INTRODUCTION

Maxillary protraction treatment using a facemask is the most common treatment for growing patients with skeletal Class III malocclusion. To mitigate tooth-borne maxillary protraction side effects, a bone-anchored maxillary protraction technique has been developed.^1–3 However, miniplates, commonly used as bone anchors, present several challenges. Placement and removal of miniplates are invasive procedures requiring general anesthesia, and their use may cause delayed treatment due to the development of permanent teeth. These factors not only increase the psychological burden on the patient and caregiver but also technical difficulty for the surgeon. Although maxillary protraction is highly effective in patients aged approximately 8 years, the placement of miniplates necessitates careful consideration of the developing dentition in the maxilla and mandible. This can often result in delayed placement, potentially missing the optimal time for maxillary protraction treatment.^2–4

As an alternative to miniplates, hybrid hyrax bone-anchored rapid palatal expansion (RPE) devices utilizing maxillary palatal anchors have been introduced.^1,5 These devices provide more stable anchorage than conventional RPE devices and minimize dental side effects.⁵ The palate offers several advantages, including a high success rate due to good cortical bone thickness, suitability for local anesthesia, and the possibility of a flapless, simple, and less invasive procedure. Consequently, various designs employing orthodontic screws in the palate have been developed to deliver orthopedic forces even in the early mixed dentition stages.^6,7 However, unlike conventional bone-anchored maxillary protraction appliances such as the miniplate, mentoplate, and c-plate, research on whether orthodontic miniscrews can deliver strong orthopedic forces to the nasomaxillary complex is limited. To understand the effect on the maxilla, it is crucial to analyze in detail the complex displacement and stress distribution occurring in the nasomaxillary complex during maxillary protraction.

Finite element analysis has been the most commonly used tool for this type of research.^8–10 Previous studies reported that stress at the circummaxillary sutures and the degree of maxillary displacement during protraction vary depending on the traction vector. Additionally, different results have been observed for various intraoral devices.^11,12 A finite element model analyzing miniscrew-assisted maxillary protraction (MAMP) showed that the maxillary complex rotates clockwise or counterclockwise and moves anteriorly and inferiorly, depending on the location and direction of the applied force.¹³

Finite element modeling faces challenges in representing the complex nasomaxillary anatomy accurately. Current models often lack precise representation of the nasomaxillary fissure or fail to capture the suture area of the posterior maxillary palate fully. These limitations can lead to discrepancies in the stress values obtained by the finite element method (FEM) within the suture area and inconsistent rankings of stress among the circummaxillary sutures.^2,7,9

Consequently, finite element analysis studies may yield varying results depending on model construction. Comparing pre- and post-procedure radiographs to ensure consistency between computer simulations and clinical outcomes is essential to validate these findings. Cantarella et al. introduced a method to measure changes in the midpalatal and pterygopalatine sutures using cone-beam computed tomography (CBCT) data before and after applying a microimplant-supported skeletal expander.¹⁴ This approach can be extended to assess changes in the circummaxillary sutures after MAMP treatment.

This study aimed to analyze the effects of MAMP on the circummaxillary sutures using CBCT data. Width changes in five circummaxillary sutures after MAMP were measured and compared. The suture exhibiting the largest change was identified and compared to the skeletal changes observed during maxillary protraction. The null hypothesis was that there would be no difference in suture width changes after MAMP.

MATERIALS AND METHODS

This study was approved by the Institutional Review Board of Pusan National University Dental Hospital (PNUDH-2022-03-011).

This study included 17 patients (three males and 14 females; mean age: 9.2 ± 0.5 years) who visited the Department of Orthodontics, Pusan National University Dental Hospital, and fulfilled the following inclusion criteria: Skeletal Class III malocclusion, anterior crossbite, actively growing stage (CVM I–IV), and no craniofacial syndrome or systemic disease. CBCT scans (Planmeca Viso, Helsinki, Finland) were obtained for all patients before treatment and at the end of treatment.

Treatment Procedure

MAMP involved placing a maxillary skeletal expander (BioMaterials Korea, Seoul, Korea) in the mid-palate. Miniscrews measuring 1.8 mm in diameter and 11–13 mm in length were inserted into the two anterior holes on both sides under infiltration anesthesia. A 0.9-mm long stainless steel wire was soldered to the buccal side to serve as a protraction hook (Figure 1). Maxillary protraction was applied with a force directed 30° downward from the occlusal plane. Patients were instructed to use the device for a minimum of 14 hours per day, with the anterior traction force gradually increased from an initial 500 g to a final 600 g. The total treatment duration was 9.7 ± 2.5 months.

