Separation of aligning and leveling stages to control mandibular incisor inclination: A randomized clinical trial

ABSTRACT Objective: To determine whether separating the alignment and leveling phases can reduce proclination of the mandibular incisors. Methods: Eligibility criteria included Class I subjects with an irregularity index of 3-5 mm, 3-4 mm curve-of-Spee (COS), and non-extraction treatment. Thirty adults were randomly allocated into two groups: (1) Control group was leveled and aligned simultaneously with flat archwires progressively to 0.016x0.022-in stainless-steel; (2) Experimental group was aligned first with 0.014-in-superelastic NiTi with mild accentuated COS, then leveled using 0.016x0.022-in beta-titanium accentuated COS archwires and gradually reduced the curve until flat. Mandibular incisor position and inclination were evaluated by cephalometric analysis. COS and irregularity index were evaluated in study models. Assessment was conducted twice after 0.016-in NiTi and after 0.016x0.022-in stainless-steel archwire placements. Dental changes from cephalograms and models were compared within group using paired t-test and between groups using independent t-test. Results: Control group: Round-wire-phase, mandibular incisors tipped labially (4.38° and 1 mm) with intrusion (-1.13 mm); Rectangular-wire-phase, mandibular incisors further intruded and proclined (-0.63 mm and 1.38°). Experimental group: During aligning with round accentuated COS archwires, mandibular incisors tipped very slightly labially (0.75° and 0.50 mm) with no significant intrusion; during leveling with rectangular archwires, incisors majorly intruded (1.75 mm) with slight proclination (1.81°). The experimental group had significant less incisor proclination (control: 5.76°, experimental: 2.56°) with more incisor intrusion (control: -1.75 mm, experimental: -2.13 mm). The COS in experimental group showed significant greater reduction (-2.88 mm) than that of the control group (-1.69 mm). Conclusion: In control group, mandibular incisor proclination was markedly observed in round archwires, with further proclination caused by rectangular archwires. In experimental group, minimal proclination was exhibited when accentuated COS round archwires were used for aligning. Leveling with rectangular archwires caused less proclination with more COS reduction.

Theerasopon P, Lindauer SJ, Charoemratrote C -Separation of aligning and leveling stages to control mandibular incisor inclination: A randomized clinical trial

INTRODUCTION
Alignment and leveling is the first stage of comprehensive orthodontic treatment. 1 Typically, superelastic archwires, such as nickel-titanium (NiTi) alloys or multi-stranded stainless steel, are used in this stage due to alignment efficiency prior to inserting stiffer archwires. 2 Results of very effective dental changes early in treatment after alignment and leveling showed that proclination of mandibular incisors was common, 3 especially in non-extraction orthodontically-treated cases. [4][5][6][7][8] Proclination of mandibular incisors may affect the esthetic outcome, surrounding periodontal tissues, 9 and treatment stability. 10 To correct crowded teeth, labial displacement of incisors is expected to gain spaces for tooth alignment. [5][6][7][8]11 To reduce this flaring, rectangular archwires, which have the ability of torque control, should be placed at the beginning stage. 12 Unfortunately, the placing of rectangular NiTi archwires with 0.016 x 0.022-in is probably not possible since the dimension of 0.022-in of the archwire cannot be forced into the disordered bracket slots on crowded teeth. Moreover, heavy forces could be expected from rectangular wire deflection. 13 In order to reduce this force with better torque control, 0.016 x 0.022inch beta-titanium archwires with multiple loops can be introduced. However, complicated archwire bending, prolonged chair time, and the difficulty of oral hygiene practice must Theerasopon P, Lindauer SJ, Charoemratrote C -Separation of aligning and leveling stages to control mandibular incisor inclination: A randomized clinical trial 5 be considered. 14 Therefore, a conventional sequence making use of the NiTi archwire for aligning is inevitable.
For tooth leveling or curve of Spee (COS) flattening, the placement of plain small round NiTi archwires can cause the incisors to flare up since intrusion forces are generated anterior to the centers of resistance of the incisors. 15 To counteract this flaring, a rectangular archwire can be applied for leveling after the crowded teeth are aligned. 16,17 The amount of incisor intrusion should be introduced little by little to avoid any heavy force.
Those problems led to the idea of changing the archwire shape to align the teeth with minimal leveling by using small round NiTi archwires with accentuated COS. Leveling can be subsequently approached using 0.016 x 0.022-in beta-titanium archwires with COS, then gradually flattened to generate optimal force. The objective of the study was to investigate the movement of mandibular incisors focusing on the inclination when conventional round and rectangular archwires were used for aligning and leveling simultaneously, compared to accentuated COS archwires used for alignment followed by rectangular archwires for leveling. Theerasopon P, Lindauer SJ, Charoemratrote C -Separation of aligning and leveling stages to control mandibular incisor inclination: A randomized clinical trial 6

TRIAL DESIGN AND ANY CHANGES AFTER TRIAL COMMENCEMENT
This was a parallel-group prospective, randomized, controlled trial with a 1:1 allocation ratio. No changes were made to the methods during the trial.

