Material and methods

This retrospective, longitudinal, split-mouth model study was approved by the Ethics in Research Committee of the Universidad Científica del Sur, Lima, Perú (Approval no. 00008). CBCT images of 27 patients (15 women and 12 men aged between 13 and 39 years) with unilateral MIC were obtained before (T0) and after (T1) canine traction. Two groups were defined regarding the type of impaction, with 14 MIC located as palatal and 13 as buccal. In addition, the nonaffected side was evaluated as a control. The CBCT scans were selected from a population of patients with a MIC diagnosis and treated by the same orthodontist (G.A.R.M).

The sample size was calculated considering 80% study power with a significance level of 0.05 (95% confidence level) to obtain a significant difference between groups of 2 mm (buccally MIC = 2.5 mm vs palatally MIC = 0.5 mm) on alveolar buccal height using a standard deviation of 2 mm (buccally MIC) vs 1.5 mm (palatally MIC), which was obtained in a previous pilot study and whose data were included in the final study. A minimum sample size of 13 was determined with the use of OpenEpi software ( http://openepi.com/SampleSize/SSMean.htm ).

The inclusion criteria consisted of patients of both sexes, older than the age of 12 years, with unilateral MIC, either buccal or palatal, treated with the same orthosurgical traction technique. The exclusion criteria were subjects with syndromes or craniofacial deformities, endoperiodontal lesions, previous orthodontic treatment, history of maxillary surgery or dentoalveolar trauma in the maxillary anterior teeth.

Some demographic data, MIC characteristics (eg, alpha angle, sector and height of impaction), and basic 2D diagnostic data were obtained from the clinical and radiographic registers. CBCT images were obtained at (T0) and (T1), defining T1 as the moment when the MIC reached the occlusal plane and traction was ended.

The measurements obtained from the CBCT images were performed by 3 trained and calibrated orthodontists (G.A.R.M, L.E.A.G, and Y.A.R.C). The intra- and interobserver agreement in location and sector of MIC was evaluated with a kappa coefficient achieving values >0.9. To evaluate the intraobserver reliability regarding the height and width change measurements, the same examiner (G.A.R.M) re-evaluated the 30% of the sample, randomly selected, after a 30-day interval. The data were analyzed with the intraclass correlation coefficient, which also provided values >0.9. In addition, the random error, estimated by the Dahlberg formula, was <1 mm.

The location of each unilateral MIC was defined as buccal or palatal following clinical criteria and CBCT axial section evaluations. The clinical criteria were (1) prominence of the MIC over the mucoperiosteum; (2) in the axial view, canine appearance over the buccal or palatal cortical region; (3) MIC crown positioned over the buccal or palatal region of the lateral incisor root; and (4) surgical access according to the surgeon.

CBCT scans of all patients were obtained with PaX-Uni 3D equipment (Vatech, Hwaseong, South Korea) set at 89 kVp; 4.7 mA; 0.125-mm voxel size; scan time, 15 seconds (mean), and field of view of 8 × 8 cm ² . Digital Imaging and Communications in Medicine images were analyzed with 3D software (version 11.8; Dolphin Imaging and Management Solutions, Chatsworth, Calif) using multiplanar and 3D reconstructions. The panoramic images derived from CBCT were used to determine the impaction sector according to Ericson and Kurol. Five sectors were considered. Sector 1 was the cusp tip of the MIC located between the mesial aspect of the first premolar to the distal aspect of the lateral incisor. Sector 2 was the cusp tip of the MIC located between the distal aspect of the lateral incisor and the long axis of the lateral incisor. Sector 3 was the cusp tip of the MIC located between the long axis of the lateral incisor and the mesial aspect of the lateral incisor. Sector 4 was the cusp tip of the MIC located between the mesial aspect of the lateral incisor and the long axis of the central incisor. Sector 5 was the cusp tip of the MIC located between the long axis of the central incisor and the interincisor midline. The impaction alpha angle was defined as the angle between the long axis of the MIC and the interincisal midline, and the canine height was measured as the perpendicular distance from the cusp tip of the MIC to the incisal plane ( Fig 1 ).

Sectors 1-5, alpha angle (α), and height of the cusp to the incisal plane (h), according to Ericson and Kurol.

Sagittal, coronal, and axial sections were evaluated using the same software (Dolphin Imaging 11.8). Using the multiplanar reformation, we identified the vertical axis located in the middle of the canine alveolar space, delimited between the lateral incisor and the first premolar. This axis was also centered on the deciduous pulp conduct axis, when present at T0, or on the pulp conduct of the permanent canine at T1 (at the level of the cervical bone crest, as a reference). This procedure allowed rotations in the 3 planes before the measurements ( red line , Figs 2 and 3 ). Palatal and buccal and mesial and distal heights were measured from the alveolar crest to the nasal floor on the sagittal and coronal views, respectively. In the sagittal view, the buccopalatal widths were measured at 3 levels (cervical, middle, and apical) every 6 mm from the alveolar crest. In the axial view, the buccopalatal cervical widths were measured mesially (proximal to the lateral incisor) and distally (proximal to the premolar) to the canine at the level of the alveolar ridge. All measurements were performed for the MIC and the control canine at T0 and T1 _.

