## 1 Introduction  
Surgery has changed significantly over the years and continues to evolve with the introduction of newer technologies and techniques. There is an increasing interest in the use of virtual, augmented and mixed reality approaches to support different medical areas, from medical teaching [1] to intra-operative guidance [2]. Recently, Mixed Reality (MR) has acquired a key role in pre-, intra-, and post-operative medical procedures [3]. MR merges both Virtual Reality (VR) and Augmented Reality (AR) to produce new environments where real and virtual objects co-exist and interact in real time [4]. This provides incredible advantages in medical applications: MR gives new ways to interact with patients [5], to accelerate diagnosis [6], to reduce intra-operative time [7], to enable treatment personalization [8] and, more in general, to improve procedure outcomes [9]. MR systems are currently used in different surgical disciplines [10]. [PERSON] et al. [11] demonstrated the clinical feasibility of a wearable Mixed Reality device for preoperative neurosurgical planning using 3D hologram reconstructions of brain tumors, achieving more precise tumor localization than the standard neuronavigation system. [PERSON] et al. [5] defined MR as the most functional technology for the orthopedic surgeon. In fact, it allows more freedom of control over the CT reconstructions for preoperative planning and intra-operative visualization. In a study by [PERSON] et al. [12], the surgeons performed a standard reverse shoulder arthroplasty aided by the MR system, remotely connected with other surgeons in the United States and the United Kingdom. Post-operative CT evaluation demonstrated proper positioning of the prosthesis, and the patient was without complications on a clinical visit 45 days after the procedure. MR systems are used also in oral and maxillofacial surgery. This clinical specialty treats diseases, injuries and defects in the area of the face, mouth and jaws [13]. Since 1992, fibula-free flaps have been used to reconstruct mandibular and maxilla defects greater than 4-6 cm [14]. However, achieving a natural and functional mandibular construction requires a precise fit between the native mandible and the modeled fibular segments, because this influences facial support, occlusal function, and masticatory movements [15]. Existing methods use customized surgical guides to remove fibula flaps to ensure a secure and precise cut. The use of guides provides good functional outcomes and correct occlusion, reducing operation time [16]. Since a guide positioning error implies an error in fibula segment shape, inaccurate guide positioning can compromise the overall outcome of the mandibular reconstruction [17]. [PERSON] et al. [18] review the current literature regarding the clinical accuracy of guided surgery and analyzed the involved clinical factors. From 1951 to 2016, fourteen case studies were analyzed and they showed that the position of guide, guide fixation, type of guide, and flap approach could influence the accuracy of computer-aided implant surgery. Moreover, a totally guided system using fixation screws with a flapless protocol demonstrated the greatest accuracy. [PERSON] et al. [19] analyzed the advantages of virtual surgical planning using customized devices. Fourteen patients undergoing maxilla reconstruction were enrolled in their study and, in all cases, flaps fitted into the defects precisely and no bone grafts were needed. They demonstrated that preoperative virtual planning enables accurate and reliable customized cutting guide positioning with better operative results. [PERSON] et al. [20] developed a novel fibula malleolus cap to overcome the sliding and rotational errors during fibula flap harvesting for oncologic jaw reconstruction, with increased accuracy in simultaneous dental implants. The use of this method significantly reduced the deviations in locations and angles of distal fibula osteotomies and increased the accuracy of implant platform locations.