The current study was designed as a randomized, controlled, two-parallel group clinical trial to investigate the efficacy of the corticotomy technique accompanying surgical exposure enhanced by a subsequent flapless corticotomy in accelerating traction movement of the palatally impacted canines (PICs) and to evaluate the associated dentoalveolar changes compared with traditional traction method . The study sample was compiled by reviewing the lists of patients recorded in the archives of the Orthodontics Department at the Faculty of Dentistry, Damascus University. The patients who were referred to the Orthodontic Department for treatment of impacted upper canines diagnosed in other departments were also examined. The approval of the Local Research Ethics Committee of Damascus University was obtaining before conducting this study (UDDS-499-01022020/SRC-2195). This study was registered at ClinicalTrials.gov and was funded by the Postgraduate Research Budget of Damascus University (Ref no: 83005751015DEN).

The desired sample size was established using Minitab ^® version 17 software (Minitab Inc., State College, Pennsylvania, USA). A proprietary free alternative is G*Power 3.1.9.7 program (the Heinrich-Heine University, Dusseldorf, Germany). It was believed that 0.25 mm/month would be the least clinically meaningful difference requiring detection in the impacted canine’s movement velocity between the two study groups. The standard deviation of this variable, based on a comparable prior study, was 0.278. ²⁹ An independent-samples t-test (significance level 5%, power 80%) revealed that each group needed a minimum of 21 patients. To account for the possibility of patient withdrawal, each group’s patient count was increased to 23.

Along with contacting and recalling 75 individuals listed in the Orthodontics Department’s archives, 52 referred patients with upper impacted canines were also examined. It was found that 66 of them satisfied the inclusion requirements. Fifty-eight patients were accepted to participate in the study after the treatment plan and the orthodontic/surgical procedures were explained to all patients. Forty-six patients were then chosen at random to enter this study ( Figure 1). The selected patient group received information sheets, and their written informed consent was obtained.

The following definitions represent the inclusion criteria: (1) patients aged 18 to 28; (2) unilateral palatally or mid-alveolar upper impaction; (3) distally placement of the impacted canine crown to the lateral incisor root midline; (4) no use of any medications that may affect the orthodontic movement. The following conditions were excluded: (1) bilateral or buccal canine impaction cases; (2) more than 45-degree angle between the canine’s longitudinal axis and the vertical facial plane; (3) presence of lateral incisors root resorption; (4) presence of contact between the canine crown and the lateral incisor root; (5) previous orthodontic treatment; (6) any medical condition that prevents oral surgery; (7) oral structural abnormality that is inherited or congenital.

The participant patients were randomly assigned to the two study groups based on a list of random numbers created using Minitab ^® software version 17 (Minitab Inc., State College, Pennsylvania, USA), with a 1:1 allocation ratio. With the use of this randomization process, patients were distributed to the two study groups in an equal numbers: the traditional traction group (TT), which consisted of 23 patients, and the corticotomy-assisted traction group (CAT), which also consisted of 23 patients. Using sequentially numbered, opaque, sealed envelopes, a researcher from the Orthodontic Department who was not involved in this study concealed the allocation.

The principal researcher (MRM) and the patients could not have been blinded during the surgical interventions. Blinding, however, was only achievable during the data analysis stage. In other words, the outcome assessors were blinded and unable to determine to which group the patients belonged.

The aligning and leveling of the maxillary dental arch were performed using the fixed orthodontic appliance (MBT prescription; 0.022-in slot, Votion™, Ortho Technology ^®, Florida, USA) in both study groups. The following wire sequence was used: (1) 0.012"- or 0.014" nickel-titanium (NiTi); (2) 0.016 × 0.022" NiTi; (3) 0.017 × 0.025" NiTi (NT3 ^® SE NiTi Wire, American Orthodontics, Sheboygan, USA); (4) 0.019 × 0.025" stainless steel (American Orthodontics, Sheboygan, WI, USA). This preparatory stage aimed to open sufficient space for aligning the impacted canine within the dental arch. A NiTi open-coil spring (made of 0.010" wire with an inner diameter of 0.030", American Orthodontics, Sheboygan, WI, USA) was used to gain additional space between the lateral incisor and the first premolar, if that distance was not sufficient to accommodate the impacted canine ( Figure 2A). Finally, in order to maintain the opened distance, a stainless steel closed-coil spring (made of 0.010" wire with an inner diameter of 0.030", American Orthodontics, Sheboygan, WI, USA) was inserted ( Figure 2B).

