## 5 Discussion  
Previous studies in VR collaboration [12] have shown that immersion and interactivity between collaborators are important factors in effective VR collaboration. From the participant behavior, one could observe that the system was sufficiently immersive and enabled effective collaboration between participants by providing comments, discussions, pointing out places, etc. The participants saw the benefits of everybody seeing the same 3D view of the medical image and being able to interactively agree on the details of the plan. Also, in the given task, the cognition of spatial arrangements was seen as being important for the planning task and it has been shown that VR collaboration is an effective tool for this [16; 17; 18]. Participants were generally satisfied with what the system offered, but there were small differences between the participants in different roles in the experiment. The \"radiologists\" gave slightly more positive evaluations than the \"surgeons\", which might be related to the less active role of the \"surgeons\" as their experience was probably less immersive. [PERSON] et al. [13] previously reported similar results. Those two participants with the most extensive planning experience gave the highest subjective evaluation values. This may have been because they were more likely to see the benefits of using a 3D environment, such as the easier perception of 3D structures. With less experience of the 2D planning process, the benefits of the prototype system were probably less obvious and the problems related to the user interface were more prevalent for these users. It was found that the interface was complicated, which affected the use of the interface and caused participants to ask for advice during the experiment. The main reason for this was the combination of the number of different functions needed in the complex procedure and the limited time to practice the interface. The complexity of the user interface was somewhat inevitable because the surgical planning task requires the user to perform many different functions. In professional use, the users are usually willing to spend a considerable amount of time training on user interfaces to master them. Still, it was recognized that the interaction design can be further improved in follow-up studies. Most of the experiment participants approved the prototype virtual reality system and expressed interest in using a similar implementation in real orthognathic-surgical operation planning cases. They saw the benefits of everybody seeing the same 3D view of the medical image and being able to interactively agree on the details of the plan. The main limitation of the study was that only four participants were used in the experiment, which inevitably introduces statistical uncertainty into the results. The reason for this choice was to collect relevant comments on the VR system, so the participants had tobe highly qualified experts in the field, which inevitably limited their number. Nevertheless, it is believed that this small cohort is still representative of potential end users. Also, as this was the first time the VR system was used in simulated surgical planning tasks, not all operations were as smooth as they would need to be in an operational system. This is inevitable in user-centered research and development, but will improve through later iterations of the systems. The \"surgeon\" who voted 1 for \"easiness\" did not elaborate on the reasons for their evaluation but considered the system too complicated. They were also less experienced with VR devices, which may have affected the given evaluation. When planning future research activities, the functionality of the virtual reality system should be limited to only those functions that are strictly necessary for the intended tasks, in order to keep the complexity of the system under control. This makes the experimental setup less real, but helps focus on the functions studied in specific experiments.