## 4 Results and Discussion  
This section reports on the results of the study conducted. Fifteen healthy subjects were recruited, five males and ten females with a mean age of \(29.3\pm 8.4\) (mean \(\pm\) standard deviation). The subjects had different backgrounds: some were computer engineers and some were biomedical. However, all subjects had some experience with HoloLens and basic knowledge about Mixed Reality. None of them reported neurological, psychiatric, or other brain-related diseases. All volunteers were informed about the experimental procedure and written consent was obtained from all participants. Firstly, the accuracy of the positioning of the guide on the fibula was studied. In order to evaluate the progress of surgical guide placement over time, distance and rotation data from all subjects were averaged (Figure 6). To highlight the trend of the data, in both cases a regression model with a polynomial curve of order 2 was used. The R-Squared (R2) value was calculated to evaluate how well the data approximated the curve. R2 can assume any value between 0 to 1:0 represents a model that does not account for any of the changes in the variable. One represents a model that is able to represent all the variations of the variable. In this case, R2 has a value of 0.98 for the distance and 0.99 for the rotation, indicating a better fit for the regression model. In both distance and rotation, the graphs show a trend that decreases over time. Moreover, in order to obtain the error, the difference between the last value obtained in the averaged session and the reference value (zero) was calculated. We obtained a distance error of 6 mm and a rotation error of 5.25 degrees. Furthermore, since the curve is almost regular, it can be concluded that the application is quite stable. In addition, a questionnaire was administered to assess the usefulness of the MR application. Questionnaires are an inexpensive and very effective tool to obtain a quantitative measure of user experience. The User Experience Questionnaire (UEQ) is a widely used evaluation tool for interactive products. The questionnaire is composed of 26 items. Each element is represented by two terms with opposite meanings. The order of the terms is randomized: half of the items on a scale begin with the positive term and the other half begin with the negative term. Each element can be rated on a 7-point Likert scale. According to [35], the Likert scale is a psychometric scale frequently used in research to represent people's opinions on a topic or subject. It consists of seven different response options from 1 to 7 used to assess the degree to which a user agrees or disagrees with a statement. The UEQ is composed of six scales: Attractiveness, Perspicuity, Efficiency, Dependability, Stimulation and Novelty. The attractiveness is the overall impression of the application,  
Figure 6: Experimental mean distance and rotation values. The polynomial curve of order 2 is overlapped showing a decreasing trend. the perspicuity is a measure of how easy the application is to learn to use. The efficiency expresses the extent to which the application can be used by the user without effort; dependability assesses how much the user feels they can control the application. Stimulation and novelty allow us to understand how stimulating and innovative the product is for the user. Usually, 3-5 min are sufficient for a participant to read the instructions and complete the questionnaire. The UEQ was provided in English, after verifying that all subjects were able to understand the language. The results obtained were analyzed. An MS Excel file was created to visualize the results and calculate some basic statistical indicators necessary for an interpretation of the data. Firstly, the data were converted by assigning the value +3 to the most positive evaluation and \(-3\) to the most negative one. The answers were grouped into the six scales (Attractiveness, Perspicuity, Efficiency, Dependability, Stimulation, and Novelty) and averaged in order to derive the value associated.