## 1 Introduction  
The use of three-dimensional (3D) data is leading to broader possibilities for daily clinical practice: digital planning may enhance diagnostic accuracy and treatment customization [1,2]. Even if 2D radiographic records such as lateral cephalospor and photographs have been routinely used in orthodontic practice, they present some limitations, as they cannot represent the depth of anatomical structures [3,4]. In orthodontics, 3D imaging can help to plan tooth movement with respect to the thickness and morphology of the alveolar bone envelop, to the roots of the contiguous teeth and their periodontal status [5]. The use of cone-beam computed tomography (CBCT) is justified in every clinical situation where more information about the spatial relationship of anatomical structures is needed: ectopic [6], impacted and supernumerary teeth [7,8], root resorption [9], cleft lip and palate [10, 11, 12], alveolar boundary conditions, placement of temporary anchorage devices [13], maxillary transverse deficiency [14], airway analyses [15], temporomandibular disorders [16] and orthognathic and cranio-facial surgery [17]. Thanks to the intraoral scanners (IOS), it is possible to have an immediate and accurate digital model of the dental arches. The advantages of a digital impression involve the absence of dimensional changes of the impression material, the efficiency of manipulation, sharing and storage as well as the possibility to realize customized virtual set-ups and CAD-CAM appliances or surgical splints for orthognathic surgery patients. As a limitation, some studies have shown that trueness and precision of intraoral scans may be controversial for full-arch digital impressions, as some imprecision may occur in not spatially defined areas. On the other hand, dental laboratory scanners (DLS) are used to digitize impressions or casts; however, any dimensional change of the impression material or plaster cast will be reproduced on three-dimensional dataset [18]. Recently, facial scanners have been introduced in the market with the purpose of acquiring the 3D image of the patients' face--this can be useful in aesthetic dentistry for planning the patient's smile [19]. The integration of 3D data from different anatomical structures (skeleton, dentition and facial soft tissues) allows to obtain a 3D virtual simulation of a patient, as well as to simulate the treatment outcome into the patients' faces, thus leading to a better communication with colleagues, technicians and patients [1,20] (Figure 1).  
Although 3D data from skeleton, dentition and facial soft tissues are currently available, it is still difficult to merge different data formats in a single planning program--the DICOM (digital imaging and communications in medicine) format for skeletal scaffold, the STL (standard tessellation language) format or.PLY (polygon file) format for dentition and.OBJ (object code) format for facial soft tissues [21]. The clinicians who approach virtual planning require an accurate, reliable and easy-to-use method for  
Figure 1: The three-dimensional workflow (from left to right): 3D image of the face, intraoral impression of the dentition and cone-beam computed tomography (CBCT) scan of the skeleton.  
the integration of 3D datasets available in different formats [4]. Despite many procedures that have been described in the recent years, there is still a lack of consensus about a gold standard protocol.  
The aim of this systematic review is to summarize the current knowledge about the superimposition methods of three-dimensional diagnostic records and to analyze their declared accuracy.