Customized Maxillary Skeletal Expander—Literature Review and Presentation of a New Digital Approach for Planning, Fabrication and Delivery

Abstract

The Maxillary Skeletal Expander (MSE) is used for maxillary expansion in adolescents and young adults. Virtual planning using 3D models, CBCT and 3D printers help in case selection, appliance design and fabrication. Using the proposed digital workflow, the accuracy of the patient selection phase and appliance delivery are increased, and the required number of visits to the clinic is decreased. The MSE serves as a guide for the insertion of mini-implants, reducing the number of appointments needed for installation. (1) Introduction: Mini-Implant-Assisted Rapid Palatal Expansion (MARPE) appliances, like the MSE, decrease the side effects that regular tooth-anchored appliances have on dental and periodontal structures, especially for skeletally mature patients, combining palatal anchorage with dental support for guidance. The digital planning of the insertion sites, length and angulation of the mini screws, and the fabrication of the 3D-printed appliance that stands as a mini-implant insertion guide give an undeniable precision. (2) Materials and methods: The laboratory steps used in the digital design and fabrication, and clinical steps needed for the insertion protocol are described. (3) Discussions: The individual assessment of the anatomical structures and the use of virtual integrated dental impressions and CBCT increase the accuracy of diagnosis, appliance fabrication and treatment progress. Implementing a digital workflow for mini-implant-supported expansion is a real advantage for both dental teams and patients. (4) Conclusions: The wide range of advantages and the ease of the process support the introduction of this digital workflow in every orthodontic practice.

1. Introduction

In recent years, the practice of orthodontics has made significant progress, especially with the introduction of intra-oral scanners and three-dimensional printers [1,2]. Also, the use of cone beam computed tomography (CBCT) allowed the better visualization of the dental and skeletal structures [3]. One of the biggest advantages of the digital era for all medical specialties was a better diagnosis, due to high-precision software, but also the thorough planning of treatment modalities, stages and final outcome. In addition to this, in the orthodontic field, new software and 3D printers allowed for the personalization of orthodontic devices, leading to the production of customized surgical guides for implant-supported appliances [4]. This has led to the ability to successfully treat extraction or even surgical cases in a more conservative way.

Maxillary expansion is an orthopedic procedure that consists of the transversal separation of the upper jaw through the mid-palatal suture [5]. The widening of the maxilla can be made slowly (slow maxillary expansion—SME) or, more commonly, rapidly (rapid maxillary expansion—RME) [6]. There are several appliances that can be used for opening the mid-palatal suture, like tooth-borne devices (hyrax) or bone-borne expanders [7]. Recently, TAD-supported expander appliances were introduced, which combine the advantages of the two previous devices (hybrid hyrax expander) [8]. In skeletally mature patients, the gain in the transversal width of the maxilla is made with the use of a fixed expansion device in conjunction with surgically assisted rapid palatal expansion (SARPE), meaning the surgical opening of the suture [9].

A large number of side effects were reported for classical palatal expansion with tooth-borne appliances, like dental tipping with consequent periodontal implications, root resorption and bone loss [10]. During this expansion, the process of hyalinization of the ligaments of the hyrax appliance anchoring teeth (usually molars or premolars) blocks most of the dental movement and produces an orthopedic effect, but dental consequences still appear [7,10,11,12].

Different studies have tried to evaluate the transverse, vertical and sagittal changes after maxillary expansion and some conducted a comparison between tooth-borne and bone-borne RME. Lagravère et al. conducted a study where they analyzed, on CBCT images, the short- and long-term changes in all three planes of space for adolescents treated by tooth-borne or bone-borne expansion or not treated (control group). The results were similar in the transversal plane for both treated groups, with a dental effect greater than the skeletal effect. The vertical and sagittal planes had almost negligible changes. The long-term results showed a greater expansion at the coronal and apical level for upper premolars in the tooth-borne expansion group [7]. In another study that compared the skeletal and dento-alveolar transversal effects on CBCT after expansion with tooth-borne expansion versus two types of micro-implant-assisted maxillary expanders, the results showed that the expansion made with a type of tooth bone-borne expander called MSE is more important at the skeletal level than the other two types of appliances. With all three appliances, the most important transversal changes were at the level of molar crowns, with the least tipping when using the bone-anchored device [13]. Similar research that analyzed the immediate results after expansion showed that the transversal widening of the maxilla using a bone-borne expander was almost twice as important when compared with hyrax appliance. Also, in the hyrax group, at the level of first premolars, important dehiscence appeared on the buccal aspect. It is interesting to mention the results that show a parallel opening of the palatal suture in the bone-borne group, compared with the bigger anterior opening in the hyrax group [14]. Another study suggested that the stability in time of the skeletal transversal response after rapid maxillary expansion is higher if the procedure is finished before the pubertal growth peak [15].

