This retrospective study was approved by the Research Ethics Committee of Sir Run Run Shaw Hospital, Zhejiang University School of Medicine. Sample size was calculated considering a mean difference of 4.5 mm between groups for the amount of total mandibular length (Co-Gn), contemplated as the primary outcome, with a previously reported standard deviation about 2.9 mm [7, 10], using 80% test power, at 5% alpha level. Then, a minimum of 9 patients was necessary in each group.
The inclusion criteria were as follows:
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1
Patients of 8–14 years of age with mixed dentition;
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2
Patients with a skeletal Class II pattern, ANB 5°-8°, SNB < 78°;
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3
Growing patients with a cervical vertebral maturation (CVM) stage between 2–4 period;
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4
Two-phase treatment consisting of Phase I RME or Twin-Block therapy, followed by Phase II fixed appliance treatment with four 1st premolar extractions;
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Patients with accurate and complete records, containing two good-quality cephalometric radiographs, taken before treatment (T1) and immediately after fixed appliance treatment (T2).
The exclusion criteria were as follows:
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1
Patients with an extremely increased vertical growth tendency, reflected by mandibular plane angle (FMA) > 36°;
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2
Patients with congenitally missing teeth;
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3
Patients with visible posterior crossbite.
A total of 29 patients were selected according to the inclusion and exclusion criteria. Fourteen patients were included in the RME group, including 4 boys and 10 girls. And the remaining fifteen patients were included in the TB group, including 9 boys and 6 girls.
The matching analysis between the two groups was based on the characteristics of chronologic age, stage of CVM, cephalometric measurements and treatment duration.
Treatment protocols
In the RME group, rapid maxillary expansion was used for phase I treatment. The bonded RME was composed of an expansion screw in the center of the palate and the acrylic splint covering the posterior teeth. The RME was activated via two turns per day (0.25 mm per turn) for a period of 2 weeks. Then the bonded RME was maintained in place at a passive state for approximately 6 months to allow for stabilization. Immediately after RME debonding, fixed appliances were placed. During phase II treatment, four 1st premolars were extracted, and mini-screws were placed in the maxilla for maximum anchorage.
In the TB group, Twin-Block was used for phase I treatment. The posterior bite blocks were ground every 4–6 weeks, until the posterior teeth established occlusion. Then, the Twin-Block was left in the mouth for 3 months to maintain stabilization. Immediately after Twin-Block removal, fixed appliances were placed. During phase II treatment, four 1st premolars were extracted, and mini-screws were placed in the maxilla for maximum anchorage.
After completion of the phase II treatment, all the patients in both groups were instructed to wear the circumferential Hawley retainer for 24 h a day.
Cephalometric analysis
T1 and T2 cephalometric radiographs were uploaded into Dolphin software (Version 11.9, Dolphin Digital Imaging, Chatsworth, Calif, USA). Two investigators simultaneously traced the anatomic contours and located landmarks. Any disagreements about landmark location were resolved by retracing the anatomic contours until the two investigators achieved the same point. Location of the landmarks and calculation of the measurement items were repeated three times. The consistency of the measurements was tested.
The analyzed angular and linear cephalometric measurements at T1 and T2 within each group included SNA, SNB, ANB, Z Angle, FMA (mandibular plane to FH), SNPog (facial plane to SN), NSGn (Y-Axis), NSAr (Sella Angle), ArGoMe (Gonial Angle), Pog-N|FH (pogonion to nasion perpendicular), PFH (posterior facial height), LAFH (lower anterior facial height), PFH/LAFH, SE (Steiner Analysis), SL (Steiner Analysis), Ar-Gn (effective mandibular length), Co-Gn (total mandibular length), Go-Gn (mandibular body length), and Co-Go (mandibular ramus length). These measurements are displayed in Figs. 1 and 2.
Statistical analysis
The data were analyzed by IBM SPSS Statistics 20.0 (IBM Corp., Armonk, NY, USA). The consistency of the three cephalometric item calculations was tested by the intraclass correlation (ICC). The average measurements of chronologic age, stage of CVM, treatment intervals, and all cephalometric items of the two groups at T1 and T2 were used for analysis.
Exploratory chi-square statistical tests were performed to examine the normal distribution of chronologic age, CVM stage, treatment intervals, and all the cephalometric measurements in the two groups.
An independent-samples T test was performed to compare chronologic age and CVM stage at T1, and treatment intervals between the two groups. Additionally, an independent-samples T test was conducted to compare cephalometric measurements between the two groups at T1 and T2. A paired-samples T test was performed to analyze the changes from T1 to T2 within each group. Finally, the changes in the cephalometric measurements from T1 to T2 between the two groups were compared using an independent-sample T test. Statistical significance was determined at the levels of P < 0.05, P < 0.01 and P < 0.001.