MIC before (T0) and after (T1) traction. Heights from the alveolar crest to the nasal floor ( dotted white line ). Mesial (M) and distal (D) height measurements in the coronal section. Palatal (P) and buccal (B) heights, and cervical (C), middle (M), and apical (A) width measurements every 6 mm from the alveolar crest in the sagittal section. Mesial (M) and distal (D) cervical width measurements at the level of the alveolar ridge in the axial section.

Control canine from the nonaffected side before (T0) and after (T1) traction. Heights from the alveolar crest to the nasal floor ( dotted white line ). Mesial (M) and distal (D) height measurements in the coronal section. Palatal (P) and buccal (B) heights, and cervical (C), middle (M), and apical (A) width measurements every 6 mm from the alveolar crest, in the sagittal section. Mesial (M) and distal (D) cervical width measurements at the level of the alveolar ridge in the axial section.

All patients were treated by an expert and trained orthodontist following the same treatment protocol (G.A.R.M). The biomechanical sequence of orthodontic treatment followed a rigorous protocol in all subjects. Fixed orthodontic appliances with 0.022 × 0.028-in (Synergy; Rocky Mountain Orthodontics, Denver, Colo) and copper nickel-titanium (Ormco, Glendora, Calif) 0.016 × 0.022-in archwires were used. The deciduous canine in the impaction side was always present during the beginning of the mechanics. Thus, once the anterior and posterior segments were aligned and leveled, the space preparation for the MIC was performed with 0.012 × 0.045-in open coil springs (Rocky Mountain Orthodontics). One month before surgery, the rigid anchorage device (1.1-mm or 1.2-mm stainless steel, Dentaurum, Ispringen, Germany) associated with a palatal acrylic button, and with occlusal-palatal-buccal stepped extensions distal to the lateral incisor and on the proximal sides of premolars and molars (performed with a 0.028-in wire), was cemented. The buccal extensions protect the tissues and avoid immersions of the Ni-TiCCS in the soft mucoperiosteal tissue. Ni-TiCCS 0.010 × 0.036-in and of 8 and 13 mm (Dentos, Daegu, Korea) were used to provide 100-150 g of force when they were activated for transalveolar traction of the MIC ( Fig 4 ).

Control of buccal and palatal traction of unilateral MIC in the x-, y-, and z-axes. Traction with Ni-TiCCS and a rigid palatal anchorage appliance with buccal extensions and palatal hooks. Distal traction on the x-axis ( white arrow ), buccal traction on the y-axis ( yellow arrow ), and extrusive traction on the z-axis ( red arrow ).

A 0.017 × 0.025-in stainless steel archwire, passively placed and distally cinched to the most distal molar, was used before traction for maxillary arch consolidation. A closed surgical technique was performed in all patients with an individualized osteotomy that never exceeded the MIC cementoenamel junction. ^{, , ,} Before the Ni-TiCCS fixation, an absolute transsurgical isolation was performed. The activation was performed immediately after the spring fixation and individualization of the anchorage ( Fig 5 ), and it was maintained during traction (6-7 mm every 6 weeks). Four to 6 months after the initiation of traction, the anchorage and traction springs were removed, and the MIC was leveled. Then, the finalization phase was initiated.

Clinical and radiographic images of palatobuccal movement in the y-axis ( yellow arrow ), mesiodistal movement in the x-axis ( white arrow ), and extrusive movement in the z-axis ( red arrow ) of palatally and buccally MIC with Ni-TiCCS activated over the rigid anchorage.

A new CBCT was taken (T1) to evaluate the 3D changes in the root and alveolar bone and to visualize the consequences of the traction on MIC and the nonimpacted teeth after the end of MIC traction. This decision was adopted according to the as low as reasonably achievable principle, international guidelines and is supported by evidence-based science. ^,

Results

The age and sex distribution of the sample and MIC characteristics by groups are summarized in Table I . In addition, the traction time in months (palatal, 5.71 ± 1.06; buccal, 5.46 ± 1.26) was similar between groups ( P = 0.579). The palatally MIC group showed significantly higher impaction than the buccally MIC group. The MIC was mainly located in sectors 3, 4, and 5 in both groups (78% of the sample; Table II ).

The alveolar height changes (T0 − T1) showed statistically significant differences between the groups (buccally vs palatally unilateral MIC groups) for all measurements. The palatally MIC height changes ranged from 2.09 to 2.79 mm, whereas the buccally MIC height changes ranged from 0.28 to 0.57 mm. Otherwise, the alveolar width changes were not statistically significantly different between groups ( Table III ).

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