## 3 Case Study  
The method presented in Section 2 has been used to develop a Mixed Reality-based application to support surgeons during mandibular reconstruction with fibula bone graft by increasing the accuracy of surgical guide positioning. In this Section, we describe the developed application in detail.  
To obtain the 3D models of the fibula and the surgical guide, CT scanning of the lower leg should be performed first in order to obtain DICOM images.  
Then, using image segmentation software, the fibula is identified and exported to Blender, an open-source 3D computer graphics software, to realize the 3D virtual model used to identify the correct position of the surgical guide. The long axis of the fibula should be aligned with the y-axis, with the backside facing the positive direction of the y-axis. In addition to the fibula, a 3D model of a surgical guide is produced for the fabrication of fibula flaps. Moreover, the guide is positioned with the long axis aligned with the y-axis. These virtual models are then customized to fit their features for our purpose, modifying the material of objects. In particular, different colors are assigned to different parts of the fibula and the surgical guide.  
These colors are helpful to distinguish the fibula and the guide from other objects and to handle illumination changes. A 3D printer is used to turn the 3D CAD models into real synthetic objects. In this way, the presence of the patient is not necessary for the development of the application.  
As a tracking system, the MR application uses Vitoria Engine, already available on the market. It enables reliable tracking and performance on a variety of hardware (including mobile devices and HMDs). Vitoria permits to position and orient virtual objects, such as 3D models and other media, to the real world when they are framed through the camera of a mobile device [32].  
It uses **Model Targets** to recognize and track particular objects in the real world based on the shape of the object. Starting from the 3D models, the Model Targets of the fibula and the guide are configured using the Model Target Generator (MTG) (Figure 4).  
From a software perspective, a Unity3D project was created. It leverages the Mixed Reality Toolkit for interfacing with the HMDs and accessing its main capabilities such as spatial anchoring and advanced interaction methods between users and the virtual content, i.e.: head movements, gestures, and voice commands.  
Figure 4: Model Target of surgical guide and fibula.  
For our purpose, the Microsoft HoloLens 2 headset was used as an MR display system. It can map the users' real environment to place virtual 3D elements in a relative position to real objects. HoloLens 2 contains the sensors, processors and hardware, including four cameras used to scan the environment, collect data and measure distance and spatial depth [33]. The displayed images are created by the reflection of two high-definition light engines on each of the user's retinas. Gaze commands and head-tracking allow the user to bring the application's attention to what the user is perceiving [34]. Therefore, being an all-in-one mobile machine, it is easily portable, hence usable both in the operating room and in wards, depending on the patient's needs.  
A feedback window has been implemented to visualize and register data giving feedback to the user (In the upper part of Figure 5). The window is a User experience (UX) control that contains a button and an input field to save the results concerning the overlap between models; and UI text which informs the user about the tracking status of the fibula and the surgical guide and provides quantitative measures of the overlap in real time.  
It follows the user's head in order to remain in the user's visual field to be easily accessible anytime.