The classical RME takes the entire force from the expansion screw and transmits it to the supporting structures, meaning the posterior teeth. The compression of the periodontal ligaments determines alveolar bone resorption, which allows tooth movement in the direction of the resorption [14]. Garib et al. conducted a study on adolescents to evaluate the periodontal effects after tooth tissue-borne and tooth-borne maxillary expansion, based on CT images. Their results showed that after RME, the buccal aspect of the alveolar bone at the level of the anchor teeth reduced in thickness with 0.6 to 0.9 mm, and dehiscence appeared on the buccal aspect of the same teeth (bone thickening 7.1 ± 4.6 mm at the level of the first premolars, 3.8 ± 4.4 mm on the mesio-buccal aspect of first molars). The adjacent teeth did not experience bone reduction on the buccal aspect, and the soft tissues did not show signs of periodontal damage in the short term [16]. Greenbaum and Zachrisson analyzed the periodontal effects after slow and rapid maxillary expansion. Although they did not find significant differences in periodontal breakdown between the two types of expansion, the lowest insertion levels for soft tissue and crestal bone were in the central aspect of the buccal surface of molar roots, after tooth-borne RME [17]. Another study by Garib et al. suggests that the expansion at the level of posterior teeth is produced by dental tipping and dental translation, and that with the traditional RME, although not banded, the second premolars were more buccal tipped than anchoring teeth [18].

In adults, the risk of the side effects mentioned before with the traditional RME are even higher, especially due to the fact that the mid-palatal suture is more difficult or impossible to open and the dento-alveolar complex is the one that receives all the force from the appliance and makes compensatory movements like buccal tipping or buccal displacement outside the bony limit [10,12]. There are studies that show that even for adolescents, sometimes it is impossible to open the mid-palatal suture and that this is dependent on its stage of ossification [19].

The mid-palatal suture consists of three segments: the anterior segment, which is the part located before the incisive foramen; the middle segment, located between the incisive foramen and the transverse palatine suture; and the posterior segment, located after the transverse palatine suture [20]. After the work Haas published in 1961, when he described an appliance with an expansion screw that was able to widen the maxilla by opening the mid-palatal suture, the procedure became a common orthopedic treatment [21]. The suture is normally open during the initial stages of growth and starts its ossification through bone spicules on the sides of the suture and masses of calcified tissues in many places along the suture. The number of calcification zones increases with maturation [22]. After the age of 10, according to Melsen, zones of interdigitation appear in the inferior part of the suture, which continue to expand [23]. The progress of ossification is from posterior to anterior [22,24].

Persson and Thilander studied in 1977 the bone changes and the intermaxillary and transverse palatal suture obliteration for young adults (from 15 to 35 years). The earliest signs of obliteration were found in a 15-year-old-female, at the posterior part of the intermaxillary suture, and the oldest subject observed with a lack of signs for the union of the suture was a 27-year-old-female [22,25]. Knaup et al. also studied changes in the palatal suture of 18- to 63-year-old specimens. The earliest signs of ossification were found in a 21-year-old man, and the oldest subject without signs of palatal fusion was a 54-year-old man [24].

Melsen described the morphological development of the suture and divided it into three stages: the first one where the suture was short, broad and Y-shaped; the second one, where the suture was squamous and wavy; and the last stage, where the suture was tortuous, with more interdigitations [23].

