Re-evaluation of the biomechanics of blunt impact injury of the head.

With blunt impacts, there may be bruising or tearing of scalp, elastic deformation of the skull with or without linear fracture, mass motions or relative movements of the brain mass--inertial stress propagation--as well as pressure gradients. Mass motions of the brain may cause concussion, vascular tears with extravasation and contusions of the cerebral surface as well as lesions in the deeper portions of the cerebral mass. Coup contusion is caused by the inbending bone at the impact site striking the immobile head. Contrecoup contusiions result from injury to the brain surface in the frontotemporal area as the moving brain impacts the rough and irregular bony interior. In the experimental laboratory, a contusion is easily produced by a moving object striking the relatively immobile head. During impact if the head is on a solid stand, such as the metal table, a contrecoup lesion is seen at the opposite end from the impact site. This contrecoup lesion is produced by the brain impacting the skull, as well as the flattening of the skull against the brain at the antipole, from the presence of the solid table. Intermediate contusions develop between the impact site and the antipole in the brain interior as a result of inertial stress propagation.