Rotational Head Injuries

Current helmet standards are based around linear acceleration, or how rapidly the head changes velocity along a straight line. These measurements have been well correlated to catastrophic head injury (see our previous post, The Science of Impact Mitigation for more information). However, numerous studies now point to the influence of rotational movements, and how angular acceleration can impart higher strains on brain tissue. If you have a snow globe, you can see this effect fairly easily by moving it back and forth in a linear motion (very little activity), then giving it a twist or rotation (resulting in LOTS of activity). 
 

Can Helmets Help This?

There are several schools of thought regarding how helmet design changes may or may not be able to improve protection against rotational impacts. One is that head rotations are largely constrained by the neck, in such a way that rotational impacts are usually the result of a linear impulse (e.g. a hit to the front of the head causes it to rotate back), and therefore improvements in linear acceleration mitigation will inherently improve rotational performance. However, this may not always be the case, as improving linear impact performance is easily done by increasing the helmet thickness and overall size, which could create a larger momentum arm, exacerbating angular accelerations when impacts occur off center.

Progress is continually being made in the understanding of brain injury mechanisms, but there is still a lack of an established rotational injury threshold to use in developing improved helmets. There is also a lack of established test method. Helmets are tested on instrumented surrogate heads (“headforms”), but there are debates on whether or not to attach the headform to a neck surrogate for rotational testing (do the accelerations occur so early in impact events that the neck doesn’t come into play?), or if using a neck surrogate, has its response been validated for the scenario being tested? 
 

Future Plans

While there are currently more questions than answers around rotational impacts and helmet design, we are continually working on advancing our understanding of how materials and helmet design effect impact kinematics, both in terms of current established standards, and new test methods yet to be implemented. In a future post we’ll cover the findings of one such study we completed on an aircrew helmet, and the types of changes in rotational impact response we found.