## 1 Introduction  
Recently, aging and low birth rates have become a serious problem in developed countries. With demographic and various social changes, such as the greater number of older persons living alone and leading an active retirement, older persons may be at increased risk for trauma, including maxillofacial fractures. Because of their diminished cognitive and motor skills, poor balance, slow reflexes, and weakened eyesight, falling has become the predominant cause of unintentional injury in Japan. Falling is also a major cause of oral and maxillofacial fractures. According to a study of older patients aged 60 years or more, who were admitted to 14 maxillofacial surgery departments worldwide, the leading cause of maxillofacial fracture was falling, accounting for 66.4% [1]. Another study for older adults also suggested that the most common cause of maxillofacial fracture was falling [2]. [PERSON] et al. indicated that falling-related maxillofacial fractures on a level surface occurred more frequently in older adults because the ability to avoid environmental hazards was reduced by age-related changes [3]. Maxillofacial fractures result in severe morbidity, loss of function, and disfigurement. Subsequent long hospital stays and disability can influence the quality of life of a patient. According to a study of bicyclists and motorcyclists, the number of fracture lines and intermaximillary fixation were independent factors for long-term hospitalization [4]. In addition, a study of patients who had fallen suggested that the number of injured teeth and intermaximillary fixation were independent predictive factors for a treatment duration of more than 1 month [5]. Therefore, preventing maxillofacial fracture and the use of intermaximillary fixation are required to maintain the daily activities of patients and maintain a reasonable quality of life. The mandible is the most common site of all maxillofacial fractures. A previous study reported the most common fracture site of older adults was the mandible (56.7%), followed by the middle face (36.4%), and the mandible and middle face (16.9%) [5]. Particularly for patients who had fallen, this trend was dominant and accounted for 70.6% to 79.7% of all cases of falling [5; 6; 7]. The anterior region of the mandible was reported to be the most frequent site impacted by falling due to its exposed position in the facial skeleton [5; 6; 7; 8]. Therefore, the mechanism of mandibular fracture caused by anterior impact has been analyzed [9]. In real-world injuries, substantial numbers of persons fall laterally, and the lateral region of the face can also be impacted. A retrospective analysis of 185 mandibular fractures resulting from falling in 109 patients showed that the condyle was the most common fractured site (92 cases), followed by the symphysis (46 cases), body (21 cases), angle (20 cases), ramus (5 cases), and coronoid process (1 case) [10]. In biomechanical research regarding lateral impacts, [PERSON] et al. applied a 2 kN force to the angle of the mandibular body and investigated the stress distribution using a finite element (FE) model [11]. In the presence of a third molar, the ipsilateral mandibular angle showed a higher risk of fracture than the condyle. In a study by [PERSON] et al., a similar force was applied to the right body of the mandible, and the effects were studied using an FE model. When a third molar was absent, the stress was evenly distributed across the internal and external oblique lines of the mandible, which reduced the risk of mandibular angle fracture [12]. [PERSON] et al. applied a force corresponding to the fall of the head from a height of 2 m on the lateral region of the mandible using an FE model [13] and found that the condyle region was susceptible to fracture. However, no studies have investigated the mechanism of mandibular fracture caused by a lateral fall using a whole-body simulation.