## 2 New Methodologies and Instruments
This planning phase is not an operational phase and is highly time consuming (preparatory time), but is needed for the achievement of excellent results for the intervention [12; 14]. The protocol developed by [4] represents a milestone for computer-assisted surgical simulation and ref. [16] stated that, in the near future, the VSP will completely replace the CMS approach for the bi-maximillary surgery. Due to the need of high technical skill, a correct VSP procedure in orthognathic surgery can be planned only by surgeons and engineers working in teams. In ref. [4], the procedure was deployed in four main steps: collection of preoperative records, data processing, surgical planning, and preparation for the surgery. Other procedures can be found in the literature, but each of them starts with the acquisition of the information; in many articles this step is closely connected with the virtual reconstruction of the head and the definition of a reference system for referencing the final occlusion parameters in the virtual environment. Some authors [17] focus on the creation of a \"composite skull model\" and on its orientation according to the natural head position and also focus on the quantification and analysis of deformities. Other authors [18] itemized the first steps in terms of 3D image acquisition, creation of a skull-dental composite model, setting up of virtual facial references, and diagnosis. In some literature [19; 20], the use of 3D cephalometry to quantify and qualify skull deformities is reported. Ref. [21] showed how the effectiveness of a 3D virtual treatment planning of orthognathic surgery was linked to a 3D creation of physical devices like splints, guides, or templates for managing the surgery. Furthermore, other researchers [15] defined a general workflow for CAS application and distinguished four steps: data acquisition, planning, surgery, and assessment. The procedure starts with a segmentation based on image pixel threshold; the segmentation is the preliminary step before the definition and manipulation of the 3D bone model, which is one of the aims of the surgical planning [22]. This segmentation can be performed in a 2D (simulated 3D) environment and in a 3D environment; the latter can be realized through an immersive responsive virtual reality (VR) workbench and can provide \"real\" feedback feelings for surgical planning and simulation, but requires extremely expensive VR devices and a high-end graphics workstation, which is not yet possible for hospital settings [22]. So, the most common devices used for performing the segmentation of images and the creation of 3D models are the 2D environment applications. Nevertheless, these, thanks to advances in digital technology, can reach a high level of precision and accuracy [10]. In the literature [23], a general methodology for a VSP/CAD/CAM system use was presented; it required the following steps: (1) data acquisition, (2) medical image analysis, (3) 3D anthropometric analysis, (4) surgical simulation, (5) implant/template design via CAD software, (6) implant/template fabrication via RP, (7) an on-line communication tool, and (8) a management system. Following this procedure, the surgeon can compare and optimize surgical options and the patient has the opportunity to visualize the complexity of achieving the desired result. Furthermore, the innovation brought in technologies for medical fields like CAD/CAM, Reverse Engineering, CT, MRI and Additive Manufacturing and allowed the availability models of parts of human anatomy and the production of personalized surgical equipment [24]. A new era of collaboration among surgeons and engineers has begun [25] and is now bringing several benefits in personalized surgery like the bi-maxilla-facial one. References [26; 27] showed how the CAD/CAM approach, supported by engineers, helps to introduce personalized devices to overcome the disadvantages in the use of standard plates, screws, and splints, especially in controlling the vertical position of the maxilla.