QUANTIFICATION OF DENTAL MOVEMENTS IN ORTHODONTIC FOLLOW-UP: A NOVEL APPROACH BASED ON REGISTRATION OF 3D MODELS OF DENTAL CASTS
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Figure 1. 3D model of a dental cast and selection of area including palatal rugae.
Figure 3. Example of chromatic map according to point-to-point distance between the two 3D models: blue areas are more vestibularized, whereas red and yellow areas are less vestibularized in the last cast than in the first one.
dental displacement due to orthodontic devices for verifying the success of therapy and provide adequate corrections. Most of these procedures are based on X-ray examinations( OPG or cephalometric radiographs. 1-3 In the last years modern 3D image acquisition systems, already applied to the study of facial modifications caused by dental displacement, 4 have been used also to detect the characteristics of dental movements. The main advantage concerns the chance of performing a three-dimensional assessment of dental morphology, more informative than the traditional radiological methods which are mainly based on the analysis of displacement of single landmarks or dental two-dimensional profile. In addition metrical measurements taken on digital models have been widely tested and proved to be reliable, with a high concordance with measurements taken directly on the plaster models. 5-7 However, surprisingly very few studies have tested 3D image acquisition systems so far for assessment of orthodontic therapies: an example was provided by Thiruvenkatachari et al. who first developed a protocol for 3D-3D superimposition of three-dimensional models of dental casts acquired through a laser scanner. 8
Figure 2. Example of registration according to area of palatal rugae: in white the earlier cast, in light blue the more recent one.
This procedure is based on the registration of 3D models of casts performed at different times and registration of surfaces according to the least point-to-point distance between the respective surfaces including palatal rugae. Displacement of molars was then assessed according to possible movements of their center of mass as calculated by the software. 8 The authors state that the use of laser scanner provides accurate and reliable measurements of dental displacement and might be a valid alternative to cephalometric radiographs. 8 However, the potential advantages which may derive from 3D-3D superimposition techniques have still to be explored: for example, the previously cited study took only the center of mass of the tooth into consideration, whereas techniques of registration may give information on the displacement of the entire surface of the dental crown, with a more anatomically adherent evaluation of dental arch modifications. In addition, the assessment of the translation of dental center of mass does not take into consideration the entire range of movements which may affect the 3D dental surface, such as rotation. A possible alternative is given by modern 3D image elaboration software, which are able to provide the registration of 3D surfaces and metrical parameters useful to assess the discordance between two models, expressed in terms of point-to-point mean and RMS( Root Mean Square) distance. In addition, the same procedure can generate a chromatic sheet able to immediately highlight areas affected by possible modifications. 9, 10 These procedures may represent a novel point of view for the assessment of dental movements in orthodontic procedures. The present article aims at exposing a protocol for 3D-3D registration of three-dimensional models of dental casts for the quantification of dental displacements, based on the calculation of pointto-point distance between two surfaces. This may provide additional information for evaluating the success of therapy and orthodontic procedures.
2. Materials and methods Casts of the upper dental were chosen from three patients aged from 10 to 15 years who underwent
56 Stoma Edu J. 2017; 4( 1): 55-61. http:// www. stomaeduj. com