## 7 Conclusions  
Three-dimensional medical images can be manipulated in VR environments for medical surgery planning. During the planning process, one needs to interact with the 3D objects and be able to make markings of high accuracy on them. We evaluated the feasibility of three different VR interaction methods: a mouse, hands, and a controller+stylus combination in virtual reality. Based on the results, we can say that the Valve Index controller and Logitech VR link stylus combination was the most feasible for tasks that require both 3D object manipulation and high marking accuracy in VR. This combination did not have issues with complex 3D objects and sensor fidelity was better than with hands interaction. Statistically significant differences were found between the controller combination and the other methods.  
Hand-based interaction was the least feasible for this kind of use. The hands and mouse methods were evaluated almost equal in feasibility by participants. Free hands usage cannot be proposed for accurate marking tasks. Mouse interaction was more accurate than controller+stylus. In detailed tasks, the mouse could replace the free hands interaction. The discrepancy between the 2D mouse and the 3D environment needs to be solved before the mouse could be considered a viable interaction method in VR.  
The results show that the users approve the easiness and fluency of the use of the controller+stylus, because of the tracking accuracy and natural interactions, such as being able to grab the object and point at it with the stylus. As sensing technologies are improving, in the future, we expect that new interaction methods will reach the same level of \"naturalness\" and become acceptable.  
Conceptualization, H.-R.R., J.K., H.M., R.R., and S.K.K.; methodology, H.-R.R. and J.K.; software, S.K.K.; validation, H.-R.R., J.K., and S.K.K.; formal analysis, H.-R.R.; investigation, H.-R.R. and J.K.; resources, R.R.; data curation, H.-R.R. and J.K.; writing--original draft preparation, H.-R.R. and J.K.; writing--review and editing, H.-R.R., J.K., S.K.K., H.M., J.J., and R.R.; visualization, J.K.; supervision, R.R.; project administration, R.R.; funding acquisition, R.R. and J.J. All authors have read and agreed to the published version of the manuscript.  
This work has been funded by Business Finland, project Digital and Physical Immersion in Radiology and Surgery (decision number 930/31/2019) and by the Academy of Finland, project Explainable AI Technologies for Segmenting 3D Imaging Data (decision number 345448).  
This study was conducted according to the guidelines of the Declaration of Helsinki. Ethical review and approval were waived for this study, due to the nature of the experimental tasks and the participant population.  
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.  
The data are available upon request.  
The authors wish to thank the dentomaxillofacial radiologists that provided the knowledge of their specialty.  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
2D two-dimensional  
3D three-dimensional  
CBCT cone beam computed tomography  
HMD head-mounted display  
VR virtual reality