The beginning and the progress of the intermaxillary suture union varies based on age and sex. Although between 20 and 25 years of age the closure of the palatal suture is very intense, some people can have no signs of palatal suture fusion in the third decade of life [22]. This important age variability in the ossification process is an impediment for planning maxillary expansion, since there is no strict limit between open and closed palatal sutures. From this reason, it is important to understand personal variability in order to identify the cases that can be treated by RME instead of more invasive surgically assisted palatal expansion [25].

In this sense, Revelo and Fishman described the individual evaluation of the palatal suture using occlusal radiographs prior to RME. Their results showed that although there is an important correlation between the start of ossification and the maturational development, there is still a significant amount of variation in the progress of suture closure [19]. Angelieri et al. disagreed with the previous study and explained that occlusal radiographs are not reliable for evaluating the suture’s degree of ossification, since other anatomical structures like the vomer and external nose could overlap with the zone of interest and modify the image’s appearance. They developed a new method of classification for the individual evaluation of the palatal suture morphology using CBCT. The classification is formed of five stages: A—a relatively straight, high-density line, with no or very few interdigitations; B—an irregular-shaped, scalloped, high-density line to two very close parallel scalloped lines separated by some low-density spaces; C—two parallel scalloped, high-density lines separated by some small low-density spaces in the maxillary and also palatine bones; D—the fusion of the palatine bones that progresses from posterior to anterior, in conjunction with two parallel lines seen in the maxillary bones; E—the fusion of the mid-palatal suture both in the maxillary and palatine bones, with continuous bone density in all regions of the palate. The age–stage distribution presented a high variability [25].

An individual assessment using CBCT is one important finding and has the potential to prevent the side effects of rapid expansion in patients with an important degree of ossification and also to eliminate unnecessary surgical interventions like SARPE in patients where the suture is still wide open. In patients where the suture has more interdigitations, if classical RME is performed, there is a risk of side effects on the periodontal tissues [26]. The dento-alveolar complex moves as a consequence of difficulty in opening the mid-palatal suture, because the expansion of the bone cannot follow the speed of the screw opening [12].

These are the reasons why, in recent years, new methods of maxillary expansion were introduced, where skeletal support through plates or mini-implants became the only or the main anchorage. Using bone anchoring systems, the expansive forces directed to the palatal suture are maximized [10]. Some designs for these appliances are as follows:

• The bone-anchored maxillary expander (BAME), with two custom-made stainless steel implants, two mini screws and no direct contact with the teeth [13].
• The C-expander, a bone-borne system supported by four mini-implants connected to the expander screw by an acrylic resin layer [14].
• The Benefit system introduced by Dr. Benedict Wilmes et al. in 2008 (Benefit system, PSM Medical Solutions, Tuttlingen, Germany) that uses two mini-implants placed in the anterior zone of the palate behind the second palatal rugae and arms on each side soldered to the molar bands [27]. An example of a 3D simulation for the anterior positioning of the two mini-implants, performed for a patient from our clinic, can be seen in Figure 1.
• The MSE system designed by Dr. Won Moon (Biomaterials Korea, Seoul, Republic of Korea) that has a jackscrew component with four parallel holes for mini-implant insertion and supporting arms on both sides soldered to the molar bands, aiming to stabilize the device’s position during expansion. Regularly, the position of the appliance is between the two zygomatic-maxillary buttresses, frequently located at the level of the first molars, so the mini screws are located just anteriorly to the soft palate [10,28]. An example of the posterior positioning of the MSE jackscrew with four mini-implants performed for a patient in our clinic, can be seen in Figure 2.

A limited number of articles address the workflow of digital planning. This article presents the steps that are needed to find the optimal candidates for the expansion procedure using mini-implant-supported appliance and it also describes in a clear and easy to follow way the digital workflow used for the fabrication and insertion of a particular type of MARPE appliance, the MSE.

2. Materials and Methods

Each case evaluation starts with full records, consisting of intra- and extra-oral photographs, dental impressions (standard or digital models), panoramic X-ray and occasionally lateral cephalogram. In late adolescent and adult patients, where the maxilla is constricted and expansion is needed, a CBCT is also mandatory. In recent years, advances in imaging technology allowed the 3D high-quality representation of bone, dental structures and soft tissues.

