3D technology to measure dental arches and create a template for lingual brackets technique

ABSTRACT Objective: This study aims at identifying anatomical dimensions of dental arches, based on landmarks currently used in the lingual orthodontic technique, and create an archwire form template to be used in orthodontic clinics. Methods: Maxillary and mandibular dental casts of 140 Caucasian individuals with natural and normal occlusion were digitized (3D), and the images were analyzed with Delcam Power ShapeTM 2010 software. The dental arch shapes and sizes were obtained from 14 landmarks selected on the lingual surface of the teeth. Points and segments defined by the software were used to create an archwire form template. Results: Various dental arch patterns were found for both maxilla and mandible. The smallest sizes were found in females, and the largest were found in male subjects. Six categories were defined for each gender, three for the maxilla and three for the mandible (Small, Medium and Large). A template was created with eighteen anatomic lingual archwire designs, nine for the maxilla and nine for the mandible, for both genders. Conclusions: Landmarks evaluated in this study showed dental arch differences between genders. This information enables making orthodontic lingual archwires that are more compatible with the anatomical forms and sizes of the maxilla and mandible. The findings also allowed the creation of a template for an anatomic lingual metallic archwire form to be used in the lingual technique.


INTRODUCTION
In the late 1970s, lingual orthodontics was introduced as a result of conventional appliances bonding to the lingual surfaces of the teeth. 1,2 The first scientific work describing brackets and the mushroom-shape of the lingual archwire was published in 1979. 3 Striking differences between the lingual and buccal techniques are observed, 4 such as the archwire form used. 5 However, few studies [6][7][8][9] have been published attempting to determine the dental arch form in the lingual technique.
Many authors have used cusp tips to outline the archwire forms, 10,11,12 while others used the medial landmarks of the crowns from a buccal perspective on the anterior and posterior teeth as references, 13 as well as lingual and occlusal landmarks, or in the long axial axis of the teeth. 7 Moreover, others researchers 14,15 used landmarks on the lingual surfaces closer to the gingival third because this site showed the smallest difference between the lingual surfaces of canines and premolars. 16 Despite the fact that there are several ways to define dental arch forms, in the lingual technique, the inter-canine distances vary substantially, 16 making it difficult to determine how many sizes of mushroom-shaped lingual archwires might exist. 3 Therefore, some authors developed the straight-wire concept in lingual orthodontics, seeking to streamline the work of the professional. 16,17 They also proposed 18 that brackets should be bonded with auxiliary blades in order to enable the use of archwires without curvatures, whereas other authors 14,15 devised a more square-shaped archwire, allowing use of a lingual straight archwire.
A previous study analyzing the shapes and dimensions of dental arches in digital 3D models for the use in lingual straightwire technique showed that more cervical archwire setting promotes smaller inter-bracket distance. 15 Although this strategy allowed reduction of the typical insets and offsets of the dental arch lingual surfaces, it hampers orthodontic mechanics in certain movements 19,20 and may cause gingival inflammation. 21 Furthermore, a specific assembly with resin pads is required to compensate the distance between the lingual surface and the base of the bracket. 4,5 In areas where compensations are made, the brackets advance more into the space occupied by the tongue, resulting in patient discomfort. 21 Furthermore, low bracket profile enables less invasion to the lingual space, therefore providing better adaptation for the patient in terms of speech and comfort. 22 In this context, the present study evaluated the dental arch shapes and sizes that are formed when the brackets are placed farther from the cervical margin of the teeth, 15 in region that keep the concave and convex form in the lingual surface, but more distant from the cervical area, to avoid gingival inflammation.
Kairalla SA, Cappellette Jr M, Velasco L, Ferreira LS, Pignatari SSN -3D technology to measure dental arches and create a template for lingual brackets technique 6 The forms and sizes of archwires for the lingual technique were also defined, and a template was created. It is believed that there is a difference in dental arch forms between genders.
