## 3 Results  
To evaluate PDApp based on operation in a real environment, the software was installed at the dental clinic of the Denistry School of Medicine and Odontology of the University of Santiago de Compostela (Spain) in February 2021. Dental experts then used PDApp in their daily work to analyze images, which required that they manually draw the retromolar space and the third molar diameter and angle. PDApp then automatically classifies the third molars in radiological images as erupted or retained. Their operations using PDApp were logged into XML files for later statistical evaluation of the machine learning algorithm (SVM classifier) incorporated in the software. We used accuracy (rate of third molars correctly classified by the software) as a measure of the classifier performance. As mentioned, the classifier included in PDApp was initially trained using a dataset of 188 molars. Thereafter, PDApp was used to classify 539 third molars from 289 patients, and 97.96% of third molars were correctly classified. Figure 3 shows an example of correct classification of the third molar by PDApp as retained. Figure 4 shows a case of a third molar that is classified as erupted by PDApp.  
The software is reliable and precise because it allows reviewing the classification of mandibular third molars. Figure 5, image (a) shows a case of a third molar that is classified as erupted by PDApp and image (b) a posterior panoramic radiograph images confirms the PDApp results. It is also easy to use and install. Another advantage that the program offers compared with manual quantification is the possibility of sharing the images and results among other experts to evaluate each case, reducing the variability among experts.  
Figure 3: Visual example showing two third molars analyzed, in which the PDApp prediction for both cases is correct as a retained tooth. The pink line represents third molar diameter, the yellow line represents retromolar space, and blue lines represent the angle of eruption. The lateral panel shows the measures for the left third molar (violet lines on the image represents the measurements realized).  
The perception of users regarding PDApp was evaluated using the system usability scale (SUS), a free questionnaire used to measure the learnability and subjective perceived usability of computer systems [26; 27]. This is a questionnaire with the following 10 items:  
1. I think that I would like to use this product frequently.
2. I found the product unnecessarily complex.
3. I thought the product was easy to use.
4. I think that I would need the support of a technical person to be able to use this product.
5. I found the various functions in the product were well integrated.
6. I thought there was too much inconsistency in the product.
7. I imagine that most people would learn to use this product very quickly.
8. I found this product very awkward to use.
9. I felt very confident using the product.
10. I needed to learn a lot of things before I could get going with this product.  
The items are scored on a 5-point scale ranging from 1 (strongly disagree) to 5 (strongly agree), resulting in a final system score between 0 and 100. A comprehensive interpretation of SUS scores [28] is as follows: <25 indicates the worst imaginable system; 25 to 39 indicates the worst imaginable to poor; 40 to 52 indicates poor to acceptable; 53 to 73 indicates acceptable to good; 74 to 85 indicates good to excellent; and above 85 indicates excellent to the best imaginable system. A small sample of 8 to 12 users is sufficient to give a robust assessment of how users perceive the software. The SUS questionnaire to evaluate users' perception of PDApp was filled out by eight dental experts and achieved a mean score of 100, indicating that the system is the best imaginable.  
Figure 4: Visual example of a third molar erupted, predicted correctly by PDApp. The pink line represents third molar diameter, the yellow line represents retromolar space, and blue lines represent the angle of eruption.