teeth. This means that there are no statistically significant differences in height between the two techniques, which is consistent with the results obtained by Hodge et al. and Aguirre et al. [11, 12] but not consistent with those reported by Koo et al. [10]. A possible explanation for that could be the fact that Boone gauges were used in the latter study, and a study by Mohammadi et al. found that height positioning gauges were more accurate than Boone gauges [20]. In the present study, the “distance from incisal edge” option was used in the OrthoAnalyzer™ software, through which the user enters the desired bracket height, and the software automatically positions the brackets accordingly. Since no differences were found between direct and indirect techniques, this could be additional validation that height positioning gauges, which were used in this study, are a precise tool for positioning brackets in the vertical dimension. This was also reported by Armstrong et al. [21].

Regarding mesio-distal positioning, the difference between the two methods was significant on teeth 13 and 15. This was not reported in the literature, but it should be said that some studies did not consider horizontal bracket positioning mistakes due to the difficulty of measuring said parameter with overlapping teeth [11, 22]. Koo et al. resorted to manually sectioning the teeth using a saw, which was a strenuous process [10]. The use of technology in our study enabled us to virtually segment the teeth, with ease and precision, using software applications. This may explain why our results are not concurrent with those found in the literature. Nevertheless, Aguirre reported that in both direct and indirect technique, linear measurements of bracket positioning tended to be more precise on the left side than that on the right side for the maxillary arch [11]. Furthermore, Dr. Tom Pitts reported that placing brackets too mesially on the premolars and too distally on the canines were common mistakes made by orthodontists during direct bonding [23].

In terms of angulation, the difference between the two methods was significant for MBT brackets on the upper right lateral, and for Damon brackets on both upper laterals. The results in the literature showed no difference regarding angulation [10,11,12]; nevertheless, it should be kept in mind that all three of these studies that compared direct and indirect bonding techniques did not include radiological support for practitioners during bracket positioning.

Panoramic radiographs have been used traditionally to assess root positioning during bracket placement. However, studies have shown that erroneous root visualization could occur, due to many factors, such as distortion from non-orthogonal X-ray beams, variable facial typology between patients or even different inclinations of the patient’s head while taking the radiograph [3]. An alternative imagery is the CBCT, which allows for a three-dimensional visualization of the roots. Due to the fact that it is a fairly irradiant technique, caution must be exerted when prescribing this radiograph [3]. In this study, an ethically justified CBCT was superimposed to the patient’s maxillary 3D model, gaining visual access to the roots during the indirect bracket placement phase. The CBCT-guided indirect positioning was compared to the panoramic-guided direct positioning, and differences in angulation were observed on the laterals, which corroborates the findings by Owens et al. that the panoramic radiograph may not faithfully reproduce the actual apical position [13]. Furthermore, Andrews and Aguirre reported that orthodontists had more trouble finding correct angulation than height while positioning brackets [11, 18]. The latter also found that most positioning errors took place in the labial segment [11].

Our study showed that a significant difference was noted between conventional and self-ligating brackets, the latter generally having a higher margin of difference for each parameter. Knowing that the cost of self-ligating brackets is higher than that of conventional twin brackets, one should keep in mind the costs associated with rebonding these pre-adjusted appliances in order to obtain a correct tooth positioning [24].

This finding is also reported by Birdsall et al., who found that self-ligating brackets were more difficult to position accurately than conventional twin brackets [14].

The mini-master brackets used in this study are larger mesio-distally than the Q2 brackets. Damon brackets are designed to have larger interbracket distance than conventional twin brackets in order to promote lower forces to the teeth [25]. Andrews pointed out that the smaller the size of the bracket, the more complicated its positioning would be [18]. This may explain the difference found in this study.

It should also be noted that the level of experience of practitioners, as well as their familiarity with this type of appliance, may have influenced the present results.

In the inclusion criteria for choosing the participating orthodontists, a minimum of 2 years of clinical experience was required: This led to a discrepancy in experience levels among different participants, as some only had two years of experience, whereas others were more qualified. This discrepancy also existed for the virtual bonding, even though the participants were all calibrated by the same technician, who would also answer any technical question they had during the procedure, without influencing the results. The large number of statistical measures in this study made it difficult to allocate orthodontists according to their experience levels, notwithstanding the fact that it would have diminished any associated bias.

Regarding direct bonding, our results are concurrent with those reported by Fowler et al., who had found that the level of experience affected angulation results, but are not in agreement with those reported by Armstrong et al., who reported that it affected the height parameter only [26, 27].

Regarding virtual bracket bonding, a study by De Oliveira et al. found that angulation was affected by the level of experience of the orthodontist, which also may explain our results regarding the angular dimension [28].

In the conclusion of their study, Koo et al. stressed the importance of finding different ways of measurement than those followed in their methodology [10]. The main difference between the present study and the previous ones is the incorporation of technology; the indirect positioning was done virtually, with a 3D visual access to the roots, which facilitated brackets’ placement at the correct angulation. Study models were scanned by a digital scanner, unlike previous studies which relied on photography, and subsequently risked errors of magnification and distortion. Furthermore, the digital software used allowed us to perform a precise superimposition of the models as well as a reproductible study of the different measurements between the two techniques.

Twenty practitioners placed the brackets on 10 different teeth, with 3 measurements taken per tooth, which makes a total of 1200 measurements carried out in our study.

Given this large number of measurements, our protocol was limited to the maxillary arch. It would be interesting to include the lower arch in an ulterior study.

Although the mannequin and models were designed in order to simulate a clinical environment, it should be kept in mind that different results may be obtained on a real patient. The difficulty of finding a sufficient number of patients with malocclusions of similar difficulty, relatively healthy teeth with an ethically justified CBCT, was a factor that made this an in vitro study. A clinical trial that involves practitioners with an equal level of experience in different types of brackets would be the golden standard.

The study was conducted on brackets from right second premolar to left second premolar. Out of the three studies in the literature that compared direct and indirect bracket positioning, two studies did not include the molars: A randomized clinical trial by Hodge et al. evaluated only the six anterior teeth because error in bracket placement on these teeth would have the most serious aesthetic consequences [12]. Koo et al. conducted a study that was limited from 15 to 25 [10]. Furthermore, we did not find molar tubes that were available in both the Lebanese market and the OrthoAnalyzer™ library. It would be interesting to include the molars in an ulterior study, since a major advantage of the indirect bonding technique is easier posterior positioning [8].

One may argue that most errors in indirect bonding occur during the transfer process. However, technological advances have increased the accuracy of said transfers. Studies have shown that using specific trays can be a very precise method of bracket transfer in the digital indirect bonding technique [29,30,31]. Therefore, we did not include the transfer process in our methodology given that the differences between the virtual 3D setup and the result after transfer would be minimal.

Since, for the majority of teeth, the differences found between the two positioning techniques were not statistically significant, one may wonder if a switch from the direct bonding system to the three-dimensional CBCT indirect world is worth it, knowing that it is a fairly irradiant technique. Nevertheless, this technique could be of interest in individualized orthodontics: The practitioner is often faced with contralateral teeth having different initial torque values. In the straight wire technique, in order to obtain a symmetrical result, he would have to either incorporate bends in the archwire, use torquing auxiliaries, or use a hybrid brackets setup [32]. As shown in this study, technology makes it possible to achieve virtual bracket positioning with a 3D acquisition of the roots by means of a CBCT. This superposition can then be followed by customization of the bracket base or of the actual bracket slot in order to obtain an adequate inclination or torque. This is only possible by means of an indirect, individualized, digitized bonding, which underlines the importance and the advantages of this technique [33].

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