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Assessment of Suture Changes

Pre- and post-treatment CBCT data of 17 patients undergoing MAMP were analyzed using OnDemand3D software (Cybermed, Seoul, Korea). Images of the sutures were acquired at identical locations before and after treatment, with superimposition on the frontal cranial base.

The circummaxillary sutures investigated in this study were the frontomaxillary, pterygomaxillary, zygomaticofrontal, zygomaticomaxillary, and zygomaticotemporal sutures (Figure 2). The suture widths were measured using ImageJ software (National Institutes of Health, Bethesda, MD, USA). The suture width was defined as the average of measurements taken at up to 20 randomly assigned points. Since all sutures were bilateral, the mean width changes on the left and right sides were compared before and after the treatment (intraclass correlation coefficient: 0.938, Dahlberg error: 0.009).

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RESULTS

In this study, the treatment effects were evaluated using CBCT-generated cephalometric analysis before and after MAMP (Table 1). The SNA angle increased significantly from 78.8° to 81.2°, and the ANB angle increased by 3.0° (P < .001). Although the SNB angle decreased slightly from 79.6° to 78.9°, this change was not statistically significant. Significant changes were also observed in the U1 to SN and IMPA angles, showing an increase of 1.9° and a decrease of 4.6°, respectively. Notably, the overjet showed the most significant change, increasing by 6.8 mm, from −1.3 to 5.5 mm (P < .001).

Table 1.Cone-Beam Computed Tomography-Generated Lateral Cephalometric Analysis Before and After Maxillary Protraction^a

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Table 2 presents the changes in the widths of each of the five circummaxillary sutures. The paired t-test revealed significant differences between the pre- and post-treatment measurements of all sutures, indicating that treatment had a substantial effect on each suture. A one-way analysis of variance with the null hypothesis of “all sutures have the same average change” demonstrated that there was a statistically significant difference in the average change among sutures (α: 0.05, P = .002).

Table 2.Comparison of Circummaxillary Suture Changes

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The zygomaticomaxillary suture width increased from a mean of 0.5 mm before treatment to 0.56 mm after treatment, with a mean difference of 0.07 mm (P < .001). Similar significant increases were observed in the other four sutures: The zygomaticotemporal suture increased from 0.45 mm to 0.53 mm; the zygomaticofrontal suture increased from 0.5 mm to 0.58 mm; the frontomaxillary suture increased from 0.49 mm to 0.55 mm; and the pterygomaxillary suture increased from 0.51 mm to 0.59 mm.

Figure 3 illustrates the frequency of the ranks of the sutures by magnitude of change after MAMP treatment. The Friedman test was used to assess the statistical significance of the difference in rankings among the five sutures. The results indicated a statistically significant difference in the magnitude of change among the sutures (Friedman chi-square test = 10.807, df = 3, P = .01282). The zygomaticofrontal suture exhibited the highest frequency for the greatest change in width, ranking first 13 times. Conversely, the frontomaxillary suture ranked fifth most frequently, showing the smallest change in width. The pterygomaxillary and zygomaticotemporal sutures showed the highest magnitudes of change after the zygomaticofrontal suture. However, the frequencies of these top three sutures did not differ significantly from each other.

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Considering only the highest rank, the zygomaticofrontal, pterygomaxillary, and zygomaticotemporal sutures showed the largest changes, without significant differences among them. A chi-square test for these sutures revealed no statistically significant difference in rank distribution (chi-square test = 4.521, P = .807). However, when considering the second to fifth rankings collectively, the zygomaticofrontal suture showed the highest frequency of significant changes.

DISCUSSION

Although maxillary protraction is a widely used technique to treat skeletal Class III malocclusion, it has associated side effects. Efforts to understand the biomechanics of maxillary protraction are ongoing to improve the technique. Finite element analysis has been particularly useful in understanding the dynamics of orthodontic forces through various skull modeling simulations.^11–13 However, finite element analysis requires validation by comparing the simulated stresses applied to the suture during maxillary protraction with the actual sutural changes. Jeon et al. demonstrated that analyzing sutural changes after skeletal changes in the maxilla is possible through CBCT image analysis.¹⁵

Previously, finite element analysis studies have inconsistently identified the sutures most stressed by maxillary protraction, citing the zygomaticomaxillary suture, zygomaticotemporal suture, and others.^2,7,9 Therefore, investigating the suture width changes after MAMP may prove valuable for biomechanical treatment planning and outcome prediction.