2.1. MSE System Description

MSE, the abbreviation of Maxillary Skeletal Expander, is an appliance designed by Dr. Won Moon in 2004 and has been continuously evolving since. It is a tooth bone-borne appliance that consists of a jackscrew for expansion with four parallel holes that serve for four self-threading mini-implant insertions (Figure 3). The expansion size of the screw varies between 8 and 12 mm, the width of the mini-implants is 1.5 or 1.8 mm and their length is either 11 or 13 mm. On both sides of the screw there are supporting arms with a width of 1.2 mm soldered to the molar bands, usually placed on the first upper permanent molars, which aim to stabilize the device position and guide the movement during rapid expansion (the appliance also serves as a placement guide for the mini-implants) [10,28,29]. The position of the appliance is just anteriorly to the soft palate. The prerequisites for a predictable result are parallelism between the 4 mini-implants and between them and the mid-line of the maxilla (Figure 4); slight contact between the expansion screw and the palatal vault (less than 1 mm recommended, no more than 2 mm) [29]; the position of the screw between the first and second molars (in order to direct the force resulted from the expansion to the buttress bones), and bicortical engagement [10,28,29]. Kumar recommends the posterior placement of the device in order to obtain a more orthopedic effect, since in that area on the buccal side the pterygoid plates exist, limiting the dental movement [30]. Also, the maximum resistance to suture opening is between the maxillary bone and the pterygoid plates, so the force should be applied at this level to eliminate their initial resistance [29].

There are two types of MSE. The MSE-I is activated using a pin, with each turn of 90° measuring 0.2 mm. The MSE-II has a different type of activation, using a spanner, and each turn of 60° measures 0.133 mm [30].

2.2. Mandatory Dental Records—Using CBCT for Treatment Planning

There are some important clinical aspects that influence the proper treatment plan and sequence using palatal expanders: the depth and width of the palatal vault, dental health in the upper posterior sectors, the position of the sinus and nasal cavities, the root dimension of the upper incisors and other anatomical landmarks. When using an MSE appliance, the main factors that need to be evaluated are the palatal vault, because the system incorporates at least 4 mini screws placed in the posterior aspect of the palate, the bone width (the segment between the palate and nasal fossa) and the dental and periodontal health of the molars.

In order to establish the right position for the appliance placement, a maxillary CBCT must be obtained, in conjunction with digital models. The present technology uses three-dimensional images obtained from CBCT and digital models resulted from scanning. After the proper evaluation of the case and treatment plan consent, the CBCT is analyzed using special software, 3Shape Dental System 2021 in our case, that incorporates both the digital impression of the patient and the CBCT. Recent advancement in the field allows for a digital workflow in the process of dental appliance manufacturing. The CBCT DICOM of the maxilla or of the entire skull are converted into an .stl file. The intra-oral digital impressions or model scans and the CBCT stereo-lithographic files (.stl) can be superimposed using anatomical corresponding points. This process allows a three-dimensional and a more in-depth visualization of the clinical case. The anatomy of the teeth and bones, the length of the roots and the width and thickness of the palatal bones can be analyzed in detail.

2.3. Digital Workflow

Decisions on whether to use a traditional or a digital approach for a certain case must be made based on the analysis of the advantages and disadvantages of each technique. Related to expansion, the traditional manufacturing of the appliance uses the CBCT for evaluation, but the insertion of the mini-implants is freehanded, followed by an impression and laboratory device fabrication. On the other hand, if a digital workflow is used, the insertion of the mini-implants and the appliance positioning is easier and more predictable and the need for an extra appointment is eliminated.

Clinicians should consider the following patients as potential candidates for expansion with this type of mini-implanted-assisted palatal expander: adolescents or young adults with type A, B or C mid-palatal suture ossification or patients with a palatal vault wider than 19 mm (the jackscrew for MSE type 1 has 16.15 mm and for MSE type 2 18.15 mm). Also, the candidates should have a constricted maxilla and transversal inter-maxillary discrepancy. Both low- and high-angle cases can benefit from this procedure, since the buccal tipping of the molars is greatly reduced.

