| The maxillofacial region is an exposed part of the body and a common location of firearm wounds, including both self-inflicted injuries and injuries caused by others. The firearm wounds of the maxillofacial region have a much higher ratio than the other parts of human body. So, it is very important to focus on trauma salvage and treatment of maxillofacial firearm wounds. The traditional injury models of firearm wounds include animal models, human cadaver models, and models that are made of artificial materials. However, the applicability of animals and human cadavers has been greatly limited due to poor reproducibility and current medical ethics concerns. The artificial material models possess biomechanical properties that are similar to human tissues, but they have high production costs and long production times. Therefore, it is desirable to develop an improved injury model using novel methods.The Finite Element Method (FEM) is a mathematical method that has been widely used in engineering science and technology. The principle of FEM is that a whole structure is assembled by many finite tiny elements, and the whole mechanical characters of structure are contributed by every elements. FEM has become an effective analytical tool for solving complicated engineering problems and widely used in different areas of biomechanics research.FEM can analysis the complicated mechanical processes among or inside the body and predict the effect of mechanical action (just like the change of shape, stress, strain, temperature and so on). It may offer the possibility to precisely measure the amount of energy or force and display the result. Moreover, FEM has the advantages of good repeatability, low cost and manageable load conditions, which makes up for the insufficiency of other firearm wounds models. Therefore, FEM will conduce to deeply researching in vulnerate mechanism of firearm wounds in the maxillofacial region.Based on our previous three-dimensional finite element (3D FE) model of firearm wounds in the pig mandible and the internal parameters of the model, we established a 3D FE model of firearm wounds to the maxilla and mandible of Chinese Visible Human (CVH). By using this model, we dynamically simulate the mechanical process of maxilla and mandibular firearm wounds in different injury conditions and investigate the preliminary biomechanical finite element analysis of the progress.Methods and Results:1. Computed Tomography (CT) data of Chinese Visible Human maxilla and mandible was imported into MIMICS software, where the three-dimension computer-aided designing surface mesh models of human maxilla and mandible were reconstituted. These surface-meshed models of the maxilla and mandible were then imported into ANSA software for automatic net-generation and then the combined hexahedral, pentahedral and tetrahedral element FE models of human maxilla and mandible were established. The final model of the maxilla was meshed with 372,501 entity elements and 751,410 nodes; the final model of the mandible was meshed with 275,216 entity elements and 1,387,101 nodes. The results showed that the FE models posessed reasonable partitioning meshes and good-quality elements, and the FE models were similar to the human anatomical structure.2. Based on our previous 3D FE model of firearm wounds in the pig mandible, we selected compatible material model, internal parameters, boundary condition and algorithm of contact in LS-DYNA software. After computation of simulation, the established 3D FE model of firearm wounds to the human mandible dynamically simulated the mechanism of mandibular firearm wounds and the stress distributions in its various regions under different injury conditions. The results showed that hexahedral elements were used almost exclusively for the mandible 3D FE model to obtain greater computational precision and meet the need of research in different injury conditions. The finite element model can dynamically simulate the injury mechanisms of firearm wounds to the human mandible. By comparing and analyzing the related data, the injury severity of the mandible and the injury efficiency of the projectiles could be investigated under different injury conditions. Moreover, we can also dynamically simulate the process of the stress spreading and the stress distribution patterns in each region of the mandible affected by the firearm wounds.3. Just like the 3D FE model of human mandible, we seted the same material model, internal parameters, algorithm of contact and compatible boundary condition in LS-DYNA software. After computation of simulation, the established 3D FE model of firearm wounds to the human maxilla dynamically simulated the process of maxillary firearm wounds and the stress distributions in its various regions using bullet or steel ball shots with varying velocities and entry position. The results showed that hexahedral, pentahedral and tetrahedral elements were used for the maxilla 3D FE model to obtain better computational efficiency and computational precision, cutting down the cost of model establishment and the time of computation. The finite element model can dynamically simulate the injury process of firearm wounds to the human maxilla. The injury severity of the maxilla could be investigated under different injury conditions. Moreover, we can also dynamically simulate the process of the stress spreading and the stress distribution patterns in each region of the maxilla and bases of skull affected by the firearm wounds.Conclusions:1. The 3D FE models for firearm wounds in the human maxilla and mandible were successfully established. The FE models of the human maxilla and mandible were similar to the real anatomical structure and had perfect meshes and elements. The quality of the mashes and elements had reached the level and the size of which were uniformity.The combined 3D FE models for firearm wounds in the human maxilla and mandible can meet the demand of dynamic simulation for biomechanical research.2. When the projectile's entry angle limited to the range of 45°to 90°and the entrance points were in angle of mandible and in centric position of mental region, we can draw the following conclusions: 1). The smaller the entry angle was, the more severe injury was caused to the mandible and the higher injury efficiency was caused by the projectile. 2). The higher the impact velocity was, the more severe the injury was caused to the mandible but the lower the injury efficiency was caused by the projectile. 3). The injury efficiency of steel ball was much higher than that of bullet. However, the injury severity to mandible of bullet was higher than that of steel ball in most of the cases. If the entry angle was close to 90°, as the impact velocity increased, the injury severity to mandible of steel ball might be higher than that of bullet. 4). When the entrance point was in angle of mandible, the injury severity to mandible and the injury efficiency of projectiles were both higher than that of centric position of mental region. 5). When the mandible was shot by projectiles, the fracture of neck of condyle in shooting side was more common than other regions. 6). The nearer the region to the wound tract of the mandible, the more stress distributed to it.3. When the projectile's entry angle was perpendicular to the sagittal plane and the two entrance points were in lateral side of maxilla, we can draw the following conclusions: 1). When the entrance point was in the forward place of maxilla, the injury severity to maxilla and the injury efficiency of projectiles were both higher than that of the backward place of maxilla. 2). The higher the impact velocity was, the more severe the injury was caused to the maxilla but the lower the injury efficiency was caused by the projectile. 3). The injury severity to maxilla of bullet was higher than that of steel ball, but the injury efficiency of steel ball were higher than that of bullet. 4). When the initial kinetic energies of the projectiles was the same, the injury severity to maxilla and the injury efficiency of steel ball were higher than those of bullet. 5). The nearer the region to the wound tract of the maxilla, the more stress distributed to it. 6). The injury severity of brain associated with firearm wounds in maxilla was related to the entry point and the nature of projectiles, but not related to the impact velocity.4?The 3D FE models could be used in investigation of biomechanical mechanism of firearm wounds in the maxillofacial region. When concerned with the investigation of biomechanical mechanism of wound ballistics, FEM can make up for the insufficiency of other traditional firearm wound models. |
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