Introduction

Esthetic expectations in the 20th century have changed in parallel to advances in orthodontics. Owing to heightened esthetic expectations, the value placed on facial and smile esthetics, and thus, interest and demand for orthodontic treatment, has increased. Orthodontic therapies are sought to achieve functional occlusion and well-being and improve the physical image. Physical image properties in relation to esthetics have an important impact on the individual’s career, social life, and personality. Although esthetics is a primary motive for orthodontic treatment, the visibility of labial brackets may cause reluctance to orthodontic treatment in adult patients [1].

In recent years, there has been a considerable increase in the number of adults undergoing orthodontic treatment [2]. This trend may be attributed, at least in part, to the introduction of “esthetic appliances”. The quality of esthetic outcome might be reduced by white-spot lesions on teeth and damage of enamel occurring during removal of labial brackets used in orthodontic treatment [3]. There is some evidence suggesting that the lingual aspect of teeth is more resistant to decay than the labial surface. Thus, orthodontics has focused on progress in the lingual area. Treatment using lingual bracket systems allows a satisfactory esthetic appearance and may increase self-confidence and interaction with others [1].

The orthodontist attempts to (1) achieve permanent outcomes with functional harmony of the teeth–jaw–face system and acceptable esthetics while he/she attempts to reduce undesired side effects of forces that emerged during active orthodontic treatment; (2) achieve maximum benefit with minimum harm, and (3) shorten the treatment duration.

Recent advances in bracket design, self-ligating brackets, the straight-wire technique, and customized wire and bracket design have facilitated the technical aspects of lingual orthodontics, resulting in improvement of patient and clinician comfort [4,5,6]. However, the main issue related to biomechanics is the short interbracket distance in the lingual technique. As a result, the archwire stiffness increases and lingual systems generate more frictional forces between the brackets and the inserted wire than labial systems [7]. In addition, a different moment of force occurs due to the bracket position [8]. Lingual brackets are positioned closer to the centers of resistance of the teeth than labial ones in the sagittal plane [7]. Another mechanical property associated with these appliances is the orientation of the slots in the brackets [9]. The above mentioned differences in biomechanical performance between lingual and labial orthodontic appliances may influence the alignment of crowded teeth.

Evaluating tooth movements to relieve crowding is needed in orthodontic treatment, particularly in the course of the leveling phase. Some studies have investigated the individual clinical effectiveness of lingual and conventional labial bracket systems, but there is a paucity of studies comparing these systems [1, 2, 4, 10, 11]. An in vitro comparison of lingual and labial brackets on tooth alignment efficacy but not on mandibular incisors has been reported to date [12]. Therefore, the aim of the present clinical study was to determine whether there were any differences between customized lingual brackets and conventional labial brackets regarding the alignment of the mandibular arch and a reduction of the irregularity index during an 18-week treatment interval. The null hypothesis of the study was that there would be no significant difference regarding changes of the irregularity index scores of patients treated with customized lingual brackets and conventional labial brackets during the initial leveling phase of treatment.

Subjects and methods

Trial design, ethical issues, and registration

This pilot study was designed as a prospective parallel-group, controlled trial randomized in a 1:1 ratio according to the Consolidated Standards of Reporting Trials (CONSORT) guidelines. The study was approved by the Ethics Committee of Aydın Adnan Menderes University, School of Medicine (2016/897). All patients and legal guardians gave written informed consent before participation. This trial was registered at ClinicalTrials.gov (NCT04357067).

Participants, randomization, and eligibility criteria

We included 20 patients (15 females, 5 males) who presented with class I malocclusion to the Orthodontics Department of Dentistry School of Aydın Adnan Menderes University for scheduled orthodontic treatment without tooth extraction. The demographic characteristics of the patients are shown in Table 1. For inclusion, the following criteria were considered: complete permanent dentition except for the third molars, absence of tooth loss or extraction, not having crossbite, crowding ≥ 6 mm according to Little’s irregularity index [13], age between 15 and 25 years, dental and skeletal class I malocclusion with normal growth pattern, lack of genetic or hormonal disorders, absence of chronic drug use, not having previous orthodontic treatment and prosthetic restoration, and good oral hygiene. During randomization, an assistant at the Orthodontic Department (who was not involved in the study) assigned the patients to groups by numbered, opaque, sealed envelopes in a 1:1 ratio.

