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.
THIS PAPER is concerned with the effects of translational and rotational acceleration on the brain in closed head injuries.
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 translational acceleration. If the impact is directed eccentrically, the result is a combined translational and rotational acceleration. 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.
The inertia effect is fundamental for all injuries caused by mechanical forces. The effect can be studied on a rigid, freely movable spherical shell filled with an incompressible, nonviscid and homogeneous fluid.
After the central impact, the shell moves only in translation, carrying with itself the contained liquid. While the blow lasts and the sphere moves, its contents are compressed at the impact side in consequence of the inertia of the liquid particles but are rarefied (expanded) at the side opposite to the impact pole (the “counterpole”). This is the basic pressure distribution picture due to an acceleration motion of the skull. Of major importance is the dependence of the pressure distribution on the time variation and duration of the blow. If the impact is not centrally directed, the skull receives an angular momentum in addition to the acceleration. The farther from the center of rotation the particle of interest is, the greater the acceleration suffered, and thus the greater the potential for injury. Therefore, points near the center of rotation are likely to be damaged only if the acceleration is severe. Rotational traumas involve considerable tensile forces between the accelerated skull and the inert brain.