Craniofacial sutures, composed of cells and fibrous tissue between the craniofacial bones and around the bony edges, play a crucial role in craniofacial growth. Significant changes in these sutures result from forces transmitted by the maxillary protraction devices. These changes are proportional to the magnitude and direction of the applied force.^16,17 Angelieri et al. categorized the zygomaticomaxillary suture into five stages based on maturity, noting that suture diameters decreased with maturity, which was also correlated with age.¹⁸

In this study, the measured changes in the suture diameters of the five circummaxillary sutures (zygomaticomaxillary, zygomaticotemporal, zygomaticofrontal, frontomaxillary, and pterygomaxillary sutures) mostly showed an increase. These findings were in contradiction to the natural maturation process in which sutural width typically decreases over time, indicating that the observed changes were most likely attributed to maxillary protraction.

Previous FEM studies identified that the pterygomaxillary suture experienced the highest von Mises stress.^2,8,19 A FEM study by Lee et al. revealed maximum stress in this suture when a miniplate was applied to the lateral nasal and infraorbital crest regions.² Tanne and Sakuda also claimed that maxillary protraction force generated the highest stresses at the pterygomaxillary suture, potentially separating the maxilla from the pterygoid process at the pterygomaxillary fissure level.¹⁷ The present study was in agreement with these findings, with the CBCT analysis revealing significant post-treatment changes in the pterygomaxillary fissure, ranking it among the most-altered circummaxillary sutures in treated patients.

However, the CBCT analysis of growing patients undergoing MAMP revealed that the zygomaticofrontal suture exhibited the most pronounced changes among the circummaxillary sutures. This discrepancy may be attributed to interpretation differences arising from the simplistic FEM modeling of the complex pterygomaxillary fissure area.³ It is important to note that FEM stress values do not account for skeletal movement over time. Nevertheless, the sutural sites showing significant width changes generally aligned with the high-stress areas reported by finite element analysis.

The frontomaxillary suture demonstrated the least width change in the present study. This suggested that an anterior traction vector of 30° downward from the occlusal plane relative to the maxillary center of resistance induced a counterclockwise rotation. Different traction directions might yield varying outcomes. Indeed, studies have shown that the frontomaxillary suture experiences the highest stresses during clockwise maxillary rotation.² Kim et al. reported the highest stress in the frontomaxillary suture when simulating miniplate application to the infraorbital crest region.¹⁹ This was probably due to the significant vertical difference between the intraoral device and miniplate positioning, allowing direct transmission of orthopedic forces from the maxillary center of resistance.

Conversely, maxillary protraction via a palatal plate, with its relatively low traction point, transmitted force more readily to the zygomaticotemporal, zygomaticomaxillary, and zygomaticofrontal sutures compared to conventional bone-anchored maxillary protraction.^4,8,19 In MAMP, the zygomaticotemporal and zygomaticofrontal sutures showed comparable changes to the pterygomaxillary suture. This pattern aligned with high-stress concentrations observed in the finite element analyses of maxillary anterior traction using a palatal miniplate. Thus, if the intraoral device design and force application points are similar to those in the present study, stress distribution and suture changes will parallel the finite element analysis prediction.⁹

Results of the current study were in agreement with previously reported finite element analysis of maxillary protraction, identifying the zygomaticofrontal, pterygomaxillary, and zygomaticotemporal sutures as the primary areas of change during treatment. Although the exact stress magnitude differences among the sutures in the finite element analyses may vary because a perfect model of the actual complex anatomy cannot be generated, the observed pattern of change at specific suture sites closely corresponded with the CBCT analysis of patients before and after treatment.

There were several challenges in measuring the irregular three-dimensional shape of the suture. Evaluating three-dimensional suture width as a single line may not fully represent actual suture expansion. Additionally, variations in the suture width could be very small. Theoretically, when obtaining CBCT images with a 0.3 mm voxel size, lengths smaller than 0.3 mm may be inaccurate. Although the suture width measurements in this study were reliable at a 0.3 mm voxel size, more accurate assessments could potentially be achieved using CBCT images with smaller voxel sizes.^20,21 However, it is also important to consider that imaging with voxel sizes smaller than 0.3 mm increases radiation exposure to the patient.

Booth et al. reported that cervical vertebrae maturation and midpalatal suture maturation could be predicted by analyzing the spheno-occipital synchondrosis stage on CBCT images.²² Similarly, the present study used CBCT images to evaluate MAMP outcomes. These methods are particularly useful for orthodontic treatment and growth prediction, as they minimize radiation exposure to patients and simplify treatment evaluation and design.

CONCLUSIONS

• This study confirms that the stress patterns at the circummaxillary sutures during MAMP, as predicted by existing finite element analysis, closely correspond to the actual suture width changes observed on CBCT.

• The null hypothesis was rejected as there was a significant difference in suture width after MAMP.

• The significant changes noted in the zygomaticofrontal, pterygomaxillary, and zygomaticotemporal sutures during MAMP are particularly important.

• Monitoring these changes can provide indirect insights into the direction of translation and rotation of the maxilla during protraction treatment.