Table 1 Tab. 1 Demographic characteristics of groupsDemographische Merkmale der Gruppen
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Interventions

The patients were randomly assigned for treatment with Incognito lingual brackets (3M Unitek, Bad Essen, Germany; lingual bracket group; n = 10) or Gemini labial brackets (3M Unitek, Monrovia, CA, USA; labial bracket group; n = 10). Gemini 0.018″ slot brackets were applied to the patients in the labial bracket group directly, while Incognito 0.018″ slot brackets were applied to the patients in the lingual bracket group indirectly. Customized Incognito brackets and tubes were applied to the first and second molar teeth, while 3M Unitek tubes (3M Unitek, Monrovia, CA, USA) were applied in the labial bracket group. Individualized superelastic nickel–titanium (NiTi) archwires were used in the lingual treatment group, while prefabricated superelastic NiTi archwires (3M Unitek, Monrovia, CA, USA) were used in the labial treatment group. No expansion appliances, extraoral appliances, elastic chains, open/closed coil springs or intraoral elastics were used throughout the study period. During the alignment phase, no stripping was applied until 0.016″ NiTi archwire removal. In both groups, elastic ligatures were used to attach the archwire to the brackets. All patients were treated by the same operator (M.K.).

In the first session (T0), the brackets and tubes were bonded. In addition, a 0.012″ NiTi archwire was applied, and a control visit was scheduled 6 weeks later. In session 2 (T1), intraoral digital scan was performed after removing the 0.012″ NiTi archwire, followed by the application of a 0.014″ NiTi archwire. In session 3 (T2), another intraoral digital scan was performed after removing the 0.014″ NiTi archwire, followed by application of a 0.016″ NiTi archwire. In session 4 (T3), the 0.016″ NiTi archwire was removed and final intraoral digital scan was performed (Fig. 1). In all sessions, digital models were obtained by a Trios Orthodontics intraoral scanning device (3Shape, Copenhagen, Denmark) after removal of the archwire. The digital models were analyzed by Ortho Analyzer software (3Shape, Copenhagen, Denmark), and the obtained measurements were stored.

Fig. 1 Abb. 1
figure 1

Labial and lingual bracket treatment photos of a patient at T0 (a), T1 (b), T2 (c), and T3 (d)

Fotos der labialen und lingualen Bracketbehandlung eines Patienten bei T0 (a), T1 (b), T2 (c) und T3 (d)

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Outcome measures

In order to analyze the changes from T0 to T1, T2, and T3, the irregularity index, intercanine width, intermolar width, and arch length measurements (Fig. 2) were performed on the digital models.

Fig. 2 Abb. 2
figure 2

Measurement of the irregularity index (a), intercanine width (b), intermolar width (c), and arch length (d) on the digital model

Messung des a Irregularitätsindexes (a), der Intereckzahnbreite (b), der Intermolarenbreite (c) und der Bogenlänge (d) auf dem digitalen Modell

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To control incidental measurement errors, all measurements were repeated by the same observer 15 days after the first measurement. Intraclass correlations were calculated.

Blinding

Blinding of the orthodontic treatment providers and patients was not possible. However, all measurements were performed blinded to the patients’ name and the time point during data analysis. Digital model measurements were performed by the same investigator (M.K.) after being given research numbers by the other investigator (Y.A.Ü.).

