Trial design and any changes after trial commencement
This was a prospective randomized clinical controlled trial (RCT), which followed the Consolidated Standards of Reporting Trials statement and guidelines [14] and did not require changes in methods after trial commencement. The methods were described in detail in our previous research [12].
Participants, eligibility criteria, and settings
The protocol of this study (PKUSSIRB-2013050) was approved by the Biomedical Ethics Committee of the School and Hospital of Stomatology, Peking University (Beijing, China) and registered at the Chinese Clinical Trial Registry (Chictr.org.cn) with the identifier ChiCTR-TRC-13003260 (15/06/2013).
This study was a serial study and its sample and methods could refer to our previous RCT study [12]. The sample consisted of 64 patients who started treatment from June 2013 to July 2014 in the orthodontic department of Peking University School and Hospital of Stomatology. The eligibility criteria for inclusion in this study were patients (1) had all erupted permanent teeth, (2) had Angle’s Class I or II malocclusion whose treatment required medium or maximum anchorage control, (3) had two upper 1st bicuspids or four 1st bicuspids extracted (4) were in good health with no chronic disease or disability. Informed consent was obtained from the patients and their parents or legal guardians before recruitment.
Interventions
The appliance and treatment philosophy used in PASS and MBT are different [15]. PASS is a modified straight wire system based on physiologic characteristics of individual patient and classical anchorage control strategy and it aims at reduction of maxillary dentoalveolar compensation and shares the similar goal of Tweed when keeping the upper curve of Spee is concerned. The appliance used in PASS is mainly composed of cross-buccal tube (XBT) and multi-level low-friction(MLF) brackets (Fig 1). The first stage of the treatment in PASS is alignment of the anterior teeth. The basic treatment procedure: bond canine to canine MLF brackets and first molar XBT. Upper and lower 0.014-inch NiTi archwires engage in −25 °auxiliary tube on the upper molar and -20°’virtual tube’ on lower molar (Fig 2A). After aligning the anterior teeth, premolar and second molar would be bonded to align posterior teeth. In the meantime, the main tube would be used for the rest of the treatment. Patients in the PASS were treated according to PASS procedure.
Patients in the MBT™ group were treated according to the MBT™ technique. 0.022-inch slot-size preadjusted brackets and buccal tubes were bonded [7] (Fig 2B). Canine lacebacks were used to assist in control of canine crown during leveling and alignment. Any auxiliary anchorage device could be used in the MBT group for patients who needed extra anchorage control
Outcomes (primary and secondary) and any changes after trial commencement
Three-dimensional change of maxillay first molar was considered as the primary outcome measurement.
Secondary outcome measurements included the 3D changes of maxillary canines, and maxillary central incisors, including tip, torque, vertical and antero-posterior movement of maxillary canine; torque, vertical and antero-posterior movement of maxillary central incisor (Table I).
All measurements were performed using Rapidform 2006 (Inus Technology Inc., Seoul, Korea) on digital dental modelsat pre-treatment(T0), 3 months(T1), and 6 months(T2). Maxillary digital models of different time points were superimposed according to Chen’s method [16] (Fig 3A). The reference coordinate system was established on the digital model of T2. The three coordinate planes included the occlusal plane, the mid-sagittal plane, and the coronal plane. The occlusal plane was formed using the mesial buccal cusps of the bilateral maxillary first molars and the midpoint of the edge of the central incisor; the mid-sagittal plane was formed using two points (P1 and P2) along the mid-palatal suture and their projections on the occlusal plane(P1’ and P2’); the coronal plane was determined by the plane passing through P1 and perpendicular to both occlusal plane and mid-sagittal plane. The angles of upper first molar, canine and central incisor were measured on digital dental model using Andrews’ method [3] (Fig 3B,C).Single tooth superimposition was used to transfer the landmark located on T0 model to T1 and T2 to minimize the error from landmark identification (Fig 3D, E).
Sample size calculation
The calculation of the sample size was based on the known variability of the mesial displacement of upper molars measured by Xu et al. [11] and was described in detail in our previous article [12].
Interim analyses and stopping guidelines
Not applicable.
Randomization (random number generation, allocation concealment, implementation)
Patients were stratified by age, sex and molar anchorage strength. Then they were allocated into the PASS or MBT™ group randomly at a 1:1 ratio with a minimization method [12,17].
Blinding
The bracket system of PASS and MBT™ was quite similar to each other, so patients were blind. However, blinding of orthodontist was impossible because the appearance and treatment procedure were different between the two systems. The judges who did the measurements and analyzed the data were blinded.
Statistical analyses (primary and secondary outcomes, subgroup analyses)
All 192 study models were measured by three orthodontic residents. SPSS 16.0 (SPSS, Chicago, IL, USA) was used for statistical analysis. The significance level was set at P<0.05.
ICC was calculated to evaluate inter-rater consistency. Every model was measured three times by the three residents and their average was used for the final measurement. The independent T-test was performed to detect any difference between the PASS group and the MBT™ group. Except for the inter-canine width and the inter-molar width, all other measurements had right and left. After comparing the right and the left measurement, no significant difference was found; thus, we combined every bilateral measurement into one measurement.