Any broadly surfaced impact imparts to the head a force by which it is accelerated. If the impact is directed at the center of mass of a freely movable object, the resulting motion is a translation acceleration. If the impact is directed eccentrically, the result is a combined translational and rotational acceleration. The magnitude of the rotational acceleration is related to the degree of eccentricity of the acting force. The magnitude of the translational acceleration is related to the distance between the point of fixation and the center of gravity of the head.
The distinction between the two types of acceleration is important in view of the different physical processes they initiate in the brain. Pure translational acceleration creates pressure gradients, while pure rotational acceleration produces rotation of the skull relative to the brain. Both processes are the effects of mass inertia of the brain.
It can be expected, according to the physical analysis of translational and rotational trauma, that different mechanisms produce different patterns of lesions.
Experiments with different animal species, which employed translational and rotational accelerations with exactly measured inputs are summarized and the morphological alterations in respect to distribution and quality are discussed.