The appliance design is influenced by the transversal expansion needed. The CBCT images can be used to determine the dental and skeletal relationship between the two arches, measure the inter-molar and inter-premolar width both in the maxilla and mandible and choose the optimal expansion screw dimension. It varies between 8 and 12 mm. The virtual screw selected from a digital library (negative MSE template) can then be superimposed on the palatal vault and its fitting to the clinical situation can be evaluated. If the maxillary constriction is too big and the MSE screw cannot fit, then a new clinical decision must be made: either the expansion will be made using several successive MSE appliances, or a different treatment plan like a surgical approach will be followed. On the other hand, if the appropriate expansion screw can be applied, the next step will be its proper positioning. The general guidelines mentioned before are followed: the central position of the MSE, with the screw parallel to the mid-line of the maxilla, and also with the mini-implants parallel between them, its position as posterior as possible, just anterior to the soft palate and between the first and second upper molars, and also its vertical level, in slight contact (less than 1 mm) with the median raphe (Figure 5).

Figure 6 shows the possibility to virtually superimpose the mini-implants in order to plan and visualize their inclination and parallelism.

The CBCT DICOM files can be used to evaluate the qualitative and the quantitative aspects of the palatal bone. The bone should be composed not only of the cortical plates, but also of some trabecular bone between them. The density of the bone must ensure proper anchorage for the appliance and primary stability of the mini-implants. This CBCT evaluation will also influence the antero-posterior placement of the MSE screw (Figure 7), and also the length and the number of the mini-implants. Traditionally, the producer recommends the use of 11 mm long implants, but if the palate is very narrow, then 13 mm long mini-implants should be used. The latest laboratory printing procedures allow even the additional use of multiple mini-implants, in cases where the palatal bone is very thin.

Virtual mini-implant stl files also exist, in different lengths. They can be superimposed on several sections of the CBCT, in order to establish the best length for bicortical engagement. As can be seen in Figure 8, in this case, the shorter mini-implants would not ensure the penetration of both cortical plates, but the use of longer screws will satisfy this demand. The CBCT sections also allow the determination of the palatal bone width in the zone where each mini-implant is placed (Figure 9), and also the length of the tip of the screw that penetrates the nasal cavity (Figure 10). Their angulation is established, keeping in mind that the parallelism of the mini-implants is very important for the suture opening, since the force is distributed between them.

After the proper positioning of the jackscrew and mini-implants, the molar bands and the connecting arms are also digitally designed. The perfect fit of the appliance to the dental structure is mandatory, since the appliance will serve, after insertion, as a “surgical guide” for implant placement. Failure to sit the device in the same position as designed digitally will consequently alter the mini-implants’ position, angulation and insertion depth. A single stl file will be created, representing the negative template of the expander. The appliance is then reproduced using a 3D metal printer. Other types of expansion appliances with bone anchorage need a surgical guide for the insertion. The guide is 3D-printed and used for the placement of the mini-implants (Figure 11). The advantage of the MSE is that the appliance itself is used for the anchorage devices’ insertion. The successful opening of the mid-palatal suture is evaluated by the use of CBCT, where on the axial images between the mini-implants the clinician must see an enlarged suture, corresponding to type A ossification.

The exact customization of the MSE appliances for the clinical situation of the case study allows a more precise fit that ensures better patient comfort, more predictable treatment progress and a diminished number of possible complications.

3. Discussions

The MSE is a type of tooth bone-borne expander that uses four mini-implants for anchorage. Its design can be made digitally, using CBCT and scans of the upper arch, that are superimposed. Nowadays, multiple software solutions allow the planning of the expansion procedure in detail. First, for the position of the mini-implants, in the MSE case: four mini-implants can be digitally moved anteriorly or posteriorly, depending on the height of the bone. Also, the length of the mini screws is important, since depending on the width of the bone and the length of the screw, there are two types of mini-implant engagement: the monocortical type, where only the palatal floor is penetrated, and the bicortical type, where the nasal wall is also engaged. Although sometimes the first type is sufficient for opening the mid-palatal suture, the second type ensures more anchorage for difficult cases. Digital CBCT planning is very important for the proper assessment of the bone width, since the length and the position of the screws is decided based on this analysis. There is individual variability in the process of mid-palatal suture ossification. CBCT analysis allows the clinician to determine which patients, adolescents or young adults, can benefit from RME and which have to undergo the surgically assisted expansion [25]. When the estimation of bone maturation is attentively made, the success rate of the suture opening increases. Also, by studying the stage of ossification and the number of interdigitations along the suture [23], the practician can decide to use shorter or longer screws, meaning monocortical or bicortical engagement. When the digital workflow is used, this step is essential in the design of the expansion device. The stl files containing different lengths of mini screws can be superimposed onto the digital model in order to ensure that the mini-implants penetrate the nasal cavity.