Statistical analysis

All statistical analyses were performed using SPSS® software version 22.0 (IBM, Armonk, NY, USA) and Microsoft Excel (Office 2016, Microsoft, Redmond, WA, USA). A p-value < 0.05 was considered statistically significant. Shapiro–Wilk test was performed for determining normality. The data of this study showed normal distribution, so they were analyzed descriptively by using means, standard deviations (SD) and minimum/maximum values for metric variables. The Student t‑test was used for comparisons within labial and lingual bracket groups. A repeated measure ANOVA was used to compare measurements at different time points between the two groups. The differences of means and standard deviations between T1–T0, T2–T1, T3–T2, and T3–T0 were calculated. The Student t‑test was used to assess whether the calculated differences between labial and lingual bracket groups were significant.

Results

Patient flow

A CONSORT diagram showing the patient flow throughout the trial is shown in Fig. 3. A total of 27 patients were screened for eligibility: 7 patients were excluded for not meeting the inclusion criteria; 20 patients were randomized in a 1:1 ratio to either the lingual brackets or labial brackets group. No patients dropped out during the trial. Patient recruitment began in May 2016 and ended in July 2016.

Fig. 3 Abb. 3
figure 3

CONSORT (Consolidated Standards of Reporting Trials) diagram showing patient flow through the trial

CONSORT(Consolidated Standards of Reporting Trials)-Diagramm zum Patientenfluss durch die Studie

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Outcomes

It was found that intraobserver reliability of digital model measurements repeated 15 days after treatment was within an acceptable range (0.9433–0.9960). When compared to T0 data, a significant reduction was detected in the irregularity index within the 18 weeks in both groups (Table 2; p < 0.001). The mean irregularity index value reduced gradually during the first 12 weeks of alignment from 9.65 mm at T0 to 6.05 mm at T1 and to 4.71 mm at T2 with a statistically significant difference in the labial bracket group. The irregularity index also reduced for the last 6 weeks of alignment from 4.71 mm at T2 to 4.20 mm at T3 but this change did not reach statistical significance. The same trend was observed in the reduction of the irregularity index values during 18 weeks of alignment in the lingual bracket group. The mean irregularity index was 9.07 mm at T0 and reduced to 7.10 mm at T1 and 5.50 mm at T2 with a statistically significant difference within the 12 weeks. After that, the index reduced to 4.23 mm at T3 but the change between T2 and T3 was as well not statistically significant in the lingual bracket group.

Table 2 Tab. 2 Intragroup comparison of labial and lingual bracket groupsIntragruppenvergleich, labiale und linguale Bracketgruppen
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Intercanine width did not change statistically significantly in the labial bracket group between T0 and T3, but in the lingual bracket group, intercanine width increased from 26.77 mm at T0 to 27.36 mm at T1 with a statistically significant difference (Table 2; p < 0.001). Thereafter, this width increased gradually between T1 and T3 to 27.78 mm at T3. However, the increase from T1 to T2 and T2 to T3 was not statistically significant. There was no statistically significant change for intermolar width for both groups. Arch length measurements increased gradually from 59.47 mm at T0 to 63.40 mm at T3 in the labial group but the increase between T1 and T2 did not reach statistical significance (Table 2; p < 0.01). A similar trend was found in the lingual bracket group (58.15 mm at T0 to 61.59 mm at T3; Table 2; p < 0.001).

When the mean values at sessions T0, T1, T2, and T3 were compared, it was found that there was no significant difference in the irregularity index, intercanine width, intermolar width, and arch length between the groups (Table 3).

Table 3 Tab. 3 Comparison of data in labial and lingual bracket groups (intergroup comparison)Vergleich der Daten in labialen und lingualen Bracketgruppen (Intergruppenvergleich)
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When means and standard deviations for the differences between T1–T0, T2–T1, T3–T2, and T3–T0 were compared between labial and lingual groups, only the change of the irregularity index for T1–T0 was statistically significant (Table 4; p < 0.001). The mean changes within the first 6 weeks (0.012″ NiTi archwire) for the irregularity index was −3.60 mm in the labial group and −1.95 mm in the lingual group.

