<p>A New Methodology for the Digital Planning of Micro-Implant-Supported Maxillary Skeletal Expansion</p>

Abstract

Introduction: Miniscrew-assisted rapid palatal expansion (MARPE) appliances utilize the skeletal anchorage to expand the maxilla. One type of MARPE device is the Maxillary Skeletal Expander (MSE), which presents four micro-implants with bicortical engagement of the palatal vault and nasal floor. MSE positioning is traditionally planned using dental stone models and 2D headfilms. This approach presents some critical issues, such as the inability to identify the MSE position relative to skeletal structures, and the potential risk of damaging anatomical structures. Methods: A novel methodology has been developed to plan MSE position using the digital model of dental arches and cone-beam computed tomography (CBCT). A virtual model of MSE appliance with the four micro-implants was created. After virtual planning, a positioning guide is virtually designed, 3D printed, and utilized to model and weld the MSE supporting arms to the molar bands. The expansion device is then cemented in the patient oral cavity and micro-implants inserted. A clinical case of a 12.9-year-old female patient presenting a Class III malocclusion with transverse and sagittal maxillary deficiency is reported. Results: The midpalatal suture was opened with a split of 3.06 mm and 2.8 mm at the anterior and posterior nasal spine, respectively. After facemask therapy, the sagittal skeletal relationship was improved, as shown by the increase in ANB, A-Na perpendicular and Wits cephalometric parameters, and the mandibular plane rotated 1.6°clockwise. Conclusion: The proposed digital methodology represents an advancement in the planning of MSE positioning, compared to the traditional approach. By evaluating the bone morphology of the palate and midface on patient CBCT, the placement of MSE is improved regarding the biomechanics of maxillary expansion and the bone thickness at micro-implants insertion sites. In the present case report, the digital planning was associated with a positive outcome of maxillary expansion and protraction in safety conditions.

Description of a Digital Work-Flow for CBCT-Guided Construction of Micro-Implant Supported Maxillary Skeletal Expander

“…Compared to other miniscrew-assisted expanders, the MSE may present some restrictions. In particular, this appliance must be placed in a more posterior position, between the maxillary first molars (barely anterior to the soft palate), in order to concentrate the forces closer to the pterygoid plates that induce great resistance to palatal expansion (Figure 1a,b) [12,23,24]. In addition, the design of the MSE requires a perfect parallelism of the four miniscrews with the nasal septum, with the body of the appliance itself firmly guiding the insertion of the miniscrews.…”

“…In addition, it is important to select the proper size of MSE since, with skeletal expansion, the MSE must be placed close to the palatal mucos to mitigate the leverage effect on the micro-implants during activations. Thus, a preliminary evaluation of palatal morphology is recommended in order to verify if the patient can be the right candidate for maxillary expansion with this appliance [15][16][17]23]. This evaluation of adaptability of the MSE can be performed during clinical examination by using a 3D printed template of the expander connected to a handle which facilitates the procedure of inspection, avoiding also discomfort to the patient (Figure 2a).…”

“…To assure that all these aspects can be satisfied, maxillary cone-beam (CBCT) scans and maxillary .stl file obtained from intra-oral or dental model scans are superimposed in order to identify the most suitable anteroposterior and vertical placement of the appliance based on the width and thickness of the palatal vault. This CBCT-dental model superimposition combines the qualitative and quantitative assessment of palatal bone for the qualitative evaluation of soft tissue characteristics, which allows clinicians to virtually place the miniscrews and the expander according to the previously defined guidelines [12,23,24]. A comprehensive description of the procedure is described below.…”

“…The original .stl files of the expander screw and the four miniscrews are jointed together by virtually inserting each miniscrew through one of the four slots of the palatal expander. A single .stl file representing the palatal expander with the four miniscrews is created (Figure 3a,b) [23]. A rectangular object (dimension16 × 20 × 16 mm) is created, and the 3d model of the palatal expander with miniscrews is placed inside, then the 3d model was subtracted in order to obtain a negative template of the MSE with miniscrews ( Figure 3c).…”

“…Cantarella et al [23] recently documented a digital workflow specifically designed for the MSE appliance. The authors planned the virtual insertion of the MSE (including the four miniscrews) to optimize the bone thickness at miniscrew insertion sites and used specific reference planes, based on CBCT midface skeletal landmarks, to improve the biomechanical effect on the midface.…”