Lee et al. analyzed the stress distribution and bone displacement in monocortical and bicortical anchorage during bone-borne palatal expansion. The stress around the implants was greater in the monocortical group. The transversal displacement was more important and more parallel in the bicortical group compared with the monocortical one [31]. As stated before, the process of ossification of the mid-palatal suture is faster in the oral cavity than at the nasal floor, and also in the posterior part than in the anterior part [22,23]. This is the reason why the position of the expansion appliance should be the most posterior aspect of the hard palate, just anterior to the soft palate. This area supports the demand of bicortical insertion for the mini screws, so that the nasal floor will also be included in the expansion process. The suitable bone thickness is 4–5 mm [31].

Palone et al. present the use of bicortical mini-implant engagement to achieve pure bone anchorage, especially when the screws are used for several purposes, not only for expansion. Also, the article emphasizes the importance of digital workflow in achieving bicortical anchorage [32]. Concerning the antero-posterior position of the screw and the length of the mini-implants, Gupta et al. performed a comparison between four groups and the results showed that the group with the jackscrew placed between the second premolar and first molar and also with mini-implants that perforated both the palatal and the nasal plate performed a better skeletal expansion [33].

Several case presentations confirm the orthopedic effect and minimal dental tipping when using tooth bone-borne expansion devices in young adults [34,35,36,37,38]. The use of MSE successfully attains a parallel skeletal expansion in adult patients, and some studies also report the widening of the nasal bone and the circum-maxillary sutures [13,33,34,39,40]. Lee et al. performed a study to evaluate the pterygopalatine suture opening after MSE expansion using pre and post CBCT comparison. The results showed a correlation between the bicortical engagement of the mini-implants and the pterygopalatine suture opening, so this type of anchorage is mandatory for important skeletal expansion and forward movement of the palato-maxillary complex [41]. Another study on 50 patients treated with the use of MSE also showed, after superimposition of the CBCT, pre and post expansion using OnDemand software that the expansion with this type of appliance is parallel and that the opening of the pterygopalatine suture was visible for almost every patient [42].

The latest technologies allow us to better inspect the clinical situation and to transmit it to the laboratory. Virtual models are easier to manipulate, since the practitioner and also the laboratory technician can measure and analyze them in three dimensions [1,2,3]. They are obtained using an easy and affordable procedure [3]. Using digital impressions allows for the design of sequential treatment appliances, for example, the expansion device and the indirect bonding tray for buccal or lingual braces or aligners [32,35,43]. The use of CBCT imaging enables a better 3D analysis compared with traditional 2D images. CBCT allows the acquisition of hard structures’ 3D images with minimal distortion. The doses of radiation are small and the main advantage is that they can be superimposed using corresponding points [44]. The ability to synchronize the previously mentioned records and incorporate them into the same software increases the accuracy of the diagnosis, treatment plan and appliance manufacturing [2,3,10,45]. Some examples of software solutions that allow for the virtual planning of orthodontic cases, in particular for an MSE appliance, are mentioned in

Table 1. Examples of software that can be used for MSE digital planning.

SOFTWARE
3SHAPE DENTAL SYSTEM
REAL GUIDE
DOLPHIN IMAGING
3D LEONE
ONYXCEPH
DELTAFACE
ONDEMAND 3D

.

