A crush injury of the larynx or cervical trachea can occur in head-on collisions when the neck is hyperextended and strikes the steering wheel or dashboard; this has been called a "dashboard injury". The larynx and cervical trachea may also be injured in front-on collisions by the seat belt.Although the mechanism is not well understood, TBI due to blunt trauma is widely thought to be caused by any combination of three possible mechanisms: an increase in pressure within the airways, shearing, and pulling apart. The first type of injury, sometimes called an "explosive rupture", may occur when the chest is violently compressed, for example when a driver strikes the steering wheel in a vehicle accident or when the chest is crushed. The pressure in the airways, especially the larger airways (the trachea and bronchi), quickly rises as a result of the compression, because the glottis reflexively closes off the airways. When this pressure exceeds the elasticity of the tissues, they burst; thus the membranous part of the trachea is more commonly affected by this mechanism of injury than cartilaginous portions.The second mechanism may occur when the chest is suddenly decelerated, as occurs in vehicle accidents, producing a shearing force. The lungs are mobile in the chest cavity but their movement is more restricted near the hilum.
If a bronchus is torn all the way around, the lung may collapse outward toward the chest wall (rather than inward, as it usually does in pneumothorax) because it loses the attachment to the bronchus which normally holds it toward the center. In a person lying face-up, the lung collapses toward the diaphragm and the back. This sign, described in 1969, is called fallen lung sign and is pathognomonic of TBI (that is, it is diagnostic for TBI because it does not occur in other conditions); however it occurs only rarely. In as many as one in five cases, people with blunt trauma and TBI have no signs of the injury on chest X-ray. CT scanning detects over 90% of TBI resulting from blunt trauma, but neither X-ray nor CT are a replacement for bronchoscopy.At least 30% of TBI are not discovered at first; this number may be as high as 50%. In about 10% of cases, TBI has no specific signs either clinically or on chest radiography, and its detection may be further complicated by concurrent injuries, since TBI tends to occur after high-energy accidents. Weeks or months may go by before the injury is diagnosed, even though the injury is better known than it was in the past. Classification
Lesions can be transverse, occurring between the rings of the trachea, longitudinal or spiral. They may occur along the membranous part of the trachea, the main bronchi, or both.