## 4 Discussion  
Several attempts have been made to achieve successful jaw reconstruction through spontaneous regeneration of the bone. However, this procedure is commonly recommended only for young patients, in whom regeneration can be achieved relatively easily [22; 25]. Therefore, the literature on R-plate reconstruction with spontaneous new bone formation in elderly patients is limited. In this retrospective study, the average patient age was 74.4 \(\pm\) 9.4 years. Nevertheless, all 14 patients showed new bone formation, indicating spontaneous regeneration, without use of graft materials. Our study consists of only the osteonecrosis on mandible, where the vascular supply is limited and somewhat dynamic compared to the maxilla. Thus, it is essential to maintain absolute immobilization in the early period of healing and obtain sufficient vascular supply for spontaneous bone healing in the mandible. Two cases with R-plate fracture occurrence might be mainly the fatigue fracture caused by the discrepancy between dynamic movement of the remaining mandibular structure and the static state of the R-plate. However, the plate fracture of both cases was found after the new bone formation was detected, indicating that the customized patient-specific R-plate reconstruction and postoperative intermaximillary fixation were sufficient to gain absolute immobilization in the early period of healing. Furthermore, the periosteum was preserved and repositioned to cover the inferior border of the mandible and R-plate in every case, which could also contribute to supplemental primary immobilization. Of the two cases of R-plate fractures, bony bridge formation was observed in one. The absence of bony bridge formation in the other may be attributed to an excessive occlusal force from the teeth opposing the surgery site [26]. It is thus critical to prevent distortion of the plate and unfavorable healing by alleviating the excessive occlusal load. Another crucial factor of spontaneous bone healing is to gain vascular supply. The preserved periosteum is essential as it can serve as a direct source of vascular supply and osteogenic cells, and can promote bone regeneration [22]. An absence of periosteum has been associated with unfavorable healing outcomes in previous studies [22; 27]. Preservation of the periosteum is impossible during mandiublectomy in cases of oral malignancy; thus, patients diagnosed with metastatic diseases were excluded. To eliminate the possible unfavorable effects of metastatic diseases on bone healing, patients taking antineoplastic or antiangiogenic drugs were also excluded. Preservation of the periosteum may have been the most important contributing factor for spontaneous new bone formation in our cases as all of them (100%) showed detectable new bone formation. Further, there were no cases of plate exposure in this study, which may also be attributed to the preservation of the periosteum and appropriate coverage by surrounding tissues such as platysma muscle, fibrous fatty tissue, and the skin. The site of the bony bridge formation is the most suitable for the rehabilitation of occlusion and mastication. The bony bridge also provides a newly formed alveolar process, enhancing the volume and strength at the site of plate reconstruction and preventing pathologic fractures from occlusive loads [28]. Therefore, when assessing new bone formation from the perspective of functional rehabilitation, bony bridge formation is an important clinical outcome factor. Most patients in the present study were female (84.6%), which is consistent with the findings of other studies on the incidence and prevalence of MRONJ. Previous studies have frequently cited patient gender (being female), age, and duration of medication intake (\(\geq\)4 years) as prognostic factors for the prevalence of MRONJ [29; 30]. Therefore, we used a cutoff of 4 years to categorize the duration of medication intake by patients.