STUDY MODEL AND CEPHALOMETRIC ANALYSES
Study models were evaluated using digital Vernier callipers set to zero before the next measurement for the amount of tooth crowding using Little's irregularity index. 19 The COS depth was assessed using a clear acrylic plate laid down from the mandibular second molars to the incisors measuring the average Theerasopon P, Lindauer SJ, Charoemratrote C -Separation of aligning and leveling stages to control mandibular incisor inclination: A randomized clinical trial depth from the acrylic plate to the most inferior cusp tip of the bilateral premolars (Fig 2). The lateral cephalometric analysis evaluated position and inclination of the mandibular incisors and rotation of the mandibular plane (Fig 3).
Theerasopon P, Lindauer SJ, Charoemratrote C -Separation of aligning and leveling stages to control mandibular incisor inclination: A randomized clinical trial

SAMPLE SIZE CALCULATION
The sample size was calculated based on a previous study 20 with alpha = 0.01 for an independent t-test and a statistical power of 90%. The sample size required 13 subjects per group and two additional participants per group were included for possible drop-outs. Thus, the total number of participants in this study was 15 per group.

INTERIM ANALYSES AND STOPPING GUIDELINES
Not applicable.

RANDOMIZATION
Patients were divided into two parallel groups using simple randomization by drawing lots. Each participant was numbered for blinding by the first author. The lots were sealed in opaque envelopes for the group assignment. One resident member who was not part of the trial shuffled and opened the envelopes for each participant in a private room to ensure that the operator was blinded.

BLINDING
Blinding of the patients and operator was not possible; however, assessment was blinded and accomplished by an examiner not involved in patient treatment using lateral cephalometric radiographs and study models that were coded to conceal patient information. Theerasopon P, Lindauer SJ, Charoemratrote C -Separation of aligning and leveling stages to control mandibular incisor inclination: A randomized clinical trial

STATISTICAL ANALYSIS
The data were analyzed using IBM SPSS Statistics 21 software (IBM Software Group, Chicago, IL, USA). Intra-examiner reliability was determined using paired t-tests by random selection of 10 cephalometric radiographs and 10 dental models after two weeks. The radiographs were retraced and measurements were repeated.
Means and standard deviations were calculated for all dental casts and lateral cephalometric radiograph parameters at T 0 , T 1, and T 2 . The distribution normality of parameters was tested by the Shapiro-Wilk test. Paired t-tests were used to analyse differences among T 0 , T 1, and T 2 of the control and experimental groups. Independent t-tests were used to analyze differences between the control and experimental groups. A significant difference level of p < 0.05 was used for all statistical tests. Intra-examiner reliability revealed no significant differences between repeated measurements (p > 0.05). Table 1 shows the demographic characteristics of the patients in each group.

RESULTS
No differences in these factors between the two groups were noted, except the treatment time in the experimental group (35.13 weeks) was significantly longer than the control group (26.63 weeks) (p < 0.0001). Group differences of dental changes from the cephalometric analyses and model measurements at all timepoints are shown in Table 2.
In the control group, mandibular incisors were significantly aligned and leveled by round archwires and it was found that the mandibular incisors moved labially 1 mm and were significantly proclined 4.38° (p < 0.0001). In the vertical dimension, the mandibular incisors were intruded 1.13 mm (p < 0.0001) with COS reduction of 1.19 mm (p < 0.0001). After changing to rectangular archwires, the mandibular incisors intruded further by 0.63 mm (p < 0.0001) and proclined slightly 1.38° (p < 0.0001).  Theerasopon P, Lindauer SJ, Charoemratrote C -Separation of aligning and leveling stages to control mandibular incisor inclination: A randomized clinical trial  However, the incisors were still proclined, but less than the control group. Thus, this technique could control the mandibular incisor inclination and be beneficial for patients who need deep COS correction whenever incisor proclination is limited.
Theerasopon P, Lindauer SJ, Charoemratrote C -Separation of aligning and leveling stages to control mandibular incisor inclination: A randomized clinical trial 20 A comparison between the two groups in the reduction of COS from mandibular incisor intrusion revealed that the control group had the most substantial proclination during the round wire phase (T 1 -T 0 ). However, in the experimental group, reduction of the COS occurred mostly during the rectangular wire phase with less proclination. This could imply that the small round archwires could be the archwire of choice for leveling when proclination is allowed.  [5][6][7][8]27,29 reported the results from total treatment with durations of more than a year and included the finishing phase, which may affect the mandibular incisor position from wire bending, torqueing or intermaxillary elastics.
Proclination is a major concern in this study due to possible risks 9,10 . The control group presented statistically significant greater incisor proclination (3.20°) compared to the experimental group. This amount of proclination difference may not be considered to be clinically significant. An additional study of surrounding bone response to these changes would be interesting to confirm whether this amount of proclination is safe for the periodontium.
The results of this study can be applied only in non-extraction patients who have similar pre-treatment characteristics of crowding in the mandibular anterior teeth of 3-5 mm and 3-4 mm COS.
Theerasopon P, Lindauer SJ, Charoemratrote C -Separation of aligning and leveling stages to control mandibular incisor inclination: A randomized clinical trial

CONCLUSIONS
In the control group, mandibular incisor proclination was markedly observed in round archwires, with further proclination caused by rectangular archwires. In experimental group, minimal proclination was exhibited when accentuated COS round archwires were used for aligning. Leveling with rectangular archwires caused less proclination with more COS reduction.

Chairat Charoemratrote (CC)
Conception or design of the study:

Writing the article:
PT, CC.

Critical revision of the article:
PT, SJL, CC.

Final approval of the article:
PT, SJL, CC.

CC.
The authors report no commercial, proprietary or financial interest in the products or companies described in this article.