Orthodontists should benefit from different sizes and shapes of archwires to perform treatments, and not be limited to a number of prefabricated archwires that are usually dictated by wire and bracket manufacturers. Using the template, the professional may choose the appropriate archwire form. Kairalla SA, Cappellette Jr M, Velasco L, Ferreira LS, Pignatari SSN -3D technology to measure dental arches and create a template for lingual brackets technique 7 angulation and inclination of the crowns (considering the long axis of the teeth) and the flat curve of spee. 23 Rotations of up to 3 degrees and diastema up to 0.5 mm were accepted.

MATERIAL AND METHODS
Sample exclusion criteria were: odontogenic abnormalities, incomplete dental eruption, and presence of erupted third molars. Kairalla SA, Cappellette Jr M, Velasco L, Ferreira LS, Pignatari SSN -3D technology to measure dental arches and create a template for lingual brackets technique 9 The model was rotated on the computer screen in a way that the lingual surfaces could be seen with a frontal view and the operator could define and locate where the marked landmarks should be placed.
By means of the X, Y, and Z coordinates, two planes were established. The X and Y axes established the horizontal plane, whereas the Y and Z axes established the vertical plane, corresponding to the median sagittal plane that passes between the central incisors and divides the model into two halves, left and right (Fig 1).
The fourteen united landmarks defined the curvature and shape of the dental lingual archwire 11 using Delcam Power SHAPE™ 2010 tools (Fig 2).
The previously chosen points were plotted in the XY plane, such that the Z axis was reset, and it was possible to obtain the XY coordinates of each landmark (Fig 2).   Kairalla SA, Cappellette Jr M, Velasco L, Ferreira LS, Pignatari SSN -3D technology to measure dental arches and create a template for lingual brackets technique these points were united two-by-two, forming small straight line segments. Previously defined points were also used on the lingual surface of the 3D model of the first and second left and right molars, and the first and second left and right premolars, which were joined two-by-two, respectively, forming small straight line segments (Fig 3).    These data can be observed in Tables 1 and 2, and the illustration is represented by Figures 6 and 7.
After completing all these steps, 0.5 mm was deducted to compensate for the base of the lingual bracket, and the final result with the template of anatomical lingual archwires can be seen in Figure 8.    showed that the measurements varied from 0.98 to 1.00, and the Bland-Altman plot showed that the largest mean variation was 0.08 mm (-0.06 to 0.23), which occurred in the measurement of the second left molars.    Table 3 shows the comparison between the means of vertical and horizontal linear measurements between the genders.
In the maxilla, we observed differences between the genders in molars, premolars and canines, whereas in the mandible, differences were observed only in the measurements of the molars and premolars.
To define the measures of the average arch, the mean was used: the 25th percentile (P25%) was used for the small arch and the 75th percentile (P75%) was used for the large arch for both males (Table 4) and females (Table 5).

DISCUSSION
This study aimed at identifying the shapes and sizes of lingual dental arches from digitized models, as well as the probable difference between genders, from the point of view of the lingual orthodontic technique. It was found that there was a difference between the lingual sizes of the maxillary and mandibular arches, which allowed to define six sizes of lingual dental arches for the maxilla and six for mandible. Three of those for females and three for males, described as S, M and L.
Another purpose was to create arch templates to assist orthodontists in bending archwires themselves and not to depend on prefabricated archwires that might not meet their needs in daily practice. It was possible to create a template composed of eighteen archwire sizes, nine for the maxilla and nine for the mandible.
3D images digitized from cast models were used, in agreement with other authors, 15,24-29 because they allow simultaneous visualization in three dimensions (horizontal, sagittal and vertical).
3D technology is currently used in several areas of Dentistry, 30,31 allowing professionals to develop studies using the same sample. Since it can be inserted in any computer compatible with the program that was used, it is maintained over time and does not occupy a physical space, in addition to the great accuracy given by the software that is needed when using several pieces of numerical data. To guarantee that the measurements remained proportional, regardless of the position of the digital models, the X, Y and Z axes were used in this study, as well as some previous studies, 15,24 allowing the reference points to be positioned in the three dimensions. However, some other studies 13,14 used only two (X and Y) coordinates, not allowing the models to be moved due to the lack of a third axis, since the Z axis does not exist in 2D models.