When using CBCT to establish the appropriate appliance and mini-implant position, 3D-printed surgical guides can serve as an accurate transfer tool during the surgical procedure [3,46,47,48]. The placement of mini-implants in orthodontics by the use of CBCT became more common in recent years, with the main advantages of anatomical area inspection and implant insertion site selection [3,4,49]. The digital planning of the MSE is important because it allows the clinician to evaluate the ossification process of the mid-palatal suture and to choose the desired length of the mini-implants, so that they penetrate one or both cortical layers between the oral and the nasal cavity [50]. Using any kind of surgical guides for implant placement increases the chances of achieving an optimal position and, when more implants are involved, of obtaining parallelism and reduces the risk of accidents or failures. Abduo and Lau compared the accuracy of freehand insertion versus fully guided (the surgical guide controls the drilling, tapping and implant placement) and pilot guided (only the drilling is made through the surgical guide) dental implant placement. The results showed that the freehand and pilot-guided protocols had an inferior accuracy [46].

The traditional approach for MSE implants uses software to print the negative position of the MSE in a resin model. After that, the MSE jackscrew is manually inserted in the exact position as established digitally, and the technician continues to adapt bands and arms and weld them together. The clinician uses the body of the appliance, after intra-oral cementation, as an insertion guide [10]. The use of 3D printing using a wider range of materials such as stainless steel, titanium and other alloys allows the direct one-piece fabrication of the primary structure of the appliance, without jackscrews [3]. Graf describes a CAD-CAM design and the 3D printing of a tooth bone-borne expansion appliance. He uses the 3Shape program to draw the molar bands with a thickness of 0.7 mm and the connecting wires with 1.2 mm diameter. This primary structure is produced by a laser melting machine using a metal alloy. After that, the expansion screw is laser welded in the designed site [51]. New technologies even create CAD implant insertion sites using virtual bushings with a cylinder inside to serve as a guide for mini screw insertion. The entire structure is digitally designed and after that reproduced by a selective laser melting machine [50].

In some cases, the vault is very thin, especially in the posterior part. The screws of some appliances like the Benefit system are placed more anteriorly, where there is a sufficient quantity of bone, but the distance from the appliance is bigger, so the stress could be more important [52]. In these cases, the digital workflow allows the incorporation of additional mini-implants in the expansion system. Cantarella describes the digital planning and CAD-CAM fabrication of an MSE with six mini-implants instead of four in cases where the thickness of the palate is thinner than 2.5 mm. The additional mini screws were incorporated in the lateral walls of the palate. The structure was obtained using the selective laser melting technique [50]. Winsauer et al. designed a four to six mini-implants supported expansion device used in adult patients [53]. The digital planning of the expansion procedure offers many advantages, such as great accuracy for the entire process and also safety during the surgical process [50].

Wilmes and Drischer compared the traditional and the digital workflow for cases needing palatal mini-implants. Their results mention as advantages for the digital workflow the insertion of the anchorage devices and the appliance in the same appointment, lower costs for the patient and clinician, a customized and improved design, but higher costs for the laboratory [54]. Hsu et al. present an adult case where the position of the MSE was established using a digital workflow and transferred using perfectly fitted bands, chosen with the use of the software. Their conclusions emphasize that a completely digital workflow for the production and optimal positioning of an MSE appliance is crucial in adult cases [55].

The digital workflow for expansion and also for other procedures has amazing advantages for the dental laboratory and especially for clinical practice. The number of appointments needed for appliance delivery and treatment start is reduced to two. During the first appointment, full records are taken, comprising digital scans and maxillary CBCT. The technician–orthodontist team creates the appliance using dedicated software (CAD); the appliance is later either produced directly on a resin model that comprises the negative template of the MSE in the exact position as established digitally [10], or it is directly produced using additive manufacturing technology (CAM) and laser-welded to the expansion screw. During the second appointment, the appliance is cemented in the oral cavity and the mini-implants are placed through the appliance’s insertion sites, making it act as a surgical guide [50,51]. The patient can then start the activation protocol after specific instructions.

4. Conclusions

Technology is evolving with a rapidity never seen before, and there is no exception for the orthodontic field. Mini-implant-assisted palatal expansion in late adolescents and young adults is a new treatment option where the bone is used as the anchorage for expansion, minimizing the dental consequences. The evaluation of CBCT- and digital impression-integrated models allows clinicians to decide which patients can benefit from non-surgical treatment and, if that is the case, design the appliance digitally with an optimal number, length and position of mini-implants that can be later transferred intra-orally. Also, the digital workflow reduces the number of visits needed until the installation of the expansion device, making it more cost effective for patients and clinicians.