## 3 Materials and Methods  
### Materials  
The device selected to implement the MR application of this study correctly is the HMD HL2, the second version of the Microsoft device. This device is a stand-alone holographic computer composed of a pair of see-through transparent lenses (also called waveguides) with a holographic resolution of 2 k 3:2 light engine and holographic density major of 2.5 k light points per radiant. The waveguides are flat optical fibers in which the light can be projected by the specific projectors in each lens. The light bounces between the interior surfaces of the display to be directly sent to the user's pupils to display the holograms directly in front of the user's eyes. HL2 is also equipped with an IR camera for eye tracking, RGB cameras, a Depth camera, and an IMU sensor that includes an accelerometer, gyroscope, and magnetometer. The device has a resolution of 2048 \(\times\) 1080 for each eye and an FOV of 52 degrees (information on the Microsoft official site). A high-performance computer with appropriate technical characteristics that allowed programming in MR was used to implement the application and to manage the distribution process on the device. Azure Kinect is the new version of the camera system developed by Microsoft. The Azure Kinect has an RGB (red, green, and blue) camera, a depth camera, IR emitters, and IMU sensors LSM6 DSMUS (gyroscope and accelerometer) are simultaneously sampled at 1.6 kHz. The samples are reported to the host at 208 Hz. Therefore, due to the presence of IMU sensors, it can measure and track the entire body in real-time and estimate 3 coordinates of every major joint of the human body in 3 planes without requiring any marker or other supplemental equipment (information available from the official site). To implement the 3D models of anatomical districts, DICOM computed tomography (CT) scans were used. The scans were achieved from different free and open-access databases, in particular:  
* Tibia and Fibula scans are from a subject in the National Cancer Institute's Clinical Proteomic Tumor Analysis Consortium Sarcomas (CPTAC-SAR) cohort;
* Femur scans are from the Cancer Imaging Archive;
* Humerus scans are from the image datasets of the Laboratory of Human Anatomy and Embryology, University of Brussels (ULB), Belgium. Humerus scans were acquired at 120 kVp, exposure of 200 mAs, and an X-ray current of 200 mA. Ulna and radius scans were acquired using 130 kVp, time of exposure of 1000 ms, current of X-ray of 70 mA, and a generator power of 10 kW. Femur scans were acquired at 80 kVp, an X-ray current of 20 mA, and a generator power of 1600 kW. Table 1 reports other key information about the CT scans grouped by the anatomical areas. 3D Slicer version 4.11.20210226, a free and open-source software for clinical and biomedical research applications, was used to develop the corresponding 3D models of the bones through segmenting the CT Scans. Subsequently, the 3D models are used in the MR application. Blender is a free and open-source software for 3D manipulation, and it was used to convert the model into a readable format with Unity 3D. Unity 3D version 2020.3.30f1 is a game engine that was combined with the Mixed Reality Toolkit package (vs. 2.7.0) (MRTK) to develop the MR application. The MRTK contains a set of basic features that added to a Unity project can implement MR behavior. Visual Studio 2019 is a free integrated development environment (IDE) that allows C# scripts to be written.