Some patients with chin trauma have this experience: after a fall and an impact on the chin, they thought it was just a chin injury, but after an examination in the hospital, they found that the temporomandibular joint (the joint that enables the jaw to open and close the mouth, located in front of the ear screen on both sides) on one side had actually fractured. Some patients cannot figure out why only the chin was hit, but the TMJ, which is far away from the impact site, was also fractured. The main reason for this is the special structure of the TMJ. The TMJ consists of the articular fossa, the articular disc and the condyle, of which the condyle is located at the end of the ascending branch of the mandible. The area where the condyle joins the mandible is called the condylar neck, and this structure is very weak. The articular end of the condyle is enlarged and forms the condylar head. When the mandible is struck by an external force on the chin, the mandible is displaced backward and the final impact is concentrated on the temporomandibular joint at the posterior end of the mandible, eventually leading to a fracture of the relatively weak condylar neck. Although the temporomandibular joint can fracture due to chin trauma, when this secondary injury actually occurs it is a protective mechanism. It is called a protective mechanism because without the cushioning of the TMJ against the impact force, the impact force may continue to be transmitted backward to damage the bones at the base of the middle cranial fossa, leading to severe craniocerebral injury.