Table 4 Tab. 4 Comparison of mandibular arch changes with Student’s t‑testVergleich der Unterkieferbogenveränderungen anhand des t‑Tests
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Discussion

In order to achieve patient motivation toward orthodontic treatment, it is important to establish an alignment of the dental arch, especially in the anterior region, as effectively as possible. It seems that all previous investigations dealing with mandibular arch changes during alignment when using individual lingual orthodontics were performed in vitro or were based on clinical observations without concrete evidence-based conclusions [12]. In addition, very few in vitro studies have compared the efficiency of nonextraction treatment with labial or conventional lingual methods [12]. Thus, this study was the first randomized controlled trial to compare conventional labial orthodontic with individual lingual orthodontic brackets in the phase of the alignment of the mandibular dental arch.

Geron et al. [8] attempted to understand the effects of lingually applied intrusive/extrusive forces on tooth movements. They realized the description of the lingual force system to be complicated and ascribed the resulting tooth movement effects primarily to bracket position and initial tooth inclination. Sifakakis et al. [14] assessed the effects of the bracket type on labiolingual momentums caused by Incognito and STb lingual brackets as well as by Gemini and In-Ovation L labial brackets. The results showed meaningful differences among the four bracket groups, excluding the Incognito and STb brackets. In addition, the lowest torque determination was observed in self-ligating lingual brackets, followed by conventional labial brackets. The Incognito and STb lingual brackets caused the highest momentums [14]. In another study, Sifakakis et al. [15] assessed the effects of different bracket types on labiolingual forces occurring in the sagittal plane. As a result, it was suggested that the bracket type was the determinant of the forces and momentums created. The created forces were different among all bracket types, and that momentums produced were different between labial and lingual brackets but similar among the lingual brackets. This clinical trial showed no significant difference in mandibular alignment between customized lingual brackets and conventional labial brackets over the observational time of 18 weeks. Thus, the null hypothesis was confirmed.

In our study, alignment was initiated by using a 0.012″ NiTi wire in both groups for standardization. The control visits were scheduled at 6‑week intervals, where 0.014″ NiTi and 0.016″ NiTi wires were used for continuation of treatment in both groups. Several studies have suggested that small archwires lead to less friction [16,17,18]. A previous study of different labial brackets revealed that alignment was affected mainly by the bracket design, bracket type, and archwire type [19]. It was also reported that 95% of the alignment occurred with the initial 0.012″ archwire [19]. From T0 to T1, in the current study the irregularity index reduced more in the labial group compared to lingual. As a result of the smaller interbracket distance, it is typically more difficult to engage the archwire in lingual brackets than in labial brackets. Therefore, the crowding might have been resolved faster in the labial than the lingual sample after the first 6 weeks of treatment. However, in this study, no difference was found for the irregularity index after 18 weeks between the labial and lingual brackets. Thus, as a result of this study, a 0.014″ superelastic NiTi wire might be the appropriate initial archwire when using customized lingual appliances. This result is corroborated by the fact that a 0.014″ superelastic NiTi is very often recommended as the starting arch wire for this technique.

The vertical orientation of the lingual brackets slots in the anterior region can generate larger rotational moments. Corrective derotation of teeth have been known as an advantage of this vertical slot in lingual brackets [6]. The full expression of this moment will occur when the lingual archwire ligated firmly. These moments reduced in the flexibly ligated labial bracket with a horizontal slot. In the present study, we used same elastic ligatures in the labial and lingual bracket group for standardization of this prospective clinical study. As a result, we could not use a lasso elastic during the first 18 weeks of the orthodontic treatment. A lasso elastic might have provided nearly fully engaged archwire into the bracket slots and this could aid in derotation of teeth. Likewise, again for the same reason, we were unable to use the additional self-retaining slots in the Incognito lower incisor lingual brackets which also might have helped the alignment of lower incisors.

