Why Most Traumatic Brain Injuries are Not Caused by Linear Acceleration but Skull Fractures are

Abstract

Injury statistics have found the most common accident situation to be an oblique impact. An oblique impact will give rise to both linear and rotational head kinematics. The human brain is most sensitive to rotational motion. The bulk modulus of brain tissue is roughly five to six orders of magnitude larger than the shear modulus so that for a given impact it tends to deform predominantly in shear. This gives a large sensitivity of the strain in the brain to rotational loading and a small sensitivity to linear kinematics. Therefore, rotational kinematics should be a better indicator of traumatic brain injury risk than linear acceleration. To illustrate the difference between radial and oblique impacts, perpendicular impacts through the center of gravity of the head and 45° oblique impacts were simulated. It is obvious that substantially higher strain levels in the brain are obtained for an oblique impact, compared to a corresponding perpendicular one, when impacted into the same padding using an identical impact velocity. It was also clearly illustrated that the radial impact causes substantially higher stresses in the skull with an associated higher risk of skull fractures, and traumatic brain injuries secondary to those.

Evaluation of the protective capacity of baseball helmets for concussive impacts

“…Rotational acceleration however, showed more significant differences between the helmet models which suggests that this metric might be the most sensitive to changes in helmet designs. This is also pertinent as rotational acceleration has often been suggested as an important contributor to the occurrence of concussion (Gennarelli et al, 1987;Kleiven, 2013;Lamy et al, 2013). The fact that maximum principal strain does not distinguish in many cases between the helmets might be a result of the combination of linear and rotational acceleration for these impacts causing a very similar amount of strain in the brain, and thus masking helmet designs that would reduce one type of motion over the other.…”

Protective Capacity of Ice Hockey Helmets against Different Impact Events

“…31,69,73 In nature, linear and rotational acceleration seldom exist independent of each other 21,50 and, as such, both forms of loading have been shown to contribute to head injury. 55 Skull fractures have been more commonly associated with linear acceleration 33,47 whereas rotational kinematics are more commonly associated with concussion and other forms of TBI. 17,21,56,74 Additionally, research examining the cause of concussion in the National Hockey League (NHL) reported concussive events due to falls account for only 7% of all concussions while collisions with an opponent accounted for 88%.…”

Head-Impact–Measurement Devices: A Systematic Review

“…24 These accelerations result in transient pressure gradients and strain fields within the soft tissue of the brain. 25 If the pressure gradients or strains exceed the tolerable limits of the brain tissue, injury occurs.…”

“…25 Whereas debate exists about whether resultant LA, resultant AA, or combined resultant LA and AA influence concussion risk, 32,33 they likely do not occur in isolation. 24 In addition to simple magnitudes, impact-severity measures quantify injury tolerance, and the original work in car impacts yielded the Wayne State Tolerance Curve (WSTC). 34 The objective of the WSTC was to inform protective material development by understanding the risk of skull fracture in moderate and severe TBI.…”