This study was approved by the Hospital Odontològic Universitat de Barcelona Ethics Committee CEIC, No. 2017/34. The first step was to obtain a digitized model of the patient’s upper maxilla. We took a conventional alginate impression and later digitized the model cast. Then, we used a 3Shape D2000 tool to scan the dental casts and the 3Shape Dental System v.2017 software to create the digital models.
Before initiating the orthodontic treatment, we took a CBCT (Planmeca series No. TFMP1015B) with a volume of 150 × 100x80 mm, which covered two thirds of the maxillofacial skeleton. Another similar CBCT was taken to compare the results immediately after the palatal expansion. Both records had a voxel size of 400 μm to achieve high-quality 3D image composition, and were developed using the Mimics Medical v20.0 software. Digital casts and CBCT records were superimposed with the 3-Matic Medical v12.0 tools.
For the sake of comfort, we used a very small maxillary expander with only two miniscrews for bone-borne anchorage. The shape of the maxillary arch, whose bottom was deep and narrow, required separating the expander from the palate vault. The device therefore consisted of a conventional Hyrax-type screw (designed by Forestadent, Pforzheim, Germany) supported by two pillars or “legs” with two anterior perforations for miniscrew placement, with a total size of 11 × 10.3 × 9.7 mm Fig. 1.
We did not suggest using molar bands or an acrylic pad for additional support. The main challenge was to obtain primary stability of the device with only two miniscrews. To this end, we considered two aspects: the characteristics of the temporary anchorage devices (TADs) and location site.
We decided their characteristics based on several studies. Walter et al. [14], determined that the use of 1.5 mm diameter titanium mini-implants may undergo major deformations with a risk of breakage; we therefore suggested the use of TADs with a diameter > 2.0 mm for bone-borne maxillary expanders. Lee et al. [15] concluded that bicortical mini-implant anchorage is recommended to avoid deformation and fracture. Since length should be proportional to the placement site, we considered the thickness of the cortical bone in the premolar area with a mean value of 1.18 mm, in line with the findings of Johari et al. [16]. Thus, we proposed a length of 8 mm.
Miniscrews were inserted with approximately 30º posterior and 20º lateral inclinations, following the natural contour of the palate vault [17]. This allowed us to maximize the amount of bone surface in contact with the miniscrews and to simplify the insertion procedure of the miniscrews using a manual straight driver Fig. 2.
Production process
We considered two options for production: 3D printing or 5 axis milling. 3D printing was not deemed appropriate since the device included smaller parts that needed further assembly, making us decant for the milling machine instead (HSC 20 linear from DMG MORI). The maxillary expander was made with grade V titanium (Ti6AI4V metal composition). Its inherent properties—e.g., biocompatibility, rigidity, lightness and resistance to the intense forces of palatal disjunction—allowed us to keep the device small while ensuring that it would not collapse during the treatment. We required a completely polished surface with a quality of Ra < 0.0025 μin to ensure comfort and to achieve the best contour and fitting accuracy. For the milling procedure, we used a titanium-aluminum-vanadium based dental alloy KERA® TI 5 disc (Eisenbacher Dentalwaren ED GmbH), a material that is normally used for dental purposes in crowns, bridges and implant-retained suprastructures.
The perforations in the anterior part of the expander were made to fit two 2.0 × 8 mm diameter miniscrews (Jeil, ref 20AT-008). They were designed to provide the correct direction by working as a screw cap system, sealed with the miniscrew heads at the end of the insertion process.
We designed a CAD/CAM surgical guide using the same 3-Matic software to facilitate expander placement and ensure correct miniscrew positioning. It surrounded the occlusal area of both premolars and molars bilaterally while safely securing the maxillary expander at the center, allowing the clinician to operate easily.
The surgical guide was manufactured with laser sintering 3D printing technology (Formiga printer P110 from EOS). We used polyamide (PA2200), a multipurpose material with a balanced property profile that is very strong and stiff, has good chemical resistance, high selectivity, detailed resolution and is biocompatible. This technique uses a fiber laser to melt and fuse fine plastic powder. We built the 3D object by layers based on the computer-aided design. Each layer was 0.1 mm thick, so that a sphere with a 1 cm diameter had 100 layers, thus providing maximum printing precision and fit—especially in the molar cusp and groove areas. The final size of the surgical guide was 34.7 × 44.5 × 14.5 mm.
Clinical example
The patient, a 13-year-old Caucasian-Spanish female, attended the Orthodontic Department of the Hospital Odontològic Universitat de Barcelona in Catalunya, Spain. Her parents were mainly concerned about her loud snoring, caused by poor nasal breathing, and difficulties in chewing due to a posterior crossbite and an augmented overjet.
The cephalometric analysis revealed a skeletal Class II with a high A point convexity and a short mandibular length. There was a moderate Class II molar and canine relationship, with an augmented overjet of 7 mm, maxillary-mandibular arch length discrepancy and vestibularized incisors. V-shaped maxillary and U-shaped mandibular arch forms were asynchronous. No temporomandibular joint disorder symptoms and signs could be observed on radiographic and clinical evaluations.
The treatment objectives were to correct the posterior crossbite and expand the maxilla, preparing it for future mandibular advancement. We decanted for rapid maxillary expansion (RME) as the most suitable choice, to even the arch length discrepancy, change the V-shaped maxillary arch and improve nasal breathing [18, 19].
The treatment started after the patient’s parents decided to participate in this study and signed a written informed consent according to ethical principles.
To ensure an effective RME, we started by correcting the negative torque of the posterior lower teeth. Using fixed braces in the lower arch and posterior build-ups to decompensate, the real posterior crossbite was obtained as a reference of how much maxillary expansion was truly needed. After placing a lower rectangular 0.019 × 0.025-inch stainless steel arch, we began production of the CAD/CAM device following the steps described in the previous section.
Once the customized palatal expander, the surgical guide and the two miniscrews were ready, we were able to place the device in just one visit.
First, we made sure that the surgical guide fitted perfectly with the device and checked that they were both stable in the patient’s mouth. To avoid patient discomfort, we applied topical gel and local anesthesia (articaine 4%—epinephrine 0,5%) in the surrounding palate area before inserting the miniscrews.
We used a conventional RME protocol for activation: a 90º turn of the central screw twice a day for a daily expansion of 0.5 mm over two weeks, to obtain a total expansion of 7 mm. At the end of this process, we asked the patient to advance the mandible, simulating the final result after the use of a functional device for Class II correction, so as to verify that palatal disjunction was adequate. Figure 3.
