## 4 Discussion  
This study showed that tailor-made mandibular resection, carefully planned three-dimensionally using multimodal image information, is a safe and less invasive method than conventional resection. Although malignant tumors were included in this study, all patients showed a disease-free bone resection margin in histopathology, and none showed local recurrence. In the case of malignant lesions, the combined use of CT and MRI information helped us to accurately determine the location and size of the lesions. Many studies have suggested that using multimodal imaging (CT, cone-beam CT, MRI, (18)F-fluoro-2-deoxy-\(D\)-glucose PET, and PET/CT) together could improve accuracy of the diagnosis of bone invasion [10; 11; 12; 13]. Particularly in radiation oncology, obtaining information from various images contributed to the successful results of radiotherapy [14]. Diagnosis and treatment based on various images have been also applied to a craniomaxillofacial surgical field with the development of CAS based on 3D images. The clinical application of this method not only diagnoses bone invasion but it also aids in surgery, as the size, location of diseases, and relationship of surrounding tissues are directly visualized in three dimensions [9; 15]. In our study, all simulation procedures including determination of surgical margins were performed by an operator under confirmation of head and neck radiologists. This was possible because CAS was effective to communicate the disease-extension and surgical margin with the radiologist through the three-dimensional image. Our study showed good results similar to those in a previous study [9]. One patient died of distant metastasis, but in this case, there was a positive margin in the soft tissue of the non-resectable area (near the skull base), but the bone margin was disease-free in histopathology. In our study, 3D lesions were extracted from MRI images and superimposed on 3D CT images for surgical simulation. Additionally, in the case of tumors, the 3D tumors were uniformly increased to 10 mm. This procedure made it easy to set the safety margins of the disease and to design various types of osteotomies. Many studies have emphasized the advantages of using CAS to save time, ensure accurate results, and obtain aesthetic results, but no study has investigated the benefits of preserving anatomical structures using various types of osteotomies [2; 4; 5; 6; 7; 16]  
Bone volume calculation in three-dimensional images is a commonly used method for the evaluation of surgical results [17]. In this study, the volume and surface of the remaining mandibular bone after mandibular resection were analyzed in conventional resection simulation and tailor-made resection simulation, and the results were statistically significant (Table 3). In addition, the lower dentition was partially preserved in six of 15 patients, and in five of those, we were able to preserve aesthetic and functionally important mandibular canines. Although many studies have attempted to restore and preserve teeth through digital technologies [18; 19; 20; 21], no attempt has been made to preserve teeth when performing segmental mandibularctomy. In this study, we proposed a method for preserving teeth even after segmental mandibularectomy. Mandibular sympphysis is an important anatomical structure as well. The extrinsic tongue muscles are attached to the inner surface of the symphysis, and airway obstruction can occur if this area is resected. To prevent this event, tracheostomy becomes one of the routine procedures. In our study, there were changes in the mandibular border defect classification in 12 patients, and in six of these patients, we were able to preserve the symphysis.