[Three-dimensional finite element analysis of traumatic mechanism of mandibular symphyseal fracture combined with bilateral intracapsular condylar fractures].

Objective: To analyze the biomechanical mechanism of mandibular symphyseal fracture combined with bilateral intracapsular condylar fractures using finite element analysis (FEA).

Methods: Maxillofacial CT scans and temporomandibular joint (TMJ) MRI were performed on a young male with normal mandible, no wisdom teeth and no history of TMJ diseases. The three-dimensional finite element model of mandible was established by Mimics and ANSYS based on the CT and MRI data. The stress distributions of mandible with different angles of traumatic loads applied on the symphyseal region were analyzed. Besides, two models with or without disc, two working conditions in occlusal or non-occlusal status were established, respectively, and the differences of stress distribution between them were compared.

Results: A three-dimensional finite element model of mandible including TMJ was established successfully with the geometry and mechanical properties to reproduce a normal mandibular structure. Following a blow to the mandibular symphysis with different angles, stress concentration areas were mainly located at condyle, anterior border of ramus and symphyseal region under all conditions. The maximum equivalent stress always appeared on condylar articular surface. As the angle between the external force and the horizontal plane gradually increased from 0° to 60°, the stress on the mandible gradually concentrated to symphysis and bilateral condyle. However, when the angle between the external force and the horizontal plane exceeded 60°, the stress tended to disperse to other parts of the mandible. Compared with the condition without simulating the disc, the stress distribution of articular surface and condylar neck decreased significantly when the disc was present. Compared with non-occlusal status, the stress on the mandible in occlusal status mainly distributed on the occlusal surface, and no stress concentration was found in other parts of the mandible.

Conclusion: When the direction of external force is 60° from the horizontal plane, the stress distribution mainly concentrates on symphyseal region and bilateral condylar surface, which explains the occurrence of symphyseal fracture and intracapsular condylar fracture. The stress distribution of condyle (including articular surface and condylar neck) decreases significantly in the presence of arti-cular disc and in stable occlusal status when mandibular symphysis is under traumatic force.