In both study groups, the impacted canines were surgically exposed using the closed flap approaches. Under the direction of one of the co-authors (OH), the same surgeon from the Department of Oral and Maxillofacial Surgery at Damascus University, Faculty of Dentistry, carried out all surgical procedures.

Traditional traction group (TT)

Under local anaesthesia (lidocaine HCL 2% - epinephrine 1:80000), a full-thickness palatal flap was raised ( Figure 3A) and the bone overlying the impacted canine crown was removed to reveal a suitable area for attachment bonding, without exposing the cemento-enamel junction (CEJ). An eyelet attachment with a hand-made twisted ligature wire was then bonded to the exposed canine crown ( Figure 3B). The flap was sutured back with the twisted ligature wire extending under the gingival mucosa towards the buccal side. A prescription of an antibiotic (Clavulanic Acid + Amoxicillin, 1000 mg, twice daily for 5 days) and a painkiller (Paracetamol 500 mg, three times daily for 4 days) was given to patients. Patients received guidelines about postoperative oral health care.

Corticotomy-assisted traction group (CAT)

During the surgical exposure phase, patients in this group underwent a corticotomy procedure by drilling a series of circular holes (1 mm in diameter, 1-2 mm deep and spaced about 1.5 mm apart) in the cortical bone. These holes drilled at the area where the impacted canine would be moved towards as well as in the mesial and distal sides of the canine crown ( Figure 4A).

Two months after the first corticotomy intervention, the second flapless corticotomy procedure was performed using piezosurgery (Mectron PIEZOSURGERY ^®, Carasco, Genova, Italy). Using a surgical scalpel, 2-3 vertical gingival incisions with a height of 8 mm were made during this intervention at the buccal side of the impaction area. Then, the cortical cuts were created using BS1 tip with a cutting depth of 2-3 mm, a 2 mm space between cuts, and a 1 mm cut width ( Figure 4B).

The traction force was applied for the first time two weeks after the surgical exposure using active power chain tied on one end to the twisted ligature wire and on the other end to the first molar band hook on the impaction side. The intensity of the applied traction force was 60 g ²⁹ measured using intraoral force gauge (Dentaurum, Ispringen, Germany). Re-activation was done every 2 weeks ³¹ until the appearance of half of the clinical crown of the impacted canine; this moment was considered as the end of the active traction phase. Then, the canine bracket was bonded to start its alignment and levelling stage. Once this stage was complete, the fixed orthodontic appliance was removed, and a vacuum-formed retainer was placed.

For all patients, two cone-beam computed tomography (CBCT) images of the maxillary jaw were obtained. The first CBCT image was obtained two weeks before the beginning of treatment while the second image was obtained right on the day the fixed appliance was removed. The CBCT system was a CBCT Picasso ^® Pro (Vatech, Seoul, South Korea) with a voxel size of 0.25 mm (FOV 70 × 120 mm), tube voltage of 70 kV, current of 6 mA, and an exposure time of 10 s.

The principal researcher (MRM) performed the measurements and evaluated all variables. The primary outcomes were (1) traction duration, (2) total treatment duration, and (3) the velocity of traction movement. The traction duration is measured as the interval between the onset of orthodontic traction on the impacted canine and the emergence of half of its clinical crown. The total treatment duration calculated as the time between the bonding of the fixed orthodontic appliance until it is removed. The velocity of traction movement is determined by dividing the depth of impaction, which is defined as the distance from the impacted canine cusp tip to the occlusal plane, by the traction duration.

The secondary outcomes were (1) bone support of the aligned canine (BS-IC), (2) bone support of the contralateral naturally erupting canine (BS-CON), (3) bone support of the adjacent lateral incisor (BS-LI), (4) bone support of the adjacent first premolar (BS-PR), and (5) root resorption of the adjacent lateral incisor (RR-LI). Bone support was evaluated on the posttreatment CBCT scans. It was assessed on the mesial, distal, buccal, and palatal aspects of the canines, lateral incisors and first premolar. The total bone support ratio was calculated by averaging the four measurements made on each tooth. The alveolar bone level was measured from the root apex of the evaluated tooth to the alveolar crest, whereas the root length of the evaluated tooth was measured from root apex to the midpoint of a line connecting the mesial and distal points on the cemento-enamel junction of this tooth. The ratio of alveolar bone level to the root length was considered the percentage of bone support. ³² The amount of root resorption of the adjacent lateral incisor was evaluated by comparing the root length measured in millimetres before and after treatment.

