# Evaluation of HMDs by QFD for Augmented Reality Applications in the Maxillofacial Surgery Domain
## 4 Discussion  
The relative importance of device specifications is compared in Figure 8. From the radar, a comparison is possible to obtain two important pieces of information: (1) the oncological and reconstructive surgery is the most demanding scenario; (2) each specification's weight is, in broad terms, similar in the three scenarios. The resulting scores are shown in the QFD tables (Tables S2-S4). They confirm, as previously stated, that the oncological scenario is the most challenging. The complexity of Oncological and reconstructive surgery is suggested by a requirement list that is longer than other scenarios. The key factor is the necessity to manage face and leg operating sites at the same time, making the presence of two medical teams in the operating room. The amount of people needed for the surgery increases the weight of many requirements. Two teams, although well-coordinated, produce more noise, hindering the voice recognition features; the hand tracking feature must deal with more visual disturbances. In the same way characteristics that make easier to move while using the device, as being battery powered or being cordless, gain importance. Two operational sites mean the number of required 3D models available during the surgery is doubled. 3D model handling is directly proportional to computing power demand. The hybrid nature of the grafts, composed of bones and soft tissues to improve vascularisation and reconstruction, require more complex modelling, when compared to the other scenarios, therefore increasing the computing power required. The orthognathic and trauma scenarios may be generally described as a scaled version of oncological surgery. Figure 8: Specification relative importance comparison for the three analysed scenarios. The specifications of the computing power group, with the resolution and the camera, are the three most important features. This fact can be explained considering how, put simply and regardless of the considered scenario, the job is to superimpose grafts 3D models on a real body. The joint work group of physicians and engineers has identified a further requirement: the possibility to track the patients using non-visual landmarks. Maxillofacial surgeries are not \"clean procedures\" where blood may make the landmarks necessary for tracking dirty, hindering the correct registration of the virtual elements onto the real scene. The evaluation charts (Figures 3, 5 and 7) show, using the standardised scores, the devices' performances for each specification groups. Table 7 aggregates the macro-specification scores into one score, which is useful to describe the devices overall performance for each scenario. Some groups of specifications (audio, files format, connectivity, and user inputs) are less demanding and so are easily compelled by most of the devices. The computing power and the display groups are very important specifications, and the devices have varied performance levels. The lower score of the XR-3(tm) device is down to the computing power, because it has a lack of on-board hardware (all elaborations are entrusted to the desktop PC connected by cables); this characteristic is a flaw for the scenarios object of this article. The power group has some scores fluctuations caused by the battery duration (with the exclusion of the XR-3(tm), which is not powered by a battery). The general features group are miscellaneous that comprehend many specifications; modularity is achieved only by the M4000 tm, which has the possibility to equip external batteries to extend the autonomy; the cordless specification is missed only by the XR-3(tm) device; mass, external dimensions, and wearability are mainly influenced by the form factor and the purpose of the device. The analysis highlights the sensors group as a weakness: Depth cameras and ambient light sensor are not always available while the cameras have an ample range of performances in terms of resolution.