Mandibular 3D position changes in miniscrew-assisted rapid palatal expansion(MARPE): a retrospective clinical trial using CBCT

DOI: https://doi.org/10.21203/rs.3.rs-1842082/v1

Abstract

Background: The aim of this study was to evaluate the immediate and short-term effects of miniscrew-assisted rapid palatal expansion (MARPE) on mandibular 3D position in treatment of skeletal Class I malocclusion with maxillary transverse deficiency in adults.

Methods: A retrospective study was conducted using cone-beam computed tomography (CBCT) scans of 20 adults with skeletal Class I malocclusion and maxillary transverse deficiency who underwent MARPE. Three CBCT scans were obtained before treatment (T0), immediately after expansion (T1) and after a six-month consolidation period (T2). Mandibular landmarks were measured with respect to axial, sagittal, and coronal reference planes respectively. Repeated measures ANOVA and LSD multiple comparison were used for statistical analysis.

Results: The mandibular rotated clockwise at T1 and relapsed to its initial position at T2. The lateral displacement was unremarkable.

Conclusions: When MARPE was used to treat skeletal Class I malocclusion with maxillary transverse deficiency, it caused a transient clockwise rotation of the mandible. However, the mandible did not show sagittal, vertical and horizontal position changes in short-term evaluation.

Background

Tooth extrusion, dental tipping, and an increase in the vertical dimension are often encountered in traditional rapid palatal expansion (RPE) devices, such as Hyrax and Hass. This may not coincide with treatment objectives[1-6]. Changes in mandibular position can affect the coordination of the entire craniofacial complex，which has always been a concern for orthodontists.

Miniscrew-assisted rapid palatal expansion (MARPE) is gradually becoming the treatment choice to correct maxillary transversal dimension in adult patients, exceeding the limitations of conventional RPE devices [7]. The incorporation of miniscrews in MARPE contributes to different stress distribution and different displacement of craniofacial structures in comparison to RPE. This enables more predictable and greater skeletal expansion, as well as less buccal tipping and alveolar height loss on anchorage teeth[8-12].

As for MARPE's effect on mandibular position, there are not enough research reports available at present and no consensus has been reached. Some studies claimed that changes in the vertical and anteroposterior dimensions were negligible[13-15]. Some revealed that MARPE showed counterclockwise rotation tendency and may be beneficial for hyperdivergent patients [1,16].

Regarding to research methods, cephalometric analysis used in previous studies has limitations, since it mainly focuses on sagittal and vertical directions[4-6,17]. Three-dimensional evaluation especially in horizontal direction is not feasible. 

Thus, this study aims to determine whether MARPE treatment will result in mandibular position changes and help orthodontists to understand the clinical effects of MARPE from a more comprehensive perspective.

Methods

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.

Results

ANOVA repeated measures showed significant differences over time for the menton and for the right gonion relative to the coronal plane (Table 1) and the Frankfurt plane (Table 2) (P=0.04, 0.02; P=0.00, 0.01 respectively). The menton measurement did not show significant changes relative to the midsagittal plane (Table 3). These indicated mandibular position changed in both vertical and sagittal directions, but not horizontally.

Multiple comparative tests revealed differences in the menton measures between T0 and T1 with respect to the axial plane (-1.46mm，table1) and to the coronal plane (1.65mm, table 2), indicating backward and downward movement of the menton. However, an upward and forward displacement of 1.39mm and 0.94mm was observed at T2 during the retention period (T1-T2) (P <0.05). There was no statistical difference between the initial and final position for the menton (T0-T2) (P >0.05). Similar changes were seen in the right gonion as well (Table 4).

Discussion

In this study, the sagittal and vertical positions of the mandible changed significantly immediately after expansion (T0-T1). The right gonion and the menton measures in relation to coronal plane decreased by 0.90mm, 1.65mm (p=0.04, 0.02), and measures in relation to axial plane increased by 0.86mm, 1.46mm (p=0.00, 0.00). A clockwise rotation of the mandible can be verified by the fact that the anterior structures (menton) changed more than the posterior parts (gonion). While both sagittal and vertical relapses of mandibular position were observed during the retention period (T1-T2). Eventually, the mandible returned to its initial position (T0-T2) (P > 0.05). 