In the current study, the improvement in irregularity indices measured after T0, T1, T2, and T3 showed significant differences within the customized lingual bracket and conventional bracket groups. However, no significant intergroup difference was found. In several previous studies, the initial irregularity index is considered the primary factor for clinical effectiveness, with minimal influence of the bracket type. Such findings are exemplified by Scott et al. [10], Fleming et al. [20], and Ong et al. [21] when comparing Damon self-ligating bracket and conventional orthodontic bracket systems, comparing mandibular arch changes during alignment by two distinct preadjusted edgewise brackets, and comparing the effectiveness of self-ligating and conventional brackets during initial alignment phase, respectively. In an experimental study comparing the forces produced by Incognito lingual and conventional labial bracket systems, Fuck et al. [22] concluded that the lingual system produced comparable forces in the anterior region. However, Alobeid et al. [12] found that labial bracket systems were more effective on alignment compared with lingual labial bracket systems in vitro. The inconsistency with our results may be due to different slot preferences in the study groups examined by Alobeid et al. [12]. In addition, Alobeid et al. [12] used a different measurement method to assess the effectiveness of labial or lingual brackets in tooth alignment and their method was claimed to be incorrect by Wiechmann et al. [23]. Furthermore, the current in vivo study considers saliva, periodontal ligament, chewing, and other oral functions that are not present in vitro.

In the present study, the increase in postalignment intercanine width was statistically significant in the customized lingual bracket group but not in the conventional labial bracket group. It is thought that bracket thickness causes a widening effect of the tongue on teeth by narrowing the tongue area, expanding the dental arch and resulting in the production of protrusive forces on mandibular teeth. It is also thought that forces used in the lingual technique predominant at the center and that the medial-to-lateral direction of the force can increase intercanine width. The small interbracket distance of lingual brackets in the anterior region may have played a role in this increase in intercanine width. Khattab et al. [24] also reported that lingual appliance caused an increase in intercanine width following maxillary dental arch alignment. The arch forms in lingual Incognito treatment group are defined by an individual set-up based for every single patient, individual arch wires may not be associated with increased intercanine width. However, the increased arch length in the labial group can be explained by using preformed archwires as they are generally too large for Caucasians. Lombardo et al. [25] compared commercially available archwires with dental arch forms in a Caucasian population and concluded that none of the commercial arch wires examined faithfully represented the shape of the dentition and found that preformed archwires were wider in the intercanine width of the lower arch.

Many problems have been overcome with individually designed lingual brackets and archwires, and treatment success has increased. As lingual treatment has been a strong alternative to labial treatment, its effectiveness should be supported with future in vivo studies with large sample size. Further studies may compare individual labial and lingual brackets with individual NiTi archwires.

Harms

No harm was detected during the study.

Limitations

There are several shortcomings in this study, most significantly its small sample size. Customized lingual orthodontic treatment is considerably more expansive than labial orthodontic treatment, we were unable to obtain a larger sample of patients due to socioeconomic reasons. Based on a similar previous study [26] in which a sample size of 21 patients in each group was calculated to detect a difference of 2 mm in irregularity index with an alpha of 0.05 and 90% power, it can be assumed that some of the nonsignificant results in the present study might be due to the limited statistical power resulting from the modest sample. Although the present study has a small sample size, to best of our knowledge this is the first prospective randomized clinical trial in the literature comparing customized lingual and labial appliance treatment of initial alignment of mandibular teeth. Therefore, the results of this study are thought to contribute to the field of lingual orthodontics. The other limitations were as follows: labial archwires were fabricated, not individualized, which may have affected the arch form response to treatment. Although blinding of the operator and patients was not possible, all measurements were performed blinded to the patients’ name and the time point during data analysis.

Generalization

The outcomes from several operators make the study results more generalizable than if this was undertaken in a single center.

Conclusion

In this pilot study, no differences between the two treatments could be detected for the initial mandibular alignment. Possible reasons could be that the study with 10 patients per treatment was slightly underpowered to detect smaller differences. A larger follow-up study could be conducted to confirm our findings.