Fourteen points were chosen on the lingual surfaces because they would represent the place where the brackets would be placed on the lingual surfaces of the teeth, and where the orthodontic wires or metallic archwires pass into the slot of these brackets. The height chosen for these points took into consideration the most concave and convex parts of the lingual surfaces, to represent the anatomical shape of the dental arches as much as possible. From these points, the measurements were obtained to define the shapes and sizes of the arches.
Kairalla SA, Cappellette Jr M, Velasco L, Ferreira LS, Pignatari SSN -3D technology to measure dental arches and create a template for lingual brackets technique 23 To accomplish this, several authors 13,14,27,33 used polynomial functions; others chose linear measurements. 12,15 In this study, fourteen linear measurements (ten horizontal and four vertical) were used to define the shapes and sizes of the dental arches, similar to an earlier study, 15 while others used four linear measurements 12 (two horizontal and two vertical). Few studies combined the two forms together, that is, polynomial functions and linear measurements: ten linear measurements (five horizontal and five vertical), 7 six horizontal linear measurements, 10 and six linear measurements (three horizontal and three vertical). 34 The linear measurements of this study were automatically defined by Delcam Power SHAPE™ 2010 software and therefore they were obtained with great precision. With this method, using methodology similar to an earlier study, 15 we noticed a difference between the measurements when points were used more at the center of the clinical crown than when these points were positioned more in the cervical region of the clinical crown. Therefore, as shown in From the measurements obtained, three sizes of dental arches were identified: S, M, and L. In another study, 14 median measurements were applied, unlike the averages found in this study and those of other authors. 15 In this way, the mean, and not the median, was used to obtain the final measurements, because mean measurements were more accurate than the medians.
In the mandible, the shape of the dental arch resembles a parabola, with a more rounded form in the anterior portion, and the posterior side is more like a straight line, with a slight deviation in the region of the premolars and molars. In the maxilla, the shape of the dental arch also resembles a parabola, with the rounded anterior portion with more pronounced curves in the canine region, and a segment of a line, with deviations, in the region of the premolars and molars (Figs 9 and 10). In other studies, 14 that were selected. In our study, those points were placed more occlusally, to allow a better adaptation to the anatomical form of the dental arch, compared to previous studies in which the placement of the points was more cervical.
As stated in a previous study, 36 to perform orthodontic treatment orthodontist should have an understanding about the shape of the dental arch. It has been observed for years that many professionals sought to find a method to reliably copy the shape of the dental arch and apply it for orthodontic treatment.
For this purpose, the first diagram in orthodontics was created by the millimeter paper method. 37 Over the years, other diagrams have emerged, 38 Some studies 42,43 have shown that professionals have a concern regarding maintenance of the dental arch shape.
Perhaps because the prefabricated archwires mostly do not correspond to the size and shape of the arches in a normal occlusion. 26 Nevertheless, it should be noted that pre- Kairalla SA, Cappellette Jr M, Velasco L, Ferreira LS, Pignatari SSN -3D technology to measure dental arches and create a template for lingual brackets technique 27 cannot substantially alter the shape of the dental arch; 25 however, they assist in the initial stages of treatment and are necessary for current orthodontics. 26 In lingual orthodontics, diagrams were proposed from photocopied models, 6 and others through computerized programs, 9,14,15,44 to allow the manufacture of individualized orthodontic archwires, because in lingual orthodontics the coordination of the archwires is a difficult or almost impossible task.
Therefore, considering that dental human arches are asymmetrical and that this characteristic is more a rule than an exception, 45  It is known that there are differences between one archwire type and another, and it is important to remember that in clinical practice, the mechanical aspect must also be considered.
Therefore, authors who compared in vitro the two types of mushroom and straight archwire observed that the advantages and disadvantages of some orthodontic movements varied for each archwire according to the treatment phase. 19,20 We believe that more studies on archwire shape should be performed (in vitro and in vivo) because it is still not possible to state which archwire shapes -mushroom, straight wire, Christmas tree or anatomical (the forms found in this study)-will be more suitable for treatment with the lingual technique. Therefore, the professional will have the opportunity to choose the shape of the archwire that better suits