Bone segment navigation is a surgical method used to find the anatomical position of displaced bone fragments in fractures, or to position surgically created fragments in craniofacial surgery. Such fragments are later fixed in position by osteosynthesis. It has been developed for use in craniofacial and oral and maxillofacial surgery. After an accident or injury, a fracture can be produced and the resulting bony fragments can be displaced. In the oral and maxillofacial area, such a displacement could have a major effect both on facial aesthetics and organ function: a fracture occurring in a bone that delimits the orbit can lead to diplopia; a mandibular fracture can induce significant modifications of the dental occlusion; in the same manner, a skull (neurocranium) fracture can produce an increased intracranial pressure. In severe congenital malformations of the facial skeleton surgical creation of usually multiple bone segments is required with precise movement of these segments to produce a more normal face. Surgical planning and surgical simulation An osteotomy is a surgical intervention that consists of cutting through bone and repositioning the resulting fragments in the correct anatomical place. To insure optimal repositioning of the bony structures by osteotomy, the intervention can be planned in advance and simulated. The surgical simulation is a key factor in reducing the actual operating time. Often, during this kind of operation, the surgical access to the bone segments is very limited by the presence of the soft tissues: muscles, fat tissue and skin - thus, the correct anatomical repositioning is very difficult to assess, or even impossible. Preoperative planning and simulation on models of the bare bony structures can be done. An alternate strategy is to plan the procedure entirely on a CT scan generated model and output the movement specifications purely numerically. Materials and devices needed for preoperative planning and simulation The osteotomies performed in orthognathic surgery are classically planned on cast models of the tooth-bearing jaws, fixed in an articulator. For edentulous patients, the surgical planning may be made by using stereolithographic models. These tridimensional models are then cut along the planned osteotomy line, slid and fixed in the new position. Since the 1990s, modern techniques of presurgical planning were developed – allowing the surgeon to plan and simulate the osteotomy in a virtual environment, based on a preoperative CT or MRI; this procedure reduces the costs and the duration of creating, positioning, cutting, repositioning and refixing