# Evaluation of HMDs by QFD for Augmented Reality Applications in the Maxillofacial Surgery Domain
## 3 Results
### Scenario n. 2--Orthognathic Surgery  
This application usually only involves one medical team. There are fewer sources of noise and visual disturbances; therefore, voice robustness and hand tracking robustness have a lower importance compared to the oncological scenario. Orthognathic surgery uses fewer standard tools than the previous scenario and so requires fewer 3D models. The complete QFD table has been supplied in the Supplementary Material (Table S3). The explanation of qualitative requirements is as follows:  
* Simple and smart user interface: smooth navigation through menus and an intuitive layout for tools and information. * Custom user interface: custom interface layout for user comfort. * Lightweight and comfortable: the device must not hinder or weary the surgeon. * Ample and unobstructed field of view: the device must not hinder the surgeon's field of view. The AR is an addition to, and not a substitution for, normal view. * Real-world colour fidelity: the device must not alter the aspect of real objects. * User feedback: using visual (colour maps, vector maps, etc.) or audio message for real-time surgical assistance. * Robust voice control: the number of people in the operating room should not condition the voice command's functionality. * Robust to occlusions: the device must keep track of operation site, surgical tools, and surgeon's hand independently from any visual hindrance. * Robust to contrast: the device must not be influenced by any light sources (e.g., surgical lights, headlights) present in the operating room. * Robust face tracking: the device must never lose the operation site tracking. The face is a sensitive site in case of head repositioning due to the number of joints in the head/neck area. * Display and navigate face DICOM: the face is important as the site of the implant; therefore, all available medical data are necessary to be displayed at the surgeon's request. * Display and handle 3D models of bones: it is fundamental that bone resection models are displayed. * Display and handle standard tools 3D models (distractors, locators): a library of standard tools used during operations must be managed. * Display and handle non-standard tools 3D models (guides, splints, plates): tailored surgical tools must be easily managed. * Display osteotomy cut lines: virtual guides to assist the surgeon to identify the position and orientation of the resection planes. * Choice for every 3D model's display style: solid, wireframe or hidden. Many objects are visualised in mixed reality; choosing the best display style for each one is, therefore, necessary. * Share live images on other devices: viewing the operation site from the user's point-of-view assists teamwork and is useful as a means of training. * Record images for training and evaluation: recorded images from the surgeon's point-of-view are useful for training and postoperative checks. * Device autonomy must be adequate for the required task: in battery-powered devices, the autonomy must be adequate to last the duration of the operation. * Cordless device: many people and devices are present in the operating room; therefore, easy-to-manoeuvre solutions are preferable. * Optical zoom: some structures and vessels are too small to be seen by the human eye, while a digital zoom may alter the images. * Add-on accessories: a modular device allows the device to be customized. Specification relative importance is shown in Figure 4. Robustness requirements are marginal compared to the previous scenario, while computing power, resolution, camera, and hand tracking features are still important but with a lower magnitude. In general terms, the area delimited by the radar is smaller than the previous scenario. The following chart (Figure 5) shows which devices are better to fulfil the requirements of the orthognathic scenario. The best-case device is represented as a white background element to improve the contrast with the market devices coloured lines; this makes it easier to identify the best-performing devices.