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