The method error was assessed by repeating the CBCT measurements for twenty patients selected by random (ten from each group) thirty days after the first measurement. The interclass correlation coefficients (ICCs) were used to determine the presence of any random error, and the paired sample t-tests were used to determine the presence of any systematic error.

Minitab ^® statistical package (version 17, State College, Pennsylvania, USA) was used to analyse data. Using Anderson-Darling normality tests, normal distributions were confirmed. Therefore, the two-sample t-test was employed for inter-group comparisons, while the paired sample t-test was used for intra-group comparisons. The two-sample t-tests were used to detect the homogeneity between the two study groups at baseline in terms of age, impaction depth, mesio-distal and buccal-palatal inclinations of the impacted canine longitudinal axis, while the chi-square test was used for gender and impaction site.

The baseline characteristics of the sample are shown in Table 1. The study sample consisted 46 patients (13 male and 33 female). There were 39 palatally impacted canines and 7 mid-alveolar impacted canines. There were 23 patients in each group, with a mean age of 20.26±2.17 years and 20.39±2.27 years, in TT and CAT group, respectively. The two study groups did not significantly differ in terms of the initial characteristics of the impacted canine’s position, and they were homogeneous concerning the patient’s age and gender.

The ICCs were in the 0.989 to 0.996 range, demonstrating the high reliability of the measurement method. The Paired-sample t-tests results showed that there was no significant difference between the two measurements, indicating the minor and insignificant systematic errors ( Table 2).

The mean duration of active traction was 9.68±3.24 months in the TT group and 6.13±1.81 months in the CAT group with a significant difference between the two groups (P=0.015, Table 3). The mean duration of total orthodontic treatment was 19.98±3.55 months in the TT group and 14.23±1.95 months in the CAT group with a significant difference between the two groups (P=0.001, Table 3). The mean velocity of traction movement was 0.70±0.33 mm/month in the TT group, while it was 1.15±0.35 mm/month in the CAT group with a significant difference between the two groups (P=0.027, Table 3). The duration of active traction decreased by 36%, while the duration of total orthodontic treatment decreased by 29% in the CAT group compared with the TT group. The velocity of traction movement was 39.21% greater in the CAT group than the TT group.

Regarding the bone support ratio, the differences between the aligned and contralateral canines were non-significant in both study groups (P≈1.000; P=0.559, in the TT and CAT group, respectively; Table 4). No significant differences were found between the two groups when comparing the posttreatment bone support ratios of the aligned canines (P=0.512, Table 5), the adjacent lateral incisors (P=0.379, Table 5), and the adjacent first premolars (P=0.622, Table 5).

The mean bone support ratio of the aligned canines in the TT group was 89% (SD: 0.04), while it was 88% (SD: 0.03) in the CAT group. As for the mean bone support ratio of the contralateral canines, the mean bone support ratio was 90% (SD: 0.03) and 89% (SD: 0.02), in the TT and the CAT group, respectively ( Table 4). The posttreatment values of the bone support ratio of the adjacent laterals was 85% (SD: 0.03), in the TT group and 86% (SD: 0.03), in the CAT group, while it was 87% (SD: 0.04) and 87% (SD: 0.03) for the adjacent first premolars in the TT and the CAT group, respectively ( Table 5).

The mean amount of root resorption of the adjacent laterals was 1.22±1.02 mm in the TT group and 1.30±1.18 mm in the CAT group, with no significant differences between the two study groups (P=0.612, Table 5).

The current study included a specific type of upper canine impactions. Therefore, the effects of different probabilities of the initial location, mesiodistal and bucco-lingual angulation of the impacted canines on the traction movement velocity were not evaluated. Secondly, only one type of mechanical traction means was used and the effectiveness of other traction techniques were not evaluated. Thirdly, this trial did not assess the effects of gender and age on the traction movement rate, for which a larger sample size would be necessary.

As the present study included particular impaction conditions with particular impacted canine’s locations and conducted on a specific patient group treated with specific mechanical orthodontic and surgical techniques in only one teaching hospital, the results of the current study can be limitedly generalized. In order to acquire results that are more generalizable, more randomized controlled trials with larger sample sizes should be carried out in diverse populations with various types of canine impaction.