Ethical approval
This retrospective study was approved by the Ethics Review Committee of Shenzhen Hospital of Southern Medical University with protocol number NYSZYYEC 20210026.
Sample size calculation
According to the study of lagravère [18], the allowable measurement error is set to be 1.5mm, the statistical test efficiency is 0.9, and the significance level is 0.05. The minimum sample size of this study is calculated to be 15 cases. Based on similar studies[14,15], 20 cases were included eventually.
Participants
From July of 2019 to March of 2022, patients were recruited at the Department of Stomatology at Shenzhen Hospital of Southern Medical University, Shenzhen, China. In total, 11 females and 9 males were involved; 9 cases of unilateral posterior crossbite, 6 cases of bilateral posterior crossbite, and 5 cases without posterior crossbite but diagnosed as maxillary transverse deficiency.
To be eligible, the patients had to meet the following inclusion criteria: (1)adults (age 19~27 years, mean age 22.35 ± 2.68 years); (2) sagittal Class I, with an ANB angle of 0~5 °; (3) underwent MARPE treatment; (4) intraoral examination: the mesial palatine cusp width of maxillary first molars was smaller than the central fossa width of mandibular first molars; (5) maxillary transverse deficiency was diagnosed on CBCT (width of maxillary basal bone - mandibular basal bone < 5mm) [19] ; (6) bilateral maxillary first molars had no large dental defects, therefore bonding was possible. (7) good periodontal health; (8) no other orthotic devices in the mouth during expansion.
Exclusion criteria were cleft lip and palate, congenital craniofacial syndromes and failure to expand the palatal suture as confirmed by CBCT images.
Interventions
A maxillary skeletal expander type II (Biomaterials Korea, Seoul, South Korea; Figure 1) was designed based on the protocol of Moon [20]. Jackscrews were positioned in the midpalate region between the first molars of the maxilla and close to the palatal tissue to allow for miniscrews insertion. Four self-drilling miniscrews (diameter 1.4mm; length 13 mm for anterior region, 11 mm for posterior region) were inserted perpendicularly under local anesthesia (Fig. 1).
Two turns a day (0.133mm/turn) were performed one week after insertion, followed by 7-day rechecks until the palatal surface of the maxillary molar contacted the buccal surface of the mandibular molar. The total time of force application did not exceed 30 days. After active expansion, the devices were maintained for a 6-month consolidation period to enable connective tissue remodeling.
CBCT protocol and analysis
CBCT images (kava 3D XAM, KaVo company, the United States) were recorded following a low dose protocol (exposure time: 20s, 3.0 mA, 120 kV, field of view [FOV]:170 × 230 mm2, voxel size: 0.30mm) before treatment (T0), immediately after expansion (T1), and after a 6-month consolidation period (T2) to ensure that the total radiation dose of repeated CBCT imaging during the experiment did not exceed the recommended annual dose limit (1 mSv).
Images were saved in DICOM format and imported into Dolphin imaging software (version 11.8 Premium, Chatsworth, CA, USA) for analysis. We standardized head orientation in Dolphin 3D software across all data sets in order to maintain the same reference planes. The 3D orientation was performed according to three reference planes obtained from stable landmarks such as porion, orbitale, and nasion. This study used Frankfurt plane as the sagittal reference plane (passing through both orbitale and porion landmarks bilaterally), transporionic plane as the coronal reference plane (passing through bilateral porion landmarks and perpendicular to Frankfurt plane), and midsagittal plane as the horizontal reference plane (passing through nasion landmark and perpendicular to transporionic plane). Oriented the head so that the Frankfurt plane, transporionic plane, and midsagittal plane lined up with the axial plane, coronal plane and sagittal plane, respectively (Figure 2).
Afterward, five landmarks (left / right condylion, left / right gonion and menton; the abbreviations were lCo, rCo, lGo, rGo, Me respectively) were localized in the 3D reconstruction view and multiplanar reconstruction view (MPR) (Figure 3 ).
The mandibular positional changes were evaluated by measuring the linear distance between the mandibular landmarks and the reference plane. Linear distance changes between the landmarks and the Frankfurt plane, transporionic plane and the midsagittal plane represented the vertical, anteroposterior and horizontal changes of the mandibular position respectively. (Figure 4).
Statistical analysis
SPSS 26.0 (SPSS, Chicago, IL, USA) was used to process the data, and `x±s was used to represent the measurement data. The same measurer randomly selected 12 CBCT data sets after 30 days and measured again to evaluate the repeatability of the measurement. The intra group correlation coefficient (ICC) of all measured values ranged from 0.931 to 0.993. After the Shapiro Wilk test was used to verify the normal distribution of the data, the single factor repeated measurement ANOVA was used to evaluate the longitudinal changes. Greenhouse-Geisser corrections were applied for data that violated sphericity assumptions. In statistically significant results, the LSD multiple comparison test was used to assess differences between time points. The statistical significance level was 0.05.