## 1 Introduction
Studying the use of VR in cranioxillofacial (CMF) CASS is interesting because current surgical planning is still carried out on two-dimensional (2D) screens, but VR has been shown to have advantages over 2D in terms of perception and visualization [28; 29; 30]. Orthopathic surgery is a type of surgical procedure that is used to correct abnormalities or defects in the facial skeleton [31]. These abnormalities may include misaligned jaws, incorrect bite, and other problems that affect the appearance and function of the face. Having a precise pre-surgical plan is essential when performing surgical treatment of dentofacial deformities, as it helps to ensure that the procedure is successful and that the desired results are achieved [32; 33]. In this experiment, to simulate a simplified mandibular bilateral sagittal split osteotomy (BSSO) operation, a scenario was created where a radiologist would make a plan together with a surgeon who is in charge of the final surgical operation. In this operation, the lower jaw is split sagittally in its posterior area [31]. An orthognathic surgery was chosen as a practical example of a collaborative VR procedure because the use of CASS is widely accepted in this field and has shown excellent results in previous studies, showing increased efficiency and more accurate osteotomies than with conventional planning methods [34]. CASS for Orthognathic Surgery is the three-dimensional (3D) integration of bone and soft tissue analysis into the execution of surgical movements to achieve a dental and skeletal harmony of tissues. The result is then transferred from the virtual environment to the surgical setting [35]. [PERSON] and [PERSON] [36] collated a review article about the use of VR in oral and maxillofacial surgery. [PERSON] et al. [37] presented a use case back in 2001. There are also several other types of cranioxillofacial surgery where CASS is increasingly being used, such as cancer surgery and including reconstructions, major trauma reconstructions, and craniosynostosis surgery. The main goal in all of these is to improve surgical accuracy, functional and morphological outcomes, and to reduce operating time by using patient-specific cutting guides and implants. Finally, CASS can also reduce costs, and the use of VR could make the visualization of anatomical structures and the final surgical outcome more accurate than a 2D approach. In earlier studies [16; 17; 18; 19], collaboration in the VR system was shown to improve performance. The study concentrated on synchronous collaboration between two participants, similar to [PERSON] and [PERSON] [38]. The two parties would make their contributions on the task simultaneously. As demonstrated by [PERSON] et al. [12], the interaction between the participants is important. In the experiment, participants were able to comment on the plan and suggest changes in a truly interactive way, and they were able to reach an agreement on the plan. The objective of the experiment was to pilot test the functionality of the collaborative VR in the operation design and, in particular, to collect the subjective experience of collaboration between experienced participants. Our contribution through the study was to show that medical professionals appreciated the capabilities of the collaborative VR environment and would be interested in using a similar system in real operations.