According to Song[21], the zygomatic maxillary complex moved to the vertical and sagittal directions immediately after MARPE, and the mandible rotated clockwise afterward (SNB decreased), which is like our results (T0-T1). The short-term effect of MARPE (>6 months) was assessed by Yılmaz [22] measuring the 2D radiographs and models, An[15] studying the contralateral radiographs, and Lagravère [13,14] evaluating the position of the mental foramen on CBCT images. All the studies mentioned above and this one indicate that the mandibular position remains unchanged after MARPE retention. The method in this study is more comprehensive and accurate by using CBCT to measure the mandibular 3D position change. 

Here are some possible explanations for this change. The MARPE device used in this study is similar to those used by Moon [12], Cantarella [23] and Guo [24]. There is a gap between miniscrews and screw tube. These two are not rigidly attached. Before the palatal suture opened during early expansion, the transversal stress is directly applied to the molars, causing them to tilt and making premature contacts. It is most apparent at the end of active expansion. However, the buccal inclined molars gradually relapse back to nearly upright during the retention period [25,26]. Previous studies reported that the inclination of molars increased by 0 ~ 3 degrees and the molars height was almost maintained after 6 months’ retention in MARPE[25,27-29], which was very slight and unnoticeable compared with the traditional RPE( >10° ) [22,26,30]. This might be the primary reason for mandibular positional changes process in sagittal and vertical plane.

Some studies [31-33] observed that the mandibular position of skeletal Class II malocclusion moved forward spontaneously after expansion. However, the sample was composed of patients with skeletal Class I malocclusions in this study, so functional mandibular retraction before treatment was very unlikely[2]. 

Regarding the horizontal effect, the main concern is whether MARPE can improve symmetry of the mandibular position[34,35]. The condylion and gonion are bilateral structures, which means that when one side moves towards the midline, the other side will move away from it. The horizontal displacement of condylion and gonion was not measured in this study because the left and right sides of the samples have different crossbite patterns before treatment. Only the linear distance between menton and midsagittal plane was measured. When the measures decrease, the mandibular position becomes more centered and symmetric; on the other hand, if the measures increase, the mandible becomes more asymmetric and deviates from the center. Results showed that lateral displacement of menton at any time was not significant.

The reason for this can be attributed to two factors. Firstly, patients with unilateral posterior crossbite were found to have a functional deviation in previous studies[36], and the mandibular position improved after the posterior crossbite was resolved [37,38]. While this study included unilateral crossbite (45%) as well as non-unilateral crossbite (55%) samples. The mandibular lateral shift might be less noticeable because of the latter component[33,39]. Furthermore, MARPE can exhibit asymmetrical expansion in the transverse plane due to initial asymmetric position of the mid-palatal suture, the differential density of the sutures and surrounding bones, and the stability of the miniscrews [29,40-42] . This causes individual occlusal changes after expansion, resulting in an unpredictable horizontal displacement of the mandible.

Finally, it is critical to understand that different MARPE designs are capable of producing different mechanical effects depending on whether they are attached to molars or not [12], mono- or bi-cortical anchorage[24], diameter and location of miniscrews [43], and the age of the patient (adolescents or adults) [44,45]. Conclusions of this study are applicable based on the specific MARPE device, research object, and force application method. Future research on different MARPE devices will be necessary.

Abbreviations

RPE: Rapid palatal expansion; MARPE: Miniscrew-assisted rapid palatal expansion; CBCT: Cone beam computed tomography; Co: right condylion; lCo: left condylion; rGo: right gonion;  lGo: left gonion; Me: menton.

Declarations

Ethics approval and consent to participate 

This study was approved by the Medical Ethics Committee of Shenzhen Hospital of Southern Medical University (No. NYSZYYEC 20210026). Consent was obtained from all participants in this study. All patients in our hospital will sign informed consent before receiving oral treatment, which includes that the patient’s medical records can be used for teaching and research. This study was performed in accordance with the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards.

Consent to publication 

Not applicable. 

Availability of data and materials 

All data generated or analyzed during this study are included in this published article.

Competing interests 

The authors have stated explicitly that there is no conflict of interests in connection with this article.

Funding 

The study was supported by a medical and health basic research project in Bao'an District, Shenzhen, Guangdong Province, China (No.2019jd440).

Author contributions 

QL executed the experiments and wrote the thesis. ZW is the project designer, guiding experimental design and the revision of the thesis. WH, QL and PX collected and analyzed experimental samples. All authors read and approved the final manuscript as submitted. 

Acknowledgements 

Not applicable.

Author details 

1 Department of Stomotology，Shenzhen Hospital of Southern Medical University，1333 Xinhu Road, Baoan Distinct, Shenzhen 510080, People’s Republic of China

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Tables

Tables 1 to 4 are available in the Supplementary Files section